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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. There are
6 separate text files for the pcregrep and pcretest commands.
7 -----------------------------------------------------------------------------
8
9
10 PCRE(3) PCRE(3)
11
12
13 NAME
14 PCRE - Perl-compatible regular expressions
15
16
17 INTRODUCTION
18
19 The PCRE library is a set of functions that implement regular expres-
20 sion pattern matching using the same syntax and semantics as Perl, with
21 just a few differences. Certain features that appeared in Python and
22 PCRE before they appeared in Perl are also available using the Python
23 syntax. There is also some support for certain .NET and Oniguruma syn-
24 tax items, and there is an option for requesting some minor changes
25 that give better JavaScript compatibility.
26
27 The current implementation of PCRE (release 7.x) corresponds approxi-
28 mately with Perl 5.10, including support for UTF-8 encoded strings and
29 Unicode general category properties. However, UTF-8 and Unicode support
30 has to be explicitly enabled; it is not the default. The Unicode tables
31 correspond to Unicode release 5.0.0.
32
33 In addition to the Perl-compatible matching function, PCRE contains an
34 alternative matching function that matches the same compiled patterns
35 in a different way. In certain circumstances, the alternative function
36 has some advantages. For a discussion of the two matching algorithms,
37 see the pcrematching page.
38
39 PCRE is written in C and released as a C library. A number of people
40 have written wrappers and interfaces of various kinds. In particular,
41 Google Inc. have provided a comprehensive C++ wrapper. This is now
42 included as part of the PCRE distribution. The pcrecpp page has details
43 of this interface. Other people's contributions can be found in the
44 Contrib directory at the primary FTP site, which is:
45
46 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
47
48 Details of exactly which Perl regular expression features are and are
49 not supported by PCRE are given in separate documents. See the pcrepat-
50 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
51 page.
52
53 Some features of PCRE can be included, excluded, or changed when the
54 library is built. The pcre_config() function makes it possible for a
55 client to discover which features are available. The features them-
56 selves are described in the pcrebuild page. Documentation about build-
57 ing PCRE for various operating systems can be found in the README file
58 in the source distribution.
59
60 The library contains a number of undocumented internal functions and
61 data tables that are used by more than one of the exported external
62 functions, but which are not intended for use by external callers.
63 Their names all begin with "_pcre_", which hopefully will not provoke
64 any name clashes. In some environments, it is possible to control which
65 external symbols are exported when a shared library is built, and in
66 these cases the undocumented symbols are not exported.
67
68
69 USER DOCUMENTATION
70
71 The user documentation for PCRE comprises a number of different sec-
72 tions. In the "man" format, each of these is a separate "man page". In
73 the HTML format, each is a separate page, linked from the index page.
74 In the plain text format, all the sections are concatenated, for ease
75 of searching. The sections are as follows:
76
77 pcre this document
78 pcre-config show PCRE installation configuration information
79 pcreapi details of PCRE's native C API
80 pcrebuild options for building PCRE
81 pcrecallout details of the callout feature
82 pcrecompat discussion of Perl compatibility
83 pcrecpp details of the C++ wrapper
84 pcregrep description of the pcregrep command
85 pcrematching discussion of the two matching algorithms
86 pcrepartial details of the partial matching facility
87 pcrepattern syntax and semantics of supported
88 regular expressions
89 pcresyntax quick syntax reference
90 pcreperform discussion of performance issues
91 pcreposix the POSIX-compatible C API
92 pcreprecompile details of saving and re-using precompiled patterns
93 pcresample discussion of the sample program
94 pcrestack discussion of stack usage
95 pcretest description of the pcretest testing command
96
97 In addition, in the "man" and HTML formats, there is a short page for
98 each C library function, listing its arguments and results.
99
100
101 LIMITATIONS
102
103 There are some size limitations in PCRE but it is hoped that they will
104 never in practice be relevant.
105
106 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
107 is compiled with the default internal linkage size of 2. If you want to
108 process regular expressions that are truly enormous, you can compile
109 PCRE with an internal linkage size of 3 or 4 (see the README file in
110 the source distribution and the pcrebuild documentation for details).
111 In these cases the limit is substantially larger. However, the speed
112 of execution is slower.
113
114 All values in repeating quantifiers must be less than 65536.
115
116 There is no limit to the number of parenthesized subpatterns, but there
117 can be no more than 65535 capturing subpatterns.
118
119 The maximum length of name for a named subpattern is 32 characters, and
120 the maximum number of named subpatterns is 10000.
121
122 The maximum length of a subject string is the largest positive number
123 that an integer variable can hold. However, when using the traditional
124 matching function, PCRE uses recursion to handle subpatterns and indef-
125 inite repetition. This means that the available stack space may limit
126 the size of a subject string that can be processed by certain patterns.
127 For a discussion of stack issues, see the pcrestack documentation.
128
129
130 UTF-8 AND UNICODE PROPERTY SUPPORT
131
132 From release 3.3, PCRE has had some support for character strings
133 encoded in the UTF-8 format. For release 4.0 this was greatly extended
134 to cover most common requirements, and in release 5.0 additional sup-
135 port for Unicode general category properties was added.
136
137 In order process UTF-8 strings, you must build PCRE to include UTF-8
138 support in the code, and, in addition, you must call pcre_compile()
139 with the PCRE_UTF8 option flag. When you do this, both the pattern and
140 any subject strings that are matched against it are treated as UTF-8
141 strings instead of just strings of bytes.
142
143 If you compile PCRE with UTF-8 support, but do not use it at run time,
144 the library will be a bit bigger, but the additional run time overhead
145 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
146 very big.
147
148 If PCRE is built with Unicode character property support (which implies
149 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
150 ported. The available properties that can be tested are limited to the
151 general category properties such as Lu for an upper case letter or Nd
152 for a decimal number, the Unicode script names such as Arabic or Han,
153 and the derived properties Any and L&. A full list is given in the
154 pcrepattern documentation. Only the short names for properties are sup-
155 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
156 ter}, is not supported. Furthermore, in Perl, many properties may
157 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
158 does not support this.
159
160 Validity of UTF-8 strings
161
162 When you set the PCRE_UTF8 flag, the strings passed as patterns and
163 subjects are (by default) checked for validity on entry to the relevant
164 functions. From release 7.3 of PCRE, the check is according the rules
165 of RFC 3629, which are themselves derived from the Unicode specifica-
166 tion. Earlier releases of PCRE followed the rules of RFC 2279, which
167 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
168 check allows only values in the range U+0 to U+10FFFF, excluding U+D800
169 to U+DFFF.
170
171 The excluded code points are the "Low Surrogate Area" of Unicode, of
172 which the Unicode Standard says this: "The Low Surrogate Area does not
173 contain any character assignments, consequently no character code
174 charts or namelists are provided for this area. Surrogates are reserved
175 for use with UTF-16 and then must be used in pairs." The code points
176 that are encoded by UTF-16 pairs are available as independent code
177 points in the UTF-8 encoding. (In other words, the whole surrogate
178 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
179
180 If an invalid UTF-8 string is passed to PCRE, an error return
181 (PCRE_ERROR_BADUTF8) is given. In some situations, you may already know
182 that your strings are valid, and therefore want to skip these checks in
183 order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag at
184 compile time or at run time, PCRE assumes that the pattern or subject
185 it is given (respectively) contains only valid UTF-8 codes. In this
186 case, it does not diagnose an invalid UTF-8 string.
187
188 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
189 what happens depends on why the string is invalid. If the string con-
190 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
191 string of characters in the range 0 to 0x7FFFFFFF. In other words,
192 apart from the initial validity test, PCRE (when in UTF-8 mode) handles
193 strings according to the more liberal rules of RFC 2279. However, if
194 the string does not even conform to RFC 2279, the result is undefined.
195 Your program may crash.
196
197 If you want to process strings of values in the full range 0 to
198 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
199 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
200 this situation, you will have to apply your own validity check.
201
202 General comments about UTF-8 mode
203
204 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
205 two-byte UTF-8 character if the value is greater than 127.
206
207 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
208 characters for values greater than \177.
209
210 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
211 vidual bytes, for example: \x{100}{3}.
212
213 4. The dot metacharacter matches one UTF-8 character instead of a sin-
214 gle byte.
215
216 5. The escape sequence \C can be used to match a single byte in UTF-8
217 mode, but its use can lead to some strange effects. This facility is
218 not available in the alternative matching function, pcre_dfa_exec().
219
220 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
221 test characters of any code value, but the characters that PCRE recog-
222 nizes as digits, spaces, or word characters remain the same set as
223 before, all with values less than 256. This remains true even when PCRE
224 includes Unicode property support, because to do otherwise would slow
225 down PCRE in many common cases. If you really want to test for a wider
226 sense of, say, "digit", you must use Unicode property tests such as
227 \p{Nd}.
228
229 7. Similarly, characters that match the POSIX named character classes
230 are all low-valued characters.
231
232 8. However, the Perl 5.10 horizontal and vertical whitespace matching
233 escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
234 acters.
235
236 9. Case-insensitive matching applies only to characters whose values
237 are less than 128, unless PCRE is built with Unicode property support.
238 Even when Unicode property support is available, PCRE still uses its
239 own character tables when checking the case of low-valued characters,
240 so as not to degrade performance. The Unicode property information is
241 used only for characters with higher values. Even when Unicode property
242 support is available, PCRE supports case-insensitive matching only when
243 there is a one-to-one mapping between a letter's cases. There are a
244 small number of many-to-one mappings in Unicode; these are not sup-
245 ported by PCRE.
246
247
248 AUTHOR
249
250 Philip Hazel
251 University Computing Service
252 Cambridge CB2 3QH, England.
253
254 Putting an actual email address here seems to have been a spam magnet,
255 so I've taken it away. If you want to email me, use my two initials,
256 followed by the two digits 10, at the domain cam.ac.uk.
257
258
259 REVISION
260
261 Last updated: 12 April 2008
262 Copyright (c) 1997-2008 University of Cambridge.
263 ------------------------------------------------------------------------------
264
265
266 PCREBUILD(3) PCREBUILD(3)
267
268
269 NAME
270 PCRE - Perl-compatible regular expressions
271
272
273 PCRE BUILD-TIME OPTIONS
274
275 This document describes the optional features of PCRE that can be
276 selected when the library is compiled. It assumes use of the configure
277 script, where the optional features are selected or deselected by pro-
278 viding options to configure before running the make command. However,
279 the same options can be selected in both Unix-like and non-Unix-like
280 environments using the GUI facility of CMakeSetup if you are using
281 CMake instead of configure to build PCRE.
282
283 The complete list of options for configure (which includes the standard
284 ones such as the selection of the installation directory) can be
285 obtained by running
286
287 ./configure --help
288
289 The following sections include descriptions of options whose names
290 begin with --enable or --disable. These settings specify changes to the
291 defaults for the configure command. Because of the way that configure
292 works, --enable and --disable always come in pairs, so the complemen-
293 tary option always exists as well, but as it specifies the default, it
294 is not described.
295
296
297 C++ SUPPORT
298
299 By default, the configure script will search for a C++ compiler and C++
300 header files. If it finds them, it automatically builds the C++ wrapper
301 library for PCRE. You can disable this by adding
302
303 --disable-cpp
304
305 to the configure command.
306
307
308 UTF-8 SUPPORT
309
310 To build PCRE with support for UTF-8 character strings, add
311
312 --enable-utf8
313
314 to the configure command. Of itself, this does not make PCRE treat
315 strings as UTF-8. As well as compiling PCRE with this option, you also
316 have have to set the PCRE_UTF8 option when you call the pcre_compile()
317 function.
318
319
320 UNICODE CHARACTER PROPERTY SUPPORT
321
322 UTF-8 support allows PCRE to process character values greater than 255
323 in the strings that it handles. On its own, however, it does not pro-
324 vide any facilities for accessing the properties of such characters. If
325 you want to be able to use the pattern escapes \P, \p, and \X, which
326 refer to Unicode character properties, you must add
327
328 --enable-unicode-properties
329
330 to the configure command. This implies UTF-8 support, even if you have
331 not explicitly requested it.
332
333 Including Unicode property support adds around 30K of tables to the
334 PCRE library. Only the general category properties such as Lu and Nd
335 are supported. Details are given in the pcrepattern documentation.
336
337
338 CODE VALUE OF NEWLINE
339
340 By default, PCRE interprets character 10 (linefeed, LF) as indicating
341 the end of a line. This is the normal newline character on Unix-like
342 systems. You can compile PCRE to use character 13 (carriage return, CR)
343 instead, by adding
344
345 --enable-newline-is-cr
346
347 to the configure command. There is also a --enable-newline-is-lf
348 option, which explicitly specifies linefeed as the newline character.
349
350 Alternatively, you can specify that line endings are to be indicated by
351 the two character sequence CRLF. If you want this, add
352
353 --enable-newline-is-crlf
354
355 to the configure command. There is a fourth option, specified by
356
357 --enable-newline-is-anycrlf
358
359 which causes PCRE to recognize any of the three sequences CR, LF, or
360 CRLF as indicating a line ending. Finally, a fifth option, specified by
361
362 --enable-newline-is-any
363
364 causes PCRE to recognize any Unicode newline sequence.
365
366 Whatever line ending convention is selected when PCRE is built can be
367 overridden when the library functions are called. At build time it is
368 conventional to use the standard for your operating system.
369
370
371 WHAT \R MATCHES
372
373 By default, the sequence \R in a pattern matches any Unicode newline
374 sequence, whatever has been selected as the line ending sequence. If
375 you specify
376
377 --enable-bsr-anycrlf
378
379 the default is changed so that \R matches only CR, LF, or CRLF. What-
380 ever is selected when PCRE is built can be overridden when the library
381 functions are called.
382
383
384 BUILDING SHARED AND STATIC LIBRARIES
385
386 The PCRE building process uses libtool to build both shared and static
387 Unix libraries by default. You can suppress one of these by adding one
388 of
389
390 --disable-shared
391 --disable-static
392
393 to the configure command, as required.
394
395
396 POSIX MALLOC USAGE
397
398 When PCRE is called through the POSIX interface (see the pcreposix doc-
399 umentation), additional working storage is required for holding the
400 pointers to capturing substrings, because PCRE requires three integers
401 per substring, whereas the POSIX interface provides only two. If the
402 number of expected substrings is small, the wrapper function uses space
403 on the stack, because this is faster than using malloc() for each call.
404 The default threshold above which the stack is no longer used is 10; it
405 can be changed by adding a setting such as
406
407 --with-posix-malloc-threshold=20
408
409 to the configure command.
410
411
412 HANDLING VERY LARGE PATTERNS
413
414 Within a compiled pattern, offset values are used to point from one
415 part to another (for example, from an opening parenthesis to an alter-
416 nation metacharacter). By default, two-byte values are used for these
417 offsets, leading to a maximum size for a compiled pattern of around
418 64K. This is sufficient to handle all but the most gigantic patterns.
419 Nevertheless, some people do want to process enormous patterns, so it
420 is possible to compile PCRE to use three-byte or four-byte offsets by
421 adding a setting such as
422
423 --with-link-size=3
424
425 to the configure command. The value given must be 2, 3, or 4. Using
426 longer offsets slows down the operation of PCRE because it has to load
427 additional bytes when handling them.
428
429
430 AVOIDING EXCESSIVE STACK USAGE
431
432 When matching with the pcre_exec() function, PCRE implements backtrack-
433 ing by making recursive calls to an internal function called match().
434 In environments where the size of the stack is limited, this can se-
435 verely limit PCRE's operation. (The Unix environment does not usually
436 suffer from this problem, but it may sometimes be necessary to increase
437 the maximum stack size. There is a discussion in the pcrestack docu-
438 mentation.) An alternative approach to recursion that uses memory from
439 the heap to remember data, instead of using recursive function calls,
440 has been implemented to work round the problem of limited stack size.
441 If you want to build a version of PCRE that works this way, add
442
443 --disable-stack-for-recursion
444
445 to the configure command. With this configuration, PCRE will use the
446 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
447 ment functions. By default these point to malloc() and free(), but you
448 can replace the pointers so that your own functions are used.
449
450 Separate functions are provided rather than using pcre_malloc and
451 pcre_free because the usage is very predictable: the block sizes
452 requested are always the same, and the blocks are always freed in
453 reverse order. A calling program might be able to implement optimized
454 functions that perform better than malloc() and free(). PCRE runs
455 noticeably more slowly when built in this way. This option affects only
456 the pcre_exec() function; it is not relevant for the the
457 pcre_dfa_exec() function.
458
459
460 LIMITING PCRE RESOURCE USAGE
461
462 Internally, PCRE has a function called match(), which it calls repeat-
463 edly (sometimes recursively) when matching a pattern with the
464 pcre_exec() function. By controlling the maximum number of times this
465 function may be called during a single matching operation, a limit can
466 be placed on the resources used by a single call to pcre_exec(). The
467 limit can be changed at run time, as described in the pcreapi documen-
468 tation. The default is 10 million, but this can be changed by adding a
469 setting such as
470
471 --with-match-limit=500000
472
473 to the configure command. This setting has no effect on the
474 pcre_dfa_exec() matching function.
475
476 In some environments it is desirable to limit the depth of recursive
477 calls of match() more strictly than the total number of calls, in order
478 to restrict the maximum amount of stack (or heap, if --disable-stack-
479 for-recursion is specified) that is used. A second limit controls this;
480 it defaults to the value that is set for --with-match-limit, which
481 imposes no additional constraints. However, you can set a lower limit
482 by adding, for example,
483
484 --with-match-limit-recursion=10000
485
486 to the configure command. This value can also be overridden at run
487 time.
488
489
490 CREATING CHARACTER TABLES AT BUILD TIME
491
492 PCRE uses fixed tables for processing characters whose code values are
493 less than 256. By default, PCRE is built with a set of tables that are
494 distributed in the file pcre_chartables.c.dist. These tables are for
495 ASCII codes only. If you add
496
497 --enable-rebuild-chartables
498
499 to the configure command, the distributed tables are no longer used.
500 Instead, a program called dftables is compiled and run. This outputs
501 the source for new set of tables, created in the default locale of your
502 C runtime system. (This method of replacing the tables does not work if
503 you are cross compiling, because dftables is run on the local host. If
504 you need to create alternative tables when cross compiling, you will
505 have to do so "by hand".)
506
507
508 USING EBCDIC CODE
509
510 PCRE assumes by default that it will run in an environment where the
511 character code is ASCII (or Unicode, which is a superset of ASCII).
512 This is the case for most computer operating systems. PCRE can, how-
513 ever, be compiled to run in an EBCDIC environment by adding
514
515 --enable-ebcdic
516
517 to the configure command. This setting implies --enable-rebuild-charta-
518 bles. You should only use it if you know that you are in an EBCDIC
519 environment (for example, an IBM mainframe operating system).
520
521
522 PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
523
524 By default, pcregrep reads all files as plain text. You can build it so
525 that it recognizes files whose names end in .gz or .bz2, and reads them
526 with libz or libbz2, respectively, by adding one or both of
527
528 --enable-pcregrep-libz
529 --enable-pcregrep-libbz2
530
531 to the configure command. These options naturally require that the rel-
532 evant libraries are installed on your system. Configuration will fail
533 if they are not.
534
535
536 PCRETEST OPTION FOR LIBREADLINE SUPPORT
537
538 If you add
539
540 --enable-pcretest-libreadline
541
542 to the configure command, pcretest is linked with the libreadline
543 library, and when its input is from a terminal, it reads it using the
544 readline() function. This provides line-editing and history facilities.
545 Note that libreadline is GPL-licenced, so if you distribute a binary of
546 pcretest linked in this way, there may be licensing issues.
547
548 Setting this option causes the -lreadline option to be added to the
549 pcretest build. In many operating environments with a sytem-installed
550 libreadline this is sufficient. However, in some environments (e.g. if
551 an unmodified distribution version of readline is in use), some extra
552 configuration may be necessary. The INSTALL file for libreadline says
553 this:
554
555 "Readline uses the termcap functions, but does not link with the
556 termcap or curses library itself, allowing applications which link
557 with readline the to choose an appropriate library."
558
559 If your environment has not been set up so that an appropriate library
560 is automatically included, you may need to add something like
561
562 LIBS="-ncurses"
563
564 immediately before the configure command.
565
566
567 SEE ALSO
568
569 pcreapi(3), pcre_config(3).
570
571
572 AUTHOR
573
574 Philip Hazel
575 University Computing Service
576 Cambridge CB2 3QH, England.
577
578
579 REVISION
580
581 Last updated: 13 April 2008
582 Copyright (c) 1997-2008 University of Cambridge.
583 ------------------------------------------------------------------------------
584
585
586 PCREMATCHING(3) PCREMATCHING(3)
587
588
589 NAME
590 PCRE - Perl-compatible regular expressions
591
592
593 PCRE MATCHING ALGORITHMS
594
595 This document describes the two different algorithms that are available
596 in PCRE for matching a compiled regular expression against a given sub-
597 ject string. The "standard" algorithm is the one provided by the
598 pcre_exec() function. This works in the same was as Perl's matching
599 function, and provides a Perl-compatible matching operation.
600
601 An alternative algorithm is provided by the pcre_dfa_exec() function;
602 this operates in a different way, and is not Perl-compatible. It has
603 advantages and disadvantages compared with the standard algorithm, and
604 these are described below.
605
606 When there is only one possible way in which a given subject string can
607 match a pattern, the two algorithms give the same answer. A difference
608 arises, however, when there are multiple possibilities. For example, if
609 the pattern
610
611 ^<.*>
612
613 is matched against the string
614
615 <something> <something else> <something further>
616
617 there are three possible answers. The standard algorithm finds only one
618 of them, whereas the alternative algorithm finds all three.
619
620
621 REGULAR EXPRESSIONS AS TREES
622
623 The set of strings that are matched by a regular expression can be rep-
624 resented as a tree structure. An unlimited repetition in the pattern
625 makes the tree of infinite size, but it is still a tree. Matching the
626 pattern to a given subject string (from a given starting point) can be
627 thought of as a search of the tree. There are two ways to search a
628 tree: depth-first and breadth-first, and these correspond to the two
629 matching algorithms provided by PCRE.
630
631
632 THE STANDARD MATCHING ALGORITHM
633
634 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
635 sions", the standard algorithm is an "NFA algorithm". It conducts a
636 depth-first search of the pattern tree. That is, it proceeds along a
637 single path through the tree, checking that the subject matches what is
638 required. When there is a mismatch, the algorithm tries any alterna-
639 tives at the current point, and if they all fail, it backs up to the
640 previous branch point in the tree, and tries the next alternative
641 branch at that level. This often involves backing up (moving to the
642 left) in the subject string as well. The order in which repetition
643 branches are tried is controlled by the greedy or ungreedy nature of
644 the quantifier.
645
646 If a leaf node is reached, a matching string has been found, and at
647 that point the algorithm stops. Thus, if there is more than one possi-
648 ble match, this algorithm returns the first one that it finds. Whether
649 this is the shortest, the longest, or some intermediate length depends
650 on the way the greedy and ungreedy repetition quantifiers are specified
651 in the pattern.
652
653 Because it ends up with a single path through the tree, it is rela-
654 tively straightforward for this algorithm to keep track of the sub-
655 strings that are matched by portions of the pattern in parentheses.
656 This provides support for capturing parentheses and back references.
657
658
659 THE ALTERNATIVE MATCHING ALGORITHM
660
661 This algorithm conducts a breadth-first search of the tree. Starting
662 from the first matching point in the subject, it scans the subject
663 string from left to right, once, character by character, and as it does
664 this, it remembers all the paths through the tree that represent valid
665 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
666 though it is not implemented as a traditional finite state machine (it
667 keeps multiple states active simultaneously).
668
669 The scan continues until either the end of the subject is reached, or
670 there are no more unterminated paths. At this point, terminated paths
671 represent the different matching possibilities (if there are none, the
672 match has failed). Thus, if there is more than one possible match,
673 this algorithm finds all of them, and in particular, it finds the long-
674 est. In PCRE, there is an option to stop the algorithm after the first
675 match (which is necessarily the shortest) has been found.
676
677 Note that all the matches that are found start at the same point in the
678 subject. If the pattern
679
680 cat(er(pillar)?)
681
682 is matched against the string "the caterpillar catchment", the result
683 will be the three strings "cat", "cater", and "caterpillar" that start
684 at the fourth character of the subject. The algorithm does not automat-
685 ically move on to find matches that start at later positions.
686
687 There are a number of features of PCRE regular expressions that are not
688 supported by the alternative matching algorithm. They are as follows:
689
690 1. Because the algorithm finds all possible matches, the greedy or
691 ungreedy nature of repetition quantifiers is not relevant. Greedy and
692 ungreedy quantifiers are treated in exactly the same way. However, pos-
693 sessive quantifiers can make a difference when what follows could also
694 match what is quantified, for example in a pattern like this:
695
696 ^a++\w!
697
698 This pattern matches "aaab!" but not "aaa!", which would be matched by
699 a non-possessive quantifier. Similarly, if an atomic group is present,
700 it is matched as if it were a standalone pattern at the current point,
701 and the longest match is then "locked in" for the rest of the overall
702 pattern.
703
704 2. When dealing with multiple paths through the tree simultaneously, it
705 is not straightforward to keep track of captured substrings for the
706 different matching possibilities, and PCRE's implementation of this
707 algorithm does not attempt to do this. This means that no captured sub-
708 strings are available.
709
710 3. Because no substrings are captured, back references within the pat-
711 tern are not supported, and cause errors if encountered.
712
713 4. For the same reason, conditional expressions that use a backrefer-
714 ence as the condition or test for a specific group recursion are not
715 supported.
716
717 5. Because many paths through the tree may be active, the \K escape
718 sequence, which resets the start of the match when encountered (but may
719 be on some paths and not on others), is not supported. It causes an
720 error if encountered.
721
722 6. Callouts are supported, but the value of the capture_top field is
723 always 1, and the value of the capture_last field is always -1.
724
725 7. The \C escape sequence, which (in the standard algorithm) matches a
726 single byte, even in UTF-8 mode, is not supported because the alterna-
727 tive algorithm moves through the subject string one character at a
728 time, for all active paths through the tree.
729
730 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
731 are not supported. (*FAIL) is supported, and behaves like a failing
732 negative assertion.
733
734
735 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
736
737 Using the alternative matching algorithm provides the following advan-
738 tages:
739
740 1. All possible matches (at a single point in the subject) are automat-
741 ically found, and in particular, the longest match is found. To find
742 more than one match using the standard algorithm, you have to do kludgy
743 things with callouts.
744
745 2. There is much better support for partial matching. The restrictions
746 on the content of the pattern that apply when using the standard algo-
747 rithm for partial matching do not apply to the alternative algorithm.
748 For non-anchored patterns, the starting position of a partial match is
749 available.
750
751 3. Because the alternative algorithm scans the subject string just
752 once, and never needs to backtrack, it is possible to pass very long
753 subject strings to the matching function in several pieces, checking
754 for partial matching each time.
755
756
757 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
758
759 The alternative algorithm suffers from a number of disadvantages:
760
761 1. It is substantially slower than the standard algorithm. This is
762 partly because it has to search for all possible matches, but is also
763 because it is less susceptible to optimization.
764
765 2. Capturing parentheses and back references are not supported.
766
767 3. Although atomic groups are supported, their use does not provide the
768 performance advantage that it does for the standard algorithm.
769
770
771 AUTHOR
772
773 Philip Hazel
774 University Computing Service
775 Cambridge CB2 3QH, England.
776
777
778 REVISION
779
780 Last updated: 19 April 2008
781 Copyright (c) 1997-2008 University of Cambridge.
782 ------------------------------------------------------------------------------
783
784
785 PCREAPI(3) PCREAPI(3)
786
787
788 NAME
789 PCRE - Perl-compatible regular expressions
790
791
792 PCRE NATIVE API
793
794 #include <pcre.h>
795
796 pcre *pcre_compile(const char *pattern, int options,
797 const char **errptr, int *erroffset,
798 const unsigned char *tableptr);
799
800 pcre *pcre_compile2(const char *pattern, int options,
801 int *errorcodeptr,
802 const char **errptr, int *erroffset,
803 const unsigned char *tableptr);
804
805 pcre_extra *pcre_study(const pcre *code, int options,
806 const char **errptr);
807
808 int pcre_exec(const pcre *code, const pcre_extra *extra,
809 const char *subject, int length, int startoffset,
810 int options, int *ovector, int ovecsize);
811
812 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
813 const char *subject, int length, int startoffset,
814 int options, int *ovector, int ovecsize,
815 int *workspace, int wscount);
816
817 int pcre_copy_named_substring(const pcre *code,
818 const char *subject, int *ovector,
819 int stringcount, const char *stringname,
820 char *buffer, int buffersize);
821
822 int pcre_copy_substring(const char *subject, int *ovector,
823 int stringcount, int stringnumber, char *buffer,
824 int buffersize);
825
826 int pcre_get_named_substring(const pcre *code,
827 const char *subject, int *ovector,
828 int stringcount, const char *stringname,
829 const char **stringptr);
830
831 int pcre_get_stringnumber(const pcre *code,
832 const char *name);
833
834 int pcre_get_stringtable_entries(const pcre *code,
835 const char *name, char **first, char **last);
836
837 int pcre_get_substring(const char *subject, int *ovector,
838 int stringcount, int stringnumber,
839 const char **stringptr);
840
841 int pcre_get_substring_list(const char *subject,
842 int *ovector, int stringcount, const char ***listptr);
843
844 void pcre_free_substring(const char *stringptr);
845
846 void pcre_free_substring_list(const char **stringptr);
847
848 const unsigned char *pcre_maketables(void);
849
850 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
851 int what, void *where);
852
853 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
854
855 int pcre_refcount(pcre *code, int adjust);
856
857 int pcre_config(int what, void *where);
858
859 char *pcre_version(void);
860
861 void *(*pcre_malloc)(size_t);
862
863 void (*pcre_free)(void *);
864
865 void *(*pcre_stack_malloc)(size_t);
866
867 void (*pcre_stack_free)(void *);
868
869 int (*pcre_callout)(pcre_callout_block *);
870
871
872 PCRE API OVERVIEW
873
874 PCRE has its own native API, which is described in this document. There
875 are also some wrapper functions that correspond to the POSIX regular
876 expression API. These are described in the pcreposix documentation.
