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 17 18 INTRODUCTION 19 20 The PCRE library is a set of functions that implement regular expres- 21 sion pattern matching using the same syntax and semantics as Perl, with 22 just a few differences. Some features that appeared in Python and PCRE 23 before they appeared in Perl are also available using the Python syn- 24 tax, there is some support for one or two .NET and Oniguruma syntax 25 items, and there is an option for requesting some minor changes that 26 give better JavaScript compatibility. 27 28 Starting with release 8.30, it is possible to compile two separate PCRE 29 libraries: the original, which supports 8-bit character strings 30 (including UTF-8 strings), and a second library that supports 16-bit 31 character strings (including UTF-16 strings). The build process allows 32 either one or both to be built. The majority of the work to make this 33 possible was done by Zoltan Herczeg. 34 35 Starting with release 8.32 it is possible to compile a third separate 36 PCRE library that supports 32-bit character strings (including UTF-32 37 strings). The build process allows any combination of the 8-, 16- and 38 32-bit libraries. The work to make this possible was done by Christian 39 Persch. 40 41 The three libraries contain identical sets of functions, except that 42 the names in the 16-bit library start with pcre16_ instead of pcre_, 43 and the names in the 32-bit library start with pcre32_ instead of 44 pcre_. To avoid over-complication and reduce the documentation mainte- 45 nance load, most of the documentation describes the 8-bit library, with 46 the differences for the 16-bit and 32-bit libraries described sepa- 47 rately in the pcre16 and pcre32 pages. References to functions or 48 structures of the form pcre[16|32]_xxx should be read as meaning 49 "pcre_xxx when using the 8-bit library, pcre16_xxx when using the 50 16-bit library, or pcre32_xxx when using the 32-bit library". 51 52 The current implementation of PCRE corresponds approximately with Perl 53 5.12, including support for UTF-8/16/32 encoded strings and Unicode 54 general category properties. However, UTF-8/16/32 and Unicode support 55 has to be explicitly enabled; it is not the default. The Unicode tables 56 correspond to Unicode release 6.3.0. 57 58 In addition to the Perl-compatible matching function, PCRE contains an 59 alternative function that matches the same compiled patterns in a dif- 60 ferent way. In certain circumstances, the alternative function has some 61 advantages. For a discussion of the two matching algorithms, see the 62 pcrematching page. 63 64 PCRE is written in C and released as a C library. A number of people 65 have written wrappers and interfaces of various kinds. In particular, 66 Google Inc. have provided a comprehensive C++ wrapper for the 8-bit 67 library. This is now included as part of the PCRE distribution. The 68 pcrecpp page has details of this interface. Other people's contribu- 69 tions can be found in the Contrib directory at the primary FTP site, 70 which is: 71 72 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre 73 74 Details of exactly which Perl regular expression features are and are 75 not supported by PCRE are given in separate documents. See the pcrepat- 76 tern and pcrecompat pages. There is a syntax summary in the pcresyntax 77 page. 78 79 Some features of PCRE can be included, excluded, or changed when the 80 library is built. The pcre_config() function makes it possible for a 81 client to discover which features are available. The features them- 82 selves are described in the pcrebuild page. Documentation about build- 83 ing PCRE for various operating systems can be found in the README and 84 NON-AUTOTOOLS_BUILD files in the source distribution. 85 86 The libraries contains a number of undocumented internal functions and 87 data tables that are used by more than one of the exported external 88 functions, but which are not intended for use by external callers. 89 Their names all begin with "_pcre_" or "_pcre16_" or "_pcre32_", which 90 hopefully will not provoke any name clashes. In some environments, it 91 is possible to control which external symbols are exported when a 92 shared library is built, and in these cases the undocumented symbols 93 are not exported. 94 95 96 SECURITY CONSIDERATIONS 97 98 If you are using PCRE in a non-UTF application that permits users to 99 supply arbitrary patterns for compilation, you should be aware of a 100 feature that allows users to turn on UTF support from within a pattern, 101 provided that PCRE was built with UTF support. For example, an 8-bit 102 pattern that begins with "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, 103 which interprets patterns and subjects as strings of UTF-8 characters 104 instead of individual 8-bit characters. This causes both the pattern 105 and any data against which it is matched to be checked for UTF-8 valid- 106 ity. If the data string is very long, such a check might use suffi- 107 ciently many resources as to cause your application to lose perfor- 108 mance. 109 110 One way of guarding against this possibility is to use the 111 pcre_fullinfo() function to check the compiled pattern's options for 112 UTF. Alternatively, from release 8.33, you can set the PCRE_NEVER_UTF 113 option at compile time. This causes an compile time error if a pattern 114 contains a UTF-setting sequence. 115 116 If your application is one that supports UTF, be aware that validity 117 checking can take time. If the same data string is to be matched many 118 times, you can use the PCRE_NO_UTF[8|16|32]_CHECK option for the second 119 and subsequent matches to save redundant checks. 120 121 Another way that performance can be hit is by running a pattern that 122 has a very large search tree against a string that will never match. 123 Nested unlimited repeats in a pattern are a common example. PCRE pro- 124 vides some protection against this: see the PCRE_EXTRA_MATCH_LIMIT fea- 125 ture in the pcreapi page. 126 127 128 USER DOCUMENTATION 129 130 The user documentation for PCRE comprises a number of different sec- 131 tions. In the "man" format, each of these is a separate "man page". In 132 the HTML format, each is a separate page, linked from the index page. 133 In the plain text format, the descriptions of the pcregrep and pcretest 134 programs are in files called pcregrep.txt and pcretest.txt, respec- 135 tively. The remaining sections, except for the pcredemo section (which 136 is a program listing), are concatenated in pcre.txt, for ease of 137 searching. The sections are as follows: 138 139 pcre this document 140 pcre-config show PCRE installation configuration information 141 pcre16 details of the 16-bit library 142 pcre32 details of the 32-bit library 143 pcreapi details of PCRE's native C API 144 pcrebuild building PCRE 145 pcrecallout details of the callout feature 146 pcrecompat discussion of Perl compatibility 147 pcrecpp details of the C++ wrapper for the 8-bit library 148 pcredemo a demonstration C program that uses PCRE 149 pcregrep description of the pcregrep command (8-bit only) 150 pcrejit discussion of the just-in-time optimization support 151 pcrelimits details of size and other limits 152 pcrematching discussion of the two matching algorithms 153 pcrepartial details of the partial matching facility 154 pcrepattern syntax and semantics of supported 155 regular expressions 156 pcreperform discussion of performance issues 157 pcreposix the POSIX-compatible C API for the 8-bit library 158 pcreprecompile details of saving and re-using precompiled patterns 159 pcresample discussion of the pcredemo program 160 pcrestack discussion of stack usage 161 pcresyntax quick syntax reference 162 pcretest description of the pcretest testing command 163 pcreunicode discussion of Unicode and UTF-8/16/32 support 164 165 In the "man" and HTML formats, there is also a short page for each C 166 library function, listing its arguments and results. 167 168 169 AUTHOR 170 171 Philip Hazel 172 University Computing Service 173 Cambridge CB2 3QH, England. 174 175 Putting an actual email address here seems to have been a spam magnet, 176 so I've taken it away. If you want to email me, use my two initials, 177 followed by the two digits 10, at the domain cam.ac.uk. 178 179 180 REVISION 181 182 Last updated: 08 January 2014 183 Copyright (c) 1997-2014 University of Cambridge. 184 ------------------------------------------------------------------------------ 185 186 187 PCRE(3) Library Functions Manual PCRE(3) 188 189 190 191 NAME 192 PCRE - Perl-compatible regular expressions 193 194 #include <pcre.h> 195 196 197 PCRE 16-BIT API BASIC FUNCTIONS 198 199 pcre16 *pcre16_compile(PCRE_SPTR16 pattern, int options, 200 const char **errptr, int *erroffset, 201 const unsigned char *tableptr); 202 203 pcre16 *pcre16_compile2(PCRE_SPTR16 pattern, int options, 204 int *errorcodeptr, 205 const char **errptr, int *erroffset, 206 const unsigned char *tableptr); 207 208 pcre16_extra *pcre16_study(const pcre16 *code, int options, 209 const char **errptr); 210 211 void pcre16_free_study(pcre16_extra *extra); 212 213 int pcre16_exec(const pcre16 *code, const pcre16_extra *extra, 214 PCRE_SPTR16 subject, int length, int startoffset, 215 int options, int *ovector, int ovecsize); 216 217 int pcre16_dfa_exec(const pcre16 *code, const pcre16_extra *extra, 218 PCRE_SPTR16 subject, int length, int startoffset, 219 int options, int *ovector, int ovecsize, 220 int *workspace, int wscount); 221 222 223 PCRE 16-BIT API STRING EXTRACTION FUNCTIONS 224 225 int pcre16_copy_named_substring(const pcre16 *code, 226 PCRE_SPTR16 subject, int *ovector, 227 int stringcount, PCRE_SPTR16 stringname, 228 PCRE_UCHAR16 *buffer, int buffersize); 229 230 int pcre16_copy_substring(PCRE_SPTR16 subject, int *ovector, 231 int stringcount, int stringnumber, PCRE_UCHAR16 *buffer, 232 int buffersize); 233 234 int pcre16_get_named_substring(const pcre16 *code, 235 PCRE_SPTR16 subject, int *ovector, 236 int stringcount, PCRE_SPTR16 stringname, 237 PCRE_SPTR16 *stringptr); 238 239 int pcre16_get_stringnumber(const pcre16 *code, 240 PCRE_SPTR16 name); 241 242 int pcre16_get_stringtable_entries(const pcre16 *code, 243 PCRE_SPTR16 name, PCRE_UCHAR16 **first, PCRE_UCHAR16 **last); 244 245 int pcre16_get_substring(PCRE_SPTR16 subject, int *ovector, 246 int stringcount, int stringnumber, 247 PCRE_SPTR16 *stringptr); 248 249 int pcre16_get_substring_list(PCRE_SPTR16 subject, 250 int *ovector, int stringcount, PCRE_SPTR16 **listptr); 251 252 void pcre16_free_substring(PCRE_SPTR16 stringptr); 253 254 void pcre16_free_substring_list(PCRE_SPTR16 *stringptr); 255 256 257 PCRE 16-BIT API AUXILIARY FUNCTIONS 258 259 pcre16_jit_stack *pcre16_jit_stack_alloc(int startsize, int maxsize); 260 261 void pcre16_jit_stack_free(pcre16_jit_stack *stack); 262 263 void pcre16_assign_jit_stack(pcre16_extra *extra, 264 pcre16_jit_callback callback, void *data); 265 266 const unsigned char *pcre16_maketables(void); 267 268 int pcre16_fullinfo(const pcre16 *code, const pcre16_extra *extra, 269 int what, void *where); 270 271 int pcre16_refcount(pcre16 *code, int adjust); 272 273 int pcre16_config(int what, void *where); 274 275 const char *pcre16_version(void); 276 277 int pcre16_pattern_to_host_byte_order(pcre16 *code, 278 pcre16_extra *extra, const unsigned char *tables); 279 280 281 PCRE 16-BIT API INDIRECTED FUNCTIONS 282 283 void *(*pcre16_malloc)(size_t); 284 285 void (*pcre16_free)(void *); 286 287 void *(*pcre16_stack_malloc)(size_t); 288 289 void (*pcre16_stack_free)(void *); 290 291 int (*pcre16_callout)(pcre16_callout_block *); 292 293 294 PCRE 16-BIT API 16-BIT-ONLY FUNCTION 295 296 int pcre16_utf16_to_host_byte_order(PCRE_UCHAR16 *output, 297 PCRE_SPTR16 input, int length, int *byte_order, 298 int keep_boms); 299 300 301 THE PCRE 16-BIT LIBRARY 302 303 Starting with release 8.30, it is possible to compile a PCRE library 304 that supports 16-bit character strings, including UTF-16 strings, as 305 well as or instead of the original 8-bit library. The majority of the 306 work to make this possible was done by Zoltan Herczeg. The two 307 libraries contain identical sets of functions, used in exactly the same 308 way. Only the names of the functions and the data types of their argu- 309 ments and results are different. To avoid over-complication and reduce 310 the documentation maintenance load, most of the PCRE documentation 311 describes the 8-bit library, with only occasional references to the 312 16-bit library. This page describes what is different when you use the 313 16-bit library. 314 315 WARNING: A single application can be linked with both libraries, but 316 you must take care when processing any particular pattern to use func- 317 tions from just one library. For example, if you want to study a pat- 318 tern that was compiled with pcre16_compile(), you must do so with 319 pcre16_study(), not pcre_study(), and you must free the study data with 320 pcre16_free_study(). 321 322 323 THE HEADER FILE 324 325 There is only one header file, pcre.h. It contains prototypes for all 326 the functions in all libraries, as well as definitions of flags, struc- 327 tures, error codes, etc. 328 329 330 THE LIBRARY NAME 331 332 In Unix-like systems, the 16-bit library is called libpcre16, and can 333 normally be accesss by adding -lpcre16 to the command for linking an 334 application that uses PCRE. 335 336 337 STRING TYPES 338 339 In the 8-bit library, strings are passed to PCRE library functions as 340 vectors of bytes with the C type "char *". In the 16-bit library, 341 strings are passed as vectors of unsigned 16-bit quantities. The macro 342 PCRE_UCHAR16 specifies an appropriate data type, and PCRE_SPTR16 is 343 defined as "const PCRE_UCHAR16 *". In very many environments, "short 344 int" is a 16-bit data type. When PCRE is built, it defines PCRE_UCHAR16 345 as "unsigned short int", but checks that it really is a 16-bit data 346 type. If it is not, the build fails with an error message telling the 347 maintainer to modify the definition appropriately. 348 349 350 STRUCTURE TYPES 351 352 The types of the opaque structures that are used for compiled 16-bit 353 patterns and JIT stacks are pcre16 and pcre16_jit_stack respectively. 354 The type of the user-accessible structure that is returned by 355 pcre16_study() is pcre16_extra, and the type of the structure that is 356 used for passing data to a callout function is pcre16_callout_block. 357 These structures contain the same fields, with the same names, as their 358 8-bit counterparts. The only difference is that pointers to character 359 strings are 16-bit instead of 8-bit types. 360 361 362 16-BIT FUNCTIONS 363 364 For every function in the 8-bit library there is a corresponding func- 365 tion in the 16-bit library with a name that starts with pcre16_ instead 366 of pcre_. The prototypes are listed above. In addition, there is one 367 extra function, pcre16_utf16_to_host_byte_order(). This is a utility 368 function that converts a UTF-16 character string to host byte order if 369 necessary. The other 16-bit functions expect the strings they are 370 passed to be in host byte order. 371 372 The input and output arguments of pcre16_utf16_to_host_byte_order() may 373 point to the same address, that is, conversion in place is supported. 374 The output buffer must be at least as long as the input. 375 376 The length argument specifies the number of 16-bit data units in the 377 input string; a negative value specifies a zero-terminated string. 378 379 If byte_order is NULL, it is assumed that the string starts off in host 380 byte order. This may be changed by byte-order marks (BOMs) anywhere in 381 the string (commonly as the first character). 382 383 If byte_order is not NULL, a non-zero value of the integer to which it 384 points means that the input starts off in host byte order, otherwise 385 the opposite order is assumed. Again, BOMs in the string can change 386 this. The final byte order is passed back at the end of processing. 387 388 If keep_boms is not zero, byte-order mark characters (0xfeff) are 389 copied into the output string. Otherwise they are discarded. 390 391 The result of the function is the number of 16-bit units placed into 392 the output buffer, including the zero terminator if the string was 393 zero-terminated. 394 395 396 SUBJECT STRING OFFSETS 397 398 The lengths and starting offsets of subject strings must be specified 399 in 16-bit data units, and the offsets within subject strings that are 400 returned by the matching functions are in also 16-bit units rather than 401 bytes. 402 403 404 NAMED SUBPATTERNS 405 406 The name-to-number translation table that is maintained for named sub- 407 patterns uses 16-bit characters. The pcre16_get_stringtable_entries() 408 function returns the length of each entry in the table as the number of 409 16-bit data units. 410 411 412 OPTION NAMES 413 414 There are two new general option names, PCRE_UTF16 and 415 PCRE_NO_UTF16_CHECK, which correspond to PCRE_UTF8 and 416 PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options 417 define the same bits in the options word. There is a discussion about 418 the validity of UTF-16 strings in the pcreunicode page. 419 420 For the pcre16_config() function there is an option PCRE_CONFIG_UTF16 421 that returns 1 if UTF-16 support is configured, otherwise 0. If this 422 option is given to pcre_config() or pcre32_config(), or if the 423 PCRE_CONFIG_UTF8 or PCRE_CONFIG_UTF32 option is given to pcre16_con- 424 fig(), the result is the PCRE_ERROR_BADOPTION error. 425 426 427 CHARACTER CODES 428 429 In 16-bit mode, when PCRE_UTF16 is not set, character values are 430 treated in the same way as in 8-bit, non UTF-8 mode, except, of course, 431 that they can range from 0 to 0xffff instead of 0 to 0xff. Character 432 types for characters less than 0xff can therefore be influenced by the 433 locale in the same way as before. Characters greater than 0xff have 434 only one case, and no "type" (such as letter or digit). 435 436 In UTF-16 mode, the character code is Unicode, in the range 0 to 437 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff 438 because those are "surrogate" values that are used in pairs to encode 439 values greater than 0xffff. 440 441 A UTF-16 string can indicate its endianness by special code knows as a 442 byte-order mark (BOM). The PCRE functions do not handle this, expecting 443 strings to be in host byte order. A utility function called 444 pcre16_utf16_to_host_byte_order() is provided to help with this (see 445 above). 446 447 448 ERROR NAMES 449 450 The errors PCRE_ERROR_BADUTF16_OFFSET and PCRE_ERROR_SHORTUTF16 corre- 451 spond to their 8-bit counterparts. The error PCRE_ERROR_BADMODE is 452 given when a compiled pattern is passed to a function that processes 453 patterns in the other mode, for example, if a pattern compiled with 454 pcre_compile() is passed to pcre16_exec(). 455 456 There are new error codes whose names begin with PCRE_UTF16_ERR for 457 invalid UTF-16 strings, corresponding to the PCRE_UTF8_ERR codes for 458 UTF-8 strings that are described in the section entitled "Reason codes 459 for invalid UTF-8 strings" in the main pcreapi page. The UTF-16 errors 460 are: 461 462 PCRE_UTF16_ERR1 Missing low surrogate at end of string 463 PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate 464 PCRE_UTF16_ERR3 Isolated low surrogate 465 PCRE_UTF16_ERR4 Non-character 466 467 468 ERROR TEXTS 469 470 If there is an error while compiling a pattern, the error text that is 471 passed back by pcre16_compile() or pcre16_compile2() is still an 8-bit 472 character string, zero-terminated. 473 474 475 CALLOUTS 476 477 The subject and mark fields in the callout block that is passed to a 478 callout function point to 16-bit vectors. 479 480 481 TESTING 482 483 The pcretest program continues to operate with 8-bit input and output 484 files, but it can be used for testing the 16-bit library. If it is run 485 with the command line option -16, patterns and subject strings are con- 486 verted from 8-bit to 16-bit before being passed to PCRE, and the 16-bit 487 library functions are used instead of the 8-bit ones. Returned 16-bit 488 strings are converted to 8-bit for output. If both the 8-bit and the 489 32-bit libraries were not compiled, pcretest defaults to 16-bit and the 490 -16 option is ignored. 491 492 When PCRE is being built, the RunTest script that is called by "make 493 check" uses the pcretest -C option to discover which of the 8-bit, 494 16-bit and 32-bit libraries has been built, and runs the tests appro- 495 priately. 496 497 498 NOT SUPPORTED IN 16-BIT MODE 499 500 Not all the features of the 8-bit library are available with the 16-bit 501 library. The C++ and POSIX wrapper functions support only the 8-bit 502 library, and the pcregrep program is at present 8-bit only. 503 504 505 AUTHOR 506 507 Philip Hazel 508 University Computing Service 509 Cambridge CB2 3QH, England. 510 511 512 REVISION 513 514 Last updated: 12 May 2013 515 Copyright (c) 1997-2013 University of Cambridge. 516 ------------------------------------------------------------------------------ 517 518 519 PCRE(3) Library Functions Manual PCRE(3) 520 521 522 523 NAME 524 PCRE - Perl-compatible regular expressions 525 526 #include <pcre.h> 527 528 529 PCRE 32-BIT API BASIC FUNCTIONS 530 531 pcre32 *pcre32_compile(PCRE_SPTR32 pattern, int options, 532 const char **errptr, int *erroffset, 533 const unsigned char *tableptr); 534 535 pcre32 *pcre32_compile2(PCRE_SPTR32 pattern, int options, 536 int *errorcodeptr, 537 const unsigned char *tableptr); 538 539 pcre32_extra *pcre32_study(const pcre32 *code, int options, 540 const char **errptr); 541 542 void pcre32_free_study(pcre32_extra *extra); 543 544 int pcre32_exec(const pcre32 *code, const pcre32_extra *extra, 545 PCRE_SPTR32 subject, int length, int startoffset, 546 int options, int *ovector, int ovecsize); 547 548 int pcre32_dfa_exec(const pcre32 *code, const pcre32_extra *extra, 549 PCRE_SPTR32 subject, int length, int startoffset, 550 int options, int *ovector, int ovecsize, 551 int *workspace, int wscount); 552 553 554 PCRE 32-BIT API STRING EXTRACTION FUNCTIONS 555 556 int pcre32_copy_named_substring(const pcre32 *code, 557 PCRE_SPTR32 subject, int *ovector, 558 int stringcount, PCRE_SPTR32 stringname, 559 PCRE_UCHAR32 *buffer, int buffersize); 560 561 int pcre32_copy_substring(PCRE_SPTR32 subject, int *ovector, 562 int stringcount, int stringnumber, PCRE_UCHAR32 *buffer, 563 int buffersize); 564 565 int pcre32_get_named_substring(const pcre32 *code, 566 PCRE_SPTR32 subject, int *ovector, 567 int stringcount, PCRE_SPTR32 stringname, 568 PCRE_SPTR32 *stringptr); 569 570 int pcre32_get_stringnumber(const pcre32 *code, 571 PCRE_SPTR32 name); 572 573 int pcre32_get_stringtable_entries(const pcre32 *code, 574 PCRE_SPTR32 name, PCRE_UCHAR32 **first, PCRE_UCHAR32 **last); 575 576 int pcre32_get_substring(PCRE_SPTR32 subject, int *ovector, 577 int stringcount, int stringnumber, 578 PCRE_SPTR32 *stringptr); 579 580 int pcre32_get_substring_list(PCRE_SPTR32 subject, 581 int *ovector, int stringcount, PCRE_SPTR32 **listptr); 582 583 void pcre32_free_substring(PCRE_SPTR32 stringptr); 584 585 void pcre32_free_substring_list(PCRE_SPTR32 *stringptr); 586 587 588 PCRE 32-BIT API AUXILIARY FUNCTIONS 589 590 pcre32_jit_stack *pcre32_jit_stack_alloc(int startsize, int maxsize); 591 592 void pcre32_jit_stack_free(pcre32_jit_stack *stack); 593 594 void pcre32_assign_jit_stack(pcre32_extra *extra, 595 pcre32_jit_callback callback, void *data); 596 597 const unsigned char *pcre32_maketables(void); 598 599 int pcre32_fullinfo(const pcre32 *code, const pcre32_extra *extra, 600 int what, void *where); 601 602 int pcre32_refcount(pcre32 *code, int adjust); 603 604 int pcre32_config(int what, void *where); 605 606 const char *pcre32_version(void); 607 608 int pcre32_pattern_to_host_byte_order(pcre32 *code, 609 pcre32_extra *extra, const unsigned char *tables); 610 611 612 PCRE 32-BIT API INDIRECTED FUNCTIONS 613 614 void *(*pcre32_malloc)(size_t); 615 616 void (*pcre32_free)(void *); 617 618 void *(*pcre32_stack_malloc)(size_t); 619 620 void (*pcre32_stack_free)(void *); 621 622 int (*pcre32_callout)(pcre32_callout_block *); 623 624 625 PCRE 32-BIT API 32-BIT-ONLY FUNCTION 626 627 int pcre32_utf32_to_host_byte_order(PCRE_UCHAR32 *output, 628 PCRE_SPTR32 input, int length, int *byte_order, 629 int keep_boms); 630 631 632 THE PCRE 32-BIT LIBRARY 633 634 Starting with release 8.32, it is possible to compile a PCRE library 635 that supports 32-bit character strings, including UTF-32 strings, as 636 well as or instead of the original 8-bit library. This work was done by 637 Christian Persch, based on the work done by Zoltan Herczeg for the 638 16-bit library. All three libraries contain identical sets of func- 639 tions, used in exactly the same way. Only the names of the functions 640 and the data types of their arguments and results are different. To 641 avoid over-complication and reduce the documentation maintenance load, 642 most of the PCRE documentation describes the 8-bit library, with only 643 occasional references to the 16-bit and 32-bit libraries. This page 644 describes what is different when you use the 32-bit library. 645 646 WARNING: A single application can be linked with all or any of the 647 three libraries, but you must take care when processing any particular 648 pattern to use functions from just one library. For example, if you 649 want to study a pattern that was compiled with pcre32_compile(), you 650 must do so with pcre32_study(), not pcre_study(), and you must free the 651 study data with pcre32_free_study(). 652 653 654 THE HEADER FILE 655 656 There is only one header file, pcre.h. It contains prototypes for all 657 the functions in all libraries, as well as definitions of flags, struc- 658 tures, error codes, etc. 659 660 661 THE LIBRARY NAME 662 663 In Unix-like systems, the 32-bit library is called libpcre32, and can 664 normally be accesss by adding -lpcre32 to the command for linking an 665 application that uses PCRE. 666 667 668 STRING TYPES 669 670 In the 8-bit library, strings are passed to PCRE library functions as 671 vectors of bytes with the C type "char *". In the 32-bit library, 672 strings are passed as vectors of unsigned 32-bit quantities. The macro 673 PCRE_UCHAR32 specifies an appropriate data type, and PCRE_SPTR32 is 674 defined as "const PCRE_UCHAR32 *". In very many environments, "unsigned 675 int" is a 32-bit data type. When PCRE is built, it defines PCRE_UCHAR32 676 as "unsigned int", but checks that it really is a 32-bit data type. If 677 it is not, the build fails with an error message telling the maintainer 678 to modify the definition appropriately. 679 680 681 STRUCTURE TYPES 682 683 The types of the opaque structures that are used for compiled 32-bit 684 patterns and JIT stacks are pcre32 and pcre32_jit_stack respectively. 685 The type of the user-accessible structure that is returned by 686 pcre32_study() is pcre32_extra, and the type of the structure that is 687 used for passing data to a callout function is pcre32_callout_block. 688 These structures contain the same fields, with the same names, as their 689 8-bit counterparts. The only difference is that pointers to character 690 strings are 32-bit instead of 8-bit types. 691 692 693 32-BIT FUNCTIONS 694 695 For every function in the 8-bit library there is a corresponding func- 696 tion in the 32-bit library with a name that starts with pcre32_ instead 697 of pcre_. The prototypes are listed above. In addition, there is one 698 extra function, pcre32_utf32_to_host_byte_order(). This is a utility 699 function that converts a UTF-32 character string to host byte order if 700 necessary. The other 32-bit functions expect the strings they are 701 passed to be in host byte order. 702 703 The input and output arguments of pcre32_utf32_to_host_byte_order() may 704 point to the same address, that is, conversion in place is supported. 705 The output buffer must be at least as long as the input. 706 707 The length argument specifies the number of 32-bit data units in the 708 input string; a negative value specifies a zero-terminated string. 709 710 If byte_order is NULL, it is assumed that the string starts off in host 711 byte order. This may be changed by byte-order marks (BOMs) anywhere in 712 the string (commonly as the first character). 713 714 If byte_order is not NULL, a non-zero value of the integer to which it 715 points means that the input starts off in host byte order, otherwise 716 the opposite order is assumed. Again, BOMs in the string can change 717 this. The final byte order is passed back at the end of processing. 718 719 If keep_boms is not zero, byte-order mark characters (0xfeff) are 720 copied into the output string. Otherwise they are discarded. 721 722 The result of the function is the number of 32-bit units placed into 723 the output buffer, including the zero terminator if the string was 724 zero-terminated. 725 726 727 SUBJECT STRING OFFSETS 728 729 The lengths and starting offsets of subject strings must be specified 730 in 32-bit data units, and the offsets within subject strings that are 731 returned by the matching functions are in also 32-bit units rather than 732 bytes. 733 734 735 NAMED SUBPATTERNS 736 737 The name-to-number translation table that is maintained for named sub- 738 patterns uses 32-bit characters. The pcre32_get_stringtable_entries() 739 function returns the length of each entry in the table as the number of 740 32-bit data units. 741 742 743 OPTION NAMES 744 745 There are two new general option names, PCRE_UTF32 and 746 PCRE_NO_UTF32_CHECK, which correspond to PCRE_UTF8 and 747 PCRE_NO_UTF8_CHECK in the 8-bit library. In fact, these new options 748 define the same bits in the options word. There is a discussion about 749 the validity of UTF-32 strings in the pcreunicode page. 750 751 For the pcre32_config() function there is an option PCRE_CONFIG_UTF32 752 that returns 1 if UTF-32 support is configured, otherwise 0. If this 753 option is given to pcre_config() or pcre16_config(), or if the 754 PCRE_CONFIG_UTF8 or PCRE_CONFIG_UTF16 option is given to pcre32_con- 755 fig(), the result is the PCRE_ERROR_BADOPTION error. 756 757 758 CHARACTER CODES 759 760 In 32-bit mode, when PCRE_UTF32 is not set, character values are 761 treated in the same way as in 8-bit, non UTF-8 mode, except, of course, 762 that they can range from 0 to 0x7fffffff instead of 0 to 0xff. Charac- 763 ter types for characters less than 0xff can therefore be influenced by 764 the locale in the same way as before. Characters greater than 0xff 765 have only one case, and no "type" (such as letter or digit). 766 767 In UTF-32 mode, the character code is Unicode, in the range 0 to 768 0x10ffff, with the exception of values in the range 0xd800 to 0xdfff 769 because those are "surrogate" values that are ill-formed in UTF-32. 770 771 A UTF-32 string can indicate its endianness by special code knows as a 772 byte-order mark (BOM). The PCRE functions do not handle this, expecting 773 strings to be in host byte order. A utility function called 774 pcre32_utf32_to_host_byte_order() is provided to help with this (see 775 above). 776 777 778 ERROR NAMES 779 780 The error PCRE_ERROR_BADUTF32 corresponds to its 8-bit counterpart. 781 The error PCRE_ERROR_BADMODE is given when a compiled pattern is passed 782 to a function that processes patterns in the other mode, for example, 783 if a pattern compiled with pcre_compile() is passed to pcre32_exec(). 784 785 There are new error codes whose names begin with PCRE_UTF32_ERR for 786 invalid UTF-32 strings, corresponding to the PCRE_UTF8_ERR codes for 787 UTF-8 strings that are described in the section entitled "Reason codes 788 for invalid UTF-8 strings" in the main pcreapi page. The UTF-32 errors 789 are: 790 791 PCRE_UTF32_ERR1 Surrogate character (range from 0xd800 to 0xdfff) 792 PCRE_UTF32_ERR2 Non-character 793 PCRE_UTF32_ERR3 Character > 0x10ffff 794 795 796 ERROR TEXTS 797 798 If there is an error while compiling a pattern, the error text that is 799 passed back by pcre32_compile() or pcre32_compile2() is still an 8-bit 800 character string, zero-terminated. 801 802 803 CALLOUTS 804 805 The subject and mark fields in the callout block that is passed to a 806 callout function point to 32-bit vectors. 807 808 809 TESTING 810 811 The pcretest program continues to operate with 8-bit input and output 812 files, but it can be used for testing the 32-bit library. If it is run 813 with the command line option -32, patterns and subject strings are con- 814 verted from 8-bit to 32-bit before being passed to PCRE, and the 32-bit 815 library functions are used instead of the 8-bit ones. Returned 32-bit 816 strings are converted to 8-bit for output. If both the 8-bit and the 817 16-bit libraries were not compiled, pcretest defaults to 32-bit and the 818 -32 option is ignored. 819 820 When PCRE is being built, the RunTest script that is called by "make 821 check" uses the pcretest -C option to discover which of the 8-bit, 822 16-bit and 32-bit libraries has been built, and runs the tests appro- 823 priately. 824 825 826 NOT SUPPORTED IN 32-BIT MODE 827 828 Not all the features of the 8-bit library are available with the 32-bit 829 library. The C++ and POSIX wrapper functions support only the 8-bit 830 library, and the pcregrep program is at present 8-bit only. 831 832 833 AUTHOR 834 835 Philip Hazel 836 University Computing Service 837 Cambridge CB2 3QH, England. 838 839 840 REVISION 841 842 Last updated: 12 May 2013 843 Copyright (c) 1997-2013 University of Cambridge. 844 ------------------------------------------------------------------------------ 845 846 847 PCREBUILD(3) Library Functions Manual PCREBUILD(3) 848 849 850 851 NAME 852 PCRE - Perl-compatible regular expressions 853 854 BUILDING PCRE 855 856 PCRE is distributed with a configure script that can be used to build 857 the library in Unix-like environments using the applications known as 858 Autotools. Also in the distribution are files to support building 859 using CMake instead of configure. The text file README contains general 860 information about building with Autotools (some of which is repeated 861 below), and also has some comments about building on various operating 862 systems. There is a lot more information about building PCRE without 863 using Autotools (including information about using CMake and building 864 "by hand") in the text file called NON-AUTOTOOLS-BUILD. You should 865 consult this file as well as the README file if you are building in a 866 non-Unix-like environment. 867 868 869 PCRE BUILD-TIME OPTIONS 870 871 The rest of this document describes the optional features of PCRE that 872 can be selected when the library is compiled. It assumes use of the 873 configure script, where the optional features are selected or dese- 874 lected by providing options to configure before running the make com- 875 mand. However, the same options can be selected in both Unix-like and 876 non-Unix-like environments using the GUI facility of cmake-gui if you 877 are using CMake instead of configure to build PCRE. 878 879 If you are not using Autotools or CMake, option selection can be done 880 by editing the config.h file, or by passing parameter settings to the 881 compiler, as described in NON-AUTOTOOLS-BUILD. 882 883 The complete list of options for configure (which includes the standard 884 ones such as the selection of the installation directory) can be 885 obtained by running 886 887 ./configure --help 888 889 The following sections include descriptions of options whose names 890 begin with --enable or --disable. These settings specify changes to the 891 defaults for the configure command. Because of the way that configure 892 works, --enable and --disable always come in pairs, so the complemen- 893 tary option always exists as well, but as it specifies the default, it 894 is not described. 895 896 897 BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES 898 899 By default, a library called libpcre is built, containing functions 900 that take string arguments contained in vectors of bytes, either as 901 single-byte characters, or interpreted as UTF-8 strings. You can also 902 build a separate library, called libpcre16, in which strings are con- 903 tained in vectors of 16-bit data units and interpreted either as sin- 904 gle-unit characters or UTF-16 strings, by adding 905 906 --enable-pcre16 907 908 to the configure command. You can also build yet another separate 909 library, called libpcre32, in which strings are contained in vectors of 910 32-bit data units and interpreted either as single-unit characters or 911 UTF-32 strings, by adding 912 913 --enable-pcre32 914 915 to the configure command. If you do not want the 8-bit library, add 916 917 --disable-pcre8 918 919 as well. At least one of the three libraries must be built. Note that 920 the C++ and POSIX wrappers are for the 8-bit library only, and that 921 pcregrep is an 8-bit program. None of these are built if you select 922 only the 16-bit or 32-bit libraries. 923 924 925 BUILDING SHARED AND STATIC LIBRARIES 926 927 The Autotools PCRE building process uses libtool to build both shared 928 and static libraries by default. You can suppress one of these by 929 adding one of 930 931 --disable-shared 932 --disable-static 933 934 to the configure command, as required. 935 936 937 C++ SUPPORT 938 939 By default, if the 8-bit library is being built, the configure script 940 will search for a C++ compiler and C++ header files. If it finds them, 941 it automatically builds the C++ wrapper library (which supports only 942 8-bit strings). You can disable this by adding 943 944 --disable-cpp 945 946 to the configure command. 947 948 949 UTF-8, UTF-16 AND UTF-32 SUPPORT 950 951 To build PCRE with support for UTF Unicode character strings, add 952 953 --enable-utf 954 955 to the configure command. This setting applies to all three libraries, 956 adding support for UTF-8 to the 8-bit library, support for UTF-16 to 957 the 16-bit library, and support for UTF-32 to the to the 32-bit 958 library. There are no separate options for enabling UTF-8, UTF-16 and 959 UTF-32 independently because that would allow ridiculous settings such 960 as requesting UTF-16 support while building only the 8-bit library. It 961 is not possible to build one library with UTF support and another with- 962 out in the same configuration. (For backwards compatibility, --enable- 963 utf8 is a synonym of --enable-utf.) 964 965 Of itself, this setting does not make PCRE treat strings as UTF-8, 966 UTF-16 or UTF-32. As well as compiling PCRE with this option, you also 967 have have to set the PCRE_UTF8, PCRE_UTF16 or PCRE_UTF32 option (as 968 appropriate) when you call one of the pattern compiling functions. 969 970 If you set --enable-utf when compiling in an EBCDIC environment, PCRE 971 expects its input to be either ASCII or UTF-8 (depending on the run- 972 time option). It is not possible to support both EBCDIC and UTF-8 codes 973 in the same version of the library. Consequently, --enable-utf and 974 --enable-ebcdic are mutually exclusive. 975 976 977 UNICODE CHARACTER PROPERTY SUPPORT 978 979 UTF support allows the libraries to process character codepoints up to 980 0x10ffff in the strings that they handle. On its own, however, it does 981 not provide any facilities for accessing the properties of such charac- 982 ters. If you want to be able to use the pattern escapes \P, \p, and \X, 983 which refer to Unicode character properties, you must add 984 985 --enable-unicode-properties 986 987 to the configure command. This implies UTF support, even if you have 988 not explicitly requested it. 989 990 Including Unicode property support adds around 30K of tables to the 991 PCRE library. Only the general category properties such as Lu and Nd 992 are supported. Details are given in the pcrepattern documentation. 993 994 995 JUST-IN-TIME COMPILER SUPPORT 996 997 Just-in-time compiler support is included in the build by specifying 998 999 --enable-jit 1000 1001 This support is available only for certain hardware architectures. If 1002 this option is set for an unsupported architecture, a compile time 1003 error occurs. See the pcrejit documentation for a discussion of JIT 1004 usage. When JIT support is enabled, pcregrep automatically makes use of 1005 it, unless you add 1006 1007 --disable-pcregrep-jit 1008 1009 to the "configure" command. 1010 1011 1012 CODE VALUE OF NEWLINE 1013 1014 By default, PCRE interprets the linefeed (LF) character as indicating 1015 the end of a line. This is the normal newline character on Unix-like 1016 systems. You can compile PCRE to use carriage return (CR) instead, by 1017 adding 1018 1019 --enable-newline-is-cr 1020 1021 to the configure command. There is also a --enable-newline-is-lf 1022 option, which explicitly specifies linefeed as the newline character. 1023 1024 Alternatively, you can specify that line endings are to be indicated by 1025 the two character sequence CRLF. If you want this, add 1026 1027 --enable-newline-is-crlf 1028 1029 to the configure command. There is a fourth option, specified by 1030 1031 --enable-newline-is-anycrlf 1032 1033 which causes PCRE to recognize any of the three sequences CR, LF, or 1034 CRLF as indicating a line ending. Finally, a fifth option, specified by 1035 1036 --enable-newline-is-any 1037 1038 causes PCRE to recognize any Unicode newline sequence. 1039 1040 Whatever line ending convention is selected when PCRE is built can be 1041 overridden when the library functions are called. At build time it is 1042 conventional to use the standard for your operating system. 1043 1044 1045 WHAT \R MATCHES 1046 1047 By default, the sequence \R in a pattern matches any Unicode newline 1048 sequence, whatever has been selected as the line ending sequence. If 1049 you specify 1050 1051 --enable-bsr-anycrlf 1052 1053 the default is changed so that \R matches only CR, LF, or CRLF. What- 1054 ever is selected when PCRE is built can be overridden when the library 1055 functions are called. 1056 1057 1058 POSIX MALLOC USAGE 1059 1060 When the 8-bit library is called through the POSIX interface (see the 1061 pcreposix documentation), additional working storage is required for 1062 holding the pointers to capturing substrings, because PCRE requires 1063 three integers per substring, whereas the POSIX interface provides only 1064 two. If the number of expected substrings is small, the wrapper func- 1065 tion uses space on the stack, because this is faster than using mal- 1066 loc() for each call. The default threshold above which the stack is no 1067 longer used is 10; it can be changed by adding a setting such as 1068 1069 --with-posix-malloc-threshold=20 1070 1071 to the configure command. 1072 1073 1074 HANDLING VERY LARGE PATTERNS 1075 1076 Within a compiled pattern, offset values are used to point from one 1077 part to another (for example, from an opening parenthesis to an alter- 1078 nation metacharacter). By default, in the 8-bit and 16-bit libraries, 1079 two-byte values are used for these offsets, leading to a maximum size 1080 for a compiled pattern of around 64K. This is sufficient to handle all 1081 but the most gigantic patterns. Nevertheless, some people do want to 1082 process truly enormous patterns, so it is possible to compile PCRE to 1083 use three-byte or four-byte offsets by adding a setting such as 1084 1085 --with-link-size=3 1086 1087 to the configure command. The value given must be 2, 3, or 4. For the 1088 16-bit library, a value of 3 is rounded up to 4. In these libraries, 1089 using longer offsets slows down the operation of PCRE because it has to 1090 load additional data when handling them. For the 32-bit library the 1091 value is always 4 and cannot be overridden; the value of --with-link- 1092 size is ignored. 1093 1094 1095 AVOIDING EXCESSIVE STACK USAGE 1096 1097 When matching with the pcre_exec() function, PCRE implements backtrack- 1098 ing by making recursive calls to an internal function called match(). 1099 In environments where the size of the stack is limited, this can se- 1100 verely limit PCRE's operation. (The Unix environment does not usually 1101 suffer from this problem, but it may sometimes be necessary to increase 1102 the maximum stack size. There is a discussion in the pcrestack docu- 1103 mentation.) An alternative approach to recursion that uses memory from 1104 the heap to remember data, instead of using recursive function calls, 1105 has been implemented to work round the problem of limited stack size. 1106 If you want to build a version of PCRE that works this way, add 1107 1108 --disable-stack-for-recursion 1109 1110 to the configure command. With this configuration, PCRE will use the 1111 pcre_stack_malloc and pcre_stack_free variables to call memory manage- 1112 ment functions. By default these point to malloc() and free(), but you 1113 can replace the pointers so that your own functions are used instead. 1114 1115 Separate functions are provided rather than using pcre_malloc and 1116 pcre_free because the usage is very predictable: the block sizes 1117 requested are always the same, and the blocks are always freed in 1118 reverse order. A calling program might be able to implement optimized 1119 functions that perform better than malloc() and free(). PCRE runs 1120 noticeably more slowly when built in this way. This option affects only 1121 the pcre_exec() function; it is not relevant for pcre_dfa_exec(). 1122 1123 1124 LIMITING PCRE RESOURCE USAGE 1125 1126 Internally, PCRE has a function called match(), which it calls repeat- 1127 edly (sometimes recursively) when matching a pattern with the 1128 pcre_exec() function. By controlling the maximum number of times this 1129 function may be called during a single matching operation, a limit can 1130 be placed on the resources used by a single call to pcre_exec(). The 1131 limit can be changed at run time, as described in the pcreapi documen- 1132 tation. The default is 10 million, but this can be changed by adding a 1133 setting such as 1134 1135 --with-match-limit=500000 1136 1137 to the configure command. This setting has no effect on the 1138 pcre_dfa_exec() matching function. 1139 1140 In some environments it is desirable to limit the depth of recursive 1141 calls of match() more strictly than the total number of calls, in order 1142 to restrict the maximum amount of stack (or heap, if --disable-stack- 1143 for-recursion is specified) that is used. A second limit controls this; 1144 it defaults to the value that is set for --with-match-limit, which 1145 imposes no additional constraints. However, you can set a lower limit 1146 by adding, for example, 1147 1148 --with-match-limit-recursion=10000 1149 1150 to the configure command. This value can also be overridden at run 1151 time. 1152 1153 1154 CREATING CHARACTER TABLES AT BUILD TIME 1155 1156 PCRE uses fixed tables for processing characters whose code values are 1157 less than 256. By default, PCRE is built with a set of tables that are 1158 distributed in the file pcre_chartables.c.dist. These tables are for 1159 ASCII codes only. If you add 1160 1161 --enable-rebuild-chartables 1162 1163 to the configure command, the distributed tables are no longer used. 1164 Instead, a program called dftables is compiled and run. This outputs 1165 the source for new set of tables, created in the default locale of your 1166 C run-time system. (This method of replacing the tables does not work 1167 if you are cross compiling, because dftables is run on the local host. 1168 If you need to create alternative tables when cross compiling, you will 1169 have to do so "by hand".) 1170 1171 1172 USING EBCDIC CODE 1173 1174 PCRE assumes by default that it will run in an environment where the 1175 character code is ASCII (or Unicode, which is a superset of ASCII). 1176 This is the case for most computer operating systems. PCRE can, how- 1177 ever, be compiled to run in an EBCDIC environment by adding 1178 1179 --enable-ebcdic 1180 1181 to the configure command. This setting implies --enable-rebuild-charta- 1182 bles. You should only use it if you know that you are in an EBCDIC 1183 environment (for example, an IBM mainframe operating system). The 1184 --enable-ebcdic option is incompatible with --enable-utf. 1185 1186 The EBCDIC character that corresponds to an ASCII LF is assumed to have 1187 the value 0x15 by default. However, in some EBCDIC environments, 0x25 1188 is used. In such an environment you should use 1189 1190 --enable-ebcdic-nl25 1191 1192 as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR 1193 has the same value as in ASCII, namely, 0x0d. Whichever of 0x15 and 1194 0x25 is not chosen as LF is made to correspond to the Unicode NEL char- 1195 acter (which, in Unicode, is 0x85). 1196 1197 The options that select newline behaviour, such as --enable-newline-is- 1198 cr, and equivalent run-time options, refer to these character values in 1199 an EBCDIC environment. 1200 1201 1202 PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT 1203 1204 By default, pcregrep reads all files as plain text. You can build it so 1205 that it recognizes files whose names end in .gz or .bz2, and reads them 1206 with libz or libbz2, respectively, by adding one or both of 1207 1208 --enable-pcregrep-libz 1209 --enable-pcregrep-libbz2 1210 1211 to the configure command. These options naturally require that the rel- 1212 evant libraries are installed on your system. Configuration will fail 1213 if they are not. 1214 1215 1216 PCREGREP BUFFER SIZE 1217 1218 pcregrep uses an internal buffer to hold a "window" on the file it is 1219 scanning, in order to be able to output "before" and "after" lines when 1220 it finds a match. The size of the buffer is controlled by a parameter 1221 whose default value is 20K. The buffer itself is three times this size, 1222 but because of the way it is used for holding "before" lines, the long- 1223 est line that is guaranteed to be processable is the parameter size. 1224 You can change the default parameter value by adding, for example, 1225 1226 --with-pcregrep-bufsize=50K 1227 1228 to the configure command. The caller of pcregrep can, however, override 1229 this value by specifying a run-time option. 1230 1231 1232 PCRETEST OPTION FOR LIBREADLINE SUPPORT 1233 1234 If you add 1235 1236 --enable-pcretest-libreadline 1237 1238 to the configure command, pcretest is linked with the libreadline 1239 library, and when its input is from a terminal, it reads it using the 1240 readline() function. This provides line-editing and history facilities. 1241 Note that libreadline is GPL-licensed, so if you distribute a binary of 1242 pcretest linked in this way, there may be licensing issues. 1243 1244 Setting this option causes the -lreadline option to be added to the 1245 pcretest build. In many operating environments with a sytem-installed 1246 libreadline this is sufficient. However, in some environments (e.g. if 1247 an unmodified distribution version of readline is in use), some extra 1248 configuration may be necessary. The INSTALL file for libreadline says 1249 this: 1250 1251 "Readline uses the termcap functions, but does not link with the 1252 termcap or curses library itself, allowing applications which link 1253 with readline the to choose an appropriate library." 1254 1255 If your environment has not been set up so that an appropriate library 1256 is automatically included, you may need to add something like 1257 1258 LIBS="-ncurses" 1259 1260 immediately before the configure command. 1261 1262 1263 DEBUGGING WITH VALGRIND SUPPORT 1264 1265 By adding the 1266 1267 --enable-valgrind 1268 1269 option to to the configure command, PCRE will use valgrind annotations 1270 to mark certain memory regions as unaddressable. This allows it to 1271 detect invalid memory accesses, and is mostly useful for debugging PCRE 1272 itself. 1273 1274 1275 CODE COVERAGE REPORTING 1276 1277 If your C compiler is gcc, you can build a version of PCRE that can 1278 generate a code coverage report for its test suite. To enable this, you 1279 must install lcov version 1.6 or above. Then specify 1280 1281 --enable-coverage 1282 1283 to the configure command and build PCRE in the usual way. 1284 1285 Note that using ccache (a caching C compiler) is incompatible with code 1286 coverage reporting. If you have configured ccache to run automatically 1287 on your system, you must set the environment variable 1288 1289 CCACHE_DISABLE=1 1290 1291 before running make to build PCRE, so that ccache is not used. 1292 1293 When --enable-coverage is used, the following addition targets are 1294 added to the Makefile: 1295 1296 make coverage 1297 1298 This creates a fresh coverage report for the PCRE test suite. It is 1299 equivalent to running "make coverage-reset", "make coverage-baseline", 1300 "make check", and then "make coverage-report". 1301 1302 make coverage-reset 1303 1304 This zeroes the coverage counters, but does nothing else. 1305 1306 make coverage-baseline 1307 1308 This captures baseline coverage information. 1309 1310 make coverage-report 1311 1312 This creates the coverage report. 1313 1314 make coverage-clean-report 1315 1316 This removes the generated coverage report without cleaning the cover- 1317 age data itself. 1318 1319 make coverage-clean-data 1320 1321 This removes the captured coverage data without removing the coverage 1322 files created at compile time (*.gcno). 1323 1324 make coverage-clean 1325 1326 This cleans all coverage data including the generated coverage report. 1327 For more information about code coverage, see the gcov and lcov docu- 1328 mentation. 1329 1330 1331 SEE ALSO 1332 1333 pcreapi(3), pcre16, pcre32, pcre_config(3). 1334 1335 1336 AUTHOR 1337 1338 Philip Hazel 1339 University Computing Service 1340 Cambridge CB2 3QH, England. 1341 1342 1343 REVISION 1344 1345 Last updated: 12 May 2013 1346 Copyright (c) 1997-2013 University of Cambridge. 1347 ------------------------------------------------------------------------------ 1348 1349 1350 PCREMATCHING(3) Library Functions Manual PCREMATCHING(3) 1351 1352 1353 1354 NAME 1355 PCRE - Perl-compatible regular expressions 1356 1357 PCRE MATCHING ALGORITHMS 1358 1359 This document describes the two different algorithms that are available 1360 in PCRE for matching a compiled regular expression against a given sub- 1361 ject string. The "standard" algorithm is the one provided by the 1362 pcre_exec(), pcre16_exec() and pcre32_exec() functions. These work in 1363 the same as as Perl's matching function, and provide a Perl-compatible 1364 matching operation. The just-in-time (JIT) optimization that is 1365 described in the pcrejit documentation is compatible with these func- 1366 tions. 1367 1368 An alternative algorithm is provided by the pcre_dfa_exec(), 1369 pcre16_dfa_exec() and pcre32_dfa_exec() functions; they operate in a 1370 different way, and are not Perl-compatible. This alternative has advan- 1371 tages and disadvantages compared with the standard algorithm, and these 1372 are described below. 1373 1374 When there is only one possible way in which a given subject string can 1375 match a pattern, the two algorithms give the same answer. A difference 1376 arises, however, when there are multiple possibilities. For example, if 1377 the pattern 1378 1379 ^<.*> 1380 1381 is matched against the string 1382 1383 <something> <something else> <something further> 1384 1385 there are three possible answers. The standard algorithm finds only one 1386 of them, whereas the alternative algorithm finds all three. 1387 1388 1389 REGULAR EXPRESSIONS AS TREES 1390 1391 The set of strings that are matched by a regular expression can be rep- 1392 resented as a tree structure. An unlimited repetition in the pattern 1393 makes the tree of infinite size, but it is still a tree. Matching the 1394 pattern to a given subject string (from a given starting point) can be 1395 thought of as a search of the tree. There are two ways to search a 1396 tree: depth-first and breadth-first, and these correspond to the two 1397 matching algorithms provided by PCRE. 1398 1399 1400 THE STANDARD MATCHING ALGORITHM 1401 1402 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- 1403 sions", the standard algorithm is an "NFA algorithm". It conducts a 1404 depth-first search of the pattern tree. That is, it proceeds along a 1405 single path through the tree, checking that the subject matches what is 1406 required. When there is a mismatch, the algorithm tries any alterna- 1407 tives at the current point, and if they all fail, it backs up to the 1408 previous branch point in the tree, and tries the next alternative 1409 branch at that level. This often involves backing up (moving to the 1410 left) in the subject string as well. The order in which repetition 1411 branches are tried is controlled by the greedy or ungreedy nature of 1412 the quantifier. 1413 1414 If a leaf node is reached, a matching string has been found, and at 1415 that point the algorithm stops. Thus, if there is more than one possi- 1416 ble match, this algorithm returns the first one that it finds. Whether 1417 this is the shortest, the longest, or some intermediate length depends 1418 on the way the greedy and ungreedy repetition quantifiers are specified 1419 in the pattern. 1420 1421 Because it ends up with a single path through the tree, it is rela- 1422 tively straightforward for this algorithm to keep track of the sub- 1423 strings that are matched by portions of the pattern in parentheses. 1424 This provides support for capturing parentheses and back references. 1425 1426 1427 THE ALTERNATIVE MATCHING ALGORITHM 1428 1429 This algorithm conducts a breadth-first search of the tree. Starting 1430 from the first matching point in the subject, it scans the subject 1431 string from left to right, once, character by character, and as it does 1432 this, it remembers all the paths through the tree that represent valid 1433 matches. In Friedl's terminology, this is a kind of "DFA algorithm", 1434 though it is not implemented as a traditional finite state machine (it 1435 keeps multiple states active simultaneously). 1436 1437 Although the general principle of this matching algorithm is that it 1438 scans the subject string only once, without backtracking, there is one 1439 exception: when a lookaround assertion is encountered, the characters 1440 following or preceding the current point have to be independently 1441 inspected. 1442 1443 The scan continues until either the end of the subject is reached, or 1444 there are no more unterminated paths. At this point, terminated paths 1445 represent the different matching possibilities (if there are none, the 1446 match has failed). Thus, if there is more than one possible match, 1447 this algorithm finds all of them, and in particular, it finds the long- 1448 est. The matches are returned in decreasing order of length. There is 1449 an option to stop the algorithm after the first match (which is neces- 1450 sarily the shortest) is found. 1451 1452 Note that all the matches that are found start at the same point in the 1453 subject. If the pattern 1454 1455 cat(er(pillar)?)? 1456 1457 is matched against the string "the caterpillar catchment", the result 1458 will be the three strings "caterpillar", "cater", and "cat" that start 1459 at the fifth character of the subject. The algorithm does not automati- 1460 cally move on to find matches that start at later positions. 