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) PCRE(3) 12 13 14 NAME 15 PCRE - Perl-compatible regular expressions 16 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 The current implementation of PCRE corresponds approximately with Perl 29 5.12, including support for UTF-8 encoded strings and Unicode general 30 category properties. However, UTF-8 and Unicode support has to be 31 explicitly enabled; it is not the default. The Unicode tables corre- 32 spond to Unicode release 5.2.0. 33 34 In addition to the Perl-compatible matching function, PCRE contains an 35 alternative function that matches the same compiled patterns in a dif- 36 ferent way. In certain circumstances, the alternative function has some 37 advantages. For a discussion of the two matching algorithms, see the 38 pcrematching page. 39 40 PCRE is written in C and released as a C library. A number of people 41 have written wrappers and interfaces of various kinds. In particular, 42 Google Inc. have provided a comprehensive C++ wrapper. This is now 43 included as part of the PCRE distribution. The pcrecpp page has details 44 of this interface. Other people's contributions can be found in the 45 Contrib directory at the primary FTP site, which is: 46 47 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre 48 49 Details of exactly which Perl regular expression features are and are 50 not supported by PCRE are given in separate documents. See the pcrepat- 51 tern and pcrecompat pages. There is a syntax summary in the pcresyntax 52 page. 53 54 Some features of PCRE can be included, excluded, or changed when the 55 library is built. The pcre_config() function makes it possible for a 56 client to discover which features are available. The features them- 57 selves are described in the pcrebuild page. Documentation about build- 58 ing PCRE for various operating systems can be found in the README and 59 NON-UNIX-USE files in the source distribution. 60 61 The library contains a number of undocumented internal functions and 62 data tables that are used by more than one of the exported external 63 functions, but which are not intended for use by external callers. 64 Their names all begin with "_pcre_", which hopefully will not provoke 65 any name clashes. In some environments, it is possible to control which 66 external symbols are exported when a shared library is built, and in 67 these cases the undocumented symbols are not exported. 68 69 70 USER DOCUMENTATION 71 72 The user documentation for PCRE comprises a number of different sec- 73 tions. In the "man" format, each of these is a separate "man page". In 74 the HTML format, each is a separate page, linked from the index page. 75 In the plain text format, all the sections, except the pcredemo sec- 76 tion, are concatenated, for ease of searching. The sections are as fol- 77 lows: 78 79 pcre this document 80 pcre-config show PCRE installation configuration information 81 pcreapi details of PCRE's native C API 82 pcrebuild options for building PCRE 83 pcrecallout details of the callout feature 84 pcrecompat discussion of Perl compatibility 85 pcrecpp details of the C++ wrapper 86 pcredemo a demonstration C program that uses PCRE 87 pcregrep description of the pcregrep command 88 pcrematching discussion of the two matching algorithms 89 pcrepartial details of the partial matching facility 90 pcrepattern syntax and semantics of supported 91 regular expressions 92 pcreperform discussion of performance issues 93 pcreposix the POSIX-compatible C API 94 pcreprecompile details of saving and re-using precompiled patterns 95 pcresample discussion of the pcredemo program 96 pcrestack discussion of stack usage 97 pcresyntax quick syntax reference 98 pcretest description of the pcretest testing command 99 100 In addition, in the "man" and HTML formats, there is a short page for 101 each C library function, listing its arguments and results. 102 103 104 LIMITATIONS 105 106 There are some size limitations in PCRE but it is hoped that they will 107 never in practice be relevant. 108 109 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE 110 is compiled with the default internal linkage size of 2. If you want to 111 process regular expressions that are truly enormous, you can compile 112 PCRE with an internal linkage size of 3 or 4 (see the README file in 113 the source distribution and the pcrebuild documentation for details). 114 In these cases the limit is substantially larger. However, the speed 115 of execution is slower. 116 117 All values in repeating quantifiers must be less than 65536. 118 119 There is no limit to the number of parenthesized subpatterns, but there 120 can be no more than 65535 capturing subpatterns. 121 122 The maximum length of name for a named subpattern is 32 characters, and 123 the maximum number of named subpatterns is 10000. 124 125 The maximum length of a subject string is the largest positive number 126 that an integer variable can hold. However, when using the traditional 127 matching function, PCRE uses recursion to handle subpatterns and indef- 128 inite repetition. This means that the available stack space may limit 129 the size of a subject string that can be processed by certain patterns. 130 For a discussion of stack issues, see the pcrestack documentation. 131 132 133 UTF-8 AND UNICODE PROPERTY SUPPORT 134 135 From release 3.3, PCRE has had some support for character strings 136 encoded in the UTF-8 format. For release 4.0 this was greatly extended 137 to cover most common requirements, and in release 5.0 additional sup- 138 port for Unicode general category properties was added. 139 140 In order process UTF-8 strings, you must build PCRE to include UTF-8 141 support in the code, and, in addition, you must call pcre_compile() 142 with the PCRE_UTF8 option flag, or the pattern must start with the 143 sequence (*UTF8). When either of these is the case, both the pattern 144 and any subject strings that are matched against it are treated as 145 UTF-8 strings instead of strings of 1-byte characters. 146 147 If you compile PCRE with UTF-8 support, but do not use it at run time, 148 the library will be a bit bigger, but the additional run time overhead 149 is limited to testing the PCRE_UTF8 flag occasionally, so should not be 150 very big. 151 152 If PCRE is built with Unicode character property support (which implies 153 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup- 154 ported. The available properties that can be tested are limited to the 155 general category properties such as Lu for an upper case letter or Nd 156 for a decimal number, the Unicode script names such as Arabic or Han, 157 and the derived properties Any and L&. A full list is given in the 158 pcrepattern documentation. Only the short names for properties are sup- 159 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let- 160 ter}, is not supported. Furthermore, in Perl, many properties may 161 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE 162 does not support this. 163 164 Validity of UTF-8 strings 165 166 When you set the PCRE_UTF8 flag, the strings passed as patterns and 167 subjects are (by default) checked for validity on entry to the relevant 168 functions. From release 7.3 of PCRE, the check is according the rules 169 of RFC 3629, which are themselves derived from the Unicode specifica- 170 tion. Earlier releases of PCRE followed the rules of RFC 2279, which 171 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current 172 check allows only values in the range U+0 to U+10FFFF, excluding U+D800 173 to U+DFFF. 174 175 The excluded code points are the "Low Surrogate Area" of Unicode, of 176 which the Unicode Standard says this: "The Low Surrogate Area does not 177 contain any character assignments, consequently no character code 178 charts or namelists are provided for this area. Surrogates are reserved 179 for use with UTF-16 and then must be used in pairs." The code points 180 that are encoded by UTF-16 pairs are available as independent code 181 points in the UTF-8 encoding. (In other words, the whole surrogate 182 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.) 183 184 If an invalid UTF-8 string is passed to PCRE, an error return 185 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know 186 that your strings are valid, and therefore want to skip these checks in 187 order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at 188 compile time or at run time, PCRE assumes that the pattern or subject 189 it is given (respectively) contains only valid UTF-8 codes. In this 190 case, it does not diagnose an invalid UTF-8 string. 191 192 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, 193 what happens depends on why the string is invalid. If the string con- 194 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a 195 string of characters in the range 0 to 0x7FFFFFFF. In other words, 196 apart from the initial validity test, PCRE (when in UTF-8 mode) handles 197 strings according to the more liberal rules of RFC 2279. However, if 198 the string does not even conform to RFC 2279, the result is undefined. 199 Your program may crash. 200 201 If you want to process strings of values in the full range 0 to 202 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can 203 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in 204 this situation, you will have to apply your own validity check. 205 206 General comments about UTF-8 mode 207 208 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a 209 two-byte UTF-8 character if the value is greater than 127. 210 211 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8 212 characters for values greater than \177. 213 214 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi- 215 vidual bytes, for example: \x{100}{3}. 216 217 4. The dot metacharacter matches one UTF-8 character instead of a sin- 218 gle byte. 219 220 5. The escape sequence \C can be used to match a single byte in UTF-8 221 mode, but its use can lead to some strange effects. This facility is 222 not available in the alternative matching function, pcre_dfa_exec(). 223 224 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly 225 test characters of any code value, but, by default, the characters that 226 PCRE recognizes as digits, spaces, or word characters remain the same 227 set as before, all with values less than 256. This remains true even 228 when PCRE is built to include Unicode property support, because to do 229 otherwise would slow down PCRE in many common cases. Note in particular 230 that this applies to \b and \B, because they are defined in terms of \w 231 and \W. If you really want to test for a wider sense of, say, "digit", 232 you can use explicit Unicode property tests such as \p{Nd}. Alterna- 233 tively, if you set the PCRE_UCP option, the way that the character 234 escapes work is changed so that Unicode properties are used to deter- 235 mine which characters match. There are more details in the section on 236 generic character types in the pcrepattern documentation. 237 238 7. Similarly, characters that match the POSIX named character classes 239 are all low-valued characters, unless the PCRE_UCP option is set. 240 241 8. However, the horizontal and vertical whitespace matching escapes 242 (\h, \H, \v, and \V) do match all the appropriate Unicode characters, 243 whether or not PCRE_UCP is set. 244 245 9. Case-insensitive matching applies only to characters whose values 246 are less than 128, unless PCRE is built with Unicode property support. 247 Even when Unicode property support is available, PCRE still uses its 248 own character tables when checking the case of low-valued characters, 249 so as not to degrade performance. The Unicode property information is 250 used only for characters with higher values. Furthermore, PCRE supports 251 case-insensitive matching only when there is a one-to-one mapping 252 between a letter's cases. There are a small number of many-to-one map- 253 pings in Unicode; these are not supported by PCRE. 254 255 256 AUTHOR 257 258 Philip Hazel 259 University Computing Service 260 Cambridge CB2 3QH, England. 261 262 Putting an actual email address here seems to have been a spam magnet, 263 so I've taken it away. If you want to email me, use my two initials, 264 followed by the two digits 10, at the domain cam.ac.uk. 265 266 267 REVISION 268 269 Last updated: 13 November 2010 270 Copyright (c) 1997-2010 University of Cambridge. 271 ------------------------------------------------------------------------------ 272 273 274 PCREBUILD(3) PCREBUILD(3) 275 276 277 NAME 278 PCRE - Perl-compatible regular expressions 279 280 281 PCRE BUILD-TIME OPTIONS 282 283 This document describes the optional features of PCRE that can be 284 selected when the library is compiled. It assumes use of the configure 285 script, where the optional features are selected or deselected by pro- 286 viding options to configure before running the make command. However, 287 the same options can be selected in both Unix-like and non-Unix-like 288 environments using the GUI facility of cmake-gui if you are using CMake 289 instead of configure to build PCRE. 290 291 There is a lot more information about building PCRE in non-Unix-like 292 environments in the file called NON_UNIX_USE, which is part of the PCRE 293 distribution. You should consult this file as well as the README file 294 if you are building in a non-Unix-like environment. 295 296 The complete list of options for configure (which includes the standard 297 ones such as the selection of the installation directory) can be 298 obtained by running 299 300 ./configure --help 301 302 The following sections include descriptions of options whose names 303 begin with --enable or --disable. These settings specify changes to the 304 defaults for the configure command. Because of the way that configure 305 works, --enable and --disable always come in pairs, so the complemen- 306 tary option always exists as well, but as it specifies the default, it 307 is not described. 308 309 310 C++ SUPPORT 311 312 By default, the configure script will search for a C++ compiler and C++ 313 header files. If it finds them, it automatically builds the C++ wrapper 314 library for PCRE. You can disable this by adding 315 316 --disable-cpp 317 318 to the configure command. 319 320 321 UTF-8 SUPPORT 322 323 To build PCRE with support for UTF-8 Unicode character strings, add 324 325 --enable-utf8 326 327 to the configure command. Of itself, this does not make PCRE treat 328 strings as UTF-8. As well as compiling PCRE with this option, you also 329 have have to set the PCRE_UTF8 option when you call the pcre_compile() 330 or pcre_compile2() functions. 331 332 If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE 333 expects its input to be either ASCII or UTF-8 (depending on the runtime 334 option). It is not possible to support both EBCDIC and UTF-8 codes in 335 the same version of the library. Consequently, --enable-utf8 and 336 --enable-ebcdic are mutually exclusive. 337 338 339 UNICODE CHARACTER PROPERTY SUPPORT 340 341 UTF-8 support allows PCRE to process character values greater than 255 342 in the strings that it handles. On its own, however, it does not pro- 343 vide any facilities for accessing the properties of such characters. If 344 you want to be able to use the pattern escapes \P, \p, and \X, which 345 refer to Unicode character properties, you must add 346 347 --enable-unicode-properties 348 349 to the configure command. This implies UTF-8 support, even if you have 350 not explicitly requested it. 351 352 Including Unicode property support adds around 30K of tables to the 353 PCRE library. Only the general category properties such as Lu and Nd 354 are supported. Details are given in the pcrepattern documentation. 355 356 357 CODE VALUE OF NEWLINE 358 359 By default, PCRE interprets the linefeed (LF) character as indicating 360 the end of a line. This is the normal newline character on Unix-like 361 systems. You can compile PCRE to use carriage return (CR) instead, by 362 adding 363 364 --enable-newline-is-cr 365 366 to the configure command. There is also a --enable-newline-is-lf 367 option, which explicitly specifies linefeed as the newline character. 368 369 Alternatively, you can specify that line endings are to be indicated by 370 the two character sequence CRLF. If you want this, add 371 372 --enable-newline-is-crlf 373 374 to the configure command. There is a fourth option, specified by 375 376 --enable-newline-is-anycrlf 377 378 which causes PCRE to recognize any of the three sequences CR, LF, or 379 CRLF as indicating a line ending. Finally, a fifth option, specified by 380 381 --enable-newline-is-any 382 383 causes PCRE to recognize any Unicode newline sequence. 384 385 Whatever line ending convention is selected when PCRE is built can be 386 overridden when the library functions are called. At build time it is 387 conventional to use the standard for your operating system. 388 389 390 WHAT \R MATCHES 391 392 By default, the sequence \R in a pattern matches any Unicode newline 393 sequence, whatever has been selected as the line ending sequence. If 394 you specify 395 396 --enable-bsr-anycrlf 397 398 the default is changed so that \R matches only CR, LF, or CRLF. What- 399 ever is selected when PCRE is built can be overridden when the library 400 functions are called. 401 402 403 BUILDING SHARED AND STATIC LIBRARIES 404 405 The PCRE building process uses libtool to build both shared and static 406 Unix libraries by default. You can suppress one of these by adding one 407 of 408 409 --disable-shared 410 --disable-static 411 412 to the configure command, as required. 413 414 415 POSIX MALLOC USAGE 416 417 When PCRE is called through the POSIX interface (see the pcreposix doc- 418 umentation), additional working storage is required for holding the 419 pointers to capturing substrings, because PCRE requires three integers 420 per substring, whereas the POSIX interface provides only two. If the 421 number of expected substrings is small, the wrapper function uses space 422 on the stack, because this is faster than using malloc() for each call. 423 The default threshold above which the stack is no longer used is 10; it 424 can be changed by adding a setting such as 425 426 --with-posix-malloc-threshold=20 427 428 to the configure command. 429 430 431 HANDLING VERY LARGE PATTERNS 432 433 Within a compiled pattern, offset values are used to point from one 434 part to another (for example, from an opening parenthesis to an alter- 435 nation metacharacter). By default, two-byte values are used for these 436 offsets, leading to a maximum size for a compiled pattern of around 437 64K. This is sufficient to handle all but the most gigantic patterns. 438 Nevertheless, some people do want to process truyl enormous patterns, 439 so it is possible to compile PCRE to use three-byte or four-byte off- 440 sets by adding a setting such as 441 442 --with-link-size=3 443 444 to the configure command. The value given must be 2, 3, or 4. Using 445 longer offsets slows down the operation of PCRE because it has to load 446 additional bytes when handling them. 447 448 449 AVOIDING EXCESSIVE STACK USAGE 450 451 When matching with the pcre_exec() function, PCRE implements backtrack- 452 ing by making recursive calls to an internal function called match(). 453 In environments where the size of the stack is limited, this can se- 454 verely limit PCRE's operation. (The Unix environment does not usually 455 suffer from this problem, but it may sometimes be necessary to increase 456 the maximum stack size. There is a discussion in the pcrestack docu- 457 mentation.) An alternative approach to recursion that uses memory from 458 the heap to remember data, instead of using recursive function calls, 459 has been implemented to work round the problem of limited stack size. 460 If you want to build a version of PCRE that works this way, add 461 462 --disable-stack-for-recursion 463 464 to the configure command. With this configuration, PCRE will use the 465 pcre_stack_malloc and pcre_stack_free variables to call memory manage- 466 ment functions. By default these point to malloc() and free(), but you 467 can replace the pointers so that your own functions are used instead. 468 469 Separate functions are provided rather than using pcre_malloc and 470 pcre_free because the usage is very predictable: the block sizes 471 requested are always the same, and the blocks are always freed in 472 reverse order. A calling program might be able to implement optimized 473 functions that perform better than malloc() and free(). PCRE runs 474 noticeably more slowly when built in this way. This option affects only 475 the pcre_exec() function; it is not relevant for pcre_dfa_exec(). 476 477 478 LIMITING PCRE RESOURCE USAGE 479 480 Internally, PCRE has a function called match(), which it calls repeat- 481 edly (sometimes recursively) when matching a pattern with the 482 pcre_exec() function. By controlling the maximum number of times this 483 function may be called during a single matching operation, a limit can 484 be placed on the resources used by a single call to pcre_exec(). The 485 limit can be changed at run time, as described in the pcreapi documen- 486 tation. The default is 10 million, but this can be changed by adding a 487 setting such as 488 489 --with-match-limit=500000 490 491 to the configure command. This setting has no effect on the 492 pcre_dfa_exec() matching function. 493 494 In some environments it is desirable to limit the depth of recursive 495 calls of match() more strictly than the total number of calls, in order 496 to restrict the maximum amount of stack (or heap, if --disable-stack- 497 for-recursion is specified) that is used. A second limit controls this; 498 it defaults to the value that is set for --with-match-limit, which 499 imposes no additional constraints. However, you can set a lower limit 500 by adding, for example, 501 502 --with-match-limit-recursion=10000 503 504 to the configure command. This value can also be overridden at run 505 time. 506 507 508 CREATING CHARACTER TABLES AT BUILD TIME 509 510 PCRE uses fixed tables for processing characters whose code values are 511 less than 256. By default, PCRE is built with a set of tables that are 512 distributed in the file pcre_chartables.c.dist. These tables are for 513 ASCII codes only. If you add 514 515 --enable-rebuild-chartables 516 517 to the configure command, the distributed tables are no longer used. 518 Instead, a program called dftables is compiled and run. This outputs 519 the source for new set of tables, created in the default locale of your 520 C runtime system. (This method of replacing the tables does not work if 521 you are cross compiling, because dftables is run on the local host. If 522 you need to create alternative tables when cross compiling, you will 523 have to do so "by hand".) 524 525 526 USING EBCDIC CODE 527 528 PCRE assumes by default that it will run in an environment where the 529 character code is ASCII (or Unicode, which is a superset of ASCII). 530 This is the case for most computer operating systems. PCRE can, how- 531 ever, be compiled to run in an EBCDIC environment by adding 532 533 --enable-ebcdic 534 535 to the configure command. This setting implies --enable-rebuild-charta- 536 bles. You should only use it if you know that you are in an EBCDIC 537 environment (for example, an IBM mainframe operating system). The 538 --enable-ebcdic option is incompatible with --enable-utf8. 539 540 541 PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT 542 543 By default, pcregrep reads all files as plain text. You can build it so 544 that it recognizes files whose names end in .gz or .bz2, and reads them 545 with libz or libbz2, respectively, by adding one or both of 546 547 --enable-pcregrep-libz 548 --enable-pcregrep-libbz2 549 550 to the configure command. These options naturally require that the rel- 551 evant libraries are installed on your system. Configuration will fail 552 if they are not. 553 554 555 PCRETEST OPTION FOR LIBREADLINE SUPPORT 556 557 If you add 558 559 --enable-pcretest-libreadline 560 561 to the configure command, pcretest is linked with the libreadline 562 library, and when its input is from a terminal, it reads it using the 563 readline() function. This provides line-editing and history facilities. 564 Note that libreadline is GPL-licensed, so if you distribute a binary of 565 pcretest linked in this way, there may be licensing issues. 566 567 Setting this option causes the -lreadline option to be added to the 568 pcretest build. In many operating environments with a sytem-installed 569 libreadline this is sufficient. However, in some environments (e.g. if 570 an unmodified distribution version of readline is in use), some extra 571 configuration may be necessary. The INSTALL file for libreadline says 572 this: 573 574 "Readline uses the termcap functions, but does not link with the 575 termcap or curses library itself, allowing applications which link 576 with readline the to choose an appropriate library." 577 578 If your environment has not been set up so that an appropriate library 579 is automatically included, you may need to add something like 580 581 LIBS="-ncurses" 582 583 immediately before the configure command. 584 585 586 SEE ALSO 587 588 pcreapi(3), pcre_config(3). 589 590 591 AUTHOR 592 593 Philip Hazel 594 University Computing Service 595 Cambridge CB2 3QH, England. 596 597 598 REVISION 599 600 Last updated: 29 September 2009 601 Copyright (c) 1997-2009 University of Cambridge. 602 ------------------------------------------------------------------------------ 603 604 605 PCREMATCHING(3) PCREMATCHING(3) 606 607 608 NAME 609 PCRE - Perl-compatible regular expressions 610 611 612 PCRE MATCHING ALGORITHMS 613 614 This document describes the two different algorithms that are available 615 in PCRE for matching a compiled regular expression against a given sub- 616 ject string. The "standard" algorithm is the one provided by the 617 pcre_exec() function. This works in the same was as Perl's matching 618 function, and provides a Perl-compatible matching operation. 619 620 An alternative algorithm is provided by the pcre_dfa_exec() function; 621 this operates in a different way, and is not Perl-compatible. It has 622 advantages and disadvantages compared with the standard algorithm, and 623 these are described below. 624 625 When there is only one possible way in which a given subject string can 626 match a pattern, the two algorithms give the same answer. A difference 627 arises, however, when there are multiple possibilities. For example, if 628 the pattern 629 630 ^<.*> 631 632 is matched against the string 633 634 <something> <something else> <something further> 635 636 there are three possible answers. The standard algorithm finds only one 637 of them, whereas the alternative algorithm finds all three. 638 639 640 REGULAR EXPRESSIONS AS TREES 641 642 The set of strings that are matched by a regular expression can be rep- 643 resented as a tree structure. An unlimited repetition in the pattern 644 makes the tree of infinite size, but it is still a tree. Matching the 645 pattern to a given subject string (from a given starting point) can be 646 thought of as a search of the tree. There are two ways to search a 647 tree: depth-first and breadth-first, and these correspond to the two 648 matching algorithms provided by PCRE. 649 650 651 THE STANDARD MATCHING ALGORITHM 652 653 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres- 654 sions", the standard algorithm is an "NFA algorithm". It conducts a 655 depth-first search of the pattern tree. That is, it proceeds along a 656 single path through the tree, checking that the subject matches what is 657 required. When there is a mismatch, the algorithm tries any alterna- 658 tives at the current point, and if they all fail, it backs up to the 659 previous branch point in the tree, and tries the next alternative 660 branch at that level. This often involves backing up (moving to the 661 left) in the subject string as well. The order in which repetition 662 branches are tried is controlled by the greedy or ungreedy nature of 663 the quantifier. 664 665 If a leaf node is reached, a matching string has been found, and at 666 that point the algorithm stops. Thus, if there is more than one possi- 667 ble match, this algorithm returns the first one that it finds. Whether 668 this is the shortest, the longest, or some intermediate length depends 669 on the way the greedy and ungreedy repetition quantifiers are specified 670 in the pattern. 671 672 Because it ends up with a single path through the tree, it is rela- 673 tively straightforward for this algorithm to keep track of the sub- 674 strings that are matched by portions of the pattern in parentheses. 675 This provides support for capturing parentheses and back references. 676 677 678 THE ALTERNATIVE MATCHING ALGORITHM 679 680 This algorithm conducts a breadth-first search of the tree. Starting 681 from the first matching point in the subject, it scans the subject 682 string from left to right, once, character by character, and as it does 683 this, it remembers all the paths through the tree that represent valid 684 matches. In Friedl's terminology, this is a kind of "DFA algorithm", 685 though it is not implemented as a traditional finite state machine (it 686 keeps multiple states active simultaneously). 687 688 Although the general principle of this matching algorithm is that it 689 scans the subject string only once, without backtracking, there is one 690 exception: when a lookaround assertion is encountered, the characters 691 following or preceding the current point have to be independently 692 inspected. 693 694 The scan continues until either the end of the subject is reached, or 695 there are no more unterminated paths. At this point, terminated paths 696 represent the different matching possibilities (if there are none, the 697 match has failed). Thus, if there is more than one possible match, 698 this algorithm finds all of them, and in particular, it finds the long- 699 est. The matches are returned in decreasing order of length. There is 700 an option to stop the algorithm after the first match (which is neces- 701 sarily the shortest) is found. 702 703 Note that all the matches that are found start at the same point in the 704 subject. If the pattern 705 706 cat(er(pillar)?)? 707 708 is matched against the string "the caterpillar catchment", the result 709 will be the three strings "caterpillar", "cater", and "cat" that start 710 at the fifth character of the subject. The algorithm does not automati- 711 cally move on to find matches that start at later positions. 712 713 There are a number of features of PCRE regular expressions that are not 714 supported by the alternative matching algorithm. They are as follows: 715 716 1. Because the algorithm finds all possible matches, the greedy or 717 ungreedy nature of repetition quantifiers is not relevant. Greedy and 718 ungreedy quantifiers are treated in exactly the same way. However, pos- 719 sessive quantifiers can make a difference when what follows could also 720 match what is quantified, for example in a pattern like this: 721 722 ^a++\w! 723 724 This pattern matches "aaab!" but not "aaa!", which would be matched by 725 a non-possessive quantifier. Similarly, if an atomic group is present, 726 it is matched as if it were a standalone pattern at the current point, 727 and the longest match is then "locked in" for the rest of the overall 728 pattern. 729 730 2. When dealing with multiple paths through the tree simultaneously, it 731 is not straightforward to keep track of captured substrings for the 732 different matching possibilities, and PCRE's implementation of this 733 algorithm does not attempt to do this. This means that no captured sub- 734 strings are available. 735 736 3. Because no substrings are captured, back references within the pat- 737 tern are not supported, and cause errors if encountered. 738 739 4. For the same reason, conditional expressions that use a backrefer- 740 ence as the condition or test for a specific group recursion are not 741 supported. 742 743 5. Because many paths through the tree may be active, the \K escape 744 sequence, which resets the start of the match when encountered (but may 745 be on some paths and not on others), is not supported. It causes an 746 error if encountered. 747 748 6. Callouts are supported, but the value of the capture_top field is 749 always 1, and the value of the capture_last field is always -1. 750 751 7. The \C escape sequence, which (in the standard algorithm) matches a 752 single byte, even in UTF-8 mode, is not supported because the alterna- 753 tive algorithm moves through the subject string one character at a 754 time, for all active paths through the tree. 755 756 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE) 757 are not supported. (*FAIL) is supported, and behaves like a failing 758 negative assertion. 759 760 761 ADVANTAGES OF THE ALTERNATIVE ALGORITHM 762 763 Using the alternative matching algorithm provides the following advan- 764 tages: 765 766 1. All possible matches (at a single point in the subject) are automat- 767 ically found, and in particular, the longest match is found. To find 768 more than one match using the standard algorithm, you have to do kludgy 769 things with callouts. 770 771 2. Because the alternative algorithm scans the subject string just 772 once, and never needs to backtrack, it is possible to pass very long 773 subject strings to the matching function in several pieces, checking 774 for partial matching each time. Although it is possible to do multi- 775 segment matching using the standard algorithm (pcre_exec()), by retain- 776 ing partially matched substrings, it is more complicated. The pcrepar- 777 tial documentation gives details of partial matching and discusses 778 multi-segment matching. 779 780 781 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM 782 783 The alternative algorithm suffers from a number of disadvantages: 784 785 1. It is substantially slower than the standard algorithm. This is 786 partly because it has to search for all possible matches, but is also 787 because it is less susceptible to optimization. 788 789 2. Capturing parentheses and back references are not supported. 790 791 3. Although atomic groups are supported, their use does not provide the 792 performance advantage that it does for the standard algorithm. 793 794 795 AUTHOR 796 797 Philip Hazel 798 University Computing Service 799 Cambridge CB2 3QH, England. 800 801 802 REVISION 803 804 Last updated: 17 November 2010 805 Copyright (c) 1997-2010 University of Cambridge. 806 ------------------------------------------------------------------------------ 807 808 809 PCREAPI(3) PCREAPI(3) 810 811 812 NAME 813 PCRE - Perl-compatible regular expressions 814 815 816 PCRE NATIVE API 817 818 #include <pcre.h> 819 820 pcre *pcre_compile(const char *pattern, int options, 821 const char **errptr, int *erroffset, 822 const unsigned char *tableptr); 823 824 pcre *pcre_compile2(const char *pattern, int options, 825 int *errorcodeptr, 826 const char **errptr, int *erroffset, 827 const unsigned char *tableptr); 828 829 pcre_extra *pcre_study(const pcre *code, int options, 830 const char **errptr); 831 832 int pcre_exec(const pcre *code, const pcre_extra *extra, 833 const char *subject, int length, int startoffset, 834 int options, int *ovector, int ovecsize); 835 836 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, 837 const char *subject, int length, int startoffset, 838 int options, int *ovector, int ovecsize, 839 int *workspace, int wscount); 840 841 int pcre_copy_named_substring(const pcre *code, 842 const char *subject, int *ovector, 843 int stringcount, const char *stringname, 844 char *buffer, int buffersize); 845 846 int pcre_copy_substring(const char *subject, int *ovector, 847 int stringcount, int stringnumber, char *buffer, 848 int buffersize); 849 850 int pcre_get_named_substring(const pcre *code, 851 const char *subject, int *ovector, 852 int stringcount, const char *stringname, 853 const char **stringptr); 854 855 int pcre_get_stringnumber(const pcre *code, 856 const char *name); 857 858 int pcre_get_stringtable_entries(const pcre *code, 859 const char *name, char **first, char **last); 860 861 int pcre_get_substring(const char *subject, int *ovector, 862 int stringcount, int stringnumber, 863 const char **stringptr); 864 865 int pcre_get_substring_list(const char *subject, 866 int *ovector, int stringcount, const char ***listptr); 867 868 void pcre_free_substring(const char *stringptr); 869 870 void pcre_free_substring_list(const char **stringptr); 871 872 const unsigned char *pcre_maketables(void); 873 874 int pcre_fullinfo(const pcre *code, const pcre_extra *extra, 875 int what, void *where); 876 877 int pcre_info(const pcre *code, int *optptr, int *firstcharptr); 878 879 int pcre_refcount(pcre *code, int adjust); 880 881 int pcre_config(int what, void *where); 882 883 char *pcre_version(void); 884 885 void *(*pcre_malloc)(size_t); 886 887 void (*pcre_free)(void *); 888 889 void *(*pcre_stack_malloc)(size_t); 890 891 void (*pcre_stack_free)(void *); 892 893 int (*pcre_callout)(pcre_callout_block *); 894 895 896 PCRE API OVERVIEW 897 898 PCRE has its own native API, which is described in this document. There 899 are also some wrapper functions that correspond to the POSIX regular 900 expression API. These are described in the pcreposix documentation. 901 Both of these APIs define a set of C function calls. A C++ wrapper is 902 distributed with PCRE. It is documented in the pcrecpp page. 903 904 The native API C function prototypes are defined in the header file 905 pcre.h, and on Unix systems the library itself is called libpcre. It 906 can normally be accessed by adding -lpcre to the command for linking an 907 application that uses PCRE. The header file defines the macros 908 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num- 909 bers for the library. Applications can use these to include support 910 for different releases of PCRE. 911 912 In a Windows environment, if you want to statically link an application 913 program against a non-dll pcre.a file, you must define PCRE_STATIC 914 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal- 915 loc() and pcre_free() exported functions will be declared 916 __declspec(dllimport), with unwanted results. 917 918 The functions pcre_compile(), pcre_compile2(), pcre_study(), and 919 pcre_exec() are used for compiling and matching regular expressions in 920 a Perl-compatible manner. A sample program that demonstrates the sim- 921 plest way of using them is provided in the file called pcredemo.c in 922 the PCRE source distribution. A listing of this program is given in the 923 pcredemo documentation, and the pcresample documentation describes how 924 to compile and run it. 925 926 A second matching function, pcre_dfa_exec(), which is not Perl-compati- 927 ble, is also provided. This uses a different algorithm for the match- 928 ing. The alternative algorithm finds all possible matches (at a given 929 point in the subject), and scans the subject just once (unless there 930 are lookbehind assertions). However, this algorithm does not return 931 captured substrings. A description of the two matching algorithms and 932 their advantages and disadvantages is given in the pcrematching docu- 933 mentation. 934 935 In addition to the main compiling and matching functions, there are 936 convenience functions for extracting captured substrings from a subject 937 string that is matched by pcre_exec(). They are: 938 939 pcre_copy_substring() 940 pcre_copy_named_substring() 941 pcre_get_substring() 942 pcre_get_named_substring() 943 pcre_get_substring_list() 944 pcre_get_stringnumber() 945 pcre_get_stringtable_entries() 946 947 pcre_free_substring() and pcre_free_substring_list() are also provided, 948 to free the memory used for extracted strings. 949 950 The function pcre_maketables() is used to build a set of character 951 tables in the current locale for passing to pcre_compile(), 952 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is 953 provided for specialist use. Most commonly, no special tables are 954 passed, in which case internal tables that are generated when PCRE is 955 built are used. 956 957 The function pcre_fullinfo() is used to find out information about a 958 compiled pattern; pcre_info() is an obsolete version that returns only 959 some of the available information, but is retained for backwards com- 960 patibility. The function pcre_version() returns a pointer to a string 961 containing the version of PCRE and its date of release. 962 963 The function pcre_refcount() maintains a reference count in a data 964 block containing a compiled pattern. This is provided for the benefit 965 of object-oriented applications. 966 967 The global variables pcre_malloc and pcre_free initially contain the 968 entry points of the standard malloc() and free() functions, respec- 969 tively. PCRE calls the memory management functions via these variables, 970 so a calling program can replace them if it wishes to intercept the 971 calls. This should be done before calling any PCRE functions. 972 973 The global variables pcre_stack_malloc and pcre_stack_free are also 974 indirections to memory management functions. These special functions 975 are used only when PCRE is compiled to use the heap for remembering 976 data, instead of recursive function calls, when running the pcre_exec() 977 function. See the pcrebuild documentation for details of how to do 978 this. It is a non-standard way of building PCRE, for use in environ- 979 ments that have limited stacks. Because of the greater use of memory 980 management, it runs more slowly. Separate functions are provided so 981 that special-purpose external code can be used for this case. When 982 used, these functions are always called in a stack-like manner (last 983 obtained, first freed), and always for memory blocks of the same size. 984 There is a discussion about PCRE's stack usage in the pcrestack docu- 985 mentation. 986 987 The global variable pcre_callout initially contains NULL. It can be set 988 by the caller to a "callout" function, which PCRE will then call at 989 specified points during a matching operation. Details are given in the 990 pcrecallout documentation. 991 992 993 NEWLINES 994 995 PCRE supports five different conventions for indicating line breaks in 996 strings: a single CR (carriage return) character, a single LF (line- 997 feed) character, the two-character sequence CRLF, any of the three pre- 998 ceding, or any Unicode newline sequence. The Unicode newline sequences 999 are the three just mentioned, plus the single characters VT (vertical 1000 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line 1001 separator, U+2028), and PS (paragraph separator, U+2029). 