877 Both of these APIs define a set of C function calls. A C++ wrapper is
878 distributed with PCRE. It is documented in the pcrecpp page.
879
880 The native API C function prototypes are defined in the header file
881 pcre.h, and on Unix systems the library itself is called libpcre. It
882 can normally be accessed by adding -lpcre to the command for linking an
883 application that uses PCRE. The header file defines the macros
884 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
885 bers for the library. Applications can use these to include support
886 for different releases of PCRE.
887
888 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
889 pcre_exec() are used for compiling and matching regular expressions in
890 a Perl-compatible manner. A sample program that demonstrates the sim-
891 plest way of using them is provided in the file called pcredemo.c in
892 the source distribution. The pcresample documentation describes how to
893 compile and run it.
894
895 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
896 ble, is also provided. This uses a different algorithm for the match-
897 ing. The alternative algorithm finds all possible matches (at a given
898 point in the subject), and scans the subject just once. However, this
899 algorithm does not return captured substrings. A description of the two
900 matching algorithms and their advantages and disadvantages is given in
901 the pcrematching documentation.
902
903 In addition to the main compiling and matching functions, there are
904 convenience functions for extracting captured substrings from a subject
905 string that is matched by pcre_exec(). They are:
906
907 pcre_copy_substring()
908 pcre_copy_named_substring()
909 pcre_get_substring()
910 pcre_get_named_substring()
911 pcre_get_substring_list()
912 pcre_get_stringnumber()
913 pcre_get_stringtable_entries()
914
915 pcre_free_substring() and pcre_free_substring_list() are also provided,
916 to free the memory used for extracted strings.
917
918 The function pcre_maketables() is used to build a set of character
919 tables in the current locale for passing to pcre_compile(),
920 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
921 provided for specialist use. Most commonly, no special tables are
922 passed, in which case internal tables that are generated when PCRE is
923 built are used.
924
925 The function pcre_fullinfo() is used to find out information about a
926 compiled pattern; pcre_info() is an obsolete version that returns only
927 some of the available information, but is retained for backwards com-
928 patibility. The function pcre_version() returns a pointer to a string
929 containing the version of PCRE and its date of release.
930
931 The function pcre_refcount() maintains a reference count in a data
932 block containing a compiled pattern. This is provided for the benefit
933 of object-oriented applications.
934
935 The global variables pcre_malloc and pcre_free initially contain the
936 entry points of the standard malloc() and free() functions, respec-
937 tively. PCRE calls the memory management functions via these variables,
938 so a calling program can replace them if it wishes to intercept the
939 calls. This should be done before calling any PCRE functions.
940
941 The global variables pcre_stack_malloc and pcre_stack_free are also
942 indirections to memory management functions. These special functions
943 are used only when PCRE is compiled to use the heap for remembering
944 data, instead of recursive function calls, when running the pcre_exec()
945 function. See the pcrebuild documentation for details of how to do
946 this. It is a non-standard way of building PCRE, for use in environ-
947 ments that have limited stacks. Because of the greater use of memory
948 management, it runs more slowly. Separate functions are provided so
949 that special-purpose external code can be used for this case. When
950 used, these functions are always called in a stack-like manner (last
951 obtained, first freed), and always for memory blocks of the same size.
952 There is a discussion about PCRE's stack usage in the pcrestack docu-
953 mentation.
954
955 The global variable pcre_callout initially contains NULL. It can be set
956 by the caller to a "callout" function, which PCRE will then call at
957 specified points during a matching operation. Details are given in the
958 pcrecallout documentation.
959
960
961 NEWLINES
962
963 PCRE supports five different conventions for indicating line breaks in
964 strings: a single CR (carriage return) character, a single LF (line-
965 feed) character, the two-character sequence CRLF, any of the three pre-
966 ceding, or any Unicode newline sequence. The Unicode newline sequences
967 are the three just mentioned, plus the single characters VT (vertical
968 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
969 separator, U+2028), and PS (paragraph separator, U+2029).
970
971 Each of the first three conventions is used by at least one operating
972 system as its standard newline sequence. When PCRE is built, a default
973 can be specified. The default default is LF, which is the Unix stan-
974 dard. When PCRE is run, the default can be overridden, either when a
975 pattern is compiled, or when it is matched.
976
977 At compile time, the newline convention can be specified by the options
978 argument of pcre_compile(), or it can be specified by special text at
979 the start of the pattern itself; this overrides any other settings. See
980 the pcrepattern page for details of the special character sequences.
981
982 In the PCRE documentation the word "newline" is used to mean "the char-
983 acter or pair of characters that indicate a line break". The choice of
984 newline convention affects the handling of the dot, circumflex, and
985 dollar metacharacters, the handling of #-comments in /x mode, and, when
986 CRLF is a recognized line ending sequence, the match position advance-
987 ment for a non-anchored pattern. There is more detail about this in the
988 section on pcre_exec() options below.
989
990 The choice of newline convention does not affect the interpretation of
991 the \n or \r escape sequences, nor does it affect what \R matches,
992 which is controlled in a similar way, but by separate options.
993
994
995 MULTITHREADING
996
997 The PCRE functions can be used in multi-threading applications, with
998 the proviso that the memory management functions pointed to by
999 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
1000 callout function pointed to by pcre_callout, are shared by all threads.
1001
1002 The compiled form of a regular expression is not altered during match-
1003 ing, so the same compiled pattern can safely be used by several threads
1004 at once.
1005
1006
1007 SAVING PRECOMPILED PATTERNS FOR LATER USE
1008
1009 The compiled form of a regular expression can be saved and re-used at a
1010 later time, possibly by a different program, and even on a host other
1011 than the one on which it was compiled. Details are given in the
1012 pcreprecompile documentation. However, compiling a regular expression
1013 with one version of PCRE for use with a different version is not guar-
1014 anteed to work and may cause crashes.
1015
1016
1017 CHECKING BUILD-TIME OPTIONS
1018
1019 int pcre_config(int what, void *where);
1020
1021 The function pcre_config() makes it possible for a PCRE client to dis-
1022 cover which optional features have been compiled into the PCRE library.
1023 The pcrebuild documentation has more details about these optional fea-
1024 tures.
1025
1026 The first argument for pcre_config() is an integer, specifying which
1027 information is required; the second argument is a pointer to a variable
1028 into which the information is placed. The following information is
1029 available:
1030
1031 PCRE_CONFIG_UTF8
1032
1033 The output is an integer that is set to one if UTF-8 support is avail-
1034 able; otherwise it is set to zero.
1035
1036 PCRE_CONFIG_UNICODE_PROPERTIES
1037
1038 The output is an integer that is set to one if support for Unicode
1039 character properties is available; otherwise it is set to zero.
1040
1041 PCRE_CONFIG_NEWLINE
1042
1043 The output is an integer whose value specifies the default character
1044 sequence that is recognized as meaning "newline". The four values that
1045 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1046 and -1 for ANY. The default should normally be the standard sequence
1047 for your operating system.
1048
1049 PCRE_CONFIG_BSR
1050
1051 The output is an integer whose value indicates what character sequences
1052 the \R escape sequence matches by default. A value of 0 means that \R
1053 matches any Unicode line ending sequence; a value of 1 means that \R
1054 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1055 tern is compiled or matched.
1056
1057 PCRE_CONFIG_LINK_SIZE
1058
1059 The output is an integer that contains the number of bytes used for
1060 internal linkage in compiled regular expressions. The value is 2, 3, or
1061 4. Larger values allow larger regular expressions to be compiled, at
1062 the expense of slower matching. The default value of 2 is sufficient
1063 for all but the most massive patterns, since it allows the compiled
1064 pattern to be up to 64K in size.
1065
1066 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1067
1068 The output is an integer that contains the threshold above which the
1069 POSIX interface uses malloc() for output vectors. Further details are
1070 given in the pcreposix documentation.
1071
1072 PCRE_CONFIG_MATCH_LIMIT
1073
1074 The output is an integer that gives the default limit for the number of
1075 internal matching function calls in a pcre_exec() execution. Further
1076 details are given with pcre_exec() below.
1077
1078 PCRE_CONFIG_MATCH_LIMIT_RECURSION
1079
1080 The output is an integer that gives the default limit for the depth of
1081 recursion when calling the internal matching function in a pcre_exec()
1082 execution. Further details are given with pcre_exec() below.
1083
1084 PCRE_CONFIG_STACKRECURSE
1085
1086 The output is an integer that is set to one if internal recursion when
1087 running pcre_exec() is implemented by recursive function calls that use
1088 the stack to remember their state. This is the usual way that PCRE is
1089 compiled. The output is zero if PCRE was compiled to use blocks of data
1090 on the heap instead of recursive function calls. In this case,
1091 pcre_stack_malloc and pcre_stack_free are called to manage memory
1092 blocks on the heap, thus avoiding the use of the stack.
1093
1094
1095 COMPILING A PATTERN
1096
1097 pcre *pcre_compile(const char *pattern, int options,
1098 const char **errptr, int *erroffset,
1099 const unsigned char *tableptr);
1100
1101 pcre *pcre_compile2(const char *pattern, int options,
1102 int *errorcodeptr,
1103 const char **errptr, int *erroffset,
1104 const unsigned char *tableptr);
1105
1106 Either of the functions pcre_compile() or pcre_compile2() can be called
1107 to compile a pattern into an internal form. The only difference between
1108 the two interfaces is that pcre_compile2() has an additional argument,
1109 errorcodeptr, via which a numerical error code can be returned.
1110
1111 The pattern is a C string terminated by a binary zero, and is passed in
1112 the pattern argument. A pointer to a single block of memory that is
1113 obtained via pcre_malloc is returned. This contains the compiled code
1114 and related data. The pcre type is defined for the returned block; this
1115 is a typedef for a structure whose contents are not externally defined.
1116 It is up to the caller to free the memory (via pcre_free) when it is no
1117 longer required.
1118
1119 Although the compiled code of a PCRE regex is relocatable, that is, it
1120 does not depend on memory location, the complete pcre data block is not
1121 fully relocatable, because it may contain a copy of the tableptr argu-
1122 ment, which is an address (see below).
1123
1124 The options argument contains various bit settings that affect the com-
1125 pilation. It should be zero if no options are required. The available
1126 options are described below. Some of them, in particular, those that
1127 are compatible with Perl, can also be set and unset from within the
1128 pattern (see the detailed description in the pcrepattern documenta-
1129 tion). For these options, the contents of the options argument speci-
1130 fies their initial settings at the start of compilation and execution.
1131 The PCRE_ANCHORED and PCRE_NEWLINE_xxx options can be set at the time
1132 of matching as well as at compile time.
1133
1134 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1135 if compilation of a pattern fails, pcre_compile() returns NULL, and
1136 sets the variable pointed to by errptr to point to a textual error mes-
1137 sage. This is a static string that is part of the library. You must not
1138 try to free it. The offset from the start of the pattern to the charac-
1139 ter where the error was discovered is placed in the variable pointed to
1140 by erroffset, which must not be NULL. If it is, an immediate error is
1141 given.
1142
1143 If pcre_compile2() is used instead of pcre_compile(), and the error-
1144 codeptr argument is not NULL, a non-zero error code number is returned
1145 via this argument in the event of an error. This is in addition to the
1146 textual error message. Error codes and messages are listed below.
1147
1148 If the final argument, tableptr, is NULL, PCRE uses a default set of
1149 character tables that are built when PCRE is compiled, using the
1150 default C locale. Otherwise, tableptr must be an address that is the
1151 result of a call to pcre_maketables(). This value is stored with the
1152 compiled pattern, and used again by pcre_exec(), unless another table
1153 pointer is passed to it. For more discussion, see the section on locale
1154 support below.
1155
1156 This code fragment shows a typical straightforward call to pcre_com-
1157 pile():
1158
1159 pcre *re;
1160 const char *error;
1161 int erroffset;
1162 re = pcre_compile(
1163 "^A.*Z", /* the pattern */
1164 0, /* default options */
1165 &error, /* for error message */
1166 &erroffset, /* for error offset */
1167 NULL); /* use default character tables */
1168
1169 The following names for option bits are defined in the pcre.h header
1170 file:
1171
1172 PCRE_ANCHORED
1173
1174 If this bit is set, the pattern is forced to be "anchored", that is, it
1175 is constrained to match only at the first matching point in the string
1176 that is being searched (the "subject string"). This effect can also be
1177 achieved by appropriate constructs in the pattern itself, which is the
1178 only way to do it in Perl.
1179
1180 PCRE_AUTO_CALLOUT
1181
1182 If this bit is set, pcre_compile() automatically inserts callout items,
1183 all with number 255, before each pattern item. For discussion of the
1184 callout facility, see the pcrecallout documentation.
1185
1186 PCRE_BSR_ANYCRLF
1187 PCRE_BSR_UNICODE
1188
1189 These options (which are mutually exclusive) control what the \R escape
1190 sequence matches. The choice is either to match only CR, LF, or CRLF,
1191 or to match any Unicode newline sequence. The default is specified when
1192 PCRE is built. It can be overridden from within the pattern, or by set-
1193 ting an option when a compiled pattern is matched.
1194
1195 PCRE_CASELESS
1196
1197 If this bit is set, letters in the pattern match both upper and lower
1198 case letters. It is equivalent to Perl's /i option, and it can be
1199 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1200 always understands the concept of case for characters whose values are
1201 less than 128, so caseless matching is always possible. For characters
1202 with higher values, the concept of case is supported if PCRE is com-
1203 piled with Unicode property support, but not otherwise. If you want to
1204 use caseless matching for characters 128 and above, you must ensure
1205 that PCRE is compiled with Unicode property support as well as with
1206 UTF-8 support.
1207
1208 PCRE_DOLLAR_ENDONLY
1209
1210 If this bit is set, a dollar metacharacter in the pattern matches only
1211 at the end of the subject string. Without this option, a dollar also
1212 matches immediately before a newline at the end of the string (but not
1213 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1214 if PCRE_MULTILINE is set. There is no equivalent to this option in
1215 Perl, and no way to set it within a pattern.
1216
1217 PCRE_DOTALL
1218
1219 If this bit is set, a dot metacharater in the pattern matches all char-
1220 acters, including those that indicate newline. Without it, a dot does
1221 not match when the current position is at a newline. This option is
1222 equivalent to Perl's /s option, and it can be changed within a pattern
1223 by a (?s) option setting. A negative class such as [^a] always matches
1224 newline characters, independent of the setting of this option.
1225
1226 PCRE_DUPNAMES
1227
1228 If this bit is set, names used to identify capturing subpatterns need
1229 not be unique. This can be helpful for certain types of pattern when it
1230 is known that only one instance of the named subpattern can ever be
1231 matched. There are more details of named subpatterns below; see also
1232 the pcrepattern documentation.
1233
1234 PCRE_EXTENDED
1235
1236 If this bit is set, whitespace data characters in the pattern are
1237 totally ignored except when escaped or inside a character class. White-
1238 space does not include the VT character (code 11). In addition, charac-
1239 ters between an unescaped # outside a character class and the next new-
1240 line, inclusive, are also ignored. This is equivalent to Perl's /x
1241 option, and it can be changed within a pattern by a (?x) option set-
1242 ting.
1243
1244 This option makes it possible to include comments inside complicated
1245 patterns. Note, however, that this applies only to data characters.
1246 Whitespace characters may never appear within special character
1247 sequences in a pattern, for example within the sequence (?( which
1248 introduces a conditional subpattern.
1249
1250 PCRE_EXTRA
1251
1252 This option was invented in order to turn on additional functionality
1253 of PCRE that is incompatible with Perl, but it is currently of very
1254 little use. When set, any backslash in a pattern that is followed by a
1255 letter that has no special meaning causes an error, thus reserving
1256 these combinations for future expansion. By default, as in Perl, a
1257 backslash followed by a letter with no special meaning is treated as a
1258 literal. (Perl can, however, be persuaded to give a warning for this.)
1259 There are at present no other features controlled by this option. It
1260 can also be set by a (?X) option setting within a pattern.
1261
1262 PCRE_FIRSTLINE
1263
1264 If this option is set, an unanchored pattern is required to match
1265 before or at the first newline in the subject string, though the
1266 matched text may continue over the newline.
1267
1268 PCRE_JAVASCRIPT_COMPAT
1269
1270 If this option is set, PCRE's behaviour is changed in some ways so that
1271 it is compatible with JavaScript rather than Perl. The changes are as
1272 follows:
1273
1274 (1) A lone closing square bracket in a pattern causes a compile-time
1275 error, because this is illegal in JavaScript (by default it is treated
1276 as a data character). Thus, the pattern AB]CD becomes illegal when this
1277 option is set.
1278
1279 (2) At run time, a back reference to an unset subpattern group matches
1280 an empty string (by default this causes the current matching alterna-
1281 tive to fail). A pattern such as (\1)(a) succeeds when this option is
1282 set (assuming it can find an "a" in the subject), whereas it fails by
1283 default, for Perl compatibility.
1284
1285 PCRE_MULTILINE
1286
1287 By default, PCRE treats the subject string as consisting of a single
1288 line of characters (even if it actually contains newlines). The "start
1289 of line" metacharacter (^) matches only at the start of the string,
1290 while the "end of line" metacharacter ($) matches only at the end of
1291 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1292 is set). This is the same as Perl.
1293
1294 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1295 constructs match immediately following or immediately before internal
1296 newlines in the subject string, respectively, as well as at the very
1297 start and end. This is equivalent to Perl's /m option, and it can be
1298 changed within a pattern by a (?m) option setting. If there are no new-
1299 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1300 setting PCRE_MULTILINE has no effect.
1301
1302 PCRE_NEWLINE_CR
1303 PCRE_NEWLINE_LF
1304 PCRE_NEWLINE_CRLF
1305 PCRE_NEWLINE_ANYCRLF
1306 PCRE_NEWLINE_ANY
1307
1308 These options override the default newline definition that was chosen
1309 when PCRE was built. Setting the first or the second specifies that a
1310 newline is indicated by a single character (CR or LF, respectively).
1311 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1312 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1313 that any of the three preceding sequences should be recognized. Setting
1314 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1315 recognized. The Unicode newline sequences are the three just mentioned,
1316 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1317 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1318 (paragraph separator, U+2029). The last two are recognized only in
1319 UTF-8 mode.
1320
1321 The newline setting in the options word uses three bits that are
1322 treated as a number, giving eight possibilities. Currently only six are
1323 used (default plus the five values above). This means that if you set
1324 more than one newline option, the combination may or may not be sensi-
1325 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1326 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1327 cause an error.
1328
1329 The only time that a line break is specially recognized when compiling
1330 a pattern is if PCRE_EXTENDED is set, and an unescaped # outside a
1331 character class is encountered. This indicates a comment that lasts
1332 until after the next line break sequence. In other circumstances, line
1333 break sequences are treated as literal data, except that in
1334 PCRE_EXTENDED mode, both CR and LF are treated as whitespace characters
1335 and are therefore ignored.
1336
1337 The newline option that is set at compile time becomes the default that
1338 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1339
1340 PCRE_NO_AUTO_CAPTURE
1341
1342 If this option is set, it disables the use of numbered capturing paren-
1343 theses in the pattern. Any opening parenthesis that is not followed by
1344 ? behaves as if it were followed by ?: but named parentheses can still
1345 be used for capturing (and they acquire numbers in the usual way).
1346 There is no equivalent of this option in Perl.
1347
1348 PCRE_UNGREEDY
1349
1350 This option inverts the "greediness" of the quantifiers so that they
1351 are not greedy by default, but become greedy if followed by "?". It is
1352 not compatible with Perl. It can also be set by a (?U) option setting
1353 within the pattern.
1354
1355 PCRE_UTF8
1356
1357 This option causes PCRE to regard both the pattern and the subject as
1358 strings of UTF-8 characters instead of single-byte character strings.
1359 However, it is available only when PCRE is built to include UTF-8 sup-
1360 port. If not, the use of this option provokes an error. Details of how
1361 this option changes the behaviour of PCRE are given in the section on
1362 UTF-8 support in the main pcre page.
1363
1364 PCRE_NO_UTF8_CHECK
1365
1366 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1367 automatically checked. There is a discussion about the validity of
1368 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1369 bytes is found, pcre_compile() returns an error. If you already know
1370 that your pattern is valid, and you want to skip this check for perfor-
1371 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1372 set, the effect of passing an invalid UTF-8 string as a pattern is
1373 undefined. It may cause your program to crash. Note that this option
1374 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1375 UTF-8 validity checking of subject strings.
1376
1377
1378 COMPILATION ERROR CODES
1379
1380 The following table lists the error codes than may be returned by
1381 pcre_compile2(), along with the error messages that may be returned by
1382 both compiling functions. As PCRE has developed, some error codes have
1383 fallen out of use. To avoid confusion, they have not been re-used.
1384
1385 0 no error
1386 1 \ at end of pattern
1387 2 \c at end of pattern
1388 3 unrecognized character follows \
1389 4 numbers out of order in {} quantifier
1390 5 number too big in {} quantifier
1391 6 missing terminating ] for character class
1392 7 invalid escape sequence in character class
1393 8 range out of order in character class
1394 9 nothing to repeat
1395 10 [this code is not in use]
1396 11 internal error: unexpected repeat
1397 12 unrecognized character after (? or (?-
1398 13 POSIX named classes are supported only within a class
1399 14 missing )
1400 15 reference to non-existent subpattern
1401 16 erroffset passed as NULL
1402 17 unknown option bit(s) set
1403 18 missing ) after comment
1404 19 [this code is not in use]
1405 20 regular expression is too large
1406 21 failed to get memory
1407 22 unmatched parentheses
1408 23 internal error: code overflow
1409 24 unrecognized character after (?<
1410 25 lookbehind assertion is not fixed length
1411 26 malformed number or name after (?(
1412 27 conditional group contains more than two branches
1413 28 assertion expected after (?(
1414 29 (?R or (?[+-]digits must be followed by )
1415 30 unknown POSIX class name
1416 31 POSIX collating elements are not supported
1417 32 this version of PCRE is not compiled with PCRE_UTF8 support
1418 33 [this code is not in use]
1419 34 character value in \x{...} sequence is too large
1420 35 invalid condition (?(0)
1421 36 \C not allowed in lookbehind assertion
1422 37 PCRE does not support \L, \l, \N, \U, or \u
1423 38 number after (?C is > 255
1424 39 closing ) for (?C expected
1425 40 recursive call could loop indefinitely
1426 41 unrecognized character after (?P
1427 42 syntax error in subpattern name (missing terminator)
1428 43 two named subpatterns have the same name
1429 44 invalid UTF-8 string
1430 45 support for \P, \p, and \X has not been compiled
1431 46 malformed \P or \p sequence
1432 47 unknown property name after \P or \p
1433 48 subpattern name is too long (maximum 32 characters)
1434 49 too many named subpatterns (maximum 10000)
1435 50 [this code is not in use]
1436 51 octal value is greater than \377 (not in UTF-8 mode)
1437 52 internal error: overran compiling workspace
1438 53 internal error: previously-checked referenced subpattern not
1439 found
1440 54 DEFINE group contains more than one branch
1441 55 repeating a DEFINE group is not allowed
1442 56 inconsistent NEWLINE options
1443 57 \g is not followed by a braced, angle-bracketed, or quoted
1444 name/number or by a plain number
1445 58 a numbered reference must not be zero
1446 59 (*VERB) with an argument is not supported
1447 60 (*VERB) not recognized
1448 61 number is too big
1449 62 subpattern name expected
1450 63 digit expected after (?+
1451 64 ] is an invalid data character in JavaScript compatibility mode
1452
1453 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1454 values may be used if the limits were changed when PCRE was built.
1455
1456
1457 STUDYING A PATTERN
1458
1459 pcre_extra *pcre_study(const pcre *code, int options
1460 const char **errptr);
1461
1462 If a compiled pattern is going to be used several times, it is worth
1463 spending more time analyzing it in order to speed up the time taken for
1464 matching. The function pcre_study() takes a pointer to a compiled pat-
1465 tern as its first argument. If studying the pattern produces additional
1466 information that will help speed up matching, pcre_study() returns a
1467 pointer to a pcre_extra block, in which the study_data field points to
1468 the results of the study.
1469
1470 The returned value from pcre_study() can be passed directly to
1471 pcre_exec(). However, a pcre_extra block also contains other fields
1472 that can be set by the caller before the block is passed; these are
1473 described below in the section on matching a pattern.
1474
1475 If studying the pattern does not produce any additional information
1476 pcre_study() returns NULL. In that circumstance, if the calling program
1477 wants to pass any of the other fields to pcre_exec(), it must set up
1478 its own pcre_extra block.
1479
1480 The second argument of pcre_study() contains option bits. At present,
1481 no options are defined, and this argument should always be zero.
1482
1483 The third argument for pcre_study() is a pointer for an error message.
1484 If studying succeeds (even if no data is returned), the variable it
1485 points to is set to NULL. Otherwise it is set to point to a textual
1486 error message. This is a static string that is part of the library. You
1487 must not try to free it. You should test the error pointer for NULL
1488 after calling pcre_study(), to be sure that it has run successfully.
1489
1490 This is a typical call to pcre_study():
1491
1492 pcre_extra *pe;
1493 pe = pcre_study(
1494 re, /* result of pcre_compile() */
1495 0, /* no options exist */
1496 &error); /* set to NULL or points to a message */
1497
1498 At present, studying a pattern is useful only for non-anchored patterns
1499 that do not have a single fixed starting character. A bitmap of possi-
1500 ble starting bytes is created.
1501
1502
1503 LOCALE SUPPORT
1504
1505 PCRE handles caseless matching, and determines whether characters are
1506 letters, digits, or whatever, by reference to a set of tables, indexed
1507 by character value. When running in UTF-8 mode, this applies only to
1508 characters with codes less than 128. Higher-valued codes never match
1509 escapes such as \w or \d, but can be tested with \p if PCRE is built
1510 with Unicode character property support. The use of locales with Uni-
1511 code is discouraged. If you are handling characters with codes greater
1512 than 128, you should either use UTF-8 and Unicode, or use locales, but
1513 not try to mix the two.
1514
1515 PCRE contains an internal set of tables that are used when the final
1516 argument of pcre_compile() is NULL. These are sufficient for many
1517 applications. Normally, the internal tables recognize only ASCII char-
1518 acters. However, when PCRE is built, it is possible to cause the inter-
1519 nal tables to be rebuilt in the default "C" locale of the local system,
1520 which may cause them to be different.
1521
1522 The internal tables can always be overridden by tables supplied by the
1523 application that calls PCRE. These may be created in a different locale
1524 from the default. As more and more applications change to using Uni-
1525 code, the need for this locale support is expected to die away.
1526
1527 External tables are built by calling the pcre_maketables() function,
1528 which has no arguments, in the relevant locale. The result can then be
1529 passed to pcre_compile() or pcre_exec() as often as necessary. For
1530 example, to build and use tables that are appropriate for the French
1531 locale (where accented characters with values greater than 128 are
1532 treated as letters), the following code could be used:
1533
1534 setlocale(LC_CTYPE, "fr_FR");
1535 tables = pcre_maketables();
1536 re = pcre_compile(..., tables);
1537
1538 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1539 if you are using Windows, the name for the French locale is "french".
1540
1541 When pcre_maketables() runs, the tables are built in memory that is
1542 obtained via pcre_malloc. It is the caller's responsibility to ensure
1543 that the memory containing the tables remains available for as long as
1544 it is needed.
1545
1546 The pointer that is passed to pcre_compile() is saved with the compiled
1547 pattern, and the same tables are used via this pointer by pcre_study()
1548 and normally also by pcre_exec(). Thus, by default, for any single pat-
1549 tern, compilation, studying and matching all happen in the same locale,
1550 but different patterns can be compiled in different locales.
