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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. Neither has
6 the pcredemo program. There are separate text files for the pcregrep and
7 pcretest commands.
8 -----------------------------------------------------------------------------
9
10
11 PCRE(3) PCRE(3)
12
13
14 NAME
15 PCRE - Perl-compatible regular expressions
16
17
18 INTRODUCTION
19
20 The PCRE library is a set of functions that implement regular expres-
21 sion pattern matching using the same syntax and semantics as Perl, with
22 just a few differences. Some features that appeared in Python and PCRE
23 before they appeared in Perl are also available using the Python syn-
24 tax, there is some support for one or two .NET and Oniguruma syntax
25 items, and there is an option for requesting some minor changes that
26 give better JavaScript compatibility.
27
28 The current implementation of PCRE corresponds approximately with Perl
29 5.12, including support for UTF-8 encoded strings and Unicode general
30 category properties. However, UTF-8 and Unicode support has to be
31 explicitly enabled; it is not the default. The Unicode tables corre-
32 spond to Unicode release 6.0.0.
33
34 In addition to the Perl-compatible matching function, PCRE contains an
35 alternative function that matches the same compiled patterns in a dif-
36 ferent way. In certain circumstances, the alternative function has some
37 advantages. For a discussion of the two matching algorithms, see the
38 pcrematching page.
39
40 PCRE is written in C and released as a C library. A number of people
41 have written wrappers and interfaces of various kinds. In particular,
42 Google Inc. have provided a comprehensive C++ wrapper. This is now
43 included as part of the PCRE distribution. The pcrecpp page has details
44 of this interface. Other people's contributions can be found in the
45 Contrib directory at the primary FTP site, which is:
46
47 ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
48
49 Details of exactly which Perl regular expression features are and are
50 not supported by PCRE are given in separate documents. See the pcrepat-
51 tern and pcrecompat pages. There is a syntax summary in the pcresyntax
52 page.
53
54 Some features of PCRE can be included, excluded, or changed when the
55 library is built. The pcre_config() function makes it possible for a
56 client to discover which features are available. The features them-
57 selves are described in the pcrebuild page. Documentation about build-
58 ing PCRE for various operating systems can be found in the README and
59 NON-UNIX-USE files in the source distribution.
60
61 The library contains a number of undocumented internal functions and
62 data tables that are used by more than one of the exported external
63 functions, but which are not intended for use by external callers.
64 Their names all begin with "_pcre_", which hopefully will not provoke
65 any name clashes. In some environments, it is possible to control which
66 external symbols are exported when a shared library is built, and in
67 these cases the undocumented symbols are not exported.
68
69
70 USER DOCUMENTATION
71
72 The user documentation for PCRE comprises a number of different sec-
73 tions. In the "man" format, each of these is a separate "man page". In
74 the HTML format, each is a separate page, linked from the index page.
75 In the plain text format, all the sections, except the pcredemo sec-
76 tion, are concatenated, for ease of searching. The sections are as fol-
77 lows:
78
79 pcre this document
80 pcre-config show PCRE installation configuration information
81 pcreapi details of PCRE's native C API
82 pcrebuild options for building PCRE
83 pcrecallout details of the callout feature
84 pcrecompat discussion of Perl compatibility
85 pcrecpp details of the C++ wrapper
86 pcredemo a demonstration C program that uses PCRE
87 pcregrep description of the pcregrep command
88 pcrematching discussion of the two matching algorithms
89 pcrepartial details of the partial matching facility
90 pcrepattern syntax and semantics of supported
91 regular expressions
92 pcreperform discussion of performance issues
93 pcreposix the POSIX-compatible C API
94 pcreprecompile details of saving and re-using precompiled patterns
95 pcresample discussion of the pcredemo program
96 pcrestack discussion of stack usage
97 pcresyntax quick syntax reference
98 pcretest description of the pcretest testing command
99
100 In addition, in the "man" and HTML formats, there is a short page for
101 each C library function, listing its arguments and results.
102
103
104 LIMITATIONS
105
106 There are some size limitations in PCRE but it is hoped that they will
107 never in practice be relevant.
108
109 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
110 is compiled with the default internal linkage size of 2. If you want to
111 process regular expressions that are truly enormous, you can compile
112 PCRE with an internal linkage size of 3 or 4 (see the README file in
113 the source distribution and the pcrebuild documentation for details).
114 In these cases the limit is substantially larger. However, the speed
115 of execution is slower.
116
117 All values in repeating quantifiers must be less than 65536.
118
119 There is no limit to the number of parenthesized subpatterns, but there
120 can be no more than 65535 capturing subpatterns.
121
122 The maximum length of name for a named subpattern is 32 characters, and
123 the maximum number of named subpatterns is 10000.
124
125 The maximum length of a subject string is the largest positive number
126 that an integer variable can hold. However, when using the traditional
127 matching function, PCRE uses recursion to handle subpatterns and indef-
128 inite repetition. This means that the available stack space may limit
129 the size of a subject string that can be processed by certain patterns.
130 For a discussion of stack issues, see the pcrestack documentation.
131
132
133 UTF-8 AND UNICODE PROPERTY SUPPORT
134
135 From release 3.3, PCRE has had some support for character strings
136 encoded in the UTF-8 format. For release 4.0 this was greatly extended
137 to cover most common requirements, and in release 5.0 additional sup-
138 port for Unicode general category properties was added.
139
140 In order process UTF-8 strings, you must build PCRE to include UTF-8
141 support in the code, and, in addition, you must call pcre_compile()
142 with the PCRE_UTF8 option flag, or the pattern must start with the
143 sequence (*UTF8). When either of these is the case, both the pattern
144 and any subject strings that are matched against it are treated as
145 UTF-8 strings instead of strings of 1-byte characters.
146
147 If you compile PCRE with UTF-8 support, but do not use it at run time,
148 the library will be a bit bigger, but the additional run time overhead
149 is limited to testing the PCRE_UTF8 flag occasionally, so should not be
150 very big.
151
152 If PCRE is built with Unicode character property support (which implies
153 UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
154 ported. The available properties that can be tested are limited to the
155 general category properties such as Lu for an upper case letter or Nd
156 for a decimal number, the Unicode script names such as Arabic or Han,
157 and the derived properties Any and L&. A full list is given in the
158 pcrepattern documentation. Only the short names for properties are sup-
159 ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
160 ter}, is not supported. Furthermore, in Perl, many properties may
161 optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
162 does not support this.
163
164 Validity of UTF-8 strings
165
166 When you set the PCRE_UTF8 flag, the strings passed as patterns and
167 subjects are (by default) checked for validity on entry to the relevant
168 functions. From release 7.3 of PCRE, the check is according the rules
169 of RFC 3629, which are themselves derived from the Unicode specifica-
170 tion. Earlier releases of PCRE followed the rules of RFC 2279, which
171 allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
172 check allows only values in the range U+0 to U+10FFFF, excluding U+D800
173 to U+DFFF.
174
175 The excluded code points are the "Low Surrogate Area" of Unicode, of
176 which the Unicode Standard says this: "The Low Surrogate Area does not
177 contain any character assignments, consequently no character code
178 charts or namelists are provided for this area. Surrogates are reserved
179 for use with UTF-16 and then must be used in pairs." The code points
180 that are encoded by UTF-16 pairs are available as independent code
181 points in the UTF-8 encoding. (In other words, the whole surrogate
182 thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
183
184 If an invalid UTF-8 string is passed to PCRE, an error return is given.
185 At compile time, the only additional information is the offset to the
186 first byte of the failing character. The runtime functions (pcre_exec()
187 and pcre_dfa_exec()), pass back this information as well as a more
188 detailed reason code if the caller has provided memory in which to do
189 this.
190
191 In some situations, you may already know that your strings are valid,
192 and therefore want to skip these checks in order to improve perfor-
193 mance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run
194 time, PCRE assumes that the pattern or subject it is given (respec-
195 tively) contains only valid UTF-8 codes. In this case, it does not
196 diagnose an invalid UTF-8 string.
197
198 If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
199 what happens depends on why the string is invalid. If the string con-
200 forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
201 string of characters in the range 0 to 0x7FFFFFFF. In other words,
202 apart from the initial validity test, PCRE (when in UTF-8 mode) handles
203 strings according to the more liberal rules of RFC 2279. However, if
204 the string does not even conform to RFC 2279, the result is undefined.
205 Your program may crash.
206
207 If you want to process strings of values in the full range 0 to
208 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
209 set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
210 this situation, you will have to apply your own validity check.
211
212 General comments about UTF-8 mode
213
214 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
215 two-byte UTF-8 character if the value is greater than 127.
216
217 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
218 characters for values greater than \177.
219
220 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
221 vidual bytes, for example: \x{100}{3}.
222
223 4. The dot metacharacter matches one UTF-8 character instead of a sin-
224 gle byte.
225
226 5. The escape sequence \C can be used to match a single byte in UTF-8
227 mode, but its use can lead to some strange effects. This facility is
228 not available in the alternative matching function, pcre_dfa_exec().
229
230 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
231 test characters of any code value, but, by default, the characters that
232 PCRE recognizes as digits, spaces, or word characters remain the same
233 set as before, all with values less than 256. This remains true even
234 when PCRE is built to include Unicode property support, because to do
235 otherwise would slow down PCRE in many common cases. Note in particular
236 that this applies to \b and \B, because they are defined in terms of \w
237 and \W. If you really want to test for a wider sense of, say, "digit",
238 you can use explicit Unicode property tests such as \p{Nd}. Alterna-
239 tively, if you set the PCRE_UCP option, the way that the character
240 escapes work is changed so that Unicode properties are used to deter-
241 mine which characters match. There are more details in the section on
242 generic character types in the pcrepattern documentation.
243
244 7. Similarly, characters that match the POSIX named character classes
245 are all low-valued characters, unless the PCRE_UCP option is set.
246
247 8. However, the horizontal and vertical whitespace matching escapes
248 (\h, \H, \v, and \V) do match all the appropriate Unicode characters,
249 whether or not PCRE_UCP is set.
250
251 9. Case-insensitive matching applies only to characters whose values
252 are less than 128, unless PCRE is built with Unicode property support.
253 Even when Unicode property support is available, PCRE still uses its
254 own character tables when checking the case of low-valued characters,
255 so as not to degrade performance. The Unicode property information is
256 used only for characters with higher values. Furthermore, PCRE supports
257 case-insensitive matching only when there is a one-to-one mapping
258 between a letter's cases. There are a small number of many-to-one map-
259 pings in Unicode; these are not supported by PCRE.
260
261
262 AUTHOR
263
264 Philip Hazel
265 University Computing Service
266 Cambridge CB2 3QH, England.
267
268 Putting an actual email address here seems to have been a spam magnet,
269 so I've taken it away. If you want to email me, use my two initials,
270 followed by the two digits 10, at the domain cam.ac.uk.
271
272
273 REVISION
274
275 Last updated: 07 May 2011
276 Copyright (c) 1997-2011 University of Cambridge.
277 ------------------------------------------------------------------------------
278
279
280 PCREBUILD(3) PCREBUILD(3)
281
282
283 NAME
284 PCRE - Perl-compatible regular expressions
285
286
287 PCRE BUILD-TIME OPTIONS
288
289 This document describes the optional features of PCRE that can be
290 selected when the library is compiled. It assumes use of the configure
291 script, where the optional features are selected or deselected by pro-
292 viding options to configure before running the make command. However,
293 the same options can be selected in both Unix-like and non-Unix-like
294 environments using the GUI facility of cmake-gui if you are using CMake
295 instead of configure to build PCRE.
296
297 There is a lot more information about building PCRE in non-Unix-like
298 environments in the file called NON_UNIX_USE, which is part of the PCRE
299 distribution. You should consult this file as well as the README file
300 if you are building in a non-Unix-like environment.
301
302 The complete list of options for configure (which includes the standard
303 ones such as the selection of the installation directory) can be
304 obtained by running
305
306 ./configure --help
307
308 The following sections include descriptions of options whose names
309 begin with --enable or --disable. These settings specify changes to the
310 defaults for the configure command. Because of the way that configure
311 works, --enable and --disable always come in pairs, so the complemen-
312 tary option always exists as well, but as it specifies the default, it
313 is not described.
314
315
316 BUILDING SHARED AND STATIC LIBRARIES
317
318 The PCRE building process uses libtool to build both shared and static
319 Unix libraries by default. You can suppress one of these by adding one
320 of
321
322 --disable-shared
323 --disable-static
324
325 to the configure command, as required.
326
327
328 C++ SUPPORT
329
330 By default, the configure script will search for a C++ compiler and C++
331 header files. If it finds them, it automatically builds the C++ wrapper
332 library for PCRE. You can disable this by adding
333
334 --disable-cpp
335
336 to the configure command.
337
338
339 UTF-8 SUPPORT
340
341 To build PCRE with support for UTF-8 Unicode character strings, add
342
343 --enable-utf8
344
345 to the configure command. Of itself, this does not make PCRE treat
346 strings as UTF-8. As well as compiling PCRE with this option, you also
347 have have to set the PCRE_UTF8 option when you call the pcre_compile()
348 or pcre_compile2() functions.
349
350 If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE
351 expects its input to be either ASCII or UTF-8 (depending on the runtime
352 option). It is not possible to support both EBCDIC and UTF-8 codes in
353 the same version of the library. Consequently, --enable-utf8 and
354 --enable-ebcdic are mutually exclusive.
355
356
357 UNICODE CHARACTER PROPERTY SUPPORT
358
359 UTF-8 support allows PCRE to process character values greater than 255
360 in the strings that it handles. On its own, however, it does not pro-
361 vide any facilities for accessing the properties of such characters. If
362 you want to be able to use the pattern escapes \P, \p, and \X, which
363 refer to Unicode character properties, you must add
364
365 --enable-unicode-properties
366
367 to the configure command. This implies UTF-8 support, even if you have
368 not explicitly requested it.
369
370 Including Unicode property support adds around 30K of tables to the
371 PCRE library. Only the general category properties such as Lu and Nd
372 are supported. Details are given in the pcrepattern documentation.
373
374
375 CODE VALUE OF NEWLINE
376
377 By default, PCRE interprets the linefeed (LF) character as indicating
378 the end of a line. This is the normal newline character on Unix-like
379 systems. You can compile PCRE to use carriage return (CR) instead, by
380 adding
381
382 --enable-newline-is-cr
383
384 to the configure command. There is also a --enable-newline-is-lf
385 option, which explicitly specifies linefeed as the newline character.
386
387 Alternatively, you can specify that line endings are to be indicated by
388 the two character sequence CRLF. If you want this, add
389
390 --enable-newline-is-crlf
391
392 to the configure command. There is a fourth option, specified by
393
394 --enable-newline-is-anycrlf
395
396 which causes PCRE to recognize any of the three sequences CR, LF, or
397 CRLF as indicating a line ending. Finally, a fifth option, specified by
398
399 --enable-newline-is-any
400
401 causes PCRE to recognize any Unicode newline sequence.
402
403 Whatever line ending convention is selected when PCRE is built can be
404 overridden when the library functions are called. At build time it is
405 conventional to use the standard for your operating system.
406
407
408 WHAT \R MATCHES
409
410 By default, the sequence \R in a pattern matches any Unicode newline
411 sequence, whatever has been selected as the line ending sequence. If
412 you specify
413
414 --enable-bsr-anycrlf
415
416 the default is changed so that \R matches only CR, LF, or CRLF. What-
417 ever is selected when PCRE is built can be overridden when the library
418 functions are called.
419
420
421 POSIX MALLOC USAGE
422
423 When PCRE is called through the POSIX interface (see the pcreposix doc-
424 umentation), additional working storage is required for holding the
425 pointers to capturing substrings, because PCRE requires three integers
426 per substring, whereas the POSIX interface provides only two. If the
427 number of expected substrings is small, the wrapper function uses space
428 on the stack, because this is faster than using malloc() for each call.
429 The default threshold above which the stack is no longer used is 10; it
430 can be changed by adding a setting such as
431
432 --with-posix-malloc-threshold=20
433
434 to the configure command.
435
436
437 HANDLING VERY LARGE PATTERNS
438
439 Within a compiled pattern, offset values are used to point from one
440 part to another (for example, from an opening parenthesis to an alter-
441 nation metacharacter). By default, two-byte values are used for these
442 offsets, leading to a maximum size for a compiled pattern of around
443 64K. This is sufficient to handle all but the most gigantic patterns.
444 Nevertheless, some people do want to process truyl enormous patterns,
445 so it is possible to compile PCRE to use three-byte or four-byte off-
446 sets by adding a setting such as
447
448 --with-link-size=3
449
450 to the configure command. The value given must be 2, 3, or 4. Using
451 longer offsets slows down the operation of PCRE because it has to load
452 additional bytes when handling them.
453
454
455 AVOIDING EXCESSIVE STACK USAGE
456
457 When matching with the pcre_exec() function, PCRE implements backtrack-
458 ing by making recursive calls to an internal function called match().
459 In environments where the size of the stack is limited, this can se-
460 verely limit PCRE's operation. (The Unix environment does not usually
461 suffer from this problem, but it may sometimes be necessary to increase
462 the maximum stack size. There is a discussion in the pcrestack docu-
463 mentation.) An alternative approach to recursion that uses memory from
464 the heap to remember data, instead of using recursive function calls,
465 has been implemented to work round the problem of limited stack size.
466 If you want to build a version of PCRE that works this way, add
467
468 --disable-stack-for-recursion
469
470 to the configure command. With this configuration, PCRE will use the
471 pcre_stack_malloc and pcre_stack_free variables to call memory manage-
472 ment functions. By default these point to malloc() and free(), but you
473 can replace the pointers so that your own functions are used instead.
474
475 Separate functions are provided rather than using pcre_malloc and
476 pcre_free because the usage is very predictable: the block sizes
477 requested are always the same, and the blocks are always freed in
478 reverse order. A calling program might be able to implement optimized
479 functions that perform better than malloc() and free(). PCRE runs
480 noticeably more slowly when built in this way. This option affects only
481 the pcre_exec() function; it is not relevant for pcre_dfa_exec().
482
483
484 LIMITING PCRE RESOURCE USAGE
485
486 Internally, PCRE has a function called match(), which it calls repeat-
487 edly (sometimes recursively) when matching a pattern with the
488 pcre_exec() function. By controlling the maximum number of times this
489 function may be called during a single matching operation, a limit can
490 be placed on the resources used by a single call to pcre_exec(). The
491 limit can be changed at run time, as described in the pcreapi documen-
492 tation. The default is 10 million, but this can be changed by adding a
493 setting such as
494
495 --with-match-limit=500000
496
497 to the configure command. This setting has no effect on the
498 pcre_dfa_exec() matching function.
499
500 In some environments it is desirable to limit the depth of recursive
501 calls of match() more strictly than the total number of calls, in order
502 to restrict the maximum amount of stack (or heap, if --disable-stack-
503 for-recursion is specified) that is used. A second limit controls this;
504 it defaults to the value that is set for --with-match-limit, which
505 imposes no additional constraints. However, you can set a lower limit
506 by adding, for example,
507
508 --with-match-limit-recursion=10000
509
510 to the configure command. This value can also be overridden at run
511 time.
512
513
514 CREATING CHARACTER TABLES AT BUILD TIME
515
516 PCRE uses fixed tables for processing characters whose code values are
517 less than 256. By default, PCRE is built with a set of tables that are
518 distributed in the file pcre_chartables.c.dist. These tables are for
519 ASCII codes only. If you add
520
521 --enable-rebuild-chartables
522
523 to the configure command, the distributed tables are no longer used.
524 Instead, a program called dftables is compiled and run. This outputs
525 the source for new set of tables, created in the default locale of your
526 C runtime system. (This method of replacing the tables does not work if
527 you are cross compiling, because dftables is run on the local host. If
528 you need to create alternative tables when cross compiling, you will
529 have to do so "by hand".)
530
531
532 USING EBCDIC CODE
533
534 PCRE assumes by default that it will run in an environment where the
535 character code is ASCII (or Unicode, which is a superset of ASCII).
536 This is the case for most computer operating systems. PCRE can, how-
537 ever, be compiled to run in an EBCDIC environment by adding
538
539 --enable-ebcdic
540
541 to the configure command. This setting implies --enable-rebuild-charta-
542 bles. You should only use it if you know that you are in an EBCDIC
543 environment (for example, an IBM mainframe operating system). The
544 --enable-ebcdic option is incompatible with --enable-utf8.
545
546
547 PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
548
549 By default, pcregrep reads all files as plain text. You can build it so
550 that it recognizes files whose names end in .gz or .bz2, and reads them
551 with libz or libbz2, respectively, by adding one or both of
552
553 --enable-pcregrep-libz
554 --enable-pcregrep-libbz2
555
556 to the configure command. These options naturally require that the rel-
557 evant libraries are installed on your system. Configuration will fail
558 if they are not.
559
560
561 PCREGREP BUFFER SIZE
562
563 pcregrep uses an internal buffer to hold a "window" on the file it is
564 scanning, in order to be able to output "before" and "after" lines when
565 it finds a match. The size of the buffer is controlled by a parameter
566 whose default value is 20K. The buffer itself is three times this size,
567 but because of the way it is used for holding "before" lines, the long-
568 est line that is guaranteed to be processable is the parameter size.
569 You can change the default parameter value by adding, for example,
570
571 --with-pcregrep-bufsize=50K
572
573 to the configure command. The caller of pcregrep can, however, override
574 this value by specifying a run-time option.
575
576
577 PCRETEST OPTION FOR LIBREADLINE SUPPORT
578
579 If you add
580
581 --enable-pcretest-libreadline
582
583 to the configure command, pcretest is linked with the libreadline
584 library, and when its input is from a terminal, it reads it using the
585 readline() function. This provides line-editing and history facilities.
586 Note that libreadline is GPL-licensed, so if you distribute a binary of
587 pcretest linked in this way, there may be licensing issues.
588
589 Setting this option causes the -lreadline option to be added to the
590 pcretest build. In many operating environments with a sytem-installed
591 libreadline this is sufficient. However, in some environments (e.g. if
592 an unmodified distribution version of readline is in use), some extra
593 configuration may be necessary. The INSTALL file for libreadline says
594 this:
595
596 "Readline uses the termcap functions, but does not link with the
597 termcap or curses library itself, allowing applications which link
598 with readline the to choose an appropriate library."
599
600 If your environment has not been set up so that an appropriate library
601 is automatically included, you may need to add something like
602
603 LIBS="-ncurses"
604
605 immediately before the configure command.
606
607
608 SEE ALSO
609
610 pcreapi(3), pcre_config(3).
611
612
613 AUTHOR
614
615 Philip Hazel
616 University Computing Service
617 Cambridge CB2 3QH, England.
618
619
620 REVISION
621
622 Last updated: 02 August 2011
623 Copyright (c) 1997-2011 University of Cambridge.
624 ------------------------------------------------------------------------------
625
626
627 PCREMATCHING(3) PCREMATCHING(3)
628
629
630 NAME
631 PCRE - Perl-compatible regular expressions
632
633
634 PCRE MATCHING ALGORITHMS
635
636 This document describes the two different algorithms that are available
637 in PCRE for matching a compiled regular expression against a given sub-
638 ject string. The "standard" algorithm is the one provided by the
639 pcre_exec() function. This works in the same was as Perl's matching
640 function, and provides a Perl-compatible matching operation.
641
642 An alternative algorithm is provided by the pcre_dfa_exec() function;
643 this operates in a different way, and is not Perl-compatible. It has
644 advantages and disadvantages compared with the standard algorithm, and
645 these are described below.
646
647 When there is only one possible way in which a given subject string can
648 match a pattern, the two algorithms give the same answer. A difference
649 arises, however, when there are multiple possibilities. For example, if
650 the pattern
651
652 ^<.*>
653
654 is matched against the string
655
656 <something> <something else> <something further>
657
658 there are three possible answers. The standard algorithm finds only one
659 of them, whereas the alternative algorithm finds all three.
660
661
662 REGULAR EXPRESSIONS AS TREES
663
664 The set of strings that are matched by a regular expression can be rep-
665 resented as a tree structure. An unlimited repetition in the pattern
666 makes the tree of infinite size, but it is still a tree. Matching the
667 pattern to a given subject string (from a given starting point) can be
668 thought of as a search of the tree. There are two ways to search a
669 tree: depth-first and breadth-first, and these correspond to the two
670 matching algorithms provided by PCRE.
671
672
673 THE STANDARD MATCHING ALGORITHM
674
675 In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
676 sions", the standard algorithm is an "NFA algorithm". It conducts a
677 depth-first search of the pattern tree. That is, it proceeds along a
678 single path through the tree, checking that the subject matches what is
679 required. When there is a mismatch, the algorithm tries any alterna-
680 tives at the current point, and if they all fail, it backs up to the
681 previous branch point in the tree, and tries the next alternative
682 branch at that level. This often involves backing up (moving to the
683 left) in the subject string as well. The order in which repetition
684 branches are tried is controlled by the greedy or ungreedy nature of
685 the quantifier.
686
687 If a leaf node is reached, a matching string has been found, and at
688 that point the algorithm stops. Thus, if there is more than one possi-
689 ble match, this algorithm returns the first one that it finds. Whether
690 this is the shortest, the longest, or some intermediate length depends
691 on the way the greedy and ungreedy repetition quantifiers are specified
692 in the pattern.
693
694 Because it ends up with a single path through the tree, it is rela-
695 tively straightforward for this algorithm to keep track of the sub-
696 strings that are matched by portions of the pattern in parentheses.
697 This provides support for capturing parentheses and back references.
698
699
700 THE ALTERNATIVE MATCHING ALGORITHM
701
702 This algorithm conducts a breadth-first search of the tree. Starting
703 from the first matching point in the subject, it scans the subject
704 string from left to right, once, character by character, and as it does
705 this, it remembers all the paths through the tree that represent valid
706 matches. In Friedl's terminology, this is a kind of "DFA algorithm",
707 though it is not implemented as a traditional finite state machine (it
708 keeps multiple states active simultaneously).
709
710 Although the general principle of this matching algorithm is that it
711 scans the subject string only once, without backtracking, there is one
712 exception: when a lookaround assertion is encountered, the characters
713 following or preceding the current point have to be independently
714 inspected.
715
716 The scan continues until either the end of the subject is reached, or
717 there are no more unterminated paths. At this point, terminated paths
718 represent the different matching possibilities (if there are none, the
719 match has failed). Thus, if there is more than one possible match,
720 this algorithm finds all of them, and in particular, it finds the long-
721 est. The matches are returned in decreasing order of length. There is
722 an option to stop the algorithm after the first match (which is neces-
723 sarily the shortest) is found.
724
725 Note that all the matches that are found start at the same point in the
726 subject. If the pattern
727
728 cat(er(pillar)?)?
729
730 is matched against the string "the caterpillar catchment", the result
731 will be the three strings "caterpillar", "cater", and "cat" that start
732 at the fifth character of the subject. The algorithm does not automati-
733 cally move on to find matches that start at later positions.
734
735 There are a number of features of PCRE regular expressions that are not
736 supported by the alternative matching algorithm. They are as follows:
737
738 1. Because the algorithm finds all possible matches, the greedy or
739 ungreedy nature of repetition quantifiers is not relevant. Greedy and
740 ungreedy quantifiers are treated in exactly the same way. However, pos-
741 sessive quantifiers can make a difference when what follows could also
742 match what is quantified, for example in a pattern like this:
743
744 ^a++\w!
745
746 This pattern matches "aaab!" but not "aaa!", which would be matched by
747 a non-possessive quantifier. Similarly, if an atomic group is present,
748 it is matched as if it were a standalone pattern at the current point,
749 and the longest match is then "locked in" for the rest of the overall
750 pattern.
751
752 2. When dealing with multiple paths through the tree simultaneously, it
753 is not straightforward to keep track of captured substrings for the
754 different matching possibilities, and PCRE's implementation of this
755 algorithm does not attempt to do this. This means that no captured sub-
756 strings are available.
757
758 3. Because no substrings are captured, back references within the pat-
759 tern are not supported, and cause errors if encountered.
760
761 4. For the same reason, conditional expressions that use a backrefer-
762 ence as the condition or test for a specific group recursion are not
763 supported.
764
765 5. Because many paths through the tree may be active, the \K escape
766 sequence, which resets the start of the match when encountered (but may
767 be on some paths and not on others), is not supported. It causes an
768 error if encountered.
769
770 6. Callouts are supported, but the value of the capture_top field is
771 always 1, and the value of the capture_last field is always -1.
772
773 7. The \C escape sequence, which (in the standard algorithm) matches a
774 single byte, even in UTF-8 mode, is not supported because the alterna-
775 tive algorithm moves through the subject string one character at a
776 time, for all active paths through the tree.
777
778 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
779 are not supported. (*FAIL) is supported, and behaves like a failing
780 negative assertion.
