1 ----------------------------------------------------------------------------- 2 This file contains a concatenation of the PCRE man pages, converted to plain 3 text format for ease of searching with a text editor, or for use on systems 4 that do not have a man page processor. The small individual files that give 5 synopses of each function in the library have not been included. Neither has 6 the pcredemo program. There are separate text files for the pcregrep and 7 pcretest commands. 8 ----------------------------------------------------------------------------- 9 10 11 PCRE(3) Library Functions Manual PCRE(3) 12 13 14 15 NAME 16 PCRE - Perl-compatible regular expressions (original API) 17 18 PLEASE TAKE NOTE 19 20 This document relates to PCRE releases that use the original API, with 21 library names libpcre, libpcre16, and libpcre32. January 2015 saw the 22 first release of a new API, known as PCRE2, with release numbers start- 23 ing at 10.00 and library names libpcre2-8, libpcre2-16, and 24 libpcre2-32. The old libraries (now called PCRE1) are still being main- 25 tained for bug fixes, but there will be no new development. New 26 projects are advised to use the new PCRE2 libraries. 27 28 29 INTRODUCTION 30 31 The PCRE library is a set of functions that implement regular expres- 32 sion pattern matching using the same syntax and semantics as Perl, with 33 just a few differences. Some features that appeared in Python and PCRE 34 before they appeared in Perl are also available using the Python syn- 35 tax, there is some support for one or two .NET and Oniguruma syntax 36 items, and there is an option for requesting some minor changes that 37 give better JavaScript compatibility. 38 39 Starting with release 8.30, it is possible to compile two separate PCRE 40 libraries: the original, which supports 8-bit character strings 41 (including UTF-8 strings), and a second library that supports 16-bit 42 character strings (including UTF-16 strings). The build process allows 43 either one or both to be built. The majority of the work to make this 44 possible was done by Zoltan Herczeg. 45 46 Starting with release 8.32 it is possible to compile a third separate 47 PCRE library that supports 32-bit character strings (including UTF-32 48 strings). The build process allows any combination of the 8-, 16- and 49 32-bit libraries. The work to make this possible was done by Christian 50 Persch. 51 52 The three libraries contain identical sets of functions, except that 53 the names in the 16-bit library start with pcre16_ instead of pcre_, 54 and the names in the 32-bit library start with pcre32_ instead of 55 pcre_. To avoid over-complication and reduce the documentation mainte- 56 nance load, most of the documentation describes the 8-bit library, with 57 the differences for the 16-bit and 32-bit libraries described sepa- 58 rately in the pcre16 and pcre32 pages. References to functions or 59 structures of the form pcre[16|32]_xxx should be read as meaning 60 "pcre_xxx when using the 8-bit library, pcre16_xxx when using the 61 16-bit library, or pcre32_xxx when using the 32-bit library". 62 63 The current implementation of PCRE corresponds approximately with Perl 64 5.12, including support for UTF-8/16/32 encoded strings and Unicode 65 general category properties. However, UTF-8/16/32 and Unicode support 66 has to be explicitly enabled; it is not the default. The Unicode tables 67 correspond to Unicode release 6.3.0. 68 69 In addition to the Perl-compatible matching function, PCRE contains an 70 alternative function that matches the same compiled patterns in a dif- 71 ferent way. In certain circumstances, the alternative function has some 72 advantages. For a discussion of the two matching algorithms, see the 73 pcrematching page. 74 75 PCRE is written in C and released as a C library. A number of people 76 have written wrappers and interfaces of various kinds. In particular, 77 Google Inc. have provided a comprehensive C++ wrapper for the 8-bit 78 library. This is now included as part of the PCRE distribution. The 79 pcrecpp page has details of this interface. Other people's contribu- 80 tions can be found in the Contrib directory at the primary FTP site, 81 which is: 82 83 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre 84 85 Details of exactly which Perl regular expression features are and are 86 not supported by PCRE are given in separate documents. See the pcrepat- 87 tern and pcrecompat pages. There is a syntax summary in the pcresyntax 88 page. 89 90 Some features of PCRE can be included, excluded, or changed when the 91 library is built. The pcre_config() function makes it possible for a 92 client to discover which features are available. The features them- 93 selves are described in the pcrebuild page. Documentation about build- 94 ing PCRE for various operating systems can be found in the README and 95 NON-AUTOTOOLS_BUILD files in the source distribution. 96 97 The libraries contains a number of undocumented internal functions and 98 data tables that are used by more than one of the exported external 99 functions, but which are not intended for use by external callers. 100 Their names all begin with "_pcre_" or "_pcre16_" or "_pcre32_", which 101 hopefully will not provoke any name clashes. In some environments, it 102 is possible to control which external symbols are exported when a 103 shared library is built, and in these cases the undocumented symbols 104 are not exported. 105 106 107 SECURITY CONSIDERATIONS 108 109 If you are using PCRE in a non-UTF application that permits users to 110 supply arbitrary patterns for compilation, you should be aware of a 111 feature that allows users to turn on UTF support from within a pattern, 112 provided that PCRE was built with UTF support. For example, an 8-bit 113 pattern that begins with "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, 114 which interprets patterns and subjects as strings of UTF-8 characters 115 instead of individual 8-bit characters. This causes both the pattern 116 and any data against which it is matched to be checked for UTF-8 valid- 117 ity. If the data string is very long, such a check might use suffi- 118 ciently many resources as to cause your application to lose perfor- 119 mance. 120 121 One way of guarding against this possibility is to use the 122 pcre_fullinfo() function to check the compiled pattern's options for 123 UTF. Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF 124 option at compile time. This causes an compile time error if a pattern 125 contains a UTF-setting sequence. 126 127 If your application is one that supports UTF, be aware that validity 128 checking can take time. If the same data string is to be matched many 129 times, you can use the PCRE_NO_UTF[8|16|32]_CHECK option for the second 130 and subsequent matches to save redundant checks. 131 132 Another way that performance can be hit is by running a pattern that 133 has a very large search tree against a string that will never match. 134 Nested unlimited repeats in a pattern are a common example. PCRE pro- 135 vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea- 136 ture in the pcreapi page. 137 138 139 USER DOCUMENTATION 140 141 The user documentation for PCRE comprises a number of different sec- 142 tions. In the "man" format, each of these is a separate "man page". In 143 the HTML format, each is a separate page, linked from the index page. 144 In the plain text format, the descriptions of the pcregrep and pcretest 145 programs are in files called pcregrep.txt and pcretest.txt, respec- 146 tively. The remaining sections, except for the pcredemo section (which 147 is a program listing), are concatenated in pcre.txt, for ease of 148 searching. The sections are as follows: 149 150 pcre this document 151 pcre-config show PCRE installation configuration information 152 pcre16 details of the 16-bit library 153 pcre32 details of the 32-bit library 154 pcreapi details of PCRE's native C API 155 pcrebuild building PCRE 156 pcrecallout details of the callout feature 157 pcrecompat discussion of Perl compatibility 158 pcrecpp details of the C++ wrapper for the 8-bit library 159 pcredemo a demonstration C program that uses PCRE 160 pcregrep description of the pcregrep command (8-bit only) 161 pcrejit discussion of the just-in-time optimization support 162 pcrelimits details of size and other limits 163 pcrematching discussion of the two matching algorithms 164 pcrepartial details of the partial matching facility 165 pcrepattern syntax and semantics of supported 166 regular expressions 167 pcreperform discussion of performance issues 168 pcreposix the POSIX-compatible C API for the 8-bit library 169 pcreprecompile details of saving and re-using precompiled patterns 170 pcresample discussion of the pcredemo program 171 pcrestack discussion of stack usage 172 pcresyntax quick syntax reference 173 pcretest description of the pcretest testing command 174 pcreunicode discussion of Unicode and UTF-8/16/32 support 175 176 In the "man" and HTML formats, there is also a short page for each C 177 library function, listing its arguments and results. 178 179 180 AUTHOR 181 182 Philip Hazel 183 University Computing Service 184 Cambridge CB2 3QH, England. 185 186 Putting an actual email address here seems to have been a spam magnet, 187 so I've taken it away. If you want to email me, use my two initials, 188 followed by the two digits 10, at the domain cam.ac.uk. 189 190 191 REVISION 192 193 Last updated: 10 February 2015 194 Copyright (c) 1997-2015 University of Cambridge. 195 ------------------------------------------------------------------------------ 196 197 198 PCRE(3) Library Functions Manual PCRE(3) 199 200 201 202 NAME 203 PCRE - Perl-compatible regular expressions 204 205 #include <pcre.h> 206 207 208 PCRE 16-BIT API BASIC FUNCTIONS 209 210 pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options, 211 const char **errptr, int *erroffset, 212 const unsigned char *tableptr); 213 214 pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options, 215 int *errorcodeptr, 216 const char **errptr, int *erroffset, 217 const unsigned char *tableptr); 218 219 pcre16_extra *pcre16_study(const pcre16 *code, int options, 220 const char **errptr); 221 222 void pcre16_free_study(pcre16_extra *extra); 223 224 int pcre16_exec(const pcre16 *code, const pcre16_extra *extra, 225 PCRE_SPTR16 subject, int length, int startoffset, 226 int options, int *ovector, int ovecsize); 227 228 int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra, 229 PCRE_SPTR16 subject, int length, int startoffset, 230 int options, int *ovector, int ovecsize, 231 int *workspace, int wscount); 232 233 234 PCRE 16-BIT API STRING EXTRACTION FUNCTIONS 235 236 int pcre16_copy_named_substring(const pcre16 *code, 237 PCRE_SPTR16 subject, int *ovector, 238 int stringcount, PCRE_SPTR16 stringname, 239 PCRE_UCHAR16 *buffer, int buffersize); 240 241 int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector, 242 int stringcount, int stringnumber, PCRE_UCHAR16 *buffer, 243 int buffersize); 244 245 int pcre16_get_named_substring(const pcre16 *code, 246 PCRE_SPTR16 subject, int *ovector, 247 int stringcount, PCRE_SPTR16 stringname, 248 PCRE_SPTR16 *stringptr); 249 250 int pcre16_get_stringnumber(const pcre16 *code, 251 PCRE_SPTR16 name); 252 253 int pcre16_get_stringtable_entries(const pcre16 *code, 254 PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last); 255 256 int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector, 257 int stringcount, int stringnumber, 258 PCRE_SPTR16 *stringptr); 259 260 int pcre16_get_substring_list(PCRE_SPTR16 subject, 261 int *ovector, int stringcount, PCRE_SPTR16 **listptr); 262 263 void pcre16_free_substring(PCRE_SPTR16 stringptr); 264 265 void pcre16_free_substring_list(PCRE_SPTR16 *stringptr); 266 267 268 PCRE 16-BIT API AUXILIARY FUNCTIONS 269 270 pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize); 271 272 void pcre16_jit_stack_free(pcre16_jit_stack *stack); 273 274 void pcre16_assign_jit_stack(pcre16_extra *extra, 275 pcre16_jit_callback callback, void *data); 276 277 const unsigned char *pcre16_maketables(void); 278 279 int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra, 280 int what, void *where); 281 282 int pcre16_refcount(pcre16 *code, int adjust); 283 284 int pcre16_config(int what, void *where); 285 286 const char *pcre16_version(void); 287 288 int pcre16_pattern_to_host_byte_order(pcre16 *code, 289 pcre16_extra *extra, const unsigned char *tables); 290 291 292 PCRE 16-BIT API INDIRECTED FUNCTIONS 293 294 void *(*pcre16_malloc)(size_t); 295 296 void (*pcre16_free)(void *); 297 298 void *(*pcre16_stack_malloc)(size_t); 299 300 void (*pcre16_stack_free)(void *); 301 302 int (*pcre16_callout)(pcre16_callout_block *); 303 304 305 PCRE 16-BIT API 16-BIT-ONLY FUNCTION 306 307 int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output, 308 PCRE_SPTR16 input, int length, int *byte_order, 309 int keep_boms); 310 311 312 THE PCRE 16-BIT LIBRARY 313 314 Starting with release 8.30, it is possible to compile a PCRE library 315 that supports 16-bit character strings, including UTF-16 strings, as 316 well as or instead of the original 8-bit library. The majority of the 317 work to make this possible was done by Zoltan Herczeg. The two 318 libraries contain identical sets of functions, used in exactly the same 319 way. Only the names of the functions and the data types of their argu- 320 ments and results are different. To avoid over-complication and reduce 321 the documentation maintenance load, most of the PCRE documentation 322 describes the 8-bit library, with only occasional references to the 323 16-bit library. This page describes what is different when you use the 324 16-bit library. 325 326 WARNING: A single application can be linked with both libraries, but 327 you must take care when processing any particular pattern to use func- 328 tions from just one library. For example, if you want to study a pat- 329 tern that was compiled with pcre16_compile(), you must do so with 330 pcre16_study(), not pcre_study(), and you must free the study data with 331 pcre16_free_study(). 332 333 334 THE HEADER FILE 335 336 There is only one header file, pcre.h. It contains prototypes for all 337 the functions in all libraries, as well as definitions of flags, struc- 338 tures, error codes, etc. 339 340 341 THE LIBRARY NAME 342 343 In Unix-like systems, the 16-bit library is called libpcre16, and can 344 normally be accesss by adding -lpcre16 to the command for linking an 345 application that uses PCRE. 346 347 348 STRING TYPES 349 350 In the 8-bit library, strings are passed to PCRE library functions as 351 vectors of bytes with the C type "char *". In the 16-bit library, 352 strings are passed as vectors of unsigned 16-bit quantities. The macro 353 PCRE_UCHAR16 specifies an appropriate data type, and PCRE_SPTR16 is 354 defined as "const PCRE_UCHAR16 *". In very many environments, "short 355 int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16 356 as "unsigned short int", but checks that it really is a 16-bit data 357 type. If it is not, the build fails with an error message telling the 358 maintainer to modify the definition appropriately. 359 360 361 STRUCTURE TYPES 362 363 The types of the opaque structures that are used for compiled 16-bit 364 patterns and JIT stacks are pcre16 and pcre16_jit_stack respectively. 365 The type of the user-accessible structure that is returned by 366 pcre16_study() is pcre16_extra, and the type of the structure that is 367 used for passing data to a callout function is pcre16_callout_block. 368 These structures contain the same fields, with the same names, as their 369 8-bit counterparts. The only difference is that pointers to character 370 strings are 16-bit instead of 8-bit types. 371 372 373 16-BIT FUNCTIONS 374 375 For every function in the 8-bit library there is a corresponding func- 376 tion in the 16-bit library with a name that starts with pcre16_ instead 377 of pcre_. The prototypes are listed above. In addition, there is one 378 extra function, pcre16_utf16_to_host_byte_order(). This is a utility 379 function that converts a UTF-16 character string to host byte order if 380 necessary. The other 16-bit functions expect the strings they are 381 passed to be in host byte order. 382 383 The input and output arguments of pcre16_utf16_to_host_byte_order() may 384 point to the same address, that is, conversion in place is supported. 385 The output buffer must be at least as long as the input. 386 387 The length argument specifies the number of 16-bit data units in the 388 input string; a negative value specifies a zero-terminated string. 389 390 If byte_order is NULL, it is assumed that the string starts off in host 391 byte order. This may be changed by byte-order marks (BOMs) anywhere in 392 the string (commonly as the first character). 393 394 If byte_order is not NULL, a non-zero value of the integer to which it 395 points means that the input starts off in host byte order, otherwise 396 the opposite order is assumed. Again, BOMs in the string can change 397 this. The final byte order is passed back at the end of processing. 398 399 If keep_boms is not zero, byte-order mark characters (0xfeff) are 400 copied into the output string. Otherwise they are discarded. 401 402 The result of the function is the number of 16-bit units placed into 403 the output buffer, including the zero terminator if the string was 404 zero-terminated. 405 406 407 SUBJECT STRING OFFSETS 408 409 The lengths and starting offsets of subject strings must be specified 410 in 16-bit data units, and the offsets within subject strings that are 411 returned by the matching functions are in also 16-bit units rather than 412 bytes. 413 414 415 NAMED SUBPATTERNS 416 417 The name-to-number translation table that is maintained for named sub- 418 patterns uses 16-bit characters. The pcre16_get_stringtable_entries() 419 function returns the length of each entry in the table as the number of 420 16-bit data units. 421 422 423 OPTION NAMES 424 425 There are two new general option names, PCRE_UTF16 and 426 PCRE_NO_UTF16_CHECK, which correspond to PCRE_UTF8 and 427 PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options 428 define the same bits in the options word. There is a discussion about 429 the validity of UTF-16 strings in the pcreunicode page. 430 431 For the pcre16_config() function there is an option PCRE_CONFIG_UTF16 432 that returns 1 if UTF-16 support is configured, otherwise 0. If this 433 option is given to pcre_config() or pcre32_config(), or if the 434 PCRE_CONFIG_UTF8 or PCRE_CONFIG_UTF32 option is given to pcre16_con- 435 fig(), the result is the PCRE_ERROR_BADOPTION error. 436 437 438 CHARACTER CODES 439 440 In 16-bit mode, when PCRE_UTF16 is not set, character values are 441 treated in the same way as in 8-bit, non UTF-8 mode, except, of course, 442 that they can range from 0 to 0xffff instead of 0 to 0xff. Character 443 types for characters less than 0xff can therefore be influenced by the 444 locale in the same way as before. Characters greater than 0xff have 445 only one case, and no "type" (such as letter or digit). 446 447 In UTF-16 mode, the character code is Unicode, in the range 0 to 448 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff 449 because those are "surrogate" values that are used in pairs to encode 450 values greater than 0xffff. 451 452 A UTF-16 string can indicate its endianness by special code knows as a 453 byte-order mark (BOM). The PCRE functions do not handle this, expecting 454 strings to be in host byte order. A utility function called 455 pcre16_utf16_to_host_byte_order() is provided to help with this (see 456 above). 457 458 459 ERROR NAMES 460 461 The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16 corre- 462 spond to their 8-bit counterparts. The error PCRE_ERROR_BADMODE is 463 given when a compiled pattern is passed to a function that processes 464 patterns in the other mode, for example, if a pattern compiled with 465 pcre_compile() is passed to pcre16_exec(). 466 467 There are new error codes whose names begin with PCRE_UTF16_ERR for 468 invalid UTF-16 strings, corresponding to the PCRE_UTF8_ERR codes for 469 UTF-8 strings that are described in the section entitled "Reason codes 470 for invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors 471 are: 472 473 PCRE_UTF16_ERR1 Missing low surrogate at end of string 474 PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate 475 PCRE_UTF16_ERR3 Isolated low surrogate 476 PCRE_UTF16_ERR4 Non-character 477 478 479 ERROR TEXTS 480 481 If there is an error while compiling a pattern, the error text that is 482 passed back by pcre16_compile() or pcre16_compile2() is still an 8-bit 483 character string, zero-terminated. 484 485 486 CALLOUTS 487 488 The subject and mark fields in the callout block that is passed to a 489 callout function point to 16-bit vectors. 490 491 492 TESTING 493 494 The pcretest program continues to operate with 8-bit input and output 495 files, but it can be used for testing the 16-bit library. If it is run 496 with the command line option -16, patterns and subject strings are con- 497 verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit 498 library functions are used instead of the 8-bit ones. Returned 16-bit 499 strings are converted to 8-bit for output. If both the 8-bit and the 500 32-bit libraries were not compiled, pcretest defaults to 16-bit and the 501 -16 option is ignored. 502 503 When PCRE is being built, the RunTest script that is called by "make 504 check" uses the pcretest -C option to discover which of the 8-bit, 505 16-bit and 32-bit libraries has been built, and runs the tests appro- 506 priately. 507 508 509 NOT SUPPORTED IN 16-BIT MODE 510 511 Not all the features of the 8-bit library are available with the 16-bit 512 library. The C++ and POSIX wrapper functions support only the 8-bit 513 library, and the pcregrep program is at present 8-bit only. 514 515 516 AUTHOR 517 518 Philip Hazel 519 University Computing Service 520 Cambridge CB2 3QH, England. 521 522 523 REVISION 524 525 Last updated: 12 May 2013 526 Copyright (c) 1997-2013 University of Cambridge. 527 ------------------------------------------------------------------------------ 528 529 530 PCRE(3) Library Functions Manual PCRE(3) 531 532 533 534 NAME 535 PCRE - Perl-compatible regular expressions 536 537 #include <pcre.h> 538 539 540 PCRE 32-BIT API BASIC FUNCTIONS 541 542 pcre32 *pcre32_compile(PCRE_SPTR32 pattern, int options, 543 const char **errptr, int *erroffset, 544 const unsigned char *tableptr); 545 546 pcre32 *pcre32_compile2(PCRE_SPTR32 pattern, int options, 547 int *errorcodeptr, 548 const unsigned char *tableptr); 549 550 pcre32_extra *pcre32_study(const pcre32 *code, int options, 551 const char **errptr); 552 553 void pcre32_free_study(pcre32_extra *extra); 554 555 int pcre32_exec(const pcre32 *code, const pcre32_extra *extra, 556 PCRE_SPTR32 subject, int length, int startoffset, 557 int options, int *ovector, int ovecsize); 558 559 int pcre32_dfa_exec(const pcre32 *code, const pcre32_extra *extra, 560 PCRE_SPTR32 subject, int length, int startoffset, 561 int options, int *ovector, int ovecsize, 562 int *workspace, int wscount); 563 564 565 PCRE 32-BIT API STRING EXTRACTION FUNCTIONS 566 567 int pcre32_copy_named_substring(const pcre32 *code, 568 PCRE_SPTR32 subject, int *ovector, 569 int stringcount, PCRE_SPTR32 stringname, 570 PCRE_UCHAR32 *buffer, int buffersize); 571 572 int pcre32_copy_substring(PCRE_SPTR32 subject, int *ovector, 573 int stringcount, int stringnumber, PCRE_UCHAR32 *buffer, 574 int buffersize); 575 576 int pcre32_get_named_substring(const pcre32 *code, 577 PCRE_SPTR32 subject, int *ovector, 578 int stringcount, PCRE_SPTR32 stringname, 579 PCRE_SPTR32 *stringptr); 580 581 int pcre32_get_stringnumber(const pcre32 *code, 582 PCRE_SPTR32 name); 583 584 int pcre32_get_stringtable_entries(const pcre32 *code, 585 PCRE_SPTR32 name, PCRE_UCHAR32 **first, PCRE_UCHAR32 **last); 586 587 int pcre32_get_substring(PCRE_SPTR32 subject, int *ovector, 588 int stringcount, int stringnumber, 589 PCRE_SPTR32 *stringptr); 590 591 int pcre32_get_substring_list(PCRE_SPTR32 subject, 592 int *ovector, int stringcount, PCRE_SPTR32 **listptr); 593 594 void pcre32_free_substring(PCRE_SPTR32 stringptr); 595 596 void pcre32_free_substring_list(PCRE_SPTR32 *stringptr); 597 598 599 PCRE 32-BIT API AUXILIARY FUNCTIONS 600 601 pcre32_jit_stack *pcre32_jit_stack_alloc(int startsize, int maxsize); 602 603 void pcre32_jit_stack_free(pcre32_jit_stack *stack); 604 605 void pcre32_assign_jit_stack(pcre32_extra *extra, 606 pcre32_jit_callback callback, void *data); 607 608 const unsigned char *pcre32_maketables(void); 609 610 int pcre32_fullinfo(const pcre32 *code, const pcre32_extra *extra, 611 int what, void *where); 612 613 int pcre32_refcount(pcre32 *code, int adjust); 614 615 int pcre32_config(int what, void *where); 616 617 const char *pcre32_version(void); 618 619 int pcre32_pattern_to_host_byte_order(pcre32 *code, 620 pcre32_extra *extra, const unsigned char *tables); 621 622 623 PCRE 32-BIT API INDIRECTED FUNCTIONS 624 625 void *(*pcre32_malloc)(size_t); 626 627 void (*pcre32_free)(void *); 628 629 void *(*pcre32_stack_malloc)(size_t); 630 631 void (*pcre32_stack_free)(void *); 632 633 int (*pcre32_callout)(pcre32_callout_block *); 634 635 636 PCRE 32-BIT API 32-BIT-ONLY FUNCTION 637 638 int pcre32_utf32_to_host_byte_order(PCRE_UCHAR32 *output, 639 PCRE_SPTR32 input, int length, int *byte_order, 640 int keep_boms); 641 642 643 THE PCRE 32-BIT LIBRARY 644 645 Starting with release 8.32, it is possible to compile a PCRE library 646 that supports 32-bit character strings, including UTF-32 strings, as 647 well as or instead of the original 8-bit library. This work was done by 648 Christian Persch, based on the work done by Zoltan Herczeg for the 649 16-bit library. All three libraries contain identical sets of func- 650 tions, used in exactly the same way. Only the names of the functions 651 and the data types of their arguments and results are different. To 652 avoid over-complication and reduce the documentation maintenance load, 653 most of the PCRE documentation describes the 8-bit library, with only 654 occasional references to the 16-bit and 32-bit libraries. This page 655 describes what is different when you use the 32-bit library. 656 657 WARNING: A single application can be linked with all or any of the 658 three libraries, but you must take care when processing any particular 659 pattern to use functions from just one library. For example, if you 660 want to study a pattern that was compiled with pcre32_compile(), you 661 must do so with pcre32_study(), not pcre_study(), and you must free the 662 study data with pcre32_free_study(). 663 664 665 THE HEADER FILE 666 667 There is only one header file, pcre.h. It contains prototypes for all 668 the functions in all libraries, as well as definitions of flags, struc- 669 tures, error codes, etc. 670 671 672 THE LIBRARY NAME 673 674 In Unix-like systems, the 32-bit library is called libpcre32, and can 675 normally be accesss by adding -lpcre32 to the command for linking an 676 application that uses PCRE. 677 678 679 STRING TYPES 680 681 In the 8-bit library, strings are passed to PCRE library functions as 682 vectors of bytes with the C type "char *". In the 32-bit library, 683 strings are passed as vectors of unsigned 32-bit quantities. The macro 684 PCRE_UCHAR32 specifies an appropriate data type, and PCRE_SPTR32 is 685 defined as "const PCRE_UCHAR32 *". In very many environments, "unsigned 686 int" is a 32-bit data type. When PCRE is built, it defines PCRE_UCHAR32 687 as "unsigned int", but checks that it really is a 32-bit data type. If 688 it is not, the build fails with an error message telling the maintainer 689 to modify the definition appropriately. 690 691 692 STRUCTURE TYPES 693 694 The types of the opaque structures that are used for compiled 32-bit 695 patterns and JIT stacks are pcre32 and pcre32_jit_stack respectively. 696 The type of the user-accessible structure that is returned by 697 pcre32_study() is pcre32_extra, and the type of the structure that is 698 used for passing data to a callout function is pcre32_callout_block. 699 These structures contain the same fields, with the same names, as their 700 8-bit counterparts. The only difference is that pointers to character 701 strings are 32-bit instead of 8-bit types. 702 703 704 32-BIT FUNCTIONS 705 706 For every function in the 8-bit library there is a corresponding func- 707 tion in the 32-bit library with a name that starts with pcre32_ instead 708 of pcre_. The prototypes are listed above. In addition, there is one 709 extra function, pcre32_utf32_to_host_byte_order(). This is a utility 710 function that converts a UTF-32 character string to host byte order if 711 necessary. The other 32-bit functions expect the strings they are 712 passed to be in host byte order. 713 714 The input and output arguments of pcre32_utf32_to_host_byte_order() may 715 point to the same address, that is, conversion in place is supported. 716 The output buffer must be at least as long as the input. 717 718 The length argument specifies the number of 32-bit data units in the 719 input string; a negative value specifies a zero-terminated string. 720 721 If byte_order is NULL, it is assumed that the string starts off in host 722 byte order. This may be changed by byte-order marks (BOMs) anywhere in 723 the string (commonly as the first character). 724 725 If byte_order is not NULL, a non-zero value of the integer to which it 726 points means that the input starts off in host byte order, otherwise 727 the opposite order is assumed. Again, BOMs in the string can change 728 this. The final byte order is passed back at the end of processing. 729 730 If keep_boms is not zero, byte-order mark characters (0xfeff) are 731 copied into the output string. Otherwise they are discarded. 732 733 The result of the function is the number of 32-bit units placed into 734 the output buffer, including the zero terminator if the string was 735 zero-terminated. 736 737 738 SUBJECT STRING OFFSETS 739 740 The lengths and starting offsets of subject strings must be specified 741 in 32-bit data units, and the offsets within subject strings that are 742 returned by the matching functions are in also 32-bit units rather than 743 bytes. 744 745 746 NAMED SUBPATTERNS 747 748 The name-to-number translation table that is maintained for named sub- 749 patterns uses 32-bit characters. The pcre32_get_stringtable_entries() 750 function returns the length of each entry in the table as the number of 751 32-bit data units. 752 753 754 OPTION NAMES 755 756 There are two new general option names, PCRE_UTF32 and 757 PCRE_NO_UTF32_CHECK, which correspond to PCRE_UTF8 and 758 PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options 759 define the same bits in the options word. There is a discussion about 760 the validity of UTF-32 strings in the pcreunicode page. 761 762 For the pcre32_config() function there is an option PCRE_CONFIG_UTF32 763 that returns 1 if UTF-32 support is configured, otherwise 0. If this 764 option is given to pcre_config() or pcre16_config(), or if the 765 PCRE_CONFIG_UTF8 or PCRE_CONFIG_UTF16 option is given to pcre32_con- 766 fig(), the result is the PCRE_ERROR_BADOPTION error. 767 768 769 CHARACTER CODES 770 771 In 32-bit mode, when PCRE_UTF32 is not set, character values are 772 treated in the same way as in 8-bit, non UTF-8 mode, except, of course, 773 that they can range from 0 to 0x7fffffff instead of 0 to 0xff. Charac- 774 ter types for characters less than 0xff can therefore be influenced by 775 the locale in the same way as before. Characters greater than 0xff 776 have only one case, and no "type" (such as letter or digit). 777 778 In UTF-32 mode, the character code is Unicode, in the range 0 to 779 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff 780 because those are "surrogate" values that are ill-formed in UTF-32. 781 782 A UTF-32 string can indicate its endianness by special code knows as a 783 byte-order mark (BOM). The PCRE functions do not handle this, expecting 784 strings to be in host byte order. A utility function called 785 pcre32_utf32_to_host_byte_order() is provided to help with this (see 786 above). 787 788 789 ERROR NAMES 790 791 The error PCRE_ERROR_BADUTF32 corresponds to its 8-bit counterpart. 792 The error PCRE_ERROR_BADMODE is given when a compiled pattern is passed 793 to a function that processes patterns in the other mode, for example, 794 if a pattern compiled with pcre_compile() is passed to pcre32_exec(). 795 796 There are new error codes whose names begin with PCRE_UTF32_ERR for 797 invalid UTF-32 strings, corresponding to the PCRE_UTF8_ERR codes for 798 UTF-8 strings that are described in the section entitled "Reason codes 799 for invalid UTF-8 strings" in the main pcreapi page. The UTF-32 errors 800 are: 801 802 PCRE_UTF32_ERR1 Surrogate character (range from 0xd800 to 0xdfff) 803 PCRE_UTF32_ERR2 Non-character 804 PCRE_UTF32_ERR3 Character > 0x10ffff 805 806 807 ERROR TEXTS 808 809 If there is an error while compiling a pattern, the error text that is 810 passed back by pcre32_compile() or pcre32_compile2() is still an 8-bit 811 character string, zero-terminated. 812 813 814 CALLOUTS 815 816 The subject and mark fields in the callout block that is passed to a 817 callout function point to 32-bit vectors. 818 819 820 TESTING 821 822 The pcretest program continues to operate with 8-bit input and output 823 files, but it can be used for testing the 32-bit library. If it is run 824 with the command line option -32, patterns and subject strings are con- 825 verted from 8-bit to 32-bit before being passed to PCRE, and the 32-bit 826 library functions are used instead of the 8-bit ones. Returned 32-bit 827 strings are converted to 8-bit for output. If both the 8-bit and the 828 16-bit libraries were not compiled, pcretest defaults to 32-bit and the 829 -32 option is ignored. 830 831 When PCRE is being built, the RunTest script that is called by "make 832 check" uses the pcretest -C option to discover which of the 8-bit, 833 16-bit and 32-bit libraries has been built, and runs the tests appro- 834 priately. 835 836 837 NOT SUPPORTED IN 32-BIT MODE 838 839 Not all the features of the 8-bit library are available with the 32-bit 840 library. The C++ and POSIX wrapper functions support only the 8-bit 841 library, and the pcregrep program is at present 8-bit only. 842 843 844 AUTHOR 845 846 Philip Hazel 847 University Computing Service 848 Cambridge CB2 3QH, England. 849 850 851 REVISION 852 853 Last updated: 12 May 2013 854 Copyright (c) 1997-2013 University of Cambridge. 855 ------------------------------------------------------------------------------ 856 857 858 PCREBUILD(3) Library Functions Manual PCREBUILD(3) 859 860 861 862 NAME 863 PCRE - Perl-compatible regular expressions 864 865 BUILDING PCRE 866 867 PCRE is distributed with a configure script that can be used to build 868 the library in Unix-like environments using the applications known as 869 Autotools. Also in the distribution are files to support building 870 using CMake instead of configure. The text file README contains general 871 information about building with Autotools (some of which is repeated 872 below), and also has some comments about building on various operating 873 systems. There is a lot more information about building PCRE without 874 using Autotools (including information about using CMake and building 875 "by hand") in the text file called NON-AUTOTOOLS-BUILD. You should 876 consult this file as well as the README file if you are building in a 877 non-Unix-like environment. 878 879 880 PCRE BUILD-TIME OPTIONS 881 882 The rest of this document describes the optional features of PCRE that 883 can be selected when the library is compiled. It assumes use of the 884 configure script, where the optional features are selected or dese- 885 lected by providing options to configure before running the make com- 886 mand. However, the same options can be selected in both Unix-like and 887 non-Unix-like environments using the GUI facility of cmake-gui if you 888 are using CMake instead of configure to build PCRE. 889 890 If you are not using Autotools or CMake, option selection can be done 891 by editing the config.h file, or by passing parameter settings to the 892 compiler, as described in NON-AUTOTOOLS-BUILD. 893 894 The complete list of options for configure (which includes the standard 895 ones such as the selection of the installation directory) can be 896 obtained by running 897 898 ./configure --help 899 900 The following sections include descriptions of options whose names 901 begin with --enable or --disable. These settings specify changes to the 902 defaults for the configure command. Because of the way that configure 903 works, --enable and --disable always come in pairs, so the complemen- 904 tary option always exists as well, but as it specifies the default, it 905 is not described. 906 907 908 BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES 909 910 By default, a library called libpcre is built, containing functions 911 that take string arguments contained in vectors of bytes, either as 912 single-byte characters, or interpreted as UTF-8 strings. You can also 913 build a separate library, called libpcre16, in which strings are con- 914 tained in vectors of 16-bit data units and interpreted either as sin- 915 gle-unit characters or UTF-16 strings, by adding 916 917 --enable-pcre16 918 919 to the configure command. You can also build yet another separate 920 library, called libpcre32, in which strings are contained in vectors of 921 32-bit data units and interpreted either as single-unit characters or 922 UTF-32 strings, by adding 923 924 --enable-pcre32 925 926 to the configure command. If you do not want the 8-bit library, add 927 928 --disable-pcre8 929 930 as well. At least one of the three libraries must be built. Note that 931 the C++ and POSIX wrappers are for the 8-bit library only, and that 932 pcregrep is an 8-bit program. None of these are built if you select 933 only the 16-bit or 32-bit libraries. 934 935 936 BUILDING SHARED AND STATIC LIBRARIES 937 938 The Autotools PCRE building process uses libtool to build both shared 939 and static libraries by default. You can suppress one of these by 940 adding one of 941 942 --disable-shared 943 --disable-static 944 945 to the configure command, as required. 946 947 948 C++ SUPPORT 949 950 By default, if the 8-bit library is being built, the configure script 951 will search for a C++ compiler and C++ header files. If it finds them, 952 it automatically builds the C++ wrapper library (which supports only 953 8-bit strings). You can disable this by adding 954 955 --disable-cpp 956 957 to the configure command. 958 959 960 UTF-8, UTF-16 AND UTF-32 SUPPORT 961 962 To build PCRE with support for UTF Unicode character strings, add 963 964 --enable-utf 965 966 to the configure command. This setting applies to all three libraries, 967 adding support for UTF-8 to the 8-bit library, support for UTF-16 to 968 the 16-bit library, and support for UTF-32 to the to the 32-bit 969 library. There are no separate options for enabling UTF-8, UTF-16 and 970 UTF-32 independently because that would allow ridiculous settings such 971 as requesting UTF-16 support while building only the 8-bit library. It 972 is not possible to build one library with UTF support and another with- 973 out in the same configuration. (For backwards compatibility, --enable- 974 utf8 is a synonym of --enable-utf.) 975 976 Of itself, this setting does not make PCRE treat strings as UTF-8, 977 UTF-16 or UTF-32. As well as compiling PCRE with this option, you also 978 have have to set the PCRE_UTF8, PCRE_UTF16 or PCRE_UTF32 option (as 979 appropriate) when you call one of the pattern compiling functions. 980 981 If you set --enable-utf when compiling in an EBCDIC environment, PCRE 982 expects its input to be either ASCII or UTF-8 (depending on the run- 983 time option). It is not possible to support both EBCDIC and UTF-8 codes 984 in the same version of the library. Consequently, --enable-utf and 985 --enable-ebcdic are mutually exclusive. 986 987 988 UNICODE CHARACTER PROPERTY SUPPORT 989 990 UTF support allows the libraries to process character codepoints up to 991 0x10ffff in the strings that they handle. On its own, however, it does 992 not provide any facilities for accessing the properties of such charac- 993 ters. If you want to be able to use the pattern escapes \P, \p, and \X, 994 which refer to Unicode character properties, you must add 995 996 --enable-unicode-properties 997 998 to the configure command. This implies UTF support, even if you have 999 not explicitly requested it. 1000 1001 Including Unicode property support adds around 30K of tables to the 1002 PCRE library. Only the general category properties such as Lu and Nd 1003 are supported. Details are given in the pcrepattern documentation. 1004 1005 1006 JUST-IN-TIME COMPILER SUPPORT 1007 1008 Just-in-time compiler support is included in the build by specifying 1009 1010 --enable-jit 1011 1012 This support is available only for certain hardware architectures. If 1013 this option is set for an unsupported architecture, a compile time 1014 error occurs. See the pcrejit documentation for a discussion of JIT 1015 usage. When JIT support is enabled, pcregrep automatically makes use of 1016 it, unless you add 1017 1018 --disable-pcregrep-jit 1019 1020 to the "configure" command. 1021 1022 1023 CODE VALUE OF NEWLINE 1024 1025 By default, PCRE interprets the linefeed (LF) character as indicating 1026 the end of a line. This is the normal newline character on Unix-like 1027 systems. You can compile PCRE to use carriage return (CR) instead, by 1028 adding 1029 1030 --enable-newline-is-cr 1031 1032 to the configure command. There is also a --enable-newline-is-lf 1033 option, which explicitly specifies linefeed as the newline character. 1034 1035 Alternatively, you can specify that line endings are to be indicated by 1036 the two character sequence CRLF. If you want this, add 1037 1038 --enable-newline-is-crlf 1039 1040 to the configure command. There is a fourth option, specified by 1041 1042 --enable-newline-is-anycrlf 1043 1044 which causes PCRE to recognize any of the three sequences CR, LF, or 1045 CRLF as indicating a line ending. Finally, a fifth option, specified by 1046 1047 --enable-newline-is-any 1048 1049 causes PCRE to recognize any Unicode newline sequence. 1050 1051 Whatever line ending convention is selected when PCRE is built can be 1052 overridden when the library functions are called. At build time it is 1053 conventional to use the standard for your operating system. 1054 1055 1056 WHAT \R MATCHES 1057 1058 By default, the sequence \R in a pattern matches any Unicode newline 1059 sequence, whatever has been selected as the line ending sequence. If 1060 you specify 1061 1062 --enable-bsr-anycrlf 1063 1064 the default is changed so that \R matches only CR, LF, or CRLF. What- 1065 ever is selected when PCRE is built can be overridden when the library 1066 functions are called. 1067 1068 1069 POSIX MALLOC USAGE 1070 1071 When the 8-bit library is called through the POSIX interface (see the 1072 pcreposix documentation), additional working storage is required for 1073 holding the pointers to capturing substrings, because PCRE requires 1074 three integers per substring, whereas the POSIX interface provides only 1075 two. If the number of expected substrings is small, the wrapper func- 1076 tion uses space on the stack, because this is faster than using mal- 1077 loc() for each call. The default threshold above which the stack is no 1078 longer used is 10; it can be changed by adding a setting such as 1079 1080 --with-posix-malloc-threshold=20 1081 1082 to the configure command. 1083 1084 1085 HANDLING VERY LARGE PATTERNS 1086 1087 Within a compiled pattern, offset values are used to point from one 1088 part to another (for example, from an opening parenthesis to an alter- 1089 nation metacharacter). By default, in the 8-bit and 16-bit libraries, 1090 two-byte values are used for these offsets, leading to a maximum size 1091 for a compiled pattern of around 64K. This is sufficient to handle all 1092 but the most gigantic patterns. Nevertheless, some people do want to 1093 process truly enormous patterns, so it is possible to compile PCRE to 1094 use three-byte or four-byte offsets by adding a setting such as 1095 1096 --with-link-size=3 1097 1098 to the configure command. The value given must be 2, 3, or 4. For the 1099 16-bit library, a value of 3 is rounded up to 4. In these libraries, 1100 using longer offsets slows down the operation of PCRE because it has to 1101 load additional data when handling them. For the 32-bit library the 1102 value is always 4 and cannot be overridden; the value of --with-link- 1103 size is ignored. 1104 1105 1106 AVOIDING EXCESSIVE STACK USAGE 1107 1108 When matching with the pcre_exec() function, PCRE implements backtrack- 1109 ing by making recursive calls to an internal function called match(). 1110 In environments where the size of the stack is limited, this can se- 1111 verely limit PCRE's operation. (The Unix environment does not usually 1112 suffer from this problem, but it may sometimes be necessary to increase 1113 the maximum stack size. There is a discussion in the pcrestack docu- 1114 mentation.) An alternative approach to recursion that uses memory from 1115 the heap to remember data, instead of using recursive function calls, 1116 has been implemented to work round the problem of limited stack size. 1117 If you want to build a version of PCRE that works this way, add 1118 1119 --disable-stack-for-recursion 1120 1121 to the configure command. With this configuration, PCRE will use the 1122 pcre_stack_malloc and pcre_stack_free variables to call memory manage- 1123 ment functions. By default these point to malloc() and free(), but you 1124 can replace the pointers so that your own functions are used instead. 1125 1126 Separate functions are provided rather than using pcre_malloc and 1127 pcre_free because the usage is very predictable: the block sizes 1128 requested are always the same, and the blocks are always freed in 1129 reverse order. A calling program might be able to implement optimized 1130 functions that perform better than malloc() and free(). PCRE runs 1131 noticeably more slowly when built in this way. This option affects only 1132 the pcre_exec() function; it is not relevant for pcre_dfa_exec(). 1133 1134 1135 LIMITING PCRE RESOURCE USAGE 1136 1137 Internally, PCRE has a function called match(), which it calls repeat- 1138 edly (sometimes recursively) when matching a pattern with the 1139 pcre_exec() function. By controlling the maximum number of times this 1140 function may be called during a single matching operation, a limit can 1141 be placed on the resources used by a single call to pcre_exec(). The 1142 limit can be changed at run time, as described in the pcreapi documen- 1143 tation. The default is 10 million, but this can be changed by adding a 1144 setting such as 1145 1146 --with-match-limit=500000 1147 1148 to the configure command. This setting has no effect on the 1149 pcre_dfa_exec() matching function. 1150 1151 In some environments it is desirable to limit the depth of recursive 1152 calls of match() more strictly than the total number of calls, in order 1153 to restrict the maximum amount of stack (or heap, if --disable-stack- 1154 for-recursion is specified) that is used. A second limit controls this; 1155 it defaults to the value that is set for --with-match-limit, which 1156 imposes no additional constraints. However, you can set a lower limit 1157 by adding, for example, 1158 1159 --with-match-limit-recursion=10000 1160 1161 to the configure command. This value can also be overridden at run 1162 time. 1163 1164 1165 CREATING CHARACTER TABLES AT BUILD TIME 1166 1167 PCRE uses fixed tables for processing characters whose code values are 1168 less than 256. By default, PCRE is built with a set of tables that are 1169 distributed in the file pcre_chartables.c.dist. These tables are for 1170 ASCII codes only. If you add 1171 1172 --enable-rebuild-chartables 1173 1174 to the configure command, the distributed tables are no longer used. 1175 Instead, a program called dftables is compiled and run. This outputs 1176 the source for new set of tables, created in the default locale of your 1177 C run-time system. (This method of replacing the tables does not work 1178 if you are cross compiling, because dftables is run on the local host. 1179 If you need to create alternative tables when cross compiling, you will 1180 have to do so "by hand".) 1181 1182 1183 USING EBCDIC CODE 1184 1185 PCRE assumes by default that it will run in an environment where the 1186 character code is ASCII (or Unicode, which is a superset of ASCII). 1187 This is the case for most computer operating systems. PCRE can, how- 1188 ever, be compiled to run in an EBCDIC environment by adding 1189 1190 --enable-ebcdic 1191 1192 to the configure command. This setting implies --enable-rebuild-charta- 1193 bles. You should only use it if you know that you are in an EBCDIC 1194 environment (for example, an IBM mainframe operating system). The 1195 --enable-ebcdic option is incompatible with --enable-utf. 1196 1197 The EBCDIC character that corresponds to an ASCII LF is assumed to have 1198 the value 0x15 by default. However, in some EBCDIC environments, 0x25 1199 is used. In such an environment you should use 1200 1201 --enable-ebcdic-nl25 1202 1203 as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR 1204 has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and 1205 0x25 is not chosen as LF is made to correspond to the Unicode NEL char- 1206 acter (which, in Unicode, is 0x85). 1207 1208 The options that select newline behaviour, such as --enable-newline-is- 1209 cr, and equivalent run-time options, refer to these character values in 1210 an EBCDIC environment. 1211 1212 1213 PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT 1214 1215 By default, pcregrep reads all files as plain text. You can build it so 1216 that it recognizes files whose names end in .gz or .bz2, and reads them 1217 with libz or libbz2, respectively, by adding one or both of 1218 1219 --enable-pcregrep-libz 1220 --enable-pcregrep-libbz2 1221 1222 to the configure command. These options naturally require that the rel- 1223 evant libraries are installed on your system. Configuration will fail 1224 if they are not. 1225 1226 1227 PCREGREP BUFFER SIZE 1228 1229 pcregrep uses an internal buffer to hold a "window" on the file it is 1230 scanning, in order to be able to output "before" and "after" lines when 1231 it finds a match. The size of the buffer is controlled by a parameter 1232 whose default value is 20K. The buffer itself is three times this size, 1233 but because of the way it is used for holding "before" lines, the long- 1234 est line that is guaranteed to be processable is the parameter size. 1235 You can change the default parameter value by adding, for example, 1236 1237 --with-pcregrep-bufsize=50K 1238 1239 to the configure command. The caller of pcregrep can, however, override 1240 this value by specifying a run-time option. 1241 1242 1243 PCRETEST OPTION FOR LIBREADLINE SUPPORT 1244 1245 If you add 1246 1247 --enable-pcretest-libreadline 1248 1249 to the configure command, pcretest is linked with the libreadline 1250 library, and when its input is from a terminal, it reads it using the 1251 readline() function. This provides line-editing and history facilities. 1252 Note that libreadline is GPL-licensed, so if you distribute a binary of 1253 pcretest linked in this way, there may be licensing issues. 1254 1255 Setting this option causes the -lreadline option to be added to the 1256 pcretest build. In many operating environments with a sytem-installed 1257 libreadline this is sufficient. However, in some environments (e.g. if 1258 an unmodified distribution version of readline is in use), some extra 1259 configuration may be necessary. The INSTALL file for libreadline says 1260 this: 1261 1262 "Readline uses the termcap functions, but does not link with the 1263 termcap or curses library itself, allowing applications which link 1264 with readline the to choose an appropriate library." 1265 1266 If your environment has not been set up so that an appropriate library 1267 is automatically included, you may need to add something like 1268 1269 LIBS="-ncurses" 1270 1271 immediately before the configure command. 1272 1273 1274 DEBUGGING WITH VALGRIND SUPPORT 1275 1276 By adding the 1277 1278 --enable-valgrind 1279 1280 option to to the configure command, PCRE will use valgrind annotations 1281 to mark certain memory regions as unaddressable. This allows it to 1282 detect invalid memory accesses, and is mostly useful for debugging PCRE 1283 itself. 1284 1285 1286 CODE COVERAGE REPORTING 1287 1288 If your C compiler is gcc, you can build a version of PCRE that can 1289 generate a code coverage report for its test suite. To enable this, you 1290 must install lcov version 1.6 or above. Then specify 1291 1292 --enable-coverage 1293 1294 to the configure command and build PCRE in the usual way. 1295 1296 Note that using ccache (a caching C compiler) is incompatible with code 1297 coverage reporting. If you have configured ccache to run automatically 1298 on your system, you must set the environment variable 1299 1300 CCACHE_DISABLE=1 1301 1302 before running make to build PCRE, so that ccache is not used. 1303 1304 When --enable-coverage is used, the following addition targets are 1305 added to the Makefile: 1306 1307 make coverage 1308 1309 This creates a fresh coverage report for the PCRE test suite. It is 1310 equivalent to running "make coverage-reset", "make coverage-baseline", 1311 "make check", and then "make coverage-report". 1312 1313 make coverage-reset 1314 1315 This zeroes the coverage counters, but does nothing else. 1316 1317 make coverage-baseline 1318 1319 This captures baseline coverage information. 1320 1321 make coverage-report 1322 1323 This creates the coverage report. 1324 1325 make coverage-clean-report 1326 1327 This removes the generated coverage report without cleaning the cover- 1328 age data itself. 1329 1330 make coverage-clean-data 1331 1332 This removes the captured coverage data without removing the coverage 1333 files created at compile time (*.gcno). 1334 1335 make coverage-clean 1336 1337 This cleans all coverage data including the generated coverage report. 1338 For more information about code coverage, see the gcov and lcov docu- 1339 mentation. 1340 1341 1342 SEE ALSO 1343 1344 pcreapi(3), pcre16, pcre32, pcre_config(3). 1345 1346 1347 AUTHOR 1348 1349 Philip Hazel 1350 University Computing Service 1351 Cambridge CB2 3QH, England. 1352 1353 1354 REVISION 1355 1356 Last updated: 12 May 2013 1357 Copyright (c) 1997-2013 University of Cambridge. 1358 ------------------------------------------------------------------------------ 1359 1360 1361 PCREMATCHING(3) Library Functions Manual PCREMATCHING(3) 1362 1363 1364 1365 NAME 1366 PCRE - Perl-compatible regular expressions 1367 1368 PCRE MATCHING ALGORITHMS 1369 1370 This document describes the two different algorithms that are available 1371 in PCRE for matching a compiled regular expression against a given sub- 1372 ject string. The "standard" algorithm is the one provided by the 1373 pcre_exec(), pcre16_exec() and pcre32_exec() functions. These work in 1374 the same as as Perl's matching function, and provide a Perl-compatible 1375 matching operation. The just-in-time (JIT) optimization that is 1376 described in the pcrejit documentation is compatible with these func- 1377 tions. 1378 1379 An alternative algorithm is provided by the pcre_dfa_exec(), 1380 pcre16_dfa_exec() and pcre32_dfa_exec() functions; they operate in a 1381 different way, and are not Perl-compatible. This alternative has advan- 1382 tages and disadvantages compared with the standard algorithm, and these 1383 are described below. 1384 1385 When there is only one possible way in which a given subject string can 1386 match a pattern, the two algorithms give the same answer. A difference 1387 arises, however, when there are multiple possibilities. For example, if 1388 the pattern 1389 1390 ^<.*> 1391 1392 is matched against the string 1393 1394 <something> <something else> <something further> 1395 1396 there are three possible answers. The standard algorithm finds only one 1397 of them, whereas the alternative algorithm finds all three. 1398 1399 1400 REGULAR EXPRESSIONS AS TREES 1401 1402 The set of strings that are matched by a regular expression can be rep- 1403 resented as a tree structure. An unlimited repetition in the pattern 1404 makes the tree of infinite size, but it is still a tree. Matching the 1405 pattern to a given subject string (from a given starting point) can be 1406 thought of as a search of the tree. There are two ways to search a 1407 tree: depth-first and breadth-first, and these correspond to the two 1408 matching algorithms provided by PCRE. 1409 1410 1411 THE STANDARD MATCHING ALGORITHM 1412 1413 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- 1414 sions", the standard algorithm is an "NFA algorithm". It conducts a 1415 depth-first search of the pattern tree. That is, it proceeds along a 1416 single path through the tree, checking that the subject matches what is 1417 required. When there is a mismatch, the algorithm tries any alterna- 1418 tives at the current point, and if they all fail, it backs up to the 1419 previous branch point in the tree, and tries the next alternative 1420 branch at that level. This often involves backing up (moving to the 1421 left) in the subject string as well. The order in which repetition 1422 branches are tried is controlled by the greedy or ungreedy nature of 1423 the quantifier. 1424 1425 If a leaf node is reached, a matching string has been found, and at 1426 that point the algorithm stops. Thus, if there is more than one possi- 1427 ble match, this algorithm returns the first one that it finds. Whether 1428 this is the shortest, the longest, or some intermediate length depends 1429 on the way the greedy and ungreedy repetition quantifiers are specified 1430 in the pattern. 1431 1432 Because it ends up with a single path through the tree, it is rela- 1433 tively straightforward for this algorithm to keep track of the sub- 1434 strings that are matched by portions of the pattern in parentheses. 1435 This provides support for capturing parentheses and back references. 1436 1437 1438 THE ALTERNATIVE MATCHING ALGORITHM 1439 1440 This algorithm conducts a breadth-first search of the tree. Starting 1441 from the first matching point in the subject, it scans the subject 1442 string from left to right, once, character by character, and as it does 1443 this, it remembers all the paths through the tree that represent valid 1444 matches. In Friedl's terminology, this is a kind of "DFA algorithm", 1445 though it is not implemented as a traditional finite state machine (it 1446 keeps multiple states active simultaneously). 1447 1448 Although the general principle of this matching algorithm is that it 1449 scans the subject string only once, without backtracking, there is one 1450 exception: when a lookaround assertion is encountered, the characters 1451 following or preceding the current point have to be independently 1452 inspected. 