1551
1552 It is possible to pass a table pointer or NULL (indicating the use of
1553 the internal tables) to pcre_exec(). Although not intended for this
1554 purpose, this facility could be used to match a pattern in a different
1555 locale from the one in which it was compiled. Passing table pointers at
1556 run time is discussed below in the section on matching a pattern.
1557
1558
1559 INFORMATION ABOUT A PATTERN
1560
1561 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1562 int what, void *where);
1563
1564 The pcre_fullinfo() function returns information about a compiled pat-
1565 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1566 less retained for backwards compability (and is documented below).
1567
1568 The first argument for pcre_fullinfo() is a pointer to the compiled
1569 pattern. The second argument is the result of pcre_study(), or NULL if
1570 the pattern was not studied. The third argument specifies which piece
1571 of information is required, and the fourth argument is a pointer to a
1572 variable to receive the data. The yield of the function is zero for
1573 success, or one of the following negative numbers:
1574
1575 PCRE_ERROR_NULL the argument code was NULL
1576 the argument where was NULL
1577 PCRE_ERROR_BADMAGIC the "magic number" was not found
1578 PCRE_ERROR_BADOPTION the value of what was invalid
1579
1580 The "magic number" is placed at the start of each compiled pattern as
1581 an simple check against passing an arbitrary memory pointer. Here is a
1582 typical call of pcre_fullinfo(), to obtain the length of the compiled
1583 pattern:
1584
1585 int rc;
1586 size_t length;
1587 rc = pcre_fullinfo(
1588 re, /* result of pcre_compile() */
1589 pe, /* result of pcre_study(), or NULL */
1590 PCRE_INFO_SIZE, /* what is required */
1591 &length); /* where to put the data */
1592
1593 The possible values for the third argument are defined in pcre.h, and
1594 are as follows:
1595
1596 PCRE_INFO_BACKREFMAX
1597
1598 Return the number of the highest back reference in the pattern. The
1599 fourth argument should point to an int variable. Zero is returned if
1600 there are no back references.
1601
1602 PCRE_INFO_CAPTURECOUNT
1603
1604 Return the number of capturing subpatterns in the pattern. The fourth
1605 argument should point to an int variable.
1606
1607 PCRE_INFO_DEFAULT_TABLES
1608
1609 Return a pointer to the internal default character tables within PCRE.
1610 The fourth argument should point to an unsigned char * variable. This
1611 information call is provided for internal use by the pcre_study() func-
1612 tion. External callers can cause PCRE to use its internal tables by
1613 passing a NULL table pointer.
1614
1615 PCRE_INFO_FIRSTBYTE
1616
1617 Return information about the first byte of any matched string, for a
1618 non-anchored pattern. The fourth argument should point to an int vari-
1619 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1620 is still recognized for backwards compatibility.)
1621
1622 If there is a fixed first byte, for example, from a pattern such as
1623 (cat|cow|coyote), its value is returned. Otherwise, if either
1624
1625 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1626 branch starts with "^", or
1627
1628 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1629 set (if it were set, the pattern would be anchored),
1630
1631 -1 is returned, indicating that the pattern matches only at the start
1632 of a subject string or after any newline within the string. Otherwise
1633 -2 is returned. For anchored patterns, -2 is returned.
1634
1635 PCRE_INFO_FIRSTTABLE
1636
1637 If the pattern was studied, and this resulted in the construction of a
1638 256-bit table indicating a fixed set of bytes for the first byte in any
1639 matching string, a pointer to the table is returned. Otherwise NULL is
1640 returned. The fourth argument should point to an unsigned char * vari-
1641 able.
1642
1643 PCRE_INFO_HASCRORLF
1644
1645 Return 1 if the pattern contains any explicit matches for CR or LF
1646 characters, otherwise 0. The fourth argument should point to an int
1647 variable. An explicit match is either a literal CR or LF character, or
1648 \r or \n.
1649
1650 PCRE_INFO_JCHANGED
1651
1652 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
1653 otherwise 0. The fourth argument should point to an int variable. (?J)
1654 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
1655
1656 PCRE_INFO_LASTLITERAL
1657
1658 Return the value of the rightmost literal byte that must exist in any
1659 matched string, other than at its start, if such a byte has been
1660 recorded. The fourth argument should point to an int variable. If there
1661 is no such byte, -1 is returned. For anchored patterns, a last literal
1662 byte is recorded only if it follows something of variable length. For
1663 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1664 /^a\dz\d/ the returned value is -1.
1665
1666 PCRE_INFO_NAMECOUNT
1667 PCRE_INFO_NAMEENTRYSIZE
1668 PCRE_INFO_NAMETABLE
1669
1670 PCRE supports the use of named as well as numbered capturing parenthe-
1671 ses. The names are just an additional way of identifying the parenthe-
1672 ses, which still acquire numbers. Several convenience functions such as
1673 pcre_get_named_substring() are provided for extracting captured sub-
1674 strings by name. It is also possible to extract the data directly, by
1675 first converting the name to a number in order to access the correct
1676 pointers in the output vector (described with pcre_exec() below). To do
1677 the conversion, you need to use the name-to-number map, which is
1678 described by these three values.
1679
1680 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1681 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1682 of each entry; both of these return an int value. The entry size
1683 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1684 a pointer to the first entry of the table (a pointer to char). The
1685 first two bytes of each entry are the number of the capturing parenthe-
1686 sis, most significant byte first. The rest of the entry is the corre-
1687 sponding name, zero terminated. The names are in alphabetical order.
1688 When PCRE_DUPNAMES is set, duplicate names are in order of their paren-
1689 theses numbers. For example, consider the following pattern (assume
1690 PCRE_EXTENDED is set, so white space - including newlines - is
1691 ignored):
1692
1693 (?<date> (?<year>(\d\d)?\d\d) -
1694 (?<month>\d\d) - (?<day>\d\d) )
1695
1696 There are four named subpatterns, so the table has four entries, and
1697 each entry in the table is eight bytes long. The table is as follows,
1698 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1699 as ??:
1700
1701 00 01 d a t e 00 ??
1702 00 05 d a y 00 ?? ??
1703 00 04 m o n t h 00
1704 00 02 y e a r 00 ??
1705
1706 When writing code to extract data from named subpatterns using the
1707 name-to-number map, remember that the length of the entries is likely
1708 to be different for each compiled pattern.
1709
1710 PCRE_INFO_OKPARTIAL
1711
1712 Return 1 if the pattern can be used for partial matching, otherwise 0.
1713 The fourth argument should point to an int variable. The pcrepartial
1714 documentation lists the restrictions that apply to patterns when par-
1715 tial matching is used.
1716
1717 PCRE_INFO_OPTIONS
1718
1719 Return a copy of the options with which the pattern was compiled. The
1720 fourth argument should point to an unsigned long int variable. These
1721 option bits are those specified in the call to pcre_compile(), modified
1722 by any top-level option settings at the start of the pattern itself. In
1723 other words, they are the options that will be in force when matching
1724 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1725 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1726 and PCRE_EXTENDED.
1727
1728 A pattern is automatically anchored by PCRE if all of its top-level
1729 alternatives begin with one of the following:
1730
1731 ^ unless PCRE_MULTILINE is set
1732 \A always
1733 \G always
1734 .* if PCRE_DOTALL is set and there are no back
1735 references to the subpattern in which .* appears
1736
1737 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1738 by pcre_fullinfo().
1739
1740 PCRE_INFO_SIZE
1741
1742 Return the size of the compiled pattern, that is, the value that was
1743 passed as the argument to pcre_malloc() when PCRE was getting memory in
1744 which to place the compiled data. The fourth argument should point to a
1745 size_t variable.
1746
1747 PCRE_INFO_STUDYSIZE
1748
1749 Return the size of the data block pointed to by the study_data field in
1750 a pcre_extra block. That is, it is the value that was passed to
1751 pcre_malloc() when PCRE was getting memory into which to place the data
1752 created by pcre_study(). The fourth argument should point to a size_t
1753 variable.
1754
1755
1756 OBSOLETE INFO FUNCTION
1757
1758 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1759
1760 The pcre_info() function is now obsolete because its interface is too
1761 restrictive to return all the available data about a compiled pattern.
1762 New programs should use pcre_fullinfo() instead. The yield of
1763 pcre_info() is the number of capturing subpatterns, or one of the fol-
1764 lowing negative numbers:
1765
1766 PCRE_ERROR_NULL the argument code was NULL
1767 PCRE_ERROR_BADMAGIC the "magic number" was not found
1768
1769 If the optptr argument is not NULL, a copy of the options with which
1770 the pattern was compiled is placed in the integer it points to (see
1771 PCRE_INFO_OPTIONS above).
1772
1773 If the pattern is not anchored and the firstcharptr argument is not
1774 NULL, it is used to pass back information about the first character of
1775 any matched string (see PCRE_INFO_FIRSTBYTE above).
1776
1777
1778 REFERENCE COUNTS
1779
1780 int pcre_refcount(pcre *code, int adjust);
1781
1782 The pcre_refcount() function is used to maintain a reference count in
1783 the data block that contains a compiled pattern. It is provided for the
1784 benefit of applications that operate in an object-oriented manner,
1785 where different parts of the application may be using the same compiled
1786 pattern, but you want to free the block when they are all done.
1787
1788 When a pattern is compiled, the reference count field is initialized to
1789 zero. It is changed only by calling this function, whose action is to
1790 add the adjust value (which may be positive or negative) to it. The
1791 yield of the function is the new value. However, the value of the count
1792 is constrained to lie between 0 and 65535, inclusive. If the new value
1793 is outside these limits, it is forced to the appropriate limit value.
1794
1795 Except when it is zero, the reference count is not correctly preserved
1796 if a pattern is compiled on one host and then transferred to a host
1797 whose byte-order is different. (This seems a highly unlikely scenario.)
1798
1799
1800 MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1801
1802 int pcre_exec(const pcre *code, const pcre_extra *extra,
1803 const char *subject, int length, int startoffset,
1804 int options, int *ovector, int ovecsize);
1805
1806 The function pcre_exec() is called to match a subject string against a
1807 compiled pattern, which is passed in the code argument. If the pattern
1808 has been studied, the result of the study should be passed in the extra
1809 argument. This function is the main matching facility of the library,
1810 and it operates in a Perl-like manner. For specialist use there is also
1811 an alternative matching function, which is described below in the sec-
1812 tion about the pcre_dfa_exec() function.
1813
1814 In most applications, the pattern will have been compiled (and option-
1815 ally studied) in the same process that calls pcre_exec(). However, it
1816 is possible to save compiled patterns and study data, and then use them
1817 later in different processes, possibly even on different hosts. For a
1818 discussion about this, see the pcreprecompile documentation.
1819
1820 Here is an example of a simple call to pcre_exec():
1821
1822 int rc;
1823 int ovector[30];
1824 rc = pcre_exec(
1825 re, /* result of pcre_compile() */
1826 NULL, /* we didn't study the pattern */
1827 "some string", /* the subject string */
1828 11, /* the length of the subject string */
1829 0, /* start at offset 0 in the subject */
1830 0, /* default options */
1831 ovector, /* vector of integers for substring information */
1832 30); /* number of elements (NOT size in bytes) */
1833
1834 Extra data for pcre_exec()
1835
1836 If the extra argument is not NULL, it must point to a pcre_extra data
1837 block. The pcre_study() function returns such a block (when it doesn't
1838 return NULL), but you can also create one for yourself, and pass addi-
1839 tional information in it. The pcre_extra block contains the following
1840 fields (not necessarily in this order):
1841
1842 unsigned long int flags;
1843 void *study_data;
1844 unsigned long int match_limit;
1845 unsigned long int match_limit_recursion;
1846 void *callout_data;
1847 const unsigned char *tables;
1848
1849 The flags field is a bitmap that specifies which of the other fields
1850 are set. The flag bits are:
1851
1852 PCRE_EXTRA_STUDY_DATA
1853 PCRE_EXTRA_MATCH_LIMIT
1854 PCRE_EXTRA_MATCH_LIMIT_RECURSION
1855 PCRE_EXTRA_CALLOUT_DATA
1856 PCRE_EXTRA_TABLES
1857
1858 Other flag bits should be set to zero. The study_data field is set in
1859 the pcre_extra block that is returned by pcre_study(), together with
1860 the appropriate flag bit. You should not set this yourself, but you may
1861 add to the block by setting the other fields and their corresponding
1862 flag bits.
1863
1864 The match_limit field provides a means of preventing PCRE from using up
1865 a vast amount of resources when running patterns that are not going to
1866 match, but which have a very large number of possibilities in their
1867 search trees. The classic example is the use of nested unlimited
1868 repeats.
1869
1870 Internally, PCRE uses a function called match() which it calls repeat-
1871 edly (sometimes recursively). The limit set by match_limit is imposed
1872 on the number of times this function is called during a match, which
1873 has the effect of limiting the amount of backtracking that can take
1874 place. For patterns that are not anchored, the count restarts from zero
1875 for each position in the subject string.
1876
1877 The default value for the limit can be set when PCRE is built; the
1878 default default is 10 million, which handles all but the most extreme
1879 cases. You can override the default by suppling pcre_exec() with a
1880 pcre_extra block in which match_limit is set, and
1881 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
1882 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
1883
1884 The match_limit_recursion field is similar to match_limit, but instead
1885 of limiting the total number of times that match() is called, it limits
1886 the depth of recursion. The recursion depth is a smaller number than
1887 the total number of calls, because not all calls to match() are recur-
1888 sive. This limit is of use only if it is set smaller than match_limit.
1889
1890 Limiting the recursion depth limits the amount of stack that can be
1891 used, or, when PCRE has been compiled to use memory on the heap instead
1892 of the stack, the amount of heap memory that can be used.
1893
1894 The default value for match_limit_recursion can be set when PCRE is
1895 built; the default default is the same value as the default for
1896 match_limit. You can override the default by suppling pcre_exec() with
1897 a pcre_extra block in which match_limit_recursion is set, and
1898 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
1899 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
1900
1901 The pcre_callout field is used in conjunction with the "callout" fea-
1902 ture, which is described in the pcrecallout documentation.
1903
1904 The tables field is used to pass a character tables pointer to
1905 pcre_exec(); this overrides the value that is stored with the compiled
1906 pattern. A non-NULL value is stored with the compiled pattern only if
1907 custom tables were supplied to pcre_compile() via its tableptr argu-
1908 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
1909 PCRE's internal tables to be used. This facility is helpful when re-
1910 using patterns that have been saved after compiling with an external
1911 set of tables, because the external tables might be at a different
1912 address when pcre_exec() is called. See the pcreprecompile documenta-
1913 tion for a discussion of saving compiled patterns for later use.
1914
1915 Option bits for pcre_exec()
1916
1917 The unused bits of the options argument for pcre_exec() must be zero.
1918 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
1919 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK and
1920 PCRE_PARTIAL.
1921
1922 PCRE_ANCHORED
1923
1924 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
1925 matching position. If a pattern was compiled with PCRE_ANCHORED, or
1926 turned out to be anchored by virtue of its contents, it cannot be made
1927 unachored at matching time.
1928
1929 PCRE_BSR_ANYCRLF
1930 PCRE_BSR_UNICODE
1931
1932 These options (which are mutually exclusive) control what the \R escape
1933 sequence matches. The choice is either to match only CR, LF, or CRLF,
1934 or to match any Unicode newline sequence. These options override the
1935 choice that was made or defaulted when the pattern was compiled.
1936
1937 PCRE_NEWLINE_CR
1938 PCRE_NEWLINE_LF
1939 PCRE_NEWLINE_CRLF
1940 PCRE_NEWLINE_ANYCRLF
1941 PCRE_NEWLINE_ANY
1942
1943 These options override the newline definition that was chosen or
1944 defaulted when the pattern was compiled. For details, see the descrip-
1945 tion of pcre_compile() above. During matching, the newline choice
1946 affects the behaviour of the dot, circumflex, and dollar metacharac-
1947 ters. It may also alter the way the match position is advanced after a
1948 match failure for an unanchored pattern.
1949
1950 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
1951 set, and a match attempt for an unanchored pattern fails when the cur-
1952 rent position is at a CRLF sequence, and the pattern contains no
1953 explicit matches for CR or LF characters, the match position is
1954 advanced by two characters instead of one, in other words, to after the
1955 CRLF.
1956
1957 The above rule is a compromise that makes the most common cases work as
1958 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
1959 option is not set), it does not match the string "\r\nA" because, after
1960 failing at the start, it skips both the CR and the LF before retrying.
1961 However, the pattern [\r\n]A does match that string, because it con-
1962 tains an explicit CR or LF reference, and so advances only by one char-
1963 acter after the first failure.
1964
1965 An explicit match for CR of LF is either a literal appearance of one of
1966 those characters, or one of the \r or \n escape sequences. Implicit
1967 matches such as [^X] do not count, nor does \s (which includes CR and
1968 LF in the characters that it matches).
1969
1970 Notwithstanding the above, anomalous effects may still occur when CRLF
1971 is a valid newline sequence and explicit \r or \n escapes appear in the
1972 pattern.
1973
1974 PCRE_NOTBOL
1975
1976 This option specifies that first character of the subject string is not
1977 the beginning of a line, so the circumflex metacharacter should not
1978 match before it. Setting this without PCRE_MULTILINE (at compile time)
1979 causes circumflex never to match. This option affects only the behav-
1980 iour of the circumflex metacharacter. It does not affect \A.
1981
1982 PCRE_NOTEOL
1983
1984 This option specifies that the end of the subject string is not the end
1985 of a line, so the dollar metacharacter should not match it nor (except
1986 in multiline mode) a newline immediately before it. Setting this with-
1987 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
1988 option affects only the behaviour of the dollar metacharacter. It does
1989 not affect \Z or \z.
1990
1991 PCRE_NOTEMPTY
1992
1993 An empty string is not considered to be a valid match if this option is
1994 set. If there are alternatives in the pattern, they are tried. If all
1995 the alternatives match the empty string, the entire match fails. For
1996 example, if the pattern
1997
1998 a?b?
1999
2000 is applied to a string not beginning with "a" or "b", it matches the
2001 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2002 match is not valid, so PCRE searches further into the string for occur-
2003 rences of "a" or "b".
2004
2005 Perl has no direct equivalent of PCRE_NOTEMPTY, but it does make a spe-
2006 cial case of a pattern match of the empty string within its split()
2007 function, and when using the /g modifier. It is possible to emulate
2008 Perl's behaviour after matching a null string by first trying the match
2009 again at the same offset with PCRE_NOTEMPTY and PCRE_ANCHORED, and then
2010 if that fails by advancing the starting offset (see below) and trying
2011 an ordinary match again. There is some code that demonstrates how to do
2012 this in the pcredemo.c sample program.
2013
2014 PCRE_NO_UTF8_CHECK
2015
2016 When PCRE_UTF8 is set at compile time, the validity of the subject as a
2017 UTF-8 string is automatically checked when pcre_exec() is subsequently
2018 called. The value of startoffset is also checked to ensure that it
2019 points to the start of a UTF-8 character. There is a discussion about
2020 the validity of UTF-8 strings in the section on UTF-8 support in the
2021 main pcre page. If an invalid UTF-8 sequence of bytes is found,
2022 pcre_exec() returns the error PCRE_ERROR_BADUTF8. If startoffset con-
2023 tains an invalid value, PCRE_ERROR_BADUTF8_OFFSET is returned.
2024
2025 If you already know that your subject is valid, and you want to skip
2026 these checks for performance reasons, you can set the
2027 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2028 do this for the second and subsequent calls to pcre_exec() if you are
2029 making repeated calls to find all the matches in a single subject
2030 string. However, you should be sure that the value of startoffset
2031 points to the start of a UTF-8 character. When PCRE_NO_UTF8_CHECK is
2032 set, the effect of passing an invalid UTF-8 string as a subject, or a
2033 value of startoffset that does not point to the start of a UTF-8 char-
2034 acter, is undefined. Your program may crash.
2035
2036 PCRE_PARTIAL
2037
2038 This option turns on the partial matching feature. If the subject
2039 string fails to match the pattern, but at some point during the match-
2040 ing process the end of the subject was reached (that is, the subject
2041 partially matches the pattern and the failure to match occurred only
2042 because there were not enough subject characters), pcre_exec() returns
2043 PCRE_ERROR_PARTIAL instead of PCRE_ERROR_NOMATCH. When PCRE_PARTIAL is
2044 used, there are restrictions on what may appear in the pattern. These
2045 are discussed in the pcrepartial documentation.
2046
2047 The string to be matched by pcre_exec()
2048
2049 The subject string is passed to pcre_exec() as a pointer in subject, a
2050 length (in bytes) in length, and a starting byte offset in startoffset.
2051 In UTF-8 mode, the byte offset must point to the start of a UTF-8 char-
2052 acter. Unlike the pattern string, the subject may contain binary zero
2053 bytes. When the starting offset is zero, the search for a match starts
2054 at the beginning of the subject, and this is by far the most common
2055 case.
2056
2057 A non-zero starting offset is useful when searching for another match
2058 in the same subject by calling pcre_exec() again after a previous suc-
2059 cess. Setting startoffset differs from just passing over a shortened
2060 string and setting PCRE_NOTBOL in the case of a pattern that begins
2061 with any kind of lookbehind. For example, consider the pattern
2062
2063 \Biss\B
2064
2065 which finds occurrences of "iss" in the middle of words. (\B matches
2066 only if the current position in the subject is not a word boundary.)
2067 When applied to the string "Mississipi" the first call to pcre_exec()
2068 finds the first occurrence. If pcre_exec() is called again with just
2069 the remainder of the subject, namely "issipi", it does not match,
2070 because \B is always false at the start of the subject, which is deemed
2071 to be a word boundary. However, if pcre_exec() is passed the entire
2072 string again, but with startoffset set to 4, it finds the second occur-
2073 rence of "iss" because it is able to look behind the starting point to
2074 discover that it is preceded by a letter.
2075
2076 If a non-zero starting offset is passed when the pattern is anchored,
2077 one attempt to match at the given offset is made. This can only succeed
2078 if the pattern does not require the match to be at the start of the
2079 subject.
2080
2081 How pcre_exec() returns captured substrings
2082
2083 In general, a pattern matches a certain portion of the subject, and in
2084 addition, further substrings from the subject may be picked out by
2085 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2086 this is called "capturing" in what follows, and the phrase "capturing
2087 subpattern" is used for a fragment of a pattern that picks out a sub-
2088 string. PCRE supports several other kinds of parenthesized subpattern
2089 that do not cause substrings to be captured.
2090
2091 Captured substrings are returned to the caller via a vector of integers
2092 whose address is passed in ovector. The number of elements in the vec-
2093 tor is passed in ovecsize, which must be a non-negative number. Note:
2094 this argument is NOT the size of ovector in bytes.
2095
2096 The first two-thirds of the vector is used to pass back captured sub-
2097 strings, each substring using a pair of integers. The remaining third
2098 of the vector is used as workspace by pcre_exec() while matching cap-
2099 turing subpatterns, and is not available for passing back information.
2100 The number passed in ovecsize should always be a multiple of three. If
2101 it is not, it is rounded down.
2102
2103 When a match is successful, information about captured substrings is
2104 returned in pairs of integers, starting at the beginning of ovector,
2105 and continuing up to two-thirds of its length at the most. The first
2106 element of each pair is set to the byte offset of the first character
2107 in a substring, and the second is set to the byte offset of the first
2108 character after the end of a substring. Note: these values are always
2109 byte offsets, even in UTF-8 mode. They are not character counts.
2110
2111 The first pair of integers, ovector[0] and ovector[1], identify the
2112 portion of the subject string matched by the entire pattern. The next
2113 pair is used for the first capturing subpattern, and so on. The value
2114 returned by pcre_exec() is one more than the highest numbered pair that
2115 has been set. For example, if two substrings have been captured, the
2116 returned value is 3. If there are no capturing subpatterns, the return
2117 value from a successful match is 1, indicating that just the first pair
2118 of offsets has been set.
2119
2120 If a capturing subpattern is matched repeatedly, it is the last portion
2121 of the string that it matched that is returned.
2122
2123 If the vector is too small to hold all the captured substring offsets,
2124 it is used as far as possible (up to two-thirds of its length), and the
2125 function returns a value of zero. If the substring offsets are not of
2126 interest, pcre_exec() may be called with ovector passed as NULL and
2127 ovecsize as zero. However, if the pattern contains back references and
2128 the ovector is not big enough to remember the related substrings, PCRE
2129 has to get additional memory for use during matching. Thus it is usu-
2130 ally advisable to supply an ovector.
2131
2132 The pcre_info() function can be used to find out how many capturing
2133 subpatterns there are in a compiled pattern. The smallest size for
2134 ovector that will allow for n captured substrings, in addition to the
2135 offsets of the substring matched by the whole pattern, is (n+1)*3.
2136
2137 It is possible for capturing subpattern number n+1 to match some part
2138 of the subject when subpattern n has not been used at all. For example,
2139 if the string "abc" is matched against the pattern (a|(z))(bc) the
2140 return from the function is 4, and subpatterns 1 and 3 are matched, but
2141 2 is not. When this happens, both values in the offset pairs corre-
2142 sponding to unused subpatterns are set to -1.
2143
2144 Offset values that correspond to unused subpatterns at the end of the
2145 expression are also set to -1. For example, if the string "abc" is
2146 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2147 matched. The return from the function is 2, because the highest used
2148 capturing subpattern number is 1. However, you can refer to the offsets
2149 for the second and third capturing subpatterns if you wish (assuming
2150 the vector is large enough, of course).
2151
2152 Some convenience functions are provided for extracting the captured
2153 substrings as separate strings. These are described below.
2154
2155 Error return values from pcre_exec()
2156
2157 If pcre_exec() fails, it returns a negative number. The following are
2158 defined in the header file:
2159
2160 PCRE_ERROR_NOMATCH (-1)
2161
2162 The subject string did not match the pattern.
2163
2164 PCRE_ERROR_NULL (-2)
2165
2166 Either code or subject was passed as NULL, or ovector was NULL and
2167 ovecsize was not zero.
2168
2169 PCRE_ERROR_BADOPTION (-3)
2170
2171 An unrecognized bit was set in the options argument.
2172
2173 PCRE_ERROR_BADMAGIC (-4)
2174
2175 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2176 to catch the case when it is passed a junk pointer and to detect when a
2177 pattern that was compiled in an environment of one endianness is run in
2178 an environment with the other endianness. This is the error that PCRE
2179 gives when the magic number is not present.
2180
2181 PCRE_ERROR_UNKNOWN_OPCODE (-5)
2182
2183 While running the pattern match, an unknown item was encountered in the
2184 compiled pattern. This error could be caused by a bug in PCRE or by
2185 overwriting of the compiled pattern.
2186
2187 PCRE_ERROR_NOMEMORY (-6)
2188
2189 If a pattern contains back references, but the ovector that is passed
2190 to pcre_exec() is not big enough to remember the referenced substrings,
2191 PCRE gets a block of memory at the start of matching to use for this
2192 purpose. If the call via pcre_malloc() fails, this error is given. The
2193 memory is automatically freed at the end of matching.
2194
2195 PCRE_ERROR_NOSUBSTRING (-7)
2196
2197 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2198 and pcre_get_substring_list() functions (see below). It is never
2199 returned by pcre_exec().
2200
2201 PCRE_ERROR_MATCHLIMIT (-8)
2202
2203 The backtracking limit, as specified by the match_limit field in a
2204 pcre_extra structure (or defaulted) was reached. See the description
2205 above.
2206
2207 PCRE_ERROR_CALLOUT (-9)
2208
2209 This error is never generated by pcre_exec() itself. It is provided for
2210 use by callout functions that want to yield a distinctive error code.
2211 See the pcrecallout documentation for details.
2212
2213 PCRE_ERROR_BADUTF8 (-10)
2214
2215 A string that contains an invalid UTF-8 byte sequence was passed as a
2216 subject.
2217
2218 PCRE_ERROR_BADUTF8_OFFSET (-11)
2219
2220 The UTF-8 byte sequence that was passed as a subject was valid, but the
2221 value of startoffset did not point to the beginning of a UTF-8 charac-
2222 ter.
2223
2224 PCRE_ERROR_PARTIAL (-12)
2225
2226 The subject string did not match, but it did match partially. See the
2227 pcrepartial documentation for details of partial matching.
2228
2229 PCRE_ERROR_BADPARTIAL (-13)
2230
2231 The PCRE_PARTIAL option was used with a compiled pattern containing
2232 items that are not supported for partial matching. See the pcrepartial
2233 documentation for details of partial matching.
2234
2235 PCRE_ERROR_INTERNAL (-14)
2236
2237 An unexpected internal error has occurred. This error could be caused
2238 by a bug in PCRE or by overwriting of the compiled pattern.
2239
2240 PCRE_ERROR_BADCOUNT (-15)
2241
2242 This error is given if the value of the ovecsize argument is negative.
2243
2244 PCRE_ERROR_RECURSIONLIMIT (-21)
2245
2246 The internal recursion limit, as specified by the match_limit_recursion
2247 field in a pcre_extra structure (or defaulted) was reached. See the
2248 description above.