1002 1003 Each of the first three conventions is used by at least one operating 1004 system as its standard newline sequence. When PCRE is built, a default 1005 can be specified. The default default is LF, which is the Unix stan- 1006 dard. When PCRE is run, the default can be overridden, either when a 1007 pattern is compiled, or when it is matched. 1008 1009 At compile time, the newline convention can be specified by the options 1010 argument of pcre_compile(), or it can be specified by special text at 1011 the start of the pattern itself; this overrides any other settings. See 1012 the pcrepattern page for details of the special character sequences. 1013 1014 In the PCRE documentation the word "newline" is used to mean "the char- 1015 acter or pair of characters that indicate a line break". The choice of 1016 newline convention affects the handling of the dot, circumflex, and 1017 dollar metacharacters, the handling of #-comments in /x mode, and, when 1018 CRLF is a recognized line ending sequence, the match position advance- 1019 ment for a non-anchored pattern. There is more detail about this in the 1020 section on pcre_exec() options below. 1021 1022 The choice of newline convention does not affect the interpretation of 1023 the \n or \r escape sequences, nor does it affect what \R matches, 1024 which is controlled in a similar way, but by separate options. 1025 1026 1027 MULTITHREADING 1028 1029 The PCRE functions can be used in multi-threading applications, with 1030 the proviso that the memory management functions pointed to by 1031 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the 1032 callout function pointed to by pcre_callout, are shared by all threads. 1033 1034 The compiled form of a regular expression is not altered during match- 1035 ing, so the same compiled pattern can safely be used by several threads 1036 at once. 1037 1038 1039 SAVING PRECOMPILED PATTERNS FOR LATER USE 1040 1041 The compiled form of a regular expression can be saved and re-used at a 1042 later time, possibly by a different program, and even on a host other 1043 than the one on which it was compiled. Details are given in the 1044 pcreprecompile documentation. However, compiling a regular expression 1045 with one version of PCRE for use with a different version is not guar- 1046 anteed to work and may cause crashes. 1047 1048 1049 CHECKING BUILD-TIME OPTIONS 1050 1051 int pcre_config(int what, void *where); 1052 1053 The function pcre_config() makes it possible for a PCRE client to dis- 1054 cover which optional features have been compiled into the PCRE library. 1055 The pcrebuild documentation has more details about these optional fea- 1056 tures. 1057 1058 The first argument for pcre_config() is an integer, specifying which 1059 information is required; the second argument is a pointer to a variable 1060 into which the information is placed. The following information is 1061 available: 1062 1063 PCRE_CONFIG_UTF8 1064 1065 The output is an integer that is set to one if UTF-8 support is avail- 1066 able; otherwise it is set to zero. 1067 1068 PCRE_CONFIG_UNICODE_PROPERTIES 1069 1070 The output is an integer that is set to one if support for Unicode 1071 character properties is available; otherwise it is set to zero. 1072 1073 PCRE_CONFIG_NEWLINE 1074 1075 The output is an integer whose value specifies the default character 1076 sequence that is recognized as meaning "newline". The four values that 1077 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF, 1078 and -1 for ANY. Though they are derived from ASCII, the same values 1079 are returned in EBCDIC environments. The default should normally corre- 1080 spond to the standard sequence for your operating system. 1081 1082 PCRE_CONFIG_BSR 1083 1084 The output is an integer whose value indicates what character sequences 1085 the \R escape sequence matches by default. A value of 0 means that \R 1086 matches any Unicode line ending sequence; a value of 1 means that \R 1087 matches only CR, LF, or CRLF. The default can be overridden when a pat- 1088 tern is compiled or matched. 1089 1090 PCRE_CONFIG_LINK_SIZE 1091 1092 The output is an integer that contains the number of bytes used for 1093 internal linkage in compiled regular expressions. The value is 2, 3, or 1094 4. Larger values allow larger regular expressions to be compiled, at 1095 the expense of slower matching. The default value of 2 is sufficient 1096 for all but the most massive patterns, since it allows the compiled 1097 pattern to be up to 64K in size. 1098 1099 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD 1100 1101 The output is an integer that contains the threshold above which the 1102 POSIX interface uses malloc() for output vectors. Further details are 1103 given in the pcreposix documentation. 1104 1105 PCRE_CONFIG_MATCH_LIMIT 1106 1107 The output is a long integer that gives the default limit for the num- 1108 ber of internal matching function calls in a pcre_exec() execution. 1109 Further details are given with pcre_exec() below. 1110 1111 PCRE_CONFIG_MATCH_LIMIT_RECURSION 1112 1113 The output is a long integer that gives the default limit for the depth 1114 of recursion when calling the internal matching function in a 1115 pcre_exec() execution. Further details are given with pcre_exec() 1116 below. 1117 1118 PCRE_CONFIG_STACKRECURSE 1119 1120 The output is an integer that is set to one if internal recursion when 1121 running pcre_exec() is implemented by recursive function calls that use 1122 the stack to remember their state. This is the usual way that PCRE is 1123 compiled. The output is zero if PCRE was compiled to use blocks of data 1124 on the heap instead of recursive function calls. In this case, 1125 pcre_stack_malloc and pcre_stack_free are called to manage memory 1126 blocks on the heap, thus avoiding the use of the stack. 1127 1128 1129 COMPILING A PATTERN 1130 1131 pcre *pcre_compile(const char *pattern, int options, 1132 const char **errptr, int *erroffset, 1133 const unsigned char *tableptr); 1134 1135 pcre *pcre_compile2(const char *pattern, int options, 1136 int *errorcodeptr, 1137 const char **errptr, int *erroffset, 1138 const unsigned char *tableptr); 1139 1140 Either of the functions pcre_compile() or pcre_compile2() can be called 1141 to compile a pattern into an internal form. The only difference between 1142 the two interfaces is that pcre_compile2() has an additional argument, 1143 errorcodeptr, via which a numerical error code can be returned. To 1144 avoid too much repetition, we refer just to pcre_compile() below, but 1145 the information applies equally to pcre_compile2(). 1146 1147 The pattern is a C string terminated by a binary zero, and is passed in 1148 the pattern argument. A pointer to a single block of memory that is 1149 obtained via pcre_malloc is returned. This contains the compiled code 1150 and related data. The pcre type is defined for the returned block; this 1151 is a typedef for a structure whose contents are not externally defined. 1152 It is up to the caller to free the memory (via pcre_free) when it is no 1153 longer required. 1154 1155 Although the compiled code of a PCRE regex is relocatable, that is, it 1156 does not depend on memory location, the complete pcre data block is not 1157 fully relocatable, because it may contain a copy of the tableptr argu- 1158 ment, which is an address (see below). 1159 1160 The options argument contains various bit settings that affect the com- 1161 pilation. It should be zero if no options are required. The available 1162 options are described below. Some of them (in particular, those that 1163 are compatible with Perl, but some others as well) can also be set and 1164 unset from within the pattern (see the detailed description in the 1165 pcrepattern documentation). For those options that can be different in 1166 different parts of the pattern, the contents of the options argument 1167 specifies their settings at the start of compilation and execution. The 1168 PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and 1169 PCRE_NO_START_OPT options can be set at the time of matching as well as 1170 at compile time. 1171 1172 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise, 1173 if compilation of a pattern fails, pcre_compile() returns NULL, and 1174 sets the variable pointed to by errptr to point to a textual error mes- 1175 sage. This is a static string that is part of the library. You must not 1176 try to free it. The offset from the start of the pattern to the byte 1177 that was being processed when the error was discovered is placed in the 1178 variable pointed to by erroffset, which must not be NULL. If it is, an 1179 immediate error is given. Some errors are not detected until checks are 1180 carried out when the whole pattern has been scanned; in this case the 1181 offset is set to the end of the pattern. 1182 1183 Note that the offset is in bytes, not characters, even in UTF-8 mode. 1184 It may point into the middle of a UTF-8 character (for example, when 1185 PCRE_ERROR_BADUTF8 is returned for an invalid UTF-8 string). 1186 1187 If pcre_compile2() is used instead of pcre_compile(), and the error- 1188 codeptr argument is not NULL, a non-zero error code number is returned 1189 via this argument in the event of an error. This is in addition to the 1190 textual error message. Error codes and messages are listed below. 1191 1192 If the final argument, tableptr, is NULL, PCRE uses a default set of 1193 character tables that are built when PCRE is compiled, using the 1194 default C locale. Otherwise, tableptr must be an address that is the 1195 result of a call to pcre_maketables(). This value is stored with the 1196 compiled pattern, and used again by pcre_exec(), unless another table 1197 pointer is passed to it. For more discussion, see the section on locale 1198 support below. 1199 1200 This code fragment shows a typical straightforward call to pcre_com- 1201 pile(): 1202 1203 pcre *re; 1204 const char *error; 1205 int erroffset; 1206 re = pcre_compile( 1207 "^A.*Z", /* the pattern */ 1208 0, /* default options */ 1209 &error, /* for error message */ 1210 &erroffset, /* for error offset */ 1211 NULL); /* use default character tables */ 1212 1213 The following names for option bits are defined in the pcre.h header 1214 file: 1215 1216 PCRE_ANCHORED 1217 1218 If this bit is set, the pattern is forced to be "anchored", that is, it 1219 is constrained to match only at the first matching point in the string 1220 that is being searched (the "subject string"). This effect can also be 1221 achieved by appropriate constructs in the pattern itself, which is the 1222 only way to do it in Perl. 1223 1224 PCRE_AUTO_CALLOUT 1225 1226 If this bit is set, pcre_compile() automatically inserts callout items, 1227 all with number 255, before each pattern item. For discussion of the 1228 callout facility, see the pcrecallout documentation. 1229 1230 PCRE_BSR_ANYCRLF 1231 PCRE_BSR_UNICODE 1232 1233 These options (which are mutually exclusive) control what the \R escape 1234 sequence matches. The choice is either to match only CR, LF, or CRLF, 1235 or to match any Unicode newline sequence. The default is specified when 1236 PCRE is built. It can be overridden from within the pattern, or by set- 1237 ting an option when a compiled pattern is matched. 1238 1239 PCRE_CASELESS 1240 1241 If this bit is set, letters in the pattern match both upper and lower 1242 case letters. It is equivalent to Perl's /i option, and it can be 1243 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE 1244 always understands the concept of case for characters whose values are 1245 less than 128, so caseless matching is always possible. For characters 1246 with higher values, the concept of case is supported if PCRE is com- 1247 piled with Unicode property support, but not otherwise. If you want to 1248 use caseless matching for characters 128 and above, you must ensure 1249 that PCRE is compiled with Unicode property support as well as with 1250 UTF-8 support. 1251 1252 PCRE_DOLLAR_ENDONLY 1253 1254 If this bit is set, a dollar metacharacter in the pattern matches only 1255 at the end of the subject string. Without this option, a dollar also 1256 matches immediately before a newline at the end of the string (but not 1257 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored 1258 if PCRE_MULTILINE is set. There is no equivalent to this option in 1259 Perl, and no way to set it within a pattern. 1260 1261 PCRE_DOTALL 1262 1263 If this bit is set, a dot metacharacter in the pattern matches a char- 1264 acter of any value, including one that indicates a newline. However, it 1265 only ever matches one character, even if newlines are coded as CRLF. 1266 Without this option, a dot does not match when the current position is 1267 at a newline. This option is equivalent to Perl's /s option, and it can 1268 be changed within a pattern by a (?s) option setting. A negative class 1269 such as [^a] always matches newline characters, independent of the set- 1270 ting of this option. 1271 1272 PCRE_DUPNAMES 1273 1274 If this bit is set, names used to identify capturing subpatterns need 1275 not be unique. This can be helpful for certain types of pattern when it 1276 is known that only one instance of the named subpattern can ever be 1277 matched. There are more details of named subpatterns below; see also 1278 the pcrepattern documentation. 1279 1280 PCRE_EXTENDED 1281 1282 If this bit is set, whitespace data characters in the pattern are 1283 totally ignored except when escaped or inside a character class. White- 1284 space does not include the VT character (code 11). In addition, charac- 1285 ters between an unescaped # outside a character class and the next new- 1286 line, inclusive, are also ignored. This is equivalent to Perl's /x 1287 option, and it can be changed within a pattern by a (?x) option set- 1288 ting. 1289 1290 Which characters are interpreted as newlines is controlled by the 1291 options passed to pcre_compile() or by a special sequence at the start 1292 of the pattern, as described in the section entitled "Newline conven- 1293 tions" in the pcrepattern documentation. Note that the end of this type 1294 of comment is a literal newline sequence in the pattern; escape 1295 sequences that happen to represent a newline do not count. 1296 1297 This option makes it possible to include comments inside complicated 1298 patterns. Note, however, that this applies only to data characters. 1299 Whitespace characters may never appear within special character 1300 sequences in a pattern, for example within the sequence (?( that intro- 1301 duces a conditional subpattern. 1302 1303 PCRE_EXTRA 1304 1305 This option was invented in order to turn on additional functionality 1306 of PCRE that is incompatible with Perl, but it is currently of very 1307 little use. When set, any backslash in a pattern that is followed by a 1308 letter that has no special meaning causes an error, thus reserving 1309 these combinations for future expansion. By default, as in Perl, a 1310 backslash followed by a letter with no special meaning is treated as a 1311 literal. (Perl can, however, be persuaded to give an error for this, by 1312 running it with the -w option.) There are at present no other features 1313 controlled by this option. It can also be set by a (?X) option setting 1314 within a pattern. 1315 1316 PCRE_FIRSTLINE 1317 1318 If this option is set, an unanchored pattern is required to match 1319 before or at the first newline in the subject string, though the 1320 matched text may continue over the newline. 1321 1322 PCRE_JAVASCRIPT_COMPAT 1323 1324 If this option is set, PCRE's behaviour is changed in some ways so that 1325 it is compatible with JavaScript rather than Perl. The changes are as 1326 follows: 1327 1328 (1) A lone closing square bracket in a pattern causes a compile-time 1329 error, because this is illegal in JavaScript (by default it is treated 1330 as a data character). Thus, the pattern AB]CD becomes illegal when this 1331 option is set. 1332 1333 (2) At run time, a back reference to an unset subpattern group matches 1334 an empty string (by default this causes the current matching alterna- 1335 tive to fail). A pattern such as (\1)(a) succeeds when this option is 1336 set (assuming it can find an "a" in the subject), whereas it fails by 1337 default, for Perl compatibility. 1338 1339 PCRE_MULTILINE 1340 1341 By default, PCRE treats the subject string as consisting of a single 1342 line of characters (even if it actually contains newlines). The "start 1343 of line" metacharacter (^) matches only at the start of the string, 1344 while the "end of line" metacharacter ($) matches only at the end of 1345 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY 1346 is set). This is the same as Perl. 1347 1348 When PCRE_MULTILINE it is set, the "start of line" and "end of line" 1349 constructs match immediately following or immediately before internal 1350 newlines in the subject string, respectively, as well as at the very 1351 start and end. This is equivalent to Perl's /m option, and it can be 1352 changed within a pattern by a (?m) option setting. If there are no new- 1353 lines in a subject string, or no occurrences of ^ or $ in a pattern, 1354 setting PCRE_MULTILINE has no effect. 1355 1356 PCRE_NEWLINE_CR 1357 PCRE_NEWLINE_LF 1358 PCRE_NEWLINE_CRLF 1359 PCRE_NEWLINE_ANYCRLF 1360 PCRE_NEWLINE_ANY 1361 1362 These options override the default newline definition that was chosen 1363 when PCRE was built. Setting the first or the second specifies that a 1364 newline is indicated by a single character (CR or LF, respectively). 1365 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the 1366 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies 1367 that any of the three preceding sequences should be recognized. Setting 1368 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be 1369 recognized. The Unicode newline sequences are the three just mentioned, 1370 plus the single characters VT (vertical tab, U+000B), FF (formfeed, 1371 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS 1372 (paragraph separator, U+2029). The last two are recognized only in 1373 UTF-8 mode. 1374 1375 The newline setting in the options word uses three bits that are 1376 treated as a number, giving eight possibilities. Currently only six are 1377 used (default plus the five values above). This means that if you set 1378 more than one newline option, the combination may or may not be sensi- 1379 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to 1380 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and 1381 cause an error. 1382 1383 The only time that a line break in a pattern is specially recognized 1384 when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace 1385 characters, and so are ignored in this mode. Also, an unescaped # out- 1386 side a character class indicates a comment that lasts until after the 1387 next line break sequence. In other circumstances, line break sequences 1388 in patterns are treated as literal data. 1389 1390 The newline option that is set at compile time becomes the default that 1391 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden. 1392 1393 PCRE_NO_AUTO_CAPTURE 1394 1395 If this option is set, it disables the use of numbered capturing paren- 1396 theses in the pattern. Any opening parenthesis that is not followed by 1397 ? behaves as if it were followed by ?: but named parentheses can still 1398 be used for capturing (and they acquire numbers in the usual way). 1399 There is no equivalent of this option in Perl. 1400 1401 NO_START_OPTIMIZE 1402 1403 This is an option that acts at matching time; that is, it is really an 1404 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile 1405 time, it is remembered with the compiled pattern and assumed at match- 1406 ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE 1407 below. 1408 1409 PCRE_UCP 1410 1411 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W, 1412 \w, and some of the POSIX character classes. By default, only ASCII 1413 characters are recognized, but if PCRE_UCP is set, Unicode properties 1414 are used instead to classify characters. More details are given in the 1415 section on generic character types in the pcrepattern page. If you set 1416 PCRE_UCP, matching one of the items it affects takes much longer. The 1417 option is available only if PCRE has been compiled with Unicode prop- 1418 erty support. 1419 1420 PCRE_UNGREEDY 1421 1422 This option inverts the "greediness" of the quantifiers so that they 1423 are not greedy by default, but become greedy if followed by "?". It is 1424 not compatible with Perl. It can also be set by a (?U) option setting 1425 within the pattern. 1426 1427 PCRE_UTF8 1428 1429 This option causes PCRE to regard both the pattern and the subject as 1430 strings of UTF-8 characters instead of single-byte character strings. 1431 However, it is available only when PCRE is built to include UTF-8 sup- 1432 port. If not, the use of this option provokes an error. Details of how 1433 this option changes the behaviour of PCRE are given in the section on 1434 UTF-8 support in the main pcre page. 1435 1436 PCRE_NO_UTF8_CHECK 1437 1438 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is 1439 automatically checked. There is a discussion about the validity of 1440 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of 1441 bytes is found, pcre_compile() returns an error. If you already know 1442 that your pattern is valid, and you want to skip this check for perfor- 1443 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is 1444 set, the effect of passing an invalid UTF-8 string as a pattern is 1445 undefined. It may cause your program to crash. Note that this option 1446 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the 1447 UTF-8 validity checking of subject strings. 1448 1449 1450 COMPILATION ERROR CODES 1451 1452 The following table lists the error codes than may be returned by 1453 pcre_compile2(), along with the error messages that may be returned by 1454 both compiling functions. As PCRE has developed, some error codes have 1455 fallen out of use. To avoid confusion, they have not been re-used. 1456 1457 0 no error 1458 1 \ at end of pattern 1459 2 \c at end of pattern 1460 3 unrecognized character follows \ 1461 4 numbers out of order in {} quantifier 1462 5 number too big in {} quantifier 1463 6 missing terminating ] for character class 1464 7 invalid escape sequence in character class 1465 8 range out of order in character class 1466 9 nothing to repeat 1467 10 [this code is not in use] 1468 11 internal error: unexpected repeat 1469 12 unrecognized character after (? or (?- 1470 13 POSIX named classes are supported only within a class 1471 14 missing ) 1472 15 reference to non-existent subpattern 1473 16 erroffset passed as NULL 1474 17 unknown option bit(s) set 1475 18 missing ) after comment 1476 19 [this code is not in use] 1477 20 regular expression is too large 1478 21 failed to get memory 1479 22 unmatched parentheses 1480 23 internal error: code overflow 1481 24 unrecognized character after (?< 1482 25 lookbehind assertion is not fixed length 1483 26 malformed number or name after (?( 1484 27 conditional group contains more than two branches 1485 28 assertion expected after (?( 1486 29 (?R or (?[+-]digits must be followed by ) 1487 30 unknown POSIX class name 1488 31 POSIX collating elements are not supported 1489 32 this version of PCRE is not compiled with PCRE_UTF8 support 1490 33 [this code is not in use] 1491 34 character value in \x{...} sequence is too large 1492 35 invalid condition (?(0) 1493 36 \C not allowed in lookbehind assertion 1494 37 PCRE does not support \L, \l, \N, \U, or \u 1495 38 number after (?C is > 255 1496 39 closing ) for (?C expected 1497 40 recursive call could loop indefinitely 1498 41 unrecognized character after (?P 1499 42 syntax error in subpattern name (missing terminator) 1500 43 two named subpatterns have the same name 1501 44 invalid UTF-8 string 1502 45 support for \P, \p, and \X has not been compiled 1503 46 malformed \P or \p sequence 1504 47 unknown property name after \P or \p 1505 48 subpattern name is too long (maximum 32 characters) 1506 49 too many named subpatterns (maximum 10000) 1507 50 [this code is not in use] 1508 51 octal value is greater than \377 (not in UTF-8 mode) 1509 52 internal error: overran compiling workspace 1510 53 internal error: previously-checked referenced subpattern 1511 not found 1512 54 DEFINE group contains more than one branch 1513 55 repeating a DEFINE group is not allowed 1514 56 inconsistent NEWLINE options 1515 57 \g is not followed by a braced, angle-bracketed, or quoted 1516 name/number or by a plain number 1517 58 a numbered reference must not be zero 1518 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT) 1519 60 (*VERB) not recognized 1520 61 number is too big 1521 62 subpattern name expected 1522 63 digit expected after (?+ 1523 64 ] is an invalid data character in JavaScript compatibility mode 1524 65 different names for subpatterns of the same number are 1525 not allowed 1526 66 (*MARK) must have an argument 1527 67 this version of PCRE is not compiled with PCRE_UCP support 1528 1529 The numbers 32 and 10000 in errors 48 and 49 are defaults; different 1530 values may be used if the limits were changed when PCRE was built. 1531 1532 1533 STUDYING A PATTERN 1534 1535 pcre_extra *pcre_study(const pcre *code, int options 1536 const char **errptr); 1537 1538 If a compiled pattern is going to be used several times, it is worth 1539 spending more time analyzing it in order to speed up the time taken for 1540 matching. The function pcre_study() takes a pointer to a compiled pat- 1541 tern as its first argument. If studying the pattern produces additional 1542 information that will help speed up matching, pcre_study() returns a 1543 pointer to a pcre_extra block, in which the study_data field points to 1544 the results of the study. 1545 1546 The returned value from pcre_study() can be passed directly to 1547 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con- 1548 tains other fields that can be set by the caller before the block is 1549 passed; these are described below in the section on matching a pattern. 1550 1551 If studying the pattern does not produce any useful information, 1552 pcre_study() returns NULL. In that circumstance, if the calling program 1553 wants to pass any of the other fields to pcre_exec() or 1554 pcre_dfa_exec(), it must set up its own pcre_extra block. 1555 1556 The second argument of pcre_study() contains option bits. At present, 1557 no options are defined, and this argument should always be zero. 1558 1559 The third argument for pcre_study() is a pointer for an error message. 1560 If studying succeeds (even if no data is returned), the variable it 1561 points to is set to NULL. Otherwise it is set to point to a textual 1562 error message. This is a static string that is part of the library. You 1563 must not try to free it. You should test the error pointer for NULL 1564 after calling pcre_study(), to be sure that it has run successfully. 1565 1566 This is a typical call to pcre_study(): 1567 1568 pcre_extra *pe; 1569 pe = pcre_study( 1570 re, /* result of pcre_compile() */ 1571 0, /* no options exist */ 1572 &error); /* set to NULL or points to a message */ 1573 1574 Studying a pattern does two things: first, a lower bound for the length 1575 of subject string that is needed to match the pattern is computed. This 1576 does not mean that there are any strings of that length that match, but 1577 it does guarantee that no shorter strings match. The value is used by 1578 pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to 1579 match strings that are shorter than the lower bound. You can find out 1580 the value in a calling program via the pcre_fullinfo() function. 1581 1582 Studying a pattern is also useful for non-anchored patterns that do not 1583 have a single fixed starting character. A bitmap of possible starting 1584 bytes is created. This speeds up finding a position in the subject at 1585 which to start matching. 1586 1587 The two optimizations just described can be disabled by setting the 1588 PCRE_NO_START_OPTIMIZE option when calling pcre_exec() or 1589 pcre_dfa_exec(). You might want to do this if your pattern contains 1590 callouts or (*MARK), and you want to make use of these facilities in 1591 cases where matching fails. See the discussion of PCRE_NO_START_OPTI- 1592 MIZE below. 1593 1594 1595 LOCALE SUPPORT 1596 1597 PCRE handles caseless matching, and determines whether characters are 1598 letters, digits, or whatever, by reference to a set of tables, indexed 1599 by character value. When running in UTF-8 mode, this applies only to 1600 characters with codes less than 128. By default, higher-valued codes 1601 never match escapes such as \w or \d, but they can be tested with \p if 1602 PCRE is built with Unicode character property support. Alternatively, 1603 the PCRE_UCP option can be set at compile time; this causes \w and 1604 friends to use Unicode property support instead of built-in tables. The 1605 use of locales with Unicode is discouraged. If you are handling charac- 1606 ters with codes greater than 128, you should either use UTF-8 and Uni- 1607 code, or use locales, but not try to mix the two. 1608 1609 PCRE contains an internal set of tables that are used when the final 1610 argument of pcre_compile() is NULL. These are sufficient for many 1611 applications. Normally, the internal tables recognize only ASCII char- 1612 acters. However, when PCRE is built, it is possible to cause the inter- 1613 nal tables to be rebuilt in the default "C" locale of the local system, 1614 which may cause them to be different. 1615 1616 The internal tables can always be overridden by tables supplied by the 1617 application that calls PCRE. These may be created in a different locale 1618 from the default. As more and more applications change to using Uni- 1619 code, the need for this locale support is expected to die away. 1620 1621 External tables are built by calling the pcre_maketables() function, 1622 which has no arguments, in the relevant locale. The result can then be 1623 passed to pcre_compile() or pcre_exec() as often as necessary. For 1624 example, to build and use tables that are appropriate for the French 1625 locale (where accented characters with values greater than 128 are 1626 treated as letters), the following code could be used: 1627 1628 setlocale(LC_CTYPE, "fr_FR"); 1629 tables = pcre_maketables(); 1630 re = pcre_compile(..., tables); 1631 1632 The locale name "fr_FR" is used on Linux and other Unix-like systems; 1633 if you are using Windows, the name for the French locale is "french". 1634 1635 When pcre_maketables() runs, the tables are built in memory that is 1636 obtained via pcre_malloc. It is the caller's responsibility to ensure 1637 that the memory containing the tables remains available for as long as 1638 it is needed. 1639 1640 The pointer that is passed to pcre_compile() is saved with the compiled 1641 pattern, and the same tables are used via this pointer by pcre_study() 1642 and normally also by pcre_exec(). Thus, by default, for any single pat- 1643 tern, compilation, studying and matching all happen in the same locale, 1644 but different patterns can be compiled in different locales. 1645 1646 It is possible to pass a table pointer or NULL (indicating the use of 1647 the internal tables) to pcre_exec(). Although not intended for this 1648 purpose, this facility could be used to match a pattern in a different 1649 locale from the one in which it was compiled. Passing table pointers at 1650 run time is discussed below in the section on matching a pattern. 1651 1652 1653 INFORMATION ABOUT A PATTERN 1654 1655 int pcre_fullinfo(const pcre *code, const pcre_extra *extra, 1656 int what, void *where); 1657 1658 The pcre_fullinfo() function returns information about a compiled pat- 1659 tern. It replaces the obsolete pcre_info() function, which is neverthe- 1660 less retained for backwards compability (and is documented below). 1661 1662 The first argument for pcre_fullinfo() is a pointer to the compiled 1663 pattern. The second argument is the result of pcre_study(), or NULL if 1664 the pattern was not studied. The third argument specifies which piece 1665 of information is required, and the fourth argument is a pointer to a 1666 variable to receive the data. The yield of the function is zero for 1667 success, or one of the following negative numbers: 1668 1669 PCRE_ERROR_NULL the argument code was NULL 1670 the argument where was NULL 1671 PCRE_ERROR_BADMAGIC the "magic number" was not found 1672 PCRE_ERROR_BADOPTION the value of what was invalid 1673 1674 The "magic number" is placed at the start of each compiled pattern as 1675 an simple check against passing an arbitrary memory pointer. Here is a 1676 typical call of pcre_fullinfo(), to obtain the length of the compiled 1677 pattern: 1678 1679 int rc; 1680 size_t length; 1681 rc = pcre_fullinfo( 1682 re, /* result of pcre_compile() */ 1683 pe, /* result of pcre_study(), or NULL */ 1684 PCRE_INFO_SIZE, /* what is required */ 1685 &length); /* where to put the data */ 1686 1687 The possible values for the third argument are defined in pcre.h, and 1688 are as follows: 1689 1690 PCRE_INFO_BACKREFMAX 1691 1692 Return the number of the highest back reference in the pattern. The 1693 fourth argument should point to an int variable. Zero is returned if 1694 there are no back references. 1695 1696 PCRE_INFO_CAPTURECOUNT 1697 1698 Return the number of capturing subpatterns in the pattern. The fourth 1699 argument should point to an int variable. 1700 1701 PCRE_INFO_DEFAULT_TABLES 1702 1703 Return a pointer to the internal default character tables within PCRE. 1704 The fourth argument should point to an unsigned char * variable. This 1705 information call is provided for internal use by the pcre_study() func- 1706 tion. External callers can cause PCRE to use its internal tables by 1707 passing a NULL table pointer. 1708 1709 PCRE_INFO_FIRSTBYTE 1710 1711 Return information about the first byte of any matched string, for a 1712 non-anchored pattern. The fourth argument should point to an int vari- 1713 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name 1714 is still recognized for backwards compatibility.) 1715 1716 If there is a fixed first byte, for example, from a pattern such as 1717 (cat|cow|coyote), its value is returned. Otherwise, if either 1718 1719 (a) the pattern was compiled with the PCRE_MULTILINE option, and every 1720 branch starts with "^", or 1721 1722 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not 1723 set (if it were set, the pattern would be anchored), 1724 1725 -1 is returned, indicating that the pattern matches only at the start 1726 of a subject string or after any newline within the string. Otherwise 1727 -2 is returned. For anchored patterns, -2 is returned. 1728 1729 PCRE_INFO_FIRSTTABLE 1730 1731 If the pattern was studied, and this resulted in the construction of a 1732 256-bit table indicating a fixed set of bytes for the first byte in any 1733 matching string, a pointer to the table is returned. Otherwise NULL is 1734 returned. The fourth argument should point to an unsigned char * vari- 1735 able. 1736 1737 PCRE_INFO_HASCRORLF 1738 1739 Return 1 if the pattern contains any explicit matches for CR or LF 1740 characters, otherwise 0. The fourth argument should point to an int 1741 variable. An explicit match is either a literal CR or LF character, or 1742 \r or \n. 1743 1744 PCRE_INFO_JCHANGED 1745 1746 Return 1 if the (?J) or (?-J) option setting is used in the pattern, 1747 otherwise 0. The fourth argument should point to an int variable. (?J) 1748 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively. 1749 1750 PCRE_INFO_LASTLITERAL 1751 1752 Return the value of the rightmost literal byte that must exist in any 1753 matched string, other than at its start, if such a byte has been 1754 recorded. The fourth argument should point to an int variable. If there 1755 is no such byte, -1 is returned. For anchored patterns, a last literal 1756 byte is recorded only if it follows something of variable length. For 1757 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for 1758 /^a\dz\d/ the returned value is -1. 1759 1760 PCRE_INFO_MINLENGTH 1761 1762 If the pattern was studied and a minimum length for matching subject 1763 strings was computed, its value is returned. Otherwise the returned 1764 value is -1. The value is a number of characters, not bytes (this may 1765 be relevant in UTF-8 mode). The fourth argument should point to an int 1766 variable. A non-negative value is a lower bound to the length of any 1767 matching string. There may not be any strings of that length that do 1768 actually match, but every string that does match is at least that long. 1769 1770 PCRE_INFO_NAMECOUNT 1771 PCRE_INFO_NAMEENTRYSIZE 1772 PCRE_INFO_NAMETABLE 1773 1774 PCRE supports the use of named as well as numbered capturing parenthe- 1775 ses. The names are just an additional way of identifying the parenthe- 1776 ses, which still acquire numbers. Several convenience functions such as 1777 pcre_get_named_substring() are provided for extracting captured sub- 1778 strings by name. It is also possible to extract the data directly, by 1779 first converting the name to a number in order to access the correct 1780 pointers in the output vector (described with pcre_exec() below). To do 1781 the conversion, you need to use the name-to-number map, which is 1782 described by these three values. 1783 1784 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT 1785 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size 1786 of each entry; both of these return an int value. The entry size 1787 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns 1788 a pointer to the first entry of the table (a pointer to char). The 1789 first two bytes of each entry are the number of the capturing parenthe- 1790 sis, most significant byte first. The rest of the entry is the corre- 1791 sponding name, zero terminated. 1792 1793 The names are in alphabetical order. Duplicate names may appear if (?| 1794 is used to create multiple groups with the same number, as described in 1795 the section on duplicate subpattern numbers in the pcrepattern page. 1796 Duplicate names for subpatterns with different numbers are permitted 1797 only if PCRE_DUPNAMES is set. In all cases of duplicate names, they 1798 appear in the table in the order in which they were found in the pat- 1799 tern. In the absence of (?| this is the order of increasing number; 1800 when (?| is used this is not necessarily the case because later subpat- 1801 terns may have lower numbers. 1802 1803 As a simple example of the name/number table, consider the following 1804 pattern (assume PCRE_EXTENDED is set, so white space - including new- 1805 lines - is ignored): 1806 1807 (?<date> (?<year>(\d\d)?\d\d) - 1808 (?<month>\d\d) - (?<day>\d\d) ) 1809 1810 There are four named subpatterns, so the table has four entries, and 1811 each entry in the table is eight bytes long. The table is as follows, 1812 with non-printing bytes shows in hexadecimal, and undefined bytes shown 1813 as ??: 1814 1815 00 01 d a t e 00 ?? 1816 00 05 d a y 00 ?? ?? 1817 00 04 m o n t h 00 1818 00 02 y e a r 00 ?? 1819 1820 When writing code to extract data from named subpatterns using the 1821 name-to-number map, remember that the length of the entries is likely 1822 to be different for each compiled pattern. 1823 1824 PCRE_INFO_OKPARTIAL 1825 1826 Return 1 if the pattern can be used for partial matching with 1827 pcre_exec(), otherwise 0. The fourth argument should point to an int 1828 variable. From release 8.00, this always returns 1, because the 1829 restrictions that previously applied to partial matching have been 1830 lifted. The pcrepartial documentation gives details of partial match- 1831 ing. 1832 1833 PCRE_INFO_OPTIONS 1834 1835 Return a copy of the options with which the pattern was compiled. The 1836 fourth argument should point to an unsigned long int variable. These 1837 option bits are those specified in the call to pcre_compile(), modified 1838 by any top-level option settings at the start of the pattern itself. In 1839 other words, they are the options that will be in force when matching 1840 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with 1841 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE, 1842 and PCRE_EXTENDED. 1843 1844 A pattern is automatically anchored by PCRE if all of its top-level 1845 alternatives begin with one of the following: 1846 1847 ^ unless PCRE_MULTILINE is set 1848 \A always 1849 \G always 1850 .* if PCRE_DOTALL is set and there are no back 1851 references to the subpattern in which .* appears 1852 1853 For such patterns, the PCRE_ANCHORED bit is set in the options returned 1854 by pcre_fullinfo(). 1855 1856 PCRE_INFO_SIZE 1857 1858 Return the size of the compiled pattern, that is, the value that was 1859 passed as the argument to pcre_malloc() when PCRE was getting memory in 1860 which to place the compiled data. The fourth argument should point to a 1861 size_t variable. 1862 1863 PCRE_INFO_STUDYSIZE 1864 1865 Return the size of the data block pointed to by the study_data field in 1866 a pcre_extra block. That is, it is the value that was passed to 1867 pcre_malloc() when PCRE was getting memory into which to place the data 1868 created by pcre_study(). If pcre_extra is NULL, or there is no study 1869 data, zero is returned. The fourth argument should point to a size_t 1870 variable. 1871 1872 1873 OBSOLETE INFO FUNCTION 1874 1875 int pcre_info(const pcre *code, int *optptr, int *firstcharptr); 1876 1877 The pcre_info() function is now obsolete because its interface is too 1878 restrictive to return all the available data about a compiled pattern. 1879 New programs should use pcre_fullinfo() instead. The yield of 1880 pcre_info() is the number of capturing subpatterns, or one of the fol- 1881 lowing negative numbers: 1882 1883 PCRE_ERROR_NULL the argument code was NULL 1884 PCRE_ERROR_BADMAGIC the "magic number" was not found 1885 1886 If the optptr argument is not NULL, a copy of the options with which 1887 the pattern was compiled is placed in the integer it points to (see 1888 PCRE_INFO_OPTIONS above). 1889 1890 If the pattern is not anchored and the firstcharptr argument is not 1891 NULL, it is used to pass back information about the first character of 1892 any matched string (see PCRE_INFO_FIRSTBYTE above). 