1461 1462 PCRE's "auto-possessification" optimization usually applies to charac- 1463 ter repeats at the end of a pattern (as well as internally). For exam- 1464 ple, the pattern "a\d+" is compiled as if it were "a\d++" because there 1465 is no point even considering the possibility of backtracking into the 1466 repeated digits. For DFA matching, this means that only one possible 1467 match is found. If you really do want multiple matches in such cases, 1468 either use an ungreedy repeat ("a\d+?") or set the PCRE_NO_AUTO_POSSESS 1469 option when compiling. 1470 1471 There are a number of features of PCRE regular expressions that are not 1472 supported by the alternative matching algorithm. They are as follows: 1473 1474 1. Because the algorithm finds all possible matches, the greedy or 1475 ungreedy nature of repetition quantifiers is not relevant. Greedy and 1476 ungreedy quantifiers are treated in exactly the same way. However, pos- 1477 sessive quantifiers can make a difference when what follows could also 1478 match what is quantified, for example in a pattern like this: 1479 1480 ^a++\w! 1481 1482 This pattern matches "aaab!" but not "aaa!", which would be matched by 1483 a non-possessive quantifier. Similarly, if an atomic group is present, 1484 it is matched as if it were a standalone pattern at the current point, 1485 and the longest match is then "locked in" for the rest of the overall 1486 pattern. 1487 1488 2. When dealing with multiple paths through the tree simultaneously, it 1489 is not straightforward to keep track of captured substrings for the 1490 different matching possibilities, and PCRE's implementation of this 1491 algorithm does not attempt to do this. This means that no captured sub- 1492 strings are available. 1493 1494 3. Because no substrings are captured, back references within the pat- 1495 tern are not supported, and cause errors if encountered. 1496 1497 4. For the same reason, conditional expressions that use a backrefer- 1498 ence as the condition or test for a specific group recursion are not 1499 supported. 1500 1501 5. Because many paths through the tree may be active, the \K escape 1502 sequence, which resets the start of the match when encountered (but may 1503 be on some paths and not on others), is not supported. It causes an 1504 error if encountered. 1505 1506 6. Callouts are supported, but the value of the capture_top field is 1507 always 1, and the value of the capture_last field is always -1. 1508 1509 7. The \C escape sequence, which (in the standard algorithm) always 1510 matches a single data unit, even in UTF-8, UTF-16 or UTF-32 modes, is 1511 not supported in these modes, because the alternative algorithm moves 1512 through the subject string one character (not data unit) at a time, for 1513 all active paths through the tree. 1514 1515 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) 1516 are not supported. (*FAIL) is supported, and behaves like a failing 1517 negative assertion. 1518 1519 1520 ADVANTAGES OF THE ALTERNATIVE ALGORITHM 1521 1522 Using the alternative matching algorithm provides the following advan- 1523 tages: 1524 1525 1. All possible matches (at a single point in the subject) are automat- 1526 ically found, and in particular, the longest match is found. To find 1527 more than one match using the standard algorithm, you have to do kludgy 1528 things with callouts. 1529 1530 2. Because the alternative algorithm scans the subject string just 1531 once, and never needs to backtrack (except for lookbehinds), it is pos- 1532 sible to pass very long subject strings to the matching function in 1533 several pieces, checking for partial matching each time. Although it is 1534 possible to do multi-segment matching using the standard algorithm by 1535 retaining partially matched substrings, it is more complicated. The 1536 pcrepartial documentation gives details of partial matching and dis- 1537 cusses multi-segment matching. 1538 1539 1540 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM 1541 1542 The alternative algorithm suffers from a number of disadvantages: 1543 1544 1. It is substantially slower than the standard algorithm. This is 1545 partly because it has to search for all possible matches, but is also 1546 because it is less susceptible to optimization. 1547 1548 2. Capturing parentheses and back references are not supported. 1549 1550 3. Although atomic groups are supported, their use does not provide the 1551 performance advantage that it does for the standard algorithm. 1552 1553 1554 AUTHOR 1555 1556 Philip Hazel 1557 University Computing Service 1558 Cambridge CB2 3QH, England. 1559 1560 1561 REVISION 1562 1563 Last updated: 12 November 2013 1564 Copyright (c) 1997-2012 University of Cambridge. 1565 ------------------------------------------------------------------------------ 1566 1567 1568 PCREAPI(3) Library Functions Manual PCREAPI(3) 1569 1570 1571 1572 NAME 1573 PCRE - Perl-compatible regular expressions 1574 1575 #include <pcre.h> 1576 1577 1578 PCRE NATIVE API BASIC FUNCTIONS 1579 1580 pcre *pcre_compile(const char *pattern, int options, 1581 const char **errptr, int *erroffset, 1582 const unsigned char *tableptr); 1583 1584 pcre *pcre_compile2(const char *pattern, int options, 1585 int *errorcodeptr, 1586 const char **errptr, int *erroffset, 1587 const unsigned char *tableptr); 1588 1589 pcre_extra *pcre_study(const pcre *code, int options, 1590 const char **errptr); 1591 1592 void pcre_free_study(pcre_extra *extra); 1593 1594 int pcre_exec(const pcre *code, const pcre_extra *extra, 1595 const char *subject, int length, int startoffset, 1596 int options, int *ovector, int ovecsize); 1597 1598 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, 1599 const char *subject, int length, int startoffset, 1600 int options, int *ovector, int ovecsize, 1601 int *workspace, int wscount); 1602 1603 1604 PCRE NATIVE API STRING EXTRACTION FUNCTIONS 1605 1606 int pcre_copy_named_substring(const pcre *code, 1607 const char *subject, int *ovector, 1608 int stringcount, const char *stringname, 1609 char *buffer, int buffersize); 1610 1611 int pcre_copy_substring(const char *subject, int *ovector, 1612 int stringcount, int stringnumber, char *buffer, 1613 int buffersize); 1614 1615 int pcre_get_named_substring(const pcre *code, 1616 const char *subject, int *ovector, 1617 int stringcount, const char *stringname, 1618 const char **stringptr); 1619 1620 int pcre_get_stringnumber(const pcre *code, 1621 const char *name); 1622 1623 int pcre_get_stringtable_entries(const pcre *code, 1624 const char *name, char **first, char **last); 1625 1626 int pcre_get_substring(const char *subject, int *ovector, 1627 int stringcount, int stringnumber, 1628 const char **stringptr); 1629 1630 int pcre_get_substring_list(const char *subject, 1631 int *ovector, int stringcount, const char ***listptr); 1632 1633 void pcre_free_substring(const char *stringptr); 1634 1635 void pcre_free_substring_list(const char **stringptr); 1636 1637 1638 PCRE NATIVE API AUXILIARY FUNCTIONS 1639 1640 int pcre_jit_exec(const pcre *code, const pcre_extra *extra, 1641 const char *subject, int length, int startoffset, 1642 int options, int *ovector, int ovecsize, 1643 pcre_jit_stack *jstack); 1644 1645 pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize); 1646 1647 void pcre_jit_stack_free(pcre_jit_stack *stack); 1648 1649 void pcre_assign_jit_stack(pcre_extra *extra, 1650 pcre_jit_callback callback, void *data); 1651 1652 const unsigned char *pcre_maketables(void); 1653 1654 int pcre_fullinfo(const pcre *code, const pcre_extra *extra, 1655 int what, void *where); 1656 1657 int pcre_refcount(pcre *code, int adjust); 1658 1659 int pcre_config(int what, void *where); 1660 1661 const char *pcre_version(void); 1662 1663 int pcre_pattern_to_host_byte_order(pcre *code, 1664 pcre_extra *extra, const unsigned char *tables); 1665 1666 1667 PCRE NATIVE API INDIRECTED FUNCTIONS 1668 1669 void *(*pcre_malloc)(size_t); 1670 1671 void (*pcre_free)(void *); 1672 1673 void *(*pcre_stack_malloc)(size_t); 1674 1675 void (*pcre_stack_free)(void *); 1676 1677 int (*pcre_callout)(pcre_callout_block *); 1678 1679 int (*pcre_stack_guard)(void); 1680 1681 1682 PCRE 8-BIT, 16-BIT, AND 32-BIT LIBRARIES 1683 1684 As well as support for 8-bit character strings, PCRE also supports 1685 16-bit strings (from release 8.30) and 32-bit strings (from release 1686 8.32), by means of two additional libraries. They can be built as well 1687 as, or instead of, the 8-bit library. To avoid too much complication, 1688 this document describes the 8-bit versions of the functions, with only 1689 occasional references to the 16-bit and 32-bit libraries. 1690 1691 The 16-bit and 32-bit functions operate in the same way as their 8-bit 1692 counterparts; they just use different data types for their arguments 1693 and results, and their names start with pcre16_ or pcre32_ instead of 1694 pcre_. For every option that has UTF8 in its name (for example, 1695 PCRE_UTF8), there are corresponding 16-bit and 32-bit names with UTF8 1696 replaced by UTF16 or UTF32, respectively. This facility is in fact just 1697 cosmetic; the 16-bit and 32-bit option names define the same bit val- 1698 ues. 1699 1700 References to bytes and UTF-8 in this document should be read as refer- 1701 ences to 16-bit data units and UTF-16 when using the 16-bit library, or 1702 32-bit data units and UTF-32 when using the 32-bit library, unless 1703 specified otherwise. More details of the specific differences for the 1704 16-bit and 32-bit libraries are given in the pcre16 and pcre32 pages. 1705 1706 1707 PCRE API OVERVIEW 1708 1709 PCRE has its own native API, which is described in this document. There 1710 are also some wrapper functions (for the 8-bit library only) that cor- 1711 respond to the POSIX regular expression API, but they do not give 1712 access to all the functionality. They are described in the pcreposix 1713 documentation. Both of these APIs define a set of C function calls. A 1714 C++ wrapper (again for the 8-bit library only) is also distributed with 1715 PCRE. It is documented in the pcrecpp page. 1716 1717 The native API C function prototypes are defined in the header file 1718 pcre.h, and on Unix-like systems the (8-bit) library itself is called 1719 libpcre. It can normally be accessed by adding -lpcre to the command 1720 for linking an application that uses PCRE. The header file defines the 1721 macros PCRE_MAJOR and PCRE_MINOR to contain the major and minor release 1722 numbers for the library. Applications can use these to include support 1723 for different releases of PCRE. 1724 1725 In a Windows environment, if you want to statically link an application 1726 program against a non-dll pcre.a file, you must define PCRE_STATIC 1727 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal- 1728 loc() and pcre_free() exported functions will be declared 1729 __declspec(dllimport), with unwanted results. 1730 1731 The functions pcre_compile(), pcre_compile2(), pcre_study(), and 1732 pcre_exec() are used for compiling and matching regular expressions in 1733 a Perl-compatible manner. A sample program that demonstrates the sim- 1734 plest way of using them is provided in the file called pcredemo.c in 1735 the PCRE source distribution. A listing of this program is given in the 1736 pcredemo documentation, and the pcresample documentation describes how 1737 to compile and run it. 1738 1739 Just-in-time compiler support is an optional feature of PCRE that can 1740 be built in appropriate hardware environments. It greatly speeds up the 1741 matching performance of many patterns. Simple programs can easily 1742 request that it be used if available, by setting an option that is 1743 ignored when it is not relevant. More complicated programs might need 1744 to make use of the functions pcre_jit_stack_alloc(), 1745 pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control 1746 the JIT code's memory usage. 1747 1748 From release 8.32 there is also a direct interface for JIT execution, 1749 which gives improved performance. The JIT-specific functions are dis- 1750 cussed in the pcrejit documentation. 1751 1752 A second matching function, pcre_dfa_exec(), which is not Perl-compati- 1753 ble, is also provided. This uses a different algorithm for the match- 1754 ing. The alternative algorithm finds all possible matches (at a given 1755 point in the subject), and scans the subject just once (unless there 1756 are lookbehind assertions). However, this algorithm does not return 1757 captured substrings. A description of the two matching algorithms and 1758 their advantages and disadvantages is given in the pcrematching docu- 1759 mentation. 1760 1761 In addition to the main compiling and matching functions, there are 1762 convenience functions for extracting captured substrings from a subject 1763 string that is matched by pcre_exec(). They are: 1764 1765 pcre_copy_substring() 1766 pcre_copy_named_substring() 1767 pcre_get_substring() 1768 pcre_get_named_substring() 1769 pcre_get_substring_list() 1770 pcre_get_stringnumber() 1771 pcre_get_stringtable_entries() 1772 1773 pcre_free_substring() and pcre_free_substring_list() are also provided, 1774 to free the memory used for extracted strings. 1775 1776 The function pcre_maketables() is used to build a set of character 1777 tables in the current locale for passing to pcre_compile(), 1778 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is 1779 provided for specialist use. Most commonly, no special tables are 1780 passed, in which case internal tables that are generated when PCRE is 1781 built are used. 1782 1783 The function pcre_fullinfo() is used to find out information about a 1784 compiled pattern. The function pcre_version() returns a pointer to a 1785 string containing the version of PCRE and its date of release. 1786 1787 The function pcre_refcount() maintains a reference count in a data 1788 block containing a compiled pattern. This is provided for the benefit 1789 of object-oriented applications. 1790 1791 The global variables pcre_malloc and pcre_free initially contain the 1792 entry points of the standard malloc() and free() functions, respec- 1793 tively. PCRE calls the memory management functions via these variables, 1794 so a calling program can replace them if it wishes to intercept the 1795 calls. This should be done before calling any PCRE functions. 1796 1797 The global variables pcre_stack_malloc and pcre_stack_free are also 1798 indirections to memory management functions. These special functions 1799 are used only when PCRE is compiled to use the heap for remembering 1800 data, instead of recursive function calls, when running the pcre_exec() 1801 function. See the pcrebuild documentation for details of how to do 1802 this. It is a non-standard way of building PCRE, for use in environ- 1803 ments that have limited stacks. Because of the greater use of memory 1804 management, it runs more slowly. Separate functions are provided so 1805 that special-purpose external code can be used for this case. When 1806 used, these functions are always called in a stack-like manner (last 1807 obtained, first freed), and always for memory blocks of the same size. 1808 There is a discussion about PCRE's stack usage in the pcrestack docu- 1809 mentation. 1810 1811 The global variable pcre_callout initially contains NULL. It can be set 1812 by the caller to a "callout" function, which PCRE will then call at 1813 specified points during a matching operation. Details are given in the 1814 pcrecallout documentation. 1815 1816 The global variable pcre_stack_guard initially contains NULL. It can be 1817 set by the caller to a function that is called by PCRE whenever it 1818 starts to compile a parenthesized part of a pattern. When parentheses 1819 are nested, PCRE uses recursive function calls, which use up the system 1820 stack. This function is provided so that applications with restricted 1821 stacks can force a compilation error if the stack runs out. The func- 1822 tion should return zero if all is well, or non-zero to force an error. 1823 1824 1825 NEWLINES 1826 1827 PCRE supports five different conventions for indicating line breaks in 1828 strings: a single CR (carriage return) character, a single LF (line- 1829 feed) character, the two-character sequence CRLF, any of the three pre- 1830 ceding, or any Unicode newline sequence. The Unicode newline sequences 1831 are the three just mentioned, plus the single characters VT (vertical 1832 tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line 1833 separator, U+2028), and PS (paragraph separator, U+2029). 1834 1835 Each of the first three conventions is used by at least one operating 1836 system as its standard newline sequence. When PCRE is built, a default 1837 can be specified. The default default is LF, which is the Unix stan- 1838 dard. When PCRE is run, the default can be overridden, either when a 1839 pattern is compiled, or when it is matched. 1840 1841 At compile time, the newline convention can be specified by the options 1842 argument of pcre_compile(), or it can be specified by special text at 1843 the start of the pattern itself; this overrides any other settings. See 1844 the pcrepattern page for details of the special character sequences. 1845 1846 In the PCRE documentation the word "newline" is used to mean "the char- 1847 acter or pair of characters that indicate a line break". The choice of 1848 newline convention affects the handling of the dot, circumflex, and 1849 dollar metacharacters, the handling of #-comments in /x mode, and, when 1850 CRLF is a recognized line ending sequence, the match position advance- 1851 ment for a non-anchored pattern. There is more detail about this in the 1852 section on pcre_exec() options below. 1853 1854 The choice of newline convention does not affect the interpretation of 1855 the \n or \r escape sequences, nor does it affect what \R matches, 1856 which is controlled in a similar way, but by separate options. 1857 1858 1859 MULTITHREADING 1860 1861 The PCRE functions can be used in multi-threading applications, with 1862 the proviso that the memory management functions pointed to by 1863 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the 1864 callout and stack-checking functions pointed to by pcre_callout and 1865 pcre_stack_guard, are shared by all threads. 1866 1867 The compiled form of a regular expression is not altered during match- 1868 ing, so the same compiled pattern can safely be used by several threads 1869 at once. 1870 1871 If the just-in-time optimization feature is being used, it needs sepa- 1872 rate memory stack areas for each thread. See the pcrejit documentation 1873 for more details. 1874 1875 1876 SAVING PRECOMPILED PATTERNS FOR LATER USE 1877 1878 The compiled form of a regular expression can be saved and re-used at a 1879 later time, possibly by a different program, and even on a host other 1880 than the one on which it was compiled. Details are given in the 1881 pcreprecompile documentation, which includes a description of the 1882 pcre_pattern_to_host_byte_order() function. However, compiling a regu- 1883 lar expression with one version of PCRE for use with a different ver- 1884 sion is not guaranteed to work and may cause crashes. 1885 1886 1887 CHECKING BUILD-TIME OPTIONS 1888 1889 int pcre_config(int what, void *where); 1890 1891 The function pcre_config() makes it possible for a PCRE client to dis- 1892 cover which optional features have been compiled into the PCRE library. 1893 The pcrebuild documentation has more details about these optional fea- 1894 tures. 1895 1896 The first argument for pcre_config() is an integer, specifying which 1897 information is required; the second argument is a pointer to a variable 1898 into which the information is placed. The returned value is zero on 1899 success, or the negative error code PCRE_ERROR_BADOPTION if the value 1900 in the first argument is not recognized. The following information is 1901 available: 1902 1903 PCRE_CONFIG_UTF8 1904 1905 The output is an integer that is set to one if UTF-8 support is avail- 1906 able; otherwise it is set to zero. This value should normally be given 1907 to the 8-bit version of this function, pcre_config(). If it is given to 1908 the 16-bit or 32-bit version of this function, the result is 1909 PCRE_ERROR_BADOPTION. 1910 1911 PCRE_CONFIG_UTF16 1912 1913 The output is an integer that is set to one if UTF-16 support is avail- 1914 able; otherwise it is set to zero. This value should normally be given 1915 to the 16-bit version of this function, pcre16_config(). If it is given 1916 to the 8-bit or 32-bit version of this function, the result is 1917 PCRE_ERROR_BADOPTION. 1918 1919 PCRE_CONFIG_UTF32 1920 1921 The output is an integer that is set to one if UTF-32 support is avail- 1922 able; otherwise it is set to zero. This value should normally be given 1923 to the 32-bit version of this function, pcre32_config(). If it is given 1924 to the 8-bit or 16-bit version of this function, the result is 1925 PCRE_ERROR_BADOPTION. 1926 1927 PCRE_CONFIG_UNICODE_PROPERTIES 1928 1929 The output is an integer that is set to one if support for Unicode 1930 character properties is available; otherwise it is set to zero. 1931 1932 PCRE_CONFIG_JIT 1933 1934 The output is an integer that is set to one if support for just-in-time 1935 compiling is available; otherwise it is set to zero. 1936 1937 PCRE_CONFIG_JITTARGET 1938 1939 The output is a pointer to a zero-terminated "const char *" string. If 1940 JIT support is available, the string contains the name of the architec- 1941 ture for which the JIT compiler is configured, for example "x86 32bit 1942 (little endian + unaligned)". If JIT support is not available, the 1943 result is NULL. 1944 1945 PCRE_CONFIG_NEWLINE 1946 1947 The output is an integer whose value specifies the default character 1948 sequence that is recognized as meaning "newline". The values that are 1949 supported in ASCII/Unicode environments are: 10 for LF, 13 for CR, 3338 1950 for CRLF, -2 for ANYCRLF, and -1 for ANY. In EBCDIC environments, CR, 1951 ANYCRLF, and ANY yield the same values. However, the value for LF is 1952 normally 21, though some EBCDIC environments use 37. The corresponding 1953 values for CRLF are 3349 and 3365. The default should normally corre- 1954 spond to the standard sequence for your operating system. 1955 1956 PCRE_CONFIG_BSR 1957 1958 The output is an integer whose value indicates what character sequences 1959 the \R escape sequence matches by default. A value of 0 means that \R 1960 matches any Unicode line ending sequence; a value of 1 means that \R 1961 matches only CR, LF, or CRLF. The default can be overridden when a pat- 1962 tern is compiled or matched. 1963 1964 PCRE_CONFIG_LINK_SIZE 1965 1966 The output is an integer that contains the number of bytes used for 1967 internal linkage in compiled regular expressions. For the 8-bit 1968 library, the value can be 2, 3, or 4. For the 16-bit library, the value 1969 is either 2 or 4 and is still a number of bytes. For the 32-bit 1970 library, the value is either 2 or 4 and is still a number of bytes. The 1971 default value of 2 is sufficient for all but the most massive patterns, 1972 since it allows the compiled pattern to be up to 64K in size. Larger 1973 values allow larger regular expressions to be compiled, at the expense 1974 of slower matching. 1975 1976 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD 1977 1978 The output is an integer that contains the threshold above which the 1979 POSIX interface uses malloc() for output vectors. Further details are 1980 given in the pcreposix documentation. 1981 1982 PCRE_CONFIG_PARENS_LIMIT 1983 1984 The output is a long integer that gives the maximum depth of nesting of 1985 parentheses (of any kind) in a pattern. This limit is imposed to cap 1986 the amount of system stack used when a pattern is compiled. It is spec- 1987 ified when PCRE is built; the default is 250. This limit does not take 1988 into account the stack that may already be used by the calling applica- 1989 tion. For finer control over compilation stack usage, you can set a 1990 pointer to an external checking function in pcre_stack_guard. 1991 1992 PCRE_CONFIG_MATCH_LIMIT 1993 1994 The output is a long integer that gives the default limit for the num- 1995 ber of internal matching function calls in a pcre_exec() execution. 1996 Further details are given with pcre_exec() below. 1997 1998 PCRE_CONFIG_MATCH_LIMIT_RECURSION 1999 2000 The output is a long integer that gives the default limit for the depth 2001 of recursion when calling the internal matching function in a 2002 pcre_exec() execution. Further details are given with pcre_exec() 2003 below. 2004 2005 PCRE_CONFIG_STACKRECURSE 2006 2007 The output is an integer that is set to one if internal recursion when 2008 running pcre_exec() is implemented by recursive function calls that use 2009 the stack to remember their state. This is the usual way that PCRE is 2010 compiled. The output is zero if PCRE was compiled to use blocks of data 2011 on the heap instead of recursive function calls. In this case, 2012 pcre_stack_malloc and pcre_stack_free are called to manage memory 2013 blocks on the heap, thus avoiding the use of the stack. 2014 2015 2016 COMPILING A PATTERN 2017 2018 pcre *pcre_compile(const char *pattern, int options, 2019 const char **errptr, int *erroffset, 2020 const unsigned char *tableptr); 2021 2022 pcre *pcre_compile2(const char *pattern, int options, 2023 int *errorcodeptr, 2024 const char **errptr, int *erroffset, 2025 const unsigned char *tableptr); 2026 2027 Either of the functions pcre_compile() or pcre_compile2() can be called 2028 to compile a pattern into an internal form. The only difference between 2029 the two interfaces is that pcre_compile2() has an additional argument, 2030 errorcodeptr, via which a numerical error code can be returned. To 2031 avoid too much repetition, we refer just to pcre_compile() below, but 2032 the information applies equally to pcre_compile2(). 2033 2034 The pattern is a C string terminated by a binary zero, and is passed in 2035 the pattern argument. A pointer to a single block of memory that is 2036 obtained via pcre_malloc is returned. This contains the compiled code 2037 and related data. The pcre type is defined for the returned block; this 2038 is a typedef for a structure whose contents are not externally defined. 2039 It is up to the caller to free the memory (via pcre_free) when it is no 2040 longer required. 2041 2042 Although the compiled code of a PCRE regex is relocatable, that is, it 2043 does not depend on memory location, the complete pcre data block is not 2044 fully relocatable, because it may contain a copy of the tableptr argu- 2045 ment, which is an address (see below). 2046 2047 The options argument contains various bit settings that affect the com- 2048 pilation. It should be zero if no options are required. The available 2049 options are described below. Some of them (in particular, those that 2050 are compatible with Perl, but some others as well) can also be set and 2051 unset from within the pattern (see the detailed description in the 2052 pcrepattern documentation). For those options that can be different in 2053 different parts of the pattern, the contents of the options argument 2054 specifies their settings at the start of compilation and execution. The 2055 PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and 2056 PCRE_NO_START_OPTIMIZE options can be set at the time of matching as 2057 well as at compile time. 2058 2059 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, 2060 if compilation of a pattern fails, pcre_compile() returns NULL, and 2061 sets the variable pointed to by errptr to point to a textual error mes- 2062 sage. This is a static string that is part of the library. You must not 2063 try to free it. Normally, the offset from the start of the pattern to 2064 the data unit that was being processed when the error was discovered is 2065 placed in the variable pointed to by erroffset, which must not be NULL 2066 (if it is, an immediate error is given). However, for an invalid UTF-8 2067 or UTF-16 string, the offset is that of the first data unit of the 2068 failing character. 2069 2070 Some errors are not detected until the whole pattern has been scanned; 2071 in these cases, the offset passed back is the length of the pattern. 2072 Note that the offset is in data units, not characters, even in a UTF 2073 mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char- 2074 acter. 2075 2076 If pcre_compile2() is used instead of pcre_compile(), and the error- 2077 codeptr argument is not NULL, a non-zero error code number is returned 2078 via this argument in the event of an error. This is in addition to the 2079 textual error message. Error codes and messages are listed below. 2080 2081 If the final argument, tableptr, is NULL, PCRE uses a default set of 2082 character tables that are built when PCRE is compiled, using the 2083 default C locale. Otherwise, tableptr must be an address that is the 2084 result of a call to pcre_maketables(). This value is stored with the 2085 compiled pattern, and used again by pcre_exec() and pcre_dfa_exec() 2086 when the pattern is matched. For more discussion, see the section on 2087 locale support below. 2088 2089 This code fragment shows a typical straightforward call to pcre_com- 2090 pile(): 2091 2092 pcre *re; 2093 const char *error; 2094 int erroffset; 2095 re = pcre_compile( 2096 "^A.*Z", /* the pattern */ 2097 0, /* default options */ 2098 &error, /* for error message */ 2099 &erroffset, /* for error offset */ 2100 NULL); /* use default character tables */ 2101 2102 The following names for option bits are defined in the pcre.h header 2103 file: 2104 2105 PCRE_ANCHORED 2106 2107 If this bit is set, the pattern is forced to be "anchored", that is, it 2108 is constrained to match only at the first matching point in the string 2109 that is being searched (the "subject string"). This effect can also be 2110 achieved by appropriate constructs in the pattern itself, which is the 2111 only way to do it in Perl. 2112 2113 PCRE_AUTO_CALLOUT 2114 2115 If this bit is set, pcre_compile() automatically inserts callout items, 2116 all with number 255, before each pattern item. For discussion of the 2117 callout facility, see the pcrecallout documentation. 2118 2119 PCRE_BSR_ANYCRLF 2120 PCRE_BSR_UNICODE 2121 2122 These options (which are mutually exclusive) control what the \R escape 2123 sequence matches. The choice is either to match only CR, LF, or CRLF, 2124 or to match any Unicode newline sequence. The default is specified when 2125 PCRE is built. It can be overridden from within the pattern, or by set- 2126 ting an option when a compiled pattern is matched. 2127 2128 PCRE_CASELESS 2129 2130 If this bit is set, letters in the pattern match both upper and lower 2131 case letters. It is equivalent to Perl's /i option, and it can be 2132 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE 2133 always understands the concept of case for characters whose values are 2134 less than 128, so caseless matching is always possible. For characters 2135 with higher values, the concept of case is supported if PCRE is com- 2136 piled with Unicode property support, but not otherwise. If you want to 2137 use caseless matching for characters 128 and above, you must ensure 2138 that PCRE is compiled with Unicode property support as well as with 2139 UTF-8 support. 2140 2141 PCRE_DOLLAR_ENDONLY 2142 2143 If this bit is set, a dollar metacharacter in the pattern matches only 2144 at the end of the subject string. Without this option, a dollar also 2145 matches immediately before a newline at the end of the string (but not 2146 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored 2147 if PCRE_MULTILINE is set. There is no equivalent to this option in 2148 Perl, and no way to set it within a pattern. 2149 2150 PCRE_DOTALL 2151 2152 If this bit is set, a dot metacharacter in the pattern matches a char- 2153 acter of any value, including one that indicates a newline. However, it 2154 only ever matches one character, even if newlines are coded as CRLF. 2155 Without this option, a dot does not match when the current position is 2156 at a newline. This option is equivalent to Perl's /s option, and it can 2157 be changed within a pattern by a (?s) option setting. A negative class 2158 such as [^a] always matches newline characters, independent of the set- 2159 ting of this option. 2160 2161 PCRE_DUPNAMES 2162 2163 If this bit is set, names used to identify capturing subpatterns need 2164 not be unique. This can be helpful for certain types of pattern when it 2165 is known that only one instance of the named subpattern can ever be 2166 matched. There are more details of named subpatterns below; see also 2167 the pcrepattern documentation. 2168 2169 PCRE_EXTENDED 2170 2171 If this bit is set, most white space characters in the pattern are 2172 totally ignored except when escaped or inside a character class. How- 2173 ever, white space is not allowed within sequences such as (?> that 2174 introduce various parenthesized subpatterns, nor within a numerical 2175 quantifier such as {1,3}. However, ignorable white space is permitted 2176 between an item and a following quantifier and between a quantifier and 2177 a following + that indicates possessiveness. 2178 2179 White space did not used to include the VT character (code 11), because 2180 Perl did not treat this character as white space. However, Perl changed 2181 at release 5.18, so PCRE followed at release 8.34, and VT is now 2182 treated as white space. 2183 2184 PCRE_EXTENDED also causes characters between an unescaped # outside a 2185 character class and the next newline, inclusive, to be ignored. 2186 PCRE_EXTENDED is equivalent to Perl's /x option, and it can be changed 2187 within a pattern by a (?x) option setting. 2188 2189 Which characters are interpreted as newlines is controlled by the 2190 options passed to pcre_compile() or by a special sequence at the start 2191 of the pattern, as described in the section entitled "Newline conven- 2192 tions" in the pcrepattern documentation. Note that the end of this type 2193 of comment is a literal newline sequence in the pattern; escape 2194 sequences that happen to represent a newline do not count. 2195 2196 This option makes it possible to include comments inside complicated 2197 patterns. Note, however, that this applies only to data characters. 2198 White space characters may never appear within special character 2199 sequences in a pattern, for example within the sequence (?( that intro- 2200 duces a conditional subpattern. 2201 2202 PCRE_EXTRA 2203 2204 This option was invented in order to turn on additional functionality 2205 of PCRE that is incompatible with Perl, but it is currently of very 2206 little use. When set, any backslash in a pattern that is followed by a 2207 letter that has no special meaning causes an error, thus reserving 2208 these combinations for future expansion. By default, as in Perl, a 2209 backslash followed by a letter with no special meaning is treated as a 2210 literal. (Perl can, however, be persuaded to give an error for this, by 2211 running it with the -w option.) There are at present no other features 2212 controlled by this option. It can also be set by a (?X) option setting 2213 within a pattern. 2214 2215 PCRE_FIRSTLINE 2216 2217 If this option is set, an unanchored pattern is required to match 2218 before or at the first newline in the subject string, though the 2219 matched text may continue over the newline. 2220 2221 PCRE_JAVASCRIPT_COMPAT 2222 2223 If this option is set, PCRE's behaviour is changed in some ways so that 2224 it is compatible with JavaScript rather than Perl. The changes are as 2225 follows: 2226 2227 (1) A lone closing square bracket in a pattern causes a compile-time 2228 error, because this is illegal in JavaScript (by default it is treated 2229 as a data character). Thus, the pattern AB]CD becomes illegal when this 2230 option is set. 2231 2232 (2) At run time, a back reference to an unset subpattern group matches 2233 an empty string (by default this causes the current matching alterna- 2234 tive to fail). A pattern such as (\1)(a) succeeds when this option is 2235 set (assuming it can find an "a" in the subject), whereas it fails by 2236 default, for Perl compatibility. 2237 2238 (3) \U matches an upper case "U" character; by default \U causes a com- 2239 pile time error (Perl uses \U to upper case subsequent characters). 2240 2241 (4) \u matches a lower case "u" character unless it is followed by four 2242 hexadecimal digits, in which case the hexadecimal number defines the 2243 code point to match. By default, \u causes a compile time error (Perl 2244 uses it to upper case the following character). 2245 2246 (5) \x matches a lower case "x" character unless it is followed by two 2247 hexadecimal digits, in which case the hexadecimal number defines the 2248 code point to match. By default, as in Perl, a hexadecimal number is 2249 always expected after \x, but it may have zero, one, or two digits (so, 2250 for example, \xz matches a binary zero character followed by z). 2251 2252 PCRE_MULTILINE 2253 2254 By default, for the purposes of matching "start of line" and "end of 2255 line", PCRE treats the subject string as consisting of a single line of 2256 characters, even if it actually contains newlines. The "start of line" 2257 metacharacter (^) matches only at the start of the string, and the "end 2258 of line" metacharacter ($) matches only at the end of the string, or 2259 before a terminating newline (except when PCRE_DOLLAR_ENDONLY is set). 2260 Note, however, that unless PCRE_DOTALL is set, the "any character" 2261 metacharacter (.) does not match at a newline. This behaviour (for ^, 2262 $, and dot) is the same as Perl. 2263 2264 When PCRE_MULTILINE it is set, the "start of line" and "end of line" 2265 constructs match immediately following or immediately before internal 2266 newlines in the subject string, respectively, as well as at the very 2267 start and end. This is equivalent to Perl's /m option, and it can be 2268 changed within a pattern by a (?m) option setting. If there are no new- 2269 lines in a subject string, or no occurrences of ^ or $ in a pattern, 2270 setting PCRE_MULTILINE has no effect. 2271 2272 PCRE_NEVER_UTF 2273 2274 This option locks out interpretation of the pattern as UTF-8 (or UTF-16 2275 or UTF-32 in the 16-bit and 32-bit libraries). In particular, it pre- 2276 vents the creator of the pattern from switching to UTF interpretation 2277 by starting the pattern with (*UTF). This may be useful in applications 2278 that process patterns from external sources. The combination of 2279 PCRE_UTF8 and PCRE_NEVER_UTF also causes an error. 2280 2281 PCRE_NEWLINE_CR 2282 PCRE_NEWLINE_LF 2283 PCRE_NEWLINE_CRLF 2284 PCRE_NEWLINE_ANYCRLF 2285 PCRE_NEWLINE_ANY 2286 2287 These options override the default newline definition that was chosen 2288 when PCRE was built. Setting the first or the second specifies that a 2289 newline is indicated by a single character (CR or LF, respectively). 2290 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the 2291 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies 2292 that any of the three preceding sequences should be recognized. Setting 2293 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be 2294 recognized. 2295 2296 In an ASCII/Unicode environment, the Unicode newline sequences are the 2297 three just mentioned, plus the single characters VT (vertical tab, 2298 U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line sep- 2299 arator, U+2028), and PS (paragraph separator, U+2029). For the 8-bit 2300 library, the last two are recognized only in UTF-8 mode. 2301 2302 When PCRE is compiled to run in an EBCDIC (mainframe) environment, the 2303 code for CR is 0x0d, the same as ASCII. However, the character code for 2304 LF is normally 0x15, though in some EBCDIC environments 0x25 is used. 2305 Whichever of these is not LF is made to correspond to Unicode's NEL 2306 character. EBCDIC codes are all less than 256. For more details, see 2307 the pcrebuild documentation. 2308 2309 The newline setting in the options word uses three bits that are 2310 treated as a number, giving eight possibilities. Currently only six are 2311 used (default plus the five values above). This means that if you set 2312 more than one newline option, the combination may or may not be sensi- 2313 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to 2314 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and 2315 cause an error. 2316 2317 The only time that a line break in a pattern is specially recognized 2318 when compiling is when PCRE_EXTENDED is set. CR and LF are white space 2319 characters, and so are ignored in this mode. Also, an unescaped # out- 2320 side a character class indicates a comment that lasts until after the 2321 next line break sequence. In other circumstances, line break sequences 2322 in patterns are treated as literal data. 2323 2324 The newline option that is set at compile time becomes the default that 2325 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. 2326 2327 PCRE_NO_AUTO_CAPTURE 2328 2329 If this option is set, it disables the use of numbered capturing paren- 2330 theses in the pattern. Any opening parenthesis that is not followed by 2331 ? behaves as if it were followed by ?: but named parentheses can still 2332 be used for capturing (and they acquire numbers in the usual way). 2333 There is no equivalent of this option in Perl. 2334 2335 PCRE_NO_AUTO_POSSESS 2336 2337 If this option is set, it disables "auto-possessification". This is an 2338 optimization that, for example, turns a+b into a++b in order to avoid 2339 backtracks into a+ that can never be successful. However, if callouts 2340 are in use, auto-possessification means that some of them are never 2341 taken. You can set this option if you want the matching functions to do 2342 a full unoptimized search and run all the callouts, but it is mainly 2343 provided for testing purposes. 2344 2345 PCRE_NO_START_OPTIMIZE 2346 2347 This is an option that acts at matching time; that is, it is really an 2348 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile 2349 time, it is remembered with the compiled pattern and assumed at match- 2350 ing time. This is necessary if you want to use JIT execution, because 2351 the JIT compiler needs to know whether or not this option is set. For 2352 details see the discussion of PCRE_NO_START_OPTIMIZE below. 2353 2354 PCRE_UCP 2355 2356 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, 2357 \w, and some of the POSIX character classes. By default, only ASCII 2358 characters are recognized, but if PCRE_UCP is set, Unicode properties 2359 are used instead to classify characters. More details are given in the 2360 section on generic character types in the pcrepattern page. If you set 2361 PCRE_UCP, matching one of the items it affects takes much longer. The 2362 option is available only if PCRE has been compiled with Unicode prop- 2363 erty support. 2364 2365 PCRE_UNGREEDY 2366 2367 This option inverts the "greediness" of the quantifiers so that they 2368 are not greedy by default, but become greedy if followed by "?". It is 2369 not compatible with Perl. It can also be set by a (?U) option setting 2370 within the pattern. 2371 2372 PCRE_UTF8 2373 2374 This option causes PCRE to regard both the pattern and the subject as 2375 strings of UTF-8 characters instead of single-byte strings. However, it 2376 is available only when PCRE is built to include UTF support. If not, 2377 the use of this option provokes an error. Details of how this option 2378 changes the behaviour of PCRE are given in the pcreunicode page. 2379 2380 PCRE_NO_UTF8_CHECK 2381 2382 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is 2383 automatically checked. There is a discussion about the validity of 2384 UTF-8 strings in the pcreunicode page. If an invalid UTF-8 sequence is 2385 found, pcre_compile() returns an error. If you already know that your 2386 pattern is valid, and you want to skip this check for performance rea- 2387 sons, you can set the PCRE_NO_UTF8_CHECK option. When it is set, the 2388 effect of passing an invalid UTF-8 string as a pattern is undefined. It 2389 may cause your program to crash or loop. Note that this option can also 2390 be passed to pcre_exec() and pcre_dfa_exec(), to suppress the validity 2391 checking of subject strings only. If the same string is being matched 2392 many times, the option can be safely set for the second and subsequent 2393 matchings to improve performance. 2394 2395 2396 COMPILATION ERROR CODES 2397 2398 The following table lists the error codes than may be returned by 2399 pcre_compile2(), along with the error messages that may be returned by 2400 both compiling functions. Note that error messages are always 8-bit 2401 ASCII strings, even in 16-bit or 32-bit mode. As PCRE has developed, 2402 some error codes have fallen out of use. To avoid confusion, they have 2403 not been re-used. 2404 2405 0 no error 2406 1 \ at end of pattern 2407 2 \c at end of pattern 2408 3 unrecognized character follows \ 2409 4 numbers out of order in {} quantifier 2410 5 number too big in {} quantifier 2411 6 missing terminating ] for character class 2412 7 invalid escape sequence in character class 2413 8 range out of order in character class 2414 9 nothing to repeat 2415 10 [this code is not in use] 2416 11 internal error: unexpected repeat 2417 12 unrecognized character after (? or (?- 2418 13 POSIX named classes are supported only within a class 2419 14 missing ) 2420 15 reference to non-existent subpattern 2421 16 erroffset passed as NULL 2422 17 unknown option bit(s) set 2423 18 missing ) after comment 2424 19 [this code is not in use] 2425 20 regular expression is too large 2426 21 failed to get memory 2427 22 unmatched parentheses 2428 23 internal error: code overflow 2429 24 unrecognized character after (?< 2430 25 lookbehind assertion is not fixed length 2431 26 malformed number or name after (?( 2432 27 conditional group contains more than two branches 2433 28 assertion expected after (?( 2434 29 (?R or (?[+-]digits must be followed by ) 2435 30 unknown POSIX class name 2436 31 POSIX collating elements are not supported 2437 32 this version of PCRE is compiled without UTF support 2438 33 [this code is not in use] 2439 34 character value in \x{} or \o{} is too large 2440 35 invalid condition (?(0) 2441 36 \C not allowed in lookbehind assertion 2442 37 PCRE does not support \L, \l, \N{name}, \U, or \u 2443 38 number after (?C is > 255 2444 39 closing ) for (?C expected 2445 40 recursive call could loop indefinitely 2446 41 unrecognized character after (?P 2447 42 syntax error in subpattern name (missing terminator) 2448 43 two named subpatterns have the same name 2449 44 invalid UTF-8 string (specifically UTF-8) 2450 45 support for \P, \p, and \X has not been compiled 2451 46 malformed \P or \p sequence 2452 47 unknown property name after \P or \p 2453 48 subpattern name is too long (maximum 32 characters) 2454 49 too many named subpatterns (maximum 10000) 2455 50 [this code is not in use] 2456 51 octal value is greater than \377 in 8-bit non-UTF-8 mode 2457 52 internal error: overran compiling workspace 2458 53 internal error: previously-checked referenced subpattern 2459 not found 2460 54 DEFINE group contains more than one branch 2461 55 repeating a DEFINE group is not allowed 2462 56 inconsistent NEWLINE options 2463 57 \g is not followed by a braced, angle-bracketed, or quoted 2464 name/number or by a plain number 2465 58 a numbered reference must not be zero 2466 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT) 2467 60 (*VERB) not recognized or malformed 2468 61 number is too big 2469 62 subpattern name expected 2470 63 digit expected after (?+ 2471 64 ] is an invalid data character in JavaScript compatibility mode 2472 65 different names for subpatterns of the same number are 2473 not allowed 2474 66 (*MARK) must have an argument 2475 67 this version of PCRE is not compiled with Unicode property 2476 support 2477 68 \c must be followed by an ASCII character 2478 69 \k is not followed by a braced, angle-bracketed, or quoted name 2479 70 internal error: unknown opcode in find_fixedlength() 2480 71 \N is not supported in a class 2481 72 too many forward references 2482 73 disallowed Unicode code point (>= 0xd800 && <= 0xdfff) 2483 74 invalid UTF-16 string (specifically UTF-16) 2484 75 name is too long in (*MARK), (*PRUNE), (*SKIP), or (*THEN) 2485 76 character value in \u.... sequence is too large 2486 77 invalid UTF-32 string (specifically UTF-32) 2487 78 setting UTF is disabled by the application 2488 79 non-hex character in \x{} (closing brace missing?) 2489 80 non-octal character in \o{} (closing brace missing?) 2490 81 missing opening brace after \o 2491 82 parentheses are too deeply nested 2492 83 invalid range in character class 2493 84 group name must start with a non-digit 2494 85 parentheses are too deeply nested (stack check) 2495 2496 The numbers 32 and 10000 in errors 48 and 49 are defaults; different 2497 values may be used if the limits were changed when PCRE was built. 2498 2499 2500 STUDYING A PATTERN 2501 2502 pcre_extra *pcre_study(const pcre *code, int options, 2503 const char **errptr); 2504 2505 If a compiled pattern is going to be used several times, it is worth 2506 spending more time analyzing it in order to speed up the time taken for 2507 matching. The function pcre_study() takes a pointer to a compiled pat- 2508 tern as its first argument. If studying the pattern produces additional 2509 information that will help speed up matching, pcre_study() returns a 2510 pointer to a pcre_extra block, in which the study_data field points to 2511 the results of the study. 2512 2513 The returned value from pcre_study() can be passed directly to 2514 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con- 2515 tains other fields that can be set by the caller before the block is 2516 passed; these are described below in the section on matching a pattern. 2517 2518 If studying the pattern does not produce any useful information, 2519 pcre_study() returns NULL by default. In that circumstance, if the 2520 calling program wants to pass any of the other fields to pcre_exec() or 2521 pcre_dfa_exec(), it must set up its own pcre_extra block. However, if 2522 pcre_study() is called with the PCRE_STUDY_EXTRA_NEEDED option, it 2523 returns a pcre_extra block even if studying did not find any additional 2524 information. It may still return NULL, however, if an error occurs in 2525 pcre_study(). 2526 2527 The second argument of pcre_study() contains option bits. There are 2528 three further options in addition to PCRE_STUDY_EXTRA_NEEDED: 2529 2530 PCRE_STUDY_JIT_COMPILE 2531 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 2532 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 2533 2534 If any of these are set, and the just-in-time compiler is available, 2535 the pattern is further compiled into machine code that executes much 2536 faster than the pcre_exec() interpretive matching function. If the 2537 just-in-time compiler is not available, these options are ignored. All 2538 undefined bits in the options argument must be zero. 2539 2540 JIT compilation is a heavyweight optimization. It can take some time 2541 for patterns to be analyzed, and for one-off matches and simple pat- 2542 terns the benefit of faster execution might be offset by a much slower 2543 study time. Not all patterns can be optimized by the JIT compiler. For 2544 those that cannot be handled, matching automatically falls back to the 2545 pcre_exec() interpreter. For more details, see the pcrejit documenta- 2546 tion. 2547 2548 The third argument for pcre_study() is a pointer for an error message. 2549 If studying succeeds (even if no data is returned), the variable it 2550 points to is set to NULL. Otherwise it is set to point to a textual 2551 error message. This is a static string that is part of the library. You 2552 must not try to free it. You should test the error pointer for NULL 2553 after calling pcre_study(), to be sure that it has run successfully. 2554 2555 When you are finished with a pattern, you can free the memory used for 2556 the study data by calling pcre_free_study(). This function was added to 2557 the API for release 8.20. For earlier versions, the memory could be 2558 freed with pcre_free(), just like the pattern itself. This will still 2559 work in cases where JIT optimization is not used, but it is advisable 2560 to change to the new function when convenient. 2561 2562 This is a typical way in which pcre_study() is used (except that in a 2563 real application there should be tests for errors): 2564 2565 int rc; 2566 pcre *re; 2567 pcre_extra *sd; 2568 re = pcre_compile("pattern", 0, &error, &erroroffset, NULL); 2569 sd = pcre_study( 2570 re, /* result of pcre_compile() */ 2571 0, /* no options */ 2572 &error); /* set to NULL or points to a message */ 2573 rc = pcre_exec( /* see below for details of pcre_exec() options */ 2574 re, sd, "subject", 7, 0, 0, ovector, 30); 2575 ... 2576 pcre_free_study(sd); 2577 pcre_free(re); 2578 2579 Studying a pattern does two things: first, a lower bound for the length 2580 of subject string that is needed to match the pattern is computed. This 2581 does not mean that there are any strings of that length that match, but 2582 it does guarantee that no shorter strings match. The value is used to 2583 avoid wasting time by trying to match strings that are shorter than the 2584 lower bound. You can find out the value in a calling program via the 2585 pcre_fullinfo() function. 2586 2587 Studying a pattern is also useful for non-anchored patterns that do not 2588 have a single fixed starting character. A bitmap of possible starting 2589 bytes is created. This speeds up finding a position in the subject at 2590 which to start matching. (In 16-bit mode, the bitmap is used for 16-bit 2591 values less than 256. In 32-bit mode, the bitmap is used for 32-bit 2592 values less than 256.) 2593 2594 These two optimizations apply to both pcre_exec() and pcre_dfa_exec(), 2595 and the information is also used by the JIT compiler. The optimiza- 2596 tions can be disabled by setting the PCRE_NO_START_OPTIMIZE option. 2597 You might want to do this if your pattern contains callouts or (*MARK) 2598 and you want to make use of these facilities in cases where matching 2599 fails. 2600 2601 PCRE_NO_START_OPTIMIZE can be specified at either compile time or exe- 2602 cution time. However, if PCRE_NO_START_OPTIMIZE is passed to 2603 pcre_exec(), (that is, after any JIT compilation has happened) JIT exe- 2604 cution is disabled. For JIT execution to work with PCRE_NO_START_OPTI- 2605 MIZE, the option must be set at compile time. 2606 2607 There is a longer discussion of PCRE_NO_START_OPTIMIZE below. 2608 2609 2610 LOCALE SUPPORT 2611 2612 PCRE handles caseless matching, and determines whether characters are 2613 letters, digits, or whatever, by reference to a set of tables, indexed 2614 by character code point. When running in UTF-8 mode, or in the 16- or 2615 32-bit libraries, this applies only to characters with code points less 2616 than 256. By default, higher-valued code points never match escapes 2617 such as \w or \d. However, if PCRE is built with Unicode property sup- 2618 port, all characters can be tested with \p and \P, or, alternatively, 2619 the PCRE_UCP option can be set when a pattern is compiled; this causes 2620 \w and friends to use Unicode property support instead of the built-in 2621 tables. 2622 2623 The use of locales with Unicode is discouraged. If you are handling 2624 characters with code points greater than 128, you should either use 2625 Unicode support, or use locales, but not try to mix the two. 2626 2627 PCRE contains an internal set of tables that are used when the final 2628 argument of pcre_compile() is NULL. These are sufficient for many 2629 applications. Normally, the internal tables recognize only ASCII char- 2630 acters. However, when PCRE is built, it is possible to cause the inter- 2631 nal tables to be rebuilt in the default "C" locale of the local system, 2632 which may cause them to be different. 2633 2634 The internal tables can always be overridden by tables supplied by the 2635 application that calls PCRE. These may be created in a different locale 2636 from the default. As more and more applications change to using Uni- 2637 code, the need for this locale support is expected to die away. 2638 2639 External tables are built by calling the pcre_maketables() function, 2640 which has no arguments, in the relevant locale. The result can then be 2641 passed to pcre_compile() as often as necessary. For example, to build 2642 and use tables that are appropriate for the French locale (where 2643 accented characters with values greater than 128 are treated as let- 2644 ters), the following code could be used: 2645 2646 setlocale(LC_CTYPE, "fr_FR"); 2647 tables = pcre_maketables(); 2648 re = pcre_compile(..., tables); 2649 2650 The locale name "fr_FR" is used on Linux and other Unix-like systems; 2651 if you are using Windows, the name for the French locale is "french". 2652 2653 When pcre_maketables() runs, the tables are built in memory that is 2654 obtained via pcre_malloc. It is the caller's responsibility to ensure 2655 that the memory containing the tables remains available for as long as 2656 it is needed. 2657 2658 The pointer that is passed to pcre_compile() is saved with the compiled 2659 pattern, and the same tables are used via this pointer by pcre_study() 2660 and also by pcre_exec() and pcre_dfa_exec(). Thus, for any single pat- 2661 tern, compilation, studying and matching all happen in the same locale, 2662 but different patterns can be processed in different locales. 2663 2664 It is possible to pass a table pointer or NULL (indicating the use of 2665 the internal tables) to pcre_exec() or pcre_dfa_exec() (see the discus- 2666 sion below in the section on matching a pattern). This facility is pro- 2667 vided for use with pre-compiled patterns that have been saved and 2668 reloaded. Character tables are not saved with patterns, so if a non- 2669 standard table was used at compile time, it must be provided again when 2670 the reloaded pattern is matched. Attempting to use this facility to 2671 match a pattern in a different locale from the one in which it was com- 2672 piled is likely to lead to anomalous (usually incorrect) results. 2673 2674 2675 INFORMATION ABOUT A PATTERN 2676 2677 int pcre_fullinfo(const pcre *code, const pcre_extra *extra, 2678 int what, void *where); 2679 2680 The pcre_fullinfo() function returns information about a compiled pat- 2681 tern. It replaces the pcre_info() function, which was removed from the 2682 library at version 8.30, after more than 10 years of obsolescence. 