2249
2250 PCRE_ERROR_BADNEWLINE (-23)
2251
2252 An invalid combination of PCRE_NEWLINE_xxx options was given.
2253
2254 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2255
2256
2257 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2258
2259 int pcre_copy_substring(const char *subject, int *ovector,
2260 int stringcount, int stringnumber, char *buffer,
2261 int buffersize);
2262
2263 int pcre_get_substring(const char *subject, int *ovector,
2264 int stringcount, int stringnumber,
2265 const char **stringptr);
2266
2267 int pcre_get_substring_list(const char *subject,
2268 int *ovector, int stringcount, const char ***listptr);
2269
2270 Captured substrings can be accessed directly by using the offsets
2271 returned by pcre_exec() in ovector. For convenience, the functions
2272 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2273 string_list() are provided for extracting captured substrings as new,
2274 separate, zero-terminated strings. These functions identify substrings
2275 by number. The next section describes functions for extracting named
2276 substrings.
2277
2278 A substring that contains a binary zero is correctly extracted and has
2279 a further zero added on the end, but the result is not, of course, a C
2280 string. However, you can process such a string by referring to the
2281 length that is returned by pcre_copy_substring() and pcre_get_sub-
2282 string(). Unfortunately, the interface to pcre_get_substring_list() is
2283 not adequate for handling strings containing binary zeros, because the
2284 end of the final string is not independently indicated.
2285
2286 The first three arguments are the same for all three of these func-
2287 tions: subject is the subject string that has just been successfully
2288 matched, ovector is a pointer to the vector of integer offsets that was
2289 passed to pcre_exec(), and stringcount is the number of substrings that
2290 were captured by the match, including the substring that matched the
2291 entire regular expression. This is the value returned by pcre_exec() if
2292 it is greater than zero. If pcre_exec() returned zero, indicating that
2293 it ran out of space in ovector, the value passed as stringcount should
2294 be the number of elements in the vector divided by three.
2295
2296 The functions pcre_copy_substring() and pcre_get_substring() extract a
2297 single substring, whose number is given as stringnumber. A value of
2298 zero extracts the substring that matched the entire pattern, whereas
2299 higher values extract the captured substrings. For pcre_copy_sub-
2300 string(), the string is placed in buffer, whose length is given by
2301 buffersize, while for pcre_get_substring() a new block of memory is
2302 obtained via pcre_malloc, and its address is returned via stringptr.
2303 The yield of the function is the length of the string, not including
2304 the terminating zero, or one of these error codes:
2305
2306 PCRE_ERROR_NOMEMORY (-6)
2307
2308 The buffer was too small for pcre_copy_substring(), or the attempt to
2309 get memory failed for pcre_get_substring().
2310
2311 PCRE_ERROR_NOSUBSTRING (-7)
2312
2313 There is no substring whose number is stringnumber.
2314
2315 The pcre_get_substring_list() function extracts all available sub-
2316 strings and builds a list of pointers to them. All this is done in a
2317 single block of memory that is obtained via pcre_malloc. The address of
2318 the memory block is returned via listptr, which is also the start of
2319 the list of string pointers. The end of the list is marked by a NULL
2320 pointer. The yield of the function is zero if all went well, or the
2321 error code
2322
2323 PCRE_ERROR_NOMEMORY (-6)
2324
2325 if the attempt to get the memory block failed.
2326
2327 When any of these functions encounter a substring that is unset, which
2328 can happen when capturing subpattern number n+1 matches some part of
2329 the subject, but subpattern n has not been used at all, they return an
2330 empty string. This can be distinguished from a genuine zero-length sub-
2331 string by inspecting the appropriate offset in ovector, which is nega-
2332 tive for unset substrings.
2333
2334 The two convenience functions pcre_free_substring() and pcre_free_sub-
2335 string_list() can be used to free the memory returned by a previous
2336 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2337 tively. They do nothing more than call the function pointed to by
2338 pcre_free, which of course could be called directly from a C program.
2339 However, PCRE is used in some situations where it is linked via a spe-
2340 cial interface to another programming language that cannot use
2341 pcre_free directly; it is for these cases that the functions are pro-
2342 vided.
2343
2344
2345 EXTRACTING CAPTURED SUBSTRINGS BY NAME
2346
2347 int pcre_get_stringnumber(const pcre *code,
2348 const char *name);
2349
2350 int pcre_copy_named_substring(const pcre *code,
2351 const char *subject, int *ovector,
2352 int stringcount, const char *stringname,
2353 char *buffer, int buffersize);
2354
2355 int pcre_get_named_substring(const pcre *code,
2356 const char *subject, int *ovector,
2357 int stringcount, const char *stringname,
2358 const char **stringptr);
2359
2360 To extract a substring by name, you first have to find associated num-
2361 ber. For example, for this pattern
2362
2363 (a+)b(?<xxx>\d+)...
2364
2365 the number of the subpattern called "xxx" is 2. If the name is known to
2366 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2367 name by calling pcre_get_stringnumber(). The first argument is the com-
2368 piled pattern, and the second is the name. The yield of the function is
2369 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2370 subpattern of that name.
2371
2372 Given the number, you can extract the substring directly, or use one of
2373 the functions described in the previous section. For convenience, there
2374 are also two functions that do the whole job.
2375
2376 Most of the arguments of pcre_copy_named_substring() and
2377 pcre_get_named_substring() are the same as those for the similarly
2378 named functions that extract by number. As these are described in the
2379 previous section, they are not re-described here. There are just two
2380 differences:
2381
2382 First, instead of a substring number, a substring name is given. Sec-
2383 ond, there is an extra argument, given at the start, which is a pointer
2384 to the compiled pattern. This is needed in order to gain access to the
2385 name-to-number translation table.
2386
2387 These functions call pcre_get_stringnumber(), and if it succeeds, they
2388 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2389 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2390 behaviour may not be what you want (see the next section).
2391
2392
2393 DUPLICATE SUBPATTERN NAMES
2394
2395 int pcre_get_stringtable_entries(const pcre *code,
2396 const char *name, char **first, char **last);
2397
2398 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2399 subpatterns are not required to be unique. Normally, patterns with
2400 duplicate names are such that in any one match, only one of the named
2401 subpatterns participates. An example is shown in the pcrepattern docu-
2402 mentation.
2403
2404 When duplicates are present, pcre_copy_named_substring() and
2405 pcre_get_named_substring() return the first substring corresponding to
2406 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2407 (-7) is returned; no data is returned. The pcre_get_stringnumber()
2408 function returns one of the numbers that are associated with the name,
2409 but it is not defined which it is.
2410
2411 If you want to get full details of all captured substrings for a given
2412 name, you must use the pcre_get_stringtable_entries() function. The
2413 first argument is the compiled pattern, and the second is the name. The
2414 third and fourth are pointers to variables which are updated by the
2415 function. After it has run, they point to the first and last entries in
2416 the name-to-number table for the given name. The function itself
2417 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2418 there are none. The format of the table is described above in the sec-
2419 tion entitled Information about a pattern. Given all the relevant
2420 entries for the name, you can extract each of their numbers, and hence
2421 the captured data, if any.
2422
2423
2424 FINDING ALL POSSIBLE MATCHES
2425
2426 The traditional matching function uses a similar algorithm to Perl,
2427 which stops when it finds the first match, starting at a given point in
2428 the subject. If you want to find all possible matches, or the longest
2429 possible match, consider using the alternative matching function (see
2430 below) instead. If you cannot use the alternative function, but still
2431 need to find all possible matches, you can kludge it up by making use
2432 of the callout facility, which is described in the pcrecallout documen-
2433 tation.
2434
2435 What you have to do is to insert a callout right at the end of the pat-
2436 tern. When your callout function is called, extract and save the cur-
2437 rent matched substring. Then return 1, which forces pcre_exec() to
2438 backtrack and try other alternatives. Ultimately, when it runs out of
2439 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2440
2441
2442 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2443
2444 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2445 const char *subject, int length, int startoffset,
2446 int options, int *ovector, int ovecsize,
2447 int *workspace, int wscount);
2448
2449 The function pcre_dfa_exec() is called to match a subject string
2450 against a compiled pattern, using a matching algorithm that scans the
2451 subject string just once, and does not backtrack. This has different
2452 characteristics to the normal algorithm, and is not compatible with
2453 Perl. Some of the features of PCRE patterns are not supported. Never-
2454 theless, there are times when this kind of matching can be useful. For
2455 a discussion of the two matching algorithms, see the pcrematching docu-
2456 mentation.
2457
2458 The arguments for the pcre_dfa_exec() function are the same as for
2459 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2460 ent way, and this is described below. The other common arguments are
2461 used in the same way as for pcre_exec(), so their description is not
2462 repeated here.
2463
2464 The two additional arguments provide workspace for the function. The
2465 workspace vector should contain at least 20 elements. It is used for
2466 keeping track of multiple paths through the pattern tree. More
2467 workspace will be needed for patterns and subjects where there are a
2468 lot of potential matches.
2469
2470 Here is an example of a simple call to pcre_dfa_exec():
2471
2472 int rc;
2473 int ovector[10];
2474 int wspace[20];
2475 rc = pcre_dfa_exec(
2476 re, /* result of pcre_compile() */
2477 NULL, /* we didn't study the pattern */
2478 "some string", /* the subject string */
2479 11, /* the length of the subject string */
2480 0, /* start at offset 0 in the subject */
2481 0, /* default options */
2482 ovector, /* vector of integers for substring information */
2483 10, /* number of elements (NOT size in bytes) */
2484 wspace, /* working space vector */
2485 20); /* number of elements (NOT size in bytes) */
2486
2487 Option bits for pcre_dfa_exec()
2488
2489 The unused bits of the options argument for pcre_dfa_exec() must be
2490 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2491 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NO_UTF8_CHECK,
2492 PCRE_PARTIAL, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2493 three of these are the same as for pcre_exec(), so their description is
2494 not repeated here.
2495
2496 PCRE_PARTIAL
2497
2498 This has the same general effect as it does for pcre_exec(), but the
2499 details are slightly different. When PCRE_PARTIAL is set for
2500 pcre_dfa_exec(), the return code PCRE_ERROR_NOMATCH is converted into
2501 PCRE_ERROR_PARTIAL if the end of the subject is reached, there have
2502 been no complete matches, but there is still at least one matching pos-
2503 sibility. The portion of the string that provided the partial match is
2504 set as the first matching string.
2505
2506 PCRE_DFA_SHORTEST
2507
2508 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2509 stop as soon as it has found one match. Because of the way the alterna-
2510 tive algorithm works, this is necessarily the shortest possible match
2511 at the first possible matching point in the subject string.
2512
2513 PCRE_DFA_RESTART
2514
2515 When pcre_dfa_exec() is called with the PCRE_PARTIAL option, and
2516 returns a partial match, it is possible to call it again, with addi-
2517 tional subject characters, and have it continue with the same match.
2518 The PCRE_DFA_RESTART option requests this action; when it is set, the
2519 workspace and wscount options must reference the same vector as before
2520 because data about the match so far is left in them after a partial
2521 match. There is more discussion of this facility in the pcrepartial
2522 documentation.
2523
2524 Successful returns from pcre_dfa_exec()
2525
2526 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2527 string in the subject. Note, however, that all the matches from one run
2528 of the function start at the same point in the subject. The shorter
2529 matches are all initial substrings of the longer matches. For example,
2530 if the pattern
2531
2532 <.*>
2533
2534 is matched against the string
2535
2536 This is <something> <something else> <something further> no more
2537
2538 the three matched strings are
2539
2540 <something>
2541 <something> <something else>
2542 <something> <something else> <something further>
2543
2544 On success, the yield of the function is a number greater than zero,
2545 which is the number of matched substrings. The substrings themselves
2546 are returned in ovector. Each string uses two elements; the first is
2547 the offset to the start, and the second is the offset to the end. In
2548 fact, all the strings have the same start offset. (Space could have
2549 been saved by giving this only once, but it was decided to retain some
2550 compatibility with the way pcre_exec() returns data, even though the
2551 meaning of the strings is different.)
2552
2553 The strings are returned in reverse order of length; that is, the long-
2554 est matching string is given first. If there were too many matches to
2555 fit into ovector, the yield of the function is zero, and the vector is
2556 filled with the longest matches.
2557
2558 Error returns from pcre_dfa_exec()
2559
2560 The pcre_dfa_exec() function returns a negative number when it fails.
2561 Many of the errors are the same as for pcre_exec(), and these are
2562 described above. There are in addition the following errors that are
2563 specific to pcre_dfa_exec():
2564
2565 PCRE_ERROR_DFA_UITEM (-16)
2566
2567 This return is given if pcre_dfa_exec() encounters an item in the pat-
2568 tern that it does not support, for instance, the use of \C or a back
2569 reference.
2570
2571 PCRE_ERROR_DFA_UCOND (-17)
2572
2573 This return is given if pcre_dfa_exec() encounters a condition item
2574 that uses a back reference for the condition, or a test for recursion
2575 in a specific group. These are not supported.
2576
2577 PCRE_ERROR_DFA_UMLIMIT (-18)
2578
2579 This return is given if pcre_dfa_exec() is called with an extra block
2580 that contains a setting of the match_limit field. This is not supported
2581 (it is meaningless).
2582
2583 PCRE_ERROR_DFA_WSSIZE (-19)
2584
2585 This return is given if pcre_dfa_exec() runs out of space in the
2586 workspace vector.
2587
2588 PCRE_ERROR_DFA_RECURSE (-20)
2589
2590 When a recursive subpattern is processed, the matching function calls
2591 itself recursively, using private vectors for ovector and workspace.
2592 This error is given if the output vector is not large enough. This
2593 should be extremely rare, as a vector of size 1000 is used.
2594
2595
2596 SEE ALSO
2597
2598 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2599 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2600
2601
2602 AUTHOR
2603
2604 Philip Hazel
2605 University Computing Service
2606 Cambridge CB2 3QH, England.
2607
2608
2609 REVISION
2610
2611 Last updated: 24 August 2008
2612 Copyright (c) 1997-2008 University of Cambridge.
2613 ------------------------------------------------------------------------------
2614
2615
2616 PCRECALLOUT(3) PCRECALLOUT(3)
2617
2618
2619 NAME
2620 PCRE - Perl-compatible regular expressions
2621
2622
2623 PCRE CALLOUTS
2624
2625 int (*pcre_callout)(pcre_callout_block *);
2626
2627 PCRE provides a feature called "callout", which is a means of temporar-
2628 ily passing control to the caller of PCRE in the middle of pattern
2629 matching. The caller of PCRE provides an external function by putting
2630 its entry point in the global variable pcre_callout. By default, this
2631 variable contains NULL, which disables all calling out.
2632
2633 Within a regular expression, (?C) indicates the points at which the
2634 external function is to be called. Different callout points can be
2635 identified by putting a number less than 256 after the letter C. The
2636 default value is zero. For example, this pattern has two callout
2637 points:
2638
2639 (?C1)abc(?C2)def
2640
2641 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() is
2642 called, PCRE automatically inserts callouts, all with number 255,
2643 before each item in the pattern. For example, if PCRE_AUTO_CALLOUT is
2644 used with the pattern
2645
2646 A(\d{2}|--)
2647
2648 it is processed as if it were
2649
2650 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
2651
2652 Notice that there is a callout before and after each parenthesis and
2653 alternation bar. Automatic callouts can be used for tracking the
2654 progress of pattern matching. The pcretest command has an option that
2655 sets automatic callouts; when it is used, the output indicates how the
2656 pattern is matched. This is useful information when you are trying to
2657 optimize the performance of a particular pattern.
2658
2659
2660 MISSING CALLOUTS
2661
2662 You should be aware that, because of optimizations in the way PCRE
2663 matches patterns, callouts sometimes do not happen. For example, if the
2664 pattern is
2665
2666 ab(?C4)cd
2667
2668 PCRE knows that any matching string must contain the letter "d". If the
2669 subject string is "abyz", the lack of "d" means that matching doesn't
2670 ever start, and the callout is never reached. However, with "abyd",
2671 though the result is still no match, the callout is obeyed.
2672
2673
2674 THE CALLOUT INTERFACE
2675
2676 During matching, when PCRE reaches a callout point, the external func-
2677 tion defined by pcre_callout is called (if it is set). This applies to
2678 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
2679 only argument to the callout function is a pointer to a pcre_callout
2680 block. This structure contains the following fields:
2681
2682 int version;
2683 int callout_number;
2684 int *offset_vector;
2685 const char *subject;
2686 int subject_length;
2687 int start_match;
2688 int current_position;
2689 int capture_top;
2690 int capture_last;
2691 void *callout_data;
2692 int pattern_position;
2693 int next_item_length;
2694
2695 The version field is an integer containing the version number of the
2696 block format. The initial version was 0; the current version is 1. The
2697 version number will change again in future if additional fields are
2698 added, but the intention is never to remove any of the existing fields.
2699
2700 The callout_number field contains the number of the callout, as com-
2701 piled into the pattern (that is, the number after ?C for manual call-
2702 outs, and 255 for automatically generated callouts).
2703
2704 The offset_vector field is a pointer to the vector of offsets that was
2705 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
2706 pcre_exec() is used, the contents can be inspected in order to extract
2707 substrings that have been matched so far, in the same way as for
2708 extracting substrings after a match has completed. For pcre_dfa_exec()
2709 this field is not useful.
2710
2711 The subject and subject_length fields contain copies of the values that
2712 were passed to pcre_exec().
2713
2714 The start_match field normally contains the offset within the subject
2715 at which the current match attempt started. However, if the escape
2716 sequence \K has been encountered, this value is changed to reflect the
2717 modified starting point. If the pattern is not anchored, the callout
2718 function may be called several times from the same point in the pattern
2719 for different starting points in the subject.
2720
2721 The current_position field contains the offset within the subject of
2722 the current match pointer.
2723
2724 When the pcre_exec() function is used, the capture_top field contains
2725 one more than the number of the highest numbered captured substring so
2726 far. If no substrings have been captured, the value of capture_top is
2727 one. This is always the case when pcre_dfa_exec() is used, because it
2728 does not support captured substrings.
2729
2730 The capture_last field contains the number of the most recently cap-
2731 tured substring. If no substrings have been captured, its value is -1.
2732 This is always the case when pcre_dfa_exec() is used.
2733
2734 The callout_data field contains a value that is passed to pcre_exec()
2735 or pcre_dfa_exec() specifically so that it can be passed back in call-
2736 outs. It is passed in the pcre_callout field of the pcre_extra data
2737 structure. If no such data was passed, the value of callout_data in a
2738 pcre_callout block is NULL. There is a description of the pcre_extra
2739 structure in the pcreapi documentation.
2740
2741 The pattern_position field is present from version 1 of the pcre_call-
2742 out structure. It contains the offset to the next item to be matched in
2743 the pattern string.
2744
2745 The next_item_length field is present from version 1 of the pcre_call-
2746 out structure. It contains the length of the next item to be matched in
2747 the pattern string. When the callout immediately precedes an alterna-
2748 tion bar, a closing parenthesis, or the end of the pattern, the length
2749 is zero. When the callout precedes an opening parenthesis, the length
2750 is that of the entire subpattern.
2751
2752 The pattern_position and next_item_length fields are intended to help
2753 in distinguishing between different automatic callouts, which all have
2754 the same callout number. However, they are set for all callouts.
2755
2756
2757 RETURN VALUES
2758
2759 The external callout function returns an integer to PCRE. If the value
2760 is zero, matching proceeds as normal. If the value is greater than
2761 zero, matching fails at the current point, but the testing of other
2762 matching possibilities goes ahead, just as if a lookahead assertion had
2763 failed. If the value is less than zero, the match is abandoned, and
2764 pcre_exec() (or pcre_dfa_exec()) returns the negative value.
2765
2766 Negative values should normally be chosen from the set of
2767 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
2768 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
2769 reserved for use by callout functions; it will never be used by PCRE
2770 itself.
2771
2772
2773 AUTHOR
2774
2775 Philip Hazel
2776 University Computing Service
2777 Cambridge CB2 3QH, England.
2778
2779
2780 REVISION
2781
2782 Last updated: 29 May 2007
2783 Copyright (c) 1997-2007 University of Cambridge.
2784 ------------------------------------------------------------------------------
2785
2786
2787 PCRECOMPAT(3) PCRECOMPAT(3)
2788
2789
2790 NAME
2791 PCRE - Perl-compatible regular expressions
2792
2793
2794 DIFFERENCES BETWEEN PCRE AND PERL
2795
2796 This document describes the differences in the ways that PCRE and Perl
2797 handle regular expressions. The differences described here are mainly
2798 with respect to Perl 5.8, though PCRE versions 7.0 and later contain
2799 some features that are expected to be in the forthcoming Perl 5.10.
2800
2801 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
2802 of what it does have are given in the section on UTF-8 support in the
2803 main pcre page.
2804
2805 2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
2806 permits them, but they do not mean what you might think. For example,
2807 (?!a){3} does not assert that the next three characters are not "a". It
2808 just asserts that the next character is not "a" three times.
2809
2810 3. Capturing subpatterns that occur inside negative lookahead asser-
2811 tions are counted, but their entries in the offsets vector are never
2812 set. Perl sets its numerical variables from any such patterns that are
2813 matched before the assertion fails to match something (thereby succeed-
2814 ing), but only if the negative lookahead assertion contains just one
2815 branch.
2816
2817 4. Though binary zero characters are supported in the subject string,
2818 they are not allowed in a pattern string because it is passed as a nor-
2819 mal C string, terminated by zero. The escape sequence \0 can be used in
2820 the pattern to represent a binary zero.
2821
2822 5. The following Perl escape sequences are not supported: \l, \u, \L,
2823 \U, and \N. In fact these are implemented by Perl's general string-han-
2824 dling and are not part of its pattern matching engine. If any of these
2825 are encountered by PCRE, an error is generated.
2826
2827 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
2828 is built with Unicode character property support. The properties that
2829 can be tested with \p and \P are limited to the general category prop-
2830 erties such as Lu and Nd, script names such as Greek or Han, and the
2831 derived properties Any and L&.
2832
2833 7. PCRE does support the \Q...\E escape for quoting substrings. Charac-
2834 ters in between are treated as literals. This is slightly different
2835 from Perl in that $ and @ are also handled as literals inside the
2836 quotes. In Perl, they cause variable interpolation (but of course PCRE
2837 does not have variables). Note the following examples:
2838
2839 Pattern PCRE matches Perl matches
2840
2841 \Qabc$xyz\E abc$xyz abc followed by the
2842 contents of $xyz
2843 \Qabc\$xyz\E abc\$xyz abc\$xyz
2844 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
2845
2846 The \Q...\E sequence is recognized both inside and outside character
2847 classes.
2848
2849 8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
2850 constructions. However, there is support for recursive patterns. This
2851 is not available in Perl 5.8, but will be in Perl 5.10. Also, the PCRE
2852 "callout" feature allows an external function to be called during pat-
2853 tern matching. See the pcrecallout documentation for details.
2854
2855 9. Subpatterns that are called recursively or as "subroutines" are
2856 always treated as atomic groups in PCRE. This is like Python, but
2857 unlike Perl.
2858
2859 10. There are some differences that are concerned with the settings of
2860 captured strings when part of a pattern is repeated. For example,
2861 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
2862 unset, but in PCRE it is set to "b".
2863
2864 11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT),
2865 (*FAIL), (*F), (*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in
2866 the forms without an argument. PCRE does not support (*MARK). If
2867 (*ACCEPT) is within capturing parentheses, PCRE does not set that cap-
2868 ture group; this is different to Perl.
2869
2870 12. PCRE provides some extensions to the Perl regular expression facil-
2871 ities. Perl 5.10 will include new features that are not in earlier
2872 versions, some of which (such as named parentheses) have been in PCRE
2873 for some time. This list is with respect to Perl 5.10:
2874
2875 (a) Although lookbehind assertions must match fixed length strings,
2876 each alternative branch of a lookbehind assertion can match a different
2877 length of string. Perl requires them all to have the same length.
2878
2879 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
2880 meta-character matches only at the very end of the string.
2881
2882 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
2883 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
2884 ignored. (Perl can be made to issue a warning.)
2885
2886 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
2887 fiers is inverted, that is, by default they are not greedy, but if fol-
2888 lowed by a question mark they are.
2889
2890 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
2891 tried only at the first matching position in the subject string.
2892
2893 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAP-
2894 TURE options for pcre_exec() have no Perl equivalents.
2895
2896 (g) The \R escape sequence can be restricted to match only CR, LF, or
2897 CRLF by the PCRE_BSR_ANYCRLF option.
2898
2899 (h) The callout facility is PCRE-specific.
2900
2901 (i) The partial matching facility is PCRE-specific.
2902
2903 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
2904 even on different hosts that have the other endianness.
2905
2906 (k) The alternative matching function (pcre_dfa_exec()) matches in a
2907 different way and is not Perl-compatible.
2908
2909 (l) PCRE recognizes some special sequences such as (*CR) at the start
2910 of a pattern that set overall options that cannot be changed within the
2911 pattern.
2912
2913
2914 AUTHOR
2915
2916 Philip Hazel
2917 University Computing Service
2918 Cambridge CB2 3QH, England.
2919
2920
2921 REVISION
2922
2923 Last updated: 11 September 2007
2924 Copyright (c) 1997-2007 University of Cambridge.
2925 ------------------------------------------------------------------------------
2926
2927
2928 PCREPATTERN(3) PCREPATTERN(3)
2929
2930
2931 NAME
2932 PCRE - Perl-compatible regular expressions
2933
2934
2935 PCRE REGULAR EXPRESSION DETAILS
2936
2937 The syntax and semantics of the regular expressions that are supported
2938 by PCRE are described in detail below. There is a quick-reference syn-
2939 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
2940 semantics as closely as it can. PCRE also supports some alternative
2941 regular expression syntax (which does not conflict with the Perl syn-
2942 tax) in order to provide some compatibility with regular expressions in
2943 Python, .NET, and Oniguruma.
2944
2945 Perl's regular expressions are described in its own documentation, and
2946 regular expressions in general are covered in a number of books, some
2947 of which have copious examples. Jeffrey Friedl's "Mastering Regular
2948 Expressions", published by O'Reilly, covers regular expressions in
2949 great detail. This description of PCRE's regular expressions is
2950 intended as reference material.
2951
2952 The original operation of PCRE was on strings of one-byte characters.
2953 However, there is now also support for UTF-8 character strings. To use
2954 this, you must build PCRE to include UTF-8 support, and then call
2955 pcre_compile() with the PCRE_UTF8 option. How this affects pattern
2956 matching is mentioned in several places below. There is also a summary
2957 of UTF-8 features in the section on UTF-8 support in the main pcre
2958 page.
2959
2960 The remainder of this document discusses the patterns that are sup-
2961 ported by PCRE when its main matching function, pcre_exec(), is used.
2962 From release 6.0, PCRE offers a second matching function,
2963 pcre_dfa_exec(), which matches using a different algorithm that is not
2964 Perl-compatible. Some of the features discussed below are not available
2965 when pcre_dfa_exec() is used. The advantages and disadvantages of the
2966 alternative function, and how it differs from the normal function, are
2967 discussed in the pcrematching page.
2968
2969
2970 NEWLINE CONVENTIONS
2971
2972 PCRE supports five different conventions for indicating line breaks in
2973 strings: a single CR (carriage return) character, a single LF (line-
2974 feed) character, the two-character sequence CRLF, any of the three pre-
2975 ceding, or any Unicode newline sequence. The pcreapi page has further
2976 discussion about newlines, and shows how to set the newline convention
2977 in the options arguments for the compiling and matching functions.
2978
2979 It is also possible to specify a newline convention by starting a pat-
2980 tern string with one of the following five sequences:
2981
2982 (*CR) carriage return
2983 (*LF) linefeed
2984 (*CRLF) carriage return, followed by linefeed
2985 (*ANYCRLF) any of the three above
2986 (*ANY) all Unicode newline sequences
2987
2988 These override the default and the options given to pcre_compile(). For
2989 example, on a Unix system where LF is the default newline sequence, the
2990 pattern
2991
2992 (*CR)a.b
2993
2994 changes the convention to CR. That pattern matches "a\nb" because LF is
2995 no longer a newline. Note that these special settings, which are not
2996 Perl-compatible, are recognized only at the very start of a pattern,
2997 and that they must be in upper case. If more than one of them is
2998 present, the last one is used.
2999
3000 The newline convention does not affect what the \R escape sequence
3001 matches. By default, this is any Unicode newline sequence, for Perl
3002 compatibility. However, this can be changed; see the description of \R
3003 in the section entitled "Newline sequences" below. A change of \R set-
3004 ting can be combined with a change of newline convention.
3005
3006
3007 CHARACTERS AND METACHARACTERS
3008
3009 A regular expression is a pattern that is matched against a subject
3010 string from left to right. Most characters stand for themselves in a
3011 pattern, and match the corresponding characters in the subject. As a
3012 trivial example, the pattern
3013
3014 The quick brown fox
3015
3016 matches a portion of a subject string that is identical to itself. When
3017 caseless matching is specified (the PCRE_CASELESS option), letters are
3018 matched independently of case. In UTF-8 mode, PCRE always understands
3019 the concept of case for characters whose values are less than 128, so
3020 caseless matching is always possible. For characters with higher val-
3021 ues, the concept of case is supported if PCRE is compiled with Unicode
3022 property support, but not otherwise. If you want to use caseless
3023 matching for characters 128 and above, you must ensure that PCRE is
3024 compiled with Unicode property support as well as with UTF-8 support.