1893 1894 1895 REFERENCE COUNTS 1896 1897 int pcre_refcount(pcre *code, int adjust); 1898 1899 The pcre_refcount() function is used to maintain a reference count in 1900 the data block that contains a compiled pattern. It is provided for the 1901 benefit of applications that operate in an object-oriented manner, 1902 where different parts of the application may be using the same compiled 1903 pattern, but you want to free the block when they are all done. 1904 1905 When a pattern is compiled, the reference count field is initialized to 1906 zero. It is changed only by calling this function, whose action is to 1907 add the adjust value (which may be positive or negative) to it. The 1908 yield of the function is the new value. However, the value of the count 1909 is constrained to lie between 0 and 65535, inclusive. If the new value 1910 is outside these limits, it is forced to the appropriate limit value. 1911 1912 Except when it is zero, the reference count is not correctly preserved 1913 if a pattern is compiled on one host and then transferred to a host 1914 whose byte-order is different. (This seems a highly unlikely scenario.) 1915 1916 1917 MATCHING A PATTERN: THE TRADITIONAL FUNCTION 1918 1919 int pcre_exec(const pcre *code, const pcre_extra *extra, 1920 const char *subject, int length, int startoffset, 1921 int options, int *ovector, int ovecsize); 1922 1923 The function pcre_exec() is called to match a subject string against a 1924 compiled pattern, which is passed in the code argument. If the pattern 1925 was studied, the result of the study should be passed in the extra 1926 argument. This function is the main matching facility of the library, 1927 and it operates in a Perl-like manner. For specialist use there is also 1928 an alternative matching function, which is described below in the sec- 1929 tion about the pcre_dfa_exec() function. 1930 1931 In most applications, the pattern will have been compiled (and option- 1932 ally studied) in the same process that calls pcre_exec(). However, it 1933 is possible to save compiled patterns and study data, and then use them 1934 later in different processes, possibly even on different hosts. For a 1935 discussion about this, see the pcreprecompile documentation. 1936 1937 Here is an example of a simple call to pcre_exec(): 1938 1939 int rc; 1940 int ovector[30]; 1941 rc = pcre_exec( 1942 re, /* result of pcre_compile() */ 1943 NULL, /* we didn't study the pattern */ 1944 "some string", /* the subject string */ 1945 11, /* the length of the subject string */ 1946 0, /* start at offset 0 in the subject */ 1947 0, /* default options */ 1948 ovector, /* vector of integers for substring information */ 1949 30); /* number of elements (NOT size in bytes) */ 1950 1951 Extra data for pcre_exec() 1952 1953 If the extra argument is not NULL, it must point to a pcre_extra data 1954 block. The pcre_study() function returns such a block (when it doesn't 1955 return NULL), but you can also create one for yourself, and pass addi- 1956 tional information in it. The pcre_extra block contains the following 1957 fields (not necessarily in this order): 1958 1959 unsigned long int flags; 1960 void *study_data; 1961 unsigned long int match_limit; 1962 unsigned long int match_limit_recursion; 1963 void *callout_data; 1964 const unsigned char *tables; 1965 unsigned char **mark; 1966 1967 The flags field is a bitmap that specifies which of the other fields 1968 are set. The flag bits are: 1969 1970 PCRE_EXTRA_STUDY_DATA 1971 PCRE_EXTRA_MATCH_LIMIT 1972 PCRE_EXTRA_MATCH_LIMIT_RECURSION 1973 PCRE_EXTRA_CALLOUT_DATA 1974 PCRE_EXTRA_TABLES 1975 PCRE_EXTRA_MARK 1976 1977 Other flag bits should be set to zero. The study_data field is set in 1978 the pcre_extra block that is returned by pcre_study(), together with 1979 the appropriate flag bit. You should not set this yourself, but you may 1980 add to the block by setting the other fields and their corresponding 1981 flag bits. 1982 1983 The match_limit field provides a means of preventing PCRE from using up 1984 a vast amount of resources when running patterns that are not going to 1985 match, but which have a very large number of possibilities in their 1986 search trees. The classic example is a pattern that uses nested unlim- 1987 ited repeats. 1988 1989 Internally, PCRE uses a function called match() which it calls repeat- 1990 edly (sometimes recursively). The limit set by match_limit is imposed 1991 on the number of times this function is called during a match, which 1992 has the effect of limiting the amount of backtracking that can take 1993 place. For patterns that are not anchored, the count restarts from zero 1994 for each position in the subject string. 1995 1996 The default value for the limit can be set when PCRE is built; the 1997 default default is 10 million, which handles all but the most extreme 1998 cases. You can override the default by suppling pcre_exec() with a 1999 pcre_extra block in which match_limit is set, and 2000 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is 2001 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT. 2002 2003 The match_limit_recursion field is similar to match_limit, but instead 2004 of limiting the total number of times that match() is called, it limits 2005 the depth of recursion. The recursion depth is a smaller number than 2006 the total number of calls, because not all calls to match() are recur- 2007 sive. This limit is of use only if it is set smaller than match_limit. 2008 2009 Limiting the recursion depth limits the amount of stack that can be 2010 used, or, when PCRE has been compiled to use memory on the heap instead 2011 of the stack, the amount of heap memory that can be used. 2012 2013 The default value for match_limit_recursion can be set when PCRE is 2014 built; the default default is the same value as the default for 2015 match_limit. You can override the default by suppling pcre_exec() with 2016 a pcre_extra block in which match_limit_recursion is set, and 2017 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the 2018 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT. 2019 2020 The callout_data field is used in conjunction with the "callout" fea- 2021 ture, and is described in the pcrecallout documentation. 2022 2023 The tables field is used to pass a character tables pointer to 2024 pcre_exec(); this overrides the value that is stored with the compiled 2025 pattern. A non-NULL value is stored with the compiled pattern only if 2026 custom tables were supplied to pcre_compile() via its tableptr argu- 2027 ment. If NULL is passed to pcre_exec() using this mechanism, it forces 2028 PCRE's internal tables to be used. This facility is helpful when re- 2029 using patterns that have been saved after compiling with an external 2030 set of tables, because the external tables might be at a different 2031 address when pcre_exec() is called. See the pcreprecompile documenta- 2032 tion for a discussion of saving compiled patterns for later use. 2033 2034 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be 2035 set to point to a char * variable. If the pattern contains any back- 2036 tracking control verbs such as (*MARK:NAME), and the execution ends up 2037 with a name to pass back, a pointer to the name string (zero termi- 2038 nated) is placed in the variable pointed to by the mark field. The 2039 names are within the compiled pattern; if you wish to retain such a 2040 name you must copy it before freeing the memory of a compiled pattern. 2041 If there is no name to pass back, the variable pointed to by the mark 2042 field set to NULL. For details of the backtracking control verbs, see 2043 the section entitled "Backtracking control" in the pcrepattern documen- 2044 tation. 2045 2046 Option bits for pcre_exec() 2047 2048 The unused bits of the options argument for pcre_exec() must be zero. 2049 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx, 2050 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, 2051 PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_SOFT, and 2052 PCRE_PARTIAL_HARD. 2053 2054 PCRE_ANCHORED 2055 2056 The PCRE_ANCHORED option limits pcre_exec() to matching at the first 2057 matching position. If a pattern was compiled with PCRE_ANCHORED, or 2058 turned out to be anchored by virtue of its contents, it cannot be made 2059 unachored at matching time. 2060 2061 PCRE_BSR_ANYCRLF 2062 PCRE_BSR_UNICODE 2063 2064 These options (which are mutually exclusive) control what the \R escape 2065 sequence matches. The choice is either to match only CR, LF, or CRLF, 2066 or to match any Unicode newline sequence. These options override the 2067 choice that was made or defaulted when the pattern was compiled. 2068 2069 PCRE_NEWLINE_CR 2070 PCRE_NEWLINE_LF 2071 PCRE_NEWLINE_CRLF 2072 PCRE_NEWLINE_ANYCRLF 2073 PCRE_NEWLINE_ANY 2074 2075 These options override the newline definition that was chosen or 2076 defaulted when the pattern was compiled. For details, see the descrip- 2077 tion of pcre_compile() above. During matching, the newline choice 2078 affects the behaviour of the dot, circumflex, and dollar metacharac- 2079 ters. It may also alter the way the match position is advanced after a 2080 match failure for an unanchored pattern. 2081 2082 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is 2083 set, and a match attempt for an unanchored pattern fails when the cur- 2084 rent position is at a CRLF sequence, and the pattern contains no 2085 explicit matches for CR or LF characters, the match position is 2086 advanced by two characters instead of one, in other words, to after the 2087 CRLF. 2088 2089 The above rule is a compromise that makes the most common cases work as 2090 expected. For example, if the pattern is .+A (and the PCRE_DOTALL 2091 option is not set), it does not match the string "\r\nA" because, after 2092 failing at the start, it skips both the CR and the LF before retrying. 2093 However, the pattern [\r\n]A does match that string, because it con- 2094 tains an explicit CR or LF reference, and so advances only by one char- 2095 acter after the first failure. 2096 2097 An explicit match for CR of LF is either a literal appearance of one of 2098 those characters, or one of the \r or \n escape sequences. Implicit 2099 matches such as [^X] do not count, nor does \s (which includes CR and 2100 LF in the characters that it matches). 2101 2102 Notwithstanding the above, anomalous effects may still occur when CRLF 2103 is a valid newline sequence and explicit \r or \n escapes appear in the 2104 pattern. 2105 2106 PCRE_NOTBOL 2107 2108 This option specifies that first character of the subject string is not 2109 the beginning of a line, so the circumflex metacharacter should not 2110 match before it. Setting this without PCRE_MULTILINE (at compile time) 2111 causes circumflex never to match. This option affects only the behav- 2112 iour of the circumflex metacharacter. It does not affect \A. 2113 2114 PCRE_NOTEOL 2115 2116 This option specifies that the end of the subject string is not the end 2117 of a line, so the dollar metacharacter should not match it nor (except 2118 in multiline mode) a newline immediately before it. Setting this with- 2119 out PCRE_MULTILINE (at compile time) causes dollar never to match. This 2120 option affects only the behaviour of the dollar metacharacter. It does 2121 not affect \Z or \z. 2122 2123 PCRE_NOTEMPTY 2124 2125 An empty string is not considered to be a valid match if this option is 2126 set. If there are alternatives in the pattern, they are tried. If all 2127 the alternatives match the empty string, the entire match fails. For 2128 example, if the pattern 2129 2130 a?b? 2131 2132 is applied to a string not beginning with "a" or "b", it matches an 2133 empty string at the start of the subject. With PCRE_NOTEMPTY set, this 2134 match is not valid, so PCRE searches further into the string for occur- 2135 rences of "a" or "b". 2136 2137 PCRE_NOTEMPTY_ATSTART 2138 2139 This is like PCRE_NOTEMPTY, except that an empty string match that is 2140 not at the start of the subject is permitted. If the pattern is 2141 anchored, such a match can occur only if the pattern contains \K. 2142 2143 Perl has no direct equivalent of PCRE_NOTEMPTY or 2144 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern 2145 match of the empty string within its split() function, and when using 2146 the /g modifier. It is possible to emulate Perl's behaviour after 2147 matching a null string by first trying the match again at the same off- 2148 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that 2149 fails, by advancing the starting offset (see below) and trying an ordi- 2150 nary match again. There is some code that demonstrates how to do this 2151 in the pcredemo sample program. In the most general case, you have to 2152 check to see if the newline convention recognizes CRLF as a newline, 2153 and if so, and the current character is CR followed by LF, advance the 2154 starting offset by two characters instead of one. 2155 2156 PCRE_NO_START_OPTIMIZE 2157 2158 There are a number of optimizations that pcre_exec() uses at the start 2159 of a match, in order to speed up the process. For example, if it is 2160 known that an unanchored match must start with a specific character, it 2161 searches the subject for that character, and fails immediately if it 2162 cannot find it, without actually running the main matching function. 2163 This means that a special item such as (*COMMIT) at the start of a pat- 2164 tern is not considered until after a suitable starting point for the 2165 match has been found. When callouts or (*MARK) items are in use, these 2166 "start-up" optimizations can cause them to be skipped if the pattern is 2167 never actually used. The start-up optimizations are in effect a pre- 2168 scan of the subject that takes place before the pattern is run. 2169 2170 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations, 2171 possibly causing performance to suffer, but ensuring that in cases 2172 where the result is "no match", the callouts do occur, and that items 2173 such as (*COMMIT) and (*MARK) are considered at every possible starting 2174 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at 2175 compile time, it cannot be unset at matching time. 2176 2177 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching 2178 operation. Consider the pattern 2179 2180 (*COMMIT)ABC 2181 2182 When this is compiled, PCRE records the fact that a match must start 2183 with the character "A". Suppose the subject string is "DEFABC". The 2184 start-up optimization scans along the subject, finds "A" and runs the 2185 first match attempt from there. The (*COMMIT) item means that the pat- 2186 tern must match the current starting position, which in this case, it 2187 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE 2188 set, the initial scan along the subject string does not happen. The 2189 first match attempt is run starting from "D" and when this fails, 2190 (*COMMIT) prevents any further matches being tried, so the overall 2191 result is "no match". If the pattern is studied, more start-up opti- 2192 mizations may be used. For example, a minimum length for the subject 2193 may be recorded. Consider the pattern 2194 2195 (*MARK:A)(X|Y) 2196 2197 The minimum length for a match is one character. If the subject is 2198 "ABC", there will be attempts to match "ABC", "BC", "C", and then 2199 finally an empty string. If the pattern is studied, the final attempt 2200 does not take place, because PCRE knows that the subject is too short, 2201 and so the (*MARK) is never encountered. In this case, studying the 2202 pattern does not affect the overall match result, which is still "no 2203 match", but it does affect the auxiliary information that is returned. 2204 2205 PCRE_NO_UTF8_CHECK 2206 2207 When PCRE_UTF8 is set at compile time, the validity of the subject as a 2208 UTF-8 string is automatically checked when pcre_exec() is subsequently 2209 called. The value of startoffset is also checked to ensure that it 2210 points to the start of a UTF-8 character. There is a discussion about 2211 the validity of UTF-8 strings in the section on UTF-8 support in the 2212 main pcre page. If an invalid UTF-8 sequence of bytes is found, 2213 pcre_exec() returns the error PCRE_ERROR_BADUTF8 or, if PCRE_PAR- 2214 TIAL_HARD is set and the problem is a truncated UTF-8 character at the 2215 end of the subject, PCRE_ERROR_SHORTUTF8. If startoffset contains a 2216 value that does not point to the start of a UTF-8 character (or to the 2217 end of the subject), PCRE_ERROR_BADUTF8_OFFSET is returned. 2218 2219 If you already know that your subject is valid, and you want to skip 2220 these checks for performance reasons, you can set the 2221 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to 2222 do this for the second and subsequent calls to pcre_exec() if you are 2223 making repeated calls to find all the matches in a single subject 2224 string. However, you should be sure that the value of startoffset 2225 points to the start of a UTF-8 character (or the end of the subject). 2226 When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8 2227 string as a subject or an invalid value of startoffset is undefined. 2228 Your program may crash. 2229 2230 PCRE_PARTIAL_HARD 2231 PCRE_PARTIAL_SOFT 2232 2233 These options turn on the partial matching feature. For backwards com- 2234 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial 2235 match occurs if the end of the subject string is reached successfully, 2236 but there are not enough subject characters to complete the match. If 2237 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set, 2238 matching continues by testing any remaining alternatives. Only if no 2239 complete match can be found is PCRE_ERROR_PARTIAL returned instead of 2240 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the 2241 caller is prepared to handle a partial match, but only if no complete 2242 match can be found. 2243 2244 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this 2245 case, if a partial match is found, pcre_exec() immediately returns 2246 PCRE_ERROR_PARTIAL, without considering any other alternatives. In 2247 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid- 2248 ered to be more important that an alternative complete match. 2249 2250 In both cases, the portion of the string that was inspected when the 2251 partial match was found is set as the first matching string. There is a 2252 more detailed discussion of partial and multi-segment matching, with 2253 examples, in the pcrepartial documentation. 2254 2255 The string to be matched by pcre_exec() 2256 2257 The subject string is passed to pcre_exec() as a pointer in subject, a 2258 length (in bytes) in length, and a starting byte offset in startoffset. 2259 If this is negative or greater than the length of the subject, 2260 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is 2261 zero, the search for a match starts at the beginning of the subject, 2262 and this is by far the most common case. In UTF-8 mode, the byte offset 2263 must point to the start of a UTF-8 character (or the end of the sub- 2264 ject). Unlike the pattern string, the subject may contain binary zero 2265 bytes. 2266 2267 A non-zero starting offset is useful when searching for another match 2268 in the same subject by calling pcre_exec() again after a previous suc- 2269 cess. Setting startoffset differs from just passing over a shortened 2270 string and setting PCRE_NOTBOL in the case of a pattern that begins 2271 with any kind of lookbehind. For example, consider the pattern 2272 2273 \Biss\B 2274 2275 which finds occurrences of "iss" in the middle of words. (\B matches 2276 only if the current position in the subject is not a word boundary.) 2277 When applied to the string "Mississipi" the first call to pcre_exec() 2278 finds the first occurrence. If pcre_exec() is called again with just 2279 the remainder of the subject, namely "issipi", it does not match, 2280 because \B is always false at the start of the subject, which is deemed 2281 to be a word boundary. However, if pcre_exec() is passed the entire 2282 string again, but with startoffset set to 4, it finds the second occur- 2283 rence of "iss" because it is able to look behind the starting point to 2284 discover that it is preceded by a letter. 2285 2286 Finding all the matches in a subject is tricky when the pattern can 2287 match an empty string. It is possible to emulate Perl's /g behaviour by 2288 first trying the match again at the same offset, with the 2289 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that 2290 fails, advancing the starting offset and trying an ordinary match 2291 again. There is some code that demonstrates how to do this in the pcre- 2292 demo sample program. In the most general case, you have to check to see 2293 if the newline convention recognizes CRLF as a newline, and if so, and 2294 the current character is CR followed by LF, advance the starting offset 2295 by two characters instead of one. 2296 2297 If a non-zero starting offset is passed when the pattern is anchored, 2298 one attempt to match at the given offset is made. This can only succeed 2299 if the pattern does not require the match to be at the start of the 2300 subject. 2301 2302 How pcre_exec() returns captured substrings 2303 2304 In general, a pattern matches a certain portion of the subject, and in 2305 addition, further substrings from the subject may be picked out by 2306 parts of the pattern. Following the usage in Jeffrey Friedl's book, 2307 this is called "capturing" in what follows, and the phrase "capturing 2308 subpattern" is used for a fragment of a pattern that picks out a sub- 2309 string. PCRE supports several other kinds of parenthesized subpattern 2310 that do not cause substrings to be captured. 2311 2312 Captured substrings are returned to the caller via a vector of integers 2313 whose address is passed in ovector. The number of elements in the vec- 2314 tor is passed in ovecsize, which must be a non-negative number. Note: 2315 this argument is NOT the size of ovector in bytes. 2316 2317 The first two-thirds of the vector is used to pass back captured sub- 2318 strings, each substring using a pair of integers. The remaining third 2319 of the vector is used as workspace by pcre_exec() while matching cap- 2320 turing subpatterns, and is not available for passing back information. 2321 The number passed in ovecsize should always be a multiple of three. If 2322 it is not, it is rounded down. 2323 2324 When a match is successful, information about captured substrings is 2325 returned in pairs of integers, starting at the beginning of ovector, 2326 and continuing up to two-thirds of its length at the most. The first 2327 element of each pair is set to the byte offset of the first character 2328 in a substring, and the second is set to the byte offset of the first 2329 character after the end of a substring. Note: these values are always 2330 byte offsets, even in UTF-8 mode. They are not character counts. 2331 2332 The first pair of integers, ovector[0] and ovector[1], identify the 2333 portion of the subject string matched by the entire pattern. The next 2334 pair is used for the first capturing subpattern, and so on. The value 2335 returned by pcre_exec() is one more than the highest numbered pair that 2336 has been set. For example, if two substrings have been captured, the 2337 returned value is 3. If there are no capturing subpatterns, the return 2338 value from a successful match is 1, indicating that just the first pair 2339 of offsets has been set. 2340 2341 If a capturing subpattern is matched repeatedly, it is the last portion 2342 of the string that it matched that is returned. 2343 2344 If the vector is too small to hold all the captured substring offsets, 2345 it is used as far as possible (up to two-thirds of its length), and the 2346 function returns a value of zero. If the substring offsets are not of 2347 interest, pcre_exec() may be called with ovector passed as NULL and 2348 ovecsize as zero. However, if the pattern contains back references and 2349 the ovector is not big enough to remember the related substrings, PCRE 2350 has to get additional memory for use during matching. Thus it is usu- 2351 ally advisable to supply an ovector. 2352 2353 The pcre_fullinfo() function can be used to find out how many capturing 2354 subpatterns there are in a compiled pattern. The smallest size for 2355 ovector that will allow for n captured substrings, in addition to the 2356 offsets of the substring matched by the whole pattern, is (n+1)*3. 2357 2358 It is possible for capturing subpattern number n+1 to match some part 2359 of the subject when subpattern n has not been used at all. For example, 2360 if the string "abc" is matched against the pattern (a|(z))(bc) the 2361 return from the function is 4, and subpatterns 1 and 3 are matched, but 2362 2 is not. When this happens, both values in the offset pairs corre- 2363 sponding to unused subpatterns are set to -1. 2364 2365 Offset values that correspond to unused subpatterns at the end of the 2366 expression are also set to -1. For example, if the string "abc" is 2367 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not 2368 matched. The return from the function is 2, because the highest used 2369 capturing subpattern number is 1, and the offsets for for the second 2370 and third capturing subpatterns (assuming the vector is large enough, 2371 of course) are set to -1. 2372 2373 Note: Elements of ovector that do not correspond to capturing parenthe- 2374 ses in the pattern are never changed. That is, if a pattern contains n 2375 capturing parentheses, no more than ovector[0] to ovector[2n+1] are set 2376 by pcre_exec(). The other elements retain whatever values they previ- 2377 ously had. 2378 2379 Some convenience functions are provided for extracting the captured 2380 substrings as separate strings. These are described below. 2381 2382 Error return values from pcre_exec() 2383 2384 If pcre_exec() fails, it returns a negative number. The following are 2385 defined in the header file: 2386 2387 PCRE_ERROR_NOMATCH (-1) 2388 2389 The subject string did not match the pattern. 2390 2391 PCRE_ERROR_NULL (-2) 2392 2393 Either code or subject was passed as NULL, or ovector was NULL and 2394 ovecsize was not zero. 2395 2396 PCRE_ERROR_BADOPTION (-3) 2397 2398 An unrecognized bit was set in the options argument. 2399 2400 PCRE_ERROR_BADMAGIC (-4) 2401 2402 PCRE stores a 4-byte "magic number" at the start of the compiled code, 2403 to catch the case when it is passed a junk pointer and to detect when a 2404 pattern that was compiled in an environment of one endianness is run in 2405 an environment with the other endianness. This is the error that PCRE 2406 gives when the magic number is not present. 2407 2408 PCRE_ERROR_UNKNOWN_OPCODE (-5) 2409 2410 While running the pattern match, an unknown item was encountered in the 2411 compiled pattern. This error could be caused by a bug in PCRE or by 2412 overwriting of the compiled pattern. 2413 2414 PCRE_ERROR_NOMEMORY (-6) 2415 2416 If a pattern contains back references, but the ovector that is passed 2417 to pcre_exec() is not big enough to remember the referenced substrings, 2418 PCRE gets a block of memory at the start of matching to use for this 2419 purpose. If the call via pcre_malloc() fails, this error is given. The 2420 memory is automatically freed at the end of matching. 2421 2422 This error is also given if pcre_stack_malloc() fails in pcre_exec(). 2423 This can happen only when PCRE has been compiled with --disable-stack- 2424 for-recursion. 2425 2426 PCRE_ERROR_NOSUBSTRING (-7) 2427 2428 This error is used by the pcre_copy_substring(), pcre_get_substring(), 2429 and pcre_get_substring_list() functions (see below). It is never 2430 returned by pcre_exec(). 2431 2432 PCRE_ERROR_MATCHLIMIT (-8) 2433 2434 The backtracking limit, as specified by the match_limit field in a 2435 pcre_extra structure (or defaulted) was reached. See the description 2436 above. 2437 2438 PCRE_ERROR_CALLOUT (-9) 2439 2440 This error is never generated by pcre_exec() itself. It is provided for 2441 use by callout functions that want to yield a distinctive error code. 2442 See the pcrecallout documentation for details. 2443 2444 PCRE_ERROR_BADUTF8 (-10) 2445 2446 A string that contains an invalid UTF-8 byte sequence was passed as a 2447 subject. However, if PCRE_PARTIAL_HARD is set and the problem is a 2448 truncated UTF-8 character at the end of the subject, PCRE_ERROR_SHORT- 2449 UTF8 is used instead. 2450 2451 PCRE_ERROR_BADUTF8_OFFSET (-11) 2452 2453 The UTF-8 byte sequence that was passed as a subject was valid, but the 2454 value of startoffset did not point to the beginning of a UTF-8 charac- 2455 ter or the end of the subject. 2456 2457 PCRE_ERROR_PARTIAL (-12) 2458 2459 The subject string did not match, but it did match partially. See the 2460 pcrepartial documentation for details of partial matching. 2461 2462 PCRE_ERROR_BADPARTIAL (-13) 2463 2464 This code is no longer in use. It was formerly returned when the 2465 PCRE_PARTIAL option was used with a compiled pattern containing items 2466 that were not supported for partial matching. From release 8.00 2467 onwards, there are no restrictions on partial matching. 2468 2469 PCRE_ERROR_INTERNAL (-14) 2470 2471 An unexpected internal error has occurred. This error could be caused 2472 by a bug in PCRE or by overwriting of the compiled pattern. 2473 2474 PCRE_ERROR_BADCOUNT (-15) 2475 2476 This error is given if the value of the ovecsize argument is negative. 2477 2478 PCRE_ERROR_RECURSIONLIMIT (-21) 2479 2480 The internal recursion limit, as specified by the match_limit_recursion 2481 field in a pcre_extra structure (or defaulted) was reached. See the 2482 description above. 2483 2484 PCRE_ERROR_BADNEWLINE (-23) 2485 2486 An invalid combination of PCRE_NEWLINE_xxx options was given. 2487 2488 PCRE_ERROR_BADOFFSET (-24) 2489 2490 The value of startoffset was negative or greater than the length of the 2491 subject, that is, the value in length. 2492 2493 PCRE_ERROR_SHORTUTF8 (-25) 2494 2495 The subject string ended with an incomplete (truncated) UTF-8 charac- 2496 ter, and the PCRE_PARTIAL_HARD option was set. Without this option, 2497 PCRE_ERROR_BADUTF8 is returned in this situation. 2498 2499 Error numbers -16 to -20 and -22 are not used by pcre_exec(). 2500 2501 2502 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER 2503 2504 int pcre_copy_substring(const char *subject, int *ovector, 2505 int stringcount, int stringnumber, char *buffer, 2506 int buffersize); 2507 2508 int pcre_get_substring(const char *subject, int *ovector, 2509 int stringcount, int stringnumber, 2510 const char **stringptr); 2511 2512 int pcre_get_substring_list(const char *subject, 2513 int *ovector, int stringcount, const char ***listptr); 2514 2515 Captured substrings can be accessed directly by using the offsets 2516 returned by pcre_exec() in ovector. For convenience, the functions 2517 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub- 2518 string_list() are provided for extracting captured substrings as new, 2519 separate, zero-terminated strings. These functions identify substrings 2520 by number. The next section describes functions for extracting named 2521 substrings. 2522 2523 A substring that contains a binary zero is correctly extracted and has 2524 a further zero added on the end, but the result is not, of course, a C 2525 string. However, you can process such a string by referring to the 2526 length that is returned by pcre_copy_substring() and pcre_get_sub- 2527 string(). Unfortunately, the interface to pcre_get_substring_list() is 2528 not adequate for handling strings containing binary zeros, because the 2529 end of the final string is not independently indicated. 2530 2531 The first three arguments are the same for all three of these func- 2532 tions: subject is the subject string that has just been successfully 2533 matched, ovector is a pointer to the vector of integer offsets that was 2534 passed to pcre_exec(), and stringcount is the number of substrings that 2535 were captured by the match, including the substring that matched the 2536 entire regular expression. This is the value returned by pcre_exec() if 2537 it is greater than zero. If pcre_exec() returned zero, indicating that 2538 it ran out of space in ovector, the value passed as stringcount should 2539 be the number of elements in the vector divided by three. 2540 2541 The functions pcre_copy_substring() and pcre_get_substring() extract a 2542 single substring, whose number is given as stringnumber. A value of 2543 zero extracts the substring that matched the entire pattern, whereas 2544 higher values extract the captured substrings. For pcre_copy_sub- 2545 string(), the string is placed in buffer, whose length is given by 2546 buffersize, while for pcre_get_substring() a new block of memory is 2547 obtained via pcre_malloc, and its address is returned via stringptr. 2548 The yield of the function is the length of the string, not including 2549 the terminating zero, or one of these error codes: 2550 2551 PCRE_ERROR_NOMEMORY (-6) 2552 2553 The buffer was too small for pcre_copy_substring(), or the attempt to 2554 get memory failed for pcre_get_substring(). 2555 2556 PCRE_ERROR_NOSUBSTRING (-7) 2557 2558 There is no substring whose number is stringnumber. 2559 2560 The pcre_get_substring_list() function extracts all available sub- 2561 strings and builds a list of pointers to them. All this is done in a 2562 single block of memory that is obtained via pcre_malloc. The address of 2563 the memory block is returned via listptr, which is also the start of 2564 the list of string pointers. The end of the list is marked by a NULL 2565 pointer. The yield of the function is zero if all went well, or the 2566 error code 2567 2568 PCRE_ERROR_NOMEMORY (-6) 2569 2570 if the attempt to get the memory block failed. 2571 2572 When any of these functions encounter a substring that is unset, which 2573 can happen when capturing subpattern number n+1 matches some part of 2574 the subject, but subpattern n has not been used at all, they return an 2575 empty string. This can be distinguished from a genuine zero-length sub- 2576 string by inspecting the appropriate offset in ovector, which is nega- 2577 tive for unset substrings. 2578 2579 The two convenience functions pcre_free_substring() and pcre_free_sub- 2580 string_list() can be used to free the memory returned by a previous 2581 call of pcre_get_substring() or pcre_get_substring_list(), respec- 2582 tively. They do nothing more than call the function pointed to by 2583 pcre_free, which of course could be called directly from a C program. 2584 However, PCRE is used in some situations where it is linked via a spe- 2585 cial interface to another programming language that cannot use 2586 pcre_free directly; it is for these cases that the functions are pro- 2587 vided. 2588 2589 2590 EXTRACTING CAPTURED SUBSTRINGS BY NAME 2591 2592 int pcre_get_stringnumber(const pcre *code, 2593 const char *name); 2594 2595 int pcre_copy_named_substring(const pcre *code, 2596 const char *subject, int *ovector, 2597 int stringcount, const char *stringname, 2598 char *buffer, int buffersize); 2599 2600 int pcre_get_named_substring(const pcre *code, 2601 const char *subject, int *ovector, 2602 int stringcount, const char *stringname, 2603 const char **stringptr); 2604 2605 To extract a substring by name, you first have to find associated num- 2606 ber. For example, for this pattern 2607 2608 (a+)b(?<xxx>\d+)... 2609 2610 the number of the subpattern called "xxx" is 2. If the name is known to 2611 be unique (PCRE_DUPNAMES was not set), you can find the number from the 2612 name by calling pcre_get_stringnumber(). The first argument is the com- 2613 piled pattern, and the second is the name. The yield of the function is 2614 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no 2615 subpattern of that name. 2616 2617 Given the number, you can extract the substring directly, or use one of 2618 the functions described in the previous section. For convenience, there 2619 are also two functions that do the whole job. 2620 2621 Most of the arguments of pcre_copy_named_substring() and 2622 pcre_get_named_substring() are the same as those for the similarly 2623 named functions that extract by number. As these are described in the 2624 previous section, they are not re-described here. There are just two 2625 differences: 2626 2627 First, instead of a substring number, a substring name is given. Sec- 2628 ond, there is an extra argument, given at the start, which is a pointer 2629 to the compiled pattern. This is needed in order to gain access to the 2630 name-to-number translation table. 2631 2632 These functions call pcre_get_stringnumber(), and if it succeeds, they 2633 then call pcre_copy_substring() or pcre_get_substring(), as appropri- 2634 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the 2635 behaviour may not be what you want (see the next section). 2636 2637 Warning: If the pattern uses the (?| feature to set up multiple subpat- 2638 terns with the same number, as described in the section on duplicate 2639 subpattern numbers in the pcrepattern page, you cannot use names to 2640 distinguish the different subpatterns, because names are not included 2641 in the compiled code. The matching process uses only numbers. For this 2642 reason, the use of different names for subpatterns of the same number 2643 causes an error at compile time. 2644 2645 2646 DUPLICATE SUBPATTERN NAMES 2647 2648 int pcre_get_stringtable_entries(const pcre *code, 2649 const char *name, char **first, char **last); 2650 2651 When a pattern is compiled with the PCRE_DUPNAMES option, names for 2652 subpatterns are not required to be unique. (Duplicate names are always 2653 allowed for subpatterns with the same number, created by using the (?| 2654 feature. Indeed, if such subpatterns are named, they are required to 2655 use the same names.) 2656 2657 Normally, patterns with duplicate names are such that in any one match, 2658 only one of the named subpatterns participates. An example is shown in 2659 the pcrepattern documentation. 2660 2661 When duplicates are present, pcre_copy_named_substring() and 2662 pcre_get_named_substring() return the first substring corresponding to 2663 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING 2664 (-7) is returned; no data is returned. The pcre_get_stringnumber() 2665 function returns one of the numbers that are associated with the name, 2666 but it is not defined which it is. 2667 2668 If you want to get full details of all captured substrings for a given 2669 name, you must use the pcre_get_stringtable_entries() function. The 2670 first argument is the compiled pattern, and the second is the name. The 2671 third and fourth are pointers to variables which are updated by the 2672 function. After it has run, they point to the first and last entries in 2673 the name-to-number table for the given name. The function itself 2674 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if 2675 there are none. The format of the table is described above in the sec- 2676 tion entitled Information about a pattern. Given all the relevant 2677 entries for the name, you can extract each of their numbers, and hence 2678 the captured data, if any. 2679 2680 2681 FINDING ALL POSSIBLE MATCHES 2682 2683 The traditional matching function uses a similar algorithm to Perl, 2684 which stops when it finds the first match, starting at a given point in 2685 the subject. If you want to find all possible matches, or the longest 2686 possible match, consider using the alternative matching function (see 2687 below) instead. If you cannot use the alternative function, but still 2688 need to find all possible matches, you can kludge it up by making use 2689 of the callout facility, which is described in the pcrecallout documen- 2690 tation. 2691 2692 What you have to do is to insert a callout right at the end of the pat- 2693 tern. When your callout function is called, extract and save the cur- 2694 rent matched substring. Then return 1, which forces pcre_exec() to 2695 backtrack and try other alternatives. Ultimately, when it runs out of 2696 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH. 2697 2698 2699 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION 2700 2701 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra, 2702 const char *subject, int length, int startoffset, 2703 int options, int *ovector, int ovecsize, 2704 int *workspace, int wscount); 2705 2706 The function pcre_dfa_exec() is called to match a subject string 2707 against a compiled pattern, using a matching algorithm that scans the 2708 subject string just once, and does not backtrack. This has different 2709 characteristics to the normal algorithm, and is not compatible with 2710 Perl. Some of the features of PCRE patterns are not supported. Never- 2711 theless, there are times when this kind of matching can be useful. For 2712 a discussion of the two matching algorithms, and a list of features 2713 that pcre_dfa_exec() does not support, see the pcrematching documenta- 2714 tion. 2715 2716 The arguments for the pcre_dfa_exec() function are the same as for 2717 pcre_exec(), plus two extras. The ovector argument is used in a differ- 2718 ent way, and this is described below. The other common arguments are 2719 used in the same way as for pcre_exec(), so their description is not 2720 repeated here. 2721 2722 The two additional arguments provide workspace for the function. The 2723 workspace vector should contain at least 20 elements. It is used for 2724 keeping track of multiple paths through the pattern tree. More 2725 workspace will be needed for patterns and subjects where there are a 2726 lot of potential matches. 2727 2728 Here is an example of a simple call to pcre_dfa_exec(): 2729 2730 int rc; 2731 int ovector[10]; 2732 int wspace[20]; 2733 rc = pcre_dfa_exec( 2734 re, /* result of pcre_compile() */ 2735 NULL, /* we didn't study the pattern */ 2736 "some string", /* the subject string */ 2737 11, /* the length of the subject string */ 2738 0, /* start at offset 0 in the subject */ 2739 0, /* default options */ 2740 ovector, /* vector of integers for substring information */ 2741 10, /* number of elements (NOT size in bytes) */ 2742 wspace, /* working space vector */ 2743 20); /* number of elements (NOT size in bytes) */ 2744 2745 Option bits for pcre_dfa_exec() 2746 2747 The unused bits of the options argument for pcre_dfa_exec() must be 2748 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW- 2749 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, 2750 PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF, 2751 PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR- 2752 TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last 2753 four of these are exactly the same as for pcre_exec(), so their 2754 description is not repeated here. 2755 2756 PCRE_PARTIAL_HARD 2757 PCRE_PARTIAL_SOFT 2758 2759 These have the same general effect as they do for pcre_exec(), but the 2760 details are slightly different. When PCRE_PARTIAL_HARD is set for 2761 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub- 2762 ject is reached and there is still at least one matching possibility 2763 that requires additional characters. This happens even if some complete 2764 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return 2765 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end 2766 of the subject is reached, there have been no complete matches, but 2767 there is still at least one matching possibility. The portion of the 2768 string that was inspected when the longest partial match was found is 2769 set as the first matching string in both cases. There is a more 2770 detailed discussion of partial and multi-segment matching, with exam- 2771 ples, in the pcrepartial documentation. 