1453 1454 The scan continues until either the end of the subject is reached, or 1455 there are no more unterminated paths. At this point, terminated paths 1456 represent the different matching possibilities (if there are none, the 1457 match has failed). Thus, if there is more than one possible match, 1458 this algorithm finds all of them, and in particular, it finds the long- 1459 est. The matches are returned in decreasing order of length. There is 1460 an option to stop the algorithm after the first match (which is neces- 1461 sarily the shortest) is found. 1462 1463 Note that all the matches that are found start at the same point in the 1464 subject. If the pattern 1465 1466 cat(er(pillar)?)? 1467 1468 is matched against the string "the caterpillar catchment", the result 1469 will be the three strings "caterpillar", "cater", and "cat" that start 1470 at the fifth character of the subject. The algorithm does not automati- 1471 cally move on to find matches that start at later positions. 1472 1473 PCRE's "auto-possessification" optimization usually applies to charac- 1474 ter repeats at the end of a pattern (as well as internally). For exam- 1475 ple, the pattern "a\d+" is compiled as if it were "a\d++" because there 1476 is no point even considering the possibility of backtracking into the 1477 repeated digits. For DFA matching, this means that only one possible 1478 match is found. If you really do want multiple matches in such cases, 1479 either use an ungreedy repeat ("a\d+?") or set the PCRE_NO_AUTO_POSSESS 1480 option when compiling. 1481 1482 There are a number of features of PCRE regular expressions that are not 1483 supported by the alternative matching algorithm. They are as follows: 1484 1485 1. Because the algorithm finds all possible matches, the greedy or 1486 ungreedy nature of repetition quantifiers is not relevant. Greedy and 1487 ungreedy quantifiers are treated in exactly the same way. However, pos- 1488 sessive quantifiers can make a difference when what follows could also 1489 match what is quantified, for example in a pattern like this: 1490 1491 ^a++\w! 1492 1493 This pattern matches "aaab!" but not "aaa!", which would be matched by 1494 a non-possessive quantifier. Similarly, if an atomic group is present, 1495 it is matched as if it were a standalone pattern at the current point, 1496 and the longest match is then "locked in" for the rest of the overall 1497 pattern. 1498 1499 2. When dealing with multiple paths through the tree simultaneously, it 1500 is not straightforward to keep track of captured substrings for the 1501 different matching possibilities, and PCRE's implementation of this 1502 algorithm does not attempt to do this. This means that no captured sub- 1503 strings are available. 1504 1505 3. Because no substrings are captured, back references within the pat- 1506 tern are not supported, and cause errors if encountered. 1507 1508 4. For the same reason, conditional expressions that use a backrefer- 1509 ence as the condition or test for a specific group recursion are not 1510 supported. 1511 1512 5. Because many paths through the tree may be active, the \K escape 1513 sequence, which resets the start of the match when encountered (but may 1514 be on some paths and not on others), is not supported. It causes an 1515 error if encountered. 1516 1517 6. Callouts are supported, but the value of the capture_top field is 1518 always 1, and the value of the capture_last field is always -1. 1519 1520 7. The \C escape sequence, which (in the standard algorithm) always 1521 matches a single data unit, even in UTF-8, UTF-16 or UTF-32 modes, is 1522 not supported in these modes, because the alternative algorithm moves 1523 through the subject string one character (not data unit) at a time, for 1524 all active paths through the tree. 1525 1526 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) 1527 are not supported. (*FAIL) is supported, and behaves like a failing 1528 negative assertion. 1529 1530 1531 ADVANTAGES OF THE ALTERNATIVE ALGORITHM 1532 1533 Using the alternative matching algorithm provides the following advan- 1534 tages: 1535 1536 1. All possible matches (at a single point in the subject) are automat- 1537 ically found, and in particular, the longest match is found. To find 1538 more than one match using the standard algorithm, you have to do kludgy 1539 things with callouts. 1540 1541 2. Because the alternative algorithm scans the subject string just 1542 once, and never needs to backtrack (except for lookbehinds), it is pos- 1543 sible to pass very long subject strings to the matching function in 1544 several pieces, checking for partial matching each time. Although it is 1545 possible to do multi-segment matching using the standard algorithm by 1546 retaining partially matched substrings, it is more complicated. The 1547 pcrepartial documentation gives details of partial matching and dis- 1548 cusses multi-segment matching. 1549 1550 1551 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM 1552 1553 The alternative algorithm suffers from a number of disadvantages: 1554 1555 1. It is substantially slower than the standard algorithm. This is 1556 partly because it has to search for all possible matches, but is also 1557 because it is less susceptible to optimization. 1558 1559 2. Capturing parentheses and back references are not supported. 1560 1561 3. Although atomic groups are supported, their use does not provide the 1562 performance advantage that it does for the standard algorithm. 1563 1564 1565 AUTHOR 1566 1567 Philip Hazel 1568 University Computing Service 1569 Cambridge CB2 3QH, England. 1570 1571 1572 REVISION 1573 1574 Last updated: 12 November 2013 1575 Copyright (c) 1997-2012 University of Cambridge. 1576 ------------------------------------------------------------------------------ 1577 1578 1579 PCREAPI(3) Library Functions Manual PCREAPI(3) 1580 1581 1582 1583 NAME 1584 PCRE - Perl-compatible regular expressions 1585 1586 #include <pcre.h> 1587 1588 1589 PCRE NATIVE API BASIC FUNCTIONS 1590 1591 pcre *pcre_compile(const char *pattern, int options, 1592 const char **errptr, int *erroffset, 1593 const unsigned char *tableptr); 1594 1595 pcre *pcre_compile2(const char *pattern, int options, 1596 int *errorcodeptr, 1597 const char **errptr, int *erroffset, 1598 const unsigned char *tableptr); 1599 1600 pcre_extra *pcre_study(const pcre *code, int options, 1601 const char **errptr); 1602 1603 void pcre_free_study(pcre_extra *extra); 1604 1605 int pcre_exec(const pcre *code, const pcre_extra *extra, 1606 const char *subject, int length, int startoffset, 1607 int options, int *ovector, int ovecsize); 1608 1609 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, 1610 const char *subject, int length, int startoffset, 1611 int options, int *ovector, int ovecsize, 1612 int *workspace, int wscount); 1613 1614 1615 PCRE NATIVE API STRING EXTRACTION FUNCTIONS 1616 1617 int pcre_copy_named_substring(const pcre *code, 1618 const char *subject, int *ovector, 1619 int stringcount, const char *stringname, 1620 char *buffer, int buffersize); 1621 1622 int pcre_copy_substring(const char *subject, int *ovector, 1623 int stringcount, int stringnumber, char *buffer, 1624 int buffersize); 1625 1626 int pcre_get_named_substring(const pcre *code, 1627 const char *subject, int *ovector, 1628 int stringcount, const char *stringname, 1629 const char **stringptr); 1630 1631 int pcre_get_stringnumber(const pcre *code, 1632 const char *name); 1633 1634 int pcre_get_stringtable_entries(const pcre *code, 1635 const char *name, char **first, char **last); 1636 1637 int pcre_get_substring(const char *subject, int *ovector, 1638 int stringcount, int stringnumber, 1639 const char **stringptr); 1640 1641 int pcre_get_substring_list(const char *subject, 1642 int *ovector, int stringcount, const char ***listptr); 1643 1644 void pcre_free_substring(const char *stringptr); 1645 1646 void pcre_free_substring_list(const char **stringptr); 1647 1648 1649 PCRE NATIVE API AUXILIARY FUNCTIONS 1650 1651 int pcre_jit_exec(const pcre *code, const pcre_extra *extra, 1652 const char *subject, int length, int startoffset, 1653 int options, int *ovector, int ovecsize, 1654 pcre_jit_stack *jstack); 1655 1656 pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize); 1657 1658 void pcre_jit_stack_free(pcre_jit_stack *stack); 1659 1660 void pcre_assign_jit_stack(pcre_extra *extra, 1661 pcre_jit_callback callback, void *data); 1662 1663 const unsigned char *pcre_maketables(void); 1664 1665 int pcre_fullinfo(const pcre *code, const pcre_extra *extra, 1666 int what, void *where); 1667 1668 int pcre_refcount(pcre *code, int adjust); 1669 1670 int pcre_config(int what, void *where); 1671 1672 const char *pcre_version(void); 1673 1674 int pcre_pattern_to_host_byte_order(pcre *code, 1675 pcre_extra *extra, const unsigned char *tables); 1676 1677 1678 PCRE NATIVE API INDIRECTED FUNCTIONS 1679 1680 void *(*pcre_malloc)(size_t); 1681 1682 void (*pcre_free)(void *); 1683 1684 void *(*pcre_stack_malloc)(size_t); 1685 1686 void (*pcre_stack_free)(void *); 1687 1688 int (*pcre_callout)(pcre_callout_block *); 1689 1690 int (*pcre_stack_guard)(void); 1691 1692 1693 PCRE 8-BIT, 16-BIT, AND 32-BIT LIBRARIES 1694 1695 As well as support for 8-bit character strings, PCRE also supports 1696 16-bit strings (from release 8.30) and 32-bit strings (from release 1697 8.32), by means of two additional libraries. They can be built as well 1698 as, or instead of, the 8-bit library. To avoid too much complication, 1699 this document describes the 8-bit versions of the functions, with only 1700 occasional references to the 16-bit and 32-bit libraries. 1701 1702 The 16-bit and 32-bit functions operate in the same way as their 8-bit 1703 counterparts; they just use different data types for their arguments 1704 and results, and their names start with pcre16_ or pcre32_ instead of 1705 pcre_. For every option that has UTF8 in its name (for example, 1706 PCRE_UTF8), there are corresponding 16-bit and 32-bit names with UTF8 1707 replaced by UTF16 or UTF32, respectively. This facility is in fact just 1708 cosmetic; the 16-bit and 32-bit option names define the same bit val- 1709 ues. 1710 1711 References to bytes and UTF-8 in this document should be read as refer- 1712 ences to 16-bit data units and UTF-16 when using the 16-bit library, or 1713 32-bit data units and UTF-32 when using the 32-bit library, unless 1714 specified otherwise. More details of the specific differences for the 1715 16-bit and 32-bit libraries are given in the pcre16 and pcre32 pages. 1716 1717 1718 PCRE API OVERVIEW 1719 1720 PCRE has its own native API, which is described in this document. There 1721 are also some wrapper functions (for the 8-bit library only) that cor- 1722 respond to the POSIX regular expression API, but they do not give 1723 access to all the functionality. They are described in the pcreposix 1724 documentation. Both of these APIs define a set of C function calls. A 1725 C++ wrapper (again for the 8-bit library only) is also distributed with 1726 PCRE. It is documented in the pcrecpp page. 1727 1728 The native API C function prototypes are defined in the header file 1729 pcre.h, and on Unix-like systems the (8-bit) library itself is called 1730 libpcre. It can normally be accessed by adding -lpcre to the command 1731 for linking an application that uses PCRE. The header file defines the 1732 macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release 1733 numbers for the library. Applications can use these to include support 1734 for different releases of PCRE. 1735 1736 In a Windows environment, if you want to statically link an application 1737 program against a non-dll pcre.a file, you must define PCRE_STATIC 1738 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal- 1739 loc() and pcre_free() exported functions will be declared 1740 __declspec(dllimport), with unwanted results. 1741 1742 The functions pcre_compile(), pcre_compile2(), pcre_study(), and 1743 pcre_exec() are used for compiling and matching regular expressions in 1744 a Perl-compatible manner. A sample program that demonstrates the sim- 1745 plest way of using them is provided in the file called pcredemo.c in 1746 the PCRE source distribution. A listing of this program is given in the 1747 pcredemo documentation, and the pcresample documentation describes how 1748 to compile and run it. 1749 1750 Just-in-time compiler support is an optional feature of PCRE that can 1751 be built in appropriate hardware environments. It greatly speeds up the 1752 matching performance of many patterns. Simple programs can easily 1753 request that it be used if available, by setting an option that is 1754 ignored when it is not relevant. More complicated programs might need 1755 to make use of the functions pcre_jit_stack_alloc(), 1756 pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control 1757 the JIT code's memory usage. 1758 1759 From release 8.32 there is also a direct interface for JIT execution, 1760 which gives improved performance. The JIT-specific functions are dis- 1761 cussed in the pcrejit documentation. 1762 1763 A second matching function, pcre_dfa_exec(), which is not Perl-compati- 1764 ble, is also provided. This uses a different algorithm for the match- 1765 ing. The alternative algorithm finds all possible matches (at a given 1766 point in the subject), and scans the subject just once (unless there 1767 are lookbehind assertions). However, this algorithm does not return 1768 captured substrings. A description of the two matching algorithms and 1769 their advantages and disadvantages is given in the pcrematching docu- 1770 mentation. 1771 1772 In addition to the main compiling and matching functions, there are 1773 convenience functions for extracting captured substrings from a subject 1774 string that is matched by pcre_exec(). They are: 1775 1776 pcre_copy_substring() 1777 pcre_copy_named_substring() 1778 pcre_get_substring() 1779 pcre_get_named_substring() 1780 pcre_get_substring_list() 1781 pcre_get_stringnumber() 1782 pcre_get_stringtable_entries() 1783 1784 pcre_free_substring() and pcre_free_substring_list() are also provided, 1785 to free the memory used for extracted strings. 1786 1787 The function pcre_maketables() is used to build a set of character 1788 tables in the current locale for passing to pcre_compile(), 1789 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is 1790 provided for specialist use. Most commonly, no special tables are 1791 passed, in which case internal tables that are generated when PCRE is 1792 built are used. 1793 1794 The function pcre_fullinfo() is used to find out information about a 1795 compiled pattern. The function pcre_version() returns a pointer to a 1796 string containing the version of PCRE and its date of release. 1797 1798 The function pcre_refcount() maintains a reference count in a data 1799 block containing a compiled pattern. This is provided for the benefit 1800 of object-oriented applications. 1801 1802 The global variables pcre_malloc and pcre_free initially contain the 1803 entry points of the standard malloc() and free() functions, respec- 1804 tively. PCRE calls the memory management functions via these variables, 1805 so a calling program can replace them if it wishes to intercept the 1806 calls. This should be done before calling any PCRE functions. 1807 1808 The global variables pcre_stack_malloc and pcre_stack_free are also 1809 indirections to memory management functions. These special functions 1810 are used only when PCRE is compiled to use the heap for remembering 1811 data, instead of recursive function calls, when running the pcre_exec() 1812 function. See the pcrebuild documentation for details of how to do 1813 this. It is a non-standard way of building PCRE, for use in environ- 1814 ments that have limited stacks. Because of the greater use of memory 1815 management, it runs more slowly. Separate functions are provided so 1816 that special-purpose external code can be used for this case. When 1817 used, these functions are always called in a stack-like manner (last 1818 obtained, first freed), and always for memory blocks of the same size. 1819 There is a discussion about PCRE's stack usage in the pcrestack docu- 1820 mentation. 1821 1822 The global variable pcre_callout initially contains NULL. It can be set 1823 by the caller to a "callout" function, which PCRE will then call at 1824 specified points during a matching operation. Details are given in the 1825 pcrecallout documentation. 1826 1827 The global variable pcre_stack_guard initially contains NULL. It can be 1828 set by the caller to a function that is called by PCRE whenever it 1829 starts to compile a parenthesized part of a pattern. When parentheses 1830 are nested, PCRE uses recursive function calls, which use up the system 1831 stack. This function is provided so that applications with restricted 1832 stacks can force a compilation error if the stack runs out. The func- 1833 tion should return zero if all is well, or non-zero to force an error. 1834 1835 1836 NEWLINES 1837 1838 PCRE supports five different conventions for indicating line breaks in 1839 strings: a single CR (carriage return) character, a single LF (line- 1840 feed) character, the two-character sequence CRLF, any of the three pre- 1841 ceding, or any Unicode newline sequence. The Unicode newline sequences 1842 are the three just mentioned, plus the single characters VT (vertical 1843 tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line 1844 separator, U+2028), and PS (paragraph separator, U+2029). 1845 1846 Each of the first three conventions is used by at least one operating 1847 system as its standard newline sequence. When PCRE is built, a default 1848 can be specified. The default default is LF, which is the Unix stan- 1849 dard. When PCRE is run, the default can be overridden, either when a 1850 pattern is compiled, or when it is matched. 1851 1852 At compile time, the newline convention can be specified by the options 1853 argument of pcre_compile(), or it can be specified by special text at 1854 the start of the pattern itself; this overrides any other settings. See 1855 the pcrepattern page for details of the special character sequences. 1856 1857 In the PCRE documentation the word "newline" is used to mean "the char- 1858 acter or pair of characters that indicate a line break". The choice of 1859 newline convention affects the handling of the dot, circumflex, and 1860 dollar metacharacters, the handling of #-comments in /x mode, and, when 1861 CRLF is a recognized line ending sequence, the match position advance- 1862 ment for a non-anchored pattern. There is more detail about this in the 1863 section on pcre_exec() options below. 1864 1865 The choice of newline convention does not affect the interpretation of 1866 the \n or \r escape sequences, nor does it affect what \R matches, 1867 which is controlled in a similar way, but by separate options. 1868 1869 1870 MULTITHREADING 1871 1872 The PCRE functions can be used in multi-threading applications, with 1873 the proviso that the memory management functions pointed to by 1874 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the 1875 callout and stack-checking functions pointed to by pcre_callout and 1876 pcre_stack_guard, are shared by all threads. 1877 1878 The compiled form of a regular expression is not altered during match- 1879 ing, so the same compiled pattern can safely be used by several threads 1880 at once. 1881 1882 If the just-in-time optimization feature is being used, it needs sepa- 1883 rate memory stack areas for each thread. See the pcrejit documentation 1884 for more details. 1885 1886 1887 SAVING PRECOMPILED PATTERNS FOR LATER USE 1888 1889 The compiled form of a regular expression can be saved and re-used at a 1890 later time, possibly by a different program, and even on a host other 1891 than the one on which it was compiled. Details are given in the 1892 pcreprecompile documentation, which includes a description of the 1893 pcre_pattern_to_host_byte_order() function. However, compiling a regu- 1894 lar expression with one version of PCRE for use with a different ver- 1895 sion is not guaranteed to work and may cause crashes. 1896 1897 1898 CHECKING BUILD-TIME OPTIONS 1899 1900 int pcre_config(int what, void *where); 1901 1902 The function pcre_config() makes it possible for a PCRE client to dis- 1903 cover which optional features have been compiled into the PCRE library. 1904 The pcrebuild documentation has more details about these optional fea- 1905 tures. 1906 1907 The first argument for pcre_config() is an integer, specifying which 1908 information is required; the second argument is a pointer to a variable 1909 into which the information is placed. The returned value is zero on 1910 success, or the negative error code PCRE_ERROR_BADOPTION if the value 1911 in the first argument is not recognized. The following information is 1912 available: 1913 1914 PCRE_CONFIG_UTF8 1915 1916 The output is an integer that is set to one if UTF-8 support is avail- 1917 able; otherwise it is set to zero. This value should normally be given 1918 to the 8-bit version of this function, pcre_config(). If it is given to 1919 the 16-bit or 32-bit version of this function, the result is 1920 PCRE_ERROR_BADOPTION. 1921 1922 PCRE_CONFIG_UTF16 1923 1924 The output is an integer that is set to one if UTF-16 support is avail- 1925 able; otherwise it is set to zero. This value should normally be given 1926 to the 16-bit version of this function, pcre16_config(). If it is given 1927 to the 8-bit or 32-bit version of this function, the result is 1928 PCRE_ERROR_BADOPTION. 1929 1930 PCRE_CONFIG_UTF32 1931 1932 The output is an integer that is set to one if UTF-32 support is avail- 1933 able; otherwise it is set to zero. This value should normally be given 1934 to the 32-bit version of this function, pcre32_config(). If it is given 1935 to the 8-bit or 16-bit version of this function, the result is 1936 PCRE_ERROR_BADOPTION. 1937 1938 PCRE_CONFIG_UNICODE_PROPERTIES 1939 1940 The output is an integer that is set to one if support for Unicode 1941 character properties is available; otherwise it is set to zero. 1942 1943 PCRE_CONFIG_JIT 1944 1945 The output is an integer that is set to one if support for just-in-time 1946 compiling is available; otherwise it is set to zero. 1947 1948 PCRE_CONFIG_JITTARGET 1949 1950 The output is a pointer to a zero-terminated "const char *" string. If 1951 JIT support is available, the string contains the name of the architec- 1952 ture for which the JIT compiler is configured, for example "x86 32bit 1953 (little endian + unaligned)". If JIT support is not available, the 1954 result is NULL. 1955 1956 PCRE_CONFIG_NEWLINE 1957 1958 The output is an integer whose value specifies the default character 1959 sequence that is recognized as meaning "newline". The values that are 1960 supported in ASCII/Unicode environments are: 10 for LF, 13 for CR, 3338 1961 for CRLF, -2 for ANYCRLF, and -1 for ANY. In EBCDIC environments, CR, 1962 ANYCRLF, and ANY yield the same values. However, the value for LF is 1963 normally 21, though some EBCDIC environments use 37. The corresponding 1964 values for CRLF are 3349 and 3365. The default should normally corre- 1965 spond to the standard sequence for your operating system. 1966 1967 PCRE_CONFIG_BSR 1968 1969 The output is an integer whose value indicates what character sequences 1970 the \R escape sequence matches by default. A value of 0 means that \R 1971 matches any Unicode line ending sequence; a value of 1 means that \R 1972 matches only CR, LF, or CRLF. The default can be overridden when a pat- 1973 tern is compiled or matched. 1974 1975 PCRE_CONFIG_LINK_SIZE 1976 1977 The output is an integer that contains the number of bytes used for 1978 internal linkage in compiled regular expressions. For the 8-bit 1979 library, the value can be 2, 3, or 4. For the 16-bit library, the value 1980 is either 2 or 4 and is still a number of bytes. For the 32-bit 1981 library, the value is either 2 or 4 and is still a number of bytes. The 1982 default value of 2 is sufficient for all but the most massive patterns, 1983 since it allows the compiled pattern to be up to 64K in size. Larger 1984 values allow larger regular expressions to be compiled, at the expense 1985 of slower matching. 1986 1987 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD 1988 1989 The output is an integer that contains the threshold above which the 1990 POSIX interface uses malloc() for output vectors. Further details are 1991 given in the pcreposix documentation. 1992 1993 PCRE_CONFIG_PARENS_LIMIT 1994 1995 The output is a long integer that gives the maximum depth of nesting of 1996 parentheses (of any kind) in a pattern. This limit is imposed to cap 1997 the amount of system stack used when a pattern is compiled. It is spec- 1998 ified when PCRE is built; the default is 250. This limit does not take 1999 into account the stack that may already be used by the calling applica- 2000 tion. For finer control over compilation stack usage, you can set a 2001 pointer to an external checking function in pcre_stack_guard. 2002 2003 PCRE_CONFIG_MATCH_LIMIT 2004 2005 The output is a long integer that gives the default limit for the num- 2006 ber of internal matching function calls in a pcre_exec() execution. 2007 Further details are given with pcre_exec() below. 2008 2009 PCRE_CONFIG_MATCH_LIMIT_RECURSION 2010 2011 The output is a long integer that gives the default limit for the depth 2012 of recursion when calling the internal matching function in a 2013 pcre_exec() execution. Further details are given with pcre_exec() 2014 below. 2015 2016 PCRE_CONFIG_STACKRECURSE 2017 2018 The output is an integer that is set to one if internal recursion when 2019 running pcre_exec() is implemented by recursive function calls that use 2020 the stack to remember their state. This is the usual way that PCRE is 2021 compiled. The output is zero if PCRE was compiled to use blocks of data 2022 on the heap instead of recursive function calls. In this case, 2023 pcre_stack_malloc and pcre_stack_free are called to manage memory 2024 blocks on the heap, thus avoiding the use of the stack. 2025 2026 2027 COMPILING A PATTERN 2028 2029 pcre *pcre_compile(const char *pattern, int options, 2030 const char **errptr, int *erroffset, 2031 const unsigned char *tableptr); 2032 2033 pcre *pcre_compile2(const char *pattern, int options, 2034 int *errorcodeptr, 2035 const char **errptr, int *erroffset, 2036 const unsigned char *tableptr); 2037 2038 Either of the functions pcre_compile() or pcre_compile2() can be called 2039 to compile a pattern into an internal form. The only difference between 2040 the two interfaces is that pcre_compile2() has an additional argument, 2041 errorcodeptr, via which a numerical error code can be returned. To 2042 avoid too much repetition, we refer just to pcre_compile() below, but 2043 the information applies equally to pcre_compile2(). 2044 2045 The pattern is a C string terminated by a binary zero, and is passed in 2046 the pattern argument. A pointer to a single block of memory that is 2047 obtained via pcre_malloc is returned. This contains the compiled code 2048 and related data. The pcre type is defined for the returned block; this 2049 is a typedef for a structure whose contents are not externally defined. 2050 It is up to the caller to free the memory (via pcre_free) when it is no 2051 longer required. 2052 2053 Although the compiled code of a PCRE regex is relocatable, that is, it 2054 does not depend on memory location, the complete pcre data block is not 2055 fully relocatable, because it may contain a copy of the tableptr argu- 2056 ment, which is an address (see below). 2057 2058 The options argument contains various bit settings that affect the com- 2059 pilation. It should be zero if no options are required. The available 2060 options are described below. Some of them (in particular, those that 2061 are compatible with Perl, but some others as well) can also be set and 2062 unset from within the pattern (see the detailed description in the 2063 pcrepattern documentation). For those options that can be different in 2064 different parts of the pattern, the contents of the options argument 2065 specifies their settings at the start of compilation and execution. The 2066 PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and 2067 PCRE_NO_START_OPTIMIZE options can be set at the time of matching as 2068 well as at compile time. 2069 2070 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, 2071 if compilation of a pattern fails, pcre_compile() returns NULL, and 2072 sets the variable pointed to by errptr to point to a textual error mes- 2073 sage. This is a static string that is part of the library. You must not 2074 try to free it. Normally, the offset from the start of the pattern to 2075 the data unit that was being processed when the error was discovered is 2076 placed in the variable pointed to by erroffset, which must not be NULL 2077 (if it is, an immediate error is given). However, for an invalid UTF-8 2078 or UTF-16 string, the offset is that of the first data unit of the 2079 failing character. 2080 2081 Some errors are not detected until the whole pattern has been scanned; 2082 in these cases, the offset passed back is the length of the pattern. 2083 Note that the offset is in data units, not characters, even in a UTF 2084 mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char- 2085 acter. 2086 2087 If pcre_compile2() is used instead of pcre_compile(), and the error- 2088 codeptr argument is not NULL, a non-zero error code number is returned 2089 via this argument in the event of an error. This is in addition to the 2090 textual error message. Error codes and messages are listed below. 2091 2092 If the final argument, tableptr, is NULL, PCRE uses a default set of 2093 character tables that are built when PCRE is compiled, using the 2094 default C locale. Otherwise, tableptr must be an address that is the 2095 result of a call to pcre_maketables(). This value is stored with the 2096 compiled pattern, and used again by pcre_exec() and pcre_dfa_exec() 2097 when the pattern is matched. For more discussion, see the section on 2098 locale support below. 2099 2100 This code fragment shows a typical straightforward call to pcre_com- 2101 pile(): 2102 2103 pcre *re; 2104 const char *error; 2105 int erroffset; 2106 re = pcre_compile( 2107 "^A.*Z", /* the pattern */ 2108 0, /* default options */ 2109 &error, /* for error message */ 2110 &erroffset, /* for error offset */ 2111 NULL); /* use default character tables */ 2112 2113 The following names for option bits are defined in the pcre.h header 2114 file: 2115 2116 PCRE_ANCHORED 2117 2118 If this bit is set, the pattern is forced to be "anchored", that is, it 2119 is constrained to match only at the first matching point in the string 2120 that is being searched (the "subject string"). This effect can also be 2121 achieved by appropriate constructs in the pattern itself, which is the 2122 only way to do it in Perl. 2123 2124 PCRE_AUTO_CALLOUT 2125 2126 If this bit is set, pcre_compile() automatically inserts callout items, 2127 all with number 255, before each pattern item. For discussion of the 2128 callout facility, see the pcrecallout documentation. 2129 2130 PCRE_BSR_ANYCRLF 2131 PCRE_BSR_UNICODE 2132 2133 These options (which are mutually exclusive) control what the \R escape 2134 sequence matches. The choice is either to match only CR, LF, or CRLF, 2135 or to match any Unicode newline sequence. The default is specified when 2136 PCRE is built. It can be overridden from within the pattern, or by set- 2137 ting an option when a compiled pattern is matched. 2138 2139 PCRE_CASELESS 2140 2141 If this bit is set, letters in the pattern match both upper and lower 2142 case letters. It is equivalent to Perl's /i option, and it can be 2143 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE 2144 always understands the concept of case for characters whose values are 2145 less than 128, so caseless matching is always possible. For characters 2146 with higher values, the concept of case is supported if PCRE is com- 2147 piled with Unicode property support, but not otherwise. If you want to 2148 use caseless matching for characters 128 and above, you must ensure 2149 that PCRE is compiled with Unicode property support as well as with 2150 UTF-8 support. 2151 2152 PCRE_DOLLAR_ENDONLY 2153 2154 If this bit is set, a dollar metacharacter in the pattern matches only 2155 at the end of the subject string. Without this option, a dollar also 2156 matches immediately before a newline at the end of the string (but not 2157 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored 2158 if PCRE_MULTILINE is set. There is no equivalent to this option in 2159 Perl, and no way to set it within a pattern. 2160 2161 PCRE_DOTALL 2162 2163 If this bit is set, a dot metacharacter in the pattern matches a char- 2164 acter of any value, including one that indicates a newline. However, it 2165 only ever matches one character, even if newlines are coded as CRLF. 2166 Without this option, a dot does not match when the current position is 2167 at a newline. This option is equivalent to Perl's /s option, and it can 2168 be changed within a pattern by a (?s) option setting. A negative class 2169 such as [^a] always matches newline characters, independent of the set- 2170 ting of this option. 2171 2172 PCRE_DUPNAMES 2173 2174 If this bit is set, names used to identify capturing subpatterns need 2175 not be unique. This can be helpful for certain types of pattern when it 2176 is known that only one instance of the named subpattern can ever be 2177 matched. There are more details of named subpatterns below; see also 2178 the pcrepattern documentation. 2179 2180 PCRE_EXTENDED 2181 2182 If this bit is set, most white space characters in the pattern are 2183 totally ignored except when escaped or inside a character class. How- 2184 ever, white space is not allowed within sequences such as (?> that 2185 introduce various parenthesized subpatterns, nor within a numerical 2186 quantifier such as {1,3}. However, ignorable white space is permitted 2187 between an item and a following quantifier and between a quantifier and 2188 a following + that indicates possessiveness. 2189 2190 White space did not used to include the VT character (code 11), because 2191 Perl did not treat this character as white space. However, Perl changed 2192 at release 5.18, so PCRE followed at release 8.34, and VT is now 2193 treated as white space. 2194 2195 PCRE_EXTENDED also causes characters between an unescaped # outside a 2196 character class and the next newline, inclusive, to be ignored. 2197 PCRE_EXTENDED is equivalent to Perl's /x option, and it can be changed 2198 within a pattern by a (?x) option setting. 2199 2200 Which characters are interpreted as newlines is controlled by the 2201 options passed to pcre_compile() or by a special sequence at the start 2202 of the pattern, as described in the section entitled "Newline conven- 2203 tions" in the pcrepattern documentation. Note that the end of this type 2204 of comment is a literal newline sequence in the pattern; escape 2205 sequences that happen to represent a newline do not count. 2206 2207 This option makes it possible to include comments inside complicated 2208 patterns. Note, however, that this applies only to data characters. 2209 White space characters may never appear within special character 2210 sequences in a pattern, for example within the sequence (?( that intro- 2211 duces a conditional subpattern. 2212 2213 PCRE_EXTRA 2214 2215 This option was invented in order to turn on additional functionality 2216 of PCRE that is incompatible with Perl, but it is currently of very 2217 little use. When set, any backslash in a pattern that is followed by a 2218 letter that has no special meaning causes an error, thus reserving 2219 these combinations for future expansion. By default, as in Perl, a 2220 backslash followed by a letter with no special meaning is treated as a 2221 literal. (Perl can, however, be persuaded to give an error for this, by 2222 running it with the -w option.) There are at present no other features 2223 controlled by this option. It can also be set by a (?X) option setting 2224 within a pattern. 2225 2226 PCRE_FIRSTLINE 2227 2228 If this option is set, an unanchored pattern is required to match 2229 before or at the first newline in the subject string, though the 2230 matched text may continue over the newline. 2231 2232 PCRE_JAVASCRIPT_COMPAT 2233 2234 If this option is set, PCRE's behaviour is changed in some ways so that 2235 it is compatible with JavaScript rather than Perl. The changes are as 2236 follows: 2237 2238 (1) A lone closing square bracket in a pattern causes a compile-time 2239 error, because this is illegal in JavaScript (by default it is treated 2240 as a data character). Thus, the pattern AB]CD becomes illegal when this 2241 option is set. 2242 2243 (2) At run time, a back reference to an unset subpattern group matches 2244 an empty string (by default this causes the current matching alterna- 2245 tive to fail). A pattern such as (\1)(a) succeeds when this option is 2246 set (assuming it can find an "a" in the subject), whereas it fails by 2247 default, for Perl compatibility. 2248 2249 (3) \U matches an upper case "U" character; by default \U causes a com- 2250 pile time error (Perl uses \U to upper case subsequent characters). 2251 2252 (4) \u matches a lower case "u" character unless it is followed by four 2253 hexadecimal digits, in which case the hexadecimal number defines the 2254 code point to match. By default, \u causes a compile time error (Perl 2255 uses it to upper case the following character). 2256 2257 (5) \x matches a lower case "x" character unless it is followed by two 2258 hexadecimal digits, in which case the hexadecimal number defines the 2259 code point to match. By default, as in Perl, a hexadecimal number is 2260 always expected after \x, but it may have zero, one, or two digits (so, 2261 for example, \xz matches a binary zero character followed by z). 2262 2263 PCRE_MULTILINE 2264 2265 By default, for the purposes of matching "start of line" and "end of 2266 line", PCRE treats the subject string as consisting of a single line of 2267 characters, even if it actually contains newlines. The "start of line" 2268 metacharacter (^) matches only at the start of the string, and the "end 2269 of line" metacharacter ($) matches only at the end of the string, or 2270 before a terminating newline (except when PCRE_DOLLAR_ENDONLY is set). 2271 Note, however, that unless PCRE_DOTALL is set, the "any character" 2272 metacharacter (.) does not match at a newline. This behaviour (for ^, 2273 $, and dot) is the same as Perl. 2274 2275 When PCRE_MULTILINE it is set, the "start of line" and "end of line" 2276 constructs match immediately following or immediately before internal 2277 newlines in the subject string, respectively, as well as at the very 2278 start and end. This is equivalent to Perl's /m option, and it can be 2279 changed within a pattern by a (?m) option setting. If there are no new- 2280 lines in a subject string, or no occurrences of ^ or $ in a pattern, 2281 setting PCRE_MULTILINE has no effect. 2282 2283 PCRE_NEVER_UTF 2284 2285 This option locks out interpretation of the pattern as UTF-8 (or UTF-16 2286 or UTF-32 in the 16-bit and 32-bit libraries). In particular, it pre- 2287 vents the creator of the pattern from switching to UTF interpretation 2288 by starting the pattern with (*UTF). This may be useful in applications 2289 that process patterns from external sources. The combination of 2290 PCRE_UTF8 and PCRE_NEVER_UTF also causes an error. 2291 2292 PCRE_NEWLINE_CR 2293 PCRE_NEWLINE_LF 2294 PCRE_NEWLINE_CRLF 2295 PCRE_NEWLINE_ANYCRLF 2296 PCRE_NEWLINE_ANY 2297 2298 These options override the default newline definition that was chosen 2299 when PCRE was built. Setting the first or the second specifies that a 2300 newline is indicated by a single character (CR or LF, respectively). 2301 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the 2302 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies 2303 that any of the three preceding sequences should be recognized. Setting 2304 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be 2305 recognized. 2306 2307 In an ASCII/Unicode environment, the Unicode newline sequences are the 2308 three just mentioned, plus the single characters VT (vertical tab, 2309 U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line sep- 2310 arator, U+2028), and PS (paragraph separator, U+2029). For the 8-bit 2311 library, the last two are recognized only in UTF-8 mode. 2312 2313 When PCRE is compiled to run in an EBCDIC (mainframe) environment, the 2314 code for CR is 0x0d, the same as ASCII. However, the character code for 2315 LF is normally 0x15, though in some EBCDIC environments 0x25 is used. 2316 Whichever of these is not LF is made to correspond to Unicode's NEL 2317 character. EBCDIC codes are all less than 256. For more details, see 2318 the pcrebuild documentation. 2319 2320 The newline setting in the options word uses three bits that are 2321 treated as a number, giving eight possibilities. Currently only six are 2322 used (default plus the five values above). This means that if you set 2323 more than one newline option, the combination may or may not be sensi- 2324 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to 2325 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and 2326 cause an error. 2327 2328 The only time that a line break in a pattern is specially recognized 2329 when compiling is when PCRE_EXTENDED is set. CR and LF are white space 2330 characters, and so are ignored in this mode. Also, an unescaped # out- 2331 side a character class indicates a comment that lasts until after the 2332 next line break sequence. In other circumstances, line break sequences 2333 in patterns are treated as literal data. 2334 2335 The newline option that is set at compile time becomes the default that 2336 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. 2337 2338 PCRE_NO_AUTO_CAPTURE 2339 2340 If this option is set, it disables the use of numbered capturing paren- 2341 theses in the pattern. Any opening parenthesis that is not followed by 2342 ? behaves as if it were followed by ?: but named parentheses can still 2343 be used for capturing (and they acquire numbers in the usual way). 2344 There is no equivalent of this option in Perl. 2345 2346 PCRE_NO_AUTO_POSSESS 2347 2348 If this option is set, it disables "auto-possessification". This is an 2349 optimization that, for example, turns a+b into a++b in order to avoid 2350 backtracks into a+ that can never be successful. However, if callouts 2351 are in use, auto-possessification means that some of them are never 2352 taken. You can set this option if you want the matching functions to do 2353 a full unoptimized search and run all the callouts, but it is mainly 2354 provided for testing purposes. 2355 2356 PCRE_NO_START_OPTIMIZE 2357 2358 This is an option that acts at matching time; that is, it is really an 2359 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile 2360 time, it is remembered with the compiled pattern and assumed at match- 2361 ing time. This is necessary if you want to use JIT execution, because 2362 the JIT compiler needs to know whether or not this option is set. For 2363 details see the discussion of PCRE_NO_START_OPTIMIZE below. 2364 2365 PCRE_UCP 2366 2367 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, 2368 \w, and some of the POSIX character classes. By default, only ASCII 2369 characters are recognized, but if PCRE_UCP is set, Unicode properties 2370 are used instead to classify characters. More details are given in the 2371 section on generic character types in the pcrepattern page. If you set 2372 PCRE_UCP, matching one of the items it affects takes much longer. The 2373 option is available only if PCRE has been compiled with Unicode prop- 2374 erty support. 2375 2376 PCRE_UNGREEDY 2377 2378 This option inverts the "greediness" of the quantifiers so that they 2379 are not greedy by default, but become greedy if followed by "?". It is 2380 not compatible with Perl. It can also be set by a (?U) option setting 2381 within the pattern. 2382 2383 PCRE_UTF8 2384 2385 This option causes PCRE to regard both the pattern and the subject as 2386 strings of UTF-8 characters instead of single-byte strings. However, it 2387 is available only when PCRE is built to include UTF support. If not, 2388 the use of this option provokes an error. Details of how this option 2389 changes the behaviour of PCRE are given in the pcreunicode page. 2390 2391 PCRE_NO_UTF8_CHECK 2392 2393 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is 2394 automatically checked. There is a discussion about the validity of 2395 UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is 2396 found, pcre_compile() returns an error. If you already know that your 2397 pattern is valid, and you want to skip this check for performance rea- 2398 sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the 2399 effect of passing an invalid UTF-8 string as a pattern is undefined. It 2400 may cause your program to crash or loop. Note that this option can also 2401 be passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity 2402 checking of subject strings only. If the same string is being matched 2403 many times, the option can be safely set for the second and subsequent 2404 matchings to improve performance. 2405 2406 2407 COMPILATION ERROR CODES 2408 2409 The following table lists the error codes than may be returned by 2410 pcre_compile2(), along with the error messages that may be returned by 2411 both compiling functions. Note that error messages are always 8-bit 2412 ASCII strings, even in 16-bit or 32-bit mode. As PCRE has developed, 2413 some error codes have fallen out of use. To avoid confusion, they have 2414 not been re-used. 2415 2416 0 no error 2417 1 \ at end of pattern 2418 2 \c at end of pattern 2419 3 unrecognized character follows \ 2420 4 numbers out of order in {} quantifier 2421 5 number too big in {} quantifier 2422 6 missing terminating ] for character class 2423 7 invalid escape sequence in character class 2424 8 range out of order in character class 2425 9 nothing to repeat 2426 10 [this code is not in use] 2427 11 internal error: unexpected repeat 2428 12 unrecognized character after (? or (?- 2429 13 POSIX named classes are supported only within a class 2430 14 missing ) 2431 15 reference to non-existent subpattern 2432 16 erroffset passed as NULL 2433 17 unknown option bit(s) set 2434 18 missing ) after comment 2435 19 [this code is not in use] 2436 20 regular expression is too large 2437 21 failed to get memory 2438 22 unmatched parentheses 2439 23 internal error: code overflow 2440 24 unrecognized character after (?< 2441 25 lookbehind assertion is not fixed length 2442 26 malformed number or name after (?( 2443 27 conditional group contains more than two branches 2444 28 assertion expected after (?( 2445 29 (?R or (?[+-]digits must be followed by ) 2446 30 unknown POSIX class name 2447 31 POSIX collating elements are not supported 2448 32 this version of PCRE is compiled without UTF support 2449 33 [this code is not in use] 2450 34 character value in \x{} or \o{} is too large 2451 35 invalid condition (?(0) 2452 36 \C not allowed in lookbehind assertion 2453 37 PCRE does not support \L, \l, \N{name}, \U, or \u 2454 38 number after (?C is > 255 2455 39 closing ) for (?C expected 2456 40 recursive call could loop indefinitely 2457 41 unrecognized character after (?P 2458 42 syntax error in subpattern name (missing terminator) 2459 43 two named subpatterns have the same name 2460 44 invalid UTF-8 string (specifically UTF-8) 2461 45 support for \P, \p, and \X has not been compiled 2462 46 malformed \P or \p sequence 2463 47 unknown property name after \P or \p 2464 48 subpattern name is too long (maximum 32 characters) 2465 49 too many named subpatterns (maximum 10000) 2466 50 [this code is not in use] 2467 51 octal value is greater than \377 in 8-bit non-UTF-8 mode 2468 52 internal error: overran compiling workspace 2469 53 internal error: previously-checked referenced subpattern 2470 not found 2471 54 DEFINE group contains more than one branch 2472 55 repeating a DEFINE group is not allowed 2473 56 inconsistent NEWLINE options 2474 57 \g is not followed by a braced, angle-bracketed, or quoted 2475 name/number or by a plain number 2476 58 a numbered reference must not be zero 2477 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT) 2478 60 (*VERB) not recognized or malformed 2479 61 number is too big 2480 62 subpattern name expected 2481 63 digit expected after (?+ 2482 64 ] is an invalid data character in JavaScript compatibility mode 2483 65 different names for subpatterns of the same number are 2484 not allowed 2485 66 (*MARK) must have an argument 2486 67 this version of PCRE is not compiled with Unicode property 2487 support 2488 68 \c must be followed by an ASCII character 2489 69 \k is not followed by a braced, angle-bracketed, or quoted name 2490 70 internal error: unknown opcode in find_fixedlength() 2491 71 \N is not supported in a class 2492 72 too many forward references 2493 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff) 2494 74 invalid UTF-16 string (specifically UTF-16) 2495 75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN) 2496 76 character value in \u.... sequence is too large 2497 77 invalid UTF-32 string (specifically UTF-32) 2498 78 setting UTF is disabled by the application 2499 79 non-hex character in \x{} (closing brace missing?) 2500 80 non-octal character in \o{} (closing brace missing?) 2501 81 missing opening brace after \o 2502 82 parentheses are too deeply nested 2503 83 invalid range in character class 2504 84 group name must start with a non-digit 2505 85 parentheses are too deeply nested (stack check) 2506 2507 The numbers 32 and 10000 in errors 48 and 49 are defaults; different 2508 values may be used if the limits were changed when PCRE was built. 2509 2510 2511 STUDYING A PATTERN 2512 2513 pcre_extra *pcre_study(const pcre *code, int options, 2514 const char **errptr); 2515 2516 If a compiled pattern is going to be used several times, it is worth 2517 spending more time analyzing it in order to speed up the time taken for 2518 matching. The function pcre_study() takes a pointer to a compiled pat- 2519 tern as its first argument. If studying the pattern produces additional 2520 information that will help speed up matching, pcre_study() returns a 2521 pointer to a pcre_extra block, in which the study_data field points to 2522 the results of the study. 2523 2524 The returned value from pcre_study() can be passed directly to 2525 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con- 2526 tains other fields that can be set by the caller before the block is 2527 passed; these are described below in the section on matching a pattern. 2528 2529 If studying the pattern does not produce any useful information, 2530 pcre_study() returns NULL by default. In that circumstance, if the 2531 calling program wants to pass any of the other fields to pcre_exec() or 2532 pcre_dfa_exec(), it must set up its own pcre_extra block. However, if 2533 pcre_study() is called with the PCRE_STUDY_EXTRA_NEEDED option, it 2534 returns a pcre_extra block even if studying did not find any additional 2535 information. It may still return NULL, however, if an error occurs in 2536 pcre_study(). 2537 2538 The second argument of pcre_study() contains option bits. There are 2539 three further options in addition to PCRE_STUDY_EXTRA_NEEDED: 2540 2541 PCRE_STUDY_JIT_COMPILE 2542 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 2543 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 2544 2545 If any of these are set, and the just-in-time compiler is available, 2546 the pattern is further compiled into machine code that executes much 2547 faster than the pcre_exec() interpretive matching function. If the 2548 just-in-time compiler is not available, these options are ignored. All 2549 undefined bits in the options argument must be zero. 2550 2551 JIT compilation is a heavyweight optimization. It can take some time 2552 for patterns to be analyzed, and for one-off matches and simple pat- 2553 terns the benefit of faster execution might be offset by a much slower 2554 study time. Not all patterns can be optimized by the JIT compiler. For 2555 those that cannot be handled, matching automatically falls back to the 2556 pcre_exec() interpreter. For more details, see the pcrejit documenta- 2557 tion. 2558 2559 The third argument for pcre_study() is a pointer for an error message. 2560 If studying succeeds (even if no data is returned), the variable it 2561 points to is set to NULL. Otherwise it is set to point to a textual 2562 error message. This is a static string that is part of the library. You 2563 must not try to free it. You should test the error pointer for NULL 2564 after calling pcre_study(), to be sure that it has run successfully. 2565 2566 When you are finished with a pattern, you can free the memory used for 2567 the study data by calling pcre_free_study(). This function was added to 2568 the API for release 8.20. For earlier versions, the memory could be 2569 freed with pcre_free(), just like the pattern itself. This will still 2570 work in cases where JIT optimization is not used, but it is advisable 2571 to change to the new function when convenient. 2572 2573 This is a typical way in which pcre_study() is used (except that in a 2574 real application there should be tests for errors): 2575 2576 int rc; 2577 pcre *re; 2578 pcre_extra *sd; 2579 re = pcre_compile("pattern", 0, &error, &erroroffset, NULL); 2580 sd = pcre_study( 2581 re, /* result of pcre_compile() */ 2582 0, /* no options */ 2583 &error); /* set to NULL or points to a message */ 2584 rc = pcre_exec( /* see below for details of pcre_exec() options */ 2585 re, sd, "subject", 7, 0, 0, ovector, 30); 2586 ... 2587 pcre_free_study(sd); 2588 pcre_free(re); 2589 2590 Studying a pattern does two things: first, a lower bound for the length 2591 of subject string that is needed to match the pattern is computed. This 2592 does not mean that there are any strings of that length that match, but 2593 it does guarantee that no shorter strings match. The value is used to 2594 avoid wasting time by trying to match strings that are shorter than the 2595 lower bound. You can find out the value in a calling program via the 2596 pcre_fullinfo() function. 2597 2598 Studying a pattern is also useful for non-anchored patterns that do not 2599 have a single fixed starting character. A bitmap of possible starting 2600 bytes is created. This speeds up finding a position in the subject at 2601 which to start matching. (In 16-bit mode, the bitmap is used for 16-bit 2602 values less than 256. In 32-bit mode, the bitmap is used for 32-bit 2603 values less than 256.) 2604 2605 These two optimizations apply to both pcre_exec() and pcre_dfa_exec(), 2606 and the information is also used by the JIT compiler. The optimiza- 2607 tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option. 2608 You might want to do this if your pattern contains callouts or (*MARK) 2609 and you want to make use of these facilities in cases where matching 2610 fails. 2611 2612 PCRE_NO_START_OPTIMIZE can be specified at either compile time or exe- 2613 cution time. However, if PCRE_NO_START_OPTIMIZE is passed to 2614 pcre_exec(), (that is, after any JIT compilation has happened) JIT exe- 2615 cution is disabled. For JIT execution to work with PCRE_NO_START_OPTI- 2616 MIZE, the option must be set at compile time. 2617 2618 There is a longer discussion of PCRE_NO_START_OPTIMIZE below. 2619 2620 2621 LOCALE SUPPORT 2622 2623 PCRE handles caseless matching, and determines whether characters are 2624 letters, digits, or whatever, by reference to a set of tables, indexed 2625 by character code point. When running in UTF-8 mode, or in the 16- or 2626 32-bit libraries, this applies only to characters with code points less 2627 than 256. By default, higher-valued code points never match escapes 2628 such as \w or \d. However, if PCRE is built with Unicode property sup- 2629 port, all characters can be tested with \p and \P, or, alternatively, 2630 the PCRE_UCP option can be set when a pattern is compiled; this causes 2631 \w and friends to use Unicode property support instead of the built-in 2632 tables. 2633 2634 The use of locales with Unicode is discouraged. If you are handling 2635 characters with code points greater than 128, you should either use 2636 Unicode support, or use locales, but not try to mix the two. 2637 2638 PCRE contains an internal set of tables that are used when the final 2639 argument of pcre_compile() is NULL. These are sufficient for many 2640 applications. Normally, the internal tables recognize only ASCII char- 2641 acters. However, when PCRE is built, it is possible to cause the inter- 2642 nal tables to be rebuilt in the default "C" locale of the local system, 2643 which may cause them to be different. 2644 2645 The internal tables can always be overridden by tables supplied by the 2646 application that calls PCRE. These may be created in a different locale 2647 from the default. As more and more applications change to using Uni- 2648 code, the need for this locale support is expected to die away. 2649 2650 External tables are built by calling the pcre_maketables() function, 2651 which has no arguments, in the relevant locale. The result can then be 2652 passed to pcre_compile() as often as necessary. For example, to build 2653 and use tables that are appropriate for the French locale (where 2654 accented characters with values greater than 128 are treated as let- 2655 ters), the following code could be used: 2656 2657 setlocale(LC_CTYPE, "fr_FR"); 2658 tables = pcre_maketables(); 2659 re = pcre_compile(..., tables); 2660 2661 The locale name "fr_FR" is used on Linux and other Unix-like systems; 2662 if you are using Windows, the name for the French locale is "french". 2663 2664 When pcre_maketables() runs, the tables are built in memory that is 2665 obtained via pcre_malloc. It is the caller's responsibility to ensure 2666 that the memory containing the tables remains available for as long as 2667 it is needed. 2668 2669 The pointer that is passed to pcre_compile() is saved with the compiled 2670 pattern, and the same tables are used via this pointer by pcre_study() 2671 and also by pcre_exec() and pcre_dfa_exec(). Thus, for any single pat- 2672 tern, compilation, studying and matching all happen in the same locale, 2673 but different patterns can be processed in different locales. 2674 2675 It is possible to pass a table pointer or NULL (indicating the use of 2676 the internal tables) to pcre_exec() or pcre_dfa_exec() (see the discus- 2677 sion below in the section on matching a pattern). This facility is pro- 2678 vided for use with pre-compiled patterns that have been saved and 2679 reloaded. Character tables are not saved with patterns, so if a non- 2680 standard table was used at compile time, it must be provided again when 2681 the reloaded pattern is matched. Attempting to use this facility to 2682 match a pattern in a different locale from the one in which it was com- 2683 piled is likely to lead to anomalous (usually incorrect) results. 