781
782
783 ADVANTAGES OF THE ALTERNATIVE ALGORITHM
784
785 Using the alternative matching algorithm provides the following advan-
786 tages:
787
788 1. All possible matches (at a single point in the subject) are automat-
789 ically found, and in particular, the longest match is found. To find
790 more than one match using the standard algorithm, you have to do kludgy
791 things with callouts.
792
793 2. Because the alternative algorithm scans the subject string just
794 once, and never needs to backtrack, it is possible to pass very long
795 subject strings to the matching function in several pieces, checking
796 for partial matching each time. Although it is possible to do multi-
797 segment matching using the standard algorithm (pcre_exec()), by retain-
798 ing partially matched substrings, it is more complicated. The pcrepar-
799 tial documentation gives details of partial matching and discusses
800 multi-segment matching.
801
802
803 DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
804
805 The alternative algorithm suffers from a number of disadvantages:
806
807 1. It is substantially slower than the standard algorithm. This is
808 partly because it has to search for all possible matches, but is also
809 because it is less susceptible to optimization.
810
811 2. Capturing parentheses and back references are not supported.
812
813 3. Although atomic groups are supported, their use does not provide the
814 performance advantage that it does for the standard algorithm.
815
816
817 AUTHOR
818
819 Philip Hazel
820 University Computing Service
821 Cambridge CB2 3QH, England.
822
823
824 REVISION
825
826 Last updated: 17 November 2010
827 Copyright (c) 1997-2010 University of Cambridge.
828 ------------------------------------------------------------------------------
829
830
831 PCREAPI(3) PCREAPI(3)
832
833
834 NAME
835 PCRE - Perl-compatible regular expressions
836
837
838 PCRE NATIVE API
839
840 #include <pcre.h>
841
842 pcre *pcre_compile(const char *pattern, int options,
843 const char **errptr, int *erroffset,
844 const unsigned char *tableptr);
845
846 pcre *pcre_compile2(const char *pattern, int options,
847 int *errorcodeptr,
848 const char **errptr, int *erroffset,
849 const unsigned char *tableptr);
850
851 pcre_extra *pcre_study(const pcre *code, int options,
852 const char **errptr);
853
854 int pcre_exec(const pcre *code, const pcre_extra *extra,
855 const char *subject, int length, int startoffset,
856 int options, int *ovector, int ovecsize);
857
858 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
859 const char *subject, int length, int startoffset,
860 int options, int *ovector, int ovecsize,
861 int *workspace, int wscount);
862
863 int pcre_copy_named_substring(const pcre *code,
864 const char *subject, int *ovector,
865 int stringcount, const char *stringname,
866 char *buffer, int buffersize);
867
868 int pcre_copy_substring(const char *subject, int *ovector,
869 int stringcount, int stringnumber, char *buffer,
870 int buffersize);
871
872 int pcre_get_named_substring(const pcre *code,
873 const char *subject, int *ovector,
874 int stringcount, const char *stringname,
875 const char **stringptr);
876
877 int pcre_get_stringnumber(const pcre *code,
878 const char *name);
879
880 int pcre_get_stringtable_entries(const pcre *code,
881 const char *name, char **first, char **last);
882
883 int pcre_get_substring(const char *subject, int *ovector,
884 int stringcount, int stringnumber,
885 const char **stringptr);
886
887 int pcre_get_substring_list(const char *subject,
888 int *ovector, int stringcount, const char ***listptr);
889
890 void pcre_free_substring(const char *stringptr);
891
892 void pcre_free_substring_list(const char **stringptr);
893
894 const unsigned char *pcre_maketables(void);
895
896 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
897 int what, void *where);
898
899 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
900
901 int pcre_refcount(pcre *code, int adjust);
902
903 int pcre_config(int what, void *where);
904
905 char *pcre_version(void);
906
907 void *(*pcre_malloc)(size_t);
908
909 void (*pcre_free)(void *);
910
911 void *(*pcre_stack_malloc)(size_t);
912
913 void (*pcre_stack_free)(void *);
914
915 int (*pcre_callout)(pcre_callout_block *);
916
917
918 PCRE API OVERVIEW
919
920 PCRE has its own native API, which is described in this document. There
921 are also some wrapper functions that correspond to the POSIX regular
922 expression API. These are described in the pcreposix documentation.
923 Both of these APIs define a set of C function calls. A C++ wrapper is
924 distributed with PCRE. It is documented in the pcrecpp page.
925
926 The native API C function prototypes are defined in the header file
927 pcre.h, and on Unix systems the library itself is called libpcre. It
928 can normally be accessed by adding -lpcre to the command for linking an
929 application that uses PCRE. The header file defines the macros
930 PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
931 bers for the library. Applications can use these to include support
932 for different releases of PCRE.
933
934 In a Windows environment, if you want to statically link an application
935 program against a non-dll pcre.a file, you must define PCRE_STATIC
936 before including pcre.h or pcrecpp.h, because otherwise the pcre_mal-
937 loc() and pcre_free() exported functions will be declared
938 __declspec(dllimport), with unwanted results.
939
940 The functions pcre_compile(), pcre_compile2(), pcre_study(), and
941 pcre_exec() are used for compiling and matching regular expressions in
942 a Perl-compatible manner. A sample program that demonstrates the sim-
943 plest way of using them is provided in the file called pcredemo.c in
944 the PCRE source distribution. A listing of this program is given in the
945 pcredemo documentation, and the pcresample documentation describes how
946 to compile and run it.
947
948 A second matching function, pcre_dfa_exec(), which is not Perl-compati-
949 ble, is also provided. This uses a different algorithm for the match-
950 ing. The alternative algorithm finds all possible matches (at a given
951 point in the subject), and scans the subject just once (unless there
952 are lookbehind assertions). However, this algorithm does not return
953 captured substrings. A description of the two matching algorithms and
954 their advantages and disadvantages is given in the pcrematching docu-
955 mentation.
956
957 In addition to the main compiling and matching functions, there are
958 convenience functions for extracting captured substrings from a subject
959 string that is matched by pcre_exec(). They are:
960
961 pcre_copy_substring()
962 pcre_copy_named_substring()
963 pcre_get_substring()
964 pcre_get_named_substring()
965 pcre_get_substring_list()
966 pcre_get_stringnumber()
967 pcre_get_stringtable_entries()
968
969 pcre_free_substring() and pcre_free_substring_list() are also provided,
970 to free the memory used for extracted strings.
971
972 The function pcre_maketables() is used to build a set of character
973 tables in the current locale for passing to pcre_compile(),
974 pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
975 provided for specialist use. Most commonly, no special tables are
976 passed, in which case internal tables that are generated when PCRE is
977 built are used.
978
979 The function pcre_fullinfo() is used to find out information about a
980 compiled pattern; pcre_info() is an obsolete version that returns only
981 some of the available information, but is retained for backwards com-
982 patibility. The function pcre_version() returns a pointer to a string
983 containing the version of PCRE and its date of release.
984
985 The function pcre_refcount() maintains a reference count in a data
986 block containing a compiled pattern. This is provided for the benefit
987 of object-oriented applications.
988
989 The global variables pcre_malloc and pcre_free initially contain the
990 entry points of the standard malloc() and free() functions, respec-
991 tively. PCRE calls the memory management functions via these variables,
992 so a calling program can replace them if it wishes to intercept the
993 calls. This should be done before calling any PCRE functions.
994
995 The global variables pcre_stack_malloc and pcre_stack_free are also
996 indirections to memory management functions. These special functions
997 are used only when PCRE is compiled to use the heap for remembering
998 data, instead of recursive function calls, when running the pcre_exec()
999 function. See the pcrebuild documentation for details of how to do
1000 this. It is a non-standard way of building PCRE, for use in environ-
1001 ments that have limited stacks. Because of the greater use of memory
1002 management, it runs more slowly. Separate functions are provided so
1003 that special-purpose external code can be used for this case. When
1004 used, these functions are always called in a stack-like manner (last
1005 obtained, first freed), and always for memory blocks of the same size.
1006 There is a discussion about PCRE's stack usage in the pcrestack docu-
1007 mentation.
1008
1009 The global variable pcre_callout initially contains NULL. It can be set
1010 by the caller to a "callout" function, which PCRE will then call at
1011 specified points during a matching operation. Details are given in the
1012 pcrecallout documentation.
1013
1014
1015 NEWLINES
1016
1017 PCRE supports five different conventions for indicating line breaks in
1018 strings: a single CR (carriage return) character, a single LF (line-
1019 feed) character, the two-character sequence CRLF, any of the three pre-
1020 ceding, or any Unicode newline sequence. The Unicode newline sequences
1021 are the three just mentioned, plus the single characters VT (vertical
1022 tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
1023 separator, U+2028), and PS (paragraph separator, U+2029).
1024
1025 Each of the first three conventions is used by at least one operating
1026 system as its standard newline sequence. When PCRE is built, a default
1027 can be specified. The default default is LF, which is the Unix stan-
1028 dard. When PCRE is run, the default can be overridden, either when a
1029 pattern is compiled, or when it is matched.
1030
1031 At compile time, the newline convention can be specified by the options
1032 argument of pcre_compile(), or it can be specified by special text at
1033 the start of the pattern itself; this overrides any other settings. See
1034 the pcrepattern page for details of the special character sequences.
1035
1036 In the PCRE documentation the word "newline" is used to mean "the char-
1037 acter or pair of characters that indicate a line break". The choice of
1038 newline convention affects the handling of the dot, circumflex, and
1039 dollar metacharacters, the handling of #-comments in /x mode, and, when
1040 CRLF is a recognized line ending sequence, the match position advance-
1041 ment for a non-anchored pattern. There is more detail about this in the
1042 section on pcre_exec() options below.
1043
1044 The choice of newline convention does not affect the interpretation of
1045 the \n or \r escape sequences, nor does it affect what \R matches,
1046 which is controlled in a similar way, but by separate options.
1047
1048
1049 MULTITHREADING
1050
1051 The PCRE functions can be used in multi-threading applications, with
1052 the proviso that the memory management functions pointed to by
1053 pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
1054 callout function pointed to by pcre_callout, are shared by all threads.
1055
1056 The compiled form of a regular expression is not altered during match-
1057 ing, so the same compiled pattern can safely be used by several threads
1058 at once.
1059
1060
1061 SAVING PRECOMPILED PATTERNS FOR LATER USE
1062
1063 The compiled form of a regular expression can be saved and re-used at a
1064 later time, possibly by a different program, and even on a host other
1065 than the one on which it was compiled. Details are given in the
1066 pcreprecompile documentation. However, compiling a regular expression
1067 with one version of PCRE for use with a different version is not guar-
1068 anteed to work and may cause crashes.
1069
1070
1071 CHECKING BUILD-TIME OPTIONS
1072
1073 int pcre_config(int what, void *where);
1074
1075 The function pcre_config() makes it possible for a PCRE client to dis-
1076 cover which optional features have been compiled into the PCRE library.
1077 The pcrebuild documentation has more details about these optional fea-
1078 tures.
1079
1080 The first argument for pcre_config() is an integer, specifying which
1081 information is required; the second argument is a pointer to a variable
1082 into which the information is placed. The following information is
1083 available:
1084
1085 PCRE_CONFIG_UTF8
1086
1087 The output is an integer that is set to one if UTF-8 support is avail-
1088 able; otherwise it is set to zero.
1089
1090 PCRE_CONFIG_UNICODE_PROPERTIES
1091
1092 The output is an integer that is set to one if support for Unicode
1093 character properties is available; otherwise it is set to zero.
1094
1095 PCRE_CONFIG_NEWLINE
1096
1097 The output is an integer whose value specifies the default character
1098 sequence that is recognized as meaning "newline". The four values that
1099 are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
1100 and -1 for ANY. Though they are derived from ASCII, the same values
1101 are returned in EBCDIC environments. The default should normally corre-
1102 spond to the standard sequence for your operating system.
1103
1104 PCRE_CONFIG_BSR
1105
1106 The output is an integer whose value indicates what character sequences
1107 the \R escape sequence matches by default. A value of 0 means that \R
1108 matches any Unicode line ending sequence; a value of 1 means that \R
1109 matches only CR, LF, or CRLF. The default can be overridden when a pat-
1110 tern is compiled or matched.
1111
1112 PCRE_CONFIG_LINK_SIZE
1113
1114 The output is an integer that contains the number of bytes used for
1115 internal linkage in compiled regular expressions. The value is 2, 3, or
1116 4. Larger values allow larger regular expressions to be compiled, at
1117 the expense of slower matching. The default value of 2 is sufficient
1118 for all but the most massive patterns, since it allows the compiled
1119 pattern to be up to 64K in size.
1120
1121 PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
1122
1123 The output is an integer that contains the threshold above which the
1124 POSIX interface uses malloc() for output vectors. Further details are
1125 given in the pcreposix documentation.
1126
1127 PCRE_CONFIG_MATCH_LIMIT
1128
1129 The output is a long integer that gives the default limit for the num-
1130 ber of internal matching function calls in a pcre_exec() execution.
1131 Further details are given with pcre_exec() below.
1132
1133 PCRE_CONFIG_MATCH_LIMIT_RECURSION
1134
1135 The output is a long integer that gives the default limit for the depth
1136 of recursion when calling the internal matching function in a
1137 pcre_exec() execution. Further details are given with pcre_exec()
1138 below.
1139
1140 PCRE_CONFIG_STACKRECURSE
1141
1142 The output is an integer that is set to one if internal recursion when
1143 running pcre_exec() is implemented by recursive function calls that use
1144 the stack to remember their state. This is the usual way that PCRE is
1145 compiled. The output is zero if PCRE was compiled to use blocks of data
1146 on the heap instead of recursive function calls. In this case,
1147 pcre_stack_malloc and pcre_stack_free are called to manage memory
1148 blocks on the heap, thus avoiding the use of the stack.
1149
1150
1151 COMPILING A PATTERN
1152
1153 pcre *pcre_compile(const char *pattern, int options,
1154 const char **errptr, int *erroffset,
1155 const unsigned char *tableptr);
1156
1157 pcre *pcre_compile2(const char *pattern, int options,
1158 int *errorcodeptr,
1159 const char **errptr, int *erroffset,
1160 const unsigned char *tableptr);
1161
1162 Either of the functions pcre_compile() or pcre_compile2() can be called
1163 to compile a pattern into an internal form. The only difference between
1164 the two interfaces is that pcre_compile2() has an additional argument,
1165 errorcodeptr, via which a numerical error code can be returned. To
1166 avoid too much repetition, we refer just to pcre_compile() below, but
1167 the information applies equally to pcre_compile2().
1168
1169 The pattern is a C string terminated by a binary zero, and is passed in
1170 the pattern argument. A pointer to a single block of memory that is
1171 obtained via pcre_malloc is returned. This contains the compiled code
1172 and related data. The pcre type is defined for the returned block; this
1173 is a typedef for a structure whose contents are not externally defined.
1174 It is up to the caller to free the memory (via pcre_free) when it is no
1175 longer required.
1176
1177 Although the compiled code of a PCRE regex is relocatable, that is, it
1178 does not depend on memory location, the complete pcre data block is not
1179 fully relocatable, because it may contain a copy of the tableptr argu-
1180 ment, which is an address (see below).
1181
1182 The options argument contains various bit settings that affect the com-
1183 pilation. It should be zero if no options are required. The available
1184 options are described below. Some of them (in particular, those that
1185 are compatible with Perl, but some others as well) can also be set and
1186 unset from within the pattern (see the detailed description in the
1187 pcrepattern documentation). For those options that can be different in
1188 different parts of the pattern, the contents of the options argument
1189 specifies their settings at the start of compilation and execution. The
1190 PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and
1191 PCRE_NO_START_OPT options can be set at the time of matching as well as
1192 at compile time.
1193
1194 If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
1195 if compilation of a pattern fails, pcre_compile() returns NULL, and
1196 sets the variable pointed to by errptr to point to a textual error mes-
1197 sage. This is a static string that is part of the library. You must not
1198 try to free it. Normally, the offset from the start of the pattern to
1199 the byte that was being processed when the error was discovered is
1200 placed in the variable pointed to by erroffset, which must not be NULL
1201 (if it is, an immediate error is given). However, for an invalid UTF-8
1202 string, the offset is that of the first byte of the failing character.
1203 Also, some errors are not detected until checks are carried out when
1204 the whole pattern has been scanned; in these cases the offset passed
1205 back is the length of the pattern.
1206
1207 Note that the offset is in bytes, not characters, even in UTF-8 mode.
1208 It may sometimes point into the middle of a UTF-8 character.
1209
1210 If pcre_compile2() is used instead of pcre_compile(), and the error-
1211 codeptr argument is not NULL, a non-zero error code number is returned
1212 via this argument in the event of an error. This is in addition to the
1213 textual error message. Error codes and messages are listed below.
1214
1215 If the final argument, tableptr, is NULL, PCRE uses a default set of
1216 character tables that are built when PCRE is compiled, using the
1217 default C locale. Otherwise, tableptr must be an address that is the
1218 result of a call to pcre_maketables(). This value is stored with the
1219 compiled pattern, and used again by pcre_exec(), unless another table
1220 pointer is passed to it. For more discussion, see the section on locale
1221 support below.
1222
1223 This code fragment shows a typical straightforward call to pcre_com-
1224 pile():
1225
1226 pcre *re;
1227 const char *error;
1228 int erroffset;
1229 re = pcre_compile(
1230 "^A.*Z", /* the pattern */
1231 0, /* default options */
1232 &error, /* for error message */
1233 &erroffset, /* for error offset */
1234 NULL); /* use default character tables */
1235
1236 The following names for option bits are defined in the pcre.h header
1237 file:
1238
1239 PCRE_ANCHORED
1240
1241 If this bit is set, the pattern is forced to be "anchored", that is, it
1242 is constrained to match only at the first matching point in the string
1243 that is being searched (the "subject string"). This effect can also be
1244 achieved by appropriate constructs in the pattern itself, which is the
1245 only way to do it in Perl.
1246
1247 PCRE_AUTO_CALLOUT
1248
1249 If this bit is set, pcre_compile() automatically inserts callout items,
1250 all with number 255, before each pattern item. For discussion of the
1251 callout facility, see the pcrecallout documentation.
1252
1253 PCRE_BSR_ANYCRLF
1254 PCRE_BSR_UNICODE
1255
1256 These options (which are mutually exclusive) control what the \R escape
1257 sequence matches. The choice is either to match only CR, LF, or CRLF,
1258 or to match any Unicode newline sequence. The default is specified when
1259 PCRE is built. It can be overridden from within the pattern, or by set-
1260 ting an option when a compiled pattern is matched.
1261
1262 PCRE_CASELESS
1263
1264 If this bit is set, letters in the pattern match both upper and lower
1265 case letters. It is equivalent to Perl's /i option, and it can be
1266 changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
1267 always understands the concept of case for characters whose values are
1268 less than 128, so caseless matching is always possible. For characters
1269 with higher values, the concept of case is supported if PCRE is com-
1270 piled with Unicode property support, but not otherwise. If you want to
1271 use caseless matching for characters 128 and above, you must ensure
1272 that PCRE is compiled with Unicode property support as well as with
1273 UTF-8 support.
1274
1275 PCRE_DOLLAR_ENDONLY
1276
1277 If this bit is set, a dollar metacharacter in the pattern matches only
1278 at the end of the subject string. Without this option, a dollar also
1279 matches immediately before a newline at the end of the string (but not
1280 before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
1281 if PCRE_MULTILINE is set. There is no equivalent to this option in
1282 Perl, and no way to set it within a pattern.
1283
1284 PCRE_DOTALL
1285
1286 If this bit is set, a dot metacharacter in the pattern matches a char-
1287 acter of any value, including one that indicates a newline. However, it
1288 only ever matches one character, even if newlines are coded as CRLF.
1289 Without this option, a dot does not match when the current position is
1290 at a newline. This option is equivalent to Perl's /s option, and it can
1291 be changed within a pattern by a (?s) option setting. A negative class
1292 such as [^a] always matches newline characters, independent of the set-
1293 ting of this option.
1294
1295 PCRE_DUPNAMES
1296
1297 If this bit is set, names used to identify capturing subpatterns need
1298 not be unique. This can be helpful for certain types of pattern when it
1299 is known that only one instance of the named subpattern can ever be
1300 matched. There are more details of named subpatterns below; see also
1301 the pcrepattern documentation.
1302
1303 PCRE_EXTENDED
1304
1305 If this bit is set, whitespace data characters in the pattern are
1306 totally ignored except when escaped or inside a character class. White-
1307 space does not include the VT character (code 11). In addition, charac-
1308 ters between an unescaped # outside a character class and the next new-
1309 line, inclusive, are also ignored. This is equivalent to Perl's /x
1310 option, and it can be changed within a pattern by a (?x) option set-
1311 ting.
1312
1313 Which characters are interpreted as newlines is controlled by the
1314 options passed to pcre_compile() or by a special sequence at the start
1315 of the pattern, as described in the section entitled "Newline conven-
1316 tions" in the pcrepattern documentation. Note that the end of this type
1317 of comment is a literal newline sequence in the pattern; escape
1318 sequences that happen to represent a newline do not count.
1319
1320 This option makes it possible to include comments inside complicated
1321 patterns. Note, however, that this applies only to data characters.
1322 Whitespace characters may never appear within special character
1323 sequences in a pattern, for example within the sequence (?( that intro-
1324 duces a conditional subpattern.
1325
1326 PCRE_EXTRA
1327
1328 This option was invented in order to turn on additional functionality
1329 of PCRE that is incompatible with Perl, but it is currently of very
1330 little use. When set, any backslash in a pattern that is followed by a
1331 letter that has no special meaning causes an error, thus reserving
1332 these combinations for future expansion. By default, as in Perl, a
1333 backslash followed by a letter with no special meaning is treated as a
1334 literal. (Perl can, however, be persuaded to give an error for this, by
1335 running it with the -w option.) There are at present no other features
1336 controlled by this option. It can also be set by a (?X) option setting
1337 within a pattern.
1338
1339 PCRE_FIRSTLINE
1340
1341 If this option is set, an unanchored pattern is required to match
1342 before or at the first newline in the subject string, though the
1343 matched text may continue over the newline.
1344
1345 PCRE_JAVASCRIPT_COMPAT
1346
1347 If this option is set, PCRE's behaviour is changed in some ways so that
1348 it is compatible with JavaScript rather than Perl. The changes are as
1349 follows:
1350
1351 (1) A lone closing square bracket in a pattern causes a compile-time
1352 error, because this is illegal in JavaScript (by default it is treated
1353 as a data character). Thus, the pattern AB]CD becomes illegal when this
1354 option is set.
1355
1356 (2) At run time, a back reference to an unset subpattern group matches
1357 an empty string (by default this causes the current matching alterna-
1358 tive to fail). A pattern such as (\1)(a) succeeds when this option is
1359 set (assuming it can find an "a" in the subject), whereas it fails by
1360 default, for Perl compatibility.
1361
1362 PCRE_MULTILINE
1363
1364 By default, PCRE treats the subject string as consisting of a single
1365 line of characters (even if it actually contains newlines). The "start
1366 of line" metacharacter (^) matches only at the start of the string,
1367 while the "end of line" metacharacter ($) matches only at the end of
1368 the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
1369 is set). This is the same as Perl.
1370
1371 When PCRE_MULTILINE it is set, the "start of line" and "end of line"
1372 constructs match immediately following or immediately before internal
1373 newlines in the subject string, respectively, as well as at the very
1374 start and end. This is equivalent to Perl's /m option, and it can be
1375 changed within a pattern by a (?m) option setting. If there are no new-
1376 lines in a subject string, or no occurrences of ^ or $ in a pattern,
1377 setting PCRE_MULTILINE has no effect.
1378
1379 PCRE_NEWLINE_CR
1380 PCRE_NEWLINE_LF
1381 PCRE_NEWLINE_CRLF
1382 PCRE_NEWLINE_ANYCRLF
1383 PCRE_NEWLINE_ANY
1384
1385 These options override the default newline definition that was chosen
1386 when PCRE was built. Setting the first or the second specifies that a
1387 newline is indicated by a single character (CR or LF, respectively).
1388 Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
1389 two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
1390 that any of the three preceding sequences should be recognized. Setting
1391 PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
1392 recognized. The Unicode newline sequences are the three just mentioned,
1393 plus the single characters VT (vertical tab, U+000B), FF (formfeed,
1394 U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
1395 (paragraph separator, U+2029). The last two are recognized only in
1396 UTF-8 mode.
1397
1398 The newline setting in the options word uses three bits that are
1399 treated as a number, giving eight possibilities. Currently only six are
1400 used (default plus the five values above). This means that if you set
1401 more than one newline option, the combination may or may not be sensi-
1402 ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
1403 PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
1404 cause an error.
1405
1406 The only time that a line break in a pattern is specially recognized
1407 when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace
1408 characters, and so are ignored in this mode. Also, an unescaped # out-
1409 side a character class indicates a comment that lasts until after the
1410 next line break sequence. In other circumstances, line break sequences
1411 in patterns are treated as literal data.
1412
1413 The newline option that is set at compile time becomes the default that
1414 is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
1415
1416 PCRE_NO_AUTO_CAPTURE
1417
1418 If this option is set, it disables the use of numbered capturing paren-
1419 theses in the pattern. Any opening parenthesis that is not followed by
1420 ? behaves as if it were followed by ?: but named parentheses can still
1421 be used for capturing (and they acquire numbers in the usual way).
1422 There is no equivalent of this option in Perl.
1423
1424 NO_START_OPTIMIZE
1425
1426 This is an option that acts at matching time; that is, it is really an
1427 option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
1428 time, it is remembered with the compiled pattern and assumed at match-
1429 ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE
1430 below.
1431
1432 PCRE_UCP
1433
1434 This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
1435 \w, and some of the POSIX character classes. By default, only ASCII
1436 characters are recognized, but if PCRE_UCP is set, Unicode properties
1437 are used instead to classify characters. More details are given in the
1438 section on generic character types in the pcrepattern page. If you set
1439 PCRE_UCP, matching one of the items it affects takes much longer. The
1440 option is available only if PCRE has been compiled with Unicode prop-
1441 erty support.
1442
1443 PCRE_UNGREEDY
1444
1445 This option inverts the "greediness" of the quantifiers so that they
1446 are not greedy by default, but become greedy if followed by "?". It is
1447 not compatible with Perl. It can also be set by a (?U) option setting
1448 within the pattern.
1449
1450 PCRE_UTF8
1451
1452 This option causes PCRE to regard both the pattern and the subject as
1453 strings of UTF-8 characters instead of single-byte character strings.
1454 However, it is available only when PCRE is built to include UTF-8 sup-
1455 port. If not, the use of this option provokes an error. Details of how
1456 this option changes the behaviour of PCRE are given in the section on
1457 UTF-8 support in the main pcre page.
1458
1459 PCRE_NO_UTF8_CHECK
1460
1461 When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
1462 automatically checked. There is a discussion about the validity of
1463 UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
1464 bytes is found, pcre_compile() returns an error. If you already know
1465 that your pattern is valid, and you want to skip this check for perfor-
1466 mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
1467 set, the effect of passing an invalid UTF-8 string as a pattern is
1468 undefined. It may cause your program to crash. Note that this option
1469 can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
1470 UTF-8 validity checking of subject strings.
1471
1472
1473 COMPILATION ERROR CODES
1474
1475 The following table lists the error codes than may be returned by
1476 pcre_compile2(), along with the error messages that may be returned by
1477 both compiling functions. As PCRE has developed, some error codes have
1478 fallen out of use. To avoid confusion, they have not been re-used.