2772 2773 PCRE_DFA_SHORTEST 2774 2775 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to 2776 stop as soon as it has found one match. Because of the way the alterna- 2777 tive algorithm works, this is necessarily the shortest possible match 2778 at the first possible matching point in the subject string. 2779 2780 PCRE_DFA_RESTART 2781 2782 When pcre_dfa_exec() returns a partial match, it is possible to call it 2783 again, with additional subject characters, and have it continue with 2784 the same match. The PCRE_DFA_RESTART option requests this action; when 2785 it is set, the workspace and wscount options must reference the same 2786 vector as before because data about the match so far is left in them 2787 after a partial match. There is more discussion of this facility in the 2788 pcrepartial documentation. 2789 2790 Successful returns from pcre_dfa_exec() 2791 2792 When pcre_dfa_exec() succeeds, it may have matched more than one sub- 2793 string in the subject. Note, however, that all the matches from one run 2794 of the function start at the same point in the subject. The shorter 2795 matches are all initial substrings of the longer matches. For example, 2796 if the pattern 2797 2798 <.*> 2799 2800 is matched against the string 2801 2802 This is <something> <something else> <something further> no more 2803 2804 the three matched strings are 2805 2806 <something> 2807 <something> <something else> 2808 <something> <something else> <something further> 2809 2810 On success, the yield of the function is a number greater than zero, 2811 which is the number of matched substrings. The substrings themselves 2812 are returned in ovector. Each string uses two elements; the first is 2813 the offset to the start, and the second is the offset to the end. In 2814 fact, all the strings have the same start offset. (Space could have 2815 been saved by giving this only once, but it was decided to retain some 2816 compatibility with the way pcre_exec() returns data, even though the 2817 meaning of the strings is different.) 2818 2819 The strings are returned in reverse order of length; that is, the long- 2820 est matching string is given first. If there were too many matches to 2821 fit into ovector, the yield of the function is zero, and the vector is 2822 filled with the longest matches. 2823 2824 Error returns from pcre_dfa_exec() 2825 2826 The pcre_dfa_exec() function returns a negative number when it fails. 2827 Many of the errors are the same as for pcre_exec(), and these are 2828 described above. There are in addition the following errors that are 2829 specific to pcre_dfa_exec(): 2830 2831 PCRE_ERROR_DFA_UITEM (-16) 2832 2833 This return is given if pcre_dfa_exec() encounters an item in the pat- 2834 tern that it does not support, for instance, the use of \C or a back 2835 reference. 2836 2837 PCRE_ERROR_DFA_UCOND (-17) 2838 2839 This return is given if pcre_dfa_exec() encounters a condition item 2840 that uses a back reference for the condition, or a test for recursion 2841 in a specific group. These are not supported. 2842 2843 PCRE_ERROR_DFA_UMLIMIT (-18) 2844 2845 This return is given if pcre_dfa_exec() is called with an extra block 2846 that contains a setting of the match_limit field. This is not supported 2847 (it is meaningless). 2848 2849 PCRE_ERROR_DFA_WSSIZE (-19) 2850 2851 This return is given if pcre_dfa_exec() runs out of space in the 2852 workspace vector. 2853 2854 PCRE_ERROR_DFA_RECURSE (-20) 2855 2856 When a recursive subpattern is processed, the matching function calls 2857 itself recursively, using private vectors for ovector and workspace. 2858 This error is given if the output vector is not large enough. This 2859 should be extremely rare, as a vector of size 1000 is used. 2860 2861 2862 SEE ALSO 2863 2864 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar- 2865 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3). 2866 2867 2868 AUTHOR 2869 2870 Philip Hazel 2871 University Computing Service 2872 Cambridge CB2 3QH, England. 2873 2874 2875 REVISION 2876 2877 Last updated: 21 November 2010 2878 Copyright (c) 1997-2010 University of Cambridge. 2879 ------------------------------------------------------------------------------ 2880 2881 2882 PCRECALLOUT(3) PCRECALLOUT(3) 2883 2884 2885 NAME 2886 PCRE - Perl-compatible regular expressions 2887 2888 2889 PCRE CALLOUTS 2890 2891 int (*pcre_callout)(pcre_callout_block *); 2892 2893 PCRE provides a feature called "callout", which is a means of temporar- 2894 ily passing control to the caller of PCRE in the middle of pattern 2895 matching. The caller of PCRE provides an external function by putting 2896 its entry point in the global variable pcre_callout. By default, this 2897 variable contains NULL, which disables all calling out. 2898 2899 Within a regular expression, (?C) indicates the points at which the 2900 external function is to be called. Different callout points can be 2901 identified by putting a number less than 256 after the letter C. The 2902 default value is zero. For example, this pattern has two callout 2903 points: 2904 2905 (?C1)abc(?C2)def 2906 2907 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or 2908 pcre_compile2() is called, PCRE automatically inserts callouts, all 2909 with number 255, before each item in the pattern. For example, if 2910 PCRE_AUTO_CALLOUT is used with the pattern 2911 2912 A(\d{2}|--) 2913 2914 it is processed as if it were 2915 2916 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255) 2917 2918 Notice that there is a callout before and after each parenthesis and 2919 alternation bar. Automatic callouts can be used for tracking the 2920 progress of pattern matching. The pcretest command has an option that 2921 sets automatic callouts; when it is used, the output indicates how the 2922 pattern is matched. This is useful information when you are trying to 2923 optimize the performance of a particular pattern. 2924 2925 2926 MISSING CALLOUTS 2927 2928 You should be aware that, because of optimizations in the way PCRE 2929 matches patterns by default, callouts sometimes do not happen. For 2930 example, if the pattern is 2931 2932 ab(?C4)cd 2933 2934 PCRE knows that any matching string must contain the letter "d". If the 2935 subject string is "abyz", the lack of "d" means that matching doesn't 2936 ever start, and the callout is never reached. However, with "abyd", 2937 though the result is still no match, the callout is obeyed. 2938 2939 If the pattern is studied, PCRE knows the minimum length of a matching 2940 string, and will immediately give a "no match" return without actually 2941 running a match if the subject is not long enough, or, for unanchored 2942 patterns, if it has been scanned far enough. 2943 2944 You can disable these optimizations by passing the PCRE_NO_START_OPTI- 2945 MIZE option to pcre_compile(), pcre_exec(), or pcre_dfa_exec(), or by 2946 starting the pattern with (*NO_START_OPT). This slows down the matching 2947 process, but does ensure that callouts such as the example above are 2948 obeyed. 2949 2950 2951 THE CALLOUT INTERFACE 2952 2953 During matching, when PCRE reaches a callout point, the external func- 2954 tion defined by pcre_callout is called (if it is set). This applies to 2955 both the pcre_exec() and the pcre_dfa_exec() matching functions. The 2956 only argument to the callout function is a pointer to a pcre_callout 2957 block. This structure contains the following fields: 2958 2959 int version; 2960 int callout_number; 2961 int *offset_vector; 2962 const char *subject; 2963 int subject_length; 2964 int start_match; 2965 int current_position; 2966 int capture_top; 2967 int capture_last; 2968 void *callout_data; 2969 int pattern_position; 2970 int next_item_length; 2971 2972 The version field is an integer containing the version number of the 2973 block format. The initial version was 0; the current version is 1. The 2974 version number will change again in future if additional fields are 2975 added, but the intention is never to remove any of the existing fields. 2976 2977 The callout_number field contains the number of the callout, as com- 2978 piled into the pattern (that is, the number after ?C for manual call- 2979 outs, and 255 for automatically generated callouts). 2980 2981 The offset_vector field is a pointer to the vector of offsets that was 2982 passed by the caller to pcre_exec() or pcre_dfa_exec(). When 2983 pcre_exec() is used, the contents can be inspected in order to extract 2984 substrings that have been matched so far, in the same way as for 2985 extracting substrings after a match has completed. For pcre_dfa_exec() 2986 this field is not useful. 2987 2988 The subject and subject_length fields contain copies of the values that 2989 were passed to pcre_exec(). 2990 2991 The start_match field normally contains the offset within the subject 2992 at which the current match attempt started. However, if the escape 2993 sequence \K has been encountered, this value is changed to reflect the 2994 modified starting point. If the pattern is not anchored, the callout 2995 function may be called several times from the same point in the pattern 2996 for different starting points in the subject. 2997 2998 The current_position field contains the offset within the subject of 2999 the current match pointer. 3000 3001 When the pcre_exec() function is used, the capture_top field contains 3002 one more than the number of the highest numbered captured substring so 3003 far. If no substrings have been captured, the value of capture_top is 3004 one. This is always the case when pcre_dfa_exec() is used, because it 3005 does not support captured substrings. 3006 3007 The capture_last field contains the number of the most recently cap- 3008 tured substring. If no substrings have been captured, its value is -1. 3009 This is always the case when pcre_dfa_exec() is used. 3010 3011 The callout_data field contains a value that is passed to pcre_exec() 3012 or pcre_dfa_exec() specifically so that it can be passed back in call- 3013 outs. It is passed in the pcre_callout field of the pcre_extra data 3014 structure. If no such data was passed, the value of callout_data in a 3015 pcre_callout block is NULL. There is a description of the pcre_extra 3016 structure in the pcreapi documentation. 3017 3018 The pattern_position field is present from version 1 of the pcre_call- 3019 out structure. It contains the offset to the next item to be matched in 3020 the pattern string. 3021 3022 The next_item_length field is present from version 1 of the pcre_call- 3023 out structure. It contains the length of the next item to be matched in 3024 the pattern string. When the callout immediately precedes an alterna- 3025 tion bar, a closing parenthesis, or the end of the pattern, the length 3026 is zero. When the callout precedes an opening parenthesis, the length 3027 is that of the entire subpattern. 3028 3029 The pattern_position and next_item_length fields are intended to help 3030 in distinguishing between different automatic callouts, which all have 3031 the same callout number. However, they are set for all callouts. 3032 3033 3034 RETURN VALUES 3035 3036 The external callout function returns an integer to PCRE. If the value 3037 is zero, matching proceeds as normal. If the value is greater than 3038 zero, matching fails at the current point, but the testing of other 3039 matching possibilities goes ahead, just as if a lookahead assertion had 3040 failed. If the value is less than zero, the match is abandoned, and 3041 pcre_exec() or pcre_dfa_exec() returns the negative value. 3042 3043 Negative values should normally be chosen from the set of 3044 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan- 3045 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is 3046 reserved for use by callout functions; it will never be used by PCRE 3047 itself. 3048 3049 3050 AUTHOR 3051 3052 Philip Hazel 3053 University Computing Service 3054 Cambridge CB2 3QH, England. 3055 3056 3057 REVISION 3058 3059 Last updated: 21 November 2010 3060 Copyright (c) 1997-2010 University of Cambridge. 3061 ------------------------------------------------------------------------------ 3062 3063 3064 PCRECOMPAT(3) PCRECOMPAT(3) 3065 3066 3067 NAME 3068 PCRE - Perl-compatible regular expressions 3069 3070 3071 DIFFERENCES BETWEEN PCRE AND PERL 3072 3073 This document describes the differences in the ways that PCRE and Perl 3074 handle regular expressions. The differences described here are with 3075 respect to Perl versions 5.10 and above. 3076 3077 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details 3078 of what it does have are given in the section on UTF-8 support in the 3079 main pcre page. 3080 3081 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl 3082 permits them, but they do not mean what you might think. For example, 3083 (?!a){3} does not assert that the next three characters are not "a". It 3084 just asserts that the next character is not "a" three times. 3085 3086 3. Capturing subpatterns that occur inside negative lookahead asser- 3087 tions are counted, but their entries in the offsets vector are never 3088 set. Perl sets its numerical variables from any such patterns that are 3089 matched before the assertion fails to match something (thereby succeed- 3090 ing), but only if the negative lookahead assertion contains just one 3091 branch. 3092 3093 4. Though binary zero characters are supported in the subject string, 3094 they are not allowed in a pattern string because it is passed as a nor- 3095 mal C string, terminated by zero. The escape sequence \0 can be used in 3096 the pattern to represent a binary zero. 3097 3098 5. The following Perl escape sequences are not supported: \l, \u, \L, 3099 \U, and \N. In fact these are implemented by Perl's general string-han- 3100 dling and are not part of its pattern matching engine. If any of these 3101 are encountered by PCRE, an error is generated. 3102 3103 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE 3104 is built with Unicode character property support. The properties that 3105 can be tested with \p and \P are limited to the general category prop- 3106 erties such as Lu and Nd, script names such as Greek or Han, and the 3107 derived properties Any and L&. PCRE does support the Cs (surrogate) 3108 property, which Perl does not; the Perl documentation says "Because 3109 Perl hides the need for the user to understand the internal representa- 3110 tion of Unicode characters, there is no need to implement the somewhat 3111 messy concept of surrogates." 3112 3113 7. PCRE does support the \Q...\E escape for quoting substrings. Charac- 3114 ters in between are treated as literals. This is slightly different 3115 from Perl in that $ and @ are also handled as literals inside the 3116 quotes. In Perl, they cause variable interpolation (but of course PCRE 3117 does not have variables). Note the following examples: 3118 3119 Pattern PCRE matches Perl matches 3120 3121 \Qabc$xyz\E abc$xyz abc followed by the 3122 contents of $xyz 3123 \Qabc\$xyz\E abc\$xyz abc\$xyz 3124 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 3125 3126 The \Q...\E sequence is recognized both inside and outside character 3127 classes. 3128 3129 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code}) 3130 constructions. However, there is support for recursive patterns. This 3131 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE 3132 "callout" feature allows an external function to be called during pat- 3133 tern matching. See the pcrecallout documentation for details. 3134 3135 9. Subpatterns that are called recursively or as "subroutines" are 3136 always treated as atomic groups in PCRE. This is like Python, but 3137 unlike Perl. There is a discussion of an example that explains this in 3138 more detail in the section on recursion differences from Perl in the 3139 pcrepattern page. 3140 3141 10. There are some differences that are concerned with the settings of 3142 captured strings when part of a pattern is repeated. For example, 3143 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2 3144 unset, but in PCRE it is set to "b". 3145 3146 11. PCRE's handling of duplicate subpattern numbers and duplicate sub- 3147 pattern names is not as general as Perl's. This is a consequence of the 3148 fact the PCRE works internally just with numbers, using an external ta- 3149 ble to translate between numbers and names. In particular, a pattern 3150 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have 3151 the same number but different names, is not supported, and causes an 3152 error at compile time. If it were allowed, it would not be possible to 3153 distinguish which parentheses matched, because both names map to cap- 3154 turing subpattern number 1. To avoid this confusing situation, an error 3155 is given at compile time. 3156 3157 12. Perl recognizes comments in some places that PCRE doesn't, for 3158 example, between the ( and ? at the start of a subpattern. 3159 3160 13. PCRE provides some extensions to the Perl regular expression facil- 3161 ities. Perl 5.10 includes new features that are not in earlier ver- 3162 sions of Perl, some of which (such as named parentheses) have been in 3163 PCRE for some time. This list is with respect to Perl 5.10: 3164 3165 (a) Although lookbehind assertions in PCRE must match fixed length 3166 strings, each alternative branch of a lookbehind assertion can match a 3167 different length of string. Perl requires them all to have the same 3168 length. 3169 3170 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $ 3171 meta-character matches only at the very end of the string. 3172 3173 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe- 3174 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly 3175 ignored. (Perl can be made to issue a warning.) 3176 3177 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti- 3178 fiers is inverted, that is, by default they are not greedy, but if fol- 3179 lowed by a question mark they are. 3180 3181 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be 3182 tried only at the first matching position in the subject string. 3183 3184 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, 3185 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva- 3186 lents. 3187 3188 (g) The \R escape sequence can be restricted to match only CR, LF, or 3189 CRLF by the PCRE_BSR_ANYCRLF option. 3190 3191 (h) The callout facility is PCRE-specific. 3192 3193 (i) The partial matching facility is PCRE-specific. 3194 3195 (j) Patterns compiled by PCRE can be saved and re-used at a later time, 3196 even on different hosts that have the other endianness. 3197 3198 (k) The alternative matching function (pcre_dfa_exec()) matches in a 3199 different way and is not Perl-compatible. 3200 3201 (l) PCRE recognizes some special sequences such as (*CR) at the start 3202 of a pattern that set overall options that cannot be changed within the 3203 pattern. 3204 3205 3206 AUTHOR 3207 3208 Philip Hazel 3209 University Computing Service 3210 Cambridge CB2 3QH, England. 3211 3212 3213 REVISION 3214 3215 Last updated: 31 October 2010 3216 Copyright (c) 1997-2010 University of Cambridge. 3217 ------------------------------------------------------------------------------ 3218 3219 3220 PCREPATTERN(3) PCREPATTERN(3) 3221 3222 3223 NAME 3224 PCRE - Perl-compatible regular expressions 3225 3226 3227 PCRE REGULAR EXPRESSION DETAILS 3228 3229 The syntax and semantics of the regular expressions that are supported 3230 by PCRE are described in detail below. There is a quick-reference syn- 3231 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and 3232 semantics as closely as it can. PCRE also supports some alternative 3233 regular expression syntax (which does not conflict with the Perl syn- 3234 tax) in order to provide some compatibility with regular expressions in 3235 Python, .NET, and Oniguruma. 3236 3237 Perl's regular expressions are described in its own documentation, and 3238 regular expressions in general are covered in a number of books, some 3239 of which have copious examples. Jeffrey Friedl's "Mastering Regular 3240 Expressions", published by O'Reilly, covers regular expressions in 3241 great detail. This description of PCRE's regular expressions is 3242 intended as reference material. 3243 3244 The original operation of PCRE was on strings of one-byte characters. 3245 However, there is now also support for UTF-8 character strings. To use 3246 this, PCRE must be built to include UTF-8 support, and you must call 3247 pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There is 3248 also a special sequence that can be given at the start of a pattern: 3249 3250 (*UTF8) 3251 3252 Starting a pattern with this sequence is equivalent to setting the 3253 PCRE_UTF8 option. This feature is not Perl-compatible. How setting 3254 UTF-8 mode affects pattern matching is mentioned in several places 3255 below. There is also a summary of UTF-8 features in the section on 3256 UTF-8 support in the main pcre page. 3257 3258 Another special sequence that may appear at the start of a pattern or 3259 in combination with (*UTF8) is: 3260 3261 (*UCP) 3262 3263 This has the same effect as setting the PCRE_UCP option: it causes 3264 sequences such as \d and \w to use Unicode properties to determine 3265 character types, instead of recognizing only characters with codes less 3266 than 128 via a lookup table. 3267 3268 If a pattern starts with (*NO_START_OPT), it has the same effect as 3269 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching 3270 time. There are also some more of these special sequences that are con- 3271 cerned with the handling of newlines; they are described below. 3272 3273 The remainder of this document discusses the patterns that are sup- 3274 ported by PCRE when its main matching function, pcre_exec(), is used. 3275 From release 6.0, PCRE offers a second matching function, 3276 pcre_dfa_exec(), which matches using a different algorithm that is not 3277 Perl-compatible. Some of the features discussed below are not available 3278 when pcre_dfa_exec() is used. The advantages and disadvantages of the 3279 alternative function, and how it differs from the normal function, are 3280 discussed in the pcrematching page. 3281 3282 3283 NEWLINE CONVENTIONS 3284 3285 PCRE supports five different conventions for indicating line breaks in 3286 strings: a single CR (carriage return) character, a single LF (line- 3287 feed) character, the two-character sequence CRLF, any of the three pre- 3288 ceding, or any Unicode newline sequence. The pcreapi page has further 3289 discussion about newlines, and shows how to set the newline convention 3290 in the options arguments for the compiling and matching functions. 3291 3292 It is also possible to specify a newline convention by starting a pat- 3293 tern string with one of the following five sequences: 3294 3295 (*CR) carriage return 3296 (*LF) linefeed 3297 (*CRLF) carriage return, followed by linefeed 3298 (*ANYCRLF) any of the three above 3299 (*ANY) all Unicode newline sequences 3300 3301 These override the default and the options given to pcre_compile() or 3302 pcre_compile2(). For example, on a Unix system where LF is the default 3303 newline sequence, the pattern 3304 3305 (*CR)a.b 3306 3307 changes the convention to CR. That pattern matches "a\nb" because LF is 3308 no longer a newline. Note that these special settings, which are not 3309 Perl-compatible, are recognized only at the very start of a pattern, 3310 and that they must be in upper case. If more than one of them is 3311 present, the last one is used. 3312 3313 The newline convention affects the interpretation of the dot metachar- 3314 acter when PCRE_DOTALL is not set, and also the behaviour of \N. How- 3315 ever, it does not affect what the \R escape sequence matches. By 3316 default, this is any Unicode newline sequence, for Perl compatibility. 3317 However, this can be changed; see the description of \R in the section 3318 entitled "Newline sequences" below. A change of \R setting can be com- 3319 bined with a change of newline convention. 3320 3321 3322 CHARACTERS AND METACHARACTERS 3323 3324 A regular expression is a pattern that is matched against a subject 3325 string from left to right. Most characters stand for themselves in a 3326 pattern, and match the corresponding characters in the subject. As a 3327 trivial example, the pattern 3328 3329 The quick brown fox 3330 3331 matches a portion of a subject string that is identical to itself. When 3332 caseless matching is specified (the PCRE_CASELESS option), letters are 3333 matched independently of case. In UTF-8 mode, PCRE always understands 3334 the concept of case for characters whose values are less than 128, so 3335 caseless matching is always possible. For characters with higher val- 3336 ues, the concept of case is supported if PCRE is compiled with Unicode 3337 property support, but not otherwise. If you want to use caseless 3338 matching for characters 128 and above, you must ensure that PCRE is 3339 compiled with Unicode property support as well as with UTF-8 support. 3340 3341 The power of regular expressions comes from the ability to include 3342 alternatives and repetitions in the pattern. These are encoded in the 3343 pattern by the use of metacharacters, which do not stand for themselves 3344 but instead are interpreted in some special way. 3345 3346 There are two different sets of metacharacters: those that are recog- 3347 nized anywhere in the pattern except within square brackets, and those 3348 that are recognized within square brackets. Outside square brackets, 3349 the metacharacters are as follows: 3350 3351 \ general escape character with several uses 3352 ^ assert start of string (or line, in multiline mode) 3353 $ assert end of string (or line, in multiline mode) 3354 . match any character except newline (by default) 3355 [ start character class definition 3356 | start of alternative branch 3357 ( start subpattern 3358 ) end subpattern 3359 ? extends the meaning of ( 3360 also 0 or 1 quantifier 3361 also quantifier minimizer 3362 * 0 or more quantifier 3363 + 1 or more quantifier 3364 also "possessive quantifier" 3365 { start min/max quantifier 3366 3367 Part of a pattern that is in square brackets is called a "character 3368 class". In a character class the only metacharacters are: 3369 3370 \ general escape character 3371 ^ negate the class, but only if the first character 3372 - indicates character range 3373 [ POSIX character class (only if followed by POSIX 3374 syntax) 3375 ] terminates the character class 3376 3377 The following sections describe the use of each of the metacharacters. 3378 3379 3380 BACKSLASH 3381 3382 The backslash character has several uses. Firstly, if it is followed by 3383 a character that is not a number or a letter, it takes away any special 3384 meaning that character may have. This use of backslash as an escape 3385 character applies both inside and outside character classes. 3386 3387 For example, if you want to match a * character, you write \* in the 3388 pattern. This escaping action applies whether or not the following 3389 character would otherwise be interpreted as a metacharacter, so it is 3390 always safe to precede a non-alphanumeric with backslash to specify 3391 that it stands for itself. In particular, if you want to match a back- 3392 slash, you write \\. 3393 3394 In UTF-8 mode, only ASCII numbers and letters have any special meaning 3395 after a backslash. All other characters (in particular, those whose 3396 codepoints are greater than 127) are treated as literals. 3397 3398 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in 3399 the pattern (other than in a character class) and characters between a 3400 # outside a character class and the next newline are ignored. An escap- 3401 ing backslash can be used to include a whitespace or # character as 3402 part of the pattern. 3403 3404 If you want to remove the special meaning from a sequence of charac- 3405 ters, you can do so by putting them between \Q and \E. This is differ- 3406 ent from Perl in that $ and @ are handled as literals in \Q...\E 3407 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola- 3408 tion. Note the following examples: 3409 3410 Pattern PCRE matches Perl matches 3411 3412 \Qabc$xyz\E abc$xyz abc followed by the 3413 contents of $xyz 3414 \Qabc\$xyz\E abc\$xyz abc\$xyz 3415 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz 3416 3417 The \Q...\E sequence is recognized both inside and outside character 3418 classes. An isolated \E that is not preceded by \Q is ignored. 3419 3420 Non-printing characters 3421 3422 A second use of backslash provides a way of encoding non-printing char- 3423 acters in patterns in a visible manner. There is no restriction on the 3424 appearance of non-printing characters, apart from the binary zero that 3425 terminates a pattern, but when a pattern is being prepared by text 3426 editing, it is often easier to use one of the following escape 3427 sequences than the binary character it represents: 3428 3429 \a alarm, that is, the BEL character (hex 07) 3430 \cx "control-x", where x is any ASCII character 3431 \e escape (hex 1B) 3432 \f formfeed (hex 0C) 3433 \n linefeed (hex 0A) 3434 \r carriage return (hex 0D) 3435 \t tab (hex 09) 3436 \ddd character with octal code ddd, or back reference 3437 \xhh character with hex code hh 3438 \x{hhh..} character with hex code hhh.. 3439 3440 The precise effect of \cx is as follows: if x is a lower case letter, 3441 it is converted to upper case. Then bit 6 of the character (hex 40) is 3442 inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({ 3443 is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c 3444 has a value greater than 127, a compile-time error occurs. This locks 3445 out non-ASCII characters in both byte mode and UTF-8 mode. (When PCRE 3446 is compiled in EBCDIC mode, all byte values are valid. A lower case 3447 letter is converted to upper case, and then the 0xc0 bits are flipped.) 3448 3449 After \x, from zero to two hexadecimal digits are read (letters can be 3450 in upper or lower case). Any number of hexadecimal digits may appear 3451 between \x{ and }, but the value of the character code must be less 3452 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, 3453 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger 3454 than the largest Unicode code point, which is 10FFFF. 3455 3456 If characters other than hexadecimal digits appear between \x{ and }, 3457 or if there is no terminating }, this form of escape is not recognized. 3458 Instead, the initial \x will be interpreted as a basic hexadecimal 3459 escape, with no following digits, giving a character whose value is 3460 zero. 3461 3462 Characters whose value is less than 256 can be defined by either of the 3463 two syntaxes for \x. There is no difference in the way they are han- 3464 dled. For example, \xdc is exactly the same as \x{dc}. 3465 3466 After \0 up to two further octal digits are read. If there are fewer 3467 than two digits, just those that are present are used. Thus the 3468 sequence \0\x\07 specifies two binary zeros followed by a BEL character 3469 (code value 7). Make sure you supply two digits after the initial zero 3470 if the pattern character that follows is itself an octal digit. 3471 3472 The handling of a backslash followed by a digit other than 0 is compli- 3473 cated. Outside a character class, PCRE reads it and any following dig- 3474 its as a decimal number. If the number is less than 10, or if there 3475 have been at least that many previous capturing left parentheses in the 3476 expression, the entire sequence is taken as a back reference. A 3477 description of how this works is given later, following the discussion 3478 of parenthesized subpatterns. 3479 3480 Inside a character class, or if the decimal number is greater than 9 3481 and there have not been that many capturing subpatterns, PCRE re-reads 3482 up to three octal digits following the backslash, and uses them to gen- 3483 erate a data character. Any subsequent digits stand for themselves. In 3484 non-UTF-8 mode, the value of a character specified in octal must be 3485 less than \400. In UTF-8 mode, values up to \777 are permitted. For 3486 example: 3487 3488 \040 is another way of writing a space 3489 \40 is the same, provided there are fewer than 40 3490 previous capturing subpatterns 3491 \7 is always a back reference 3492 \11 might be a back reference, or another way of 3493 writing a tab 3494 \011 is always a tab 3495 \0113 is a tab followed by the character "3" 3496 \113 might be a back reference, otherwise the 3497 character with octal code 113 3498 \377 might be a back reference, otherwise 3499 the byte consisting entirely of 1 bits 3500 \81 is either a back reference, or a binary zero 3501 followed by the two characters "8" and "1" 3502 3503 Note that octal values of 100 or greater must not be introduced by a 3504 leading zero, because no more than three octal digits are ever read. 3505 3506 All the sequences that define a single character value can be used both 3507 inside and outside character classes. In addition, inside a character 3508 class, the sequence \b is interpreted as the backspace character (hex 3509 08). The sequences \B, \N, \R, and \X are not special inside a charac- 3510 ter class. Like any other unrecognized escape sequences, they are 3511 treated as the literal characters "B", "N", "R", and "X" by default, 3512 but cause an error if the PCRE_EXTRA option is set. Outside a character 3513 class, these sequences have different meanings. 3514 3515 Absolute and relative back references 3516 3517 The sequence \g followed by an unsigned or a negative number, option- 3518 ally enclosed in braces, is an absolute or relative back reference. A 3519 named back reference can be coded as \g{name}. Back references are dis- 3520 cussed later, following the discussion of parenthesized subpatterns. 3521 3522 Absolute and relative subroutine calls 3523 3524 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 3525 name or a number enclosed either in angle brackets or single quotes, is 3526 an alternative syntax for referencing a subpattern as a "subroutine". 3527 Details are discussed later. Note that \g{...} (Perl syntax) and 3528 \g<...> (Oniguruma syntax) are not synonymous. The former is a back 3529 reference; the latter is a subroutine call. 3530 3531 Generic character types 3532 3533 Another use of backslash is for specifying generic character types: 3534 3535 \d any decimal digit 3536 \D any character that is not a decimal digit 3537 \h any horizontal whitespace character 3538 \H any character that is not a horizontal whitespace character 3539 \s any whitespace character 3540 \S any character that is not a whitespace character 3541 \v any vertical whitespace character 3542 \V any character that is not a vertical whitespace character 3543 \w any "word" character 3544 \W any "non-word" character 3545 3546 There is also the single sequence \N, which matches a non-newline char- 3547 acter. This is the same as the "." metacharacter when PCRE_DOTALL is 3548 not set. 3549 3550 Each pair of lower and upper case escape sequences partitions the com- 3551 plete set of characters into two disjoint sets. Any given character 3552 matches one, and only one, of each pair. The sequences can appear both 3553 inside and outside character classes. They each match one character of 3554 the appropriate type. If the current matching point is at the end of 3555 the subject string, all of them fail, because there is no character to 3556 match. 3557 3558 For compatibility with Perl, \s does not match the VT character (code 3559 11). This makes it different from the the POSIX "space" class. The \s 3560 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If 3561 "use locale;" is included in a Perl script, \s may match the VT charac- 3562 ter. In PCRE, it never does. 3563 3564 A "word" character is an underscore or any character that is a letter 3565 or digit. By default, the definition of letters and digits is con- 3566 trolled by PCRE's low-valued character tables, and may vary if locale- 3567 specific matching is taking place (see "Locale support" in the pcreapi 3568 page). For example, in a French locale such as "fr_FR" in Unix-like 3569 systems, or "french" in Windows, some character codes greater than 128 3570 are used for accented letters, and these are then matched by \w. The 3571 use of locales with Unicode is discouraged. 3572 3573 By default, in UTF-8 mode, characters with values greater than 128 3574 never match \d, \s, or \w, and always match \D, \S, and \W. These 3575 sequences retain their original meanings from before UTF-8 support was 3576 available, mainly for efficiency reasons. However, if PCRE is compiled 3577 with Unicode property support, and the PCRE_UCP option is set, the be- 3578 haviour is changed so that Unicode properties are used to determine 3579 character types, as follows: 3580 3581 \d any character that \p{Nd} matches (decimal digit) 3582 \s any character that \p{Z} matches, plus HT, LF, FF, CR 3583 \w any character that \p{L} or \p{N} matches, plus underscore 3584 3585 The upper case escapes match the inverse sets of characters. Note that 3586 \d matches only decimal digits, whereas \w matches any Unicode digit, 3587 as well as any Unicode letter, and underscore. Note also that PCRE_UCP 3588 affects \b, and \B because they are defined in terms of \w and \W. 3589 Matching these sequences is noticeably slower when PCRE_UCP is set. 3590 3591 The sequences \h, \H, \v, and \V are features that were added to Perl 3592 at release 5.10. In contrast to the other sequences, which match only 3593 ASCII characters by default, these always match certain high-valued 3594 codepoints in UTF-8 mode, whether or not PCRE_UCP is set. The horizon- 3595 tal space characters are: 3596 3597 U+0009 Horizontal tab 3598 U+0020 Space 3599 U+00A0 Non-break space 3600 U+1680 Ogham space mark 3601 U+180E Mongolian vowel separator 3602 U+2000 En quad 3603 U+2001 Em quad 3604 U+2002 En space 3605 U+2003 Em space 3606 U+2004 Three-per-em space 3607 U+2005 Four-per-em space 3608 U+2006 Six-per-em space 3609 U+2007 Figure space 3610 U+2008 Punctuation space 3611 U+2009 Thin space 3612 U+200A Hair space 3613 U+202F Narrow no-break space 3614 U+205F Medium mathematical space 3615 U+3000 Ideographic space 3616 3617 The vertical space characters are: 3618 3619 U+000A Linefeed 3620 U+000B Vertical tab 3621 U+000C Formfeed 3622 U+000D Carriage return 3623 U+0085 Next line 3624 U+2028 Line separator 3625 U+2029 Paragraph separator 3626 3627 Newline sequences 3628 3629 Outside a character class, by default, the escape sequence \R matches 3630 any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the 3631 following: 3632 3633 (?>\r\n|\n|\x0b|\f|\r|\x85) 3634 3635 This is an example of an "atomic group", details of which are given 3636 below. This particular group matches either the two-character sequence 3637 CR followed by LF, or one of the single characters LF (linefeed, 3638 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage 3639 return, U+000D), or NEL (next line, U+0085). The two-character sequence 3640 is treated as a single unit that cannot be split. 3641 3642 In UTF-8 mode, two additional characters whose codepoints are greater 3643 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa- 3644 rator, U+2029). Unicode character property support is not needed for 3645 these characters to be recognized. 3646 3647 It is possible to restrict \R to match only CR, LF, or CRLF (instead of 3648 the complete set of Unicode line endings) by setting the option 3649 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched. 3650 (BSR is an abbrevation for "backslash R".) This can be made the default 3651 when PCRE is built; if this is the case, the other behaviour can be 3652 requested via the PCRE_BSR_UNICODE option. It is also possible to 3653 specify these settings by starting a pattern string with one of the 3654 following sequences: 3655 3656 (*BSR_ANYCRLF) CR, LF, or CRLF only 3657 (*BSR_UNICODE) any Unicode newline sequence 3658 3659 These override the default and the options given to pcre_compile() or 3660 pcre_compile2(), but they can be overridden by options given to 3661 pcre_exec() or pcre_dfa_exec(). Note that these special settings, which 3662 are not Perl-compatible, are recognized only at the very start of a 3663 pattern, and that they must be in upper case. If more than one of them 3664 is present, the last one is used. They can be combined with a change of 3665 newline convention; for example, a pattern can start with: 3666 3667 (*ANY)(*BSR_ANYCRLF) 3668 3669 They can also be combined with the (*UTF8) or (*UCP) special sequences. 3670 Inside a character class, \R is treated as an unrecognized escape 3671 sequence, and so matches the letter "R" by default, but causes an error 3672 if PCRE_EXTRA is set. 3673 3674 Unicode character properties 3675 3676 When PCRE is built with Unicode character property support, three addi- 3677 tional escape sequences that match characters with specific properties 3678 are available. When not in UTF-8 mode, these sequences are of course 3679 limited to testing characters whose codepoints are less than 256, but 3680 they do work in this mode. The extra escape sequences are: 3681 3682 \p{xx} a character with the xx property 3683 \P{xx} a character without the xx property 3684 \X an extended Unicode sequence 3685 3686 The property names represented by xx above are limited to the Unicode 3687 script names, the general category properties, "Any", which matches any 3688 character (including newline), and some special PCRE properties 3689 (described in the next section). Other Perl properties such as "InMu- 3690 sicalSymbols" are not currently supported by PCRE. Note that \P{Any} 3691 does not match any characters, so always causes a match failure. 3692 3693 Sets of Unicode characters are defined as belonging to certain scripts. 