2684 2685 2686 INFORMATION ABOUT A PATTERN 2687 2688 int pcre_fullinfo(const pcre *code, const pcre_extra *extra, 2689 int what, void *where); 2690 2691 The pcre_fullinfo() function returns information about a compiled pat- 2692 tern. It replaces the pcre_info() function, which was removed from the 2693 library at version 8.30, after more than 10 years of obsolescence. 2694 2695 The first argument for pcre_fullinfo() is a pointer to the compiled 2696 pattern. The second argument is the result of pcre_study(), or NULL if 2697 the pattern was not studied. The third argument specifies which piece 2698 of information is required, and the fourth argument is a pointer to a 2699 variable to receive the data. The yield of the function is zero for 2700 success, or one of the following negative numbers: 2701 2702 PCRE_ERROR_NULL the argument code was NULL 2703 the argument where was NULL 2704 PCRE_ERROR_BADMAGIC the "magic number" was not found 2705 PCRE_ERROR_BADENDIANNESS the pattern was compiled with different 2706 endianness 2707 PCRE_ERROR_BADOPTION the value of what was invalid 2708 PCRE_ERROR_UNSET the requested field is not set 2709 2710 The "magic number" is placed at the start of each compiled pattern as 2711 an simple check against passing an arbitrary memory pointer. The endi- 2712 anness error can occur if a compiled pattern is saved and reloaded on a 2713 different host. Here is a typical call of pcre_fullinfo(), to obtain 2714 the length of the compiled pattern: 2715 2716 int rc; 2717 size_t length; 2718 rc = pcre_fullinfo( 2719 re, /* result of pcre_compile() */ 2720 sd, /* result of pcre_study(), or NULL */ 2721 PCRE_INFO_SIZE, /* what is required */ 2722 &length); /* where to put the data */ 2723 2724 The possible values for the third argument are defined in pcre.h, and 2725 are as follows: 2726 2727 PCRE_INFO_BACKREFMAX 2728 2729 Return the number of the highest back reference in the pattern. The 2730 fourth argument should point to an int variable. Zero is returned if 2731 there are no back references. 2732 2733 PCRE_INFO_CAPTURECOUNT 2734 2735 Return the number of capturing subpatterns in the pattern. The fourth 2736 argument should point to an int variable. 2737 2738 PCRE_INFO_DEFAULT_TABLES 2739 2740 Return a pointer to the internal default character tables within PCRE. 2741 The fourth argument should point to an unsigned char * variable. This 2742 information call is provided for internal use by the pcre_study() func- 2743 tion. External callers can cause PCRE to use its internal tables by 2744 passing a NULL table pointer. 2745 2746 PCRE_INFO_FIRSTBYTE (deprecated) 2747 2748 Return information about the first data unit of any matched string, for 2749 a non-anchored pattern. The name of this option refers to the 8-bit 2750 library, where data units are bytes. The fourth argument should point 2751 to an int variable. Negative values are used for special cases. How- 2752 ever, this means that when the 32-bit library is in non-UTF-32 mode, 2753 the full 32-bit range of characters cannot be returned. For this rea- 2754 son, this value is deprecated; use PCRE_INFO_FIRSTCHARACTERFLAGS and 2755 PCRE_INFO_FIRSTCHARACTER instead. 2756 2757 If there is a fixed first value, for example, the letter "c" from a 2758 pattern such as (cat|cow|coyote), its value is returned. In the 8-bit 2759 library, the value is always less than 256. In the 16-bit library the 2760 value can be up to 0xffff. In the 32-bit library the value can be up to 2761 0x10ffff. 2762 2763 If there is no fixed first value, and if either 2764 2765 (a) the pattern was compiled with the PCRE_MULTILINE option, and every 2766 branch starts with "^", or 2767 2768 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not 2769 set (if it were set, the pattern would be anchored), 2770 2771 -1 is returned, indicating that the pattern matches only at the start 2772 of a subject string or after any newline within the string. Otherwise 2773 -2 is returned. For anchored patterns, -2 is returned. 2774 2775 PCRE_INFO_FIRSTCHARACTER 2776 2777 Return the value of the first data unit (non-UTF character) of any 2778 matched string in the situation where PCRE_INFO_FIRSTCHARACTERFLAGS 2779 returns 1; otherwise return 0. The fourth argument should point to an 2780 uint_t variable. 2781 2782 In the 8-bit library, the value is always less than 256. In the 16-bit 2783 library the value can be up to 0xffff. In the 32-bit library in UTF-32 2784 mode the value can be up to 0x10ffff, and up to 0xffffffff when not 2785 using UTF-32 mode. 2786 2787 PCRE_INFO_FIRSTCHARACTERFLAGS 2788 2789 Return information about the first data unit of any matched string, for 2790 a non-anchored pattern. The fourth argument should point to an int 2791 variable. 2792 2793 If there is a fixed first value, for example, the letter "c" from a 2794 pattern such as (cat|cow|coyote), 1 is returned, and the character 2795 value can be retrieved using PCRE_INFO_FIRSTCHARACTER. If there is no 2796 fixed first value, and if either 2797 2798 (a) the pattern was compiled with the PCRE_MULTILINE option, and every 2799 branch starts with "^", or 2800 2801 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not 2802 set (if it were set, the pattern would be anchored), 2803 2804 2 is returned, indicating that the pattern matches only at the start of 2805 a subject string or after any newline within the string. Otherwise 0 is 2806 returned. For anchored patterns, 0 is returned. 2807 2808 PCRE_INFO_FIRSTTABLE 2809 2810 If the pattern was studied, and this resulted in the construction of a 2811 256-bit table indicating a fixed set of values for the first data unit 2812 in any matching string, a pointer to the table is returned. Otherwise 2813 NULL is returned. The fourth argument should point to an unsigned char 2814 * variable. 2815 2816 PCRE_INFO_HASCRORLF 2817 2818 Return 1 if the pattern contains any explicit matches for CR or LF 2819 characters, otherwise 0. The fourth argument should point to an int 2820 variable. An explicit match is either a literal CR or LF character, or 2821 \r or \n. 2822 2823 PCRE_INFO_JCHANGED 2824 2825 Return 1 if the (?J) or (?-J) option setting is used in the pattern, 2826 otherwise 0. The fourth argument should point to an int variable. (?J) 2827 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. 2828 2829 PCRE_INFO_JIT 2830 2831 Return 1 if the pattern was studied with one of the JIT options, and 2832 just-in-time compiling was successful. The fourth argument should point 2833 to an int variable. A return value of 0 means that JIT support is not 2834 available in this version of PCRE, or that the pattern was not studied 2835 with a JIT option, or that the JIT compiler could not handle this par- 2836 ticular pattern. See the pcrejit documentation for details of what can 2837 and cannot be handled. 2838 2839 PCRE_INFO_JITSIZE 2840 2841 If the pattern was successfully studied with a JIT option, return the 2842 size of the JIT compiled code, otherwise return zero. The fourth argu- 2843 ment should point to a size_t variable. 2844 2845 PCRE_INFO_LASTLITERAL 2846 2847 Return the value of the rightmost literal data unit that must exist in 2848 any matched string, other than at its start, if such a value has been 2849 recorded. The fourth argument should point to an int variable. If there 2850 is no such value, -1 is returned. For anchored patterns, a last literal 2851 value is recorded only if it follows something of variable length. For 2852 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for 2853 /^a\dz\d/ the returned value is -1. 2854 2855 Since for the 32-bit library using the non-UTF-32 mode, this function 2856 is unable to return the full 32-bit range of characters, this value is 2857 deprecated; instead the PCRE_INFO_REQUIREDCHARFLAGS and 2858 PCRE_INFO_REQUIREDCHAR values should be used. 2859 2860 PCRE_INFO_MATCH_EMPTY 2861 2862 Return 1 if the pattern can match an empty string, otherwise 0. The 2863 fourth argument should point to an int variable. 2864 2865 PCRE_INFO_MATCHLIMIT 2866 2867 If the pattern set a match limit by including an item of the form 2868 (*LIMIT_MATCH=nnnn) at the start, the value is returned. The fourth 2869 argument should point to an unsigned 32-bit integer. If no such value 2870 has been set, the call to pcre_fullinfo() returns the error 2871 PCRE_ERROR_UNSET. 2872 2873 PCRE_INFO_MAXLOOKBEHIND 2874 2875 Return the number of characters (NB not data units) in the longest 2876 lookbehind assertion in the pattern. This information is useful when 2877 doing multi-segment matching using the partial matching facilities. 2878 Note that the simple assertions \b and \B require a one-character look- 2879 behind. \A also registers a one-character lookbehind, though it does 2880 not actually inspect the previous character. This is to ensure that at 2881 least one character from the old segment is retained when a new segment 2882 is processed. Otherwise, if there are no lookbehinds in the pattern, \A 2883 might match incorrectly at the start of a new segment. 2884 2885 PCRE_INFO_MINLENGTH 2886 2887 If the pattern was studied and a minimum length for matching subject 2888 strings was computed, its value is returned. Otherwise the returned 2889 value is -1. The value is a number of characters, which in UTF mode may 2890 be different from the number of data units. The fourth argument should 2891 point to an int variable. A non-negative value is a lower bound to the 2892 length of any matching string. There may not be any strings of that 2893 length that do actually match, but every string that does match is at 2894 least that long. 2895 2896 PCRE_INFO_NAMECOUNT 2897 PCRE_INFO_NAMEENTRYSIZE 2898 PCRE_INFO_NAMETABLE 2899 2900 PCRE supports the use of named as well as numbered capturing parenthe- 2901 ses. The names are just an additional way of identifying the parenthe- 2902 ses, which still acquire numbers. Several convenience functions such as 2903 pcre_get_named_substring() are provided for extracting captured sub- 2904 strings by name. It is also possible to extract the data directly, by 2905 first converting the name to a number in order to access the correct 2906 pointers in the output vector (described with pcre_exec() below). To do 2907 the conversion, you need to use the name-to-number map, which is 2908 described by these three values. 2909 2910 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT 2911 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size 2912 of each entry; both of these return an int value. The entry size 2913 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns 2914 a pointer to the first entry of the table. This is a pointer to char in 2915 the 8-bit library, where the first two bytes of each entry are the num- 2916 ber of the capturing parenthesis, most significant byte first. In the 2917 16-bit library, the pointer points to 16-bit data units, the first of 2918 which contains the parenthesis number. In the 32-bit library, the 2919 pointer points to 32-bit data units, the first of which contains the 2920 parenthesis number. The rest of the entry is the corresponding name, 2921 zero terminated. 2922 2923 The names are in alphabetical order. If (?| is used to create multiple 2924 groups with the same number, as described in the section on duplicate 2925 subpattern numbers in the pcrepattern page, the groups may be given the 2926 same name, but there is only one entry in the table. Different names 2927 for groups of the same number are not permitted. Duplicate names for 2928 subpatterns with different numbers are permitted, but only if PCRE_DUP- 2929 NAMES is set. They appear in the table in the order in which they were 2930 found in the pattern. In the absence of (?| this is the order of 2931 increasing number; when (?| is used this is not necessarily the case 2932 because later subpatterns may have lower numbers. 2933 2934 As a simple example of the name/number table, consider the following 2935 pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is 2936 set, so white space - including newlines - is ignored): 2937 2938 (?<date> (?<year>(\d\d)?\d\d) - 2939 (?<month>\d\d) - (?<day>\d\d) ) 2940 2941 There are four named subpatterns, so the table has four entries, and 2942 each entry in the table is eight bytes long. The table is as follows, 2943 with non-printing bytes shows in hexadecimal, and undefined bytes shown 2944 as ??: 2945 2946 00 01 d a t e 00 ?? 2947 00 05 d a y 00 ?? ?? 2948 00 04 m o n t h 00 2949 00 02 y e a r 00 ?? 2950 2951 When writing code to extract data from named subpatterns using the 2952 name-to-number map, remember that the length of the entries is likely 2953 to be different for each compiled pattern. 2954 2955 PCRE_INFO_OKPARTIAL 2956 2957 Return 1 if the pattern can be used for partial matching with 2958 pcre_exec(), otherwise 0. The fourth argument should point to an int 2959 variable. From release 8.00, this always returns 1, because the 2960 restrictions that previously applied to partial matching have been 2961 lifted. The pcrepartial documentation gives details of partial match- 2962 ing. 2963 2964 PCRE_INFO_OPTIONS 2965 2966 Return a copy of the options with which the pattern was compiled. The 2967 fourth argument should point to an unsigned long int variable. These 2968 option bits are those specified in the call to pcre_compile(), modified 2969 by any top-level option settings at the start of the pattern itself. In 2970 other words, they are the options that will be in force when matching 2971 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with 2972 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, 2973 and PCRE_EXTENDED. 2974 2975 A pattern is automatically anchored by PCRE if all of its top-level 2976 alternatives begin with one of the following: 2977 2978 ^ unless PCRE_MULTILINE is set 2979 \A always 2980 \G always 2981 .* if PCRE_DOTALL is set and there are no back 2982 references to the subpattern in which .* appears 2983 2984 For such patterns, the PCRE_ANCHORED bit is set in the options returned 2985 by pcre_fullinfo(). 2986 2987 PCRE_INFO_RECURSIONLIMIT 2988 2989 If the pattern set a recursion limit by including an item of the form 2990 (*LIMIT_RECURSION=nnnn) at the start, the value is returned. The fourth 2991 argument should point to an unsigned 32-bit integer. If no such value 2992 has been set, the call to pcre_fullinfo() returns the error 2993 PCRE_ERROR_UNSET. 2994 2995 PCRE_INFO_SIZE 2996 2997 Return the size of the compiled pattern in bytes (for all three 2998 libraries). The fourth argument should point to a size_t variable. This 2999 value does not include the size of the pcre structure that is returned 3000 by pcre_compile(). The value that is passed as the argument to 3001 pcre_malloc() when pcre_compile() is getting memory in which to place 3002 the compiled data is the value returned by this option plus the size of 3003 the pcre structure. Studying a compiled pattern, with or without JIT, 3004 does not alter the value returned by this option. 3005 3006 PCRE_INFO_STUDYSIZE 3007 3008 Return the size in bytes (for all three libraries) of the data block 3009 pointed to by the study_data field in a pcre_extra block. If pcre_extra 3010 is NULL, or there is no study data, zero is returned. The fourth argu- 3011 ment should point to a size_t variable. The study_data field is set by 3012 pcre_study() to record information that will speed up matching (see the 3013 section entitled "Studying a pattern" above). The format of the 3014 study_data block is private, but its length is made available via this 3015 option so that it can be saved and restored (see the pcreprecompile 3016 documentation for details). 3017 3018 PCRE_INFO_REQUIREDCHARFLAGS 3019 3020 Returns 1 if there is a rightmost literal data unit that must exist in 3021 any matched string, other than at its start. The fourth argument should 3022 point to an int variable. If there is no such value, 0 is returned. If 3023 returning 1, the character value itself can be retrieved using 3024 PCRE_INFO_REQUIREDCHAR. 3025 3026 For anchored patterns, a last literal value is recorded only if it fol- 3027 lows something of variable length. For example, for the pattern 3028 /^a\d+z\d+/ the returned value 1 (with "z" returned from 3029 PCRE_INFO_REQUIREDCHAR), but for /^a\dz\d/ the returned value is 0. 3030 3031 PCRE_INFO_REQUIREDCHAR 3032 3033 Return the value of the rightmost literal data unit that must exist in 3034 any matched string, other than at its start, if such a value has been 3035 recorded. The fourth argument should point to an uint32_t variable. If 3036 there is no such value, 0 is returned. 3037 3038 3039 REFERENCE COUNTS 3040 3041 int pcre_refcount(pcre *code, int adjust); 3042 3043 The pcre_refcount() function is used to maintain a reference count in 3044 the data block that contains a compiled pattern. It is provided for the 3045 benefit of applications that operate in an object-oriented manner, 3046 where different parts of the application may be using the same compiled 3047 pattern, but you want to free the block when they are all done. 3048 3049 When a pattern is compiled, the reference count field is initialized to 3050 zero. It is changed only by calling this function, whose action is to 3051 add the adjust value (which may be positive or negative) to it. The 3052 yield of the function is the new value. However, the value of the count 3053 is constrained to lie between 0 and 65535, inclusive. If the new value 3054 is outside these limits, it is forced to the appropriate limit value. 3055 3056 Except when it is zero, the reference count is not correctly preserved 3057 if a pattern is compiled on one host and then transferred to a host 3058 whose byte-order is different. (This seems a highly unlikely scenario.) 3059 3060 3061 MATCHING A PATTERN: THE TRADITIONAL FUNCTION 3062 3063 int pcre_exec(const pcre *code, const pcre_extra *extra, 3064 const char *subject, int length, int startoffset, 3065 int options, int *ovector, int ovecsize); 3066 3067 The function pcre_exec() is called to match a subject string against a 3068 compiled pattern, which is passed in the code argument. If the pattern 3069 was studied, the result of the study should be passed in the extra 3070 argument. You can call pcre_exec() with the same code and extra argu- 3071 ments as many times as you like, in order to match different subject 3072 strings with the same pattern. 3073 3074 This function is the main matching facility of the library, and it 3075 operates in a Perl-like manner. For specialist use there is also an 3076 alternative matching function, which is described below in the section 3077 about the pcre_dfa_exec() function. 3078 3079 In most applications, the pattern will have been compiled (and option- 3080 ally studied) in the same process that calls pcre_exec(). However, it 3081 is possible to save compiled patterns and study data, and then use them 3082 later in different processes, possibly even on different hosts. For a 3083 discussion about this, see the pcreprecompile documentation. 3084 3085 Here is an example of a simple call to pcre_exec(): 3086 3087 int rc; 3088 int ovector[30]; 3089 rc = pcre_exec( 3090 re, /* result of pcre_compile() */ 3091 NULL, /* we didn't study the pattern */ 3092 "some string", /* the subject string */ 3093 11, /* the length of the subject string */ 3094 0, /* start at offset 0 in the subject */ 3095 0, /* default options */ 3096 ovector, /* vector of integers for substring information */ 3097 30); /* number of elements (NOT size in bytes) */ 3098 3099 Extra data for pcre_exec() 3100 3101 If the extra argument is not NULL, it must point to a pcre_extra data 3102 block. The pcre_study() function returns such a block (when it doesn't 3103 return NULL), but you can also create one for yourself, and pass addi- 3104 tional information in it. The pcre_extra block contains the following 3105 fields (not necessarily in this order): 3106 3107 unsigned long int flags; 3108 void *study_data; 3109 void *executable_jit; 3110 unsigned long int match_limit; 3111 unsigned long int match_limit_recursion; 3112 void *callout_data; 3113 const unsigned char *tables; 3114 unsigned char **mark; 3115 3116 In the 16-bit version of this structure, the mark field has type 3117 "PCRE_UCHAR16 **". 3118 3119 In the 32-bit version of this structure, the mark field has type 3120 "PCRE_UCHAR32 **". 3121 3122 The flags field is used to specify which of the other fields are set. 3123 The flag bits are: 3124 3125 PCRE_EXTRA_CALLOUT_DATA 3126 PCRE_EXTRA_EXECUTABLE_JIT 3127 PCRE_EXTRA_MARK 3128 PCRE_EXTRA_MATCH_LIMIT 3129 PCRE_EXTRA_MATCH_LIMIT_RECURSION 3130 PCRE_EXTRA_STUDY_DATA 3131 PCRE_EXTRA_TABLES 3132 3133 Other flag bits should be set to zero. The study_data field and some- 3134 times the executable_jit field are set in the pcre_extra block that is 3135 returned by pcre_study(), together with the appropriate flag bits. You 3136 should not set these yourself, but you may add to the block by setting 3137 other fields and their corresponding flag bits. 3138 3139 The match_limit field provides a means of preventing PCRE from using up 3140 a vast amount of resources when running patterns that are not going to 3141 match, but which have a very large number of possibilities in their 3142 search trees. The classic example is a pattern that uses nested unlim- 3143 ited repeats. 3144 3145 Internally, pcre_exec() uses a function called match(), which it calls 3146 repeatedly (sometimes recursively). The limit set by match_limit is 3147 imposed on the number of times this function is called during a match, 3148 which has the effect of limiting the amount of backtracking that can 3149 take place. For patterns that are not anchored, the count restarts from 3150 zero for each position in the subject string. 3151 3152 When pcre_exec() is called with a pattern that was successfully studied 3153 with a JIT option, the way that the matching is executed is entirely 3154 different. However, there is still the possibility of runaway matching 3155 that goes on for a very long time, and so the match_limit value is also 3156 used in this case (but in a different way) to limit how long the match- 3157 ing can continue. 3158 3159 The default value for the limit can be set when PCRE is built; the 3160 default default is 10 million, which handles all but the most extreme 3161 cases. You can override the default by suppling pcre_exec() with a 3162 pcre_extra block in which match_limit is set, and 3163 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is 3164 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. 3165 3166 A value for the match limit may also be supplied by an item at the 3167 start of a pattern of the form 3168 3169 (*LIMIT_MATCH=d) 3170 3171 where d is a decimal number. However, such a setting is ignored unless 3172 d is less than the limit set by the caller of pcre_exec() or, if no 3173 such limit is set, less than the default. 3174 3175 The match_limit_recursion field is similar to match_limit, but instead 3176 of limiting the total number of times that match() is called, it limits 3177 the depth of recursion. The recursion depth is a smaller number than 3178 the total number of calls, because not all calls to match() are recur- 3179 sive. This limit is of use only if it is set smaller than match_limit. 3180 3181 Limiting the recursion depth limits the amount of machine stack that 3182 can be used, or, when PCRE has been compiled to use memory on the heap 3183 instead of the stack, the amount of heap memory that can be used. This 3184 limit is not relevant, and is ignored, when matching is done using JIT 3185 compiled code. 3186 3187 The default value for match_limit_recursion can be set when PCRE is 3188 built; the default default is the same value as the default for 3189 match_limit. You can override the default by suppling pcre_exec() with 3190 a pcre_extra block in which match_limit_recursion is set, and 3191 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the 3192 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. 3193 3194 A value for the recursion limit may also be supplied by an item at the 3195 start of a pattern of the form 3196 3197 (*LIMIT_RECURSION=d) 3198 3199 where d is a decimal number. However, such a setting is ignored unless 3200 d is less than the limit set by the caller of pcre_exec() or, if no 3201 such limit is set, less than the default. 3202 3203 The callout_data field is used in conjunction with the "callout" fea- 3204 ture, and is described in the pcrecallout documentation. 3205 3206 The tables field is provided for use with patterns that have been pre- 3207 compiled using custom character tables, saved to disc or elsewhere, and 3208 then reloaded, because the tables that were used to compile a pattern 3209 are not saved with it. See the pcreprecompile documentation for a dis- 3210 cussion of saving compiled patterns for later use. If NULL is passed 3211 using this mechanism, it forces PCRE's internal tables to be used. 3212 3213 Warning: The tables that pcre_exec() uses must be the same as those 3214 that were used when the pattern was compiled. If this is not the case, 3215 the behaviour of pcre_exec() is undefined. Therefore, when a pattern is 3216 compiled and matched in the same process, this field should never be 3217 set. In this (the most common) case, the correct table pointer is auto- 3218 matically passed with the compiled pattern from pcre_compile() to 3219 pcre_exec(). 3220 3221 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be 3222 set to point to a suitable variable. If the pattern contains any back- 3223 tracking control verbs such as (*MARK:NAME), and the execution ends up 3224 with a name to pass back, a pointer to the name string (zero termi- 3225 nated) is placed in the variable pointed to by the mark field. The 3226 names are within the compiled pattern; if you wish to retain such a 3227 name you must copy it before freeing the memory of a compiled pattern. 3228 If there is no name to pass back, the variable pointed to by the mark 3229 field is set to NULL. For details of the backtracking control verbs, 3230 see the section entitled "Backtracking control" in the pcrepattern doc- 3231 umentation. 3232 3233 Option bits for pcre_exec() 3234 3235 The unused bits of the options argument for pcre_exec() must be zero. 3236 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, 3237 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, 3238 PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and 3239 PCRE_PARTIAL_SOFT. 3240 3241 If the pattern was successfully studied with one of the just-in-time 3242 (JIT) compile options, the only supported options for JIT execution are 3243 PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, 3244 PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If an 3245 unsupported option is used, JIT execution is disabled and the normal 3246 interpretive code in pcre_exec() is run. 3247 3248 PCRE_ANCHORED 3249 3250 The PCRE_ANCHORED option limits pcre_exec() to matching at the first 3251 matching position. If a pattern was compiled with PCRE_ANCHORED, or 3252 turned out to be anchored by virtue of its contents, it cannot be made 3253 unachored at matching time. 3254 3255 PCRE_BSR_ANYCRLF 3256 PCRE_BSR_UNICODE 3257 3258 These options (which are mutually exclusive) control what the \R escape 3259 sequence matches. The choice is either to match only CR, LF, or CRLF, 3260 or to match any Unicode newline sequence. These options override the 3261 choice that was made or defaulted when the pattern was compiled. 3262 3263 PCRE_NEWLINE_CR 3264 PCRE_NEWLINE_LF 3265 PCRE_NEWLINE_CRLF 3266 PCRE_NEWLINE_ANYCRLF 3267 PCRE_NEWLINE_ANY 3268 3269 These options override the newline definition that was chosen or 3270 defaulted when the pattern was compiled. For details, see the descrip- 3271 tion of pcre_compile() above. During matching, the newline choice 3272 affects the behaviour of the dot, circumflex, and dollar metacharac- 3273 ters. It may also alter the way the match position is advanced after a 3274 match failure for an unanchored pattern. 3275 3276 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is 3277 set, and a match attempt for an unanchored pattern fails when the cur- 3278 rent position is at a CRLF sequence, and the pattern contains no 3279 explicit matches for CR or LF characters, the match position is 3280 advanced by two characters instead of one, in other words, to after the 3281 CRLF. 3282 3283 The above rule is a compromise that makes the most common cases work as 3284 expected. For example, if the pattern is .+A (and the PCRE_DOTALL 3285 option is not set), it does not match the string "\r\nA" because, after 3286 failing at the start, it skips both the CR and the LF before retrying. 3287 However, the pattern [\r\n]A does match that string, because it con- 3288 tains an explicit CR or LF reference, and so advances only by one char- 3289 acter after the first failure. 3290 3291 An explicit match for CR of LF is either a literal appearance of one of 3292 those characters, or one of the \r or \n escape sequences. Implicit 3293 matches such as [^X] do not count, nor does \s (which includes CR and 3294 LF in the characters that it matches). 3295 3296 Notwithstanding the above, anomalous effects may still occur when CRLF 3297 is a valid newline sequence and explicit \r or \n escapes appear in the 3298 pattern. 3299 3300 PCRE_NOTBOL 3301 3302 This option specifies that first character of the subject string is not 3303 the beginning of a line, so the circumflex metacharacter should not 3304 match before it. Setting this without PCRE_MULTILINE (at compile time) 3305 causes circumflex never to match. This option affects only the behav- 3306 iour of the circumflex metacharacter. It does not affect \A. 3307 3308 PCRE_NOTEOL 3309 3310 This option specifies that the end of the subject string is not the end 3311 of a line, so the dollar metacharacter should not match it nor (except 3312 in multiline mode) a newline immediately before it. Setting this with- 3313 out PCRE_MULTILINE (at compile time) causes dollar never to match. This 3314 option affects only the behaviour of the dollar metacharacter. It does 3315 not affect \Z or \z. 3316 3317 PCRE_NOTEMPTY 3318 3319 An empty string is not considered to be a valid match if this option is 3320 set. If there are alternatives in the pattern, they are tried. If all 3321 the alternatives match the empty string, the entire match fails. For 3322 example, if the pattern 3323 3324 a?b? 3325 3326 is applied to a string not beginning with "a" or "b", it matches an 3327 empty string at the start of the subject. With PCRE_NOTEMPTY set, this 3328 match is not valid, so PCRE searches further into the string for occur- 3329 rences of "a" or "b". 3330 3331 PCRE_NOTEMPTY_ATSTART 3332 3333 This is like PCRE_NOTEMPTY, except that an empty string match that is 3334 not at the start of the subject is permitted. If the pattern is 3335 anchored, such a match can occur only if the pattern contains \K. 3336 3337 Perl has no direct equivalent of PCRE_NOTEMPTY or 3338 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern 3339 match of the empty string within its split() function, and when using 3340 the /g modifier. It is possible to emulate Perl's behaviour after 3341 matching a null string by first trying the match again at the same off- 3342 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that 3343 fails, by advancing the starting offset (see below) and trying an ordi- 3344 nary match again. There is some code that demonstrates how to do this 3345 in the pcredemo sample program. In the most general case, you have to 3346 check to see if the newline convention recognizes CRLF as a newline, 3347 and if so, and the current character is CR followed by LF, advance the 3348 starting offset by two characters instead of one. 3349 3350 PCRE_NO_START_OPTIMIZE 3351 3352 There are a number of optimizations that pcre_exec() uses at the start 3353 of a match, in order to speed up the process. For example, if it is 3354 known that an unanchored match must start with a specific character, it 3355 searches the subject for that character, and fails immediately if it 3356 cannot find it, without actually running the main matching function. 3357 This means that a special item such as (*COMMIT) at the start of a pat- 3358 tern is not considered until after a suitable starting point for the 3359 match has been found. Also, when callouts or (*MARK) items are in use, 3360 these "start-up" optimizations can cause them to be skipped if the pat- 3361 tern is never actually used. The start-up optimizations are in effect a 3362 pre-scan of the subject that takes place before the pattern is run. 3363 3364 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations, 3365 possibly causing performance to suffer, but ensuring that in cases 3366 where the result is "no match", the callouts do occur, and that items 3367 such as (*COMMIT) and (*MARK) are considered at every possible starting 3368 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at 3369 compile time, it cannot be unset at matching time. The use of 3370 PCRE_NO_START_OPTIMIZE at matching time (that is, passing it to 3371 pcre_exec()) disables JIT execution; in this situation, matching is 3372 always done using interpretively. 3373 3374 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching 3375 operation. Consider the pattern 3376 3377 (*COMMIT)ABC 3378 3379 When this is compiled, PCRE records the fact that a match must start 3380 with the character "A". Suppose the subject string is "DEFABC". The 3381 start-up optimization scans along the subject, finds "A" and runs the 3382 first match attempt from there. The (*COMMIT) item means that the pat- 3383 tern must match the current starting position, which in this case, it 3384 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE 3385 set, the initial scan along the subject string does not happen. The 3386 first match attempt is run starting from "D" and when this fails, 3387 (*COMMIT) prevents any further matches being tried, so the overall 3388 result is "no match". If the pattern is studied, more start-up opti- 3389 mizations may be used. For example, a minimum length for the subject 3390 may be recorded. Consider the pattern 3391 3392 (*MARK:A)(X|Y) 3393 3394 The minimum length for a match is one character. If the subject is 3395 "ABC", there will be attempts to match "ABC", "BC", "C", and then 3396 finally an empty string. If the pattern is studied, the final attempt 3397 does not take place, because PCRE knows that the subject is too short, 3398 and so the (*MARK) is never encountered. In this case, studying the 3399 pattern does not affect the overall match result, which is still "no 3400 match", but it does affect the auxiliary information that is returned. 3401 3402 PCRE_NO_UTF8_CHECK 3403 3404 When PCRE_UTF8 is set at compile time, the validity of the subject as a 3405 UTF-8 string is automatically checked when pcre_exec() is subsequently 3406 called. The entire string is checked before any other processing takes 3407 place. The value of startoffset is also checked to ensure that it 3408 points to the start of a UTF-8 character. There is a discussion about 3409 the validity of UTF-8 strings in the pcreunicode page. If an invalid 3410 sequence of bytes is found, pcre_exec() returns the error 3411 PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a 3412 truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In 3413 both cases, information about the precise nature of the error may also 3414 be returned (see the descriptions of these errors in the section enti- 3415 tled Error return values from pcre_exec() below). If startoffset con- 3416 tains a value that does not point to the start of a UTF-8 character (or 3417 to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned. 3418 3419 If you already know that your subject is valid, and you want to skip 3420 these checks for performance reasons, you can set the 3421 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to 3422 do this for the second and subsequent calls to pcre_exec() if you are 3423 making repeated calls to find all the matches in a single subject 3424 string. However, you should be sure that the value of startoffset 3425 points to the start of a character (or the end of the subject). When 3426 PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a 3427 subject or an invalid value of startoffset is undefined. Your program 3428 may crash or loop. 3429 3430 PCRE_PARTIAL_HARD 3431 PCRE_PARTIAL_SOFT 3432 3433 These options turn on the partial matching feature. For backwards com- 3434 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial 3435 match occurs if the end of the subject string is reached successfully, 3436 but there are not enough subject characters to complete the match. If 3437 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set, 3438 matching continues by testing any remaining alternatives. Only if no 3439 complete match can be found is PCRE_ERROR_PARTIAL returned instead of 3440 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the 3441 caller is prepared to handle a partial match, but only if no complete 3442 match can be found. 3443 3444 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this 3445 case, if a partial match is found, pcre_exec() immediately returns 3446 PCRE_ERROR_PARTIAL, without considering any other alternatives. In 3447 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid- 3448 ered to be more important that an alternative complete match. 3449 3450 In both cases, the portion of the string that was inspected when the 3451 partial match was found is set as the first matching string. There is a 3452 more detailed discussion of partial and multi-segment matching, with 3453 examples, in the pcrepartial documentation. 3454 3455 The string to be matched by pcre_exec() 3456 3457 The subject string is passed to pcre_exec() as a pointer in subject, a 3458 length in length, and a starting offset in startoffset. The units for 3459 length and startoffset are bytes for the 8-bit library, 16-bit data 3460 items for the 16-bit library, and 32-bit data items for the 32-bit 3461 library. 3462 3463 If startoffset is negative or greater than the length of the subject, 3464 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is 3465 zero, the search for a match starts at the beginning of the subject, 3466 and this is by far the most common case. In UTF-8 or UTF-16 mode, the 3467 offset must point to the start of a character, or the end of the sub- 3468 ject (in UTF-32 mode, one data unit equals one character, so all off- 3469 sets are valid). Unlike the pattern string, the subject may contain 3470 binary zeroes. 3471 3472 A non-zero starting offset is useful when searching for another match 3473 in the same subject by calling pcre_exec() again after a previous suc- 3474 cess. Setting startoffset differs from just passing over a shortened 3475 string and setting PCRE_NOTBOL in the case of a pattern that begins 3476 with any kind of lookbehind. For example, consider the pattern 3477 3478 \Biss\B 3479 3480 which finds occurrences of "iss" in the middle of words. (\B matches 3481 only if the current position in the subject is not a word boundary.) 3482 When applied to the string "Mississipi" the first call to pcre_exec() 3483 finds the first occurrence. If pcre_exec() is called again with just 3484 the remainder of the subject, namely "issipi", it does not match, 3485 because \B is always false at the start of the subject, which is deemed 3486 to be a word boundary. However, if pcre_exec() is passed the entire 3487 string again, but with startoffset set to 4, it finds the second occur- 3488 rence of "iss" because it is able to look behind the starting point to 3489 discover that it is preceded by a letter. 3490 3491 Finding all the matches in a subject is tricky when the pattern can 3492 match an empty string. It is possible to emulate Perl's /g behaviour by 3493 first trying the match again at the same offset, with the 3494 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that 3495 fails, advancing the starting offset and trying an ordinary match 3496 again. There is some code that demonstrates how to do this in the pcre- 3497 demo sample program. In the most general case, you have to check to see 3498 if the newline convention recognizes CRLF as a newline, and if so, and 3499 the current character is CR followed by LF, advance the starting offset 3500 by two characters instead of one. 3501 3502 If a non-zero starting offset is passed when the pattern is anchored, 3503 one attempt to match at the given offset is made. This can only succeed 3504 if the pattern does not require the match to be at the start of the 3505 subject. 3506 3507 How pcre_exec() returns captured substrings 3508 3509 In general, a pattern matches a certain portion of the subject, and in 3510 addition, further substrings from the subject may be picked out by 3511 parts of the pattern. Following the usage in Jeffrey Friedl's book, 3512 this is called "capturing" in what follows, and the phrase "capturing 3513 subpattern" is used for a fragment of a pattern that picks out a sub- 3514 string. PCRE supports several other kinds of parenthesized subpattern 3515 that do not cause substrings to be captured. 3516 3517 Captured substrings are returned to the caller via a vector of integers 3518 whose address is passed in ovector. The number of elements in the vec- 3519 tor is passed in ovecsize, which must be a non-negative number. Note: 3520 this argument is NOT the size of ovector in bytes. 3521 3522 The first two-thirds of the vector is used to pass back captured sub- 3523 strings, each substring using a pair of integers. The remaining third 3524 of the vector is used as workspace by pcre_exec() while matching cap- 3525 turing subpatterns, and is not available for passing back information. 3526 The number passed in ovecsize should always be a multiple of three. If 3527 it is not, it is rounded down. 3528 3529 When a match is successful, information about captured substrings is 3530 returned in pairs of integers, starting at the beginning of ovector, 3531 and continuing up to two-thirds of its length at the most. The first 3532 element of each pair is set to the offset of the first character in a 3533 substring, and the second is set to the offset of the first character 3534 after the end of a substring. These values are always data unit off- 3535 sets, even in UTF mode. They are byte offsets in the 8-bit library, 3536 16-bit data item offsets in the 16-bit library, and 32-bit data item 3537 offsets in the 32-bit library. Note: they are not character counts. 3538 3539 The first pair of integers, ovector[0] and ovector[1], identify the 3540 portion of the subject string matched by the entire pattern. The next 3541 pair is used for the first capturing subpattern, and so on. The value 3542 returned by pcre_exec() is one more than the highest numbered pair that 3543 has been set. For example, if two substrings have been captured, the 3544 returned value is 3. If there are no capturing subpatterns, the return 3545 value from a successful match is 1, indicating that just the first pair 3546 of offsets has been set. 3547 3548 If a capturing subpattern is matched repeatedly, it is the last portion 3549 of the string that it matched that is returned. 3550 3551 If the vector is too small to hold all the captured substring offsets, 3552 it is used as far as possible (up to two-thirds of its length), and the 3553 function returns a value of zero. If neither the actual string matched 3554 nor any captured substrings are of interest, pcre_exec() may be called 3555 with ovector passed as NULL and ovecsize as zero. However, if the pat- 3556 tern contains back references and the ovector is not big enough to 3557 remember the related substrings, PCRE has to get additional memory for 3558 use during matching. Thus it is usually advisable to supply an ovector 3559 of reasonable size. 3560 3561 There are some cases where zero is returned (indicating vector over- 3562 flow) when in fact the vector is exactly the right size for the final 3563 match. For example, consider the pattern 3564 3565 (a)(?:(b)c|bd) 3566 3567 If a vector of 6 elements (allowing for only 1 captured substring) is 3568 given with subject string "abd", pcre_exec() will try to set the second 3569 captured string, thereby recording a vector overflow, before failing to 3570 match "c" and backing up to try the second alternative. The zero 3571 return, however, does correctly indicate that the maximum number of 3572 slots (namely 2) have been filled. In similar cases where there is tem- 3573 porary overflow, but the final number of used slots is actually less 3574 than the maximum, a non-zero value is returned. 3575 3576 The pcre_fullinfo() function can be used to find out how many capturing 3577 subpatterns there are in a compiled pattern. The smallest size for 3578 ovector that will allow for n captured substrings, in addition to the 3579 offsets of the substring matched by the whole pattern, is (n+1)*3. 3580 3581 It is possible for capturing subpattern number n+1 to match some part 3582 of the subject when subpattern n has not been used at all. For example, 3583 if the string "abc" is matched against the pattern (a|(z))(bc) the 3584 return from the function is 4, and subpatterns 1 and 3 are matched, but 3585 2 is not. When this happens, both values in the offset pairs corre- 3586 sponding to unused subpatterns are set to -1. 3587 3588 Offset values that correspond to unused subpatterns at the end of the 3589 expression are also set to -1. For example, if the string "abc" is 3590 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not 3591 matched. The return from the function is 2, because the highest used 3592 capturing subpattern number is 1, and the offsets for for the second 3593 and third capturing subpatterns (assuming the vector is large enough, 3594 of course) are set to -1. 3595 3596 Note: Elements in the first two-thirds of ovector that do not corre- 3597 spond to capturing parentheses in the pattern are never changed. That 3598 is, if a pattern contains n capturing parentheses, no more than ovec- 3599 tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in 3600 the first two-thirds) retain whatever values they previously had. 3601 3602 Some convenience functions are provided for extracting the captured 3603 substrings as separate strings. These are described below. 3604 3605 Error return values from pcre_exec() 3606 3607 If pcre_exec() fails, it returns a negative number. The following are 3608 defined in the header file: 3609 3610 PCRE_ERROR_NOMATCH (-1) 3611 3612 The subject string did not match the pattern. 3613 3614 PCRE_ERROR_NULL (-2) 3615 3616 Either code or subject was passed as NULL, or ovector was NULL and 3617 ovecsize was not zero. 3618 3619 PCRE_ERROR_BADOPTION (-3) 3620 3621 An unrecognized bit was set in the options argument. 3622 3623 PCRE_ERROR_BADMAGIC (-4) 3624 3625 PCRE stores a 4-byte "magic number" at the start of the compiled code, 3626 to catch the case when it is passed a junk pointer and to detect when a 3627 pattern that was compiled in an environment of one endianness is run in 3628 an environment with the other endianness. This is the error that PCRE 3629 gives when the magic number is not present. 3630 3631 PCRE_ERROR_UNKNOWN_OPCODE (-5) 3632 3633 While running the pattern match, an unknown item was encountered in the 3634 compiled pattern. This error could be caused by a bug in PCRE or by 3635 overwriting of the compiled pattern. 3636 3637 PCRE_ERROR_NOMEMORY (-6) 3638 3639 If a pattern contains back references, but the ovector that is passed 3640 to pcre_exec() is not big enough to remember the referenced substrings, 3641 PCRE gets a block of memory at the start of matching to use for this 3642 purpose. If the call via pcre_malloc() fails, this error is given. The 3643 memory is automatically freed at the end of matching. 3644 3645 This error is also given if pcre_stack_malloc() fails in pcre_exec(). 3646 This can happen only when PCRE has been compiled with --disable-stack- 3647 for-recursion. 3648 3649 PCRE_ERROR_NOSUBSTRING (-7) 3650 3651 This error is used by the pcre_copy_substring(), pcre_get_substring(), 3652 and pcre_get_substring_list() functions (see below). It is never 3653 returned by pcre_exec(). 3654 3655 PCRE_ERROR_MATCHLIMIT (-8) 3656 3657 The backtracking limit, as specified by the match_limit field in a 3658 pcre_extra structure (or defaulted) was reached. See the description 3659 above. 3660 3661 PCRE_ERROR_CALLOUT (-9) 3662 3663 This error is never generated by pcre_exec() itself. It is provided for 3664 use by callout functions that want to yield a distinctive error code. 3665 See the pcrecallout documentation for details. 3666 3667 PCRE_ERROR_BADUTF8 (-10) 3668 3669 A string that contains an invalid UTF-8 byte sequence was passed as a 3670 subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of 3671 the output vector (ovecsize) is at least 2, the byte offset to the 3672 start of the the invalid UTF-8 character is placed in the first ele- 3673 ment, and a reason code is placed in the second element. The reason 3674 codes are listed in the following section. For backward compatibility, 3675 if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char- 3676 acter at the end of the subject (reason codes 1 to 5), 3677 PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8. 3678 3679 PCRE_ERROR_BADUTF8_OFFSET (-11) 3680 3681 The UTF-8 byte sequence that was passed as a subject was checked and 3682 found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the 3683 value of startoffset did not point to the beginning of a UTF-8 charac- 3684 ter or the end of the subject. 3685 3686 PCRE_ERROR_PARTIAL (-12) 3687 3688 The subject string did not match, but it did match partially. See the 3689 pcrepartial documentation for details of partial matching. 3690 3691 PCRE_ERROR_BADPARTIAL (-13) 3692 3693 This code is no longer in use. It was formerly returned when the 3694 PCRE_PARTIAL option was used with a compiled pattern containing items 3695 that were not supported for partial matching. From release 8.00 3696 onwards, there are no restrictions on partial matching. 3697 3698 PCRE_ERROR_INTERNAL (-14) 3699 3700 An unexpected internal error has occurred. This error could be caused 3701 by a bug in PCRE or by overwriting of the compiled pattern. 3702 3703 PCRE_ERROR_BADCOUNT (-15) 3704 3705 This error is given if the value of the ovecsize argument is negative. 3706 3707 PCRE_ERROR_RECURSIONLIMIT (-21) 3708 3709 The internal recursion limit, as specified by the match_limit_recursion 3710 field in a pcre_extra structure (or defaulted) was reached. See the 3711 description above. 3712 3713 PCRE_ERROR_BADNEWLINE (-23) 3714 3715 An invalid combination of PCRE_NEWLINE_xxx options was given. 3716 3717 PCRE_ERROR_BADOFFSET (-24) 3718 3719 The value of startoffset was negative or greater than the length of the 3720 subject, that is, the value in length. 3721 3722 PCRE_ERROR_SHORTUTF8 (-25) 3723 3724 This error is returned instead of PCRE_ERROR_BADUTF8 when the subject 3725 string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD 3726 option is set. Information about the failure is returned as for 3727 PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but 3728 this special error code for PCRE_PARTIAL_HARD precedes the implementa- 3729 tion of returned information; it is retained for backwards compatibil- 3730 ity. 3731 3732 PCRE_ERROR_RECURSELOOP (-26) 3733 3734 This error is returned when pcre_exec() detects a recursion loop within 3735 the pattern. Specifically, it means that either the whole pattern or a 3736 subpattern has been called recursively for the second time at the same 3737 position in the subject string. Some simple patterns that might do this 3738 are detected and faulted at compile time, but more complicated cases, 3739 in particular mutual recursions between two different subpatterns, can- 3740 not be detected until run time. 3741 3742 PCRE_ERROR_JIT_STACKLIMIT (-27) 3743 3744 This error is returned when a pattern that was successfully studied 3745 using a JIT compile option is being matched, but the memory available 3746 for the just-in-time processing stack is not large enough. See the 3747 pcrejit documentation for more details. 3748 3749 PCRE_ERROR_BADMODE (-28) 3750 3751 This error is given if a pattern that was compiled by the 8-bit library 3752 is passed to a 16-bit or 32-bit library function, or vice versa. 3753 3754 PCRE_ERROR_BADENDIANNESS (-29) 3755 3756 This error is given if a pattern that was compiled and saved is 3757 reloaded on a host with different endianness. The utility function 3758 pcre_pattern_to_host_byte_order() can be used to convert such a pattern 3759 so that it runs on the new host. 3760 3761 PCRE_ERROR_JIT_BADOPTION 3762 3763 This error is returned when a pattern that was successfully studied 3764 using a JIT compile option is being matched, but the matching mode 3765 (partial or complete match) does not correspond to any JIT compilation 3766 mode. When the JIT fast path function is used, this error may be also 3767 given for invalid options. See the pcrejit documentation for more 3768 details. 3769 3770 PCRE_ERROR_BADLENGTH (-32) 3771 3772 This error is given if pcre_exec() is called with a negative value for 3773 the length argument. 3774 3775 Error numbers -16 to -20, -22, and 30 are not used by pcre_exec(). 3776 3777 Reason codes for invalid UTF-8 strings 3778 3779 This section applies only to the 8-bit library. The corresponding 3780 information for the 16-bit and 32-bit libraries is given in the pcre16 3781 and pcre32 pages. 3782 3783 When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT- 3784 UTF8, and the size of the output vector (ovecsize) is at least 2, the 3785 offset of the start of the invalid UTF-8 character is placed in the 3786 first output vector element (ovector[0]) and a reason code is placed in 3787 the second element (ovector[1]). The reason codes are given names in 3788 the pcre.h header file: 3789 3790 PCRE_UTF8_ERR1 3791 PCRE_UTF8_ERR2 3792 PCRE_UTF8_ERR3 3793 PCRE_UTF8_ERR4 3794 PCRE_UTF8_ERR5 3795 3796 The string ends with a truncated UTF-8 character; the code specifies 3797 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 3798 characters to be no longer than 4 bytes, the encoding scheme (origi- 3799 nally defined by RFC 2279) allows for up to 6 bytes, and this is 3800 checked first; hence the possibility of 4 or 5 missing bytes. 3801 3802 PCRE_UTF8_ERR6 3803 PCRE_UTF8_ERR7 3804 PCRE_UTF8_ERR8 3805 PCRE_UTF8_ERR9 3806 PCRE_UTF8_ERR10 3807 3808 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of 3809 the character do not have the binary value 0b10 (that is, either the 3810 most significant bit is 0, or the next bit is 1). 3811 3812 PCRE_UTF8_ERR11 3813 PCRE_UTF8_ERR12 3814 3815 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes 3816 long; these code points are excluded by RFC 3629. 3817 3818 PCRE_UTF8_ERR13 3819 3820 A 4-byte character has a value greater than 0x10fff; these code points 3821 are excluded by RFC 3629. 3822 3823 PCRE_UTF8_ERR14 3824 3825 A 3-byte character has a value in the range 0xd800 to 0xdfff; this 3826 range of code points are reserved by RFC 3629 for use with UTF-16, and 3827 so are excluded from UTF-8. 3828 3829 PCRE_UTF8_ERR15 3830 PCRE_UTF8_ERR16 3831 PCRE_UTF8_ERR17 3832 PCRE_UTF8_ERR18 3833 PCRE_UTF8_ERR19 3834 3835 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes 3836 for a value that can be represented by fewer bytes, which is invalid. 3837 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- 3838 rect coding uses just one byte. 3839 3840 PCRE_UTF8_ERR20 3841 3842 The two most significant bits of the first byte of a character have the 3843 binary value 0b10 (that is, the most significant bit is 1 and the sec- 3844 ond is 0). Such a byte can only validly occur as the second or subse- 3845 quent byte of a multi-byte character. 3846 3847 PCRE_UTF8_ERR21 3848 3849 The first byte of a character has the value 0xfe or 0xff. These values 3850 can never occur in a valid UTF-8 string. 3851 3852 PCRE_UTF8_ERR22 3853 3854 This error code was formerly used when the presence of a so-called 3855 "non-character" caused an error. Unicode corrigendum #9 makes it clear 3856 that such characters should not cause a string to be rejected, and so 3857 this code is no longer in use and is never returned. 3858 3859 3860 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER 3861 3862 int pcre_copy_substring(const char *subject, int *ovector, 3863 int stringcount, int stringnumber, char *buffer, 3864 int buffersize); 3865 3866 int pcre_get_substring(const char *subject, int *ovector, 3867 int stringcount, int stringnumber, 3868 const char **stringptr); 3869 3870 int pcre_get_substring_list(const char *subject, 3871 int *ovector, int stringcount, const char ***listptr); 3872 3873 Captured substrings can be accessed directly by using the offsets 3874 returned by pcre_exec() in ovector. For convenience, the functions 3875 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- 3876 string_list() are provided for extracting captured substrings as new, 3877 separate, zero-terminated strings. These functions identify substrings 3878 by number. The next section describes functions for extracting named 3879 substrings. 3880 3881 A substring that contains a binary zero is correctly extracted and has 3882 a further zero added on the end, but the result is not, of course, a C 3883 string. However, you can process such a string by referring to the 3884 length that is returned by pcre_copy_substring() and pcre_get_sub- 3885 string(). Unfortunately, the interface to pcre_get_substring_list() is 3886 not adequate for handling strings containing binary zeros, because the 3887 end of the final string is not independently indicated. 3888 3889 The first three arguments are the same for all three of these func- 3890 tions: subject is the subject string that has just been successfully 3891 matched, ovector is a pointer to the vector of integer offsets that was 3892 passed to pcre_exec(), and stringcount is the number of substrings that 3893 were captured by the match, including the substring that matched the 3894 entire regular expression. This is the value returned by pcre_exec() if 3895 it is greater than zero. If pcre_exec() returned zero, indicating that 3896 it ran out of space in ovector, the value passed as stringcount should 3897 be the number of elements in the vector divided by three. 3898 3899 The functions pcre_copy_substring() and pcre_get_substring() extract a 3900 single substring, whose number is given as stringnumber. A value of 3901 zero extracts the substring that matched the entire pattern, whereas 3902 higher values extract the captured substrings. For pcre_copy_sub- 3903 string(), the string is placed in buffer, whose length is given by 3904 buffersize, while for pcre_get_substring() a new block of memory is 3905 obtained via pcre_malloc, and its address is returned via stringptr. 3906 The yield of the function is the length of the string, not including 3907 the terminating zero, or one of these error codes: 3908 3909 PCRE_ERROR_NOMEMORY (-6) 3910 3911 The buffer was too small for pcre_copy_substring(), or the attempt to 3912 get memory failed for pcre_get_substring(). 