3025
3026 The power of regular expressions comes from the ability to include
3027 alternatives and repetitions in the pattern. These are encoded in the
3028 pattern by the use of metacharacters, which do not stand for themselves
3029 but instead are interpreted in some special way.
3030
3031 There are two different sets of metacharacters: those that are recog-
3032 nized anywhere in the pattern except within square brackets, and those
3033 that are recognized within square brackets. Outside square brackets,
3034 the metacharacters are as follows:
3035
3036 \ general escape character with several uses
3037 ^ assert start of string (or line, in multiline mode)
3038 $ assert end of string (or line, in multiline mode)
3039 . match any character except newline (by default)
3040 [ start character class definition
3041 | start of alternative branch
3042 ( start subpattern
3043 ) end subpattern
3044 ? extends the meaning of (
3045 also 0 or 1 quantifier
3046 also quantifier minimizer
3047 * 0 or more quantifier
3048 + 1 or more quantifier
3049 also "possessive quantifier"
3050 { start min/max quantifier
3051
3052 Part of a pattern that is in square brackets is called a "character
3053 class". In a character class the only metacharacters are:
3054
3055 \ general escape character
3056 ^ negate the class, but only if the first character
3057 - indicates character range
3058 [ POSIX character class (only if followed by POSIX
3059 syntax)
3060 ] terminates the character class
3061
3062 The following sections describe the use of each of the metacharacters.
3063
3064
3065 BACKSLASH
3066
3067 The backslash character has several uses. Firstly, if it is followed by
3068 a non-alphanumeric character, it takes away any special meaning that
3069 character may have. This use of backslash as an escape character
3070 applies both inside and outside character classes.
3071
3072 For example, if you want to match a * character, you write \* in the
3073 pattern. This escaping action applies whether or not the following
3074 character would otherwise be interpreted as a metacharacter, so it is
3075 always safe to precede a non-alphanumeric with backslash to specify
3076 that it stands for itself. In particular, if you want to match a back-
3077 slash, you write \\.
3078
3079 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
3080 the pattern (other than in a character class) and characters between a
3081 # outside a character class and the next newline are ignored. An escap-
3082 ing backslash can be used to include a whitespace or # character as
3083 part of the pattern.
3084
3085 If you want to remove the special meaning from a sequence of charac-
3086 ters, you can do so by putting them between \Q and \E. This is differ-
3087 ent from Perl in that $ and @ are handled as literals in \Q...\E
3088 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
3089 tion. Note the following examples:
3090
3091 Pattern PCRE matches Perl matches
3092
3093 \Qabc$xyz\E abc$xyz abc followed by the
3094 contents of $xyz
3095 \Qabc\$xyz\E abc\$xyz abc\$xyz
3096 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3097
3098 The \Q...\E sequence is recognized both inside and outside character
3099 classes.
3100
3101 Non-printing characters
3102
3103 A second use of backslash provides a way of encoding non-printing char-
3104 acters in patterns in a visible manner. There is no restriction on the
3105 appearance of non-printing characters, apart from the binary zero that
3106 terminates a pattern, but when a pattern is being prepared by text
3107 editing, it is usually easier to use one of the following escape
3108 sequences than the binary character it represents:
3109
3110 \a alarm, that is, the BEL character (hex 07)
3111 \cx "control-x", where x is any character
3112 \e escape (hex 1B)
3113 \f formfeed (hex 0C)
3114 \n linefeed (hex 0A)
3115 \r carriage return (hex 0D)
3116 \t tab (hex 09)
3117 \ddd character with octal code ddd, or backreference
3118 \xhh character with hex code hh
3119 \x{hhh..} character with hex code hhh..
3120
3121 The precise effect of \cx is as follows: if x is a lower case letter,
3122 it is converted to upper case. Then bit 6 of the character (hex 40) is
3123 inverted. Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c;
3124 becomes hex 7B.
3125
3126 After \x, from zero to two hexadecimal digits are read (letters can be
3127 in upper or lower case). Any number of hexadecimal digits may appear
3128 between \x{ and }, but the value of the character code must be less
3129 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3130 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
3131 than the largest Unicode code point, which is 10FFFF.
3132
3133 If characters other than hexadecimal digits appear between \x{ and },
3134 or if there is no terminating }, this form of escape is not recognized.
3135 Instead, the initial \x will be interpreted as a basic hexadecimal
3136 escape, with no following digits, giving a character whose value is
3137 zero.
3138
3139 Characters whose value is less than 256 can be defined by either of the
3140 two syntaxes for \x. There is no difference in the way they are han-
3141 dled. For example, \xdc is exactly the same as \x{dc}.
3142
3143 After \0 up to two further octal digits are read. If there are fewer
3144 than two digits, just those that are present are used. Thus the
3145 sequence \0\x\07 specifies two binary zeros followed by a BEL character
3146 (code value 7). Make sure you supply two digits after the initial zero
3147 if the pattern character that follows is itself an octal digit.
3148
3149 The handling of a backslash followed by a digit other than 0 is compli-
3150 cated. Outside a character class, PCRE reads it and any following dig-
3151 its as a decimal number. If the number is less than 10, or if there
3152 have been at least that many previous capturing left parentheses in the
3153 expression, the entire sequence is taken as a back reference. A
3154 description of how this works is given later, following the discussion
3155 of parenthesized subpatterns.
3156
3157 Inside a character class, or if the decimal number is greater than 9
3158 and there have not been that many capturing subpatterns, PCRE re-reads
3159 up to three octal digits following the backslash, and uses them to gen-
3160 erate a data character. Any subsequent digits stand for themselves. In
3161 non-UTF-8 mode, the value of a character specified in octal must be
3162 less than \400. In UTF-8 mode, values up to \777 are permitted. For
3163 example:
3164
3165 \040 is another way of writing a space
3166 \40 is the same, provided there are fewer than 40
3167 previous capturing subpatterns
3168 \7 is always a back reference
3169 \11 might be a back reference, or another way of
3170 writing a tab
3171 \011 is always a tab
3172 \0113 is a tab followed by the character "3"
3173 \113 might be a back reference, otherwise the
3174 character with octal code 113
3175 \377 might be a back reference, otherwise
3176 the byte consisting entirely of 1 bits
3177 \81 is either a back reference, or a binary zero
3178 followed by the two characters "8" and "1"
3179
3180 Note that octal values of 100 or greater must not be introduced by a
3181 leading zero, because no more than three octal digits are ever read.
3182
3183 All the sequences that define a single character value can be used both
3184 inside and outside character classes. In addition, inside a character
3185 class, the sequence \b is interpreted as the backspace character (hex
3186 08), and the sequences \R and \X are interpreted as the characters "R"
3187 and "X", respectively. Outside a character class, these sequences have
3188 different meanings (see below).
3189
3190 Absolute and relative back references
3191
3192 The sequence \g followed by an unsigned or a negative number, option-
3193 ally enclosed in braces, is an absolute or relative back reference. A
3194 named back reference can be coded as \g{name}. Back references are dis-
3195 cussed later, following the discussion of parenthesized subpatterns.
3196
3197 Absolute and relative subroutine calls
3198
3199 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
3200 name or a number enclosed either in angle brackets or single quotes, is
3201 an alternative syntax for referencing a subpattern as a "subroutine".
3202 Details are discussed later. Note that \g{...} (Perl syntax) and
3203 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
3204 reference; the latter is a subroutine call.
3205
3206 Generic character types
3207
3208 Another use of backslash is for specifying generic character types. The
3209 following are always recognized:
3210
3211 \d any decimal digit
3212 \D any character that is not a decimal digit
3213 \h any horizontal whitespace character
3214 \H any character that is not a horizontal whitespace character
3215 \s any whitespace character
3216 \S any character that is not a whitespace character
3217 \v any vertical whitespace character
3218 \V any character that is not a vertical whitespace character
3219 \w any "word" character
3220 \W any "non-word" character
3221
3222 Each pair of escape sequences partitions the complete set of characters
3223 into two disjoint sets. Any given character matches one, and only one,
3224 of each pair.
3225
3226 These character type sequences can appear both inside and outside char-
3227 acter classes. They each match one character of the appropriate type.
3228 If the current matching point is at the end of the subject string, all
3229 of them fail, since there is no character to match.
3230
3231 For compatibility with Perl, \s does not match the VT character (code
3232 11). This makes it different from the the POSIX "space" class. The \s
3233 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3234 "use locale;" is included in a Perl script, \s may match the VT charac-
3235 ter. In PCRE, it never does.
3236
3237 In UTF-8 mode, characters with values greater than 128 never match \d,
3238 \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3239 code character property support is available. These sequences retain
3240 their original meanings from before UTF-8 support was available, mainly
3241 for efficiency reasons.
3242
3243 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3244 the other sequences, these do match certain high-valued codepoints in
3245 UTF-8 mode. The horizontal space characters are:
3246
3247 U+0009 Horizontal tab
3248 U+0020 Space
3249 U+00A0 Non-break space
3250 U+1680 Ogham space mark
3251 U+180E Mongolian vowel separator
3252 U+2000 En quad
3253 U+2001 Em quad
3254 U+2002 En space
3255 U+2003 Em space
3256 U+2004 Three-per-em space
3257 U+2005 Four-per-em space
3258 U+2006 Six-per-em space
3259 U+2007 Figure space
3260 U+2008 Punctuation space
3261 U+2009 Thin space
3262 U+200A Hair space
3263 U+202F Narrow no-break space
3264 U+205F Medium mathematical space
3265 U+3000 Ideographic space
3266
3267 The vertical space characters are:
3268
3269 U+000A Linefeed
3270 U+000B Vertical tab
3271 U+000C Formfeed
3272 U+000D Carriage return
3273 U+0085 Next line
3274 U+2028 Line separator
3275 U+2029 Paragraph separator
3276
3277 A "word" character is an underscore or any character less than 256 that
3278 is a letter or digit. The definition of letters and digits is con-
3279 trolled by PCRE's low-valued character tables, and may vary if locale-
3280 specific matching is taking place (see "Locale support" in the pcreapi
3281 page). For example, in a French locale such as "fr_FR" in Unix-like
3282 systems, or "french" in Windows, some character codes greater than 128
3283 are used for accented letters, and these are matched by \w. The use of
3284 locales with Unicode is discouraged.
3285
3286 Newline sequences
3287
3288 Outside a character class, by default, the escape sequence \R matches
3289 any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
3290 mode \R is equivalent to the following:
3291
3292 (?>\r\n|\n|\x0b|\f|\r|\x85)
3293
3294 This is an example of an "atomic group", details of which are given
3295 below. This particular group matches either the two-character sequence
3296 CR followed by LF, or one of the single characters LF (linefeed,
3297 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3298 return, U+000D), or NEL (next line, U+0085). The two-character sequence
3299 is treated as a single unit that cannot be split.
3300
3301 In UTF-8 mode, two additional characters whose codepoints are greater
3302 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3303 rator, U+2029). Unicode character property support is not needed for
3304 these characters to be recognized.
3305
3306 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3307 the complete set of Unicode line endings) by setting the option
3308 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3309 (BSR is an abbrevation for "backslash R".) This can be made the default
3310 when PCRE is built; if this is the case, the other behaviour can be
3311 requested via the PCRE_BSR_UNICODE option. It is also possible to
3312 specify these settings by starting a pattern string with one of the
3313 following sequences:
3314
3315 (*BSR_ANYCRLF) CR, LF, or CRLF only
3316 (*BSR_UNICODE) any Unicode newline sequence
3317
3318 These override the default and the options given to pcre_compile(), but
3319 they can be overridden by options given to pcre_exec(). Note that these
3320 special settings, which are not Perl-compatible, are recognized only at
3321 the very start of a pattern, and that they must be in upper case. If
3322 more than one of them is present, the last one is used. They can be
3323 combined with a change of newline convention, for example, a pattern
3324 can start with:
3325
3326 (*ANY)(*BSR_ANYCRLF)
3327
3328 Inside a character class, \R matches the letter "R".
3329
3330 Unicode character properties
3331
3332 When PCRE is built with Unicode character property support, three addi-
3333 tional escape sequences that match characters with specific properties
3334 are available. When not in UTF-8 mode, these sequences are of course
3335 limited to testing characters whose codepoints are less than 256, but
3336 they do work in this mode. The extra escape sequences are:
3337
3338 \p{xx} a character with the xx property
3339 \P{xx} a character without the xx property
3340 \X an extended Unicode sequence
3341
3342 The property names represented by xx above are limited to the Unicode
3343 script names, the general category properties, and "Any", which matches
3344 any character (including newline). Other properties such as "InMusical-
3345 Symbols" are not currently supported by PCRE. Note that \P{Any} does
3346 not match any characters, so always causes a match failure.
3347
3348 Sets of Unicode characters are defined as belonging to certain scripts.
3349 A character from one of these sets can be matched using a script name.
3350 For example:
3351
3352 \p{Greek}
3353 \P{Han}
3354
3355 Those that are not part of an identified script are lumped together as
3356 "Common". The current list of scripts is:
3357
3358 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
3359 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
3360 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3361 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
3362 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
3363 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
3364 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
3365 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
3366 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3367
3368 Each character has exactly one general category property, specified by
3369 a two-letter abbreviation. For compatibility with Perl, negation can be
3370 specified by including a circumflex between the opening brace and the
3371 property name. For example, \p{^Lu} is the same as \P{Lu}.
3372
3373 If only one letter is specified with \p or \P, it includes all the gen-
3374 eral category properties that start with that letter. In this case, in
3375 the absence of negation, the curly brackets in the escape sequence are
3376 optional; these two examples have the same effect:
3377
3378 \p{L}
3379 \pL
3380
3381 The following general category property codes are supported:
3382
3383 C Other
3384 Cc Control
3385 Cf Format
3386 Cn Unassigned
3387 Co Private use
3388 Cs Surrogate
3389
3390 L Letter
3391 Ll Lower case letter
3392 Lm Modifier letter
3393 Lo Other letter
3394 Lt Title case letter
3395 Lu Upper case letter
3396
3397 M Mark
3398 Mc Spacing mark
3399 Me Enclosing mark
3400 Mn Non-spacing mark
3401
3402 N Number
3403 Nd Decimal number
3404 Nl Letter number
3405 No Other number
3406
3407 P Punctuation
3408 Pc Connector punctuation
3409 Pd Dash punctuation
3410 Pe Close punctuation
3411 Pf Final punctuation
3412 Pi Initial punctuation
3413 Po Other punctuation
3414 Ps Open punctuation
3415
3416 S Symbol
3417 Sc Currency symbol
3418 Sk Modifier symbol
3419 Sm Mathematical symbol
3420 So Other symbol
3421
3422 Z Separator
3423 Zl Line separator
3424 Zp Paragraph separator
3425 Zs Space separator
3426
3427 The special property L& is also supported: it matches a character that
3428 has the Lu, Ll, or Lt property, in other words, a letter that is not
3429 classified as a modifier or "other".
3430
3431 The Cs (Surrogate) property applies only to characters in the range
3432 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3433 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3434 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3435 the pcreapi page).
3436
3437 The long synonyms for these properties that Perl supports (such as
3438 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3439 any of these properties with "Is".
3440
3441 No character that is in the Unicode table has the Cn (unassigned) prop-
3442 erty. Instead, this property is assumed for any code point that is not
3443 in the Unicode table.
3444
3445 Specifying caseless matching does not affect these escape sequences.
3446 For example, \p{Lu} always matches only upper case letters.
3447
3448 The \X escape matches any number of Unicode characters that form an
3449 extended Unicode sequence. \X is equivalent to
3450
3451 (?>\PM\pM*)
3452
3453 That is, it matches a character without the "mark" property, followed
3454 by zero or more characters with the "mark" property, and treats the
3455 sequence as an atomic group (see below). Characters with the "mark"
3456 property are typically accents that affect the preceding character.
3457 None of them have codepoints less than 256, so in non-UTF-8 mode \X
3458 matches any one character.
3459
3460 Matching characters by Unicode property is not fast, because PCRE has
3461 to search a structure that contains data for over fifteen thousand
3462 characters. That is why the traditional escape sequences such as \d and
3463 \w do not use Unicode properties in PCRE.
3464
3465 Resetting the match start
3466
3467 The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3468 ously matched characters not to be included in the final matched
3469 sequence. For example, the pattern:
3470
3471 foo\Kbar
3472
3473 matches "foobar", but reports that it has matched "bar". This feature
3474 is similar to a lookbehind assertion (described below). However, in
3475 this case, the part of the subject before the real match does not have
3476 to be of fixed length, as lookbehind assertions do. The use of \K does
3477 not interfere with the setting of captured substrings. For example,
3478 when the pattern
3479
3480 (foo)\Kbar
3481
3482 matches "foobar", the first substring is still set to "foo".
3483
3484 Simple assertions
3485
3486 The final use of backslash is for certain simple assertions. An asser-
3487 tion specifies a condition that has to be met at a particular point in
3488 a match, without consuming any characters from the subject string. The
3489 use of subpatterns for more complicated assertions is described below.
3490 The backslashed assertions are:
3491
3492 \b matches at a word boundary
3493 \B matches when not at a word boundary
3494 \A matches at the start of the subject
3495 \Z matches at the end of the subject
3496 also matches before a newline at the end of the subject
3497 \z matches only at the end of the subject
3498 \G matches at the first matching position in the subject
3499
3500 These assertions may not appear in character classes (but note that \b
3501 has a different meaning, namely the backspace character, inside a char-
3502 acter class).
3503
3504 A word boundary is a position in the subject string where the current
3505 character and the previous character do not both match \w or \W (i.e.
3506 one matches \w and the other matches \W), or the start or end of the
3507 string if the first or last character matches \w, respectively.
3508
3509 The \A, \Z, and \z assertions differ from the traditional circumflex
3510 and dollar (described in the next section) in that they only ever match
3511 at the very start and end of the subject string, whatever options are
3512 set. Thus, they are independent of multiline mode. These three asser-
3513 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3514 affect only the behaviour of the circumflex and dollar metacharacters.
3515 However, if the startoffset argument of pcre_exec() is non-zero, indi-
3516 cating that matching is to start at a point other than the beginning of
3517 the subject, \A can never match. The difference between \Z and \z is
3518 that \Z matches before a newline at the end of the string as well as at
3519 the very end, whereas \z matches only at the end.
3520
3521 The \G assertion is true only when the current matching position is at
3522 the start point of the match, as specified by the startoffset argument
3523 of pcre_exec(). It differs from \A when the value of startoffset is
3524 non-zero. By calling pcre_exec() multiple times with appropriate argu-
3525 ments, you can mimic Perl's /g option, and it is in this kind of imple-
3526 mentation where \G can be useful.
3527
3528 Note, however, that PCRE's interpretation of \G, as the start of the
3529 current match, is subtly different from Perl's, which defines it as the
3530 end of the previous match. In Perl, these can be different when the
3531 previously matched string was empty. Because PCRE does just one match
3532 at a time, it cannot reproduce this behaviour.
3533
3534 If all the alternatives of a pattern begin with \G, the expression is
3535 anchored to the starting match position, and the "anchored" flag is set
3536 in the compiled regular expression.
3537
3538
3539 CIRCUMFLEX AND DOLLAR
3540
3541 Outside a character class, in the default matching mode, the circumflex
3542 character is an assertion that is true only if the current matching
3543 point is at the start of the subject string. If the startoffset argu-
3544 ment of pcre_exec() is non-zero, circumflex can never match if the
3545 PCRE_MULTILINE option is unset. Inside a character class, circumflex
3546 has an entirely different meaning (see below).
3547
3548 Circumflex need not be the first character of the pattern if a number
3549 of alternatives are involved, but it should be the first thing in each
3550 alternative in which it appears if the pattern is ever to match that
3551 branch. If all possible alternatives start with a circumflex, that is,
3552 if the pattern is constrained to match only at the start of the sub-
3553 ject, it is said to be an "anchored" pattern. (There are also other
3554 constructs that can cause a pattern to be anchored.)
3555
3556 A dollar character is an assertion that is true only if the current
3557 matching point is at the end of the subject string, or immediately
3558 before a newline at the end of the string (by default). Dollar need not
3559 be the last character of the pattern if a number of alternatives are
3560 involved, but it should be the last item in any branch in which it
3561 appears. Dollar has no special meaning in a character class.
3562
3563 The meaning of dollar can be changed so that it matches only at the
3564 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
3565 compile time. This does not affect the \Z assertion.
3566
3567 The meanings of the circumflex and dollar characters are changed if the
3568 PCRE_MULTILINE option is set. When this is the case, a circumflex
3569 matches immediately after internal newlines as well as at the start of
3570 the subject string. It does not match after a newline that ends the
3571 string. A dollar matches before any newlines in the string, as well as
3572 at the very end, when PCRE_MULTILINE is set. When newline is specified
3573 as the two-character sequence CRLF, isolated CR and LF characters do
3574 not indicate newlines.
3575
3576 For example, the pattern /^abc$/ matches the subject string "def\nabc"
3577 (where \n represents a newline) in multiline mode, but not otherwise.
3578 Consequently, patterns that are anchored in single line mode because
3579 all branches start with ^ are not anchored in multiline mode, and a
3580 match for circumflex is possible when the startoffset argument of
3581 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
3582 PCRE_MULTILINE is set.
3583
3584 Note that the sequences \A, \Z, and \z can be used to match the start
3585 and end of the subject in both modes, and if all branches of a pattern
3586 start with \A it is always anchored, whether or not PCRE_MULTILINE is
3587 set.
3588
3589
3590 FULL STOP (PERIOD, DOT)
3591
3592 Outside a character class, a dot in the pattern matches any one charac-
3593 ter in the subject string except (by default) a character that signi-
3594 fies the end of a line. In UTF-8 mode, the matched character may be
3595 more than one byte long.
3596
3597 When a line ending is defined as a single character, dot never matches
3598 that character; when the two-character sequence CRLF is used, dot does
3599 not match CR if it is immediately followed by LF, but otherwise it
3600 matches all characters (including isolated CRs and LFs). When any Uni-
3601 code line endings are being recognized, dot does not match CR or LF or
3602 any of the other line ending characters.
3603
3604 The behaviour of dot with regard to newlines can be changed. If the
3605 PCRE_DOTALL option is set, a dot matches any one character, without
3606 exception. If the two-character sequence CRLF is present in the subject
3607 string, it takes two dots to match it.
3608
3609 The handling of dot is entirely independent of the handling of circum-
3610 flex and dollar, the only relationship being that they both involve
3611 newlines. Dot has no special meaning in a character class.
3612
3613
3614 MATCHING A SINGLE BYTE
3615
3616 Outside a character class, the escape sequence \C matches any one byte,
3617 both in and out of UTF-8 mode. Unlike a dot, it always matches any
3618 line-ending characters. The feature is provided in Perl in order to
3619 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
3620 acters into individual bytes, what remains in the string may be a mal-
3621 formed UTF-8 string. For this reason, the \C escape sequence is best
3622 avoided.
3623
3624 PCRE does not allow \C to appear in lookbehind assertions (described
3625 below), because in UTF-8 mode this would make it impossible to calcu-
3626 late the length of the lookbehind.
3627
3628
3629 SQUARE BRACKETS AND CHARACTER CLASSES
3630
3631 An opening square bracket introduces a character class, terminated by a
3632 closing square bracket. A closing square bracket on its own is not spe-
3633 cial. If a closing square bracket is required as a member of the class,
3634 it should be the first data character in the class (after an initial
3635 circumflex, if present) or escaped with a backslash.
3636
3637 A character class matches a single character in the subject. In UTF-8
3638 mode, the character may occupy more than one byte. A matched character
3639 must be in the set of characters defined by the class, unless the first
3640 character in the class definition is a circumflex, in which case the
3641 subject character must not be in the set defined by the class. If a
3642 circumflex is actually required as a member of the class, ensure it is
3643 not the first character, or escape it with a backslash.
3644
3645 For example, the character class [aeiou] matches any lower case vowel,
3646 while [^aeiou] matches any character that is not a lower case vowel.
3647 Note that a circumflex is just a convenient notation for specifying the
3648 characters that are in the class by enumerating those that are not. A
3649 class that starts with a circumflex is not an assertion: it still con-
3650 sumes a character from the subject string, and therefore it fails if
3651 the current pointer is at the end of the string.
3652
3653 In UTF-8 mode, characters with values greater than 255 can be included
3654 in a class as a literal string of bytes, or by using the \x{ escaping
3655 mechanism.
3656
3657 When caseless matching is set, any letters in a class represent both
3658 their upper case and lower case versions, so for example, a caseless
3659 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
3660 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
3661 understands the concept of case for characters whose values are less
3662 than 128, so caseless matching is always possible. For characters with
3663 higher values, the concept of case is supported if PCRE is compiled
3664 with Unicode property support, but not otherwise. If you want to use
3665 caseless matching for characters 128 and above, you must ensure that
3666 PCRE is compiled with Unicode property support as well as with UTF-8
3667 support.
3668
3669 Characters that might indicate line breaks are never treated in any
3670 special way when matching character classes, whatever line-ending
3671 sequence is in use, and whatever setting of the PCRE_DOTALL and
3672 PCRE_MULTILINE options is used. A class such as [^a] always matches one
3673 of these characters.
3674
3675 The minus (hyphen) character can be used to specify a range of charac-
3676 ters in a character class. For example, [d-m] matches any letter
3677 between d and m, inclusive. If a minus character is required in a
3678 class, it must be escaped with a backslash or appear in a position
3679 where it cannot be interpreted as indicating a range, typically as the
3680 first or last character in the class.
3681
3682 It is not possible to have the literal character "]" as the end charac-
3683 ter of a range. A pattern such as [W-]46] is interpreted as a class of
3684 two characters ("W" and "-") followed by a literal string "46]", so it
3685 would match "W46]" or "-46]". However, if the "]" is escaped with a
3686 backslash it is interpreted as the end of range, so [W-\]46] is inter-
3687 preted as a class containing a range followed by two other characters.
3688 The octal or hexadecimal representation of "]" can also be used to end
3689 a range.
3690
3691 Ranges operate in the collating sequence of character values. They can
3692 also be used for characters specified numerically, for example
3693 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
3694 are greater than 255, for example [\x{100}-\x{2ff}].
3695
3696 If a range that includes letters is used when caseless matching is set,
3697 it matches the letters in either case. For example, [W-c] is equivalent
3698 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
3699 character tables for a French locale are in use, [\xc8-\xcb] matches
3700 accented E characters in both cases. In UTF-8 mode, PCRE supports the
3701 concept of case for characters with values greater than 128 only when
3702 it is compiled with Unicode property support.
3703
3704 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
3705 in a character class, and add the characters that they match to the
3706 class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3707 flex can conveniently be used with the upper case character types to
3708 specify a more restricted set of characters than the matching lower
3709 case type. For example, the class [^\W_] matches any letter or digit,
3710 but not underscore.
3711
3712 The only metacharacters that are recognized in character classes are
3713 backslash, hyphen (only where it can be interpreted as specifying a
3714 range), circumflex (only at the start), opening square bracket (only
3715 when it can be interpreted as introducing a POSIX class name - see the
3716 next section), and the terminating closing square bracket. However,
3717 escaping other non-alphanumeric characters does no harm.
3718
3719
3720 POSIX CHARACTER CLASSES
3721
3722 Perl supports the POSIX notation for character classes. This uses names
3723 enclosed by [: and :] within the enclosing square brackets. PCRE also
3724 supports this notation. For example,
3725
3726 [01[:alpha:]%]
3727
3728 matches "0", "1", any alphabetic character, or "%". The supported class
3729 names are
3730
3731 alnum letters and digits
3732 alpha letters
3733 ascii character codes 0 - 127
3734 blank space or tab only
3735 cntrl control characters
3736 digit decimal digits (same as \d)
3737 graph printing characters, excluding space
3738 lower lower case letters
3739 print printing characters, including space
3740 punct printing characters, excluding letters and digits
3741 space white space (not quite the same as \s)
3742 upper upper case letters
3743 word "word" characters (same as \w)
3744 xdigit hexadecimal digits
3745
3746 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
3747 and space (32). Notice that this list includes the VT character (code
3748 11). This makes "space" different to \s, which does not include VT (for
3749 Perl compatibility).
3750
3751 The name "word" is a Perl extension, and "blank" is a GNU extension
3752 from Perl 5.8. Another Perl extension is negation, which is indicated
3753 by a ^ character after the colon. For example,
3754
3755 [12[:^digit:]]
3756
3757 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3758 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3759 these are not supported, and an error is given if they are encountered.
3760
3761 In UTF-8 mode, characters with values greater than 128 do not match any
3762 of the POSIX character classes.