1479
1480 0 no error
1481 1 \ at end of pattern
1482 2 \c at end of pattern
1483 3 unrecognized character follows \
1484 4 numbers out of order in {} quantifier
1485 5 number too big in {} quantifier
1486 6 missing terminating ] for character class
1487 7 invalid escape sequence in character class
1488 8 range out of order in character class
1489 9 nothing to repeat
1490 10 [this code is not in use]
1491 11 internal error: unexpected repeat
1492 12 unrecognized character after (? or (?-
1493 13 POSIX named classes are supported only within a class
1494 14 missing )
1495 15 reference to non-existent subpattern
1496 16 erroffset passed as NULL
1497 17 unknown option bit(s) set
1498 18 missing ) after comment
1499 19 [this code is not in use]
1500 20 regular expression is too large
1501 21 failed to get memory
1502 22 unmatched parentheses
1503 23 internal error: code overflow
1504 24 unrecognized character after (?<
1505 25 lookbehind assertion is not fixed length
1506 26 malformed number or name after (?(
1507 27 conditional group contains more than two branches
1508 28 assertion expected after (?(
1509 29 (?R or (?[+-]digits must be followed by )
1510 30 unknown POSIX class name
1511 31 POSIX collating elements are not supported
1512 32 this version of PCRE is not compiled with PCRE_UTF8 support
1513 33 [this code is not in use]
1514 34 character value in \x{...} sequence is too large
1515 35 invalid condition (?(0)
1516 36 \C not allowed in lookbehind assertion
1517 37 PCRE does not support \L, \l, \N{name}, \U, or \u
1518 38 number after (?C is > 255
1519 39 closing ) for (?C expected
1520 40 recursive call could loop indefinitely
1521 41 unrecognized character after (?P
1522 42 syntax error in subpattern name (missing terminator)
1523 43 two named subpatterns have the same name
1524 44 invalid UTF-8 string
1525 45 support for \P, \p, and \X has not been compiled
1526 46 malformed \P or \p sequence
1527 47 unknown property name after \P or \p
1528 48 subpattern name is too long (maximum 32 characters)
1529 49 too many named subpatterns (maximum 10000)
1530 50 [this code is not in use]
1531 51 octal value is greater than \377 (not in UTF-8 mode)
1532 52 internal error: overran compiling workspace
1533 53 internal error: previously-checked referenced subpattern
1534 not found
1535 54 DEFINE group contains more than one branch
1536 55 repeating a DEFINE group is not allowed
1537 56 inconsistent NEWLINE options
1538 57 \g is not followed by a braced, angle-bracketed, or quoted
1539 name/number or by a plain number
1540 58 a numbered reference must not be zero
1541 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
1542 60 (*VERB) not recognized
1543 61 number is too big
1544 62 subpattern name expected
1545 63 digit expected after (?+
1546 64 ] is an invalid data character in JavaScript compatibility mode
1547 65 different names for subpatterns of the same number are
1548 not allowed
1549 66 (*MARK) must have an argument
1550 67 this version of PCRE is not compiled with PCRE_UCP support
1551 68 \c must be followed by an ASCII character
1552 69 \k is not followed by a braced, angle-bracketed, or quoted name
1553
1554 The numbers 32 and 10000 in errors 48 and 49 are defaults; different
1555 values may be used if the limits were changed when PCRE was built.
1556
1557
1558 STUDYING A PATTERN
1559
1560 pcre_extra *pcre_study(const pcre *code, int options
1561 const char **errptr);
1562
1563 If a compiled pattern is going to be used several times, it is worth
1564 spending more time analyzing it in order to speed up the time taken for
1565 matching. The function pcre_study() takes a pointer to a compiled pat-
1566 tern as its first argument. If studying the pattern produces additional
1567 information that will help speed up matching, pcre_study() returns a
1568 pointer to a pcre_extra block, in which the study_data field points to
1569 the results of the study.
1570
1571 The returned value from pcre_study() can be passed directly to
1572 pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
1573 tains other fields that can be set by the caller before the block is
1574 passed; these are described below in the section on matching a pattern.
1575
1576 If studying the pattern does not produce any useful information,
1577 pcre_study() returns NULL. In that circumstance, if the calling program
1578 wants to pass any of the other fields to pcre_exec() or
1579 pcre_dfa_exec(), it must set up its own pcre_extra block.
1580
1581 The second argument of pcre_study() contains option bits. At present,
1582 no options are defined, and this argument should always be zero.
1583
1584 The third argument for pcre_study() is a pointer for an error message.
1585 If studying succeeds (even if no data is returned), the variable it
1586 points to is set to NULL. Otherwise it is set to point to a textual
1587 error message. This is a static string that is part of the library. You
1588 must not try to free it. You should test the error pointer for NULL
1589 after calling pcre_study(), to be sure that it has run successfully.
1590
1591 This is a typical call to pcre_study():
1592
1593 pcre_extra *pe;
1594 pe = pcre_study(
1595 re, /* result of pcre_compile() */
1596 0, /* no options exist */
1597 &error); /* set to NULL or points to a message */
1598
1599 Studying a pattern does two things: first, a lower bound for the length
1600 of subject string that is needed to match the pattern is computed. This
1601 does not mean that there are any strings of that length that match, but
1602 it does guarantee that no shorter strings match. The value is used by
1603 pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
1604 match strings that are shorter than the lower bound. You can find out
1605 the value in a calling program via the pcre_fullinfo() function.
1606
1607 Studying a pattern is also useful for non-anchored patterns that do not
1608 have a single fixed starting character. A bitmap of possible starting
1609 bytes is created. This speeds up finding a position in the subject at
1610 which to start matching.
1611
1612 The two optimizations just described can be disabled by setting the
1613 PCRE_NO_START_OPTIMIZE option when calling pcre_exec() or
1614 pcre_dfa_exec(). You might want to do this if your pattern contains
1615 callouts or (*MARK), and you want to make use of these facilities in
1616 cases where matching fails. See the discussion of PCRE_NO_START_OPTI-
1617 MIZE below.
1618
1619
1620 LOCALE SUPPORT
1621
1622 PCRE handles caseless matching, and determines whether characters are
1623 letters, digits, or whatever, by reference to a set of tables, indexed
1624 by character value. When running in UTF-8 mode, this applies only to
1625 characters with codes less than 128. By default, higher-valued codes
1626 never match escapes such as \w or \d, but they can be tested with \p if
1627 PCRE is built with Unicode character property support. Alternatively,
1628 the PCRE_UCP option can be set at compile time; this causes \w and
1629 friends to use Unicode property support instead of built-in tables. The
1630 use of locales with Unicode is discouraged. If you are handling charac-
1631 ters with codes greater than 128, you should either use UTF-8 and Uni-
1632 code, or use locales, but not try to mix the two.
1633
1634 PCRE contains an internal set of tables that are used when the final
1635 argument of pcre_compile() is NULL. These are sufficient for many
1636 applications. Normally, the internal tables recognize only ASCII char-
1637 acters. However, when PCRE is built, it is possible to cause the inter-
1638 nal tables to be rebuilt in the default "C" locale of the local system,
1639 which may cause them to be different.
1640
1641 The internal tables can always be overridden by tables supplied by the
1642 application that calls PCRE. These may be created in a different locale
1643 from the default. As more and more applications change to using Uni-
1644 code, the need for this locale support is expected to die away.
1645
1646 External tables are built by calling the pcre_maketables() function,
1647 which has no arguments, in the relevant locale. The result can then be
1648 passed to pcre_compile() or pcre_exec() as often as necessary. For
1649 example, to build and use tables that are appropriate for the French
1650 locale (where accented characters with values greater than 128 are
1651 treated as letters), the following code could be used:
1652
1653 setlocale(LC_CTYPE, "fr_FR");
1654 tables = pcre_maketables();
1655 re = pcre_compile(..., tables);
1656
1657 The locale name "fr_FR" is used on Linux and other Unix-like systems;
1658 if you are using Windows, the name for the French locale is "french".
1659
1660 When pcre_maketables() runs, the tables are built in memory that is
1661 obtained via pcre_malloc. It is the caller's responsibility to ensure
1662 that the memory containing the tables remains available for as long as
1663 it is needed.
1664
1665 The pointer that is passed to pcre_compile() is saved with the compiled
1666 pattern, and the same tables are used via this pointer by pcre_study()
1667 and normally also by pcre_exec(). Thus, by default, for any single pat-
1668 tern, compilation, studying and matching all happen in the same locale,
1669 but different patterns can be compiled in different locales.
1670
1671 It is possible to pass a table pointer or NULL (indicating the use of
1672 the internal tables) to pcre_exec(). Although not intended for this
1673 purpose, this facility could be used to match a pattern in a different
1674 locale from the one in which it was compiled. Passing table pointers at
1675 run time is discussed below in the section on matching a pattern.
1676
1677
1678 INFORMATION ABOUT A PATTERN
1679
1680 int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
1681 int what, void *where);
1682
1683 The pcre_fullinfo() function returns information about a compiled pat-
1684 tern. It replaces the obsolete pcre_info() function, which is neverthe-
1685 less retained for backwards compability (and is documented below).
1686
1687 The first argument for pcre_fullinfo() is a pointer to the compiled
1688 pattern. The second argument is the result of pcre_study(), or NULL if
1689 the pattern was not studied. The third argument specifies which piece
1690 of information is required, and the fourth argument is a pointer to a
1691 variable to receive the data. The yield of the function is zero for
1692 success, or one of the following negative numbers:
1693
1694 PCRE_ERROR_NULL the argument code was NULL
1695 the argument where was NULL
1696 PCRE_ERROR_BADMAGIC the "magic number" was not found
1697 PCRE_ERROR_BADOPTION the value of what was invalid
1698
1699 The "magic number" is placed at the start of each compiled pattern as
1700 an simple check against passing an arbitrary memory pointer. Here is a
1701 typical call of pcre_fullinfo(), to obtain the length of the compiled
1702 pattern:
1703
1704 int rc;
1705 size_t length;
1706 rc = pcre_fullinfo(
1707 re, /* result of pcre_compile() */
1708 pe, /* result of pcre_study(), or NULL */
1709 PCRE_INFO_SIZE, /* what is required */
1710 &length); /* where to put the data */
1711
1712 The possible values for the third argument are defined in pcre.h, and
1713 are as follows:
1714
1715 PCRE_INFO_BACKREFMAX
1716
1717 Return the number of the highest back reference in the pattern. The
1718 fourth argument should point to an int variable. Zero is returned if
1719 there are no back references.
1720
1721 PCRE_INFO_CAPTURECOUNT
1722
1723 Return the number of capturing subpatterns in the pattern. The fourth
1724 argument should point to an int variable.
1725
1726 PCRE_INFO_DEFAULT_TABLES
1727
1728 Return a pointer to the internal default character tables within PCRE.
1729 The fourth argument should point to an unsigned char * variable. This
1730 information call is provided for internal use by the pcre_study() func-
1731 tion. External callers can cause PCRE to use its internal tables by
1732 passing a NULL table pointer.
1733
1734 PCRE_INFO_FIRSTBYTE
1735
1736 Return information about the first byte of any matched string, for a
1737 non-anchored pattern. The fourth argument should point to an int vari-
1738 able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
1739 is still recognized for backwards compatibility.)
1740
1741 If there is a fixed first byte, for example, from a pattern such as
1742 (cat|cow|coyote), its value is returned. Otherwise, if either
1743
1744 (a) the pattern was compiled with the PCRE_MULTILINE option, and every
1745 branch starts with "^", or
1746
1747 (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
1748 set (if it were set, the pattern would be anchored),
1749
1750 -1 is returned, indicating that the pattern matches only at the start
1751 of a subject string or after any newline within the string. Otherwise
1752 -2 is returned. For anchored patterns, -2 is returned.
1753
1754 PCRE_INFO_FIRSTTABLE
1755
1756 If the pattern was studied, and this resulted in the construction of a
1757 256-bit table indicating a fixed set of bytes for the first byte in any
1758 matching string, a pointer to the table is returned. Otherwise NULL is
1759 returned. The fourth argument should point to an unsigned char * vari-
1760 able.
1761
1762 PCRE_INFO_HASCRORLF
1763
1764 Return 1 if the pattern contains any explicit matches for CR or LF
1765 characters, otherwise 0. The fourth argument should point to an int
1766 variable. An explicit match is either a literal CR or LF character, or
1767 \r or \n.
1768
1769 PCRE_INFO_JCHANGED
1770
1771 Return 1 if the (?J) or (?-J) option setting is used in the pattern,
1772 otherwise 0. The fourth argument should point to an int variable. (?J)
1773 and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
1774
1775 PCRE_INFO_LASTLITERAL
1776
1777 Return the value of the rightmost literal byte that must exist in any
1778 matched string, other than at its start, if such a byte has been
1779 recorded. The fourth argument should point to an int variable. If there
1780 is no such byte, -1 is returned. For anchored patterns, a last literal
1781 byte is recorded only if it follows something of variable length. For
1782 example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
1783 /^a\dz\d/ the returned value is -1.
1784
1785 PCRE_INFO_MINLENGTH
1786
1787 If the pattern was studied and a minimum length for matching subject
1788 strings was computed, its value is returned. Otherwise the returned
1789 value is -1. The value is a number of characters, not bytes (this may
1790 be relevant in UTF-8 mode). The fourth argument should point to an int
1791 variable. A non-negative value is a lower bound to the length of any
1792 matching string. There may not be any strings of that length that do
1793 actually match, but every string that does match is at least that long.
1794
1795 PCRE_INFO_NAMECOUNT
1796 PCRE_INFO_NAMEENTRYSIZE
1797 PCRE_INFO_NAMETABLE
1798
1799 PCRE supports the use of named as well as numbered capturing parenthe-
1800 ses. The names are just an additional way of identifying the parenthe-
1801 ses, which still acquire numbers. Several convenience functions such as
1802 pcre_get_named_substring() are provided for extracting captured sub-
1803 strings by name. It is also possible to extract the data directly, by
1804 first converting the name to a number in order to access the correct
1805 pointers in the output vector (described with pcre_exec() below). To do
1806 the conversion, you need to use the name-to-number map, which is
1807 described by these three values.
1808
1809 The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
1810 gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
1811 of each entry; both of these return an int value. The entry size
1812 depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
1813 a pointer to the first entry of the table (a pointer to char). The
1814 first two bytes of each entry are the number of the capturing parenthe-
1815 sis, most significant byte first. The rest of the entry is the corre-
1816 sponding name, zero terminated.
1817
1818 The names are in alphabetical order. Duplicate names may appear if (?|
1819 is used to create multiple groups with the same number, as described in
1820 the section on duplicate subpattern numbers in the pcrepattern page.
1821 Duplicate names for subpatterns with different numbers are permitted
1822 only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
1823 appear in the table in the order in which they were found in the pat-
1824 tern. In the absence of (?| this is the order of increasing number;
1825 when (?| is used this is not necessarily the case because later subpat-
1826 terns may have lower numbers.
1827
1828 As a simple example of the name/number table, consider the following
1829 pattern (assume PCRE_EXTENDED is set, so white space - including new-
1830 lines - is ignored):
1831
1832 (?<date> (?<year>(\d\d)?\d\d) -
1833 (?<month>\d\d) - (?<day>\d\d) )
1834
1835 There are four named subpatterns, so the table has four entries, and
1836 each entry in the table is eight bytes long. The table is as follows,
1837 with non-printing bytes shows in hexadecimal, and undefined bytes shown
1838 as ??:
1839
1840 00 01 d a t e 00 ??
1841 00 05 d a y 00 ?? ??
1842 00 04 m o n t h 00
1843 00 02 y e a r 00 ??
1844
1845 When writing code to extract data from named subpatterns using the
1846 name-to-number map, remember that the length of the entries is likely
1847 to be different for each compiled pattern.
1848
1849 PCRE_INFO_OKPARTIAL
1850
1851 Return 1 if the pattern can be used for partial matching with
1852 pcre_exec(), otherwise 0. The fourth argument should point to an int
1853 variable. From release 8.00, this always returns 1, because the
1854 restrictions that previously applied to partial matching have been
1855 lifted. The pcrepartial documentation gives details of partial match-
1856 ing.
1857
1858 PCRE_INFO_OPTIONS
1859
1860 Return a copy of the options with which the pattern was compiled. The
1861 fourth argument should point to an unsigned long int variable. These
1862 option bits are those specified in the call to pcre_compile(), modified
1863 by any top-level option settings at the start of the pattern itself. In
1864 other words, they are the options that will be in force when matching
1865 starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
1866 the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
1867 and PCRE_EXTENDED.
1868
1869 A pattern is automatically anchored by PCRE if all of its top-level
1870 alternatives begin with one of the following:
1871
1872 ^ unless PCRE_MULTILINE is set
1873 \A always
1874 \G always
1875 .* if PCRE_DOTALL is set and there are no back
1876 references to the subpattern in which .* appears
1877
1878 For such patterns, the PCRE_ANCHORED bit is set in the options returned
1879 by pcre_fullinfo().
1880
1881 PCRE_INFO_SIZE
1882
1883 Return the size of the compiled pattern, that is, the value that was
1884 passed as the argument to pcre_malloc() when PCRE was getting memory in
1885 which to place the compiled data. The fourth argument should point to a
1886 size_t variable.
1887
1888 PCRE_INFO_STUDYSIZE
1889
1890 Return the size of the data block pointed to by the study_data field in
1891 a pcre_extra block. If pcre_extra is NULL, or there is no study data,
1892 zero is returned. The fourth argument should point to a size_t vari-
1893 able. The study_data field is set by pcre_study() to record informa-
1894 tion that will speed up matching (see the section entitled "Studying a
1895 pattern" above). The format of the study_data block is private, but its
1896 length is made available via this option so that it can be saved and
1897 restored (see the pcreprecompile documentation for details).
1898
1899
1900 OBSOLETE INFO FUNCTION
1901
1902 int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
1903
1904 The pcre_info() function is now obsolete because its interface is too
1905 restrictive to return all the available data about a compiled pattern.
1906 New programs should use pcre_fullinfo() instead. The yield of
1907 pcre_info() is the number of capturing subpatterns, or one of the fol-
1908 lowing negative numbers:
1909
1910 PCRE_ERROR_NULL the argument code was NULL
1911 PCRE_ERROR_BADMAGIC the "magic number" was not found
1912
1913 If the optptr argument is not NULL, a copy of the options with which
1914 the pattern was compiled is placed in the integer it points to (see
1915 PCRE_INFO_OPTIONS above).
1916
1917 If the pattern is not anchored and the firstcharptr argument is not
1918 NULL, it is used to pass back information about the first character of
1919 any matched string (see PCRE_INFO_FIRSTBYTE above).
1920
1921
1922 REFERENCE COUNTS
1923
1924 int pcre_refcount(pcre *code, int adjust);
1925
1926 The pcre_refcount() function is used to maintain a reference count in
1927 the data block that contains a compiled pattern. It is provided for the
1928 benefit of applications that operate in an object-oriented manner,
1929 where different parts of the application may be using the same compiled
1930 pattern, but you want to free the block when they are all done.
1931
1932 When a pattern is compiled, the reference count field is initialized to
1933 zero. It is changed only by calling this function, whose action is to
1934 add the adjust value (which may be positive or negative) to it. The
1935 yield of the function is the new value. However, the value of the count
1936 is constrained to lie between 0 and 65535, inclusive. If the new value
1937 is outside these limits, it is forced to the appropriate limit value.
1938
1939 Except when it is zero, the reference count is not correctly preserved
1940 if a pattern is compiled on one host and then transferred to a host
1941 whose byte-order is different. (This seems a highly unlikely scenario.)
1942
1943
1944 MATCHING A PATTERN: THE TRADITIONAL FUNCTION
1945
1946 int pcre_exec(const pcre *code, const pcre_extra *extra,
1947 const char *subject, int length, int startoffset,
1948 int options, int *ovector, int ovecsize);
1949
1950 The function pcre_exec() is called to match a subject string against a
1951 compiled pattern, which is passed in the code argument. If the pattern
1952 was studied, the result of the study should be passed in the extra
1953 argument. This function is the main matching facility of the library,
1954 and it operates in a Perl-like manner. For specialist use there is also
1955 an alternative matching function, which is described below in the sec-
1956 tion about the pcre_dfa_exec() function.
1957
1958 In most applications, the pattern will have been compiled (and option-
1959 ally studied) in the same process that calls pcre_exec(). However, it
1960 is possible to save compiled patterns and study data, and then use them
1961 later in different processes, possibly even on different hosts. For a
1962 discussion about this, see the pcreprecompile documentation.
1963
1964 Here is an example of a simple call to pcre_exec():
1965
1966 int rc;
1967 int ovector[30];
1968 rc = pcre_exec(
1969 re, /* result of pcre_compile() */
1970 NULL, /* we didn't study the pattern */
1971 "some string", /* the subject string */
1972 11, /* the length of the subject string */
1973 0, /* start at offset 0 in the subject */
1974 0, /* default options */
1975 ovector, /* vector of integers for substring information */
1976 30); /* number of elements (NOT size in bytes) */
1977
1978 Extra data for pcre_exec()
1979
1980 If the extra argument is not NULL, it must point to a pcre_extra data
1981 block. The pcre_study() function returns such a block (when it doesn't
1982 return NULL), but you can also create one for yourself, and pass addi-
1983 tional information in it. The pcre_extra block contains the following
1984 fields (not necessarily in this order):
1985
1986 unsigned long int flags;
1987 void *study_data;
1988 unsigned long int match_limit;
1989 unsigned long int match_limit_recursion;
1990 void *callout_data;
1991 const unsigned char *tables;
1992 unsigned char **mark;
1993
1994 The flags field is a bitmap that specifies which of the other fields
1995 are set. The flag bits are:
1996
1997 PCRE_EXTRA_STUDY_DATA
1998 PCRE_EXTRA_MATCH_LIMIT
1999 PCRE_EXTRA_MATCH_LIMIT_RECURSION
2000 PCRE_EXTRA_CALLOUT_DATA
2001 PCRE_EXTRA_TABLES
2002 PCRE_EXTRA_MARK
2003
2004 Other flag bits should be set to zero. The study_data field is set in
2005 the pcre_extra block that is returned by pcre_study(), together with
2006 the appropriate flag bit. You should not set this yourself, but you may
2007 add to the block by setting the other fields and their corresponding
2008 flag bits.
2009
2010 The match_limit field provides a means of preventing PCRE from using up
2011 a vast amount of resources when running patterns that are not going to
2012 match, but which have a very large number of possibilities in their
2013 search trees. The classic example is a pattern that uses nested unlim-
2014 ited repeats.
2015
2016 Internally, PCRE uses a function called match() which it calls repeat-
2017 edly (sometimes recursively). The limit set by match_limit is imposed
2018 on the number of times this function is called during a match, which
2019 has the effect of limiting the amount of backtracking that can take
2020 place. For patterns that are not anchored, the count restarts from zero
2021 for each position in the subject string.
2022
2023 The default value for the limit can be set when PCRE is built; the
2024 default default is 10 million, which handles all but the most extreme
2025 cases. You can override the default by suppling pcre_exec() with a
2026 pcre_extra block in which match_limit is set, and
2027 PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
2028 exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
2029
2030 The match_limit_recursion field is similar to match_limit, but instead
2031 of limiting the total number of times that match() is called, it limits
2032 the depth of recursion. The recursion depth is a smaller number than
2033 the total number of calls, because not all calls to match() are recur-
2034 sive. This limit is of use only if it is set smaller than match_limit.
2035
2036 Limiting the recursion depth limits the amount of stack that can be
2037 used, or, when PCRE has been compiled to use memory on the heap instead
2038 of the stack, the amount of heap memory that can be used.
2039
2040 The default value for match_limit_recursion can be set when PCRE is
2041 built; the default default is the same value as the default for
2042 match_limit. You can override the default by suppling pcre_exec() with
2043 a pcre_extra block in which match_limit_recursion is set, and
2044 PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
2045 limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
2046
2047 The callout_data field is used in conjunction with the "callout" fea-
2048 ture, and is described in the pcrecallout documentation.
2049
2050 The tables field is used to pass a character tables pointer to
2051 pcre_exec(); this overrides the value that is stored with the compiled
2052 pattern. A non-NULL value is stored with the compiled pattern only if
2053 custom tables were supplied to pcre_compile() via its tableptr argu-
2054 ment. If NULL is passed to pcre_exec() using this mechanism, it forces
2055 PCRE's internal tables to be used. This facility is helpful when re-
2056 using patterns that have been saved after compiling with an external
2057 set of tables, because the external tables might be at a different
2058 address when pcre_exec() is called. See the pcreprecompile documenta-
2059 tion for a discussion of saving compiled patterns for later use.
2060
2061 If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
2062 set to point to a char * variable. If the pattern contains any back-
2063 tracking control verbs such as (*MARK:NAME), and the execution ends up
2064 with a name to pass back, a pointer to the name string (zero termi-
2065 nated) is placed in the variable pointed to by the mark field. The
2066 names are within the compiled pattern; if you wish to retain such a
2067 name you must copy it before freeing the memory of a compiled pattern.
2068 If there is no name to pass back, the variable pointed to by the mark
2069 field set to NULL. For details of the backtracking control verbs, see
2070 the section entitled "Backtracking control" in the pcrepattern documen-
2071 tation.
2072
2073 Option bits for pcre_exec()
2074
2075 The unused bits of the options argument for pcre_exec() must be zero.
2076 The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
2077 PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
2078 PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_SOFT, and
2079 PCRE_PARTIAL_HARD.
2080
2081 PCRE_ANCHORED
2082
2083 The PCRE_ANCHORED option limits pcre_exec() to matching at the first
2084 matching position. If a pattern was compiled with PCRE_ANCHORED, or
2085 turned out to be anchored by virtue of its contents, it cannot be made
2086 unachored at matching time.
2087
2088 PCRE_BSR_ANYCRLF
2089 PCRE_BSR_UNICODE
2090
2091 These options (which are mutually exclusive) control what the \R escape
2092 sequence matches. The choice is either to match only CR, LF, or CRLF,
2093 or to match any Unicode newline sequence. These options override the
2094 choice that was made or defaulted when the pattern was compiled.
2095
2096 PCRE_NEWLINE_CR
2097 PCRE_NEWLINE_LF
2098 PCRE_NEWLINE_CRLF
2099 PCRE_NEWLINE_ANYCRLF
2100 PCRE_NEWLINE_ANY
2101
2102 These options override the newline definition that was chosen or
2103 defaulted when the pattern was compiled. For details, see the descrip-
2104 tion of pcre_compile() above. During matching, the newline choice
2105 affects the behaviour of the dot, circumflex, and dollar metacharac-
2106 ters. It may also alter the way the match position is advanced after a
2107 match failure for an unanchored pattern.
2108
2109 When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
2110 set, and a match attempt for an unanchored pattern fails when the cur-
2111 rent position is at a CRLF sequence, and the pattern contains no
2112 explicit matches for CR or LF characters, the match position is
2113 advanced by two characters instead of one, in other words, to after the
2114 CRLF.
2115
2116 The above rule is a compromise that makes the most common cases work as
2117 expected. For example, if the pattern is .+A (and the PCRE_DOTALL
2118 option is not set), it does not match the string "\r\nA" because, after
2119 failing at the start, it skips both the CR and the LF before retrying.
2120 However, the pattern [\r\n]A does match that string, because it con-
2121 tains an explicit CR or LF reference, and so advances only by one char-
2122 acter after the first failure.
2123
2124 An explicit match for CR of LF is either a literal appearance of one of
2125 those characters, or one of the \r or \n escape sequences. Implicit
2126 matches such as [^X] do not count, nor does \s (which includes CR and
2127 LF in the characters that it matches).
2128
2129 Notwithstanding the above, anomalous effects may still occur when CRLF
2130 is a valid newline sequence and explicit \r or \n escapes appear in the
2131 pattern.
2132
2133 PCRE_NOTBOL
2134
2135 This option specifies that first character of the subject string is not
2136 the beginning of a line, so the circumflex metacharacter should not
2137 match before it. Setting this without PCRE_MULTILINE (at compile time)
2138 causes circumflex never to match. This option affects only the behav-
2139 iour of the circumflex metacharacter. It does not affect \A.
2140
2141 PCRE_NOTEOL
2142
2143 This option specifies that the end of the subject string is not the end
2144 of a line, so the dollar metacharacter should not match it nor (except
2145 in multiline mode) a newline immediately before it. Setting this with-
2146 out PCRE_MULTILINE (at compile time) causes dollar never to match. This
2147 option affects only the behaviour of the dollar metacharacter. It does
2148 not affect \Z or \z.
2149
2150 PCRE_NOTEMPTY
2151
2152 An empty string is not considered to be a valid match if this option is
2153 set. If there are alternatives in the pattern, they are tried. If all
2154 the alternatives match the empty string, the entire match fails. For
2155 example, if the pattern
2156
2157 a?b?
2158
2159 is applied to a string not beginning with "a" or "b", it matches an
2160 empty string at the start of the subject. With PCRE_NOTEMPTY set, this
2161 match is not valid, so PCRE searches further into the string for occur-
2162 rences of "a" or "b".
2163
2164 PCRE_NOTEMPTY_ATSTART
2165
2166 This is like PCRE_NOTEMPTY, except that an empty string match that is
2167 not at the start of the subject is permitted. If the pattern is
2168 anchored, such a match can occur only if the pattern contains \K.
2169
2170 Perl has no direct equivalent of PCRE_NOTEMPTY or
2171 PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
2172 match of the empty string within its split() function, and when using
2173 the /g modifier. It is possible to emulate Perl's behaviour after
2174 matching a null string by first trying the match again at the same off-
2175 set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
2176 fails, by advancing the starting offset (see below) and trying an ordi-
2177 nary match again. There is some code that demonstrates how to do this
2178 in the pcredemo sample program. In the most general case, you have to
2179 check to see if the newline convention recognizes CRLF as a newline,
2180 and if so, and the current character is CR followed by LF, advance the
2181 starting offset by two characters instead of one.
2182
2183 PCRE_NO_START_OPTIMIZE
2184
2185 There are a number of optimizations that pcre_exec() uses at the start
2186 of a match, in order to speed up the process. For example, if it is
2187 known that an unanchored match must start with a specific character, it
2188 searches the subject for that character, and fails immediately if it
2189 cannot find it, without actually running the main matching function.