3913 3914 PCRE_ERROR_NOSUBSTRING (-7) 3915 3916 There is no substring whose number is stringnumber. 3917 3918 The pcre_get_substring_list() function extracts all available sub- 3919 strings and builds a list of pointers to them. All this is done in a 3920 single block of memory that is obtained via pcre_malloc. The address of 3921 the memory block is returned via listptr, which is also the start of 3922 the list of string pointers. The end of the list is marked by a NULL 3923 pointer. The yield of the function is zero if all went well, or the 3924 error code 3925 3926 PCRE_ERROR_NOMEMORY (-6) 3927 3928 if the attempt to get the memory block failed. 3929 3930 When any of these functions encounter a substring that is unset, which 3931 can happen when capturing subpattern number n+1 matches some part of 3932 the subject, but subpattern n has not been used at all, they return an 3933 empty string. This can be distinguished from a genuine zero-length sub- 3934 string by inspecting the appropriate offset in ovector, which is nega- 3935 tive for unset substrings. 3936 3937 The two convenience functions pcre_free_substring() and pcre_free_sub- 3938 string_list() can be used to free the memory returned by a previous 3939 call of pcre_get_substring() or pcre_get_substring_list(), respec- 3940 tively. They do nothing more than call the function pointed to by 3941 pcre_free, which of course could be called directly from a C program. 3942 However, PCRE is used in some situations where it is linked via a spe- 3943 cial interface to another programming language that cannot use 3944 pcre_free directly; it is for these cases that the functions are pro- 3945 vided. 3946 3947 3948 EXTRACTING CAPTURED SUBSTRINGS BY NAME 3949 3950 int pcre_get_stringnumber(const pcre *code, 3951 const char *name); 3952 3953 int pcre_copy_named_substring(const pcre *code, 3954 const char *subject, int *ovector, 3955 int stringcount, const char *stringname, 3956 char *buffer, int buffersize); 3957 3958 int pcre_get_named_substring(const pcre *code, 3959 const char *subject, int *ovector, 3960 int stringcount, const char *stringname, 3961 const char **stringptr); 3962 3963 To extract a substring by name, you first have to find associated num- 3964 ber. For example, for this pattern 3965 3966 (a+)b(?<xxx>\d+)... 3967 3968 the number of the subpattern called "xxx" is 2. If the name is known to 3969 be unique (PCRE_DUPNAMES was not set), you can find the number from the 3970 name by calling pcre_get_stringnumber(). The first argument is the com- 3971 piled pattern, and the second is the name. The yield of the function is 3972 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no 3973 subpattern of that name. 3974 3975 Given the number, you can extract the substring directly, or use one of 3976 the functions described in the previous section. For convenience, there 3977 are also two functions that do the whole job. 3978 3979 Most of the arguments of pcre_copy_named_substring() and 3980 pcre_get_named_substring() are the same as those for the similarly 3981 named functions that extract by number. As these are described in the 3982 previous section, they are not re-described here. There are just two 3983 differences: 3984 3985 First, instead of a substring number, a substring name is given. Sec- 3986 ond, there is an extra argument, given at the start, which is a pointer 3987 to the compiled pattern. This is needed in order to gain access to the 3988 name-to-number translation table. 3989 3990 These functions call pcre_get_stringnumber(), and if it succeeds, they 3991 then call pcre_copy_substring() or pcre_get_substring(), as appropri- 3992 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the 3993 behaviour may not be what you want (see the next section). 3994 3995 Warning: If the pattern uses the (?| feature to set up multiple subpat- 3996 terns with the same number, as described in the section on duplicate 3997 subpattern numbers in the pcrepattern page, you cannot use names to 3998 distinguish the different subpatterns, because names are not included 3999 in the compiled code. The matching process uses only numbers. For this 4000 reason, the use of different names for subpatterns of the same number 4001 causes an error at compile time. 4002 4003 4004 DUPLICATE SUBPATTERN NAMES 4005 4006 int pcre_get_stringtable_entries(const pcre *code, 4007 const char *name, char **first, char **last); 4008 4009 When a pattern is compiled with the PCRE_DUPNAMES option, names for 4010 subpatterns are not required to be unique. (Duplicate names are always 4011 allowed for subpatterns with the same number, created by using the (?| 4012 feature. Indeed, if such subpatterns are named, they are required to 4013 use the same names.) 4014 4015 Normally, patterns with duplicate names are such that in any one match, 4016 only one of the named subpatterns participates. An example is shown in 4017 the pcrepattern documentation. 4018 4019 When duplicates are present, pcre_copy_named_substring() and 4020 pcre_get_named_substring() return the first substring corresponding to 4021 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING 4022 (-7) is returned; no data is returned. The pcre_get_stringnumber() 4023 function returns one of the numbers that are associated with the name, 4024 but it is not defined which it is. 4025 4026 If you want to get full details of all captured substrings for a given 4027 name, you must use the pcre_get_stringtable_entries() function. The 4028 first argument is the compiled pattern, and the second is the name. The 4029 third and fourth are pointers to variables which are updated by the 4030 function. After it has run, they point to the first and last entries in 4031 the name-to-number table for the given name. The function itself 4032 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if 4033 there are none. The format of the table is described above in the sec- 4034 tion entitled Information about a pattern above. Given all the rele- 4035 vant entries for the name, you can extract each of their numbers, and 4036 hence the captured data, if any. 4037 4038 4039 FINDING ALL POSSIBLE MATCHES 4040 4041 The traditional matching function uses a similar algorithm to Perl, 4042 which stops when it finds the first match, starting at a given point in 4043 the subject. If you want to find all possible matches, or the longest 4044 possible match, consider using the alternative matching function (see 4045 below) instead. If you cannot use the alternative function, but still 4046 need to find all possible matches, you can kludge it up by making use 4047 of the callout facility, which is described in the pcrecallout documen- 4048 tation. 4049 4050 What you have to do is to insert a callout right at the end of the pat- 4051 tern. When your callout function is called, extract and save the cur- 4052 rent matched substring. Then return 1, which forces pcre_exec() to 4053 backtrack and try other alternatives. Ultimately, when it runs out of 4054 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. 4055 4056 4057 OBTAINING AN ESTIMATE OF STACK USAGE 4058 4059 Matching certain patterns using pcre_exec() can use a lot of process 4060 stack, which in certain environments can be rather limited in size. 4061 Some users find it helpful to have an estimate of the amount of stack 4062 that is used by pcre_exec(), to help them set recursion limits, as 4063 described in the pcrestack documentation. The estimate that is output 4064 by pcretest when called with the -m and -C options is obtained by call- 4065 ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its 4066 first five arguments. 4067 4068 Normally, if its first argument is NULL, pcre_exec() immediately 4069 returns the negative error code PCRE_ERROR_NULL, but with this special 4070 combination of arguments, it returns instead a negative number whose 4071 absolute value is the approximate stack frame size in bytes. (A nega- 4072 tive number is used so that it is clear that no match has happened.) 4073 The value is approximate because in some cases, recursive calls to 4074 pcre_exec() occur when there are one or two additional variables on the 4075 stack. 4076 4077 If PCRE has been compiled to use the heap instead of the stack for 4078 recursion, the value returned is the size of each block that is 4079 obtained from the heap. 4080 4081 4082 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION 4083 4084 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, 4085 const char *subject, int length, int startoffset, 4086 int options, int *ovector, int ovecsize, 4087 int *workspace, int wscount); 4088 4089 The function pcre_dfa_exec() is called to match a subject string 4090 against a compiled pattern, using a matching algorithm that scans the 4091 subject string just once, and does not backtrack. This has different 4092 characteristics to the normal algorithm, and is not compatible with 4093 Perl. Some of the features of PCRE patterns are not supported. Never- 4094 theless, there are times when this kind of matching can be useful. For 4095 a discussion of the two matching algorithms, and a list of features 4096 that pcre_dfa_exec() does not support, see the pcrematching documenta- 4097 tion. 4098 4099 The arguments for the pcre_dfa_exec() function are the same as for 4100 pcre_exec(), plus two extras. The ovector argument is used in a differ- 4101 ent way, and this is described below. The other common arguments are 4102 used in the same way as for pcre_exec(), so their description is not 4103 repeated here. 4104 4105 The two additional arguments provide workspace for the function. The 4106 workspace vector should contain at least 20 elements. It is used for 4107 keeping track of multiple paths through the pattern tree. More 4108 workspace will be needed for patterns and subjects where there are a 4109 lot of potential matches. 4110 4111 Here is an example of a simple call to pcre_dfa_exec(): 4112 4113 int rc; 4114 int ovector[10]; 4115 int wspace[20]; 4116 rc = pcre_dfa_exec( 4117 re, /* result of pcre_compile() */ 4118 NULL, /* we didn't study the pattern */ 4119 "some string", /* the subject string */ 4120 11, /* the length of the subject string */ 4121 0, /* start at offset 0 in the subject */ 4122 0, /* default options */ 4123 ovector, /* vector of integers for substring information */ 4124 10, /* number of elements (NOT size in bytes) */ 4125 wspace, /* working space vector */ 4126 20); /* number of elements (NOT size in bytes) */ 4127 4128 Option bits for pcre_dfa_exec() 4129 4130 The unused bits of the options argument for pcre_dfa_exec() must be 4131 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- 4132 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, 4133 PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF, 4134 PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR- 4135 TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last 4136 four of these are exactly the same as for pcre_exec(), so their 4137 description is not repeated here. 4138 4139 PCRE_PARTIAL_HARD 4140 PCRE_PARTIAL_SOFT 4141 4142 These have the same general effect as they do for pcre_exec(), but the 4143 details are slightly different. When PCRE_PARTIAL_HARD is set for 4144 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub- 4145 ject is reached and there is still at least one matching possibility 4146 that requires additional characters. This happens even if some complete 4147 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return 4148 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end 4149 of the subject is reached, there have been no complete matches, but 4150 there is still at least one matching possibility. The portion of the 4151 string that was inspected when the longest partial match was found is 4152 set as the first matching string in both cases. There is a more 4153 detailed discussion of partial and multi-segment matching, with exam- 4154 ples, in the pcrepartial documentation. 4155 4156 PCRE_DFA_SHORTEST 4157 4158 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to 4159 stop as soon as it has found one match. Because of the way the alterna- 4160 tive algorithm works, this is necessarily the shortest possible match 4161 at the first possible matching point in the subject string. 4162 4163 PCRE_DFA_RESTART 4164 4165 When pcre_dfa_exec() returns a partial match, it is possible to call it 4166 again, with additional subject characters, and have it continue with 4167 the same match. The PCRE_DFA_RESTART option requests this action; when 4168 it is set, the workspace and wscount options must reference the same 4169 vector as before because data about the match so far is left in them 4170 after a partial match. There is more discussion of this facility in the 4171 pcrepartial documentation. 4172 4173 Successful returns from pcre_dfa_exec() 4174 4175 When pcre_dfa_exec() succeeds, it may have matched more than one sub- 4176 string in the subject. Note, however, that all the matches from one run 4177 of the function start at the same point in the subject. The shorter 4178 matches are all initial substrings of the longer matches. For example, 4179 if the pattern 4180 4181 <.*> 4182 4183 is matched against the string 4184 4185 This is <something> <something else> <something further> no more 4186 4187 the three matched strings are 4188 4189 <something> 4190 <something> <something else> 4191 <something> <something else> <something further> 4192 4193 On success, the yield of the function is a number greater than zero, 4194 which is the number of matched substrings. The substrings themselves 4195 are returned in ovector. Each string uses two elements; the first is 4196 the offset to the start, and the second is the offset to the end. In 4197 fact, all the strings have the same start offset. (Space could have 4198 been saved by giving this only once, but it was decided to retain some 4199 compatibility with the way pcre_exec() returns data, even though the 4200 meaning of the strings is different.) 4201 4202 The strings are returned in reverse order of length; that is, the long- 4203 est matching string is given first. If there were too many matches to 4204 fit into ovector, the yield of the function is zero, and the vector is 4205 filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec() 4206 can use the entire ovector for returning matched strings. 4207 4208 NOTE: PCRE's "auto-possessification" optimization usually applies to 4209 character repeats at the end of a pattern (as well as internally). For 4210 example, the pattern "a\d+" is compiled as if it were "a\d++" because 4211 there is no point even considering the possibility of backtracking into 4212 the repeated digits. For DFA matching, this means that only one possi- 4213 ble match is found. If you really do want multiple matches in such 4214 cases, either use an ungreedy repeat ("a\d+?") or set the 4215 PCRE_NO_AUTO_POSSESS option when compiling. 4216 4217 Error returns from pcre_dfa_exec() 4218 4219 The pcre_dfa_exec() function returns a negative number when it fails. 4220 Many of the errors are the same as for pcre_exec(), and these are 4221 described above. There are in addition the following errors that are 4222 specific to pcre_dfa_exec(): 4223 4224 PCRE_ERROR_DFA_UITEM (-16) 4225 4226 This return is given if pcre_dfa_exec() encounters an item in the pat- 4227 tern that it does not support, for instance, the use of \C or a back 4228 reference. 4229 4230 PCRE_ERROR_DFA_UCOND (-17) 4231 4232 This return is given if pcre_dfa_exec() encounters a condition item 4233 that uses a back reference for the condition, or a test for recursion 4234 in a specific group. These are not supported. 4235 4236 PCRE_ERROR_DFA_UMLIMIT (-18) 4237 4238 This return is given if pcre_dfa_exec() is called with an extra block 4239 that contains a setting of the match_limit or match_limit_recursion 4240 fields. This is not supported (these fields are meaningless for DFA 4241 matching). 4242 4243 PCRE_ERROR_DFA_WSSIZE (-19) 4244 4245 This return is given if pcre_dfa_exec() runs out of space in the 4246 workspace vector. 4247 4248 PCRE_ERROR_DFA_RECURSE (-20) 4249 4250 When a recursive subpattern is processed, the matching function calls 4251 itself recursively, using private vectors for ovector and workspace. 4252 This error is given if the output vector is not large enough. This 4253 should be extremely rare, as a vector of size 1000 is used. 4254 4255 PCRE_ERROR_DFA_BADRESTART (-30) 4256 4257 When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some 4258 plausibility checks are made on the contents of the workspace, which 4259 should contain data about the previous partial match. If any of these 4260 checks fail, this error is given. 4261 4262 4263 SEE ALSO 4264 4265 pcre16(3), pcre32(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), 4266 pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcre- 4267 sample(3), pcrestack(3). 4268 4269 4270 AUTHOR 4271 4272 Philip Hazel 4273 University Computing Service 4274 Cambridge CB2 3QH, England. 4275 4276 4277 REVISION 4278 4279 Last updated: 09 February 2014 4280 Copyright (c) 1997-2014 University of Cambridge. 4281 ------------------------------------------------------------------------------ 4282 4283 4284 PCRECALLOUT(3) Library Functions Manual PCRECALLOUT(3) 4285 4286 4287 4288 NAME 4289 PCRE - Perl-compatible regular expressions 4290 4291 SYNOPSIS 4292 4293 #include <pcre.h> 4294 4295 int (*pcre_callout)(pcre_callout_block *); 4296 4297 int (*pcre16_callout)(pcre16_callout_block *); 4298 4299 int (*pcre32_callout)(pcre32_callout_block *); 4300 4301 4302 DESCRIPTION 4303 4304 PCRE provides a feature called "callout", which is a means of temporar- 4305 ily passing control to the caller of PCRE in the middle of pattern 4306 matching. The caller of PCRE provides an external function by putting 4307 its entry point in the global variable pcre_callout (pcre16_callout for 4308 the 16-bit library, pcre32_callout for the 32-bit library). By default, 4309 this variable contains NULL, which disables all calling out. 4310 4311 Within a regular expression, (?C) indicates the points at which the 4312 external function is to be called. Different callout points can be 4313 identified by putting a number less than 256 after the letter C. The 4314 default value is zero. For example, this pattern has two callout 4315 points: 4316 4317 (?C1)abc(?C2)def 4318 4319 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, 4320 PCRE automatically inserts callouts, all with number 255, before each 4321 item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the 4322 pattern 4323 4324 A(\d{2}|--) 4325 4326 it is processed as if it were 4327 4328 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) 4329 4330 Notice that there is a callout before and after each parenthesis and 4331 alternation bar. If the pattern contains a conditional group whose con- 4332 dition is an assertion, an automatic callout is inserted immediately 4333 before the condition. Such a callout may also be inserted explicitly, 4334 for example: 4335 4336 (?(?C9)(?=a)ab|de) 4337 4338 This applies only to assertion conditions (because they are themselves 4339 independent groups). 4340 4341 Automatic callouts can be used for tracking the progress of pattern 4342 matching. The pcretest program has a pattern qualifier (/C) that sets 4343 automatic callouts; when it is used, the output indicates how the pat- 4344 tern is being matched. This is useful information when you are trying 4345 to optimize the performance of a particular pattern. 4346 4347 4348 MISSING CALLOUTS 4349 4350 You should be aware that, because of optimizations in the way PCRE com- 4351 piles and matches patterns, callouts sometimes do not happen exactly as 4352 you might expect. 4353 4354 At compile time, PCRE "auto-possessifies" repeated items when it knows 4355 that what follows cannot be part of the repeat. For example, a+[bc] is 4356 compiled as if it were a++[bc]. The pcretest output when this pattern 4357 is anchored and then applied with automatic callouts to the string 4358 "aaaa" is: 4359 4360 --->aaaa 4361 +0 ^ ^ 4362 +1 ^ a+ 4363 +3 ^ ^ [bc] 4364 No match 4365 4366 This indicates that when matching [bc] fails, there is no backtracking 4367 into a+ and therefore the callouts that would be taken for the back- 4368 tracks do not occur. You can disable the auto-possessify feature by 4369 passing PCRE_NO_AUTO_POSSESS to pcre_compile(), or starting the pattern 4370 with (*NO_AUTO_POSSESS). If this is done in pcretest (using the /O 4371 qualifier), the output changes to this: 4372 4373 --->aaaa 4374 +0 ^ ^ 4375 +1 ^ a+ 4376 +3 ^ ^ [bc] 4377 +3 ^ ^ [bc] 4378 +3 ^ ^ [bc] 4379 +3 ^^ [bc] 4380 No match 4381 4382 This time, when matching [bc] fails, the matcher backtracks into a+ and 4383 tries again, repeatedly, until a+ itself fails. 4384 4385 Other optimizations that provide fast "no match" results also affect 4386 callouts. For example, if the pattern is 4387 4388 ab(?C4)cd 4389 4390 PCRE knows that any matching string must contain the letter "d". If the 4391 subject string is "abyz", the lack of "d" means that matching doesn't 4392 ever start, and the callout is never reached. However, with "abyd", 4393 though the result is still no match, the callout is obeyed. 4394 4395 If the pattern is studied, PCRE knows the minimum length of a matching 4396 string, and will immediately give a "no match" return without actually 4397 running a match if the subject is not long enough, or, for unanchored 4398 patterns, if it has been scanned far enough. 4399 4400 You can disable these optimizations by passing the PCRE_NO_START_OPTI- 4401 MIZE option to the matching function, or by starting the pattern with 4402 (*NO_START_OPT). This slows down the matching process, but does ensure 4403 that callouts such as the example above are obeyed. 4404 4405 4406 THE CALLOUT INTERFACE 4407 4408 During matching, when PCRE reaches a callout point, the external func- 4409 tion defined by pcre_callout or pcre[16|32]_callout is called (if it is 4410 set). This applies to both normal and DFA matching. The only argument 4411 to the callout function is a pointer to a pcre_callout or 4412 pcre[16|32]_callout block. These structures contains the following 4413 fields: 4414 4415 int version; 4416 int callout_number; 4417 int *offset_vector; 4418 const char *subject; (8-bit version) 4419 PCRE_SPTR16 subject; (16-bit version) 4420 PCRE_SPTR32 subject; (32-bit version) 4421 int subject_length; 4422 int start_match; 4423 int current_position; 4424 int capture_top; 4425 int capture_last; 4426 void *callout_data; 4427 int pattern_position; 4428 int next_item_length; 4429 const unsigned char *mark; (8-bit version) 4430 const PCRE_UCHAR16 *mark; (16-bit version) 4431 const PCRE_UCHAR32 *mark; (32-bit version) 4432 4433 The version field is an integer containing the version number of the 4434 block format. The initial version was 0; the current version is 2. The 4435 version number will change again in future if additional fields are 4436 added, but the intention is never to remove any of the existing fields. 4437 4438 The callout_number field contains the number of the callout, as com- 4439 piled into the pattern (that is, the number after ?C for manual call- 4440 outs, and 255 for automatically generated callouts). 4441 4442 The offset_vector field is a pointer to the vector of offsets that was 4443 passed by the caller to the matching function. When pcre_exec() or 4444 pcre[16|32]_exec() is used, the contents can be inspected, in order to 4445 extract substrings that have been matched so far, in the same way as 4446 for extracting substrings after a match has completed. For the DFA 4447 matching functions, this field is not useful. 4448 4449 The subject and subject_length fields contain copies of the values that 4450 were passed to the matching function. 4451 4452 The start_match field normally contains the offset within the subject 4453 at which the current match attempt started. However, if the escape 4454 sequence \K has been encountered, this value is changed to reflect the 4455 modified starting point. If the pattern is not anchored, the callout 4456 function may be called several times from the same point in the pattern 4457 for different starting points in the subject. 4458 4459 The current_position field contains the offset within the subject of 4460 the current match pointer. 4461 4462 When the pcre_exec() or pcre[16|32]_exec() is used, the capture_top 4463 field contains one more than the number of the highest numbered cap- 4464 tured substring so far. If no substrings have been captured, the value 4465 of capture_top is one. This is always the case when the DFA functions 4466 are used, because they do not support captured substrings. 4467 4468 The capture_last field contains the number of the most recently cap- 4469 tured substring. However, when a recursion exits, the value reverts to 4470 what it was outside the recursion, as do the values of all captured 4471 substrings. If no substrings have been captured, the value of cap- 4472 ture_last is -1. This is always the case for the DFA matching func- 4473 tions. 4474 4475 The callout_data field contains a value that is passed to a matching 4476 function specifically so that it can be passed back in callouts. It is 4477 passed in the callout_data field of a pcre_extra or pcre[16|32]_extra 4478 data structure. If no such data was passed, the value of callout_data 4479 in a callout block is NULL. There is a description of the pcre_extra 4480 structure in the pcreapi documentation. 4481 4482 The pattern_position field is present from version 1 of the callout 4483 structure. It contains the offset to the next item to be matched in the 4484 pattern string. 4485 4486 The next_item_length field is present from version 1 of the callout 4487 structure. It contains the length of the next item to be matched in the 4488 pattern string. When the callout immediately precedes an alternation 4489 bar, a closing parenthesis, or the end of the pattern, the length is 4490 zero. When the callout precedes an opening parenthesis, the length is 4491 that of the entire subpattern. 4492 4493 The pattern_position and next_item_length fields are intended to help 4494 in distinguishing between different automatic callouts, which all have 4495 the same callout number. However, they are set for all callouts. 4496 4497 The mark field is present from version 2 of the callout structure. In 4498 callouts from pcre_exec() or pcre[16|32]_exec() it contains a pointer 4499 to the zero-terminated name of the most recently passed (*MARK), 4500 (*PRUNE), or (*THEN) item in the match, or NULL if no such items have 4501 been passed. Instances of (*PRUNE) or (*THEN) without a name do not 4502 obliterate a previous (*MARK). In callouts from the DFA matching func- 4503 tions this field always contains NULL. 4504 4505 4506 RETURN VALUES 4507 4508 The external callout function returns an integer to PCRE. If the value 4509 is zero, matching proceeds as normal. If the value is greater than 4510 zero, matching fails at the current point, but the testing of other 4511 matching possibilities goes ahead, just as if a lookahead assertion had 4512 failed. If the value is less than zero, the match is abandoned, the 4513 matching function returns the negative value. 4514 4515 Negative values should normally be chosen from the set of 4516 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- 4517 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is 4518 reserved for use by callout functions; it will never be used by PCRE 4519 itself. 4520 4521 4522 AUTHOR 4523 4524 Philip Hazel 4525 University Computing Service 4526 Cambridge CB2 3QH, England. 4527 4528 4529 REVISION 4530 4531 Last updated: 12 November 2013 4532 Copyright (c) 1997-2013 University of Cambridge. 4533 ------------------------------------------------------------------------------ 4534 4535 4536 PCRECOMPAT(3) Library Functions Manual PCRECOMPAT(3) 4537 4538 4539 4540 NAME 4541 PCRE - Perl-compatible regular expressions 4542 4543 DIFFERENCES BETWEEN PCRE AND PERL 4544 4545 This document describes the differences in the ways that PCRE and Perl 4546 handle regular expressions. The differences described here are with 4547 respect to Perl versions 5.10 and above. 4548 4549 1. PCRE has only a subset of Perl's Unicode support. Details of what it 4550 does have are given in the pcreunicode page. 4551 4552 2. PCRE allows repeat quantifiers only on parenthesized assertions, but 4553 they do not mean what you might think. For example, (?!a){3} does not 4554 assert that the next three characters are not "a". It just asserts that 4555 the next character is not "a" three times (in principle: PCRE optimizes 4556 this to run the assertion just once). Perl allows repeat quantifiers on 4557 other assertions such as \b, but these do not seem to have any use. 4558 4559 3. Capturing subpatterns that occur inside negative lookahead asser- 4560 tions are counted, but their entries in the offsets vector are never 4561 set. Perl sometimes (but not always) sets its numerical variables from 4562 inside negative assertions. 4563 4564 4. Though binary zero characters are supported in the subject string, 4565 they are not allowed in a pattern string because it is passed as a nor- 4566 mal C string, terminated by zero. The escape sequence \0 can be used in 4567 the pattern to represent a binary zero. 4568 4569 5. The following Perl escape sequences are not supported: \l, \u, \L, 4570 \U, and \N when followed by a character name or Unicode value. (\N on 4571 its own, matching a non-newline character, is supported.) In fact these 4572 are implemented by Perl's general string-handling and are not part of 4573 its pattern matching engine. If any of these are encountered by PCRE, 4574 an error is generated by default. However, if the PCRE_JAVASCRIPT_COM- 4575 PAT option is set, \U and \u are interpreted as JavaScript interprets 4576 them. 4577 4578 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE 4579 is built with Unicode character property support. The properties that 4580 can be tested with \p and \P are limited to the general category prop- 4581 erties such as Lu and Nd, script names such as Greek or Han, and the 4582 derived properties Any and L&. PCRE does support the Cs (surrogate) 4583 property, which Perl does not; the Perl documentation says "Because 4584 Perl hides the need for the user to understand the internal representa- 4585 tion of Unicode characters, there is no need to implement the somewhat 4586 messy concept of surrogates." 4587 4588 7. PCRE does support the \Q...\E escape for quoting substrings. Charac- 4589 ters in between are treated as literals. This is slightly different 4590 from Perl in that $ and @ are also handled as literals inside the 4591 quotes. In Perl, they cause variable interpolation (but of course PCRE 4592 does not have variables). Note the following examples: 4593 4594 Pattern PCRE matches Perl matches 4595 4596 \Qabc$xyz\E abc$xyz abc followed by the 4597 contents of $xyz 4598 \Qabc\$xyz\E abc\$xyz abc\$xyz 4599 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 4600 4601 The \Q...\E sequence is recognized both inside and outside character 4602 classes. 4603 4604 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) 4605 constructions. However, there is support for recursive patterns. This 4606 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE 4607 "callout" feature allows an external function to be called during pat- 4608 tern matching. See the pcrecallout documentation for details. 4609 4610 9. Subpatterns that are called as subroutines (whether or not recur- 4611 sively) are always treated as atomic groups in PCRE. This is like 4612 Python, but unlike Perl. Captured values that are set outside a sub- 4613 routine call can be reference from inside in PCRE, but not in Perl. 4614 There is a discussion that explains these differences in more detail in 4615 the section on recursion differences from Perl in the pcrepattern page. 4616 4617 10. If any of the backtracking control verbs are used in a subpattern 4618 that is called as a subroutine (whether or not recursively), their 4619 effect is confined to that subpattern; it does not extend to the sur- 4620 rounding pattern. This is not always the case in Perl. In particular, 4621 if (*THEN) is present in a group that is called as a subroutine, its 4622 action is limited to that group, even if the group does not contain any 4623 | characters. Note that such subpatterns are processed as anchored at 4624 the point where they are tested. 4625 4626 11. If a pattern contains more than one backtracking control verb, the 4627 first one that is backtracked onto acts. For example, in the pattern 4628 A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure 4629 in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases 4630 it is the same as PCRE, but there are examples where it differs. 4631 4632 12. Most backtracking verbs in assertions have their normal actions. 4633 They are not confined to the assertion. 4634 4635 13. There are some differences that are concerned with the settings of 4636 captured strings when part of a pattern is repeated. For example, 4637 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 4638 unset, but in PCRE it is set to "b". 4639 4640 14. PCRE's handling of duplicate subpattern numbers and duplicate sub- 4641 pattern names is not as general as Perl's. This is a consequence of the 4642 fact the PCRE works internally just with numbers, using an external ta- 4643 ble to translate between numbers and names. In particular, a pattern 4644 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have 4645 the same number but different names, is not supported, and causes an 4646 error at compile time. If it were allowed, it would not be possible to 4647 distinguish which parentheses matched, because both names map to cap- 4648 turing subpattern number 1. To avoid this confusing situation, an error 4649 is given at compile time. 4650 4651 15. Perl recognizes comments in some places that PCRE does not, for 4652 example, between the ( and ? at the start of a subpattern. If the /x 4653 modifier is set, Perl allows white space between ( and ? (though cur- 4654 rent Perls warn that this is deprecated) but PCRE never does, even if 4655 the PCRE_EXTENDED option is set. 4656 4657 16. Perl, when in warning mode, gives warnings for character classes 4658 such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter- 4659 als. PCRE has no warning features, so it gives an error in these cases 4660 because they are almost certainly user mistakes. 4661 4662 17. In PCRE, the upper/lower case character properties Lu and Ll are 4663 not affected when case-independent matching is specified. For example, 4664 \p{Lu} always matches an upper case letter. I think Perl has changed in 4665 this respect; in the release at the time of writing (5.16), \p{Lu} and 4666 \p{Ll} match all letters, regardless of case, when case independence is 4667 specified. 4668 4669 18. PCRE provides some extensions to the Perl regular expression facil- 4670 ities. Perl 5.10 includes new features that are not in earlier ver- 4671 sions of Perl, some of which (such as named parentheses) have been in 4672 PCRE for some time. This list is with respect to Perl 5.10: 4673 4674 (a) Although lookbehind assertions in PCRE must match fixed length 4675 strings, each alternative branch of a lookbehind assertion can match a 4676 different length of string. Perl requires them all to have the same 4677 length. 4678 4679 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ 4680 meta-character matches only at the very end of the string. 4681 4682 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- 4683 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly 4684 ignored. (Perl can be made to issue a warning.) 4685 4686 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- 4687 fiers is inverted, that is, by default they are not greedy, but if fol- 4688 lowed by a question mark they are. 4689 4690 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be 4691 tried only at the first matching position in the subject string. 4692 4693 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, 4694 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva- 4695 lents. 4696 4697 (g) The \R escape sequence can be restricted to match only CR, LF, or 4698 CRLF by the PCRE_BSR_ANYCRLF option. 4699 4700 (h) The callout facility is PCRE-specific. 4701 4702 (i) The partial matching facility is PCRE-specific. 4703 4704 (j) Patterns compiled by PCRE can be saved and re-used at a later time, 4705 even on different hosts that have the other endianness. However, this 4706 does not apply to optimized data created by the just-in-time compiler. 4707 4708 (k) The alternative matching functions (pcre_dfa_exec(), 4709 pcre16_dfa_exec() and pcre32_dfa_exec(),) match in a different way and 4710 are not Perl-compatible. 4711 4712 (l) PCRE recognizes some special sequences such as (*CR) at the start 4713 of a pattern that set overall options that cannot be changed within the 4714 pattern. 4715 4716 4717 AUTHOR 4718 4719 Philip Hazel 4720 University Computing Service 4721 Cambridge CB2 3QH, England. 4722 4723 4724 REVISION 4725 4726 Last updated: 10 November 2013 4727 Copyright (c) 1997-2013 University of Cambridge. 4728 ------------------------------------------------------------------------------ 4729 4730 4731 PCREPATTERN(3) Library Functions Manual PCREPATTERN(3) 4732 4733 4734 4735 NAME 4736 PCRE - Perl-compatible regular expressions 4737 4738 PCRE REGULAR EXPRESSION DETAILS 4739 4740 The syntax and semantics of the regular expressions that are supported 4741 by PCRE are described in detail below. There is a quick-reference syn- 4742 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and 4743 semantics as closely as it can. PCRE also supports some alternative 4744 regular expression syntax (which does not conflict with the Perl syn- 4745 tax) in order to provide some compatibility with regular expressions in 4746 Python, .NET, and Oniguruma. 4747 4748 Perl's regular expressions are described in its own documentation, and 4749 regular expressions in general are covered in a number of books, some 4750 of which have copious examples. Jeffrey Friedl's "Mastering Regular 4751 Expressions", published by O'Reilly, covers regular expressions in 4752 great detail. This description of PCRE's regular expressions is 4753 intended as reference material. 4754 4755 This document discusses the patterns that are supported by PCRE when 4756 one its main matching functions, pcre_exec() (8-bit) or 4757 pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has alternative 4758 matching functions, pcre_dfa_exec() and pcre[16|32_dfa_exec(), which 4759 match using a different algorithm that is not Perl-compatible. Some of 4760 the features discussed below are not available when DFA matching is 4761 used. The advantages and disadvantages of the alternative functions, 4762 and how they differ from the normal functions, are discussed in the 4763 pcrematching page. 4764 4765 4766 SPECIAL START-OF-PATTERN ITEMS 4767 4768 A number of options that can be passed to pcre_compile() can also be 4769 set by special items at the start of a pattern. These are not Perl-com- 4770 patible, but are provided to make these options accessible to pattern 4771 writers who are not able to change the program that processes the pat- 4772 tern. Any number of these items may appear, but they must all be 4773 together right at the start of the pattern string, and the letters must 4774 be in upper case. 4775 4776 UTF support 4777 4778 The original operation of PCRE was on strings of one-byte characters. 4779 However, there is now also support for UTF-8 strings in the original 4780 library, an extra library that supports 16-bit and UTF-16 character 4781 strings, and a third library that supports 32-bit and UTF-32 character 4782 strings. To use these features, PCRE must be built to include appropri- 4783 ate support. When using UTF strings you must either call the compiling 4784 function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option, or the 4785 pattern must start with one of these special sequences: 4786 4787 (*UTF8) 4788 (*UTF16) 4789 (*UTF32) 4790 (*UTF) 4791 4792 (*UTF) is a generic sequence that can be used with any of the 4793 libraries. Starting a pattern with such a sequence is equivalent to 4794 setting the relevant option. How setting a UTF mode affects pattern 4795 matching is mentioned in several places below. There is also a summary 4796 of features in the pcreunicode page. 4797 4798 Some applications that allow their users to supply patterns may wish to 4799 restrict them to non-UTF data for security reasons. If the 4800 PCRE_NEVER_UTF option is set at compile time, (*UTF) etc. are not 4801 allowed, and their appearance causes an error. 4802 4803 Unicode property support 4804 4805 Another special sequence that may appear at the start of a pattern is 4806 (*UCP). This has the same effect as setting the PCRE_UCP option: it 4807 causes sequences such as \d and \w to use Unicode properties to deter- 4808 mine character types, instead of recognizing only characters with codes 4809 less than 128 via a lookup table. 4810 4811 Disabling auto-possessification 4812 4813 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as 4814 setting the PCRE_NO_AUTO_POSSESS option at compile time. This stops 4815 PCRE from making quantifiers possessive when what follows cannot match 4816 the repeated item. For example, by default a+b is treated as a++b. For 4817 more details, see the pcreapi documentation. 4818 4819 Disabling start-up optimizations 4820 4821 If a pattern starts with (*NO_START_OPT), it has the same effect as 4822 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching 4823 time. This disables several optimizations for quickly reaching "no 4824 match" results. For more details, see the pcreapi documentation. 4825 4826 Newline conventions 4827 4828 PCRE supports five different conventions for indicating line breaks in 4829 strings: a single CR (carriage return) character, a single LF (line- 4830 feed) character, the two-character sequence CRLF, any of the three pre- 4831 ceding, or any Unicode newline sequence. The pcreapi page has further 4832 discussion about newlines, and shows how to set the newline convention 4833 in the options arguments for the compiling and matching functions. 4834 4835 It is also possible to specify a newline convention by starting a pat- 4836 tern string with one of the following five sequences: 4837 4838 (*CR) carriage return 4839 (*LF) linefeed 4840 (*CRLF) carriage return, followed by linefeed 4841 (*ANYCRLF) any of the three above 4842 (*ANY) all Unicode newline sequences 4843 4844 These override the default and the options given to the compiling func- 4845 tion. For example, on a Unix system where LF is the default newline 4846 sequence, the pattern 4847 4848 (*CR)a.b 4849 4850 changes the convention to CR. That pattern matches "a\nb" because LF is 4851 no longer a newline. If more than one of these settings is present, the 4852 last one is used. 4853 4854 The newline convention affects where the circumflex and dollar asser- 4855 tions are true. It also affects the interpretation of the dot metachar- 4856 acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it 4857 does not affect what the \R escape sequence matches. By default, this 4858 is any Unicode newline sequence, for Perl compatibility. However, this 4859 can be changed; see the description of \R in the section entitled "New- 4860 line sequences" below. A change of \R setting can be combined with a 4861 change of newline convention. 4862 4863 Setting match and recursion limits 4864 4865 The caller of pcre_exec() can set a limit on the number of times the 4866 internal match() function is called and on the maximum depth of recur- 4867 sive calls. These facilities are provided to catch runaway matches that 4868 are provoked by patterns with huge matching trees (a typical example is 4869 a pattern with nested unlimited repeats) and to avoid running out of 4870 system stack by too much recursion. When one of these limits is 4871 reached, pcre_exec() gives an error return. The limits can also be set 4872 by items at the start of the pattern of the form 4873 4874 (*LIMIT_MATCH=d) 4875 (*LIMIT_RECURSION=d) 4876 4877 where d is any number of decimal digits. However, the value of the set- 4878 ting must be less than the value set (or defaulted) by the caller of 4879 pcre_exec() for it to have any effect. In other words, the pattern 4880 writer can lower the limits set by the programmer, but not raise them. 4881 If there is more than one setting of one of these limits, the lower 4882 value is used. 4883 4884 4885 EBCDIC CHARACTER CODES 4886 4887 PCRE can be compiled to run in an environment that uses EBCDIC as its 4888 character code rather than ASCII or Unicode (typically a mainframe sys- 4889 tem). In the sections below, character code values are ASCII or Uni- 4890 code; in an EBCDIC environment these characters may have different code 4891 values, and there are no code points greater than 255. 4892 4893 4894 CHARACTERS AND METACHARACTERS 4895 4896 A regular expression is a pattern that is matched against a subject 4897 string from left to right. Most characters stand for themselves in a 4898 pattern, and match the corresponding characters in the subject. As a 4899 trivial example, the pattern 4900 4901 The quick brown fox 4902 4903 matches a portion of a subject string that is identical to itself. When 4904 caseless matching is specified (the PCRE_CASELESS option), letters are 4905 matched independently of case. In a UTF mode, PCRE always understands 4906 the concept of case for characters whose values are less than 128, so 4907 caseless matching is always possible. For characters with higher val- 4908 ues, the concept of case is supported if PCRE is compiled with Unicode 4909 property support, but not otherwise. If you want to use caseless 4910 matching for characters 128 and above, you must ensure that PCRE is 4911 compiled with Unicode property support as well as with UTF support. 4912 4913 The power of regular expressions comes from the ability to include 4914 alternatives and repetitions in the pattern. These are encoded in the 4915 pattern by the use of metacharacters, which do not stand for themselves 4916 but instead are interpreted in some special way. 4917 4918 There are two different sets of metacharacters: those that are recog- 4919 nized anywhere in the pattern except within square brackets, and those 4920 that are recognized within square brackets. Outside square brackets, 4921 the metacharacters are as follows: 4922 4923 \ general escape character with several uses 4924 ^ assert start of string (or line, in multiline mode) 4925 $ assert end of string (or line, in multiline mode) 4926 . match any character except newline (by default) 4927 [ start character class definition 4928 | start of alternative branch 4929 ( start subpattern 4930 ) end subpattern 4931 ? extends the meaning of ( 4932 also 0 or 1 quantifier 4933 also quantifier minimizer 4934 * 0 or more quantifier 4935 + 1 or more quantifier 4936 also "possessive quantifier" 4937 { start min/max quantifier 4938 4939 Part of a pattern that is in square brackets is called a "character 4940 class". In a character class the only metacharacters are: 4941 4942 \ general escape character 4943 ^ negate the class, but only if the first character 4944 - indicates character range 4945 [ POSIX character class (only if followed by POSIX 4946 syntax) 4947 ] terminates the character class 4948 4949 The following sections describe the use of each of the metacharacters. 4950 4951 4952 BACKSLASH 4953 4954 The backslash character has several uses. Firstly, if it is followed by 4955 a character that is not a number or a letter, it takes away any special 4956 meaning that character may have. This use of backslash as an escape 4957 character applies both inside and outside character classes. 4958 4959 For example, if you want to match a * character, you write \* in the 4960 pattern. This escaping action applies whether or not the following 4961 character would otherwise be interpreted as a metacharacter, so it is 4962 always safe to precede a non-alphanumeric with backslash to specify 4963 that it stands for itself. In particular, if you want to match a back- 4964 slash, you write \\. 4965 4966 In a UTF mode, only ASCII numbers and letters have any special meaning 4967 after a backslash. All other characters (in particular, those whose 4968 codepoints are greater than 127) are treated as literals. 4969 4970 If a pattern is compiled with the PCRE_EXTENDED option, most white 4971 space in the pattern (other than in a character class), and characters 4972 between a # outside a character class and the next newline, inclusive, 4973 are ignored. An escaping backslash can be used to include a white space 4974 or # character as part of the pattern. 4975 4976 If you want to remove the special meaning from a sequence of charac- 4977 ters, you can do so by putting them between \Q and \E. This is differ- 4978 ent from Perl in that $ and @ are handled as literals in \Q...\E 4979 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- 4980 tion. Note the following examples: 4981 4982 Pattern PCRE matches Perl matches 4983 4984 \Qabc$xyz\E abc$xyz abc followed by the 4985 contents of $xyz 4986 \Qabc\$xyz\E abc\$xyz abc\$xyz 4987 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 4988 4989 The \Q...\E sequence is recognized both inside and outside character 4990 classes. An isolated \E that is not preceded by \Q is ignored. If \Q 4991 is not followed by \E later in the pattern, the literal interpretation 4992 continues to the end of the pattern (that is, \E is assumed at the 4993 end). If the isolated \Q is inside a character class, this causes an 4994 error, because the character class is not terminated. 4995 4996 Non-printing characters 4997 4998 A second use of backslash provides a way of encoding non-printing char- 4999 acters in patterns in a visible manner. There is no restriction on the 5000 appearance of non-printing characters, apart from the binary zero that 5001 terminates a pattern, but when a pattern is being prepared by text 5002 editing, it is often easier to use one of the following escape 5003 sequences than the binary character it represents. In an ASCII or Uni- 5004 code environment, these escapes are as follows: 5005 5006 \a alarm, that is, the BEL character (hex 07) 5007 \cx "control-x", where x is any ASCII character 5008 \e escape (hex 1B) 5009 \f form feed (hex 0C) 5010 \n linefeed (hex 0A) 5011 \r carriage return (hex 0D) 5012 \t tab (hex 09) 5013 \0dd character with octal code 0dd 5014 \ddd character with octal code ddd, or back reference 5015 \o{ddd..} character with octal code ddd.. 5016 \xhh character with hex code hh 5017 \x{hhh..} character with hex code hhh.. (non-JavaScript mode) 5018 \uhhhh character with hex code hhhh (JavaScript mode only) 5019 5020 The precise effect of \cx on ASCII characters is as follows: if x is a 5021 lower case letter, it is converted to upper case. Then bit 6 of the 5022 character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A 5023 (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes 5024 hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c 5025 has a value greater than 127, a compile-time error occurs. This locks 5026 out non-ASCII characters in all modes. 5027 5028 When PCRE is compiled in EBCDIC mode, \a, \e, \f, \n, \r, and \t gener- 5029 ate the appropriate EBCDIC code values. The \c escape is processed as 5030 specified for Perl in the perlebcdic document. The only characters that 5031 are allowed after \c are A-Z, a-z, or one of @, [, \, ], ^, _, or ?. 5032 Any other character provokes a compile-time error. The sequence \@ 5033 encodes character code 0; the letters (in either case) encode charac- 5034 ters 1-26 (hex 01 to hex 1A); [, \, ], ^, and _ encode characters 27-31 5035 (hex 1B to hex 1F), and \? becomes either 255 (hex FF) or 95 (hex 5F). 5036 5037 Thus, apart from \?, these escapes generate the same character code 5038 values as they do in an ASCII environment, though the meanings of the 5039 values mostly differ. For example, \G always generates code value 7, 5040 which is BEL in ASCII but DEL in EBCDIC. 5041 5042 The sequence \? generates DEL (127, hex 7F) in an ASCII environment, 5043 but because 127 is not a control character in EBCDIC, Perl makes it 5044 generate the APC character. Unfortunately, there are several variants 5045 of EBCDIC. In most of them the APC character has the value 255 (hex 5046 FF), but in the one Perl calls POSIX-BC its value is 95 (hex 5F). If 5047 certain other characters have POSIX-BC values, PCRE makes \? generate 5048 95; otherwise it generates 255. 5049 5050 After \0 up to two further octal digits are read. If there are fewer 5051 than two digits, just those that are present are used. Thus the 5052 sequence \0\x\015 specifies two binary zeros followed by a CR character 5053 (code value 13). Make sure you supply two digits after the initial zero 5054 if the pattern character that follows is itself an octal digit. 5055 5056 The escape \o must be followed by a sequence of octal digits, enclosed 5057 in braces. An error occurs if this is not the case. This escape is a 5058 recent addition to Perl; it provides way of specifying character code 5059 points as octal numbers greater than 0777, and it also allows octal 5060 numbers and back references to be unambiguously specified. 5061 5062 For greater clarity and unambiguity, it is best to avoid following \ by 5063 a digit greater than zero. Instead, use \o{} or \x{} to specify charac- 5064 ter numbers, and \g{} to specify back references. The following para- 5065 graphs describe the old, ambiguous syntax. 5066 5067 The handling of a backslash followed by a digit other than 0 is compli- 5068 cated, and Perl has changed in recent releases, causing PCRE also to 5069 change. Outside a character class, PCRE reads the digit and any follow- 5070 ing digits as a decimal number. If the number is less than 8, or if 5071 there have been at least that many previous capturing left parentheses 5072 in the expression, the entire sequence is taken as a back reference. A 5073 description of how this works is given later, following the discussion 5074 of parenthesized subpatterns. 5075 5076 Inside a character class, or if the decimal number following \ is 5077 greater than 7 and there have not been that many capturing subpatterns, 5078 PCRE handles \8 and \9 as the literal characters "8" and "9", and oth- 5079 erwise re-reads up to three octal digits following the backslash, using 5080 them to generate a data character. Any subsequent digits stand for 5081 themselves. For example: 5082 5083 \040 is another way of writing an ASCII space 5084 \40 is the same, provided there are fewer than 40 5085 previous capturing subpatterns 5086 \7 is always a back reference 5087 \11 might be a back reference, or another way of 5088 writing a tab 5089 \011 is always a tab 5090 \0113 is a tab followed by the character "3" 5091 \113 might be a back reference, otherwise the 5092 character with octal code 113 5093 \377 might be a back reference, otherwise 5094 the value 255 (decimal) 5095 \81 is either a back reference, or the two 5096 characters "8" and "1" 5097 5098 Note that octal values of 100 or greater that are specified using this 5099 syntax must not be introduced by a leading zero, because no more than 5100 three octal digits are ever read. 5101 5102 By default, after \x that is not followed by {, from zero to two hexa- 5103 decimal digits are read (letters can be in upper or lower case). Any 5104 number of hexadecimal digits may appear between \x{ and }. If a charac- 5105 ter other than a hexadecimal digit appears between \x{ and }, or if 5106 there is no terminating }, an error occurs. 5107 5108 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x 5109 is as just described only when it is followed by two hexadecimal dig- 5110 its. Otherwise, it matches a literal "x" character. In JavaScript 5111 mode, support for code points greater than 256 is provided by \u, which 5112 must be followed by four hexadecimal digits; otherwise it matches a 5113 literal "u" character. 5114 5115 Characters whose value is less than 256 can be defined by either of the 5116 two syntaxes for \x (or by \u in JavaScript mode). There is no differ- 5117 ence in the way they are handled. For example, \xdc is exactly the same 5118 as \x{dc} (or \u00dc in JavaScript mode). 5119 5120 Constraints on character values 5121 5122 Characters that are specified using octal or hexadecimal numbers are 5123 limited to certain values, as follows: 5124 5125 8-bit non-UTF mode less than 0x100 5126 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint 5127 16-bit non-UTF mode less than 0x10000 5128 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint 5129 32-bit non-UTF mode less than 0x100000000 5130 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint 5131 5132 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so- 5133 called "surrogate" codepoints), and 0xffef. 5134 5135 Escape sequences in character classes 5136 5137 All the sequences that define a single character value can be used both 5138 inside and outside character classes. In addition, inside a character 5139 class, \b is interpreted as the backspace character (hex 08). 5140 5141 \N is not allowed in a character class. \B, \R, and \X are not special 5142 inside a character class. Like other unrecognized escape sequences, 5143 they are treated as the literal characters "B", "R", and "X" by 5144 default, but cause an error if the PCRE_EXTRA option is set. Outside a 5145 character class, these sequences have different meanings. 5146 5147 Unsupported escape sequences 5148 5149 In Perl, the sequences \l, \L, \u, and \U are recognized by its string 5150 handler and used to modify the case of following characters. By 5151 default, PCRE does not support these escape sequences. However, if the 5152 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and 5153 \u can be used to define a character by code point, as described in the 5154 previous section. 5155 5156 Absolute and relative back references 5157 5158 The sequence \g followed by an unsigned or a negative number, option- 5159 ally enclosed in braces, is an absolute or relative back reference. A 5160 named back reference can be coded as \g{name}. Back references are dis- 5161 cussed later, following the discussion of parenthesized subpatterns. 5162 5163 Absolute and relative subroutine calls 5164 5165 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 5166 name or a number enclosed either in angle brackets or single quotes, is 5167 an alternative syntax for referencing a subpattern as a "subroutine". 5168 Details are discussed later. Note that \g{...} (Perl syntax) and 5169 \g<...