2683 2684 The first argument for pcre_fullinfo() is a pointer to the compiled 2685 pattern. The second argument is the result of pcre_study(), or NULL if 2686 the pattern was not studied. The third argument specifies which piece 2687 of information is required, and the fourth argument is a pointer to a 2688 variable to receive the data. The yield of the function is zero for 2689 success, or one of the following negative numbers: 2690 2691 PCRE_ERROR_NULL the argument code was NULL 2692 the argument where was NULL 2693 PCRE_ERROR_BADMAGIC the "magic number" was not found 2694 PCRE_ERROR_BADENDIANNESS the pattern was compiled with different 2695 endianness 2696 PCRE_ERROR_BADOPTION the value of what was invalid 2697 PCRE_ERROR_UNSET the requested field is not set 2698 2699 The "magic number" is placed at the start of each compiled pattern as 2700 an simple check against passing an arbitrary memory pointer. The endi- 2701 anness error can occur if a compiled pattern is saved and reloaded on a 2702 different host. Here is a typical call of pcre_fullinfo(), to obtain 2703 the length of the compiled pattern: 2704 2705 int rc; 2706 size_t length; 2707 rc = pcre_fullinfo( 2708 re, /* result of pcre_compile() */ 2709 sd, /* result of pcre_study(), or NULL */ 2710 PCRE_INFO_SIZE, /* what is required */ 2711 &length); /* where to put the data */ 2712 2713 The possible values for the third argument are defined in pcre.h, and 2714 are as follows: 2715 2716 PCRE_INFO_BACKREFMAX 2717 2718 Return the number of the highest back reference in the pattern. The 2719 fourth argument should point to an int variable. Zero is returned if 2720 there are no back references. 2721 2722 PCRE_INFO_CAPTURECOUNT 2723 2724 Return the number of capturing subpatterns in the pattern. The fourth 2725 argument should point to an int variable. 2726 2727 PCRE_INFO_DEFAULT_TABLES 2728 2729 Return a pointer to the internal default character tables within PCRE. 2730 The fourth argument should point to an unsigned char * variable. This 2731 information call is provided for internal use by the pcre_study() func- 2732 tion. External callers can cause PCRE to use its internal tables by 2733 passing a NULL table pointer. 2734 2735 PCRE_INFO_FIRSTBYTE (deprecated) 2736 2737 Return information about the first data unit of any matched string, for 2738 a non-anchored pattern. The name of this option refers to the 8-bit 2739 library, where data units are bytes. The fourth argument should point 2740 to an int variable. Negative values are used for special cases. How- 2741 ever, this means that when the 32-bit library is in non-UTF-32 mode, 2742 the full 32-bit range of characters cannot be returned. For this rea- 2743 son, this value is deprecated; use PCRE_INFO_FIRSTCHARACTERFLAGS and 2744 PCRE_INFO_FIRSTCHARACTER instead. 2745 2746 If there is a fixed first value, for example, the letter "c" from a 2747 pattern such as (cat|cow|coyote), its value is returned. In the 8-bit 2748 library, the value is always less than 256. In the 16-bit library the 2749 value can be up to 0xffff. In the 32-bit library the value can be up to 2750 0x10ffff. 2751 2752 If there is no fixed first value, and if either 2753 2754 (a) the pattern was compiled with the PCRE_MULTILINE option, and every 2755 branch starts with "^", or 2756 2757 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not 2758 set (if it were set, the pattern would be anchored), 2759 2760 -1 is returned, indicating that the pattern matches only at the start 2761 of a subject string or after any newline within the string. Otherwise 2762 -2 is returned. For anchored patterns, -2 is returned. 2763 2764 PCRE_INFO_FIRSTCHARACTER 2765 2766 Return the value of the first data unit (non-UTF character) of any 2767 matched string in the situation where PCRE_INFO_FIRSTCHARACTERFLAGS 2768 returns 1; otherwise return 0. The fourth argument should point to an 2769 uint_t variable. 2770 2771 In the 8-bit library, the value is always less than 256. In the 16-bit 2772 library the value can be up to 0xffff. In the 32-bit library in UTF-32 2773 mode the value can be up to 0x10ffff, and up to 0xffffffff when not 2774 using UTF-32 mode. 2775 2776 PCRE_INFO_FIRSTCHARACTERFLAGS 2777 2778 Return information about the first data unit of any matched string, for 2779 a non-anchored pattern. The fourth argument should point to an int 2780 variable. 2781 2782 If there is a fixed first value, for example, the letter "c" from a 2783 pattern such as (cat|cow|coyote), 1 is returned, and the character 2784 value can be retrieved using PCRE_INFO_FIRSTCHARACTER. If there is no 2785 fixed first value, and if either 2786 2787 (a) the pattern was compiled with the PCRE_MULTILINE option, and every 2788 branch starts with "^", or 2789 2790 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not 2791 set (if it were set, the pattern would be anchored), 2792 2793 2 is returned, indicating that the pattern matches only at the start of 2794 a subject string or after any newline within the string. Otherwise 0 is 2795 returned. For anchored patterns, 0 is returned. 2796 2797 PCRE_INFO_FIRSTTABLE 2798 2799 If the pattern was studied, and this resulted in the construction of a 2800 256-bit table indicating a fixed set of values for the first data unit 2801 in any matching string, a pointer to the table is returned. Otherwise 2802 NULL is returned. The fourth argument should point to an unsigned char 2803 * variable. 2804 2805 PCRE_INFO_HASCRORLF 2806 2807 Return 1 if the pattern contains any explicit matches for CR or LF 2808 characters, otherwise 0. The fourth argument should point to an int 2809 variable. An explicit match is either a literal CR or LF character, or 2810 \r or \n. 2811 2812 PCRE_INFO_JCHANGED 2813 2814 Return 1 if the (?J) or (?-J) option setting is used in the pattern, 2815 otherwise 0. The fourth argument should point to an int variable. (?J) 2816 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. 2817 2818 PCRE_INFO_JIT 2819 2820 Return 1 if the pattern was studied with one of the JIT options, and 2821 just-in-time compiling was successful. The fourth argument should point 2822 to an int variable. A return value of 0 means that JIT support is not 2823 available in this version of PCRE, or that the pattern was not studied 2824 with a JIT option, or that the JIT compiler could not handle this par- 2825 ticular pattern. See the pcrejit documentation for details of what can 2826 and cannot be handled. 2827 2828 PCRE_INFO_JITSIZE 2829 2830 If the pattern was successfully studied with a JIT option, return the 2831 size of the JIT compiled code, otherwise return zero. The fourth argu- 2832 ment should point to a size_t variable. 2833 2834 PCRE_INFO_LASTLITERAL 2835 2836 Return the value of the rightmost literal data unit that must exist in 2837 any matched string, other than at its start, if such a value has been 2838 recorded. The fourth argument should point to an int variable. If there 2839 is no such value, -1 is returned. For anchored patterns, a last literal 2840 value is recorded only if it follows something of variable length. For 2841 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for 2842 /^a\dz\d/ the returned value is -1. 2843 2844 Since for the 32-bit library using the non-UTF-32 mode, this function 2845 is unable to return the full 32-bit range of characters, this value is 2846 deprecated; instead the PCRE_INFO_REQUIREDCHARFLAGS and 2847 PCRE_INFO_REQUIREDCHAR values should be used. 2848 2849 PCRE_INFO_MATCH_EMPTY 2850 2851 Return 1 if the pattern can match an empty string, otherwise 0. The 2852 fourth argument should point to an int variable. 2853 2854 PCRE_INFO_MATCHLIMIT 2855 2856 If the pattern set a match limit by including an item of the form 2857 (*LIMIT_MATCH=nnnn) at the start, the value is returned. The fourth 2858 argument should point to an unsigned 32-bit integer. If no such value 2859 has been set, the call to pcre_fullinfo() returns the error 2860 PCRE_ERROR_UNSET. 2861 2862 PCRE_INFO_MAXLOOKBEHIND 2863 2864 Return the number of characters (NB not data units) in the longest 2865 lookbehind assertion in the pattern. This information is useful when 2866 doing multi-segment matching using the partial matching facilities. 2867 Note that the simple assertions \b and \B require a one-character look- 2868 behind. \A also registers a one-character lookbehind, though it does 2869 not actually inspect the previous character. This is to ensure that at 2870 least one character from the old segment is retained when a new segment 2871 is processed. Otherwise, if there are no lookbehinds in the pattern, \A 2872 might match incorrectly at the start of a new segment. 2873 2874 PCRE_INFO_MINLENGTH 2875 2876 If the pattern was studied and a minimum length for matching subject 2877 strings was computed, its value is returned. Otherwise the returned 2878 value is -1. The value is a number of characters, which in UTF mode may 2879 be different from the number of data units. The fourth argument should 2880 point to an int variable. A non-negative value is a lower bound to the 2881 length of any matching string. There may not be any strings of that 2882 length that do actually match, but every string that does match is at 2883 least that long. 2884 2885 PCRE_INFO_NAMECOUNT 2886 PCRE_INFO_NAMEENTRYSIZE 2887 PCRE_INFO_NAMETABLE 2888 2889 PCRE supports the use of named as well as numbered capturing parenthe- 2890 ses. The names are just an additional way of identifying the parenthe- 2891 ses, which still acquire numbers. Several convenience functions such as 2892 pcre_get_named_substring() are provided for extracting captured sub- 2893 strings by name. It is also possible to extract the data directly, by 2894 first converting the name to a number in order to access the correct 2895 pointers in the output vector (described with pcre_exec() below). To do 2896 the conversion, you need to use the name-to-number map, which is 2897 described by these three values. 2898 2899 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT 2900 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size 2901 of each entry; both of these return an int value. The entry size 2902 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns 2903 a pointer to the first entry of the table. This is a pointer to char in 2904 the 8-bit library, where the first two bytes of each entry are the num- 2905 ber of the capturing parenthesis, most significant byte first. In the 2906 16-bit library, the pointer points to 16-bit data units, the first of 2907 which contains the parenthesis number. In the 32-bit library, the 2908 pointer points to 32-bit data units, the first of which contains the 2909 parenthesis number. The rest of the entry is the corresponding name, 2910 zero terminated. 2911 2912 The names are in alphabetical order. If (?| is used to create multiple 2913 groups with the same number, as described in the section on duplicate 2914 subpattern numbers in the pcrepattern page, the groups may be given the 2915 same name, but there is only one entry in the table. Different names 2916 for groups of the same number are not permitted. Duplicate names for 2917 subpatterns with different numbers are permitted, but only if PCRE_DUP- 2918 NAMES is set. They appear in the table in the order in which they were 2919 found in the pattern. In the absence of (?| this is the order of 2920 increasing number; when (?| is used this is not necessarily the case 2921 because later subpatterns may have lower numbers. 2922 2923 As a simple example of the name/number table, consider the following 2924 pattern after compilation by the 8-bit library (assume PCRE_EXTENDED is 2925 set, so white space - including newlines - is ignored): 2926 2927 (?<date> (?<year>(\d\d)?\d\d) - 2928 (?<month>\d\d) - (?<day>\d\d) ) 2929 2930 There are four named subpatterns, so the table has four entries, and 2931 each entry in the table is eight bytes long. The table is as follows, 2932 with non-printing bytes shows in hexadecimal, and undefined bytes shown 2933 as ??: 2934 2935 00 01 d a t e 00 ?? 2936 00 05 d a y 00 ?? ?? 2937 00 04 m o n t h 00 2938 00 02 y e a r 00 ?? 2939 2940 When writing code to extract data from named subpatterns using the 2941 name-to-number map, remember that the length of the entries is likely 2942 to be different for each compiled pattern. 2943 2944 PCRE_INFO_OKPARTIAL 2945 2946 Return 1 if the pattern can be used for partial matching with 2947 pcre_exec(), otherwise 0. The fourth argument should point to an int 2948 variable. From release 8.00, this always returns 1, because the 2949 restrictions that previously applied to partial matching have been 2950 lifted. The pcrepartial documentation gives details of partial match- 2951 ing. 2952 2953 PCRE_INFO_OPTIONS 2954 2955 Return a copy of the options with which the pattern was compiled. The 2956 fourth argument should point to an unsigned long int variable. These 2957 option bits are those specified in the call to pcre_compile(), modified 2958 by any top-level option settings at the start of the pattern itself. In 2959 other words, they are the options that will be in force when matching 2960 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with 2961 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, 2962 and PCRE_EXTENDED. 2963 2964 A pattern is automatically anchored by PCRE if all of its top-level 2965 alternatives begin with one of the following: 2966 2967 ^ unless PCRE_MULTILINE is set 2968 \A always 2969 \G always 2970 .* if PCRE_DOTALL is set and there are no back 2971 references to the subpattern in which .* appears 2972 2973 For such patterns, the PCRE_ANCHORED bit is set in the options returned 2974 by pcre_fullinfo(). 2975 2976 PCRE_INFO_RECURSIONLIMIT 2977 2978 If the pattern set a recursion limit by including an item of the form 2979 (*LIMIT_RECURSION=nnnn) at the start, the value is returned. The fourth 2980 argument should point to an unsigned 32-bit integer. If no such value 2981 has been set, the call to pcre_fullinfo() returns the error 2982 PCRE_ERROR_UNSET. 2983 2984 PCRE_INFO_SIZE 2985 2986 Return the size of the compiled pattern in bytes (for all three 2987 libraries). The fourth argument should point to a size_t variable. This 2988 value does not include the size of the pcre structure that is returned 2989 by pcre_compile(). The value that is passed as the argument to 2990 pcre_malloc() when pcre_compile() is getting memory in which to place 2991 the compiled data is the value returned by this option plus the size of 2992 the pcre structure. Studying a compiled pattern, with or without JIT, 2993 does not alter the value returned by this option. 2994 2995 PCRE_INFO_STUDYSIZE 2996 2997 Return the size in bytes (for all three libraries) of the data block 2998 pointed to by the study_data field in a pcre_extra block. If pcre_extra 2999 is NULL, or there is no study data, zero is returned. The fourth argu- 3000 ment should point to a size_t variable. The study_data field is set by 3001 pcre_study() to record information that will speed up matching (see the 3002 section entitled "Studying a pattern" above). The format of the 3003 study_data block is private, but its length is made available via this 3004 option so that it can be saved and restored (see the pcreprecompile 3005 documentation for details). 3006 3007 PCRE_INFO_REQUIREDCHARFLAGS 3008 3009 Returns 1 if there is a rightmost literal data unit that must exist in 3010 any matched string, other than at its start. The fourth argument should 3011 point to an int variable. If there is no such value, 0 is returned. If 3012 returning 1, the character value itself can be retrieved using 3013 PCRE_INFO_REQUIREDCHAR. 3014 3015 For anchored patterns, a last literal value is recorded only if it fol- 3016 lows something of variable length. For example, for the pattern 3017 /^a\d+z\d+/ the returned value 1 (with "z" returned from 3018 PCRE_INFO_REQUIREDCHAR), but for /^a\dz\d/ the returned value is 0. 3019 3020 PCRE_INFO_REQUIREDCHAR 3021 3022 Return the value of the rightmost literal data unit that must exist in 3023 any matched string, other than at its start, if such a value has been 3024 recorded. The fourth argument should point to an uint32_t variable. If 3025 there is no such value, 0 is returned. 3026 3027 3028 REFERENCE COUNTS 3029 3030 int pcre_refcount(pcre *code, int adjust); 3031 3032 The pcre_refcount() function is used to maintain a reference count in 3033 the data block that contains a compiled pattern. It is provided for the 3034 benefit of applications that operate in an object-oriented manner, 3035 where different parts of the application may be using the same compiled 3036 pattern, but you want to free the block when they are all done. 3037 3038 When a pattern is compiled, the reference count field is initialized to 3039 zero. It is changed only by calling this function, whose action is to 3040 add the adjust value (which may be positive or negative) to it. The 3041 yield of the function is the new value. However, the value of the count 3042 is constrained to lie between 0 and 65535, inclusive. If the new value 3043 is outside these limits, it is forced to the appropriate limit value. 3044 3045 Except when it is zero, the reference count is not correctly preserved 3046 if a pattern is compiled on one host and then transferred to a host 3047 whose byte-order is different. (This seems a highly unlikely scenario.) 3048 3049 3050 MATCHING A PATTERN: THE TRADITIONAL FUNCTION 3051 3052 int pcre_exec(const pcre *code, const pcre_extra *extra, 3053 const char *subject, int length, int startoffset, 3054 int options, int *ovector, int ovecsize); 3055 3056 The function pcre_exec() is called to match a subject string against a 3057 compiled pattern, which is passed in the code argument. If the pattern 3058 was studied, the result of the study should be passed in the extra 3059 argument. You can call pcre_exec() with the same code and extra argu- 3060 ments as many times as you like, in order to match different subject 3061 strings with the same pattern. 3062 3063 This function is the main matching facility of the library, and it 3064 operates in a Perl-like manner. For specialist use there is also an 3065 alternative matching function, which is described below in the section 3066 about the pcre_dfa_exec() function. 3067 3068 In most applications, the pattern will have been compiled (and option- 3069 ally studied) in the same process that calls pcre_exec(). However, it 3070 is possible to save compiled patterns and study data, and then use them 3071 later in different processes, possibly even on different hosts. For a 3072 discussion about this, see the pcreprecompile documentation. 3073 3074 Here is an example of a simple call to pcre_exec(): 3075 3076 int rc; 3077 int ovector[30]; 3078 rc = pcre_exec( 3079 re, /* result of pcre_compile() */ 3080 NULL, /* we didn't study the pattern */ 3081 "some string", /* the subject string */ 3082 11, /* the length of the subject string */ 3083 0, /* start at offset 0 in the subject */ 3084 0, /* default options */ 3085 ovector, /* vector of integers for substring information */ 3086 30); /* number of elements (NOT size in bytes) */ 3087 3088 Extra data for pcre_exec() 3089 3090 If the extra argument is not NULL, it must point to a pcre_extra data 3091 block. The pcre_study() function returns such a block (when it doesn't 3092 return NULL), but you can also create one for yourself, and pass addi- 3093 tional information in it. The pcre_extra block contains the following 3094 fields (not necessarily in this order): 3095 3096 unsigned long int flags; 3097 void *study_data; 3098 void *executable_jit; 3099 unsigned long int match_limit; 3100 unsigned long int match_limit_recursion; 3101 void *callout_data; 3102 const unsigned char *tables; 3103 unsigned char **mark; 3104 3105 In the 16-bit version of this structure, the mark field has type 3106 "PCRE_UCHAR16 **". 3107 3108 In the 32-bit version of this structure, the mark field has type 3109 "PCRE_UCHAR32 **". 3110 3111 The flags field is used to specify which of the other fields are set. 3112 The flag bits are: 3113 3114 PCRE_EXTRA_CALLOUT_DATA 3115 PCRE_EXTRA_EXECUTABLE_JIT 3116 PCRE_EXTRA_MARK 3117 PCRE_EXTRA_MATCH_LIMIT 3118 PCRE_EXTRA_MATCH_LIMIT_RECURSION 3119 PCRE_EXTRA_STUDY_DATA 3120 PCRE_EXTRA_TABLES 3121 3122 Other flag bits should be set to zero. The study_data field and some- 3123 times the executable_jit field are set in the pcre_extra block that is 3124 returned by pcre_study(), together with the appropriate flag bits. You 3125 should not set these yourself, but you may add to the block by setting 3126 other fields and their corresponding flag bits. 3127 3128 The match_limit field provides a means of preventing PCRE from using up 3129 a vast amount of resources when running patterns that are not going to 3130 match, but which have a very large number of possibilities in their 3131 search trees. The classic example is a pattern that uses nested unlim- 3132 ited repeats. 3133 3134 Internally, pcre_exec() uses a function called match(), which it calls 3135 repeatedly (sometimes recursively). The limit set by match_limit is 3136 imposed on the number of times this function is called during a match, 3137 which has the effect of limiting the amount of backtracking that can 3138 take place. For patterns that are not anchored, the count restarts from 3139 zero for each position in the subject string. 3140 3141 When pcre_exec() is called with a pattern that was successfully studied 3142 with a JIT option, the way that the matching is executed is entirely 3143 different. However, there is still the possibility of runaway matching 3144 that goes on for a very long time, and so the match_limit value is also 3145 used in this case (but in a different way) to limit how long the match- 3146 ing can continue. 3147 3148 The default value for the limit can be set when PCRE is built; the 3149 default default is 10 million, which handles all but the most extreme 3150 cases. You can override the default by suppling pcre_exec() with a 3151 pcre_extra block in which match_limit is set, and 3152 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is 3153 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. 3154 3155 A value for the match limit may also be supplied by an item at the 3156 start of a pattern of the form 3157 3158 (*LIMIT_MATCH=d) 3159 3160 where d is a decimal number. However, such a setting is ignored unless 3161 d is less than the limit set by the caller of pcre_exec() or, if no 3162 such limit is set, less than the default. 3163 3164 The match_limit_recursion field is similar to match_limit, but instead 3165 of limiting the total number of times that match() is called, it limits 3166 the depth of recursion. The recursion depth is a smaller number than 3167 the total number of calls, because not all calls to match() are recur- 3168 sive. This limit is of use only if it is set smaller than match_limit. 3169 3170 Limiting the recursion depth limits the amount of machine stack that 3171 can be used, or, when PCRE has been compiled to use memory on the heap 3172 instead of the stack, the amount of heap memory that can be used. This 3173 limit is not relevant, and is ignored, when matching is done using JIT 3174 compiled code. 3175 3176 The default value for match_limit_recursion can be set when PCRE is 3177 built; the default default is the same value as the default for 3178 match_limit. You can override the default by suppling pcre_exec() with 3179 a pcre_extra block in which match_limit_recursion is set, and 3180 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the 3181 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. 3182 3183 A value for the recursion limit may also be supplied by an item at the 3184 start of a pattern of the form 3185 3186 (*LIMIT_RECURSION=d) 3187 3188 where d is a decimal number. However, such a setting is ignored unless 3189 d is less than the limit set by the caller of pcre_exec() or, if no 3190 such limit is set, less than the default. 3191 3192 The callout_data field is used in conjunction with the "callout" fea- 3193 ture, and is described in the pcrecallout documentation. 3194 3195 The tables field is provided for use with patterns that have been pre- 3196 compiled using custom character tables, saved to disc or elsewhere, and 3197 then reloaded, because the tables that were used to compile a pattern 3198 are not saved with it. See the pcreprecompile documentation for a dis- 3199 cussion of saving compiled patterns for later use. If NULL is passed 3200 using this mechanism, it forces PCRE's internal tables to be used. 3201 3202 Warning: The tables that pcre_exec() uses must be the same as those 3203 that were used when the pattern was compiled. If this is not the case, 3204 the behaviour of pcre_exec() is undefined. Therefore, when a pattern is 3205 compiled and matched in the same process, this field should never be 3206 set. In this (the most common) case, the correct table pointer is auto- 3207 matically passed with the compiled pattern from pcre_compile() to 3208 pcre_exec(). 3209 3210 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be 3211 set to point to a suitable variable. If the pattern contains any back- 3212 tracking control verbs such as (*MARK:NAME), and the execution ends up 3213 with a name to pass back, a pointer to the name string (zero termi- 3214 nated) is placed in the variable pointed to by the mark field. The 3215 names are within the compiled pattern; if you wish to retain such a 3216 name you must copy it before freeing the memory of a compiled pattern. 3217 If there is no name to pass back, the variable pointed to by the mark 3218 field is set to NULL. For details of the backtracking control verbs, 3219 see the section entitled "Backtracking control" in the pcrepattern doc- 3220 umentation. 3221 3222 Option bits for pcre_exec() 3223 3224 The unused bits of the options argument for pcre_exec() must be zero. 3225 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, 3226 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, 3227 PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_HARD, and 3228 PCRE_PARTIAL_SOFT. 3229 3230 If the pattern was successfully studied with one of the just-in-time 3231 (JIT) compile options, the only supported options for JIT execution are 3232 PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, 3233 PCRE_NOTEMPTY_ATSTART, PCRE_PARTIAL_HARD, and PCRE_PARTIAL_SOFT. If an 3234 unsupported option is used, JIT execution is disabled and the normal 3235 interpretive code in pcre_exec() is run. 3236 3237 PCRE_ANCHORED 3238 3239 The PCRE_ANCHORED option limits pcre_exec() to matching at the first 3240 matching position. If a pattern was compiled with PCRE_ANCHORED, or 3241 turned out to be anchored by virtue of its contents, it cannot be made 3242 unachored at matching time. 3243 3244 PCRE_BSR_ANYCRLF 3245 PCRE_BSR_UNICODE 3246 3247 These options (which are mutually exclusive) control what the \R escape 3248 sequence matches. The choice is either to match only CR, LF, or CRLF, 3249 or to match any Unicode newline sequence. These options override the 3250 choice that was made or defaulted when the pattern was compiled. 3251 3252 PCRE_NEWLINE_CR 3253 PCRE_NEWLINE_LF 3254 PCRE_NEWLINE_CRLF 3255 PCRE_NEWLINE_ANYCRLF 3256 PCRE_NEWLINE_ANY 3257 3258 These options override the newline definition that was chosen or 3259 defaulted when the pattern was compiled. For details, see the descrip- 3260 tion of pcre_compile() above. During matching, the newline choice 3261 affects the behaviour of the dot, circumflex, and dollar metacharac- 3262 ters. It may also alter the way the match position is advanced after a 3263 match failure for an unanchored pattern. 3264 3265 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is 3266 set, and a match attempt for an unanchored pattern fails when the cur- 3267 rent position is at a CRLF sequence, and the pattern contains no 3268 explicit matches for CR or LF characters, the match position is 3269 advanced by two characters instead of one, in other words, to after the 3270 CRLF. 3271 3272 The above rule is a compromise that makes the most common cases work as 3273 expected. For example, if the pattern is .+A (and the PCRE_DOTALL 3274 option is not set), it does not match the string "\r\nA" because, after 3275 failing at the start, it skips both the CR and the LF before retrying. 3276 However, the pattern [\r\n]A does match that string, because it con- 3277 tains an explicit CR or LF reference, and so advances only by one char- 3278 acter after the first failure. 3279 3280 An explicit match for CR of LF is either a literal appearance of one of 3281 those characters, or one of the \r or \n escape sequences. Implicit 3282 matches such as [^X] do not count, nor does \s (which includes CR and 3283 LF in the characters that it matches). 3284 3285 Notwithstanding the above, anomalous effects may still occur when CRLF 3286 is a valid newline sequence and explicit \r or \n escapes appear in the 3287 pattern. 3288 3289 PCRE_NOTBOL 3290 3291 This option specifies that first character of the subject string is not 3292 the beginning of a line, so the circumflex metacharacter should not 3293 match before it. Setting this without PCRE_MULTILINE (at compile time) 3294 causes circumflex never to match. This option affects only the behav- 3295 iour of the circumflex metacharacter. It does not affect \A. 3296 3297 PCRE_NOTEOL 3298 3299 This option specifies that the end of the subject string is not the end 3300 of a line, so the dollar metacharacter should not match it nor (except 3301 in multiline mode) a newline immediately before it. Setting this with- 3302 out PCRE_MULTILINE (at compile time) causes dollar never to match. This 3303 option affects only the behaviour of the dollar metacharacter. It does 3304 not affect \Z or \z. 3305 3306 PCRE_NOTEMPTY 3307 3308 An empty string is not considered to be a valid match if this option is 3309 set. If there are alternatives in the pattern, they are tried. If all 3310 the alternatives match the empty string, the entire match fails. For 3311 example, if the pattern 3312 3313 a?b? 3314 3315 is applied to a string not beginning with "a" or "b", it matches an 3316 empty string at the start of the subject. With PCRE_NOTEMPTY set, this 3317 match is not valid, so PCRE searches further into the string for occur- 3318 rences of "a" or "b". 3319 3320 PCRE_NOTEMPTY_ATSTART 3321 3322 This is like PCRE_NOTEMPTY, except that an empty string match that is 3323 not at the start of the subject is permitted. If the pattern is 3324 anchored, such a match can occur only if the pattern contains \K. 3325 3326 Perl has no direct equivalent of PCRE_NOTEMPTY or 3327 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern 3328 match of the empty string within its split() function, and when using 3329 the /g modifier. It is possible to emulate Perl's behaviour after 3330 matching a null string by first trying the match again at the same off- 3331 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that 3332 fails, by advancing the starting offset (see below) and trying an ordi- 3333 nary match again. There is some code that demonstrates how to do this 3334 in the pcredemo sample program. In the most general case, you have to 3335 check to see if the newline convention recognizes CRLF as a newline, 3336 and if so, and the current character is CR followed by LF, advance the 3337 starting offset by two characters instead of one. 3338 3339 PCRE_NO_START_OPTIMIZE 3340 3341 There are a number of optimizations that pcre_exec() uses at the start 3342 of a match, in order to speed up the process. For example, if it is 3343 known that an unanchored match must start with a specific character, it 3344 searches the subject for that character, and fails immediately if it 3345 cannot find it, without actually running the main matching function. 3346 This means that a special item such as (*COMMIT) at the start of a pat- 3347 tern is not considered until after a suitable starting point for the 3348 match has been found. Also, when callouts or (*MARK) items are in use, 3349 these "start-up" optimizations can cause them to be skipped if the pat- 3350 tern is never actually used. The start-up optimizations are in effect a 3351 pre-scan of the subject that takes place before the pattern is run. 3352 3353 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations, 3354 possibly causing performance to suffer, but ensuring that in cases 3355 where the result is "no match", the callouts do occur, and that items 3356 such as (*COMMIT) and (*MARK) are considered at every possible starting 3357 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at 3358 compile time, it cannot be unset at matching time. The use of 3359 PCRE_NO_START_OPTIMIZE at matching time (that is, passing it to 3360 pcre_exec()) disables JIT execution; in this situation, matching is 3361 always done using interpretively. 3362 3363 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching 3364 operation. Consider the pattern 3365 3366 (*COMMIT)ABC 3367 3368 When this is compiled, PCRE records the fact that a match must start 3369 with the character "A". Suppose the subject string is "DEFABC". The 3370 start-up optimization scans along the subject, finds "A" and runs the 3371 first match attempt from there. The (*COMMIT) item means that the pat- 3372 tern must match the current starting position, which in this case, it 3373 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE 3374 set, the initial scan along the subject string does not happen. The 3375 first match attempt is run starting from "D" and when this fails, 3376 (*COMMIT) prevents any further matches being tried, so the overall 3377 result is "no match". If the pattern is studied, more start-up opti- 3378 mizations may be used. For example, a minimum length for the subject 3379 may be recorded. Consider the pattern 3380 3381 (*MARK:A)(X|Y) 3382 3383 The minimum length for a match is one character. If the subject is 3384 "ABC", there will be attempts to match "ABC", "BC", "C", and then 3385 finally an empty string. If the pattern is studied, the final attempt 3386 does not take place, because PCRE knows that the subject is too short, 3387 and so the (*MARK) is never encountered. In this case, studying the 3388 pattern does not affect the overall match result, which is still "no 3389 match", but it does affect the auxiliary information that is returned. 3390 3391 PCRE_NO_UTF8_CHECK 3392 3393 When PCRE_UTF8 is set at compile time, the validity of the subject as a 3394 UTF-8 string is automatically checked when pcre_exec() is subsequently 3395 called. The entire string is checked before any other processing takes 3396 place. The value of startoffset is also checked to ensure that it 3397 points to the start of a UTF-8 character. There is a discussion about 3398 the validity of UTF-8 strings in the pcreunicode page. If an invalid 3399 sequence of bytes is found, pcre_exec() returns the error 3400 PCRE_ERROR_BADUTF8 or, if PCRE_PARTIAL_HARD is set and the problem is a 3401 truncated character at the end of the subject, PCRE_ERROR_SHORTUTF8. In 3402 both cases, information about the precise nature of the error may also 3403 be returned (see the descriptions of these errors in the section enti- 3404 tled Error return values from pcre_exec() below). If startoffset con- 3405 tains a value that does not point to the start of a UTF-8 character (or 3406 to the end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned. 3407 3408 If you already know that your subject is valid, and you want to skip 3409 these checks for performance reasons, you can set the 3410 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to 3411 do this for the second and subsequent calls to pcre_exec() if you are 3412 making repeated calls to find all the matches in a single subject 3413 string. However, you should be sure that the value of startoffset 3414 points to the start of a character (or the end of the subject). When 3415 PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid string as a 3416 subject or an invalid value of startoffset is undefined. Your program 3417 may crash or loop. 3418 3419 PCRE_PARTIAL_HARD 3420 PCRE_PARTIAL_SOFT 3421 3422 These options turn on the partial matching feature. For backwards com- 3423 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial 3424 match occurs if the end of the subject string is reached successfully, 3425 but there are not enough subject characters to complete the match. If 3426 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set, 3427 matching continues by testing any remaining alternatives. Only if no 3428 complete match can be found is PCRE_ERROR_PARTIAL returned instead of 3429 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the 3430 caller is prepared to handle a partial match, but only if no complete 3431 match can be found. 3432 3433 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this 3434 case, if a partial match is found, pcre_exec() immediately returns 3435 PCRE_ERROR_PARTIAL, without considering any other alternatives. In 3436 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid- 3437 ered to be more important that an alternative complete match. 3438 3439 In both cases, the portion of the string that was inspected when the 3440 partial match was found is set as the first matching string. There is a 3441 more detailed discussion of partial and multi-segment matching, with 3442 examples, in the pcrepartial documentation. 3443 3444 The string to be matched by pcre_exec() 3445 3446 The subject string is passed to pcre_exec() as a pointer in subject, a 3447 length in length, and a starting offset in startoffset. The units for 3448 length and startoffset are bytes for the 8-bit library, 16-bit data 3449 items for the 16-bit library, and 32-bit data items for the 32-bit 3450 library. 3451 3452 If startoffset is negative or greater than the length of the subject, 3453 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is 3454 zero, the search for a match starts at the beginning of the subject, 3455 and this is by far the most common case. In UTF-8 or UTF-16 mode, the 3456 offset must point to the start of a character, or the end of the sub- 3457 ject (in UTF-32 mode, one data unit equals one character, so all off- 3458 sets are valid). Unlike the pattern string, the subject may contain 3459 binary zeroes. 3460 3461 A non-zero starting offset is useful when searching for another match 3462 in the same subject by calling pcre_exec() again after a previous suc- 3463 cess. Setting startoffset differs from just passing over a shortened 3464 string and setting PCRE_NOTBOL in the case of a pattern that begins 3465 with any kind of lookbehind. For example, consider the pattern 3466 3467 \Biss\B 3468 3469 which finds occurrences of "iss" in the middle of words. (\B matches 3470 only if the current position in the subject is not a word boundary.) 3471 When applied to the string "Mississipi" the first call to pcre_exec() 3472 finds the first occurrence. If pcre_exec() is called again with just 3473 the remainder of the subject, namely "issipi", it does not match, 3474 because \B is always false at the start of the subject, which is deemed 3475 to be a word boundary. However, if pcre_exec() is passed the entire 3476 string again, but with startoffset set to 4, it finds the second occur- 3477 rence of "iss" because it is able to look behind the starting point to 3478 discover that it is preceded by a letter. 3479 3480 Finding all the matches in a subject is tricky when the pattern can 3481 match an empty string. It is possible to emulate Perl's /g behaviour by 3482 first trying the match again at the same offset, with the 3483 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that 3484 fails, advancing the starting offset and trying an ordinary match 3485 again. There is some code that demonstrates how to do this in the pcre- 3486 demo sample program. In the most general case, you have to check to see 3487 if the newline convention recognizes CRLF as a newline, and if so, and 3488 the current character is CR followed by LF, advance the starting offset 3489 by two characters instead of one. 3490 3491 If a non-zero starting offset is passed when the pattern is anchored, 3492 one attempt to match at the given offset is made. This can only succeed 3493 if the pattern does not require the match to be at the start of the 3494 subject. 3495 3496 How pcre_exec() returns captured substrings 3497 3498 In general, a pattern matches a certain portion of the subject, and in 3499 addition, further substrings from the subject may be picked out by 3500 parts of the pattern. Following the usage in Jeffrey Friedl's book, 3501 this is called "capturing" in what follows, and the phrase "capturing 3502 subpattern" is used for a fragment of a pattern that picks out a sub- 3503 string. PCRE supports several other kinds of parenthesized subpattern 3504 that do not cause substrings to be captured. 3505 3506 Captured substrings are returned to the caller via a vector of integers 3507 whose address is passed in ovector. The number of elements in the vec- 3508 tor is passed in ovecsize, which must be a non-negative number. Note: 3509 this argument is NOT the size of ovector in bytes. 3510 3511 The first two-thirds of the vector is used to pass back captured sub- 3512 strings, each substring using a pair of integers. The remaining third 3513 of the vector is used as workspace by pcre_exec() while matching cap- 3514 turing subpatterns, and is not available for passing back information. 3515 The number passed in ovecsize should always be a multiple of three. If 3516 it is not, it is rounded down. 3517 3518 When a match is successful, information about captured substrings is 3519 returned in pairs of integers, starting at the beginning of ovector, 3520 and continuing up to two-thirds of its length at the most. The first 3521 element of each pair is set to the offset of the first character in a 3522 substring, and the second is set to the offset of the first character 3523 after the end of a substring. These values are always data unit off- 3524 sets, even in UTF mode. They are byte offsets in the 8-bit library, 3525 16-bit data item offsets in the 16-bit library, and 32-bit data item 3526 offsets in the 32-bit library. Note: they are not character counts. 3527 3528 The first pair of integers, ovector[0] and ovector[1], identify the 3529 portion of the subject string matched by the entire pattern. The next 3530 pair is used for the first capturing subpattern, and so on. The value 3531 returned by pcre_exec() is one more than the highest numbered pair that 3532 has been set. For example, if two substrings have been captured, the 3533 returned value is 3. If there are no capturing subpatterns, the return 3534 value from a successful match is 1, indicating that just the first pair 3535 of offsets has been set. 3536 3537 If a capturing subpattern is matched repeatedly, it is the last portion 3538 of the string that it matched that is returned. 3539 3540 If the vector is too small to hold all the captured substring offsets, 3541 it is used as far as possible (up to two-thirds of its length), and the 3542 function returns a value of zero. If neither the actual string matched 3543 nor any captured substrings are of interest, pcre_exec() may be called 3544 with ovector passed as NULL and ovecsize as zero. However, if the pat- 3545 tern contains back references and the ovector is not big enough to 3546 remember the related substrings, PCRE has to get additional memory for 3547 use during matching. Thus it is usually advisable to supply an ovector 3548 of reasonable size. 3549 3550 There are some cases where zero is returned (indicating vector over- 3551 flow) when in fact the vector is exactly the right size for the final 3552 match. For example, consider the pattern 3553 3554 (a)(?:(b)c|bd) 3555 3556 If a vector of 6 elements (allowing for only 1 captured substring) is 3557 given with subject string "abd", pcre_exec() will try to set the second 3558 captured string, thereby recording a vector overflow, before failing to 3559 match "c" and backing up to try the second alternative. The zero 3560 return, however, does correctly indicate that the maximum number of 3561 slots (namely 2) have been filled. In similar cases where there is tem- 3562 porary overflow, but the final number of used slots is actually less 3563 than the maximum, a non-zero value is returned. 3564 3565 The pcre_fullinfo() function can be used to find out how many capturing 3566 subpatterns there are in a compiled pattern. The smallest size for 3567 ovector that will allow for n captured substrings, in addition to the 3568 offsets of the substring matched by the whole pattern, is (n+1)*3. 3569 3570 It is possible for capturing subpattern number n+1 to match some part 3571 of the subject when subpattern n has not been used at all. For example, 3572 if the string "abc" is matched against the pattern (a|(z))(bc) the 3573 return from the function is 4, and subpatterns 1 and 3 are matched, but 3574 2 is not. When this happens, both values in the offset pairs corre- 3575 sponding to unused subpatterns are set to -1. 3576 3577 Offset values that correspond to unused subpatterns at the end of the 3578 expression are also set to -1. For example, if the string "abc" is 3579 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not 3580 matched. The return from the function is 2, because the highest used 3581 capturing subpattern number is 1, and the offsets for for the second 3582 and third capturing subpatterns (assuming the vector is large enough, 3583 of course) are set to -1. 3584 3585 Note: Elements in the first two-thirds of ovector that do not corre- 3586 spond to capturing parentheses in the pattern are never changed. That 3587 is, if a pattern contains n capturing parentheses, no more than ovec- 3588 tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in 3589 the first two-thirds) retain whatever values they previously had. 3590 3591 Some convenience functions are provided for extracting the captured 3592 substrings as separate strings. These are described below. 3593 3594 Error return values from pcre_exec() 3595 3596 If pcre_exec() fails, it returns a negative number. The following are 3597 defined in the header file: 3598 3599 PCRE_ERROR_NOMATCH (-1) 3600 3601 The subject string did not match the pattern. 3602 3603 PCRE_ERROR_NULL (-2) 3604 3605 Either code or subject was passed as NULL, or ovector was NULL and 3606 ovecsize was not zero. 3607 3608 PCRE_ERROR_BADOPTION (-3) 3609 3610 An unrecognized bit was set in the options argument. 3611 3612 PCRE_ERROR_BADMAGIC (-4) 3613 3614 PCRE stores a 4-byte "magic number" at the start of the compiled code, 3615 to catch the case when it is passed a junk pointer and to detect when a 3616 pattern that was compiled in an environment of one endianness is run in 3617 an environment with the other endianness. This is the error that PCRE 3618 gives when the magic number is not present. 3619 3620 PCRE_ERROR_UNKNOWN_OPCODE (-5) 3621 3622 While running the pattern match, an unknown item was encountered in the 3623 compiled pattern. This error could be caused by a bug in PCRE or by 3624 overwriting of the compiled pattern. 3625 3626 PCRE_ERROR_NOMEMORY (-6) 3627 3628 If a pattern contains back references, but the ovector that is passed 3629 to pcre_exec() is not big enough to remember the referenced substrings, 3630 PCRE gets a block of memory at the start of matching to use for this 3631 purpose. If the call via pcre_malloc() fails, this error is given. The 3632 memory is automatically freed at the end of matching. 3633 3634 This error is also given if pcre_stack_malloc() fails in pcre_exec(). 3635 This can happen only when PCRE has been compiled with --disable-stack- 3636 for-recursion. 3637 3638 PCRE_ERROR_NOSUBSTRING (-7) 3639 3640 This error is used by the pcre_copy_substring(), pcre_get_substring(), 3641 and pcre_get_substring_list() functions (see below). It is never 3642 returned by pcre_exec(). 3643 3644 PCRE_ERROR_MATCHLIMIT (-8) 3645 3646 The backtracking limit, as specified by the match_limit field in a 3647 pcre_extra structure (or defaulted) was reached. See the description 3648 above. 3649 3650 PCRE_ERROR_CALLOUT (-9) 3651 3652 This error is never generated by pcre_exec() itself. It is provided for 3653 use by callout functions that want to yield a distinctive error code. 3654 See the pcrecallout documentation for details. 3655 3656 PCRE_ERROR_BADUTF8 (-10) 3657 3658 A string that contains an invalid UTF-8 byte sequence was passed as a 3659 subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of 3660 the output vector (ovecsize) is at least 2, the byte offset to the 3661 start of the the invalid UTF-8 character is placed in the first ele- 3662 ment, and a reason code is placed in the second element. The reason 3663 codes are listed in the following section. For backward compatibility, 3664 if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char- 3665 acter at the end of the subject (reason codes 1 to 5), 3666 PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8. 3667 3668 PCRE_ERROR_BADUTF8_OFFSET (-11) 3669 3670 The UTF-8 byte sequence that was passed as a subject was checked and 3671 found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the 3672 value of startoffset did not point to the beginning of a UTF-8 charac- 3673 ter or the end of the subject. 3674 3675 PCRE_ERROR_PARTIAL (-12) 3676 3677 The subject string did not match, but it did match partially. See the 3678 pcrepartial documentation for details of partial matching. 3679 3680 PCRE_ERROR_BADPARTIAL (-13) 3681 3682 This code is no longer in use. It was formerly returned when the 3683 PCRE_PARTIAL option was used with a compiled pattern containing items 3684 that were not supported for partial matching. From release 8.00 3685 onwards, there are no restrictions on partial matching. 3686 3687 PCRE_ERROR_INTERNAL (-14) 3688 3689 An unexpected internal error has occurred. This error could be caused 3690 by a bug in PCRE or by overwriting of the compiled pattern. 3691 3692 PCRE_ERROR_BADCOUNT (-15) 3693 3694 This error is given if the value of the ovecsize argument is negative. 3695 3696 PCRE_ERROR_RECURSIONLIMIT (-21) 3697 3698 The internal recursion limit, as specified by the match_limit_recursion 3699 field in a pcre_extra structure (or defaulted) was reached. See the 3700 description above. 3701 3702 PCRE_ERROR_BADNEWLINE (-23) 3703 3704 An invalid combination of PCRE_NEWLINE_xxx options was given. 3705 3706 PCRE_ERROR_BADOFFSET (-24) 3707 3708 The value of startoffset was negative or greater than the length of the 3709 subject, that is, the value in length. 3710 3711 PCRE_ERROR_SHORTUTF8 (-25) 3712 3713 This error is returned instead of PCRE_ERROR_BADUTF8 when the subject 3714 string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD 3715 option is set. Information about the failure is returned as for 3716 PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but 3717 this special error code for PCRE_PARTIAL_HARD precedes the implementa- 3718 tion of returned information; it is retained for backwards compatibil- 3719 ity. 3720 3721 PCRE_ERROR_RECURSELOOP (-26) 3722 3723 This error is returned when pcre_exec() detects a recursion loop within 3724 the pattern. Specifically, it means that either the whole pattern or a 3725 subpattern has been called recursively for the second time at the same 3726 position in the subject string. Some simple patterns that might do this 3727 are detected and faulted at compile time, but more complicated cases, 3728 in particular mutual recursions between two different subpatterns, can- 3729 not be detected until run time. 3730 3731 PCRE_ERROR_JIT_STACKLIMIT (-27) 3732 3733 This error is returned when a pattern that was successfully studied 3734 using a JIT compile option is being matched, but the memory available 3735 for the just-in-time processing stack is not large enough. See the 3736 pcrejit documentation for more details. 3737 3738 PCRE_ERROR_BADMODE (-28) 3739 3740 This error is given if a pattern that was compiled by the 8-bit library 3741 is passed to a 16-bit or 32-bit library function, or vice versa. 3742 3743 PCRE_ERROR_BADENDIANNESS (-29) 3744 3745 This error is given if a pattern that was compiled and saved is 3746 reloaded on a host with different endianness. The utility function 3747 pcre_pattern_to_host_byte_order() can be used to convert such a pattern 3748 so that it runs on the new host. 3749 3750 PCRE_ERROR_JIT_BADOPTION 3751 3752 This error is returned when a pattern that was successfully studied 3753 using a JIT compile option is being matched, but the matching mode 3754 (partial or complete match) does not correspond to any JIT compilation 3755 mode. When the JIT fast path function is used, this error may be also 3756 given for invalid options. See the pcrejit documentation for more 3757 details. 3758 3759 PCRE_ERROR_BADLENGTH (-32) 3760 3761 This error is given if pcre_exec() is called with a negative value for 3762 the length argument. 3763 3764 Error numbers -16 to -20, -22, and 30 are not used by pcre_exec(). 3765 3766 Reason codes for invalid UTF-8 strings 3767 3768 This section applies only to the 8-bit library. The corresponding 3769 information for the 16-bit and 32-bit libraries is given in the pcre16 3770 and pcre32 pages. 3771 3772 When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT- 3773 UTF8, and the size of the output vector (ovecsize) is at least 2, the 3774 offset of the start of the invalid UTF-8 character is placed in the 3775 first output vector element (ovector[0]) and a reason code is placed in 3776 the second element (ovector[1]). The reason codes are given names in 3777 the pcre.h header file: 3778 3779 PCRE_UTF8_ERR1 3780 PCRE_UTF8_ERR2 3781 PCRE_UTF8_ERR3 3782 PCRE_UTF8_ERR4 3783 PCRE_UTF8_ERR5 3784 3785 The string ends with a truncated UTF-8 character; the code specifies 3786 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8 3787 characters to be no longer than 4 bytes, the encoding scheme (origi- 3788 nally defined by RFC 2279) allows for up to 6 bytes, and this is 3789 checked first; hence the possibility of 4 or 5 missing bytes. 3790 3791 PCRE_UTF8_ERR6 3792 PCRE_UTF8_ERR7 3793 PCRE_UTF8_ERR8 3794 PCRE_UTF8_ERR9 3795 PCRE_UTF8_ERR10 3796 3797 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of 3798 the character do not have the binary value 0b10 (that is, either the 3799 most significant bit is 0, or the next bit is 1). 3800 3801 PCRE_UTF8_ERR11 3802 PCRE_UTF8_ERR12 3803 3804 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes 3805 long; these code points are excluded by RFC 3629. 3806 3807 PCRE_UTF8_ERR13 3808 3809 A 4-byte character has a value greater than 0x10fff; these code points 3810 are excluded by RFC 3629. 3811 3812 PCRE_UTF8_ERR14 3813 3814 A 3-byte character has a value in the range 0xd800 to 0xdfff; this 3815 range of code points are reserved by RFC 3629 for use with UTF-16, and 3816 so are excluded from UTF-8. 3817 3818 PCRE_UTF8_ERR15 3819 PCRE_UTF8_ERR16 3820 PCRE_UTF8_ERR17 3821 PCRE_UTF8_ERR18 3822 PCRE_UTF8_ERR19 3823 3824 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes 3825 for a value that can be represented by fewer bytes, which is invalid. 3826 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor- 3827 rect coding uses just one byte. 3828 3829 PCRE_UTF8_ERR20 3830 3831 The two most significant bits of the first byte of a character have the 3832 binary value 0b10 (that is, the most significant bit is 1 and the sec- 3833 ond is 0). Such a byte can only validly occur as the second or subse- 3834 quent byte of a multi-byte character. 3835 3836 PCRE_UTF8_ERR21 3837 3838 The first byte of a character has the value 0xfe or 0xff. These values 3839 can never occur in a valid UTF-8 string. 3840 3841 PCRE_UTF8_ERR22 3842 3843 This error code was formerly used when the presence of a so-called 3844 "non-character" caused an error. Unicode corrigendum #9 makes it clear 3845 that such characters should not cause a string to be rejected, and so 3846 this code is no longer in use and is never returned. 3847 3848 3849 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER 3850 3851 int pcre_copy_substring(const char *subject, int *ovector, 3852 int stringcount, int stringnumber, char *buffer, 3853 int buffersize); 3854 3855 int pcre_get_substring(const char *subject, int *ovector, 3856 int stringcount, int stringnumber, 3857 const char **stringptr); 3858 3859 int pcre_get_substring_list(const char *subject, 3860 int *ovector, int stringcount, const char ***listptr); 3861 3862 Captured substrings can be accessed directly by using the offsets 3863 returned by pcre_exec() in ovector. For convenience, the functions 3864 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- 3865 string_list() are provided for extracting captured substrings as new, 3866 separate, zero-terminated strings. These functions identify substrings 3867 by number. The next section describes functions for extracting named 3868 substrings. 3869 3870 A substring that contains a binary zero is correctly extracted and has 3871 a further zero added on the end, but the result is not, of course, a C 3872 string. However, you can process such a string by referring to the 3873 length that is returned by pcre_copy_substring() and pcre_get_sub- 3874 string(). Unfortunately, the interface to pcre_get_substring_list() is 3875 not adequate for handling strings containing binary zeros, because the 3876 end of the final string is not independently indicated. 3877 3878 The first three arguments are the same for all three of these func- 3879 tions: subject is the subject string that has just been successfully 3880 matched, ovector is a pointer to the vector of integer offsets that was 3881 passed to pcre_exec(), and stringcount is the number of substrings that 3882 were captured by the match, including the substring that matched the 3883 entire regular expression. This is the value returned by pcre_exec() if 3884 it is greater than zero. If pcre_exec() returned zero, indicating that 3885 it ran out of space in ovector, the value passed as stringcount should 3886 be the number of elements in the vector divided by three. 3887 3888 The functions pcre_copy_substring() and pcre_get_substring() extract a 3889 single substring, whose number is given as stringnumber. A value of 3890 zero extracts the substring that matched the entire pattern, whereas 3891 higher values extract the captured substrings. For pcre_copy_sub- 3892 string(), the string is placed in buffer, whose length is given by 3893 buffersize, while for pcre_get_substring() a new block of memory is 3894 obtained via pcre_malloc, and its address is returned via stringptr. 3895 The yield of the function is the length of the string, not including 3896 the terminating zero, or one of these error codes: 3897 3898 PCRE_ERROR_NOMEMORY (-6) 3899 3900 The buffer was too small for pcre_copy_substring(), or the attempt to 3901 get memory failed for pcre_get_substring(). 3902 3903 PCRE_ERROR_NOSUBSTRING (-7) 3904 3905 There is no substring whose number is stringnumber. 3906 3907 The pcre_get_substring_list() function extracts all available sub- 3908 strings and builds a list of pointers to them. All this is done in a 3909 single block of memory that is obtained via pcre_malloc. The address of 3910 the memory block is returned via listptr, which is also the start of 3911 the list of string pointers. The end of the list is marked by a NULL 3912 pointer. The yield of the function is zero if all went well, or the 3913 error code 3914 3915 PCRE_ERROR_NOMEMORY (-6) 3916 3917 if the attempt to get the memory block failed. 