2190 This means that a special item such as (*COMMIT) at the start of a pat-
2191 tern is not considered until after a suitable starting point for the
2192 match has been found. When callouts or (*MARK) items are in use, these
2193 "start-up" optimizations can cause them to be skipped if the pattern is
2194 never actually used. The start-up optimizations are in effect a pre-
2195 scan of the subject that takes place before the pattern is run.
2196
2197 The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
2198 possibly causing performance to suffer, but ensuring that in cases
2199 where the result is "no match", the callouts do occur, and that items
2200 such as (*COMMIT) and (*MARK) are considered at every possible starting
2201 position in the subject string. If PCRE_NO_START_OPTIMIZE is set at
2202 compile time, it cannot be unset at matching time.
2203
2204 Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
2205 operation. Consider the pattern
2206
2207 (*COMMIT)ABC
2208
2209 When this is compiled, PCRE records the fact that a match must start
2210 with the character "A". Suppose the subject string is "DEFABC". The
2211 start-up optimization scans along the subject, finds "A" and runs the
2212 first match attempt from there. The (*COMMIT) item means that the pat-
2213 tern must match the current starting position, which in this case, it
2214 does. However, if the same match is run with PCRE_NO_START_OPTIMIZE
2215 set, the initial scan along the subject string does not happen. The
2216 first match attempt is run starting from "D" and when this fails,
2217 (*COMMIT) prevents any further matches being tried, so the overall
2218 result is "no match". If the pattern is studied, more start-up opti-
2219 mizations may be used. For example, a minimum length for the subject
2220 may be recorded. Consider the pattern
2221
2222 (*MARK:A)(X|Y)
2223
2224 The minimum length for a match is one character. If the subject is
2225 "ABC", there will be attempts to match "ABC", "BC", "C", and then
2226 finally an empty string. If the pattern is studied, the final attempt
2227 does not take place, because PCRE knows that the subject is too short,
2228 and so the (*MARK) is never encountered. In this case, studying the
2229 pattern does not affect the overall match result, which is still "no
2230 match", but it does affect the auxiliary information that is returned.
2231
2232 PCRE_NO_UTF8_CHECK
2233
2234 When PCRE_UTF8 is set at compile time, the validity of the subject as a
2235 UTF-8 string is automatically checked when pcre_exec() is subsequently
2236 called. The value of startoffset is also checked to ensure that it
2237 points to the start of a UTF-8 character. There is a discussion about
2238 the validity of UTF-8 strings in the section on UTF-8 support in the
2239 main pcre page. If an invalid UTF-8 sequence of bytes is found,
2240 pcre_exec() returns the error PCRE_ERROR_BADUTF8 or, if PCRE_PAR-
2241 TIAL_HARD is set and the problem is a truncated UTF-8 character at the
2242 end of the subject, PCRE_ERROR_SHORTUTF8. In both cases, information
2243 about the precise nature of the error may also be returned (see the
2244 descriptions of these errors in the section entitled Error return val-
2245 ues from pcre_exec() below). If startoffset contains a value that does
2246 not point to the start of a UTF-8 character (or to the end of the sub-
2247 ject), PCRE_ERROR_BADUTF8_OFFSET is returned.
2248
2249 If you already know that your subject is valid, and you want to skip
2250 these checks for performance reasons, you can set the
2251 PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
2252 do this for the second and subsequent calls to pcre_exec() if you are
2253 making repeated calls to find all the matches in a single subject
2254 string. However, you should be sure that the value of startoffset
2255 points to the start of a UTF-8 character (or the end of the subject).
2256 When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8
2257 string as a subject or an invalid value of startoffset is undefined.
2258 Your program may crash.
2259
2260 PCRE_PARTIAL_HARD
2261 PCRE_PARTIAL_SOFT
2262
2263 These options turn on the partial matching feature. For backwards com-
2264 patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
2265 match occurs if the end of the subject string is reached successfully,
2266 but there are not enough subject characters to complete the match. If
2267 this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
2268 matching continues by testing any remaining alternatives. Only if no
2269 complete match can be found is PCRE_ERROR_PARTIAL returned instead of
2270 PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the
2271 caller is prepared to handle a partial match, but only if no complete
2272 match can be found.
2273
2274 If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
2275 case, if a partial match is found, pcre_exec() immediately returns
2276 PCRE_ERROR_PARTIAL, without considering any other alternatives. In
2277 other words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
2278 ered to be more important that an alternative complete match.
2279
2280 In both cases, the portion of the string that was inspected when the
2281 partial match was found is set as the first matching string. There is a
2282 more detailed discussion of partial and multi-segment matching, with
2283 examples, in the pcrepartial documentation.
2284
2285 The string to be matched by pcre_exec()
2286
2287 The subject string is passed to pcre_exec() as a pointer in subject, a
2288 length (in bytes) in length, and a starting byte offset in startoffset.
2289 If this is negative or greater than the length of the subject,
2290 pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
2291 zero, the search for a match starts at the beginning of the subject,
2292 and this is by far the most common case. In UTF-8 mode, the byte offset
2293 must point to the start of a UTF-8 character (or the end of the sub-
2294 ject). Unlike the pattern string, the subject may contain binary zero
2295 bytes.
2296
2297 A non-zero starting offset is useful when searching for another match
2298 in the same subject by calling pcre_exec() again after a previous suc-
2299 cess. Setting startoffset differs from just passing over a shortened
2300 string and setting PCRE_NOTBOL in the case of a pattern that begins
2301 with any kind of lookbehind. For example, consider the pattern
2302
2303 \Biss\B
2304
2305 which finds occurrences of "iss" in the middle of words. (\B matches
2306 only if the current position in the subject is not a word boundary.)
2307 When applied to the string "Mississipi" the first call to pcre_exec()
2308 finds the first occurrence. If pcre_exec() is called again with just
2309 the remainder of the subject, namely "issipi", it does not match,
2310 because \B is always false at the start of the subject, which is deemed
2311 to be a word boundary. However, if pcre_exec() is passed the entire
2312 string again, but with startoffset set to 4, it finds the second occur-
2313 rence of "iss" because it is able to look behind the starting point to
2314 discover that it is preceded by a letter.
2315
2316 Finding all the matches in a subject is tricky when the pattern can
2317 match an empty string. It is possible to emulate Perl's /g behaviour by
2318 first trying the match again at the same offset, with the
2319 PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
2320 fails, advancing the starting offset and trying an ordinary match
2321 again. There is some code that demonstrates how to do this in the pcre-
2322 demo sample program. In the most general case, you have to check to see
2323 if the newline convention recognizes CRLF as a newline, and if so, and
2324 the current character is CR followed by LF, advance the starting offset
2325 by two characters instead of one.
2326
2327 If a non-zero starting offset is passed when the pattern is anchored,
2328 one attempt to match at the given offset is made. This can only succeed
2329 if the pattern does not require the match to be at the start of the
2330 subject.
2331
2332 How pcre_exec() returns captured substrings
2333
2334 In general, a pattern matches a certain portion of the subject, and in
2335 addition, further substrings from the subject may be picked out by
2336 parts of the pattern. Following the usage in Jeffrey Friedl's book,
2337 this is called "capturing" in what follows, and the phrase "capturing
2338 subpattern" is used for a fragment of a pattern that picks out a sub-
2339 string. PCRE supports several other kinds of parenthesized subpattern
2340 that do not cause substrings to be captured.
2341
2342 Captured substrings are returned to the caller via a vector of integers
2343 whose address is passed in ovector. The number of elements in the vec-
2344 tor is passed in ovecsize, which must be a non-negative number. Note:
2345 this argument is NOT the size of ovector in bytes.
2346
2347 The first two-thirds of the vector is used to pass back captured sub-
2348 strings, each substring using a pair of integers. The remaining third
2349 of the vector is used as workspace by pcre_exec() while matching cap-
2350 turing subpatterns, and is not available for passing back information.
2351 The number passed in ovecsize should always be a multiple of three. If
2352 it is not, it is rounded down.
2353
2354 When a match is successful, information about captured substrings is
2355 returned in pairs of integers, starting at the beginning of ovector,
2356 and continuing up to two-thirds of its length at the most. The first
2357 element of each pair is set to the byte offset of the first character
2358 in a substring, and the second is set to the byte offset of the first
2359 character after the end of a substring. Note: these values are always
2360 byte offsets, even in UTF-8 mode. They are not character counts.
2361
2362 The first pair of integers, ovector[0] and ovector[1], identify the
2363 portion of the subject string matched by the entire pattern. The next
2364 pair is used for the first capturing subpattern, and so on. The value
2365 returned by pcre_exec() is one more than the highest numbered pair that
2366 has been set. For example, if two substrings have been captured, the
2367 returned value is 3. If there are no capturing subpatterns, the return
2368 value from a successful match is 1, indicating that just the first pair
2369 of offsets has been set.
2370
2371 If a capturing subpattern is matched repeatedly, it is the last portion
2372 of the string that it matched that is returned.
2373
2374 If the vector is too small to hold all the captured substring offsets,
2375 it is used as far as possible (up to two-thirds of its length), and the
2376 function returns a value of zero. If the substring offsets are not of
2377 interest, pcre_exec() may be called with ovector passed as NULL and
2378 ovecsize as zero. However, if the pattern contains back references and
2379 the ovector is not big enough to remember the related substrings, PCRE
2380 has to get additional memory for use during matching. Thus it is usu-
2381 ally advisable to supply an ovector.
2382
2383 The pcre_fullinfo() function can be used to find out how many capturing
2384 subpatterns there are in a compiled pattern. The smallest size for
2385 ovector that will allow for n captured substrings, in addition to the
2386 offsets of the substring matched by the whole pattern, is (n+1)*3.
2387
2388 It is possible for capturing subpattern number n+1 to match some part
2389 of the subject when subpattern n has not been used at all. For example,
2390 if the string "abc" is matched against the pattern (a|(z))(bc) the
2391 return from the function is 4, and subpatterns 1 and 3 are matched, but
2392 2 is not. When this happens, both values in the offset pairs corre-
2393 sponding to unused subpatterns are set to -1.
2394
2395 Offset values that correspond to unused subpatterns at the end of the
2396 expression are also set to -1. For example, if the string "abc" is
2397 matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
2398 matched. The return from the function is 2, because the highest used
2399 capturing subpattern number is 1, and the offsets for for the second
2400 and third capturing subpatterns (assuming the vector is large enough,
2401 of course) are set to -1.
2402
2403 Note: Elements of ovector that do not correspond to capturing parenthe-
2404 ses in the pattern are never changed. That is, if a pattern contains n
2405 capturing parentheses, no more than ovector[0] to ovector[2n+1] are set
2406 by pcre_exec(). The other elements retain whatever values they previ-
2407 ously had.
2408
2409 Some convenience functions are provided for extracting the captured
2410 substrings as separate strings. These are described below.
2411
2412 Error return values from pcre_exec()
2413
2414 If pcre_exec() fails, it returns a negative number. The following are
2415 defined in the header file:
2416
2417 PCRE_ERROR_NOMATCH (-1)
2418
2419 The subject string did not match the pattern.
2420
2421 PCRE_ERROR_NULL (-2)
2422
2423 Either code or subject was passed as NULL, or ovector was NULL and
2424 ovecsize was not zero.
2425
2426 PCRE_ERROR_BADOPTION (-3)
2427
2428 An unrecognized bit was set in the options argument.
2429
2430 PCRE_ERROR_BADMAGIC (-4)
2431
2432 PCRE stores a 4-byte "magic number" at the start of the compiled code,
2433 to catch the case when it is passed a junk pointer and to detect when a
2434 pattern that was compiled in an environment of one endianness is run in
2435 an environment with the other endianness. This is the error that PCRE
2436 gives when the magic number is not present.
2437
2438 PCRE_ERROR_UNKNOWN_OPCODE (-5)
2439
2440 While running the pattern match, an unknown item was encountered in the
2441 compiled pattern. This error could be caused by a bug in PCRE or by
2442 overwriting of the compiled pattern.
2443
2444 PCRE_ERROR_NOMEMORY (-6)
2445
2446 If a pattern contains back references, but the ovector that is passed
2447 to pcre_exec() is not big enough to remember the referenced substrings,
2448 PCRE gets a block of memory at the start of matching to use for this
2449 purpose. If the call via pcre_malloc() fails, this error is given. The
2450 memory is automatically freed at the end of matching.
2451
2452 This error is also given if pcre_stack_malloc() fails in pcre_exec().
2453 This can happen only when PCRE has been compiled with --disable-stack-
2454 for-recursion.
2455
2456 PCRE_ERROR_NOSUBSTRING (-7)
2457
2458 This error is used by the pcre_copy_substring(), pcre_get_substring(),
2459 and pcre_get_substring_list() functions (see below). It is never
2460 returned by pcre_exec().
2461
2462 PCRE_ERROR_MATCHLIMIT (-8)
2463
2464 The backtracking limit, as specified by the match_limit field in a
2465 pcre_extra structure (or defaulted) was reached. See the description
2466 above.
2467
2468 PCRE_ERROR_CALLOUT (-9)
2469
2470 This error is never generated by pcre_exec() itself. It is provided for
2471 use by callout functions that want to yield a distinctive error code.
2472 See the pcrecallout documentation for details.
2473
2474 PCRE_ERROR_BADUTF8 (-10)
2475
2476 A string that contains an invalid UTF-8 byte sequence was passed as a
2477 subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of
2478 the output vector (ovecsize) is at least 2, the byte offset to the
2479 start of the the invalid UTF-8 character is placed in the first ele-
2480 ment, and a reason code is placed in the second element. The reason
2481 codes are listed in the following section. For backward compatibility,
2482 if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
2483 acter at the end of the subject (reason codes 1 to 5),
2484 PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
2485
2486 PCRE_ERROR_BADUTF8_OFFSET (-11)
2487
2488 The UTF-8 byte sequence that was passed as a subject was checked and
2489 found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
2490 value of startoffset did not point to the beginning of a UTF-8 charac-
2491 ter or the end of the subject.
2492
2493 PCRE_ERROR_PARTIAL (-12)
2494
2495 The subject string did not match, but it did match partially. See the
2496 pcrepartial documentation for details of partial matching.
2497
2498 PCRE_ERROR_BADPARTIAL (-13)
2499
2500 This code is no longer in use. It was formerly returned when the
2501 PCRE_PARTIAL option was used with a compiled pattern containing items
2502 that were not supported for partial matching. From release 8.00
2503 onwards, there are no restrictions on partial matching.
2504
2505 PCRE_ERROR_INTERNAL (-14)
2506
2507 An unexpected internal error has occurred. This error could be caused
2508 by a bug in PCRE or by overwriting of the compiled pattern.
2509
2510 PCRE_ERROR_BADCOUNT (-15)
2511
2512 This error is given if the value of the ovecsize argument is negative.
2513
2514 PCRE_ERROR_RECURSIONLIMIT (-21)
2515
2516 The internal recursion limit, as specified by the match_limit_recursion
2517 field in a pcre_extra structure (or defaulted) was reached. See the
2518 description above.
2519
2520 PCRE_ERROR_BADNEWLINE (-23)
2521
2522 An invalid combination of PCRE_NEWLINE_xxx options was given.
2523
2524 PCRE_ERROR_BADOFFSET (-24)
2525
2526 The value of startoffset was negative or greater than the length of the
2527 subject, that is, the value in length.
2528
2529 PCRE_ERROR_SHORTUTF8 (-25)
2530
2531 This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
2532 string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
2533 option is set. Information about the failure is returned as for
2534 PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but
2535 this special error code for PCRE_PARTIAL_HARD precedes the implementa-
2536 tion of returned information; it is retained for backwards compatibil-
2537 ity.
2538
2539 PCRE_ERROR_RECURSELOOP (-26)
2540
2541 This error is returned when pcre_exec() detects a recursion loop within
2542 the pattern. Specifically, it means that either the whole pattern or a
2543 subpattern has been called recursively for the second time at the same
2544 position in the subject string. Some simple patterns that might do this
2545 are detected and faulted at compile time, but more complicated cases,
2546 in particular mutual recursions between two different subpatterns, can-
2547 not be detected until run time.
2548
2549 Error numbers -16 to -20 and -22 are not used by pcre_exec().
2550
2551 Reason codes for invalid UTF-8 strings
2552
2553 When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
2554 UTF8, and the size of the output vector (ovecsize) is at least 2, the
2555 offset of the start of the invalid UTF-8 character is placed in the
2556 first output vector element (ovector[0]) and a reason code is placed in
2557 the second element (ovector[1]). The reason codes are given names in
2558 the pcre.h header file:
2559
2560 PCRE_UTF8_ERR1
2561 PCRE_UTF8_ERR2
2562 PCRE_UTF8_ERR3
2563 PCRE_UTF8_ERR4
2564 PCRE_UTF8_ERR5
2565
2566 The string ends with a truncated UTF-8 character; the code specifies
2567 how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
2568 characters to be no longer than 4 bytes, the encoding scheme (origi-
2569 nally defined by RFC 2279) allows for up to 6 bytes, and this is
2570 checked first; hence the possibility of 4 or 5 missing bytes.
2571
2572 PCRE_UTF8_ERR6
2573 PCRE_UTF8_ERR7
2574 PCRE_UTF8_ERR8
2575 PCRE_UTF8_ERR9
2576 PCRE_UTF8_ERR10
2577
2578 The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
2579 the character do not have the binary value 0b10 (that is, either the
2580 most significant bit is 0, or the next bit is 1).
2581
2582 PCRE_UTF8_ERR11
2583 PCRE_UTF8_ERR12
2584
2585 A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
2586 long; these code points are excluded by RFC 3629.
2587
2588 PCRE_UTF8_ERR13
2589
2590 A 4-byte character has a value greater than 0x10fff; these code points
2591 are excluded by RFC 3629.
2592
2593 PCRE_UTF8_ERR14
2594
2595 A 3-byte character has a value in the range 0xd800 to 0xdfff; this
2596 range of code points are reserved by RFC 3629 for use with UTF-16, and
2597 so are excluded from UTF-8.
2598
2599 PCRE_UTF8_ERR15
2600 PCRE_UTF8_ERR16
2601 PCRE_UTF8_ERR17
2602 PCRE_UTF8_ERR18
2603 PCRE_UTF8_ERR19
2604
2605 A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
2606 for a value that can be represented by fewer bytes, which is invalid.
2607 For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
2608 rect coding uses just one byte.
2609
2610 PCRE_UTF8_ERR20
2611
2612 The two most significant bits of the first byte of a character have the
2613 binary value 0b10 (that is, the most significant bit is 1 and the sec-
2614 ond is 0). Such a byte can only validly occur as the second or subse-
2615 quent byte of a multi-byte character.
2616
2617 PCRE_UTF8_ERR21
2618
2619 The first byte of a character has the value 0xfe or 0xff. These values
2620 can never occur in a valid UTF-8 string.
2621
2622
2623 EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
2624
2625 int pcre_copy_substring(const char *subject, int *ovector,
2626 int stringcount, int stringnumber, char *buffer,
2627 int buffersize);
2628
2629 int pcre_get_substring(const char *subject, int *ovector,
2630 int stringcount, int stringnumber,
2631 const char **stringptr);
2632
2633 int pcre_get_substring_list(const char *subject,
2634 int *ovector, int stringcount, const char ***listptr);
2635
2636 Captured substrings can be accessed directly by using the offsets
2637 returned by pcre_exec() in ovector. For convenience, the functions
2638 pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
2639 string_list() are provided for extracting captured substrings as new,
2640 separate, zero-terminated strings. These functions identify substrings
2641 by number. The next section describes functions for extracting named
2642 substrings.
2643
2644 A substring that contains a binary zero is correctly extracted and has
2645 a further zero added on the end, but the result is not, of course, a C
2646 string. However, you can process such a string by referring to the
2647 length that is returned by pcre_copy_substring() and pcre_get_sub-
2648 string(). Unfortunately, the interface to pcre_get_substring_list() is
2649 not adequate for handling strings containing binary zeros, because the
2650 end of the final string is not independently indicated.
2651
2652 The first three arguments are the same for all three of these func-
2653 tions: subject is the subject string that has just been successfully
2654 matched, ovector is a pointer to the vector of integer offsets that was
2655 passed to pcre_exec(), and stringcount is the number of substrings that
2656 were captured by the match, including the substring that matched the
2657 entire regular expression. This is the value returned by pcre_exec() if
2658 it is greater than zero. If pcre_exec() returned zero, indicating that
2659 it ran out of space in ovector, the value passed as stringcount should
2660 be the number of elements in the vector divided by three.
2661
2662 The functions pcre_copy_substring() and pcre_get_substring() extract a
2663 single substring, whose number is given as stringnumber. A value of
2664 zero extracts the substring that matched the entire pattern, whereas
2665 higher values extract the captured substrings. For pcre_copy_sub-
2666 string(), the string is placed in buffer, whose length is given by
2667 buffersize, while for pcre_get_substring() a new block of memory is
2668 obtained via pcre_malloc, and its address is returned via stringptr.
2669 The yield of the function is the length of the string, not including
2670 the terminating zero, or one of these error codes:
2671
2672 PCRE_ERROR_NOMEMORY (-6)
2673
2674 The buffer was too small for pcre_copy_substring(), or the attempt to
2675 get memory failed for pcre_get_substring().
2676
2677 PCRE_ERROR_NOSUBSTRING (-7)
2678
2679 There is no substring whose number is stringnumber.
2680
2681 The pcre_get_substring_list() function extracts all available sub-
2682 strings and builds a list of pointers to them. All this is done in a
2683 single block of memory that is obtained via pcre_malloc. The address of
2684 the memory block is returned via listptr, which is also the start of
2685 the list of string pointers. The end of the list is marked by a NULL
2686 pointer. The yield of the function is zero if all went well, or the
2687 error code
2688
2689 PCRE_ERROR_NOMEMORY (-6)
2690
2691 if the attempt to get the memory block failed.
2692
2693 When any of these functions encounter a substring that is unset, which
2694 can happen when capturing subpattern number n+1 matches some part of
2695 the subject, but subpattern n has not been used at all, they return an
2696 empty string. This can be distinguished from a genuine zero-length sub-
2697 string by inspecting the appropriate offset in ovector, which is nega-
2698 tive for unset substrings.
2699
2700 The two convenience functions pcre_free_substring() and pcre_free_sub-
2701 string_list() can be used to free the memory returned by a previous
2702 call of pcre_get_substring() or pcre_get_substring_list(), respec-
2703 tively. They do nothing more than call the function pointed to by
2704 pcre_free, which of course could be called directly from a C program.
2705 However, PCRE is used in some situations where it is linked via a spe-
2706 cial interface to another programming language that cannot use
2707 pcre_free directly; it is for these cases that the functions are pro-
2708 vided.
2709
2710
2711 EXTRACTING CAPTURED SUBSTRINGS BY NAME
2712
2713 int pcre_get_stringnumber(const pcre *code,
2714 const char *name);
2715
2716 int pcre_copy_named_substring(const pcre *code,
2717 const char *subject, int *ovector,
2718 int stringcount, const char *stringname,
2719 char *buffer, int buffersize);
2720
2721 int pcre_get_named_substring(const pcre *code,
2722 const char *subject, int *ovector,
2723 int stringcount, const char *stringname,
2724 const char **stringptr);
2725
2726 To extract a substring by name, you first have to find associated num-
2727 ber. For example, for this pattern
2728
2729 (a+)b(?<xxx>\d+)...
2730
2731 the number of the subpattern called "xxx" is 2. If the name is known to
2732 be unique (PCRE_DUPNAMES was not set), you can find the number from the
2733 name by calling pcre_get_stringnumber(). The first argument is the com-
2734 piled pattern, and the second is the name. The yield of the function is
2735 the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
2736 subpattern of that name.
2737
2738 Given the number, you can extract the substring directly, or use one of
2739 the functions described in the previous section. For convenience, there
2740 are also two functions that do the whole job.
2741
2742 Most of the arguments of pcre_copy_named_substring() and
2743 pcre_get_named_substring() are the same as those for the similarly
2744 named functions that extract by number. As these are described in the
2745 previous section, they are not re-described here. There are just two
2746 differences:
2747
2748 First, instead of a substring number, a substring name is given. Sec-
2749 ond, there is an extra argument, given at the start, which is a pointer
2750 to the compiled pattern. This is needed in order to gain access to the
2751 name-to-number translation table.
2752
2753 These functions call pcre_get_stringnumber(), and if it succeeds, they
2754 then call pcre_copy_substring() or pcre_get_substring(), as appropri-
2755 ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
2756 behaviour may not be what you want (see the next section).
2757
2758 Warning: If the pattern uses the (?| feature to set up multiple subpat-
2759 terns with the same number, as described in the section on duplicate
2760 subpattern numbers in the pcrepattern page, you cannot use names to
2761 distinguish the different subpatterns, because names are not included
2762 in the compiled code. The matching process uses only numbers. For this
2763 reason, the use of different names for subpatterns of the same number
2764 causes an error at compile time.
2765
2766
2767 DUPLICATE SUBPATTERN NAMES
2768
2769 int pcre_get_stringtable_entries(const pcre *code,
2770 const char *name, char **first, char **last);
2771
2772 When a pattern is compiled with the PCRE_DUPNAMES option, names for
2773 subpatterns are not required to be unique. (Duplicate names are always
2774 allowed for subpatterns with the same number, created by using the (?|
2775 feature. Indeed, if such subpatterns are named, they are required to
2776 use the same names.)
2777
2778 Normally, patterns with duplicate names are such that in any one match,
2779 only one of the named subpatterns participates. An example is shown in
2780 the pcrepattern documentation.
2781
2782 When duplicates are present, pcre_copy_named_substring() and
2783 pcre_get_named_substring() return the first substring corresponding to
2784 the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
2785 (-7) is returned; no data is returned. The pcre_get_stringnumber()
2786 function returns one of the numbers that are associated with the name,
2787 but it is not defined which it is.
2788
2789 If you want to get full details of all captured substrings for a given
2790 name, you must use the pcre_get_stringtable_entries() function. The
2791 first argument is the compiled pattern, and the second is the name. The
2792 third and fourth are pointers to variables which are updated by the
2793 function. After it has run, they point to the first and last entries in
2794 the name-to-number table for the given name. The function itself
2795 returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
2796 there are none. The format of the table is described above in the sec-
2797 tion entitled Information about a pattern above. Given all the rele-
2798 vant entries for the name, you can extract each of their numbers, and
2799 hence the captured data, if any.
2800
2801
2802 FINDING ALL POSSIBLE MATCHES
2803
2804 The traditional matching function uses a similar algorithm to Perl,
2805 which stops when it finds the first match, starting at a given point in
2806 the subject. If you want to find all possible matches, or the longest
2807 possible match, consider using the alternative matching function (see
2808 below) instead. If you cannot use the alternative function, but still
2809 need to find all possible matches, you can kludge it up by making use
2810 of the callout facility, which is described in the pcrecallout documen-
2811 tation.
2812
2813 What you have to do is to insert a callout right at the end of the pat-
2814 tern. When your callout function is called, extract and save the cur-
2815 rent matched substring. Then return 1, which forces pcre_exec() to
2816 backtrack and try other alternatives. Ultimately, when it runs out of
2817 matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
2818
2819
2820 MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
2821
2822 int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
2823 const char *subject, int length, int startoffset,
2824 int options, int *ovector, int ovecsize,
2825 int *workspace, int wscount);
2826
2827 The function pcre_dfa_exec() is called to match a subject string
2828 against a compiled pattern, using a matching algorithm that scans the
2829 subject string just once, and does not backtrack. This has different
2830 characteristics to the normal algorithm, and is not compatible with
2831 Perl. Some of the features of PCRE patterns are not supported. Never-
2832 theless, there are times when this kind of matching can be useful. For
2833 a discussion of the two matching algorithms, and a list of features
2834 that pcre_dfa_exec() does not support, see the pcrematching documenta-
2835 tion.
2836
2837 The arguments for the pcre_dfa_exec() function are the same as for
2838 pcre_exec(), plus two extras. The ovector argument is used in a differ-
2839 ent way, and this is described below. The other common arguments are
2840 used in the same way as for pcre_exec(), so their description is not
2841 repeated here.
2842
2843 The two additional arguments provide workspace for the function. The
2844 workspace vector should contain at least 20 elements. It is used for
2845 keeping track of multiple paths through the pattern tree. More
2846 workspace will be needed for patterns and subjects where there are a
2847 lot of potential matches.