> (Oniguruma syntax) are not synonymous. The former is a back 5170 reference; the latter is a subroutine call. 5171 5172 Generic character types 5173 5174 Another use of backslash is for specifying generic character types: 5175 5176 \d any decimal digit 5177 \D any character that is not a decimal digit 5178 \h any horizontal white space character 5179 \H any character that is not a horizontal white space character 5180 \s any white space character 5181 \S any character that is not a white space character 5182 \v any vertical white space character 5183 \V any character that is not a vertical white space character 5184 \w any "word" character 5185 \W any "non-word" character 5186 5187 There is also the single sequence \N, which matches a non-newline char- 5188 acter. This is the same as the "." metacharacter when PCRE_DOTALL is 5189 not set. Perl also uses \N to match characters by name; PCRE does not 5190 support this. 5191 5192 Each pair of lower and upper case escape sequences partitions the com- 5193 plete set of characters into two disjoint sets. Any given character 5194 matches one, and only one, of each pair. The sequences can appear both 5195 inside and outside character classes. They each match one character of 5196 the appropriate type. If the current matching point is at the end of 5197 the subject string, all of them fail, because there is no character to 5198 match. 5199 5200 For compatibility with Perl, \s did not used to match the VT character 5201 (code 11), which made it different from the the POSIX "space" class. 5202 However, Perl added VT at release 5.18, and PCRE followed suit at 5203 release 8.34. The default \s characters are now HT (9), LF (10), VT 5204 (11), FF (12), CR (13), and space (32), which are defined as white 5205 space in the "C" locale. This list may vary if locale-specific matching 5206 is taking place. For example, in some locales the "non-breaking space" 5207 character (\xA0) is recognized as white space, and in others the VT 5208 character is not. 5209 5210 A "word" character is an underscore or any character that is a letter 5211 or digit. By default, the definition of letters and digits is con- 5212 trolled by PCRE's low-valued character tables, and may vary if locale- 5213 specific matching is taking place (see "Locale support" in the pcreapi 5214 page). For example, in a French locale such as "fr_FR" in Unix-like 5215 systems, or "french" in Windows, some character codes greater than 127 5216 are used for accented letters, and these are then matched by \w. The 5217 use of locales with Unicode is discouraged. 5218 5219 By default, characters whose code points are greater than 127 never 5220 match \d, \s, or \w, and always match \D, \S, and \W, although this may 5221 vary for characters in the range 128-255 when locale-specific matching 5222 is happening. These escape sequences retain their original meanings 5223 from before Unicode support was available, mainly for efficiency rea- 5224 sons. If PCRE is compiled with Unicode property support, and the 5225 PCRE_UCP option is set, the behaviour is changed so that Unicode prop- 5226 erties are used to determine character types, as follows: 5227 5228 \d any character that matches \p{Nd} (decimal digit) 5229 \s any character that matches \p{Z} or \h or \v 5230 \w any character that matches \p{L} or \p{N}, plus underscore 5231 5232 The upper case escapes match the inverse sets of characters. Note that 5233 \d matches only decimal digits, whereas \w matches any Unicode digit, 5234 as well as any Unicode letter, and underscore. Note also that PCRE_UCP 5235 affects \b, and \B because they are defined in terms of \w and \W. 5236 Matching these sequences is noticeably slower when PCRE_UCP is set. 5237 5238 The sequences \h, \H, \v, and \V are features that were added to Perl 5239 at release 5.10. In contrast to the other sequences, which match only 5240 ASCII characters by default, these always match certain high-valued 5241 code points, whether or not PCRE_UCP is set. The horizontal space char- 5242 acters are: 5243 5244 U+0009 Horizontal tab (HT) 5245 U+0020 Space 5246 U+00A0 Non-break space 5247 U+1680 Ogham space mark 5248 U+180E Mongolian vowel separator 5249 U+2000 En quad 5250 U+2001 Em quad 5251 U+2002 En space 5252 U+2003 Em space 5253 U+2004 Three-per-em space 5254 U+2005 Four-per-em space 5255 U+2006 Six-per-em space 5256 U+2007 Figure space 5257 U+2008 Punctuation space 5258 U+2009 Thin space 5259 U+200A Hair space 5260 U+202F Narrow no-break space 5261 U+205F Medium mathematical space 5262 U+3000 Ideographic space 5263 5264 The vertical space characters are: 5265 5266 U+000A Linefeed (LF) 5267 U+000B Vertical tab (VT) 5268 U+000C Form feed (FF) 5269 U+000D Carriage return (CR) 5270 U+0085 Next line (NEL) 5271 U+2028 Line separator 5272 U+2029 Paragraph separator 5273 5274 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 5275 256 are relevant. 5276 5277 Newline sequences 5278 5279 Outside a character class, by default, the escape sequence \R matches 5280 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent 5281 to the following: 5282 5283 (?>\r\n|\n|\x0b|\f|\r|\x85) 5284 5285 This is an example of an "atomic group", details of which are given 5286 below. This particular group matches either the two-character sequence 5287 CR followed by LF, or one of the single characters LF (linefeed, 5288 U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car- 5289 riage return, U+000D), or NEL (next line, U+0085). The two-character 5290 sequence is treated as a single unit that cannot be split. 5291 5292 In other modes, two additional characters whose codepoints are greater 5293 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- 5294 rator, U+2029). Unicode character property support is not needed for 5295 these characters to be recognized. 5296 5297 It is possible to restrict \R to match only CR, LF, or CRLF (instead of 5298 the complete set of Unicode line endings) by setting the option 5299 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. 5300 (BSR is an abbrevation for "backslash R".) This can be made the default 5301 when PCRE is built; if this is the case, the other behaviour can be 5302 requested via the PCRE_BSR_UNICODE option. It is also possible to 5303 specify these settings by starting a pattern string with one of the 5304 following sequences: 5305 5306 (*BSR_ANYCRLF) CR, LF, or CRLF only 5307 (*BSR_UNICODE) any Unicode newline sequence 5308 5309 These override the default and the options given to the compiling func- 5310 tion, but they can themselves be overridden by options given to a 5311 matching function. Note that these special settings, which are not 5312 Perl-compatible, are recognized only at the very start of a pattern, 5313 and that they must be in upper case. If more than one of them is 5314 present, the last one is used. They can be combined with a change of 5315 newline convention; for example, a pattern can start with: 5316 5317 (*ANY)(*BSR_ANYCRLF) 5318 5319 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) 5320 or (*UCP) special sequences. Inside a character class, \R is treated as 5321 an unrecognized escape sequence, and so matches the letter "R" by 5322 default, but causes an error if PCRE_EXTRA is set. 5323 5324 Unicode character properties 5325 5326 When PCRE is built with Unicode character property support, three addi- 5327 tional escape sequences that match characters with specific properties 5328 are available. When in 8-bit non-UTF-8 mode, these sequences are of 5329 course limited to testing characters whose codepoints are less than 5330 256, but they do work in this mode. The extra escape sequences are: 5331 5332 \p{xx} a character with the xx property 5333 \P{xx} a character without the xx property 5334 \X a Unicode extended grapheme cluster 5335 5336 The property names represented by xx above are limited to the Unicode 5337 script names, the general category properties, "Any", which matches any 5338 character (including newline), and some special PCRE properties 5339 (described in the next section). Other Perl properties such as "InMu- 5340 sicalSymbols" are not currently supported by PCRE. Note that \P{Any} 5341 does not match any characters, so always causes a match failure. 5342 5343 Sets of Unicode characters are defined as belonging to certain scripts. 5344 A character from one of these sets can be matched using a script name. 5345 For example: 5346 5347 \p{Greek} 5348 \P{Han} 5349 5350 Those that are not part of an identified script are lumped together as 5351 "Common". The current list of scripts is: 5352 5353 Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, 5354 Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Car- 5355 ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei- 5356 form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero- 5357 glyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, 5358 Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, 5359 Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- 5360 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, 5361 Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Lin- 5362 ear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic, 5363 Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, 5364 Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, 5365 New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, 5366 Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, 5367 Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, 5368 Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha- 5369 vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, 5370 Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, 5371 Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, 5372 Yi. 5373 5374 Each character has exactly one Unicode general category property, spec- 5375 ified by a two-letter abbreviation. For compatibility with Perl, nega- 5376 tion can be specified by including a circumflex between the opening 5377 brace and the property name. For example, \p{^Lu} is the same as 5378 \P{Lu}. 5379 5380 If only one letter is specified with \p or \P, it includes all the gen- 5381 eral category properties that start with that letter. In this case, in 5382 the absence of negation, the curly brackets in the escape sequence are 5383 optional; these two examples have the same effect: 5384 5385 \p{L} 5386 \pL 5387 5388 The following general category property codes are supported: 5389 5390 C Other 5391 Cc Control 5392 Cf Format 5393 Cn Unassigned 5394 Co Private use 5395 Cs Surrogate 5396 5397 L Letter 5398 Ll Lower case letter 5399 Lm Modifier letter 5400 Lo Other letter 5401 Lt Title case letter 5402 Lu Upper case letter 5403 5404 M Mark 5405 Mc Spacing mark 5406 Me Enclosing mark 5407 Mn Non-spacing mark 5408 5409 N Number 5410 Nd Decimal number 5411 Nl Letter number 5412 No Other number 5413 5414 P Punctuation 5415 Pc Connector punctuation 5416 Pd Dash punctuation 5417 Pe Close punctuation 5418 Pf Final punctuation 5419 Pi Initial punctuation 5420 Po Other punctuation 5421 Ps Open punctuation 5422 5423 S Symbol 5424 Sc Currency symbol 5425 Sk Modifier symbol 5426 Sm Mathematical symbol 5427 So Other symbol 5428 5429 Z Separator 5430 Zl Line separator 5431 Zp Paragraph separator 5432 Zs Space separator 5433 5434 The special property L& is also supported: it matches a character that 5435 has the Lu, Ll, or Lt property, in other words, a letter that is not 5436 classified as a modifier or "other". 5437 5438 The Cs (Surrogate) property applies only to characters in the range 5439 U+D800 to U+DFFF. Such characters are not valid in Unicode strings and 5440 so cannot be tested by PCRE, unless UTF validity checking has been 5441 turned off (see the discussion of PCRE_NO_UTF8_CHECK, 5442 PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl 5443 does not support the Cs property. 5444 5445 The long synonyms for property names that Perl supports (such as 5446 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix 5447 any of these properties with "Is". 5448 5449 No character that is in the Unicode table has the Cn (unassigned) prop- 5450 erty. Instead, this property is assumed for any code point that is not 5451 in the Unicode table. 5452 5453 Specifying caseless matching does not affect these escape sequences. 5454 For example, \p{Lu} always matches only upper case letters. This is 5455 different from the behaviour of current versions of Perl. 5456 5457 Matching characters by Unicode property is not fast, because PCRE has 5458 to do a multistage table lookup in order to find a character's prop- 5459 erty. That is why the traditional escape sequences such as \d and \w do 5460 not use Unicode properties in PCRE by default, though you can make them 5461 do so by setting the PCRE_UCP option or by starting the pattern with 5462 (*UCP). 5463 5464 Extended grapheme clusters 5465 5466 The \X escape matches any number of Unicode characters that form an 5467 "extended grapheme cluster", and treats the sequence as an atomic group 5468 (see below). Up to and including release 8.31, PCRE matched an ear- 5469 lier, simpler definition that was equivalent to 5470 5471 (?>\PM\pM*) 5472 5473 That is, it matched a character without the "mark" property, followed 5474 by zero or more characters with the "mark" property. Characters with 5475 the "mark" property are typically non-spacing accents that affect the 5476 preceding character. 5477 5478 This simple definition was extended in Unicode to include more compli- 5479 cated kinds of composite character by giving each character a grapheme 5480 breaking property, and creating rules that use these properties to 5481 define the boundaries of extended grapheme clusters. In releases of 5482 PCRE later than 8.31, \X matches one of these clusters. 5483 5484 \X always matches at least one character. Then it decides whether to 5485 add additional characters according to the following rules for ending a 5486 cluster: 5487 5488 1. End at the end of the subject string. 5489 5490 2. Do not end between CR and LF; otherwise end after any control char- 5491 acter. 5492 5493 3. Do not break Hangul (a Korean script) syllable sequences. Hangul 5494 characters are of five types: L, V, T, LV, and LVT. An L character may 5495 be followed by an L, V, LV, or LVT character; an LV or V character may 5496 be followed by a V or T character; an LVT or T character may be follwed 5497 only by a T character. 5498 5499 4. Do not end before extending characters or spacing marks. Characters 5500 with the "mark" property always have the "extend" grapheme breaking 5501 property. 5502 5503 5. Do not end after prepend characters. 5504 5505 6. Otherwise, end the cluster. 5506 5507 PCRE's additional properties 5508 5509 As well as the standard Unicode properties described above, PCRE sup- 5510 ports four more that make it possible to convert traditional escape 5511 sequences such as \w and \s to use Unicode properties. PCRE uses these 5512 non-standard, non-Perl properties internally when PCRE_UCP is set. How- 5513 ever, they may also be used explicitly. These properties are: 5514 5515 Xan Any alphanumeric character 5516 Xps Any POSIX space character 5517 Xsp Any Perl space character 5518 Xwd Any Perl "word" character 5519 5520 Xan matches characters that have either the L (letter) or the N (num- 5521 ber) property. Xps matches the characters tab, linefeed, vertical tab, 5522 form feed, or carriage return, and any other character that has the Z 5523 (separator) property. Xsp is the same as Xps; it used to exclude ver- 5524 tical tab, for Perl compatibility, but Perl changed, and so PCRE fol- 5525 lowed at release 8.34. Xwd matches the same characters as Xan, plus 5526 underscore. 5527 5528 There is another non-standard property, Xuc, which matches any charac- 5529 ter that can be represented by a Universal Character Name in C++ and 5530 other programming languages. These are the characters $, @, ` (grave 5531 accent), and all characters with Unicode code points greater than or 5532 equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that 5533 most base (ASCII) characters are excluded. (Universal Character Names 5534 are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit. 5535 Note that the Xuc property does not match these sequences but the char- 5536 acters that they represent.) 5537 5538 Resetting the match start 5539 5540 The escape sequence \K causes any previously matched characters not to 5541 be included in the final matched sequence. For example, the pattern: 5542 5543 foo\Kbar 5544 5545 matches "foobar", but reports that it has matched "bar". This feature 5546 is similar to a lookbehind assertion (described below). However, in 5547 this case, the part of the subject before the real match does not have 5548 to be of fixed length, as lookbehind assertions do. The use of \K does 5549 not interfere with the setting of captured substrings. For example, 5550 when the pattern 5551 5552 (foo)\Kbar 5553 5554 matches "foobar", the first substring is still set to "foo". 5555 5556 Perl documents that the use of \K within assertions is "not well 5557 defined". In PCRE, \K is acted upon when it occurs inside positive 5558 assertions, but is ignored in negative assertions. Note that when a 5559 pattern such as (?=ab\K) matches, the reported start of the match can 5560 be greater than the end of the match. 5561 5562 Simple assertions 5563 5564 The final use of backslash is for certain simple assertions. An asser- 5565 tion specifies a condition that has to be met at a particular point in 5566 a match, without consuming any characters from the subject string. The 5567 use of subpatterns for more complicated assertions is described below. 5568 The backslashed assertions are: 5569 5570 \b matches at a word boundary 5571 \B matches when not at a word boundary 5572 \A matches at the start of the subject 5573 \Z matches at the end of the subject 5574 also matches before a newline at the end of the subject 5575 \z matches only at the end of the subject 5576 \G matches at the first matching position in the subject 5577 5578 Inside a character class, \b has a different meaning; it matches the 5579 backspace character. If any other of these assertions appears in a 5580 character class, by default it matches the corresponding literal char- 5581 acter (for example, \B matches the letter B). However, if the 5582 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener- 5583 ated instead. 5584 5585 A word boundary is a position in the subject string where the current 5586 character and the previous character do not both match \w or \W (i.e. 5587 one matches \w and the other matches \W), or the start or end of the 5588 string if the first or last character matches \w, respectively. In a 5589 UTF mode, the meanings of \w and \W can be changed by setting the 5590 PCRE_UCP option. When this is done, it also affects \b and \B. Neither 5591 PCRE nor Perl has a separate "start of word" or "end of word" metase- 5592 quence. However, whatever follows \b normally determines which it is. 5593 For example, the fragment \ba matches "a" at the start of a word. 5594 5595 The \A, \Z, and \z assertions differ from the traditional circumflex 5596 and dollar (described in the next section) in that they only ever match 5597 at the very start and end of the subject string, whatever options are 5598 set. Thus, they are independent of multiline mode. These three asser- 5599 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which 5600 affect only the behaviour of the circumflex and dollar metacharacters. 5601 However, if the startoffset argument of pcre_exec() is non-zero, indi- 5602 cating that matching is to start at a point other than the beginning of 5603 the subject, \A can never match. The difference between \Z and \z is 5604 that \Z matches before a newline at the end of the string as well as at 5605 the very end, whereas \z matches only at the end. 5606 5607 The \G assertion is true only when the current matching position is at 5608 the start point of the match, as specified by the startoffset argument 5609 of pcre_exec(). It differs from \A when the value of startoffset is 5610 non-zero. By calling pcre_exec() multiple times with appropriate argu- 5611 ments, you can mimic Perl's /g option, and it is in this kind of imple- 5612 mentation where \G can be useful. 5613 5614 Note, however, that PCRE's interpretation of \G, as the start of the 5615 current match, is subtly different from Perl's, which defines it as the 5616 end of the previous match. In Perl, these can be different when the 5617 previously matched string was empty. Because PCRE does just one match 5618 at a time, it cannot reproduce this behaviour. 5619 5620 If all the alternatives of a pattern begin with \G, the expression is 5621 anchored to the starting match position, and the "anchored" flag is set 5622 in the compiled regular expression. 5623 5624 5625 CIRCUMFLEX AND DOLLAR 5626 5627 The circumflex and dollar metacharacters are zero-width assertions. 5628 That is, they test for a particular condition being true without con- 5629 suming any characters from the subject string. 5630 5631 Outside a character class, in the default matching mode, the circumflex 5632 character is an assertion that is true only if the current matching 5633 point is at the start of the subject string. If the startoffset argu- 5634 ment of pcre_exec() is non-zero, circumflex can never match if the 5635 PCRE_MULTILINE option is unset. Inside a character class, circumflex 5636 has an entirely different meaning (see below). 5637 5638 Circumflex need not be the first character of the pattern if a number 5639 of alternatives are involved, but it should be the first thing in each 5640 alternative in which it appears if the pattern is ever to match that 5641 branch. If all possible alternatives start with a circumflex, that is, 5642 if the pattern is constrained to match only at the start of the sub- 5643 ject, it is said to be an "anchored" pattern. (There are also other 5644 constructs that can cause a pattern to be anchored.) 5645 5646 The dollar character is an assertion that is true only if the current 5647 matching point is at the end of the subject string, or immediately 5648 before a newline at the end of the string (by default). Note, however, 5649 that it does not actually match the newline. Dollar need not be the 5650 last character of the pattern if a number of alternatives are involved, 5651 but it should be the last item in any branch in which it appears. Dol- 5652 lar has no special meaning in a character class. 5653 5654 The meaning of dollar can be changed so that it matches only at the 5655 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at 5656 compile time. This does not affect the \Z assertion. 5657 5658 The meanings of the circumflex and dollar characters are changed if the 5659 PCRE_MULTILINE option is set. When this is the case, a circumflex 5660 matches immediately after internal newlines as well as at the start of 5661 the subject string. It does not match after a newline that ends the 5662 string. A dollar matches before any newlines in the string, as well as 5663 at the very end, when PCRE_MULTILINE is set. When newline is specified 5664 as the two-character sequence CRLF, isolated CR and LF characters do 5665 not indicate newlines. 5666 5667 For example, the pattern /^abc$/ matches the subject string "def\nabc" 5668 (where \n represents a newline) in multiline mode, but not otherwise. 5669 Consequently, patterns that are anchored in single line mode because 5670 all branches start with ^ are not anchored in multiline mode, and a 5671 match for circumflex is possible when the startoffset argument of 5672 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if 5673 PCRE_MULTILINE is set. 5674 5675 Note that the sequences \A, \Z, and \z can be used to match the start 5676 and end of the subject in both modes, and if all branches of a pattern 5677 start with \A it is always anchored, whether or not PCRE_MULTILINE is 5678 set. 5679 5680 5681 FULL STOP (PERIOD, DOT) AND \N 5682 5683 Outside a character class, a dot in the pattern matches any one charac- 5684 ter in the subject string except (by default) a character that signi- 5685 fies the end of a line. 5686 5687 When a line ending is defined as a single character, dot never matches 5688 that character; when the two-character sequence CRLF is used, dot does 5689 not match CR if it is immediately followed by LF, but otherwise it 5690 matches all characters (including isolated CRs and LFs). When any Uni- 5691 code line endings are being recognized, dot does not match CR or LF or 5692 any of the other line ending characters. 5693 5694 The behaviour of dot with regard to newlines can be changed. If the 5695 PCRE_DOTALL option is set, a dot matches any one character, without 5696 exception. If the two-character sequence CRLF is present in the subject 5697 string, it takes two dots to match it. 5698 5699 The handling of dot is entirely independent of the handling of circum- 5700 flex and dollar, the only relationship being that they both involve 5701 newlines. Dot has no special meaning in a character class. 5702 5703 The escape sequence \N behaves like a dot, except that it is not 5704 affected by the PCRE_DOTALL option. In other words, it matches any 5705 character except one that signifies the end of a line. Perl also uses 5706 \N to match characters by name; PCRE does not support this. 5707 5708 5709 MATCHING A SINGLE DATA UNIT 5710 5711 Outside a character class, the escape sequence \C matches any one data 5712 unit, whether or not a UTF mode is set. In the 8-bit library, one data 5713 unit is one byte; in the 16-bit library it is a 16-bit unit; in the 5714 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches 5715 line-ending characters. The feature is provided in Perl in order to 5716 match individual bytes in UTF-8 mode, but it is unclear how it can use- 5717 fully be used. Because \C breaks up characters into individual data 5718 units, matching one unit with \C in a UTF mode means that the rest of 5719 the string may start with a malformed UTF character. This has undefined 5720 results, because PCRE assumes that it is dealing with valid UTF strings 5721 (and by default it checks this at the start of processing unless the 5722 PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK option 5723 is used). 5724 5725 PCRE does not allow \C to appear in lookbehind assertions (described 5726 below) in a UTF mode, because this would make it impossible to calcu- 5727 late the length of the lookbehind. 5728 5729 In general, the \C escape sequence is best avoided. However, one way of 5730 using it that avoids the problem of malformed UTF characters is to use 5731 a lookahead to check the length of the next character, as in this pat- 5732 tern, which could be used with a UTF-8 string (ignore white space and 5733 line breaks): 5734 5735 (?| (?=[\x00-\x7f])(\C) | 5736 (?=[\x80-\x{7ff}])(\C)(\C) | 5737 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | 5738 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) 5739 5740 A group that starts with (?| resets the capturing parentheses numbers 5741 in each alternative (see "Duplicate Subpattern Numbers" below). The 5742 assertions at the start of each branch check the next UTF-8 character 5743 for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The 5744 character's individual bytes are then captured by the appropriate num- 5745 ber of groups. 5746 5747 5748 SQUARE BRACKETS AND CHARACTER CLASSES 5749 5750 An opening square bracket introduces a character class, terminated by a 5751 closing square bracket. A closing square bracket on its own is not spe- 5752 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, 5753 a lone closing square bracket causes a compile-time error. If a closing 5754 square bracket is required as a member of the class, it should be the 5755 first data character in the class (after an initial circumflex, if 5756 present) or escaped with a backslash. 5757 5758 A character class matches a single character in the subject. In a UTF 5759 mode, the character may be more than one data unit long. A matched 5760 character must be in the set of characters defined by the class, unless 5761 the first character in the class definition is a circumflex, in which 5762 case the subject character must not be in the set defined by the class. 5763 If a circumflex is actually required as a member of the class, ensure 5764 it is not the first character, or escape it with a backslash. 5765 5766 For example, the character class [aeiou] matches any lower case vowel, 5767 while [^aeiou] matches any character that is not a lower case vowel. 5768 Note that a circumflex is just a convenient notation for specifying the 5769 characters that are in the class by enumerating those that are not. A 5770 class that starts with a circumflex is not an assertion; it still con- 5771 sumes a character from the subject string, and therefore it fails if 5772 the current pointer is at the end of the string. 5773 5774 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 5775 (0xffff) can be included in a class as a literal string of data units, 5776 or by using the \x{ escaping mechanism. 5777 5778 When caseless matching is set, any letters in a class represent both 5779 their upper case and lower case versions, so for example, a caseless 5780 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not 5781 match "A", whereas a caseful version would. In a UTF mode, PCRE always 5782 understands the concept of case for characters whose values are less 5783 than 128, so caseless matching is always possible. For characters with 5784 higher values, the concept of case is supported if PCRE is compiled 5785 with Unicode property support, but not otherwise. If you want to use 5786 caseless matching in a UTF mode for characters 128 and above, you must 5787 ensure that PCRE is compiled with Unicode property support as well as 5788 with UTF support. 5789 5790 Characters that might indicate line breaks are never treated in any 5791 special way when matching character classes, whatever line-ending 5792 sequence is in use, and whatever setting of the PCRE_DOTALL and 5793 PCRE_MULTILINE options is used. A class such as [^a] always matches one 5794 of these characters. 5795 5796 The minus (hyphen) character can be used to specify a range of charac- 5797 ters in a character class. For example, [d-m] matches any letter 5798 between d and m, inclusive. If a minus character is required in a 5799 class, it must be escaped with a backslash or appear in a position 5800 where it cannot be interpreted as indicating a range, typically as the 5801 first or last character in the class, or immediately after a range. For 5802 example, [b-d-z] matches letters in the range b to d, a hyphen charac- 5803 ter, or z. 5804 5805 It is not possible to have the literal character "]" as the end charac- 5806 ter of a range. A pattern such as [W-]46] is interpreted as a class of 5807 two characters ("W" and "-") followed by a literal string "46]", so it 5808 would match "W46]" or "-46]". However, if the "]" is escaped with a 5809 backslash it is interpreted as the end of range, so [W-\]46] is inter- 5810 preted as a class containing a range followed by two other characters. 5811 The octal or hexadecimal representation of "]" can also be used to end 5812 a range. 5813 5814 An error is generated if a POSIX character class (see below) or an 5815 escape sequence other than one that defines a single character appears 5816 at a point where a range ending character is expected. For example, 5817 [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not. 5818 5819 Ranges operate in the collating sequence of character values. They can 5820 also be used for characters specified numerically, for example 5821 [\000-\037]. Ranges can include any characters that are valid for the 5822 current mode. 5823 5824 If a range that includes letters is used when caseless matching is set, 5825 it matches the letters in either case. For example, [W-c] is equivalent 5826 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if 5827 character tables for a French locale are in use, [\xc8-\xcb] matches 5828 accented E characters in both cases. In UTF modes, PCRE supports the 5829 concept of case for characters with values greater than 128 only when 5830 it is compiled with Unicode property support. 5831 5832 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, 5833 \w, and \W may appear in a character class, and add the characters that 5834 they match to the class. For example, [\dABCDEF] matches any hexadeci- 5835 mal digit. In UTF modes, the PCRE_UCP option affects the meanings of 5836 \d, \s, \w and their upper case partners, just as it does when they 5837 appear outside a character class, as described in the section entitled 5838 "Generic character types" above. The escape sequence \b has a different 5839 meaning inside a character class; it matches the backspace character. 5840 The sequences \B, \N, \R, and \X are not special inside a character 5841 class. Like any other unrecognized escape sequences, they are treated 5842 as the literal characters "B", "N", "R", and "X" by default, but cause 5843 an error if the PCRE_EXTRA option is set. 5844 5845 A circumflex can conveniently be used with the upper case character 5846 types to specify a more restricted set of characters than the matching 5847 lower case type. For example, the class [^\W_] matches any letter or 5848 digit, but not underscore, whereas [\w] includes underscore. A positive 5849 character class should be read as "something OR something OR ..." and a 5850 negative class as "NOT something AND NOT something AND NOT ...". 5851 5852 The only metacharacters that are recognized in character classes are 5853 backslash, hyphen (only where it can be interpreted as specifying a 5854 range), circumflex (only at the start), opening square bracket (only 5855 when it can be interpreted as introducing a POSIX class name, or for a 5856 special compatibility feature - see the next two sections), and the 5857 terminating closing square bracket. However, escaping other non- 5858 alphanumeric characters does no harm. 5859 5860 5861 POSIX CHARACTER CLASSES 5862 5863 Perl supports the POSIX notation for character classes. This uses names 5864 enclosed by [: and :] within the enclosing square brackets. PCRE also 5865 supports this notation. For example, 5866 5867 [01[:alpha:]%] 5868 5869 matches "0", "1", any alphabetic character, or "%". The supported class 5870 names are: 5871 5872 alnum letters and digits 5873 alpha letters 5874 ascii character codes 0 - 127 5875 blank space or tab only 5876 cntrl control characters 5877 digit decimal digits (same as \d) 5878 graph printing characters, excluding space 5879 lower lower case letters 5880 print printing characters, including space 5881 punct printing characters, excluding letters and digits and space 5882 space white space (the same as \s from PCRE 8.34) 5883 upper upper case letters 5884 word "word" characters (same as \w) 5885 xdigit hexadecimal digits 5886 5887 The default "space" characters are HT (9), LF (10), VT (11), FF (12), 5888 CR (13), and space (32). If locale-specific matching is taking place, 5889 the list of space characters may be different; there may be fewer or 5890 more of them. "Space" used to be different to \s, which did not include 5891 VT, for Perl compatibility. However, Perl changed at release 5.18, and 5892 PCRE followed at release 8.34. "Space" and \s now match the same set 5893 of characters. 5894 5895 The name "word" is a Perl extension, and "blank" is a GNU extension 5896 from Perl 5.8. Another Perl extension is negation, which is indicated 5897 by a ^ character after the colon. For example, 5898 5899 [12[:^digit:]] 5900 5901 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the 5902 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but 5903 these are not supported, and an error is given if they are encountered. 5904 5905 By default, characters with values greater than 128 do not match any of 5906 the POSIX character classes. However, if the PCRE_UCP option is passed 5907 to pcre_compile(), some of the classes are changed so that Unicode 5908 character properties are used. This is achieved by replacing certain 5909 POSIX classes by other sequences, as follows: 5910 5911 [:alnum:] becomes \p{Xan} 5912 [:alpha:] becomes \p{L} 5913 [:blank:] becomes \h 5914 [:digit:] becomes \p{Nd} 5915 [:lower:] becomes \p{Ll} 5916 [:space:] becomes \p{Xps} 5917 [:upper:] becomes \p{Lu} 5918 [:word:] becomes \p{Xwd} 5919 5920 Negated versions, such as [:^alpha:] use \P instead of \p. Three other 5921 POSIX classes are handled specially in UCP mode: 5922 5923 [:graph:] This matches characters that have glyphs that mark the page 5924 when printed. In Unicode property terms, it matches all char- 5925 acters with the L, M, N, P, S, or Cf properties, except for: 5926 5927 U+061C Arabic Letter Mark 5928 U+180E Mongolian Vowel Separator 5929 U+2066 - U+2069 Various "isolate"s 5930 5931 5932 [:print:] This matches the same characters as [:graph:] plus space 5933 characters that are not controls, that is, characters with 5934 the Zs property. 5935 5936 [:punct:] This matches all characters that have the Unicode P (punctua- 5937 tion) property, plus those characters whose code points are 5938 less than 128 that have the S (Symbol) property. 5939 5940 The other POSIX classes are unchanged, and match only characters with 5941 code points less than 128. 5942 5943 5944 COMPATIBILITY FEATURE FOR WORD BOUNDARIES 5945 5946 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the 5947 ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" 5948 and "end of word". PCRE treats these items as follows: 5949 5950 [[:<:]] is converted to \b(?=\w) 5951 [[:>:]] is converted to \b(?<=\w) 5952 5953 Only these exact character sequences are recognized. A sequence such as 5954 [a[:<:]b] provokes error for an unrecognized POSIX class name. This 5955 support is not compatible with Perl. It is provided to help migrations 5956 from other environments, and is best not used in any new patterns. Note 5957 that \b matches at the start and the end of a word (see "Simple asser- 5958 tions" above), and in a Perl-style pattern the preceding or following 5959 character normally shows which is wanted, without the need for the 5960 assertions that are used above in order to give exactly the POSIX be- 5961 haviour. 5962 5963 5964 VERTICAL BAR 5965 5966 Vertical bar characters are used to separate alternative patterns. For 5967 example, the pattern 5968 5969 gilbert|sullivan 5970 5971 matches either "gilbert" or "sullivan". Any number of alternatives may 5972 appear, and an empty alternative is permitted (matching the empty 5973 string). The matching process tries each alternative in turn, from left 5974 to right, and the first one that succeeds is used. If the alternatives 5975 are within a subpattern (defined below), "succeeds" means matching the 5976 rest of the main pattern as well as the alternative in the subpattern. 5977 5978 5979 INTERNAL OPTION SETTING 5980 5981 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 5982 PCRE_EXTENDED options (which are Perl-compatible) can be changed from 5983 within the pattern by a sequence of Perl option letters enclosed 5984 between "(?" and ")". The option letters are 5985 5986 i for PCRE_CASELESS 5987 m for PCRE_MULTILINE 5988 s for PCRE_DOTALL 5989 x for PCRE_EXTENDED 5990 5991 For example, (?im) sets caseless, multiline matching. It is also possi- 5992 ble to unset these options by preceding the letter with a hyphen, and a 5993 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- 5994 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, 5995 is also permitted. If a letter appears both before and after the 5996 hyphen, the option is unset. 5997 5998 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA 5999 can be changed in the same way as the Perl-compatible options by using 6000 the characters J, U and X respectively. 6001 6002 When one of these option changes occurs at top level (that is, not 6003 inside subpattern parentheses), the change applies to the remainder of 6004 the pattern that follows. If the change is placed right at the start of 6005 a pattern, PCRE extracts it into the global options (and it will there- 6006 fore show up in data extracted by the pcre_fullinfo() function). 6007 6008 An option change within a subpattern (see below for a description of 6009 subpatterns) affects only that part of the subpattern that follows it, 6010 so 6011 6012 (a(?i)b)c 6013 6014 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not 6015 used). By this means, options can be made to have different settings 6016 in different parts of the pattern. Any changes made in one alternative 6017 do carry on into subsequent branches within the same subpattern. For 6018 example, 6019 6020 (a(?i)b|c) 6021 6022 matches "ab", "aB", "c", and "C", even though when matching "C" the 6023 first branch is abandoned before the option setting. This is because 6024 the effects of option settings happen at compile time. There would be 6025 some very weird behaviour otherwise. 6026 6027 Note: There are other PCRE-specific options that can be set by the 6028 application when the compiling or matching functions are called. In 6029 some cases the pattern can contain special leading sequences such as 6030 (*CRLF) to override what the application has set or what has been 6031 defaulted. Details are given in the section entitled "Newline 6032 sequences" above. There are also the (*UTF8), (*UTF16),(*UTF32), and 6033 (*UCP) leading sequences that can be used to set UTF and Unicode prop- 6034 erty modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, 6035 PCRE_UTF32 and the PCRE_UCP options, respectively. The (*UTF) sequence 6036 is a generic version that can be used with any of the libraries. How- 6037 ever, the application can set the PCRE_NEVER_UTF option, which locks 6038 out the use of the (*UTF) sequences. 6039 6040 6041 SUBPATTERNS 6042 6043 Subpatterns are delimited by parentheses (round brackets), which can be 6044 nested. Turning part of a pattern into a subpattern does two things: 6045 6046 1. It localizes a set of alternatives. For example, the pattern 6047 6048 cat(aract|erpillar|) 6049 6050 matches "cataract", "caterpillar", or "cat". Without the parentheses, 6051 it would match "cataract", "erpillar" or an empty string. 6052 6053 2. It sets up the subpattern as a capturing subpattern. This means 6054 that, when the whole pattern matches, that portion of the subject 6055 string that matched the subpattern is passed back to the caller via the 6056 ovector argument of the matching function. (This applies only to the 6057 traditional matching functions; the DFA matching functions do not sup- 6058 port capturing.) 6059 6060 Opening parentheses are counted from left to right (starting from 1) to 6061 obtain numbers for the capturing subpatterns. For example, if the 6062 string "the red king" is matched against the pattern 6063 6064 the ((red|white) (king|queen)) 6065 6066 the captured substrings are "red king", "red", and "king", and are num- 6067 bered 1, 2, and 3, respectively. 6068 6069 The fact that plain parentheses fulfil two functions is not always 6070 helpful. There are often times when a grouping subpattern is required 6071 without a capturing requirement. If an opening parenthesis is followed 6072 by a question mark and a colon, the subpattern does not do any captur- 6073 ing, and is not counted when computing the number of any subsequent 6074 capturing subpatterns. For example, if the string "the white queen" is 6075 matched against the pattern 6076 6077 the ((?:red|white) (king|queen)) 6078 6079 the captured substrings are "white queen" and "queen", and are numbered 6080 1 and 2. The maximum number of capturing subpatterns is 65535. 6081 6082 As a convenient shorthand, if any option settings are required at the 6083 start of a non-capturing subpattern, the option letters may appear 6084 between the "?" and the ":". Thus the two patterns 6085 6086 (?i:saturday|sunday) 6087 (?:(?i)saturday|sunday) 6088 6089 match exactly the same set of strings. Because alternative branches are 6090 tried from left to right, and options are not reset until the end of 6091 the subpattern is reached, an option setting in one branch does affect 6092 subsequent branches, so the above patterns match "SUNDAY" as well as 6093 "Saturday". 6094 6095 6096 DUPLICATE SUBPATTERN NUMBERS 6097 6098 Perl 5.10 introduced a feature whereby each alternative in a subpattern 6099 uses the same numbers for its capturing parentheses. Such a subpattern 6100 starts with (?| and is itself a non-capturing subpattern. For example, 6101 consider this pattern: 6102 6103 (?|(Sat)ur|(Sun))day 6104 6105 Because the two alternatives are inside a (?| group, both sets of cap- 6106 turing parentheses are numbered one. Thus, when the pattern matches, 6107 you can look at captured substring number one, whichever alternative 6108 matched. This construct is useful when you want to capture part, but 6109 not all, of one of a number of alternatives. Inside a (?| group, paren- 6110 theses are numbered as usual, but the number is reset at the start of 6111 each branch. The numbers of any capturing parentheses that follow the 6112 subpattern start after the highest number used in any branch. The fol- 6113 lowing example is taken from the Perl documentation. The numbers under- 6114 neath show in which buffer the captured content will be stored. 6115 6116 # before ---------------branch-reset----------- after 6117 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 6118 # 1 2 2 3 2 3 4 6119 6120 A back reference to a numbered subpattern uses the most recent value 6121 that is set for that number by any subpattern. The following pattern 6122 matches "abcabc" or "defdef": 6123 6124 /(?|(abc)|(def))\1/ 6125 6126 In contrast, a subroutine call to a numbered subpattern always refers 6127 to the first one in the pattern with the given number. The following 6128 pattern matches "abcabc" or "defabc": 6129 6130 /(?|(abc)|(def))(?1)/ 6131 6132 If a condition test for a subpattern's having matched refers to a non- 6133 unique number, the test is true if any of the subpatterns of that num- 6134 ber have matched. 6135 6136 An alternative approach to using this "branch reset" feature is to use 6137 duplicate named subpatterns, as described in the next section. 6138 6139 6140 NAMED SUBPATTERNS 6141 6142 Identifying capturing parentheses by number is simple, but it can be 6143 very hard to keep track of the numbers in complicated regular expres- 6144 sions. Furthermore, if an expression is modified, the numbers may 6145 change. To help with this difficulty, PCRE supports the naming of sub- 6146 patterns. This feature was not added to Perl until release 5.10. Python 6147 had the feature earlier, and PCRE introduced it at release 4.0, using 6148 the Python syntax. PCRE now supports both the Perl and the Python syn- 6149 tax. Perl allows identically numbered subpatterns to have different 6150 names, but PCRE does not. 6151 6152 In PCRE, a subpattern can be named in one of three ways: (?<name>...) 6153 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References 6154 to capturing parentheses from other parts of the pattern, such as back 6155 references, recursion, and conditions, can be made by name as well as 6156 by number. 6157 6158 Names consist of up to 32 alphanumeric characters and underscores, but 6159 must start with a non-digit. Named capturing parentheses are still 6160 allocated numbers as well as names, exactly as if the names were not 6161 present. The PCRE API provides function calls for extracting the name- 6162 to-number translation table from a compiled pattern. There is also a 6163 convenience function for extracting a captured substring by name. 6164 6165 By default, a name must be unique within a pattern, but it is possible 6166 to relax this constraint by setting the PCRE_DUPNAMES option at compile 6167 time. (Duplicate names are also always permitted for subpatterns with 6168 the same number, set up as described in the previous section.) Dupli- 6169 cate names can be useful for patterns where only one instance of the 6170 named parentheses can match. Suppose you want to match the name of a 6171 weekday, either as a 3-letter abbreviation or as the full name, and in 6172 both cases you want to extract the abbreviation. This pattern (ignoring 6173 the line breaks) does the job: 6174 6175 (?<DN>Mon|Fri|Sun)(?:day)?| 6176 (?<DN>Tue)(?:sday)?| 6177 (?<DN>Wed)(?:nesday)?| 6178 (?<DN>Thu)(?:rsday)?| 6179 (?<DN>Sat)(?:urday)? 6180 6181 There are five capturing substrings, but only one is ever set after a 6182 match. (An alternative way of solving this problem is to use a "branch 6183 reset" subpattern, as described in the previous section.) 6184 6185 The convenience function for extracting the data by name returns the 6186 substring for the first (and in this example, the only) subpattern of 6187 that name that matched. This saves searching to find which numbered 6188 subpattern it was. 6189 6190 If you make a back reference to a non-unique named subpattern from 6191 elsewhere in the pattern, the subpatterns to which the name refers are 6192 checked in the order in which they appear in the overall pattern. The 6193 first one that is set is used for the reference. For example, this pat- 6194 tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo": 6195 6196 (?:(?<n>foo)|(?<n>bar))\k<n> 6197 6198 6199 If you make a subroutine call to a non-unique named subpattern, the one 6200 that corresponds to the first occurrence of the name is used. In the 6201 absence of duplicate numbers (see the previous section) this is the one 6202 with the lowest number. 6203 6204 If you use a named reference in a condition test (see the section about 6205 conditions below), either to check whether a subpattern has matched, or 6206 to check for recursion, all subpatterns with the same name are tested. 6207 If the condition is true for any one of them, the overall condition is 6208 true. This is the same behaviour as testing by number. For further 6209 details of the interfaces for handling named subpatterns, see the 6210 pcreapi documentation. 6211 6212 Warning: You cannot use different names to distinguish between two sub- 6213 patterns with the same number because PCRE uses only the numbers when 6214 matching. For this reason, an error is given at compile time if differ- 6215 ent names are given to subpatterns with the same number. However, you 6216 can always give the same name to subpatterns with the same number, even 6217 when PCRE_DUPNAMES is not set. 6218 6219 6220 REPETITION 6221 6222 Repetition is specified by quantifiers, which can follow any of the 6223 following items: 6224 6225 a literal data character 6226 the dot metacharacter 6227 the \C escape sequence 6228 the \X escape sequence 6229 the \R escape sequence 6230 an escape such as \d or \pL that matches a single character 6231 a character class 6232 a back reference (see next section) 6233 a parenthesized subpattern (including assertions) 6234 a subroutine call to a subpattern (recursive or otherwise) 6235 6236 The general repetition quantifier specifies a minimum and maximum num- 6237 ber of permitted matches, by giving the two numbers in curly brackets 6238 (braces), separated by a comma. The numbers must be less than 65536, 6239 and the first must be less than or equal to the second. For example: 6240 6241 z{2,4} 6242 6243 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a 6244 special character. If the second number is omitted, but the comma is 6245 present, there is no upper limit; if the second number and the comma 6246 are both omitted, the quantifier specifies an exact number of required 6247 matches. Thus 6248 6249 [aeiou]{3,} 6250 6251 matches at least 3 successive vowels, but may match many more, while 6252 6253 \d{8} 6254 6255 matches exactly 8 digits. An opening curly bracket that appears in a 6256 position where a quantifier is not allowed, or one that does not match 6257 the syntax of a quantifier, is taken as a literal character. For exam- 6258 ple, {,6} is not a quantifier, but a literal string of four characters. 6259 6260 In UTF modes, quantifiers apply to characters rather than to individual 6261 data units. Thus, for example, \x{100}{2} matches two characters, each 6262 of which is represented by a two-byte sequence in a UTF-8 string. Simi- 6263 larly, \X{3} matches three Unicode extended grapheme clusters, each of 6264 which may be several data units long (and they may be of different 6265 lengths). 6266 6267 The quantifier {0} is permitted, causing the expression to behave as if 6268 the previous item and the quantifier were not present. This may be use- 6269 ful for subpatterns that are referenced as subroutines from elsewhere 6270 in the pattern (but see also the section entitled "Defining subpatterns 6271 for use by reference only" below). Items other than subpatterns that 6272 have a {0} quantifier are omitted from the compiled pattern. 6273 6274 For convenience, the three most common quantifiers have single-charac- 6275 ter abbreviations: 6276 6277 * is equivalent to {0,} 6278 + is equivalent to {1,} 6279 ? is equivalent to {0,1} 6280 6281 It is possible to construct infinite loops by following a subpattern 6282 that can match no characters with a quantifier that has no upper limit, 6283 for example: 6284 6285 (a?)* 6286 6287 Earlier versions of Perl and PCRE used to give an error at compile time 6288 for such patterns. However, because there are cases where this can be 6289 useful, such patterns are now accepted, but if any repetition of the 6290 subpattern does in fact match no characters, the loop is forcibly bro- 6291 ken. 6292 6293 By default, the quantifiers are "greedy", that is, they match as much 6294 as possible (up to the maximum number of permitted times), without 6295 causing the rest of the pattern to fail. The classic example of where 6296 this gives problems is in trying to match comments in C programs. These 6297 appear between /* and */ and within the comment, individual * and / 6298 characters may appear. An attempt to match C comments by applying the 6299 pattern 6300 6301 /\*.*\*/ 6302 6303 to the string 6304 6305 /* first comment */ not comment /* second comment */ 6306 6307 fails, because it matches the entire string owing to the greediness of 6308 the .* item. 6309 6310 However, if a quantifier is followed by a question mark, it ceases to 6311 be greedy, and instead matches the minimum number of times possible, so 6312 the pattern 6313 6314 /\*.*?\*/ 6315 6316 does the right thing with the C comments. The meaning of the various 6317 quantifiers is not otherwise changed, just the preferred number of 6318 matches. Do not confuse this use of question mark with its use as a 6319 quantifier in its own right. Because it has two uses, it can sometimes 6320 appear doubled, as in 6321 6322 \d??\d 6323 6324 which matches one digit by preference, but can match two if that is the 6325 only way the rest of the pattern matches. 6326 6327 If the PCRE_UNGREEDY option is set (an option that is not available in 6328 Perl), the quantifiers are not greedy by default, but individual ones 6329 can be made greedy by following them with a question mark. In other 6330 words, it inverts the default behaviour. 6331 6332 When a parenthesized subpattern is quantified with a minimum repeat 6333 count that is greater than 1 or with a limited maximum, more memory is 6334 required for the compiled pattern, in proportion to the size of the 6335 minimum or maximum. 6336 6337 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- 6338 alent to Perl's /s) is set, thus allowing the dot to match newlines, 6339 the pattern is implicitly anchored, because whatever follows will be 6340 tried against every character position in the subject string, so there 6341 is no point in retrying the overall match at any position after the 6342 first. PCRE normally treats such a pattern as though it were preceded 6343 by \A. 6344 6345 In cases where it is known that the subject string contains no new- 6346 lines, it is worth setting PCRE_DOTALL in order to obtain this opti- 6347 mization, or alternatively using ^ to indicate anchoring explicitly. 6348 6349 However, there are some cases where the optimization cannot be used. 6350 When .* is inside capturing parentheses that are the subject of a back 6351 reference elsewhere in the pattern, a match at the start may fail where 6352 a later one succeeds. Consider, for example: 6353 6354 (.*)abc\1 6355 6356 If the subject is "xyz123abc123" the match point is the fourth charac- 6357 ter. For this reason, such a pattern is not implicitly anchored. 6358 6359 Another case where implicit anchoring is not applied is when the lead- 6360 ing .* is inside an atomic group. Once again, a match at the start may 6361 fail where a later one succeeds. Consider this pattern: 6362 6363 (?>.*?a)b 6364 6365 It matches "ab" in the subject "aab". The use of the backtracking con- 6366 trol verbs (*PRUNE) and (*SKIP) also disable this optimization. 6367 6368 When a capturing subpattern is repeated, the value captured is the sub- 6369 string that matched the final iteration. For example, after 6370 6371 (tweedle[dume]{3}\s*)+ 6372 6373 has matched "tweedledum tweedledee" the value of the captured substring 6374 is "tweedledee". However, if there are nested capturing subpatterns, 6375 the corresponding captured values may have been set in previous itera- 6376 tions. For example, after 6377 6378 /(a|(b))+/ 6379 6380 matches "aba" the value of the second captured substring is "b". 6381 6382 6383 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS 6384 6385 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 6386 repetition, failure of what follows normally causes the repeated item 6387 to be re-evaluated to see if a different number of repeats allows the 6388 rest of the pattern to match. Sometimes it is useful to prevent this, 6389 either to change the nature of the match, or to cause it fail earlier 6390 than it otherwise might, when the author of the pattern knows there is 6391 no point in carrying on. 6392 6393 Consider, for example, the pattern \d+foo when applied to the subject 6394 line 6395 6396 123456bar 6397 6398 After matching all 6 digits and then failing to match "foo", the normal 6399 action of the matcher is to try again with only 5 digits matching the 6400 \d+ item, and then with 4, and so on, before ultimately failing. 6401 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides 6402 the means for specifying that once a subpattern has matched, it is not 6403 to be re-evaluated in this way. 6404 6405 If we use atomic grouping for the previous example, the matcher gives 6406 up immediately on failing to match "foo" the first time. The notation 6407 is a kind of special parenthesis, starting with (?> as in this example: 6408 6409 (?>\d+)foo 6410 6411 This kind of parenthesis "locks up" the part of the pattern it con- 6412 tains once it has matched, and a failure further into the pattern is 6413 prevented from backtracking into it. Backtracking past it to previous 6414 items, however, works as normal. 6415 6416 An alternative description is that a subpattern of this type matches 6417 the string of characters that an identical standalone pattern would 6418 match, if anchored at the current point in the subject string. 6419 6420 Atomic grouping subpatterns are not capturing subpatterns. Simple cases 6421 such as the above example can be thought of as a maximizing repeat that 6422 must swallow everything it can. So, while both \d+ and \d+? are pre- 6423 pared to adjust the number of digits they match in order to make the 6424 rest of the pattern match, (?>\d+) can only match an entire sequence of 6425 digits. 