3918 3919 When any of these functions encounter a substring that is unset, which 3920 can happen when capturing subpattern number n+1 matches some part of 3921 the subject, but subpattern n has not been used at all, they return an 3922 empty string. This can be distinguished from a genuine zero-length sub- 3923 string by inspecting the appropriate offset in ovector, which is nega- 3924 tive for unset substrings. 3925 3926 The two convenience functions pcre_free_substring() and pcre_free_sub- 3927 string_list() can be used to free the memory returned by a previous 3928 call of pcre_get_substring() or pcre_get_substring_list(), respec- 3929 tively. They do nothing more than call the function pointed to by 3930 pcre_free, which of course could be called directly from a C program. 3931 However, PCRE is used in some situations where it is linked via a spe- 3932 cial interface to another programming language that cannot use 3933 pcre_free directly; it is for these cases that the functions are pro- 3934 vided. 3935 3936 3937 EXTRACTING CAPTURED SUBSTRINGS BY NAME 3938 3939 int pcre_get_stringnumber(const pcre *code, 3940 const char *name); 3941 3942 int pcre_copy_named_substring(const pcre *code, 3943 const char *subject, int *ovector, 3944 int stringcount, const char *stringname, 3945 char *buffer, int buffersize); 3946 3947 int pcre_get_named_substring(const pcre *code, 3948 const char *subject, int *ovector, 3949 int stringcount, const char *stringname, 3950 const char **stringptr); 3951 3952 To extract a substring by name, you first have to find associated num- 3953 ber. For example, for this pattern 3954 3955 (a+)b(?<xxx>\d+)... 3956 3957 the number of the subpattern called "xxx" is 2. If the name is known to 3958 be unique (PCRE_DUPNAMES was not set), you can find the number from the 3959 name by calling pcre_get_stringnumber(). The first argument is the com- 3960 piled pattern, and the second is the name. The yield of the function is 3961 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no 3962 subpattern of that name. 3963 3964 Given the number, you can extract the substring directly, or use one of 3965 the functions described in the previous section. For convenience, there 3966 are also two functions that do the whole job. 3967 3968 Most of the arguments of pcre_copy_named_substring() and 3969 pcre_get_named_substring() are the same as those for the similarly 3970 named functions that extract by number. As these are described in the 3971 previous section, they are not re-described here. There are just two 3972 differences: 3973 3974 First, instead of a substring number, a substring name is given. Sec- 3975 ond, there is an extra argument, given at the start, which is a pointer 3976 to the compiled pattern. This is needed in order to gain access to the 3977 name-to-number translation table. 3978 3979 These functions call pcre_get_stringnumber(), and if it succeeds, they 3980 then call pcre_copy_substring() or pcre_get_substring(), as appropri- 3981 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the 3982 behaviour may not be what you want (see the next section). 3983 3984 Warning: If the pattern uses the (?| feature to set up multiple subpat- 3985 terns with the same number, as described in the section on duplicate 3986 subpattern numbers in the pcrepattern page, you cannot use names to 3987 distinguish the different subpatterns, because names are not included 3988 in the compiled code. The matching process uses only numbers. For this 3989 reason, the use of different names for subpatterns of the same number 3990 causes an error at compile time. 3991 3992 3993 DUPLICATE SUBPATTERN NAMES 3994 3995 int pcre_get_stringtable_entries(const pcre *code, 3996 const char *name, char **first, char **last); 3997 3998 When a pattern is compiled with the PCRE_DUPNAMES option, names for 3999 subpatterns are not required to be unique. (Duplicate names are always 4000 allowed for subpatterns with the same number, created by using the (?| 4001 feature. Indeed, if such subpatterns are named, they are required to 4002 use the same names.) 4003 4004 Normally, patterns with duplicate names are such that in any one match, 4005 only one of the named subpatterns participates. An example is shown in 4006 the pcrepattern documentation. 4007 4008 When duplicates are present, pcre_copy_named_substring() and 4009 pcre_get_named_substring() return the first substring corresponding to 4010 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING 4011 (-7) is returned; no data is returned. The pcre_get_stringnumber() 4012 function returns one of the numbers that are associated with the name, 4013 but it is not defined which it is. 4014 4015 If you want to get full details of all captured substrings for a given 4016 name, you must use the pcre_get_stringtable_entries() function. The 4017 first argument is the compiled pattern, and the second is the name. The 4018 third and fourth are pointers to variables which are updated by the 4019 function. After it has run, they point to the first and last entries in 4020 the name-to-number table for the given name. The function itself 4021 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if 4022 there are none. The format of the table is described above in the sec- 4023 tion entitled Information about a pattern above. Given all the rele- 4024 vant entries for the name, you can extract each of their numbers, and 4025 hence the captured data, if any. 4026 4027 4028 FINDING ALL POSSIBLE MATCHES 4029 4030 The traditional matching function uses a similar algorithm to Perl, 4031 which stops when it finds the first match, starting at a given point in 4032 the subject. If you want to find all possible matches, or the longest 4033 possible match, consider using the alternative matching function (see 4034 below) instead. If you cannot use the alternative function, but still 4035 need to find all possible matches, you can kludge it up by making use 4036 of the callout facility, which is described in the pcrecallout documen- 4037 tation. 4038 4039 What you have to do is to insert a callout right at the end of the pat- 4040 tern. When your callout function is called, extract and save the cur- 4041 rent matched substring. Then return 1, which forces pcre_exec() to 4042 backtrack and try other alternatives. Ultimately, when it runs out of 4043 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. 4044 4045 4046 OBTAINING AN ESTIMATE OF STACK USAGE 4047 4048 Matching certain patterns using pcre_exec() can use a lot of process 4049 stack, which in certain environments can be rather limited in size. 4050 Some users find it helpful to have an estimate of the amount of stack 4051 that is used by pcre_exec(), to help them set recursion limits, as 4052 described in the pcrestack documentation. The estimate that is output 4053 by pcretest when called with the -m and -C options is obtained by call- 4054 ing pcre_exec with the values NULL, NULL, NULL, -999, and -999 for its 4055 first five arguments. 4056 4057 Normally, if its first argument is NULL, pcre_exec() immediately 4058 returns the negative error code PCRE_ERROR_NULL, but with this special 4059 combination of arguments, it returns instead a negative number whose 4060 absolute value is the approximate stack frame size in bytes. (A nega- 4061 tive number is used so that it is clear that no match has happened.) 4062 The value is approximate because in some cases, recursive calls to 4063 pcre_exec() occur when there are one or two additional variables on the 4064 stack. 4065 4066 If PCRE has been compiled to use the heap instead of the stack for 4067 recursion, the value returned is the size of each block that is 4068 obtained from the heap. 4069 4070 4071 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION 4072 4073 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, 4074 const char *subject, int length, int startoffset, 4075 int options, int *ovector, int ovecsize, 4076 int *workspace, int wscount); 4077 4078 The function pcre_dfa_exec() is called to match a subject string 4079 against a compiled pattern, using a matching algorithm that scans the 4080 subject string just once, and does not backtrack. This has different 4081 characteristics to the normal algorithm, and is not compatible with 4082 Perl. Some of the features of PCRE patterns are not supported. Never- 4083 theless, there are times when this kind of matching can be useful. For 4084 a discussion of the two matching algorithms, and a list of features 4085 that pcre_dfa_exec() does not support, see the pcrematching documenta- 4086 tion. 4087 4088 The arguments for the pcre_dfa_exec() function are the same as for 4089 pcre_exec(), plus two extras. The ovector argument is used in a differ- 4090 ent way, and this is described below. The other common arguments are 4091 used in the same way as for pcre_exec(), so their description is not 4092 repeated here. 4093 4094 The two additional arguments provide workspace for the function. The 4095 workspace vector should contain at least 20 elements. It is used for 4096 keeping track of multiple paths through the pattern tree. More 4097 workspace will be needed for patterns and subjects where there are a 4098 lot of potential matches. 4099 4100 Here is an example of a simple call to pcre_dfa_exec(): 4101 4102 int rc; 4103 int ovector[10]; 4104 int wspace[20]; 4105 rc = pcre_dfa_exec( 4106 re, /* result of pcre_compile() */ 4107 NULL, /* we didn't study the pattern */ 4108 "some string", /* the subject string */ 4109 11, /* the length of the subject string */ 4110 0, /* start at offset 0 in the subject */ 4111 0, /* default options */ 4112 ovector, /* vector of integers for substring information */ 4113 10, /* number of elements (NOT size in bytes) */ 4114 wspace, /* working space vector */ 4115 20); /* number of elements (NOT size in bytes) */ 4116 4117 Option bits for pcre_dfa_exec() 4118 4119 The unused bits of the options argument for pcre_dfa_exec() must be 4120 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- 4121 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, 4122 PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF, 4123 PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR- 4124 TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last 4125 four of these are exactly the same as for pcre_exec(), so their 4126 description is not repeated here. 4127 4128 PCRE_PARTIAL_HARD 4129 PCRE_PARTIAL_SOFT 4130 4131 These have the same general effect as they do for pcre_exec(), but the 4132 details are slightly different. When PCRE_PARTIAL_HARD is set for 4133 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub- 4134 ject is reached and there is still at least one matching possibility 4135 that requires additional characters. This happens even if some complete 4136 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return 4137 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end 4138 of the subject is reached, there have been no complete matches, but 4139 there is still at least one matching possibility. The portion of the 4140 string that was inspected when the longest partial match was found is 4141 set as the first matching string in both cases. There is a more 4142 detailed discussion of partial and multi-segment matching, with exam- 4143 ples, in the pcrepartial documentation. 4144 4145 PCRE_DFA_SHORTEST 4146 4147 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to 4148 stop as soon as it has found one match. Because of the way the alterna- 4149 tive algorithm works, this is necessarily the shortest possible match 4150 at the first possible matching point in the subject string. 4151 4152 PCRE_DFA_RESTART 4153 4154 When pcre_dfa_exec() returns a partial match, it is possible to call it 4155 again, with additional subject characters, and have it continue with 4156 the same match. The PCRE_DFA_RESTART option requests this action; when 4157 it is set, the workspace and wscount options must reference the same 4158 vector as before because data about the match so far is left in them 4159 after a partial match. There is more discussion of this facility in the 4160 pcrepartial documentation. 4161 4162 Successful returns from pcre_dfa_exec() 4163 4164 When pcre_dfa_exec() succeeds, it may have matched more than one sub- 4165 string in the subject. Note, however, that all the matches from one run 4166 of the function start at the same point in the subject. The shorter 4167 matches are all initial substrings of the longer matches. For example, 4168 if the pattern 4169 4170 <.*> 4171 4172 is matched against the string 4173 4174 This is <something> <something else> <something further> no more 4175 4176 the three matched strings are 4177 4178 <something> 4179 <something> <something else> 4180 <something> <something else> <something further> 4181 4182 On success, the yield of the function is a number greater than zero, 4183 which is the number of matched substrings. The substrings themselves 4184 are returned in ovector. Each string uses two elements; the first is 4185 the offset to the start, and the second is the offset to the end. In 4186 fact, all the strings have the same start offset. (Space could have 4187 been saved by giving this only once, but it was decided to retain some 4188 compatibility with the way pcre_exec() returns data, even though the 4189 meaning of the strings is different.) 4190 4191 The strings are returned in reverse order of length; that is, the long- 4192 est matching string is given first. If there were too many matches to 4193 fit into ovector, the yield of the function is zero, and the vector is 4194 filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec() 4195 can use the entire ovector for returning matched strings. 4196 4197 NOTE: PCRE's "auto-possessification" optimization usually applies to 4198 character repeats at the end of a pattern (as well as internally). For 4199 example, the pattern "a\d+" is compiled as if it were "a\d++" because 4200 there is no point even considering the possibility of backtracking into 4201 the repeated digits. For DFA matching, this means that only one possi- 4202 ble match is found. If you really do want multiple matches in such 4203 cases, either use an ungreedy repeat ("a\d+?") or set the 4204 PCRE_NO_AUTO_POSSESS option when compiling. 4205 4206 Error returns from pcre_dfa_exec() 4207 4208 The pcre_dfa_exec() function returns a negative number when it fails. 4209 Many of the errors are the same as for pcre_exec(), and these are 4210 described above. There are in addition the following errors that are 4211 specific to pcre_dfa_exec(): 4212 4213 PCRE_ERROR_DFA_UITEM (-16) 4214 4215 This return is given if pcre_dfa_exec() encounters an item in the pat- 4216 tern that it does not support, for instance, the use of \C or a back 4217 reference. 4218 4219 PCRE_ERROR_DFA_UCOND (-17) 4220 4221 This return is given if pcre_dfa_exec() encounters a condition item 4222 that uses a back reference for the condition, or a test for recursion 4223 in a specific group. These are not supported. 4224 4225 PCRE_ERROR_DFA_UMLIMIT (-18) 4226 4227 This return is given if pcre_dfa_exec() is called with an extra block 4228 that contains a setting of the match_limit or match_limit_recursion 4229 fields. This is not supported (these fields are meaningless for DFA 4230 matching). 4231 4232 PCRE_ERROR_DFA_WSSIZE (-19) 4233 4234 This return is given if pcre_dfa_exec() runs out of space in the 4235 workspace vector. 4236 4237 PCRE_ERROR_DFA_RECURSE (-20) 4238 4239 When a recursive subpattern is processed, the matching function calls 4240 itself recursively, using private vectors for ovector and workspace. 4241 This error is given if the output vector is not large enough. This 4242 should be extremely rare, as a vector of size 1000 is used. 4243 4244 PCRE_ERROR_DFA_BADRESTART (-30) 4245 4246 When pcre_dfa_exec() is called with the PCRE_DFA_RESTART option, some 4247 plausibility checks are made on the contents of the workspace, which 4248 should contain data about the previous partial match. If any of these 4249 checks fail, this error is given. 4250 4251 4252 SEE ALSO 4253 4254 pcre16(3), pcre32(3), pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), 4255 pcrematching(3), pcrepartial(3), pcreposix(3), pcreprecompile(3), pcre- 4256 sample(3), pcrestack(3). 4257 4258 4259 AUTHOR 4260 4261 Philip Hazel 4262 University Computing Service 4263 Cambridge CB2 3QH, England. 4264 4265 4266 REVISION 4267 4268 Last updated: 09 February 2014 4269 Copyright (c) 1997-2014 University of Cambridge. 4270 ------------------------------------------------------------------------------ 4271 4272 4273 PCRECALLOUT(3) Library Functions Manual PCRECALLOUT(3) 4274 4275 4276 4277 NAME 4278 PCRE - Perl-compatible regular expressions 4279 4280 SYNOPSIS 4281 4282 #include <pcre.h> 4283 4284 int (*pcre_callout)(pcre_callout_block *); 4285 4286 int (*pcre16_callout)(pcre16_callout_block *); 4287 4288 int (*pcre32_callout)(pcre32_callout_block *); 4289 4290 4291 DESCRIPTION 4292 4293 PCRE provides a feature called "callout", which is a means of temporar- 4294 ily passing control to the caller of PCRE in the middle of pattern 4295 matching. The caller of PCRE provides an external function by putting 4296 its entry point in the global variable pcre_callout (pcre16_callout for 4297 the 16-bit library, pcre32_callout for the 32-bit library). By default, 4298 this variable contains NULL, which disables all calling out. 4299 4300 Within a regular expression, (?C) indicates the points at which the 4301 external function is to be called. Different callout points can be 4302 identified by putting a number less than 256 after the letter C. The 4303 default value is zero. For example, this pattern has two callout 4304 points: 4305 4306 (?C1)abc(?C2)def 4307 4308 If the PCRE_AUTO_CALLOUT option bit is set when a pattern is compiled, 4309 PCRE automatically inserts callouts, all with number 255, before each 4310 item in the pattern. For example, if PCRE_AUTO_CALLOUT is used with the 4311 pattern 4312 4313 A(\d{2}|--) 4314 4315 it is processed as if it were 4316 4317 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) 4318 4319 Notice that there is a callout before and after each parenthesis and 4320 alternation bar. If the pattern contains a conditional group whose con- 4321 dition is an assertion, an automatic callout is inserted immediately 4322 before the condition. Such a callout may also be inserted explicitly, 4323 for example: 4324 4325 (?(?C9)(?=a)ab|de) 4326 4327 This applies only to assertion conditions (because they are themselves 4328 independent groups). 4329 4330 Automatic callouts can be used for tracking the progress of pattern 4331 matching. The pcretest program has a pattern qualifier (/C) that sets 4332 automatic callouts; when it is used, the output indicates how the pat- 4333 tern is being matched. This is useful information when you are trying 4334 to optimize the performance of a particular pattern. 4335 4336 4337 MISSING CALLOUTS 4338 4339 You should be aware that, because of optimizations in the way PCRE com- 4340 piles and matches patterns, callouts sometimes do not happen exactly as 4341 you might expect. 4342 4343 At compile time, PCRE "auto-possessifies" repeated items when it knows 4344 that what follows cannot be part of the repeat. For example, a+[bc] is 4345 compiled as if it were a++[bc]. The pcretest output when this pattern 4346 is anchored and then applied with automatic callouts to the string 4347 "aaaa" is: 4348 4349 --->aaaa 4350 +0 ^ ^ 4351 +1 ^ a+ 4352 +3 ^ ^ [bc] 4353 No match 4354 4355 This indicates that when matching [bc] fails, there is no backtracking 4356 into a+ and therefore the callouts that would be taken for the back- 4357 tracks do not occur. You can disable the auto-possessify feature by 4358 passing PCRE_NO_AUTO_POSSESS to pcre_compile(), or starting the pattern 4359 with (*NO_AUTO_POSSESS). If this is done in pcretest (using the /O 4360 qualifier), the output changes to this: 4361 4362 --->aaaa 4363 +0 ^ ^ 4364 +1 ^ a+ 4365 +3 ^ ^ [bc] 4366 +3 ^ ^ [bc] 4367 +3 ^ ^ [bc] 4368 +3 ^^ [bc] 4369 No match 4370 4371 This time, when matching [bc] fails, the matcher backtracks into a+ and 4372 tries again, repeatedly, until a+ itself fails. 4373 4374 Other optimizations that provide fast "no match" results also affect 4375 callouts. For example, if the pattern is 4376 4377 ab(?C4)cd 4378 4379 PCRE knows that any matching string must contain the letter "d". If the 4380 subject string is "abyz", the lack of "d" means that matching doesn't 4381 ever start, and the callout is never reached. However, with "abyd", 4382 though the result is still no match, the callout is obeyed. 4383 4384 If the pattern is studied, PCRE knows the minimum length of a matching 4385 string, and will immediately give a "no match" return without actually 4386 running a match if the subject is not long enough, or, for unanchored 4387 patterns, if it has been scanned far enough. 4388 4389 You can disable these optimizations by passing the PCRE_NO_START_OPTI- 4390 MIZE option to the matching function, or by starting the pattern with 4391 (*NO_START_OPT). This slows down the matching process, but does ensure 4392 that callouts such as the example above are obeyed. 4393 4394 4395 THE CALLOUT INTERFACE 4396 4397 During matching, when PCRE reaches a callout point, the external func- 4398 tion defined by pcre_callout or pcre[16|32]_callout is called (if it is 4399 set). This applies to both normal and DFA matching. The only argument 4400 to the callout function is a pointer to a pcre_callout or 4401 pcre[16|32]_callout block. These structures contains the following 4402 fields: 4403 4404 int version; 4405 int callout_number; 4406 int *offset_vector; 4407 const char *subject; (8-bit version) 4408 PCRE_SPTR16 subject; (16-bit version) 4409 PCRE_SPTR32 subject; (32-bit version) 4410 int subject_length; 4411 int start_match; 4412 int current_position; 4413 int capture_top; 4414 int capture_last; 4415 void *callout_data; 4416 int pattern_position; 4417 int next_item_length; 4418 const unsigned char *mark; (8-bit version) 4419 const PCRE_UCHAR16 *mark; (16-bit version) 4420 const PCRE_UCHAR32 *mark; (32-bit version) 4421 4422 The version field is an integer containing the version number of the 4423 block format. The initial version was 0; the current version is 2. The 4424 version number will change again in future if additional fields are 4425 added, but the intention is never to remove any of the existing fields. 4426 4427 The callout_number field contains the number of the callout, as com- 4428 piled into the pattern (that is, the number after ?C for manual call- 4429 outs, and 255 for automatically generated callouts). 4430 4431 The offset_vector field is a pointer to the vector of offsets that was 4432 passed by the caller to the matching function. When pcre_exec() or 4433 pcre[16|32]_exec() is used, the contents can be inspected, in order to 4434 extract substrings that have been matched so far, in the same way as 4435 for extracting substrings after a match has completed. For the DFA 4436 matching functions, this field is not useful. 4437 4438 The subject and subject_length fields contain copies of the values that 4439 were passed to the matching function. 4440 4441 The start_match field normally contains the offset within the subject 4442 at which the current match attempt started. However, if the escape 4443 sequence \K has been encountered, this value is changed to reflect the 4444 modified starting point. If the pattern is not anchored, the callout 4445 function may be called several times from the same point in the pattern 4446 for different starting points in the subject. 4447 4448 The current_position field contains the offset within the subject of 4449 the current match pointer. 4450 4451 When the pcre_exec() or pcre[16|32]_exec() is used, the capture_top 4452 field contains one more than the number of the highest numbered cap- 4453 tured substring so far. If no substrings have been captured, the value 4454 of capture_top is one. This is always the case when the DFA functions 4455 are used, because they do not support captured substrings. 4456 4457 The capture_last field contains the number of the most recently cap- 4458 tured substring. However, when a recursion exits, the value reverts to 4459 what it was outside the recursion, as do the values of all captured 4460 substrings. If no substrings have been captured, the value of cap- 4461 ture_last is -1. This is always the case for the DFA matching func- 4462 tions. 4463 4464 The callout_data field contains a value that is passed to a matching 4465 function specifically so that it can be passed back in callouts. It is 4466 passed in the callout_data field of a pcre_extra or pcre[16|32]_extra 4467 data structure. If no such data was passed, the value of callout_data 4468 in a callout block is NULL. There is a description of the pcre_extra 4469 structure in the pcreapi documentation. 4470 4471 The pattern_position field is present from version 1 of the callout 4472 structure. It contains the offset to the next item to be matched in the 4473 pattern string. 4474 4475 The next_item_length field is present from version 1 of the callout 4476 structure. It contains the length of the next item to be matched in the 4477 pattern string. When the callout immediately precedes an alternation 4478 bar, a closing parenthesis, or the end of the pattern, the length is 4479 zero. When the callout precedes an opening parenthesis, the length is 4480 that of the entire subpattern. 4481 4482 The pattern_position and next_item_length fields are intended to help 4483 in distinguishing between different automatic callouts, which all have 4484 the same callout number. However, they are set for all callouts. 4485 4486 The mark field is present from version 2 of the callout structure. In 4487 callouts from pcre_exec() or pcre[16|32]_exec() it contains a pointer 4488 to the zero-terminated name of the most recently passed (*MARK), 4489 (*PRUNE), or (*THEN) item in the match, or NULL if no such items have 4490 been passed. Instances of (*PRUNE) or (*THEN) without a name do not 4491 obliterate a previous (*MARK). In callouts from the DFA matching func- 4492 tions this field always contains NULL. 4493 4494 4495 RETURN VALUES 4496 4497 The external callout function returns an integer to PCRE. If the value 4498 is zero, matching proceeds as normal. If the value is greater than 4499 zero, matching fails at the current point, but the testing of other 4500 matching possibilities goes ahead, just as if a lookahead assertion had 4501 failed. If the value is less than zero, the match is abandoned, the 4502 matching function returns the negative value. 4503 4504 Negative values should normally be chosen from the set of 4505 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- 4506 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is 4507 reserved for use by callout functions; it will never be used by PCRE 4508 itself. 4509 4510 4511 AUTHOR 4512 4513 Philip Hazel 4514 University Computing Service 4515 Cambridge CB2 3QH, England. 4516 4517 4518 REVISION 4519 4520 Last updated: 12 November 2013 4521 Copyright (c) 1997-2013 University of Cambridge. 4522 ------------------------------------------------------------------------------ 4523 4524 4525 PCRECOMPAT(3) Library Functions Manual PCRECOMPAT(3) 4526 4527 4528 4529 NAME 4530 PCRE - Perl-compatible regular expressions 4531 4532 DIFFERENCES BETWEEN PCRE AND PERL 4533 4534 This document describes the differences in the ways that PCRE and Perl 4535 handle regular expressions. The differences described here are with 4536 respect to Perl versions 5.10 and above. 4537 4538 1. PCRE has only a subset of Perl's Unicode support. Details of what it 4539 does have are given in the pcreunicode page. 4540 4541 2. PCRE allows repeat quantifiers only on parenthesized assertions, but 4542 they do not mean what you might think. For example, (?!a){3} does not 4543 assert that the next three characters are not "a". It just asserts that 4544 the next character is not "a" three times (in principle: PCRE optimizes 4545 this to run the assertion just once). Perl allows repeat quantifiers on 4546 other assertions such as \b, but these do not seem to have any use. 4547 4548 3. Capturing subpatterns that occur inside negative lookahead asser- 4549 tions are counted, but their entries in the offsets vector are never 4550 set. Perl sometimes (but not always) sets its numerical variables from 4551 inside negative assertions. 4552 4553 4. Though binary zero characters are supported in the subject string, 4554 they are not allowed in a pattern string because it is passed as a nor- 4555 mal C string, terminated by zero. The escape sequence \0 can be used in 4556 the pattern to represent a binary zero. 4557 4558 5. The following Perl escape sequences are not supported: \l, \u, \L, 4559 \U, and \N when followed by a character name or Unicode value. (\N on 4560 its own, matching a non-newline character, is supported.) In fact these 4561 are implemented by Perl's general string-handling and are not part of 4562 its pattern matching engine. If any of these are encountered by PCRE, 4563 an error is generated by default. However, if the PCRE_JAVASCRIPT_COM- 4564 PAT option is set, \U and \u are interpreted as JavaScript interprets 4565 them. 4566 4567 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE 4568 is built with Unicode character property support. The properties that 4569 can be tested with \p and \P are limited to the general category prop- 4570 erties such as Lu and Nd, script names such as Greek or Han, and the 4571 derived properties Any and L&. PCRE does support the Cs (surrogate) 4572 property, which Perl does not; the Perl documentation says "Because 4573 Perl hides the need for the user to understand the internal representa- 4574 tion of Unicode characters, there is no need to implement the somewhat 4575 messy concept of surrogates." 4576 4577 7. PCRE does support the \Q...\E escape for quoting substrings. Charac- 4578 ters in between are treated as literals. This is slightly different 4579 from Perl in that $ and @ are also handled as literals inside the 4580 quotes. In Perl, they cause variable interpolation (but of course PCRE 4581 does not have variables). Note the following examples: 4582 4583 Pattern PCRE matches Perl matches 4584 4585 \Qabc$xyz\E abc$xyz abc followed by the 4586 contents of $xyz 4587 \Qabc\$xyz\E abc\$xyz abc\$xyz 4588 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 4589 4590 The \Q...\E sequence is recognized both inside and outside character 4591 classes. 4592 4593 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) 4594 constructions. However, there is support for recursive patterns. This 4595 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE 4596 "callout" feature allows an external function to be called during pat- 4597 tern matching. See the pcrecallout documentation for details. 4598 4599 9. Subpatterns that are called as subroutines (whether or not recur- 4600 sively) are always treated as atomic groups in PCRE. This is like 4601 Python, but unlike Perl. Captured values that are set outside a sub- 4602 routine call can be reference from inside in PCRE, but not in Perl. 4603 There is a discussion that explains these differences in more detail in 4604 the section on recursion differences from Perl in the pcrepattern page. 4605 4606 10. If any of the backtracking control verbs are used in a subpattern 4607 that is called as a subroutine (whether or not recursively), their 4608 effect is confined to that subpattern; it does not extend to the sur- 4609 rounding pattern. This is not always the case in Perl. In particular, 4610 if (*THEN) is present in a group that is called as a subroutine, its 4611 action is limited to that group, even if the group does not contain any 4612 | characters. Note that such subpatterns are processed as anchored at 4613 the point where they are tested. 4614 4615 11. If a pattern contains more than one backtracking control verb, the 4616 first one that is backtracked onto acts. For example, in the pattern 4617 A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure 4618 in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases 4619 it is the same as PCRE, but there are examples where it differs. 4620 4621 12. Most backtracking verbs in assertions have their normal actions. 4622 They are not confined to the assertion. 4623 4624 13. There are some differences that are concerned with the settings of 4625 captured strings when part of a pattern is repeated. For example, 4626 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 4627 unset, but in PCRE it is set to "b". 4628 4629 14. PCRE's handling of duplicate subpattern numbers and duplicate sub- 4630 pattern names is not as general as Perl's. This is a consequence of the 4631 fact the PCRE works internally just with numbers, using an external ta- 4632 ble to translate between numbers and names. In particular, a pattern 4633 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have 4634 the same number but different names, is not supported, and causes an 4635 error at compile time. If it were allowed, it would not be possible to 4636 distinguish which parentheses matched, because both names map to cap- 4637 turing subpattern number 1. To avoid this confusing situation, an error 4638 is given at compile time. 4639 4640 15. Perl recognizes comments in some places that PCRE does not, for 4641 example, between the ( and ? at the start of a subpattern. If the /x 4642 modifier is set, Perl allows white space between ( and ? (though cur- 4643 rent Perls warn that this is deprecated) but PCRE never does, even if 4644 the PCRE_EXTENDED option is set. 4645 4646 16. Perl, when in warning mode, gives warnings for character classes 4647 such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as liter- 4648 als. PCRE has no warning features, so it gives an error in these cases 4649 because they are almost certainly user mistakes. 4650 4651 17. In PCRE, the upper/lower case character properties Lu and Ll are 4652 not affected when case-independent matching is specified. For example, 4653 \p{Lu} always matches an upper case letter. I think Perl has changed in 4654 this respect; in the release at the time of writing (5.16), \p{Lu} and 4655 \p{Ll} match all letters, regardless of case, when case independence is 4656 specified. 4657 4658 18. PCRE provides some extensions to the Perl regular expression facil- 4659 ities. Perl 5.10 includes new features that are not in earlier ver- 4660 sions of Perl, some of which (such as named parentheses) have been in 4661 PCRE for some time. This list is with respect to Perl 5.10: 4662 4663 (a) Although lookbehind assertions in PCRE must match fixed length 4664 strings, each alternative branch of a lookbehind assertion can match a 4665 different length of string. Perl requires them all to have the same 4666 length. 4667 4668 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ 4669 meta-character matches only at the very end of the string. 4670 4671 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- 4672 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly 4673 ignored. (Perl can be made to issue a warning.) 4674 4675 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- 4676 fiers is inverted, that is, by default they are not greedy, but if fol- 4677 lowed by a question mark they are. 4678 4679 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be 4680 tried only at the first matching position in the subject string. 4681 4682 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, 4683 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva- 4684 lents. 4685 4686 (g) The \R escape sequence can be restricted to match only CR, LF, or 4687 CRLF by the PCRE_BSR_ANYCRLF option. 4688 4689 (h) The callout facility is PCRE-specific. 4690 4691 (i) The partial matching facility is PCRE-specific. 4692 4693 (j) Patterns compiled by PCRE can be saved and re-used at a later time, 4694 even on different hosts that have the other endianness. However, this 4695 does not apply to optimized data created by the just-in-time compiler. 4696 4697 (k) The alternative matching functions (pcre_dfa_exec(), 4698 pcre16_dfa_exec() and pcre32_dfa_exec(),) match in a different way and 4699 are not Perl-compatible. 4700 4701 (l) PCRE recognizes some special sequences such as (*CR) at the start 4702 of a pattern that set overall options that cannot be changed within the 4703 pattern. 4704 4705 4706 AUTHOR 4707 4708 Philip Hazel 4709 University Computing Service 4710 Cambridge CB2 3QH, England. 4711 4712 4713 REVISION 4714 4715 Last updated: 10 November 2013 4716 Copyright (c) 1997-2013 University of Cambridge. 4717 ------------------------------------------------------------------------------ 4718 4719 4720 PCREPATTERN(3) Library Functions Manual PCREPATTERN(3) 4721 4722 4723 4724 NAME 4725 PCRE - Perl-compatible regular expressions 4726 4727 PCRE REGULAR EXPRESSION DETAILS 4728 4729 The syntax and semantics of the regular expressions that are supported 4730 by PCRE are described in detail below. There is a quick-reference syn- 4731 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and 4732 semantics as closely as it can. PCRE also supports some alternative 4733 regular expression syntax (which does not conflict with the Perl syn- 4734 tax) in order to provide some compatibility with regular expressions in 4735 Python, .NET, and Oniguruma. 4736 4737 Perl's regular expressions are described in its own documentation, and 4738 regular expressions in general are covered in a number of books, some 4739 of which have copious examples. Jeffrey Friedl's "Mastering Regular 4740 Expressions", published by O'Reilly, covers regular expressions in 4741 great detail. This description of PCRE's regular expressions is 4742 intended as reference material. 4743 4744 This document discusses the patterns that are supported by PCRE when 4745 one its main matching functions, pcre_exec() (8-bit) or 4746 pcre[16|32]_exec() (16- or 32-bit), is used. PCRE also has alternative 4747 matching functions, pcre_dfa_exec() and pcre[16|32_dfa_exec(), which 4748 match using a different algorithm that is not Perl-compatible. Some of 4749 the features discussed below are not available when DFA matching is 4750 used. The advantages and disadvantages of the alternative functions, 4751 and how they differ from the normal functions, are discussed in the 4752 pcrematching page. 4753 4754 4755 SPECIAL START-OF-PATTERN ITEMS 4756 4757 A number of options that can be passed to pcre_compile() can also be 4758 set by special items at the start of a pattern. These are not Perl-com- 4759 patible, but are provided to make these options accessible to pattern 4760 writers who are not able to change the program that processes the pat- 4761 tern. Any number of these items may appear, but they must all be 4762 together right at the start of the pattern string, and the letters must 4763 be in upper case. 4764 4765 UTF support 4766 4767 The original operation of PCRE was on strings of one-byte characters. 4768 However, there is now also support for UTF-8 strings in the original 4769 library, an extra library that supports 16-bit and UTF-16 character 4770 strings, and a third library that supports 32-bit and UTF-32 character 4771 strings. To use these features, PCRE must be built to include appropri- 4772 ate support. When using UTF strings you must either call the compiling 4773 function with the PCRE_UTF8, PCRE_UTF16, or PCRE_UTF32 option, or the 4774 pattern must start with one of these special sequences: 4775 4776 (*UTF8) 4777 (*UTF16) 4778 (*UTF32) 4779 (*UTF) 4780 4781 (*UTF) is a generic sequence that can be used with any of the 4782 libraries. Starting a pattern with such a sequence is equivalent to 4783 setting the relevant option. How setting a UTF mode affects pattern 4784 matching is mentioned in several places below. There is also a summary 4785 of features in the pcreunicode page. 4786 4787 Some applications that allow their users to supply patterns may wish to 4788 restrict them to non-UTF data for security reasons. If the 4789 PCRE_NEVER_UTF option is set at compile time, (*UTF) etc. are not 4790 allowed, and their appearance causes an error. 4791 4792 Unicode property support 4793 4794 Another special sequence that may appear at the start of a pattern is 4795 (*UCP). This has the same effect as setting the PCRE_UCP option: it 4796 causes sequences such as \d and \w to use Unicode properties to deter- 4797 mine character types, instead of recognizing only characters with codes 4798 less than 128 via a lookup table. 4799 4800 Disabling auto-possessification 4801 4802 If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as 4803 setting the PCRE_NO_AUTO_POSSESS option at compile time. This stops 4804 PCRE from making quantifiers possessive when what follows cannot match 4805 the repeated item. For example, by default a+b is treated as a++b. For 4806 more details, see the pcreapi documentation. 4807 4808 Disabling start-up optimizations 4809 4810 If a pattern starts with (*NO_START_OPT), it has the same effect as 4811 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching 4812 time. This disables several optimizations for quickly reaching "no 4813 match" results. For more details, see the pcreapi documentation. 4814 4815 Newline conventions 4816 4817 PCRE supports five different conventions for indicating line breaks in 4818 strings: a single CR (carriage return) character, a single LF (line- 4819 feed) character, the two-character sequence CRLF, any of the three pre- 4820 ceding, or any Unicode newline sequence. The pcreapi page has further 4821 discussion about newlines, and shows how to set the newline convention 4822 in the options arguments for the compiling and matching functions. 4823 4824 It is also possible to specify a newline convention by starting a pat- 4825 tern string with one of the following five sequences: 4826 4827 (*CR) carriage return 4828 (*LF) linefeed 4829 (*CRLF) carriage return, followed by linefeed 4830 (*ANYCRLF) any of the three above 4831 (*ANY) all Unicode newline sequences 4832 4833 These override the default and the options given to the compiling func- 4834 tion. For example, on a Unix system where LF is the default newline 4835 sequence, the pattern 4836 4837 (*CR)a.b 4838 4839 changes the convention to CR. That pattern matches "a\nb" because LF is 4840 no longer a newline. If more than one of these settings is present, the 4841 last one is used. 4842 4843 The newline convention affects where the circumflex and dollar asser- 4844 tions are true. It also affects the interpretation of the dot metachar- 4845 acter when PCRE_DOTALL is not set, and the behaviour of \N. However, it 4846 does not affect what the \R escape sequence matches. By default, this 4847 is any Unicode newline sequence, for Perl compatibility. However, this 4848 can be changed; see the description of \R in the section entitled "New- 4849 line sequences" below. A change of \R setting can be combined with a 4850 change of newline convention. 4851 4852 Setting match and recursion limits 4853 4854 The caller of pcre_exec() can set a limit on the number of times the 4855 internal match() function is called and on the maximum depth of recur- 4856 sive calls. These facilities are provided to catch runaway matches that 4857 are provoked by patterns with huge matching trees (a typical example is 4858 a pattern with nested unlimited repeats) and to avoid running out of 4859 system stack by too much recursion. When one of these limits is 4860 reached, pcre_exec() gives an error return. The limits can also be set 4861 by items at the start of the pattern of the form 4862 4863 (*LIMIT_MATCH=d) 4864 (*LIMIT_RECURSION=d) 4865 4866 where d is any number of decimal digits. However, the value of the set- 4867 ting must be less than the value set (or defaulted) by the caller of 4868 pcre_exec() for it to have any effect. In other words, the pattern 4869 writer can lower the limits set by the programmer, but not raise them. 4870 If there is more than one setting of one of these limits, the lower 4871 value is used. 4872 4873 4874 EBCDIC CHARACTER CODES 4875 4876 PCRE can be compiled to run in an environment that uses EBCDIC as its 4877 character code rather than ASCII or Unicode (typically a mainframe sys- 4878 tem). In the sections below, character code values are ASCII or Uni- 4879 code; in an EBCDIC environment these characters may have different code 4880 values, and there are no code points greater than 255. 4881 4882 4883 CHARACTERS AND METACHARACTERS 4884 4885 A regular expression is a pattern that is matched against a subject 4886 string from left to right. Most characters stand for themselves in a 4887 pattern, and match the corresponding characters in the subject. As a 4888 trivial example, the pattern 4889 4890 The quick brown fox 4891 4892 matches a portion of a subject string that is identical to itself. When 4893 caseless matching is specified (the PCRE_CASELESS option), letters are 4894 matched independently of case. In a UTF mode, PCRE always understands 4895 the concept of case for characters whose values are less than 128, so 4896 caseless matching is always possible. For characters with higher val- 4897 ues, the concept of case is supported if PCRE is compiled with Unicode 4898 property support, but not otherwise. If you want to use caseless 4899 matching for characters 128 and above, you must ensure that PCRE is 4900 compiled with Unicode property support as well as with UTF support. 4901 4902 The power of regular expressions comes from the ability to include 4903 alternatives and repetitions in the pattern. These are encoded in the 4904 pattern by the use of metacharacters, which do not stand for themselves 4905 but instead are interpreted in some special way. 4906 4907 There are two different sets of metacharacters: those that are recog- 4908 nized anywhere in the pattern except within square brackets, and those 4909 that are recognized within square brackets. Outside square brackets, 4910 the metacharacters are as follows: 4911 4912 \ general escape character with several uses 4913 ^ assert start of string (or line, in multiline mode) 4914 $ assert end of string (or line, in multiline mode) 4915 . match any character except newline (by default) 4916 [ start character class definition 4917 | start of alternative branch 4918 ( start subpattern 4919 ) end subpattern 4920 ? extends the meaning of ( 4921 also 0 or 1 quantifier 4922 also quantifier minimizer 4923 * 0 or more quantifier 4924 + 1 or more quantifier 4925 also "possessive quantifier" 4926 { start min/max quantifier 4927 4928 Part of a pattern that is in square brackets is called a "character 4929 class". In a character class the only metacharacters are: 4930 4931 \ general escape character 4932 ^ negate the class, but only if the first character 4933 - indicates character range 4934 [ POSIX character class (only if followed by POSIX 4935 syntax) 4936 ] terminates the character class 4937 4938 The following sections describe the use of each of the metacharacters. 4939 4940 4941 BACKSLASH 4942 4943 The backslash character has several uses. Firstly, if it is followed by 4944 a character that is not a number or a letter, it takes away any special 4945 meaning that character may have. This use of backslash as an escape 4946 character applies both inside and outside character classes. 4947 4948 For example, if you want to match a * character, you write \* in the 4949 pattern. This escaping action applies whether or not the following 4950 character would otherwise be interpreted as a metacharacter, so it is 4951 always safe to precede a non-alphanumeric with backslash to specify 4952 that it stands for itself. In particular, if you want to match a back- 4953 slash, you write \\. 4954 4955 In a UTF mode, only ASCII numbers and letters have any special meaning 4956 after a backslash. All other characters (in particular, those whose 4957 codepoints are greater than 127) are treated as literals. 4958 4959 If a pattern is compiled with the PCRE_EXTENDED option, most white 4960 space in the pattern (other than in a character class), and characters 4961 between a # outside a character class and the next newline, inclusive, 4962 are ignored. An escaping backslash can be used to include a white space 4963 or # character as part of the pattern. 4964 4965 If you want to remove the special meaning from a sequence of charac- 4966 ters, you can do so by putting them between \Q and \E. This is differ- 4967 ent from Perl in that $ and @ are handled as literals in \Q...\E 4968 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- 4969 tion. Note the following examples: 4970 4971 Pattern PCRE matches Perl matches 4972 4973 \Qabc$xyz\E abc$xyz abc followed by the 4974 contents of $xyz 4975 \Qabc\$xyz\E abc\$xyz abc\$xyz 4976 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 4977 4978 The \Q...\E sequence is recognized both inside and outside character 4979 classes. An isolated \E that is not preceded by \Q is ignored. If \Q 4980 is not followed by \E later in the pattern, the literal interpretation 4981 continues to the end of the pattern (that is, \E is assumed at the 4982 end). If the isolated \Q is inside a character class, this causes an 4983 error, because the character class is not terminated. 4984 4985 Non-printing characters 4986 4987 A second use of backslash provides a way of encoding non-printing char- 4988 acters in patterns in a visible manner. There is no restriction on the 4989 appearance of non-printing characters, apart from the binary zero that 4990 terminates a pattern, but when a pattern is being prepared by text 4991 editing, it is often easier to use one of the following escape 4992 sequences than the binary character it represents: 4993 4994 \a alarm, that is, the BEL character (hex 07) 4995 \cx "control-x", where x is any ASCII character 4996 \e escape (hex 1B) 4997 \f form feed (hex 0C) 4998 \n linefeed (hex 0A) 4999 \r carriage return (hex 0D) 5000 \t tab (hex 09) 5001 \0dd character with octal code 0dd 5002 \ddd character with octal code ddd, or back reference 5003 \o{ddd..} character with octal code ddd.. 5004 \xhh character with hex code hh 5005 \x{hhh..} character with hex code hhh.. (non-JavaScript mode) 5006 \uhhhh character with hex code hhhh (JavaScript mode only) 5007 5008 The precise effect of \cx on ASCII characters is as follows: if x is a 5009 lower case letter, it is converted to upper case. Then bit 6 of the 5010 character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A 5011 (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and \c; becomes 5012 hex 7B (; is 3B). If the data item (byte or 16-bit value) following \c 5013 has a value greater than 127, a compile-time error occurs. This locks 5014 out non-ASCII characters in all modes. 5015 5016 The \c facility was designed for use with ASCII characters, but with 5017 the extension to Unicode it is even less useful than it once was. It 5018 is, however, recognized when PCRE is compiled in EBCDIC mode, where 5019 data items are always bytes. In this mode, all values are valid after 5020 \c. If the next character is a lower case letter, it is converted to 5021 upper case. Then the 0xc0 bits of the byte are inverted. Thus \cA 5022 becomes hex 01, as in ASCII (A is C1), but because the EBCDIC letters 5023 are disjoint, \cZ becomes hex 29 (Z is E9), and other characters also 5024 generate different values. 5025 5026 After \0 up to two further octal digits are read. If there are fewer 5027 than two digits, just those that are present are used. Thus the 5028 sequence \0\x\07 specifies two binary zeros followed by a BEL character 5029 (code value 7). Make sure you supply two digits after the initial zero 5030 if the pattern character that follows is itself an octal digit. 5031 5032 The escape \o must be followed by a sequence of octal digits, enclosed 5033 in braces. An error occurs if this is not the case. This escape is a 5034 recent addition to Perl; it provides way of specifying character code 5035 points as octal numbers greater than 0777, and it also allows octal 5036 numbers and back references to be unambiguously specified. 5037 5038 For greater clarity and unambiguity, it is best to avoid following \ by 5039 a digit greater than zero. Instead, use \o{} or \x{} to specify charac- 5040 ter numbers, and \g{} to specify back references. The following para- 5041 graphs describe the old, ambiguous syntax. 5042 5043 The handling of a backslash followed by a digit other than 0 is compli- 5044 cated, and Perl has changed in recent releases, causing PCRE also to 5045 change. Outside a character class, PCRE reads the digit and any follow- 5046 ing digits as a decimal number. If the number is less than 8, or if 5047 there have been at least that many previous capturing left parentheses 5048 in the expression, the entire sequence is taken as a back reference. A 5049 description of how this works is given later, following the discussion 5050 of parenthesized subpatterns. 5051 5052 Inside a character class, or if the decimal number following \ is 5053 greater than 7 and there have not been that many capturing subpatterns, 5054 PCRE handles \8 and \9 as the literal characters "8" and "9", and oth- 5055 erwise re-reads up to three octal digits following the backslash, using 5056 them to generate a data character. Any subsequent digits stand for 5057 themselves. For example: 5058 5059 \040 is another way of writing an ASCII space 5060 \40 is the same, provided there are fewer than 40 5061 previous capturing subpatterns 5062 \7 is always a back reference 5063 \11 might be a back reference, or another way of 5064 writing a tab 5065 \011 is always a tab 5066 \0113 is a tab followed by the character "3" 5067 \113 might be a back reference, otherwise the 5068 character with octal code 113 5069 \377 might be a back reference, otherwise 5070 the value 255 (decimal) 5071 \81 is either a back reference, or the two 5072 characters "8" and "1" 5073 5074 Note that octal values of 100 or greater that are specified using this 5075 syntax must not be introduced by a leading zero, because no more than 5076 three octal digits are ever read. 5077 5078 By default, after \x that is not followed by {, from zero to two hexa- 5079 decimal digits are read (letters can be in upper or lower case). Any 5080 number of hexadecimal digits may appear between \x{ and }. If a charac- 5081 ter other than a hexadecimal digit appears between \x{ and }, or if 5082 there is no terminating }, an error occurs. 5083 5084 If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x 5085 is as just described only when it is followed by two hexadecimal dig- 5086 its. Otherwise, it matches a literal "x" character. In JavaScript 5087 mode, support for code points greater than 256 is provided by \u, which 5088 must be followed by four hexadecimal digits; otherwise it matches a 5089 literal "u" character. 5090 5091 Characters whose value is less than 256 can be defined by either of the 5092 two syntaxes for \x (or by \u in JavaScript mode). There is no differ- 5093 ence in the way they are handled. For example, \xdc is exactly the same 5094 as \x{dc} (or \u00dc in JavaScript mode). 5095 5096 Constraints on character values 5097 5098 Characters that are specified using octal or hexadecimal numbers are 5099 limited to certain values, as follows: 5100 5101 8-bit non-UTF mode less than 0x100 5102 8-bit UTF-8 mode less than 0x10ffff and a valid codepoint 5103 16-bit non-UTF mode less than 0x10000 5104 16-bit UTF-16 mode less than 0x10ffff and a valid codepoint 5105 32-bit non-UTF mode less than 0x100000000 5106 32-bit UTF-32 mode less than 0x10ffff and a valid codepoint 5107 5108 Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so- 5109 called "surrogate" codepoints), and 0xffef. 5110 5111 Escape sequences in character classes 5112 5113 All the sequences that define a single character value can be used both 5114 inside and outside character classes. In addition, inside a character 5115 class, \b is interpreted as the backspace character (hex 08). 5116 5117 \N is not allowed in a character class. \B, \R, and \X are not special 5118 inside a character class. Like other unrecognized escape sequences, 5119 they are treated as the literal characters "B", "R", and "X" by 5120 default, but cause an error if the PCRE_EXTRA option is set. Outside a 5121 character class, these sequences have different meanings. 5122 5123 Unsupported escape sequences 5124 5125 In Perl, the sequences \l, \L, \u, and \U are recognized by its string 5126 handler and used to modify the case of following characters. By 5127 default, PCRE does not support these escape sequences. However, if the 5128 PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and 5129 \u can be used to define a character by code point, as described in the 5130 previous section. 5131 5132 Absolute and relative back references 5133 5134 The sequence \g followed by an unsigned or a negative number, option- 5135 ally enclosed in braces, is an absolute or relative back reference. A 5136 named back reference can be coded as \g{name}. Back references are dis- 5137 cussed later, following the discussion of parenthesized subpatterns. 5138 5139 Absolute and relative subroutine calls 5140 5141 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 5142 name or a number enclosed either in angle brackets or single quotes, is 5143 an alternative syntax for referencing a subpattern as a "subroutine". 5144 Details are discussed later. Note that \g{...} (Perl syntax) and 5145 \g<...> (Oniguruma syntax) are not synonymous. The former is a back 5146 reference; the latter is a subroutine call. 5147 5148 Generic character types 5149 5150 Another use of backslash is for specifying generic character types: 5151 5152 \d any decimal digit 5153 \D any character that is not a decimal digit 5154 \h any horizontal white space character 5155 \H any character that is not a horizontal white space character 5156 \s any white space character 5157 \S any character that is not a white space character 5158 \v any vertical white space character 5159 \V any character that is not a vertical white space character 5160 \w any "word" character 5161 \W any "non-word" character 5162 5163 There is also the single sequence \N, which matches a non-newline char- 5164 acter. This is the same as the "." metacharacter when PCRE_DOTALL is 5165 not set. Perl also uses \N to match characters by name; PCRE does not 5166 support this. 5167 5168 Each pair of lower and upper case escape sequences partitions the com- 5169 plete set of characters into two disjoint sets. Any given character 5170 matches one, and only one, of each pair. The sequences can appear both 5171 inside and outside character classes. They each match one character of 5172 the appropriate type. If the current matching point is at the end of 5173 the subject string, all of them fail, because there is no character to 5174 match. 5175 5176 For compatibility with Perl, \s did not used to match the VT character 5177 (code 11), which made it different from the the POSIX "space" class. 