2848
2849 Here is an example of a simple call to pcre_dfa_exec():
2850
2851 int rc;
2852 int ovector[10];
2853 int wspace[20];
2854 rc = pcre_dfa_exec(
2855 re, /* result of pcre_compile() */
2856 NULL, /* we didn't study the pattern */
2857 "some string", /* the subject string */
2858 11, /* the length of the subject string */
2859 0, /* start at offset 0 in the subject */
2860 0, /* default options */
2861 ovector, /* vector of integers for substring information */
2862 10, /* number of elements (NOT size in bytes) */
2863 wspace, /* working space vector */
2864 20); /* number of elements (NOT size in bytes) */
2865
2866 Option bits for pcre_dfa_exec()
2867
2868 The unused bits of the options argument for pcre_dfa_exec() must be
2869 zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
2870 LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
2871 PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF,
2872 PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR-
2873 TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
2874 four of these are exactly the same as for pcre_exec(), so their
2875 description is not repeated here.
2876
2877 PCRE_PARTIAL_HARD
2878 PCRE_PARTIAL_SOFT
2879
2880 These have the same general effect as they do for pcre_exec(), but the
2881 details are slightly different. When PCRE_PARTIAL_HARD is set for
2882 pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
2883 ject is reached and there is still at least one matching possibility
2884 that requires additional characters. This happens even if some complete
2885 matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
2886 code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
2887 of the subject is reached, there have been no complete matches, but
2888 there is still at least one matching possibility. The portion of the
2889 string that was inspected when the longest partial match was found is
2890 set as the first matching string in both cases. There is a more
2891 detailed discussion of partial and multi-segment matching, with exam-
2892 ples, in the pcrepartial documentation.
2893
2894 PCRE_DFA_SHORTEST
2895
2896 Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
2897 stop as soon as it has found one match. Because of the way the alterna-
2898 tive algorithm works, this is necessarily the shortest possible match
2899 at the first possible matching point in the subject string.
2900
2901 PCRE_DFA_RESTART
2902
2903 When pcre_dfa_exec() returns a partial match, it is possible to call it
2904 again, with additional subject characters, and have it continue with
2905 the same match. The PCRE_DFA_RESTART option requests this action; when
2906 it is set, the workspace and wscount options must reference the same
2907 vector as before because data about the match so far is left in them
2908 after a partial match. There is more discussion of this facility in the
2909 pcrepartial documentation.
2910
2911 Successful returns from pcre_dfa_exec()
2912
2913 When pcre_dfa_exec() succeeds, it may have matched more than one sub-
2914 string in the subject. Note, however, that all the matches from one run
2915 of the function start at the same point in the subject. The shorter
2916 matches are all initial substrings of the longer matches. For example,
2917 if the pattern
2918
2919 <.*>
2920
2921 is matched against the string
2922
2923 This is <something> <something else> <something further> no more
2924
2925 the three matched strings are
2926
2927 <something>
2928 <something> <something else>
2929 <something> <something else> <something further>
2930
2931 On success, the yield of the function is a number greater than zero,
2932 which is the number of matched substrings. The substrings themselves
2933 are returned in ovector. Each string uses two elements; the first is
2934 the offset to the start, and the second is the offset to the end. In
2935 fact, all the strings have the same start offset. (Space could have
2936 been saved by giving this only once, but it was decided to retain some
2937 compatibility with the way pcre_exec() returns data, even though the
2938 meaning of the strings is different.)
2939
2940 The strings are returned in reverse order of length; that is, the long-
2941 est matching string is given first. If there were too many matches to
2942 fit into ovector, the yield of the function is zero, and the vector is
2943 filled with the longest matches.
2944
2945 Error returns from pcre_dfa_exec()
2946
2947 The pcre_dfa_exec() function returns a negative number when it fails.
2948 Many of the errors are the same as for pcre_exec(), and these are
2949 described above. There are in addition the following errors that are
2950 specific to pcre_dfa_exec():
2951
2952 PCRE_ERROR_DFA_UITEM (-16)
2953
2954 This return is given if pcre_dfa_exec() encounters an item in the pat-
2955 tern that it does not support, for instance, the use of \C or a back
2956 reference.
2957
2958 PCRE_ERROR_DFA_UCOND (-17)
2959
2960 This return is given if pcre_dfa_exec() encounters a condition item
2961 that uses a back reference for the condition, or a test for recursion
2962 in a specific group. These are not supported.
2963
2964 PCRE_ERROR_DFA_UMLIMIT (-18)
2965
2966 This return is given if pcre_dfa_exec() is called with an extra block
2967 that contains a setting of the match_limit field. This is not supported
2968 (it is meaningless).
2969
2970 PCRE_ERROR_DFA_WSSIZE (-19)
2971
2972 This return is given if pcre_dfa_exec() runs out of space in the
2973 workspace vector.
2974
2975 PCRE_ERROR_DFA_RECURSE (-20)
2976
2977 When a recursive subpattern is processed, the matching function calls
2978 itself recursively, using private vectors for ovector and workspace.
2979 This error is given if the output vector is not large enough. This
2980 should be extremely rare, as a vector of size 1000 is used.
2981
2982
2983 SEE ALSO
2984
2985 pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
2986 tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
2987
2988
2989 AUTHOR
2990
2991 Philip Hazel
2992 University Computing Service
2993 Cambridge CB2 3QH, England.
2994
2995
2996 REVISION
2997
2998 Last updated: 13 August 2011
2999 Copyright (c) 1997-2011 University of Cambridge.
3000 ------------------------------------------------------------------------------
3001
3002
3003 PCRECALLOUT(3) PCRECALLOUT(3)
3004
3005
3006 NAME
3007 PCRE - Perl-compatible regular expressions
3008
3009
3010 PCRE CALLOUTS
3011
3012 int (*pcre_callout)(pcre_callout_block *);
3013
3014 PCRE provides a feature called "callout", which is a means of temporar-
3015 ily passing control to the caller of PCRE in the middle of pattern
3016 matching. The caller of PCRE provides an external function by putting
3017 its entry point in the global variable pcre_callout. By default, this
3018 variable contains NULL, which disables all calling out.
3019
3020 Within a regular expression, (?C) indicates the points at which the
3021 external function is to be called. Different callout points can be
3022 identified by putting a number less than 256 after the letter C. The
3023 default value is zero. For example, this pattern has two callout
3024 points:
3025
3026 (?C1)abc(?C2)def
3027
3028 If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or
3029 pcre_compile2() is called, PCRE automatically inserts callouts, all
3030 with number 255, before each item in the pattern. For example, if
3031 PCRE_AUTO_CALLOUT is used with the pattern
3032
3033 A(\d{2}|--)
3034
3035 it is processed as if it were
3036
3037 (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
3038
3039 Notice that there is a callout before and after each parenthesis and
3040 alternation bar. Automatic callouts can be used for tracking the
3041 progress of pattern matching. The pcretest command has an option that
3042 sets automatic callouts; when it is used, the output indicates how the
3043 pattern is matched. This is useful information when you are trying to
3044 optimize the performance of a particular pattern.
3045
3046
3047 MISSING CALLOUTS
3048
3049 You should be aware that, because of optimizations in the way PCRE
3050 matches patterns by default, callouts sometimes do not happen. For
3051 example, if the pattern is
3052
3053 ab(?C4)cd
3054
3055 PCRE knows that any matching string must contain the letter "d". If the
3056 subject string is "abyz", the lack of "d" means that matching doesn't
3057 ever start, and the callout is never reached. However, with "abyd",
3058 though the result is still no match, the callout is obeyed.
3059
3060 If the pattern is studied, PCRE knows the minimum length of a matching
3061 string, and will immediately give a "no match" return without actually
3062 running a match if the subject is not long enough, or, for unanchored
3063 patterns, if it has been scanned far enough.
3064
3065 You can disable these optimizations by passing the PCRE_NO_START_OPTI-
3066 MIZE option to pcre_compile(), pcre_exec(), or pcre_dfa_exec(), or by
3067 starting the pattern with (*NO_START_OPT). This slows down the matching
3068 process, but does ensure that callouts such as the example above are
3069 obeyed.
3070
3071
3072 THE CALLOUT INTERFACE
3073
3074 During matching, when PCRE reaches a callout point, the external func-
3075 tion defined by pcre_callout is called (if it is set). This applies to
3076 both the pcre_exec() and the pcre_dfa_exec() matching functions. The
3077 only argument to the callout function is a pointer to a pcre_callout
3078 block. This structure contains the following fields:
3079
3080 int version;
3081 int callout_number;
3082 int *offset_vector;
3083 const char *subject;
3084 int subject_length;
3085 int start_match;
3086 int current_position;
3087 int capture_top;
3088 int capture_last;
3089 void *callout_data;
3090 int pattern_position;
3091 int next_item_length;
3092 const unsigned char *mark;
3093
3094 The version field is an integer containing the version number of the
3095 block format. The initial version was 0; the current version is 2. The
3096 version number will change again in future if additional fields are
3097 added, but the intention is never to remove any of the existing fields.
3098
3099 The callout_number field contains the number of the callout, as com-
3100 piled into the pattern (that is, the number after ?C for manual call-
3101 outs, and 255 for automatically generated callouts).
3102
3103 The offset_vector field is a pointer to the vector of offsets that was
3104 passed by the caller to pcre_exec() or pcre_dfa_exec(). When
3105 pcre_exec() is used, the contents can be inspected in order to extract
3106 substrings that have been matched so far, in the same way as for
3107 extracting substrings after a match has completed. For pcre_dfa_exec()
3108 this field is not useful.
3109
3110 The subject and subject_length fields contain copies of the values that
3111 were passed to pcre_exec().
3112
3113 The start_match field normally contains the offset within the subject
3114 at which the current match attempt started. However, if the escape
3115 sequence \K has been encountered, this value is changed to reflect the
3116 modified starting point. If the pattern is not anchored, the callout
3117 function may be called several times from the same point in the pattern
3118 for different starting points in the subject.
3119
3120 The current_position field contains the offset within the subject of
3121 the current match pointer.
3122
3123 When the pcre_exec() function is used, the capture_top field contains
3124 one more than the number of the highest numbered captured substring so
3125 far. If no substrings have been captured, the value of capture_top is
3126 one. This is always the case when pcre_dfa_exec() is used, because it
3127 does not support captured substrings.
3128
3129 The capture_last field contains the number of the most recently cap-
3130 tured substring. If no substrings have been captured, its value is -1.
3131 This is always the case when pcre_dfa_exec() is used.
3132
3133 The callout_data field contains a value that is passed to pcre_exec()
3134 or pcre_dfa_exec() specifically so that it can be passed back in call-
3135 outs. It is passed in the pcre_callout field of the pcre_extra data
3136 structure. If no such data was passed, the value of callout_data in a
3137 pcre_callout block is NULL. There is a description of the pcre_extra
3138 structure in the pcreapi documentation.
3139
3140 The pattern_position field is present from version 1 of the pcre_call-
3141 out structure. It contains the offset to the next item to be matched in
3142 the pattern string.
3143
3144 The next_item_length field is present from version 1 of the pcre_call-
3145 out structure. It contains the length of the next item to be matched in
3146 the pattern string. When the callout immediately precedes an alterna-
3147 tion bar, a closing parenthesis, or the end of the pattern, the length
3148 is zero. When the callout precedes an opening parenthesis, the length
3149 is that of the entire subpattern.
3150
3151 The pattern_position and next_item_length fields are intended to help
3152 in distinguishing between different automatic callouts, which all have
3153 the same callout number. However, they are set for all callouts.
3154
3155 The mark field is present from version 2 of the pcre_callout structure.
3156 In callouts from pcre_exec() it contains a pointer to the zero-termi-
3157 nated name of the most recently passed (*MARK) item in the match, or
3158 NULL if there are no (*MARK)s in the current matching path. In callouts
3159 from pcre_dfa_exec() this field always contains NULL.
3160
3161
3162 RETURN VALUES
3163
3164 The external callout function returns an integer to PCRE. If the value
3165 is zero, matching proceeds as normal. If the value is greater than
3166 zero, matching fails at the current point, but the testing of other
3167 matching possibilities goes ahead, just as if a lookahead assertion had
3168 failed. If the value is less than zero, the match is abandoned, and
3169 pcre_exec() or pcre_dfa_exec() returns the negative value.
3170
3171 Negative values should normally be chosen from the set of
3172 PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
3173 dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
3174 reserved for use by callout functions; it will never be used by PCRE
3175 itself.
3176
3177
3178 AUTHOR
3179
3180 Philip Hazel
3181 University Computing Service
3182 Cambridge CB2 3QH, England.
3183
3184
3185 REVISION
3186
3187 Last updated: 31 July 2011
3188 Copyright (c) 1997-2011 University of Cambridge.
3189 ------------------------------------------------------------------------------
3190
3191
3192 PCRECOMPAT(3) PCRECOMPAT(3)
3193
3194
3195 NAME
3196 PCRE - Perl-compatible regular expressions
3197
3198
3199 DIFFERENCES BETWEEN PCRE AND PERL
3200
3201 This document describes the differences in the ways that PCRE and Perl
3202 handle regular expressions. The differences described here are with
3203 respect to Perl versions 5.10 and above.
3204
3205 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
3206 of what it does have are given in the section on UTF-8 support in the
3207 main pcre page.
3208
3209 2. PCRE allows repeat quantifiers only on parenthesized assertions, but
3210 they do not mean what you might think. For example, (?!a){3} does not
3211 assert that the next three characters are not "a". It just asserts that
3212 the next character is not "a" three times (in principle: PCRE optimizes
3213 this to run the assertion just once). Perl allows repeat quantifiers on
3214 other assertions such as \b, but these do not seem to have any use.
3215
3216 3. Capturing subpatterns that occur inside negative lookahead asser-
3217 tions are counted, but their entries in the offsets vector are never
3218 set. Perl sets its numerical variables from any such patterns that are
3219 matched before the assertion fails to match something (thereby succeed-
3220 ing), but only if the negative lookahead assertion contains just one
3221 branch.
3222
3223 4. Though binary zero characters are supported in the subject string,
3224 they are not allowed in a pattern string because it is passed as a nor-
3225 mal C string, terminated by zero. The escape sequence \0 can be used in
3226 the pattern to represent a binary zero.
3227
3228 5. The following Perl escape sequences are not supported: \l, \u, \L,
3229 \U, and \N when followed by a character name or Unicode value. (\N on
3230 its own, matching a non-newline character, is supported.) In fact these
3231 are implemented by Perl's general string-handling and are not part of
3232 its pattern matching engine. If any of these are encountered by PCRE,
3233 an error is generated.
3234
3235 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
3236 is built with Unicode character property support. The properties that
3237 can be tested with \p and \P are limited to the general category prop-
3238 erties such as Lu and Nd, script names such as Greek or Han, and the
3239 derived properties Any and L&. PCRE does support the Cs (surrogate)
3240 property, which Perl does not; the Perl documentation says "Because
3241 Perl hides the need for the user to understand the internal representa-
3242 tion of Unicode characters, there is no need to implement the somewhat
3243 messy concept of surrogates."
3244
3245 7. PCRE implements a simpler version of \X than Perl, which changed to
3246 make \X match what Unicode calls an "extended grapheme cluster". This
3247 is more complicated than an extended Unicode sequence, which is what
3248 PCRE matches.
3249
3250 8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
3251 ters in between are treated as literals. This is slightly different
3252 from Perl in that $ and @ are also handled as literals inside the
3253 quotes. In Perl, they cause variable interpolation (but of course PCRE
3254 does not have variables). Note the following examples:
3255
3256 Pattern PCRE matches Perl matches
3257
3258 \Qabc$xyz\E abc$xyz abc followed by the
3259 contents of $xyz
3260 \Qabc\$xyz\E abc\$xyz abc\$xyz
3261 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3262
3263 The \Q...\E sequence is recognized both inside and outside character
3264 classes.
3265
3266 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
3267 constructions. However, there is support for recursive patterns. This
3268 is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
3269 "callout" feature allows an external function to be called during pat-
3270 tern matching. See the pcrecallout documentation for details.
3271
3272 10. Subpatterns that are called recursively or as "subroutines" are
3273 always treated as atomic groups in PCRE. This is like Python, but
3274 unlike Perl. There is a discussion of an example that explains this in
3275 more detail in the section on recursion differences from Perl in the
3276 pcrepattern page.
3277
3278 11. There are some differences that are concerned with the settings of
3279 captured strings when part of a pattern is repeated. For example,
3280 matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
3281 unset, but in PCRE it is set to "b".
3282
3283 12. PCRE's handling of duplicate subpattern numbers and duplicate sub-
3284 pattern names is not as general as Perl's. This is a consequence of the
3285 fact the PCRE works internally just with numbers, using an external ta-
3286 ble to translate between numbers and names. In particular, a pattern
3287 such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
3288 the same number but different names, is not supported, and causes an
3289 error at compile time. If it were allowed, it would not be possible to
3290 distinguish which parentheses matched, because both names map to cap-
3291 turing subpattern number 1. To avoid this confusing situation, an error
3292 is given at compile time.
3293
3294 13. Perl recognizes comments in some places that PCRE does not, for
3295 example, between the ( and ? at the start of a subpattern. If the /x
3296 modifier is set, Perl allows whitespace between ( and ? but PCRE never
3297 does, even if the PCRE_EXTENDED option is set.
3298
3299 14. PCRE provides some extensions to the Perl regular expression facil-
3300 ities. Perl 5.10 includes new features that are not in earlier ver-
3301 sions of Perl, some of which (such as named parentheses) have been in
3302 PCRE for some time. This list is with respect to Perl 5.10:
3303
3304 (a) Although lookbehind assertions in PCRE must match fixed length
3305 strings, each alternative branch of a lookbehind assertion can match a
3306 different length of string. Perl requires them all to have the same
3307 length.
3308
3309 (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
3310 meta-character matches only at the very end of the string.
3311
3312 (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
3313 cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
3314 ignored. (Perl can be made to issue a warning.)
3315
3316 (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
3317 fiers is inverted, that is, by default they are not greedy, but if fol-
3318 lowed by a question mark they are.
3319
3320 (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
3321 tried only at the first matching position in the subject string.
3322
3323 (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
3324 and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
3325 lents.
3326
3327 (g) The \R escape sequence can be restricted to match only CR, LF, or
3328 CRLF by the PCRE_BSR_ANYCRLF option.
3329
3330 (h) The callout facility is PCRE-specific.
3331
3332 (i) The partial matching facility is PCRE-specific.
3333
3334 (j) Patterns compiled by PCRE can be saved and re-used at a later time,
3335 even on different hosts that have the other endianness.
3336
3337 (k) The alternative matching function (pcre_dfa_exec()) matches in a
3338 different way and is not Perl-compatible.
3339
3340 (l) PCRE recognizes some special sequences such as (*CR) at the start
3341 of a pattern that set overall options that cannot be changed within the
3342 pattern.
3343
3344
3345 AUTHOR
3346
3347 Philip Hazel
3348 University Computing Service
3349 Cambridge CB2 3QH, England.
3350
3351
3352 REVISION
3353
3354 Last updated: 24 July 2011
3355 Copyright (c) 1997-2011 University of Cambridge.
3356 ------------------------------------------------------------------------------
3357
3358
3359 PCREPATTERN(3) PCREPATTERN(3)
3360
3361
3362 NAME
3363 PCRE - Perl-compatible regular expressions
3364
3365
3366 PCRE REGULAR EXPRESSION DETAILS
3367
3368 The syntax and semantics of the regular expressions that are supported
3369 by PCRE are described in detail below. There is a quick-reference syn-
3370 tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
3371 semantics as closely as it can. PCRE also supports some alternative
3372 regular expression syntax (which does not conflict with the Perl syn-
3373 tax) in order to provide some compatibility with regular expressions in
3374 Python, .NET, and Oniguruma.
3375
3376 Perl's regular expressions are described in its own documentation, and
3377 regular expressions in general are covered in a number of books, some
3378 of which have copious examples. Jeffrey Friedl's "Mastering Regular
3379 Expressions", published by O'Reilly, covers regular expressions in
3380 great detail. This description of PCRE's regular expressions is
3381 intended as reference material.
3382
3383 The original operation of PCRE was on strings of one-byte characters.
3384 However, there is now also support for UTF-8 character strings. To use
3385 this, PCRE must be built to include UTF-8 support, and you must call
3386 pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There is
3387 also a special sequence that can be given at the start of a pattern:
3388
3389 (*UTF8)
3390
3391 Starting a pattern with this sequence is equivalent to setting the
3392 PCRE_UTF8 option. This feature is not Perl-compatible. How setting
3393 UTF-8 mode affects pattern matching is mentioned in several places
3394 below. There is also a summary of UTF-8 features in the section on
3395 UTF-8 support in the main pcre page.
3396
3397 Another special sequence that may appear at the start of a pattern or
3398 in combination with (*UTF8) is:
3399
3400 (*UCP)
3401
3402 This has the same effect as setting the PCRE_UCP option: it causes
3403 sequences such as \d and \w to use Unicode properties to determine
3404 character types, instead of recognizing only characters with codes less
3405 than 128 via a lookup table.
3406
3407 If a pattern starts with (*NO_START_OPT), it has the same effect as
3408 setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
3409 time. There are also some more of these special sequences that are con-
3410 cerned with the handling of newlines; they are described below.
3411
3412 The remainder of this document discusses the patterns that are sup-
3413 ported by PCRE when its main matching function, pcre_exec(), is used.
3414 From release 6.0, PCRE offers a second matching function,
3415 pcre_dfa_exec(), which matches using a different algorithm that is not
3416 Perl-compatible. Some of the features discussed below are not available
3417 when pcre_dfa_exec() is used. The advantages and disadvantages of the
3418 alternative function, and how it differs from the normal function, are
3419 discussed in the pcrematching page.
3420
3421
3422 NEWLINE CONVENTIONS
3423
3424 PCRE supports five different conventions for indicating line breaks in
3425 strings: a single CR (carriage return) character, a single LF (line-
3426 feed) character, the two-character sequence CRLF, any of the three pre-
3427 ceding, or any Unicode newline sequence. The pcreapi page has further
3428 discussion about newlines, and shows how to set the newline convention
3429 in the options arguments for the compiling and matching functions.
3430
3431 It is also possible to specify a newline convention by starting a pat-
3432 tern string with one of the following five sequences:
3433
3434 (*CR) carriage return
3435 (*LF) linefeed
3436 (*CRLF) carriage return, followed by linefeed
3437 (*ANYCRLF) any of the three above
3438 (*ANY) all Unicode newline sequences
3439
3440 These override the default and the options given to pcre_compile() or
3441 pcre_compile2(). For example, on a Unix system where LF is the default
3442 newline sequence, the pattern
3443
3444 (*CR)a.b
3445
3446 changes the convention to CR. That pattern matches "a\nb" because LF is
3447 no longer a newline. Note that these special settings, which are not
3448 Perl-compatible, are recognized only at the very start of a pattern,
3449 and that they must be in upper case. If more than one of them is
3450 present, the last one is used.
3451
3452 The newline convention affects the interpretation of the dot metachar-
3453 acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
3454 ever, it does not affect what the \R escape sequence matches. By
3455 default, this is any Unicode newline sequence, for Perl compatibility.
3456 However, this can be changed; see the description of \R in the section
3457 entitled "Newline sequences" below. A change of \R setting can be com-
3458 bined with a change of newline convention.
3459
3460
3461 CHARACTERS AND METACHARACTERS
3462
3463 A regular expression is a pattern that is matched against a subject
3464 string from left to right. Most characters stand for themselves in a
3465 pattern, and match the corresponding characters in the subject. As a
3466 trivial example, the pattern
3467
3468 The quick brown fox
3469
3470 matches a portion of a subject string that is identical to itself. When
3471 caseless matching is specified (the PCRE_CASELESS option), letters are
3472 matched independently of case. In UTF-8 mode, PCRE always understands
3473 the concept of case for characters whose values are less than 128, so
3474 caseless matching is always possible. For characters with higher val-
3475 ues, the concept of case is supported if PCRE is compiled with Unicode
3476 property support, but not otherwise. If you want to use caseless
3477 matching for characters 128 and above, you must ensure that PCRE is
3478 compiled with Unicode property support as well as with UTF-8 support.
3479
3480 The power of regular expressions comes from the ability to include
3481 alternatives and repetitions in the pattern. These are encoded in the
3482 pattern by the use of metacharacters, which do not stand for themselves
3483 but instead are interpreted in some special way.
3484
3485 There are two different sets of metacharacters: those that are recog-
3486 nized anywhere in the pattern except within square brackets, and those
3487 that are recognized within square brackets. Outside square brackets,
3488 the metacharacters are as follows:
3489
3490 \ general escape character with several uses
3491 ^ assert start of string (or line, in multiline mode)
3492 $ assert end of string (or line, in multiline mode)
3493 . match any character except newline (by default)
3494 [ start character class definition
3495 | start of alternative branch
3496 ( start subpattern
3497 ) end subpattern
3498 ? extends the meaning of (
3499 also 0 or 1 quantifier
3500 also quantifier minimizer
3501 * 0 or more quantifier
3502 + 1 or more quantifier
3503 also "possessive quantifier"
3504 { start min/max quantifier
3505
3506 Part of a pattern that is in square brackets is called a "character
3507 class". In a character class the only metacharacters are:
3508
3509 \ general escape character
3510 ^ negate the class, but only if the first character
3511 - indicates character range
3512 [ POSIX character class (only if followed by POSIX
3513 syntax)
3514 ] terminates the character class
3515
3516 The following sections describe the use of each of the metacharacters.
3517
3518
3519 BACKSLASH
3520
3521 The backslash character has several uses. Firstly, if it is followed by
3522 a character that is not a number or a letter, it takes away any special
3523 meaning that character may have. This use of backslash as an escape
3524 character applies both inside and outside character classes.
3525
3526 For example, if you want to match a * character, you write \* in the
3527 pattern. This escaping action applies whether or not the following
3528 character would otherwise be interpreted as a metacharacter, so it is
3529 always safe to precede a non-alphanumeric with backslash to specify
3530 that it stands for itself. In particular, if you want to match a back-
3531 slash, you write \\.
3532
3533 In UTF-8 mode, only ASCII numbers and letters have any special meaning
3534 after a backslash. All other characters (in particular, those whose
3535 codepoints are greater than 127) are treated as literals.
3536
3537 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
3538 the pattern (other than in a character class) and characters between a
3539 # outside a character class and the next newline are ignored. An escap-
3540 ing backslash can be used to include a whitespace or # character as
3541 part of the pattern.
3542
3543 If you want to remove the special meaning from a sequence of charac-
3544 ters, you can do so by putting them between \Q and \E. This is differ-
3545 ent from Perl in that $ and @ are handled as literals in \Q...\E
3546 sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
3547 tion. Note the following examples:
3548
3549 Pattern PCRE matches Perl matches
3550
3551 \Qabc$xyz\E abc$xyz abc followed by the
3552 contents of $xyz
3553 \Qabc\$xyz\E abc\$xyz abc\$xyz
3554 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
3555
3556 The \Q...\E sequence is recognized both inside and outside character
3557 classes. An isolated \E that is not preceded by \Q is ignored. If \Q
3558 is not followed by \E later in the pattern, the literal interpretation
3559 continues to the end of the pattern (that is, \E is assumed at the
3560 end). If the isolated \Q is inside a character class, this causes an
3561 error, because the character class is not terminated.
3562
3563 Non-printing characters
3564
3565 A second use of backslash provides a way of encoding non-printing char-
3566 acters in patterns in a visible manner. There is no restriction on the
3567 appearance of non-printing characters, apart from the binary zero that
3568 terminates a pattern, but when a pattern is being prepared by text
3569 editing, it is often easier to use one of the following escape
3570 sequences than the binary character it represents:
3571
3572 \a alarm, that is, the BEL character (hex 07)
3573 \cx "control-x", where x is any ASCII character
3574 \e escape (hex 1B)
3575 \f formfeed (hex 0C)
3576 \n linefeed (hex 0A)
3577 \r carriage return (hex 0D)
3578 \t tab (hex 09)
3579 \ddd character with octal code ddd, or back reference
3580 \xhh character with hex code hh
3581 \x{hhh..} character with hex code hhh..
3582
3583 The precise effect of \cx is as follows: if x is a lower case letter,
3584 it is converted to upper case. Then bit 6 of the character (hex 40) is
3585 inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
3586 is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c
3587 has a value greater than 127, a compile-time error occurs. This locks
3588 out non-ASCII characters in both byte mode and UTF-8 mode. (When PCRE
3589 is compiled in EBCDIC mode, all byte values are valid. A lower case
3590 letter is converted to upper case, and then the 0xc0 bits are flipped.)
3591
3592 After \x, from zero to two hexadecimal digits are read (letters can be
3593 in upper or lower case). Any number of hexadecimal digits may appear
3594 between \x{ and }, but the value of the character code must be less
3595 than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is,
3596 the maximum value in hexadecimal is 7FFFFFFF. Note that this is bigger
3597 than the largest Unicode code point, which is 10FFFF.
3598
3599 If characters other than hexadecimal digits appear between \x{ and },
3600 or if there is no terminating }, this form of escape is not recognized.