6426 6427 Atomic groups in general can of course contain arbitrarily complicated 6428 subpatterns, and can be nested. However, when the subpattern for an 6429 atomic group is just a single repeated item, as in the example above, a 6430 simpler notation, called a "possessive quantifier" can be used. This 6431 consists of an additional + character following a quantifier. Using 6432 this notation, the previous example can be rewritten as 6433 6434 \d++foo 6435 6436 Note that a possessive quantifier can be used with an entire group, for 6437 example: 6438 6439 (abc|xyz){2,3}+ 6440 6441 Possessive quantifiers are always greedy; the setting of the 6442 PCRE_UNGREEDY option is ignored. They are a convenient notation for the 6443 simpler forms of atomic group. However, there is no difference in the 6444 meaning of a possessive quantifier and the equivalent atomic group, 6445 though there may be a performance difference; possessive quantifiers 6446 should be slightly faster. 6447 6448 The possessive quantifier syntax is an extension to the Perl 5.8 syn- 6449 tax. Jeffrey Friedl originated the idea (and the name) in the first 6450 edition of his book. Mike McCloskey liked it, so implemented it when he 6451 built Sun's Java package, and PCRE copied it from there. It ultimately 6452 found its way into Perl at release 5.10. 6453 6454 PCRE has an optimization that automatically "possessifies" certain sim- 6455 ple pattern constructs. For example, the sequence A+B is treated as 6456 A++B because there is no point in backtracking into a sequence of A's 6457 when B must follow. 6458 6459 When a pattern contains an unlimited repeat inside a subpattern that 6460 can itself be repeated an unlimited number of times, the use of an 6461 atomic group is the only way to avoid some failing matches taking a 6462 very long time indeed. The pattern 6463 6464 (\D+|<\d+>)*[!?] 6465 6466 matches an unlimited number of substrings that either consist of non- 6467 digits, or digits enclosed in <>, followed by either ! or ?. When it 6468 matches, it runs quickly. However, if it is applied to 6469 6470 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 6471 6472 it takes a long time before reporting failure. This is because the 6473 string can be divided between the internal \D+ repeat and the external 6474 * repeat in a large number of ways, and all have to be tried. (The 6475 example uses [!?] rather than a single character at the end, because 6476 both PCRE and Perl have an optimization that allows for fast failure 6477 when a single character is used. They remember the last single charac- 6478 ter that is required for a match, and fail early if it is not present 6479 in the string.) If the pattern is changed so that it uses an atomic 6480 group, like this: 6481 6482 ((?>\D+)|<\d+>)*[!?] 6483 6484 sequences of non-digits cannot be broken, and failure happens quickly. 6485 6486 6487 BACK REFERENCES 6488 6489 Outside a character class, a backslash followed by a digit greater than 6490 0 (and possibly further digits) is a back reference to a capturing sub- 6491 pattern earlier (that is, to its left) in the pattern, provided there 6492 have been that many previous capturing left parentheses. 6493 6494 However, if the decimal number following the backslash is less than 10, 6495 it is always taken as a back reference, and causes an error only if 6496 there are not that many capturing left parentheses in the entire pat- 6497 tern. In other words, the parentheses that are referenced need not be 6498 to the left of the reference for numbers less than 10. A "forward back 6499 reference" of this type can make sense when a repetition is involved 6500 and the subpattern to the right has participated in an earlier itera- 6501 tion. 6502 6503 It is not possible to have a numerical "forward back reference" to a 6504 subpattern whose number is 10 or more using this syntax because a 6505 sequence such as \50 is interpreted as a character defined in octal. 6506 See the subsection entitled "Non-printing characters" above for further 6507 details of the handling of digits following a backslash. There is no 6508 such problem when named parentheses are used. A back reference to any 6509 subpattern is possible using named parentheses (see below). 6510 6511 Another way of avoiding the ambiguity inherent in the use of digits 6512 following a backslash is to use the \g escape sequence. This escape 6513 must be followed by an unsigned number or a negative number, optionally 6514 enclosed in braces. These examples are all identical: 6515 6516 (ring), \1 6517 (ring), \g1 6518 (ring), \g{1} 6519 6520 An unsigned number specifies an absolute reference without the ambigu- 6521 ity that is present in the older syntax. It is also useful when literal 6522 digits follow the reference. A negative number is a relative reference. 6523 Consider this example: 6524 6525 (abc(def)ghi)\g{-1} 6526 6527 The sequence \g{-1} is a reference to the most recently started captur- 6528 ing subpattern before \g, that is, is it equivalent to \2 in this exam- 6529 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative 6530 references can be helpful in long patterns, and also in patterns that 6531 are created by joining together fragments that contain references 6532 within themselves. 6533 6534 A back reference matches whatever actually matched the capturing sub- 6535 pattern in the current subject string, rather than anything matching 6536 the subpattern itself (see "Subpatterns as subroutines" below for a way 6537 of doing that). So the pattern 6538 6539 (sens|respons)e and \1ibility 6540 6541 matches "sense and sensibility" and "response and responsibility", but 6542 not "sense and responsibility". If caseful matching is in force at the 6543 time of the back reference, the case of letters is relevant. For exam- 6544 ple, 6545 6546 ((?i)rah)\s+\1 6547 6548 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the 6549 original capturing subpattern is matched caselessly. 6550 6551 There are several different ways of writing back references to named 6552 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or 6553 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's 6554 unified back reference syntax, in which \g can be used for both numeric 6555 and named references, is also supported. We could rewrite the above 6556 example in any of the following ways: 6557 6558 (?<p1>(?i)rah)\s+\k<p1> 6559 (?'p1'(?i)rah)\s+\k{p1} 6560 (?P<p1>(?i)rah)\s+(?P=p1) 6561 (?<p1>(?i)rah)\s+\g{p1} 6562 6563 A subpattern that is referenced by name may appear in the pattern 6564 before or after the reference. 6565 6566 There may be more than one back reference to the same subpattern. If a 6567 subpattern has not actually been used in a particular match, any back 6568 references to it always fail by default. For example, the pattern 6569 6570 (a|(bc))\2 6571 6572 always fails if it starts to match "a" rather than "bc". However, if 6573 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer- 6574 ence to an unset value matches an empty string. 6575 6576 Because there may be many capturing parentheses in a pattern, all dig- 6577 its following a backslash are taken as part of a potential back refer- 6578 ence number. If the pattern continues with a digit character, some 6579 delimiter must be used to terminate the back reference. If the 6580 PCRE_EXTENDED option is set, this can be white space. Otherwise, the 6581 \g{ syntax or an empty comment (see "Comments" below) can be used. 6582 6583 Recursive back references 6584 6585 A back reference that occurs inside the parentheses to which it refers 6586 fails when the subpattern is first used, so, for example, (a\1) never 6587 matches. However, such references can be useful inside repeated sub- 6588 patterns. For example, the pattern 6589 6590 (a|b\1)+ 6591 6592 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- 6593 ation of the subpattern, the back reference matches the character 6594 string corresponding to the previous iteration. In order for this to 6595 work, the pattern must be such that the first iteration does not need 6596 to match the back reference. This can be done using alternation, as in 6597 the example above, or by a quantifier with a minimum of zero. 6598 6599 Back references of this type cause the group that they reference to be 6600 treated as an atomic group. Once the whole group has been matched, a 6601 subsequent matching failure cannot cause backtracking into the middle 6602 of the group. 6603 6604 6605 ASSERTIONS 6606 6607 An assertion is a test on the characters following or preceding the 6608 current matching point that does not actually consume any characters. 6609 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are 6610 described above. 6611 6612 More complicated assertions are coded as subpatterns. There are two 6613 kinds: those that look ahead of the current position in the subject 6614 string, and those that look behind it. An assertion subpattern is 6615 matched in the normal way, except that it does not cause the current 6616 matching position to be changed. 6617 6618 Assertion subpatterns are not capturing subpatterns. If such an asser- 6619 tion contains capturing subpatterns within it, these are counted for 6620 the purposes of numbering the capturing subpatterns in the whole pat- 6621 tern. However, substring capturing is carried out only for positive 6622 assertions. (Perl sometimes, but not always, does do capturing in nega- 6623 tive assertions.) 6624 6625 For compatibility with Perl, assertion subpatterns may be repeated; 6626 though it makes no sense to assert the same thing several times, the 6627 side effect of capturing parentheses may occasionally be useful. In 6628 practice, there only three cases: 6629 6630 (1) If the quantifier is {0}, the assertion is never obeyed during 6631 matching. However, it may contain internal capturing parenthesized 6632 groups that are called from elsewhere via the subroutine mechanism. 6633 6634 (2) If quantifier is {0,n} where n is greater than zero, it is treated 6635 as if it were {0,1}. At run time, the rest of the pattern match is 6636 tried with and without the assertion, the order depending on the greed- 6637 iness of the quantifier. 6638 6639 (3) If the minimum repetition is greater than zero, the quantifier is 6640 ignored. The assertion is obeyed just once when encountered during 6641 matching. 6642 6643 Lookahead assertions 6644 6645 Lookahead assertions start with (?= for positive assertions and (?! for 6646 negative assertions. For example, 6647 6648 \w+(?=;) 6649 6650 matches a word followed by a semicolon, but does not include the semi- 6651 colon in the match, and 6652 6653 foo(?!bar) 6654 6655 matches any occurrence of "foo" that is not followed by "bar". Note 6656 that the apparently similar pattern 6657 6658 (?!foo)bar 6659 6660 does not find an occurrence of "bar" that is preceded by something 6661 other than "foo"; it finds any occurrence of "bar" whatsoever, because 6662 the assertion (?!foo) is always true when the next three characters are 6663 "bar". A lookbehind assertion is needed to achieve the other effect. 6664 6665 If you want to force a matching failure at some point in a pattern, the 6666 most convenient way to do it is with (?!) because an empty string 6667 always matches, so an assertion that requires there not to be an empty 6668 string must always fail. The backtracking control verb (*FAIL) or (*F) 6669 is a synonym for (?!). 6670 6671 Lookbehind assertions 6672 6673 Lookbehind assertions start with (?<= for positive assertions and (?<! 6674 for negative assertions. For example, 6675 6676 (?<!foo)bar 6677 6678 does find an occurrence of "bar" that is not preceded by "foo". The 6679 contents of a lookbehind assertion are restricted such that all the 6680 strings it matches must have a fixed length. However, if there are sev- 6681 eral top-level alternatives, they do not all have to have the same 6682 fixed length. Thus 6683 6684 (?<=bullock|donkey) 6685 6686 is permitted, but 6687 6688 (?<!dogs?|cats?) 6689 6690 causes an error at compile time. Branches that match different length 6691 strings are permitted only at the top level of a lookbehind assertion. 6692 This is an extension compared with Perl, which requires all branches to 6693 match the same length of string. An assertion such as 6694 6695 (?<=ab(c|de)) 6696 6697 is not permitted, because its single top-level branch can match two 6698 different lengths, but it is acceptable to PCRE if rewritten to use two 6699 top-level branches: 6700 6701 (?<=abc|abde) 6702 6703 In some cases, the escape sequence \K (see above) can be used instead 6704 of a lookbehind assertion to get round the fixed-length restriction. 6705 6706 The implementation of lookbehind assertions is, for each alternative, 6707 to temporarily move the current position back by the fixed length and 6708 then try to match. If there are insufficient characters before the cur- 6709 rent position, the assertion fails. 6710 6711 In a UTF mode, PCRE does not allow the \C escape (which matches a sin- 6712 gle data unit even in a UTF mode) to appear in lookbehind assertions, 6713 because it makes it impossible to calculate the length of the lookbe- 6714 hind. The \X and \R escapes, which can match different numbers of data 6715 units, are also not permitted. 6716 6717 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in 6718 lookbehinds, as long as the subpattern matches a fixed-length string. 6719 Recursion, however, is not supported. 6720 6721 Possessive quantifiers can be used in conjunction with lookbehind 6722 assertions to specify efficient matching of fixed-length strings at the 6723 end of subject strings. Consider a simple pattern such as 6724 6725 abcd$ 6726 6727 when applied to a long string that does not match. Because matching 6728 proceeds from left to right, PCRE will look for each "a" in the subject 6729 and then see if what follows matches the rest of the pattern. If the 6730 pattern is specified as 6731 6732 ^.*abcd$ 6733 6734 the initial .* matches the entire string at first, but when this fails 6735 (because there is no following "a"), it backtracks to match all but the 6736 last character, then all but the last two characters, and so on. Once 6737 again the search for "a" covers the entire string, from right to left, 6738 so we are no better off. However, if the pattern is written as 6739 6740 ^.*+(?<=abcd) 6741 6742 there can be no backtracking for the .*+ item; it can match only the 6743 entire string. The subsequent lookbehind assertion does a single test 6744 on the last four characters. If it fails, the match fails immediately. 6745 For long strings, this approach makes a significant difference to the 6746 processing time. 6747 6748 Using multiple assertions 6749 6750 Several assertions (of any sort) may occur in succession. For example, 6751 6752 (?<=\d{3})(?<!999)foo 6753 6754 matches "foo" preceded by three digits that are not "999". Notice that 6755 each of the assertions is applied independently at the same point in 6756 the subject string. First there is a check that the previous three 6757 characters are all digits, and then there is a check that the same 6758 three characters are not "999". This pattern does not match "foo" pre- 6759 ceded by six characters, the first of which are digits and the last 6760 three of which are not "999". For example, it doesn't match "123abc- 6761 foo". A pattern to do that is 6762 6763 (?<=\d{3}...)(?<!999)foo 6764 6765 This time the first assertion looks at the preceding six characters, 6766 checking that the first three are digits, and then the second assertion 6767 checks that the preceding three characters are not "999". 6768 6769 Assertions can be nested in any combination. For example, 6770 6771 (?<=(?<!foo)bar)baz 6772 6773 matches an occurrence of "baz" that is preceded by "bar" which in turn 6774 is not preceded by "foo", while 6775 6776 (?<=\d{3}(?!999)...)foo 6777 6778 is another pattern that matches "foo" preceded by three digits and any 6779 three characters that are not "999". 6780 6781 6782 CONDITIONAL SUBPATTERNS 6783 6784 It is possible to cause the matching process to obey a subpattern con- 6785 ditionally or to choose between two alternative subpatterns, depending 6786 on the result of an assertion, or whether a specific capturing subpat- 6787 tern has already been matched. The two possible forms of conditional 6788 subpattern are: 6789 6790 (?(condition)yes-pattern) 6791 (?(condition)yes-pattern|no-pattern) 6792 6793 If the condition is satisfied, the yes-pattern is used; otherwise the 6794 no-pattern (if present) is used. If there are more than two alterna- 6795 tives in the subpattern, a compile-time error occurs. Each of the two 6796 alternatives may itself contain nested subpatterns of any form, includ- 6797 ing conditional subpatterns; the restriction to two alternatives 6798 applies only at the level of the condition. This pattern fragment is an 6799 example where the alternatives are complex: 6800 6801 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 6802 6803 6804 There are four kinds of condition: references to subpatterns, refer- 6805 ences to recursion, a pseudo-condition called DEFINE, and assertions. 6806 6807 Checking for a used subpattern by number 6808 6809 If the text between the parentheses consists of a sequence of digits, 6810 the condition is true if a capturing subpattern of that number has pre- 6811 viously matched. If there is more than one capturing subpattern with 6812 the same number (see the earlier section about duplicate subpattern 6813 numbers), the condition is true if any of them have matched. An alter- 6814 native notation is to precede the digits with a plus or minus sign. In 6815 this case, the subpattern number is relative rather than absolute. The 6816 most recently opened parentheses can be referenced by (?(-1), the next 6817 most recent by (?(-2), and so on. Inside loops it can also make sense 6818 to refer to subsequent groups. The next parentheses to be opened can be 6819 referenced as (?(+1), and so on. (The value zero in any of these forms 6820 is not used; it provokes a compile-time error.) 6821 6822 Consider the following pattern, which contains non-significant white 6823 space to make it more readable (assume the PCRE_EXTENDED option) and to 6824 divide it into three parts for ease of discussion: 6825 6826 ( \( )? [^()]+ (?(1) \) ) 6827 6828 The first part matches an optional opening parenthesis, and if that 6829 character is present, sets it as the first captured substring. The sec- 6830 ond part matches one or more characters that are not parentheses. The 6831 third part is a conditional subpattern that tests whether or not the 6832 first set of parentheses matched. If they did, that is, if subject 6833 started with an opening parenthesis, the condition is true, and so the 6834 yes-pattern is executed and a closing parenthesis is required. Other- 6835 wise, since no-pattern is not present, the subpattern matches nothing. 6836 In other words, this pattern matches a sequence of non-parentheses, 6837 optionally enclosed in parentheses. 6838 6839 If you were embedding this pattern in a larger one, you could use a 6840 relative reference: 6841 6842 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 6843 6844 This makes the fragment independent of the parentheses in the larger 6845 pattern. 6846 6847 Checking for a used subpattern by name 6848 6849 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a 6850 used subpattern by name. For compatibility with earlier versions of 6851 PCRE, which had this facility before Perl, the syntax (?(name)...) is 6852 also recognized. 6853 6854 Rewriting the above example to use a named subpattern gives this: 6855 6856 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 6857 6858 If the name used in a condition of this kind is a duplicate, the test 6859 is applied to all subpatterns of the same name, and is true if any one 6860 of them has matched. 6861 6862 Checking for pattern recursion 6863 6864 If the condition is the string (R), and there is no subpattern with the 6865 name R, the condition is true if a recursive call to the whole pattern 6866 or any subpattern has been made. If digits or a name preceded by amper- 6867 sand follow the letter R, for example: 6868 6869 (?(R3)...) or (?(R&name)...) 6870 6871 the condition is true if the most recent recursion is into a subpattern 6872 whose number or name is given. This condition does not check the entire 6873 recursion stack. If the name used in a condition of this kind is a 6874 duplicate, the test is applied to all subpatterns of the same name, and 6875 is true if any one of them is the most recent recursion. 6876 6877 At "top level", all these recursion test conditions are false. The 6878 syntax for recursive patterns is described below. 6879 6880 Defining subpatterns for use by reference only 6881 6882 If the condition is the string (DEFINE), and there is no subpattern 6883 with the name DEFINE, the condition is always false. In this case, 6884 there may be only one alternative in the subpattern. It is always 6885 skipped if control reaches this point in the pattern; the idea of 6886 DEFINE is that it can be used to define subroutines that can be refer- 6887 enced from elsewhere. (The use of subroutines is described below.) For 6888 example, a pattern to match an IPv4 address such as "192.168.23.245" 6889 could be written like this (ignore white space and line breaks): 6890 6891 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 6892 \b (?&byte) (\.(?&byte)){3} \b 6893 6894 The first part of the pattern is a DEFINE group inside which a another 6895 group named "byte" is defined. This matches an individual component of 6896 an IPv4 address (a number less than 256). When matching takes place, 6897 this part of the pattern is skipped because DEFINE acts like a false 6898 condition. The rest of the pattern uses references to the named group 6899 to match the four dot-separated components of an IPv4 address, insist- 6900 ing on a word boundary at each end. 6901 6902 Assertion conditions 6903 6904 If the condition is not in any of the above formats, it must be an 6905 assertion. This may be a positive or negative lookahead or lookbehind 6906 assertion. Consider this pattern, again containing non-significant 6907 white space, and with the two alternatives on the second line: 6908 6909 (?(?=[^a-z]*[a-z]) 6910 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 6911 6912 The condition is a positive lookahead assertion that matches an 6913 optional sequence of non-letters followed by a letter. In other words, 6914 it tests for the presence of at least one letter in the subject. If a 6915 letter is found, the subject is matched against the first alternative; 6916 otherwise it is matched against the second. This pattern matches 6917 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are 6918 letters and dd are digits. 6919 6920 6921 COMMENTS 6922 6923 There are two ways of including comments in patterns that are processed 6924 by PCRE. In both cases, the start of the comment must not be in a char- 6925 acter class, nor in the middle of any other sequence of related charac- 6926 ters such as (?: or a subpattern name or number. The characters that 6927 make up a comment play no part in the pattern matching. 6928 6929 The sequence (?# marks the start of a comment that continues up to the 6930 next closing parenthesis. Nested parentheses are not permitted. If the 6931 PCRE_EXTENDED option is set, an unescaped # character also introduces a 6932 comment, which in this case continues to immediately after the next 6933 newline character or character sequence in the pattern. Which charac- 6934 ters are interpreted as newlines is controlled by the options passed to 6935 a compiling function or by a special sequence at the start of the pat- 6936 tern, as described in the section entitled "Newline conventions" above. 6937 Note that the end of this type of comment is a literal newline sequence 6938 in the pattern; escape sequences that happen to represent a newline do 6939 not count. For example, consider this pattern when PCRE_EXTENDED is 6940 set, and the default newline convention is in force: 6941 6942 abc #comment \n still comment 6943 6944 On encountering the # character, pcre_compile() skips along, looking 6945 for a newline in the pattern. The sequence \n is still literal at this 6946 stage, so it does not terminate the comment. Only an actual character 6947 with the code value 0x0a (the default newline) does so. 6948 6949 6950 RECURSIVE PATTERNS 6951 6952 Consider the problem of matching a string in parentheses, allowing for 6953 unlimited nested parentheses. Without the use of recursion, the best 6954 that can be done is to use a pattern that matches up to some fixed 6955 depth of nesting. It is not possible to handle an arbitrary nesting 6956 depth. 6957 6958 For some time, Perl has provided a facility that allows regular expres- 6959 sions to recurse (amongst other things). It does this by interpolating 6960 Perl code in the expression at run time, and the code can refer to the 6961 expression itself. A Perl pattern using code interpolation to solve the 6962 parentheses problem can be created like this: 6963 6964 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 6965 6966 The (?p{...}) item interpolates Perl code at run time, and in this case 6967 refers recursively to the pattern in which it appears. 6968 6969 Obviously, PCRE cannot support the interpolation of Perl code. Instead, 6970 it supports special syntax for recursion of the entire pattern, and 6971 also for individual subpattern recursion. After its introduction in 6972 PCRE and Python, this kind of recursion was subsequently introduced 6973 into Perl at release 5.10. 6974 6975 A special item that consists of (? followed by a number greater than 6976 zero and a closing parenthesis is a recursive subroutine call of the 6977 subpattern of the given number, provided that it occurs inside that 6978 subpattern. (If not, it is a non-recursive subroutine call, which is 6979 described in the next section.) The special item (?R) or (?0) is a 6980 recursive call of the entire regular expression. 6981 6982 This PCRE pattern solves the nested parentheses problem (assume the 6983 PCRE_EXTENDED option is set so that white space is ignored): 6984 6985 \( ( [^()]++ | (?R) )* \) 6986 6987 First it matches an opening parenthesis. Then it matches any number of 6988 substrings which can either be a sequence of non-parentheses, or a 6989 recursive match of the pattern itself (that is, a correctly parenthe- 6990 sized substring). Finally there is a closing parenthesis. Note the use 6991 of a possessive quantifier to avoid backtracking into sequences of non- 6992 parentheses. 6993 6994 If this were part of a larger pattern, you would not want to recurse 6995 the entire pattern, so instead you could use this: 6996 6997 ( \( ( [^()]++ | (?1) )* \) ) 6998 6999 We have put the pattern into parentheses, and caused the recursion to 7000 refer to them instead of the whole pattern. 7001 7002 In a larger pattern, keeping track of parenthesis numbers can be 7003 tricky. This is made easier by the use of relative references. Instead 7004 of (?1) in the pattern above you can write (?-2) to refer to the second 7005 most recently opened parentheses preceding the recursion. In other 7006 words, a negative number counts capturing parentheses leftwards from 7007 the point at which it is encountered. 7008 7009 It is also possible to refer to subsequently opened parentheses, by 7010 writing references such as (?+2). However, these cannot be recursive 7011 because the reference is not inside the parentheses that are refer- 7012 enced. They are always non-recursive subroutine calls, as described in 7013 the next section. 7014 7015 An alternative approach is to use named parentheses instead. The Perl 7016 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also 7017 supported. We could rewrite the above example as follows: 7018 7019 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 7020 7021 If there is more than one subpattern with the same name, the earliest 7022 one is used. 7023 7024 This particular example pattern that we have been looking at contains 7025 nested unlimited repeats, and so the use of a possessive quantifier for 7026 matching strings of non-parentheses is important when applying the pat- 7027 tern to strings that do not match. For example, when this pattern is 7028 applied to 7029 7030 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 7031 7032 it yields "no match" quickly. However, if a possessive quantifier is 7033 not used, the match runs for a very long time indeed because there are 7034 so many different ways the + and * repeats can carve up the subject, 7035 and all have to be tested before failure can be reported. 7036 7037 At the end of a match, the values of capturing parentheses are those 7038 from the outermost level. If you want to obtain intermediate values, a 7039 callout function can be used (see below and the pcrecallout documenta- 7040 tion). If the pattern above is matched against 7041 7042 (ab(cd)ef) 7043 7044 the value for the inner capturing parentheses (numbered 2) is "ef", 7045 which is the last value taken on at the top level. If a capturing sub- 7046 pattern is not matched at the top level, its final captured value is 7047 unset, even if it was (temporarily) set at a deeper level during the 7048 matching process. 7049 7050 If there are more than 15 capturing parentheses in a pattern, PCRE has 7051 to obtain extra memory to store data during a recursion, which it does 7052 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory 7053 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 7054 7055 Do not confuse the (?R) item with the condition (R), which tests for 7056 recursion. Consider this pattern, which matches text in angle brack- 7057 ets, allowing for arbitrary nesting. Only digits are allowed in nested 7058 brackets (that is, when recursing), whereas any characters are permit- 7059 ted at the outer level. 7060 7061 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 7062 7063 In this pattern, (?(R) is the start of a conditional subpattern, with 7064 two different alternatives for the recursive and non-recursive cases. 7065 The (?R) item is the actual recursive call. 7066 7067 Differences in recursion processing between PCRE and Perl 7068 7069 Recursion processing in PCRE differs from Perl in two important ways. 7070 In PCRE (like Python, but unlike Perl), a recursive subpattern call is 7071 always treated as an atomic group. That is, once it has matched some of 7072 the subject string, it is never re-entered, even if it contains untried 7073 alternatives and there is a subsequent matching failure. This can be 7074 illustrated by the following pattern, which purports to match a palin- 7075 dromic string that contains an odd number of characters (for example, 7076 "a", "aba", "abcba", "abcdcba"): 7077 7078 ^(.|(.)(?1)\2)$ 7079 7080 The idea is that it either matches a single character, or two identical 7081 characters surrounding a sub-palindrome. In Perl, this pattern works; 7082 in PCRE it does not if the pattern is longer than three characters. 7083 Consider the subject string "abcba": 7084 7085 At the top level, the first character is matched, but as it is not at 7086 the end of the string, the first alternative fails; the second alterna- 7087 tive is taken and the recursion kicks in. The recursive call to subpat- 7088 tern 1 successfully matches the next character ("b"). (Note that the 7089 beginning and end of line tests are not part of the recursion). 7090 7091 Back at the top level, the next character ("c") is compared with what 7092 subpattern 2 matched, which was "a". This fails. Because the recursion 7093 is treated as an atomic group, there are now no backtracking points, 7094 and so the entire match fails. (Perl is able, at this point, to re- 7095 enter the recursion and try the second alternative.) However, if the 7096 pattern is written with the alternatives in the other order, things are 7097 different: 7098 7099 ^((.)(?1)\2|.)$ 7100 7101 This time, the recursing alternative is tried first, and continues to 7102 recurse until it runs out of characters, at which point the recursion 7103 fails. But this time we do have another alternative to try at the 7104 higher level. That is the big difference: in the previous case the 7105 remaining alternative is at a deeper recursion level, which PCRE cannot 7106 use. 7107 7108 To change the pattern so that it matches all palindromic strings, not 7109 just those with an odd number of characters, it is tempting to change 7110 the pattern to this: 7111 7112 ^((.)(?1)\2|.?)$ 7113 7114 Again, this works in Perl, but not in PCRE, and for the same reason. 7115 When a deeper recursion has matched a single character, it cannot be 7116 entered again in order to match an empty string. The solution is to 7117 separate the two cases, and write out the odd and even cases as alter- 7118 natives at the higher level: 7119 7120 ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) 7121 7122 If you want to match typical palindromic phrases, the pattern has to 7123 ignore all non-word characters, which can be done like this: 7124 7125 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ 7126 7127 If run with the PCRE_CASELESS option, this pattern matches phrases such 7128 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and 7129 Perl. Note the use of the possessive quantifier *+ to avoid backtrack- 7130 ing into sequences of non-word characters. Without this, PCRE takes a 7131 great deal longer (ten times or more) to match typical phrases, and 7132 Perl takes so long that you think it has gone into a loop. 7133 7134 WARNING: The palindrome-matching patterns above work only if the sub- 7135 ject string does not start with a palindrome that is shorter than the 7136 entire string. For example, although "abcba" is correctly matched, if 7137 the subject is "ababa", PCRE finds the palindrome "aba" at the start, 7138 then fails at top level because the end of the string does not follow. 7139 Once again, it cannot jump back into the recursion to try other alter- 7140 natives, so the entire match fails. 7141 7142 The second way in which PCRE and Perl differ in their recursion pro- 7143 cessing is in the handling of captured values. In Perl, when a subpat- 7144 tern is called recursively or as a subpattern (see the next section), 7145 it has no access to any values that were captured outside the recur- 7146 sion, whereas in PCRE these values can be referenced. Consider this 7147 pattern: 7148 7149 ^(.)(\1|a(?2)) 7150 7151 In PCRE, this pattern matches "bab". The first capturing parentheses 7152 match "b", then in the second group, when the back reference \1 fails 7153 to match "b", the second alternative matches "a" and then recurses. In 7154 the recursion, \1 does now match "b" and so the whole match succeeds. 7155 In Perl, the pattern fails to match because inside the recursive call 7156 \1 cannot access the externally set value. 7157 7158 7159 SUBPATTERNS AS SUBROUTINES 7160 7161 If the syntax for a recursive subpattern call (either by number or by 7162 name) is used outside the parentheses to which it refers, it operates 7163 like a subroutine in a programming language. The called subpattern may 7164 be defined before or after the reference. A numbered reference can be 7165 absolute or relative, as in these examples: 7166 7167 (...(absolute)...)...(?2)... 7168 (...(relative)...)...(?-1)... 7169 (...(?+1)...(relative)... 7170 7171 An earlier example pointed out that the pattern 7172 7173 (sens|respons)e and \1ibility 7174 7175 matches "sense and sensibility" and "response and responsibility", but 7176 not "sense and responsibility". If instead the pattern 7177 7178 (sens|respons)e and (?1)ibility 7179 7180 is used, it does match "sense and responsibility" as well as the other 7181 two strings. Another example is given in the discussion of DEFINE 7182 above. 7183 7184 All subroutine calls, whether recursive or not, are always treated as 7185 atomic groups. That is, once a subroutine has matched some of the sub- 7186 ject string, it is never re-entered, even if it contains untried alter- 7187 natives and there is a subsequent matching failure. Any capturing 7188 parentheses that are set during the subroutine call revert to their 7189 previous values afterwards. 7190 7191 Processing options such as case-independence are fixed when a subpat- 7192 tern is defined, so if it is used as a subroutine, such options cannot 7193 be changed for different calls. For example, consider this pattern: 7194 7195 (abc)(?i:(?-1)) 7196 7197 It matches "abcabc". It does not match "abcABC" because the change of 7198 processing option does not affect the called subpattern. 7199 7200 7201 ONIGURUMA SUBROUTINE SYNTAX 7202 7203 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 7204 name or a number enclosed either in angle brackets or single quotes, is 7205 an alternative syntax for referencing a subpattern as a subroutine, 7206 possibly recursively. Here are two of the examples used above, rewrit- 7207 ten using this syntax: 7208 7209 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 7210 (sens|respons)e and \g'1'ibility 7211 7212 PCRE supports an extension to Oniguruma: if a number is preceded by a 7213 plus or a minus sign it is taken as a relative reference. For example: 7214 7215 (abc)(?i:\g<-1>) 7216 7217 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not 7218 synonymous. The former is a back reference; the latter is a subroutine 7219 call. 7220 7221 7222 CALLOUTS 7223 7224 Perl has a feature whereby using the sequence (?{...}) causes arbitrary 7225 Perl code to be obeyed in the middle of matching a regular expression. 7226 This makes it possible, amongst other things, to extract different sub- 7227 strings that match the same pair of parentheses when there is a repeti- 7228 tion. 7229 7230 PCRE provides a similar feature, but of course it cannot obey arbitrary 7231 Perl code. The feature is called "callout". The caller of PCRE provides 7232 an external function by putting its entry point in the global variable 7233 pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit 7234 library). By default, this variable contains NULL, which disables all 7235 calling out. 7236 7237 Within a regular expression, (?C) indicates the points at which the 7238 external function is to be called. If you want to identify different 7239 callout points, you can put a number less than 256 after the letter C. 7240 The default value is zero. For example, this pattern has two callout 7241 points: 7242 7243 (?C1)abc(?C2)def 7244 7245 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call- 7246 outs are automatically installed before each item in the pattern. They 7247 are all numbered 255. If there is a conditional group in the pattern 7248 whose condition is an assertion, an additional callout is inserted just 7249 before the condition. An explicit callout may also be set at this posi- 7250 tion, as in this example: 7251 7252 (?(?C9)(?=a)abc|def) 7253 7254 Note that this applies only to assertion conditions, not to other types 7255 of condition. 7256 7257 During matching, when PCRE reaches a callout point, the external func- 7258 tion is called. It is provided with the number of the callout, the 7259 position in the pattern, and, optionally, one item of data originally 7260 supplied by the caller of the matching function. The callout function 7261 may cause matching to proceed, to backtrack, or to fail altogether. 7262 7263 By default, PCRE implements a number of optimizations at compile time 7264 and matching time, and one side-effect is that sometimes callouts are 7265 skipped. If you need all possible callouts to happen, you need to set 7266 options that disable the relevant optimizations. More details, and a 7267 complete description of the interface to the callout function, are 7268 given in the pcrecallout documentation. 7269 7270 7271 BACKTRACKING CONTROL 7272 7273 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", 7274 which are still described in the Perl documentation as "experimental 7275 and subject to change or removal in a future version of Perl". It goes 7276 on to say: "Their usage in production code should be noted to avoid 7277 problems during upgrades." The same remarks apply to the PCRE features 7278 described in this section. 7279 7280 The new verbs make use of what was previously invalid syntax: an open- 7281 ing parenthesis followed by an asterisk. They are generally of the form 7282 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving 7283 differently depending on whether or not a name is present. A name is 7284 any sequence of characters that does not include a closing parenthesis. 7285 The maximum length of name is 255 in the 8-bit library and 65535 in the 7286 16-bit and 32-bit libraries. If the name is empty, that is, if the 7287 closing parenthesis immediately follows the colon, the effect is as if 7288 the colon were not there. Any number of these verbs may occur in a 7289 pattern. 7290 7291 Since these verbs are specifically related to backtracking, most of 7292 them can be used only when the pattern is to be matched using one of 7293 the traditional matching functions, because these use a backtracking 7294 algorithm. With the exception of (*FAIL), which behaves like a failing 7295 negative assertion, the backtracking control verbs cause an error if 7296 encountered by a DFA matching function. 7297 7298 The behaviour of these verbs in repeated groups, assertions, and in 7299 subpatterns called as subroutines (whether or not recursively) is docu- 7300 mented below. 7301 7302 Optimizations that affect backtracking verbs 7303 7304 PCRE contains some optimizations that are used to speed up matching by 7305 running some checks at the start of each match attempt. For example, it 7306 may know the minimum length of matching subject, or that a particular 7307 character must be present. When one of these optimizations bypasses the 7308 running of a match, any included backtracking verbs will not, of 7309 course, be processed. You can suppress the start-of-match optimizations 7310 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com- 7311 pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT). 7312 There is more discussion of this option in the section entitled "Option 7313 bits for pcre_exec()" in the pcreapi documentation. 7314 7315 Experiments with Perl suggest that it too has similar optimizations, 7316 sometimes leading to anomalous results. 7317 7318 Verbs that act immediately 7319 7320 The following verbs act as soon as they are encountered. They may not 7321 be followed by a name. 7322 7323 (*ACCEPT) 7324 7325 This verb causes the match to end successfully, skipping the remainder 7326 of the pattern. However, when it is inside a subpattern that is called 7327 as a subroutine, only that subpattern is ended successfully. Matching 7328 then continues at the outer level. If (*ACCEPT) in triggered in a posi- 7329 tive assertion, the assertion succeeds; in a negative assertion, the 7330 assertion fails. 7331 7332 If (*ACCEPT) is inside capturing parentheses, the data so far is cap- 7333 tured. For example: 7334 7335 A((?:A|B(*ACCEPT)|C)D) 7336 7337 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- 7338 tured by the outer parentheses. 7339 7340 (*FAIL) or (*F) 7341 7342 This verb causes a matching failure, forcing backtracking to occur. It 7343 is equivalent to (?!) but easier to read. The Perl documentation notes 7344 that it is probably useful only when combined with (?{}) or (??{}). 7345 Those are, of course, Perl features that are not present in PCRE. The 7346 nearest equivalent is the callout feature, as for example in this pat- 7347 tern: 7348 7349 a+(?C)(*FAIL) 7350 7351 A match with the string "aaaa" always fails, but the callout is taken 7352 before each backtrack happens (in this example, 10 times). 7353 7354 Recording which path was taken 7355 7356 There is one verb whose main purpose is to track how a match was 7357 arrived at, though it also has a secondary use in conjunction with 7358 advancing the match starting point (see (*SKIP) below). 7359 7360 (*MARK:NAME) or (*:NAME) 7361 7362 A name is always required with this verb. There may be as many 7363 instances of (*MARK) as you like in a pattern, and their names do not 7364 have to be unique. 7365 7366 When a match succeeds, the name of the last-encountered (*MARK:NAME), 7367 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to 7368 the caller as described in the section entitled "Extra data for 7369 pcre_exec()" in the pcreapi documentation. Here is an example of 7370 pcretest output, where the /K modifier requests the retrieval and out- 7371 putting of (*MARK) data: 7372 7373 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 7374 data> XY 7375 0: XY 7376 MK: A 7377 XZ 7378 0: XZ 7379 MK: B 7380 7381 The (*MARK) name is tagged with "MK:" in this output, and in this exam- 7382 ple it indicates which of the two alternatives matched. This is a more 7383 efficient way of obtaining this information than putting each alterna- 7384 tive in its own capturing parentheses. 7385 7386 If a verb with a name is encountered in a positive assertion that is 7387 true, the name is recorded and passed back if it is the last-encoun- 7388 tered. This does not happen for negative assertions or failing positive 7389 assertions. 7390 7391 After a partial match or a failed match, the last encountered name in 7392 the entire match process is returned. For example: 7393 7394 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 7395 data> XP 7396 No match, mark = B 7397 7398 Note that in this unanchored example the mark is retained from the 7399 match attempt that started at the letter "X" in the subject. Subsequent 7400 match attempts starting at "P" and then with an empty string do not get 7401 as far as the (*MARK) item, but nevertheless do not reset it. 7402 7403 If you are interested in (*MARK) values after failed matches, you 7404 should probably set the PCRE_NO_START_OPTIMIZE option (see above) to 7405 ensure that the match is always attempted. 7406 7407 Verbs that act after backtracking 7408 7409 The following verbs do nothing when they are encountered. Matching con- 7410 tinues with what follows, but if there is no subsequent match, causing 7411 a backtrack to the verb, a failure is forced. That is, backtracking 7412 cannot pass to the left of the verb. However, when one of these verbs 7413 appears inside an atomic group or an assertion that is true, its effect 7414 is confined to that group, because once the group has been matched, 7415 there is never any backtracking into it. In this situation, backtrack- 7416 ing can "jump back" to the left of the entire atomic group or asser- 7417 tion. (Remember also, as stated above, that this localization also 7418 applies in subroutine calls.) 7419 7420 These verbs differ in exactly what kind of failure occurs when back- 7421 tracking reaches them. The behaviour described below is what happens 7422 when the verb is not in a subroutine or an assertion. Subsequent sec- 7423 tions cover these special cases. 7424 7425 (*COMMIT) 7426 7427 This verb, which may not be followed by a name, causes the whole match 7428 to fail outright if there is a later matching failure that causes back- 7429 tracking to reach it. Even if the pattern is unanchored, no further 7430 attempts to find a match by advancing the starting point take place. If 7431 (*COMMIT) is the only backtracking verb that is encountered, once it 7432 has been passed pcre_exec() is committed to finding a match at the cur- 7433 rent starting point, or not at all. For example: 7434 7435 a+(*COMMIT)b 7436 7437 This matches "xxaab" but not "aacaab". It can be thought of as a kind 7438 of dynamic anchor, or "I've started, so I must finish." The name of the 7439 most recently passed (*MARK) in the path is passed back when (*COMMIT) 7440 forces a match failure. 7441 7442 If there is more than one backtracking verb in a pattern, a different 7443 one that follows (*COMMIT) may be triggered first, so merely passing 7444 (*COMMIT) during a match does not always guarantee that a match must be 7445 at this starting point. 7446 7447 Note that (*COMMIT) at the start of a pattern is not the same as an 7448 anchor, unless PCRE's start-of-match optimizations are turned off, as 7449 shown in this output from pcretest: 7450 7451 re> /(*COMMIT)abc/ 7452 data> xyzabc 7453 0: abc 7454 data> xyzabc\Y 7455 No match 7456 7457 For this pattern, PCRE knows that any match must start with "a", so the 7458 optimization skips along the subject to "a" before applying the pattern 7459 to the first set of data. The match attempt then succeeds. In the sec- 7460 ond set of data, the escape sequence \Y is interpreted by the pcretest 7461 program. It causes the PCRE_NO_START_OPTIMIZE option to be set when 7462 pcre_exec() is called. This disables the optimization that skips along 7463 to the first character. The pattern is now applied starting at "x", and 7464 so the (*COMMIT) causes the match to fail without trying any other 7465 starting points. 7466 7467 (*PRUNE) or (*PRUNE:NAME) 7468 7469 This verb causes the match to fail at the current starting position in 7470 the subject if there is a later matching failure that causes backtrack- 7471 ing to reach it. If the pattern is unanchored, the normal "bumpalong" 7472 advance to the next starting character then happens. Backtracking can 7473 occur as usual to the left of (*PRUNE), before it is reached, or when 7474 matching to the right of (*PRUNE), but if there is no match to the 7475 right, backtracking cannot cross (*PRUNE). In simple cases, the use of 7476 (*PRUNE) is just an alternative to an atomic group or possessive quan- 7477 tifier, but there are some uses of (*PRUNE) that cannot be expressed in 7478 any other way. In an anchored pattern (*PRUNE) has the same effect as 7479 (*COMMIT). 7480 7481 The behaviour of (*PRUNE:NAME) is the not the same as 7482 (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is 7483 remembered for passing back to the caller. However, (*SKIP:NAME) 7484 searches only for names set with (*MARK). 7485 7486 (*SKIP) 7487 7488 This verb, when given without a name, is like (*PRUNE), except that if 7489 the pattern is unanchored, the "bumpalong" advance is not to the next 7490 character, but to the position in the subject where (*SKIP) was encoun- 7491 tered. (*SKIP) signifies that whatever text was matched leading up to 7492 it cannot be part of a successful match. Consider: 7493 7494 a+(*SKIP)b 7495 7496 If the subject is "aaaac...", after the first match attempt fails 7497 (starting at the first character in the string), the starting point 7498 skips on to start the next attempt at "c". Note that a possessive quan- 7499 tifer does not have the same effect as this example; although it would 7500 suppress backtracking during the first match attempt, the second 7501 attempt would start at the second character instead of skipping on to 7502 "c". 7503 7504 (*SKIP:NAME) 7505 7506 When (*SKIP) has an associated name, its behaviour is modified. When it 7507 is triggered, the previous path through the pattern is searched for the 7508 most recent (*MARK) that has the same name. If one is found, the 7509 "bumpalong" advance is to the subject position that corresponds to that 7510 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with 7511 a matching name is found, the (*SKIP) is ignored. 7512 7513 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It 7514 ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME). 7515 7516 (*THEN) or (*THEN:NAME) 7517 7518 This verb causes a skip to the next innermost alternative when back- 7519 tracking reaches it. That is, it cancels any further backtracking 7520 within the current alternative. Its name comes from the observation 7521 that it can be used for a pattern-based if-then-else block: 7522 7523 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 7524 7525 If the COND1 pattern matches, FOO is tried (and possibly further items 7526 after the end of the group if FOO succeeds); on failure, the matcher 7527 skips to the second alternative and tries COND2, without backtracking 7528 into COND1. If that succeeds and BAR fails, COND3 is tried. If subse- 7529 quently BAZ fails, there are no more alternatives, so there is a back- 7530 track to whatever came before the entire group. If (*THEN) is not 7531 inside an alternation, it acts like (*PRUNE). 7532 7533 The behaviour of (*THEN:NAME) is the not the same as 7534 (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is 7535 remembered for passing back to the caller. However, (*SKIP:NAME) 7536 searches only for names set with (*MARK). 7537 7538 A subpattern that does not contain a | character is just a part of the 7539 enclosing alternative; it is not a nested alternation with only one 7540 alternative. The effect of (*THEN) extends beyond such a subpattern to 7541 the enclosing alternative. Consider this pattern, where A, B, etc. are 7542 complex pattern fragments that do not contain any | characters at this 7543 level: 7544 7545 A (B(*THEN)C) | D 7546 7547 If A and B are matched, but there is a failure in C, matching does not 7548 backtrack into A; instead it moves to the next alternative, that is, D. 7549 However, if the subpattern containing (*THEN) is given an alternative, 7550 it behaves differently: 7551 7552 A (B(*THEN)C | (*FAIL)) | D 7553 7554 The effect of (*THEN) is now confined to the inner subpattern. After a 7555 failure in C, matching moves to (*FAIL), which causes the whole subpat- 7556 tern to fail because there are no more alternatives to try. In this 7557 case, matching does now backtrack into A. 7558 7559 Note that a conditional subpattern is not considered as having two 7560 alternatives, because only one is ever used. In other words, the | 7561 character in a conditional subpattern has a different meaning. Ignoring 7562 white space, consider: 7563 7564 ^.*? (?(?=a) a | b(*THEN)c ) 7565 7566 If the subject is "ba", this pattern does not match. Because .*? is 7567 ungreedy, it initially matches zero characters. The condition (?=a) 7568 then fails, the character "b" is matched, but "c" is not. At this 7569 point, matching does not backtrack to .*? as might perhaps be expected 7570 from the presence of the | character. The conditional subpattern is 7571 part of the single alternative that comprises the whole pattern, and so 7572 the match fails. (If there was a backtrack into .*?, allowing it to 7573 match "b", the match would succeed.) 7574 7575 The verbs just described provide four different "strengths" of control 7576 when subsequent matching fails. (*THEN) is the weakest, carrying on the 7577 match at the next alternative. (*PRUNE) comes next, failing the match 7578 at the current starting position, but allowing an advance to the next 7579 character (for an unanchored pattern). (*SKIP) is similar, except that 7580 the advance may be more than one character. (*COMMIT) is the strongest, 7581 causing the entire match to fail. 7582 7583 More than one backtracking verb 7584 7585 If more than one backtracking verb is present in a pattern, the one 7586 that is backtracked onto first acts. For example, consider this pat- 7587 tern, where A, B, etc. are complex pattern fragments: 7588 7589 (A(*COMMIT)B(*THEN)C|ABD) 7590 7591 If A matches but B fails, the backtrack to (*COMMIT) causes the entire 7592 match to fail. However, if A and B match, but C fails, the backtrack to 7593 (*THEN) causes the next alternative (ABD) to be tried. This behaviour 7594 is consistent, but is not always the same as Perl's. It means that if 7595 two or more backtracking verbs appear in succession, all the the last 7596 of them has no effect. Consider this example: 7597 7598 ...(*COMMIT)(*PRUNE)... 7599 7600 If there is a matching failure to the right, backtracking onto (*PRUNE) 7601 causes it to be triggered, and its action is taken. There can never be 7602 a backtrack onto (*COMMIT). 7603 7604 Backtracking verbs in repeated groups 7605 7606 PCRE differs from Perl in its handling of backtracking verbs in 7607 repeated groups. For example, consider: 7608 7609 /(a(*COMMIT)b)+ac/ 7610 7611 If the subject is "abac", Perl matches, but PCRE fails because the 7612 (*COMMIT) in the second repeat of the group acts. 7613 7614 Backtracking verbs in assertions 7615 7616 (*FAIL) in an assertion has its normal effect: it forces an immediate 7617 backtrack. 7618 7619 (*ACCEPT) in a positive assertion causes the assertion to succeed with- 7620 out any further processing. In a negative assertion, (*ACCEPT) causes 7621 the assertion to fail without any further processing. 7622 7623 The other backtracking verbs are not treated specially if they appear 7624 in a positive assertion. In particular, (*THEN) skips to the next 7625 alternative in the innermost enclosing group that has alternations, 7626 whether or not this is within the assertion. 7627 7628 Negative assertions are, however, different, in order to ensure that 7629 changing a positive assertion into a negative assertion changes its 7630 result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg- 7631 ative assertion to be true, without considering any further alternative 7632 branches in the assertion. Backtracking into (*THEN) causes it to skip 7633 to the next enclosing alternative within the assertion (the normal be- 7634 haviour), but if the assertion does not have such an alternative, 7635 (*THEN) behaves like (*PRUNE). 7636 7637 Backtracking verbs in subroutines 7638 7639 These behaviours occur whether or not the subpattern is called recur- 7640 sively. Perl's treatment of subroutines is different in some cases. 7641 7642 (*FAIL) in a subpattern called as a subroutine has its normal effect: 7643 it forces an immediate backtrack. 7644 7645 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine 7646 match to succeed without any further processing. Matching then contin- 7647 ues after the subroutine call. 7648 7649 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine 7650 cause the subroutine match to fail. 7651 7652 (*THEN) skips to the next alternative in the innermost enclosing group 7653 within the subpattern that has alternatives. If there is no such group 7654 within the subpattern, (*THEN) causes the subroutine match to fail. 7655 7656 7657 SEE ALSO 7658 7659 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3), 7660 pcre16(3), pcre32(3). 7661 7662 7663 AUTHOR 7664 7665 Philip Hazel 7666 University Computing Service 7667 Cambridge CB2 3QH, England. 7668 7669 7670 REVISION 7671 7672 Last updated: 14 June 2015 7673 Copyright (c) 1997-2015 University of Cambridge. 7674 ------------------------------------------------------------------------------ 7675 7676 7677 PCRESYNTAX(3) Library Functions Manual PCRESYNTAX(3) 7678 7679 7680 7681 NAME 7682 PCRE - Perl-compatible regular expressions 7683 7684 PCRE REGULAR EXPRESSION SYNTAX SUMMARY 7685 7686 The full syntax and semantics of the regular expressions that are sup- 7687 ported by PCRE are described in the pcrepattern documentation. This 7688 document contains a quick-reference summary of the syntax. 7689 7690 7691 QUOTING 7692 7693 \x where x is non-alphanumeric is a literal x 7694 \Q...\E treat enclosed characters as literal 7695 7696 7697 CHARACTERS 7698 7699 \a alarm, that is, the BEL character (hex 07) 7700 \cx "control-x", where x is any ASCII character 7701 \e escape (hex 1B) 7702 \f form feed (hex 0C) 7703 \n newline (hex 0A) 7704 \r carriage return (hex 0D) 7705 \t tab (hex 09) 7706 \0dd character with octal code 0dd 7707 \ddd character with octal code ddd, or backreference 7708 \o{ddd..