3694 A character from one of these sets can be matched using a script name. 3695 For example: 3696 3697 \p{Greek} 3698 \P{Han} 3699 3700 Those that are not part of an identified script are lumped together as 3701 "Common". The current list of scripts is: 3702 3703 Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille, 3704 Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common, 3705 Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp- 3706 tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek, 3707 Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe- 3708 rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, 3709 Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, 3710 Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam, 3711 Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, 3712 Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya, 3713 Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian, 3714 Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, 3715 Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, 3716 Ugaritic, Vai, Yi. 3717 3718 Each character has exactly one Unicode general category property, spec- 3719 ified by a two-letter abbreviation. For compatibility with Perl, nega- 3720 tion can be specified by including a circumflex between the opening 3721 brace and the property name. For example, \p{^Lu} is the same as 3722 \P{Lu}. 3723 3724 If only one letter is specified with \p or \P, it includes all the gen- 3725 eral category properties that start with that letter. In this case, in 3726 the absence of negation, the curly brackets in the escape sequence are 3727 optional; these two examples have the same effect: 3728 3729 \p{L} 3730 \pL 3731 3732 The following general category property codes are supported: 3733 3734 C Other 3735 Cc Control 3736 Cf Format 3737 Cn Unassigned 3738 Co Private use 3739 Cs Surrogate 3740 3741 L Letter 3742 Ll Lower case letter 3743 Lm Modifier letter 3744 Lo Other letter 3745 Lt Title case letter 3746 Lu Upper case letter 3747 3748 M Mark 3749 Mc Spacing mark 3750 Me Enclosing mark 3751 Mn Non-spacing mark 3752 3753 N Number 3754 Nd Decimal number 3755 Nl Letter number 3756 No Other number 3757 3758 P Punctuation 3759 Pc Connector punctuation 3760 Pd Dash punctuation 3761 Pe Close punctuation 3762 Pf Final punctuation 3763 Pi Initial punctuation 3764 Po Other punctuation 3765 Ps Open punctuation 3766 3767 S Symbol 3768 Sc Currency symbol 3769 Sk Modifier symbol 3770 Sm Mathematical symbol 3771 So Other symbol 3772 3773 Z Separator 3774 Zl Line separator 3775 Zp Paragraph separator 3776 Zs Space separator 3777 3778 The special property L& is also supported: it matches a character that 3779 has the Lu, Ll, or Lt property, in other words, a letter that is not 3780 classified as a modifier or "other". 3781 3782 The Cs (Surrogate) property applies only to characters in the range 3783 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see 3784 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check- 3785 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in 3786 the pcreapi page). Perl does not support the Cs property. 3787 3788 The long synonyms for property names that Perl supports (such as 3789 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix 3790 any of these properties with "Is". 3791 3792 No character that is in the Unicode table has the Cn (unassigned) prop- 3793 erty. Instead, this property is assumed for any code point that is not 3794 in the Unicode table. 3795 3796 Specifying caseless matching does not affect these escape sequences. 3797 For example, \p{Lu} always matches only upper case letters. 3798 3799 The \X escape matches any number of Unicode characters that form an 3800 extended Unicode sequence. \X is equivalent to 3801 3802 (?>\PM\pM*) 3803 3804 That is, it matches a character without the "mark" property, followed 3805 by zero or more characters with the "mark" property, and treats the 3806 sequence as an atomic group (see below). Characters with the "mark" 3807 property are typically accents that affect the preceding character. 3808 None of them have codepoints less than 256, so in non-UTF-8 mode \X 3809 matches any one character. 3810 3811 Matching characters by Unicode property is not fast, because PCRE has 3812 to search a structure that contains data for over fifteen thousand 3813 characters. That is why the traditional escape sequences such as \d and 3814 \w do not use Unicode properties in PCRE by default, though you can 3815 make them do so by setting the PCRE_UCP option for pcre_compile() or by 3816 starting the pattern with (*UCP). 3817 3818 PCRE's additional properties 3819 3820 As well as the standard Unicode properties described in the previous 3821 section, PCRE supports four more that make it possible to convert tra- 3822 ditional escape sequences such as \w and \s and POSIX character classes 3823 to use Unicode properties. PCRE uses these non-standard, non-Perl prop- 3824 erties internally when PCRE_UCP is set. They are: 3825 3826 Xan Any alphanumeric character 3827 Xps Any POSIX space character 3828 Xsp Any Perl space character 3829 Xwd Any Perl "word" character 3830 3831 Xan matches characters that have either the L (letter) or the N (num- 3832 ber) property. Xps matches the characters tab, linefeed, vertical tab, 3833 formfeed, or carriage return, and any other character that has the Z 3834 (separator) property. Xsp is the same as Xps, except that vertical tab 3835 is excluded. Xwd matches the same characters as Xan, plus underscore. 3836 3837 Resetting the match start 3838 3839 The escape sequence \K causes any previously matched characters not to 3840 be included in the final matched sequence. For example, the pattern: 3841 3842 foo\Kbar 3843 3844 matches "foobar", but reports that it has matched "bar". This feature 3845 is similar to a lookbehind assertion (described below). However, in 3846 this case, the part of the subject before the real match does not have 3847 to be of fixed length, as lookbehind assertions do. The use of \K does 3848 not interfere with the setting of captured substrings. For example, 3849 when the pattern 3850 3851 (foo)\Kbar 3852 3853 matches "foobar", the first substring is still set to "foo". 3854 3855 Perl documents that the use of \K within assertions is "not well 3856 defined". In PCRE, \K is acted upon when it occurs inside positive 3857 assertions, but is ignored in negative assertions. 3858 3859 Simple assertions 3860 3861 The final use of backslash is for certain simple assertions. An asser- 3862 tion specifies a condition that has to be met at a particular point in 3863 a match, without consuming any characters from the subject string. The 3864 use of subpatterns for more complicated assertions is described below. 3865 The backslashed assertions are: 3866 3867 \b matches at a word boundary 3868 \B matches when not at a word boundary 3869 \A matches at the start of the subject 3870 \Z matches at the end of the subject 3871 also matches before a newline at the end of the subject 3872 \z matches only at the end of the subject 3873 \G matches at the first matching position in the subject 3874 3875 Inside a character class, \b has a different meaning; it matches the 3876 backspace character. If any other of these assertions appears in a 3877 character class, by default it matches the corresponding literal char- 3878 acter (for example, \B matches the letter B). However, if the 3879 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener- 3880 ated instead. 3881 3882 A word boundary is a position in the subject string where the current 3883 character and the previous character do not both match \w or \W (i.e. 3884 one matches \w and the other matches \W), or the start or end of the 3885 string if the first or last character matches \w, respectively. In 3886 UTF-8 mode, the meanings of \w and \W can be changed by setting the 3887 PCRE_UCP option. When this is done, it also affects \b and \B. Neither 3888 PCRE nor Perl has a separate "start of word" or "end of word" metase- 3889 quence. However, whatever follows \b normally determines which it is. 3890 For example, the fragment \ba matches "a" at the start of a word. 3891 3892 The \A, \Z, and \z assertions differ from the traditional circumflex 3893 and dollar (described in the next section) in that they only ever match 3894 at the very start and end of the subject string, whatever options are 3895 set. Thus, they are independent of multiline mode. These three asser- 3896 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which 3897 affect only the behaviour of the circumflex and dollar metacharacters. 3898 However, if the startoffset argument of pcre_exec() is non-zero, indi- 3899 cating that matching is to start at a point other than the beginning of 3900 the subject, \A can never match. The difference between \Z and \z is 3901 that \Z matches before a newline at the end of the string as well as at 3902 the very end, whereas \z matches only at the end. 3903 3904 The \G assertion is true only when the current matching position is at 3905 the start point of the match, as specified by the startoffset argument 3906 of pcre_exec(). It differs from \A when the value of startoffset is 3907 non-zero. By calling pcre_exec() multiple times with appropriate argu- 3908 ments, you can mimic Perl's /g option, and it is in this kind of imple- 3909 mentation where \G can be useful. 3910 3911 Note, however, that PCRE's interpretation of \G, as the start of the 3912 current match, is subtly different from Perl's, which defines it as the 3913 end of the previous match. In Perl, these can be different when the 3914 previously matched string was empty. Because PCRE does just one match 3915 at a time, it cannot reproduce this behaviour. 3916 3917 If all the alternatives of a pattern begin with \G, the expression is 3918 anchored to the starting match position, and the "anchored" flag is set 3919 in the compiled regular expression. 3920 3921 3922 CIRCUMFLEX AND DOLLAR 3923 3924 Outside a character class, in the default matching mode, the circumflex 3925 character is an assertion that is true only if the current matching 3926 point is at the start of the subject string. If the startoffset argu- 3927 ment of pcre_exec() is non-zero, circumflex can never match if the 3928 PCRE_MULTILINE option is unset. Inside a character class, circumflex 3929 has an entirely different meaning (see below). 3930 3931 Circumflex need not be the first character of the pattern if a number 3932 of alternatives are involved, but it should be the first thing in each 3933 alternative in which it appears if the pattern is ever to match that 3934 branch. If all possible alternatives start with a circumflex, that is, 3935 if the pattern is constrained to match only at the start of the sub- 3936 ject, it is said to be an "anchored" pattern. (There are also other 3937 constructs that can cause a pattern to be anchored.) 3938 3939 A dollar character is an assertion that is true only if the current 3940 matching point is at the end of the subject string, or immediately 3941 before a newline at the end of the string (by default). Dollar need not 3942 be the last character of the pattern if a number of alternatives are 3943 involved, but it should be the last item in any branch in which it 3944 appears. Dollar has no special meaning in a character class. 3945 3946 The meaning of dollar can be changed so that it matches only at the 3947 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at 3948 compile time. This does not affect the \Z assertion. 3949 3950 The meanings of the circumflex and dollar characters are changed if the 3951 PCRE_MULTILINE option is set. When this is the case, a circumflex 3952 matches immediately after internal newlines as well as at the start of 3953 the subject string. It does not match after a newline that ends the 3954 string. A dollar matches before any newlines in the string, as well as 3955 at the very end, when PCRE_MULTILINE is set. When newline is specified 3956 as the two-character sequence CRLF, isolated CR and LF characters do 3957 not indicate newlines. 3958 3959 For example, the pattern /^abc$/ matches the subject string "def\nabc" 3960 (where \n represents a newline) in multiline mode, but not otherwise. 3961 Consequently, patterns that are anchored in single line mode because 3962 all branches start with ^ are not anchored in multiline mode, and a 3963 match for circumflex is possible when the startoffset argument of 3964 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if 3965 PCRE_MULTILINE is set. 3966 3967 Note that the sequences \A, \Z, and \z can be used to match the start 3968 and end of the subject in both modes, and if all branches of a pattern 3969 start with \A it is always anchored, whether or not PCRE_MULTILINE is 3970 set. 3971 3972 3973 FULL STOP (PERIOD, DOT) AND \N 3974 3975 Outside a character class, a dot in the pattern matches any one charac- 3976 ter in the subject string except (by default) a character that signi- 3977 fies the end of a line. In UTF-8 mode, the matched character may be 3978 more than one byte long. 3979 3980 When a line ending is defined as a single character, dot never matches 3981 that character; when the two-character sequence CRLF is used, dot does 3982 not match CR if it is immediately followed by LF, but otherwise it 3983 matches all characters (including isolated CRs and LFs). When any Uni- 3984 code line endings are being recognized, dot does not match CR or LF or 3985 any of the other line ending characters. 3986 3987 The behaviour of dot with regard to newlines can be changed. If the 3988 PCRE_DOTALL option is set, a dot matches any one character, without 3989 exception. If the two-character sequence CRLF is present in the subject 3990 string, it takes two dots to match it. 3991 3992 The handling of dot is entirely independent of the handling of circum- 3993 flex and dollar, the only relationship being that they both involve 3994 newlines. Dot has no special meaning in a character class. 3995 3996 The escape sequence \N behaves like a dot, except that it is not 3997 affected by the PCRE_DOTALL option. In other words, it matches any 3998 character except one that signifies the end of a line. 3999 4000 4001 MATCHING A SINGLE BYTE 4002 4003 Outside a character class, the escape sequence \C matches any one byte, 4004 both in and out of UTF-8 mode. Unlike a dot, it always matches any 4005 line-ending characters. The feature is provided in Perl in order to 4006 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char- 4007 acters into individual bytes, the rest of the string may start with a 4008 malformed UTF-8 character. For this reason, the \C escape sequence is 4009 best avoided. 4010 4011 PCRE does not allow \C to appear in lookbehind assertions (described 4012 below), because in UTF-8 mode this would make it impossible to calcu- 4013 late the length of the lookbehind. 4014 4015 4016 SQUARE BRACKETS AND CHARACTER CLASSES 4017 4018 An opening square bracket introduces a character class, terminated by a 4019 closing square bracket. A closing square bracket on its own is not spe- 4020 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set, 4021 a lone closing square bracket causes a compile-time error. If a closing 4022 square bracket is required as a member of the class, it should be the 4023 first data character in the class (after an initial circumflex, if 4024 present) or escaped with a backslash. 4025 4026 A character class matches a single character in the subject. In UTF-8 4027 mode, the character may be more than one byte long. A matched character 4028 must be in the set of characters defined by the class, unless the first 4029 character in the class definition is a circumflex, in which case the 4030 subject character must not be in the set defined by the class. If a 4031 circumflex is actually required as a member of the class, ensure it is 4032 not the first character, or escape it with a backslash. 4033 4034 For example, the character class [aeiou] matches any lower case vowel, 4035 while [^aeiou] matches any character that is not a lower case vowel. 4036 Note that a circumflex is just a convenient notation for specifying the 4037 characters that are in the class by enumerating those that are not. A 4038 class that starts with a circumflex is not an assertion; it still con- 4039 sumes a character from the subject string, and therefore it fails if 4040 the current pointer is at the end of the string. 4041 4042 In UTF-8 mode, characters with values greater than 255 can be included 4043 in a class as a literal string of bytes, or by using the \x{ escaping 4044 mechanism. 4045 4046 When caseless matching is set, any letters in a class represent both 4047 their upper case and lower case versions, so for example, a caseless 4048 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not 4049 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always 4050 understands the concept of case for characters whose values are less 4051 than 128, so caseless matching is always possible. For characters with 4052 higher values, the concept of case is supported if PCRE is compiled 4053 with Unicode property support, but not otherwise. If you want to use 4054 caseless matching in UTF8-mode for characters 128 and above, you must 4055 ensure that PCRE is compiled with Unicode property support as well as 4056 with UTF-8 support. 4057 4058 Characters that might indicate line breaks are never treated in any 4059 special way when matching character classes, whatever line-ending 4060 sequence is in use, and whatever setting of the PCRE_DOTALL and 4061 PCRE_MULTILINE options is used. A class such as [^a] always matches one 4062 of these characters. 4063 4064 The minus (hyphen) character can be used to specify a range of charac- 4065 ters in a character class. For example, [d-m] matches any letter 4066 between d and m, inclusive. If a minus character is required in a 4067 class, it must be escaped with a backslash or appear in a position 4068 where it cannot be interpreted as indicating a range, typically as the 4069 first or last character in the class. 4070 4071 It is not possible to have the literal character "]" as the end charac- 4072 ter of a range. A pattern such as [W-]46] is interpreted as a class of 4073 two characters ("W" and "-") followed by a literal string "46]", so it 4074 would match "W46]" or "-46]". However, if the "]" is escaped with a 4075 backslash it is interpreted as the end of range, so [W-\]46] is inter- 4076 preted as a class containing a range followed by two other characters. 4077 The octal or hexadecimal representation of "]" can also be used to end 4078 a range. 4079 4080 Ranges operate in the collating sequence of character values. They can 4081 also be used for characters specified numerically, for example 4082 [\000-\037]. In UTF-8 mode, ranges can include characters whose values 4083 are greater than 255, for example [\x{100}-\x{2ff}]. 4084 4085 If a range that includes letters is used when caseless matching is set, 4086 it matches the letters in either case. For example, [W-c] is equivalent 4087 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if 4088 character tables for a French locale are in use, [\xc8-\xcb] matches 4089 accented E characters in both cases. In UTF-8 mode, PCRE supports the 4090 concept of case for characters with values greater than 128 only when 4091 it is compiled with Unicode property support. 4092 4093 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V, 4094 \w, and \W may appear in a character class, and add the characters that 4095 they match to the class. For example, [\dABCDEF] matches any hexadeci- 4096 mal digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of 4097 \d, \s, \w and their upper case partners, just as it does when they 4098 appear outside a character class, as described in the section entitled 4099 "Generic character types" above. The escape sequence \b has a different 4100 meaning inside a character class; it matches the backspace character. 4101 The sequences \B, \N, \R, and \X are not special inside a character 4102 class. Like any other unrecognized escape sequences, they are treated 4103 as the literal characters "B", "N", "R", and "X" by default, but cause 4104 an error if the PCRE_EXTRA option is set. 4105 4106 A circumflex can conveniently be used with the upper case character 4107 types to specify a more restricted set of characters than the matching 4108 lower case type. For example, the class [^\W_] matches any letter or 4109 digit, but not underscore, whereas [\w] includes underscore. A positive 4110 character class should be read as "something OR something OR ..." and a 4111 negative class as "NOT something AND NOT something AND NOT ...". 4112 4113 The only metacharacters that are recognized in character classes are 4114 backslash, hyphen (only where it can be interpreted as specifying a 4115 range), circumflex (only at the start), opening square bracket (only 4116 when it can be interpreted as introducing a POSIX class name - see the 4117 next section), and the terminating closing square bracket. However, 4118 escaping other non-alphanumeric characters does no harm. 4119 4120 4121 POSIX CHARACTER CLASSES 4122 4123 Perl supports the POSIX notation for character classes. This uses names 4124 enclosed by [: and :] within the enclosing square brackets. PCRE also 4125 supports this notation. For example, 4126 4127 [01[:alpha:]%] 4128 4129 matches "0", "1", any alphabetic character, or "%". The supported class 4130 names are: 4131 4132 alnum letters and digits 4133 alpha letters 4134 ascii character codes 0 - 127 4135 blank space or tab only 4136 cntrl control characters 4137 digit decimal digits (same as \d) 4138 graph printing characters, excluding space 4139 lower lower case letters 4140 print printing characters, including space 4141 punct printing characters, excluding letters and digits and space 4142 space white space (not quite the same as \s) 4143 upper upper case letters 4144 word "word" characters (same as \w) 4145 xdigit hexadecimal digits 4146 4147 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), 4148 and space (32). Notice that this list includes the VT character (code 4149 11). This makes "space" different to \s, which does not include VT (for 4150 Perl compatibility). 4151 4152 The name "word" is a Perl extension, and "blank" is a GNU extension 4153 from Perl 5.8. Another Perl extension is negation, which is indicated 4154 by a ^ character after the colon. For example, 4155 4156 [12[:^digit:]] 4157 4158 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the 4159 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but 4160 these are not supported, and an error is given if they are encountered. 4161 4162 By default, in UTF-8 mode, characters with values greater than 128 do 4163 not match any of the POSIX character classes. However, if the PCRE_UCP 4164 option is passed to pcre_compile(), some of the classes are changed so 4165 that Unicode character properties are used. This is achieved by replac- 4166 ing the POSIX classes by other sequences, as follows: 4167 4168 [:alnum:] becomes \p{Xan} 4169 [:alpha:] becomes \p{L} 4170 [:blank:] becomes \h 4171 [:digit:] becomes \p{Nd} 4172 [:lower:] becomes \p{Ll} 4173 [:space:] becomes \p{Xps} 4174 [:upper:] becomes \p{Lu} 4175 [:word:] becomes \p{Xwd} 4176 4177 Negated versions, such as [:^alpha:] use \P instead of \p. The other 4178 POSIX classes are unchanged, and match only characters with code points 4179 less than 128. 4180 4181 4182 VERTICAL BAR 4183 4184 Vertical bar characters are used to separate alternative patterns. For 4185 example, the pattern 4186 4187 gilbert|sullivan 4188 4189 matches either "gilbert" or "sullivan". Any number of alternatives may 4190 appear, and an empty alternative is permitted (matching the empty 4191 string). The matching process tries each alternative in turn, from left 4192 to right, and the first one that succeeds is used. If the alternatives 4193 are within a subpattern (defined below), "succeeds" means matching the 4194 rest of the main pattern as well as the alternative in the subpattern. 4195 4196 4197 INTERNAL OPTION SETTING 4198 4199 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and 4200 PCRE_EXTENDED options (which are Perl-compatible) can be changed from 4201 within the pattern by a sequence of Perl option letters enclosed 4202 between "(?" and ")". The option letters are 4203 4204 i for PCRE_CASELESS 4205 m for PCRE_MULTILINE 4206 s for PCRE_DOTALL 4207 x for PCRE_EXTENDED 4208 4209 For example, (?im) sets caseless, multiline matching. It is also possi- 4210 ble to unset these options by preceding the letter with a hyphen, and a 4211 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE- 4212 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, 4213 is also permitted. If a letter appears both before and after the 4214 hyphen, the option is unset. 4215 4216 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA 4217 can be changed in the same way as the Perl-compatible options by using 4218 the characters J, U and X respectively. 4219 4220 When one of these option changes occurs at top level (that is, not 4221 inside subpattern parentheses), the change applies to the remainder of 4222 the pattern that follows. If the change is placed right at the start of 4223 a pattern, PCRE extracts it into the global options (and it will there- 4224 fore show up in data extracted by the pcre_fullinfo() function). 4225 4226 An option change within a subpattern (see below for a description of 4227 subpatterns) affects only that part of the subpattern that follows it, 4228 so 4229 4230 (a(?i)b)c 4231 4232 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not 4233 used). By this means, options can be made to have different settings 4234 in different parts of the pattern. Any changes made in one alternative 4235 do carry on into subsequent branches within the same subpattern. For 4236 example, 4237 4238 (a(?i)b|c) 4239 4240 matches "ab", "aB", "c", and "C", even though when matching "C" the 4241 first branch is abandoned before the option setting. This is because 4242 the effects of option settings happen at compile time. There would be 4243 some very weird behaviour otherwise. 4244 4245 Note: There are other PCRE-specific options that can be set by the 4246 application when the compile or match functions are called. In some 4247 cases the pattern can contain special leading sequences such as (*CRLF) 4248 to override what the application has set or what has been defaulted. 4249 Details are given in the section entitled "Newline sequences" above. 4250 There are also the (*UTF8) and (*UCP) leading sequences that can be 4251 used to set UTF-8 and Unicode property modes; they are equivalent to 4252 setting the PCRE_UTF8 and the PCRE_UCP options, respectively. 4253 4254 4255 SUBPATTERNS 4256 4257 Subpatterns are delimited by parentheses (round brackets), which can be 4258 nested. Turning part of a pattern into a subpattern does two things: 4259 4260 1. It localizes a set of alternatives. For example, the pattern 4261 4262 cat(aract|erpillar|) 4263 4264 matches "cataract", "caterpillar", or "cat". Without the parentheses, 4265 it would match "cataract", "erpillar" or an empty string. 4266 4267 2. It sets up the subpattern as a capturing subpattern. This means 4268 that, when the whole pattern matches, that portion of the subject 4269 string that matched the subpattern is passed back to the caller via the 4270 ovector argument of pcre_exec(). Opening parentheses are counted from 4271 left to right (starting from 1) to obtain numbers for the capturing 4272 subpatterns. For example, if the string "the red king" is matched 4273 against the pattern 4274 4275 the ((red|white) (king|queen)) 4276 4277 the captured substrings are "red king", "red", and "king", and are num- 4278 bered 1, 2, and 3, respectively. 4279 4280 The fact that plain parentheses fulfil two functions is not always 4281 helpful. There are often times when a grouping subpattern is required 4282 without a capturing requirement. If an opening parenthesis is followed 4283 by a question mark and a colon, the subpattern does not do any captur- 4284 ing, and is not counted when computing the number of any subsequent 4285 capturing subpatterns. For example, if the string "the white queen" is 4286 matched against the pattern 4287 4288 the ((?:red|white) (king|queen)) 4289 4290 the captured substrings are "white queen" and "queen", and are numbered 4291 1 and 2. The maximum number of capturing subpatterns is 65535. 4292 4293 As a convenient shorthand, if any option settings are required at the 4294 start of a non-capturing subpattern, the option letters may appear 4295 between the "?" and the ":". Thus the two patterns 4296 4297 (?i:saturday|sunday) 4298 (?:(?i)saturday|sunday) 4299 4300 match exactly the same set of strings. Because alternative branches are 4301 tried from left to right, and options are not reset until the end of 4302 the subpattern is reached, an option setting in one branch does affect 4303 subsequent branches, so the above patterns match "SUNDAY" as well as 4304 "Saturday". 4305 4306 4307 DUPLICATE SUBPATTERN NUMBERS 4308 4309 Perl 5.10 introduced a feature whereby each alternative in a subpattern 4310 uses the same numbers for its capturing parentheses. Such a subpattern 4311 starts with (?| and is itself a non-capturing subpattern. For example, 4312 consider this pattern: 4313 4314 (?|(Sat)ur|(Sun))day 4315 4316 Because the two alternatives are inside a (?| group, both sets of cap- 4317 turing parentheses are numbered one. Thus, when the pattern matches, 4318 you can look at captured substring number one, whichever alternative 4319 matched. This construct is useful when you want to capture part, but 4320 not all, of one of a number of alternatives. Inside a (?| group, paren- 4321 theses are numbered as usual, but the number is reset at the start of 4322 each branch. The numbers of any capturing parentheses that follow the 4323 subpattern start after the highest number used in any branch. The fol- 4324 lowing example is taken from the Perl documentation. The numbers under- 4325 neath show in which buffer the captured content will be stored. 4326 4327 # before ---------------branch-reset----------- after 4328 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x 4329 # 1 2 2 3 2 3 4 4330 4331 A back reference to a numbered subpattern uses the most recent value 4332 that is set for that number by any subpattern. The following pattern 4333 matches "abcabc" or "defdef": 4334 4335 /(?|(abc)|(def))\1/ 4336 4337 In contrast, a recursive or "subroutine" call to a numbered subpattern 4338 always refers to the first one in the pattern with the given number. 4339 The following pattern matches "abcabc" or "defabc": 4340 4341 /(?|(abc)|(def))(?1)/ 4342 4343 If a condition test for a subpattern's having matched refers to a non- 4344 unique number, the test is true if any of the subpatterns of that num- 4345 ber have matched. 4346 4347 An alternative approach to using this "branch reset" feature is to use 4348 duplicate named subpatterns, as described in the next section. 4349 4350 4351 NAMED SUBPATTERNS 4352 4353 Identifying capturing parentheses by number is simple, but it can be 4354 very hard to keep track of the numbers in complicated regular expres- 4355 sions. Furthermore, if an expression is modified, the numbers may 4356 change. To help with this difficulty, PCRE supports the naming of sub- 4357 patterns. This feature was not added to Perl until release 5.10. Python 4358 had the feature earlier, and PCRE introduced it at release 4.0, using 4359 the Python syntax. PCRE now supports both the Perl and the Python syn- 4360 tax. Perl allows identically numbered subpatterns to have different 4361 names, but PCRE does not. 4362 4363 In PCRE, a subpattern can be named in one of three ways: (?<name>...) 4364 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References 4365 to capturing parentheses from other parts of the pattern, such as back 4366 references, recursion, and conditions, can be made by name as well as 4367 by number. 4368 4369 Names consist of up to 32 alphanumeric characters and underscores. 4370 Named capturing parentheses are still allocated numbers as well as 4371 names, exactly as if the names were not present. The PCRE API provides 4372 function calls for extracting the name-to-number translation table from 4373 a compiled pattern. There is also a convenience function for extracting 4374 a captured substring by name. 4375 4376 By default, a name must be unique within a pattern, but it is possible 4377 to relax this constraint by setting the PCRE_DUPNAMES option at compile 4378 time. (Duplicate names are also always permitted for subpatterns with 4379 the same number, set up as described in the previous section.) Dupli- 4380 cate names can be useful for patterns where only one instance of the 4381 named parentheses can match. Suppose you want to match the name of a 4382 weekday, either as a 3-letter abbreviation or as the full name, and in 4383 both cases you want to extract the abbreviation. This pattern (ignoring 4384 the line breaks) does the job: 4385 4386 (?<DN>Mon|Fri|Sun)(?:day)?| 4387 (?<DN>Tue)(?:sday)?| 4388 (?<DN>Wed)(?:nesday)?| 4389 (?<DN>Thu)(?:rsday)?| 4390 (?<DN>Sat)(?:urday)? 4391 4392 There are five capturing substrings, but only one is ever set after a 4393 match. (An alternative way of solving this problem is to use a "branch 4394 reset" subpattern, as described in the previous section.) 4395 4396 The convenience function for extracting the data by name returns the 4397 substring for the first (and in this example, the only) subpattern of 4398 that name that matched. This saves searching to find which numbered 4399 subpattern it was. 4400 4401 If you make a back reference to a non-unique named subpattern from 4402 elsewhere in the pattern, the one that corresponds to the first occur- 4403 rence of the name is used. In the absence of duplicate numbers (see the 4404 previous section) this is the one with the lowest number. If you use a 4405 named reference in a condition test (see the section about conditions 4406 below), either to check whether a subpattern has matched, or to check 4407 for recursion, all subpatterns with the same name are tested. If the 4408 condition is true for any one of them, the overall condition is true. 4409 This is the same behaviour as testing by number. For further details of 4410 the interfaces for handling named subpatterns, see the pcreapi documen- 4411 tation. 4412 4413 Warning: You cannot use different names to distinguish between two sub- 4414 patterns with the same number because PCRE uses only the numbers when 4415 matching. For this reason, an error is given at compile time if differ- 4416 ent names are given to subpatterns with the same number. However, you 4417 can give the same name to subpatterns with the same number, even when 4418 PCRE_DUPNAMES is not set. 4419 4420 4421 REPETITION 4422 4423 Repetition is specified by quantifiers, which can follow any of the 4424 following items: 4425 4426 a literal data character 4427 the dot metacharacter 4428 the \C escape sequence 4429 the \X escape sequence (in UTF-8 mode with Unicode properties) 4430 the \R escape sequence 4431 an escape such as \d or \pL that matches a single character 4432 a character class 4433 a back reference (see next section) 4434 a parenthesized subpattern (unless it is an assertion) 4435 a recursive or "subroutine" call to a subpattern 4436 4437 The general repetition quantifier specifies a minimum and maximum num- 4438 ber of permitted matches, by giving the two numbers in curly brackets 4439 (braces), separated by a comma. The numbers must be less than 65536, 4440 and the first must be less than or equal to the second. For example: 4441 4442 z{2,4} 4443 4444 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a 4445 special character. If the second number is omitted, but the comma is 4446 present, there is no upper limit; if the second number and the comma 4447 are both omitted, the quantifier specifies an exact number of required 4448 matches. Thus 4449 4450 [aeiou]{3,} 4451 4452 matches at least 3 successive vowels, but may match many more, while 4453 4454 \d{8} 4455 4456 matches exactly 8 digits. An opening curly bracket that appears in a 4457 position where a quantifier is not allowed, or one that does not match 4458 the syntax of a quantifier, is taken as a literal character. For exam- 4459 ple, {,6} is not a quantifier, but a literal string of four characters. 4460 4461 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to 4462 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char- 4463 acters, each of which is represented by a two-byte sequence. Similarly, 4464 when Unicode property support is available, \X{3} matches three Unicode 4465 extended sequences, each of which may be several bytes long (and they 4466 may be of different lengths). 4467 4468 The quantifier {0} is permitted, causing the expression to behave as if 4469 the previous item and the quantifier were not present. This may be use- 4470 ful for subpatterns that are referenced as subroutines from elsewhere 4471 in the pattern (but see also the section entitled "Defining subpatterns 4472 for use by reference only" below). Items other than subpatterns that 4473 have a {0} quantifier are omitted from the compiled pattern. 4474 4475 For convenience, the three most common quantifiers have single-charac- 4476 ter abbreviations: 4477 4478 * is equivalent to {0,} 4479 + is equivalent to {1,} 4480 ? is equivalent to {0,1} 4481 4482 It is possible to construct infinite loops by following a subpattern 4483 that can match no characters with a quantifier that has no upper limit, 4484 for example: 4485 4486 (a?)* 4487 4488 Earlier versions of Perl and PCRE used to give an error at compile time 4489 for such patterns. However, because there are cases where this can be 4490 useful, such patterns are now accepted, but if any repetition of the 4491 subpattern does in fact match no characters, the loop is forcibly bro- 4492 ken. 4493 4494 By default, the quantifiers are "greedy", that is, they match as much 4495 as possible (up to the maximum number of permitted times), without 4496 causing the rest of the pattern to fail. The classic example of where 4497 this gives problems is in trying to match comments in C programs. These 4498 appear between /* and */ and within the comment, individual * and / 4499 characters may appear. An attempt to match C comments by applying the 4500 pattern 4501 4502 /\*.*\*/ 4503 4504 to the string 4505 4506 /* first comment */ not comment /* second comment */ 4507 4508 fails, because it matches the entire string owing to the greediness of 4509 the .* item. 4510 4511 However, if a quantifier is followed by a question mark, it ceases to 4512 be greedy, and instead matches the minimum number of times possible, so 4513 the pattern 4514 4515 /\*.*?\*/ 4516 4517 does the right thing with the C comments. The meaning of the various 4518 quantifiers is not otherwise changed, just the preferred number of 4519 matches. Do not confuse this use of question mark with its use as a 4520 quantifier in its own right. Because it has two uses, it can sometimes 4521 appear doubled, as in 4522 4523 \d??\d 4524 4525 which matches one digit by preference, but can match two if that is the 4526 only way the rest of the pattern matches. 4527 4528 If the PCRE_UNGREEDY option is set (an option that is not available in 4529 Perl), the quantifiers are not greedy by default, but individual ones 4530 can be made greedy by following them with a question mark. In other 4531 words, it inverts the default behaviour. 4532 4533 When a parenthesized subpattern is quantified with a minimum repeat 4534 count that is greater than 1 or with a limited maximum, more memory is 4535 required for the compiled pattern, in proportion to the size of the 4536 minimum or maximum. 4537 4538 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv- 4539 alent to Perl's /s) is set, thus allowing the dot to match newlines, 4540 the pattern is implicitly anchored, because whatever follows will be 4541 tried against every character position in the subject string, so there 4542 is no point in retrying the overall match at any position after the 4543 first. PCRE normally treats such a pattern as though it were preceded 4544 by \A. 4545 4546 In cases where it is known that the subject string contains no new- 4547 lines, it is worth setting PCRE_DOTALL in order to obtain this opti- 4548 mization, or alternatively using ^ to indicate anchoring explicitly. 4549 4550 However, there is one situation where the optimization cannot be used. 4551 When .* is inside capturing parentheses that are the subject of a back 4552 reference elsewhere in the pattern, a match at the start may fail where 4553 a later one succeeds. Consider, for example: 4554 4555 (.*)abc\1 4556 4557 If the subject is "xyz123abc123" the match point is the fourth charac- 4558 ter. For this reason, such a pattern is not implicitly anchored. 4559 4560 When a capturing subpattern is repeated, the value captured is the sub- 4561 string that matched the final iteration. For example, after 4562 4563 (tweedle[dume]{3}\s*)+ 4564 4565 has matched "tweedledum tweedledee" the value of the captured substring 4566 is "tweedledee". However, if there are nested capturing subpatterns, 4567 the corresponding captured values may have been set in previous itera- 4568 tions. For example, after 4569 4570 /(a|(b))+/ 4571 4572 matches "aba" the value of the second captured substring is "b". 4573 4574 4575 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS 4576 4577 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy") 4578 repetition, failure of what follows normally causes the repeated item 4579 to be re-evaluated to see if a different number of repeats allows the 4580 rest of the pattern to match. Sometimes it is useful to prevent this, 4581 either to change the nature of the match, or to cause it fail earlier 4582 than it otherwise might, when the author of the pattern knows there is 4583 no point in carrying on. 4584 4585 Consider, for example, the pattern \d+foo when applied to the subject 4586 line 4587 4588 123456bar 4589 4590 After matching all 6 digits and then failing to match "foo", the normal 4591 action of the matcher is to try again with only 5 digits matching the 4592 \d+ item, and then with 4, and so on, before ultimately failing. 4593 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides 4594 the means for specifying that once a subpattern has matched, it is not 4595 to be re-evaluated in this way. 4596 4597 If we use atomic grouping for the previous example, the matcher gives 4598 up immediately on failing to match "foo" the first time. The notation 4599 is a kind of special parenthesis, starting with (?> as in this example: 4600 4601 (?>\d+)foo 4602 4603 This kind of parenthesis "locks up" the part of the pattern it con- 4604 tains once it has matched, and a failure further into the pattern is 4605 prevented from backtracking into it. Backtracking past it to previous 4606 items, however, works as normal. 