5178 However, Perl added VT at release 5.18, and PCRE followed suit at 5179 release 8.34. The default \s characters are now HT (9), LF (10), VT 5180 (11), FF (12), CR (13), and space (32), which are defined as white 5181 space in the "C" locale. This list may vary if locale-specific matching 5182 is taking place. For example, in some locales the "non-breaking space" 5183 character (\xA0) is recognized as white space, and in others the VT 5184 character is not. 5185 5186 A "word" character is an underscore or any character that is a letter 5187 or digit. By default, the definition of letters and digits is con- 5188 trolled by PCRE's low-valued character tables, and may vary if locale- 5189 specific matching is taking place (see "Locale support" in the pcreapi 5190 page). For example, in a French locale such as "fr_FR" in Unix-like 5191 systems, or "french" in Windows, some character codes greater than 127 5192 are used for accented letters, and these are then matched by \w. The 5193 use of locales with Unicode is discouraged. 5194 5195 By default, characters whose code points are greater than 127 never 5196 match \d, \s, or \w, and always match \D, \S, and \W, although this may 5197 vary for characters in the range 128-255 when locale-specific matching 5198 is happening. These escape sequences retain their original meanings 5199 from before Unicode support was available, mainly for efficiency rea- 5200 sons. If PCRE is compiled with Unicode property support, and the 5201 PCRE_UCP option is set, the behaviour is changed so that Unicode prop- 5202 erties are used to determine character types, as follows: 5203 5204 \d any character that matches \p{Nd} (decimal digit) 5205 \s any character that matches \p{Z} or \h or \v 5206 \w any character that matches \p{L} or \p{N}, plus underscore 5207 5208 The upper case escapes match the inverse sets of characters. Note that 5209 \d matches only decimal digits, whereas \w matches any Unicode digit, 5210 as well as any Unicode letter, and underscore. Note also that PCRE_UCP 5211 affects \b, and \B because they are defined in terms of \w and \W. 5212 Matching these sequences is noticeably slower when PCRE_UCP is set. 5213 5214 The sequences \h, \H, \v, and \V are features that were added to Perl 5215 at release 5.10. In contrast to the other sequences, which match only 5216 ASCII characters by default, these always match certain high-valued 5217 code points, whether or not PCRE_UCP is set. The horizontal space char- 5218 acters are: 5219 5220 U+0009 Horizontal tab (HT) 5221 U+0020 Space 5222 U+00A0 Non-break space 5223 U+1680 Ogham space mark 5224 U+180E Mongolian vowel separator 5225 U+2000 En quad 5226 U+2001 Em quad 5227 U+2002 En space 5228 U+2003 Em space 5229 U+2004 Three-per-em space 5230 U+2005 Four-per-em space 5231 U+2006 Six-per-em space 5232 U+2007 Figure space 5233 U+2008 Punctuation space 5234 U+2009 Thin space 5235 U+200A Hair space 5236 U+202F Narrow no-break space 5237 U+205F Medium mathematical space 5238 U+3000 Ideographic space 5239 5240 The vertical space characters are: 5241 5242 U+000A Linefeed (LF) 5243 U+000B Vertical tab (VT) 5244 U+000C Form feed (FF) 5245 U+000D Carriage return (CR) 5246 U+0085 Next line (NEL) 5247 U+2028 Line separator 5248 U+2029 Paragraph separator 5249 5250 In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 5251 256 are relevant. 5252 5253 Newline sequences 5254 5255 Outside a character class, by default, the escape sequence \R matches 5256 any Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent 5257 to the following: 5258 5259 (?>\r\n|\n|\x0b|\f|\r|\x85) 5260 5261 This is an example of an "atomic group", details of which are given 5262 below. This particular group matches either the two-character sequence 5263 CR followed by LF, or one of the single characters LF (linefeed, 5264 U+000A), VT (vertical tab, U+000B), FF (form feed, U+000C), CR (car- 5265 riage return, U+000D), or NEL (next line, U+0085). The two-character 5266 sequence is treated as a single unit that cannot be split. 5267 5268 In other modes, two additional characters whose codepoints are greater 5269 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- 5270 rator, U+2029). Unicode character property support is not needed for 5271 these characters to be recognized. 5272 5273 It is possible to restrict \R to match only CR, LF, or CRLF (instead of 5274 the complete set of Unicode line endings) by setting the option 5275 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. 5276 (BSR is an abbrevation for "backslash R".) This can be made the default 5277 when PCRE is built; if this is the case, the other behaviour can be 5278 requested via the PCRE_BSR_UNICODE option. It is also possible to 5279 specify these settings by starting a pattern string with one of the 5280 following sequences: 5281 5282 (*BSR_ANYCRLF) CR, LF, or CRLF only 5283 (*BSR_UNICODE) any Unicode newline sequence 5284 5285 These override the default and the options given to the compiling func- 5286 tion, but they can themselves be overridden by options given to a 5287 matching function. Note that these special settings, which are not 5288 Perl-compatible, are recognized only at the very start of a pattern, 5289 and that they must be in upper case. If more than one of them is 5290 present, the last one is used. They can be combined with a change of 5291 newline convention; for example, a pattern can start with: 5292 5293 (*ANY)(*BSR_ANYCRLF) 5294 5295 They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) 5296 or (*UCP) special sequences. Inside a character class, \R is treated as 5297 an unrecognized escape sequence, and so matches the letter "R" by 5298 default, but causes an error if PCRE_EXTRA is set. 5299 5300 Unicode character properties 5301 5302 When PCRE is built with Unicode character property support, three addi- 5303 tional escape sequences that match characters with specific properties 5304 are available. When in 8-bit non-UTF-8 mode, these sequences are of 5305 course limited to testing characters whose codepoints are less than 5306 256, but they do work in this mode. The extra escape sequences are: 5307 5308 \p{xx} a character with the xx property 5309 \P{xx} a character without the xx property 5310 \X a Unicode extended grapheme cluster 5311 5312 The property names represented by xx above are limited to the Unicode 5313 script names, the general category properties, "Any", which matches any 5314 character (including newline), and some special PCRE properties 5315 (described in the next section). Other Perl properties such as "InMu- 5316 sicalSymbols" are not currently supported by PCRE. Note that \P{Any} 5317 does not match any characters, so always causes a match failure. 5318 5319 Sets of Unicode characters are defined as belonging to certain scripts. 5320 A character from one of these sets can be matched using a script name. 5321 For example: 5322 5323 \p{Greek} 5324 \P{Han} 5325 5326 Those that are not part of an identified script are lumped together as 5327 "Common". The current list of scripts is: 5328 5329 Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, 5330 Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Car- 5331 ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei- 5332 form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero- 5333 glyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, 5334 Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, 5335 Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- 5336 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, 5337 Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Lin- 5338 ear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic, 5339 Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, 5340 Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, 5341 New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, 5342 Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, 5343 Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, 5344 Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha- 5345 vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, 5346 Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, 5347 Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, 5348 Yi. 5349 5350 Each character has exactly one Unicode general category property, spec- 5351 ified by a two-letter abbreviation. For compatibility with Perl, nega- 5352 tion can be specified by including a circumflex between the opening 5353 brace and the property name. For example, \p{^Lu} is the same as 5354 \P{Lu}. 5355 5356 If only one letter is specified with \p or \P, it includes all the gen- 5357 eral category properties that start with that letter. In this case, in 5358 the absence of negation, the curly brackets in the escape sequence are 5359 optional; these two examples have the same effect: 5360 5361 \p{L} 5362 \pL 5363 5364 The following general category property codes are supported: 5365 5366 C Other 5367 Cc Control 5368 Cf Format 5369 Cn Unassigned 5370 Co Private use 5371 Cs Surrogate 5372 5373 L Letter 5374 Ll Lower case letter 5375 Lm Modifier letter 5376 Lo Other letter 5377 Lt Title case letter 5378 Lu Upper case letter 5379 5380 M Mark 5381 Mc Spacing mark 5382 Me Enclosing mark 5383 Mn Non-spacing mark 5384 5385 N Number 5386 Nd Decimal number 5387 Nl Letter number 5388 No Other number 5389 5390 P Punctuation 5391 Pc Connector punctuation 5392 Pd Dash punctuation 5393 Pe Close punctuation 5394 Pf Final punctuation 5395 Pi Initial punctuation 5396 Po Other punctuation 5397 Ps Open punctuation 5398 5399 S Symbol 5400 Sc Currency symbol 5401 Sk Modifier symbol 5402 Sm Mathematical symbol 5403 So Other symbol 5404 5405 Z Separator 5406 Zl Line separator 5407 Zp Paragraph separator 5408 Zs Space separator 5409 5410 The special property L& is also supported: it matches a character that 5411 has the Lu, Ll, or Lt property, in other words, a letter that is not 5412 classified as a modifier or "other". 5413 5414 The Cs (Surrogate) property applies only to characters in the range 5415 U+D800 to U+DFFF. Such characters are not valid in Unicode strings and 5416 so cannot be tested by PCRE, unless UTF validity checking has been 5417 turned off (see the discussion of PCRE_NO_UTF8_CHECK, 5418 PCRE_NO_UTF16_CHECK and PCRE_NO_UTF32_CHECK in the pcreapi page). Perl 5419 does not support the Cs property. 5420 5421 The long synonyms for property names that Perl supports (such as 5422 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix 5423 any of these properties with "Is". 5424 5425 No character that is in the Unicode table has the Cn (unassigned) prop- 5426 erty. Instead, this property is assumed for any code point that is not 5427 in the Unicode table. 5428 5429 Specifying caseless matching does not affect these escape sequences. 5430 For example, \p{Lu} always matches only upper case letters. This is 5431 different from the behaviour of current versions of Perl. 5432 5433 Matching characters by Unicode property is not fast, because PCRE has 5434 to do a multistage table lookup in order to find a character's prop- 5435 erty. That is why the traditional escape sequences such as \d and \w do 5436 not use Unicode properties in PCRE by default, though you can make them 5437 do so by setting the PCRE_UCP option or by starting the pattern with 5438 (*UCP). 5439 5440 Extended grapheme clusters 5441 5442 The \X escape matches any number of Unicode characters that form an 5443 "extended grapheme cluster", and treats the sequence as an atomic group 5444 (see below). Up to and including release 8.31, PCRE matched an ear- 5445 lier, simpler definition that was equivalent to 5446 5447 (?>\PM\pM*) 5448 5449 That is, it matched a character without the "mark" property, followed 5450 by zero or more characters with the "mark" property. Characters with 5451 the "mark" property are typically non-spacing accents that affect the 5452 preceding character. 5453 5454 This simple definition was extended in Unicode to include more compli- 5455 cated kinds of composite character by giving each character a grapheme 5456 breaking property, and creating rules that use these properties to 5457 define the boundaries of extended grapheme clusters. In releases of 5458 PCRE later than 8.31, \X matches one of these clusters. 5459 5460 \X always matches at least one character. Then it decides whether to 5461 add additional characters according to the following rules for ending a 5462 cluster: 5463 5464 1. End at the end of the subject string. 5465 5466 2. Do not end between CR and LF; otherwise end after any control char- 5467 acter. 5468 5469 3. Do not break Hangul (a Korean script) syllable sequences. Hangul 5470 characters are of five types: L, V, T, LV, and LVT. An L character may 5471 be followed by an L, V, LV, or LVT character; an LV or V character may 5472 be followed by a V or T character; an LVT or T character may be follwed 5473 only by a T character. 5474 5475 4. Do not end before extending characters or spacing marks. Characters 5476 with the "mark" property always have the "extend" grapheme breaking 5477 property. 5478 5479 5. Do not end after prepend characters. 5480 5481 6. Otherwise, end the cluster. 5482 5483 PCRE's additional properties 5484 5485 As well as the standard Unicode properties described above, PCRE sup- 5486 ports four more that make it possible to convert traditional escape 5487 sequences such as \w and \s to use Unicode properties. PCRE uses these 5488 non-standard, non-Perl properties internally when PCRE_UCP is set. How- 5489 ever, they may also be used explicitly. These properties are: 5490 5491 Xan Any alphanumeric character 5492 Xps Any POSIX space character 5493 Xsp Any Perl space character 5494 Xwd Any Perl "word" character 5495 5496 Xan matches characters that have either the L (letter) or the N (num- 5497 ber) property. Xps matches the characters tab, linefeed, vertical tab, 5498 form feed, or carriage return, and any other character that has the Z 5499 (separator) property. Xsp is the same as Xps; it used to exclude ver- 5500 tical tab, for Perl compatibility, but Perl changed, and so PCRE fol- 5501 lowed at release 8.34. Xwd matches the same characters as Xan, plus 5502 underscore. 5503 5504 There is another non-standard property, Xuc, which matches any charac- 5505 ter that can be represented by a Universal Character Name in C++ and 5506 other programming languages. These are the characters $, @, ` (grave 5507 accent), and all characters with Unicode code points greater than or 5508 equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note that 5509 most base (ASCII) characters are excluded. (Universal Character Names 5510 are of the form \uHHHH or \UHHHHHHHH where H is a hexadecimal digit. 5511 Note that the Xuc property does not match these sequences but the char- 5512 acters that they represent.) 5513 5514 Resetting the match start 5515 5516 The escape sequence \K causes any previously matched characters not to 5517 be included in the final matched sequence. For example, the pattern: 5518 5519 foo\Kbar 5520 5521 matches "foobar", but reports that it has matched "bar". This feature 5522 is similar to a lookbehind assertion (described below). However, in 5523 this case, the part of the subject before the real match does not have 5524 to be of fixed length, as lookbehind assertions do. The use of \K does 5525 not interfere with the setting of captured substrings. For example, 5526 when the pattern 5527 5528 (foo)\Kbar 5529 5530 matches "foobar", the first substring is still set to "foo". 5531 5532 Perl documents that the use of \K within assertions is "not well 5533 defined". In PCRE, \K is acted upon when it occurs inside positive 5534 assertions, but is ignored in negative assertions. Note that when a 5535 pattern such as (?=ab\K) matches, the reported start of the match can 5536 be greater than the end of the match. 5537 5538 Simple assertions 5539 5540 The final use of backslash is for certain simple assertions. An asser- 5541 tion specifies a condition that has to be met at a particular point in 5542 a match, without consuming any characters from the subject string. The 5543 use of subpatterns for more complicated assertions is described below. 5544 The backslashed assertions are: 5545 5546 \b matches at a word boundary 5547 \B matches when not at a word boundary 5548 \A matches at the start of the subject 5549 \Z matches at the end of the subject 5550 also matches before a newline at the end of the subject 5551 \z matches only at the end of the subject 5552 \G matches at the first matching position in the subject 5553 5554 Inside a character class, \b has a different meaning; it matches the 5555 backspace character. If any other of these assertions appears in a 5556 character class, by default it matches the corresponding literal char- 5557 acter (for example, \B matches the letter B). However, if the 5558 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener- 5559 ated instead. 5560 5561 A word boundary is a position in the subject string where the current 5562 character and the previous character do not both match \w or \W (i.e. 5563 one matches \w and the other matches \W), or the start or end of the 5564 string if the first or last character matches \w, respectively. In a 5565 UTF mode, the meanings of \w and \W can be changed by setting the 5566 PCRE_UCP option. When this is done, it also affects \b and \B. Neither 5567 PCRE nor Perl has a separate "start of word" or "end of word" metase- 5568 quence. However, whatever follows \b normally determines which it is. 5569 For example, the fragment \ba matches "a" at the start of a word. 5570 5571 The \A, \Z, and \z assertions differ from the traditional circumflex 5572 and dollar (described in the next section) in that they only ever match 5573 at the very start and end of the subject string, whatever options are 5574 set. Thus, they are independent of multiline mode. These three asser- 5575 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which 5576 affect only the behaviour of the circumflex and dollar metacharacters. 5577 However, if the startoffset argument of pcre_exec() is non-zero, indi- 5578 cating that matching is to start at a point other than the beginning of 5579 the subject, \A can never match. The difference between \Z and \z is 5580 that \Z matches before a newline at the end of the string as well as at 5581 the very end, whereas \z matches only at the end. 5582 5583 The \G assertion is true only when the current matching position is at 5584 the start point of the match, as specified by the startoffset argument 5585 of pcre_exec(). It differs from \A when the value of startoffset is 5586 non-zero. By calling pcre_exec() multiple times with appropriate argu- 5587 ments, you can mimic Perl's /g option, and it is in this kind of imple- 5588 mentation where \G can be useful. 5589 5590 Note, however, that PCRE's interpretation of \G, as the start of the 5591 current match, is subtly different from Perl's, which defines it as the 5592 end of the previous match. In Perl, these can be different when the 5593 previously matched string was empty. Because PCRE does just one match 5594 at a time, it cannot reproduce this behaviour. 5595 5596 If all the alternatives of a pattern begin with \G, the expression is 5597 anchored to the starting match position, and the "anchored" flag is set 5598 in the compiled regular expression. 5599 5600 5601 CIRCUMFLEX AND DOLLAR 5602 5603 The circumflex and dollar metacharacters are zero-width assertions. 5604 That is, they test for a particular condition being true without con- 5605 suming any characters from the subject string. 5606 5607 Outside a character class, in the default matching mode, the circumflex 5608 character is an assertion that is true only if the current matching 5609 point is at the start of the subject string. If the startoffset argu- 5610 ment of pcre_exec() is non-zero, circumflex can never match if the 5611 PCRE_MULTILINE option is unset. Inside a character class, circumflex 5612 has an entirely different meaning (see below). 5613 5614 Circumflex need not be the first character of the pattern if a number 5615 of alternatives are involved, but it should be the first thing in each 5616 alternative in which it appears if the pattern is ever to match that 5617 branch. If all possible alternatives start with a circumflex, that is, 5618 if the pattern is constrained to match only at the start of the sub- 5619 ject, it is said to be an "anchored" pattern. (There are also other 5620 constructs that can cause a pattern to be anchored.) 5621 5622 The dollar character is an assertion that is true only if the current 5623 matching point is at the end of the subject string, or immediately 5624 before a newline at the end of the string (by default). Note, however, 5625 that it does not actually match the newline. Dollar need not be the 5626 last character of the pattern if a number of alternatives are involved, 5627 but it should be the last item in any branch in which it appears. Dol- 5628 lar has no special meaning in a character class. 5629 5630 The meaning of dollar can be changed so that it matches only at the 5631 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at 5632 compile time. This does not affect the \Z assertion. 5633 5634 The meanings of the circumflex and dollar characters are changed if the 5635 PCRE_MULTILINE option is set. When this is the case, a circumflex 5636 matches immediately after internal newlines as well as at the start of 5637 the subject string. It does not match after a newline that ends the 5638 string. A dollar matches before any newlines in the string, as well as 5639 at the very end, when PCRE_MULTILINE is set. When newline is specified 5640 as the two-character sequence CRLF, isolated CR and LF characters do 5641 not indicate newlines. 5642 5643 For example, the pattern /^abc$/ matches the subject string "def\nabc" 5644 (where \n represents a newline) in multiline mode, but not otherwise. 5645 Consequently, patterns that are anchored in single line mode because 5646 all branches start with ^ are not anchored in multiline mode, and a 5647 match for circumflex is possible when the startoffset argument of 5648 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if 5649 PCRE_MULTILINE is set. 5650 5651 Note that the sequences \A, \Z, and \z can be used to match the start 5652 and end of the subject in both modes, and if all branches of a pattern 5653 start with \A it is always anchored, whether or not PCRE_MULTILINE is 5654 set. 5655 5656 5657 FULL STOP (PERIOD, DOT) AND \N 5658 5659 Outside a character class, a dot in the pattern matches any one charac- 5660 ter in the subject string except (by default) a character that signi- 5661 fies the end of a line. 5662 5663 When a line ending is defined as a single character, dot never matches 5664 that character; when the two-character sequence CRLF is used, dot does 5665 not match CR if it is immediately followed by LF, but otherwise it 5666 matches all characters (including isolated CRs and LFs). When any Uni- 5667 code line endings are being recognized, dot does not match CR or LF or 5668 any of the other line ending characters. 5669 5670 The behaviour of dot with regard to newlines can be changed. If the 5671 PCRE_DOTALL option is set, a dot matches any one character, without 5672 exception. If the two-character sequence CRLF is present in the subject 5673 string, it takes two dots to match it. 5674 5675 The handling of dot is entirely independent of the handling of circum- 5676 flex and dollar, the only relationship being that they both involve 5677 newlines. Dot has no special meaning in a character class. 5678 5679 The escape sequence \N behaves like a dot, except that it is not 5680 affected by the PCRE_DOTALL option. In other words, it matches any 5681 character except one that signifies the end of a line. Perl also uses 5682 \N to match characters by name; PCRE does not support this. 5683 5684 5685 MATCHING A SINGLE DATA UNIT 5686 5687 Outside a character class, the escape sequence \C matches any one data 5688 unit, whether or not a UTF mode is set. In the 8-bit library, one data 5689 unit is one byte; in the 16-bit library it is a 16-bit unit; in the 5690 32-bit library it is a 32-bit unit. Unlike a dot, \C always matches 5691 line-ending characters. The feature is provided in Perl in order to 5692 match individual bytes in UTF-8 mode, but it is unclear how it can use- 5693 fully be used. Because \C breaks up characters into individual data 5694 units, matching one unit with \C in a UTF mode means that the rest of 5695 the string may start with a malformed UTF character. This has undefined 5696 results, because PCRE assumes that it is dealing with valid UTF strings 5697 (and by default it checks this at the start of processing unless the 5698 PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or PCRE_NO_UTF32_CHECK option 5699 is used). 5700 5701 PCRE does not allow \C to appear in lookbehind assertions (described 5702 below) in a UTF mode, because this would make it impossible to calcu- 5703 late the length of the lookbehind. 5704 5705 In general, the \C escape sequence is best avoided. However, one way of 5706 using it that avoids the problem of malformed UTF characters is to use 5707 a lookahead to check the length of the next character, as in this pat- 5708 tern, which could be used with a UTF-8 string (ignore white space and 5709 line breaks): 5710 5711 (?| (?=[\x00-\x7f])(\C) | 5712 (?=[\x80-\x{7ff}])(\C)(\C) | 5713 (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) | 5714 (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C)) 5715 5716 A group that starts with (?| resets the capturing parentheses numbers 5717 in each alternative (see "Duplicate Subpattern Numbers" below). The 5718 assertions at the start of each branch check the next UTF-8 character 5719 for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The 5720 character's individual bytes are then captured by the appropriate num- 5721 ber of groups. 5722 5723 5724 SQUARE BRACKETS AND CHARACTER CLASSES 5725 5726 An opening square bracket introduces a character class, terminated by a 5727 closing square bracket. A closing square bracket on its own is not spe- 5728 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, 5729 a lone closing square bracket causes a compile-time error. If a closing 5730 square bracket is required as a member of the class, it should be the 5731 first data character in the class (after an initial circumflex, if 5732 present) or escaped with a backslash. 5733 5734 A character class matches a single character in the subject. In a UTF 5735 mode, the character may be more than one data unit long. A matched 5736 character must be in the set of characters defined by the class, unless 5737 the first character in the class definition is a circumflex, in which 5738 case the subject character must not be in the set defined by the class. 5739 If a circumflex is actually required as a member of the class, ensure 5740 it is not the first character, or escape it with a backslash. 5741 5742 For example, the character class [aeiou] matches any lower case vowel, 5743 while [^aeiou] matches any character that is not a lower case vowel. 5744 Note that a circumflex is just a convenient notation for specifying the 5745 characters that are in the class by enumerating those that are not. A 5746 class that starts with a circumflex is not an assertion; it still con- 5747 sumes a character from the subject string, and therefore it fails if 5748 the current pointer is at the end of the string. 5749 5750 In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 5751 (0xffff) can be included in a class as a literal string of data units, 5752 or by using the \x{ escaping mechanism. 5753 5754 When caseless matching is set, any letters in a class represent both 5755 their upper case and lower case versions, so for example, a caseless 5756 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not 5757 match "A", whereas a caseful version would. In a UTF mode, PCRE always 5758 understands the concept of case for characters whose values are less 5759 than 128, so caseless matching is always possible. For characters with 5760 higher values, the concept of case is supported if PCRE is compiled 5761 with Unicode property support, but not otherwise. If you want to use 5762 caseless matching in a UTF mode for characters 128 and above, you must 5763 ensure that PCRE is compiled with Unicode property support as well as 5764 with UTF support. 5765 5766 Characters that might indicate line breaks are never treated in any 5767 special way when matching character classes, whatever line-ending 5768 sequence is in use, and whatever setting of the PCRE_DOTALL and 5769 PCRE_MULTILINE options is used. A class such as [^a] always matches one 5770 of these characters. 5771 5772 The minus (hyphen) character can be used to specify a range of charac- 5773 ters in a character class. For example, [d-m] matches any letter 5774 between d and m, inclusive. If a minus character is required in a 5775 class, it must be escaped with a backslash or appear in a position 5776 where it cannot be interpreted as indicating a range, typically as the 5777 first or last character in the class, or immediately after a range. For 5778 example, [b-d-z] matches letters in the range b to d, a hyphen charac- 5779 ter, or z. 5780 5781 It is not possible to have the literal character "]" as the end charac- 5782 ter of a range. A pattern such as [W-]46] is interpreted as a class of 5783 two characters ("W" and "-") followed by a literal string "46]", so it 5784 would match "W46]" or "-46]". However, if the "]" is escaped with a 5785 backslash it is interpreted as the end of range, so [W-\]46] is inter- 5786 preted as a class containing a range followed by two other characters. 5787 The octal or hexadecimal representation of "]" can also be used to end 5788 a range. 5789 5790 An error is generated if a POSIX character class (see below) or an 5791 escape sequence other than one that defines a single character appears 5792 at a point where a range ending character is expected. For example, 5793 [z-\xff] is valid, but [A-\d] and [A-[:digit:]] are not. 5794 5795 Ranges operate in the collating sequence of character values. They can 5796 also be used for characters specified numerically, for example 5797 [\000-\037]. Ranges can include any characters that are valid for the 5798 current mode. 5799 5800 If a range that includes letters is used when caseless matching is set, 5801 it matches the letters in either case. For example, [W-c] is equivalent 5802 to [][\\^_`wxyzabc], matched caselessly, and in a non-UTF mode, if 5803 character tables for a French locale are in use, [\xc8-\xcb] matches 5804 accented E characters in both cases. In UTF modes, PCRE supports the 5805 concept of case for characters with values greater than 128 only when 5806 it is compiled with Unicode property support. 5807 5808 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, 5809 \w, and \W may appear in a character class, and add the characters that 5810 they match to the class. For example, [\dABCDEF] matches any hexadeci- 5811 mal digit. In UTF modes, the PCRE_UCP option affects the meanings of 5812 \d, \s, \w and their upper case partners, just as it does when they 5813 appear outside a character class, as described in the section entitled 5814 "Generic character types" above. The escape sequence \b has a different 5815 meaning inside a character class; it matches the backspace character. 5816 The sequences \B, \N, \R, and \X are not special inside a character 5817 class. Like any other unrecognized escape sequences, they are treated 5818 as the literal characters "B", "N", "R", and "X" by default, but cause 5819 an error if the PCRE_EXTRA option is set. 5820 5821 A circumflex can conveniently be used with the upper case character 5822 types to specify a more restricted set of characters than the matching 5823 lower case type. For example, the class [^\W_] matches any letter or 5824 digit, but not underscore, whereas [\w] includes underscore. A positive 5825 character class should be read as "something OR something OR ..." and a 5826 negative class as "NOT something AND NOT something AND NOT ...". 5827 5828 The only metacharacters that are recognized in character classes are 5829 backslash, hyphen (only where it can be interpreted as specifying a 5830 range), circumflex (only at the start), opening square bracket (only 5831 when it can be interpreted as introducing a POSIX class name, or for a 5832 special compatibility feature - see the next two sections), and the 5833 terminating closing square bracket. However, escaping other non- 5834 alphanumeric characters does no harm. 5835 5836 5837 POSIX CHARACTER CLASSES 5838 5839 Perl supports the POSIX notation for character classes. This uses names 5840 enclosed by [: and :] within the enclosing square brackets. PCRE also 5841 supports this notation. For example, 5842 5843 [01[:alpha:]%] 5844 5845 matches "0", "1", any alphabetic character, or "%". The supported class 5846 names are: 5847 5848 alnum letters and digits 5849 alpha letters 5850 ascii character codes 0 - 127 5851 blank space or tab only 5852 cntrl control characters 5853 digit decimal digits (same as \d) 5854 graph printing characters, excluding space 5855 lower lower case letters 5856 print printing characters, including space 5857 punct printing characters, excluding letters and digits and space 5858 space white space (the same as \s from PCRE 8.34) 5859 upper upper case letters 5860 word "word" characters (same as \w) 5861 xdigit hexadecimal digits 5862 5863 The default "space" characters are HT (9), LF (10), VT (11), FF (12), 5864 CR (13), and space (32). If locale-specific matching is taking place, 5865 the list of space characters may be different; there may be fewer or 5866 more of them. "Space" used to be different to \s, which did not include 5867 VT, for Perl compatibility. However, Perl changed at release 5.18, and 5868 PCRE followed at release 8.34. "Space" and \s now match the same set 5869 of characters. 5870 5871 The name "word" is a Perl extension, and "blank" is a GNU extension 5872 from Perl 5.8. Another Perl extension is negation, which is indicated 5873 by a ^ character after the colon. For example, 5874 5875 [12[:^digit:]] 5876 5877 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the 5878 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but 5879 these are not supported, and an error is given if they are encountered. 5880 5881 By default, characters with values greater than 128 do not match any of 5882 the POSIX character classes. However, if the PCRE_UCP option is passed 5883 to pcre_compile(), some of the classes are changed so that Unicode 5884 character properties are used. This is achieved by replacing certain 5885 POSIX classes by other sequences, as follows: 5886 5887 [:alnum:] becomes \p{Xan} 5888 [:alpha:] becomes \p{L} 5889 [:blank:] becomes \h 5890 [:digit:] becomes \p{Nd} 5891 [:lower:] becomes \p{Ll} 5892 [:space:] becomes \p{Xps} 5893 [:upper:] becomes \p{Lu} 5894 [:word:] becomes \p{Xwd} 5895 5896 Negated versions, such as [:^alpha:] use \P instead of \p. Three other 5897 POSIX classes are handled specially in UCP mode: 5898 5899 [:graph:] This matches characters that have glyphs that mark the page 5900 when printed. In Unicode property terms, it matches all char- 5901 acters with the L, M, N, P, S, or Cf properties, except for: 5902 5903 U+061C Arabic Letter Mark 5904 U+180E Mongolian Vowel Separator 5905 U+2066 - U+2069 Various "isolate"s 5906 5907 5908 [:print:] This matches the same characters as [:graph:] plus space 5909 characters that are not controls, that is, characters with 5910 the Zs property. 5911 5912 [:punct:] This matches all characters that have the Unicode P (punctua- 5913 tion) property, plus those characters whose code points are 5914 less than 128 that have the S (Symbol) property. 5915 5916 The other POSIX classes are unchanged, and match only characters with 5917 code points less than 128. 5918 5919 5920 COMPATIBILITY FEATURE FOR WORD BOUNDARIES 5921 5922 In the POSIX.2 compliant library that was included in 4.4BSD Unix, the 5923 ugly syntax [[:<:]] and [[:>:]] is used for matching "start of word" 5924 and "end of word". PCRE treats these items as follows: 5925 5926 [[:<:]] is converted to \b(?=\w) 5927 [[:>:]] is converted to \b(?<=\w) 5928 5929 Only these exact character sequences are recognized. A sequence such as 5930 [a[:<:]b] provokes error for an unrecognized POSIX class name. This 5931 support is not compatible with Perl. It is provided to help migrations 5932 from other environments, and is best not used in any new patterns. Note 5933 that \b matches at the start and the end of a word (see "Simple asser- 5934 tions" above), and in a Perl-style pattern the preceding or following 5935 character normally shows which is wanted, without the need for the 5936 assertions that are used above in order to give exactly the POSIX be- 5937 haviour. 5938 5939 5940 VERTICAL BAR 5941 5942 Vertical bar characters are used to separate alternative patterns. For 5943 example, the pattern 5944 5945 gilbert|sullivan 5946 5947 matches either "gilbert" or "sullivan". Any number of alternatives may 5948 appear, and an empty alternative is permitted (matching the empty 5949 string). The matching process tries each alternative in turn, from left 5950 to right, and the first one that succeeds is used. If the alternatives 5951 are within a subpattern (defined below), "succeeds" means matching the 5952 rest of the main pattern as well as the alternative in the subpattern. 5953 5954 5955 INTERNAL OPTION SETTING 5956 5957 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 5958 PCRE_EXTENDED options (which are Perl-compatible) can be changed from 5959 within the pattern by a sequence of Perl option letters enclosed 5960 between "(?" and ")". The option letters are 5961 5962 i for PCRE_CASELESS 5963 m for PCRE_MULTILINE 5964 s for PCRE_DOTALL 5965 x for PCRE_EXTENDED 5966 5967 For example, (?im) sets caseless, multiline matching. It is also possi- 5968 ble to unset these options by preceding the letter with a hyphen, and a 5969 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- 5970 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, 5971 is also permitted. If a letter appears both before and after the 5972 hyphen, the option is unset. 5973 5974 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA 5975 can be changed in the same way as the Perl-compatible options by using 5976 the characters J, U and X respectively. 5977 5978 When one of these option changes occurs at top level (that is, not 5979 inside subpattern parentheses), the change applies to the remainder of 5980 the pattern that follows. If the change is placed right at the start of 5981 a pattern, PCRE extracts it into the global options (and it will there- 5982 fore show up in data extracted by the pcre_fullinfo() function). 5983 5984 An option change within a subpattern (see below for a description of 5985 subpatterns) affects only that part of the subpattern that follows it, 5986 so 5987 5988 (a(?i)b)c 5989 5990 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not 5991 used). By this means, options can be made to have different settings 5992 in different parts of the pattern. Any changes made in one alternative 5993 do carry on into subsequent branches within the same subpattern. For 5994 example, 5995 5996 (a(?i)b|c) 5997 5998 matches "ab", "aB", "c", and "C", even though when matching "C" the 5999 first branch is abandoned before the option setting. This is because 6000 the effects of option settings happen at compile time. There would be 6001 some very weird behaviour otherwise. 6002 6003 Note: There are other PCRE-specific options that can be set by the 6004 application when the compiling or matching functions are called. In 6005 some cases the pattern can contain special leading sequences such as 6006 (*CRLF) to override what the application has set or what has been 6007 defaulted. Details are given in the section entitled "Newline 6008 sequences" above. There are also the (*UTF8), (*UTF16),(*UTF32), and 6009 (*UCP) leading sequences that can be used to set UTF and Unicode prop- 6010 erty modes; they are equivalent to setting the PCRE_UTF8, PCRE_UTF16, 6011 PCRE_UTF32 and the PCRE_UCP options, respectively. The (*UTF) sequence 6012 is a generic version that can be used with any of the libraries. How- 6013 ever, the application can set the PCRE_NEVER_UTF option, which locks 6014 out the use of the (*UTF) sequences. 6015 6016 6017 SUBPATTERNS 6018 6019 Subpatterns are delimited by parentheses (round brackets), which can be 6020 nested. Turning part of a pattern into a subpattern does two things: 6021 6022 1. It localizes a set of alternatives. For example, the pattern 6023 6024 cat(aract|erpillar|) 6025 6026 matches "cataract", "caterpillar", or "cat". Without the parentheses, 6027 it would match "cataract", "erpillar" or an empty string. 6028 6029 2. It sets up the subpattern as a capturing subpattern. This means 6030 that, when the whole pattern matches, that portion of the subject 6031 string that matched the subpattern is passed back to the caller via the 6032 ovector argument of the matching function. (This applies only to the 6033 traditional matching functions; the DFA matching functions do not sup- 6034 port capturing.) 6035 6036 Opening parentheses are counted from left to right (starting from 1) to 6037 obtain numbers for the capturing subpatterns. For example, if the 6038 string "the red king" is matched against the pattern 6039 6040 the ((red|white) (king|queen)) 6041 6042 the captured substrings are "red king", "red", and "king", and are num- 6043 bered 1, 2, and 3, respectively. 6044 6045 The fact that plain parentheses fulfil two functions is not always 6046 helpful. There are often times when a grouping subpattern is required 6047 without a capturing requirement. If an opening parenthesis is followed 6048 by a question mark and a colon, the subpattern does not do any captur- 6049 ing, and is not counted when computing the number of any subsequent 6050 capturing subpatterns. For example, if the string "the white queen" is 6051 matched against the pattern 6052 6053 the ((?:red|white) (king|queen)) 6054 6055 the captured substrings are "white queen" and "queen", and are numbered 6056 1 and 2. The maximum number of capturing subpatterns is 65535. 6057 6058 As a convenient shorthand, if any option settings are required at the 6059 start of a non-capturing subpattern, the option letters may appear 6060 between the "?" and the ":". Thus the two patterns 6061 6062 (?i:saturday|sunday) 6063 (?:(?i)saturday|sunday) 6064 6065 match exactly the same set of strings. Because alternative branches are 6066 tried from left to right, and options are not reset until the end of 6067 the subpattern is reached, an option setting in one branch does affect 6068 subsequent branches, so the above patterns match "SUNDAY" as well as 6069 "Saturday". 6070 6071 6072 DUPLICATE SUBPATTERN NUMBERS 6073 6074 Perl 5.10 introduced a feature whereby each alternative in a subpattern 6075 uses the same numbers for its capturing parentheses. Such a subpattern 6076 starts with (?| and is itself a non-capturing subpattern. For example, 6077 consider this pattern: 6078 6079 (?|(Sat)ur|(Sun))day 6080 6081 Because the two alternatives are inside a (?| group, both sets of cap- 6082 turing parentheses are numbered one. Thus, when the pattern matches, 6083 you can look at captured substring number one, whichever alternative 6084 matched. This construct is useful when you want to capture part, but 6085 not all, of one of a number of alternatives. Inside a (?| group, paren- 6086 theses are numbered as usual, but the number is reset at the start of 6087 each branch. The numbers of any capturing parentheses that follow the 6088 subpattern start after the highest number used in any branch. The fol- 6089 lowing example is taken from the Perl documentation. The numbers under- 6090 neath show in which buffer the captured content will be stored. 6091 6092 # before ---------------branch-reset----------- after 6093 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 6094 # 1 2 2 3 2 3 4 6095 6096 A back reference to a numbered subpattern uses the most recent value 6097 that is set for that number by any subpattern. The following pattern 6098 matches "abcabc" or "defdef": 6099 6100 /(?|(abc)|(def))\1/ 6101 6102 In contrast, a subroutine call to a numbered subpattern always refers 6103 to the first one in the pattern with the given number. The following 6104 pattern matches "abcabc" or "defabc": 6105 6106 /(?|(abc)|(def))(?1)/ 6107 6108 If a condition test for a subpattern's having matched refers to a non- 6109 unique number, the test is true if any of the subpatterns of that num- 6110 ber have matched. 6111 6112 An alternative approach to using this "branch reset" feature is to use 6113 duplicate named subpatterns, as described in the next section. 6114 6115 6116 NAMED SUBPATTERNS 6117 6118 Identifying capturing parentheses by number is simple, but it can be 6119 very hard to keep track of the numbers in complicated regular expres- 6120 sions. Furthermore, if an expression is modified, the numbers may 6121 change. To help with this difficulty, PCRE supports the naming of sub- 6122 patterns. This feature was not added to Perl until release 5.10. Python 6123 had the feature earlier, and PCRE introduced it at release 4.0, using 6124 the Python syntax. PCRE now supports both the Perl and the Python syn- 6125 tax. Perl allows identically numbered subpatterns to have different 6126 names, but PCRE does not. 6127 6128 In PCRE, a subpattern can be named in one of three ways: (?<name>...) 6129 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References 6130 to capturing parentheses from other parts of the pattern, such as back 6131 references, recursion, and conditions, can be made by name as well as 6132 by number. 6133 6134 Names consist of up to 32 alphanumeric characters and underscores, but 6135 must start with a non-digit. Named capturing parentheses are still 6136 allocated numbers as well as names, exactly as if the names were not 6137 present. The PCRE API provides function calls for extracting the name- 6138 to-number translation table from a compiled pattern. There is also a 6139 convenience function for extracting a captured substring by name. 6140 6141 By default, a name must be unique within a pattern, but it is possible 6142 to relax this constraint by setting the PCRE_DUPNAMES option at compile 6143 time. (Duplicate names are also always permitted for subpatterns with 6144 the same number, set up as described in the previous section.) Dupli- 6145 cate names can be useful for patterns where only one instance of the 6146 named parentheses can match. Suppose you want to match the name of a 6147 weekday, either as a 3-letter abbreviation or as the full name, and in 6148 both cases you want to extract the abbreviation. This pattern (ignoring 6149 the line breaks) does the job: 6150 6151 (?<DN>Mon|Fri|Sun)(?:day)?| 6152 (?<DN>Tue)(?:sday)?| 6153 (?<DN>Wed)(?:nesday)?| 6154 (?<DN>Thu)(?:rsday)?| 6155 (?<DN>Sat)(?:urday)? 6156 6157 There are five capturing substrings, but only one is ever set after a 6158 match. (An alternative way of solving this problem is to use a "branch 6159 reset" subpattern, as described in the previous section.) 6160 6161 The convenience function for extracting the data by name returns the 6162 substring for the first (and in this example, the only) subpattern of 6163 that name that matched. This saves searching to find which numbered 6164 subpattern it was. 6165 6166 If you make a back reference to a non-unique named subpattern from 6167 elsewhere in the pattern, the subpatterns to which the name refers are 6168 checked in the order in which they appear in the overall pattern. The 6169 first one that is set is used for the reference. For example, this pat- 6170 tern matches both "foofoo" and "barbar" but not "foobar" or "barfoo": 6171 6172 (?:(?<n>foo)|(?<n>bar))\k<n> 6173 6174 6175 If you make a subroutine call to a non-unique named subpattern, the one 6176 that corresponds to the first occurrence of the name is used. In the 6177 absence of duplicate numbers (see the previous section) this is the one 6178 with the lowest number. 6179 6180 If you use a named reference in a condition test (see the section about 6181 conditions below), either to check whether a subpattern has matched, or 6182 to check for recursion, all subpatterns with the same name are tested. 6183 If the condition is true for any one of them, the overall condition is 6184 true. This is the same behaviour as testing by number. For further 6185 details of the interfaces for handling named subpatterns, see the 6186 pcreapi documentation. 6187 6188 Warning: You cannot use different names to distinguish between two sub- 6189 patterns with the same number because PCRE uses only the numbers when 6190 matching. For this reason, an error is given at compile time if differ- 6191 ent names are given to subpatterns with the same number. However, you 6192 can always give the same name to subpatterns with the same number, even 6193 when PCRE_DUPNAMES is not set. 6194 6195 6196 REPETITION 6197 6198 Repetition is specified by quantifiers, which can follow any of the 6199 following items: 6200 6201 a literal data character 6202 the dot metacharacter 6203 the \C escape sequence 6204 the \X escape sequence 6205 the \R escape sequence 6206 an escape such as \d or \pL that matches a single character 6207 a character class 6208 a back reference (see next section) 6209 a parenthesized subpattern (including assertions) 6210 a subroutine call to a subpattern (recursive or otherwise) 6211 6212 The general repetition quantifier specifies a minimum and maximum num- 6213 ber of permitted matches, by giving the two numbers in curly brackets 6214 (braces), separated by a comma. The numbers must be less than 65536, 6215 and the first must be less than or equal to the second. For example: 6216 6217 z{2,4} 6218 6219 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a 6220 special character. If the second number is omitted, but the comma is 6221 present, there is no upper limit; if the second number and the comma 6222 are both omitted, the quantifier specifies an exact number of required 6223 matches. Thus 6224 6225 [aeiou]{3,} 6226 6227 matches at least 3 successive vowels, but may match many more, while 6228 6229 \d{8} 6230 6231 matches exactly 8 digits. An opening curly bracket that appears in a 6232 position where a quantifier is not allowed, or one that does not match 6233 the syntax of a quantifier, is taken as a literal character. For exam- 6234 ple, {,6} is not a quantifier, but a literal string of four characters. 6235 6236 In UTF modes, quantifiers apply to characters rather than to individual 6237 data units. Thus, for example, \x{100}{2} matches two characters, each 6238 of which is represented by a two-byte sequence in a UTF-8 string. Simi- 6239 larly, \X{3} matches three Unicode extended grapheme clusters, each of 6240 which may be several data units long (and they may be of different 6241 lengths). 6242 6243 The quantifier {0} is permitted, causing the expression to behave as if 6244 the previous item and the quantifier were not present. This may be use- 6245 ful for subpatterns that are referenced as subroutines from elsewhere 6246 in the pattern (but see also the section entitled "Defining subpatterns 6247 for use by reference only" below). Items other than subpatterns that 6248 have a {0} quantifier are omitted from the compiled pattern. 6249 6250 For convenience, the three most common quantifiers have single-charac- 6251 ter abbreviations: 6252 6253 * is equivalent to {0,} 6254 + is equivalent to {1,} 6255 ? is equivalent to {0,1} 6256 6257 It is possible to construct infinite loops by following a subpattern 6258 that can match no characters with a quantifier that has no upper limit, 6259 for example: 6260 6261 (a?)* 6262 6263 Earlier versions of Perl and PCRE used to give an error at compile time 6264 for such patterns. However, because there are cases where this can be 6265 useful, such patterns are now accepted, but if any repetition of the 6266 subpattern does in fact match no characters, the loop is forcibly bro- 6267 ken. 6268 6269 By default, the quantifiers are "greedy", that is, they match as much 6270 as possible (up to the maximum number of permitted times), without 6271 causing the rest of the pattern to fail. The classic example of where 6272 this gives problems is in trying to match comments in C programs. These 6273 appear between /* and */ and within the comment, individual * and / 6274 characters may appear. An attempt to match C comments by applying the 6275 pattern 6276 6277 /\*.*\*/ 6278 6279 to the string 6280 6281 /* first comment */ not comment /* second comment */ 6282 6283 fails, because it matches the entire string owing to the greediness of 6284 the .* item. 6285 6286 However, if a quantifier is followed by a question mark, it ceases to 6287 be greedy, and instead matches the minimum number of times possible, so 6288 the pattern 6289 6290 /\*.*?\*/ 6291 6292 does the right thing with the C comments. The meaning of the various 6293 quantifiers is not otherwise changed, just the preferred number of 6294 matches. Do not confuse this use of question mark with its use as a 6295 quantifier in its own right. Because it has two uses, it can sometimes 6296 appear doubled, as in 6297 6298 \d??\d 6299 6300 which matches one digit by preference, but can match two if that is the 6301 only way the rest of the pattern matches. 6302 6303 If the PCRE_UNGREEDY option is set (an option that is not available in 6304 Perl), the quantifiers are not greedy by default, but individual ones 6305 can be made greedy by following them with a question mark. In other 6306 words, it inverts the default behaviour. 6307 6308 When a parenthesized subpattern is quantified with a minimum repeat 6309 count that is greater than 1 or with a limited maximum, more memory is 6310 required for the compiled pattern, in proportion to the size of the 6311 minimum or maximum. 6312 6313 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- 6314 alent to Perl's /s) is set, thus allowing the dot to match newlines, 6315 the pattern is implicitly anchored, because whatever follows will be 6316 tried against every character position in the subject string, so there 6317 is no point in retrying the overall match at any position after the 6318 first. PCRE normally treats such a pattern as though it were preceded 6319 by \A. 6320 6321 In cases where it is known that the subject string contains no new- 6322 lines, it is worth setting PCRE_DOTALL in order to obtain this opti- 6323 mization, or alternatively using ^ to indicate anchoring explicitly. 6324 6325 However, there are some cases where the optimization cannot be used. 6326 When .* is inside capturing parentheses that are the subject of a back 6327 reference elsewhere in the pattern, a match at the start may fail where 6328 a later one succeeds. Consider, for example: 6329 6330 (.*)abc\1 6331 6332 If the subject is "xyz123abc123" the match point is the fourth charac- 6333 ter. For this reason, such a pattern is not implicitly anchored. 6334 6335 Another case where implicit anchoring is not applied is when the lead- 6336 ing .* is inside an atomic group. Once again, a match at the start may 6337 fail where a later one succeeds. Consider this pattern: 6338 6339 (?>.*?a)b 6340 6341 It matches "ab" in the subject "aab". The use of the backtracking con- 6342 trol verbs (*PRUNE) and (*SKIP) also disable this optimization. 6343 6344 When a capturing subpattern is repeated, the value captured is the sub- 6345 string that matched the final iteration. For example, after 6346 6347 (tweedle[dume]{3}\s*)+ 6348 6349 has matched "tweedledum tweedledee" the value of the captured substring 6350 is "tweedledee". However, if there are nested capturing subpatterns, 6351 the corresponding captured values may have been set in previous itera- 6352 tions. For example, after 6353 6354 /(a|(b))+/ 6355 6356 matches "aba" the value of the second captured substring is "b". 6357 6358 6359 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS 6360 6361 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 6362 repetition, failure of what follows normally causes the repeated item 6363 to be re-evaluated to see if a different number of repeats allows the 6364 rest of the pattern to match. Sometimes it is useful to prevent this, 6365 either to change the nature of the match, or to cause it fail earlier 6366 than it otherwise might, when the author of the pattern knows there is 6367 no point in carrying on. 6368 6369 Consider, for example, the pattern \d+foo when applied to the subject 6370 line 6371 6372 123456bar 6373 6374 After matching all 6 digits and then failing to match "foo", the normal 6375 action of the matcher is to try again with only 5 digits matching the 6376 \d+ item, and then with 4, and so on, before ultimately failing. 6377 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides 6378 the means for specifying that once a subpattern has matched, it is not 6379 to be re-evaluated in this way. 6380 6381 If we use atomic grouping for the previous example, the matcher gives 6382 up immediately on failing to match "foo" the first time. The notation 6383 is a kind of special parenthesis, starting with (?> as in this example: 6384 6385 (?>\d+)foo 6386 6387 This kind of parenthesis "locks up" the part of the pattern it con- 6388 tains once it has matched, and a failure further into the pattern is 6389 prevented from backtracking into it. Backtracking past it to previous 6390 items, however, works as normal. 6391 6392 An alternative description is that a subpattern of this type matches 6393 the string of characters that an identical standalone pattern would 6394 match, if anchored at the current point in the subject string. 6395 6396 Atomic grouping subpatterns are not capturing subpatterns. Simple cases 6397 such as the above example can be thought of as a maximizing repeat that 6398 must swallow everything it can. So, while both \d+ and \d+? are pre- 6399 pared to adjust the number of digits they match in order to make the 6400 rest of the pattern match, (?>\d+) can only match an entire sequence of 6401 digits. 6402 6403 Atomic groups in general can of course contain arbitrarily complicated 6404 subpatterns, and can be nested. However, when the subpattern for an 6405 atomic group is just a single repeated item, as in the example above, a 6406 simpler notation, called a "possessive quantifier" can be used. This 6407 consists of an additional + character following a quantifier. Using 6408 this notation, the previous example can be rewritten as 6409 6410 \d++foo 6411 6412 Note that a possessive quantifier can be used with an entire group, for 6413 example: 6414 6415 (abc|xyz){2,3}+ 6416 6417 Possessive quantifiers are always greedy; the setting of the 6418 PCRE_UNGREEDY option is ignored. They are a convenient notation for the 6419 simpler forms of atomic group. However, there is no difference in the 6420 meaning of a possessive quantifier and the equivalent atomic group, 6421 though there may be a performance difference; possessive quantifiers 6422 should be slightly faster. 