} character with octal code ddd.. 7709 \xhh character with hex code hh 7710 \x{hhh..} character with hex code hhh.. 7711 7712 Note that \0dd is always an octal code, and that \8 and \9 are the lit- 7713 eral characters "8" and "9". 7714 7715 7716 CHARACTER TYPES 7717 7718 . any character except newline; 7719 in dotall mode, any character whatsoever 7720 \C one data unit, even in UTF mode (best avoided) 7721 \d a decimal digit 7722 \D a character that is not a decimal digit 7723 \h a horizontal white space character 7724 \H a character that is not a horizontal white space character 7725 \N a character that is not a newline 7726 \p{xx} a character with the xx property 7727 \P{xx} a character without the xx property 7728 \R a newline sequence 7729 \s a white space character 7730 \S a character that is not a white space character 7731 \v a vertical white space character 7732 \V a character that is not a vertical white space character 7733 \w a "word" character 7734 \W a "non-word" character 7735 \X a Unicode extended grapheme cluster 7736 7737 By default, \d, \s, and \w match only ASCII characters, even in UTF-8 7738 mode or in the 16- bit and 32-bit libraries. However, if locale-spe- 7739 cific matching is happening, \s and \w may also match characters with 7740 code points in the range 128-255. If the PCRE_UCP option is set, the 7741 behaviour of these escape sequences is changed to use Unicode proper- 7742 ties and they match many more characters. 7743 7744 7745 GENERAL CATEGORY PROPERTIES FOR \p and \P 7746 7747 C Other 7748 Cc Control 7749 Cf Format 7750 Cn Unassigned 7751 Co Private use 7752 Cs Surrogate 7753 7754 L Letter 7755 Ll Lower case letter 7756 Lm Modifier letter 7757 Lo Other letter 7758 Lt Title case letter 7759 Lu Upper case letter 7760 L& Ll, Lu, or Lt 7761 7762 M Mark 7763 Mc Spacing mark 7764 Me Enclosing mark 7765 Mn Non-spacing mark 7766 7767 N Number 7768 Nd Decimal number 7769 Nl Letter number 7770 No Other number 7771 7772 P Punctuation 7773 Pc Connector punctuation 7774 Pd Dash punctuation 7775 Pe Close punctuation 7776 Pf Final punctuation 7777 Pi Initial punctuation 7778 Po Other punctuation 7779 Ps Open punctuation 7780 7781 S Symbol 7782 Sc Currency symbol 7783 Sk Modifier symbol 7784 Sm Mathematical symbol 7785 So Other symbol 7786 7787 Z Separator 7788 Zl Line separator 7789 Zp Paragraph separator 7790 Zs Space separator 7791 7792 7793 PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P 7794 7795 Xan Alphanumeric: union of properties L and N 7796 Xps POSIX space: property Z or tab, NL, VT, FF, CR 7797 Xsp Perl space: property Z or tab, NL, VT, FF, CR 7798 Xuc Univerally-named character: one that can be 7799 represented by a Universal Character Name 7800 Xwd Perl word: property Xan or underscore 7801 7802 Perl and POSIX space are now the same. Perl added VT to its space char- 7803 acter set at release 5.18 and PCRE changed at release 8.34. 7804 7805 7806 SCRIPT NAMES FOR \p AND \P 7807 7808 Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, 7809 Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Car- 7810 ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei- 7811 form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero- 7812 glyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, 7813 Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, 7814 Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- 7815 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, 7816 Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Lin- 7817 ear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic, 7818 Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, 7819 Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, 7820 New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, 7821 Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, 7822 Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, 7823 Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha- 7824 vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, 7825 Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, 7826 Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, 7827 Yi. 7828 7829 7830 CHARACTER CLASSES 7831 7832 [...] positive character class 7833 [^...] negative character class 7834 [x-y] range (can be used for hex characters) 7835 [[:xxx:]] positive POSIX named set 7836 [[:^xxx:]] negative POSIX named set 7837 7838 alnum alphanumeric 7839 alpha alphabetic 7840 ascii 0-127 7841 blank space or tab 7842 cntrl control character 7843 digit decimal digit 7844 graph printing, excluding space 7845 lower lower case letter 7846 print printing, including space 7847 punct printing, excluding alphanumeric 7848 space white space 7849 upper upper case letter 7850 word same as \w 7851 xdigit hexadecimal digit 7852 7853 In PCRE, POSIX character set names recognize only ASCII characters by 7854 default, but some of them use Unicode properties if PCRE_UCP is set. 7855 You can use \Q...\E inside a character class. 7856 7857 7858 QUANTIFIERS 7859 7860 ? 0 or 1, greedy 7861 ?+ 0 or 1, possessive 7862 ?? 0 or 1, lazy 7863 * 0 or more, greedy 7864 *+ 0 or more, possessive 7865 *? 0 or more, lazy 7866 + 1 or more, greedy 7867 ++ 1 or more, possessive 7868 +? 1 or more, lazy 7869 {n} exactly n 7870 {n,m} at least n, no more than m, greedy 7871 {n,m}+ at least n, no more than m, possessive 7872 {n,m}? at least n, no more than m, lazy 7873 {n,} n or more, greedy 7874 {n,}+ n or more, possessive 7875 {n,}? n or more, lazy 7876 7877 7878 ANCHORS AND SIMPLE ASSERTIONS 7879 7880 \b word boundary 7881 \B not a word boundary 7882 ^ start of subject 7883 also after internal newline in multiline mode 7884 \A start of subject 7885 $ end of subject 7886 also before newline at end of subject 7887 also before internal newline in multiline mode 7888 \Z end of subject 7889 also before newline at end of subject 7890 \z end of subject 7891 \G first matching position in subject 7892 7893 7894 MATCH POINT RESET 7895 7896 \K reset start of match 7897 7898 \K is honoured in positive assertions, but ignored in negative ones. 7899 7900 7901 ALTERNATION 7902 7903 expr|expr|expr... 7904 7905 7906 CAPTURING 7907 7908 (...) capturing group 7909 (?<name>...) named capturing group (Perl) 7910 (?'name'...) named capturing group (Perl) 7911 (?P<name>...) named capturing group (Python) 7912 (?:...) non-capturing group 7913 (?|...) non-capturing group; reset group numbers for 7914 capturing groups in each alternative 7915 7916 7917 ATOMIC GROUPS 7918 7919 (?>...) atomic, non-capturing group 7920 7921 7922 COMMENT 7923 7924 (?#....) comment (not nestable) 7925 7926 7927 OPTION SETTING 7928 7929 (?i) caseless 7930 (?J) allow duplicate names 7931 (?m) multiline 7932 (?s) single line (dotall) 7933 (?U) default ungreedy (lazy) 7934 (?x) extended (ignore white space) 7935 (?-...) unset option(s) 7936 7937 The following are recognized only at the very start of a pattern or 7938 after one of the newline or \R options with similar syntax. More than 7939 one of them may appear. 7940 7941 (*LIMIT_MATCH=d) set the match limit to d (decimal number) 7942 (*LIMIT_RECURSION=d) set the recursion limit to d (decimal number) 7943 (*NO_AUTO_POSSESS) no auto-possessification (PCRE_NO_AUTO_POSSESS) 7944 (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE) 7945 (*UTF8) set UTF-8 mode: 8-bit library (PCRE_UTF8) 7946 (*UTF16) set UTF-16 mode: 16-bit library (PCRE_UTF16) 7947 (*UTF32) set UTF-32 mode: 32-bit library (PCRE_UTF32) 7948 (*UTF) set appropriate UTF mode for the library in use 7949 (*UCP) set PCRE_UCP (use Unicode properties for \d etc) 7950 7951 Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of 7952 the limits set by the caller of pcre_exec(), not increase them. 7953 7954 7955 NEWLINE CONVENTION 7956 7957 These are recognized only at the very start of the pattern or after 7958 option settings with a similar syntax. 7959 7960 (*CR) carriage return only 7961 (*LF) linefeed only 7962 (*CRLF) carriage return followed by linefeed 7963 (*ANYCRLF) all three of the above 7964 (*ANY) any Unicode newline sequence 7965 7966 7967 WHAT \R MATCHES 7968 7969 These are recognized only at the very start of the pattern or after 7970 option setting with a similar syntax. 7971 7972 (*BSR_ANYCRLF) CR, LF, or CRLF 7973 (*BSR_UNICODE) any Unicode newline sequence 7974 7975 7976 LOOKAHEAD AND LOOKBEHIND ASSERTIONS 7977 7978 (?=...) positive look ahead 7979 (?!...) negative look ahead 7980 (?<=...) positive look behind 7981 (?<!...) negative look behind 7982 7983 Each top-level branch of a look behind must be of a fixed length. 7984 7985 7986 BACKREFERENCES 7987 7988 \n reference by number (can be ambiguous) 7989 \gn reference by number 7990 \g{n} reference by number 7991 \g{-n} relative reference by number 7992 \k<name> reference by name (Perl) 7993 \k'name' reference by name (Perl) 7994 \g{name} reference by name (Perl) 7995 \k{name} reference by name (.NET) 7996 (?P=name) reference by name (Python) 7997 7998 7999 SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) 8000 8001 (?R) recurse whole pattern 8002 (?n) call subpattern by absolute number 8003 (?+n) call subpattern by relative number 8004 (?-n) call subpattern by relative number 8005 (?&name) call subpattern by name (Perl) 8006 (?P>name) call subpattern by name (Python) 8007 \g<name> call subpattern by name (Oniguruma) 8008 \g'name' call subpattern by name (Oniguruma) 8009 \g<n> call subpattern by absolute number (Oniguruma) 8010 \g'n' call subpattern by absolute number (Oniguruma) 8011 \g<+n> call subpattern by relative number (PCRE extension) 8012 \g'+n' call subpattern by relative number (PCRE extension) 8013 \g<-n> call subpattern by relative number (PCRE extension) 8014 \g'-n' call subpattern by relative number (PCRE extension) 8015 8016 8017 CONDITIONAL PATTERNS 8018 8019 (?(condition)yes-pattern) 8020 (?(condition)yes-pattern|no-pattern) 8021 8022 (?(n)... absolute reference condition 8023 (?(+n)... relative reference condition 8024 (?(-n)... relative reference condition 8025 (?(<name>)... named reference condition (Perl) 8026 (?('name')... named reference condition (Perl) 8027 (?(name)... named reference condition (PCRE) 8028 (?(R)... overall recursion condition 8029 (?(Rn)... specific group recursion condition 8030 (?(R&name)... specific recursion condition 8031 (?(DEFINE)... define subpattern for reference 8032 (?(assert)... assertion condition 8033 8034 8035 BACKTRACKING CONTROL 8036 8037 The following act immediately they are reached: 8038 8039 (*ACCEPT) force successful match 8040 (*FAIL) force backtrack; synonym (*F) 8041 (*MARK:NAME) set name to be passed back; synonym (*:NAME) 8042 8043 The following act only when a subsequent match failure causes a back- 8044 track to reach them. They all force a match failure, but they differ in 8045 what happens afterwards. Those that advance the start-of-match point do 8046 so only if the pattern is not anchored. 8047 8048 (*COMMIT) overall failure, no advance of starting point 8049 (*PRUNE) advance to next starting character 8050 (*PRUNE:NAME) equivalent to (*MARK:NAME)(*PRUNE) 8051 (*SKIP) advance to current matching position 8052 (*SKIP:NAME) advance to position corresponding to an earlier 8053 (*MARK:NAME); if not found, the (*SKIP) is ignored 8054 (*THEN) local failure, backtrack to next alternation 8055 (*THEN:NAME) equivalent to (*MARK:NAME)(*THEN) 8056 8057 8058 CALLOUTS 8059 8060 (?C) callout 8061 (?Cn) callout with data n 8062 8063 8064 SEE ALSO 8065 8066 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). 8067 8068 8069 AUTHOR 8070 8071 Philip Hazel 8072 University Computing Service 8073 Cambridge CB2 3QH, England. 8074 8075 8076 REVISION 8077 8078 Last updated: 08 January 2014 8079 Copyright (c) 1997-2014 University of Cambridge. 8080 ------------------------------------------------------------------------------ 8081 8082 8083 PCREUNICODE(3) Library Functions Manual PCREUNICODE(3) 8084 8085 8086 8087 NAME 8088 PCRE - Perl-compatible regular expressions 8089 8090 UTF-8, UTF-16, UTF-32, AND UNICODE PROPERTY SUPPORT 8091 8092 As well as UTF-8 support, PCRE also supports UTF-16 (from release 8.30) 8093 and UTF-32 (from release 8.32), by means of two additional libraries. 8094 They can be built as well as, or instead of, the 8-bit library. 8095 8096 8097 UTF-8 SUPPORT 8098 8099 In order process UTF-8 strings, you must build PCRE's 8-bit library 8100 with UTF support, and, in addition, you must call pcre_compile() with 8101 the PCRE_UTF8 option flag, or the pattern must start with the sequence 8102 (*UTF8) or (*UTF). When either of these is the case, both the pattern 8103 and any subject strings that are matched against it are treated as 8104 UTF-8 strings instead of strings of individual 1-byte characters. 8105 8106 8107 UTF-16 AND UTF-32 SUPPORT 8108 8109 In order process UTF-16 or UTF-32 strings, you must build PCRE's 16-bit 8110 or 32-bit library with UTF support, and, in addition, you must call 8111 pcre16_compile() or pcre32_compile() with the PCRE_UTF16 or PCRE_UTF32 8112 option flag, as appropriate. Alternatively, the pattern must start with 8113 the sequence (*UTF16), (*UTF32), as appropriate, or (*UTF), which can 8114 be used with either library. When UTF mode is set, both the pattern and 8115 any subject strings that are matched against it are treated as UTF-16 8116 or UTF-32 strings instead of strings of individual 16-bit or 32-bit 8117 characters. 8118 8119 8120 UTF SUPPORT OVERHEAD 8121 8122 If you compile PCRE with UTF support, but do not use it at run time, 8123 the library will be a bit bigger, but the additional run time overhead 8124 is limited to testing the PCRE_UTF[8|16|32] flag occasionally, so 8125 should not be very big. 8126 8127 8128 UNICODE PROPERTY SUPPORT 8129 8130 If PCRE is built with Unicode character property support (which implies 8131 UTF support), the escape sequences \p{..}, \P{..}, and \X can be used. 8132 The available properties that can be tested are limited to the general 8133 category properties such as Lu for an upper case letter or Nd for a 8134 decimal number, the Unicode script names such as Arabic or Han, and the 8135 derived properties Any and L&. Full lists is given in the pcrepattern 8136 and pcresyntax documentation. Only the short names for properties are 8137 supported. For example, \p{L} matches a letter. Its Perl synonym, 8138 \p{Letter}, is not supported. Furthermore, in Perl, many properties 8139 may optionally be prefixed by "Is", for compatibility with Perl 5.6. 8140 PCRE does not support this. 8141 8142 Validity of UTF-8 strings 8143 8144 When you set the PCRE_UTF8 flag, the byte strings passed as patterns 8145 and subjects are (by default) checked for validity on entry to the rel- 8146 evant functions. The entire string is checked before any other process- 8147 ing takes place. From release 7.3 of PCRE, the check is according the 8148 rules of RFC 3629, which are themselves derived from the Unicode speci- 8149 fication. Earlier releases of PCRE followed the rules of RFC 2279, 8150 which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The 8151 current check allows only values in the range U+0 to U+10FFFF, exclud- 8152 ing the surrogate area. (From release 8.33 the so-called "non-charac- 8153 ter" code points are no longer excluded because Unicode corrigendum #9 8154 makes it clear that they should not be.) 8155 8156 Characters in the "Surrogate Area" of Unicode are reserved for use by 8157 UTF-16, where they are used in pairs to encode codepoints with values 8158 greater than 0xFFFF. The code points that are encoded by UTF-16 pairs 8159 are available independently in the UTF-8 and UTF-32 encodings. (In 8160 other words, the whole surrogate thing is a fudge for UTF-16 which 8161 unfortunately messes up UTF-8 and UTF-32.) 8162 8163 If an invalid UTF-8 string is passed to PCRE, an error return is given. 8164 At compile time, the only additional information is the offset to the 8165 first byte of the failing character. The run-time functions pcre_exec() 8166 and pcre_dfa_exec() also pass back this information, as well as a more 8167 detailed reason code if the caller has provided memory in which to do 8168 this. 8169 8170 In some situations, you may already know that your strings are valid, 8171 and therefore want to skip these checks in order to improve perfor- 8172 mance, for example in the case of a long subject string that is being 8173 scanned repeatedly. If you set the PCRE_NO_UTF8_CHECK flag at compile 8174 time or at run time, PCRE assumes that the pattern or subject it is 8175 given (respectively) contains only valid UTF-8 codes. In this case, it 8176 does not diagnose an invalid UTF-8 string. 8177 8178 Note that passing PCRE_NO_UTF8_CHECK to pcre_compile() just disables 8179 the check for the pattern; it does not also apply to subject strings. 8180 If you want to disable the check for a subject string you must pass 8181 this option to pcre_exec() or pcre_dfa_exec(). 8182 8183 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, the 8184 result is undefined and your program may crash. 8185 8186 Validity of UTF-16 strings 8187 8188 When you set the PCRE_UTF16 flag, the strings of 16-bit data units that 8189 are passed as patterns and subjects are (by default) checked for valid- 8190 ity on entry to the relevant functions. Values other than those in the 8191 surrogate range U+D800 to U+DFFF are independent code points. Values in 8192 the surrogate range must be used in pairs in the correct manner. 8193 8194 If an invalid UTF-16 string is passed to PCRE, an error return is 8195 given. At compile time, the only additional information is the offset 8196 to the first data unit of the failing character. The run-time functions 8197 pcre16_exec() and pcre16_dfa_exec() also pass back this information, as 8198 well as a more detailed reason code if the caller has provided memory 8199 in which to do this. 8200 8201 In some situations, you may already know that your strings are valid, 8202 and therefore want to skip these checks in order to improve perfor- 8203 mance. If you set the PCRE_NO_UTF16_CHECK flag at compile time or at 8204 run time, PCRE assumes that the pattern or subject it is given (respec- 8205 tively) contains only valid UTF-16 sequences. In this case, it does not 8206 diagnose an invalid UTF-16 string. However, if an invalid string is 8207 passed, the result is undefined. 8208 8209 Validity of UTF-32 strings 8210 8211 When you set the PCRE_UTF32 flag, the strings of 32-bit data units that 8212 are passed as patterns and subjects are (by default) checked for valid- 8213 ity on entry to the relevant functions. This check allows only values 8214 in the range U+0 to U+10FFFF, excluding the surrogate area U+D800 to 8215 U+DFFF. 8216 8217 If an invalid UTF-32 string is passed to PCRE, an error return is 8218 given. At compile time, the only additional information is the offset 8219 to the first data unit of the failing character. The run-time functions 8220 pcre32_exec() and pcre32_dfa_exec() also pass back this information, as 8221 well as a more detailed reason code if the caller has provided memory 8222 in which to do this. 8223 8224 In some situations, you may already know that your strings are valid, 8225 and therefore want to skip these checks in order to improve perfor- 8226 mance. If you set the PCRE_NO_UTF32_CHECK flag at compile time or at 8227 run time, PCRE assumes that the pattern or subject it is given (respec- 8228 tively) contains only valid UTF-32 sequences. In this case, it does not 8229 diagnose an invalid UTF-32 string. However, if an invalid string is 8230 passed, the result is undefined. 8231 8232 General comments about UTF modes 8233 8234 1. Codepoints less than 256 can be specified in patterns by either 8235 braced or unbraced hexadecimal escape sequences (for example, \x{b3} or 8236 \xb3). Larger values have to use braced sequences. 8237 8238 2. Octal numbers up to \777 are recognized, and in UTF-8 mode they 8239 match two-byte characters for values greater than \177. 8240 8241 3. Repeat quantifiers apply to complete UTF characters, not to individ- 8242 ual data units, for example: \x{100}{3}. 8243 8244 4. The dot metacharacter matches one UTF character instead of a single 8245 data unit. 8246 8247 5. The escape sequence \C can be used to match a single byte in UTF-8 8248 mode, or a single 16-bit data unit in UTF-16 mode, or a single 32-bit 8249 data unit in UTF-32 mode, but its use can lead to some strange effects 8250 because it breaks up multi-unit characters (see the description of \C 8251 in the pcrepattern documentation). The use of \C is not supported in 8252 the alternative matching function pcre[16|32]_dfa_exec(), nor is it 8253 supported in UTF mode by the JIT optimization of pcre[16|32]_exec(). If 8254 JIT optimization is requested for a UTF pattern that contains \C, it 8255 will not succeed, and so the matching will be carried out by the normal 8256 interpretive function. 8257 8258 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly 8259 test characters of any code value, but, by default, the characters that 8260 PCRE recognizes as digits, spaces, or word characters remain the same 8261 set as in non-UTF mode, all with values less than 256. This remains 8262 true even when PCRE is built to include Unicode property support, 8263 because to do otherwise would slow down PCRE in many common cases. Note 8264 in particular that this applies to \b and \B, because they are defined 8265 in terms of \w and \W. If you really want to test for a wider sense of, 8266 say, "digit", you can use explicit Unicode property tests such as 8267 \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the 8268 character escapes work is changed so that Unicode properties are used 8269 to determine which characters match. There are more details in the sec- 8270 tion on generic character types in the pcrepattern documentation. 8271 8272 7. Similarly, characters that match the POSIX named character classes 8273 are all low-valued characters, unless the PCRE_UCP option is set. 8274 8275 8. However, the horizontal and vertical white space matching escapes 8276 (\h, \H, \v, and \V) do match all the appropriate Unicode characters, 8277 whether or not PCRE_UCP is set. 8278 8279 9. Case-insensitive matching applies only to characters whose values 8280 are less than 128, unless PCRE is built with Unicode property support. 8281 A few Unicode characters such as Greek sigma have more than two code- 8282 points that are case-equivalent. Up to and including PCRE release 8.31, 8283 only one-to-one case mappings were supported, but later releases (with 8284 Unicode property support) do treat as case-equivalent all versions of 8285 characters such as Greek sigma. 8286 8287 8288 AUTHOR 8289 8290 Philip Hazel 8291 University Computing Service 8292 Cambridge CB2 3QH, England. 8293 8294 8295 REVISION 8296 8297 Last updated: 27 February 2013 8298 Copyright (c) 1997-2013 University of Cambridge. 8299 ------------------------------------------------------------------------------ 8300 8301 8302 PCREJIT(3) Library Functions Manual PCREJIT(3) 8303 8304 8305 8306 NAME 8307 PCRE - Perl-compatible regular expressions 8308 8309 PCRE JUST-IN-TIME COMPILER SUPPORT 8310 8311 Just-in-time compiling is a heavyweight optimization that can greatly 8312 speed up pattern matching. However, it comes at the cost of extra pro- 8313 cessing before the match is performed. Therefore, it is of most benefit 8314 when the same pattern is going to be matched many times. This does not 8315 necessarily mean many calls of a matching function; if the pattern is 8316 not anchored, matching attempts may take place many times at various 8317 positions in the subject, even for a single call. Therefore, if the 8318 subject string is very long, it may still pay to use JIT for one-off 8319 matches. 8320 8321 JIT support applies only to the traditional Perl-compatible matching 8322 function. It does not apply when the DFA matching function is being 8323 used. The code for this support was written by Zoltan Herczeg. 8324 8325 8326 8-BIT, 16-BIT AND 32-BIT SUPPORT 8327 8328 JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE 8329 libraries. To keep this documentation simple, only the 8-bit interface 8330 is described in what follows. If you are using the 16-bit library, sub- 8331 stitute the 16-bit functions and 16-bit structures (for example, 8332 pcre16_jit_stack instead of pcre_jit_stack). If you are using the 8333 32-bit library, substitute the 32-bit functions and 32-bit structures 8334 (for example, pcre32_jit_stack instead of pcre_jit_stack). 8335 8336 8337 AVAILABILITY OF JIT SUPPORT 8338 8339 JIT support is an optional feature of PCRE. The "configure" option 8340 --enable-jit (or equivalent CMake option) must be set when PCRE is 8341 built if you want to use JIT. The support is limited to the following 8342 hardware platforms: 8343 8344 ARM v5, v7, and Thumb2 8345 Intel x86 32-bit and 64-bit 8346 MIPS 32-bit 8347 Power PC 32-bit and 64-bit 8348 SPARC 32-bit (experimental) 8349 8350 If --enable-jit is set on an unsupported platform, compilation fails. 8351 8352 A program that is linked with PCRE 8.20 or later can tell if JIT sup- 8353 port is available by calling pcre_config() with the PCRE_CONFIG_JIT 8354 option. The result is 1 when JIT is available, and 0 otherwise. How- 8355 ever, a simple program does not need to check this in order to use JIT. 8356 The normal API is implemented in a way that falls back to the interpre- 8357 tive code if JIT is not available. For programs that need the best pos- 8358 sible performance, there is also a "fast path" API that is JIT-spe- 8359 cific. 8360 8361 If your program may sometimes be linked with versions of PCRE that are 8362 older than 8.20, but you want to use JIT when it is available, you can 8363 test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT 8364 macro such as PCRE_CONFIG_JIT, for compile-time control of your code. 8365 8366 8367 SIMPLE USE OF JIT 8368 8369 You have to do two things to make use of the JIT support in the sim- 8370 plest way: 8371 8372 (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for 8373 each compiled pattern, and pass the resulting pcre_extra block to 8374 pcre_exec(). 8375 8376 (2) Use pcre_free_study() to free the pcre_extra block when it is 8377 no longer needed, instead of just freeing it yourself. This 8378 ensures that 8379 any JIT data is also freed. 8380 8381 For a program that may be linked with pre-8.20 versions of PCRE, you 8382 can insert 8383 8384 #ifndef PCRE_STUDY_JIT_COMPILE 8385 #define PCRE_STUDY_JIT_COMPILE 0 8386 #endif 8387 8388 so that no option is passed to pcre_study(), and then use something 8389 like this to free the study data: 8390 8391 #ifdef PCRE_CONFIG_JIT 8392 pcre_free_study(study_ptr); 8393 #else 8394 pcre_free(study_ptr); 8395 #endif 8396 8397 PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for 8398 complete matches. If you want to run partial matches using the 8399 PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options of pcre_exec(), you 8400 should set one or both of the following options in addition to, or 8401 instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study(): 8402 8403 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 8404 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 8405 8406 The JIT compiler generates different optimized code for each of the 8407 three modes (normal, soft partial, hard partial). When pcre_exec() is 8408 called, the appropriate code is run if it is available. Otherwise, the 8409 pattern is matched using interpretive code. 8410 8411 In some circumstances you may need to call additional functions. These 8412 are described in the section entitled "Controlling the JIT stack" 8413 below. 8414 8415 If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are 8416 ignored, and no JIT data is created. Otherwise, the compiled pattern is 8417 passed to the JIT compiler, which turns it into machine code that exe- 8418 cutes much faster than the normal interpretive code. When pcre_exec() 8419 is passed a pcre_extra block containing a pointer to JIT code of the 8420 appropriate mode (normal or hard/soft partial), it obeys that code 8421 instead of running the interpreter. The result is identical, but the 8422 compiled JIT code runs much faster. 8423 8424 There are some pcre_exec() options that are not supported for JIT exe- 8425 cution. There are also some pattern items that JIT cannot handle. 8426 Details are given below. In both cases, execution automatically falls 8427 back to the interpretive code. If you want to know whether JIT was 8428 actually used for a particular match, you should arrange for a JIT 8429 callback function to be set up as described in the section entitled 8430 "Controlling the JIT stack" below, even if you do not need to supply a 8431 non-default JIT stack. Such a callback function is called whenever JIT 8432 code is about to be obeyed. If the execution options are not right for 8433 JIT execution, the callback function is not obeyed. 8434 8435 If the JIT compiler finds an unsupported item, no JIT data is gener- 8436 ated. You can find out if JIT execution is available after studying a 8437 pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A 8438 result of 1 means that JIT compilation was successful. A result of 0 8439 means that JIT support is not available, or the pattern was not studied 8440 with PCRE_STUDY_JIT_COMPILE etc., or the JIT compiler was not able to 8441 handle the pattern. 8442 8443 Once a pattern has been studied, with or without JIT, it can be used as 8444 many times as you like for matching different subject strings. 8445 8446 8447 UNSUPPORTED OPTIONS AND PATTERN ITEMS 8448 8449 The only pcre_exec() options that are supported for JIT execution are 8450 PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK, PCRE_NOT- 8451 BOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PAR- 8452 TIAL_HARD, and PCRE_PARTIAL_SOFT. 8453 8454 The only unsupported pattern items are \C (match a single data unit) 8455 when running in a UTF mode, and a callout immediately before an asser- 8456 tion condition in a conditional group. 8457 8458 8459 RETURN VALUES FROM JIT EXECUTION 8460 8461 When a pattern is matched using JIT execution, the return values are 8462 the same as those given by the interpretive pcre_exec() code, with the 8463 addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means 8464 that the memory used for the JIT stack was insufficient. See "Control- 8465 ling the JIT stack" below for a discussion of JIT stack usage. For com- 8466 patibility with the interpretive pcre_exec() code, no more than two- 8467 thirds of the ovector argument is used for passing back captured sub- 8468 strings. 8469 8470 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if 8471 searching a very large pattern tree goes on for too long, as it is in 8472 the same circumstance when JIT is not used, but the details of exactly 8473 what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error 8474 code is never returned by JIT execution. 8475 8476 8477 SAVING AND RESTORING COMPILED PATTERNS 8478 8479 The code that is generated by the JIT compiler is architecture-spe- 8480 cific, and is also position dependent. For those reasons it cannot be 8481 saved (in a file or database) and restored later like the bytecode and 8482 other data of a compiled pattern. Saving and restoring compiled pat- 8483 terns is not something many people do. More detail about this facility 8484 is given in the pcreprecompile documentation. It should be possible to 8485 run pcre_study() on a saved and restored pattern, and thereby recreate 8486 the JIT data, but because JIT compilation uses significant resources, 8487 it is probably not worth doing this; you might as well recompile the 8488 original pattern. 8489 8490 8491 CONTROLLING THE JIT STACK 8492 8493 When the compiled JIT code runs, it needs a block of memory to use as a 8494 stack. By default, it uses 32K on the machine stack. However, some 8495 large or complicated patterns need more than this. The error 8496 PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack. 8497 Three functions are provided for managing blocks of memory for use as 8498 JIT stacks. There is further discussion about the use of JIT stacks in 8499 the section entitled "JIT stack FAQ" below. 8500 8501 The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments 8502 are a starting size and a maximum size, and it returns a pointer to an 8503 opaque structure of type pcre_jit_stack, or NULL if there is an error. 8504 The pcre_jit_stack_free() function can be used to free a stack that is 8505 no longer needed. (For the technically minded: the address space is 8506 allocated by mmap or VirtualAlloc.) 8507 8508 JIT uses far less memory for recursion than the interpretive code, and 8509 a maximum stack size of 512K to 1M should be more than enough for any 8510 pattern. 8511 8512 The pcre_assign_jit_stack() function specifies which stack JIT code 8513 should use. Its arguments are as follows: 8514 8515 pcre_extra *extra 8516 pcre_jit_callback callback 8517 void *data 8518 8519 The extra argument must be the result of studying a pattern with 8520 PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the 8521 other two options: 8522 8523 (1) If callback is NULL and data is NULL, an internal 32K block 8524 on the machine stack is used. 8525 8526 (2) If callback is NULL and data is not NULL, data must be 8527 a valid JIT stack, the result of calling pcre_jit_stack_alloc(). 8528 8529 (3) If callback is not NULL, it must point to a function that is 8530 called with data as an argument at the start of matching, in 8531 order to set up a JIT stack. If the return from the callback 8532 function is NULL, the internal 32K stack is used; otherwise the 8533 return value must be a valid JIT stack, the result of calling 8534 pcre_jit_stack_alloc(). 8535 8536 A callback function is obeyed whenever JIT code is about to be run; it 8537 is not obeyed when pcre_exec() is called with options that are incom- 8538 patible for JIT execution. A callback function can therefore be used to 8539 determine whether a match operation was executed by JIT or by the 8540 interpreter. 8541 8542 You may safely use the same JIT stack for more than one pattern (either 8543 by assigning directly or by callback), as long as the patterns are all 8544 matched sequentially in the same thread. In a multithread application, 8545 if you do not specify a JIT stack, or if you assign or pass back NULL 8546 from a callback, that is thread-safe, because each thread has its own 8547 machine stack. However, if you assign or pass back a non-NULL JIT 8548 stack, this must be a different stack for each thread so that the 8549 application is thread-safe. 8550 8551 Strictly speaking, even more is allowed. You can assign the same non- 8552 NULL stack to any number of patterns as long as they are not used for 8553 matching by multiple threads at the same time. For example, you can 8554 assign the same stack to all compiled patterns, and use a global mutex 8555 in the callback to wait until the stack is available for use. However, 8556 this is an inefficient solution, and not recommended. 8557 8558 This is a suggestion for how a multithreaded program that needs to set 8559 up non-default JIT stacks might operate: 8560 8561 During thread initalization 8562 thread_local_var = pcre_jit_stack_alloc(...) 8563 8564 During thread exit 8565 pcre_jit_stack_free(thread_local_var) 8566 8567 Use a one-line callback function 8568 return thread_local_var 8569 8570 All the functions described in this section do nothing if JIT is not 8571 available, and pcre_assign_jit_stack() does nothing unless the extra 8572 argument is non-NULL and points to a pcre_extra block that is the 8573 result of a successful study with PCRE_STUDY_JIT_COMPILE etc. 8574 8575 8576 JIT STACK FAQ 8577 8578 (1) Why do we need JIT stacks? 8579 8580 PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack 8581 where the local data of the current node is pushed before checking its 8582 child nodes. Allocating real machine stack on some platforms is diffi- 8583 cult. For example, the stack chain needs to be updated every time if we 8584 extend the stack on PowerPC. Although it is possible, its updating 8585 time overhead decreases performance. So we do the recursion in memory. 8586 8587 (2) Why don't we simply allocate blocks of memory with malloc()? 8588 8589 Modern operating systems have a nice feature: they can reserve an 8590 address space instead of allocating memory. We can safely allocate mem- 8591 ory pages inside this address space, so the stack could grow without 8592 moving memory data (this is important because of pointers). Thus we can 8593 allocate 1M address space, and use only a single memory page (usually 8594 4K) if that is enough. However, we can still grow up to 1M anytime if 8595 needed. 8596 8597 (3) Who "owns" a JIT stack? 8598 8599 The owner of the stack is the user program, not the JIT studied pattern 8600 or anything else. The user program must ensure that if a stack is used 8601 by pcre_exec(), (that is, it is assigned to the pattern currently run- 8602 ning), that stack must not be used by any other threads (to avoid over- 8603 writing the same memory area). The best practice for multithreaded pro- 8604 grams is to allocate a stack for each thread, and return this stack 8605 through the JIT callback function. 8606 8607 (4) When should a JIT stack be freed? 8608 8609 You can free a JIT stack at any time, as long as it will not be used by 8610 pcre_exec() again. When you assign the stack to a pattern, only a 8611 pointer is set. There is no reference counting or any other magic. You 8612 can free the patterns and stacks in any order, anytime. Just do not 8613 call pcre_exec() with a pattern pointing to an already freed stack, as 8614 that will cause SEGFAULT. (Also, do not free a stack currently used by 8615 pcre_exec() in another thread). You can also replace the stack for a 8616 pattern at any time. You can even free the previous stack before 8617 assigning a replacement. 8618 8619 (5) Should I allocate/free a stack every time before/after calling 8620 pcre_exec()? 8621 8622 No, because this is too costly in terms of resources. However, you 8623 could implement some clever idea which release the stack if it is not 8624 used in let's say two minutes. The JIT callback can help to achieve 8625 this without keeping a list of the currently JIT studied patterns. 8626 8627 (6) OK, the stack is for long term memory allocation. But what happens 8628 if a pattern causes stack overflow with a stack of 1M? Is that 1M kept 8629 until the stack is freed? 8630 8631 Especially on embedded sytems, it might be a good idea to release mem- 8632 ory sometimes without freeing the stack. There is no API for this at 8633 the moment. Probably a function call which returns with the currently 8634 allocated memory for any stack and another which allows releasing mem- 8635 ory (shrinking the stack) would be a good idea if someone needs this. 8636 8637 (7) This is too much of a headache. Isn't there any better solution for 8638 JIT stack handling? 8639 8640 No, thanks to Windows. If POSIX threads were used everywhere, we could 8641 throw out this complicated API. 8642 8643 8644 EXAMPLE CODE 8645 8646 This is a single-threaded example that specifies a JIT stack without 8647 using a callback. 8648 8649 int rc; 8650 int ovector[30]; 8651 pcre *re; 8652 pcre_extra *extra; 8653 pcre_jit_stack *jit_stack; 8654 8655 re = pcre_compile(pattern, 0, &error, &erroffset, NULL); 8656 /* Check for errors */ 8657 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error); 8658 jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024); 8659 /* Check for error (NULL) */ 8660 pcre_assign_jit_stack(extra, NULL, jit_stack); 8661 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30); 8662 /* Check results */ 8663 pcre_free(re); 8664 pcre_free_study(extra); 8665 pcre_jit_stack_free(jit_stack); 8666 8667 8668 JIT FAST PATH API 8669 8670 Because the API described above falls back to interpreted execution 8671 when JIT is not available, it is convenient for programs that are writ- 8672 ten for general use in many environments. However, calling JIT via 8673 pcre_exec() does have a performance impact. Programs that are written 8674 for use where JIT is known to be available, and which need the best 8675 possible performance, can instead use a "fast path" API to call JIT 8676 execution directly instead of calling pcre_exec() (obviously only for 8677 patterns that have been successfully studied by JIT). 8678 8679 The fast path function is called pcre_jit_exec(), and it takes exactly 8680 the same arguments as pcre_exec(), plus one additional argument that 8681 must point to a JIT stack. The JIT stack arrangements described above 8682 do not apply. The return values are the same as for pcre_exec(). 8683 8684 When you call pcre_exec(), as well as testing for invalid options, a 8685 number of other sanity checks are performed on the arguments. For exam- 8686 ple, if the subject pointer is NULL, or its length is negative, an 8687 immediate error is given. Also, unless PCRE_NO_UTF[8|16|32] is set, a 8688 UTF subject string is tested for validity. In the interests of speed, 8689 these checks do not happen on the JIT fast path, and if invalid data is 8690 passed, the result is undefined. 8691 8692 Bypassing the sanity checks and the pcre_exec() wrapping can give 8693 speedups of more than 10%. 8694 8695 8696 SEE ALSO 8697 8698 pcreapi(3) 8699 8700 8701 AUTHOR 8702 8703 Philip Hazel (FAQ by Zoltan Herczeg) 8704 University Computing Service 8705 Cambridge CB2 3QH, England. 8706 8707 8708 REVISION 8709 8710 Last updated: 17 March 2013 8711 Copyright (c) 1997-2013 University of Cambridge. 8712 ------------------------------------------------------------------------------ 8713 8714 8715 PCREPARTIAL(3) Library Functions Manual PCREPARTIAL(3) 8716 8717 8718 8719 NAME 8720 PCRE - Perl-compatible regular expressions 8721 8722 PARTIAL MATCHING IN PCRE 8723 8724 In normal use of PCRE, if the subject string that is passed to a match- 8725 ing function matches as far as it goes, but is too short to match the 8726 entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances 8727 where it might be helpful to distinguish this case from other cases in 8728 which there is no match. 8729 8730 Consider, for example, an application where a human is required to type 8731 in data for a field with specific formatting requirements. An example 8732 might be a date in the form ddmmmyy, defined by this pattern: 8733 8734 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ 8735 8736 If the application sees the user's keystrokes one by one, and can check 8737 that what has been typed so far is potentially valid, it is able to 8738 raise an error as soon as a mistake is made, by beeping and not 8739 reflecting the character that has been typed, for example. This immedi- 8740 ate feedback is likely to be a better user interface than a check that 8741 is delayed until the entire string has been entered. Partial matching 8742 can also be useful when the subject string is very long and is not all 8743 available at once. 8744 8745 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and 8746 PCRE_PARTIAL_HARD options, which can be set when calling any of the 8747 matching functions. For backwards compatibility, PCRE_PARTIAL is a syn- 8748 onym for PCRE_PARTIAL_SOFT. The essential difference between the two 8749 options is whether or not a partial match is preferred to an alterna- 8750 tive complete match, though the details differ between the two types of 8751 matching function. If both options are set, PCRE_PARTIAL_HARD takes 8752 precedence. 8753 8754 If you want to use partial matching with just-in-time optimized code, 8755 you must call pcre_study(), pcre16_study() or pcre32_study() with one 8756 or both of these options: 8757 8758 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 8759 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 8760 8761 PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non- 8762 partial matches on the same pattern. If the appropriate JIT study mode 8763 has not been set for a match, the interpretive matching code is used. 8764 8765 Setting a partial matching option disables two of PCRE's standard opti- 8766 mizations. PCRE remembers the last literal data unit in a pattern, and 8767 abandons matching immediately if it is not present in the subject 8768 string. This optimization cannot be used for a subject string that 8769 might match only partially. If the pattern was studied, PCRE knows the 8770 minimum length of a matching string, and does not bother to run the 8771 matching function on shorter strings. This optimization is also dis- 8772 abled for partial matching. 8773 8774 8775 PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec() 8776 8777 A partial match occurs during a call to pcre_exec() or 8778 pcre[16|32]_exec() when the end of the subject string is reached suc- 8779 cessfully, but matching cannot continue because more characters are 8780 needed. However, at least one character in the subject must have been 8781 inspected. This character need not form part of the final matched 8782 string; lookbehind assertions and the \K escape sequence provide ways 8783 of inspecting characters before the start of a matched substring. The 8784 requirement for inspecting at least one character exists because an 8785 empty string can always be matched; without such a restriction there 8786 would always be a partial match of an empty string at the end of the 8787 subject. 8788 8789 If there are at least two slots in the offsets vector when a partial 8790 match is returned, the first slot is set to the offset of the earliest 8791 character that was inspected. For convenience, the second offset points 8792 to the end of the subject so that a substring can easily be identified. 8793 If there are at least three slots in the offsets vector, the third slot 8794 is set to the offset of the character where matching started. 8795 8796 For the majority of patterns, the contents of the first and third slots 8797 will be the same. However, for patterns that contain lookbehind asser- 8798 tions, or begin with \b or \B, characters before the one where matching 8799 started may have been inspected while carrying out the match. For exam- 8800 ple, consider this pattern: 8801 8802 /(?<=abc)123/ 8803 8804 This pattern matches "123", but only if it is preceded by "abc". If the 8805 subject string is "xyzabc12", the first two offsets after a partial 8806 match are for the substring "abc12", because all these characters were 8807 inspected. However, the third offset is set to 6, because that is the 8808 offset where matching began. 8809 8810 What happens when a partial match is identified depends on which of the 8811 two partial matching options are set. 8812 8813 PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec() 8814 8815 If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre[16|32]_exec() 8816 identifies a partial match, the partial match is remembered, but match- 8817 ing continues as normal, and other alternatives in the pattern are 8818 tried. If no complete match can be found, PCRE_ERROR_PARTIAL is 8819 returned instead of PCRE_ERROR_NOMATCH. 8820 8821 This option is "soft" because it prefers a complete match over a par- 8822 tial match. All the various matching items in a pattern behave as if 8823 the subject string is potentially complete. For example, \z, \Z, and $ 8824 match at the end of the subject, as normal, and for \b and \B the end 8825 of the subject is treated as a non-alphanumeric. 8826 8827 If there is more than one partial match, the first one that was found 8828 provides the data that is returned. Consider this pattern: 8829 8830 /123\w+X|dogY/ 8831 8832 If this is matched against the subject string "abc123dog", both alter- 8833 natives fail to match, but the end of the subject is reached during 8834 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 8835 and 9, identifying "123dog" as the first partial match that was found. 8836 (In this example, there are two partial matches, because "dog" on its 8837 own partially matches the second alternative.) 8838 8839 PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec() 8840 8841 If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre[16|32]_exec(), 8842 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, 8843 without continuing to search for possible complete matches. This option 8844 is "hard" because it prefers an earlier partial match over a later com- 8845 plete match. For this reason, the assumption is made that the end of 8846 the supplied subject string may not be the true end of the available 8847 data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the 8848 subject, the result is PCRE_ERROR_PARTIAL, provided that at least one 8849 character in the subject has been inspected. 8850 8851 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject 8852 strings are checked for validity. Normally, an invalid sequence causes 8853 the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the 8854 special case of a truncated character at the end of the subject, 8855 PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when 8856 PCRE_PARTIAL_HARD is set. 8857 8858 Comparing hard and soft partial matching 8859 8860 The difference between the two partial matching options can be illus- 8861 trated by a pattern such as: 8862 8863 /dog(sbody)?/ 8864 8865 This matches either "dog" or "dogsbody", greedily (that is, it prefers 8866 the longer string if possible). If it is matched against the string 8867 "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". 8868 However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. 8869 On the other hand, if the pattern is made ungreedy the result is dif- 8870 ferent: 8871 8872 /dog(sbody)??/ 8873 8874 In this case the result is always a complete match because that is 8875 found first, and matching never continues after finding a complete 8876 match. It might be easier to follow this explanation by thinking of the 8877 two patterns like this: 8878 8879 /dog(sbody)?/ is the same as /dogsbody|dog/ 8880 /dog(sbody)??/ is the same as /dog|dogsbody/ 8881 8882 The second pattern will never match "dogsbody", because it will always 8883 find the shorter match first. 8884 8885 8886 PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec() 8887 8888 The DFA functions move along the subject string character by character, 8889 without backtracking, searching for all possible matches simultane- 8890 ously. If the end of the subject is reached before the end of the pat- 8891 tern, there is the possibility of a partial match, again provided that 8892 at least one character has been inspected. 8893 8894 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if 8895 there have been no complete matches. Otherwise, the complete matches 8896 are returned. However, if PCRE_PARTIAL_HARD is set, a partial match 8897 takes precedence over any complete matches. The portion of the string 8898 that was inspected when the longest partial match was found is set as 8899 the first matching string, provided there are at least two slots in the 8900 offsets vector. 8901 8902 Because the DFA functions always search for all possible matches, and 8903 there is no difference between greedy and ungreedy repetition, their 8904 behaviour is different from the standard functions when PCRE_PAR- 8905 TIAL_HARD is set. Consider the string "dog" matched against the 8906 ungreedy pattern shown above: 8907 8908 /dog(sbody)??/ 8909 8910 Whereas the standard functions stop as soon as they find the complete 8911 match for "dog", the DFA functions also find the partial match for 8912 "dogsbody", and so return that when PCRE_PARTIAL_HARD is set. 8913 8914 8915 PARTIAL MATCHING AND WORD BOUNDARIES 8916 8917 If a pattern ends with one of sequences \b or \B, which test for word 8918 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter- 8919 intuitive results. Consider this pattern: 8920 8921 /\bcat\b/ 8922 8923 This matches "cat", provided there is a word boundary at either end. If 8924 the subject string is "the cat", the comparison of the final "t" with a 8925 following character cannot take place, so a partial match is found. 8926 However, normal matching carries on, and \b matches at the end of the 8927 subject when the last character is a letter, so a complete match is 8928 found. The result, therefore, is not PCRE_ERROR_PARTIAL. Using 8929 PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because 8930 then the partial match takes precedence. 8931 8932 8933 FORMERLY RESTRICTED PATTERNS 8934 8935 For releases of PCRE prior to 8.00, because of the way certain internal 8936 optimizations were implemented in the pcre_exec() function, the 8937 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be 8938 used with all patterns. From release 8.00 onwards, the restrictions no 8939 longer apply, and partial matching with can be requested for any pat- 8940 tern. 8941 8942 Items that were formerly restricted were repeated single characters and 8943 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did 8944 not conform to the restrictions, pcre_exec() returned the error code 8945 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The 8946 PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled 8947 pattern can be used for partial matching now always returns 1. 8948 8949 8950 EXAMPLE OF PARTIAL MATCHING USING PCRETEST 8951 8952 If the escape sequence \P is present in a pcretest data line, the 8953 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of 8954 pcretest that uses the date example quoted above: 8955 8956 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 8957 data> 25jun04\P 8958 0: 25jun04 8959 1: jun 8960 data> 25dec3\P 8961 Partial match: 23dec3 8962 data> 3ju\P 8963 Partial match: 3ju 8964 data> 3juj\P 8965 No match 8966 data> j\P 8967 No match 8968 8969 The first data string is matched completely, so pcretest shows the 8970 matched substrings. The remaining four strings do not match the com- 8971 plete pattern, but the first two are partial matches. Similar output is 8972 obtained if DFA matching is used. 8973 8974 If the escape sequence \P is present more than once in a pcretest data 8975 line, the PCRE_PARTIAL_HARD option is set for the match. 8976 8977 8978 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec() 8979 8980 When a partial match has been found using a DFA matching function, it 8981 is possible to continue the match by providing additional subject data 8982 and calling the function again with the same compiled regular expres- 8983 sion, this time setting the PCRE_DFA_RESTART option. You must pass the 8984 same working space as before, because this is where details of the pre- 8985 vious partial match are stored. Here is an example using pcretest, 8986 using the \R escape sequence to set the PCRE_DFA_RESTART option (\D 8987 specifies the use of the DFA matching function): 8988 8989 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 8990 data> 23ja\P\D 8991 Partial match: 23ja 8992 data> n05\R\D 8993 0: n05 8994 8995 The first call has "23ja" as the subject, and requests partial match- 8996 ing; the second call has "n05" as the subject for the continued 8997 (restarted) match. Notice that when the match is complete, only the 8998 last part is shown; PCRE does not retain the previously partially- 8999 matched string. It is up to the calling program to do that if it needs 9000 to. 9001 9002 That means that, for an unanchored pattern, if a continued match fails, 9003 it is not possible to try again at a new starting point. All this 9004 facility is capable of doing is continuing with the previous match 9005 attempt. In the previous example, if the second set of data is "ug23" 9006 the result is no match, even though there would be a match for "aug23" 9007 if the entire string were given at once. Depending on the application, 9008 this may or may not be what you want. The only way to allow for start- 9009 ing again at the next character is to retain the matched part of the 9010 subject and try a new complete match. 9011 9012 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with 9013 PCRE_DFA_RESTART to continue partial matching over multiple segments. 9014 This facility can be used to pass very long subject strings to the DFA 9015 matching functions. 9016 9017 9018 MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec() 9019 9020 From release 8.00, the standard matching functions can also be used to 9021 do multi-segment matching. Unlike the DFA functions, it is not possible 9022 to restart the previous match with a new segment of data. Instead, new 9023 data must be added to the previous subject string, and the entire match 9024 re-run, starting from the point where the partial match occurred. Ear- 9025 lier data can be discarded. 9026 9027 It is best to use PCRE_PARTIAL_HARD in this situation, because it does 9028 not treat the end of a segment as the end of the subject when matching 9029 \z, \Z, \b, \B, and $. Consider an unanchored pattern that matches 9030 dates: 9031 9032 re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ 9033 data> The date is 23ja\P\P 9034 Partial match: 23ja 9035 9036 At this stage, an application could discard the text preceding "23ja", 9037 add on text from the next segment, and call the matching function 9038 again. Unlike the DFA matching functions, the entire matching string 9039 must always be available, and the complete matching process occurs for 9040 each call, so more memory and more processing time is needed. 9041 9042 Note: If the pattern contains lookbehind assertions, or \K, or starts 9043 with \b or \B, the string that is returned for a partial match includes 9044 characters that precede the start of what would be returned for a com- 9045 plete match, because it contains all the characters that were inspected 9046 during the partial match. 9047 9048 9049 ISSUES WITH MULTI-SEGMENT MATCHING 9050 9051 Certain types of pattern may give problems with multi-segment matching, 9052 whichever matching function is used. 9053 9054 1. If the pattern contains a test for the beginning of a line, you need 9055 to pass the PCRE_NOTBOL option when the subject string for any call 9056 does start at the beginning of a line. There is also a PCRE_NOTEOL 9057 option, but in practice when doing multi-segment matching you should be 9058 using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL. 9059 9060 2. Lookbehind assertions that have already been obeyed are catered for 9061 in the offsets that are returned for a partial match. However a lookbe- 9062 hind assertion later in the pattern could require even earlier charac- 9063 ters to be inspected. You can handle this case by using the 9064 PCRE_INFO_MAXLOOKBEHIND option of the pcre_fullinfo() or 9065 pcre[16|32]_fullinfo() functions to obtain the length of the longest 9066 lookbehind in the pattern. This length is given in characters, not 9067 bytes. If you always retain at least that many characters before the 9068 partially matched string, all should be well. (Of course, near the 9069 start of the subject, fewer characters may be present; in that case all 9070 characters should be retained.) 9071 9072 From release 8.33, there is a more accurate way of deciding which char- 9073 acters to retain. Instead of subtracting the length of the longest 9074 lookbehind from the earliest inspected character (offsets[0]), the 9075 match start position (offsets[2]) should be used, and the next match 9076 attempt started at the offsets[2] character by setting the startoffset 9077 argument of pcre_exec() or pcre_dfa_exec(). 