3601 Instead, the initial \x will be interpreted as a basic hexadecimal
3602 escape, with no following digits, giving a character whose value is
3603 zero.
3604
3605 Characters whose value is less than 256 can be defined by either of the
3606 two syntaxes for \x. There is no difference in the way they are han-
3607 dled. For example, \xdc is exactly the same as \x{dc}.
3608
3609 After \0 up to two further octal digits are read. If there are fewer
3610 than two digits, just those that are present are used. Thus the
3611 sequence \0\x\07 specifies two binary zeros followed by a BEL character
3612 (code value 7). Make sure you supply two digits after the initial zero
3613 if the pattern character that follows is itself an octal digit.
3614
3615 The handling of a backslash followed by a digit other than 0 is compli-
3616 cated. Outside a character class, PCRE reads it and any following dig-
3617 its as a decimal number. If the number is less than 10, or if there
3618 have been at least that many previous capturing left parentheses in the
3619 expression, the entire sequence is taken as a back reference. A
3620 description of how this works is given later, following the discussion
3621 of parenthesized subpatterns.
3622
3623 Inside a character class, or if the decimal number is greater than 9
3624 and there have not been that many capturing subpatterns, PCRE re-reads
3625 up to three octal digits following the backslash, and uses them to gen-
3626 erate a data character. Any subsequent digits stand for themselves. In
3627 non-UTF-8 mode, the value of a character specified in octal must be
3628 less than \400. In UTF-8 mode, values up to \777 are permitted. For
3629 example:
3630
3631 \040 is another way of writing a space
3632 \40 is the same, provided there are fewer than 40
3633 previous capturing subpatterns
3634 \7 is always a back reference
3635 \11 might be a back reference, or another way of
3636 writing a tab
3637 \011 is always a tab
3638 \0113 is a tab followed by the character "3"
3639 \113 might be a back reference, otherwise the
3640 character with octal code 113
3641 \377 might be a back reference, otherwise
3642 the byte consisting entirely of 1 bits
3643 \81 is either a back reference, or a binary zero
3644 followed by the two characters "8" and "1"
3645
3646 Note that octal values of 100 or greater must not be introduced by a
3647 leading zero, because no more than three octal digits are ever read.
3648
3649 All the sequences that define a single character value can be used both
3650 inside and outside character classes. In addition, inside a character
3651 class, the sequence \b is interpreted as the backspace character (hex
3652 08). The sequences \B, \N, \R, and \X are not special inside a charac-
3653 ter class. Like any other unrecognized escape sequences, they are
3654 treated as the literal characters "B", "N", "R", and "X" by default,
3655 but cause an error if the PCRE_EXTRA option is set. Outside a character
3656 class, these sequences have different meanings.
3657
3658 Absolute and relative back references
3659
3660 The sequence \g followed by an unsigned or a negative number, option-
3661 ally enclosed in braces, is an absolute or relative back reference. A
3662 named back reference can be coded as \g{name}. Back references are dis-
3663 cussed later, following the discussion of parenthesized subpatterns.
3664
3665 Absolute and relative subroutine calls
3666
3667 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
3668 name or a number enclosed either in angle brackets or single quotes, is
3669 an alternative syntax for referencing a subpattern as a "subroutine".
3670 Details are discussed later. Note that \g{...} (Perl syntax) and
3671 \g<...> (Oniguruma syntax) are not synonymous. The former is a back
3672 reference; the latter is a subroutine call.
3673
3674 Generic character types
3675
3676 Another use of backslash is for specifying generic character types:
3677
3678 \d any decimal digit
3679 \D any character that is not a decimal digit
3680 \h any horizontal whitespace character
3681 \H any character that is not a horizontal whitespace character
3682 \s any whitespace character
3683 \S any character that is not a whitespace character
3684 \v any vertical whitespace character
3685 \V any character that is not a vertical whitespace character
3686 \w any "word" character
3687 \W any "non-word" character
3688
3689 There is also the single sequence \N, which matches a non-newline char-
3690 acter. This is the same as the "." metacharacter when PCRE_DOTALL is
3691 not set.
3692
3693 Each pair of lower and upper case escape sequences partitions the com-
3694 plete set of characters into two disjoint sets. Any given character
3695 matches one, and only one, of each pair. The sequences can appear both
3696 inside and outside character classes. They each match one character of
3697 the appropriate type. If the current matching point is at the end of
3698 the subject string, all of them fail, because there is no character to
3699 match.
3700
3701 For compatibility with Perl, \s does not match the VT character (code
3702 11). This makes it different from the the POSIX "space" class. The \s
3703 characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
3704 "use locale;" is included in a Perl script, \s may match the VT charac-
3705 ter. In PCRE, it never does.
3706
3707 A "word" character is an underscore or any character that is a letter
3708 or digit. By default, the definition of letters and digits is con-
3709 trolled by PCRE's low-valued character tables, and may vary if locale-
3710 specific matching is taking place (see "Locale support" in the pcreapi
3711 page). For example, in a French locale such as "fr_FR" in Unix-like
3712 systems, or "french" in Windows, some character codes greater than 128
3713 are used for accented letters, and these are then matched by \w. The
3714 use of locales with Unicode is discouraged.
3715
3716 By default, in UTF-8 mode, characters with values greater than 128
3717 never match \d, \s, or \w, and always match \D, \S, and \W. These
3718 sequences retain their original meanings from before UTF-8 support was
3719 available, mainly for efficiency reasons. However, if PCRE is compiled
3720 with Unicode property support, and the PCRE_UCP option is set, the be-
3721 haviour is changed so that Unicode properties are used to determine
3722 character types, as follows:
3723
3724 \d any character that \p{Nd} matches (decimal digit)
3725 \s any character that \p{Z} matches, plus HT, LF, FF, CR
3726 \w any character that \p{L} or \p{N} matches, plus underscore
3727
3728 The upper case escapes match the inverse sets of characters. Note that
3729 \d matches only decimal digits, whereas \w matches any Unicode digit,
3730 as well as any Unicode letter, and underscore. Note also that PCRE_UCP
3731 affects \b, and \B because they are defined in terms of \w and \W.
3732 Matching these sequences is noticeably slower when PCRE_UCP is set.
3733
3734 The sequences \h, \H, \v, and \V are features that were added to Perl
3735 at release 5.10. In contrast to the other sequences, which match only
3736 ASCII characters by default, these always match certain high-valued
3737 codepoints in UTF-8 mode, whether or not PCRE_UCP is set. The horizon-
3738 tal space characters are:
3739
3740 U+0009 Horizontal tab
3741 U+0020 Space
3742 U+00A0 Non-break space
3743 U+1680 Ogham space mark
3744 U+180E Mongolian vowel separator
3745 U+2000 En quad
3746 U+2001 Em quad
3747 U+2002 En space
3748 U+2003 Em space
3749 U+2004 Three-per-em space
3750 U+2005 Four-per-em space
3751 U+2006 Six-per-em space
3752 U+2007 Figure space
3753 U+2008 Punctuation space
3754 U+2009 Thin space
3755 U+200A Hair space
3756 U+202F Narrow no-break space
3757 U+205F Medium mathematical space
3758 U+3000 Ideographic space
3759
3760 The vertical space characters are:
3761
3762 U+000A Linefeed
3763 U+000B Vertical tab
3764 U+000C Formfeed
3765 U+000D Carriage return
3766 U+0085 Next line
3767 U+2028 Line separator
3768 U+2029 Paragraph separator
3769
3770 Newline sequences
3771
3772 Outside a character class, by default, the escape sequence \R matches
3773 any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the
3774 following:
3775
3776 (?>\r\n|\n|\x0b|\f|\r|\x85)
3777
3778 This is an example of an "atomic group", details of which are given
3779 below. This particular group matches either the two-character sequence
3780 CR followed by LF, or one of the single characters LF (linefeed,
3781 U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3782 return, U+000D), or NEL (next line, U+0085). The two-character sequence
3783 is treated as a single unit that cannot be split.
3784
3785 In UTF-8 mode, two additional characters whose codepoints are greater
3786 than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3787 rator, U+2029). Unicode character property support is not needed for
3788 these characters to be recognized.
3789
3790 It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3791 the complete set of Unicode line endings) by setting the option
3792 PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3793 (BSR is an abbrevation for "backslash R".) This can be made the default
3794 when PCRE is built; if this is the case, the other behaviour can be
3795 requested via the PCRE_BSR_UNICODE option. It is also possible to
3796 specify these settings by starting a pattern string with one of the
3797 following sequences:
3798
3799 (*BSR_ANYCRLF) CR, LF, or CRLF only
3800 (*BSR_UNICODE) any Unicode newline sequence
3801
3802 These override the default and the options given to pcre_compile() or
3803 pcre_compile2(), but they can be overridden by options given to
3804 pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
3805 are not Perl-compatible, are recognized only at the very start of a
3806 pattern, and that they must be in upper case. If more than one of them
3807 is present, the last one is used. They can be combined with a change of
3808 newline convention; for example, a pattern can start with:
3809
3810 (*ANY)(*BSR_ANYCRLF)
3811
3812 They can also be combined with the (*UTF8) or (*UCP) special sequences.
3813 Inside a character class, \R is treated as an unrecognized escape
3814 sequence, and so matches the letter "R" by default, but causes an error
3815 if PCRE_EXTRA is set.
3816
3817 Unicode character properties
3818
3819 When PCRE is built with Unicode character property support, three addi-
3820 tional escape sequences that match characters with specific properties
3821 are available. When not in UTF-8 mode, these sequences are of course
3822 limited to testing characters whose codepoints are less than 256, but
3823 they do work in this mode. The extra escape sequences are:
3824
3825 \p{xx} a character with the xx property
3826 \P{xx} a character without the xx property
3827 \X an extended Unicode sequence
3828
3829 The property names represented by xx above are limited to the Unicode
3830 script names, the general category properties, "Any", which matches any
3831 character (including newline), and some special PCRE properties
3832 (described in the next section). Other Perl properties such as "InMu-
3833 sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
3834 does not match any characters, so always causes a match failure.
3835
3836 Sets of Unicode characters are defined as belonging to certain scripts.
3837 A character from one of these sets can be matched using a script name.
3838 For example:
3839
3840 \p{Greek}
3841 \P{Han}
3842
3843 Those that are not part of an identified script are lumped together as
3844 "Common". The current list of scripts is:
3845
3846 Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
3847 Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
3848 Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
3849 tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
3850 Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
3851 rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
3852 Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
3853 Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
3854 Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
3855 Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
3856 Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
3857 Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
3858 Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
3859 Ugaritic, Vai, Yi.
3860
3861 Each character has exactly one Unicode general category property, spec-
3862 ified by a two-letter abbreviation. For compatibility with Perl, nega-
3863 tion can be specified by including a circumflex between the opening
3864 brace and the property name. For example, \p{^Lu} is the same as
3865 \P{Lu}.
3866
3867 If only one letter is specified with \p or \P, it includes all the gen-
3868 eral category properties that start with that letter. In this case, in
3869 the absence of negation, the curly brackets in the escape sequence are
3870 optional; these two examples have the same effect:
3871
3872 \p{L}
3873 \pL
3874
3875 The following general category property codes are supported:
3876
3877 C Other
3878 Cc Control
3879 Cf Format
3880 Cn Unassigned
3881 Co Private use
3882 Cs Surrogate
3883
3884 L Letter
3885 Ll Lower case letter
3886 Lm Modifier letter
3887 Lo Other letter
3888 Lt Title case letter
3889 Lu Upper case letter
3890
3891 M Mark
3892 Mc Spacing mark
3893 Me Enclosing mark
3894 Mn Non-spacing mark
3895
3896 N Number
3897 Nd Decimal number
3898 Nl Letter number
3899 No Other number
3900
3901 P Punctuation
3902 Pc Connector punctuation
3903 Pd Dash punctuation
3904 Pe Close punctuation
3905 Pf Final punctuation
3906 Pi Initial punctuation
3907 Po Other punctuation
3908 Ps Open punctuation
3909
3910 S Symbol
3911 Sc Currency symbol
3912 Sk Modifier symbol
3913 Sm Mathematical symbol
3914 So Other symbol
3915
3916 Z Separator
3917 Zl Line separator
3918 Zp Paragraph separator
3919 Zs Space separator
3920
3921 The special property L& is also supported: it matches a character that
3922 has the Lu, Ll, or Lt property, in other words, a letter that is not
3923 classified as a modifier or "other".
3924
3925 The Cs (Surrogate) property applies only to characters in the range
3926 U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
3927 RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3928 ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
3929 the pcreapi page). Perl does not support the Cs property.
3930
3931 The long synonyms for property names that Perl supports (such as
3932 \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
3933 any of these properties with "Is".
3934
3935 No character that is in the Unicode table has the Cn (unassigned) prop-
3936 erty. Instead, this property is assumed for any code point that is not
3937 in the Unicode table.
3938
3939 Specifying caseless matching does not affect these escape sequences.
3940 For example, \p{Lu} always matches only upper case letters.
3941
3942 The \X escape matches any number of Unicode characters that form an
3943 extended Unicode sequence. \X is equivalent to
3944
3945 (?>\PM\pM*)
3946
3947 That is, it matches a character without the "mark" property, followed
3948 by zero or more characters with the "mark" property, and treats the
3949 sequence as an atomic group (see below). Characters with the "mark"
3950 property are typically accents that affect the preceding character.
3951 None of them have codepoints less than 256, so in non-UTF-8 mode \X
3952 matches any one character.
3953
3954 Note that recent versions of Perl have changed \X to match what Unicode
3955 calls an "extended grapheme cluster", which has a more complicated def-
3956 inition.
3957
3958 Matching characters by Unicode property is not fast, because PCRE has
3959 to search a structure that contains data for over fifteen thousand
3960 characters. That is why the traditional escape sequences such as \d and
3961 \w do not use Unicode properties in PCRE by default, though you can
3962 make them do so by setting the PCRE_UCP option for pcre_compile() or by
3963 starting the pattern with (*UCP).
3964
3965 PCRE's additional properties
3966
3967 As well as the standard Unicode properties described in the previous
3968 section, PCRE supports four more that make it possible to convert tra-
3969 ditional escape sequences such as \w and \s and POSIX character classes
3970 to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
3971 erties internally when PCRE_UCP is set. They are:
3972
3973 Xan Any alphanumeric character
3974 Xps Any POSIX space character
3975 Xsp Any Perl space character
3976 Xwd Any Perl "word" character
3977
3978 Xan matches characters that have either the L (letter) or the N (num-
3979 ber) property. Xps matches the characters tab, linefeed, vertical tab,
3980 formfeed, or carriage return, and any other character that has the Z
3981 (separator) property. Xsp is the same as Xps, except that vertical tab
3982 is excluded. Xwd matches the same characters as Xan, plus underscore.
3983
3984 Resetting the match start
3985
3986 The escape sequence \K causes any previously matched characters not to
3987 be included in the final matched sequence. For example, the pattern:
3988
3989 foo\Kbar
3990
3991 matches "foobar", but reports that it has matched "bar". This feature
3992 is similar to a lookbehind assertion (described below). However, in
3993 this case, the part of the subject before the real match does not have
3994 to be of fixed length, as lookbehind assertions do. The use of \K does
3995 not interfere with the setting of captured substrings. For example,
3996 when the pattern
3997
3998 (foo)\Kbar
3999
4000 matches "foobar", the first substring is still set to "foo".
4001
4002 Perl documents that the use of \K within assertions is "not well
4003 defined". In PCRE, \K is acted upon when it occurs inside positive
4004 assertions, but is ignored in negative assertions.
4005
4006 Simple assertions
4007
4008 The final use of backslash is for certain simple assertions. An asser-
4009 tion specifies a condition that has to be met at a particular point in
4010 a match, without consuming any characters from the subject string. The
4011 use of subpatterns for more complicated assertions is described below.
4012 The backslashed assertions are:
4013
4014 \b matches at a word boundary
4015 \B matches when not at a word boundary
4016 \A matches at the start of the subject
4017 \Z matches at the end of the subject
4018 also matches before a newline at the end of the subject
4019 \z matches only at the end of the subject
4020 \G matches at the first matching position in the subject
4021
4022 Inside a character class, \b has a different meaning; it matches the
4023 backspace character. If any other of these assertions appears in a
4024 character class, by default it matches the corresponding literal char-
4025 acter (for example, \B matches the letter B). However, if the
4026 PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
4027 ated instead.
4028
4029 A word boundary is a position in the subject string where the current
4030 character and the previous character do not both match \w or \W (i.e.
4031 one matches \w and the other matches \W), or the start or end of the
4032 string if the first or last character matches \w, respectively. In
4033 UTF-8 mode, the meanings of \w and \W can be changed by setting the
4034 PCRE_UCP option. When this is done, it also affects \b and \B. Neither
4035 PCRE nor Perl has a separate "start of word" or "end of word" metase-
4036 quence. However, whatever follows \b normally determines which it is.
4037 For example, the fragment \ba matches "a" at the start of a word.
4038
4039 The \A, \Z, and \z assertions differ from the traditional circumflex
4040 and dollar (described in the next section) in that they only ever match
4041 at the very start and end of the subject string, whatever options are
4042 set. Thus, they are independent of multiline mode. These three asser-
4043 tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
4044 affect only the behaviour of the circumflex and dollar metacharacters.
4045 However, if the startoffset argument of pcre_exec() is non-zero, indi-
4046 cating that matching is to start at a point other than the beginning of
4047 the subject, \A can never match. The difference between \Z and \z is
4048 that \Z matches before a newline at the end of the string as well as at
4049 the very end, whereas \z matches only at the end.
4050
4051 The \G assertion is true only when the current matching position is at
4052 the start point of the match, as specified by the startoffset argument
4053 of pcre_exec(). It differs from \A when the value of startoffset is
4054 non-zero. By calling pcre_exec() multiple times with appropriate argu-
4055 ments, you can mimic Perl's /g option, and it is in this kind of imple-
4056 mentation where \G can be useful.
4057
4058 Note, however, that PCRE's interpretation of \G, as the start of the
4059 current match, is subtly different from Perl's, which defines it as the
4060 end of the previous match. In Perl, these can be different when the
4061 previously matched string was empty. Because PCRE does just one match
4062 at a time, it cannot reproduce this behaviour.
4063
4064 If all the alternatives of a pattern begin with \G, the expression is
4065 anchored to the starting match position, and the "anchored" flag is set
4066 in the compiled regular expression.
4067
4068
4069 CIRCUMFLEX AND DOLLAR
4070
4071 Outside a character class, in the default matching mode, the circumflex
4072 character is an assertion that is true only if the current matching
4073 point is at the start of the subject string. If the startoffset argu-
4074 ment of pcre_exec() is non-zero, circumflex can never match if the
4075 PCRE_MULTILINE option is unset. Inside a character class, circumflex
4076 has an entirely different meaning (see below).
4077
4078 Circumflex need not be the first character of the pattern if a number
4079 of alternatives are involved, but it should be the first thing in each
4080 alternative in which it appears if the pattern is ever to match that
4081 branch. If all possible alternatives start with a circumflex, that is,
4082 if the pattern is constrained to match only at the start of the sub-
4083 ject, it is said to be an "anchored" pattern. (There are also other
4084 constructs that can cause a pattern to be anchored.)
4085
4086 A dollar character is an assertion that is true only if the current
4087 matching point is at the end of the subject string, or immediately
4088 before a newline at the end of the string (by default). Dollar need not
4089 be the last character of the pattern if a number of alternatives are
4090 involved, but it should be the last item in any branch in which it
4091 appears. Dollar has no special meaning in a character class.
4092
4093 The meaning of dollar can be changed so that it matches only at the
4094 very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
4095 compile time. This does not affect the \Z assertion.
4096
4097 The meanings of the circumflex and dollar characters are changed if the
4098 PCRE_MULTILINE option is set. When this is the case, a circumflex
4099 matches immediately after internal newlines as well as at the start of
4100 the subject string. It does not match after a newline that ends the
4101 string. A dollar matches before any newlines in the string, as well as
4102 at the very end, when PCRE_MULTILINE is set. When newline is specified
4103 as the two-character sequence CRLF, isolated CR and LF characters do
4104 not indicate newlines.
4105
4106 For example, the pattern /^abc$/ matches the subject string "def\nabc"
4107 (where \n represents a newline) in multiline mode, but not otherwise.
4108 Consequently, patterns that are anchored in single line mode because
4109 all branches start with ^ are not anchored in multiline mode, and a
4110 match for circumflex is possible when the startoffset argument of
4111 pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
4112 PCRE_MULTILINE is set.
4113
4114 Note that the sequences \A, \Z, and \z can be used to match the start
4115 and end of the subject in both modes, and if all branches of a pattern
4116 start with \A it is always anchored, whether or not PCRE_MULTILINE is
4117 set.
4118
4119
4120 FULL STOP (PERIOD, DOT) AND \N
4121
4122 Outside a character class, a dot in the pattern matches any one charac-
4123 ter in the subject string except (by default) a character that signi-
4124 fies the end of a line. In UTF-8 mode, the matched character may be
4125 more than one byte long.
4126
4127 When a line ending is defined as a single character, dot never matches
4128 that character; when the two-character sequence CRLF is used, dot does
4129 not match CR if it is immediately followed by LF, but otherwise it
4130 matches all characters (including isolated CRs and LFs). When any Uni-
4131 code line endings are being recognized, dot does not match CR or LF or
4132 any of the other line ending characters.
4133
4134 The behaviour of dot with regard to newlines can be changed. If the
4135 PCRE_DOTALL option is set, a dot matches any one character, without
4136 exception. If the two-character sequence CRLF is present in the subject
4137 string, it takes two dots to match it.
4138
4139 The handling of dot is entirely independent of the handling of circum-
4140 flex and dollar, the only relationship being that they both involve
4141 newlines. Dot has no special meaning in a character class.
4142
4143 The escape sequence \N behaves like a dot, except that it is not
4144 affected by the PCRE_DOTALL option. In other words, it matches any
4145 character except one that signifies the end of a line.
4146
4147
4148 MATCHING A SINGLE BYTE
4149
4150 Outside a character class, the escape sequence \C matches any one byte,
4151 both in and out of UTF-8 mode. Unlike a dot, it always matches any
4152 line-ending characters. The feature is provided in Perl in order to
4153 match individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-
4154 acters into individual bytes, the rest of the string may start with a
4155 malformed UTF-8 character. For this reason, the \C escape sequence is
4156 best avoided.
4157
4158 PCRE does not allow \C to appear in lookbehind assertions (described
4159 below), because in UTF-8 mode this would make it impossible to calcu-
4160 late the length of the lookbehind.
4161
4162
4163 SQUARE BRACKETS AND CHARACTER CLASSES
4164
4165 An opening square bracket introduces a character class, terminated by a
4166 closing square bracket. A closing square bracket on its own is not spe-
4167 cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
4168 a lone closing square bracket causes a compile-time error. If a closing
4169 square bracket is required as a member of the class, it should be the
4170 first data character in the class (after an initial circumflex, if
4171 present) or escaped with a backslash.
4172
4173 A character class matches a single character in the subject. In UTF-8
4174 mode, the character may be more than one byte long. A matched character
4175 must be in the set of characters defined by the class, unless the first
4176 character in the class definition is a circumflex, in which case the
4177 subject character must not be in the set defined by the class. If a
4178 circumflex is actually required as a member of the class, ensure it is
4179 not the first character, or escape it with a backslash.
4180
4181 For example, the character class [aeiou] matches any lower case vowel,
4182 while [^aeiou] matches any character that is not a lower case vowel.
4183 Note that a circumflex is just a convenient notation for specifying the
4184 characters that are in the class by enumerating those that are not. A
4185 class that starts with a circumflex is not an assertion; it still con-
4186 sumes a character from the subject string, and therefore it fails if
4187 the current pointer is at the end of the string.
4188
4189 In UTF-8 mode, characters with values greater than 255 can be included
4190 in a class as a literal string of bytes, or by using the \x{ escaping
4191 mechanism.
4192
4193 When caseless matching is set, any letters in a class represent both
4194 their upper case and lower case versions, so for example, a caseless
4195 [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
4196 match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
4197 understands the concept of case for characters whose values are less
4198 than 128, so caseless matching is always possible. For characters with
4199 higher values, the concept of case is supported if PCRE is compiled
4200 with Unicode property support, but not otherwise. If you want to use
4201 caseless matching in UTF8-mode for characters 128 and above, you must
4202 ensure that PCRE is compiled with Unicode property support as well as
4203 with UTF-8 support.
4204
4205 Characters that might indicate line breaks are never treated in any
4206 special way when matching character classes, whatever line-ending
4207 sequence is in use, and whatever setting of the PCRE_DOTALL and
4208 PCRE_MULTILINE options is used. A class such as [^a] always matches one
4209 of these characters.
4210
4211 The minus (hyphen) character can be used to specify a range of charac-
4212 ters in a character class. For example, [d-m] matches any letter
4213 between d and m, inclusive. If a minus character is required in a
4214 class, it must be escaped with a backslash or appear in a position
4215 where it cannot be interpreted as indicating a range, typically as the
4216 first or last character in the class.
4217
4218 It is not possible to have the literal character "]" as the end charac-
4219 ter of a range. A pattern such as [W-]46] is interpreted as a class of
4220 two characters ("W" and "-") followed by a literal string "46]", so it
4221 would match "W46]" or "-46]". However, if the "]" is escaped with a
4222 backslash it is interpreted as the end of range, so [W-\]46] is inter-
4223 preted as a class containing a range followed by two other characters.
4224 The octal or hexadecimal representation of "]" can also be used to end
4225 a range.
4226
4227 Ranges operate in the collating sequence of character values. They can
4228 also be used for characters specified numerically, for example
4229 [\000-\037]. In UTF-8 mode, ranges can include characters whose values
4230 are greater than 255, for example [\x{100}-\x{2ff}].
4231
4232 If a range that includes letters is used when caseless matching is set,
4233 it matches the letters in either case. For example, [W-c] is equivalent
4234 to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
4235 character tables for a French locale are in use, [\xc8-\xcb] matches
4236 accented E characters in both cases. In UTF-8 mode, PCRE supports the
4237 concept of case for characters with values greater than 128 only when
4238 it is compiled with Unicode property support.
4239
4240 The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
4241 \w, and \W may appear in a character class, and add the characters that
4242 they match to the class. For example, [\dABCDEF] matches any hexadeci-
4243 mal digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of
4244 \d, \s, \w and their upper case partners, just as it does when they
4245 appear outside a character class, as described in the section entitled
4246 "Generic character types" above. The escape sequence \b has a different
4247 meaning inside a character class; it matches the backspace character.
4248 The sequences \B, \N, \R, and \X are not special inside a character
4249 class. Like any other unrecognized escape sequences, they are treated
4250 as the literal characters "B", "N", "R", and "X" by default, but cause
4251 an error if the PCRE_EXTRA option is set.
4252
4253 A circumflex can conveniently be used with the upper case character
4254 types to specify a more restricted set of characters than the matching
4255 lower case type. For example, the class [^\W_] matches any letter or
4256 digit, but not underscore, whereas [\w] includes underscore. A positive
4257 character class should be read as "something OR something OR ..." and a
4258 negative class as "NOT something AND NOT something AND NOT ...".
4259
4260 The only metacharacters that are recognized in character classes are
4261 backslash, hyphen (only where it can be interpreted as specifying a
4262 range), circumflex (only at the start), opening square bracket (only
4263 when it can be interpreted as introducing a POSIX class name - see the
4264 next section), and the terminating closing square bracket. However,
4265 escaping other non-alphanumeric characters does no harm.
4266
4267
4268 POSIX CHARACTER CLASSES
4269
4270 Perl supports the POSIX notation for character classes. This uses names
4271 enclosed by [: and :] within the enclosing square brackets. PCRE also
4272 supports this notation. For example,
4273
4274 [01[:alpha:]%]
4275
4276 matches "0", "1", any alphabetic character, or "%". The supported class
4277 names are:
4278
4279 alnum letters and digits
4280 alpha letters
4281 ascii character codes 0 - 127
4282 blank space or tab only
4283 cntrl control characters
4284 digit decimal digits (same as \d)
4285 graph printing characters, excluding space
4286 lower lower case letters
4287 print printing characters, including space
4288 punct printing characters, excluding letters and digits and space
4289 space white space (not quite the same as \s)
4290 upper upper case letters
4291 word "word" characters (same as \w)
4292 xdigit hexadecimal digits
4293
4294 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
4295 and space (32). Notice that this list includes the VT character (code
4296 11). This makes "space" different to \s, which does not include VT (for
4297 Perl compatibility).
4298
4299 The name "word" is a Perl extension, and "blank" is a GNU extension
4300 from Perl 5.8. Another Perl extension is negation, which is indicated
4301 by a ^ character after the colon. For example,
4302
4303 [12[:^digit:]]
4304
4305 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
4306 POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
4307 these are not supported, and an error is given if they are encountered.