3763
3764
3765 VERTICAL BAR
3766
3767 Vertical bar characters are used to separate alternative patterns. For
3768 example, the pattern
3769
3770 gilbert|sullivan
3771
3772 matches either "gilbert" or "sullivan". Any number of alternatives may
3773 appear, and an empty alternative is permitted (matching the empty
3774 string). The matching process tries each alternative in turn, from left
3775 to right, and the first one that succeeds is used. If the alternatives
3776 are within a subpattern (defined below), "succeeds" means matching the
3777 rest of the main pattern as well as the alternative in the subpattern.
3778
3779
3780 INTERNAL OPTION SETTING
3781
3782 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
3783 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
3784 within the pattern by a sequence of Perl option letters enclosed
3785 between "(?" and ")". The option letters are
3786
3787 i for PCRE_CASELESS
3788 m for PCRE_MULTILINE
3789 s for PCRE_DOTALL
3790 x for PCRE_EXTENDED
3791
3792 For example, (?im) sets caseless, multiline matching. It is also possi-
3793 ble to unset these options by preceding the letter with a hyphen, and a
3794 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3795 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
3796 is also permitted. If a letter appears both before and after the
3797 hyphen, the option is unset.
3798
3799 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
3800 can be changed in the same way as the Perl-compatible options by using
3801 the characters J, U and X respectively.
3802
3803 When an option change occurs at top level (that is, not inside subpat-
3804 tern parentheses), the change applies to the remainder of the pattern
3805 that follows. If the change is placed right at the start of a pattern,
3806 PCRE extracts it into the global options (and it will therefore show up
3807 in data extracted by the pcre_fullinfo() function).
3808
3809 An option change within a subpattern (see below for a description of
3810 subpatterns) affects only that part of the current pattern that follows
3811 it, so
3812
3813 (a(?i)b)c
3814
3815 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3816 used). By this means, options can be made to have different settings
3817 in different parts of the pattern. Any changes made in one alternative
3818 do carry on into subsequent branches within the same subpattern. For
3819 example,
3820
3821 (a(?i)b|c)
3822
3823 matches "ab", "aB", "c", and "C", even though when matching "C" the
3824 first branch is abandoned before the option setting. This is because
3825 the effects of option settings happen at compile time. There would be
3826 some very weird behaviour otherwise.
3827
3828 Note: There are other PCRE-specific options that can be set by the
3829 application when the compile or match functions are called. In some
3830 cases the pattern can contain special leading sequences to override
3831 what the application has set or what has been defaulted. Details are
3832 given in the section entitled "Newline sequences" above.
3833
3834
3835 SUBPATTERNS
3836
3837 Subpatterns are delimited by parentheses (round brackets), which can be
3838 nested. Turning part of a pattern into a subpattern does two things:
3839
3840 1. It localizes a set of alternatives. For example, the pattern
3841
3842 cat(aract|erpillar|)
3843
3844 matches one of the words "cat", "cataract", or "caterpillar". Without
3845 the parentheses, it would match "cataract", "erpillar" or an empty
3846 string.
3847
3848 2. It sets up the subpattern as a capturing subpattern. This means
3849 that, when the whole pattern matches, that portion of the subject
3850 string that matched the subpattern is passed back to the caller via the
3851 ovector argument of pcre_exec(). Opening parentheses are counted from
3852 left to right (starting from 1) to obtain numbers for the capturing
3853 subpatterns.
3854
3855 For example, if the string "the red king" is matched against the pat-
3856 tern
3857
3858 the ((red|white) (king|queen))
3859
3860 the captured substrings are "red king", "red", and "king", and are num-
3861 bered 1, 2, and 3, respectively.
3862
3863 The fact that plain parentheses fulfil two functions is not always
3864 helpful. There are often times when a grouping subpattern is required
3865 without a capturing requirement. If an opening parenthesis is followed
3866 by a question mark and a colon, the subpattern does not do any captur-
3867 ing, and is not counted when computing the number of any subsequent
3868 capturing subpatterns. For example, if the string "the white queen" is
3869 matched against the pattern
3870
3871 the ((?:red|white) (king|queen))
3872
3873 the captured substrings are "white queen" and "queen", and are numbered
3874 1 and 2. The maximum number of capturing subpatterns is 65535.
3875
3876 As a convenient shorthand, if any option settings are required at the
3877 start of a non-capturing subpattern, the option letters may appear
3878 between the "?" and the ":". Thus the two patterns
3879
3880 (?i:saturday|sunday)
3881 (?:(?i)saturday|sunday)
3882
3883 match exactly the same set of strings. Because alternative branches are
3884 tried from left to right, and options are not reset until the end of
3885 the subpattern is reached, an option setting in one branch does affect
3886 subsequent branches, so the above patterns match "SUNDAY" as well as
3887 "Saturday".
3888
3889
3890 DUPLICATE SUBPATTERN NUMBERS
3891
3892 Perl 5.10 introduced a feature whereby each alternative in a subpattern
3893 uses the same numbers for its capturing parentheses. Such a subpattern
3894 starts with (?| and is itself a non-capturing subpattern. For example,
3895 consider this pattern:
3896
3897 (?|(Sat)ur|(Sun))day
3898
3899 Because the two alternatives are inside a (?| group, both sets of cap-
3900 turing parentheses are numbered one. Thus, when the pattern matches,
3901 you can look at captured substring number one, whichever alternative
3902 matched. This construct is useful when you want to capture part, but
3903 not all, of one of a number of alternatives. Inside a (?| group, paren-
3904 theses are numbered as usual, but the number is reset at the start of
3905 each branch. The numbers of any capturing buffers that follow the sub-
3906 pattern start after the highest number used in any branch. The follow-
3907 ing example is taken from the Perl documentation. The numbers under-
3908 neath show in which buffer the captured content will be stored.
3909
3910 # before ---------------branch-reset----------- after
3911 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
3912 # 1 2 2 3 2 3 4
3913
3914 A backreference or a recursive call to a numbered subpattern always
3915 refers to the first one in the pattern with the given number.
3916
3917 An alternative approach to using this "branch reset" feature is to use
3918 duplicate named subpatterns, as described in the next section.
3919
3920
3921 NAMED SUBPATTERNS
3922
3923 Identifying capturing parentheses by number is simple, but it can be
3924 very hard to keep track of the numbers in complicated regular expres-
3925 sions. Furthermore, if an expression is modified, the numbers may
3926 change. To help with this difficulty, PCRE supports the naming of sub-
3927 patterns. This feature was not added to Perl until release 5.10. Python
3928 had the feature earlier, and PCRE introduced it at release 4.0, using
3929 the Python syntax. PCRE now supports both the Perl and the Python syn-
3930 tax.
3931
3932 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
3933 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
3934 to capturing parentheses from other parts of the pattern, such as back-
3935 references, recursion, and conditions, can be made by name as well as
3936 by number.
3937
3938 Names consist of up to 32 alphanumeric characters and underscores.
3939 Named capturing parentheses are still allocated numbers as well as
3940 names, exactly as if the names were not present. The PCRE API provides
3941 function calls for extracting the name-to-number translation table from
3942 a compiled pattern. There is also a convenience function for extracting
3943 a captured substring by name.
3944
3945 By default, a name must be unique within a pattern, but it is possible
3946 to relax this constraint by setting the PCRE_DUPNAMES option at compile
3947 time. This can be useful for patterns where only one instance of the
3948 named parentheses can match. Suppose you want to match the name of a
3949 weekday, either as a 3-letter abbreviation or as the full name, and in
3950 both cases you want to extract the abbreviation. This pattern (ignoring
3951 the line breaks) does the job:
3952
3953 (?<DN>Mon|Fri|Sun)(?:day)?|
3954 (?<DN>Tue)(?:sday)?|
3955 (?<DN>Wed)(?:nesday)?|
3956 (?<DN>Thu)(?:rsday)?|
3957 (?<DN>Sat)(?:urday)?
3958
3959 There are five capturing substrings, but only one is ever set after a
3960 match. (An alternative way of solving this problem is to use a "branch
3961 reset" subpattern, as described in the previous section.)
3962
3963 The convenience function for extracting the data by name returns the
3964 substring for the first (and in this example, the only) subpattern of
3965 that name that matched. This saves searching to find which numbered
3966 subpattern it was. If you make a reference to a non-unique named sub-
3967 pattern from elsewhere in the pattern, the one that corresponds to the
3968 lowest number is used. For further details of the interfaces for han-
3969 dling named subpatterns, see the pcreapi documentation.
3970
3971
3972 REPETITION
3973
3974 Repetition is specified by quantifiers, which can follow any of the
3975 following items:
3976
3977 a literal data character
3978 the dot metacharacter
3979 the \C escape sequence
3980 the \X escape sequence (in UTF-8 mode with Unicode properties)
3981 the \R escape sequence
3982 an escape such as \d that matches a single character
3983 a character class
3984 a back reference (see next section)
3985 a parenthesized subpattern (unless it is an assertion)
3986
3987 The general repetition quantifier specifies a minimum and maximum num-
3988 ber of permitted matches, by giving the two numbers in curly brackets
3989 (braces), separated by a comma. The numbers must be less than 65536,
3990 and the first must be less than or equal to the second. For example:
3991
3992 z{2,4}
3993
3994 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
3995 special character. If the second number is omitted, but the comma is
3996 present, there is no upper limit; if the second number and the comma
3997 are both omitted, the quantifier specifies an exact number of required
3998 matches. Thus
3999
4000 [aeiou]{3,}
4001
4002 matches at least 3 successive vowels, but may match many more, while
4003
4004 \d{8}
4005
4006 matches exactly 8 digits. An opening curly bracket that appears in a
4007 position where a quantifier is not allowed, or one that does not match
4008 the syntax of a quantifier, is taken as a literal character. For exam-
4009 ple, {,6} is not a quantifier, but a literal string of four characters.
4010
4011 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
4012 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
4013 acters, each of which is represented by a two-byte sequence. Similarly,
4014 when Unicode property support is available, \X{3} matches three Unicode
4015 extended sequences, each of which may be several bytes long (and they
4016 may be of different lengths).
4017
4018 The quantifier {0} is permitted, causing the expression to behave as if
4019 the previous item and the quantifier were not present. This may be use-
4020 ful for subpatterns that are referenced as subroutines from elsewhere
4021 in the pattern. Items other than subpatterns that have a {0} quantifier
4022 are omitted from the compiled pattern.
4023
4024 For convenience, the three most common quantifiers have single-charac-
4025 ter abbreviations:
4026
4027 * is equivalent to {0,}
4028 + is equivalent to {1,}
4029 ? is equivalent to {0,1}
4030
4031 It is possible to construct infinite loops by following a subpattern
4032 that can match no characters with a quantifier that has no upper limit,
4033 for example:
4034
4035 (a?)*
4036
4037 Earlier versions of Perl and PCRE used to give an error at compile time
4038 for such patterns. However, because there are cases where this can be
4039 useful, such patterns are now accepted, but if any repetition of the
4040 subpattern does in fact match no characters, the loop is forcibly bro-
4041 ken.
4042
4043 By default, the quantifiers are "greedy", that is, they match as much
4044 as possible (up to the maximum number of permitted times), without
4045 causing the rest of the pattern to fail. The classic example of where
4046 this gives problems is in trying to match comments in C programs. These
4047 appear between /* and */ and within the comment, individual * and /
4048 characters may appear. An attempt to match C comments by applying the
4049 pattern
4050
4051 /\*.*\*/
4052
4053 to the string
4054
4055 /* first comment */ not comment /* second comment */
4056
4057 fails, because it matches the entire string owing to the greediness of
4058 the .* item.
4059
4060 However, if a quantifier is followed by a question mark, it ceases to
4061 be greedy, and instead matches the minimum number of times possible, so
4062 the pattern
4063
4064 /\*.*?\*/
4065
4066 does the right thing with the C comments. The meaning of the various
4067 quantifiers is not otherwise changed, just the preferred number of
4068 matches. Do not confuse this use of question mark with its use as a
4069 quantifier in its own right. Because it has two uses, it can sometimes
4070 appear doubled, as in
4071
4072 \d??\d
4073
4074 which matches one digit by preference, but can match two if that is the
4075 only way the rest of the pattern matches.
4076
4077 If the PCRE_UNGREEDY option is set (an option that is not available in
4078 Perl), the quantifiers are not greedy by default, but individual ones
4079 can be made greedy by following them with a question mark. In other
4080 words, it inverts the default behaviour.
4081
4082 When a parenthesized subpattern is quantified with a minimum repeat
4083 count that is greater than 1 or with a limited maximum, more memory is
4084 required for the compiled pattern, in proportion to the size of the
4085 minimum or maximum.
4086
4087 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4088 alent to Perl's /s) is set, thus allowing the dot to match newlines,
4089 the pattern is implicitly anchored, because whatever follows will be
4090 tried against every character position in the subject string, so there
4091 is no point in retrying the overall match at any position after the
4092 first. PCRE normally treats such a pattern as though it were preceded
4093 by \A.
4094
4095 In cases where it is known that the subject string contains no new-
4096 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
4097 mization, or alternatively using ^ to indicate anchoring explicitly.
4098
4099 However, there is one situation where the optimization cannot be used.
4100 When .* is inside capturing parentheses that are the subject of a
4101 backreference elsewhere in the pattern, a match at the start may fail
4102 where a later one succeeds. Consider, for example:
4103
4104 (.*)abc\1
4105
4106 If the subject is "xyz123abc123" the match point is the fourth charac-
4107 ter. For this reason, such a pattern is not implicitly anchored.
4108
4109 When a capturing subpattern is repeated, the value captured is the sub-
4110 string that matched the final iteration. For example, after
4111
4112 (tweedle[dume]{3}\s*)+
4113
4114 has matched "tweedledum tweedledee" the value of the captured substring
4115 is "tweedledee". However, if there are nested capturing subpatterns,
4116 the corresponding captured values may have been set in previous itera-
4117 tions. For example, after
4118
4119 /(a|(b))+/
4120
4121 matches "aba" the value of the second captured substring is "b".
4122
4123
4124 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4125
4126 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4127 repetition, failure of what follows normally causes the repeated item
4128 to be re-evaluated to see if a different number of repeats allows the
4129 rest of the pattern to match. Sometimes it is useful to prevent this,
4130 either to change the nature of the match, or to cause it fail earlier
4131 than it otherwise might, when the author of the pattern knows there is
4132 no point in carrying on.
4133
4134 Consider, for example, the pattern \d+foo when applied to the subject
4135 line
4136
4137 123456bar
4138
4139 After matching all 6 digits and then failing to match "foo", the normal
4140 action of the matcher is to try again with only 5 digits matching the
4141 \d+ item, and then with 4, and so on, before ultimately failing.
4142 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4143 the means for specifying that once a subpattern has matched, it is not
4144 to be re-evaluated in this way.
4145
4146 If we use atomic grouping for the previous example, the matcher gives
4147 up immediately on failing to match "foo" the first time. The notation
4148 is a kind of special parenthesis, starting with (?> as in this example:
4149
4150 (?>\d+)foo
4151
4152 This kind of parenthesis "locks up" the part of the pattern it con-
4153 tains once it has matched, and a failure further into the pattern is
4154 prevented from backtracking into it. Backtracking past it to previous
4155 items, however, works as normal.
4156
4157 An alternative description is that a subpattern of this type matches
4158 the string of characters that an identical standalone pattern would
4159 match, if anchored at the current point in the subject string.
4160
4161 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4162 such as the above example can be thought of as a maximizing repeat that
4163 must swallow everything it can. So, while both \d+ and \d+? are pre-
4164 pared to adjust the number of digits they match in order to make the
4165 rest of the pattern match, (?>\d+) can only match an entire sequence of
4166 digits.
4167
4168 Atomic groups in general can of course contain arbitrarily complicated
4169 subpatterns, and can be nested. However, when the subpattern for an
4170 atomic group is just a single repeated item, as in the example above, a
4171 simpler notation, called a "possessive quantifier" can be used. This
4172 consists of an additional + character following a quantifier. Using
4173 this notation, the previous example can be rewritten as
4174
4175 \d++foo
4176
4177 Note that a possessive quantifier can be used with an entire group, for
4178 example:
4179
4180 (abc|xyz){2,3}+
4181
4182 Possessive quantifiers are always greedy; the setting of the
4183 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4184 simpler forms of atomic group. However, there is no difference in the
4185 meaning of a possessive quantifier and the equivalent atomic group,
4186 though there may be a performance difference; possessive quantifiers
4187 should be slightly faster.
4188
4189 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4190 tax. Jeffrey Friedl originated the idea (and the name) in the first
4191 edition of his book. Mike McCloskey liked it, so implemented it when he
4192 built Sun's Java package, and PCRE copied it from there. It ultimately
4193 found its way into Perl at release 5.10.
4194
4195 PCRE has an optimization that automatically "possessifies" certain sim-
4196 ple pattern constructs. For example, the sequence A+B is treated as
4197 A++B because there is no point in backtracking into a sequence of A's
4198 when B must follow.
4199
4200 When a pattern contains an unlimited repeat inside a subpattern that
4201 can itself be repeated an unlimited number of times, the use of an
4202 atomic group is the only way to avoid some failing matches taking a
4203 very long time indeed. The pattern
4204
4205 (\D+|<\d+>)*[!?]
4206
4207 matches an unlimited number of substrings that either consist of non-
4208 digits, or digits enclosed in <>, followed by either ! or ?. When it
4209 matches, it runs quickly. However, if it is applied to
4210
4211 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4212
4213 it takes a long time before reporting failure. This is because the
4214 string can be divided between the internal \D+ repeat and the external
4215 * repeat in a large number of ways, and all have to be tried. (The
4216 example uses [!?] rather than a single character at the end, because
4217 both PCRE and Perl have an optimization that allows for fast failure
4218 when a single character is used. They remember the last single charac-
4219 ter that is required for a match, and fail early if it is not present
4220 in the string.) If the pattern is changed so that it uses an atomic
4221 group, like this:
4222
4223 ((?>\D+)|<\d+>)*[!?]
4224
4225 sequences of non-digits cannot be broken, and failure happens quickly.
4226
4227
4228 BACK REFERENCES
4229
4230 Outside a character class, a backslash followed by a digit greater than
4231 0 (and possibly further digits) is a back reference to a capturing sub-
4232 pattern earlier (that is, to its left) in the pattern, provided there
4233 have been that many previous capturing left parentheses.
4234
4235 However, if the decimal number following the backslash is less than 10,
4236 it is always taken as a back reference, and causes an error only if
4237 there are not that many capturing left parentheses in the entire pat-
4238 tern. In other words, the parentheses that are referenced need not be
4239 to the left of the reference for numbers less than 10. A "forward back
4240 reference" of this type can make sense when a repetition is involved
4241 and the subpattern to the right has participated in an earlier itera-
4242 tion.
4243
4244 It is not possible to have a numerical "forward back reference" to a
4245 subpattern whose number is 10 or more using this syntax because a
4246 sequence such as \50 is interpreted as a character defined in octal.
4247 See the subsection entitled "Non-printing characters" above for further
4248 details of the handling of digits following a backslash. There is no
4249 such problem when named parentheses are used. A back reference to any
4250 subpattern is possible using named parentheses (see below).
4251
4252 Another way of avoiding the ambiguity inherent in the use of digits
4253 following a backslash is to use the \g escape sequence, which is a fea-
4254 ture introduced in Perl 5.10. This escape must be followed by an
4255 unsigned number or a negative number, optionally enclosed in braces.
4256 These examples are all identical:
4257
4258 (ring), \1
4259 (ring), \g1
4260 (ring), \g{1}
4261
4262 An unsigned number specifies an absolute reference without the ambigu-
4263 ity that is present in the older syntax. It is also useful when literal
4264 digits follow the reference. A negative number is a relative reference.
4265 Consider this example:
4266
4267 (abc(def)ghi)\g{-1}
4268
4269 The sequence \g{-1} is a reference to the most recently started captur-
4270 ing subpattern before \g, that is, is it equivalent to \2. Similarly,
4271 \g{-2} would be equivalent to \1. The use of relative references can be
4272 helpful in long patterns, and also in patterns that are created by
4273 joining together fragments that contain references within themselves.
4274
4275 A back reference matches whatever actually matched the capturing sub-
4276 pattern in the current subject string, rather than anything matching
4277 the subpattern itself (see "Subpatterns as subroutines" below for a way
4278 of doing that). So the pattern
4279
4280 (sens|respons)e and \1ibility
4281
4282 matches "sense and sensibility" and "response and responsibility", but
4283 not "sense and responsibility". If caseful matching is in force at the
4284 time of the back reference, the case of letters is relevant. For exam-
4285 ple,
4286
4287 ((?i)rah)\s+\1
4288
4289 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4290 original capturing subpattern is matched caselessly.
4291
4292 There are several different ways of writing back references to named
4293 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4294 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4295 unified back reference syntax, in which \g can be used for both numeric
4296 and named references, is also supported. We could rewrite the above
4297 example in any of the following ways:
4298
4299 (?<p1>(?i)rah)\s+\k<p1>
4300 (?'p1'(?i)rah)\s+\k{p1}
4301 (?P<p1>(?i)rah)\s+(?P=p1)
4302 (?<p1>(?i)rah)\s+\g{p1}
4303
4304 A subpattern that is referenced by name may appear in the pattern
4305 before or after the reference.
4306
4307 There may be more than one back reference to the same subpattern. If a
4308 subpattern has not actually been used in a particular match, any back
4309 references to it always fail. For example, the pattern
4310
4311 (a|(bc))\2
4312
4313 always fails if it starts to match "a" rather than "bc". Because there
4314 may be many capturing parentheses in a pattern, all digits following
4315 the backslash are taken as part of a potential back reference number.
4316 If the pattern continues with a digit character, some delimiter must be
4317 used to terminate the back reference. If the PCRE_EXTENDED option is
4318 set, this can be whitespace. Otherwise an empty comment (see "Com-
4319 ments" below) can be used.
4320
4321 A back reference that occurs inside the parentheses to which it refers
4322 fails when the subpattern is first used, so, for example, (a\1) never
4323 matches. However, such references can be useful inside repeated sub-
4324 patterns. For example, the pattern
4325
4326 (a|b\1)+
4327
4328 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4329 ation of the subpattern, the back reference matches the character
4330 string corresponding to the previous iteration. In order for this to
4331 work, the pattern must be such that the first iteration does not need
4332 to match the back reference. This can be done using alternation, as in
4333 the example above, or by a quantifier with a minimum of zero.
4334
4335
4336 ASSERTIONS
4337
4338 An assertion is a test on the characters following or preceding the
4339 current matching point that does not actually consume any characters.
4340 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
4341 described above.
4342
4343 More complicated assertions are coded as subpatterns. There are two
4344 kinds: those that look ahead of the current position in the subject
4345 string, and those that look behind it. An assertion subpattern is
4346 matched in the normal way, except that it does not cause the current
4347 matching position to be changed.
4348
4349 Assertion subpatterns are not capturing subpatterns, and may not be
4350 repeated, because it makes no sense to assert the same thing several
4351 times. If any kind of assertion contains capturing subpatterns within
4352 it, these are counted for the purposes of numbering the capturing sub-
4353 patterns in the whole pattern. However, substring capturing is carried
4354 out only for positive assertions, because it does not make sense for
4355 negative assertions.
4356
4357 Lookahead assertions
4358
4359 Lookahead assertions start with (?= for positive assertions and (?! for
4360 negative assertions. For example,
4361
4362 \w+(?=;)
4363
4364 matches a word followed by a semicolon, but does not include the semi-
4365 colon in the match, and
4366
4367 foo(?!bar)
4368
4369 matches any occurrence of "foo" that is not followed by "bar". Note
4370 that the apparently similar pattern
4371
4372 (?!foo)bar
4373
4374 does not find an occurrence of "bar" that is preceded by something
4375 other than "foo"; it finds any occurrence of "bar" whatsoever, because
4376 the assertion (?!foo) is always true when the next three characters are
4377 "bar". A lookbehind assertion is needed to achieve the other effect.
4378
4379 If you want to force a matching failure at some point in a pattern, the
4380 most convenient way to do it is with (?!) because an empty string
4381 always matches, so an assertion that requires there not to be an empty
4382 string must always fail.
4383
4384 Lookbehind assertions
4385
4386 Lookbehind assertions start with (?<= for positive assertions and (?<!
4387 for negative assertions. For example,
4388
4389 (?<!foo)bar
4390
4391 does find an occurrence of "bar" that is not preceded by "foo". The
4392 contents of a lookbehind assertion are restricted such that all the
4393 strings it matches must have a fixed length. However, if there are sev-
4394 eral top-level alternatives, they do not all have to have the same
4395 fixed length. Thus
4396
4397 (?<=bullock|donkey)
4398
4399 is permitted, but
4400
4401 (?<!dogs?|cats?)
4402
4403 causes an error at compile time. Branches that match different length
4404 strings are permitted only at the top level of a lookbehind assertion.
4405 This is an extension compared with Perl (at least for 5.8), which
4406 requires all branches to match the same length of string. An assertion
4407 such as
4408
4409 (?<=ab(c|de))
4410
4411 is not permitted, because its single top-level branch can match two
4412 different lengths, but it is acceptable if rewritten to use two top-
4413 level branches:
4414
4415 (?<=abc|abde)
4416
4417 In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4418 instead of a lookbehind assertion; this is not restricted to a fixed-
4419 length.
4420
4421 The implementation of lookbehind assertions is, for each alternative,
4422 to temporarily move the current position back by the fixed length and
4423 then try to match. If there are insufficient characters before the cur-
4424 rent position, the assertion fails.
4425
4426 PCRE does not allow the \C escape (which matches a single byte in UTF-8
4427 mode) to appear in lookbehind assertions, because it makes it impossi-
4428 ble to calculate the length of the lookbehind. The \X and \R escapes,
4429 which can match different numbers of bytes, are also not permitted.
4430
4431 Possessive quantifiers can be used in conjunction with lookbehind
4432 assertions to specify efficient matching at the end of the subject
4433 string. Consider a simple pattern such as
4434
4435 abcd$
4436
4437 when applied to a long string that does not match. Because matching
4438 proceeds from left to right, PCRE will look for each "a" in the subject
4439 and then see if what follows matches the rest of the pattern. If the
4440 pattern is specified as
4441
4442 ^.*abcd$
4443
4444 the initial .* matches the entire string at first, but when this fails
4445 (because there is no following "a"), it backtracks to match all but the
4446 last character, then all but the last two characters, and so on. Once
4447 again the search for "a" covers the entire string, from right to left,
4448 so we are no better off. However, if the pattern is written as
4449
4450 ^.*+(?<=abcd)
4451
4452 there can be no backtracking for the .*+ item; it can match only the
4453 entire string. The subsequent lookbehind assertion does a single test
4454 on the last four characters. If it fails, the match fails immediately.
4455 For long strings, this approach makes a significant difference to the
4456 processing time.
4457
4458 Using multiple assertions
4459
4460 Several assertions (of any sort) may occur in succession. For example,
4461
4462 (?<=\d{3})(?<!999)foo
4463
4464 matches "foo" preceded by three digits that are not "999". Notice that
4465 each of the assertions is applied independently at the same point in
4466 the subject string. First there is a check that the previous three
4467 characters are all digits, and then there is a check that the same
4468 three characters are not "999". This pattern does not match "foo" pre-
4469 ceded by six characters, the first of which are digits and the last
4470 three of which are not "999". For example, it doesn't match "123abc-
4471 foo". A pattern to do that is
4472
4473 (?<=\d{3}...)(?<!999)foo
4474
4475 This time the first assertion looks at the preceding six characters,
4476 checking that the first three are digits, and then the second assertion
4477 checks that the preceding three characters are not "999".
4478
4479 Assertions can be nested in any combination. For example,
4480
4481 (?<=(?<!foo)bar)baz
4482
4483 matches an occurrence of "baz" that is preceded by "bar" which in turn
4484 is not preceded by "foo", while
4485
4486 (?<=\d{3}(?!999)...)foo
4487
4488 is another pattern that matches "foo" preceded by three digits and any
4489 three characters that are not "999".
4490
4491
4492 CONDITIONAL SUBPATTERNS
4493
4494 It is possible to cause the matching process to obey a subpattern con-
4495 ditionally or to choose between two alternative subpatterns, depending
4496 on the result of an assertion, or whether a previous capturing subpat-
4497 tern matched or not. The two possible forms of conditional subpattern
4498 are
4499
4500 (?(condition)yes-pattern)
4501 (?(condition)yes-pattern|no-pattern)
4502
4503 If the condition is satisfied, the yes-pattern is used; otherwise the
4504 no-pattern (if present) is used. If there are more than two alterna-
4505 tives in the subpattern, a compile-time error occurs.
4506
4507 There are four kinds of condition: references to subpatterns, refer-
4508 ences to recursion, a pseudo-condition called DEFINE, and assertions.
4509
4510 Checking for a used subpattern by number
4511
4512 If the text between the parentheses consists of a sequence of digits,
4513 the condition is true if the capturing subpattern of that number has
4514 previously matched. An alternative notation is to precede the digits
4515 with a plus or minus sign. In this case, the subpattern number is rela-
4516 tive rather than absolute. The most recently opened parentheses can be
4517 referenced by (?(-1), the next most recent by (?(-2), and so on. In
4518 looping constructs it can also make sense to refer to subsequent groups
4519 with constructs such as (?(+2).