9078 9079 For example, if the pattern "(?<=123)abc" is partially matched against 9080 the string "xx123a", the three offset values returned are 2, 6, and 5. 9081 This indicates that the matching process that gave a partial match 9082 started at offset 5, but the characters "123a" were all inspected. The 9083 maximum lookbehind for that pattern is 3, so taking that away from 5 9084 shows that we need only keep "123a", and the next match attempt can be 9085 started at offset 3 (that is, at "a") when further characters have been 9086 added. When the match start is not the earliest inspected character, 9087 pcretest shows it explicitly: 9088 9089 re> "(?<=123)abc" 9090 data> xx123a\P\P 9091 Partial match at offset 5: 123a 9092 9093 3. Because a partial match must always contain at least one character, 9094 what might be considered a partial match of an empty string actually 9095 gives a "no match" result. For example: 9096 9097 re> /c(?<=abc)x/ 9098 data> ab\P 9099 No match 9100 9101 If the next segment begins "cx", a match should be found, but this will 9102 only happen if characters from the previous segment are retained. For 9103 this reason, a "no match" result should be interpreted as "partial 9104 match of an empty string" when the pattern contains lookbehinds. 9105 9106 4. Matching a subject string that is split into multiple segments may 9107 not always produce exactly the same result as matching over one single 9108 long string, especially when PCRE_PARTIAL_SOFT is used. The section 9109 "Partial Matching and Word Boundaries" above describes an issue that 9110 arises if the pattern ends with \b or \B. Another kind of difference 9111 may occur when there are multiple matching possibilities, because (for 9112 PCRE_PARTIAL_SOFT) a partial match result is given only when there are 9113 no completed matches. This means that as soon as the shortest match has 9114 been found, continuation to a new subject segment is no longer possi- 9115 ble. Consider again this pcretest example: 9116 9117 re> /dog(sbody)?/ 9118 data> dogsb\P 9119 0: dog 9120 data> do\P\D 9121 Partial match: do 9122 data> gsb\R\P\D 9123 0: g 9124 data> dogsbody\D 9125 0: dogsbody 9126 1: dog 9127 9128 The first data line passes the string "dogsb" to a standard matching 9129 function, setting the PCRE_PARTIAL_SOFT option. Although the string is 9130 a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL, 9131 because the shorter string "dog" is a complete match. Similarly, when 9132 the subject is presented to a DFA matching function in several parts 9133 ("do" and "gsb" being the first two) the match stops when "dog" has 9134 been found, and it is not possible to continue. On the other hand, if 9135 "dogsbody" is presented as a single string, a DFA matching function 9136 finds both matches. 9137 9138 Because of these problems, it is best to use PCRE_PARTIAL_HARD when 9139 matching multi-segment data. The example above then behaves differ- 9140 ently: 9141 9142 re> /dog(sbody)?/ 9143 data> dogsb\P\P 9144 Partial match: dogsb 9145 data> do\P\D 9146 Partial match: do 9147 data> gsb\R\P\P\D 9148 Partial match: gsb 9149 9150 5. Patterns that contain alternatives at the top level which do not all 9151 start with the same pattern item may not work as expected when 9152 PCRE_DFA_RESTART is used. For example, consider this pattern: 9153 9154 1234|3789 9155 9156 If the first part of the subject is "ABC123", a partial match of the 9157 first alternative is found at offset 3. There is no partial match for 9158 the second alternative, because such a match does not start at the same 9159 point in the subject string. Attempting to continue with the string 9160 "7890" does not yield a match because only those alternatives that 9161 match at one point in the subject are remembered. The problem arises 9162 because the start of the second alternative matches within the first 9163 alternative. There is no problem with anchored patterns or patterns 9164 such as: 9165 9166 1234|ABCD 9167 9168 where no string can be a partial match for both alternatives. This is 9169 not a problem if a standard matching function is used, because the 9170 entire match has to be rerun each time: 9171 9172 re> /1234|3789/ 9173 data> ABC123\P\P 9174 Partial match: 123 9175 data> 1237890 9176 0: 3789 9177 9178 Of course, instead of using PCRE_DFA_RESTART, the same technique of re- 9179 running the entire match can also be used with the DFA matching func- 9180 tions. Another possibility is to work with two buffers. If a partial 9181 match at offset n in the first buffer is followed by "no match" when 9182 PCRE_DFA_RESTART is used on the second buffer, you can then try a new 9183 match starting at offset n+1 in the first buffer. 9184 9185 9186 AUTHOR 9187 9188 Philip Hazel 9189 University Computing Service 9190 Cambridge CB2 3QH, England. 9191 9192 9193 REVISION 9194 9195 Last updated: 02 July 2013 9196 Copyright (c) 1997-2013 University of Cambridge. 9197 ------------------------------------------------------------------------------ 9198 9199 9200 PCREPRECOMPILE(3) Library Functions Manual PCREPRECOMPILE(3) 9201 9202 9203 9204 NAME 9205 PCRE - Perl-compatible regular expressions 9206 9207 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS 9208 9209 If you are running an application that uses a large number of regular 9210 expression patterns, it may be useful to store them in a precompiled 9211 form instead of having to compile them every time the application is 9212 run. If you are not using any private character tables (see the 9213 pcre_maketables() documentation), this is relatively straightforward. 9214 If you are using private tables, it is a little bit more complicated. 9215 However, if you are using the just-in-time optimization feature, it is 9216 not possible to save and reload the JIT data. 9217 9218 If you save compiled patterns to a file, you can copy them to a differ- 9219 ent host and run them there. If the two hosts have different endianness 9220 (byte order), you should run the pcre[16|32]_pat- 9221 tern_to_host_byte_order() function on the new host before trying to 9222 match the pattern. The matching functions return PCRE_ERROR_BADENDIAN- 9223 NESS if they detect a pattern with the wrong endianness. 9224 9225 Compiling regular expressions with one version of PCRE for use with a 9226 different version is not guaranteed to work and may cause crashes, and 9227 saving and restoring a compiled pattern loses any JIT optimization 9228 data. 9229 9230 9231 SAVING A COMPILED PATTERN 9232 9233 The value returned by pcre[16|32]_compile() points to a single block of 9234 memory that holds the compiled pattern and associated data. You can 9235 find the length of this block in bytes by calling 9236 pcre[16|32]_fullinfo() with an argument of PCRE_INFO_SIZE. You can then 9237 save the data in any appropriate manner. Here is sample code for the 9238 8-bit library that compiles a pattern and writes it to a file. It 9239 assumes that the variable fd refers to a file that is open for output: 9240 9241 int erroroffset, rc, size; 9242 char *error; 9243 pcre *re; 9244 9245 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); 9246 if (re == NULL) { ... handle errors ... } 9247 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); 9248 if (rc < 0) { ... handle errors ... } 9249 rc = fwrite(re, 1, size, fd); 9250 if (rc != size) { ... handle errors ... } 9251 9252 In this example, the bytes that comprise the compiled pattern are 9253 copied exactly. Note that this is binary data that may contain any of 9254 the 256 possible byte values. On systems that make a distinction 9255 between binary and non-binary data, be sure that the file is opened for 9256 binary output. 9257 9258 If you want to write more than one pattern to a file, you will have to 9259 devise a way of separating them. For binary data, preceding each pat- 9260 tern with its length is probably the most straightforward approach. 9261 Another possibility is to write out the data in hexadecimal instead of 9262 binary, one pattern to a line. 9263 9264 Saving compiled patterns in a file is only one possible way of storing 9265 them for later use. They could equally well be saved in a database, or 9266 in the memory of some daemon process that passes them via sockets to 9267 the processes that want them. 9268 9269 If the pattern has been studied, it is also possible to save the normal 9270 study data in a similar way to the compiled pattern itself. However, if 9271 the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre- 9272 ated cannot be saved because it is too dependent on the current envi- 9273 ronment. When studying generates additional information, 9274 pcre[16|32]_study() returns a pointer to a pcre[16|32]_extra data 9275 block. Its format is defined in the section on matching a pattern in 9276 the pcreapi documentation. The study_data field points to the binary 9277 study data, and this is what you must save (not the pcre[16|32]_extra 9278 block itself). The length of the study data can be obtained by calling 9279 pcre[16|32]_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. Remem- 9280 ber to check that pcre[16|32]_study() did return a non-NULL value 9281 before trying to save the study data. 9282 9283 9284 RE-USING A PRECOMPILED PATTERN 9285 9286 Re-using a precompiled pattern is straightforward. Having reloaded it 9287 into main memory, called pcre[16|32]_pattern_to_host_byte_order() if 9288 necessary, you pass its pointer to pcre[16|32]_exec() or 9289 pcre[16|32]_dfa_exec() in the usual way. 9290 9291 However, if you passed a pointer to custom character tables when the 9292 pattern was compiled (the tableptr argument of pcre[16|32]_compile()), 9293 you must now pass a similar pointer to pcre[16|32]_exec() or 9294 pcre[16|32]_dfa_exec(), because the value saved with the compiled pat- 9295 tern will obviously be nonsense. A field in a pcre[16|32]_extra() block 9296 is used to pass this data, as described in the section on matching a 9297 pattern in the pcreapi documentation. 9298 9299 Warning: The tables that pcre_exec() and pcre_dfa_exec() use must be 9300 the same as those that were used when the pattern was compiled. If this 9301 is not the case, the behaviour is undefined. 9302 9303 If you did not provide custom character tables when the pattern was 9304 compiled, the pointer in the compiled pattern is NULL, which causes the 9305 matching functions to use PCRE's internal tables. Thus, you do not need 9306 to take any special action at run time in this case. 9307 9308 If you saved study data with the compiled pattern, you need to create 9309 your own pcre[16|32]_extra data block and set the study_data field to 9310 point to the reloaded study data. You must also set the 9311 PCRE_EXTRA_STUDY_DATA bit in the flags field to indicate that study 9312 data is present. Then pass the pcre[16|32]_extra block to the matching 9313 function in the usual way. If the pattern was studied for just-in-time 9314 optimization, that data cannot be saved, and so is lost by a 9315 save/restore cycle. 9316 9317 9318 COMPATIBILITY WITH DIFFERENT PCRE RELEASES 9319 9320 In general, it is safest to recompile all saved patterns when you 9321 update to a new PCRE release, though not all updates actually require 9322 this. 9323 9324 9325 AUTHOR 9326 9327 Philip Hazel 9328 University Computing Service 9329 Cambridge CB2 3QH, England. 9330 9331 9332 REVISION 9333 9334 Last updated: 12 November 2013 9335 Copyright (c) 1997-2013 University of Cambridge. 9336 ------------------------------------------------------------------------------ 9337 9338 9339 PCREPERFORM(3) Library Functions Manual PCREPERFORM(3) 9340 9341 9342 9343 NAME 9344 PCRE - Perl-compatible regular expressions 9345 9346 PCRE PERFORMANCE 9347 9348 Two aspects of performance are discussed below: memory usage and pro- 9349 cessing time. The way you express your pattern as a regular expression 9350 can affect both of them. 9351 9352 9353 COMPILED PATTERN MEMORY USAGE 9354 9355 Patterns are compiled by PCRE into a reasonably efficient interpretive 9356 code, so that most simple patterns do not use much memory. However, 9357 there is one case where the memory usage of a compiled pattern can be 9358 unexpectedly large. If a parenthesized subpattern has a quantifier with 9359 a minimum greater than 1 and/or a limited maximum, the whole subpattern 9360 is repeated in the compiled code. For example, the pattern 9361 9362 (abc|def){2,4} 9363 9364 is compiled as if it were 9365 9366 (abc|def)(abc|def)((abc|def)(abc|def)?)? 9367 9368 (Technical aside: It is done this way so that backtrack points within 9369 each of the repetitions can be independently maintained.) 9370 9371 For regular expressions whose quantifiers use only small numbers, this 9372 is not usually a problem. However, if the numbers are large, and par- 9373 ticularly if such repetitions are nested, the memory usage can become 9374 an embarrassment. For example, the very simple pattern 9375 9376 ((ab){1,1000}c){1,3} 9377 9378 uses 51K bytes when compiled using the 8-bit library. When PCRE is com- 9379 piled with its default internal pointer size of two bytes, the size 9380 limit on a compiled pattern is 64K data units, and this is reached with 9381 the above pattern if the outer repetition is increased from 3 to 4. 9382 PCRE can be compiled to use larger internal pointers and thus handle 9383 larger compiled patterns, but it is better to try to rewrite your pat- 9384 tern to use less memory if you can. 9385 9386 One way of reducing the memory usage for such patterns is to make use 9387 of PCRE's "subroutine" facility. Re-writing the above pattern as 9388 9389 ((ab)(?2){0,999}c)(?1){0,2} 9390 9391 reduces the memory requirements to 18K, and indeed it remains under 20K 9392 even with the outer repetition increased to 100. However, this pattern 9393 is not exactly equivalent, because the "subroutine" calls are treated 9394 as atomic groups into which there can be no backtracking if there is a 9395 subsequent matching failure. Therefore, PCRE cannot do this kind of 9396 rewriting automatically. Furthermore, there is a noticeable loss of 9397 speed when executing the modified pattern. Nevertheless, if the atomic 9398 grouping is not a problem and the loss of speed is acceptable, this 9399 kind of rewriting will allow you to process patterns that PCRE cannot 9400 otherwise handle. 9401 9402 9403 STACK USAGE AT RUN TIME 9404 9405 When pcre_exec() or pcre[16|32]_exec() is used for matching, certain 9406 kinds of pattern can cause it to use large amounts of the process 9407 stack. In some environments the default process stack is quite small, 9408 and if it runs out the result is often SIGSEGV. This issue is probably 9409 the most frequently raised problem with PCRE. Rewriting your pattern 9410 can often help. The pcrestack documentation discusses this issue in 9411 detail. 9412 9413 9414 PROCESSING TIME 9415 9416 Certain items in regular expression patterns are processed more effi- 9417 ciently than others. It is more efficient to use a character class like 9418 [aeiou] than a set of single-character alternatives such as 9419 (a|e|i|o|u). In general, the simplest construction that provides the 9420 required behaviour is usually the most efficient. Jeffrey Friedl's book 9421 contains a lot of useful general discussion about optimizing regular 9422 expressions for efficient performance. This document contains a few 9423 observations about PCRE. 9424 9425 Using Unicode character properties (the \p, \P, and \X escapes) is 9426 slow, because PCRE has to use a multi-stage table lookup whenever it 9427 needs a character's property. If you can find an alternative pattern 9428 that does not use character properties, it will probably be faster. 9429 9430 By default, the escape sequences \b, \d, \s, and \w, and the POSIX 9431 character classes such as [:alpha:] do not use Unicode properties, 9432 partly for backwards compatibility, and partly for performance reasons. 9433 However, you can set PCRE_UCP if you want Unicode character properties 9434 to be used. This can double the matching time for items such as \d, 9435 when matched with a traditional matching function; the performance loss 9436 is less with a DFA matching function, and in both cases there is not 9437 much difference for \b. 9438 9439 When a pattern begins with .* not in parentheses, or in parentheses 9440 that are not the subject of a backreference, and the PCRE_DOTALL option 9441 is set, the pattern is implicitly anchored by PCRE, since it can match 9442 only at the start of a subject string. However, if PCRE_DOTALL is not 9443 set, PCRE cannot make this optimization, because the . metacharacter 9444 does not then match a newline, and if the subject string contains new- 9445 lines, the pattern may match from the character immediately following 9446 one of them instead of from the very start. For example, the pattern 9447 9448 .*second 9449 9450 matches the subject "first\nand second" (where \n stands for a newline 9451 character), with the match starting at the seventh character. In order 9452 to do this, PCRE has to retry the match starting after every newline in 9453 the subject. 9454 9455 If you are using such a pattern with subject strings that do not con- 9456 tain newlines, the best performance is obtained by setting PCRE_DOTALL, 9457 or starting the pattern with ^.* or ^.*? to indicate explicit anchor- 9458 ing. That saves PCRE from having to scan along the subject looking for 9459 a newline to restart at. 9460 9461 Beware of patterns that contain nested indefinite repeats. These can 9462 take a long time to run when applied to a string that does not match. 9463 Consider the pattern fragment 9464 9465 ^(a+)* 9466 9467 This can match "aaaa" in 16 different ways, and this number increases 9468 very rapidly as the string gets longer. (The * repeat can match 0, 1, 9469 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + 9470 repeats can match different numbers of times.) When the remainder of 9471 the pattern is such that the entire match is going to fail, PCRE has in 9472 principle to try every possible variation, and this can take an 9473 extremely long time, even for relatively short strings. 9474 9475 An optimization catches some of the more simple cases such as 9476 9477 (a+)*b 9478 9479 where a literal character follows. Before embarking on the standard 9480 matching procedure, PCRE checks that there is a "b" later in the sub- 9481 ject string, and if there is not, it fails the match immediately. How- 9482 ever, when there is no following literal this optimization cannot be 9483 used. You can see the difference by comparing the behaviour of 9484 9485 (a+)*\d 9486 9487 with the pattern above. The former gives a failure almost instantly 9488 when applied to a whole line of "a" characters, whereas the latter 9489 takes an appreciable time with strings longer than about 20 characters. 9490 9491 In many cases, the solution to this kind of performance issue is to use 9492 an atomic group or a possessive quantifier. 9493 9494 9495 AUTHOR 9496 9497 Philip Hazel 9498 University Computing Service 9499 Cambridge CB2 3QH, England. 9500 9501 9502 REVISION 9503 9504 Last updated: 25 August 2012 9505 Copyright (c) 1997-2012 University of Cambridge. 9506 ------------------------------------------------------------------------------ 9507 9508 9509 PCREPOSIX(3) Library Functions Manual PCREPOSIX(3) 9510 9511 9512 9513 NAME 9514 PCRE - Perl-compatible regular expressions. 9515 9516 SYNOPSIS 9517 9518 #include <pcreposix.h> 9519 9520 int regcomp(regex_t *preg, const char *pattern, 9521 int cflags); 9522 9523 int regexec(regex_t *preg, const char *string, 9524 size_t nmatch, regmatch_t pmatch[], int eflags); 9525 size_t regerror(int errcode, const regex_t *preg, 9526 char *errbuf, size_t errbuf_size); 9527 9528 void regfree(regex_t *preg); 9529 9530 9531 DESCRIPTION 9532 9533 This set of functions provides a POSIX-style API for the PCRE regular 9534 expression 8-bit library. See the pcreapi documentation for a descrip- 9535 tion of PCRE's native API, which contains much additional functional- 9536 ity. There is no POSIX-style wrapper for PCRE's 16-bit and 32-bit 9537 library. 9538 9539 The functions described here are just wrapper functions that ultimately 9540 call the PCRE native API. Their prototypes are defined in the 9541 pcreposix.h header file, and on Unix systems the library itself is 9542 called pcreposix.a, so can be accessed by adding -lpcreposix to the 9543 command for linking an application that uses them. Because the POSIX 9544 functions call the native ones, it is also necessary to add -lpcre. 9545 9546 I have implemented only those POSIX option bits that can be reasonably 9547 mapped to PCRE native options. In addition, the option REG_EXTENDED is 9548 defined with the value zero. This has no effect, but since programs 9549 that are written to the POSIX interface often use it, this makes it 9550 easier to slot in PCRE as a replacement library. Other POSIX options 9551 are not even defined. 9552 9553 There are also some other options that are not defined by POSIX. These 9554 have been added at the request of users who want to make use of certain 9555 PCRE-specific features via the POSIX calling interface. 9556 9557 When PCRE is called via these functions, it is only the API that is 9558 POSIX-like in style. The syntax and semantics of the regular expres- 9559 sions themselves are still those of Perl, subject to the setting of 9560 various PCRE options, as described below. "POSIX-like in style" means 9561 that the API approximates to the POSIX definition; it is not fully 9562 POSIX-compatible, and in multi-byte encoding domains it is probably 9563 even less compatible. 9564 9565 The header for these functions is supplied as pcreposix.h to avoid any 9566 potential clash with other POSIX libraries. It can, of course, be 9567 renamed or aliased as regex.h, which is the "correct" name. It provides 9568 two structure types, regex_t for compiled internal forms, and reg- 9569 match_t for returning captured substrings. It also defines some con- 9570 stants whose names start with "REG_"; these are used for setting 9571 options and identifying error codes. 9572 9573 9574 COMPILING A PATTERN 9575 9576 The function regcomp() is called to compile a pattern into an internal 9577 form. The pattern is a C string terminated by a binary zero, and is 9578 passed in the argument pattern. The preg argument is a pointer to a 9579 regex_t structure that is used as a base for storing information about 9580 the compiled regular expression. 9581 9582 The argument cflags is either zero, or contains one or more of the bits 9583 defined by the following macros: 9584 9585 REG_DOTALL 9586 9587 The PCRE_DOTALL option is set when the regular expression is passed for 9588 compilation to the native function. Note that REG_DOTALL is not part of 9589 the POSIX standard. 9590 9591 REG_ICASE 9592 9593 The PCRE_CASELESS option is set when the regular expression is passed 9594 for compilation to the native function. 9595 9596 REG_NEWLINE 9597 9598 The PCRE_MULTILINE option is set when the regular expression is passed 9599 for compilation to the native function. Note that this does not mimic 9600 the defined POSIX behaviour for REG_NEWLINE (see the following sec- 9601 tion). 9602 9603 REG_NOSUB 9604 9605 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is 9606 passed for compilation to the native function. In addition, when a pat- 9607 tern that is compiled with this flag is passed to regexec() for match- 9608 ing, the nmatch and pmatch arguments are ignored, and no captured 9609 strings are returned. 9610 9611 REG_UCP 9612 9613 The PCRE_UCP option is set when the regular expression is passed for 9614 compilation to the native function. This causes PCRE to use Unicode 9615 properties when matchine \d, \w, etc., instead of just recognizing 9616 ASCII values. Note that REG_UTF8 is not part of the POSIX standard. 9617 9618 REG_UNGREEDY 9619 9620 The PCRE_UNGREEDY option is set when the regular expression is passed 9621 for compilation to the native function. Note that REG_UNGREEDY is not 9622 part of the POSIX standard. 9623 9624 REG_UTF8 9625 9626 The PCRE_UTF8 option is set when the regular expression is passed for 9627 compilation to the native function. This causes the pattern itself and 9628 all data strings used for matching it to be treated as UTF-8 strings. 9629 Note that REG_UTF8 is not part of the POSIX standard. 9630 9631 In the absence of these flags, no options are passed to the native 9632 function. This means the the regex is compiled with PCRE default 9633 semantics. In particular, the way it handles newline characters in the 9634 subject string is the Perl way, not the POSIX way. Note that setting 9635 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. 9636 It does not affect the way newlines are matched by . (they are not) or 9637 by a negative class such as [^a] (they are). 9638 9639 The yield of regcomp() is zero on success, and non-zero otherwise. The 9640 preg structure is filled in on success, and one member of the structure 9641 is public: re_nsub contains the number of capturing subpatterns in the 9642 regular expression. Various error codes are defined in the header file. 9643 9644 NOTE: If the yield of regcomp() is non-zero, you must not attempt to 9645 use the contents of the preg structure. If, for example, you pass it to 9646 regexec(), the result is undefined and your program is likely to crash. 9647 9648 9649 MATCHING NEWLINE CHARACTERS 9650 9651 This area is not simple, because POSIX and Perl take different views of 9652 things. It is not possible to get PCRE to obey POSIX semantics, but 9653 then PCRE was never intended to be a POSIX engine. The following table 9654 lists the different possibilities for matching newline characters in 9655 PCRE: 9656 9657 Default Change with 9658 9659 . matches newline no PCRE_DOTALL 9660 newline matches [^a] yes not changeable 9661 $ matches \n at end yes PCRE_DOLLARENDONLY 9662 $ matches \n in middle no PCRE_MULTILINE 9663 ^ matches \n in middle no PCRE_MULTILINE 9664 9665 This is the equivalent table for POSIX: 9666 9667 Default Change with 9668 9669 . matches newline yes REG_NEWLINE 9670 newline matches [^a] yes REG_NEWLINE 9671 $ matches \n at end no REG_NEWLINE 9672 $ matches \n in middle no REG_NEWLINE 9673 ^ matches \n in middle no REG_NEWLINE 9674 9675 PCRE's behaviour is the same as Perl's, except that there is no equiva- 9676 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is 9677 no way to stop newline from matching [^a]. 9678 9679 The default POSIX newline handling can be obtained by setting 9680 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE 9681 behave exactly as for the REG_NEWLINE action. 9682 9683 9684 MATCHING A PATTERN 9685 9686 The function regexec() is called to match a compiled pattern preg 9687 against a given string, which is by default terminated by a zero byte 9688 (but see REG_STARTEND below), subject to the options in eflags. These 9689 can be: 9690 9691 REG_NOTBOL 9692 9693 The PCRE_NOTBOL option is set when calling the underlying PCRE matching 9694 function. 9695 9696 REG_NOTEMPTY 9697 9698 The PCRE_NOTEMPTY option is set when calling the underlying PCRE match- 9699 ing function. Note that REG_NOTEMPTY is not part of the POSIX standard. 9700 However, setting this option can give more POSIX-like behaviour in some 9701 situations. 9702 9703 REG_NOTEOL 9704 9705 The PCRE_NOTEOL option is set when calling the underlying PCRE matching 9706 function. 9707 9708 REG_STARTEND 9709 9710 The string is considered to start at string + pmatch[0].rm_so and to 9711 have a terminating NUL located at string + pmatch[0].rm_eo (there need 9712 not actually be a NUL at that location), regardless of the value of 9713 nmatch. This is a BSD extension, compatible with but not specified by 9714 IEEE Standard 1003.2 (POSIX.2), and should be used with caution in 9715 software intended to be portable to other systems. Note that a non-zero 9716 rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location 9717 of the string, not how it is matched. 9718 9719 If the pattern was compiled with the REG_NOSUB flag, no data about any 9720 matched strings is returned. The nmatch and pmatch arguments of 9721 regexec() are ignored. 9722 9723 If the value of nmatch is zero, or if the value pmatch is NULL, no data 9724 about any matched strings is returned. 9725 9726 Otherwise,the portion of the string that was matched, and also any cap- 9727 tured substrings, are returned via the pmatch argument, which points to 9728 an array of nmatch structures of type regmatch_t, containing the mem- 9729 bers rm_so and rm_eo. These contain the offset to the first character 9730 of each substring and the offset to the first character after the end 9731 of each substring, respectively. The 0th element of the vector relates 9732 to the entire portion of string that was matched; subsequent elements 9733 relate to the capturing subpatterns of the regular expression. Unused 9734 entries in the array have both structure members set to -1. 9735 9736 A successful match yields a zero return; various error codes are 9737 defined in the header file, of which REG_NOMATCH is the "expected" 9738 failure code. 9739 9740 9741 ERROR MESSAGES 9742 9743 The regerror() function maps a non-zero errorcode from either regcomp() 9744 or regexec() to a printable message. If preg is not NULL, the error 9745 should have arisen from the use of that structure. A message terminated 9746 by a binary zero is placed in errbuf. The length of the message, 9747 including the zero, is limited to errbuf_size. The yield of the func- 9748 tion is the size of buffer needed to hold the whole message. 9749 9750 9751 MEMORY USAGE 9752 9753 Compiling a regular expression causes memory to be allocated and asso- 9754 ciated with the preg structure. The function regfree() frees all such 9755 memory, after which preg may no longer be used as a compiled expres- 9756 sion. 9757 9758 9759 AUTHOR 9760 9761 Philip Hazel 9762 University Computing Service 9763 Cambridge CB2 3QH, England. 9764 9765 9766 REVISION 9767 9768 Last updated: 09 January 2012 9769 Copyright (c) 1997-2012 University of Cambridge. 9770 ------------------------------------------------------------------------------ 9771 9772 9773 PCRECPP(3) Library Functions Manual PCRECPP(3) 9774 9775 9776 9777 NAME 9778 PCRE - Perl-compatible regular expressions. 9779 9780 SYNOPSIS OF C++ WRAPPER 9781 9782 #include <pcrecpp.h> 9783 9784 9785 DESCRIPTION 9786 9787 The C++ wrapper for PCRE was provided by Google Inc. Some additional 9788 functionality was added by Giuseppe Maxia. This brief man page was con- 9789 structed from the notes in the pcrecpp.h file, which should be con- 9790 sulted for further details. Note that the C++ wrapper supports only the 9791 original 8-bit PCRE library. There is no 16-bit or 32-bit support at 9792 present. 9793 9794 9795 MATCHING INTERFACE 9796 9797 The "FullMatch" operation checks that supplied text matches a supplied 9798 pattern exactly. If pointer arguments are supplied, it copies matched 9799 sub-strings that match sub-patterns into them. 9800 9801 Example: successful match 9802 pcrecpp::RE re("h.*o"); 9803 re.FullMatch("hello"); 9804 9805 Example: unsuccessful match (requires full match): 9806 pcrecpp::RE re("e"); 9807 !re.FullMatch("hello"); 9808 9809 Example: creating a temporary RE object: 9810 pcrecpp::RE("h.*o").FullMatch("hello"); 9811 9812 You can pass in a "const char*" or a "string" for "text". The examples 9813 below tend to use a const char*. You can, as in the different examples 9814 above, store the RE object explicitly in a variable or use a temporary 9815 RE object. The examples below use one mode or the other arbitrarily. 9816 Either could correctly be used for any of these examples. 9817 9818 You must supply extra pointer arguments to extract matched subpieces. 9819 9820 Example: extracts "ruby" into "s" and 1234 into "i" 9821 int i; 9822 string s; 9823 pcrecpp::RE re("(\\w+):(\\d+)"); 9824 re.FullMatch("ruby:1234", &s, &i); 9825 9826 Example: does not try to extract any extra sub-patterns 9827 re.FullMatch("ruby:1234", &s); 9828 9829 Example: does not try to extract into NULL 9830 re.FullMatch("ruby:1234", NULL, &i); 9831 9832 Example: integer overflow causes failure 9833 !re.FullMatch("ruby:1234567891234", NULL, &i); 9834 9835 Example: fails because there aren't enough sub-patterns: 9836 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); 9837 9838 Example: fails because string cannot be stored in integer 9839 !pcrecpp::RE("(.*)").FullMatch("ruby", &i); 9840 9841 The provided pointer arguments can be pointers to any scalar numeric 9842 type, or one of: 9843 9844 string (matched piece is copied to string) 9845 StringPiece (StringPiece is mutated to point to matched piece) 9846 T (where "bool T::ParseFrom(const char*, int)" exists) 9847 NULL (the corresponding matched sub-pattern is not copied) 9848 9849 The function returns true iff all of the following conditions are sat- 9850 isfied: 9851 9852 a. "text" matches "pattern" exactly; 9853 9854 b. The number of matched sub-patterns is >= number of supplied 9855 pointers; 9856 9857 c. The "i"th argument has a suitable type for holding the 9858 string captured as the "i"th sub-pattern. If you pass in 9859 void * NULL for the "i"th argument, or a non-void * NULL 9860 of the correct type, or pass fewer arguments than the 9861 number of sub-patterns, "i"th captured sub-pattern is 9862 ignored. 9863 9864 CAVEAT: An optional sub-pattern that does not exist in the matched 9865 string is assigned the empty string. Therefore, the following will 9866 return false (because the empty string is not a valid number): 9867 9868 int number; 9869 pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); 9870 9871 The matching interface supports at most 16 arguments per call. If you 9872 need more, consider using the more general interface 9873 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. 9874 9875 NOTE: Do not use no_arg, which is used internally to mark the end of a 9876 list of optional arguments, as a placeholder for missing arguments, as 9877 this can lead to segfaults. 9878 9879 9880 QUOTING METACHARACTERS 9881 9882 You can use the "QuoteMeta" operation to insert backslashes before all 9883 potentially meaningful characters in a string. The returned string, 9884 used as a regular expression, will exactly match the original string. 9885 9886 Example: 9887 string quoted = RE::QuoteMeta(unquoted); 9888 9889 Note that it's legal to escape a character even if it has no special 9890 meaning in a regular expression -- so this function does that. (This 9891 also makes it identical to the perl function of the same name; see 9892 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes 9893 "1\.5\-2\.0\?". 9894 9895 9896 PARTIAL MATCHES 9897 9898 You can use the "PartialMatch" operation when you want the pattern to 9899 match any substring of the text. 9900 9901 Example: simple search for a string: 9902 pcrecpp::RE("ell").PartialMatch("hello"); 9903 9904 Example: find first number in a string: 9905 int number; 9906 pcrecpp::RE re("(\\d+)"); 9907 re.PartialMatch("x*100 + 20", &number); 9908 assert(number == 100); 9909 9910 9911 UTF-8 AND THE MATCHING INTERFACE 9912 9913 By default, pattern and text are plain text, one byte per character. 9914 The UTF8 flag, passed to the constructor, causes both pattern and 9915 string to be treated as UTF-8 text, still a byte stream but potentially 9916 multiple bytes per character. In practice, the text is likelier to be 9917 UTF-8 than the pattern, but the match returned may depend on the UTF8 9918 flag, so always use it when matching UTF8 text. For example, "." will 9919 match one byte normally but with UTF8 set may match up to three bytes 9920 of a multi-byte character. 9921 9922 Example: 9923 pcrecpp::RE_Options options; 9924 options.set_utf8(); 9925 pcrecpp::RE re(utf8_pattern, options); 9926 re.FullMatch(utf8_string); 9927 9928 Example: using the convenience function UTF8(): 9929 pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); 9930 re.FullMatch(utf8_string); 9931 9932 NOTE: The UTF8 flag is ignored if pcre was not configured with the 9933 --enable-utf8 flag. 9934 9935 9936 PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE 9937 9938 PCRE defines some modifiers to change the behavior of the regular 9939 expression engine. The C++ wrapper defines an auxiliary class, 9940 RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- 9941 rently, the following modifiers are supported: 9942 9943 modifier description Perl corresponding 9944 9945 PCRE_CASELESS case insensitive match /i 9946 PCRE_MULTILINE multiple lines match /m 9947 PCRE_DOTALL dot matches newlines /s 9948 PCRE_DOLLAR_ENDONLY $ matches only at end N/A 9949 PCRE_EXTRA strict escape parsing N/A 9950 PCRE_EXTENDED ignore white spaces /x 9951 PCRE_UTF8 handles UTF8 chars built-in 9952 PCRE_UNGREEDY reverses * and *? N/A 9953 PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) 9954 9955 (*) Both Perl and PCRE allow non capturing parentheses by means of the 9956 "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- 9957 ture, while (ab|cd) does. 9958 9959 For a full account on how each modifier works, please check the PCRE 9960 API reference page. 9961 9962 For each modifier, there are two member functions whose name is made 9963 out of the modifier in lowercase, without the "PCRE_" prefix. For 9964 instance, PCRE_CASELESS is handled by 9965 9966 bool caseless() 9967 9968 which returns true if the modifier is set, and 9969 9970 RE_Options & set_caseless(bool) 9971 9972 which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can 9973 be accessed through the set_match_limit() and match_limit() member 9974 functions. Setting match_limit to a non-zero value will limit the exe- 9975 cution of pcre to keep it from doing bad things like blowing the stack 9976 or taking an eternity to return a result. A value of 5000 is good 9977 enough to stop stack blowup in a 2MB thread stack. Setting match_limit 9978 to zero disables match limiting. Alternatively, you can call 9979 match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to 9980 limit how much PCRE recurses. match_limit() limits the number of 9981 matches PCRE does; match_limit_recursion() limits the depth of internal 9982 recursion, and therefore the amount of stack that is used. 9983 9984 Normally, to pass one or more modifiers to a RE class, you declare a 9985 RE_Options object, set the appropriate options, and pass this object to 9986 a RE constructor. Example: 9987 9988 RE_Options opt; 9989 opt.set_caseless(true); 9990 if (RE("HELLO", opt).PartialMatch("hello world")) ... 9991 9992 RE_options has two constructors. The default constructor takes no argu- 9993 ments and creates a set of flags that are off by default. The optional 9994 parameter option_flags is to facilitate transfer of legacy code from C 9995 programs. This lets you do 9996 9997 RE(pattern, 9998 RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); 9999 10000 However, new code is better off doing 10001 10002 RE(pattern, 10003 RE_Options().set_caseless(true).set_multiline(true)) 10004 .PartialMatch(str); 10005 10006 If you are going to pass one of the most used modifiers, there are some 10007 convenience functions that return a RE_Options class with the appropri- 10008 ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), 10009 and EXTENDED(). 10010 10011 If you need to set several options at once, and you don't want to go 10012 through the pains of declaring a RE_Options object and setting several 10013 options, there is a parallel method that give you such ability on the 10014 fly. You can concatenate several set_xxxxx() member functions, since 10015 each of them returns a reference to its class object. For example, to 10016 pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one 10017 statement, you may write: 10018 10019 RE(" ^ xyz \\s+ .* blah$", 10020 RE_Options() 10021 .set_caseless(true) 10022 .set_extended(true) 10023 .set_multiline(true)).PartialMatch(sometext); 10024 10025 10026 SCANNING TEXT INCREMENTALLY 10027 10028 The "Consume" operation may be useful if you want to repeatedly match 10029 regular expressions at the front of a string and skip over them as they 10030 match. This requires use of the "StringPiece" type, which represents a 10031 sub-range of a real string. Like RE, StringPiece is defined in the 10032 pcrecpp namespace. 10033 10034 Example: read lines of the form "var = value" from a string. 10035 string contents = ...; // Fill string somehow 10036 pcrecpp::StringPiece input(contents); // Wrap in a StringPiece 10037 10038 string var; 10039 int value; 10040 pcrecpp::RE re("(\\w+) = (\\d+)\n"); 10041 while (re.Consume(&input, &var, &value)) { 10042 ...; 10043 } 10044 10045 Each successful call to "Consume" will set "var/value", and also 10046 advance "input" so it points past the matched text. 10047 10048 The "FindAndConsume" operation is similar to "Consume" but does not 10049 anchor your match at the beginning of the string. For example, you 10050 could extract all words from a string by repeatedly calling 10051 10052 pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) 10053 10054 10055 PARSING HEX/OCTAL/C-RADIX NUMBERS 10056 10057 By default, if you pass a pointer to a numeric value, the corresponding 10058 text is interpreted as a base-10 number. You can instead wrap the 10059 pointer with a call to one of the operators Hex(), Octal(), or CRadix() 10060 to interpret the text in another base. The CRadix operator interprets 10061 C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to 10062 base-10. 10063 10064 Example: 10065 int a, b, c, d; 10066 pcrecpp::RE re("(.*) (.*) (.*) (.*)"); 10067 re.FullMatch("100 40 0100 0x40", 10068 pcrecpp::Octal(&a), pcrecpp::Hex(&b), 10069 pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); 10070 10071 will leave 64 in a, b, c, and d. 10072 10073 10074 REPLACING PARTS OF STRINGS 10075 10076 You can replace the first match of "pattern" in "str" with "rewrite". 10077 Within "rewrite", backslash-escaped digits (\1 to \9) can be used to 10078 insert text matching corresponding parenthesized group from the pat- 10079 tern. \0 in "rewrite" refers to the entire matching text. For example: 10080 10081 string s = "yabba dabba doo"; 10082 pcrecpp::RE("b+").Replace("d", &s); 10083 10084 will leave "s" containing "yada dabba doo". The result is true if the 10085 pattern matches and a replacement occurs, false otherwise. 10086 10087 GlobalReplace is like Replace except that it replaces all occurrences 10088 of the pattern in the string with the rewrite. Replacements are not 10089 subject to re-matching. For example: 10090 10091 string s = "yabba dabba doo"; 10092 pcrecpp::RE("b+").GlobalReplace("d", &s); 10093 10094 will leave "s" containing "yada dada doo". It returns the number of 10095 replacements made. 10096 10097 Extract is like Replace, except that if the pattern matches, "rewrite" 10098 is copied into "out" (an additional argument) with substitutions. The 10099 non-matching portions of "text" are ignored. Returns true iff a match 10100 occurred and the extraction happened successfully; if no match occurs, 10101 the string is left unaffected. 10102 10103 10104 AUTHOR 10105 10106 The C++ wrapper was contributed by Google Inc. 10107 Copyright (c) 2007 Google Inc. 10108 10109 10110 REVISION 10111 10112 Last updated: 08 January 2012 10113 ------------------------------------------------------------------------------ 10114 10115 10116 PCRESAMPLE(3) Library Functions Manual PCRESAMPLE(3) 10117 10118 10119 10120 NAME 10121 PCRE - Perl-compatible regular expressions 10122 10123 PCRE SAMPLE PROGRAM 10124 10125 A simple, complete demonstration program, to get you started with using 10126 PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A 10127 listing of this program is given in the pcredemo documentation. If you 10128 do not have a copy of the PCRE distribution, you can save this listing 10129 to re-create pcredemo.c. 10130 10131 The demonstration program, which uses the original PCRE 8-bit library, 10132 compiles the regular expression that is its first argument, and matches 10133 it against the subject string in its second argument. No PCRE options 10134 are set, and default character tables are used. If matching succeeds, 10135 the program outputs the portion of the subject that matched, together 10136 with the contents of any captured substrings. 10137 10138 If the -g option is given on the command line, the program then goes on 10139 to check for further matches of the same regular expression in the same 10140 subject string. The logic is a little bit tricky because of the possi- 10141 bility of matching an empty string. Comments in the code explain what 10142 is going on. 10143 10144 If PCRE is installed in the standard include and library directories 10145 for your operating system, you should be able to compile the demonstra- 10146 tion program using this command: 10147 10148 gcc -o pcredemo pcredemo.c -lpcre 10149 10150 If PCRE is installed elsewhere, you may need to add additional options 10151 to the command line. For example, on a Unix-like system that has PCRE 10152 installed in /usr/local, you can compile the demonstration program 10153 using a command like this: 10154 10155 gcc -o pcredemo -I/usr/local/include pcredemo.c \ 10156 -L/usr/local/lib -lpcre 10157 10158 In a Windows environment, if you want to statically link the program 10159 against a non-dll pcre.a file, you must uncomment the line that defines 10160 PCRE_STATIC before including pcre.h, because otherwise the pcre_mal- 10161 loc() and pcre_free() exported functions will be declared 10162 __declspec(dllimport), with unwanted results. 10163 10164 Once you have compiled and linked the demonstration program, you can 10165 run simple tests like this: 10166 10167 ./pcredemo 'cat|dog' 'the cat sat on the mat' 10168 ./pcredemo -g 'cat|dog' 'the dog sat on the cat' 10169 10170 Note that there is a much more comprehensive test program, called 10171 pcretest, which supports many more facilities for testing regular 10172 expressions and both PCRE libraries. The pcredemo program is provided 10173 as a simple coding example. 10174 10175 If you try to run pcredemo when PCRE is not installed in the standard 10176 library directory, you may get an error like this on some operating 10177 systems (e.g. Solaris): 10178 10179 ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or 10180 directory 10181 10182 This is caused by the way shared library support works on those sys- 10183 tems. You need to add 10184 10185 -R/usr/local/lib 10186 10187 (for example) to the compile command to get round this problem. 10188 10189 10190 AUTHOR 10191 10192 Philip Hazel 10193 University Computing Service 10194 Cambridge CB2 3QH, England. 10195 10196 10197 REVISION 10198 10199 Last updated: 10 January 2012 10200 Copyright (c) 1997-2012 University of Cambridge. 10201 ------------------------------------------------------------------------------ 10202 PCRELIMITS(3) Library Functions Manual PCRELIMITS(3) 10203 10204 10205 10206 NAME 10207 PCRE - Perl-compatible regular expressions 10208 10209 SIZE AND OTHER LIMITATIONS 10210 10211 There are some size limitations in PCRE but it is hoped that they will 10212 never in practice be relevant. 10213 10214 The maximum length of a compiled pattern is approximately 64K data 10215 units (bytes for the 8-bit library, 16-bit units for the 16-bit 10216 library, and 32-bit units for the 32-bit library) if PCRE is compiled 10217 with the default internal linkage size, which is 2 bytes for the 8-bit 10218 and 16-bit libraries, and 4 bytes for the 32-bit library. If you want 10219 to process regular expressions that are truly enormous, you can compile 10220 PCRE with an internal linkage size of 3 or 4 (when building the 16-bit 10221 or 32-bit library, 3 is rounded up to 4). See the README file in the 10222 source distribution and the pcrebuild documentation for details. In 10223 these cases the limit is substantially larger. However, the speed of 10224 execution is slower. 10225 10226 All values in repeating quantifiers must be less than 65536. 10227 10228 There is no limit to the number of parenthesized subpatterns, but there 10229 can be no more than 65535 capturing subpatterns. There is, however, a 10230 limit to the depth of nesting of parenthesized subpatterns of all 10231 kinds. This is imposed in order to limit the amount of system stack 10232 used at compile time. The limit can be specified when PCRE is built; 10233 the default is 250. 10234 10235 There is a limit to the number of forward references to subsequent sub- 10236 patterns of around 200,000. Repeated forward references with fixed 10237 upper limits, for example, (?2){0,100} when subpattern number 2 is to 10238 the right, are included in the count. There is no limit to the number 10239 of backward references. 10240 10241 The maximum length of name for a named subpattern is 32 characters, and 10242 the maximum number of named subpatterns is 10000. 10243 10244 The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or 10245 (*THEN) verb is 255 for the 8-bit library and 65535 for the 16-bit and 10246 32-bit libraries. 10247 10248 The maximum length of a subject string is the largest positive number 10249 that an integer variable can hold. However, when using the traditional 10250 matching function, PCRE uses recursion to handle subpatterns and indef- 10251 inite repetition. This means that the available stack space may limit 10252 the size of a subject string that can be processed by certain patterns. 10253 For a discussion of stack issues, see the pcrestack documentation. 10254 10255 10256 AUTHOR 10257 10258 Philip Hazel 10259 University Computing Service 10260 Cambridge CB2 3QH, England. 10261 10262 10263 REVISION 10264 10265 Last updated: 05 November 2013 10266 Copyright (c) 1997-2013 University of Cambridge. 10267 ------------------------------------------------------------------------------ 10268 10269 10270 PCRESTACK(3) Library Functions Manual PCRESTACK(3) 10271 10272 10273 10274 NAME 10275 PCRE - Perl-compatible regular expressions 10276 10277 PCRE DISCUSSION OF STACK USAGE 10278 10279 When you call pcre[16|32]_exec(), it makes use of an internal function 10280 called match(). This calls itself recursively at branch points in the 10281 pattern, in order to remember the state of the match so that it can 10282 back up and try a different alternative if the first one fails. As 10283 matching proceeds deeper and deeper into the tree of possibilities, the 10284 recursion depth increases. The match() function is also called in other 10285 circumstances, for example, whenever a parenthesized sub-pattern is 10286 entered, and in certain cases of repetition. 10287 10288 Not all calls of match() increase the recursion depth; for an item such 10289 as a* it may be called several times at the same level, after matching 10290 different numbers of a's. Furthermore, in a number of cases where the 10291 result of the recursive call would immediately be passed back as the 10292 result of the current call (a "tail recursion"), the function is just 10293 restarted instead. 10294 10295 The above comments apply when pcre[16|32]_exec() is run in its normal 10296 interpretive manner. If the pattern was studied with the 10297 PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was success- 10298 ful, and the options passed to pcre[16|32]_exec() were not incompati- 10299 ble, the matching process uses the JIT-compiled code instead of the 10300 match() function. In this case, the memory requirements are handled 10301 entirely differently. See the pcrejit documentation for details. 10302 10303 The pcre[16|32]_dfa_exec() function operates in an entirely different 10304 way, and uses recursion only when there is a regular expression recur- 10305 sion or subroutine call in the pattern. This includes the processing of 10306 assertion and "once-only" subpatterns, which are handled like subrou- 10307 tine calls. Normally, these are never very deep, and the limit on the 10308 complexity of pcre[16|32]_dfa_exec() is controlled by the amount of 10309 workspace it is given. However, it is possible to write patterns with 10310 runaway infinite recursions; such patterns will cause 10311 pcre[16|32]_dfa_exec() to run out of stack. At present, there is no 10312 protection against this. 10313 10314 The comments that follow do NOT apply to pcre[16|32]_dfa_exec(); they 10315 are relevant only for pcre[16|32]_exec() without the JIT optimization. 10316 10317 Reducing pcre[16|32]_exec()'s stack usage 10318 10319 Each time that match() is actually called recursively, it uses memory 10320 from the process stack. For certain kinds of pattern and data, very 10321 large amounts of stack may be needed, despite the recognition of "tail 10322 recursion". You can often reduce the amount of recursion, and there- 10323 fore the amount of stack used, by modifying the pattern that is being 10324 matched. Consider, for example, this pattern: 10325 10326 ([^<]|<(?!inet))+ 10327 10328 It matches from wherever it starts until it encounters "<inet" or the 10329 end of the data, and is the kind of pattern that might be used when 10330 processing an XML file. Each iteration of the outer parentheses matches 10331 either one character that is not "<" or a "<" that is not followed by 10332 "inet". However, each time a parenthesis is processed, a recursion 10333 occurs, so this formulation uses a stack frame for each matched charac- 10334 ter. For a long string, a lot of stack is required. Consider now this 10335 rewritten pattern, which matches exactly the same strings: 10336 10337 ([^<]++|<(?!inet))+ 10338 10339 This uses very much less stack, because runs of characters that do not 10340 contain "<" are "swallowed" in one item inside the parentheses. Recur- 10341 sion happens only when a "<" character that is not followed by "inet" 10342 is encountered (and we assume this is relatively rare). A possessive 10343 quantifier is used to stop any backtracking into the runs of non-"<" 10344 characters, but that is not related to stack usage. 10345 10346 This example shows that one way of avoiding stack problems when match- 10347 ing long subject strings is to write repeated parenthesized subpatterns 10348 to match more than one character whenever possible. 10349 10350 Compiling PCRE to use heap instead of stack for pcre[16|32]_exec() 10351 10352 In environments where stack memory is constrained, you might want to 10353 compile PCRE to use heap memory instead of stack for remembering back- 10354 up points when pcre[16|32]_exec() is running. This makes it run a lot 10355 more slowly, however. Details of how to do this are given in the pcre- 10356 build documentation. When built in this way, instead of using the 10357 stack, PCRE obtains and frees memory by calling the functions that are 10358 pointed to by the pcre[16|32]_stack_malloc and pcre[16|32]_stack_free 10359 variables. By default, these point to malloc() and free(), but you can 10360 replace the pointers to cause PCRE to use your own functions. Since the 10361 block sizes are always the same, and are always freed in reverse order, 10362 it may be possible to implement customized memory handlers that are 10363 more efficient than the standard functions. 10364 10365 Limiting pcre[16|32]_exec()'s stack usage 10366 10367 You can set limits on the number of times that match() is called, both 10368 in total and recursively. If a limit is exceeded, pcre[16|32]_exec() 10369 returns an error code. Setting suitable limits should prevent it from 10370 running out of stack. The default values of the limits are very large, 10371 and unlikely ever to operate. They can be changed when PCRE is built, 10372 and they can also be set when pcre[16|32]_exec() is called. For details 10373 of these interfaces, see the pcrebuild documentation and the section on 10374 extra data for pcre[16|32]_exec() in the pcreapi documentation. 10375 10376 As a very rough rule of thumb, you should reckon on about 500 bytes per 10377 recursion. Thus, if you want to limit your stack usage to 8Mb, you 10378 should set the limit at 16000 recursions. A 64Mb stack, on the other 10379 hand, can support around 128000 recursions. 10380 10381 In Unix-like environments, the pcretest test program has a command line 10382 option (-S) that can be used to increase the size of its stack. As long 10383 as the stack is large enough, another option (-M) can be used to find 10384 the smallest limits that allow a particular pattern to match a given 10385 subject string. This is done by calling pcre[16|32]_exec() repeatedly 10386 with different limits. 10387 10388 Obtaining an estimate of stack usage 10389 10390 The actual amount of stack used per recursion can vary quite a lot, 10391 depending on the compiler that was used to build PCRE and the optimiza- 10392 tion or debugging options that were set for it. The rule of thumb value 10393 of 500 bytes mentioned above may be larger or smaller than what is 10394 actually needed. A better approximation can be obtained by running this 10395 command: 10396 10397 pcretest -m -C 10398 10399 The -C option causes pcretest to output information about the options 10400 with which PCRE was compiled. When -m is also given (before -C), infor- 10401 mation about stack use is given in a line like this: 10402 10403 Match recursion uses stack: approximate frame size = 640 bytes 10404 10405 The value is approximate because some recursions need a bit more (up to 10406 perhaps 16 more bytes). 10407 10408 If the above command is given when PCRE is compiled to use the heap 10409 instead of the stack for recursion, the value that is output is the 10410 size of each block that is obtained from the heap. 10411 10412 Changing stack size in Unix-like systems 10413 10414 In Unix-like environments, there is not often a problem with the stack 10415 unless very long strings are involved, though the default limit on 10416 stack size varies from system to system. Values from 8Mb to 64Mb are 10417 common. You can find your default limit by running the command: 10418 10419 ulimit -s 10420 10421 Unfortunately, the effect of running out of stack is often SIGSEGV, 10422 though sometimes a more explicit error message is given. You can nor- 10423 mally increase the limit on stack size by code such as this: 10424 10425 struct rlimit rlim; 10426 getrlimit(RLIMIT_STACK, &rlim); 10427 rlim.rlim_cur = 100*1024*1024; 10428 setrlimit(RLIMIT_STACK, &rlim); 10429 10430 This reads the current limits (soft and hard) using getrlimit(), then 10431 attempts to increase the soft limit to 100Mb using setrlimit(). You 10432 must do this before calling pcre[16|32]_exec(). 10433 10434 Changing stack size in Mac OS X 10435 10436 Using setrlimit(), as described above, should also work on Mac OS X. It 10437 is also possible to set a stack size when linking a program. There is a 10438 discussion about stack sizes in Mac OS X at this web site: 10439 http://developer.apple.com/qa/qa2005/qa1419.html. 10440 10441 10442 AUTHOR 10443 10444 Philip Hazel 10445 University Computing Service 10446 Cambridge CB2 3QH, England. 10447 10448 10449 REVISION 10450 10451 Last updated: 24 June 2012 10452 Copyright (c) 1997-2012 University of Cambridge. 10453 ------------------------------------------------------------------------------ 10454 10455 10456