6423 6424 The possessive quantifier syntax is an extension to the Perl 5.8 syn- 6425 tax. Jeffrey Friedl originated the idea (and the name) in the first 6426 edition of his book. Mike McCloskey liked it, so implemented it when he 6427 built Sun's Java package, and PCRE copied it from there. It ultimately 6428 found its way into Perl at release 5.10. 6429 6430 PCRE has an optimization that automatically "possessifies" certain sim- 6431 ple pattern constructs. For example, the sequence A+B is treated as 6432 A++B because there is no point in backtracking into a sequence of A's 6433 when B must follow. 6434 6435 When a pattern contains an unlimited repeat inside a subpattern that 6436 can itself be repeated an unlimited number of times, the use of an 6437 atomic group is the only way to avoid some failing matches taking a 6438 very long time indeed. The pattern 6439 6440 (\D+|<\d+>)*[!?] 6441 6442 matches an unlimited number of substrings that either consist of non- 6443 digits, or digits enclosed in <>, followed by either ! or ?. When it 6444 matches, it runs quickly. However, if it is applied to 6445 6446 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 6447 6448 it takes a long time before reporting failure. This is because the 6449 string can be divided between the internal \D+ repeat and the external 6450 * repeat in a large number of ways, and all have to be tried. (The 6451 example uses [!?] rather than a single character at the end, because 6452 both PCRE and Perl have an optimization that allows for fast failure 6453 when a single character is used. They remember the last single charac- 6454 ter that is required for a match, and fail early if it is not present 6455 in the string.) If the pattern is changed so that it uses an atomic 6456 group, like this: 6457 6458 ((?>\D+)|<\d+>)*[!?] 6459 6460 sequences of non-digits cannot be broken, and failure happens quickly. 6461 6462 6463 BACK REFERENCES 6464 6465 Outside a character class, a backslash followed by a digit greater than 6466 0 (and possibly further digits) is a back reference to a capturing sub- 6467 pattern earlier (that is, to its left) in the pattern, provided there 6468 have been that many previous capturing left parentheses. 6469 6470 However, if the decimal number following the backslash is less than 10, 6471 it is always taken as a back reference, and causes an error only if 6472 there are not that many capturing left parentheses in the entire pat- 6473 tern. In other words, the parentheses that are referenced need not be 6474 to the left of the reference for numbers less than 10. A "forward back 6475 reference" of this type can make sense when a repetition is involved 6476 and the subpattern to the right has participated in an earlier itera- 6477 tion. 6478 6479 It is not possible to have a numerical "forward back reference" to a 6480 subpattern whose number is 10 or more using this syntax because a 6481 sequence such as \50 is interpreted as a character defined in octal. 6482 See the subsection entitled "Non-printing characters" above for further 6483 details of the handling of digits following a backslash. There is no 6484 such problem when named parentheses are used. A back reference to any 6485 subpattern is possible using named parentheses (see below). 6486 6487 Another way of avoiding the ambiguity inherent in the use of digits 6488 following a backslash is to use the \g escape sequence. This escape 6489 must be followed by an unsigned number or a negative number, optionally 6490 enclosed in braces. These examples are all identical: 6491 6492 (ring), \1 6493 (ring), \g1 6494 (ring), \g{1} 6495 6496 An unsigned number specifies an absolute reference without the ambigu- 6497 ity that is present in the older syntax. It is also useful when literal 6498 digits follow the reference. A negative number is a relative reference. 6499 Consider this example: 6500 6501 (abc(def)ghi)\g{-1} 6502 6503 The sequence \g{-1} is a reference to the most recently started captur- 6504 ing subpattern before \g, that is, is it equivalent to \2 in this exam- 6505 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative 6506 references can be helpful in long patterns, and also in patterns that 6507 are created by joining together fragments that contain references 6508 within themselves. 6509 6510 A back reference matches whatever actually matched the capturing sub- 6511 pattern in the current subject string, rather than anything matching 6512 the subpattern itself (see "Subpatterns as subroutines" below for a way 6513 of doing that). So the pattern 6514 6515 (sens|respons)e and \1ibility 6516 6517 matches "sense and sensibility" and "response and responsibility", but 6518 not "sense and responsibility". If caseful matching is in force at the 6519 time of the back reference, the case of letters is relevant. For exam- 6520 ple, 6521 6522 ((?i)rah)\s+\1 6523 6524 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the 6525 original capturing subpattern is matched caselessly. 6526 6527 There are several different ways of writing back references to named 6528 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or 6529 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's 6530 unified back reference syntax, in which \g can be used for both numeric 6531 and named references, is also supported. We could rewrite the above 6532 example in any of the following ways: 6533 6534 (?<p1>(?i)rah)\s+\k<p1> 6535 (?'p1'(?i)rah)\s+\k{p1} 6536 (?P<p1>(?i)rah)\s+(?P=p1) 6537 (?<p1>(?i)rah)\s+\g{p1} 6538 6539 A subpattern that is referenced by name may appear in the pattern 6540 before or after the reference. 6541 6542 There may be more than one back reference to the same subpattern. If a 6543 subpattern has not actually been used in a particular match, any back 6544 references to it always fail by default. For example, the pattern 6545 6546 (a|(bc))\2 6547 6548 always fails if it starts to match "a" rather than "bc". However, if 6549 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer- 6550 ence to an unset value matches an empty string. 6551 6552 Because there may be many capturing parentheses in a pattern, all dig- 6553 its following a backslash are taken as part of a potential back refer- 6554 ence number. If the pattern continues with a digit character, some 6555 delimiter must be used to terminate the back reference. If the 6556 PCRE_EXTENDED option is set, this can be white space. Otherwise, the 6557 \g{ syntax or an empty comment (see "Comments" below) can be used. 6558 6559 Recursive back references 6560 6561 A back reference that occurs inside the parentheses to which it refers 6562 fails when the subpattern is first used, so, for example, (a\1) never 6563 matches. However, such references can be useful inside repeated sub- 6564 patterns. For example, the pattern 6565 6566 (a|b\1)+ 6567 6568 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- 6569 ation of the subpattern, the back reference matches the character 6570 string corresponding to the previous iteration. In order for this to 6571 work, the pattern must be such that the first iteration does not need 6572 to match the back reference. This can be done using alternation, as in 6573 the example above, or by a quantifier with a minimum of zero. 6574 6575 Back references of this type cause the group that they reference to be 6576 treated as an atomic group. Once the whole group has been matched, a 6577 subsequent matching failure cannot cause backtracking into the middle 6578 of the group. 6579 6580 6581 ASSERTIONS 6582 6583 An assertion is a test on the characters following or preceding the 6584 current matching point that does not actually consume any characters. 6585 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are 6586 described above. 6587 6588 More complicated assertions are coded as subpatterns. There are two 6589 kinds: those that look ahead of the current position in the subject 6590 string, and those that look behind it. An assertion subpattern is 6591 matched in the normal way, except that it does not cause the current 6592 matching position to be changed. 6593 6594 Assertion subpatterns are not capturing subpatterns. If such an asser- 6595 tion contains capturing subpatterns within it, these are counted for 6596 the purposes of numbering the capturing subpatterns in the whole pat- 6597 tern. However, substring capturing is carried out only for positive 6598 assertions. (Perl sometimes, but not always, does do capturing in nega- 6599 tive assertions.) 6600 6601 For compatibility with Perl, assertion subpatterns may be repeated; 6602 though it makes no sense to assert the same thing several times, the 6603 side effect of capturing parentheses may occasionally be useful. In 6604 practice, there only three cases: 6605 6606 (1) If the quantifier is {0}, the assertion is never obeyed during 6607 matching. However, it may contain internal capturing parenthesized 6608 groups that are called from elsewhere via the subroutine mechanism. 6609 6610 (2) If quantifier is {0,n} where n is greater than zero, it is treated 6611 as if it were {0,1}. At run time, the rest of the pattern match is 6612 tried with and without the assertion, the order depending on the greed- 6613 iness of the quantifier. 6614 6615 (3) If the minimum repetition is greater than zero, the quantifier is 6616 ignored. The assertion is obeyed just once when encountered during 6617 matching. 6618 6619 Lookahead assertions 6620 6621 Lookahead assertions start with (?= for positive assertions and (?! for 6622 negative assertions. For example, 6623 6624 \w+(?=;) 6625 6626 matches a word followed by a semicolon, but does not include the semi- 6627 colon in the match, and 6628 6629 foo(?!bar) 6630 6631 matches any occurrence of "foo" that is not followed by "bar". Note 6632 that the apparently similar pattern 6633 6634 (?!foo)bar 6635 6636 does not find an occurrence of "bar" that is preceded by something 6637 other than "foo"; it finds any occurrence of "bar" whatsoever, because 6638 the assertion (?!foo) is always true when the next three characters are 6639 "bar". A lookbehind assertion is needed to achieve the other effect. 6640 6641 If you want to force a matching failure at some point in a pattern, the 6642 most convenient way to do it is with (?!) because an empty string 6643 always matches, so an assertion that requires there not to be an empty 6644 string must always fail. The backtracking control verb (*FAIL) or (*F) 6645 is a synonym for (?!). 6646 6647 Lookbehind assertions 6648 6649 Lookbehind assertions start with (?<= for positive assertions and (?<! 6650 for negative assertions. For example, 6651 6652 (?<!foo)bar 6653 6654 does find an occurrence of "bar" that is not preceded by "foo". The 6655 contents of a lookbehind assertion are restricted such that all the 6656 strings it matches must have a fixed length. However, if there are sev- 6657 eral top-level alternatives, they do not all have to have the same 6658 fixed length. Thus 6659 6660 (?<=bullock|donkey) 6661 6662 is permitted, but 6663 6664 (?<!dogs?|cats?) 6665 6666 causes an error at compile time. Branches that match different length 6667 strings are permitted only at the top level of a lookbehind assertion. 6668 This is an extension compared with Perl, which requires all branches to 6669 match the same length of string. An assertion such as 6670 6671 (?<=ab(c|de)) 6672 6673 is not permitted, because its single top-level branch can match two 6674 different lengths, but it is acceptable to PCRE if rewritten to use two 6675 top-level branches: 6676 6677 (?<=abc|abde) 6678 6679 In some cases, the escape sequence \K (see above) can be used instead 6680 of a lookbehind assertion to get round the fixed-length restriction. 6681 6682 The implementation of lookbehind assertions is, for each alternative, 6683 to temporarily move the current position back by the fixed length and 6684 then try to match. If there are insufficient characters before the cur- 6685 rent position, the assertion fails. 6686 6687 In a UTF mode, PCRE does not allow the \C escape (which matches a sin- 6688 gle data unit even in a UTF mode) to appear in lookbehind assertions, 6689 because it makes it impossible to calculate the length of the lookbe- 6690 hind. The \X and \R escapes, which can match different numbers of data 6691 units, are also not permitted. 6692 6693 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in 6694 lookbehinds, as long as the subpattern matches a fixed-length string. 6695 Recursion, however, is not supported. 6696 6697 Possessive quantifiers can be used in conjunction with lookbehind 6698 assertions to specify efficient matching of fixed-length strings at the 6699 end of subject strings. Consider a simple pattern such as 6700 6701 abcd$ 6702 6703 when applied to a long string that does not match. Because matching 6704 proceeds from left to right, PCRE will look for each "a" in the subject 6705 and then see if what follows matches the rest of the pattern. If the 6706 pattern is specified as 6707 6708 ^.*abcd$ 6709 6710 the initial .* matches the entire string at first, but when this fails 6711 (because there is no following "a"), it backtracks to match all but the 6712 last character, then all but the last two characters, and so on. Once 6713 again the search for "a" covers the entire string, from right to left, 6714 so we are no better off. However, if the pattern is written as 6715 6716 ^.*+(?<=abcd) 6717 6718 there can be no backtracking for the .*+ item; it can match only the 6719 entire string. The subsequent lookbehind assertion does a single test 6720 on the last four characters. If it fails, the match fails immediately. 6721 For long strings, this approach makes a significant difference to the 6722 processing time. 6723 6724 Using multiple assertions 6725 6726 Several assertions (of any sort) may occur in succession. For example, 6727 6728 (?<=\d{3})(?<!999)foo 6729 6730 matches "foo" preceded by three digits that are not "999". Notice that 6731 each of the assertions is applied independently at the same point in 6732 the subject string. First there is a check that the previous three 6733 characters are all digits, and then there is a check that the same 6734 three characters are not "999". This pattern does not match "foo" pre- 6735 ceded by six characters, the first of which are digits and the last 6736 three of which are not "999". For example, it doesn't match "123abc- 6737 foo". A pattern to do that is 6738 6739 (?<=\d{3}...)(?<!999)foo 6740 6741 This time the first assertion looks at the preceding six characters, 6742 checking that the first three are digits, and then the second assertion 6743 checks that the preceding three characters are not "999". 6744 6745 Assertions can be nested in any combination. For example, 6746 6747 (?<=(?<!foo)bar)baz 6748 6749 matches an occurrence of "baz" that is preceded by "bar" which in turn 6750 is not preceded by "foo", while 6751 6752 (?<=\d{3}(?!999)...)foo 6753 6754 is another pattern that matches "foo" preceded by three digits and any 6755 three characters that are not "999". 6756 6757 6758 CONDITIONAL SUBPATTERNS 6759 6760 It is possible to cause the matching process to obey a subpattern con- 6761 ditionally or to choose between two alternative subpatterns, depending 6762 on the result of an assertion, or whether a specific capturing subpat- 6763 tern has already been matched. The two possible forms of conditional 6764 subpattern are: 6765 6766 (?(condition)yes-pattern) 6767 (?(condition)yes-pattern|no-pattern) 6768 6769 If the condition is satisfied, the yes-pattern is used; otherwise the 6770 no-pattern (if present) is used. If there are more than two alterna- 6771 tives in the subpattern, a compile-time error occurs. Each of the two 6772 alternatives may itself contain nested subpatterns of any form, includ- 6773 ing conditional subpatterns; the restriction to two alternatives 6774 applies only at the level of the condition. This pattern fragment is an 6775 example where the alternatives are complex: 6776 6777 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 6778 6779 6780 There are four kinds of condition: references to subpatterns, refer- 6781 ences to recursion, a pseudo-condition called DEFINE, and assertions. 6782 6783 Checking for a used subpattern by number 6784 6785 If the text between the parentheses consists of a sequence of digits, 6786 the condition is true if a capturing subpattern of that number has pre- 6787 viously matched. If there is more than one capturing subpattern with 6788 the same number (see the earlier section about duplicate subpattern 6789 numbers), the condition is true if any of them have matched. An alter- 6790 native notation is to precede the digits with a plus or minus sign. In 6791 this case, the subpattern number is relative rather than absolute. The 6792 most recently opened parentheses can be referenced by (?(-1), the next 6793 most recent by (?(-2), and so on. Inside loops it can also make sense 6794 to refer to subsequent groups. The next parentheses to be opened can be 6795 referenced as (?(+1), and so on. (The value zero in any of these forms 6796 is not used; it provokes a compile-time error.) 6797 6798 Consider the following pattern, which contains non-significant white 6799 space to make it more readable (assume the PCRE_EXTENDED option) and to 6800 divide it into three parts for ease of discussion: 6801 6802 ( \( )? [^()]+ (?(1) \) ) 6803 6804 The first part matches an optional opening parenthesis, and if that 6805 character is present, sets it as the first captured substring. The sec- 6806 ond part matches one or more characters that are not parentheses. The 6807 third part is a conditional subpattern that tests whether or not the 6808 first set of parentheses matched. If they did, that is, if subject 6809 started with an opening parenthesis, the condition is true, and so the 6810 yes-pattern is executed and a closing parenthesis is required. Other- 6811 wise, since no-pattern is not present, the subpattern matches nothing. 6812 In other words, this pattern matches a sequence of non-parentheses, 6813 optionally enclosed in parentheses. 6814 6815 If you were embedding this pattern in a larger one, you could use a 6816 relative reference: 6817 6818 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 6819 6820 This makes the fragment independent of the parentheses in the larger 6821 pattern. 6822 6823 Checking for a used subpattern by name 6824 6825 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a 6826 used subpattern by name. For compatibility with earlier versions of 6827 PCRE, which had this facility before Perl, the syntax (?(name)...) is 6828 also recognized. 6829 6830 Rewriting the above example to use a named subpattern gives this: 6831 6832 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 6833 6834 If the name used in a condition of this kind is a duplicate, the test 6835 is applied to all subpatterns of the same name, and is true if any one 6836 of them has matched. 6837 6838 Checking for pattern recursion 6839 6840 If the condition is the string (R), and there is no subpattern with the 6841 name R, the condition is true if a recursive call to the whole pattern 6842 or any subpattern has been made. If digits or a name preceded by amper- 6843 sand follow the letter R, for example: 6844 6845 (?(R3)...) or (?(R&name)...) 6846 6847 the condition is true if the most recent recursion is into a subpattern 6848 whose number or name is given. This condition does not check the entire 6849 recursion stack. If the name used in a condition of this kind is a 6850 duplicate, the test is applied to all subpatterns of the same name, and 6851 is true if any one of them is the most recent recursion. 6852 6853 At "top level", all these recursion test conditions are false. The 6854 syntax for recursive patterns is described below. 6855 6856 Defining subpatterns for use by reference only 6857 6858 If the condition is the string (DEFINE), and there is no subpattern 6859 with the name DEFINE, the condition is always false. In this case, 6860 there may be only one alternative in the subpattern. It is always 6861 skipped if control reaches this point in the pattern; the idea of 6862 DEFINE is that it can be used to define subroutines that can be refer- 6863 enced from elsewhere. (The use of subroutines is described below.) For 6864 example, a pattern to match an IPv4 address such as "192.168.23.245" 6865 could be written like this (ignore white space and line breaks): 6866 6867 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 6868 \b (?&byte) (\.(?&byte)){3} \b 6869 6870 The first part of the pattern is a DEFINE group inside which a another 6871 group named "byte" is defined. This matches an individual component of 6872 an IPv4 address (a number less than 256). When matching takes place, 6873 this part of the pattern is skipped because DEFINE acts like a false 6874 condition. The rest of the pattern uses references to the named group 6875 to match the four dot-separated components of an IPv4 address, insist- 6876 ing on a word boundary at each end. 6877 6878 Assertion conditions 6879 6880 If the condition is not in any of the above formats, it must be an 6881 assertion. This may be a positive or negative lookahead or lookbehind 6882 assertion. Consider this pattern, again containing non-significant 6883 white space, and with the two alternatives on the second line: 6884 6885 (?(?=[^a-z]*[a-z]) 6886 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 6887 6888 The condition is a positive lookahead assertion that matches an 6889 optional sequence of non-letters followed by a letter. In other words, 6890 it tests for the presence of at least one letter in the subject. If a 6891 letter is found, the subject is matched against the first alternative; 6892 otherwise it is matched against the second. This pattern matches 6893 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are 6894 letters and dd are digits. 6895 6896 6897 COMMENTS 6898 6899 There are two ways of including comments in patterns that are processed 6900 by PCRE. In both cases, the start of the comment must not be in a char- 6901 acter class, nor in the middle of any other sequence of related charac- 6902 ters such as (?: or a subpattern name or number. The characters that 6903 make up a comment play no part in the pattern matching. 6904 6905 The sequence (?# marks the start of a comment that continues up to the 6906 next closing parenthesis. Nested parentheses are not permitted. If the 6907 PCRE_EXTENDED option is set, an unescaped # character also introduces a 6908 comment, which in this case continues to immediately after the next 6909 newline character or character sequence in the pattern. Which charac- 6910 ters are interpreted as newlines is controlled by the options passed to 6911 a compiling function or by a special sequence at the start of the pat- 6912 tern, as described in the section entitled "Newline conventions" above. 6913 Note that the end of this type of comment is a literal newline sequence 6914 in the pattern; escape sequences that happen to represent a newline do 6915 not count. For example, consider this pattern when PCRE_EXTENDED is 6916 set, and the default newline convention is in force: 6917 6918 abc #comment \n still comment 6919 6920 On encountering the # character, pcre_compile() skips along, looking 6921 for a newline in the pattern. The sequence \n is still literal at this 6922 stage, so it does not terminate the comment. Only an actual character 6923 with the code value 0x0a (the default newline) does so. 6924 6925 6926 RECURSIVE PATTERNS 6927 6928 Consider the problem of matching a string in parentheses, allowing for 6929 unlimited nested parentheses. Without the use of recursion, the best 6930 that can be done is to use a pattern that matches up to some fixed 6931 depth of nesting. It is not possible to handle an arbitrary nesting 6932 depth. 6933 6934 For some time, Perl has provided a facility that allows regular expres- 6935 sions to recurse (amongst other things). It does this by interpolating 6936 Perl code in the expression at run time, and the code can refer to the 6937 expression itself. A Perl pattern using code interpolation to solve the 6938 parentheses problem can be created like this: 6939 6940 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 6941 6942 The (?p{...}) item interpolates Perl code at run time, and in this case 6943 refers recursively to the pattern in which it appears. 6944 6945 Obviously, PCRE cannot support the interpolation of Perl code. Instead, 6946 it supports special syntax for recursion of the entire pattern, and 6947 also for individual subpattern recursion. After its introduction in 6948 PCRE and Python, this kind of recursion was subsequently introduced 6949 into Perl at release 5.10. 6950 6951 A special item that consists of (? followed by a number greater than 6952 zero and a closing parenthesis is a recursive subroutine call of the 6953 subpattern of the given number, provided that it occurs inside that 6954 subpattern. (If not, it is a non-recursive subroutine call, which is 6955 described in the next section.) The special item (?R) or (?0) is a 6956 recursive call of the entire regular expression. 6957 6958 This PCRE pattern solves the nested parentheses problem (assume the 6959 PCRE_EXTENDED option is set so that white space is ignored): 6960 6961 \( ( [^()]++ | (?R) )* \) 6962 6963 First it matches an opening parenthesis. Then it matches any number of 6964 substrings which can either be a sequence of non-parentheses, or a 6965 recursive match of the pattern itself (that is, a correctly parenthe- 6966 sized substring). Finally there is a closing parenthesis. Note the use 6967 of a possessive quantifier to avoid backtracking into sequences of non- 6968 parentheses. 6969 6970 If this were part of a larger pattern, you would not want to recurse 6971 the entire pattern, so instead you could use this: 6972 6973 ( \( ( [^()]++ | (?1) )* \) ) 6974 6975 We have put the pattern into parentheses, and caused the recursion to 6976 refer to them instead of the whole pattern. 6977 6978 In a larger pattern, keeping track of parenthesis numbers can be 6979 tricky. This is made easier by the use of relative references. Instead 6980 of (?1) in the pattern above you can write (?-2) to refer to the second 6981 most recently opened parentheses preceding the recursion. In other 6982 words, a negative number counts capturing parentheses leftwards from 6983 the point at which it is encountered. 6984 6985 It is also possible to refer to subsequently opened parentheses, by 6986 writing references such as (?+2). However, these cannot be recursive 6987 because the reference is not inside the parentheses that are refer- 6988 enced. They are always non-recursive subroutine calls, as described in 6989 the next section. 6990 6991 An alternative approach is to use named parentheses instead. The Perl 6992 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also 6993 supported. We could rewrite the above example as follows: 6994 6995 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 6996 6997 If there is more than one subpattern with the same name, the earliest 6998 one is used. 6999 7000 This particular example pattern that we have been looking at contains 7001 nested unlimited repeats, and so the use of a possessive quantifier for 7002 matching strings of non-parentheses is important when applying the pat- 7003 tern to strings that do not match. For example, when this pattern is 7004 applied to 7005 7006 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 7007 7008 it yields "no match" quickly. However, if a possessive quantifier is 7009 not used, the match runs for a very long time indeed because there are 7010 so many different ways the + and * repeats can carve up the subject, 7011 and all have to be tested before failure can be reported. 7012 7013 At the end of a match, the values of capturing parentheses are those 7014 from the outermost level. If you want to obtain intermediate values, a 7015 callout function can be used (see below and the pcrecallout documenta- 7016 tion). If the pattern above is matched against 7017 7018 (ab(cd)ef) 7019 7020 the value for the inner capturing parentheses (numbered 2) is "ef", 7021 which is the last value taken on at the top level. If a capturing sub- 7022 pattern is not matched at the top level, its final captured value is 7023 unset, even if it was (temporarily) set at a deeper level during the 7024 matching process. 7025 7026 If there are more than 15 capturing parentheses in a pattern, PCRE has 7027 to obtain extra memory to store data during a recursion, which it does 7028 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory 7029 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 7030 7031 Do not confuse the (?R) item with the condition (R), which tests for 7032 recursion. Consider this pattern, which matches text in angle brack- 7033 ets, allowing for arbitrary nesting. Only digits are allowed in nested 7034 brackets (that is, when recursing), whereas any characters are permit- 7035 ted at the outer level. 7036 7037 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 7038 7039 In this pattern, (?(R) is the start of a conditional subpattern, with 7040 two different alternatives for the recursive and non-recursive cases. 7041 The (?R) item is the actual recursive call. 7042 7043 Differences in recursion processing between PCRE and Perl 7044 7045 Recursion processing in PCRE differs from Perl in two important ways. 7046 In PCRE (like Python, but unlike Perl), a recursive subpattern call is 7047 always treated as an atomic group. That is, once it has matched some of 7048 the subject string, it is never re-entered, even if it contains untried 7049 alternatives and there is a subsequent matching failure. This can be 7050 illustrated by the following pattern, which purports to match a palin- 7051 dromic string that contains an odd number of characters (for example, 7052 "a", "aba", "abcba", "abcdcba"): 7053 7054 ^(.|(.)(?1)\2)$ 7055 7056 The idea is that it either matches a single character, or two identical 7057 characters surrounding a sub-palindrome. In Perl, this pattern works; 7058 in PCRE it does not if the pattern is longer than three characters. 7059 Consider the subject string "abcba": 7060 7061 At the top level, the first character is matched, but as it is not at 7062 the end of the string, the first alternative fails; the second alterna- 7063 tive is taken and the recursion kicks in. The recursive call to subpat- 7064 tern 1 successfully matches the next character ("b"). (Note that the 7065 beginning and end of line tests are not part of the recursion). 7066 7067 Back at the top level, the next character ("c") is compared with what 7068 subpattern 2 matched, which was "a". This fails. Because the recursion 7069 is treated as an atomic group, there are now no backtracking points, 7070 and so the entire match fails. (Perl is able, at this point, to re- 7071 enter the recursion and try the second alternative.) However, if the 7072 pattern is written with the alternatives in the other order, things are 7073 different: 7074 7075 ^((.)(?1)\2|.)$ 7076 7077 This time, the recursing alternative is tried first, and continues to 7078 recurse until it runs out of characters, at which point the recursion 7079 fails. But this time we do have another alternative to try at the 7080 higher level. That is the big difference: in the previous case the 7081 remaining alternative is at a deeper recursion level, which PCRE cannot 7082 use. 7083 7084 To change the pattern so that it matches all palindromic strings, not 7085 just those with an odd number of characters, it is tempting to change 7086 the pattern to this: 7087 7088 ^((.)(?1)\2|.?)$ 7089 7090 Again, this works in Perl, but not in PCRE, and for the same reason. 7091 When a deeper recursion has matched a single character, it cannot be 7092 entered again in order to match an empty string. The solution is to 7093 separate the two cases, and write out the odd and even cases as alter- 7094 natives at the higher level: 7095 7096 ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) 7097 7098 If you want to match typical palindromic phrases, the pattern has to 7099 ignore all non-word characters, which can be done like this: 7100 7101 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ 7102 7103 If run with the PCRE_CASELESS option, this pattern matches phrases such 7104 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and 7105 Perl. Note the use of the possessive quantifier *+ to avoid backtrack- 7106 ing into sequences of non-word characters. Without this, PCRE takes a 7107 great deal longer (ten times or more) to match typical phrases, and 7108 Perl takes so long that you think it has gone into a loop. 7109 7110 WARNING: The palindrome-matching patterns above work only if the sub- 7111 ject string does not start with a palindrome that is shorter than the 7112 entire string. For example, although "abcba" is correctly matched, if 7113 the subject is "ababa", PCRE finds the palindrome "aba" at the start, 7114 then fails at top level because the end of the string does not follow. 7115 Once again, it cannot jump back into the recursion to try other alter- 7116 natives, so the entire match fails. 7117 7118 The second way in which PCRE and Perl differ in their recursion pro- 7119 cessing is in the handling of captured values. In Perl, when a subpat- 7120 tern is called recursively or as a subpattern (see the next section), 7121 it has no access to any values that were captured outside the recur- 7122 sion, whereas in PCRE these values can be referenced. Consider this 7123 pattern: 7124 7125 ^(.)(\1|a(?2)) 7126 7127 In PCRE, this pattern matches "bab". The first capturing parentheses 7128 match "b", then in the second group, when the back reference \1 fails 7129 to match "b", the second alternative matches "a" and then recurses. In 7130 the recursion, \1 does now match "b" and so the whole match succeeds. 7131 In Perl, the pattern fails to match because inside the recursive call 7132 \1 cannot access the externally set value. 7133 7134 7135 SUBPATTERNS AS SUBROUTINES 7136 7137 If the syntax for a recursive subpattern call (either by number or by 7138 name) is used outside the parentheses to which it refers, it operates 7139 like a subroutine in a programming language. The called subpattern may 7140 be defined before or after the reference. A numbered reference can be 7141 absolute or relative, as in these examples: 7142 7143 (...(absolute)...)...(?2)... 7144 (...(relative)...)...(?-1)... 7145 (...(?+1)...(relative)... 7146 7147 An earlier example pointed out that the pattern 7148 7149 (sens|respons)e and \1ibility 7150 7151 matches "sense and sensibility" and "response and responsibility", but 7152 not "sense and responsibility". If instead the pattern 7153 7154 (sens|respons)e and (?1)ibility 7155 7156 is used, it does match "sense and responsibility" as well as the other 7157 two strings. Another example is given in the discussion of DEFINE 7158 above. 7159 7160 All subroutine calls, whether recursive or not, are always treated as 7161 atomic groups. That is, once a subroutine has matched some of the sub- 7162 ject string, it is never re-entered, even if it contains untried alter- 7163 natives and there is a subsequent matching failure. Any capturing 7164 parentheses that are set during the subroutine call revert to their 7165 previous values afterwards. 7166 7167 Processing options such as case-independence are fixed when a subpat- 7168 tern is defined, so if it is used as a subroutine, such options cannot 7169 be changed for different calls. For example, consider this pattern: 7170 7171 (abc)(?i:(?-1)) 7172 7173 It matches "abcabc". It does not match "abcABC" because the change of 7174 processing option does not affect the called subpattern. 7175 7176 7177 ONIGURUMA SUBROUTINE SYNTAX 7178 7179 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 7180 name or a number enclosed either in angle brackets or single quotes, is 7181 an alternative syntax for referencing a subpattern as a subroutine, 7182 possibly recursively. Here are two of the examples used above, rewrit- 7183 ten using this syntax: 7184 7185 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 7186 (sens|respons)e and \g'1'ibility 7187 7188 PCRE supports an extension to Oniguruma: if a number is preceded by a 7189 plus or a minus sign it is taken as a relative reference. For example: 7190 7191 (abc)(?i:\g<-1>) 7192 7193 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not 7194 synonymous. The former is a back reference; the latter is a subroutine 7195 call. 7196 7197 7198 CALLOUTS 7199 7200 Perl has a feature whereby using the sequence (?{...}) causes arbitrary 7201 Perl code to be obeyed in the middle of matching a regular expression. 7202 This makes it possible, amongst other things, to extract different sub- 7203 strings that match the same pair of parentheses when there is a repeti- 7204 tion. 7205 7206 PCRE provides a similar feature, but of course it cannot obey arbitrary 7207 Perl code. The feature is called "callout". The caller of PCRE provides 7208 an external function by putting its entry point in the global variable 7209 pcre_callout (8-bit library) or pcre[16|32]_callout (16-bit or 32-bit 7210 library). By default, this variable contains NULL, which disables all 7211 calling out. 7212 7213 Within a regular expression, (?C) indicates the points at which the 7214 external function is to be called. If you want to identify different 7215 callout points, you can put a number less than 256 after the letter C. 7216 The default value is zero. For example, this pattern has two callout 7217 points: 7218 7219 (?C1)abc(?C2)def 7220 7221 If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, call- 7222 outs are automatically installed before each item in the pattern. They 7223 are all numbered 255. If there is a conditional group in the pattern 7224 whose condition is an assertion, an additional callout is inserted just 7225 before the condition. An explicit callout may also be set at this posi- 7226 tion, as in this example: 7227 7228 (?(?C9)(?=a)abc|def) 7229 7230 Note that this applies only to assertion conditions, not to other types 7231 of condition. 7232 7233 During matching, when PCRE reaches a callout point, the external func- 7234 tion is called. It is provided with the number of the callout, the 7235 position in the pattern, and, optionally, one item of data originally 7236 supplied by the caller of the matching function. The callout function 7237 may cause matching to proceed, to backtrack, or to fail altogether. 7238 7239 By default, PCRE implements a number of optimizations at compile time 7240 and matching time, and one side-effect is that sometimes callouts are 7241 skipped. If you need all possible callouts to happen, you need to set 7242 options that disable the relevant optimizations. More details, and a 7243 complete description of the interface to the callout function, are 7244 given in the pcrecallout documentation. 7245 7246 7247 BACKTRACKING CONTROL 7248 7249 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", 7250 which are still described in the Perl documentation as "experimental 7251 and subject to change or removal in a future version of Perl". It goes 7252 on to say: "Their usage in production code should be noted to avoid 7253 problems during upgrades." The same remarks apply to the PCRE features 7254 described in this section. 7255 7256 The new verbs make use of what was previously invalid syntax: an open- 7257 ing parenthesis followed by an asterisk. They are generally of the form 7258 (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving 7259 differently depending on whether or not a name is present. A name is 7260 any sequence of characters that does not include a closing parenthesis. 7261 The maximum length of name is 255 in the 8-bit library and 65535 in the 7262 16-bit and 32-bit libraries. If the name is empty, that is, if the 7263 closing parenthesis immediately follows the colon, the effect is as if 7264 the colon were not there. Any number of these verbs may occur in a 7265 pattern. 7266 7267 Since these verbs are specifically related to backtracking, most of 7268 them can be used only when the pattern is to be matched using one of 7269 the traditional matching functions, because these use a backtracking 7270 algorithm. With the exception of (*FAIL), which behaves like a failing 7271 negative assertion, the backtracking control verbs cause an error if 7272 encountered by a DFA matching function. 7273 7274 The behaviour of these verbs in repeated groups, assertions, and in 7275 subpatterns called as subroutines (whether or not recursively) is docu- 7276 mented below. 7277 7278 Optimizations that affect backtracking verbs 7279 7280 PCRE contains some optimizations that are used to speed up matching by 7281 running some checks at the start of each match attempt. For example, it 7282 may know the minimum length of matching subject, or that a particular 7283 character must be present. When one of these optimizations bypasses the 7284 running of a match, any included backtracking verbs will not, of 7285 course, be processed. You can suppress the start-of-match optimizations 7286 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com- 7287 pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT). 7288 There is more discussion of this option in the section entitled "Option 7289 bits for pcre_exec()" in the pcreapi documentation. 7290 7291 Experiments with Perl suggest that it too has similar optimizations, 7292 sometimes leading to anomalous results. 7293 7294 Verbs that act immediately 7295 7296 The following verbs act as soon as they are encountered. They may not 7297 be followed by a name. 7298 7299 (*ACCEPT) 7300 7301 This verb causes the match to end successfully, skipping the remainder 7302 of the pattern. However, when it is inside a subpattern that is called 7303 as a subroutine, only that subpattern is ended successfully. Matching 7304 then continues at the outer level. If (*ACCEPT) in triggered in a posi- 7305 tive assertion, the assertion succeeds; in a negative assertion, the 7306 assertion fails. 7307 7308 If (*ACCEPT) is inside capturing parentheses, the data so far is cap- 7309 tured. For example: 7310 7311 A((?:A|B(*ACCEPT)|C)D) 7312 7313 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- 7314 tured by the outer parentheses. 7315 7316 (*FAIL) or (*F) 7317 7318 This verb causes a matching failure, forcing backtracking to occur. It 7319 is equivalent to (?!) but easier to read. The Perl documentation notes 7320 that it is probably useful only when combined with (?{}) or (??{}). 7321 Those are, of course, Perl features that are not present in PCRE. The 7322 nearest equivalent is the callout feature, as for example in this pat- 7323 tern: 7324 7325 a+(?C)(*FAIL) 7326 7327 A match with the string "aaaa" always fails, but the callout is taken 7328 before each backtrack happens (in this example, 10 times). 7329 7330 Recording which path was taken 7331 7332 There is one verb whose main purpose is to track how a match was 7333 arrived at, though it also has a secondary use in conjunction with 7334 advancing the match starting point (see (*SKIP) below). 7335 7336 (*MARK:NAME) or (*:NAME) 7337 7338 A name is always required with this verb. There may be as many 7339 instances of (*MARK) as you like in a pattern, and their names do not 7340 have to be unique. 7341 7342 When a match succeeds, the name of the last-encountered (*MARK:NAME), 7343 (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to 7344 the caller as described in the section entitled "Extra data for 7345 pcre_exec()" in the pcreapi documentation. Here is an example of 7346 pcretest output, where the /K modifier requests the retrieval and out- 7347 putting of (*MARK) data: 7348 7349 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 7350 data> XY 7351 0: XY 7352 MK: A 7353 XZ 7354 0: XZ 7355 MK: B 7356 7357 The (*MARK) name is tagged with "MK:" in this output, and in this exam- 7358 ple it indicates which of the two alternatives matched. This is a more 7359 efficient way of obtaining this information than putting each alterna- 7360 tive in its own capturing parentheses. 7361 7362 If a verb with a name is encountered in a positive assertion that is 7363 true, the name is recorded and passed back if it is the last-encoun- 7364 tered. This does not happen for negative assertions or failing positive 7365 assertions. 7366 7367 After a partial match or a failed match, the last encountered name in 7368 the entire match process is returned. For example: 7369 7370 re> /X(*MARK:A)Y|X(*MARK:B)Z/K 7371 data> XP 7372 No match, mark = B 7373 7374 Note that in this unanchored example the mark is retained from the 7375 match attempt that started at the letter "X" in the subject. Subsequent 7376 match attempts starting at "P" and then with an empty string do not get 7377 as far as the (*MARK) item, but nevertheless do not reset it. 7378 7379 If you are interested in (*MARK) values after failed matches, you 7380 should probably set the PCRE_NO_START_OPTIMIZE option (see above) to 7381 ensure that the match is always attempted. 7382 7383 Verbs that act after backtracking 7384 7385 The following verbs do nothing when they are encountered. Matching con- 7386 tinues with what follows, but if there is no subsequent match, causing 7387 a backtrack to the verb, a failure is forced. That is, backtracking 7388 cannot pass to the left of the verb. However, when one of these verbs 7389 appears inside an atomic group or an assertion that is true, its effect 7390 is confined to that group, because once the group has been matched, 7391 there is never any backtracking into it. In this situation, backtrack- 7392 ing can "jump back" to the left of the entire atomic group or asser- 7393 tion. (Remember also, as stated above, that this localization also 7394 applies in subroutine calls.) 7395 7396 These verbs differ in exactly what kind of failure occurs when back- 7397 tracking reaches them. The behaviour described below is what happens 7398 when the verb is not in a subroutine or an assertion. Subsequent sec- 7399 tions cover these special cases. 7400 7401 (*COMMIT) 7402 7403 This verb, which may not be followed by a name, causes the whole match 7404 to fail outright if there is a later matching failure that causes back- 7405 tracking to reach it. Even if the pattern is unanchored, no further 7406 attempts to find a match by advancing the starting point take place. If 7407 (*COMMIT) is the only backtracking verb that is encountered, once it 7408 has been passed pcre_exec() is committed to finding a match at the cur- 7409 rent starting point, or not at all. For example: 7410 7411 a+(*COMMIT)b 7412 7413 This matches "xxaab" but not "aacaab". It can be thought of as a kind 7414 of dynamic anchor, or "I've started, so I must finish." The name of the 7415 most recently passed (*MARK) in the path is passed back when (*COMMIT) 7416 forces a match failure. 7417 7418 If there is more than one backtracking verb in a pattern, a different 7419 one that follows (*COMMIT) may be triggered first, so merely passing 7420 (*COMMIT) during a match does not always guarantee that a match must be 7421 at this starting point. 7422 7423 Note that (*COMMIT) at the start of a pattern is not the same as an 7424 anchor, unless PCRE's start-of-match optimizations are turned off, as 7425 shown in this output from pcretest: 7426 7427 re> /(*COMMIT)abc/ 7428 data> xyzabc 7429 0: abc 7430 data> xyzabc\Y 7431 No match 7432 7433 For this pattern, PCRE knows that any match must start with "a", so the 7434 optimization skips along the subject to "a" before applying the pattern 7435 to the first set of data. The match attempt then succeeds. In the sec- 7436 ond set of data, the escape sequence \Y is interpreted by the pcretest 7437 program. It causes the PCRE_NO_START_OPTIMIZE option to be set when 7438 pcre_exec() is called. This disables the optimization that skips along 7439 to the first character. The pattern is now applied starting at "x", and 7440 so the (*COMMIT) causes the match to fail without trying any other 7441 starting points. 7442 7443 (*PRUNE) or (*PRUNE:NAME) 7444 7445 This verb causes the match to fail at the current starting position in 7446 the subject if there is a later matching failure that causes backtrack- 7447 ing to reach it. If the pattern is unanchored, the normal "bumpalong" 7448 advance to the next starting character then happens. Backtracking can 7449 occur as usual to the left of (*PRUNE), before it is reached, or when 7450 matching to the right of (*PRUNE), but if there is no match to the 7451 right, backtracking cannot cross (*PRUNE). In simple cases, the use of 7452 (*PRUNE) is just an alternative to an atomic group or possessive quan- 7453 tifier, but there are some uses of (*PRUNE) that cannot be expressed in 7454 any other way. In an anchored pattern (*PRUNE) has the same effect as 7455 (*COMMIT). 7456 7457 The behaviour of (*PRUNE:NAME) is the not the same as 7458 (*MARK:NAME)(*PRUNE). It is like (*MARK:NAME) in that the name is 7459 remembered for passing back to the caller. However, (*SKIP:NAME) 7460 searches only for names set with (*MARK). 7461 7462 (*SKIP) 7463 7464 This verb, when given without a name, is like (*PRUNE), except that if 7465 the pattern is unanchored, the "bumpalong" advance is not to the next 7466 character, but to the position in the subject where (*SKIP) was encoun- 7467 tered. (*SKIP) signifies that whatever text was matched leading up to 7468 it cannot be part of a successful match. Consider: 7469 7470 a+(*SKIP)b 7471 7472 If the subject is "aaaac...", after the first match attempt fails 7473 (starting at the first character in the string), the starting point 7474 skips on to start the next attempt at "c". Note that a possessive quan- 7475 tifer does not have the same effect as this example; although it would 7476 suppress backtracking during the first match attempt, the second 7477 attempt would start at the second character instead of skipping on to 7478 "c". 7479 7480 (*SKIP:NAME) 7481 7482 When (*SKIP) has an associated name, its behaviour is modified. When it 7483 is triggered, the previous path through the pattern is searched for the 7484 most recent (*MARK) that has the same name. If one is found, the 7485 "bumpalong" advance is to the subject position that corresponds to that 7486 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with 7487 a matching name is found, the (*SKIP) is ignored. 7488 7489 Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It 7490 ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME). 7491 7492 (*THEN) or (*THEN:NAME) 7493 7494 This verb causes a skip to the next innermost alternative when back- 7495 tracking reaches it. That is, it cancels any further backtracking 7496 within the current alternative. Its name comes from the observation 7497 that it can be used for a pattern-based if-then-else block: 7498 7499 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 7500 7501 If the COND1 pattern matches, FOO is tried (and possibly further items 7502 after the end of the group if FOO succeeds); on failure, the matcher 7503 skips to the second alternative and tries COND2, without backtracking 7504 into COND1. If that succeeds and BAR fails, COND3 is tried. If subse- 7505 quently BAZ fails, there are no more alternatives, so there is a back- 7506 track to whatever came before the entire group. If (*THEN) is not 7507 inside an alternation, it acts like (*PRUNE). 7508 7509 The behaviour of (*THEN:NAME) is the not the same as 7510 (*MARK:NAME)(*THEN). It is like (*MARK:NAME) in that the name is 7511 remembered for passing back to the caller. However, (*SKIP:NAME) 7512 searches only for names set with (*MARK). 7513 7514 A subpattern that does not contain a | character is just a part of the 7515 enclosing alternative; it is not a nested alternation with only one 7516 alternative. The effect of (*THEN) extends beyond such a subpattern to 7517 the enclosing alternative. Consider this pattern, where A, B, etc. are 7518 complex pattern fragments that do not contain any | characters at this 7519 level: 7520 7521 A (B(*THEN)C) | D 7522 7523 If A and B are matched, but there is a failure in C, matching does not 7524 backtrack into A; instead it moves to the next alternative, that is, D. 7525 However, if the subpattern containing (*THEN) is given an alternative, 7526 it behaves differently: 7527 7528 A (B(*THEN)C | (*FAIL)) | D 7529 7530 The effect of (*THEN) is now confined to the inner subpattern. After a 7531 failure in C, matching moves to (*FAIL), which causes the whole subpat- 7532 tern to fail because there are no more alternatives to try. In this 7533 case, matching does now backtrack into A. 7534 7535 Note that a conditional subpattern is not considered as having two 7536 alternatives, because only one is ever used. In other words, the | 7537 character in a conditional subpattern has a different meaning. Ignoring 7538 white space, consider: 7539 7540 ^.*? (?(?=a) a | b(*THEN)c ) 7541 7542 If the subject is "ba", this pattern does not match. Because .*? is 7543 ungreedy, it initially matches zero characters. The condition (?=a) 7544 then fails, the character "b" is matched, but "c" is not. At this 7545 point, matching does not backtrack to .*? as might perhaps be expected 7546 from the presence of the | character. The conditional subpattern is 7547 part of the single alternative that comprises the whole pattern, and so 7548 the match fails. (If there was a backtrack into .*?, allowing it to 7549 match "b", the match would succeed.) 7550 7551 The verbs just described provide four different "strengths" of control 7552 when subsequent matching fails. (*THEN) is the weakest, carrying on the 7553 match at the next alternative. (*PRUNE) comes next, failing the match 7554 at the current starting position, but allowing an advance to the next 7555 character (for an unanchored pattern). (*SKIP) is similar, except that 7556 the advance may be more than one character. (*COMMIT) is the strongest, 7557 causing the entire match to fail. 7558 7559 More than one backtracking verb 7560 7561 If more than one backtracking verb is present in a pattern, the one 7562 that is backtracked onto first acts. For example, consider this pat- 7563 tern, where A, B, etc. are complex pattern fragments: 7564 7565 (A(*COMMIT)B(*THEN)C|ABD) 7566 7567 If A matches but B fails, the backtrack to (*COMMIT) causes the entire 7568 match to fail. However, if A and B match, but C fails, the backtrack to 7569 (*THEN) causes the next alternative (ABD) to be tried. This behaviour 7570 is consistent, but is not always the same as Perl's. It means that if 7571 two or more backtracking verbs appear in succession, all the the last 7572 of them has no effect. Consider this example: 7573 7574 ...(*COMMIT)(*PRUNE)... 7575 7576 If there is a matching failure to the right, backtracking onto (*PRUNE) 7577 causes it to be triggered, and its action is taken. There can never be 7578 a backtrack onto (*COMMIT). 7579 7580 Backtracking verbs in repeated groups 7581 7582 PCRE differs from Perl in its handling of backtracking verbs in 7583 repeated groups. For example, consider: 7584 7585 /(a(*COMMIT)b)+ac/ 7586 7587 If the subject is "abac", Perl matches, but PCRE fails because the 7588 (*COMMIT) in the second repeat of the group acts. 7589 7590 Backtracking verbs in assertions 7591 7592 (*FAIL) in an assertion has its normal effect: it forces an immediate 7593 backtrack. 7594 7595 (*ACCEPT) in a positive assertion causes the assertion to succeed with- 7596 out any further processing. In a negative assertion, (*ACCEPT) causes 7597 the assertion to fail without any further processing. 7598 7599 The other backtracking verbs are not treated specially if they appear 7600 in a positive assertion. In particular, (*THEN) skips to the next 7601 alternative in the innermost enclosing group that has alternations, 7602 whether or not this is within the assertion. 7603 7604 Negative assertions are, however, different, in order to ensure that 7605 changing a positive assertion into a negative assertion changes its 7606 result. Backtracking into (*COMMIT), (*SKIP), or (*PRUNE) causes a neg- 7607 ative assertion to be true, without considering any further alternative 7608 branches in the assertion. Backtracking into (*THEN) causes it to skip 7609 to the next enclosing alternative within the assertion (the normal be- 7610 haviour), but if the assertion does not have such an alternative, 7611 (*THEN) behaves like (*PRUNE). 7612 7613 Backtracking verbs in subroutines 7614 7615 These behaviours occur whether or not the subpattern is called recur- 7616 sively. Perl's treatment of subroutines is different in some cases. 7617 7618 (*FAIL) in a subpattern called as a subroutine has its normal effect: 7619 it forces an immediate backtrack. 7620 7621 (*ACCEPT) in a subpattern called as a subroutine causes the subroutine 7622 match to succeed without any further processing. Matching then contin- 7623 ues after the subroutine call. 7624 7625 (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine 7626 cause the subroutine match to fail. 7627 7628 (*THEN) skips to the next alternative in the innermost enclosing group 7629 within the subpattern that has alternatives. If there is no such group 7630 within the subpattern, (*THEN) causes the subroutine match to fail. 7631 7632 7633 SEE ALSO 7634 7635 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3), 7636 pcre16(3), pcre32(3). 7637 7638 7639 AUTHOR 7640 7641 Philip Hazel 7642 University Computing Service 7643 Cambridge CB2 3QH, England. 7644 7645 7646 REVISION 7647 7648 Last updated: 08 January 2014 7649 Copyright (c) 1997-2014 University of Cambridge. 7650 ------------------------------------------------------------------------------ 7651 7652 7653 PCRESYNTAX(3) Library Functions Manual PCRESYNTAX(3) 7654 7655 7656 7657 NAME 7658 PCRE - Perl-compatible regular expressions 7659 7660 PCRE REGULAR EXPRESSION SYNTAX SUMMARY 7661 7662 The full syntax and semantics of the regular expressions that are sup- 7663 ported by PCRE are described in the pcrepattern documentation. This 7664 document contains a quick-reference summary of the syntax. 7665 7666 7667 QUOTING 7668 7669 \x where x is non-alphanumeric is a literal x 7670 \Q...\E treat enclosed characters as literal 7671 7672 7673 CHARACTERS 7674 7675 \a alarm, that is, the BEL character (hex 07) 7676 \cx "control-x", where x is any ASCII character 7677 \e escape (hex 1B) 7678 \f form feed (hex 0C) 7679 \n newline (hex 0A) 7680 \r carriage return (hex 0D) 7681 \t tab (hex 09) 7682 \0dd character with octal code 0dd 7683 \ddd character with octal code ddd, or backreference 7684 \o{ddd..} character with octal code ddd.. 7685 \xhh character with hex code hh 7686 \x{hhh..} character with hex code hhh.. 7687 7688 Note that \0dd is always an octal code, and that \8 and \9 are the lit- 7689 eral characters "8" and "9". 