4607 4608 An alternative description is that a subpattern of this type matches 4609 the string of characters that an identical standalone pattern would 4610 match, if anchored at the current point in the subject string. 4611 4612 Atomic grouping subpatterns are not capturing subpatterns. Simple cases 4613 such as the above example can be thought of as a maximizing repeat that 4614 must swallow everything it can. So, while both \d+ and \d+? are pre- 4615 pared to adjust the number of digits they match in order to make the 4616 rest of the pattern match, (?>\d+) can only match an entire sequence of 4617 digits. 4618 4619 Atomic groups in general can of course contain arbitrarily complicated 4620 subpatterns, and can be nested. However, when the subpattern for an 4621 atomic group is just a single repeated item, as in the example above, a 4622 simpler notation, called a "possessive quantifier" can be used. This 4623 consists of an additional + character following a quantifier. Using 4624 this notation, the previous example can be rewritten as 4625 4626 \d++foo 4627 4628 Note that a possessive quantifier can be used with an entire group, for 4629 example: 4630 4631 (abc|xyz){2,3}+ 4632 4633 Possessive quantifiers are always greedy; the setting of the 4634 PCRE_UNGREEDY option is ignored. They are a convenient notation for the 4635 simpler forms of atomic group. However, there is no difference in the 4636 meaning of a possessive quantifier and the equivalent atomic group, 4637 though there may be a performance difference; possessive quantifiers 4638 should be slightly faster. 4639 4640 The possessive quantifier syntax is an extension to the Perl 5.8 syn- 4641 tax. Jeffrey Friedl originated the idea (and the name) in the first 4642 edition of his book. Mike McCloskey liked it, so implemented it when he 4643 built Sun's Java package, and PCRE copied it from there. It ultimately 4644 found its way into Perl at release 5.10. 4645 4646 PCRE has an optimization that automatically "possessifies" certain sim- 4647 ple pattern constructs. For example, the sequence A+B is treated as 4648 A++B because there is no point in backtracking into a sequence of A's 4649 when B must follow. 4650 4651 When a pattern contains an unlimited repeat inside a subpattern that 4652 can itself be repeated an unlimited number of times, the use of an 4653 atomic group is the only way to avoid some failing matches taking a 4654 very long time indeed. The pattern 4655 4656 (\D+|<\d+>)*[!?] 4657 4658 matches an unlimited number of substrings that either consist of non- 4659 digits, or digits enclosed in <>, followed by either ! or ?. When it 4660 matches, it runs quickly. However, if it is applied to 4661 4662 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa 4663 4664 it takes a long time before reporting failure. This is because the 4665 string can be divided between the internal \D+ repeat and the external 4666 * repeat in a large number of ways, and all have to be tried. (The 4667 example uses [!?] rather than a single character at the end, because 4668 both PCRE and Perl have an optimization that allows for fast failure 4669 when a single character is used. They remember the last single charac- 4670 ter that is required for a match, and fail early if it is not present 4671 in the string.) If the pattern is changed so that it uses an atomic 4672 group, like this: 4673 4674 ((?>\D+)|<\d+>)*[!?] 4675 4676 sequences of non-digits cannot be broken, and failure happens quickly. 4677 4678 4679 BACK REFERENCES 4680 4681 Outside a character class, a backslash followed by a digit greater than 4682 0 (and possibly further digits) is a back reference to a capturing sub- 4683 pattern earlier (that is, to its left) in the pattern, provided there 4684 have been that many previous capturing left parentheses. 4685 4686 However, if the decimal number following the backslash is less than 10, 4687 it is always taken as a back reference, and causes an error only if 4688 there are not that many capturing left parentheses in the entire pat- 4689 tern. In other words, the parentheses that are referenced need not be 4690 to the left of the reference for numbers less than 10. A "forward back 4691 reference" of this type can make sense when a repetition is involved 4692 and the subpattern to the right has participated in an earlier itera- 4693 tion. 4694 4695 It is not possible to have a numerical "forward back reference" to a 4696 subpattern whose number is 10 or more using this syntax because a 4697 sequence such as \50 is interpreted as a character defined in octal. 4698 See the subsection entitled "Non-printing characters" above for further 4699 details of the handling of digits following a backslash. There is no 4700 such problem when named parentheses are used. A back reference to any 4701 subpattern is possible using named parentheses (see below). 4702 4703 Another way of avoiding the ambiguity inherent in the use of digits 4704 following a backslash is to use the \g escape sequence. This escape 4705 must be followed by an unsigned number or a negative number, optionally 4706 enclosed in braces. These examples are all identical: 4707 4708 (ring), \1 4709 (ring), \g1 4710 (ring), \g{1} 4711 4712 An unsigned number specifies an absolute reference without the ambigu- 4713 ity that is present in the older syntax. It is also useful when literal 4714 digits follow the reference. A negative number is a relative reference. 4715 Consider this example: 4716 4717 (abc(def)ghi)\g{-1} 4718 4719 The sequence \g{-1} is a reference to the most recently started captur- 4720 ing subpattern before \g, that is, is it equivalent to \2 in this exam- 4721 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative 4722 references can be helpful in long patterns, and also in patterns that 4723 are created by joining together fragments that contain references 4724 within themselves. 4725 4726 A back reference matches whatever actually matched the capturing sub- 4727 pattern in the current subject string, rather than anything matching 4728 the subpattern itself (see "Subpatterns as subroutines" below for a way 4729 of doing that). So the pattern 4730 4731 (sens|respons)e and \1ibility 4732 4733 matches "sense and sensibility" and "response and responsibility", but 4734 not "sense and responsibility". If caseful matching is in force at the 4735 time of the back reference, the case of letters is relevant. For exam- 4736 ple, 4737 4738 ((?i)rah)\s+\1 4739 4740 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the 4741 original capturing subpattern is matched caselessly. 4742 4743 There are several different ways of writing back references to named 4744 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or 4745 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's 4746 unified back reference syntax, in which \g can be used for both numeric 4747 and named references, is also supported. We could rewrite the above 4748 example in any of the following ways: 4749 4750 (?<p1>(?i)rah)\s+\k<p1> 4751 (?'p1'(?i)rah)\s+\k{p1} 4752 (?P<p1>(?i)rah)\s+(?P=p1) 4753 (?<p1>(?i)rah)\s+\g{p1} 4754 4755 A subpattern that is referenced by name may appear in the pattern 4756 before or after the reference. 4757 4758 There may be more than one back reference to the same subpattern. If a 4759 subpattern has not actually been used in a particular match, any back 4760 references to it always fail by default. For example, the pattern 4761 4762 (a|(bc))\2 4763 4764 always fails if it starts to match "a" rather than "bc". However, if 4765 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer- 4766 ence to an unset value matches an empty string. 4767 4768 Because there may be many capturing parentheses in a pattern, all dig- 4769 its following a backslash are taken as part of a potential back refer- 4770 ence number. If the pattern continues with a digit character, some 4771 delimiter must be used to terminate the back reference. If the 4772 PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{ 4773 syntax or an empty comment (see "Comments" below) can be used. 4774 4775 Recursive back references 4776 4777 A back reference that occurs inside the parentheses to which it refers 4778 fails when the subpattern is first used, so, for example, (a\1) never 4779 matches. However, such references can be useful inside repeated sub- 4780 patterns. For example, the pattern 4781 4782 (a|b\1)+ 4783 4784 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter- 4785 ation of the subpattern, the back reference matches the character 4786 string corresponding to the previous iteration. In order for this to 4787 work, the pattern must be such that the first iteration does not need 4788 to match the back reference. This can be done using alternation, as in 4789 the example above, or by a quantifier with a minimum of zero. 4790 4791 Back references of this type cause the group that they reference to be 4792 treated as an atomic group. Once the whole group has been matched, a 4793 subsequent matching failure cannot cause backtracking into the middle 4794 of the group. 4795 4796 4797 ASSERTIONS 4798 4799 An assertion is a test on the characters following or preceding the 4800 current matching point that does not actually consume any characters. 4801 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are 4802 described above. 4803 4804 More complicated assertions are coded as subpatterns. There are two 4805 kinds: those that look ahead of the current position in the subject 4806 string, and those that look behind it. An assertion subpattern is 4807 matched in the normal way, except that it does not cause the current 4808 matching position to be changed. 4809 4810 Assertion subpatterns are not capturing subpatterns, and may not be 4811 repeated, because it makes no sense to assert the same thing several 4812 times. If any kind of assertion contains capturing subpatterns within 4813 it, these are counted for the purposes of numbering the capturing sub- 4814 patterns in the whole pattern. However, substring capturing is carried 4815 out only for positive assertions, because it does not make sense for 4816 negative assertions. 4817 4818 Lookahead assertions 4819 4820 Lookahead assertions start with (?= for positive assertions and (?! for 4821 negative assertions. For example, 4822 4823 \w+(?=;) 4824 4825 matches a word followed by a semicolon, but does not include the semi- 4826 colon in the match, and 4827 4828 foo(?!bar) 4829 4830 matches any occurrence of "foo" that is not followed by "bar". Note 4831 that the apparently similar pattern 4832 4833 (?!foo)bar 4834 4835 does not find an occurrence of "bar" that is preceded by something 4836 other than "foo"; it finds any occurrence of "bar" whatsoever, because 4837 the assertion (?!foo) is always true when the next three characters are 4838 "bar". A lookbehind assertion is needed to achieve the other effect. 4839 4840 If you want to force a matching failure at some point in a pattern, the 4841 most convenient way to do it is with (?!) because an empty string 4842 always matches, so an assertion that requires there not to be an empty 4843 string must always fail. The backtracking control verb (*FAIL) or (*F) 4844 is a synonym for (?!). 4845 4846 Lookbehind assertions 4847 4848 Lookbehind assertions start with (?<= for positive assertions and (?<! 4849 for negative assertions. For example, 4850 4851 (?<!foo)bar 4852 4853 does find an occurrence of "bar" that is not preceded by "foo". The 4854 contents of a lookbehind assertion are restricted such that all the 4855 strings it matches must have a fixed length. However, if there are sev- 4856 eral top-level alternatives, they do not all have to have the same 4857 fixed length. Thus 4858 4859 (?<=bullock|donkey) 4860 4861 is permitted, but 4862 4863 (?<!dogs?|cats?) 4864 4865 causes an error at compile time. Branches that match different length 4866 strings are permitted only at the top level of a lookbehind assertion. 4867 This is an extension compared with Perl, which requires all branches to 4868 match the same length of string. An assertion such as 4869 4870 (?<=ab(c|de)) 4871 4872 is not permitted, because its single top-level branch can match two 4873 different lengths, but it is acceptable to PCRE if rewritten to use two 4874 top-level branches: 4875 4876 (?<=abc|abde) 4877 4878 In some cases, the escape sequence \K (see above) can be used instead 4879 of a lookbehind assertion to get round the fixed-length restriction. 4880 4881 The implementation of lookbehind assertions is, for each alternative, 4882 to temporarily move the current position back by the fixed length and 4883 then try to match. If there are insufficient characters before the cur- 4884 rent position, the assertion fails. 4885 4886 PCRE does not allow the \C escape (which matches a single byte in UTF-8 4887 mode) to appear in lookbehind assertions, because it makes it impossi- 4888 ble to calculate the length of the lookbehind. The \X and \R escapes, 4889 which can match different numbers of bytes, are also not permitted. 4890 4891 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in 4892 lookbehinds, as long as the subpattern matches a fixed-length string. 4893 Recursion, however, is not supported. 4894 4895 Possessive quantifiers can be used in conjunction with lookbehind 4896 assertions to specify efficient matching of fixed-length strings at the 4897 end of subject strings. Consider a simple pattern such as 4898 4899 abcd$ 4900 4901 when applied to a long string that does not match. Because matching 4902 proceeds from left to right, PCRE will look for each "a" in the subject 4903 and then see if what follows matches the rest of the pattern. If the 4904 pattern is specified as 4905 4906 ^.*abcd$ 4907 4908 the initial .* matches the entire string at first, but when this fails 4909 (because there is no following "a"), it backtracks to match all but the 4910 last character, then all but the last two characters, and so on. Once 4911 again the search for "a" covers the entire string, from right to left, 4912 so we are no better off. However, if the pattern is written as 4913 4914 ^.*+(?<=abcd) 4915 4916 there can be no backtracking for the .*+ item; it can match only the 4917 entire string. The subsequent lookbehind assertion does a single test 4918 on the last four characters. If it fails, the match fails immediately. 4919 For long strings, this approach makes a significant difference to the 4920 processing time. 4921 4922 Using multiple assertions 4923 4924 Several assertions (of any sort) may occur in succession. For example, 4925 4926 (?<=\d{3})(?<!999)foo 4927 4928 matches "foo" preceded by three digits that are not "999". Notice that 4929 each of the assertions is applied independently at the same point in 4930 the subject string. First there is a check that the previous three 4931 characters are all digits, and then there is a check that the same 4932 three characters are not "999". This pattern does not match "foo" pre- 4933 ceded by six characters, the first of which are digits and the last 4934 three of which are not "999". For example, it doesn't match "123abc- 4935 foo". A pattern to do that is 4936 4937 (?<=\d{3}...)(?<!999)foo 4938 4939 This time the first assertion looks at the preceding six characters, 4940 checking that the first three are digits, and then the second assertion 4941 checks that the preceding three characters are not "999". 4942 4943 Assertions can be nested in any combination. For example, 4944 4945 (?<=(?<!foo)bar)baz 4946 4947 matches an occurrence of "baz" that is preceded by "bar" which in turn 4948 is not preceded by "foo", while 4949 4950 (?<=\d{3}(?!999)...)foo 4951 4952 is another pattern that matches "foo" preceded by three digits and any 4953 three characters that are not "999". 4954 4955 4956 CONDITIONAL SUBPATTERNS 4957 4958 It is possible to cause the matching process to obey a subpattern con- 4959 ditionally or to choose between two alternative subpatterns, depending 4960 on the result of an assertion, or whether a specific capturing subpat- 4961 tern has already been matched. The two possible forms of conditional 4962 subpattern are: 4963 4964 (?(condition)yes-pattern) 4965 (?(condition)yes-pattern|no-pattern) 4966 4967 If the condition is satisfied, the yes-pattern is used; otherwise the 4968 no-pattern (if present) is used. If there are more than two alterna- 4969 tives in the subpattern, a compile-time error occurs. Each of the two 4970 alternatives may itself contain nested subpatterns of any form, includ- 4971 ing conditional subpatterns; the restriction to two alternatives 4972 applies only at the level of the condition. This pattern fragment is an 4973 example where the alternatives are complex: 4974 4975 (?(1) (A|B|C) | (D | (?(2)E|F) | E) ) 4976 4977 4978 There are four kinds of condition: references to subpatterns, refer- 4979 ences to recursion, a pseudo-condition called DEFINE, and assertions. 4980 4981 Checking for a used subpattern by number 4982 4983 If the text between the parentheses consists of a sequence of digits, 4984 the condition is true if a capturing subpattern of that number has pre- 4985 viously matched. If there is more than one capturing subpattern with 4986 the same number (see the earlier section about duplicate subpattern 4987 numbers), the condition is true if any of them have matched. An alter- 4988 native notation is to precede the digits with a plus or minus sign. In 4989 this case, the subpattern number is relative rather than absolute. The 4990 most recently opened parentheses can be referenced by (?(-1), the next 4991 most recent by (?(-2), and so on. Inside loops it can also make sense 4992 to refer to subsequent groups. The next parentheses to be opened can be 4993 referenced as (?(+1), and so on. (The value zero in any of these forms 4994 is not used; it provokes a compile-time error.) 4995 4996 Consider the following pattern, which contains non-significant white 4997 space to make it more readable (assume the PCRE_EXTENDED option) and to 4998 divide it into three parts for ease of discussion: 4999 5000 ( \( )? [^()]+ (?(1) \) ) 5001 5002 The first part matches an optional opening parenthesis, and if that 5003 character is present, sets it as the first captured substring. The sec- 5004 ond part matches one or more characters that are not parentheses. The 5005 third part is a conditional subpattern that tests whether or not the 5006 first set of parentheses matched. If they did, that is, if subject 5007 started with an opening parenthesis, the condition is true, and so the 5008 yes-pattern is executed and a closing parenthesis is required. Other- 5009 wise, since no-pattern is not present, the subpattern matches nothing. 5010 In other words, this pattern matches a sequence of non-parentheses, 5011 optionally enclosed in parentheses. 5012 5013 If you were embedding this pattern in a larger one, you could use a 5014 relative reference: 5015 5016 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ... 5017 5018 This makes the fragment independent of the parentheses in the larger 5019 pattern. 5020 5021 Checking for a used subpattern by name 5022 5023 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a 5024 used subpattern by name. For compatibility with earlier versions of 5025 PCRE, which had this facility before Perl, the syntax (?(name)...) is 5026 also recognized. However, there is a possible ambiguity with this syn- 5027 tax, because subpattern names may consist entirely of digits. PCRE 5028 looks first for a named subpattern; if it cannot find one and the name 5029 consists entirely of digits, PCRE looks for a subpattern of that num- 5030 ber, which must be greater than zero. Using subpattern names that con- 5031 sist entirely of digits is not recommended. 5032 5033 Rewriting the above example to use a named subpattern gives this: 5034 5035 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) ) 5036 5037 If the name used in a condition of this kind is a duplicate, the test 5038 is applied to all subpatterns of the same name, and is true if any one 5039 of them has matched. 5040 5041 Checking for pattern recursion 5042 5043 If the condition is the string (R), and there is no subpattern with the 5044 name R, the condition is true if a recursive call to the whole pattern 5045 or any subpattern has been made. If digits or a name preceded by amper- 5046 sand follow the letter R, for example: 5047 5048 (?(R3)...) or (?(R&name)...) 5049 5050 the condition is true if the most recent recursion is into a subpattern 5051 whose number or name is given. This condition does not check the entire 5052 recursion stack. If the name used in a condition of this kind is a 5053 duplicate, the test is applied to all subpatterns of the same name, and 5054 is true if any one of them is the most recent recursion. 5055 5056 At "top level", all these recursion test conditions are false. The 5057 syntax for recursive patterns is described below. 5058 5059 Defining subpatterns for use by reference only 5060 5061 If the condition is the string (DEFINE), and there is no subpattern 5062 with the name DEFINE, the condition is always false. In this case, 5063 there may be only one alternative in the subpattern. It is always 5064 skipped if control reaches this point in the pattern; the idea of 5065 DEFINE is that it can be used to define "subroutines" that can be ref- 5066 erenced from elsewhere. (The use of "subroutines" is described below.) 5067 For example, a pattern to match an IPv4 address such as 5068 "192.168.23.245" could be written like this (ignore whitespace and line 5069 breaks): 5070 5071 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) ) 5072 \b (?&byte) (\.(?&byte)){3} \b 5073 5074 The first part of the pattern is a DEFINE group inside which a another 5075 group named "byte" is defined. This matches an individual component of 5076 an IPv4 address (a number less than 256). When matching takes place, 5077 this part of the pattern is skipped because DEFINE acts like a false 5078 condition. The rest of the pattern uses references to the named group 5079 to match the four dot-separated components of an IPv4 address, insist- 5080 ing on a word boundary at each end. 5081 5082 Assertion conditions 5083 5084 If the condition is not in any of the above formats, it must be an 5085 assertion. This may be a positive or negative lookahead or lookbehind 5086 assertion. Consider this pattern, again containing non-significant 5087 white space, and with the two alternatives on the second line: 5088 5089 (?(?=[^a-z]*[a-z]) 5090 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} ) 5091 5092 The condition is a positive lookahead assertion that matches an 5093 optional sequence of non-letters followed by a letter. In other words, 5094 it tests for the presence of at least one letter in the subject. If a 5095 letter is found, the subject is matched against the first alternative; 5096 otherwise it is matched against the second. This pattern matches 5097 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are 5098 letters and dd are digits. 5099 5100 5101 COMMENTS 5102 5103 There are two ways of including comments in patterns that are processed 5104 by PCRE. In both cases, the start of the comment must not be in a char- 5105 acter class, nor in the middle of any other sequence of related charac- 5106 ters such as (?: or a subpattern name or number. The characters that 5107 make up a comment play no part in the pattern matching. 5108 5109 The sequence (?# marks the start of a comment that continues up to the 5110 next closing parenthesis. Nested parentheses are not permitted. If the 5111 PCRE_EXTENDED option is set, an unescaped # character also introduces a 5112 comment, which in this case continues to immediately after the next 5113 newline character or character sequence in the pattern. Which charac- 5114 ters are interpreted as newlines is controlled by the options passed to 5115 pcre_compile() or by a special sequence at the start of the pattern, as 5116 described in the section entitled "Newline conventions" above. Note 5117 that the end of this type of comment is a literal newline sequence in 5118 the pattern; escape sequences that happen to represent a newline do not 5119 count. For example, consider this pattern when PCRE_EXTENDED is set, 5120 and the default newline convention is in force: 5121 5122 abc #comment \n still comment 5123 5124 On encountering the # character, pcre_compile() skips along, looking 5125 for a newline in the pattern. The sequence \n is still literal at this 5126 stage, so it does not terminate the comment. Only an actual character 5127 with the code value 0x0a (the default newline) does so. 5128 5129 5130 RECURSIVE PATTERNS 5131 5132 Consider the problem of matching a string in parentheses, allowing for 5133 unlimited nested parentheses. Without the use of recursion, the best 5134 that can be done is to use a pattern that matches up to some fixed 5135 depth of nesting. It is not possible to handle an arbitrary nesting 5136 depth. 5137 5138 For some time, Perl has provided a facility that allows regular expres- 5139 sions to recurse (amongst other things). It does this by interpolating 5140 Perl code in the expression at run time, and the code can refer to the 5141 expression itself. A Perl pattern using code interpolation to solve the 5142 parentheses problem can be created like this: 5143 5144 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x; 5145 5146 The (?p{...}) item interpolates Perl code at run time, and in this case 5147 refers recursively to the pattern in which it appears. 5148 5149 Obviously, PCRE cannot support the interpolation of Perl code. Instead, 5150 it supports special syntax for recursion of the entire pattern, and 5151 also for individual subpattern recursion. After its introduction in 5152 PCRE and Python, this kind of recursion was subsequently introduced 5153 into Perl at release 5.10. 5154 5155 A special item that consists of (? followed by a number greater than 5156 zero and a closing parenthesis is a recursive call of the subpattern of 5157 the given number, provided that it occurs inside that subpattern. (If 5158 not, it is a "subroutine" call, which is described in the next sec- 5159 tion.) The special item (?R) or (?0) is a recursive call of the entire 5160 regular expression. 5161 5162 This PCRE pattern solves the nested parentheses problem (assume the 5163 PCRE_EXTENDED option is set so that white space is ignored): 5164 5165 \( ( [^()]++ | (?R) )* \) 5166 5167 First it matches an opening parenthesis. Then it matches any number of 5168 substrings which can either be a sequence of non-parentheses, or a 5169 recursive match of the pattern itself (that is, a correctly parenthe- 5170 sized substring). Finally there is a closing parenthesis. Note the use 5171 of a possessive quantifier to avoid backtracking into sequences of non- 5172 parentheses. 5173 5174 If this were part of a larger pattern, you would not want to recurse 5175 the entire pattern, so instead you could use this: 5176 5177 ( \( ( [^()]++ | (?1) )* \) ) 5178 5179 We have put the pattern into parentheses, and caused the recursion to 5180 refer to them instead of the whole pattern. 5181 5182 In a larger pattern, keeping track of parenthesis numbers can be 5183 tricky. This is made easier by the use of relative references. Instead 5184 of (?1) in the pattern above you can write (?-2) to refer to the second 5185 most recently opened parentheses preceding the recursion. In other 5186 words, a negative number counts capturing parentheses leftwards from 5187 the point at which it is encountered. 5188 5189 It is also possible to refer to subsequently opened parentheses, by 5190 writing references such as (?+2). However, these cannot be recursive 5191 because the reference is not inside the parentheses that are refer- 5192 enced. They are always "subroutine" calls, as described in the next 5193 section. 5194 5195 An alternative approach is to use named parentheses instead. The Perl 5196 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also 5197 supported. We could rewrite the above example as follows: 5198 5199 (?<pn> \( ( [^()]++ | (?&pn) )* \) ) 5200 5201 If there is more than one subpattern with the same name, the earliest 5202 one is used. 5203 5204 This particular example pattern that we have been looking at contains 5205 nested unlimited repeats, and so the use of a possessive quantifier for 5206 matching strings of non-parentheses is important when applying the pat- 5207 tern to strings that do not match. For example, when this pattern is 5208 applied to 5209 5210 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa() 5211 5212 it yields "no match" quickly. However, if a possessive quantifier is 5213 not used, the match runs for a very long time indeed because there are 5214 so many different ways the + and * repeats can carve up the subject, 5215 and all have to be tested before failure can be reported. 5216 5217 At the end of a match, the values of capturing parentheses are those 5218 from the outermost level. If you want to obtain intermediate values, a 5219 callout function can be used (see below and the pcrecallout documenta- 5220 tion). If the pattern above is matched against 5221 5222 (ab(cd)ef) 5223 5224 the value for the inner capturing parentheses (numbered 2) is "ef", 5225 which is the last value taken on at the top level. If a capturing sub- 5226 pattern is not matched at the top level, its final value is unset, even 5227 if it is (temporarily) set at a deeper level. 5228 5229 If there are more than 15 capturing parentheses in a pattern, PCRE has 5230 to obtain extra memory to store data during a recursion, which it does 5231 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory 5232 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error. 5233 5234 Do not confuse the (?R) item with the condition (R), which tests for 5235 recursion. Consider this pattern, which matches text in angle brack- 5236 ets, allowing for arbitrary nesting. Only digits are allowed in nested 5237 brackets (that is, when recursing), whereas any characters are permit- 5238 ted at the outer level. 5239 5240 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * > 5241 5242 In this pattern, (?(R) is the start of a conditional subpattern, with 5243 two different alternatives for the recursive and non-recursive cases. 5244 The (?R) item is the actual recursive call. 5245 5246 Recursion difference from Perl 5247 5248 In PCRE (like Python, but unlike Perl), a recursive subpattern call is 5249 always treated as an atomic group. That is, once it has matched some of 5250 the subject string, it is never re-entered, even if it contains untried 5251 alternatives and there is a subsequent matching failure. This can be 5252 illustrated by the following pattern, which purports to match a palin- 5253 dromic string that contains an odd number of characters (for example, 5254 "a", "aba", "abcba", "abcdcba"): 5255 5256 ^(.|(.)(?1)\2)$ 5257 5258 The idea is that it either matches a single character, or two identical 5259 characters surrounding a sub-palindrome. In Perl, this pattern works; 5260 in PCRE it does not if the pattern is longer than three characters. 5261 Consider the subject string "abcba": 5262 5263 At the top level, the first character is matched, but as it is not at 5264 the end of the string, the first alternative fails; the second alterna- 5265 tive is taken and the recursion kicks in. The recursive call to subpat- 5266 tern 1 successfully matches the next character ("b"). (Note that the 5267 beginning and end of line tests are not part of the recursion). 5268 5269 Back at the top level, the next character ("c") is compared with what 5270 subpattern 2 matched, which was "a". This fails. Because the recursion 5271 is treated as an atomic group, there are now no backtracking points, 5272 and so the entire match fails. (Perl is able, at this point, to re- 5273 enter the recursion and try the second alternative.) However, if the 5274 pattern is written with the alternatives in the other order, things are 5275 different: 5276 5277 ^((.)(?1)\2|.)$ 5278 5279 This time, the recursing alternative is tried first, and continues to 5280 recurse until it runs out of characters, at which point the recursion 5281 fails. But this time we do have another alternative to try at the 5282 higher level. That is the big difference: in the previous case the 5283 remaining alternative is at a deeper recursion level, which PCRE cannot 5284 use. 5285 5286 To change the pattern so that it matches all palindromic strings, not 5287 just those with an odd number of characters, it is tempting to change 5288 the pattern to this: 5289 5290 ^((.)(?1)\2|.?)$ 5291 5292 Again, this works in Perl, but not in PCRE, and for the same reason. 5293 When a deeper recursion has matched a single character, it cannot be 5294 entered again in order to match an empty string. The solution is to 5295 separate the two cases, and write out the odd and even cases as alter- 5296 natives at the higher level: 5297 5298 ^(?:((.)(?1)\2|)|((.)(?3)\4|.)) 5299 5300 If you want to match typical palindromic phrases, the pattern has to 5301 ignore all non-word characters, which can be done like this: 5302 5303 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$ 5304 5305 If run with the PCRE_CASELESS option, this pattern matches phrases such 5306 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and 5307 Perl. Note the use of the possessive quantifier *+ to avoid backtrack- 5308 ing into sequences of non-word characters. Without this, PCRE takes a 5309 great deal longer (ten times or more) to match typical phrases, and 5310 Perl takes so long that you think it has gone into a loop. 5311 5312 WARNING: The palindrome-matching patterns above work only if the sub- 5313 ject string does not start with a palindrome that is shorter than the 5314 entire string. For example, although "abcba" is correctly matched, if 5315 the subject is "ababa", PCRE finds the palindrome "aba" at the start, 5316 then fails at top level because the end of the string does not follow. 5317 Once again, it cannot jump back into the recursion to try other alter- 5318 natives, so the entire match fails. 5319 5320 5321 SUBPATTERNS AS SUBROUTINES 5322 5323 If the syntax for a recursive subpattern reference (either by number or 5324 by name) is used outside the parentheses to which it refers, it oper- 5325 ates like a subroutine in a programming language. The "called" subpat- 5326 tern may be defined before or after the reference. A numbered reference 5327 can be absolute or relative, as in these examples: 5328 5329 (...(absolute)...)...(?2)... 5330 (...(relative)...)...(?-1)... 5331 (...(?+1)...(relative)... 5332 5333 An earlier example pointed out that the pattern 5334 5335 (sens|respons)e and \1ibility 5336 5337 matches "sense and sensibility" and "response and responsibility", but 5338 not "sense and responsibility". If instead the pattern 5339 5340 (sens|respons)e and (?1)ibility 5341 5342 is used, it does match "sense and responsibility" as well as the other 5343 two strings. Another example is given in the discussion of DEFINE 5344 above. 5345 5346 Like recursive subpatterns, a subroutine call is always treated as an 5347 atomic group. That is, once it has matched some of the subject string, 5348 it is never re-entered, even if it contains untried alternatives and 5349 there is a subsequent matching failure. Any capturing parentheses that 5350 are set during the subroutine call revert to their previous values 5351 afterwards. 5352 5353 When a subpattern is used as a subroutine, processing options such as 5354 case-independence are fixed when the subpattern is defined. They cannot 5355 be changed for different calls. For example, consider this pattern: 5356 5357 (abc)(?i:(?-1)) 5358 5359 It matches "abcabc". It does not match "abcABC" because the change of 5360 processing option does not affect the called subpattern. 5361 5362 5363 ONIGURUMA SUBROUTINE SYNTAX 5364 5365 For compatibility with Oniguruma, the non-Perl syntax \g followed by a 5366 name or a number enclosed either in angle brackets or single quotes, is 5367 an alternative syntax for referencing a subpattern as a subroutine, 5368 possibly recursively. Here are two of the examples used above, rewrit- 5369 ten using this syntax: 5370 5371 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) ) 5372 (sens|respons)e and \g'1'ibility 5373 5374 PCRE supports an extension to Oniguruma: if a number is preceded by a 5375 plus or a minus sign it is taken as a relative reference. For example: 5376 5377 (abc)(?i:\g<-1>) 5378 5379 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not 5380 synonymous. The former is a back reference; the latter is a subroutine 5381 call. 5382 5383 5384 CALLOUTS 5385 5386 Perl has a feature whereby using the sequence (?{...}) causes arbitrary 5387 Perl code to be obeyed in the middle of matching a regular expression. 5388 This makes it possible, amongst other things, to extract different sub- 5389 strings that match the same pair of parentheses when there is a repeti- 5390 tion. 5391 5392 PCRE provides a similar feature, but of course it cannot obey arbitrary 5393 Perl code. The feature is called "callout". The caller of PCRE provides 5394 an external function by putting its entry point in the global variable 5395 pcre_callout. By default, this variable contains NULL, which disables 5396 all calling out. 5397 5398 Within a regular expression, (?C) indicates the points at which the 5399 external function is to be called. If you want to identify different 5400 callout points, you can put a number less than 256 after the letter C. 5401 The default value is zero. For example, this pattern has two callout 5402 points: 5403 5404 (?C1)abc(?C2)def 5405 5406 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are 5407 automatically installed before each item in the pattern. They are all 5408 numbered 255. 5409 5410 During matching, when PCRE reaches a callout point (and pcre_callout is 5411 set), the external function is called. It is provided with the number 5412 of the callout, the position in the pattern, and, optionally, one item 5413 of data originally supplied by the caller of pcre_exec(). The callout 5414 function may cause matching to proceed, to backtrack, or to fail alto- 5415 gether. A complete description of the interface to the callout function 5416 is given in the pcrecallout documentation. 5417 5418 5419 BACKTRACKING CONTROL 5420 5421 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", 5422 which are described in the Perl documentation as "experimental and sub- 5423 ject to change or removal in a future version of Perl". It goes on to 5424 say: "Their usage in production code should be noted to avoid problems 5425 during upgrades." The same remarks apply to the PCRE features described 5426 in this section. 5427 5428 Since these verbs are specifically related to backtracking, most of 5429 them can be used only when the pattern is to be matched using 5430 pcre_exec(), which uses a backtracking algorithm. With the exception of 5431 (*FAIL), which behaves like a failing negative assertion, they cause an 5432 error if encountered by pcre_dfa_exec(). 5433 5434 If any of these verbs are used in an assertion or subroutine subpattern 5435 (including recursive subpatterns), their effect is confined to that 5436 subpattern; it does not extend to the surrounding pattern. Note that 5437 such subpatterns are processed as anchored at the point where they are 5438 tested. 5439 5440 The new verbs make use of what was previously invalid syntax: an open- 5441 ing parenthesis followed by an asterisk. They are generally of the form 5442 (*VERB) or (*VERB:NAME). Some may take either form, with differing be- 5443 haviour, depending on whether or not an argument is present. An name is 5444 a sequence of letters, digits, and underscores. If the name is empty, 5445 that is, if the closing parenthesis immediately follows the colon, the 5446 effect is as if the colon were not there. Any number of these verbs may 5447 occur in a pattern. 5448 5449 PCRE contains some optimizations that are used to speed up matching by 5450 running some checks at the start of each match attempt. For example, it 5451 may know the minimum length of matching subject, or that a particular 5452 character must be present. When one of these optimizations suppresses 5453 the running of a match, any included backtracking verbs will not, of 5454 course, be processed. You can suppress the start-of-match optimizations 5455 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com- 5456 pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT). 5457 5458 Verbs that act immediately 5459 5460 The following verbs act as soon as they are encountered. They may not 5461 be followed by a name. 5462 5463 (*ACCEPT) 5464 5465 This verb causes the match to end successfully, skipping the remainder 5466 of the pattern. When inside a recursion, only the innermost pattern is 5467 ended immediately. If (*ACCEPT) is inside capturing parentheses, the 5468 data so far is captured. (This feature was added to PCRE at release 5469 8.00.) For example: 5470 5471 A((?:A|B(*ACCEPT)|C)D) 5472 5473 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap- 5474 tured by the outer parentheses. 5475 5476 (*FAIL) or (*F) 5477 5478 This verb causes the match to fail, forcing backtracking to occur. It 5479 is equivalent to (?!) but easier to read. The Perl documentation notes 5480 that it is probably useful only when combined with (?{}) or (??{}). 5481 Those are, of course, Perl features that are not present in PCRE. The 5482 nearest equivalent is the callout feature, as for example in this pat- 5483 tern: 5484 5485 a+(?C)(*FAIL) 5486 5487 A match with the string "aaaa" always fails, but the callout is taken 5488 before each backtrack happens (in this example, 10 times). 5489 5490 Recording which path was taken 5491 5492 There is one verb whose main purpose is to track how a match was 5493 arrived at, though it also has a secondary use in conjunction with 5494 advancing the match starting point (see (*SKIP) below). 5495 5496 (*MARK:NAME) or (*:NAME) 5497 5498 A name is always required with this verb. There may be as many 5499 instances of (*MARK) as you like in a pattern, and their names do not 5500 have to be unique. 5501 5502 When a match succeeds, the name of the last-encountered (*MARK) is 5503 passed back to the caller via the pcre_extra data structure, as 5504 described in the section on pcre_extra in the pcreapi documentation. No 5505 data is returned for a partial match. Here is an example of pcretest 5506 output, where the /K modifier requests the retrieval and outputting of 5507 (*MARK) data: 5508 5509 /X(*MARK:A)Y|X(*MARK:B)Z/K 5510 XY 5511 0: XY 5512 MK: A 5513 XZ 5514 0: XZ 5515 MK: B 5516 5517 The (*MARK) name is tagged with "MK:" in this output, and in this exam- 5518 ple it indicates which of the two alternatives matched. This is a more 5519 efficient way of obtaining this information than putting each alterna- 5520 tive in its own capturing parentheses. 