4520
4521 Consider the following pattern, which contains non-significant white
4522 space to make it more readable (assume the PCRE_EXTENDED option) and to
4523 divide it into three parts for ease of discussion:
4524
4525 ( \( )? [^()]+ (?(1) \) )
4526
4527 The first part matches an optional opening parenthesis, and if that
4528 character is present, sets it as the first captured substring. The sec-
4529 ond part matches one or more characters that are not parentheses. The
4530 third part is a conditional subpattern that tests whether the first set
4531 of parentheses matched or not. If they did, that is, if subject started
4532 with an opening parenthesis, the condition is true, and so the yes-pat-
4533 tern is executed and a closing parenthesis is required. Otherwise,
4534 since no-pattern is not present, the subpattern matches nothing. In
4535 other words, this pattern matches a sequence of non-parentheses,
4536 optionally enclosed in parentheses.
4537
4538 If you were embedding this pattern in a larger one, you could use a
4539 relative reference:
4540
4541 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
4542
4543 This makes the fragment independent of the parentheses in the larger
4544 pattern.
4545
4546 Checking for a used subpattern by name
4547
4548 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
4549 used subpattern by name. For compatibility with earlier versions of
4550 PCRE, which had this facility before Perl, the syntax (?(name)...) is
4551 also recognized. However, there is a possible ambiguity with this syn-
4552 tax, because subpattern names may consist entirely of digits. PCRE
4553 looks first for a named subpattern; if it cannot find one and the name
4554 consists entirely of digits, PCRE looks for a subpattern of that num-
4555 ber, which must be greater than zero. Using subpattern names that con-
4556 sist entirely of digits is not recommended.
4557
4558 Rewriting the above example to use a named subpattern gives this:
4559
4560 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
4561
4562
4563 Checking for pattern recursion
4564
4565 If the condition is the string (R), and there is no subpattern with the
4566 name R, the condition is true if a recursive call to the whole pattern
4567 or any subpattern has been made. If digits or a name preceded by amper-
4568 sand follow the letter R, for example:
4569
4570 (?(R3)...) or (?(R&name)...)
4571
4572 the condition is true if the most recent recursion is into the subpat-
4573 tern whose number or name is given. This condition does not check the
4574 entire recursion stack.
4575
4576 At "top level", all these recursion test conditions are false. Recur-
4577 sive patterns are described below.
4578
4579 Defining subpatterns for use by reference only
4580
4581 If the condition is the string (DEFINE), and there is no subpattern
4582 with the name DEFINE, the condition is always false. In this case,
4583 there may be only one alternative in the subpattern. It is always
4584 skipped if control reaches this point in the pattern; the idea of
4585 DEFINE is that it can be used to define "subroutines" that can be ref-
4586 erenced from elsewhere. (The use of "subroutines" is described below.)
4587 For example, a pattern to match an IPv4 address could be written like
4588 this (ignore whitespace and line breaks):
4589
4590 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4591 \b (?&byte) (\.(?&byte)){3} \b
4592
4593 The first part of the pattern is a DEFINE group inside which a another
4594 group named "byte" is defined. This matches an individual component of
4595 an IPv4 address (a number less than 256). When matching takes place,
4596 this part of the pattern is skipped because DEFINE acts like a false
4597 condition.
4598
4599 The rest of the pattern uses references to the named group to match the
4600 four dot-separated components of an IPv4 address, insisting on a word
4601 boundary at each end.
4602
4603 Assertion conditions
4604
4605 If the condition is not in any of the above formats, it must be an
4606 assertion. This may be a positive or negative lookahead or lookbehind
4607 assertion. Consider this pattern, again containing non-significant
4608 white space, and with the two alternatives on the second line:
4609
4610 (?(?=[^a-z]*[a-z])
4611 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
4612
4613 The condition is a positive lookahead assertion that matches an
4614 optional sequence of non-letters followed by a letter. In other words,
4615 it tests for the presence of at least one letter in the subject. If a
4616 letter is found, the subject is matched against the first alternative;
4617 otherwise it is matched against the second. This pattern matches
4618 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4619 letters and dd are digits.
4620
4621
4622 COMMENTS
4623
4624 The sequence (?# marks the start of a comment that continues up to the
4625 next closing parenthesis. Nested parentheses are not permitted. The
4626 characters that make up a comment play no part in the pattern matching
4627 at all.
4628
4629 If the PCRE_EXTENDED option is set, an unescaped # character outside a
4630 character class introduces a comment that continues to immediately
4631 after the next newline in the pattern.
4632
4633
4634 RECURSIVE PATTERNS
4635
4636 Consider the problem of matching a string in parentheses, allowing for
4637 unlimited nested parentheses. Without the use of recursion, the best
4638 that can be done is to use a pattern that matches up to some fixed
4639 depth of nesting. It is not possible to handle an arbitrary nesting
4640 depth.
4641
4642 For some time, Perl has provided a facility that allows regular expres-
4643 sions to recurse (amongst other things). It does this by interpolating
4644 Perl code in the expression at run time, and the code can refer to the
4645 expression itself. A Perl pattern using code interpolation to solve the
4646 parentheses problem can be created like this:
4647
4648 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
4649
4650 The (?p{...}) item interpolates Perl code at run time, and in this case
4651 refers recursively to the pattern in which it appears.
4652
4653 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4654 it supports special syntax for recursion of the entire pattern, and
4655 also for individual subpattern recursion. After its introduction in
4656 PCRE and Python, this kind of recursion was introduced into Perl at
4657 release 5.10.
4658
4659 A special item that consists of (? followed by a number greater than
4660 zero and a closing parenthesis is a recursive call of the subpattern of
4661 the given number, provided that it occurs inside that subpattern. (If
4662 not, it is a "subroutine" call, which is described in the next sec-
4663 tion.) The special item (?R) or (?0) is a recursive call of the entire
4664 regular expression.
4665
4666 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
4667 always treated as an atomic group. That is, once it has matched some of
4668 the subject string, it is never re-entered, even if it contains untried
4669 alternatives and there is a subsequent matching failure.
4670
4671 This PCRE pattern solves the nested parentheses problem (assume the
4672 PCRE_EXTENDED option is set so that white space is ignored):
4673
4674 \( ( (?>[^()]+) | (?R) )* \)
4675
4676 First it matches an opening parenthesis. Then it matches any number of
4677 substrings which can either be a sequence of non-parentheses, or a
4678 recursive match of the pattern itself (that is, a correctly parenthe-
4679 sized substring). Finally there is a closing parenthesis.
4680
4681 If this were part of a larger pattern, you would not want to recurse
4682 the entire pattern, so instead you could use this:
4683
4684 ( \( ( (?>[^()]+) | (?1) )* \) )
4685
4686 We have put the pattern into parentheses, and caused the recursion to
4687 refer to them instead of the whole pattern.
4688
4689 In a larger pattern, keeping track of parenthesis numbers can be
4690 tricky. This is made easier by the use of relative references. (A Perl
4691 5.10 feature.) Instead of (?1) in the pattern above you can write
4692 (?-2) to refer to the second most recently opened parentheses preceding
4693 the recursion. In other words, a negative number counts capturing
4694 parentheses leftwards from the point at which it is encountered.
4695
4696 It is also possible to refer to subsequently opened parentheses, by
4697 writing references such as (?+2). However, these cannot be recursive
4698 because the reference is not inside the parentheses that are refer-
4699 enced. They are always "subroutine" calls, as described in the next
4700 section.
4701
4702 An alternative approach is to use named parentheses instead. The Perl
4703 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
4704 supported. We could rewrite the above example as follows:
4705
4706 (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4707
4708 If there is more than one subpattern with the same name, the earliest
4709 one is used.
4710
4711 This particular example pattern that we have been looking at contains
4712 nested unlimited repeats, and so the use of atomic grouping for match-
4713 ing strings of non-parentheses is important when applying the pattern
4714 to strings that do not match. For example, when this pattern is applied
4715 to
4716
4717 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4718
4719 it yields "no match" quickly. However, if atomic grouping is not used,
4720 the match runs for a very long time indeed because there are so many
4721 different ways the + and * repeats can carve up the subject, and all
4722 have to be tested before failure can be reported.
4723
4724 At the end of a match, the values set for any capturing subpatterns are
4725 those from the outermost level of the recursion at which the subpattern
4726 value is set. If you want to obtain intermediate values, a callout
4727 function can be used (see below and the pcrecallout documentation). If
4728 the pattern above is matched against
4729
4730 (ab(cd)ef)
4731
4732 the value for the capturing parentheses is "ef", which is the last
4733 value taken on at the top level. If additional parentheses are added,
4734 giving
4735
4736 \( ( ( (?>[^()]+) | (?R) )* ) \)
4737 ^ ^
4738 ^ ^
4739
4740 the string they capture is "ab(cd)ef", the contents of the top level
4741 parentheses. If there are more than 15 capturing parentheses in a pat-
4742 tern, PCRE has to obtain extra memory to store data during a recursion,
4743 which it does by using pcre_malloc, freeing it via pcre_free after-
4744 wards. If no memory can be obtained, the match fails with the
4745 PCRE_ERROR_NOMEMORY error.
4746
4747 Do not confuse the (?R) item with the condition (R), which tests for
4748 recursion. Consider this pattern, which matches text in angle brack-
4749 ets, allowing for arbitrary nesting. Only digits are allowed in nested
4750 brackets (that is, when recursing), whereas any characters are permit-
4751 ted at the outer level.
4752
4753 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
4754
4755 In this pattern, (?(R) is the start of a conditional subpattern, with
4756 two different alternatives for the recursive and non-recursive cases.
4757 The (?R) item is the actual recursive call.
4758
4759
4760 SUBPATTERNS AS SUBROUTINES
4761
4762 If the syntax for a recursive subpattern reference (either by number or
4763 by name) is used outside the parentheses to which it refers, it oper-
4764 ates like a subroutine in a programming language. The "called" subpat-
4765 tern may be defined before or after the reference. A numbered reference
4766 can be absolute or relative, as in these examples:
4767
4768 (...(absolute)...)...(?2)...
4769 (...(relative)...)...(?-1)...
4770 (...(?+1)...(relative)...
4771
4772 An earlier example pointed out that the pattern
4773
4774 (sens|respons)e and \1ibility
4775
4776 matches "sense and sensibility" and "response and responsibility", but
4777 not "sense and responsibility". If instead the pattern
4778
4779 (sens|respons)e and (?1)ibility
4780
4781 is used, it does match "sense and responsibility" as well as the other
4782 two strings. Another example is given in the discussion of DEFINE
4783 above.
4784
4785 Like recursive subpatterns, a "subroutine" call is always treated as an
4786 atomic group. That is, once it has matched some of the subject string,
4787 it is never re-entered, even if it contains untried alternatives and
4788 there is a subsequent matching failure.
4789
4790 When a subpattern is used as a subroutine, processing options such as
4791 case-independence are fixed when the subpattern is defined. They cannot
4792 be changed for different calls. For example, consider this pattern:
4793
4794 (abc)(?i:(?-1))
4795
4796 It matches "abcabc". It does not match "abcABC" because the change of
4797 processing option does not affect the called subpattern.
4798
4799
4800 ONIGURUMA SUBROUTINE SYNTAX
4801
4802 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
4803 name or a number enclosed either in angle brackets or single quotes, is
4804 an alternative syntax for referencing a subpattern as a subroutine,
4805 possibly recursively. Here are two of the examples used above, rewrit-
4806 ten using this syntax:
4807
4808 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
4809 (sens|respons)e and \g'1'ibility
4810
4811 PCRE supports an extension to Oniguruma: if a number is preceded by a
4812 plus or a minus sign it is taken as a relative reference. For example:
4813
4814 (abc)(?i:\g<-1>)
4815
4816 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
4817 synonymous. The former is a back reference; the latter is a subroutine
4818 call.
4819
4820
4821 CALLOUTS
4822
4823 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4824 Perl code to be obeyed in the middle of matching a regular expression.
4825 This makes it possible, amongst other things, to extract different sub-
4826 strings that match the same pair of parentheses when there is a repeti-
4827 tion.
4828
4829 PCRE provides a similar feature, but of course it cannot obey arbitrary
4830 Perl code. The feature is called "callout". The caller of PCRE provides
4831 an external function by putting its entry point in the global variable
4832 pcre_callout. By default, this variable contains NULL, which disables
4833 all calling out.
4834
4835 Within a regular expression, (?C) indicates the points at which the
4836 external function is to be called. If you want to identify different
4837 callout points, you can put a number less than 256 after the letter C.
4838 The default value is zero. For example, this pattern has two callout
4839 points:
4840
4841 (?C1)abc(?C2)def
4842
4843 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4844 automatically installed before each item in the pattern. They are all
4845 numbered 255.
4846
4847 During matching, when PCRE reaches a callout point (and pcre_callout is
4848 set), the external function is called. It is provided with the number
4849 of the callout, the position in the pattern, and, optionally, one item
4850 of data originally supplied by the caller of pcre_exec(). The callout
4851 function may cause matching to proceed, to backtrack, or to fail alto-
4852 gether. A complete description of the interface to the callout function
4853 is given in the pcrecallout documentation.
4854
4855
4856 BACKTRACKING CONTROL
4857
4858 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
4859 which are described in the Perl documentation as "experimental and sub-
4860 ject to change or removal in a future version of Perl". It goes on to
4861 say: "Their usage in production code should be noted to avoid problems
4862 during upgrades." The same remarks apply to the PCRE features described
4863 in this section.
4864
4865 Since these verbs are specifically related to backtracking, most of
4866 them can be used only when the pattern is to be matched using
4867 pcre_exec(), which uses a backtracking algorithm. With the exception of
4868 (*FAIL), which behaves like a failing negative assertion, they cause an
4869 error if encountered by pcre_dfa_exec().
4870
4871 The new verbs make use of what was previously invalid syntax: an open-
4872 ing parenthesis followed by an asterisk. In Perl, they are generally of
4873 the form (*VERB:ARG) but PCRE does not support the use of arguments, so
4874 its general form is just (*VERB). Any number of these verbs may occur
4875 in a pattern. There are two kinds:
4876
4877 Verbs that act immediately
4878
4879 The following verbs act as soon as they are encountered:
4880
4881 (*ACCEPT)
4882
4883 This verb causes the match to end successfully, skipping the remainder
4884 of the pattern. When inside a recursion, only the innermost pattern is
4885 ended immediately. PCRE differs from Perl in what happens if the
4886 (*ACCEPT) is inside capturing parentheses. In Perl, the data so far is
4887 captured: in PCRE no data is captured. For example:
4888
4889 A(A|B(*ACCEPT)|C)D
4890
4891 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data
4892 is captured.
4893
4894 (*FAIL) or (*F)
4895
4896 This verb causes the match to fail, forcing backtracking to occur. It
4897 is equivalent to (?!) but easier to read. The Perl documentation notes
4898 that it is probably useful only when combined with (?{}) or (??{}).
4899 Those are, of course, Perl features that are not present in PCRE. The
4900 nearest equivalent is the callout feature, as for example in this pat-
4901 tern:
4902
4903 a+(?C)(*FAIL)
4904
4905 A match with the string "aaaa" always fails, but the callout is taken
4906 before each backtrack happens (in this example, 10 times).
4907
4908 Verbs that act after backtracking
4909
4910 The following verbs do nothing when they are encountered. Matching con-
4911 tinues with what follows, but if there is no subsequent match, a fail-
4912 ure is forced. The verbs differ in exactly what kind of failure
4913 occurs.
4914
4915 (*COMMIT)
4916
4917 This verb causes the whole match to fail outright if the rest of the
4918 pattern does not match. Even if the pattern is unanchored, no further
4919 attempts to find a match by advancing the start point take place. Once
4920 (*COMMIT) has been passed, pcre_exec() is committed to finding a match
4921 at the current starting point, or not at all. For example:
4922
4923 a+(*COMMIT)b
4924
4925 This matches "xxaab" but not "aacaab". It can be thought of as a kind
4926 of dynamic anchor, or "I've started, so I must finish."
4927
4928 (*PRUNE)
4929
4930 This verb causes the match to fail at the current position if the rest
4931 of the pattern does not match. If the pattern is unanchored, the normal
4932 "bumpalong" advance to the next starting character then happens. Back-
4933 tracking can occur as usual to the left of (*PRUNE), or when matching
4934 to the right of (*PRUNE), but if there is no match to the right, back-
4935 tracking cannot cross (*PRUNE). In simple cases, the use of (*PRUNE)
4936 is just an alternative to an atomic group or possessive quantifier, but
4937 there are some uses of (*PRUNE) that cannot be expressed in any other
4938 way.
4939
4940 (*SKIP)
4941
4942 This verb is like (*PRUNE), except that if the pattern is unanchored,
4943 the "bumpalong" advance is not to the next character, but to the posi-
4944 tion in the subject where (*SKIP) was encountered. (*SKIP) signifies
4945 that whatever text was matched leading up to it cannot be part of a
4946 successful match. Consider:
4947
4948 a+(*SKIP)b
4949
4950 If the subject is "aaaac...", after the first match attempt fails
4951 (starting at the first character in the string), the starting point
4952 skips on to start the next attempt at "c". Note that a possessive quan-
4953 tifer does not have the same effect in this example; although it would
4954 suppress backtracking during the first match attempt, the second
4955 attempt would start at the second character instead of skipping on to
4956 "c".
4957
4958 (*THEN)
4959
4960 This verb causes a skip to the next alternation if the rest of the pat-
4961 tern does not match. That is, it cancels pending backtracking, but only
4962 within the current alternation. Its name comes from the observation
4963 that it can be used for a pattern-based if-then-else block:
4964
4965 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
4966
4967 If the COND1 pattern matches, FOO is tried (and possibly further items
4968 after the end of the group if FOO succeeds); on failure the matcher
4969 skips to the second alternative and tries COND2, without backtracking
4970 into COND1. If (*THEN) is used outside of any alternation, it acts
4971 exactly like (*PRUNE).
4972
4973
4974 SEE ALSO
4975
4976 pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
4977
4978
4979 AUTHOR
4980
4981 Philip Hazel
4982 University Computing Service
4983 Cambridge CB2 3QH, England.
4984
4985
4986 REVISION
4987
4988 Last updated: 19 April 2008
4989 Copyright (c) 1997-2008 University of Cambridge.
4990 ------------------------------------------------------------------------------
4991
4992
4993 PCRESYNTAX(3) PCRESYNTAX(3)
4994
4995
4996 NAME
4997 PCRE - Perl-compatible regular expressions
4998
4999
5000 PCRE REGULAR EXPRESSION SYNTAX SUMMARY
5001
5002 The full syntax and semantics of the regular expressions that are sup-
5003 ported by PCRE are described in the pcrepattern documentation. This
5004 document contains just a quick-reference summary of the syntax.
5005
5006
5007 QUOTING
5008
5009 \x where x is non-alphanumeric is a literal x
5010 \Q...\E treat enclosed characters as literal
5011
5012
5013 CHARACTERS
5014
5015 \a alarm, that is, the BEL character (hex 07)
5016 \cx "control-x", where x is any character
5017 \e escape (hex 1B)
5018 \f formfeed (hex 0C)
5019 \n newline (hex 0A)
5020 \r carriage return (hex 0D)
5021 \t tab (hex 09)
5022 \ddd character with octal code ddd, or backreference
5023 \xhh character with hex code hh
5024 \x{hhh..} character with hex code hhh..
5025
5026
5027 CHARACTER TYPES
5028
5029 . any character except newline;
5030 in dotall mode, any character whatsoever
5031 \C one byte, even in UTF-8 mode (best avoided)
5032 \d a decimal digit
5033 \D a character that is not a decimal digit
5034 \h a horizontal whitespace character
5035 \H a character that is not a horizontal whitespace character
5036 \p{xx} a character with the xx property
5037 \P{xx} a character without the xx property
5038 \R a newline sequence
5039 \s a whitespace character
5040 \S a character that is not a whitespace character
5041 \v a vertical whitespace character
5042 \V a character that is not a vertical whitespace character
5043 \w a "word" character
5044 \W a "non-word" character
5045 \X an extended Unicode sequence
5046
5047 In PCRE, \d, \D, \s, \S, \w, and \W recognize only ASCII characters.
5048
5049
5050 GENERAL CATEGORY PROPERTY CODES FOR \p and \P
5051
5052 C Other
5053 Cc Control
5054 Cf Format
5055 Cn Unassigned
5056 Co Private use
5057 Cs Surrogate
5058
5059 L Letter
5060 Ll Lower case letter
5061 Lm Modifier letter
5062 Lo Other letter
5063 Lt Title case letter
5064 Lu Upper case letter
5065 L& Ll, Lu, or Lt
5066
5067 M Mark
5068 Mc Spacing mark
5069 Me Enclosing mark
5070 Mn Non-spacing mark
5071
5072 N Number
5073 Nd Decimal number
5074 Nl Letter number
5075 No Other number
5076
5077 P Punctuation
5078 Pc Connector punctuation
5079 Pd Dash punctuation
5080 Pe Close punctuation
5081 Pf Final punctuation
5082 Pi Initial punctuation
5083 Po Other punctuation
5084 Ps Open punctuation
5085
5086 S Symbol
5087 Sc Currency symbol
5088 Sk Modifier symbol
5089 Sm Mathematical symbol
5090 So Other symbol
5091
5092 Z Separator
5093 Zl Line separator
5094 Zp Paragraph separator
5095 Zs Space separator
5096
5097
5098 SCRIPT NAMES FOR \p AND \P
5099
5100 Arabic, Armenian, Balinese, Bengali, Bopomofo, Braille, Buginese,
5101 Buhid, Canadian_Aboriginal, Cherokee, Common, Coptic, Cuneiform,
5102 Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
5103 Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-
5104 gana, Inherited, Kannada, Katakana, Kharoshthi, Khmer, Lao, Latin,
5105 Limbu, Linear_B, Malayalam, Mongolian, Myanmar, New_Tai_Lue, Nko,
5106 Ogham, Old_Italic, Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
5107 Runic, Shavian, Sinhala, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
5108 Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
5109
5110
5111 CHARACTER CLASSES
5112
5113 [...] positive character class
5114 [^...] negative character class
5115 [x-y] range (can be used for hex characters)
5116 [[:xxx:]] positive POSIX named set
5117 [[:^xxx:]] negative POSIX named set
5118
5119 alnum alphanumeric
5120 alpha alphabetic
5121 ascii 0-127
5122 blank space or tab
5123 cntrl control character
5124 digit decimal digit
5125 graph printing, excluding space
5126 lower lower case letter
5127 print printing, including space
5128 punct printing, excluding alphanumeric
5129 space whitespace
5130 upper upper case letter
5131 word same as \w
5132 xdigit hexadecimal digit
5133
5134 In PCRE, POSIX character set names recognize only ASCII characters. You
5135 can use \Q...\E inside a character class.
5136
5137
5138 QUANTIFIERS
5139
5140 ? 0 or 1, greedy
5141 ?+ 0 or 1, possessive
5142 ?? 0 or 1, lazy
5143 * 0 or more, greedy
5144 *+ 0 or more, possessive
5145 *? 0 or more, lazy
5146 + 1 or more, greedy
5147 ++ 1 or more, possessive
5148 +? 1 or more, lazy
5149 {n} exactly n
5150 {n,m} at least n, no more than m, greedy
5151 {n,m}+ at least n, no more than m, possessive
5152 {n,m}? at least n, no more than m, lazy
5153 {n,} n or more, greedy
5154 {n,}+ n or more, possessive
5155 {n,}? n or more, lazy
5156
5157
5158 ANCHORS AND SIMPLE ASSERTIONS
5159
5160 \b word boundary
5161 \B not a word boundary
5162 ^ start of subject
5163 also after internal newline in multiline mode
5164 \A start of subject
5165 $ end of subject
5166 also before newline at end of subject
5167 also before internal newline in multiline mode
5168 \Z end of subject
5169 also before newline at end of subject
5170 \z end of subject
5171 \G first matching position in subject
5172
5173
5174 MATCH POINT RESET
5175
5176 \K reset start of match
5177
5178
5179 ALTERNATION
5180
5181 expr|expr|expr...
5182
5183
5184 CAPTURING
5185
5186 (...) capturing group
5187 (?<name>...) named capturing group (Perl)
5188 (?'name'...) named capturing group (Perl)
5189 (?P<name>...) named capturing group (Python)
5190 (?:...) non-capturing group
5191 (?|...) non-capturing group; reset group numbers for
5192 capturing groups in each alternative
5193
5194
5195 ATOMIC GROUPS
5196
5197 (?>...) atomic, non-capturing group
5198
5199
5200 COMMENT
5201
5202 (?#....) comment (not nestable)
5203
5204
5205 OPTION SETTING
5206
5207 (?i) caseless
5208 (?J) allow duplicate names
5209 (?m) multiline
5210 (?s) single line (dotall)
5211 (?U) default ungreedy (lazy)
5212 (?x) extended (ignore white space)
5213 (?-...) unset option(s)
5214
5215
5216 LOOKAHEAD AND LOOKBEHIND ASSERTIONS
5217
5218 (?=...) positive look ahead
5219 (?!...) negative look ahead
5220 (?<=...) positive look behind
5221 (?<!...) negative look behind
5222
5223 Each top-level branch of a look behind must be of a fixed length.
5224
5225
5226 BACKREFERENCES
5227
5228 \n reference by number (can be ambiguous)
5229 \gn reference by number
5230 \g{n} reference by number
5231 \g{-n} relative reference by number
5232 \k<name> reference by name (Perl)
5233 \k'name' reference by name (Perl)
5234 \g{name} reference by name (Perl)
5235 \k{name} reference by name (.NET)
5236 (?P=name) reference by name (Python)
5237
5238
5239 SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
5240
5241 (?R) recurse whole pattern
5242 (?n) call subpattern by absolute number
5243 (?+n) call subpattern by relative number
5244 (?-n) call subpattern by relative number
5245 (?&name) call subpattern by name (Perl)
5246 (?P>name) call subpattern by name (Python)
5247 \g<name> call subpattern by name (Oniguruma)
5248 \g'name' call subpattern by name (Oniguruma)
5249 \g<n> call subpattern by absolute number (Oniguruma)
5250 \g'n' call subpattern by absolute number (Oniguruma)
5251 \g<+n> call subpattern by relative number (PCRE extension)
5252 \g'+n' call subpattern by relative number (PCRE extension)
5253 \g<-n> call subpattern by relative number (PCRE extension)
5254 \g'-n' call subpattern by relative number (PCRE extension)
5255
5256
5257 CONDITIONAL PATTERNS
5258
5259 (?(condition)yes-pattern)
5260 (?(condition)yes-pattern|no-pattern)
5261
5262 (?(n)... absolute reference condition
5263 (?(+n)... relative reference condition
5264 (?(-n)... relative reference condition
5265 (?(<name>)... named reference condition (Perl)
5266 (?('name')... named reference condition (Perl)
5267 (?(name)... named reference condition (PCRE)
5268 (?(R)... overall recursion condition
5269 (?(Rn)... specific group recursion condition
5270 (?(R&name)... specific recursion condition
5271 (?(DEFINE)... define subpattern for reference
5272 (?(assert)... assertion condition
5273
5274
5275 BACKTRACKING CONTROL
5276
5277 The following act immediately they are reached:
5278
5279 (*ACCEPT) force successful match
5280 (*FAIL) force backtrack; synonym (*F)
5281
5282 The following act only when a subsequent match failure causes a back-
5283 track to reach them. They all force a match failure, but they differ in
5284 what happens afterwards. Those that advance the start-of-match point do
5285 so only if the pattern is not anchored.
5286
5287 (*COMMIT) overall failure, no advance of starting point
5288 (*PRUNE) advance to next starting character
5289 (*SKIP) advance start to current matching position
5290 (*THEN) local failure, backtrack to next alternation
5291
5292
5293 NEWLINE CONVENTIONS
5294
5295 These are recognized only at the very start of the pattern or after a
5296 (*BSR_...) option.
5297
5298 (*CR)
5299 (*LF)
5300 (*CRLF)
5301 (*ANYCRLF)
5302 (*ANY)
5303
5304
5305 WHAT \R MATCHES
5306
5307 These are recognized only at the very start of the pattern or after a
5308 (*...) option that sets the newline convention.
5309
5310 (*BSR_ANYCRLF)
5311 (*BSR_UNICODE)
5312
5313
5314 CALLOUTS
5315
5316 (?C) callout
5317 (?Cn) callout with data n
5318
5319
5320 SEE ALSO
5321
5322 pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
5323
5324
5325 AUTHOR
5326
5327 Philip Hazel
5328 University Computing Service
5329 Cambridge CB2 3QH, England.
5330
5331
5332 REVISION
5333
5334 Last updated: 09 April 2008
5335 Copyright (c) 1997-2008 University of Cambridge.
5336 ------------------------------------------------------------------------------
5337
5338
5339 PCREPARTIAL(3) PCREPARTIAL(3)
5340
5341
5342 NAME
5343 PCRE - Perl-compatible regular expressions
5344
5345
5346 PARTIAL MATCHING IN PCRE
5347
5348 In normal use of PCRE, if the subject string that is passed to
5349 pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
5350 short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
5351 There are circumstances where it might be helpful to distinguish this
5352 case from other cases in which there is no match.