7690 7691 7692 CHARACTER TYPES 7693 7694 . any character except newline; 7695 in dotall mode, any character whatsoever 7696 \C one data unit, even in UTF mode (best avoided) 7697 \d a decimal digit 7698 \D a character that is not a decimal digit 7699 \h a horizontal white space character 7700 \H a character that is not a horizontal white space character 7701 \N a character that is not a newline 7702 \p{xx} a character with the xx property 7703 \P{xx} a character without the xx property 7704 \R a newline sequence 7705 \s a white space character 7706 \S a character that is not a white space character 7707 \v a vertical white space character 7708 \V a character that is not a vertical white space character 7709 \w a "word" character 7710 \W a "non-word" character 7711 \X a Unicode extended grapheme cluster 7712 7713 By default, \d, \s, and \w match only ASCII characters, even in UTF-8 7714 mode or in the 16- bit and 32-bit libraries. However, if locale-spe- 7715 cific matching is happening, \s and \w may also match characters with 7716 code points in the range 128-255. If the PCRE_UCP option is set, the 7717 behaviour of these escape sequences is changed to use Unicode proper- 7718 ties and they match many more characters. 7719 7720 7721 GENERAL CATEGORY PROPERTIES FOR \p and \P 7722 7723 C Other 7724 Cc Control 7725 Cf Format 7726 Cn Unassigned 7727 Co Private use 7728 Cs Surrogate 7729 7730 L Letter 7731 Ll Lower case letter 7732 Lm Modifier letter 7733 Lo Other letter 7734 Lt Title case letter 7735 Lu Upper case letter 7736 L& Ll, Lu, or Lt 7737 7738 M Mark 7739 Mc Spacing mark 7740 Me Enclosing mark 7741 Mn Non-spacing mark 7742 7743 N Number 7744 Nd Decimal number 7745 Nl Letter number 7746 No Other number 7747 7748 P Punctuation 7749 Pc Connector punctuation 7750 Pd Dash punctuation 7751 Pe Close punctuation 7752 Pf Final punctuation 7753 Pi Initial punctuation 7754 Po Other punctuation 7755 Ps Open punctuation 7756 7757 S Symbol 7758 Sc Currency symbol 7759 Sk Modifier symbol 7760 Sm Mathematical symbol 7761 So Other symbol 7762 7763 Z Separator 7764 Zl Line separator 7765 Zp Paragraph separator 7766 Zs Space separator 7767 7768 7769 PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P 7770 7771 Xan Alphanumeric: union of properties L and N 7772 Xps POSIX space: property Z or tab, NL, VT, FF, CR 7773 Xsp Perl space: property Z or tab, NL, VT, FF, CR 7774 Xuc Univerally-named character: one that can be 7775 represented by a Universal Character Name 7776 Xwd Perl word: property Xan or underscore 7777 7778 Perl and POSIX space are now the same. Perl added VT to its space char- 7779 acter set at release 5.18 and PCRE changed at release 8.34. 7780 7781 7782 SCRIPT NAMES FOR \p AND \P 7783 7784 Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, 7785 Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Car- 7786 ian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cunei- 7787 form, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hiero- 7788 glyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, 7789 Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, 7790 Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscrip- 7791 tional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, 7792 Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Lin- 7793 ear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic, 7794 Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, 7795 Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, 7796 New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, 7797 Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, 7798 Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, 7799 Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Sha- 7800 vian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, 7801 Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, 7802 Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, 7803 Yi. 7804 7805 7806 CHARACTER CLASSES 7807 7808 [...] positive character class 7809 [^...] negative character class 7810 [x-y] range (can be used for hex characters) 7811 [[:xxx:]] positive POSIX named set 7812 [[:^xxx:]] negative POSIX named set 7813 7814 alnum alphanumeric 7815 alpha alphabetic 7816 ascii 0-127 7817 blank space or tab 7818 cntrl control character 7819 digit decimal digit 7820 graph printing, excluding space 7821 lower lower case letter 7822 print printing, including space 7823 punct printing, excluding alphanumeric 7824 space white space 7825 upper upper case letter 7826 word same as \w 7827 xdigit hexadecimal digit 7828 7829 In PCRE, POSIX character set names recognize only ASCII characters by 7830 default, but some of them use Unicode properties if PCRE_UCP is set. 7831 You can use \Q...\E inside a character class. 7832 7833 7834 QUANTIFIERS 7835 7836 ? 0 or 1, greedy 7837 ?+ 0 or 1, possessive 7838 ?? 0 or 1, lazy 7839 * 0 or more, greedy 7840 *+ 0 or more, possessive 7841 *? 0 or more, lazy 7842 + 1 or more, greedy 7843 ++ 1 or more, possessive 7844 +? 1 or more, lazy 7845 {n} exactly n 7846 {n,m} at least n, no more than m, greedy 7847 {n,m}+ at least n, no more than m, possessive 7848 {n,m}? at least n, no more than m, lazy 7849 {n,} n or more, greedy 7850 {n,}+ n or more, possessive 7851 {n,}? n or more, lazy 7852 7853 7854 ANCHORS AND SIMPLE ASSERTIONS 7855 7856 \b word boundary 7857 \B not a word boundary 7858 ^ start of subject 7859 also after internal newline in multiline mode 7860 \A start of subject 7861 $ end of subject 7862 also before newline at end of subject 7863 also before internal newline in multiline mode 7864 \Z end of subject 7865 also before newline at end of subject 7866 \z end of subject 7867 \G first matching position in subject 7868 7869 7870 MATCH POINT RESET 7871 7872 \K reset start of match 7873 7874 \K is honoured in positive assertions, but ignored in negative ones. 7875 7876 7877 ALTERNATION 7878 7879 expr|expr|expr... 7880 7881 7882 CAPTURING 7883 7884 (...) capturing group 7885 (?<name>...) named capturing group (Perl) 7886 (?'name'...) named capturing group (Perl) 7887 (?P<name>...) named capturing group (Python) 7888 (?:...) non-capturing group 7889 (?|...) non-capturing group; reset group numbers for 7890 capturing groups in each alternative 7891 7892 7893 ATOMIC GROUPS 7894 7895 (?>...) atomic, non-capturing group 7896 7897 7898 COMMENT 7899 7900 (?#....) comment (not nestable) 7901 7902 7903 OPTION SETTING 7904 7905 (?i) caseless 7906 (?J) allow duplicate names 7907 (?m) multiline 7908 (?s) single line (dotall) 7909 (?U) default ungreedy (lazy) 7910 (?x) extended (ignore white space) 7911 (?-...) unset option(s) 7912 7913 The following are recognized only at the very start of a pattern or 7914 after one of the newline or \R options with similar syntax. More than 7915 one of them may appear. 7916 7917 (*LIMIT_MATCH=d) set the match limit to d (decimal number) 7918 (*LIMIT_RECURSION=d) set the recursion limit to d (decimal number) 7919 (*NO_AUTO_POSSESS) no auto-possessification (PCRE_NO_AUTO_POSSESS) 7920 (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE) 7921 (*UTF8) set UTF-8 mode: 8-bit library (PCRE_UTF8) 7922 (*UTF16) set UTF-16 mode: 16-bit library (PCRE_UTF16) 7923 (*UTF32) set UTF-32 mode: 32-bit library (PCRE_UTF32) 7924 (*UTF) set appropriate UTF mode for the library in use 7925 (*UCP) set PCRE_UCP (use Unicode properties for \d etc) 7926 7927 Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of 7928 the limits set by the caller of pcre_exec(), not increase them. 7929 7930 7931 NEWLINE CONVENTION 7932 7933 These are recognized only at the very start of the pattern or after 7934 option settings with a similar syntax. 7935 7936 (*CR) carriage return only 7937 (*LF) linefeed only 7938 (*CRLF) carriage return followed by linefeed 7939 (*ANYCRLF) all three of the above 7940 (*ANY) any Unicode newline sequence 7941 7942 7943 WHAT \R MATCHES 7944 7945 These are recognized only at the very start of the pattern or after 7946 option setting with a similar syntax. 7947 7948 (*BSR_ANYCRLF) CR, LF, or CRLF 7949 (*BSR_UNICODE) any Unicode newline sequence 7950 7951 7952 LOOKAHEAD AND LOOKBEHIND ASSERTIONS 7953 7954 (?=...) positive look ahead 7955 (?!...) negative look ahead 7956 (?<=...) positive look behind 7957 (?<!...) negative look behind 7958 7959 Each top-level branch of a look behind must be of a fixed length. 7960 7961 7962 BACKREFERENCES 7963 7964 \n reference by number (can be ambiguous) 7965 \gn reference by number 7966 \g{n} reference by number 7967 \g{-n} relative reference by number 7968 \k<name> reference by name (Perl) 7969 \k'name' reference by name (Perl) 7970 \g{name} reference by name (Perl) 7971 \k{name} reference by name (.NET) 7972 (?P=name) reference by name (Python) 7973 7974 7975 SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) 7976 7977 (?R) recurse whole pattern 7978 (?n) call subpattern by absolute number 7979 (?+n) call subpattern by relative number 7980 (?-n) call subpattern by relative number 7981 (?&name) call subpattern by name (Perl) 7982 (?P>name) call subpattern by name (Python) 7983 \g<name> call subpattern by name (Oniguruma) 7984 \g'name' call subpattern by name (Oniguruma) 7985 \g<n> call subpattern by absolute number (Oniguruma) 7986 \g'n' call subpattern by absolute number (Oniguruma) 7987 \g<+n> call subpattern by relative number (PCRE extension) 7988 \g'+n' call subpattern by relative number (PCRE extension) 7989 \g<-n> call subpattern by relative number (PCRE extension) 7990 \g'-n' call subpattern by relative number (PCRE extension) 7991 7992 7993 CONDITIONAL PATTERNS 7994 7995 (?(condition)yes-pattern) 7996 (?(condition)yes-pattern|no-pattern) 7997 7998 (?(n)... absolute reference condition 7999 (?(+n)... relative reference condition 8000 (?(-n)... relative reference condition 8001 (?(<name>)... named reference condition (Perl) 8002 (?('name')... named reference condition (Perl) 8003 (?(name)... named reference condition (PCRE) 8004 (?(R)... overall recursion condition 8005 (?(Rn)... specific group recursion condition 8006 (?(R&name)... specific recursion condition 8007 (?(DEFINE)... define subpattern for reference 8008 (?(assert)... assertion condition 8009 8010 8011 BACKTRACKING CONTROL 8012 8013 The following act immediately they are reached: 8014 8015 (*ACCEPT) force successful match 8016 (*FAIL) force backtrack; synonym (*F) 8017 (*MARK:NAME) set name to be passed back; synonym (*:NAME) 8018 8019 The following act only when a subsequent match failure causes a back- 8020 track to reach them. They all force a match failure, but they differ in 8021 what happens afterwards. Those that advance the start-of-match point do 8022 so only if the pattern is not anchored. 8023 8024 (*COMMIT) overall failure, no advance of starting point 8025 (*PRUNE) advance to next starting character 8026 (*PRUNE:NAME) equivalent to (*MARK:NAME)(*PRUNE) 8027 (*SKIP) advance to current matching position 8028 (*SKIP:NAME) advance to position corresponding to an earlier 8029 (*MARK:NAME); if not found, the (*SKIP) is ignored 8030 (*THEN) local failure, backtrack to next alternation 8031 (*THEN:NAME) equivalent to (*MARK:NAME)(*THEN) 8032 8033 8034 CALLOUTS 8035 8036 (?C) callout 8037 (?Cn) callout with data n 8038 8039 8040 SEE ALSO 8041 8042 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). 8043 8044 8045 AUTHOR 8046 8047 Philip Hazel 8048 University Computing Service 8049 Cambridge CB2 3QH, England. 8050 8051 8052 REVISION 8053 8054 Last updated: 08 January 2014 8055 Copyright (c) 1997-2014 University of Cambridge. 8056 ------------------------------------------------------------------------------ 8057 8058 8059 PCREUNICODE(3) Library Functions Manual PCREUNICODE(3) 8060 8061 8062 8063 NAME 8064 PCRE - Perl-compatible regular expressions 8065 8066 UTF-8, UTF-16, UTF-32, AND UNICODE PROPERTY SUPPORT 8067 8068 As well as UTF-8 support, PCRE also supports UTF-16 (from release 8.30) 8069 and UTF-32 (from release 8.32), by means of two additional libraries. 8070 They can be built as well as, or instead of, the 8-bit library. 8071 8072 8073 UTF-8 SUPPORT 8074 8075 In order process UTF-8 strings, you must build PCRE's 8-bit library 8076 with UTF support, and, in addition, you must call pcre_compile() with 8077 the PCRE_UTF8 option flag, or the pattern must start with the sequence 8078 (*UTF8) or (*UTF). When either of these is the case, both the pattern 8079 and any subject strings that are matched against it are treated as 8080 UTF-8 strings instead of strings of individual 1-byte characters. 8081 8082 8083 UTF-16 AND UTF-32 SUPPORT 8084 8085 In order process UTF-16 or UTF-32 strings, you must build PCRE's 16-bit 8086 or 32-bit library with UTF support, and, in addition, you must call 8087 pcre16_compile() or pcre32_compile() with the PCRE_UTF16 or PCRE_UTF32 8088 option flag, as appropriate. Alternatively, the pattern must start with 8089 the sequence (*UTF16), (*UTF32), as appropriate, or (*UTF), which can 8090 be used with either library. When UTF mode is set, both the pattern and 8091 any subject strings that are matched against it are treated as UTF-16 8092 or UTF-32 strings instead of strings of individual 16-bit or 32-bit 8093 characters. 8094 8095 8096 UTF SUPPORT OVERHEAD 8097 8098 If you compile PCRE with UTF support, but do not use it at run time, 8099 the library will be a bit bigger, but the additional run time overhead 8100 is limited to testing the PCRE_UTF[8|16|32] flag occasionally, so 8101 should not be very big. 8102 8103 8104 UNICODE PROPERTY SUPPORT 8105 8106 If PCRE is built with Unicode character property support (which implies 8107 UTF support), the escape sequences \p{..}, \P{..}, and \X can be used. 8108 The available properties that can be tested are limited to the general 8109 category properties such as Lu for an upper case letter or Nd for a 8110 decimal number, the Unicode script names such as Arabic or Han, and the 8111 derived properties Any and L&. Full lists is given in the pcrepattern 8112 and pcresyntax documentation. Only the short names for properties are 8113 supported. For example, \p{L} matches a letter. Its Perl synonym, 8114 \p{Letter}, is not supported. Furthermore, in Perl, many properties 8115 may optionally be prefixed by "Is", for compatibility with Perl 5.6. 8116 PCRE does not support this. 8117 8118 Validity of UTF-8 strings 8119 8120 When you set the PCRE_UTF8 flag, the byte strings passed as patterns 8121 and subjects are (by default) checked for validity on entry to the rel- 8122 evant functions. The entire string is checked before any other process- 8123 ing takes place. From release 7.3 of PCRE, the check is according the 8124 rules of RFC 3629, which are themselves derived from the Unicode speci- 8125 fication. Earlier releases of PCRE followed the rules of RFC 2279, 8126 which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The 8127 current check allows only values in the range U+0 to U+10FFFF, exclud- 8128 ing the surrogate area. (From release 8.33 the so-called "non-charac- 8129 ter" code points are no longer excluded because Unicode corrigendum #9 8130 makes it clear that they should not be.) 8131 8132 Characters in the "Surrogate Area" of Unicode are reserved for use by 8133 UTF-16, where they are used in pairs to encode codepoints with values 8134 greater than 0xFFFF. The code points that are encoded by UTF-16 pairs 8135 are available independently in the UTF-8 and UTF-32 encodings. (In 8136 other words, the whole surrogate thing is a fudge for UTF-16 which 8137 unfortunately messes up UTF-8 and UTF-32.) 8138 8139 If an invalid UTF-8 string is passed to PCRE, an error return is given. 8140 At compile time, the only additional information is the offset to the 8141 first byte of the failing character. The run-time functions pcre_exec() 8142 and pcre_dfa_exec() also pass back this information, as well as a more 8143 detailed reason code if the caller has provided memory in which to do 8144 this. 8145 8146 In some situations, you may already know that your strings are valid, 8147 and therefore want to skip these checks in order to improve perfor- 8148 mance, for example in the case of a long subject string that is being 8149 scanned repeatedly. If you set the PCRE_NO_UTF8_CHECK flag at compile 8150 time or at run time, PCRE assumes that the pattern or subject it is 8151 given (respectively) contains only valid UTF-8 codes. In this case, it 8152 does not diagnose an invalid UTF-8 string. 8153 8154 Note that passing PCRE_NO_UTF8_CHECK to pcre_compile() just disables 8155 the check for the pattern; it does not also apply to subject strings. 8156 If you want to disable the check for a subject string you must pass 8157 this option to pcre_exec() or pcre_dfa_exec(). 8158 8159 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, the 8160 result is undefined and your program may crash. 8161 8162 Validity of UTF-16 strings 8163 8164 When you set the PCRE_UTF16 flag, the strings of 16-bit data units that 8165 are passed as patterns and subjects are (by default) checked for valid- 8166 ity on entry to the relevant functions. Values other than those in the 8167 surrogate range U+D800 to U+DFFF are independent code points. Values in 8168 the surrogate range must be used in pairs in the correct manner. 8169 8170 If an invalid UTF-16 string is passed to PCRE, an error return is 8171 given. At compile time, the only additional information is the offset 8172 to the first data unit of the failing character. The run-time functions 8173 pcre16_exec() and pcre16_dfa_exec() also pass back this information, as 8174 well as a more detailed reason code if the caller has provided memory 8175 in which to do this. 8176 8177 In some situations, you may already know that your strings are valid, 8178 and therefore want to skip these checks in order to improve perfor- 8179 mance. If you set the PCRE_NO_UTF16_CHECK flag at compile time or at 8180 run time, PCRE assumes that the pattern or subject it is given (respec- 8181 tively) contains only valid UTF-16 sequences. In this case, it does not 8182 diagnose an invalid UTF-16 string. However, if an invalid string is 8183 passed, the result is undefined. 8184 8185 Validity of UTF-32 strings 8186 8187 When you set the PCRE_UTF32 flag, the strings of 32-bit data units that 8188 are passed as patterns and subjects are (by default) checked for valid- 8189 ity on entry to the relevant functions. This check allows only values 8190 in the range U+0 to U+10FFFF, excluding the surrogate area U+D800 to 8191 U+DFFF. 8192 8193 If an invalid UTF-32 string is passed to PCRE, an error return is 8194 given. At compile time, the only additional information is the offset 8195 to the first data unit of the failing character. The run-time functions 8196 pcre32_exec() and pcre32_dfa_exec() also pass back this information, as 8197 well as a more detailed reason code if the caller has provided memory 8198 in which to do this. 8199 8200 In some situations, you may already know that your strings are valid, 8201 and therefore want to skip these checks in order to improve perfor- 8202 mance. If you set the PCRE_NO_UTF32_CHECK flag at compile time or at 8203 run time, PCRE assumes that the pattern or subject it is given (respec- 8204 tively) contains only valid UTF-32 sequences. In this case, it does not 8205 diagnose an invalid UTF-32 string. However, if an invalid string is 8206 passed, the result is undefined. 8207 8208 General comments about UTF modes 8209 8210 1. Codepoints less than 256 can be specified in patterns by either 8211 braced or unbraced hexadecimal escape sequences (for example, \x{b3} or 8212 \xb3). Larger values have to use braced sequences. 8213 8214 2. Octal numbers up to \777 are recognized, and in UTF-8 mode they 8215 match two-byte characters for values greater than \177. 8216 8217 3. Repeat quantifiers apply to complete UTF characters, not to individ- 8218 ual data units, for example: \x{100}{3}. 8219 8220 4. The dot metacharacter matches one UTF character instead of a single 8221 data unit. 8222 8223 5. The escape sequence \C can be used to match a single byte in UTF-8 8224 mode, or a single 16-bit data unit in UTF-16 mode, or a single 32-bit 8225 data unit in UTF-32 mode, but its use can lead to some strange effects 8226 because it breaks up multi-unit characters (see the description of \C 8227 in the pcrepattern documentation). The use of \C is not supported in 8228 the alternative matching function pcre[16|32]_dfa_exec(), nor is it 8229 supported in UTF mode by the JIT optimization of pcre[16|32]_exec(). If 8230 JIT optimization is requested for a UTF pattern that contains \C, it 8231 will not succeed, and so the matching will be carried out by the normal 8232 interpretive function. 8233 8234 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly 8235 test characters of any code value, but, by default, the characters that 8236 PCRE recognizes as digits, spaces, or word characters remain the same 8237 set as in non-UTF mode, all with values less than 256. This remains 8238 true even when PCRE is built to include Unicode property support, 8239 because to do otherwise would slow down PCRE in many common cases. Note 8240 in particular that this applies to \b and \B, because they are defined 8241 in terms of \w and \W. If you really want to test for a wider sense of, 8242 say, "digit", you can use explicit Unicode property tests such as 8243 \p{Nd}. Alternatively, if you set the PCRE_UCP option, the way that the 8244 character escapes work is changed so that Unicode properties are used 8245 to determine which characters match. There are more details in the sec- 8246 tion on generic character types in the pcrepattern documentation. 8247 8248 7. Similarly, characters that match the POSIX named character classes 8249 are all low-valued characters, unless the PCRE_UCP option is set. 8250 8251 8. However, the horizontal and vertical white space matching escapes 8252 (\h, \H, \v, and \V) do match all the appropriate Unicode characters, 8253 whether or not PCRE_UCP is set. 8254 8255 9. Case-insensitive matching applies only to characters whose values 8256 are less than 128, unless PCRE is built with Unicode property support. 8257 A few Unicode characters such as Greek sigma have more than two code- 8258 points that are case-equivalent. Up to and including PCRE release 8.31, 8259 only one-to-one case mappings were supported, but later releases (with 8260 Unicode property support) do treat as case-equivalent all versions of 8261 characters such as Greek sigma. 8262 8263 8264 AUTHOR 8265 8266 Philip Hazel 8267 University Computing Service 8268 Cambridge CB2 3QH, England. 8269 8270 8271 REVISION 8272 8273 Last updated: 27 February 2013 8274 Copyright (c) 1997-2013 University of Cambridge. 8275 ------------------------------------------------------------------------------ 8276 8277 8278 PCREJIT(3) Library Functions Manual PCREJIT(3) 8279 8280 8281 8282 NAME 8283 PCRE - Perl-compatible regular expressions 8284 8285 PCRE JUST-IN-TIME COMPILER SUPPORT 8286 8287 Just-in-time compiling is a heavyweight optimization that can greatly 8288 speed up pattern matching. However, it comes at the cost of extra pro- 8289 cessing before the match is performed. Therefore, it is of most benefit 8290 when the same pattern is going to be matched many times. This does not 8291 necessarily mean many calls of a matching function; if the pattern is 8292 not anchored, matching attempts may take place many times at various 8293 positions in the subject, even for a single call. Therefore, if the 8294 subject string is very long, it may still pay to use JIT for one-off 8295 matches. 8296 8297 JIT support applies only to the traditional Perl-compatible matching 8298 function. It does not apply when the DFA matching function is being 8299 used. The code for this support was written by Zoltan Herczeg. 8300 8301 8302 8-BIT, 16-BIT AND 32-BIT SUPPORT 8303 8304 JIT support is available for all of the 8-bit, 16-bit and 32-bit PCRE 8305 libraries. To keep this documentation simple, only the 8-bit interface 8306 is described in what follows. If you are using the 16-bit library, sub- 8307 stitute the 16-bit functions and 16-bit structures (for example, 8308 pcre16_jit_stack instead of pcre_jit_stack). If you are using the 8309 32-bit library, substitute the 32-bit functions and 32-bit structures 8310 (for example, pcre32_jit_stack instead of pcre_jit_stack). 8311 8312 8313 AVAILABILITY OF JIT SUPPORT 8314 8315 JIT support is an optional feature of PCRE. The "configure" option 8316 --enable-jit (or equivalent CMake option) must be set when PCRE is 8317 built if you want to use JIT. The support is limited to the following 8318 hardware platforms: 8319 8320 ARM v5, v7, and Thumb2 8321 Intel x86 32-bit and 64-bit 8322 MIPS 32-bit 8323 Power PC 32-bit and 64-bit 8324 SPARC 32-bit (experimental) 8325 8326 If --enable-jit is set on an unsupported platform, compilation fails. 8327 8328 A program that is linked with PCRE 8.20 or later can tell if JIT sup- 8329 port is available by calling pcre_config() with the PCRE_CONFIG_JIT 8330 option. The result is 1 when JIT is available, and 0 otherwise. How- 8331 ever, a simple program does not need to check this in order to use JIT. 8332 The normal API is implemented in a way that falls back to the interpre- 8333 tive code if JIT is not available. For programs that need the best pos- 8334 sible performance, there is also a "fast path" API that is JIT-spe- 8335 cific. 8336 8337 If your program may sometimes be linked with versions of PCRE that are 8338 older than 8.20, but you want to use JIT when it is available, you can 8339 test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT 8340 macro such as PCRE_CONFIG_JIT, for compile-time control of your code. 8341 8342 8343 SIMPLE USE OF JIT 8344 8345 You have to do two things to make use of the JIT support in the sim- 8346 plest way: 8347 8348 (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for 8349 each compiled pattern, and pass the resulting pcre_extra block to 8350 pcre_exec(). 8351 8352 (2) Use pcre_free_study() to free the pcre_extra block when it is 8353 no longer needed, instead of just freeing it yourself. This 8354 ensures that 8355 any JIT data is also freed. 8356 8357 For a program that may be linked with pre-8.20 versions of PCRE, you 8358 can insert 8359 8360 #ifndef PCRE_STUDY_JIT_COMPILE 8361 #define PCRE_STUDY_JIT_COMPILE 0 8362 #endif 8363 8364 so that no option is passed to pcre_study(), and then use something 8365 like this to free the study data: 8366 8367 #ifdef PCRE_CONFIG_JIT 8368 pcre_free_study(study_ptr); 8369 #else 8370 pcre_free(study_ptr); 8371 #endif 8372 8373 PCRE_STUDY_JIT_COMPILE requests the JIT compiler to generate code for 8374 complete matches. If you want to run partial matches using the 8375 PCRE_PARTIAL_HARD or PCRE_PARTIAL_SOFT options of pcre_exec(), you 8376 should set one or both of the following options in addition to, or 8377 instead of, PCRE_STUDY_JIT_COMPILE when you call pcre_study(): 8378 8379 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 8380 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 8381 8382 The JIT compiler generates different optimized code for each of the 8383 three modes (normal, soft partial, hard partial). When pcre_exec() is 8384 called, the appropriate code is run if it is available. Otherwise, the 8385 pattern is matched using interpretive code. 8386 8387 In some circumstances you may need to call additional functions. These 8388 are described in the section entitled "Controlling the JIT stack" 8389 below. 8390 8391 If JIT support is not available, PCRE_STUDY_JIT_COMPILE etc. are 8392 ignored, and no JIT data is created. Otherwise, the compiled pattern is 8393 passed to the JIT compiler, which turns it into machine code that exe- 8394 cutes much faster than the normal interpretive code. When pcre_exec() 8395 is passed a pcre_extra block containing a pointer to JIT code of the 8396 appropriate mode (normal or hard/soft partial), it obeys that code 8397 instead of running the interpreter. The result is identical, but the 8398 compiled JIT code runs much faster. 8399 8400 There are some pcre_exec() options that are not supported for JIT exe- 8401 cution. There are also some pattern items that JIT cannot handle. 8402 Details are given below. In both cases, execution automatically falls 8403 back to the interpretive code. If you want to know whether JIT was 8404 actually used for a particular match, you should arrange for a JIT 8405 callback function to be set up as described in the section entitled 8406 "Controlling the JIT stack" below, even if you do not need to supply a 8407 non-default JIT stack. Such a callback function is called whenever JIT 8408 code is about to be obeyed. If the execution options are not right for 8409 JIT execution, the callback function is not obeyed. 8410 8411 If the JIT compiler finds an unsupported item, no JIT data is gener- 8412 ated. You can find out if JIT execution is available after studying a 8413 pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A 8414 result of 1 means that JIT compilation was successful. A result of 0 8415 means that JIT support is not available, or the pattern was not studied 8416 with PCRE_STUDY_JIT_COMPILE etc., or the JIT compiler was not able to 8417 handle the pattern. 8418 8419 Once a pattern has been studied, with or without JIT, it can be used as 8420 many times as you like for matching different subject strings. 8421 8422 8423 UNSUPPORTED OPTIONS AND PATTERN ITEMS 8424 8425 The only pcre_exec() options that are supported for JIT execution are 8426 PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK, PCRE_NO_UTF32_CHECK, PCRE_NOT- 8427 BOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, PCRE_PAR- 8428 TIAL_HARD, and PCRE_PARTIAL_SOFT. 8429 8430 The only unsupported pattern items are \C (match a single data unit) 8431 when running in a UTF mode, and a callout immediately before an asser- 8432 tion condition in a conditional group. 8433 8434 8435 RETURN VALUES FROM JIT EXECUTION 8436 8437 When a pattern is matched using JIT execution, the return values are 8438 the same as those given by the interpretive pcre_exec() code, with the 8439 addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means 8440 that the memory used for the JIT stack was insufficient. See "Control- 8441 ling the JIT stack" below for a discussion of JIT stack usage. For com- 8442 patibility with the interpretive pcre_exec() code, no more than two- 8443 thirds of the ovector argument is used for passing back captured sub- 8444 strings. 8445 8446 The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if 8447 searching a very large pattern tree goes on for too long, as it is in 8448 the same circumstance when JIT is not used, but the details of exactly 8449 what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error 8450 code is never returned by JIT execution. 8451 8452 8453 SAVING AND RESTORING COMPILED PATTERNS 8454 8455 The code that is generated by the JIT compiler is architecture-spe- 8456 cific, and is also position dependent. For those reasons it cannot be 8457 saved (in a file or database) and restored later like the bytecode and 8458 other data of a compiled pattern. Saving and restoring compiled pat- 8459 terns is not something many people do. More detail about this facility 8460 is given in the pcreprecompile documentation. It should be possible to 8461 run pcre_study() on a saved and restored pattern, and thereby recreate 8462 the JIT data, but because JIT compilation uses significant resources, 8463 it is probably not worth doing this; you might as well recompile the 8464 original pattern. 8465 8466 8467 CONTROLLING THE JIT STACK 8468 8469 When the compiled JIT code runs, it needs a block of memory to use as a 8470 stack. By default, it uses 32K on the machine stack. However, some 8471 large or complicated patterns need more than this. The error 8472 PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack. 8473 Three functions are provided for managing blocks of memory for use as 8474 JIT stacks. There is further discussion about the use of JIT stacks in 8475 the section entitled "JIT stack FAQ" below. 8476 8477 The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments 8478 are a starting size and a maximum size, and it returns a pointer to an 8479 opaque structure of type pcre_jit_stack, or NULL if there is an error. 8480 The pcre_jit_stack_free() function can be used to free a stack that is 8481 no longer needed. (For the technically minded: the address space is 8482 allocated by mmap or VirtualAlloc.) 8483 8484 JIT uses far less memory for recursion than the interpretive code, and 8485 a maximum stack size of 512K to 1M should be more than enough for any 8486 pattern. 8487 8488 The pcre_assign_jit_stack() function specifies which stack JIT code 8489 should use. Its arguments are as follows: 8490 8491 pcre_extra *extra 8492 pcre_jit_callback callback 8493 void *data 8494 8495 The extra argument must be the result of studying a pattern with 8496 PCRE_STUDY_JIT_COMPILE etc. There are three cases for the values of the 8497 other two options: 8498 8499 (1) If callback is NULL and data is NULL, an internal 32K block 8500 on the machine stack is used. 8501 8502 (2) If callback is NULL and data is not NULL, data must be 8503 a valid JIT stack, the result of calling pcre_jit_stack_alloc(). 8504 8505 (3) If callback is not NULL, it must point to a function that is 8506 called with data as an argument at the start of matching, in 8507 order to set up a JIT stack. If the return from the callback 8508 function is NULL, the internal 32K stack is used; otherwise the 8509 return value must be a valid JIT stack, the result of calling 8510 pcre_jit_stack_alloc(). 8511 8512 A callback function is obeyed whenever JIT code is about to be run; it 8513 is not obeyed when pcre_exec() is called with options that are incom- 8514 patible for JIT execution. A callback function can therefore be used to 8515 determine whether a match operation was executed by JIT or by the 8516 interpreter. 8517 8518 You may safely use the same JIT stack for more than one pattern (either 8519 by assigning directly or by callback), as long as the patterns are all 8520 matched sequentially in the same thread. In a multithread application, 8521 if you do not specify a JIT stack, or if you assign or pass back NULL 8522 from a callback, that is thread-safe, because each thread has its own 8523 machine stack. However, if you assign or pass back a non-NULL JIT 8524 stack, this must be a different stack for each thread so that the 8525 application is thread-safe. 8526 8527 Strictly speaking, even more is allowed. You can assign the same non- 8528 NULL stack to any number of patterns as long as they are not used for 8529 matching by multiple threads at the same time. For example, you can 8530 assign the same stack to all compiled patterns, and use a global mutex 8531 in the callback to wait until the stack is available for use. However, 8532 this is an inefficient solution, and not recommended. 8533 8534 This is a suggestion for how a multithreaded program that needs to set 8535 up non-default JIT stacks might operate: 8536 8537 During thread initalization 8538 thread_local_var = pcre_jit_stack_alloc(...) 8539 8540 During thread exit 8541 pcre_jit_stack_free(thread_local_var) 8542 8543 Use a one-line callback function 8544 return thread_local_var 8545 8546 All the functions described in this section do nothing if JIT is not 8547 available, and pcre_assign_jit_stack() does nothing unless the extra 8548 argument is non-NULL and points to a pcre_extra block that is the 8549 result of a successful study with PCRE_STUDY_JIT_COMPILE etc. 8550 8551 8552 JIT STACK FAQ 8553 8554 (1) Why do we need JIT stacks? 8555 8556 PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack 8557 where the local data of the current node is pushed before checking its 8558 child nodes. Allocating real machine stack on some platforms is diffi- 8559 cult. For example, the stack chain needs to be updated every time if we 8560 extend the stack on PowerPC. Although it is possible, its updating 8561 time overhead decreases performance. So we do the recursion in memory. 8562 8563 (2) Why don't we simply allocate blocks of memory with malloc()? 8564 8565 Modern operating systems have a nice feature: they can reserve an 8566 address space instead of allocating memory. We can safely allocate mem- 8567 ory pages inside this address space, so the stack could grow without 8568 moving memory data (this is important because of pointers). Thus we can 8569 allocate 1M address space, and use only a single memory page (usually 8570 4K) if that is enough. However, we can still grow up to 1M anytime if 8571 needed. 8572 8573 (3) Who "owns" a JIT stack? 8574 8575 The owner of the stack is the user program, not the JIT studied pattern 8576 or anything else. The user program must ensure that if a stack is used 8577 by pcre_exec(), (that is, it is assigned to the pattern currently run- 8578 ning), that stack must not be used by any other threads (to avoid over- 8579 writing the same memory area). The best practice for multithreaded pro- 8580 grams is to allocate a stack for each thread, and return this stack 8581 through the JIT callback function. 8582 8583 (4) When should a JIT stack be freed? 8584 8585 You can free a JIT stack at any time, as long as it will not be used by 8586 pcre_exec() again. When you assign the stack to a pattern, only a 8587 pointer is set. There is no reference counting or any other magic. You 8588 can free the patterns and stacks in any order, anytime. Just do not 8589 call pcre_exec() with a pattern pointing to an already freed stack, as 8590 that will cause SEGFAULT. (Also, do not free a stack currently used by 8591 pcre_exec() in another thread). You can also replace the stack for a 8592 pattern at any time. You can even free the previous stack before 8593 assigning a replacement. 8594 8595 (5) Should I allocate/free a stack every time before/after calling 8596 pcre_exec()? 8597 8598 No, because this is too costly in terms of resources. However, you 8599 could implement some clever idea which release the stack if it is not 8600 used in let's say two minutes. The JIT callback can help to achieve 8601 this without keeping a list of the currently JIT studied patterns. 8602 8603 (6) OK, the stack is for long term memory allocation. But what happens 8604 if a pattern causes stack overflow with a stack of 1M? Is that 1M kept 8605 until the stack is freed? 8606 8607 Especially on embedded sytems, it might be a good idea to release mem- 8608 ory sometimes without freeing the stack. There is no API for this at 8609 the moment. Probably a function call which returns with the currently 8610 allocated memory for any stack and another which allows releasing mem- 8611 ory (shrinking the stack) would be a good idea if someone needs this. 8612 8613 (7) This is too much of a headache. Isn't there any better solution for 8614 JIT stack handling? 8615 8616 No, thanks to Windows. If POSIX threads were used everywhere, we could 8617 throw out this complicated API. 8618 8619 8620 EXAMPLE CODE 8621 8622 This is a single-threaded example that specifies a JIT stack without 8623 using a callback. 8624 8625 int rc; 8626 int ovector[30]; 8627 pcre *re; 8628 pcre_extra *extra; 8629 pcre_jit_stack *jit_stack; 8630 8631 re = pcre_compile(pattern, 0, &error, &erroffset, NULL); 8632 /* Check for errors */ 8633 extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error); 8634 jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024); 8635 /* Check for error (NULL) */ 8636 pcre_assign_jit_stack(extra, NULL, jit_stack); 8637 rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30); 8638 /* Check results */ 8639 pcre_free(re); 8640 pcre_free_study(extra); 8641 pcre_jit_stack_free(jit_stack); 8642 8643 8644 JIT FAST PATH API 8645 8646 Because the API described above falls back to interpreted execution 8647 when JIT is not available, it is convenient for programs that are writ- 8648 ten for general use in many environments. However, calling JIT via 8649 pcre_exec() does have a performance impact. Programs that are written 8650 for use where JIT is known to be available, and which need the best 8651 possible performance, can instead use a "fast path" API to call JIT 8652 execution directly instead of calling pcre_exec() (obviously only for 8653 patterns that have been successfully studied by JIT). 8654 8655 The fast path function is called pcre_jit_exec(), and it takes exactly 8656 the same arguments as pcre_exec(), plus one additional argument that 8657 must point to a JIT stack. The JIT stack arrangements described above 8658 do not apply. The return values are the same as for pcre_exec(). 8659 8660 When you call pcre_exec(), as well as testing for invalid options, a 8661 number of other sanity checks are performed on the arguments. For exam- 8662 ple, if the subject pointer is NULL, or its length is negative, an 8663 immediate error is given. Also, unless PCRE_NO_UTF[8|16|32] is set, a 8664 UTF subject string is tested for validity. In the interests of speed, 8665 these checks do not happen on the JIT fast path, and if invalid data is 8666 passed, the result is undefined. 8667 8668 Bypassing the sanity checks and the pcre_exec() wrapping can give 8669 speedups of more than 10%. 8670 8671 8672 SEE ALSO 8673 8674 pcreapi(3) 8675 8676 8677 AUTHOR 8678 8679 Philip Hazel (FAQ by Zoltan Herczeg) 8680 University Computing Service 8681 Cambridge CB2 3QH, England. 8682 8683 8684 REVISION 8685 8686 Last updated: 17 March 2013 8687 Copyright (c) 1997-2013 University of Cambridge. 8688 ------------------------------------------------------------------------------ 8689 8690 8691 PCREPARTIAL(3) Library Functions Manual PCREPARTIAL(3) 8692 8693 8694 8695 NAME 8696 PCRE - Perl-compatible regular expressions 8697 8698 PARTIAL MATCHING IN PCRE 8699 8700 In normal use of PCRE, if the subject string that is passed to a match- 8701 ing function matches as far as it goes, but is too short to match the 8702 entire pattern, PCRE_ERROR_NOMATCH is returned. There are circumstances 8703 where it might be helpful to distinguish this case from other cases in 8704 which there is no match. 8705 8706 Consider, for example, an application where a human is required to type 8707 in data for a field with specific formatting requirements. An example 8708 might be a date in the form ddmmmyy, defined by this pattern: 8709 8710 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ 8711 8712 If the application sees the user's keystrokes one by one, and can check 8713 that what has been typed so far is potentially valid, it is able to 8714 raise an error as soon as a mistake is made, by beeping and not 8715 reflecting the character that has been typed, for example. This immedi- 8716 ate feedback is likely to be a better user interface than a check that 8717 is delayed until the entire string has been entered. Partial matching 8718 can also be useful when the subject string is very long and is not all 8719 available at once. 8720 8721 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and 8722 PCRE_PARTIAL_HARD options, which can be set when calling any of the 8723 matching functions. For backwards compatibility, PCRE_PARTIAL is a syn- 8724 onym for PCRE_PARTIAL_SOFT. The essential difference between the two 8725 options is whether or not a partial match is preferred to an alterna- 8726 tive complete match, though the details differ between the two types of 8727 matching function. If both options are set, PCRE_PARTIAL_HARD takes 8728 precedence. 8729 8730 If you want to use partial matching with just-in-time optimized code, 8731 you must call pcre_study(), pcre16_study() or pcre32_study() with one 8732 or both of these options: 8733 8734 PCRE_STUDY_JIT_PARTIAL_SOFT_COMPILE 8735 PCRE_STUDY_JIT_PARTIAL_HARD_COMPILE 8736 8737 PCRE_STUDY_JIT_COMPILE should also be set if you are going to run non- 8738 partial matches on the same pattern. If the appropriate JIT study mode 8739 has not been set for a match, the interpretive matching code is used. 8740 8741 Setting a partial matching option disables two of PCRE's standard opti- 8742 mizations. PCRE remembers the last literal data unit in a pattern, and 8743 abandons matching immediately if it is not present in the subject 8744 string. This optimization cannot be used for a subject string that 8745 might match only partially. If the pattern was studied, PCRE knows the 8746 minimum length of a matching string, and does not bother to run the 8747 matching function on shorter strings. This optimization is also dis- 8748 abled for partial matching. 8749 8750 8751 PARTIAL MATCHING USING pcre_exec() OR pcre[16|32]_exec() 8752 8753 A partial match occurs during a call to pcre_exec() or 8754 pcre[16|32]_exec() when the end of the subject string is reached suc- 8755 cessfully, but matching cannot continue because more characters are 8756 needed. However, at least one character in the subject must have been 8757 inspected. This character need not form part of the final matched 8758 string; lookbehind assertions and the \K escape sequence provide ways 8759 of inspecting characters before the start of a matched substring. The 8760 requirement for inspecting at least one character exists because an 8761 empty string can always be matched; without such a restriction there 8762 would always be a partial match of an empty string at the end of the 8763 subject. 8764 8765 If there are at least two slots in the offsets vector when a partial 8766 match is returned, the first slot is set to the offset of the earliest 8767 character that was inspected. For convenience, the second offset points 8768 to the end of the subject so that a substring can easily be identified. 8769 If there are at least three slots in the offsets vector, the third slot 8770 is set to the offset of the character where matching started. 8771 8772 For the majority of patterns, the contents of the first and third slots 8773 will be the same. However, for patterns that contain lookbehind asser- 8774 tions, or begin with \b or \B, characters before the one where matching 8775 started may have been inspected while carrying out the match. For exam- 8776 ple, consider this pattern: 8777 8778 /(?<=abc)123/ 8779 8780 This pattern matches "123", but only if it is preceded by "abc". If the 8781 subject string is "xyzabc12", the first two offsets after a partial 8782 match are for the substring "abc12", because all these characters were 8783 inspected. However, the third offset is set to 6, because that is the 8784 offset where matching began. 8785 8786 What happens when a partial match is identified depends on which of the 8787 two partial matching options are set. 8788 8789 PCRE_PARTIAL_SOFT WITH pcre_exec() OR pcre[16|32]_exec() 8790 8791 If PCRE_PARTIAL_SOFT is set when pcre_exec() or pcre[16|32]_exec() 8792 identifies a partial match, the partial match is remembered, but match- 8793 ing continues as normal, and other alternatives in the pattern are 8794 tried. If no complete match can be found, PCRE_ERROR_PARTIAL is 8795 returned instead of PCRE_ERROR_NOMATCH. 8796 8797 This option is "soft" because it prefers a complete match over a par- 8798 tial match. All the various matching items in a pattern behave as if 8799 the subject string is potentially complete. For example, \z, \Z, and $ 8800 match at the end of the subject, as normal, and for \b and \B the end 8801 of the subject is treated as a non-alphanumeric. 8802 8803 If there is more than one partial match, the first one that was found 8804 provides the data that is returned. Consider this pattern: 8805 8806 /123\w+X|dogY/ 8807 8808 If this is matched against the subject string "abc123dog", both alter- 8809 natives fail to match, but the end of the subject is reached during 8810 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 8811 and 9, identifying "123dog" as the first partial match that was found. 8812 (In this example, there are two partial matches, because "dog" on its 8813 own partially matches the second alternative.) 8814 8815 PCRE_PARTIAL_HARD WITH pcre_exec() OR pcre[16|32]_exec() 8816 8817 If PCRE_PARTIAL_HARD is set for pcre_exec() or pcre[16|32]_exec(), 8818 PCRE_ERROR_PARTIAL is returned as soon as a partial match is found, 8819 without continuing to search for possible complete matches. This option 8820 is "hard" because it prefers an earlier partial match over a later com- 8821 plete match. For this reason, the assumption is made that the end of 8822 the supplied subject string may not be the true end of the available 8823 data, and so, if \z, \Z, \b, \B, or $ are encountered at the end of the 8824 subject, the result is PCRE_ERROR_PARTIAL, provided that at least one 8825 character in the subject has been inspected. 8826 8827 Setting PCRE_PARTIAL_HARD also affects the way UTF-8 and UTF-16 subject 8828 strings are checked for validity. Normally, an invalid sequence causes 8829 the error PCRE_ERROR_BADUTF8 or PCRE_ERROR_BADUTF16. However, in the 8830 special case of a truncated character at the end of the subject, 8831 PCRE_ERROR_SHORTUTF8 or PCRE_ERROR_SHORTUTF16 is returned when 8832 PCRE_PARTIAL_HARD is set. 8833 8834 Comparing hard and soft partial matching 8835 8836 The difference between the two partial matching options can be illus- 8837 trated by a pattern such as: 8838 8839 /dog(sbody)?/ 8840 8841 This matches either "dog" or "dogsbody", greedily (that is, it prefers 8842 the longer string if possible). If it is matched against the string 8843 "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". 8844 However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. 8845 On the other hand, if the pattern is made ungreedy the result is dif- 8846 ferent: 8847 8848 /dog(sbody)??/ 8849 8850 In this case the result is always a complete match because that is 8851 found first, and matching never continues after finding a complete 8852 match. It might be easier to follow this explanation by thinking of the 8853 two patterns like this: 8854 8855 /dog(sbody)?/ is the same as /dogsbody|dog/ 8856 /dog(sbody)??/ is the same as /dog|dogsbody/ 8857 8858 The second pattern will never match "dogsbody", because it will always 8859 find the shorter match first. 8860 8861 8862 PARTIAL MATCHING USING pcre_dfa_exec() OR pcre[16|32]_dfa_exec() 8863 8864 The DFA functions move along the subject string character by character, 8865 without backtracking, searching for all possible matches simultane- 8866 ously. If the end of the subject is reached before the end of the pat- 8867 tern, there is the possibility of a partial match, again provided that 8868 at least one character has been inspected. 8869 8870 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if 8871 there have been no complete matches. Otherwise, the complete matches 8872 are returned. However, if PCRE_PARTIAL_HARD is set, a partial match 8873 takes precedence over any complete matches. The portion of the string 8874 that was inspected when the longest partial match was found is set as 8875 the first matching string, provided there are at least two slots in the 8876 offsets vector. 8877 8878 Because the DFA functions always search for all possible matches, and 8879 there is no difference between greedy and ungreedy repetition, their 8880 behaviour is different from the standard functions when PCRE_PAR- 8881 TIAL_HARD is set. Consider the string "dog" matched against the 8882 ungreedy pattern shown above: 8883 8884 /dog(sbody)??/ 8885 8886 Whereas the standard functions stop as soon as they find the complete 8887 match for "dog", the DFA functions also find the partial match for 8888 "dogsbody", and so return that when PCRE_PARTIAL_HARD is set. 8889 8890 8891 PARTIAL MATCHING AND WORD BOUNDARIES 8892 8893 If a pattern ends with one of sequences \b or \B, which test for word 8894 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter- 8895 intuitive results. Consider this pattern: 8896 8897 /\bcat\b/ 8898 8899 This matches "cat", provided there is a word boundary at either end. If 8900 the subject string is "the cat", the comparison of the final "t" with a 8901 following character cannot take place, so a partial match is found. 8902 However, normal matching carries on, and \b matches at the end of the 8903 subject when the last character is a letter, so a complete match is 8904 found. The result, therefore, is not PCRE_ERROR_PARTIAL. Using 8905 PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, because 8906 then the partial match takes precedence. 8907 8908 8909 FORMERLY RESTRICTED PATTERNS 8910 8911 For releases of PCRE prior to 8.00, because of the way certain internal 8912 optimizations were implemented in the pcre_exec() function, the 8913 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be 8914 used with all patterns. From release 8.00 onwards, the restrictions no 8915 longer apply, and partial matching with can be requested for any pat- 8916 tern. 8917 8918 Items that were formerly restricted were repeated single characters and 8919 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did 8920 not conform to the restrictions, pcre_exec() returned the error code 8921 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The 8922 PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled 8923 pattern can be used for partial matching now always returns 1. 8924 8925 8926 EXAMPLE OF PARTIAL MATCHING USING PCRETEST 8927 8928 If the escape sequence \P is present in a pcretest data line, the 8929 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of 8930 pcretest that uses the date example quoted above: 8931 8932 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 8933 data> 25jun04\P 8934 0: 25jun04 8935 1: jun 8936 data> 25dec3\P 8937 Partial match: 23dec3 8938 data> 3ju\P 8939 Partial match: 3ju 8940 data> 3juj\P 8941 No match 8942 data> j\P 8943 No match 8944 8945 The first data string is matched completely, so pcretest shows the 8946 matched substrings. The remaining four strings do not match the com- 8947 plete pattern, but the first two are partial matches. Similar output is 8948 obtained if DFA matching is used. 8949 8950 If the escape sequence \P is present more than once in a pcretest data 8951 line, the PCRE_PARTIAL_HARD option is set for the match. 8952 8953 8954 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() OR pcre[16|32]_dfa_exec() 8955 8956 When a partial match has been found using a DFA matching function, it 8957 is possible to continue the match by providing additional subject data 8958 and calling the function again with the same compiled regular expres- 8959 sion, this time setting the PCRE_DFA_RESTART option. You must pass the 8960 same working space as before, because this is where details of the pre- 8961 vious partial match are stored. Here is an example using pcretest, 8962 using the \R escape sequence to set the PCRE_DFA_RESTART option (\D 8963 specifies the use of the DFA matching function): 8964 8965 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 8966 data> 23ja\P\D 8967 Partial match: 23ja 8968 data> n05\R\D 8969 0: n05 8970 8971 The first call has "23ja" as the subject, and requests partial match- 8972 ing; the second call has "n05" as the subject for the continued 8973 (restarted) match. Notice that when the match is complete, only the 8974 last part is shown; PCRE does not retain the previously partially- 8975 matched string. It is up to the calling program to do that if it needs 8976 to. 8977 8978 That means that, for an unanchored pattern, if a continued match fails, 8979 it is not possible to try again at a new starting point. All this 8980 facility is capable of doing is continuing with the previous match 8981 attempt. In the previous example, if the second set of data is "ug23" 8982 the result is no match, even though there would be a match for "aug23" 8983 if the entire string were given at once. Depending on the application, 8984 this may or may not be what you want. The only way to allow for start- 8985 ing again at the next character is to retain the matched part of the 8986 subject and try a new complete match. 8987 8988 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with 8989 PCRE_DFA_RESTART to continue partial matching over multiple segments. 8990 This facility can be used to pass very long subject strings to the DFA 8991 matching functions. 8992 8993 8994 MULTI-SEGMENT MATCHING WITH pcre_exec() OR pcre[16|32]_exec() 8995 8996 From release 8.00, the standard matching functions can also be used to 8997 do multi-segment matching. Unlike the DFA functions, it is not possible 8998 to restart the previous match with a new segment of data. Instead, new 8999 data must be added to the previous subject string, and the entire match 9000 re-run, starting from the point where the partial match occurred. Ear- 9001 lier data can be discarded. 9002 9003 It is best to use PCRE_PARTIAL_HARD in this situation, because it does 9004 not treat the end of a segment as the end of the subject when matching 9005 \z, \Z, \b, \B, and $. Consider an unanchored pattern that matches 9006 dates: 9007 9008 re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ 9009 data> The date is 23ja\P\P 9010 Partial match: 23ja 9011 9012 At this stage, an application could discard the text preceding "23ja", 9013 add on text from the next segment, and call the matching function 9014 again. Unlike the DFA matching functions, the entire matching string 9015 must always be available, and the complete matching process occurs for 9016 each call, so more memory and more processing time is needed. 9017 9018 Note: If the pattern contains lookbehind assertions, or \K, or starts 9019 with \b or \B, the string that is returned for a partial match includes 9020 characters that precede the start of what would be returned for a com- 9021 plete match, because it contains all the characters that were inspected 9022 during the partial match. 9023 9024 9025 ISSUES WITH MULTI-SEGMENT MATCHING 9026 9027 Certain types of pattern may give problems with multi-segment matching, 9028 whichever matching function is used. 9029 9030 1. If the pattern contains a test for the beginning of a line, you need 9031 to pass the PCRE_NOTBOL option when the subject string for any call 9032 does start at the beginning of a line. There is also a PCRE_NOTEOL 9033 option, but in practice when doing multi-segment matching you should be 9034 using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL. 9035 9036 2. Lookbehind assertions that have already been obeyed are catered for 9037 in the offsets that are returned for a partial match. However a lookbe- 9038 hind assertion later in the pattern could require even earlier charac- 9039 ters to be inspected. You can handle this case by using the 9040 PCRE_INFO_MAXLOOKBEHIND option of the pcre_fullinfo() or 9041 pcre[16|32]_fullinfo() functions to obtain the length of the longest 9042 lookbehind in the pattern. This length is given in characters, not 9043 bytes. If you always retain at least that many characters before the 9044 partially matched string, all should be well. (Of course, near the 9045 start of the subject, fewer characters may be present; in that case all 9046 characters should be retained.) 9047 9048 From release 8.33, there is a more accurate way of deciding which char- 9049 acters to retain. Instead of subtracting the length of the longest 9050 lookbehind from the earliest inspected character (offsets[0]), the 9051 match start position (offsets[2]) should be used, and the next match 9052 attempt started at the offsets[2] character by setting the startoffset 9053 argument of pcre_exec() or pcre_dfa_exec(). 9054 9055 For example, if the pattern "(?