4308
4309 By default, in UTF-8 mode, characters with values greater than 128 do
4310 not match any of the POSIX character classes. However, if the PCRE_UCP
4311 option is passed to pcre_compile(), some of the classes are changed so
4312 that Unicode character properties are used. This is achieved by replac-
4313 ing the POSIX classes by other sequences, as follows:
4314
4315 [:alnum:] becomes \p{Xan}
4316 [:alpha:] becomes \p{L}
4317 [:blank:] becomes \h
4318 [:digit:] becomes \p{Nd}
4319 [:lower:] becomes \p{Ll}
4320 [:space:] becomes \p{Xps}
4321 [:upper:] becomes \p{Lu}
4322 [:word:] becomes \p{Xwd}
4323
4324 Negated versions, such as [:^alpha:] use \P instead of \p. The other
4325 POSIX classes are unchanged, and match only characters with code points
4326 less than 128.
4327
4328
4329 VERTICAL BAR
4330
4331 Vertical bar characters are used to separate alternative patterns. For
4332 example, the pattern
4333
4334 gilbert|sullivan
4335
4336 matches either "gilbert" or "sullivan". Any number of alternatives may
4337 appear, and an empty alternative is permitted (matching the empty
4338 string). The matching process tries each alternative in turn, from left
4339 to right, and the first one that succeeds is used. If the alternatives
4340 are within a subpattern (defined below), "succeeds" means matching the
4341 rest of the main pattern as well as the alternative in the subpattern.
4342
4343
4344 INTERNAL OPTION SETTING
4345
4346 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
4347 PCRE_EXTENDED options (which are Perl-compatible) can be changed from
4348 within the pattern by a sequence of Perl option letters enclosed
4349 between "(?" and ")". The option letters are
4350
4351 i for PCRE_CASELESS
4352 m for PCRE_MULTILINE
4353 s for PCRE_DOTALL
4354 x for PCRE_EXTENDED
4355
4356 For example, (?im) sets caseless, multiline matching. It is also possi-
4357 ble to unset these options by preceding the letter with a hyphen, and a
4358 combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
4359 LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
4360 is also permitted. If a letter appears both before and after the
4361 hyphen, the option is unset.
4362
4363 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
4364 can be changed in the same way as the Perl-compatible options by using
4365 the characters J, U and X respectively.
4366
4367 When one of these option changes occurs at top level (that is, not
4368 inside subpattern parentheses), the change applies to the remainder of
4369 the pattern that follows. If the change is placed right at the start of
4370 a pattern, PCRE extracts it into the global options (and it will there-
4371 fore show up in data extracted by the pcre_fullinfo() function).
4372
4373 An option change within a subpattern (see below for a description of
4374 subpatterns) affects only that part of the subpattern that follows it,
4375 so
4376
4377 (a(?i)b)c
4378
4379 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
4380 used). By this means, options can be made to have different settings
4381 in different parts of the pattern. Any changes made in one alternative
4382 do carry on into subsequent branches within the same subpattern. For
4383 example,
4384
4385 (a(?i)b|c)
4386
4387 matches "ab", "aB", "c", and "C", even though when matching "C" the
4388 first branch is abandoned before the option setting. This is because
4389 the effects of option settings happen at compile time. There would be
4390 some very weird behaviour otherwise.
4391
4392 Note: There are other PCRE-specific options that can be set by the
4393 application when the compile or match functions are called. In some
4394 cases the pattern can contain special leading sequences such as (*CRLF)
4395 to override what the application has set or what has been defaulted.
4396 Details are given in the section entitled "Newline sequences" above.
4397 There are also the (*UTF8) and (*UCP) leading sequences that can be
4398 used to set UTF-8 and Unicode property modes; they are equivalent to
4399 setting the PCRE_UTF8 and the PCRE_UCP options, respectively.
4400
4401
4402 SUBPATTERNS
4403
4404 Subpatterns are delimited by parentheses (round brackets), which can be
4405 nested. Turning part of a pattern into a subpattern does two things:
4406
4407 1. It localizes a set of alternatives. For example, the pattern
4408
4409 cat(aract|erpillar|)
4410
4411 matches "cataract", "caterpillar", or "cat". Without the parentheses,
4412 it would match "cataract", "erpillar" or an empty string.
4413
4414 2. It sets up the subpattern as a capturing subpattern. This means
4415 that, when the whole pattern matches, that portion of the subject
4416 string that matched the subpattern is passed back to the caller via the
4417 ovector argument of pcre_exec(). Opening parentheses are counted from
4418 left to right (starting from 1) to obtain numbers for the capturing
4419 subpatterns. For example, if the string "the red king" is matched
4420 against the pattern
4421
4422 the ((red|white) (king|queen))
4423
4424 the captured substrings are "red king", "red", and "king", and are num-
4425 bered 1, 2, and 3, respectively.
4426
4427 The fact that plain parentheses fulfil two functions is not always
4428 helpful. There are often times when a grouping subpattern is required
4429 without a capturing requirement. If an opening parenthesis is followed
4430 by a question mark and a colon, the subpattern does not do any captur-
4431 ing, and is not counted when computing the number of any subsequent
4432 capturing subpatterns. For example, if the string "the white queen" is
4433 matched against the pattern
4434
4435 the ((?:red|white) (king|queen))
4436
4437 the captured substrings are "white queen" and "queen", and are numbered
4438 1 and 2. The maximum number of capturing subpatterns is 65535.
4439
4440 As a convenient shorthand, if any option settings are required at the
4441 start of a non-capturing subpattern, the option letters may appear
4442 between the "?" and the ":". Thus the two patterns
4443
4444 (?i:saturday|sunday)
4445 (?:(?i)saturday|sunday)
4446
4447 match exactly the same set of strings. Because alternative branches are
4448 tried from left to right, and options are not reset until the end of
4449 the subpattern is reached, an option setting in one branch does affect
4450 subsequent branches, so the above patterns match "SUNDAY" as well as
4451 "Saturday".
4452
4453
4454 DUPLICATE SUBPATTERN NUMBERS
4455
4456 Perl 5.10 introduced a feature whereby each alternative in a subpattern
4457 uses the same numbers for its capturing parentheses. Such a subpattern
4458 starts with (?| and is itself a non-capturing subpattern. For example,
4459 consider this pattern:
4460
4461 (?|(Sat)ur|(Sun))day
4462
4463 Because the two alternatives are inside a (?| group, both sets of cap-
4464 turing parentheses are numbered one. Thus, when the pattern matches,
4465 you can look at captured substring number one, whichever alternative
4466 matched. This construct is useful when you want to capture part, but
4467 not all, of one of a number of alternatives. Inside a (?| group, paren-
4468 theses are numbered as usual, but the number is reset at the start of
4469 each branch. The numbers of any capturing parentheses that follow the
4470 subpattern start after the highest number used in any branch. The fol-
4471 lowing example is taken from the Perl documentation. The numbers under-
4472 neath show in which buffer the captured content will be stored.
4473
4474 # before ---------------branch-reset----------- after
4475 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
4476 # 1 2 2 3 2 3 4
4477
4478 A back reference to a numbered subpattern uses the most recent value
4479 that is set for that number by any subpattern. The following pattern
4480 matches "abcabc" or "defdef":
4481
4482 /(?|(abc)|(def))\1/
4483
4484 In contrast, a recursive or "subroutine" call to a numbered subpattern
4485 always refers to the first one in the pattern with the given number.
4486 The following pattern matches "abcabc" or "defabc":
4487
4488 /(?|(abc)|(def))(?1)/
4489
4490 If a condition test for a subpattern's having matched refers to a non-
4491 unique number, the test is true if any of the subpatterns of that num-
4492 ber have matched.
4493
4494 An alternative approach to using this "branch reset" feature is to use
4495 duplicate named subpatterns, as described in the next section.
4496
4497
4498 NAMED SUBPATTERNS
4499
4500 Identifying capturing parentheses by number is simple, but it can be
4501 very hard to keep track of the numbers in complicated regular expres-
4502 sions. Furthermore, if an expression is modified, the numbers may
4503 change. To help with this difficulty, PCRE supports the naming of sub-
4504 patterns. This feature was not added to Perl until release 5.10. Python
4505 had the feature earlier, and PCRE introduced it at release 4.0, using
4506 the Python syntax. PCRE now supports both the Perl and the Python syn-
4507 tax. Perl allows identically numbered subpatterns to have different
4508 names, but PCRE does not.
4509
4510 In PCRE, a subpattern can be named in one of three ways: (?<name>...)
4511 or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
4512 to capturing parentheses from other parts of the pattern, such as back
4513 references, recursion, and conditions, can be made by name as well as
4514 by number.
4515
4516 Names consist of up to 32 alphanumeric characters and underscores.
4517 Named capturing parentheses are still allocated numbers as well as
4518 names, exactly as if the names were not present. The PCRE API provides
4519 function calls for extracting the name-to-number translation table from
4520 a compiled pattern. There is also a convenience function for extracting
4521 a captured substring by name.
4522
4523 By default, a name must be unique within a pattern, but it is possible
4524 to relax this constraint by setting the PCRE_DUPNAMES option at compile
4525 time. (Duplicate names are also always permitted for subpatterns with
4526 the same number, set up as described in the previous section.) Dupli-
4527 cate names can be useful for patterns where only one instance of the
4528 named parentheses can match. Suppose you want to match the name of a
4529 weekday, either as a 3-letter abbreviation or as the full name, and in
4530 both cases you want to extract the abbreviation. This pattern (ignoring
4531 the line breaks) does the job:
4532
4533 (?<DN>Mon|Fri|Sun)(?:day)?|
4534 (?<DN>Tue)(?:sday)?|
4535 (?<DN>Wed)(?:nesday)?|
4536 (?<DN>Thu)(?:rsday)?|
4537 (?<DN>Sat)(?:urday)?
4538
4539 There are five capturing substrings, but only one is ever set after a
4540 match. (An alternative way of solving this problem is to use a "branch
4541 reset" subpattern, as described in the previous section.)
4542
4543 The convenience function for extracting the data by name returns the
4544 substring for the first (and in this example, the only) subpattern of
4545 that name that matched. This saves searching to find which numbered
4546 subpattern it was.
4547
4548 If you make a back reference to a non-unique named subpattern from
4549 elsewhere in the pattern, the one that corresponds to the first occur-
4550 rence of the name is used. In the absence of duplicate numbers (see the
4551 previous section) this is the one with the lowest number. If you use a
4552 named reference in a condition test (see the section about conditions
4553 below), either to check whether a subpattern has matched, or to check
4554 for recursion, all subpatterns with the same name are tested. If the
4555 condition is true for any one of them, the overall condition is true.
4556 This is the same behaviour as testing by number. For further details of
4557 the interfaces for handling named subpatterns, see the pcreapi documen-
4558 tation.
4559
4560 Warning: You cannot use different names to distinguish between two sub-
4561 patterns with the same number because PCRE uses only the numbers when
4562 matching. For this reason, an error is given at compile time if differ-
4563 ent names are given to subpatterns with the same number. However, you
4564 can give the same name to subpatterns with the same number, even when
4565 PCRE_DUPNAMES is not set.
4566
4567
4568 REPETITION
4569
4570 Repetition is specified by quantifiers, which can follow any of the
4571 following items:
4572
4573 a literal data character
4574 the dot metacharacter
4575 the \C escape sequence
4576 the \X escape sequence (in UTF-8 mode with Unicode properties)
4577 the \R escape sequence
4578 an escape such as \d or \pL that matches a single character
4579 a character class
4580 a back reference (see next section)
4581 a parenthesized subpattern (including assertions)
4582 a recursive or "subroutine" call to a subpattern
4583
4584 The general repetition quantifier specifies a minimum and maximum num-
4585 ber of permitted matches, by giving the two numbers in curly brackets
4586 (braces), separated by a comma. The numbers must be less than 65536,
4587 and the first must be less than or equal to the second. For example:
4588
4589 z{2,4}
4590
4591 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
4592 special character. If the second number is omitted, but the comma is
4593 present, there is no upper limit; if the second number and the comma
4594 are both omitted, the quantifier specifies an exact number of required
4595 matches. Thus
4596
4597 [aeiou]{3,}
4598
4599 matches at least 3 successive vowels, but may match many more, while
4600
4601 \d{8}
4602
4603 matches exactly 8 digits. An opening curly bracket that appears in a
4604 position where a quantifier is not allowed, or one that does not match
4605 the syntax of a quantifier, is taken as a literal character. For exam-
4606 ple, {,6} is not a quantifier, but a literal string of four characters.
4607
4608 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
4609 individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
4610 acters, each of which is represented by a two-byte sequence. Similarly,
4611 when Unicode property support is available, \X{3} matches three Unicode
4612 extended sequences, each of which may be several bytes long (and they
4613 may be of different lengths).
4614
4615 The quantifier {0} is permitted, causing the expression to behave as if
4616 the previous item and the quantifier were not present. This may be use-
4617 ful for subpatterns that are referenced as subroutines from elsewhere
4618 in the pattern (but see also the section entitled "Defining subpatterns
4619 for use by reference only" below). Items other than subpatterns that
4620 have a {0} quantifier are omitted from the compiled pattern.
4621
4622 For convenience, the three most common quantifiers have single-charac-
4623 ter abbreviations:
4624
4625 * is equivalent to {0,}
4626 + is equivalent to {1,}
4627 ? is equivalent to {0,1}
4628
4629 It is possible to construct infinite loops by following a subpattern
4630 that can match no characters with a quantifier that has no upper limit,
4631 for example:
4632
4633 (a?)*
4634
4635 Earlier versions of Perl and PCRE used to give an error at compile time
4636 for such patterns. However, because there are cases where this can be
4637 useful, such patterns are now accepted, but if any repetition of the
4638 subpattern does in fact match no characters, the loop is forcibly bro-
4639 ken.
4640
4641 By default, the quantifiers are "greedy", that is, they match as much
4642 as possible (up to the maximum number of permitted times), without
4643 causing the rest of the pattern to fail. The classic example of where
4644 this gives problems is in trying to match comments in C programs. These
4645 appear between /* and */ and within the comment, individual * and /
4646 characters may appear. An attempt to match C comments by applying the
4647 pattern
4648
4649 /\*.*\*/
4650
4651 to the string
4652
4653 /* first comment */ not comment /* second comment */
4654
4655 fails, because it matches the entire string owing to the greediness of
4656 the .* item.
4657
4658 However, if a quantifier is followed by a question mark, it ceases to
4659 be greedy, and instead matches the minimum number of times possible, so
4660 the pattern
4661
4662 /\*.*?\*/
4663
4664 does the right thing with the C comments. The meaning of the various
4665 quantifiers is not otherwise changed, just the preferred number of
4666 matches. Do not confuse this use of question mark with its use as a
4667 quantifier in its own right. Because it has two uses, it can sometimes
4668 appear doubled, as in
4669
4670 \d??\d
4671
4672 which matches one digit by preference, but can match two if that is the
4673 only way the rest of the pattern matches.
4674
4675 If the PCRE_UNGREEDY option is set (an option that is not available in
4676 Perl), the quantifiers are not greedy by default, but individual ones
4677 can be made greedy by following them with a question mark. In other
4678 words, it inverts the default behaviour.
4679
4680 When a parenthesized subpattern is quantified with a minimum repeat
4681 count that is greater than 1 or with a limited maximum, more memory is
4682 required for the compiled pattern, in proportion to the size of the
4683 minimum or maximum.
4684
4685 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4686 alent to Perl's /s) is set, thus allowing the dot to match newlines,
4687 the pattern is implicitly anchored, because whatever follows will be
4688 tried against every character position in the subject string, so there
4689 is no point in retrying the overall match at any position after the
4690 first. PCRE normally treats such a pattern as though it were preceded
4691 by \A.
4692
4693 In cases where it is known that the subject string contains no new-
4694 lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
4695 mization, or alternatively using ^ to indicate anchoring explicitly.
4696
4697 However, there is one situation where the optimization cannot be used.
4698 When .* is inside capturing parentheses that are the subject of a back
4699 reference elsewhere in the pattern, a match at the start may fail where
4700 a later one succeeds. Consider, for example:
4701
4702 (.*)abc\1
4703
4704 If the subject is "xyz123abc123" the match point is the fourth charac-
4705 ter. For this reason, such a pattern is not implicitly anchored.
4706
4707 When a capturing subpattern is repeated, the value captured is the sub-
4708 string that matched the final iteration. For example, after
4709
4710 (tweedle[dume]{3}\s*)+
4711
4712 has matched "tweedledum tweedledee" the value of the captured substring
4713 is "tweedledee". However, if there are nested capturing subpatterns,
4714 the corresponding captured values may have been set in previous itera-
4715 tions. For example, after
4716
4717 /(a|(b))+/
4718
4719 matches "aba" the value of the second captured substring is "b".
4720
4721
4722 ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4723
4724 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4725 repetition, failure of what follows normally causes the repeated item
4726 to be re-evaluated to see if a different number of repeats allows the
4727 rest of the pattern to match. Sometimes it is useful to prevent this,
4728 either to change the nature of the match, or to cause it fail earlier
4729 than it otherwise might, when the author of the pattern knows there is
4730 no point in carrying on.
4731
4732 Consider, for example, the pattern \d+foo when applied to the subject
4733 line
4734
4735 123456bar
4736
4737 After matching all 6 digits and then failing to match "foo", the normal
4738 action of the matcher is to try again with only 5 digits matching the
4739 \d+ item, and then with 4, and so on, before ultimately failing.
4740 "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
4741 the means for specifying that once a subpattern has matched, it is not
4742 to be re-evaluated in this way.
4743
4744 If we use atomic grouping for the previous example, the matcher gives
4745 up immediately on failing to match "foo" the first time. The notation
4746 is a kind of special parenthesis, starting with (?> as in this example:
4747
4748 (?>\d+)foo
4749
4750 This kind of parenthesis "locks up" the part of the pattern it con-
4751 tains once it has matched, and a failure further into the pattern is
4752 prevented from backtracking into it. Backtracking past it to previous
4753 items, however, works as normal.
4754
4755 An alternative description is that a subpattern of this type matches
4756 the string of characters that an identical standalone pattern would
4757 match, if anchored at the current point in the subject string.
4758
4759 Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4760 such as the above example can be thought of as a maximizing repeat that
4761 must swallow everything it can. So, while both \d+ and \d+? are pre-
4762 pared to adjust the number of digits they match in order to make the
4763 rest of the pattern match, (?>\d+) can only match an entire sequence of
4764 digits.
4765
4766 Atomic groups in general can of course contain arbitrarily complicated
4767 subpatterns, and can be nested. However, when the subpattern for an
4768 atomic group is just a single repeated item, as in the example above, a
4769 simpler notation, called a "possessive quantifier" can be used. This
4770 consists of an additional + character following a quantifier. Using
4771 this notation, the previous example can be rewritten as
4772
4773 \d++foo
4774
4775 Note that a possessive quantifier can be used with an entire group, for
4776 example:
4777
4778 (abc|xyz){2,3}+
4779
4780 Possessive quantifiers are always greedy; the setting of the
4781 PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4782 simpler forms of atomic group. However, there is no difference in the
4783 meaning of a possessive quantifier and the equivalent atomic group,
4784 though there may be a performance difference; possessive quantifiers
4785 should be slightly faster.
4786
4787 The possessive quantifier syntax is an extension to the Perl 5.8 syn-
4788 tax. Jeffrey Friedl originated the idea (and the name) in the first
4789 edition of his book. Mike McCloskey liked it, so implemented it when he
4790 built Sun's Java package, and PCRE copied it from there. It ultimately
4791 found its way into Perl at release 5.10.
4792
4793 PCRE has an optimization that automatically "possessifies" certain sim-
4794 ple pattern constructs. For example, the sequence A+B is treated as
4795 A++B because there is no point in backtracking into a sequence of A's
4796 when B must follow.
4797
4798 When a pattern contains an unlimited repeat inside a subpattern that
4799 can itself be repeated an unlimited number of times, the use of an
4800 atomic group is the only way to avoid some failing matches taking a
4801 very long time indeed. The pattern
4802
4803 (\D+|<\d+>)*[!?]
4804
4805 matches an unlimited number of substrings that either consist of non-
4806 digits, or digits enclosed in <>, followed by either ! or ?. When it
4807 matches, it runs quickly. However, if it is applied to
4808
4809 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4810
4811 it takes a long time before reporting failure. This is because the
4812 string can be divided between the internal \D+ repeat and the external
4813 * repeat in a large number of ways, and all have to be tried. (The
4814 example uses [!?] rather than a single character at the end, because
4815 both PCRE and Perl have an optimization that allows for fast failure
4816 when a single character is used. They remember the last single charac-
4817 ter that is required for a match, and fail early if it is not present
4818 in the string.) If the pattern is changed so that it uses an atomic
4819 group, like this:
4820
4821 ((?>\D+)|<\d+>)*[!?]
4822
4823 sequences of non-digits cannot be broken, and failure happens quickly.
4824
4825
4826 BACK REFERENCES
4827
4828 Outside a character class, a backslash followed by a digit greater than
4829 0 (and possibly further digits) is a back reference to a capturing sub-
4830 pattern earlier (that is, to its left) in the pattern, provided there
4831 have been that many previous capturing left parentheses.
4832
4833 However, if the decimal number following the backslash is less than 10,
4834 it is always taken as a back reference, and causes an error only if
4835 there are not that many capturing left parentheses in the entire pat-
4836 tern. In other words, the parentheses that are referenced need not be
4837 to the left of the reference for numbers less than 10. A "forward back
4838 reference" of this type can make sense when a repetition is involved
4839 and the subpattern to the right has participated in an earlier itera-
4840 tion.
4841
4842 It is not possible to have a numerical "forward back reference" to a
4843 subpattern whose number is 10 or more using this syntax because a
4844 sequence such as \50 is interpreted as a character defined in octal.
4845 See the subsection entitled "Non-printing characters" above for further
4846 details of the handling of digits following a backslash. There is no
4847 such problem when named parentheses are used. A back reference to any
4848 subpattern is possible using named parentheses (see below).
4849
4850 Another way of avoiding the ambiguity inherent in the use of digits
4851 following a backslash is to use the \g escape sequence. This escape
4852 must be followed by an unsigned number or a negative number, optionally
4853 enclosed in braces. These examples are all identical:
4854
4855 (ring), \1
4856 (ring), \g1
4857 (ring), \g{1}
4858
4859 An unsigned number specifies an absolute reference without the ambigu-
4860 ity that is present in the older syntax. It is also useful when literal
4861 digits follow the reference. A negative number is a relative reference.
4862 Consider this example:
4863
4864 (abc(def)ghi)\g{-1}
4865
4866 The sequence \g{-1} is a reference to the most recently started captur-
4867 ing subpattern before \g, that is, is it equivalent to \2 in this exam-
4868 ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
4869 references can be helpful in long patterns, and also in patterns that
4870 are created by joining together fragments that contain references
4871 within themselves.
4872
4873 A back reference matches whatever actually matched the capturing sub-
4874 pattern in the current subject string, rather than anything matching
4875 the subpattern itself (see "Subpatterns as subroutines" below for a way
4876 of doing that). So the pattern
4877
4878 (sens|respons)e and \1ibility
4879
4880 matches "sense and sensibility" and "response and responsibility", but
4881 not "sense and responsibility". If caseful matching is in force at the
4882 time of the back reference, the case of letters is relevant. For exam-
4883 ple,
4884
4885 ((?i)rah)\s+\1
4886
4887 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
4888 original capturing subpattern is matched caselessly.
4889
4890 There are several different ways of writing back references to named
4891 subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
4892 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
4893 unified back reference syntax, in which \g can be used for both numeric
4894 and named references, is also supported. We could rewrite the above
4895 example in any of the following ways:
4896
4897 (?<p1>(?i)rah)\s+\k<p1>
4898 (?'p1'(?i)rah)\s+\k{p1}
4899 (?P<p1>(?i)rah)\s+(?P=p1)
4900 (?<p1>(?i)rah)\s+\g{p1}
4901
4902 A subpattern that is referenced by name may appear in the pattern
4903 before or after the reference.
4904
4905 There may be more than one back reference to the same subpattern. If a
4906 subpattern has not actually been used in a particular match, any back
4907 references to it always fail by default. For example, the pattern
4908
4909 (a|(bc))\2
4910
4911 always fails if it starts to match "a" rather than "bc". However, if
4912 the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
4913 ence to an unset value matches an empty string.
4914
4915 Because there may be many capturing parentheses in a pattern, all dig-
4916 its following a backslash are taken as part of a potential back refer-
4917 ence number. If the pattern continues with a digit character, some
4918 delimiter must be used to terminate the back reference. If the
4919 PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
4920 syntax or an empty comment (see "Comments" below) can be used.
4921
4922 Recursive back references
4923
4924 A back reference that occurs inside the parentheses to which it refers
4925 fails when the subpattern is first used, so, for example, (a\1) never
4926 matches. However, such references can be useful inside repeated sub-
4927 patterns. For example, the pattern
4928
4929 (a|b\1)+
4930
4931 matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4932 ation of the subpattern, the back reference matches the character
4933 string corresponding to the previous iteration. In order for this to
4934 work, the pattern must be such that the first iteration does not need
4935 to match the back reference. This can be done using alternation, as in
4936 the example above, or by a quantifier with a minimum of zero.
4937
4938 Back references of this type cause the group that they reference to be
4939 treated as an atomic group. Once the whole group has been matched, a
4940 subsequent matching failure cannot cause backtracking into the middle
4941 of the group.
4942
4943
4944 ASSERTIONS
4945
4946 An assertion is a test on the characters following or preceding the
4947 current matching point that does not actually consume any characters.
4948 The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
4949 described above.
4950
4951 More complicated assertions are coded as subpatterns. There are two
4952 kinds: those that look ahead of the current position in the subject
4953 string, and those that look behind it. An assertion subpattern is
4954 matched in the normal way, except that it does not cause the current
4955 matching position to be changed.
4956
4957 Assertion subpatterns are not capturing subpatterns. If such an asser-
4958 tion contains capturing subpatterns within it, these are counted for
4959 the purposes of numbering the capturing subpatterns in the whole pat-
4960 tern. However, substring capturing is carried out only for positive
4961 assertions, because it does not make sense for negative assertions.
4962
4963 For compatibility with Perl, assertion subpatterns may be repeated;
4964 though it makes no sense to assert the same thing several times, the
4965 side effect of capturing parentheses may occasionally be useful. In
4966 practice, there only three cases:
4967
4968 (1) If the quantifier is {0}, the assertion is never obeyed during
4969 matching. However, it may contain internal capturing parenthesized
4970 groups that are called from elsewhere via the subroutine mechanism.
4971
4972 (2) If quantifier is {0,n} where n is greater than zero, it is treated
4973 as if it were {0,1}. At run time, the rest of the pattern match is
4974 tried with and without the assertion, the order depending on the greed-
4975 iness of the quantifier.
4976
4977 (3) If the minimum repetition is greater than zero, the quantifier is
4978 ignored. The assertion is obeyed just once when encountered during
4979 matching.
4980
4981 Lookahead assertions
4982
4983 Lookahead assertions start with (?= for positive assertions and (?! for
4984 negative assertions. For example,
4985
4986 \w+(?=;)
4987
4988 matches a word followed by a semicolon, but does not include the semi-
4989 colon in the match, and
4990
4991 foo(?!bar)
4992
4993 matches any occurrence of "foo" that is not followed by "bar". Note
4994 that the apparently similar pattern
4995
4996 (?!foo)bar
4997
4998 does not find an occurrence of "bar" that is preceded by something
4999 other than "foo"; it finds any occurrence of "bar" whatsoever, because
5000 the assertion (?!foo) is always true when the next three characters are
5001 "bar". A lookbehind assertion is needed to achieve the other effect.
5002
5003 If you want to force a matching failure at some point in a pattern, the
5004 most convenient way to do it is with (?!) because an empty string
5005 always matches, so an assertion that requires there not to be an empty
5006 string must always fail. The backtracking control verb (*FAIL) or (*F)
5007 is a synonym for (?!).
5008
5009 Lookbehind assertions
5010
5011 Lookbehind assertions start with (?<= for positive assertions and (?<!
5012 for negative assertions. For example,
5013
5014 (?<!foo)bar
5015
5016 does find an occurrence of "bar" that is not preceded by "foo". The
5017 contents of a lookbehind assertion are restricted such that all the
5018 strings it matches must have a fixed length. However, if there are sev-
5019 eral top-level alternatives, they do not all have to have the same
5020 fixed length. Thus
5021
5022 (?<=bullock|donkey)
5023
5024 is permitted, but
5025
5026 (?<!dogs?|cats?)
5027
5028 causes an error at compile time. Branches that match different length
5029 strings are permitted only at the top level of a lookbehind assertion.