5521 5522 A name may also be returned after a failed match if the final path 5523 through the pattern involves (*MARK). However, unless (*MARK) used in 5524 conjunction with (*COMMIT), this is unlikely to happen for an unan- 5525 chored pattern because, as the starting point for matching is advanced, 5526 the final check is often with an empty string, causing a failure before 5527 (*MARK) is reached. For example: 5528 5529 /X(*MARK:A)Y|X(*MARK:B)Z/K 5530 XP 5531 No match 5532 5533 There are three potential starting points for this match (starting with 5534 X, starting with P, and with an empty string). If the pattern is 5535 anchored, the result is different: 5536 5537 /^X(*MARK:A)Y|^X(*MARK:B)Z/K 5538 XP 5539 No match, mark = B 5540 5541 PCRE's start-of-match optimizations can also interfere with this. For 5542 example, if, as a result of a call to pcre_study(), it knows the mini- 5543 mum subject length for a match, a shorter subject will not be scanned 5544 at all. 5545 5546 Note that similar anomalies (though different in detail) exist in Perl, 5547 no doubt for the same reasons. The use of (*MARK) data after a failed 5548 match of an unanchored pattern is not recommended, unless (*COMMIT) is 5549 involved. 5550 5551 Verbs that act after backtracking 5552 5553 The following verbs do nothing when they are encountered. Matching con- 5554 tinues with what follows, but if there is no subsequent match, causing 5555 a backtrack to the verb, a failure is forced. That is, backtracking 5556 cannot pass to the left of the verb. However, when one of these verbs 5557 appears inside an atomic group, its effect is confined to that group, 5558 because once the group has been matched, there is never any backtrack- 5559 ing into it. In this situation, backtracking can "jump back" to the 5560 left of the entire atomic group. (Remember also, as stated above, that 5561 this localization also applies in subroutine calls and assertions.) 5562 5563 These verbs differ in exactly what kind of failure occurs when back- 5564 tracking reaches them. 5565 5566 (*COMMIT) 5567 5568 This verb, which may not be followed by a name, causes the whole match 5569 to fail outright if the rest of the pattern does not match. Even if the 5570 pattern is unanchored, no further attempts to find a match by advancing 5571 the starting point take place. Once (*COMMIT) has been passed, 5572 pcre_exec() is committed to finding a match at the current starting 5573 point, or not at all. For example: 5574 5575 a+(*COMMIT)b 5576 5577 This matches "xxaab" but not "aacaab". It can be thought of as a kind 5578 of dynamic anchor, or "I've started, so I must finish." The name of the 5579 most recently passed (*MARK) in the path is passed back when (*COMMIT) 5580 forces a match failure. 5581 5582 Note that (*COMMIT) at the start of a pattern is not the same as an 5583 anchor, unless PCRE's start-of-match optimizations are turned off, as 5584 shown in this pcretest example: 5585 5586 /(*COMMIT)abc/ 5587 xyzabc 5588 0: abc 5589 xyzabc\Y 5590 No match 5591 5592 PCRE knows that any match must start with "a", so the optimization 5593 skips along the subject to "a" before running the first match attempt, 5594 which succeeds. When the optimization is disabled by the \Y escape in 5595 the second subject, the match starts at "x" and so the (*COMMIT) causes 5596 it to fail without trying any other starting points. 5597 5598 (*PRUNE) or (*PRUNE:NAME) 5599 5600 This verb causes the match to fail at the current starting position in 5601 the subject if the rest of the pattern does not match. If the pattern 5602 is unanchored, the normal "bumpalong" advance to the next starting 5603 character then happens. Backtracking can occur as usual to the left of 5604 (*PRUNE), before it is reached, or when matching to the right of 5605 (*PRUNE), but if there is no match to the right, backtracking cannot 5606 cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter- 5607 native to an atomic group or possessive quantifier, but there are some 5608 uses of (*PRUNE) that cannot be expressed in any other way. The behav- 5609 iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the 5610 match fails completely; the name is passed back if this is the final 5611 attempt. (*PRUNE:NAME) does not pass back a name if the match suc- 5612 ceeds. In an anchored pattern (*PRUNE) has the same effect as (*COM- 5613 MIT). 5614 5615 (*SKIP) 5616 5617 This verb, when given without a name, is like (*PRUNE), except that if 5618 the pattern is unanchored, the "bumpalong" advance is not to the next 5619 character, but to the position in the subject where (*SKIP) was encoun- 5620 tered. (*SKIP) signifies that whatever text was matched leading up to 5621 it cannot be part of a successful match. Consider: 5622 5623 a+(*SKIP)b 5624 5625 If the subject is "aaaac...", after the first match attempt fails 5626 (starting at the first character in the string), the starting point 5627 skips on to start the next attempt at "c". Note that a possessive quan- 5628 tifer does not have the same effect as this example; although it would 5629 suppress backtracking during the first match attempt, the second 5630 attempt would start at the second character instead of skipping on to 5631 "c". 5632 5633 (*SKIP:NAME) 5634 5635 When (*SKIP) has an associated name, its behaviour is modified. If the 5636 following pattern fails to match, the previous path through the pattern 5637 is searched for the most recent (*MARK) that has the same name. If one 5638 is found, the "bumpalong" advance is to the subject position that cor- 5639 responds to that (*MARK) instead of to where (*SKIP) was encountered. 5640 If no (*MARK) with a matching name is found, normal "bumpalong" of one 5641 character happens (the (*SKIP) is ignored). 5642 5643 (*THEN) or (*THEN:NAME) 5644 5645 This verb causes a skip to the next alternation in the innermost 5646 enclosing group if the rest of the pattern does not match. That is, it 5647 cancels pending backtracking, but only within the current alternation. 5648 Its name comes from the observation that it can be used for a pattern- 5649 based if-then-else block: 5650 5651 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ... 5652 5653 If the COND1 pattern matches, FOO is tried (and possibly further items 5654 after the end of the group if FOO succeeds); on failure the matcher 5655 skips to the second alternative and tries COND2, without backtracking 5656 into COND1. The behaviour of (*THEN:NAME) is exactly the same as 5657 (*MARK:NAME)(*THEN) if the overall match fails. If (*THEN) is not 5658 directly inside an alternation, it acts like (*PRUNE). 5659 5660 The above verbs provide four different "strengths" of control when sub- 5661 sequent matching fails. (*THEN) is the weakest, carrying on the match 5662 at the next alternation. (*PRUNE) comes next, failing the match at the 5663 current starting position, but allowing an advance to the next charac- 5664 ter (for an unanchored pattern). (*SKIP) is similar, except that the 5665 advance may be more than one character. (*COMMIT) is the strongest, 5666 causing the entire match to fail. 5667 5668 If more than one is present in a pattern, the "stongest" one wins. For 5669 example, consider this pattern, where A, B, etc. are complex pattern 5670 fragments: 5671 5672 (A(*COMMIT)B(*THEN)C|D) 5673 5674 Once A has matched, PCRE is committed to this match, at the current 5675 starting position. If subsequently B matches, but C does not, the nor- 5676 mal (*THEN) action of trying the next alternation (that is, D) does not 5677 happen because (*COMMIT) overrides. 5678 5679 5680 SEE ALSO 5681 5682 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3). 5683 5684 5685 AUTHOR 5686 5687 Philip Hazel 5688 University Computing Service 5689 Cambridge CB2 3QH, England. 5690 5691 5692 REVISION 5693 5694 Last updated: 21 November 2010 5695 Copyright (c) 1997-2010 University of Cambridge. 5696 ------------------------------------------------------------------------------ 5697 5698 5699 PCRESYNTAX(3) PCRESYNTAX(3) 5700 5701 5702 NAME 5703 PCRE - Perl-compatible regular expressions 5704 5705 5706 PCRE REGULAR EXPRESSION SYNTAX SUMMARY 5707 5708 The full syntax and semantics of the regular expressions that are sup- 5709 ported by PCRE are described in the pcrepattern documentation. This 5710 document contains just a quick-reference summary of the syntax. 5711 5712 5713 QUOTING 5714 5715 \x where x is non-alphanumeric is a literal x 5716 \Q...\E treat enclosed characters as literal 5717 5718 5719 CHARACTERS 5720 5721 \a alarm, that is, the BEL character (hex 07) 5722 \cx "control-x", where x is any ASCII character 5723 \e escape (hex 1B) 5724 \f formfeed (hex 0C) 5725 \n newline (hex 0A) 5726 \r carriage return (hex 0D) 5727 \t tab (hex 09) 5728 \ddd character with octal code ddd, or backreference 5729 \xhh character with hex code hh 5730 \x{hhh..} character with hex code hhh.. 5731 5732 5733 CHARACTER TYPES 5734 5735 . any character except newline; 5736 in dotall mode, any character whatsoever 5737 \C one byte, even in UTF-8 mode (best avoided) 5738 \d a decimal digit 5739 \D a character that is not a decimal digit 5740 \h a horizontal whitespace character 5741 \H a character that is not a horizontal whitespace character 5742 \N a character that is not a newline 5743 \p{xx} a character with the xx property 5744 \P{xx} a character without the xx property 5745 \R a newline sequence 5746 \s a whitespace character 5747 \S a character that is not a whitespace character 5748 \v a vertical whitespace character 5749 \V a character that is not a vertical whitespace character 5750 \w a "word" character 5751 \W a "non-word" character 5752 \X an extended Unicode sequence 5753 5754 In PCRE, by default, \d, \D, \s, \S, \w, and \W recognize only ASCII 5755 characters, even in UTF-8 mode. However, this can be changed by setting 5756 the PCRE_UCP option. 5757 5758 5759 GENERAL CATEGORY PROPERTIES FOR \p and \P 5760 5761 C Other 5762 Cc Control 5763 Cf Format 5764 Cn Unassigned 5765 Co Private use 5766 Cs Surrogate 5767 5768 L Letter 5769 Ll Lower case letter 5770 Lm Modifier letter 5771 Lo Other letter 5772 Lt Title case letter 5773 Lu Upper case letter 5774 L& Ll, Lu, or Lt 5775 5776 M Mark 5777 Mc Spacing mark 5778 Me Enclosing mark 5779 Mn Non-spacing mark 5780 5781 N Number 5782 Nd Decimal number 5783 Nl Letter number 5784 No Other number 5785 5786 P Punctuation 5787 Pc Connector punctuation 5788 Pd Dash punctuation 5789 Pe Close punctuation 5790 Pf Final punctuation 5791 Pi Initial punctuation 5792 Po Other punctuation 5793 Ps Open punctuation 5794 5795 S Symbol 5796 Sc Currency symbol 5797 Sk Modifier symbol 5798 Sm Mathematical symbol 5799 So Other symbol 5800 5801 Z Separator 5802 Zl Line separator 5803 Zp Paragraph separator 5804 Zs Space separator 5805 5806 5807 PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P 5808 5809 Xan Alphanumeric: union of properties L and N 5810 Xps POSIX space: property Z or tab, NL, VT, FF, CR 5811 Xsp Perl space: property Z or tab, NL, FF, CR 5812 Xwd Perl word: property Xan or underscore 5813 5814 5815 SCRIPT NAMES FOR \p AND \P 5816 5817 Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille, 5818 Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common, 5819 Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp- 5820 tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek, 5821 Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe- 5822 rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, 5823 Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao, 5824 Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam, 5825 Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic, 5826 Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya, 5827 Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian, 5828 Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, 5829 Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, 5830 Ugaritic, Vai, Yi. 5831 5832 5833 CHARACTER CLASSES 5834 5835 [...] positive character class 5836 [^...] negative character class 5837 [x-y] range (can be used for hex characters) 5838 [[:xxx:]] positive POSIX named set 5839 [[:^xxx:]] negative POSIX named set 5840 5841 alnum alphanumeric 5842 alpha alphabetic 5843 ascii 0-127 5844 blank space or tab 5845 cntrl control character 5846 digit decimal digit 5847 graph printing, excluding space 5848 lower lower case letter 5849 print printing, including space 5850 punct printing, excluding alphanumeric 5851 space whitespace 5852 upper upper case letter 5853 word same as \w 5854 xdigit hexadecimal digit 5855 5856 In PCRE, POSIX character set names recognize only ASCII characters by 5857 default, but some of them use Unicode properties if PCRE_UCP is set. 5858 You can use \Q...\E inside a character class. 5859 5860 5861 QUANTIFIERS 5862 5863 ? 0 or 1, greedy 5864 ?+ 0 or 1, possessive 5865 ?? 0 or 1, lazy 5866 * 0 or more, greedy 5867 *+ 0 or more, possessive 5868 *? 0 or more, lazy 5869 + 1 or more, greedy 5870 ++ 1 or more, possessive 5871 +? 1 or more, lazy 5872 {n} exactly n 5873 {n,m} at least n, no more than m, greedy 5874 {n,m}+ at least n, no more than m, possessive 5875 {n,m}? at least n, no more than m, lazy 5876 {n,} n or more, greedy 5877 {n,}+ n or more, possessive 5878 {n,}? n or more, lazy 5879 5880 5881 ANCHORS AND SIMPLE ASSERTIONS 5882 5883 \b word boundary 5884 \B not a word boundary 5885 ^ start of subject 5886 also after internal newline in multiline mode 5887 \A start of subject 5888 $ end of subject 5889 also before newline at end of subject 5890 also before internal newline in multiline mode 5891 \Z end of subject 5892 also before newline at end of subject 5893 \z end of subject 5894 \G first matching position in subject 5895 5896 5897 MATCH POINT RESET 5898 5899 \K reset start of match 5900 5901 5902 ALTERNATION 5903 5904 expr|expr|expr... 5905 5906 5907 CAPTURING 5908 5909 (...) capturing group 5910 (?<name>...) named capturing group (Perl) 5911 (?'name'...) named capturing group (Perl) 5912 (?P<name>...) named capturing group (Python) 5913 (?:...) non-capturing group 5914 (?|...) non-capturing group; reset group numbers for 5915 capturing groups in each alternative 5916 5917 5918 ATOMIC GROUPS 5919 5920 (?>...) atomic, non-capturing group 5921 5922 5923 COMMENT 5924 5925 (?#....) comment (not nestable) 5926 5927 5928 OPTION SETTING 5929 5930 (?i) caseless 5931 (?J) allow duplicate names 5932 (?m) multiline 5933 (?s) single line (dotall) 5934 (?U) default ungreedy (lazy) 5935 (?x) extended (ignore white space) 5936 (?-...) unset option(s) 5937 5938 The following are recognized only at the start of a pattern or after 5939 one of the newline-setting options with similar syntax: 5940 5941 (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE) 5942 (*UTF8) set UTF-8 mode (PCRE_UTF8) 5943 (*UCP) set PCRE_UCP (use Unicode properties for \d etc) 5944 5945 5946 LOOKAHEAD AND LOOKBEHIND ASSERTIONS 5947 5948 (?=...) positive look ahead 5949 (?!...) negative look ahead 5950 (?<=...) positive look behind 5951 (?<!...) negative look behind 5952 5953 Each top-level branch of a look behind must be of a fixed length. 5954 5955 5956 BACKREFERENCES 5957 5958 \n reference by number (can be ambiguous) 5959 \gn reference by number 5960 \g{n} reference by number 5961 \g{-n} relative reference by number 5962 \k<name> reference by name (Perl) 5963 \k'name' reference by name (Perl) 5964 \g{name} reference by name (Perl) 5965 \k{name} reference by name (.NET) 5966 (?P=name) reference by name (Python) 5967 5968 5969 SUBROUTINE REFERENCES (POSSIBLY RECURSIVE) 5970 5971 (?R) recurse whole pattern 5972 (?n) call subpattern by absolute number 5973 (?+n) call subpattern by relative number 5974 (?-n) call subpattern by relative number 5975 (?&name) call subpattern by name (Perl) 5976 (?P>name) call subpattern by name (Python) 5977 \g<name> call subpattern by name (Oniguruma) 5978 \g'name' call subpattern by name (Oniguruma) 5979 \g<n> call subpattern by absolute number (Oniguruma) 5980 \g'n' call subpattern by absolute number (Oniguruma) 5981 \g<+n> call subpattern by relative number (PCRE extension) 5982 \g'+n' call subpattern by relative number (PCRE extension) 5983 \g<-n> call subpattern by relative number (PCRE extension) 5984 \g'-n' call subpattern by relative number (PCRE extension) 5985 5986 5987 CONDITIONAL PATTERNS 5988 5989 (?(condition)yes-pattern) 5990 (?(condition)yes-pattern|no-pattern) 5991 5992 (?(n)... absolute reference condition 5993 (?(+n)... relative reference condition 5994 (?(-n)... relative reference condition 5995 (?(<name>)... named reference condition (Perl) 5996 (?('name')... named reference condition (Perl) 5997 (?(name)... named reference condition (PCRE) 5998 (?(R)... overall recursion condition 5999 (?(Rn)... specific group recursion condition 6000 (?(R&name)... specific recursion condition 6001 (?(DEFINE)... define subpattern for reference 6002 (?(assert)... assertion condition 6003 6004 6005 BACKTRACKING CONTROL 6006 6007 The following act immediately they are reached: 6008 6009 (*ACCEPT) force successful match 6010 (*FAIL) force backtrack; synonym (*F) 6011 6012 The following act only when a subsequent match failure causes a back- 6013 track to reach them. They all force a match failure, but they differ in 6014 what happens afterwards. Those that advance the start-of-match point do 6015 so only if the pattern is not anchored. 6016 6017 (*COMMIT) overall failure, no advance of starting point 6018 (*PRUNE) advance to next starting character 6019 (*SKIP) advance start to current matching position 6020 (*THEN) local failure, backtrack to next alternation 6021 6022 6023 NEWLINE CONVENTIONS 6024 6025 These are recognized only at the very start of the pattern or after a 6026 (*BSR_...) or (*UTF8) or (*UCP) option. 6027 6028 (*CR) carriage return only 6029 (*LF) linefeed only 6030 (*CRLF) carriage return followed by linefeed 6031 (*ANYCRLF) all three of the above 6032 (*ANY) any Unicode newline sequence 6033 6034 6035 WHAT \R MATCHES 6036 6037 These are recognized only at the very start of the pattern or after a 6038 (*...) option that sets the newline convention or UTF-8 or UCP mode. 6039 6040 (*BSR_ANYCRLF) CR, LF, or CRLF 6041 (*BSR_UNICODE) any Unicode newline sequence 6042 6043 6044 CALLOUTS 6045 6046 (?C) callout 6047 (?Cn) callout with data n 6048 6049 6050 SEE ALSO 6051 6052 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3). 6053 6054 6055 AUTHOR 6056 6057 Philip Hazel 6058 University Computing Service 6059 Cambridge CB2 3QH, England. 6060 6061 6062 REVISION 6063 6064 Last updated: 21 November 2010 6065 Copyright (c) 1997-2010 University of Cambridge. 6066 ------------------------------------------------------------------------------ 6067 6068 6069 PCREPARTIAL(3) PCREPARTIAL(3) 6070 6071 6072 NAME 6073 PCRE - Perl-compatible regular expressions 6074 6075 6076 PARTIAL MATCHING IN PCRE 6077 6078 In normal use of PCRE, if the subject string that is passed to 6079 pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too 6080 short to match the entire pattern, PCRE_ERROR_NOMATCH is returned. 6081 There are circumstances where it might be helpful to distinguish this 6082 case from other cases in which there is no match. 6083 6084 Consider, for example, an application where a human is required to type 6085 in data for a field with specific formatting requirements. An example 6086 might be a date in the form ddmmmyy, defined by this pattern: 6087 6088 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$ 6089 6090 If the application sees the user's keystrokes one by one, and can check 6091 that what has been typed so far is potentially valid, it is able to 6092 raise an error as soon as a mistake is made, by beeping and not 6093 reflecting the character that has been typed, for example. This immedi- 6094 ate feedback is likely to be a better user interface than a check that 6095 is delayed until the entire string has been entered. Partial matching 6096 can also be useful when the subject string is very long and is not all 6097 available at once. 6098 6099 PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and 6100 PCRE_PARTIAL_HARD options, which can be set when calling pcre_exec() or 6101 pcre_dfa_exec(). For backwards compatibility, PCRE_PARTIAL is a synonym 6102 for PCRE_PARTIAL_SOFT. The essential difference between the two options 6103 is whether or not a partial match is preferred to an alternative com- 6104 plete match, though the details differ between the two matching func- 6105 tions. If both options are set, PCRE_PARTIAL_HARD takes precedence. 6106 6107 Setting a partial matching option disables two of PCRE's optimizations. 6108 PCRE remembers the last literal byte in a pattern, and abandons match- 6109 ing immediately if such a byte is not present in the subject string. 6110 This optimization cannot be used for a subject string that might match 6111 only partially. If the pattern was studied, PCRE knows the minimum 6112 length of a matching string, and does not bother to run the matching 6113 function on shorter strings. This optimization is also disabled for 6114 partial matching. 6115 6116 6117 PARTIAL MATCHING USING pcre_exec() 6118 6119 A partial match occurs during a call to pcre_exec() when the end of the 6120 subject string is reached successfully, but matching cannot continue 6121 because more characters are needed. However, at least one character in 6122 the subject must have been inspected. This character need not form part 6123 of the final matched string; lookbehind assertions and the \K escape 6124 sequence provide ways of inspecting characters before the start of a 6125 matched substring. The requirement for inspecting at least one charac- 6126 ter exists because an empty string can always be matched; without such 6127 a restriction there would always be a partial match of an empty string 6128 at the end of the subject. 6129 6130 If there are at least two slots in the offsets vector when pcre_exec() 6131 returns with a partial match, the first slot is set to the offset of 6132 the earliest character that was inspected when the partial match was 6133 found. For convenience, the second offset points to the end of the sub- 6134 ject so that a substring can easily be identified. 6135 6136 For the majority of patterns, the first offset identifies the start of 6137 the partially matched string. However, for patterns that contain look- 6138 behind assertions, or \K, or begin with \b or \B, earlier characters 6139 have been inspected while carrying out the match. For example: 6140 6141 /(?<=abc)123/ 6142 6143 This pattern matches "123", but only if it is preceded by "abc". If the 6144 subject string is "xyzabc12", the offsets after a partial match are for 6145 the substring "abc12", because all these characters are needed if 6146 another match is tried with extra characters added to the subject. 6147 6148 What happens when a partial match is identified depends on which of the 6149 two partial matching options are set. 6150 6151 PCRE_PARTIAL_SOFT with pcre_exec() 6152 6153 If PCRE_PARTIAL_SOFT is set when pcre_exec() identifies a partial 6154 match, the partial match is remembered, but matching continues as nor- 6155 mal, and other alternatives in the pattern are tried. If no complete 6156 match can be found, pcre_exec() returns PCRE_ERROR_PARTIAL instead of 6157 PCRE_ERROR_NOMATCH. 6158 6159 This option is "soft" because it prefers a complete match over a par- 6160 tial match. All the various matching items in a pattern behave as if 6161 the subject string is potentially complete. For example, \z, \Z, and $ 6162 match at the end of the subject, as normal, and for \b and \B the end 6163 of the subject is treated as a non-alphanumeric. 6164 6165 If there is more than one partial match, the first one that was found 6166 provides the data that is returned. Consider this pattern: 6167 6168 /123\w+X|dogY/ 6169 6170 If this is matched against the subject string "abc123dog", both alter- 6171 natives fail to match, but the end of the subject is reached during 6172 matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3 6173 and 9, identifying "123dog" as the first partial match that was found. 6174 (In this example, there are two partial matches, because "dog" on its 6175 own partially matches the second alternative.) 6176 6177 PCRE_PARTIAL_HARD with pcre_exec() 6178 6179 If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PAR- 6180 TIAL as soon as a partial match is found, without continuing to search 6181 for possible complete matches. This option is "hard" because it prefers 6182 an earlier partial match over a later complete match. For this reason, 6183 the assumption is made that the end of the supplied subject string may 6184 not be the true end of the available data, and so, if \z, \Z, \b, \B, 6185 or $ are encountered at the end of the subject, the result is 6186 PCRE_ERROR_PARTIAL. 6187 6188 Setting PCRE_PARTIAL_HARD also affects the way pcre_exec() checks UTF-8 6189 subject strings for validity. Normally, an invalid UTF-8 sequence 6190 causes the error PCRE_ERROR_BADUTF8. However, in the special case of a 6191 truncated UTF-8 character at the end of the subject, PCRE_ERROR_SHORT- 6192 UTF8 is returned when PCRE_PARTIAL_HARD is set. 6193 6194 Comparing hard and soft partial matching 6195 6196 The difference between the two partial matching options can be illus- 6197 trated by a pattern such as: 6198 6199 /dog(sbody)?/ 6200 6201 This matches either "dog" or "dogsbody", greedily (that is, it prefers 6202 the longer string if possible). If it is matched against the string 6203 "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog". 6204 However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL. 6205 On the other hand, if the pattern is made ungreedy the result is dif- 6206 ferent: 6207 6208 /dog(sbody)??/ 6209 6210 In this case the result is always a complete match because pcre_exec() 6211 finds that first, and it never continues after finding a match. It 6212 might be easier to follow this explanation by thinking of the two pat- 6213 terns like this: 6214 6215 /dog(sbody)?/ is the same as /dogsbody|dog/ 6216 /dog(sbody)??/ is the same as /dog|dogsbody/ 6217 6218 The second pattern will never match "dogsbody" when pcre_exec() is 6219 used, because it will always find the shorter match first. 6220 6221 6222 PARTIAL MATCHING USING pcre_dfa_exec() 6223 6224 The pcre_dfa_exec() function moves along the subject string character 6225 by character, without backtracking, searching for all possible matches 6226 simultaneously. If the end of the subject is reached before the end of 6227 the pattern, there is the possibility of a partial match, again pro- 6228 vided that at least one character has been inspected. 6229 6230 When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if 6231 there have been no complete matches. Otherwise, the complete matches 6232 are returned. However, if PCRE_PARTIAL_HARD is set, a partial match 6233 takes precedence over any complete matches. The portion of the string 6234 that was inspected when the longest partial match was found is set as 6235 the first matching string, provided there are at least two slots in the 6236 offsets vector. 6237 6238 Because pcre_dfa_exec() always searches for all possible matches, and 6239 there is no difference between greedy and ungreedy repetition, its be- 6240 haviour is different from pcre_exec when PCRE_PARTIAL_HARD is set. Con- 6241 sider the string "dog" matched against the ungreedy pattern shown 6242 above: 6243 6244 /dog(sbody)??/ 6245 6246 Whereas pcre_exec() stops as soon as it finds the complete match for 6247 "dog", pcre_dfa_exec() also finds the partial match for "dogsbody", and 6248 so returns that when PCRE_PARTIAL_HARD is set. 6249 6250 6251 PARTIAL MATCHING AND WORD BOUNDARIES 6252 6253 If a pattern ends with one of sequences \b or \B, which test for word 6254 boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter- 6255 intuitive results. Consider this pattern: 6256 6257 /\bcat\b/ 6258 6259 This matches "cat", provided there is a word boundary at either end. If 6260 the subject string is "the cat", the comparison of the final "t" with a 6261 following character cannot take place, so a partial match is found. 6262 However, pcre_exec() carries on with normal matching, which matches \b 6263 at the end of the subject when the last character is a letter, thus 6264 finding a complete match. The result, therefore, is not PCRE_ERROR_PAR- 6265 TIAL. The same thing happens with pcre_dfa_exec(), because it also 6266 finds the complete match. 6267 6268 Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL, 6269 because then the partial match takes precedence. 6270 6271 6272 FORMERLY RESTRICTED PATTERNS 6273 6274 For releases of PCRE prior to 8.00, because of the way certain internal 6275 optimizations were implemented in the pcre_exec() function, the 6276 PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be 6277 used with all patterns. From release 8.00 onwards, the restrictions no 6278 longer apply, and partial matching with pcre_exec() can be requested 6279 for any pattern. 6280 6281 Items that were formerly restricted were repeated single characters and 6282 repeated metasequences. If PCRE_PARTIAL was set for a pattern that did 6283 not conform to the restrictions, pcre_exec() returned the error code 6284 PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The 6285 PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled 6286 pattern can be used for partial matching now always returns 1. 6287 6288 6289 EXAMPLE OF PARTIAL MATCHING USING PCRETEST 6290 6291 If the escape sequence \P is present in a pcretest data line, the 6292 PCRE_PARTIAL_SOFT option is used for the match. Here is a run of 6293 pcretest that uses the date example quoted above: 6294 6295 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 6296 data> 25jun04\P 6297 0: 25jun04 6298 1: jun 6299 data> 25dec3\P 6300 Partial match: 23dec3 6301 data> 3ju\P 6302 Partial match: 3ju 6303 data> 3juj\P 6304 No match 6305 data> j\P 6306 No match 6307 6308 The first data string is matched completely, so pcretest shows the 6309 matched substrings. The remaining four strings do not match the com- 6310 plete pattern, but the first two are partial matches. Similar output is 6311 obtained when pcre_dfa_exec() is used. 6312 6313 If the escape sequence \P is present more than once in a pcretest data 6314 line, the PCRE_PARTIAL_HARD option is set for the match. 6315 6316 6317 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec() 6318 6319 When a partial match has been found using pcre_dfa_exec(), it is possi- 6320 ble to continue the match by providing additional subject data and 6321 calling pcre_dfa_exec() again with the same compiled regular expres- 6322 sion, this time setting the PCRE_DFA_RESTART option. You must pass the 6323 same working space as before, because this is where details of the pre- 6324 vious partial match are stored. Here is an example using pcretest, 6325 using the \R escape sequence to set the PCRE_DFA_RESTART option (\D 6326 specifies the use of pcre_dfa_exec()): 6327 6328 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/ 6329 data> 23ja\P\D 6330 Partial match: 23ja 6331 data> n05\R\D 6332 0: n05 6333 6334 The first call has "23ja" as the subject, and requests partial match- 6335 ing; the second call has "n05" as the subject for the continued 6336 (restarted) match. Notice that when the match is complete, only the 6337 last part is shown; PCRE does not retain the previously partially- 6338 matched string. It is up to the calling program to do that if it needs 6339 to. 6340 6341 You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with 6342 PCRE_DFA_RESTART to continue partial matching over multiple segments. 6343 This facility can be used to pass very long subject strings to 6344 pcre_dfa_exec(). 6345 6346 6347 MULTI-SEGMENT MATCHING WITH pcre_exec() 6348 6349 From release 8.00, pcre_exec() can also be used to do multi-segment 6350 matching. Unlike pcre_dfa_exec(), it is not possible to restart the 6351 previous match with a new segment of data. Instead, new data must be 6352 added to the previous subject string, and the entire match re-run, 6353 starting from the point where the partial match occurred. Earlier data 6354 can be discarded. It is best to use PCRE_PARTIAL_HARD in this situa- 6355 tion, because it does not treat the end of a segment as the end of the 6356 subject when matching \z, \Z, \b, \B, and $. Consider an unanchored 6357 pattern that matches dates: 6358 6359 re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/ 6360 data> The date is 23ja\P\P 6361 Partial match: 23ja 6362 6363 At this stage, an application could discard the text preceding "23ja", 6364 add on text from the next segment, and call pcre_exec() again. Unlike 6365 pcre_dfa_exec(), the entire matching string must always be available, 6366 and the complete matching process occurs for each call, so more memory 6367 and more processing time is needed. 6368 6369 Note: If the pattern contains lookbehind assertions, or \K, or starts 6370 with \b or \B, the string that is returned for a partial match will 6371 include characters that precede the partially matched string itself, 6372 because these must be retained when adding on more characters for a 6373 subsequent matching attempt. 6374 6375 6376 ISSUES WITH MULTI-SEGMENT MATCHING 6377 6378 Certain types of pattern may give problems with multi-segment matching, 6379 whichever matching function is used. 6380 6381 1. If the pattern contains a test for the beginning of a line, you need 6382 to pass the PCRE_NOTBOL option when the subject string for any call 6383 does start at the beginning of a line. There is also a PCRE_NOTEOL 6384 option, but in practice when doing multi-segment matching you should be 6385 using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL. 6386 6387 2. Lookbehind assertions at the start of a pattern are catered for in 6388 the offsets that are returned for a partial match. However, in theory, 6389 a lookbehind assertion later in the pattern could require even earlier 6390 characters to be inspected, and it might not have been reached when a 6391 partial match occurs. This is probably an extremely unlikely case; you 6392 could guard against it to a certain extent by always including extra 6393 characters at the start. 6394 6395 3. Matching a subject string that is split into multiple segments may 6396 not always produce exactly the same result as matching over one single 6397 long string, especially when PCRE_PARTIAL_SOFT is used. The section 6398 "Partial Matching and Word Boundaries" above describes an issue that 6399 arises if the pattern ends with \b or \B. Another kind of difference 6400 may occur when there are multiple matching possibilities, because (for 6401 PCRE_PARTIAL_SOFT) a partial match result is given only when there are 6402 no completed matches. This means that as soon as the shortest match has 6403 been found, continuation to a new subject segment is no longer possi- 6404 ble. Consider again this pcretest example: 6405 6406 re> /dog(sbody)?/ 6407 data> dogsb\P 6408 0: dog 6409 data> do\P\D 6410 Partial match: do 6411 data> gsb\R\P\D 6412 0: g 6413 data> dogsbody\D 6414 0: dogsbody 6415 1: dog 6416 6417 The first data line passes the string "dogsb" to pcre_exec(), setting 6418 the PCRE_PARTIAL_SOFT option. Although the string is a partial match 6419 for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the 6420 shorter string "dog" is a complete match. Similarly, when the subject 6421 is presented to pcre_dfa_exec() in several parts ("do" and "gsb" being 6422 the first two) the match stops when "dog" has been found, and it is not 6423 possible to continue. On the other hand, if "dogsbody" is presented as 6424 a single string, pcre_dfa_exec() finds both matches. 6425 6426 Because of these problems, it is best to use PCRE_PARTIAL_HARD when 6427 matching multi-segment data. The example above then behaves differ- 6428 ently: 6429 6430 re> /dog(sbody)?/ 6431 data> dogsb\P\P 6432 Partial match: dogsb 6433 data> do\P\D 6434 Partial match: do 6435 data> gsb\R\P\P\D 6436 Partial match: gsb 6437 6438 4. Patterns that contain alternatives at the top level which do not all 6439 start with the same pattern item may not work as expected when 6440 PCRE_DFA_RESTART is used with pcre_dfa_exec(). For example, consider 6441 this pattern: 6442 6443 1234|3789 6444 6445 If the first part of the subject is "ABC123", a partial match of the 6446 first alternative is found at offset 3. There is no partial match for 6447 the second alternative, because such a match does not start at the same 6448 point in the subject string. Attempting to continue with the string 6449 "7890" does not yield a match because only those alternatives that 6450 match at one point in the subject are remembered. The problem arises 6451 because the start of the second alternative matches within the first 6452 alternative. There is no problem with anchored patterns or patterns 6453 such as: 6454 6455 1234|ABCD 6456 6457 where no string can be a partial match for both alternatives. This is 6458 not a problem if pcre_exec() is used, because the entire match has to 6459 be rerun each time: 6460 6461 re> /1234|3789/ 6462 data> ABC123\P\P 6463 Partial match: 123 6464 data> 1237890 6465 0: 3789 6466 6467 Of course, instead of using PCRE_DFA_RESTART, the same technique of re- 6468 running the entire match can also be used with pcre_dfa_exec(). Another 6469 possibility is to work with two buffers. If a partial match at offset n 6470 in the first buffer is followed by "no match" when PCRE_DFA_RESTART is 6471 used on the second buffer, you can then try a new match starting at 6472 offset n+1 in the first buffer. 6473 6474 6475 AUTHOR 6476 6477 Philip Hazel 6478 University Computing Service 6479 Cambridge CB2 3QH, England. 6480 6481 6482 REVISION 6483 6484 Last updated: 07 November 2010 6485 Copyright (c) 1997-2010 University of Cambridge. 6486 ------------------------------------------------------------------------------ 6487 6488 6489 PCREPRECOMPILE(3) PCREPRECOMPILE(3) 6490 6491 6492 NAME 6493 PCRE - Perl-compatible regular expressions 6494 6495 6496 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS 6497 6498 If you are running an application that uses a large number of regular 6499 expression patterns, it may be useful to store them in a precompiled 6500 form instead of having to compile them every time the application is 6501 run. If you are not using any private character tables (see the 6502 pcre_maketables() documentation), this is relatively straightforward. 6503 If you are using private tables, it is a little bit more complicated. 6504 6505 If you save compiled patterns to a file, you can copy them to a differ- 6506 ent host and run them there. This works even if the new host has the 6507 opposite endianness to the one on which the patterns were compiled. 6508 There may be a small performance penalty, but it should be insignifi- 6509 cant. However, compiling regular expressions with one version of PCRE 6510 for use with a different version is not guaranteed to work and may 6511 cause crashes. 6512 6513 6514 SAVING A COMPILED PATTERN 6515 6516 The value returned by pcre_compile() points to a single block of memory 6517 that holds the compiled pattern and associated data. You can find the 6518 length of this block in bytes by calling pcre_fullinfo() with an argu- 6519 ment of PCRE_INFO_SIZE. You can then save the data in any appropriate 6520 manner. Here is sample code that compiles a pattern and writes it to a 6521 file. It assumes that the variable fd refers to a file that is open for 6522 output: 6523 6524 int erroroffset, rc, size; 6525 char *error; 6526 pcre *re; 6527 6528 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL); 6529 if (re == NULL) { ... handle errors ... } 6530 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size); 6531 if (rc < 0) { ... handle errors ... } 6532 rc = fwrite(re, 1, size, fd); 6533 if (rc != size) { ... handle errors ... } 6534 6535 In this example, the bytes that comprise the compiled pattern are 6536 copied exactly. Note that this is binary data that may contain any of 6537 the 256 possible byte values. On systems that make a distinction 6538 between binary and non-binary data, be sure that the file is opened for 6539 binary output. 6540 6541 If you want to write more than one pattern to a file, you will have to 6542 devise a way of separating them. For binary data, preceding each pat- 6543 tern with its length is probably the most straightforward approach. 6544 Another possibility is to write out the data in hexadecimal instead of 6545 binary, one pattern to a line. 6546 6547 Saving compiled patterns in a file is only one possible way of storing 6548 them for later use. They could equally well be saved in a database, or 6549 in the memory of some daemon process that passes them via sockets to 6550 the processes that want them. 6551 6552 If the pattern has been studied, it is also possible to save the study 6553 data in a similar way to the compiled pattern itself. When studying 6554 generates additional information, pcre_study() returns a pointer to a 6555 pcre_extra data block. Its format is defined in the section on matching 6556 a pattern in the pcreapi documentation. The study_data field points to 6557 the binary study data, and this is what you must save (not the 6558 pcre_extra block itself). The length of the study data can be obtained 6559 by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE. 6560 Remember to check that pcre_study() did return a non-NULL value before 6561 trying to save the study data. 6562 6563 6564 RE-USING A PRECOMPILED PATTERN 6565 6566 Re-using a precompiled pattern is straightforward. Having reloaded it 6567 into main memory, you pass its pointer to pcre_exec() or 6568 pcre_dfa_exec() in the usual way. This should work even on another 6569 host, and even if that host has the opposite endianness to the one 6570 where the pattern was compiled. 6571 6572 However, if you passed a pointer to custom character tables when the 6573 pattern was compiled (the tableptr argument of pcre_compile()), you 6574 must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(), 6575 because the value saved with the compiled pattern will obviously be 6576 nonsense. A field in a pcre_extra() block is used to pass this data, as 6577 described in the section on matching a pattern in the pcreapi documen- 6578 tation. 