<=123)abc" is partially matched against 9056 the string "xx123a", the three offset values returned are 2, 6, and 5. 9057 This indicates that the matching process that gave a partial match 9058 started at offset 5, but the characters "123a" were all inspected. The 9059 maximum lookbehind for that pattern is 3, so taking that away from 5 9060 shows that we need only keep "123a", and the next match attempt can be 9061 started at offset 3 (that is, at "a") when further characters have been 9062 added. When the match start is not the earliest inspected character, 9063 pcretest shows it explicitly: 9064 9065 re> "(?<=123)abc" 9066 data> xx123a\P\P 9067 Partial match at offset 5: 123a 9068 9069 3. Because a partial match must always contain at least one character, 9070 what might be considered a partial match of an empty string actually 9071 gives a "no match" result. For example: 9072 9073 re> /c(?<=abc)x/ 9074 data> ab\P 9075 No match 9076 9077 If the next segment begins "cx", a match should be found, but this will 9078 only happen if characters from the previous segment are retained. For 9079 this reason, a "no match" result should be interpreted as "partial 9080 match of an empty string" when the pattern contains lookbehinds. 9081 9082 4. Matching a subject string that is split into multiple segments may 9083 not always produce exactly the same result as matching over one single 9084 long string, especially when PCRE_PARTIAL_SOFT is used. The section 9085 "Partial Matching and Word Boundaries" above describes an issue that 9086 arises if the pattern ends with \b or \B. Another kind of difference 9087 may occur when there are multiple matching possibilities, because (for 9088 PCRE_PARTIAL_SOFT) a partial match result is given only when there are 9089 no completed matches. This means that as soon as the shortest match has 9090 been found, continuation to a new subject segment is no longer possi- 9091 ble. Consider again this pcretest example: 9092 9093 re> /dog(sbody)?/ 9094 data> dogsb\P 9095 0: dog 9096 data> do\P\D 9097 Partial match: do 9098 data> gsb\R\P\D 9099 0: g 9100 data> dogsbody\D 9101 0: dogsbody 9102 1: dog 9103 9104 The first data line passes the string "dogsb" to a standard matching 9105 function, setting the PCRE_PARTIAL_SOFT option. Although the string is 9106 a partial match for "dogsbody", the result is not PCRE_ERROR_PARTIAL, 9107 because the shorter string "dog" is a complete match. Similarly, when 9108 the subject is presented to a DFA matching function in several parts 9109 ("do" and "gsb" being the first two) the match stops when "dog" has 9110 been found, and it is not possible to continue. On the other hand, if 9111 "dogsbody" is presented as a single string, a DFA matching function 9112 finds both matches. 9113 9114 Because of these problems, it is best to use PCRE_PARTIAL_HARD when 9115 matching multi-segment data. The example above then behaves differ- 9116 ently: 9117 9118 re> /dog(sbody)?/ 9119 data> dogsb\P\P 9120 Partial match: dogsb 9121 data> do\P\D 9122 Partial match: do 9123 data> gsb\R\P\P\D 9124 Partial match: gsb 9125 9126 5. Patterns that contain alternatives at the top level which do not all 9127 start with the same pattern item may not work as expected when 9128 PCRE_DFA_RESTART is used. For example, consider this pattern: 9129 9130 1234|3789 9131 9132 If the first part of the subject is "ABC123", a partial match of the 9133 first alternative is found at offset 3. There is no partial match for 9134 the second alternative, because such a match does not start at the same 9135 point in the subject string. Attempting to continue with the string 9136 "7890" does not yield a match because only those alternatives that 9137 match at one point in the subject are remembered. The problem arises 9138 because the start of the second alternative matches within the first 9139 alternative. There is no problem with anchored patterns or patterns 9140 such as: 9141 9142 1234|ABCD 9143 9144 where no string can be a partial match for both alternatives. This is 9145 not a problem if a standard matching function is used, because the 9146 entire match has to be rerun each time: 9147 9148 re> /1234|3789/ 9149 data> ABC123\P\P 9150 Partial match: 123 9151 data> 1237890 9152 0: 3789 9153 9154 Of course, instead of using PCRE_DFA_RESTART, the same technique of re- 9155 running the entire match can also be used with the DFA matching func- 9156 tions. Another possibility is to work with two buffers. If a partial 9157 match at offset n in the first buffer is followed by "no match" when 9158 PCRE_DFA_RESTART is used on the second buffer, you can then try a new 9159 match starting at offset n+1 in the first buffer. 9160 9161 9162 AUTHOR 9163 9164 Philip Hazel 9165 University Computing Service 9166 Cambridge CB2 3QH, England. 9167 9168 9169 REVISION 9170 9171 Last updated: 02 July 2013 9172 Copyright (c) 1997-2013 University of Cambridge. 9173 ------------------------------------------------------------------------------ 9174 9175 9176 PCREPRECOMPILE(3) Library Functions Manual PCREPRECOMPILE(3) 9177 9178 9179 9180 NAME 9181 PCRE - Perl-compatible regular expressions 9182 9183 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS 9184 9185 If you are running an application that uses a large number of regular 9186 expression patterns, it may be useful to store them in a precompiled 9187 form instead of having to compile them every time the application is 9188 run. If you are not using any private character tables (see the 9189 pcre_maketables() documentation), this is relatively straightforward. 9190 If you are using private tables, it is a little bit more complicated. 9191 However, if you are using the just-in-time optimization feature, it is 9192 not possible to save and reload the JIT data. 9193 9194 If you save compiled patterns to a file, you can copy them to a differ- 9195 ent host and run them there. If the two hosts have different endianness 9196 (byte order), you should run the pcre[16|32]_pat- 9197 tern_to_host_byte_order() function on the new host before trying to 9198 match the pattern. The matching functions return PCRE_ERROR_BADENDIAN- 9199 NESS if they detect a pattern with the wrong endianness. 9200 9201 Compiling regular expressions with one version of PCRE for use with a 9202 different version is not guaranteed to work and may cause crashes, and 9203 saving and restoring a compiled pattern loses any JIT optimization 9204 data. 9205 9206 9207 SAVING A COMPILED PATTERN 9208 9209 The value returned by pcre[16|32]_compile() points to a single block of 9210 memory that holds the compiled pattern and associated data. You can 9211 find the length of this block in bytes by calling 9212 pcre[16|32]_fullinfo() with an argument of PCRE_INFO_SIZE. You can then 9213 save the data in any appropriate manner. Here is sample code for the 9214 8-bit library that compiles a pattern and writes it to a file. It 9215 assumes that the variable fd refers to a file that is open for output: 9216 9217 int erroroffset, rc, size; 9218 char *error; 9219 pcre *re; 9220 9221 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); 9222 if (re == NULL) { ... handle errors ... } 9223 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); 9224 if (rc < 0) { ... handle errors ... } 9225 rc = fwrite(re, 1, size, fd); 9226 if (rc != size) { ... handle errors ... } 9227 9228 In this example, the bytes that comprise the compiled pattern are 9229 copied exactly. Note that this is binary data that may contain any of 9230 the 256 possible byte values. On systems that make a distinction 9231 between binary and non-binary data, be sure that the file is opened for 9232 binary output. 9233 9234 If you want to write more than one pattern to a file, you will have to 9235 devise a way of separating them. For binary data, preceding each pat- 9236 tern with its length is probably the most straightforward approach. 9237 Another possibility is to write out the data in hexadecimal instead of 9238 binary, one pattern to a line. 9239 9240 Saving compiled patterns in a file is only one possible way of storing 9241 them for later use. They could equally well be saved in a database, or 9242 in the memory of some daemon process that passes them via sockets to 9243 the processes that want them. 9244 9245 If the pattern has been studied, it is also possible to save the normal 9246 study data in a similar way to the compiled pattern itself. However, if 9247 the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre- 9248 ated cannot be saved because it is too dependent on the current envi- 9249 ronment. When studying generates additional information, 9250 pcre[16|32]_study() returns a pointer to a pcre[16|32]_extra data 9251 block. Its format is defined in the section on matching a pattern in 9252 the pcreapi documentation. The study_data field points to the binary 9253 study data, and this is what you must save (not the pcre[16|32]_extra 9254 block itself). The length of the study data can be obtained by calling 9255 pcre[16|32]_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. Remem- 9256 ber to check that pcre[16|32]_study() did return a non-NULL value 9257 before trying to save the study data. 9258 9259 9260 RE-USING A PRECOMPILED PATTERN 9261 9262 Re-using a precompiled pattern is straightforward. Having reloaded it 9263 into main memory, called pcre[16|32]_pattern_to_host_byte_order() if 9264 necessary, you pass its pointer to pcre[16|32]_exec() or 9265 pcre[16|32]_dfa_exec() in the usual way. 9266 9267 However, if you passed a pointer to custom character tables when the 9268 pattern was compiled (the tableptr argument of pcre[16|32]_compile()), 9269 you must now pass a similar pointer to pcre[16|32]_exec() or 9270 pcre[16|32]_dfa_exec(), because the value saved with the compiled pat- 9271 tern will obviously be nonsense. A field in a pcre[16|32]_extra() block 9272 is used to pass this data, as described in the section on matching a 9273 pattern in the pcreapi documentation. 9274 9275 Warning: The tables that pcre_exec() and pcre_dfa_exec() use must be 9276 the same as those that were used when the pattern was compiled. If this 9277 is not the case, the behaviour is undefined. 9278 9279 If you did not provide custom character tables when the pattern was 9280 compiled, the pointer in the compiled pattern is NULL, which causes the 9281 matching functions to use PCRE's internal tables. Thus, you do not need 9282 to take any special action at run time in this case. 9283 9284 If you saved study data with the compiled pattern, you need to create 9285 your own pcre[16|32]_extra data block and set the study_data field to 9286 point to the reloaded study data. You must also set the 9287 PCRE_EXTRA_STUDY_DATA bit in the flags field to indicate that study 9288 data is present. Then pass the pcre[16|32]_extra block to the matching 9289 function in the usual way. If the pattern was studied for just-in-time 9290 optimization, that data cannot be saved, and so is lost by a 9291 save/restore cycle. 9292 9293 9294 COMPATIBILITY WITH DIFFERENT PCRE RELEASES 9295 9296 In general, it is safest to recompile all saved patterns when you 9297 update to a new PCRE release, though not all updates actually require 9298 this. 9299 9300 9301 AUTHOR 9302 9303 Philip Hazel 9304 University Computing Service 9305 Cambridge CB2 3QH, England. 9306 9307 9308 REVISION 9309 9310 Last updated: 12 November 2013 9311 Copyright (c) 1997-2013 University of Cambridge. 9312 ------------------------------------------------------------------------------ 9313 9314 9315 PCREPERFORM(3) Library Functions Manual PCREPERFORM(3) 9316 9317 9318 9319 NAME 9320 PCRE - Perl-compatible regular expressions 9321 9322 PCRE PERFORMANCE 9323 9324 Two aspects of performance are discussed below: memory usage and pro- 9325 cessing time. The way you express your pattern as a regular expression 9326 can affect both of them. 9327 9328 9329 COMPILED PATTERN MEMORY USAGE 9330 9331 Patterns are compiled by PCRE into a reasonably efficient interpretive 9332 code, so that most simple patterns do not use much memory. However, 9333 there is one case where the memory usage of a compiled pattern can be 9334 unexpectedly large. If a parenthesized subpattern has a quantifier with 9335 a minimum greater than 1 and/or a limited maximum, the whole subpattern 9336 is repeated in the compiled code. For example, the pattern 9337 9338 (abc|def){2,4} 9339 9340 is compiled as if it were 9341 9342 (abc|def)(abc|def)((abc|def)(abc|def)?)? 9343 9344 (Technical aside: It is done this way so that backtrack points within 9345 each of the repetitions can be independently maintained.) 9346 9347 For regular expressions whose quantifiers use only small numbers, this 9348 is not usually a problem. However, if the numbers are large, and par- 9349 ticularly if such repetitions are nested, the memory usage can become 9350 an embarrassment. For example, the very simple pattern 9351 9352 ((ab){1,1000}c){1,3} 9353 9354 uses 51K bytes when compiled using the 8-bit library. When PCRE is com- 9355 piled with its default internal pointer size of two bytes, the size 9356 limit on a compiled pattern is 64K data units, and this is reached with 9357 the above pattern if the outer repetition is increased from 3 to 4. 9358 PCRE can be compiled to use larger internal pointers and thus handle 9359 larger compiled patterns, but it is better to try to rewrite your pat- 9360 tern to use less memory if you can. 9361 9362 One way of reducing the memory usage for such patterns is to make use 9363 of PCRE's "subroutine" facility. Re-writing the above pattern as 9364 9365 ((ab)(?2){0,999}c)(?1){0,2} 9366 9367 reduces the memory requirements to 18K, and indeed it remains under 20K 9368 even with the outer repetition increased to 100. However, this pattern 9369 is not exactly equivalent, because the "subroutine" calls are treated 9370 as atomic groups into which there can be no backtracking if there is a 9371 subsequent matching failure. Therefore, PCRE cannot do this kind of 9372 rewriting automatically. Furthermore, there is a noticeable loss of 9373 speed when executing the modified pattern. Nevertheless, if the atomic 9374 grouping is not a problem and the loss of speed is acceptable, this 9375 kind of rewriting will allow you to process patterns that PCRE cannot 9376 otherwise handle. 9377 9378 9379 STACK USAGE AT RUN TIME 9380 9381 When pcre_exec() or pcre[16|32]_exec() is used for matching, certain 9382 kinds of pattern can cause it to use large amounts of the process 9383 stack. In some environments the default process stack is quite small, 9384 and if it runs out the result is often SIGSEGV. This issue is probably 9385 the most frequently raised problem with PCRE. Rewriting your pattern 9386 can often help. The pcrestack documentation discusses this issue in 9387 detail. 9388 9389 9390 PROCESSING TIME 9391 9392 Certain items in regular expression patterns are processed more effi- 9393 ciently than others. It is more efficient to use a character class like 9394 [aeiou] than a set of single-character alternatives such as 9395 (a|e|i|o|u). In general, the simplest construction that provides the 9396 required behaviour is usually the most efficient. Jeffrey Friedl's book 9397 contains a lot of useful general discussion about optimizing regular 9398 expressions for efficient performance. This document contains a few 9399 observations about PCRE. 9400 9401 Using Unicode character properties (the \p, \P, and \X escapes) is 9402 slow, because PCRE has to use a multi-stage table lookup whenever it 9403 needs a character's property. If you can find an alternative pattern 9404 that does not use character properties, it will probably be faster. 9405 9406 By default, the escape sequences \b, \d, \s, and \w, and the POSIX 9407 character classes such as [:alpha:] do not use Unicode properties, 9408 partly for backwards compatibility, and partly for performance reasons. 9409 However, you can set PCRE_UCP if you want Unicode character properties 9410 to be used. This can double the matching time for items such as \d, 9411 when matched with a traditional matching function; the performance loss 9412 is less with a DFA matching function, and in both cases there is not 9413 much difference for \b. 9414 9415 When a pattern begins with .* not in parentheses, or in parentheses 9416 that are not the subject of a backreference, and the PCRE_DOTALL option 9417 is set, the pattern is implicitly anchored by PCRE, since it can match 9418 only at the start of a subject string. However, if PCRE_DOTALL is not 9419 set, PCRE cannot make this optimization, because the . metacharacter 9420 does not then match a newline, and if the subject string contains new- 9421 lines, the pattern may match from the character immediately following 9422 one of them instead of from the very start. For example, the pattern 9423 9424 .*second 9425 9426 matches the subject "first\nand second" (where \n stands for a newline 9427 character), with the match starting at the seventh character. In order 9428 to do this, PCRE has to retry the match starting after every newline in 9429 the subject. 9430 9431 If you are using such a pattern with subject strings that do not con- 9432 tain newlines, the best performance is obtained by setting PCRE_DOTALL, 9433 or starting the pattern with ^.* or ^.*? to indicate explicit anchor- 9434 ing. That saves PCRE from having to scan along the subject looking for 9435 a newline to restart at. 9436 9437 Beware of patterns that contain nested indefinite repeats. These can 9438 take a long time to run when applied to a string that does not match. 9439 Consider the pattern fragment 9440 9441 ^(a+)* 9442 9443 This can match "aaaa" in 16 different ways, and this number increases 9444 very rapidly as the string gets longer. (The * repeat can match 0, 1, 9445 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + 9446 repeats can match different numbers of times.) When the remainder of 9447 the pattern is such that the entire match is going to fail, PCRE has in 9448 principle to try every possible variation, and this can take an 9449 extremely long time, even for relatively short strings. 9450 9451 An optimization catches some of the more simple cases such as 9452 9453 (a+)*b 9454 9455 where a literal character follows. Before embarking on the standard 9456 matching procedure, PCRE checks that there is a "b" later in the sub- 9457 ject string, and if there is not, it fails the match immediately. How- 9458 ever, when there is no following literal this optimization cannot be 9459 used. You can see the difference by comparing the behaviour of 9460 9461 (a+)*\d 9462 9463 with the pattern above. The former gives a failure almost instantly 9464 when applied to a whole line of "a" characters, whereas the latter 9465 takes an appreciable time with strings longer than about 20 characters. 9466 9467 In many cases, the solution to this kind of performance issue is to use 9468 an atomic group or a possessive quantifier. 9469 9470 9471 AUTHOR 9472 9473 Philip Hazel 9474 University Computing Service 9475 Cambridge CB2 3QH, England. 9476 9477 9478 REVISION 9479 9480 Last updated: 25 August 2012 9481 Copyright (c) 1997-2012 University of Cambridge. 9482 ------------------------------------------------------------------------------ 9483 9484 9485 PCREPOSIX(3) Library Functions Manual PCREPOSIX(3) 9486 9487 9488 9489 NAME 9490 PCRE - Perl-compatible regular expressions. 9491 9492 SYNOPSIS 9493 9494 #include <pcreposix.h> 9495 9496 int regcomp(regex_t *preg, const char *pattern, 9497 int cflags); 9498 9499 int regexec(regex_t *preg, const char *string, 9500 size_t nmatch, regmatch_t pmatch[], int eflags); 9501 size_t regerror(int errcode, const regex_t *preg, 9502 char *errbuf, size_t errbuf_size); 9503 9504 void regfree(regex_t *preg); 9505 9506 9507 DESCRIPTION 9508 9509 This set of functions provides a POSIX-style API for the PCRE regular 9510 expression 8-bit library. See the pcreapi documentation for a descrip- 9511 tion of PCRE's native API, which contains much additional functional- 9512 ity. There is no POSIX-style wrapper for PCRE's 16-bit and 32-bit 9513 library. 9514 9515 The functions described here are just wrapper functions that ultimately 9516 call the PCRE native API. Their prototypes are defined in the 9517 pcreposix.h header file, and on Unix systems the library itself is 9518 called pcreposix.a, so can be accessed by adding -lpcreposix to the 9519 command for linking an application that uses them. Because the POSIX 9520 functions call the native ones, it is also necessary to add -lpcre. 9521 9522 I have implemented only those POSIX option bits that can be reasonably 9523 mapped to PCRE native options. In addition, the option REG_EXTENDED is 9524 defined with the value zero. This has no effect, but since programs 9525 that are written to the POSIX interface often use it, this makes it 9526 easier to slot in PCRE as a replacement library. Other POSIX options 9527 are not even defined. 9528 9529 There are also some other options that are not defined by POSIX. These 9530 have been added at the request of users who want to make use of certain 9531 PCRE-specific features via the POSIX calling interface. 9532 9533 When PCRE is called via these functions, it is only the API that is 9534 POSIX-like in style. The syntax and semantics of the regular expres- 9535 sions themselves are still those of Perl, subject to the setting of 9536 various PCRE options, as described below. "POSIX-like in style" means 9537 that the API approximates to the POSIX definition; it is not fully 9538 POSIX-compatible, and in multi-byte encoding domains it is probably 9539 even less compatible. 9540 9541 The header for these functions is supplied as pcreposix.h to avoid any 9542 potential clash with other POSIX libraries. It can, of course, be 9543 renamed or aliased as regex.h, which is the "correct" name. It provides 9544 two structure types, regex_t for compiled internal forms, and reg- 9545 match_t for returning captured substrings. It also defines some con- 9546 stants whose names start with "REG_"; these are used for setting 9547 options and identifying error codes. 9548 9549 9550 COMPILING A PATTERN 9551 9552 The function regcomp() is called to compile a pattern into an internal 9553 form. The pattern is a C string terminated by a binary zero, and is 9554 passed in the argument pattern. The preg argument is a pointer to a 9555 regex_t structure that is used as a base for storing information about 9556 the compiled regular expression. 9557 9558 The argument cflags is either zero, or contains one or more of the bits 9559 defined by the following macros: 9560 9561 REG_DOTALL 9562 9563 The PCRE_DOTALL option is set when the regular expression is passed for 9564 compilation to the native function. Note that REG_DOTALL is not part of 9565 the POSIX standard. 9566 9567 REG_ICASE 9568 9569 The PCRE_CASELESS option is set when the regular expression is passed 9570 for compilation to the native function. 9571 9572 REG_NEWLINE 9573 9574 The PCRE_MULTILINE option is set when the regular expression is passed 9575 for compilation to the native function. Note that this does not mimic 9576 the defined POSIX behaviour for REG_NEWLINE (see the following sec- 9577 tion). 9578 9579 REG_NOSUB 9580 9581 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is 9582 passed for compilation to the native function. In addition, when a pat- 9583 tern that is compiled with this flag is passed to regexec() for match- 9584 ing, the nmatch and pmatch arguments are ignored, and no captured 9585 strings are returned. 9586 9587 REG_UCP 9588 9589 The PCRE_UCP option is set when the regular expression is passed for 9590 compilation to the native function. This causes PCRE to use Unicode 9591 properties when matchine \d, \w, etc., instead of just recognizing 9592 ASCII values. Note that REG_UTF8 is not part of the POSIX standard. 9593 9594 REG_UNGREEDY 9595 9596 The PCRE_UNGREEDY option is set when the regular expression is passed 9597 for compilation to the native function. Note that REG_UNGREEDY is not 9598 part of the POSIX standard. 9599 9600 REG_UTF8 9601 9602 The PCRE_UTF8 option is set when the regular expression is passed for 9603 compilation to the native function. This causes the pattern itself and 9604 all data strings used for matching it to be treated as UTF-8 strings. 9605 Note that REG_UTF8 is not part of the POSIX standard. 9606 9607 In the absence of these flags, no options are passed to the native 9608 function. This means the the regex is compiled with PCRE default 9609 semantics. In particular, the way it handles newline characters in the 9610 subject string is the Perl way, not the POSIX way. Note that setting 9611 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. 9612 It does not affect the way newlines are matched by . (they are not) or 9613 by a negative class such as [^a] (they are). 9614 9615 The yield of regcomp() is zero on success, and non-zero otherwise. The 9616 preg structure is filled in on success, and one member of the structure 9617 is public: re_nsub contains the number of capturing subpatterns in the 9618 regular expression. Various error codes are defined in the header file. 9619 9620 NOTE: If the yield of regcomp() is non-zero, you must not attempt to 9621 use the contents of the preg structure. If, for example, you pass it to 9622 regexec(), the result is undefined and your program is likely to crash. 9623 9624 9625 MATCHING NEWLINE CHARACTERS 9626 9627 This area is not simple, because POSIX and Perl take different views of 9628 things. It is not possible to get PCRE to obey POSIX semantics, but 9629 then PCRE was never intended to be a POSIX engine. The following table 9630 lists the different possibilities for matching newline characters in 9631 PCRE: 9632 9633 Default Change with 9634 9635 . matches newline no PCRE_DOTALL 9636 newline matches [^a] yes not changeable 9637 $ matches \n at end yes PCRE_DOLLARENDONLY 9638 $ matches \n in middle no PCRE_MULTILINE 9639 ^ matches \n in middle no PCRE_MULTILINE 9640 9641 This is the equivalent table for POSIX: 9642 9643 Default Change with 9644 9645 . matches newline yes REG_NEWLINE 9646 newline matches [^a] yes REG_NEWLINE 9647 $ matches \n at end no REG_NEWLINE 9648 $ matches \n in middle no REG_NEWLINE 9649 ^ matches \n in middle no REG_NEWLINE 9650 9651 PCRE's behaviour is the same as Perl's, except that there is no equiva- 9652 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is 9653 no way to stop newline from matching [^a]. 9654 9655 The default POSIX newline handling can be obtained by setting 9656 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE 9657 behave exactly as for the REG_NEWLINE action. 9658 9659 9660 MATCHING A PATTERN 9661 9662 The function regexec() is called to match a compiled pattern preg 9663 against a given string, which is by default terminated by a zero byte 9664 (but see REG_STARTEND below), subject to the options in eflags. These 9665 can be: 9666 9667 REG_NOTBOL 9668 9669 The PCRE_NOTBOL option is set when calling the underlying PCRE matching 9670 function. 9671 9672 REG_NOTEMPTY 9673 9674 The PCRE_NOTEMPTY option is set when calling the underlying PCRE match- 9675 ing function. Note that REG_NOTEMPTY is not part of the POSIX standard. 9676 However, setting this option can give more POSIX-like behaviour in some 9677 situations. 9678 9679 REG_NOTEOL 9680 9681 The PCRE_NOTEOL option is set when calling the underlying PCRE matching 9682 function. 9683 9684 REG_STARTEND 9685 9686 The string is considered to start at string + pmatch[0].rm_so and to 9687 have a terminating NUL located at string + pmatch[0].rm_eo (there need 9688 not actually be a NUL at that location), regardless of the value of 9689 nmatch. This is a BSD extension, compatible with but not specified by 9690 IEEE Standard 1003.2 (POSIX.2), and should be used with caution in 9691 software intended to be portable to other systems. Note that a non-zero 9692 rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location 9693 of the string, not how it is matched. 9694 9695 If the pattern was compiled with the REG_NOSUB flag, no data about any 9696 matched strings is returned. The nmatch and pmatch arguments of 9697 regexec() are ignored. 9698 9699 If the value of nmatch is zero, or if the value pmatch is NULL, no data 9700 about any matched strings is returned. 9701 9702 Otherwise,the portion of the string that was matched, and also any cap- 9703 tured substrings, are returned via the pmatch argument, which points to 9704 an array of nmatch structures of type regmatch_t, containing the mem- 9705 bers rm_so and rm_eo. These contain the offset to the first character 9706 of each substring and the offset to the first character after the end 9707 of each substring, respectively. The 0th element of the vector relates 9708 to the entire portion of string that was matched; subsequent elements 9709 relate to the capturing subpatterns of the regular expression. Unused 9710 entries in the array have both structure members set to -1. 9711 9712 A successful match yields a zero return; various error codes are 9713 defined in the header file, of which REG_NOMATCH is the "expected" 9714 failure code. 9715 9716 9717 ERROR MESSAGES 9718 9719 The regerror() function maps a non-zero errorcode from either regcomp() 9720 or regexec() to a printable message. If preg is not NULL, the error 9721 should have arisen from the use of that structure. A message terminated 9722 by a binary zero is placed in errbuf. The length of the message, 9723 including the zero, is limited to errbuf_size. The yield of the func- 9724 tion is the size of buffer needed to hold the whole message. 9725 9726 9727 MEMORY USAGE 9728 9729 Compiling a regular expression causes memory to be allocated and asso- 9730 ciated with the preg structure. The function regfree() frees all such 9731 memory, after which preg may no longer be used as a compiled expres- 9732 sion. 9733 9734 9735 AUTHOR 9736 9737 Philip Hazel 9738 University Computing Service 9739 Cambridge CB2 3QH, England. 9740 9741 9742 REVISION 9743 9744 Last updated: 09 January 2012 9745 Copyright (c) 1997-2012 University of Cambridge. 9746 ------------------------------------------------------------------------------ 9747 9748 9749 PCRECPP(3) Library Functions Manual PCRECPP(3) 9750 9751 9752 9753 NAME 9754 PCRE - Perl-compatible regular expressions. 9755 9756 SYNOPSIS OF C++ WRAPPER 9757 9758 #include <pcrecpp.h> 9759 9760 9761 DESCRIPTION 9762 9763 The C++ wrapper for PCRE was provided by Google Inc. Some additional 9764 functionality was added by Giuseppe Maxia. This brief man page was con- 9765 structed from the notes in the pcrecpp.h file, which should be con- 9766 sulted for further details. Note that the C++ wrapper supports only the 9767 original 8-bit PCRE library. There is no 16-bit or 32-bit support at 9768 present. 9769 9770 9771 MATCHING INTERFACE 9772 9773 The "FullMatch" operation checks that supplied text matches a supplied 9774 pattern exactly. If pointer arguments are supplied, it copies matched 9775 sub-strings that match sub-patterns into them. 9776 9777 Example: successful match 9778 pcrecpp::RE re("h.*o"); 9779 re.FullMatch("hello"); 9780 9781 Example: unsuccessful match (requires full match): 9782 pcrecpp::RE re("e"); 9783 !re.FullMatch("hello"); 9784 9785 Example: creating a temporary RE object: 9786 pcrecpp::RE("h.*o").FullMatch("hello"); 9787 9788 You can pass in a "const char*" or a "string" for "text". The examples 9789 below tend to use a const char*. You can, as in the different examples 9790 above, store the RE object explicitly in a variable or use a temporary 9791 RE object. The examples below use one mode or the other arbitrarily. 9792 Either could correctly be used for any of these examples. 9793 9794 You must supply extra pointer arguments to extract matched subpieces. 9795 9796 Example: extracts "ruby" into "s" and 1234 into "i" 9797 int i; 9798 string s; 9799 pcrecpp::RE re("(\\w+):(\\d+)"); 9800 re.FullMatch("ruby:1234", &s, &i); 9801 9802 Example: does not try to extract any extra sub-patterns 9803 re.FullMatch("ruby:1234", &s); 9804 9805 Example: does not try to extract into NULL 9806 re.FullMatch("ruby:1234", NULL, &i); 9807 9808 Example: integer overflow causes failure 9809 !re.FullMatch("ruby:1234567891234", NULL, &i); 9810 9811 Example: fails because there aren't enough sub-patterns: 9812 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); 9813 9814 Example: fails because string cannot be stored in integer 9815 !pcrecpp::RE("(.*)").FullMatch("ruby", &i); 9816 9817 The provided pointer arguments can be pointers to any scalar numeric 9818 type, or one of: 9819 9820 string (matched piece is copied to string) 9821 StringPiece (StringPiece is mutated to point to matched piece) 9822 T (where "bool T::ParseFrom(const char*, int)" exists) 9823 NULL (the corresponding matched sub-pattern is not copied) 9824 9825 The function returns true iff all of the following conditions are sat- 9826 isfied: 9827 9828 a. "text" matches "pattern" exactly; 9829 9830 b. The number of matched sub-patterns is >= number of supplied 9831 pointers; 9832 9833 c. The "i"th argument has a suitable type for holding the 9834 string captured as the "i"th sub-pattern. If you pass in 9835 void * NULL for the "i"th argument, or a non-void * NULL 9836 of the correct type, or pass fewer arguments than the 9837 number of sub-patterns, "i"th captured sub-pattern is 9838 ignored. 9839 9840 CAVEAT: An optional sub-pattern that does not exist in the matched 9841 string is assigned the empty string. Therefore, the following will 9842 return false (because the empty string is not a valid number): 9843 9844 int number; 9845 pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); 9846 9847 The matching interface supports at most 16 arguments per call. If you 9848 need more, consider using the more general interface 9849 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. 9850 9851 NOTE: Do not use no_arg, which is used internally to mark the end of a 9852 list of optional arguments, as a placeholder for missing arguments, as 9853 this can lead to segfaults. 9854 9855 9856 QUOTING METACHARACTERS 9857 9858 You can use the "QuoteMeta" operation to insert backslashes before all 9859 potentially meaningful characters in a string. The returned string, 9860 used as a regular expression, will exactly match the original string. 9861 9862 Example: 9863 string quoted = RE::QuoteMeta(unquoted); 9864 9865 Note that it's legal to escape a character even if it has no special 9866 meaning in a regular expression -- so this function does that. (This 9867 also makes it identical to the perl function of the same name; see 9868 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes 9869 "1\.5\-2\.0\?". 9870 9871 9872 PARTIAL MATCHES 9873 9874 You can use the "PartialMatch" operation when you want the pattern to 9875 match any substring of the text. 9876 9877 Example: simple search for a string: 9878 pcrecpp::RE("ell").PartialMatch("hello"); 9879 9880 Example: find first number in a string: 9881 int number; 9882 pcrecpp::RE re("(\\d+)"); 9883 re.PartialMatch("x*100 + 20", &number); 9884 assert(number == 100); 9885 9886 9887 UTF-8 AND THE MATCHING INTERFACE 9888 9889 By default, pattern and text are plain text, one byte per character. 9890 The UTF8 flag, passed to the constructor, causes both pattern and 9891 string to be treated as UTF-8 text, still a byte stream but potentially 9892 multiple bytes per character. In practice, the text is likelier to be 9893 UTF-8 than the pattern, but the match returned may depend on the UTF8 9894 flag, so always use it when matching UTF8 text. For example, "." will 9895 match one byte normally but with UTF8 set may match up to three bytes 9896 of a multi-byte character. 9897 9898 Example: 9899 pcrecpp::RE_Options options; 9900 options.set_utf8(); 9901 pcrecpp::RE re(utf8_pattern, options); 9902 re.FullMatch(utf8_string); 9903 9904 Example: using the convenience function UTF8(): 9905 pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); 9906 re.FullMatch(utf8_string); 9907 9908 NOTE: The UTF8 flag is ignored if pcre was not configured with the 9909 --enable-utf8 flag. 9910 9911 9912 PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE 9913 9914 PCRE defines some modifiers to change the behavior of the regular 9915 expression engine. The C++ wrapper defines an auxiliary class, 9916 RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- 9917 rently, the following modifiers are supported: 9918 9919 modifier description Perl corresponding 9920 9921 PCRE_CASELESS case insensitive match /i 9922 PCRE_MULTILINE multiple lines match /m 9923 PCRE_DOTALL dot matches newlines /s 9924 PCRE_DOLLAR_ENDONLY $ matches only at end N/A 9925 PCRE_EXTRA strict escape parsing N/A 9926 PCRE_EXTENDED ignore white spaces /x 9927 PCRE_UTF8 handles UTF8 chars built-in 9928 PCRE_UNGREEDY reverses * and *? N/A 9929 PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) 9930 9931 (*) Both Perl and PCRE allow non capturing parentheses by means of the 9932 "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- 9933 ture, while (ab|cd) does. 9934 9935 For a full account on how each modifier works, please check the PCRE 9936 API reference page. 9937 9938 For each modifier, there are two member functions whose name is made 9939 out of the modifier in lowercase, without the "PCRE_" prefix. For 9940 instance, PCRE_CASELESS is handled by 9941 9942 bool caseless() 9943 9944 which returns true if the modifier is set, and 9945 9946 RE_Options & set_caseless(bool) 9947 9948 which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can 9949 be accessed through the set_match_limit() and match_limit() member 9950 functions. Setting match_limit to a non-zero value will limit the exe- 9951 cution of pcre to keep it from doing bad things like blowing the stack 9952 or taking an eternity to return a result. A value of 5000 is good 9953 enough to stop stack blowup in a 2MB thread stack. Setting match_limit 9954 to zero disables match limiting. Alternatively, you can call 9955 match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to 9956 limit how much PCRE recurses. match_limit() limits the number of 9957 matches PCRE does; match_limit_recursion() limits the depth of internal 9958 recursion, and therefore the amount of stack that is used. 9959 9960 Normally, to pass one or more modifiers to a RE class, you declare a 9961 RE_Options object, set the appropriate options, and pass this object to 9962 a RE constructor. Example: 9963 9964 RE_Options opt; 9965 opt.set_caseless(true); 9966 if (RE("HELLO", opt).PartialMatch("hello world")) ... 9967 9968 RE_options has two constructors. The default constructor takes no argu- 9969 ments and creates a set of flags that are off by default. The optional 9970 parameter option_flags is to facilitate transfer of legacy code from C 9971 programs. This lets you do 9972 9973 RE(pattern, 9974 RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); 9975 9976 However, new code is better off doing 9977 9978 RE(pattern, 9979 RE_Options().set_caseless(true).set_multiline(true)) 9980 .PartialMatch(str); 9981 9982 If you are going to pass one of the most used modifiers, there are some 9983 convenience functions that return a RE_Options class with the appropri- 9984 ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), 9985 and EXTENDED(). 9986 9987 If you need to set several options at once, and you don't want to go 9988 through the pains of declaring a RE_Options object and setting several 9989 options, there is a parallel method that give you such ability on the 9990 fly. You can concatenate several set_xxxxx() member functions, since 9991 each of them returns a reference to its class object. For example, to 9992 pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one 9993 statement, you may write: 9994 9995 RE(" ^ xyz \\s+ .* blah$", 9996 RE_Options() 9997 .set_caseless(true) 9998 .set_extended(true) 9999 .set_multiline(true)).PartialMatch(sometext); 10000 10001 10002 SCANNING TEXT INCREMENTALLY 10003 10004 The "Consume" operation may be useful if you want to repeatedly match 10005 regular expressions at the front of a string and skip over them as they 10006 match. This requires use of the "StringPiece" type, which represents a 10007 sub-range of a real string. Like RE, StringPiece is defined in the 10008 pcrecpp namespace. 10009 10010 Example: read lines of the form "var = value" from a string. 10011 string contents = ...; // Fill string somehow 10012 pcrecpp::StringPiece input(contents); // Wrap in a StringPiece 10013 10014 string var; 10015 int value; 10016 pcrecpp::RE re("(\\w+) = (\\d+)\n"); 10017 while (re.Consume(&input, &var, &value)) { 10018 ...; 10019 } 10020 10021 Each successful call to "Consume" will set "var/value", and also 10022 advance "input" so it points past the matched text. 10023 10024 The "FindAndConsume" operation is similar to "Consume" but does not 10025 anchor your match at the beginning of the string. For example, you 10026 could extract all words from a string by repeatedly calling 10027 10028 pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) 10029 10030 10031 PARSING HEX/OCTAL/C-RADIX NUMBERS 10032 10033 By default, if you pass a pointer to a numeric value, the corresponding 10034 text is interpreted as a base-10 number. You can instead wrap the 10035 pointer with a call to one of the operators Hex(), Octal(), or CRadix() 10036 to interpret the text in another base. The CRadix operator interprets 10037 C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to 10038 base-10. 10039 10040 Example: 10041 int a, b, c, d; 10042 pcrecpp::RE re("(.*) (.*) (.*) (.*)"); 10043 re.FullMatch("100 40 0100 0x40", 10044 pcrecpp::Octal(&a), pcrecpp::Hex(&b), 10045 pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); 10046 10047 will leave 64 in a, b, c, and d. 10048 10049 10050 REPLACING PARTS OF STRINGS 10051 10052 You can replace the first match of "pattern" in "str" with "rewrite". 10053 Within "rewrite", backslash-escaped digits (\1 to \9) can be used to 10054 insert text matching corresponding parenthesized group from the pat- 10055 tern. \0 in "rewrite" refers to the entire matching text. For example: 10056 10057 string s = "yabba dabba doo"; 10058 pcrecpp::RE("b+").Replace("d", &s); 10059 10060 will leave "s" containing "yada dabba doo". The result is true if the 10061 pattern matches and a replacement occurs, false otherwise. 10062 10063 GlobalReplace is like Replace except that it replaces all occurrences 10064 of the pattern in the string with the rewrite. Replacements are not 10065 subject to re-matching. For example: 10066 10067 string s = "yabba dabba doo"; 10068 pcrecpp::RE("b+").GlobalReplace("d", &s); 10069 10070 will leave "s" containing "yada dada doo". It returns the number of 10071 replacements made. 10072 10073 Extract is like Replace, except that if the pattern matches, "rewrite" 10074 is copied into "out" (an additional argument) with substitutions. The 10075 non-matching portions of "text" are ignored. Returns true iff a match 10076 occurred and the extraction happened successfully; if no match occurs, 10077 the string is left unaffected. 10078 10079 10080 AUTHOR 10081 10082 The C++ wrapper was contributed by Google Inc. 10083 Copyright (c) 2007 Google Inc. 10084 10085 10086 REVISION 10087 10088 Last updated: 08 January 2012 10089 ------------------------------------------------------------------------------ 10090 10091 10092 PCRESAMPLE(3) Library Functions Manual PCRESAMPLE(3) 10093 10094 10095 10096 NAME 10097 PCRE - Perl-compatible regular expressions 10098 10099 PCRE SAMPLE PROGRAM 10100 10101 A simple, complete demonstration program, to get you started with using 10102 PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A 10103 listing of this program is given in the pcredemo documentation. If you 10104 do not have a copy of the PCRE distribution, you can save this listing 10105 to re-create pcredemo.c. 10106 10107 The demonstration program, which uses the original PCRE 8-bit library, 10108 compiles the regular expression that is its first argument, and matches 10109 it against the subject string in its second argument. No PCRE options 10110 are set, and default character tables are used. If matching succeeds, 10111 the program outputs the portion of the subject that matched, together 10112 with the contents of any captured substrings. 10113 10114 If the -g option is given on the command line, the program then goes on 10115 to check for further matches of the same regular expression in the same 10116 subject string. The logic is a little bit tricky because of the possi- 10117 bility of matching an empty string. Comments in the code explain what 10118 is going on. 10119 10120 If PCRE is installed in the standard include and library directories 10121 for your operating system, you should be able to compile the demonstra- 10122 tion program using this command: 10123 10124 gcc -o pcredemo pcredemo.c -lpcre 10125 10126 If PCRE is installed elsewhere, you may need to add additional options 10127 to the command line. For example, on a Unix-like system that has PCRE 10128 installed in /usr/local, you can compile the demonstration program 10129 using a command like this: 10130 10131 gcc -o pcredemo -I/usr/local/include pcredemo.c \ 10132 -L/usr/local/lib -lpcre 10133 10134 In a Windows environment, if you want to statically link the program 10135 against a non-dll pcre.a file, you must uncomment the line that defines 10136 PCRE_STATIC before including pcre.h, because otherwise the pcre_mal- 10137 loc() and pcre_free() exported functions will be declared 10138 __declspec(dllimport), with unwanted results. 10139 10140 Once you have compiled and linked the demonstration program, you can 10141 run simple tests like this: 10142 10143 ./pcredemo 'cat|dog' 'the cat sat on the mat' 10144 ./pcredemo -g 'cat|dog' 'the dog sat on the cat' 10145 10146 Note that there is a much more comprehensive test program, called 10147 pcretest, which supports many more facilities for testing regular 10148 expressions and both PCRE libraries. The pcredemo program is provided 10149 as a simple coding example. 10150 10151 If you try to run pcredemo when PCRE is not installed in the standard 10152 library directory, you may get an error like this on some operating 10153 systems (e.g. Solaris): 10154 10155 ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or 10156 directory 10157 10158 This is caused by the way shared library support works on those sys- 10159 tems. You need to add 10160 10161 -R/usr/local/lib 10162 10163 (for example) to the compile command to get round this problem. 10164 10165 10166 AUTHOR 10167 10168 Philip Hazel 10169 University Computing Service 10170 Cambridge CB2 3QH, England. 10171 10172 10173 REVISION 10174 10175 Last updated: 10 January 2012 10176 Copyright (c) 1997-2012 University of Cambridge. 10177 ------------------------------------------------------------------------------ 10178 PCRELIMITS(3) Library Functions Manual PCRELIMITS(3) 10179 10180 10181 10182 NAME 10183 PCRE - Perl-compatible regular expressions 10184 10185 SIZE AND OTHER LIMITATIONS 10186 10187 There are some size limitations in PCRE but it is hoped that they will 10188 never in practice be relevant. 10189 10190 The maximum length of a compiled pattern is approximately 64K data 10191 units (bytes for the 8-bit library, 16-bit units for the 16-bit 10192 library, and 32-bit units for the 32-bit library) if PCRE is compiled 10193 with the default internal linkage size, which is 2 bytes for the 8-bit 10194 and 16-bit libraries, and 4 bytes for the 32-bit library. If you want 10195 to process regular expressions that are truly enormous, you can compile 10196 PCRE with an internal linkage size of 3 or 4 (when building the 16-bit 10197 or 32-bit library, 3 is rounded up to 4). See the README file in the 10198 source distribution and the pcrebuild documentation for details. In 10199 these cases the limit is substantially larger. However, the speed of 10200 execution is slower. 10201 10202 All values in repeating quantifiers must be less than 65536. 10203 10204 There is no limit to the number of parenthesized subpatterns, but there 10205 can be no more than 65535 capturing subpatterns. There is, however, a 10206 limit to the depth of nesting of parenthesized subpatterns of all 10207 kinds. This is imposed in order to limit the amount of system stack 10208 used at compile time. The limit can be specified when PCRE is built; 10209 the default is 250. 10210 10211 There is a limit to the number of forward references to subsequent sub- 10212 patterns of around 200,000. Repeated forward references with fixed 10213 upper limits, for example, (?2){0,100} when subpattern number 2 is to 10214 the right, are included in the count. There is no limit to the number 10215 of backward references. 10216 10217 The maximum length of name for a named subpattern is 32 characters, and 10218 the maximum number of named subpatterns is 10000. 10219 10220 The maximum length of a name in a (*MARK), (*PRUNE), (*SKIP), or 10221 (*THEN) verb is 255 for the 8-bit library and 65535 for the 16-bit and 10222 32-bit libraries. 10223 10224 The maximum length of a subject string is the largest positive number 10225 that an integer variable can hold. However, when using the traditional 10226 matching function, PCRE uses recursion to handle subpatterns and indef- 10227 inite repetition. This means that the available stack space may limit 10228 the size of a subject string that can be processed by certain patterns. 10229 For a discussion of stack issues, see the pcrestack documentation. 10230 10231 10232 AUTHOR 10233 10234 Philip Hazel 10235 University Computing Service 10236 Cambridge CB2 3QH, England. 10237 10238 10239 REVISION 10240 10241 Last updated: 05 November 2013 10242 Copyright (c) 1997-2013 University of Cambridge. 10243 ------------------------------------------------------------------------------ 10244 10245 10246 PCRESTACK(3) Library Functions Manual PCRESTACK(3) 10247 10248 10249 10250 NAME 10251 PCRE - Perl-compatible regular expressions 10252 10253 PCRE DISCUSSION OF STACK USAGE 10254 10255 When you call pcre[16|32]_exec(), it makes use of an internal function 10256 called match(). This calls itself recursively at branch points in the 10257 pattern, in order to remember the state of the match so that it can 10258 back up and try a different alternative if the first one fails. As 10259 matching proceeds deeper and deeper into the tree of possibilities, the 10260 recursion depth increases. The match() function is also called in other 10261 circumstances, for example, whenever a parenthesized sub-pattern is 10262 entered, and in certain cases of repetition. 10263 10264 Not all calls of match() increase the recursion depth; for an item such 10265 as a* it may be called several times at the same level, after matching 10266 different numbers of a's. Furthermore, in a number of cases where the 10267 result of the recursive call would immediately be passed back as the 10268 result of the current call (a "tail recursion"), the function is just 10269 restarted instead. 10270 10271 The above comments apply when pcre[16|32]_exec() is run in its normal 10272 interpretive manner. If the pattern was studied with the 10273 PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling was success- 10274 ful, and the options passed to pcre[16|32]_exec() were not incompati- 10275 ble, the matching process uses the JIT-compiled code instead of the 10276 match() function. In this case, the memory requirements are handled 10277 entirely differently. See the pcrejit documentation for details. 10278 10279 The pcre[16|32]_dfa_exec() function operates in an entirely different 10280 way, and uses recursion only when there is a regular expression recur- 10281 sion or subroutine call in the pattern. This includes the processing of 10282 assertion and "once-only" subpatterns, which are handled like subrou- 10283 tine calls. Normally, these are never very deep, and the limit on the 10284 complexity of pcre[16|32]_dfa_exec() is controlled by the amount of 10285 workspace it is given. However, it is possible to write patterns with 10286 runaway infinite recursions; such patterns will cause 10287 pcre[16|32]_dfa_exec() to run out of stack. At present, there is no 10288 protection against this. 10289 10290 The comments that follow do NOT apply to pcre[16|32]_dfa_exec(); they 10291 are relevant only for pcre[16|32]_exec() without the JIT optimization. 10292 10293 Reducing pcre[16|32]_exec()'s stack usage 10294 10295 Each time that match() is actually called recursively, it uses memory 10296 from the process stack. For certain kinds of pattern and data, very 10297 large amounts of stack may be needed, despite the recognition of "tail 10298 recursion". You can often reduce the amount of recursion, and there- 10299 fore the amount of stack used, by modifying the pattern that is being 10300 matched. Consider, for example, this pattern: 10301 10302 ([^<]|<(?!inet))+ 10303 10304 It matches from wherever it starts until it encounters "<inet" or the 10305 end of the data, and is the kind of pattern that might be used when 10306 processing an XML file. Each iteration of the outer parentheses matches 10307 either one character that is not "<" or a "<" that is not followed by 10308 "inet". However, each time a parenthesis is processed, a recursion 10309 occurs, so this formulation uses a stack frame for each matched charac- 10310 ter. For a long string, a lot of stack is required. Consider now this 10311 rewritten pattern, which matches exactly the same strings: 10312 10313 ([^<]++|<(?!inet))+ 10314 10315 This uses very much less stack, because runs of characters that do not 10316 contain "<" are "swallowed" in one item inside the parentheses. Recur- 10317 sion happens only when a "<" character that is not followed by "inet" 10318 is encountered (and we assume this is relatively rare). A possessive 10319 quantifier is used to stop any backtracking into the runs of non-"<" 10320 characters, but that is not related to stack usage. 10321 10322 This example shows that one way of avoiding stack problems when match- 10323 ing long subject strings is to write repeated parenthesized subpatterns 10324 to match more than one character whenever possible. 10325 10326 Compiling PCRE to use heap instead of stack for pcre[16|32]_exec() 10327 10328 In environments where stack memory is constrained, you might want to 10329 compile PCRE to use heap memory instead of stack for remembering back- 10330 up points when pcre[16|32]_exec() is running. This makes it run a lot 10331 more slowly, however. Details of how to do this are given in the pcre- 10332 build documentation. When built in this way, instead of using the 10333 stack, PCRE obtains and frees memory by calling the functions that are 10334 pointed to by the pcre[16|32]_stack_malloc and pcre[16|32]_stack_free 10335 variables. By default, these point to malloc() and free(), but you can 10336 replace the pointers to cause PCRE to use your own functions. Since the 10337 block sizes are always the same, and are always freed in reverse order, 10338 it may be possible to implement customized memory handlers that are 10339 more efficient than the standard functions. 10340 10341 Limiting pcre[16|32]_exec()'s stack usage 10342 10343 You can set limits on the number of times that match() is called, both 10344 in total and recursively. If a limit is exceeded, pcre[16|32]_exec() 10345 returns an error code. Setting suitable limits should prevent it from 10346 running out of stack. The default values of the limits are very large, 10347 and unlikely ever to operate. They can be changed when PCRE is built, 10348 and they can also be set when pcre[16|32]_exec() is called. For details 10349 of these interfaces, see the pcrebuild documentation and the section on 10350 extra data for pcre[16|32]_exec() in the pcreapi documentation. 10351 10352 As a very rough rule of thumb, you should reckon on about 500 bytes per 10353 recursion. Thus, if you want to limit your stack usage to 8Mb, you 10354 should set the limit at 16000 recursions. A 64Mb stack, on the other 10355 hand, can support around 128000 recursions. 10356 10357 In Unix-like environments, the pcretest test program has a command line 10358 option (-S) that can be used to increase the size of its stack. As long 10359 as the stack is large enough, another option (-M) can be used to find 10360 the smallest limits that allow a particular pattern to match a given 10361 subject string. This is done by calling pcre[16|32]_exec() repeatedly 10362 with different limits. 10363 10364 Obtaining an estimate of stack usage 10365 10366 The actual amount of stack used per recursion can vary quite a lot, 10367 depending on the compiler that was used to build PCRE and the optimiza- 10368 tion or debugging options that were set for it. The rule of thumb value 10369 of 500 bytes mentioned above may be larger or smaller than what is 10370 actually needed. A better approximation can be obtained by running this 10371 command: 10372 10373 pcretest -m -C 10374 10375 The -C option causes pcretest to output information about the options 10376 with which PCRE was compiled. When -m is also given (before -C), infor- 10377 mation about stack use is given in a line like this: 10378 10379 Match recursion uses stack: approximate frame size = 640 bytes 10380 10381 The value is approximate because some recursions need a bit more (up to 10382 perhaps 16 more bytes). 10383 10384 If the above command is given when PCRE is compiled to use the heap 10385 instead of the stack for recursion, the value that is output is the 10386 size of each block that is obtained from the heap. 10387 10388 Changing stack size in Unix-like systems 10389 10390 In Unix-like environments, there is not often a problem with the stack 10391 unless very long strings are involved, though the default limit on 10392 stack size varies from system to system. Values from 8Mb to 64Mb are 10393 common. You can find your default limit by running the command: 10394 10395 ulimit -s 10396 10397 Unfortunately, the effect of running out of stack is often SIGSEGV, 10398 though sometimes a more explicit error message is given. You can nor- 10399 mally increase the limit on stack size by code such as this: 10400 10401 struct rlimit rlim; 10402 getrlimit(RLIMIT_STACK, &rlim); 10403 rlim.rlim_cur = 100*1024*1024; 10404 setrlimit(RLIMIT_STACK, &rlim); 10405 10406 This reads the current limits (soft and hard) using getrlimit(), then 10407 attempts to increase the soft limit to 100Mb using setrlimit(). You 10408 must do this before calling pcre[16|32]_exec(). 10409 10410 Changing stack size in Mac OS X 10411 10412 Using setrlimit(), as described above, should also work on Mac OS X. It 10413 is also possible to set a stack size when linking a program. There is a 10414 discussion about stack sizes in Mac OS X at this web site: 10415 http://developer.apple.com/qa/qa2005/qa1419.html. 10416 10417 10418 AUTHOR 10419 10420 Philip Hazel 10421 University Computing Service 10422 Cambridge CB2 3QH, England. 10423 10424 10425 REVISION 10426 10427 Last updated: 24 June 2012 10428 Copyright (c) 1997-2012 University of Cambridge. 10429 ------------------------------------------------------------------------------ 10430 10431 10432