5353
5354 Consider, for example, an application where a human is required to type
5355 in data for a field with specific formatting requirements. An example
5356 might be a date in the form ddmmmyy, defined by this pattern:
5357
5358 ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
5359
5360 If the application sees the user's keystrokes one by one, and can check
5361 that what has been typed so far is potentially valid, it is able to
5362 raise an error as soon as a mistake is made, possibly beeping and not
5363 reflecting the character that has been typed. This immediate feedback
5364 is likely to be a better user interface than a check that is delayed
5365 until the entire string has been entered.
5366
5367 PCRE supports the concept of partial matching by means of the PCRE_PAR-
5368 TIAL option, which can be set when calling pcre_exec() or
5369 pcre_dfa_exec(). When this flag is set for pcre_exec(), the return code
5370 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if at any time
5371 during the matching process the last part of the subject string matched
5372 part of the pattern. Unfortunately, for non-anchored matching, it is
5373 not possible to obtain the position of the start of the partial match.
5374 No captured data is set when PCRE_ERROR_PARTIAL is returned.
5375
5376 When PCRE_PARTIAL is set for pcre_dfa_exec(), the return code
5377 PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end of
5378 the subject is reached, there have been no complete matches, but there
5379 is still at least one matching possibility. The portion of the string
5380 that provided the partial match is set as the first matching string.
5381
5382 Using PCRE_PARTIAL disables one of PCRE's optimizations. PCRE remembers
5383 the last literal byte in a pattern, and abandons matching immediately
5384 if such a byte is not present in the subject string. This optimization
5385 cannot be used for a subject string that might match only partially.
5386
5387
5388 RESTRICTED PATTERNS FOR PCRE_PARTIAL
5389
5390 Because of the way certain internal optimizations are implemented in
5391 the pcre_exec() function, the PCRE_PARTIAL option cannot be used with
5392 all patterns. These restrictions do not apply when pcre_dfa_exec() is
5393 used. For pcre_exec(), repeated single characters such as
5394
5395 a{2,4}
5396
5397 and repeated single metasequences such as
5398
5399 \d+
5400
5401 are not permitted if the maximum number of occurrences is greater than
5402 one. Optional items such as \d? (where the maximum is one) are permit-
5403 ted. Quantifiers with any values are permitted after parentheses, so
5404 the invalid examples above can be coded thus:
5405
5406 (a){2,4}
5407 (\d)+
5408
5409 These constructions run more slowly, but for the kinds of application
5410 that are envisaged for this facility, this is not felt to be a major
5411 restriction.
5412
5413 If PCRE_PARTIAL is set for a pattern that does not conform to the
5414 restrictions, pcre_exec() returns the error code PCRE_ERROR_BADPARTIAL
5415 (-13). You can use the PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to
5416 find out if a compiled pattern can be used for partial matching.
5417
5418
5419 EXAMPLE OF PARTIAL MATCHING USING PCRETEST
5420
5421 If the escape sequence \P is present in a pcretest data line, the
5422 PCRE_PARTIAL flag is used for the match. Here is a run of pcretest that
5423 uses the date example quoted above:
5424
5425 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5426 data> 25jun04\P
5427 0: 25jun04
5428 1: jun
5429 data> 25dec3\P
5430 Partial match
5431 data> 3ju\P
5432 Partial match
5433 data> 3juj\P
5434 No match
5435 data> j\P
5436 No match
5437
5438 The first data string is matched completely, so pcretest shows the
5439 matched substrings. The remaining four strings do not match the com-
5440 plete pattern, but the first two are partial matches. The same test,
5441 using pcre_dfa_exec() matching (by means of the \D escape sequence),
5442 produces the following output:
5443
5444 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5445 data> 25jun04\P\D
5446 0: 25jun04
5447 data> 23dec3\P\D
5448 Partial match: 23dec3
5449 data> 3ju\P\D
5450 Partial match: 3ju
5451 data> 3juj\P\D
5452 No match
5453 data> j\P\D
5454 No match
5455
5456 Notice that in this case the portion of the string that was matched is
5457 made available.
5458
5459
5460 MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
5461
5462 When a partial match has been found using pcre_dfa_exec(), it is possi-
5463 ble to continue the match by providing additional subject data and
5464 calling pcre_dfa_exec() again with the same compiled regular expres-
5465 sion, this time setting the PCRE_DFA_RESTART option. You must also pass
5466 the same working space as before, because this is where details of the
5467 previous partial match are stored. Here is an example using pcretest,
5468 using the \R escape sequence to set the PCRE_DFA_RESTART option (\P and
5469 \D are as above):
5470
5471 re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
5472 data> 23ja\P\D
5473 Partial match: 23ja
5474 data> n05\R\D
5475 0: n05
5476
5477 The first call has "23ja" as the subject, and requests partial match-
5478 ing; the second call has "n05" as the subject for the continued
5479 (restarted) match. Notice that when the match is complete, only the
5480 last part is shown; PCRE does not retain the previously partially-
5481 matched string. It is up to the calling program to do that if it needs
5482 to.
5483
5484 You can set PCRE_PARTIAL with PCRE_DFA_RESTART to continue partial
5485 matching over multiple segments. This facility can be used to pass very
5486 long subject strings to pcre_dfa_exec(). However, some care is needed
5487 for certain types of pattern.
5488
5489 1. If the pattern contains tests for the beginning or end of a line,
5490 you need to pass the PCRE_NOTBOL or PCRE_NOTEOL options, as appropri-
5491 ate, when the subject string for any call does not contain the begin-
5492 ning or end of a line.
5493
5494 2. If the pattern contains backward assertions (including \b or \B),
5495 you need to arrange for some overlap in the subject strings to allow
5496 for this. For example, you could pass the subject in chunks that are
5497 500 bytes long, but in a buffer of 700 bytes, with the starting offset
5498 set to 200 and the previous 200 bytes at the start of the buffer.
5499
5500 3. Matching a subject string that is split into multiple segments does
5501 not always produce exactly the same result as matching over one single
5502 long string. The difference arises when there are multiple matching
5503 possibilities, because a partial match result is given only when there
5504 are no completed matches in a call to pcre_dfa_exec(). This means that
5505 as soon as the shortest match has been found, continuation to a new
5506 subject segment is no longer possible. Consider this pcretest example:
5507
5508 re> /dog(sbody)?/
5509 data> do\P\D
5510 Partial match: do
5511 data> gsb\R\P\D
5512 0: g
5513 data> dogsbody\D
5514 0: dogsbody
5515 1: dog
5516
5517 The pattern matches the words "dog" or "dogsbody". When the subject is
5518 presented in several parts ("do" and "gsb" being the first two) the
5519 match stops when "dog" has been found, and it is not possible to con-
5520 tinue. On the other hand, if "dogsbody" is presented as a single
5521 string, both matches are found.
5522
5523 Because of this phenomenon, it does not usually make sense to end a
5524 pattern that is going to be matched in this way with a variable repeat.
5525
5526 4. Patterns that contain alternatives at the top level which do not all
5527 start with the same pattern item may not work as expected. For example,
5528 consider this pattern:
5529
5530 1234|3789
5531
5532 If the first part of the subject is "ABC123", a partial match of the
5533 first alternative is found at offset 3. There is no partial match for
5534 the second alternative, because such a match does not start at the same
5535 point in the subject string. Attempting to continue with the string
5536 "789" does not yield a match because only those alternatives that match
5537 at one point in the subject are remembered. The problem arises because
5538 the start of the second alternative matches within the first alterna-
5539 tive. There is no problem with anchored patterns or patterns such as:
5540
5541 1234|ABCD
5542
5543 where no string can be a partial match for both alternatives.
5544
5545
5546 AUTHOR
5547
5548 Philip Hazel
5549 University Computing Service
5550 Cambridge CB2 3QH, England.
5551
5552
5553 REVISION
5554
5555 Last updated: 04 June 2007
5556 Copyright (c) 1997-2007 University of Cambridge.
5557 ------------------------------------------------------------------------------
5558
5559
5560 PCREPRECOMPILE(3) PCREPRECOMPILE(3)
5561
5562
5563 NAME
5564 PCRE - Perl-compatible regular expressions
5565
5566
5567 SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
5568
5569 If you are running an application that uses a large number of regular
5570 expression patterns, it may be useful to store them in a precompiled
5571 form instead of having to compile them every time the application is
5572 run. If you are not using any private character tables (see the
5573 pcre_maketables() documentation), this is relatively straightforward.
5574 If you are using private tables, it is a little bit more complicated.
5575
5576 If you save compiled patterns to a file, you can copy them to a differ-
5577 ent host and run them there. This works even if the new host has the
5578 opposite endianness to the one on which the patterns were compiled.
5579 There may be a small performance penalty, but it should be insignifi-
5580 cant. However, compiling regular expressions with one version of PCRE
5581 for use with a different version is not guaranteed to work and may
5582 cause crashes.
5583
5584
5585 SAVING A COMPILED PATTERN
5586 The value returned by pcre_compile() points to a single block of memory
5587 that holds the compiled pattern and associated data. You can find the
5588 length of this block in bytes by calling pcre_fullinfo() with an argu-
5589 ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
5590 manner. Here is sample code that compiles a pattern and writes it to a
5591 file. It assumes that the variable fd refers to a file that is open for
5592 output:
5593
5594 int erroroffset, rc, size;
5595 char *error;
5596 pcre *re;
5597
5598 re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
5599 if (re == NULL) { ... handle errors ... }
5600 rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
5601 if (rc < 0) { ... handle errors ... }
5602 rc = fwrite(re, 1, size, fd);
5603 if (rc != size) { ... handle errors ... }
5604
5605 In this example, the bytes that comprise the compiled pattern are
5606 copied exactly. Note that this is binary data that may contain any of
5607 the 256 possible byte values. On systems that make a distinction
5608 between binary and non-binary data, be sure that the file is opened for
5609 binary output.
5610
5611 If you want to write more than one pattern to a file, you will have to
5612 devise a way of separating them. For binary data, preceding each pat-
5613 tern with its length is probably the most straightforward approach.
5614 Another possibility is to write out the data in hexadecimal instead of
5615 binary, one pattern to a line.
5616
5617 Saving compiled patterns in a file is only one possible way of storing
5618 them for later use. They could equally well be saved in a database, or
5619 in the memory of some daemon process that passes them via sockets to
5620 the processes that want them.
5621
5622 If the pattern has been studied, it is also possible to save the study
5623 data in a similar way to the compiled pattern itself. When studying
5624 generates additional information, pcre_study() returns a pointer to a
5625 pcre_extra data block. Its format is defined in the section on matching
5626 a pattern in the pcreapi documentation. The study_data field points to
5627 the binary study data, and this is what you must save (not the
5628 pcre_extra block itself). The length of the study data can be obtained
5629 by calling pcre_fullinfo() with an argument of PCRE_INFO_STUDYSIZE.
5630 Remember to check that pcre_study() did return a non-NULL value before
5631 trying to save the study data.
5632
5633
5634 RE-USING A PRECOMPILED PATTERN
5635
5636 Re-using a precompiled pattern is straightforward. Having reloaded it
5637 into main memory, you pass its pointer to pcre_exec() or
5638 pcre_dfa_exec() in the usual way. This should work even on another
5639 host, and even if that host has the opposite endianness to the one
5640 where the pattern was compiled.
5641
5642 However, if you passed a pointer to custom character tables when the
5643 pattern was compiled (the tableptr argument of pcre_compile()), you
5644 must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
5645 because the value saved with the compiled pattern will obviously be
5646 nonsense. A field in a pcre_extra() block is used to pass this data, as
5647 described in the section on matching a pattern in the pcreapi documen-
5648 tation.
5649
5650 If you did not provide custom character tables when the pattern was
5651 compiled, the pointer in the compiled pattern is NULL, which causes
5652 pcre_exec() to use PCRE's internal tables. Thus, you do not need to
5653 take any special action at run time in this case.
5654
5655 If you saved study data with the compiled pattern, you need to create
5656 your own pcre_extra data block and set the study_data field to point to
5657 the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
5658 bit in the flags field to indicate that study data is present. Then
5659 pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
5660 usual way.
5661
5662
5663 COMPATIBILITY WITH DIFFERENT PCRE RELEASES
5664
5665 In general, it is safest to recompile all saved patterns when you
5666 update to a new PCRE release, though not all updates actually require
5667 this. Recompiling is definitely needed for release 7.2.
5668
5669
5670 AUTHOR
5671
5672 Philip Hazel
5673 University Computing Service
5674 Cambridge CB2 3QH, England.
5675
5676
5677 REVISION
5678
5679 Last updated: 13 June 2007
5680 Copyright (c) 1997-2007 University of Cambridge.
5681 ------------------------------------------------------------------------------
5682
5683
5684 PCREPERFORM(3) PCREPERFORM(3)
5685
5686
5687 NAME
5688 PCRE - Perl-compatible regular expressions
5689
5690
5691 PCRE PERFORMANCE
5692
5693 Two aspects of performance are discussed below: memory usage and pro-
5694 cessing time. The way you express your pattern as a regular expression
5695 can affect both of them.
5696
5697
5698 MEMORY USAGE
5699
5700 Patterns are compiled by PCRE into a reasonably efficient byte code, so
5701 that most simple patterns do not use much memory. However, there is one
5702 case where memory usage can be unexpectedly large. When a parenthesized
5703 subpattern has a quantifier with a minimum greater than 1 and/or a lim-
5704 ited maximum, the whole subpattern is repeated in the compiled code.
5705 For example, the pattern
5706
5707 (abc|def){2,4}
5708
5709 is compiled as if it were
5710
5711 (abc|def)(abc|def)((abc|def)(abc|def)?)?
5712
5713 (Technical aside: It is done this way so that backtrack points within
5714 each of the repetitions can be independently maintained.)
5715
5716 For regular expressions whose quantifiers use only small numbers, this
5717 is not usually a problem. However, if the numbers are large, and par-
5718 ticularly if such repetitions are nested, the memory usage can become
5719 an embarrassment. For example, the very simple pattern
5720
5721 ((ab){1,1000}c){1,3}
5722
5723 uses 51K bytes when compiled. When PCRE is compiled with its default
5724 internal pointer size of two bytes, the size limit on a compiled pat-
5725 tern is 64K, and this is reached with the above pattern if the outer
5726 repetition is increased from 3 to 4. PCRE can be compiled to use larger
5727 internal pointers and thus handle larger compiled patterns, but it is
5728 better to try to rewrite your pattern to use less memory if you can.
5729
5730 One way of reducing the memory usage for such patterns is to make use
5731 of PCRE's "subroutine" facility. Re-writing the above pattern as
5732
5733 ((ab)(?2){0,999}c)(?1){0,2}
5734
5735 reduces the memory requirements to 18K, and indeed it remains under 20K
5736 even with the outer repetition increased to 100. However, this pattern
5737 is not exactly equivalent, because the "subroutine" calls are treated
5738 as atomic groups into which there can be no backtracking if there is a
5739 subsequent matching failure. Therefore, PCRE cannot do this kind of
5740 rewriting automatically. Furthermore, there is a noticeable loss of
5741 speed when executing the modified pattern. Nevertheless, if the atomic
5742 grouping is not a problem and the loss of speed is acceptable, this
5743 kind of rewriting will allow you to process patterns that PCRE cannot
5744 otherwise handle.
5745
5746
5747 PROCESSING TIME
5748
5749 Certain items in regular expression patterns are processed more effi-
5750 ciently than others. It is more efficient to use a character class like
5751 [aeiou] than a set of single-character alternatives such as
5752 (a|e|i|o|u). In general, the simplest construction that provides the
5753 required behaviour is usually the most efficient. Jeffrey Friedl's book
5754 contains a lot of useful general discussion about optimizing regular
5755 expressions for efficient performance. This document contains a few
5756 observations about PCRE.
5757
5758 Using Unicode character properties (the \p, \P, and \X escapes) is
5759 slow, because PCRE has to scan a structure that contains data for over
5760 fifteen thousand characters whenever it needs a character's property.
5761 If you can find an alternative pattern that does not use character
5762 properties, it will probably be faster.
5763
5764 When a pattern begins with .* not in parentheses, or in parentheses
5765 that are not the subject of a backreference, and the PCRE_DOTALL option
5766 is set, the pattern is implicitly anchored by PCRE, since it can match
5767 only at the start of a subject string. However, if PCRE_DOTALL is not
5768 set, PCRE cannot make this optimization, because the . metacharacter
5769 does not then match a newline, and if the subject string contains new-
5770 lines, the pattern may match from the character immediately following
5771 one of them instead of from the very start. For example, the pattern
5772
5773 .*second
5774
5775 matches the subject "first\nand second" (where \n stands for a newline
5776 character), with the match starting at the seventh character. In order
5777 to do this, PCRE has to retry the match starting after every newline in
5778 the subject.
5779
5780 If you are using such a pattern with subject strings that do not con-
5781 tain newlines, the best performance is obtained by setting PCRE_DOTALL,
5782 or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
5783 ing. That saves PCRE from having to scan along the subject looking for
5784 a newline to restart at.
5785
5786 Beware of patterns that contain nested indefinite repeats. These can
5787 take a long time to run when applied to a string that does not match.
5788 Consider the pattern fragment
5789
5790 ^(a+)*
5791
5792 This can match "aaaa" in 16 different ways, and this number increases
5793 very rapidly as the string gets longer. (The * repeat can match 0, 1,
5794 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
5795 repeats can match different numbers of times.) When the remainder of
5796 the pattern is such that the entire match is going to fail, PCRE has in
5797 principle to try every possible variation, and this can take an
5798 extremely long time, even for relatively short strings.
5799
5800 An optimization catches some of the more simple cases such as
5801
5802 (a+)*b
5803
5804 where a literal character follows. Before embarking on the standard
5805 matching procedure, PCRE checks that there is a "b" later in the sub-
5806 ject string, and if there is not, it fails the match immediately. How-
5807 ever, when there is no following literal this optimization cannot be
5808 used. You can see the difference by comparing the behaviour of
5809
5810 (a+)*\d
5811
5812 with the pattern above. The former gives a failure almost instantly
5813 when applied to a whole line of "a" characters, whereas the latter
5814 takes an appreciable time with strings longer than about 20 characters.
5815
5816 In many cases, the solution to this kind of performance issue is to use
5817 an atomic group or a possessive quantifier.
5818
5819
5820 AUTHOR
5821
5822 Philip Hazel
5823 University Computing Service
5824 Cambridge CB2 3QH, England.
5825
5826
5827 REVISION
5828
5829 Last updated: 06 March 2007
5830 Copyright (c) 1997-2007 University of Cambridge.
5831 ------------------------------------------------------------------------------
5832
5833
5834 PCREPOSIX(3) PCREPOSIX(3)
5835
5836
5837 NAME
5838 PCRE - Perl-compatible regular expressions.
5839
5840
5841 SYNOPSIS OF POSIX API
5842
5843 #include <pcreposix.h>
5844
5845 int regcomp(regex_t *preg, const char *pattern,
5846 int cflags);
5847
5848 int regexec(regex_t *preg, const char *string,
5849 size_t nmatch, regmatch_t pmatch[], int eflags);
5850
5851 size_t regerror(int errcode, const regex_t *preg,
5852 char *errbuf, size_t errbuf_size);
5853
5854 void regfree(regex_t *preg);
5855
5856
5857 DESCRIPTION
5858
5859 This set of functions provides a POSIX-style API to the PCRE regular
5860 expression package. See the pcreapi documentation for a description of
5861 PCRE's native API, which contains much additional functionality.
5862
5863 The functions described here are just wrapper functions that ultimately
5864 call the PCRE native API. Their prototypes are defined in the
5865 pcreposix.h header file, and on Unix systems the library itself is
5866 called pcreposix.a, so can be accessed by adding -lpcreposix to the
5867 command for linking an application that uses them. Because the POSIX
5868 functions call the native ones, it is also necessary to add -lpcre.
5869
5870 I have implemented only those option bits that can be reasonably mapped
5871 to PCRE native options. In addition, the option REG_EXTENDED is defined
5872 with the value zero. This has no effect, but since programs that are
5873 written to the POSIX interface often use it, this makes it easier to
5874 slot in PCRE as a replacement library. Other POSIX options are not even
5875 defined.
5876
5877 When PCRE is called via these functions, it is only the API that is
5878 POSIX-like in style. The syntax and semantics of the regular expres-
5879 sions themselves are still those of Perl, subject to the setting of
5880 various PCRE options, as described below. "POSIX-like in style" means
5881 that the API approximates to the POSIX definition; it is not fully
5882 POSIX-compatible, and in multi-byte encoding domains it is probably
5883 even less compatible.
5884
5885 The header for these functions is supplied as pcreposix.h to avoid any
5886 potential clash with other POSIX libraries. It can, of course, be
5887 renamed or aliased as regex.h, which is the "correct" name. It provides
5888 two structure types, regex_t for compiled internal forms, and reg-
5889 match_t for returning captured substrings. It also defines some con-
5890 stants whose names start with "REG_"; these are used for setting
5891 options and identifying error codes.
5892
5893
5894 COMPILING A PATTERN
5895
5896 The function regcomp() is called to compile a pattern into an internal
5897 form. The pattern is a C string terminated by a binary zero, and is
5898 passed in the argument pattern. The preg argument is a pointer to a
5899 regex_t structure that is used as a base for storing information about
5900 the compiled regular expression.
5901
5902 The argument cflags is either zero, or contains one or more of the bits
5903 defined by the following macros:
5904
5905 REG_DOTALL
5906
5907 The PCRE_DOTALL option is set when the regular expression is passed for
5908 compilation to the native function. Note that REG_DOTALL is not part of
5909 the POSIX standard.
5910
5911 REG_ICASE
5912
5913 The PCRE_CASELESS option is set when the regular expression is passed
5914 for compilation to the native function.
5915
5916 REG_NEWLINE
5917
5918 The PCRE_MULTILINE option is set when the regular expression is passed
5919 for compilation to the native function. Note that this does not mimic
5920 the defined POSIX behaviour for REG_NEWLINE (see the following sec-
5921 tion).
5922
5923 REG_NOSUB
5924
5925 The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
5926 passed for compilation to the native function. In addition, when a pat-
5927 tern that is compiled with this flag is passed to regexec() for match-
5928 ing, the nmatch and pmatch arguments are ignored, and no captured
5929 strings are returned.
5930
5931 REG_UTF8
5932
5933 The PCRE_UTF8 option is set when the regular expression is passed for
5934 compilation to the native function. This causes the pattern itself and
5935 all data strings used for matching it to be treated as UTF-8 strings.
5936 Note that REG_UTF8 is not part of the POSIX standard.
5937
5938 In the absence of these flags, no options are passed to the native
5939 function. This means the the regex is compiled with PCRE default
5940 semantics. In particular, the way it handles newline characters in the
5941 subject string is the Perl way, not the POSIX way. Note that setting
5942 PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
5943 It does not affect the way newlines are matched by . (they aren't) or
5944 by a negative class such as [^a] (they are).
5945
5946 The yield of regcomp() is zero on success, and non-zero otherwise. The
5947 preg structure is filled in on success, and one member of the structure
5948 is public: re_nsub contains the number of capturing subpatterns in the
5949 regular expression. Various error codes are defined in the header file.
5950
5951
5952 MATCHING NEWLINE CHARACTERS
5953
5954 This area is not simple, because POSIX and Perl take different views of
5955 things. It is not possible to get PCRE to obey POSIX semantics, but
5956 then PCRE was never intended to be a POSIX engine. The following table
5957 lists the different possibilities for matching newline characters in
5958 PCRE:
5959
5960 Default Change with
5961
5962 . matches newline no PCRE_DOTALL
5963 newline matches [^a] yes not changeable
5964 $ matches \n at end yes PCRE_DOLLARENDONLY
5965 $ matches \n in middle no PCRE_MULTILINE
5966 ^ matches \n in middle no PCRE_MULTILINE
5967
5968 This is the equivalent table for POSIX:
5969
5970 Default Change with
5971
5972 . matches newline yes REG_NEWLINE
5973 newline matches [^a] yes REG_NEWLINE
5974 $ matches \n at end no REG_NEWLINE
5975 $ matches \n in middle no REG_NEWLINE
5976 ^ matches \n in middle no REG_NEWLINE
5977
5978 PCRE's behaviour is the same as Perl's, except that there is no equiva-
5979 lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
5980 no way to stop newline from matching [^a].
5981
5982 The default POSIX newline handling can be obtained by setting
5983 PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
5984 behave exactly as for the REG_NEWLINE action.
5985
5986
5987 MATCHING A PATTERN
5988
5989 The function regexec() is called to match a compiled pattern preg
5990 against a given string, which is by default terminated by a zero byte
5991 (but see REG_STARTEND below), subject to the options in eflags. These
5992 can be:
5993
5994 REG_NOTBOL
5995
5996 The PCRE_NOTBOL option is set when calling the underlying PCRE matching
5997 function.
5998
5999 REG_NOTEOL
6000
6001 The PCRE_NOTEOL option is set when calling the underlying PCRE matching
6002 function.
6003
6004 REG_STARTEND
6005
6006 The string is considered to start at string + pmatch[0].rm_so and to
6007 have a terminating NUL located at string + pmatch[0].rm_eo (there need
6008 not actually be a NUL at that location), regardless of the value of
6009 nmatch. This is a BSD extension, compatible with but not specified by
6010 IEEE Standard 1003.2 (POSIX.2), and should be used with caution in
6011 software intended to be portable to other systems. Note that a non-zero
6012 rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
6013 of the string, not how it is matched.
6014
6015 If the pattern was compiled with the REG_NOSUB flag, no data about any
6016 matched strings is returned. The nmatch and pmatch arguments of
6017 regexec() are ignored.
6018
6019 Otherwise,the portion of the string that was matched, and also any cap-
6020 tured substrings, are returned via the pmatch argument, which points to
6021 an array of nmatch structures of type regmatch_t, containing the mem-
6022 bers rm_so and rm_eo. These contain the offset to the first character
6023 of each substring and the offset to the first character after the end
6024 of each substring, respectively. The 0th element of the vector relates
6025 to the entire portion of string that was matched; subsequent elements
6026 relate to the capturing subpatterns of the regular expression. Unused
6027 entries in the array have both structure members set to -1.
6028
6029 A successful match yields a zero return; various error codes are
6030 defined in the header file, of which REG_NOMATCH is the "expected"
6031 failure code.
6032
6033
6034 ERROR MESSAGES
6035
6036 The regerror() function maps a non-zero errorcode from either regcomp()
6037 or regexec() to a printable message. If preg is not NULL, the error
6038 should have arisen from the use of that structure. A message terminated
6039 by a binary zero is placed in errbuf. The length of the message,
6040 including the zero, is limited to errbuf_size. The yield of the func-
6041 tion is the size of buffer needed to hold the whole message.
6042
6043
6044 MEMORY USAGE
6045
6046 Compiling a regular expression causes memory to be allocated and asso-
6047 ciated with the preg structure. The function regfree() frees all such
6048 memory, after which preg may no longer be used as a compiled expres-
6049 sion.
6050
6051
6052 AUTHOR
6053
6054 Philip Hazel
6055 University Computing Service
6056 Cambridge CB2 3QH, England.
6057
6058
6059 REVISION
6060
6061 Last updated: 05 April 2008
6062 Copyright (c) 1997-2008 University of Cambridge.
6063 ------------------------------------------------------------------------------
6064
6065
6066 PCRECPP(3) PCRECPP(3)
6067
6068
6069 NAME
6070 PCRE - Perl-compatible regular expressions.
6071
6072
6073 SYNOPSIS OF C++ WRAPPER
6074
6075 #include <pcrecpp.h>
6076
6077
6078 DESCRIPTION
6079
6080 The C++ wrapper for PCRE was provided by Google Inc. Some additional
6081 functionality was added by Giuseppe Maxia. This brief man page was con-
6082 structed from the notes in the pcrecpp.h file, which should be con-
6083 sulted for further details.
6084
6085
6086 MATCHING INTERFACE
6087
6088 The "FullMatch" operation checks that supplied text matches a supplied
6089 pattern exactly. If pointer arguments are supplied, it copies matched
6090 sub-strings that match sub-patterns into them.
6091
6092 Example: successful match
6093 pcrecpp::RE re("h.*o");
6094 re.FullMatch("hello");
6095
6096 Example: unsuccessful match (requires full match):
6097 pcrecpp::RE re("e");
6098 !re.FullMatch("hello");
6099
6100 Example: creating a temporary RE object:
6101 pcrecpp::RE("h.*o").FullMatch("hello");
6102
6103 You can pass in a "const char*" or a "string" for "text". The examples
6104 below tend to use a const char*. You can, as in the different examples
6105 above, store the RE object explicitly in a variable or use a temporary
6106 RE object. The examples below use one mode or the