6579 6580 If you did not provide custom character tables when the pattern was 6581 compiled, the pointer in the compiled pattern is NULL, which causes 6582 pcre_exec() to use PCRE's internal tables. Thus, you do not need to 6583 take any special action at run time in this case. 6584 6585 If you saved study data with the compiled pattern, you need to create 6586 your own pcre_extra data block and set the study_data field to point to 6587 the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA 6588 bit in the flags field to indicate that study data is present. Then 6589 pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the 6590 usual way. 6591 6592 6593 COMPATIBILITY WITH DIFFERENT PCRE RELEASES 6594 6595 In general, it is safest to recompile all saved patterns when you 6596 update to a new PCRE release, though not all updates actually require 6597 this. 6598 6599 6600 AUTHOR 6601 6602 Philip Hazel 6603 University Computing Service 6604 Cambridge CB2 3QH, England. 6605 6606 6607 REVISION 6608 6609 Last updated: 17 November 2010 6610 Copyright (c) 1997-2010 University of Cambridge. 6611 ------------------------------------------------------------------------------ 6612 6613 6614 PCREPERFORM(3) PCREPERFORM(3) 6615 6616 6617 NAME 6618 PCRE - Perl-compatible regular expressions 6619 6620 6621 PCRE PERFORMANCE 6622 6623 Two aspects of performance are discussed below: memory usage and pro- 6624 cessing time. The way you express your pattern as a regular expression 6625 can affect both of them. 6626 6627 6628 COMPILED PATTERN MEMORY USAGE 6629 6630 Patterns are compiled by PCRE into a reasonably efficient byte code, so 6631 that most simple patterns do not use much memory. However, there is one 6632 case where the memory usage of a compiled pattern can be unexpectedly 6633 large. If a parenthesized subpattern has a quantifier with a minimum 6634 greater than 1 and/or a limited maximum, the whole subpattern is 6635 repeated in the compiled code. For example, the pattern 6636 6637 (abc|def){2,4} 6638 6639 is compiled as if it were 6640 6641 (abc|def)(abc|def)((abc|def)(abc|def)?)? 6642 6643 (Technical aside: It is done this way so that backtrack points within 6644 each of the repetitions can be independently maintained.) 6645 6646 For regular expressions whose quantifiers use only small numbers, this 6647 is not usually a problem. However, if the numbers are large, and par- 6648 ticularly if such repetitions are nested, the memory usage can become 6649 an embarrassment. For example, the very simple pattern 6650 6651 ((ab){1,1000}c){1,3} 6652 6653 uses 51K bytes when compiled. When PCRE is compiled with its default 6654 internal pointer size of two bytes, the size limit on a compiled pat- 6655 tern is 64K, and this is reached with the above pattern if the outer 6656 repetition is increased from 3 to 4. PCRE can be compiled to use larger 6657 internal pointers and thus handle larger compiled patterns, but it is 6658 better to try to rewrite your pattern to use less memory if you can. 6659 6660 One way of reducing the memory usage for such patterns is to make use 6661 of PCRE's "subroutine" facility. Re-writing the above pattern as 6662 6663 ((ab)(?2){0,999}c)(?1){0,2} 6664 6665 reduces the memory requirements to 18K, and indeed it remains under 20K 6666 even with the outer repetition increased to 100. However, this pattern 6667 is not exactly equivalent, because the "subroutine" calls are treated 6668 as atomic groups into which there can be no backtracking if there is a 6669 subsequent matching failure. Therefore, PCRE cannot do this kind of 6670 rewriting automatically. Furthermore, there is a noticeable loss of 6671 speed when executing the modified pattern. Nevertheless, if the atomic 6672 grouping is not a problem and the loss of speed is acceptable, this 6673 kind of rewriting will allow you to process patterns that PCRE cannot 6674 otherwise handle. 6675 6676 6677 STACK USAGE AT RUN TIME 6678 6679 When pcre_exec() is used for matching, certain kinds of pattern can 6680 cause it to use large amounts of the process stack. In some environ- 6681 ments the default process stack is quite small, and if it runs out the 6682 result is often SIGSEGV. This issue is probably the most frequently 6683 raised problem with PCRE. Rewriting your pattern can often help. The 6684 pcrestack documentation discusses this issue in detail. 6685 6686 6687 PROCESSING TIME 6688 6689 Certain items in regular expression patterns are processed more effi- 6690 ciently than others. It is more efficient to use a character class like 6691 [aeiou] than a set of single-character alternatives such as 6692 (a|e|i|o|u). In general, the simplest construction that provides the 6693 required behaviour is usually the most efficient. Jeffrey Friedl's book 6694 contains a lot of useful general discussion about optimizing regular 6695 expressions for efficient performance. This document contains a few 6696 observations about PCRE. 6697 6698 Using Unicode character properties (the \p, \P, and \X escapes) is 6699 slow, because PCRE has to scan a structure that contains data for over 6700 fifteen thousand characters whenever it needs a character's property. 6701 If you can find an alternative pattern that does not use character 6702 properties, it will probably be faster. 6703 6704 By default, the escape sequences \b, \d, \s, and \w, and the POSIX 6705 character classes such as [:alpha:] do not use Unicode properties, 6706 partly for backwards compatibility, and partly for performance reasons. 6707 However, you can set PCRE_UCP if you want Unicode character properties 6708 to be used. This can double the matching time for items such as \d, 6709 when matched with pcre_exec(); the performance loss is less with 6710 pcre_dfa_exec(), and in both cases there is not much difference for \b. 6711 6712 When a pattern begins with .* not in parentheses, or in parentheses 6713 that are not the subject of a backreference, and the PCRE_DOTALL option 6714 is set, the pattern is implicitly anchored by PCRE, since it can match 6715 only at the start of a subject string. However, if PCRE_DOTALL is not 6716 set, PCRE cannot make this optimization, because the . metacharacter 6717 does not then match a newline, and if the subject string contains new- 6718 lines, the pattern may match from the character immediately following 6719 one of them instead of from the very start. For example, the pattern 6720 6721 .*second 6722 6723 matches the subject "first\nand second" (where \n stands for a newline 6724 character), with the match starting at the seventh character. In order 6725 to do this, PCRE has to retry the match starting after every newline in 6726 the subject. 6727 6728 If you are using such a pattern with subject strings that do not con- 6729 tain newlines, the best performance is obtained by setting PCRE_DOTALL, 6730 or starting the pattern with ^.* or ^.*? to indicate explicit anchor- 6731 ing. That saves PCRE from having to scan along the subject looking for 6732 a newline to restart at. 6733 6734 Beware of patterns that contain nested indefinite repeats. These can 6735 take a long time to run when applied to a string that does not match. 6736 Consider the pattern fragment 6737 6738 ^(a+)* 6739 6740 This can match "aaaa" in 16 different ways, and this number increases 6741 very rapidly as the string gets longer. (The * repeat can match 0, 1, 6742 2, 3, or 4 times, and for each of those cases other than 0 or 4, the + 6743 repeats can match different numbers of times.) When the remainder of 6744 the pattern is such that the entire match is going to fail, PCRE has in 6745 principle to try every possible variation, and this can take an 6746 extremely long time, even for relatively short strings. 6747 6748 An optimization catches some of the more simple cases such as 6749 6750 (a+)*b 6751 6752 where a literal character follows. Before embarking on the standard 6753 matching procedure, PCRE checks that there is a "b" later in the sub- 6754 ject string, and if there is not, it fails the match immediately. How- 6755 ever, when there is no following literal this optimization cannot be 6756 used. You can see the difference by comparing the behaviour of 6757 6758 (a+)*\d 6759 6760 with the pattern above. The former gives a failure almost instantly 6761 when applied to a whole line of "a" characters, whereas the latter 6762 takes an appreciable time with strings longer than about 20 characters. 6763 6764 In many cases, the solution to this kind of performance issue is to use 6765 an atomic group or a possessive quantifier. 6766 6767 6768 AUTHOR 6769 6770 Philip Hazel 6771 University Computing Service 6772 Cambridge CB2 3QH, England. 6773 6774 6775 REVISION 6776 6777 Last updated: 16 May 2010 6778 Copyright (c) 1997-2010 University of Cambridge. 6779 ------------------------------------------------------------------------------ 6780 6781 6782 PCREPOSIX(3) PCREPOSIX(3) 6783 6784 6785 NAME 6786 PCRE - Perl-compatible regular expressions. 6787 6788 6789 SYNOPSIS OF POSIX API 6790 6791 #include <pcreposix.h> 6792 6793 int regcomp(regex_t *preg, const char *pattern, 6794 int cflags); 6795 6796 int regexec(regex_t *preg, const char *string, 6797 size_t nmatch, regmatch_t pmatch[], int eflags); 6798 6799 size_t regerror(int errcode, const regex_t *preg, 6800 char *errbuf, size_t errbuf_size); 6801 6802 void regfree(regex_t *preg); 6803 6804 6805 DESCRIPTION 6806 6807 This set of functions provides a POSIX-style API to the PCRE regular 6808 expression package. See the pcreapi documentation for a description of 6809 PCRE's native API, which contains much additional functionality. 6810 6811 The functions described here are just wrapper functions that ultimately 6812 call the PCRE native API. Their prototypes are defined in the 6813 pcreposix.h header file, and on Unix systems the library itself is 6814 called pcreposix.a, so can be accessed by adding -lpcreposix to the 6815 command for linking an application that uses them. Because the POSIX 6816 functions call the native ones, it is also necessary to add -lpcre. 6817 6818 I have implemented only those POSIX option bits that can be reasonably 6819 mapped to PCRE native options. In addition, the option REG_EXTENDED is 6820 defined with the value zero. This has no effect, but since programs 6821 that are written to the POSIX interface often use it, this makes it 6822 easier to slot in PCRE as a replacement library. Other POSIX options 6823 are not even defined. 6824 6825 There are also some other options that are not defined by POSIX. These 6826 have been added at the request of users who want to make use of certain 6827 PCRE-specific features via the POSIX calling interface. 6828 6829 When PCRE is called via these functions, it is only the API that is 6830 POSIX-like in style. The syntax and semantics of the regular expres- 6831 sions themselves are still those of Perl, subject to the setting of 6832 various PCRE options, as described below. "POSIX-like in style" means 6833 that the API approximates to the POSIX definition; it is not fully 6834 POSIX-compatible, and in multi-byte encoding domains it is probably 6835 even less compatible. 6836 6837 The header for these functions is supplied as pcreposix.h to avoid any 6838 potential clash with other POSIX libraries. It can, of course, be 6839 renamed or aliased as regex.h, which is the "correct" name. It provides 6840 two structure types, regex_t for compiled internal forms, and reg- 6841 match_t for returning captured substrings. It also defines some con- 6842 stants whose names start with "REG_"; these are used for setting 6843 options and identifying error codes. 6844 6845 6846 COMPILING A PATTERN 6847 6848 The function regcomp() is called to compile a pattern into an internal 6849 form. The pattern is a C string terminated by a binary zero, and is 6850 passed in the argument pattern. The preg argument is a pointer to a 6851 regex_t structure that is used as a base for storing information about 6852 the compiled regular expression. 6853 6854 The argument cflags is either zero, or contains one or more of the bits 6855 defined by the following macros: 6856 6857 REG_DOTALL 6858 6859 The PCRE_DOTALL option is set when the regular expression is passed for 6860 compilation to the native function. Note that REG_DOTALL is not part of 6861 the POSIX standard. 6862 6863 REG_ICASE 6864 6865 The PCRE_CASELESS option is set when the regular expression is passed 6866 for compilation to the native function. 6867 6868 REG_NEWLINE 6869 6870 The PCRE_MULTILINE option is set when the regular expression is passed 6871 for compilation to the native function. Note that this does not mimic 6872 the defined POSIX behaviour for REG_NEWLINE (see the following sec- 6873 tion). 6874 6875 REG_NOSUB 6876 6877 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is 6878 passed for compilation to the native function. In addition, when a pat- 6879 tern that is compiled with this flag is passed to regexec() for match- 6880 ing, the nmatch and pmatch arguments are ignored, and no captured 6881 strings are returned. 6882 6883 REG_UCP 6884 6885 The PCRE_UCP option is set when the regular expression is passed for 6886 compilation to the native function. This causes PCRE to use Unicode 6887 properties when matchine \d, \w, etc., instead of just recognizing 6888 ASCII values. Note that REG_UTF8 is not part of the POSIX standard. 6889 6890 REG_UNGREEDY 6891 6892 The PCRE_UNGREEDY option is set when the regular expression is passed 6893 for compilation to the native function. Note that REG_UNGREEDY is not 6894 part of the POSIX standard. 6895 6896 REG_UTF8 6897 6898 The PCRE_UTF8 option is set when the regular expression is passed for 6899 compilation to the native function. This causes the pattern itself and 6900 all data strings used for matching it to be treated as UTF-8 strings. 6901 Note that REG_UTF8 is not part of the POSIX standard. 6902 6903 In the absence of these flags, no options are passed to the native 6904 function. This means the the regex is compiled with PCRE default 6905 semantics. In particular, the way it handles newline characters in the 6906 subject string is the Perl way, not the POSIX way. Note that setting 6907 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE. 6908 It does not affect the way newlines are matched by . (they are not) or 6909 by a negative class such as [^a] (they are). 6910 6911 The yield of regcomp() is zero on success, and non-zero otherwise. The 6912 preg structure is filled in on success, and one member of the structure 6913 is public: re_nsub contains the number of capturing subpatterns in the 6914 regular expression. Various error codes are defined in the header file. 6915 6916 NOTE: If the yield of regcomp() is non-zero, you must not attempt to 6917 use the contents of the preg structure. If, for example, you pass it to 6918 regexec(), the result is undefined and your program is likely to crash. 6919 6920 6921 MATCHING NEWLINE CHARACTERS 6922 6923 This area is not simple, because POSIX and Perl take different views of 6924 things. It is not possible to get PCRE to obey POSIX semantics, but 6925 then PCRE was never intended to be a POSIX engine. The following table 6926 lists the different possibilities for matching newline characters in 6927 PCRE: 6928 6929 Default Change with 6930 6931 . matches newline no PCRE_DOTALL 6932 newline matches [^a] yes not changeable 6933 $ matches \n at end yes PCRE_DOLLARENDONLY 6934 $ matches \n in middle no PCRE_MULTILINE 6935 ^ matches \n in middle no PCRE_MULTILINE 6936 6937 This is the equivalent table for POSIX: 6938 6939 Default Change with 6940 6941 . matches newline yes REG_NEWLINE 6942 newline matches [^a] yes REG_NEWLINE 6943 $ matches \n at end no REG_NEWLINE 6944 $ matches \n in middle no REG_NEWLINE 6945 ^ matches \n in middle no REG_NEWLINE 6946 6947 PCRE's behaviour is the same as Perl's, except that there is no equiva- 6948 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is 6949 no way to stop newline from matching [^a]. 6950 6951 The default POSIX newline handling can be obtained by setting 6952 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE 6953 behave exactly as for the REG_NEWLINE action. 6954 6955 6956 MATCHING A PATTERN 6957 6958 The function regexec() is called to match a compiled pattern preg 6959 against a given string, which is by default terminated by a zero byte 6960 (but see REG_STARTEND below), subject to the options in eflags. These 6961 can be: 6962 6963 REG_NOTBOL 6964 6965 The PCRE_NOTBOL option is set when calling the underlying PCRE matching 6966 function. 6967 6968 REG_NOTEMPTY 6969 6970 The PCRE_NOTEMPTY option is set when calling the underlying PCRE match- 6971 ing function. Note that REG_NOTEMPTY is not part of the POSIX standard. 6972 However, setting this option can give more POSIX-like behaviour in some 6973 situations. 6974 6975 REG_NOTEOL 6976 6977 The PCRE_NOTEOL option is set when calling the underlying PCRE matching 6978 function. 6979 6980 REG_STARTEND 6981 6982 The string is considered to start at string + pmatch[0].rm_so and to 6983 have a terminating NUL located at string + pmatch[0].rm_eo (there need 6984 not actually be a NUL at that location), regardless of the value of 6985 nmatch. This is a BSD extension, compatible with but not specified by 6986 IEEE Standard 1003.2 (POSIX.2), and should be used with caution in 6987 software intended to be portable to other systems. Note that a non-zero 6988 rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location 6989 of the string, not how it is matched. 6990 6991 If the pattern was compiled with the REG_NOSUB flag, no data about any 6992 matched strings is returned. The nmatch and pmatch arguments of 6993 regexec() are ignored. 6994 6995 If the value of nmatch is zero, or if the value pmatch is NULL, no data 6996 about any matched strings is returned. 6997 6998 Otherwise,the portion of the string that was matched, and also any cap- 6999 tured substrings, are returned via the pmatch argument, which points to 7000 an array of nmatch structures of type regmatch_t, containing the mem- 7001 bers rm_so and rm_eo. These contain the offset to the first character 7002 of each substring and the offset to the first character after the end 7003 of each substring, respectively. The 0th element of the vector relates 7004 to the entire portion of string that was matched; subsequent elements 7005 relate to the capturing subpatterns of the regular expression. Unused 7006 entries in the array have both structure members set to -1. 7007 7008 A successful match yields a zero return; various error codes are 7009 defined in the header file, of which REG_NOMATCH is the "expected" 7010 failure code. 7011 7012 7013 ERROR MESSAGES 7014 7015 The regerror() function maps a non-zero errorcode from either regcomp() 7016 or regexec() to a printable message. If preg is not NULL, the error 7017 should have arisen from the use of that structure. A message terminated 7018 by a binary zero is placed in errbuf. The length of the message, 7019 including the zero, is limited to errbuf_size. The yield of the func- 7020 tion is the size of buffer needed to hold the whole message. 7021 7022 7023 MEMORY USAGE 7024 7025 Compiling a regular expression causes memory to be allocated and asso- 7026 ciated with the preg structure. The function regfree() frees all such 7027 memory, after which preg may no longer be used as a compiled expres- 7028 sion. 7029 7030 7031 AUTHOR 7032 7033 Philip Hazel 7034 University Computing Service 7035 Cambridge CB2 3QH, England. 7036 7037 7038 REVISION 7039 7040 Last updated: 16 May 2010 7041 Copyright (c) 1997-2010 University of Cambridge. 7042 ------------------------------------------------------------------------------ 7043 7044 7045 PCRECPP(3) PCRECPP(3) 7046 7047 7048 NAME 7049 PCRE - Perl-compatible regular expressions. 7050 7051 7052 SYNOPSIS OF C++ WRAPPER 7053 7054 #include <pcrecpp.h> 7055 7056 7057 DESCRIPTION 7058 7059 The C++ wrapper for PCRE was provided by Google Inc. Some additional 7060 functionality was added by Giuseppe Maxia. This brief man page was con- 7061 structed from the notes in the pcrecpp.h file, which should be con- 7062 sulted for further details. 7063 7064 7065 MATCHING INTERFACE 7066 7067 The "FullMatch" operation checks that supplied text matches a supplied 7068 pattern exactly. If pointer arguments are supplied, it copies matched 7069 sub-strings that match sub-patterns into them. 7070 7071 Example: successful match 7072 pcrecpp::RE re("h.*o"); 7073 re.FullMatch("hello"); 7074 7075 Example: unsuccessful match (requires full match): 7076 pcrecpp::RE re("e"); 7077 !re.FullMatch("hello"); 7078 7079 Example: creating a temporary RE object: 7080 pcrecpp::RE("h.*o").FullMatch("hello"); 7081 7082 You can pass in a "const char*" or a "string" for "text". The examples 7083 below tend to use a const char*. You can, as in the different examples 7084 above, store the RE object explicitly in a variable or use a temporary 7085 RE object. The examples below use one mode or the other arbitrarily. 7086 Either could correctly be used for any of these examples. 7087 7088 You must supply extra pointer arguments to extract matched subpieces. 7089 7090 Example: extracts "ruby" into "s" and 1234 into "i" 7091 int i; 7092 string s; 7093 pcrecpp::RE re("(\\w+):(\\d+)"); 7094 re.FullMatch("ruby:1234", &s, &i); 7095 7096 Example: does not try to extract any extra sub-patterns 7097 re.FullMatch("ruby:1234", &s); 7098 7099 Example: does not try to extract into NULL 7100 re.FullMatch("ruby:1234", NULL, &i); 7101 7102 Example: integer overflow causes failure 7103 !re.FullMatch("ruby:1234567891234", NULL, &i); 7104 7105 Example: fails because there aren't enough sub-patterns: 7106 !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s); 7107 7108 Example: fails because string cannot be stored in integer 7109 !pcrecpp::RE("(.*)").FullMatch("ruby", &i); 7110 7111 The provided pointer arguments can be pointers to any scalar numeric 7112 type, or one of: 7113 7114 string (matched piece is copied to string) 7115 StringPiece (StringPiece is mutated to point to matched piece) 7116 T (where "bool T::ParseFrom(const char*, int)" exists) 7117 NULL (the corresponding matched sub-pattern is not copied) 7118 7119 The function returns true iff all of the following conditions are sat- 7120 isfied: 7121 7122 a. "text" matches "pattern" exactly; 7123 7124 b. The number of matched sub-patterns is >= number of supplied 7125 pointers; 7126 7127 c. The "i"th argument has a suitable type for holding the 7128 string captured as the "i"th sub-pattern. If you pass in 7129 void * NULL for the "i"th argument, or a non-void * NULL 7130 of the correct type, or pass fewer arguments than the 7131 number of sub-patterns, "i"th captured sub-pattern is 7132 ignored. 7133 7134 CAVEAT: An optional sub-pattern that does not exist in the matched 7135 string is assigned the empty string. Therefore, the following will 7136 return false (because the empty string is not a valid number): 7137 7138 int number; 7139 pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number); 7140 7141 The matching interface supports at most 16 arguments per call. If you 7142 need more, consider using the more general interface 7143 pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch. 7144 7145 NOTE: Do not use no_arg, which is used internally to mark the end of a 7146 list of optional arguments, as a placeholder for missing arguments, as 7147 this can lead to segfaults. 7148 7149 7150 QUOTING METACHARACTERS 7151 7152 You can use the "QuoteMeta" operation to insert backslashes before all 7153 potentially meaningful characters in a string. The returned string, 7154 used as a regular expression, will exactly match the original string. 7155 7156 Example: 7157 string quoted = RE::QuoteMeta(unquoted); 7158 7159 Note that it's legal to escape a character even if it has no special 7160 meaning in a regular expression -- so this function does that. (This 7161 also makes it identical to the perl function of the same name; see 7162 "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes 7163 "1\.5\-2\.0\?". 7164 7165 7166 PARTIAL MATCHES 7167 7168 You can use the "PartialMatch" operation when you want the pattern to 7169 match any substring of the text. 7170 7171 Example: simple search for a string: 7172 pcrecpp::RE("ell").PartialMatch("hello"); 7173 7174 Example: find first number in a string: 7175 int number; 7176 pcrecpp::RE re("(\\d+)"); 7177 re.PartialMatch("x*100 + 20", &number); 7178 assert(number == 100); 7179 7180 7181 UTF-8 AND THE MATCHING INTERFACE 7182 7183 By default, pattern and text are plain text, one byte per character. 7184 The UTF8 flag, passed to the constructor, causes both pattern and 7185 string to be treated as UTF-8 text, still a byte stream but potentially 7186 multiple bytes per character. In practice, the text is likelier to be 7187 UTF-8 than the pattern, but the match returned may depend on the UTF8 7188 flag, so always use it when matching UTF8 text. For example, "." will 7189 match one byte normally but with UTF8 set may match up to three bytes 7190 of a multi-byte character. 7191 7192 Example: 7193 pcrecpp::RE_Options options; 7194 options.set_utf8(); 7195 pcrecpp::RE re(utf8_pattern, options); 7196 re.FullMatch(utf8_string); 7197 7198 Example: using the convenience function UTF8(): 7199 pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8()); 7200 re.FullMatch(utf8_string); 7201 7202 NOTE: The UTF8 flag is ignored if pcre was not configured with the 7203 --enable-utf8 flag. 7204 7205 7206 PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE 7207 7208 PCRE defines some modifiers to change the behavior of the regular 7209 expression engine. The C++ wrapper defines an auxiliary class, 7210 RE_Options, as a vehicle to pass such modifiers to a RE class. Cur- 7211 rently, the following modifiers are supported: 7212 7213 modifier description Perl corresponding 7214 7215 PCRE_CASELESS case insensitive match /i 7216 PCRE_MULTILINE multiple lines match /m 7217 PCRE_DOTALL dot matches newlines /s 7218 PCRE_DOLLAR_ENDONLY $ matches only at end N/A 7219 PCRE_EXTRA strict escape parsing N/A 7220 PCRE_EXTENDED ignore whitespaces /x 7221 PCRE_UTF8 handles UTF8 chars built-in 7222 PCRE_UNGREEDY reverses * and *? N/A 7223 PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*) 7224 7225 (*) Both Perl and PCRE allow non capturing parentheses by means of the 7226 "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap- 7227 ture, while (ab|cd) does. 7228 7229 For a full account on how each modifier works, please check the PCRE 7230 API reference page. 7231 7232 For each modifier, there are two member functions whose name is made 7233 out of the modifier in lowercase, without the "PCRE_" prefix. For 7234 instance, PCRE_CASELESS is handled by 7235 7236 bool caseless() 7237 7238 which returns true if the modifier is set, and 7239 7240 RE_Options & set_caseless(bool) 7241 7242 which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can 7243 be accessed through the set_match_limit() and match_limit() member 7244 functions. Setting match_limit to a non-zero value will limit the exe- 7245 cution of pcre to keep it from doing bad things like blowing the stack 7246 or taking an eternity to return a result. A value of 5000 is good 7247 enough to stop stack blowup in a 2MB thread stack. Setting match_limit 7248 to zero disables match limiting. Alternatively, you can call 7249 match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to 7250 limit how much PCRE recurses. match_limit() limits the number of 7251 matches PCRE does; match_limit_recursion() limits the depth of internal 7252 recursion, and therefore the amount of stack that is used. 7253 7254 Normally, to pass one or more modifiers to a RE class, you declare a 7255 RE_Options object, set the appropriate options, and pass this object to 7256 a RE constructor. Example: 7257 7258 RE_options opt; 7259 opt.set_caseless(true); 7260 if (RE("HELLO", opt).PartialMatch("hello world")) ... 7261 7262 RE_options has two constructors. The default constructor takes no argu- 7263 ments and creates a set of flags that are off by default. The optional 7264 parameter option_flags is to facilitate transfer of legacy code from C 7265 programs. This lets you do 7266 7267 RE(pattern, 7268 RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str); 7269 7270 However, new code is better off doing 7271 7272 RE(pattern, 7273 RE_Options().set_caseless(true).set_multiline(true)) 7274 .PartialMatch(str); 7275 7276 If you are going to pass one of the most used modifiers, there are some 7277 convenience functions that return a RE_Options class with the appropri- 7278 ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(), 7279 and EXTENDED(). 7280 7281 If you need to set several options at once, and you don't want to go 7282 through the pains of declaring a RE_Options object and setting several 7283 options, there is a parallel method that give you such ability on the 7284 fly. You can concatenate several set_xxxxx() member functions, since 7285 each of them returns a reference to its class object. For example, to 7286 pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one 7287 statement, you may write: 7288 7289 RE(" ^ xyz \\s+ .* blah$", 7290 RE_Options() 7291 .set_caseless(true) 7292 .set_extended(true) 7293 .set_multiline(true)).PartialMatch(sometext); 7294 7295 7296 SCANNING TEXT INCREMENTALLY 7297 7298 The "Consume" operation may be useful if you want to repeatedly match 7299 regular expressions at the front of a string and skip over them as they 7300 match. This requires use of the "StringPiece" type, which represents a 7301 sub-range of a real string. Like RE, StringPiece is defined in the 7302 pcrecpp namespace. 7303 7304 Example: read lines of the form "var = value" from a string. 7305 string contents = ...; // Fill string somehow 7306 pcrecpp::StringPiece input(contents); // Wrap in a StringPiece 7307 7308 string var; 7309 int value; 7310 pcrecpp::RE re("(\\w+) = (\\d+)\n"); 7311 while (re.Consume(&input, &var, &value)) { 7312 ...; 7313 } 7314 7315 Each successful call to "Consume" will set "var/value", and also 7316 advance "input" so it points past the matched text. 7317 7318 The "FindAndConsume" operation is similar to "Consume" but does not 7319 anchor your match at the beginning of the string. For example, you 7320 could extract all words from a string by repeatedly calling 7321 7322 pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word) 7323 7324 7325 PARSING HEX/OCTAL/C-RADIX NUMBERS 7326 7327 By default, if you pass a pointer to a numeric value, the corresponding 7328 text is interpreted as a base-10 number. You can instead wrap the 7329 pointer with a call to one of the operators Hex(), Octal(), or CRadix() 7330 to interpret the text in another base. The CRadix operator interprets 7331 C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to 7332 base-10. 7333 7334 Example: 7335 int a, b, c, d; 7336 pcrecpp::RE re("(.*) (.*) (.*) (.*)"); 7337 re.FullMatch("100 40 0100 0x40", 7338 pcrecpp::Octal(&a), pcrecpp::Hex(&b), 7339 pcrecpp::CRadix(&c), pcrecpp::CRadix(&d)); 7340 7341 will leave 64 in a, b, c, and d. 7342 7343 7344 REPLACING PARTS OF STRINGS 7345 7346 You can replace the first match of "pattern" in "str" with "rewrite". 7347 Within "rewrite", backslash-escaped digits (\1 to \9) can be used to 7348 insert text matching corresponding parenthesized group from the pat- 7349 tern. \0 in "rewrite" refers to the entire matching text. For example: 7350 7351 string s = "yabba dabba doo"; 7352 pcrecpp::RE("b+").Replace("d", &s); 7353 7354 will leave "s" containing "yada dabba doo". The result is true if the 7355 pattern matches and a replacement occurs, false otherwise. 7356 7357 GlobalReplace is like Replace except that it replaces all occurrences 7358 of the pattern in the string with the rewrite. Replacements are not 7359 subject to re-matching. For example: 7360 7361 string s = "yabba dabba doo"; 7362 pcrecpp::RE("b+").GlobalReplace("d", &s); 7363 7364 will leave "s" containing "yada dada doo". It returns the number of 7365 replacements made. 7366 7367 Extract is like Replace, except that if the pattern matches, "rewrite" 7368 is copied into "out" (an additional argument) with substitutions. The 7369 non-matching portions of "text" are ignored. Returns true iff a match 7370 occurred and the extraction happened successfully; if no match occurs, 7371 the string is left unaffected. 7372 7373 7374 AUTHOR 7375 7376 The C++ wrapper was contributed by Google Inc. 7377 Copyright (c) 2007 Google Inc. 7378 7379 7380 REVISION 7381 7382 Last updated: 17 March 2009 7383 ------------------------------------------------------------------------------ 7384 7385 7386 PCRESAMPLE(3) PCRESAMPLE(3) 7387 7388 7389 NAME 7390 PCRE - Perl-compatible regular expressions 7391 7392 7393 PCRE SAMPLE PROGRAM 7394 7395 A simple, complete demonstration program, to get you started with using 7396 PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A 7397 listing of this program is given in the pcredemo documentation. If you 7398 do not have a copy of the PCRE distribution, you can save this listing 7399 to re-create pcredemo.c. 7400 7401 The program compiles the regular expression that is its first argument, 7402 and matches it against the subject string in its second argument. No 7403 PCRE options are set, and default character tables are used. If match- 7404 ing succeeds, the program outputs the portion of the subject that 7405 matched, together with the contents of any captured substrings. 7406 7407 If the -g option is given on the command line, the program then goes on 7408 to check for further matches of the same regular expression in the same 7409 subject string. The logic is a little bit tricky because of the possi- 7410 bility of matching an empty string. Comments in the code explain what 7411 is going on. 7412 7413 If PCRE is installed in the standard include and library directories 7414 for your operating system, you should be able to compile the demonstra- 7415 tion program using this command: 7416 7417 gcc -o pcredemo pcredemo.c -lpcre 7418 7419 If PCRE is installed elsewhere, you may need to add additional options 7420 to the command line. For example, on a Unix-like system that has PCRE 7421 installed in /usr/local, you can compile the demonstration program 7422 using a command like this: 7423 7424 gcc -o pcredemo -I/usr/local/include pcredemo.c \ 7425 -L/usr/local/lib -lpcre 7426 7427 In a Windows environment, if you want to statically link the program 7428 against a non-dll pcre.a file, you must uncomment the line that defines 7429 PCRE_STATIC before including pcre.h, because otherwise the pcre_mal- 7430 loc() and pcre_free() exported functions will be declared 7431 __declspec(dllimport), with unwanted results. 7432 7433 Once you have compiled and linked the demonstration program, you can 7434 run simple tests like this: 7435 7436 ./pcredemo 'cat|dog' 'the cat sat on the mat' 7437 ./pcredemo -g 'cat|dog' 'the dog sat on the cat' 7438 7439 Note that there is a much more comprehensive test program, called 7440 pcretest, which supports many more facilities for testing regular 7441 expressions and the PCRE library. The pcredemo program is provided as a 7442 simple coding example. 7443 7444 If you try to run pcredemo when PCRE is not installed in the standard 7445 library directory, you may get an error like this on some operating 7446 systems (e.g. Solaris): 7447 7448 ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or 7449 directory 7450 7451 This is caused by the way shared library support works on those sys- 7452 tems. You need to add 7453 7454 -R/usr/local/lib 7455 7456 (for example) to the compile command to get round this problem. 7457 7458 7459 AUTHOR 7460 7461 Philip Hazel 7462 University Computing Service 7463 Cambridge CB2 3QH, England. 7464 7465 7466 REVISION 7467 7468 Last updated: 17 November 2010 7469 Copyright (c) 1997-2010 University of Cambridge. 7470 ------------------------------------------------------------------------------ 7471 PCRESTACK(3) PCRESTACK(3) 7472 7473 7474 NAME 7475 PCRE - Perl-compatible regular expressions 7476 7477 7478 PCRE DISCUSSION OF STACK USAGE 7479 7480 When you call pcre_exec(), it makes use of an internal function called 7481 match(). This calls itself recursively at branch points in the pattern, 7482 in order to remember the state of the match so that it can back up and 7483 try a different alternative if the first one fails. As matching pro- 7484 ceeds deeper and deeper into the tree of possibilities, the recursion 7485 depth increases. 7486 7487 Not all calls of match() increase the recursion depth; for an item such 7488 as a* it may be called several times at the same level, after matching 7489 different numbers of a's. Furthermore, in a number of cases where the 7490 result of the recursive call would immediately be passed back as the 7491 result of the current call (a "tail recursion"), the function is just 7492 restarted instead. 7493 7494 The pcre_dfa_exec() function operates in an entirely different way, and 7495 uses recursion only when there is a regular expression recursion or 7496 subroutine call in the pattern. This includes the processing of asser- 7497 tion and "once-only" subpatterns, which are handled like subroutine 7498 calls. Normally, these are never very deep, and the limit on the com- 7499 plexity of pcre_dfa_exec() is controlled by the amount of workspace it 7500 is given. However, it is possible to write patterns with runaway infi- 7501 nite recursions; such patterns will cause pcre_dfa_exec() to run out of 7502 stack. At present, there is no protection against this. 7503 7504 The comments that follow do NOT apply to pcre_dfa_exec(); they are rel- 7505 evant only for pcre_exec(). 7506 7507 Reducing pcre_exec()'s stack usage 7508 7509 Each time that match() is actually called recursively, it uses memory 7510 from the process stack. For certain kinds of pattern and data, very 7511 large amounts of stack may be needed, despite the recognition of "tail 7512 recursion". You can often reduce the amount of recursion, and there- 7513 fore the amount of stack used, by modifying the pattern that is being 7514 matched. Consider, for example, this pattern: 7515 7516 ([^<]|<(?!inet))+ 7517 7518 It matches from wherever it starts until it encounters "<inet" or the 7519 end of the data, and is the kind of pattern that might be used when 7520 processing an XML file. Each iteration of the outer parentheses matches 7521 either one character that is not "<" or a "<" that is not followed by 7522 "inet". However, each time a parenthesis is processed, a recursion 7523 occurs, so this formulation uses a stack frame for each matched charac- 7524 ter. For a long string, a lot of stack is required. Consider now this 7525 rewritten pattern, which matches exactly the same strings: 7526 7527 ([^<]++|<(?!inet))+ 7528 7529 This uses very much less stack, because runs of characters that do not 7530 contain "<" are "swallowed" in one item inside the parentheses. Recur- 7531 sion happens only when a "<" character that is not followed by "inet" 7532 is encountered (and we assume this is relatively rare). A possessive 7533 quantifier is used to stop any backtracking into the runs of non-"<" 7534 characters, but that is not related to stack usage. 7535 7536 This example shows that one way of avoiding stack problems when match- 7537 ing long subject strings is to write repeated parenthesized subpatterns 7538 to match more than one character whenever possible. 7539 7540 Compiling PCRE to use heap instead of stack for pcre_exec() 7541 7542 In environments where stack memory is constrained, you might want to 7543 compile PCRE to use heap memory instead of stack for remembering back- 7544 up points when pcre_exec() is running. This makes it run a lot more 7545 slowly, however. Details of how to do this are given in the pcrebuild 7546 documentation. When built in this way, instead of using the stack, PCRE 7547 obtains and frees memory by calling the functions that are pointed to 7548 by the pcre_stack_malloc and pcre_stack_free variables. By default, 7549 these point to malloc() and free(), but you can replace the pointers to 7550 cause PCRE to use your own functions. Since the block sizes are always 7551 the same, and are always freed in reverse order, it may be possible to 7552 implement customized memory handlers that are more efficient than the 7553 standard functions. 7554 7555 Limiting pcre_exec()'s stack usage 7556 7557 You can set limits on the number of times that match() is called, both 7558 in total and recursively. If a limit is exceeded, pcre_exec() returns 7559 an error code. Setting suitable limits should prevent it from running 7560 out of stack. The default values of the limits are very large, and 7561 unlikely ever to operate. They can be changed when PCRE is built, and 7562 they can also be set when pcre_exec() is called. For details of these 7563 interfaces, see the pcrebuild documentation and the section on extra 7564 data for pcre_exec() in the pcreapi documentation. 7565 7566 As a very rough rule of thumb, you should reckon on about 500 bytes per 7567 recursion. Thus, if you want to limit your stack usage to 8Mb, you 7568 should set the limit at 16000 recursions. A 64Mb stack, on the other 7569 hand, can support around 128000 recursions. 7570 7571 In Unix-like environments, the pcretest test program has a command line 7572 option (-S) that can be used to increase the size of its stack. As long 7573 as the stack is large enough, another option (-M) can be used to find 7574 the smallest limits that allow a particular pattern to match a given 7575 subject string. This is done by calling pcre_exec() repeatedly with 7576 different limits. 7577 7578 Changing stack size in Unix-like systems 7579 7580 In Unix-like environments, there is not often a problem with the stack 7581 unless very long strings are involved, though the default limit on 7582 stack size varies from system to system. Values from 8Mb to 64Mb are 7583 common. You can find your default limit by running the command: 7584 7585 ulimit -s 7586 7587 Unfortunately, the effect of running out of stack is often SIGSEGV, 7588 though sometimes a more explicit error message is given. You can nor- 7589 mally increase the limit on stack size by code such as this: 7590 7591 struct rlimit rlim; 7592 getrlimit(RLIMIT_STACK, &rlim); 7593 rlim.rlim_cur = 100*1024*1024; 7594 setrlimit(RLIMIT_STACK, &rlim); 7595 7596 This reads the current limits (soft and hard) using getrlimit(), then 7597 attempts to increase the soft limit to 100Mb using setrlimit(). You 7598 must do this before calling pcre_exec(). 7599 7600 Changing stack size in Mac OS X 7601 7602 Using setrlimit(), as described above, should also work on Mac OS X. It 7603 is also possible to set a stack size when linking a program. There is a 7604 discussion about stack sizes in Mac OS X at this web site: 7605 http://developer.apple.com/qa/qa2005/qa1419.html. 7606 7607 7608 AUTHOR 7609 7610 Philip Hazel 7611 University Computing Service 7612 Cambridge CB2 3QH, England. 7613 7614 7615 REVISION 7616 7617 Last updated: 03 January 2010 7618 Copyright (c) 1997-2010 University of Cambridge. 7619 ------------------------------------------------------------------------------ 7620 7621 7622