5030 This is an extension compared with Perl, which requires all branches to
5031 match the same length of string. An assertion such as
5032
5033 (?<=ab(c|de))
5034
5035 is not permitted, because its single top-level branch can match two
5036 different lengths, but it is acceptable to PCRE if rewritten to use two
5037 top-level branches:
5038
5039 (?<=abc|abde)
5040
5041 In some cases, the escape sequence \K (see above) can be used instead
5042 of a lookbehind assertion to get round the fixed-length restriction.
5043
5044 The implementation of lookbehind assertions is, for each alternative,
5045 to temporarily move the current position back by the fixed length and
5046 then try to match. If there are insufficient characters before the cur-
5047 rent position, the assertion fails.
5048
5049 PCRE does not allow the \C escape (which matches a single byte in UTF-8
5050 mode) to appear in lookbehind assertions, because it makes it impossi-
5051 ble to calculate the length of the lookbehind. The \X and \R escapes,
5052 which can match different numbers of bytes, are also not permitted.
5053
5054 "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
5055 lookbehinds, as long as the subpattern matches a fixed-length string.
5056 Recursion, however, is not supported.
5057
5058 Possessive quantifiers can be used in conjunction with lookbehind
5059 assertions to specify efficient matching of fixed-length strings at the
5060 end of subject strings. Consider a simple pattern such as
5061
5062 abcd$
5063
5064 when applied to a long string that does not match. Because matching
5065 proceeds from left to right, PCRE will look for each "a" in the subject
5066 and then see if what follows matches the rest of the pattern. If the
5067 pattern is specified as
5068
5069 ^.*abcd$
5070
5071 the initial .* matches the entire string at first, but when this fails
5072 (because there is no following "a"), it backtracks to match all but the
5073 last character, then all but the last two characters, and so on. Once
5074 again the search for "a" covers the entire string, from right to left,
5075 so we are no better off. However, if the pattern is written as
5076
5077 ^.*+(?<=abcd)
5078
5079 there can be no backtracking for the .*+ item; it can match only the
5080 entire string. The subsequent lookbehind assertion does a single test
5081 on the last four characters. If it fails, the match fails immediately.
5082 For long strings, this approach makes a significant difference to the
5083 processing time.
5084
5085 Using multiple assertions
5086
5087 Several assertions (of any sort) may occur in succession. For example,
5088
5089 (?<=\d{3})(?<!999)foo
5090
5091 matches "foo" preceded by three digits that are not "999". Notice that
5092 each of the assertions is applied independently at the same point in
5093 the subject string. First there is a check that the previous three
5094 characters are all digits, and then there is a check that the same
5095 three characters are not "999". This pattern does not match "foo" pre-
5096 ceded by six characters, the first of which are digits and the last
5097 three of which are not "999". For example, it doesn't match "123abc-
5098 foo". A pattern to do that is
5099
5100 (?<=\d{3}...)(?<!999)foo
5101
5102 This time the first assertion looks at the preceding six characters,
5103 checking that the first three are digits, and then the second assertion
5104 checks that the preceding three characters are not "999".
5105
5106 Assertions can be nested in any combination. For example,
5107
5108 (?<=(?<!foo)bar)baz
5109
5110 matches an occurrence of "baz" that is preceded by "bar" which in turn
5111 is not preceded by "foo", while
5112
5113 (?<=\d{3}(?!999)...)foo
5114
5115 is another pattern that matches "foo" preceded by three digits and any
5116 three characters that are not "999".
5117
5118
5119 CONDITIONAL SUBPATTERNS
5120
5121 It is possible to cause the matching process to obey a subpattern con-
5122 ditionally or to choose between two alternative subpatterns, depending
5123 on the result of an assertion, or whether a specific capturing subpat-
5124 tern has already been matched. The two possible forms of conditional
5125 subpattern are:
5126
5127 (?(condition)yes-pattern)
5128 (?(condition)yes-pattern|no-pattern)
5129
5130 If the condition is satisfied, the yes-pattern is used; otherwise the
5131 no-pattern (if present) is used. If there are more than two alterna-
5132 tives in the subpattern, a compile-time error occurs. Each of the two
5133 alternatives may itself contain nested subpatterns of any form, includ-
5134 ing conditional subpatterns; the restriction to two alternatives
5135 applies only at the level of the condition. This pattern fragment is an
5136 example where the alternatives are complex:
5137
5138 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
5139
5140
5141 There are four kinds of condition: references to subpatterns, refer-
5142 ences to recursion, a pseudo-condition called DEFINE, and assertions.
5143
5144 Checking for a used subpattern by number
5145
5146 If the text between the parentheses consists of a sequence of digits,
5147 the condition is true if a capturing subpattern of that number has pre-
5148 viously matched. If there is more than one capturing subpattern with
5149 the same number (see the earlier section about duplicate subpattern
5150 numbers), the condition is true if any of them have matched. An alter-
5151 native notation is to precede the digits with a plus or minus sign. In
5152 this case, the subpattern number is relative rather than absolute. The
5153 most recently opened parentheses can be referenced by (?(-1), the next
5154 most recent by (?(-2), and so on. Inside loops it can also make sense
5155 to refer to subsequent groups. The next parentheses to be opened can be
5156 referenced as (?(+1), and so on. (The value zero in any of these forms
5157 is not used; it provokes a compile-time error.)
5158
5159 Consider the following pattern, which contains non-significant white
5160 space to make it more readable (assume the PCRE_EXTENDED option) and to
5161 divide it into three parts for ease of discussion:
5162
5163 ( \( )? [^()]+ (?(1) \) )
5164
5165 The first part matches an optional opening parenthesis, and if that
5166 character is present, sets it as the first captured substring. The sec-
5167 ond part matches one or more characters that are not parentheses. The
5168 third part is a conditional subpattern that tests whether or not the
5169 first set of parentheses matched. If they did, that is, if subject
5170 started with an opening parenthesis, the condition is true, and so the
5171 yes-pattern is executed and a closing parenthesis is required. Other-
5172 wise, since no-pattern is not present, the subpattern matches nothing.
5173 In other words, this pattern matches a sequence of non-parentheses,
5174 optionally enclosed in parentheses.
5175
5176 If you were embedding this pattern in a larger one, you could use a
5177 relative reference:
5178
5179 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
5180
5181 This makes the fragment independent of the parentheses in the larger
5182 pattern.
5183
5184 Checking for a used subpattern by name
5185
5186 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
5187 used subpattern by name. For compatibility with earlier versions of
5188 PCRE, which had this facility before Perl, the syntax (?(name)...) is
5189 also recognized. However, there is a possible ambiguity with this syn-
5190 tax, because subpattern names may consist entirely of digits. PCRE
5191 looks first for a named subpattern; if it cannot find one and the name
5192 consists entirely of digits, PCRE looks for a subpattern of that num-
5193 ber, which must be greater than zero. Using subpattern names that con-
5194 sist entirely of digits is not recommended.
5195
5196 Rewriting the above example to use a named subpattern gives this:
5197
5198 (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
5199
5200 If the name used in a condition of this kind is a duplicate, the test
5201 is applied to all subpatterns of the same name, and is true if any one
5202 of them has matched.
5203
5204 Checking for pattern recursion
5205
5206 If the condition is the string (R), and there is no subpattern with the
5207 name R, the condition is true if a recursive call to the whole pattern
5208 or any subpattern has been made. If digits or a name preceded by amper-
5209 sand follow the letter R, for example:
5210
5211 (?(R3)...) or (?(R&name)...)
5212
5213 the condition is true if the most recent recursion is into a subpattern
5214 whose number or name is given. This condition does not check the entire
5215 recursion stack. If the name used in a condition of this kind is a
5216 duplicate, the test is applied to all subpatterns of the same name, and
5217 is true if any one of them is the most recent recursion.
5218
5219 At "top level", all these recursion test conditions are false. The
5220 syntax for recursive patterns is described below.
5221
5222 Defining subpatterns for use by reference only
5223
5224 If the condition is the string (DEFINE), and there is no subpattern
5225 with the name DEFINE, the condition is always false. In this case,
5226 there may be only one alternative in the subpattern. It is always
5227 skipped if control reaches this point in the pattern; the idea of
5228 DEFINE is that it can be used to define "subroutines" that can be ref-
5229 erenced from elsewhere. (The use of "subroutines" is described below.)
5230 For example, a pattern to match an IPv4 address such as
5231 "192.168.23.245" could be written like this (ignore whitespace and line
5232 breaks):
5233
5234 (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
5235 \b (?&byte) (\.(?&byte)){3} \b
5236
5237 The first part of the pattern is a DEFINE group inside which a another
5238 group named "byte" is defined. This matches an individual component of
5239 an IPv4 address (a number less than 256). When matching takes place,
5240 this part of the pattern is skipped because DEFINE acts like a false
5241 condition. The rest of the pattern uses references to the named group
5242 to match the four dot-separated components of an IPv4 address, insist-
5243 ing on a word boundary at each end.
5244
5245 Assertion conditions
5246
5247 If the condition is not in any of the above formats, it must be an
5248 assertion. This may be a positive or negative lookahead or lookbehind
5249 assertion. Consider this pattern, again containing non-significant
5250 white space, and with the two alternatives on the second line:
5251
5252 (?(?=[^a-z]*[a-z])
5253 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
5254
5255 The condition is a positive lookahead assertion that matches an
5256 optional sequence of non-letters followed by a letter. In other words,
5257 it tests for the presence of at least one letter in the subject. If a
5258 letter is found, the subject is matched against the first alternative;
5259 otherwise it is matched against the second. This pattern matches
5260 strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
5261 letters and dd are digits.
5262
5263
5264 COMMENTS
5265
5266 There are two ways of including comments in patterns that are processed
5267 by PCRE. In both cases, the start of the comment must not be in a char-
5268 acter class, nor in the middle of any other sequence of related charac-
5269 ters such as (?: or a subpattern name or number. The characters that
5270 make up a comment play no part in the pattern matching.
5271
5272 The sequence (?# marks the start of a comment that continues up to the
5273 next closing parenthesis. Nested parentheses are not permitted. If the
5274 PCRE_EXTENDED option is set, an unescaped # character also introduces a
5275 comment, which in this case continues to immediately after the next
5276 newline character or character sequence in the pattern. Which charac-
5277 ters are interpreted as newlines is controlled by the options passed to
5278 pcre_compile() or by a special sequence at the start of the pattern, as
5279 described in the section entitled "Newline conventions" above. Note
5280 that the end of this type of comment is a literal newline sequence in
5281 the pattern; escape sequences that happen to represent a newline do not
5282 count. For example, consider this pattern when PCRE_EXTENDED is set,
5283 and the default newline convention is in force:
5284
5285 abc #comment \n still comment
5286
5287 On encountering the # character, pcre_compile() skips along, looking
5288 for a newline in the pattern. The sequence \n is still literal at this
5289 stage, so it does not terminate the comment. Only an actual character
5290 with the code value 0x0a (the default newline) does so.
5291
5292
5293 RECURSIVE PATTERNS
5294
5295 Consider the problem of matching a string in parentheses, allowing for
5296 unlimited nested parentheses. Without the use of recursion, the best
5297 that can be done is to use a pattern that matches up to some fixed
5298 depth of nesting. It is not possible to handle an arbitrary nesting
5299 depth.
5300
5301 For some time, Perl has provided a facility that allows regular expres-
5302 sions to recurse (amongst other things). It does this by interpolating
5303 Perl code in the expression at run time, and the code can refer to the
5304 expression itself. A Perl pattern using code interpolation to solve the
5305 parentheses problem can be created like this:
5306
5307 $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
5308
5309 The (?p{...}) item interpolates Perl code at run time, and in this case
5310 refers recursively to the pattern in which it appears.
5311
5312 Obviously, PCRE cannot support the interpolation of Perl code. Instead,
5313 it supports special syntax for recursion of the entire pattern, and
5314 also for individual subpattern recursion. After its introduction in
5315 PCRE and Python, this kind of recursion was subsequently introduced
5316 into Perl at release 5.10.
5317
5318 A special item that consists of (? followed by a number greater than
5319 zero and a closing parenthesis is a recursive call of the subpattern of
5320 the given number, provided that it occurs inside that subpattern. (If
5321 not, it is a "subroutine" call, which is described in the next sec-
5322 tion.) The special item (?R) or (?0) is a recursive call of the entire
5323 regular expression.
5324
5325 This PCRE pattern solves the nested parentheses problem (assume the
5326 PCRE_EXTENDED option is set so that white space is ignored):
5327
5328 \( ( [^()]++ | (?R) )* \)
5329
5330 First it matches an opening parenthesis. Then it matches any number of
5331 substrings which can either be a sequence of non-parentheses, or a
5332 recursive match of the pattern itself (that is, a correctly parenthe-
5333 sized substring). Finally there is a closing parenthesis. Note the use
5334 of a possessive quantifier to avoid backtracking into sequences of non-
5335 parentheses.
5336
5337 If this were part of a larger pattern, you would not want to recurse
5338 the entire pattern, so instead you could use this:
5339
5340 ( \( ( [^()]++ | (?1) )* \) )
5341
5342 We have put the pattern into parentheses, and caused the recursion to
5343 refer to them instead of the whole pattern.
5344
5345 In a larger pattern, keeping track of parenthesis numbers can be
5346 tricky. This is made easier by the use of relative references. Instead
5347 of (?1) in the pattern above you can write (?-2) to refer to the second
5348 most recently opened parentheses preceding the recursion. In other
5349 words, a negative number counts capturing parentheses leftwards from
5350 the point at which it is encountered.
5351
5352 It is also possible to refer to subsequently opened parentheses, by
5353 writing references such as (?+2). However, these cannot be recursive
5354 because the reference is not inside the parentheses that are refer-
5355 enced. They are always "subroutine" calls, as described in the next
5356 section.
5357
5358 An alternative approach is to use named parentheses instead. The Perl
5359 syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
5360 supported. We could rewrite the above example as follows:
5361
5362 (?<pn> \( ( [^()]++ | (?&pn) )* \) )
5363
5364 If there is more than one subpattern with the same name, the earliest
5365 one is used.
5366
5367 This particular example pattern that we have been looking at contains
5368 nested unlimited repeats, and so the use of a possessive quantifier for
5369 matching strings of non-parentheses is important when applying the pat-
5370 tern to strings that do not match. For example, when this pattern is
5371 applied to
5372
5373 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
5374
5375 it yields "no match" quickly. However, if a possessive quantifier is
5376 not used, the match runs for a very long time indeed because there are
5377 so many different ways the + and * repeats can carve up the subject,
5378 and all have to be tested before failure can be reported.
5379
5380 At the end of a match, the values of capturing parentheses are those
5381 from the outermost level. If you want to obtain intermediate values, a
5382 callout function can be used (see below and the pcrecallout documenta-
5383 tion). If the pattern above is matched against
5384
5385 (ab(cd)ef)
5386
5387 the value for the inner capturing parentheses (numbered 2) is "ef",
5388 which is the last value taken on at the top level. If a capturing sub-
5389 pattern is not matched at the top level, its final value is unset, even
5390 if it is (temporarily) set at a deeper level.
5391
5392 If there are more than 15 capturing parentheses in a pattern, PCRE has
5393 to obtain extra memory to store data during a recursion, which it does
5394 by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
5395 can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
5396
5397 Do not confuse the (?R) item with the condition (R), which tests for
5398 recursion. Consider this pattern, which matches text in angle brack-
5399 ets, allowing for arbitrary nesting. Only digits are allowed in nested
5400 brackets (that is, when recursing), whereas any characters are permit-
5401 ted at the outer level.
5402
5403 < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
5404
5405 In this pattern, (?(R) is the start of a conditional subpattern, with
5406 two different alternatives for the recursive and non-recursive cases.
5407 The (?R) item is the actual recursive call.
5408
5409 Recursion difference from Perl
5410
5411 In PCRE (like Python, but unlike Perl), a recursive subpattern call is
5412 always treated as an atomic group. That is, once it has matched some of
5413 the subject string, it is never re-entered, even if it contains untried
5414 alternatives and there is a subsequent matching failure. This can be
5415 illustrated by the following pattern, which purports to match a palin-
5416 dromic string that contains an odd number of characters (for example,
5417 "a", "aba", "abcba", "abcdcba"):
5418
5419 ^(.|(.)(?1)\2)$
5420
5421 The idea is that it either matches a single character, or two identical
5422 characters surrounding a sub-palindrome. In Perl, this pattern works;
5423 in PCRE it does not if the pattern is longer than three characters.
5424 Consider the subject string "abcba":
5425
5426 At the top level, the first character is matched, but as it is not at
5427 the end of the string, the first alternative fails; the second alterna-
5428 tive is taken and the recursion kicks in. The recursive call to subpat-
5429 tern 1 successfully matches the next character ("b"). (Note that the
5430 beginning and end of line tests are not part of the recursion).
5431
5432 Back at the top level, the next character ("c") is compared with what
5433 subpattern 2 matched, which was "a". This fails. Because the recursion
5434 is treated as an atomic group, there are now no backtracking points,
5435 and so the entire match fails. (Perl is able, at this point, to re-
5436 enter the recursion and try the second alternative.) However, if the
5437 pattern is written with the alternatives in the other order, things are
5438 different:
5439
5440 ^((.)(?1)\2|.)$
5441
5442 This time, the recursing alternative is tried first, and continues to
5443 recurse until it runs out of characters, at which point the recursion
5444 fails. But this time we do have another alternative to try at the
5445 higher level. That is the big difference: in the previous case the
5446 remaining alternative is at a deeper recursion level, which PCRE cannot
5447 use.
5448
5449 To change the pattern so that it matches all palindromic strings, not
5450 just those with an odd number of characters, it is tempting to change
5451 the pattern to this:
5452
5453 ^((.)(?1)\2|.?)$
5454
5455 Again, this works in Perl, but not in PCRE, and for the same reason.
5456 When a deeper recursion has matched a single character, it cannot be
5457 entered again in order to match an empty string. The solution is to
5458 separate the two cases, and write out the odd and even cases as alter-
5459 natives at the higher level:
5460
5461 ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
5462
5463 If you want to match typical palindromic phrases, the pattern has to
5464 ignore all non-word characters, which can be done like this:
5465
5466 ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
5467
5468 If run with the PCRE_CASELESS option, this pattern matches phrases such
5469 as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
5470 Perl. Note the use of the possessive quantifier *+ to avoid backtrack-
5471 ing into sequences of non-word characters. Without this, PCRE takes a
5472 great deal longer (ten times or more) to match typical phrases, and
5473 Perl takes so long that you think it has gone into a loop.
5474
5475 WARNING: The palindrome-matching patterns above work only if the sub-
5476 ject string does not start with a palindrome that is shorter than the
5477 entire string. For example, although "abcba" is correctly matched, if
5478 the subject is "ababa", PCRE finds the palindrome "aba" at the start,
5479 then fails at top level because the end of the string does not follow.
5480 Once again, it cannot jump back into the recursion to try other alter-
5481 natives, so the entire match fails.
5482
5483
5484 SUBPATTERNS AS SUBROUTINES
5485
5486 If the syntax for a recursive subpattern reference (either by number or
5487 by name) is used outside the parentheses to which it refers, it oper-
5488 ates like a subroutine in a programming language. The "called" subpat-
5489 tern may be defined before or after the reference. A numbered reference
5490 can be absolute or relative, as in these examples:
5491
5492 (...(absolute)...)...(?2)...
5493 (...(relative)...)...(?-1)...
5494 (...(?+1)...(relative)...
5495
5496 An earlier example pointed out that the pattern
5497
5498 (sens|respons)e and \1ibility
5499
5500 matches "sense and sensibility" and "response and responsibility", but
5501 not "sense and responsibility". If instead the pattern
5502
5503 (sens|respons)e and (?1)ibility
5504
5505 is used, it does match "sense and responsibility" as well as the other
5506 two strings. Another example is given in the discussion of DEFINE
5507 above.
5508
5509 Like recursive subpatterns, a subroutine call is always treated as an
5510 atomic group. That is, once it has matched some of the subject string,
5511 it is never re-entered, even if it contains untried alternatives and
5512 there is a subsequent matching failure. Any capturing parentheses that
5513 are set during the subroutine call revert to their previous values
5514 afterwards.
5515
5516 When a subpattern is used as a subroutine, processing options such as
5517 case-independence are fixed when the subpattern is defined. They cannot
5518 be changed for different calls. For example, consider this pattern:
5519
5520 (abc)(?i:(?-1))
5521
5522 It matches "abcabc". It does not match "abcABC" because the change of
5523 processing option does not affect the called subpattern.
5524
5525
5526 ONIGURUMA SUBROUTINE SYNTAX
5527
5528 For compatibility with Oniguruma, the non-Perl syntax \g followed by a
5529 name or a number enclosed either in angle brackets or single quotes, is
5530 an alternative syntax for referencing a subpattern as a subroutine,
5531 possibly recursively. Here are two of the examples used above, rewrit-
5532 ten using this syntax:
5533
5534 (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
5535 (sens|respons)e and \g'1'ibility
5536
5537 PCRE supports an extension to Oniguruma: if a number is preceded by a
5538 plus or a minus sign it is taken as a relative reference. For example:
5539
5540 (abc)(?i:\g<-1>)
5541
5542 Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
5543 synonymous. The former is a back reference; the latter is a subroutine
5544 call.
5545
5546
5547 CALLOUTS
5548
5549 Perl has a feature whereby using the sequence (?{...}) causes arbitrary
5550 Perl code to be obeyed in the middle of matching a regular expression.
5551 This makes it possible, amongst other things, to extract different sub-
5552 strings that match the same pair of parentheses when there is a repeti-
5553 tion.
5554
5555 PCRE provides a similar feature, but of course it cannot obey arbitrary
5556 Perl code. The feature is called "callout". The caller of PCRE provides
5557 an external function by putting its entry point in the global variable
5558 pcre_callout. By default, this variable contains NULL, which disables
5559 all calling out.
5560
5561 Within a regular expression, (?C) indicates the points at which the
5562 external function is to be called. If you want to identify different
5563 callout points, you can put a number less than 256 after the letter C.
5564 The default value is zero. For example, this pattern has two callout
5565 points:
5566
5567 (?C1)abc(?C2)def
5568
5569 If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
5570 automatically installed before each item in the pattern. They are all
5571 numbered 255.
5572
5573 During matching, when PCRE reaches a callout point (and pcre_callout is
5574 set), the external function is called. It is provided with the number
5575 of the callout, the position in the pattern, and, optionally, one item
5576 of data originally supplied by the caller of pcre_exec(). The callout
5577 function may cause matching to proceed, to backtrack, or to fail alto-
5578 gether. A complete description of the interface to the callout function
5579 is given in the pcrecallout documentation.
5580
5581
5582 BACKTRACKING CONTROL
5583
5584 Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
5585 which are described in the Perl documentation as "experimental and sub-
5586 ject to change or removal in a future version of Perl". It goes on to
5587 say: "Their usage in production code should be noted to avoid problems
5588 during upgrades." The same remarks apply to the PCRE features described
5589 in this section.
5590
5591 Since these verbs are specifically related to backtracking, most of
5592 them can be used only when the pattern is to be matched using
5593 pcre_exec(), which uses a backtracking algorithm. With the exception of
5594 (*FAIL), which behaves like a failing negative assertion, they cause an
5595 error if encountered by pcre_dfa_exec().
5596
5597 If any of these verbs are used in an assertion or subroutine subpattern
5598 (including recursive subpatterns), their effect is confined to that
5599 subpattern; it does not extend to the surrounding pattern, with one
5600 exception: a *MARK that is encountered in a positive assertion is
5601 passed back (compare capturing parentheses in assertions). Note that
5602 such subpatterns are processed as anchored at the point where they are
5603 tested.
5604
5605 The new verbs make use of what was previously invalid syntax: an open-
5606 ing parenthesis followed by an asterisk. They are generally of the form
5607 (*VERB) or (*VERB:NAME). Some may take either form, with differing be-
5608 haviour, depending on whether or not an argument is present. An name is
5609 a sequence of letters, digits, and underscores. If the name is empty,
5610 that is, if the closing parenthesis immediately follows the colon, the
5611 effect is as if the colon were not there. Any number of these verbs may
5612 occur in a pattern.
5613
5614 PCRE contains some optimizations that are used to speed up matching by
5615 running some checks at the start of each match attempt. For example, it
5616 may know the minimum length of matching subject, or that a particular
5617 character must be present. When one of these optimizations suppresses
5618 the running of a match, any included backtracking verbs will not, of
5619 course, be processed. You can suppress the start-of-match optimizations
5620 by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com-
5621 pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
5622
5623 Verbs that act immediately
5624
5625 The following verbs act as soon as they are encountered. They may not
5626 be followed by a name.
5627
5628 (*ACCEPT)
5629
5630 This verb causes the match to end successfully, skipping the remainder
5631 of the pattern. When inside a recursion, only the innermost pattern is
5632 ended immediately. If (*ACCEPT) is inside capturing parentheses, the
5633 data so far is captured. (This feature was added to PCRE at release
5634 8.00.) For example:
5635
5636 A((?:A|B(*ACCEPT)|C)D)
5637
5638 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
5639 tured by the outer parentheses.
5640
5641 (*FAIL) or (*F)
5642
5643 This verb causes the match to fail, forcing backtracking to occur. It
5644 is equivalent to (?!) but easier to read. The Perl documentation notes
5645 that it is probably useful only when combined with (?{}) or (??{}).
5646 Those are, of course, Perl features that are not present in PCRE. The
5647 nearest equivalent is the callout feature, as for example in this pat-
5648 tern:
5649
5650 a+(?C)(*FAIL)
5651
5652 A match with the string "aaaa" always fails, but the callout is taken
5653 before each backtrack happens (in this example, 10 times).
5654
5655 Recording which path was taken
5656
5657 There is one verb whose main purpose is to track how a match was
5658 arrived at, though it also has a secondary use in conjunction with
5659 advancing the match starting point (see (*SKIP) below).
5660
5661 (*MARK:NAME) or (*:NAME)
5662
5663 A name is always required with this verb. There may be as many
5664 instances of (*MARK) as you like in a pattern, and their names do not
5665 have to be unique.
5666
5667 When a match succeeds, the name of the last-encountered (*MARK) is
5668 passed back to the caller via the pcre_extra data structure, as
5669 described in the section on pcre_extra in the pcreapi documentation. No
5670 data is returned for a partial match. Here is an example of pcretest
5671 output, where the /K modifier requests the retrieval and outputting of
5672 (*MARK) data:
5673
5674 /X(*MARK:A)Y|X(*MARK:B)Z/K
5675 XY
5676 0: XY
5677 MK: A
5678 XZ
5679 0: XZ
5680 MK: B
5681
5682 The (*MARK) name is tagged with "MK:" in this output, and in this exam-
5683 ple it indicates which of the two alternatives matched. This is a more
5684 efficient way of obtaining this information than putting each alterna-
5685 tive in its own capturing parentheses.
5686
5687 If (*MARK) is encountered in a positive assertion, its name is recorded
5688 and passed back if it is the last-encountered. This does not happen for
5689 negative assetions.
5690
5691 A name may also be returned after a failed match if the final path
5692 through the pattern involves (*MARK). However, unless (*MARK) used in
5693 conjunction with (*COMMIT), this is unlikely to happen for an unan-
5694 chored pattern because, as the starting point for matching is advanced,
5695 the final check is often with an empty string, causing a failure before
5696 (*MARK) is reached. For example:
5697
5698 /X(*MARK:A)Y|X(*MARK:B)Z/K
5699 XP
5700 No match
5701
5702 There are three potential starting points for this match (starting with
5703 X, starting with P, and with an empty string). If the pattern is
5704 anchored, the result is different:
5705
5706 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
5707 XP
5708 No match, mark = B
5709
5710 PCRE's start-of-match optimizations can also interfere with this. For
5711 example, if, as a result of a call to pcre_study(), it knows the mini-
5712 mum subject length for a match, a shorter subject will not be scanned
5713 at all.
5714
5715 Note that similar anomalies (though different in detail) exist in Perl,
5716 no doubt for the same reasons. The use of (*MARK) data after a failed
5717 match of an unanchored pattern is not recommended, unless (*COMMIT) is
5718 involved.
5719
5720 Verbs that act after backtracking
5721
5722 The following verbs do nothing when they are encountered. Matching con-
5723 tinues with what follows, but if there is no subsequent match, causing
5724 a backtrack to the verb, a failure is forced. That is, backtracking
5725 cannot pass to the left of the verb. However, when one of these verbs
5726 appears inside an atomic group, its effect is confined to that group,
5727 because once the group has been matched, there is never any backtrack-
5728 ing into it. In this situation, backtracking can "jump back" to the
5729 left of the entire atomic group. (Remember also, as stated above, that
5730 this localization also applies in subroutine calls and assertions.)
5731
5732 These verbs differ in exactly what kind of failure occurs when back-
5733 tracking reaches them.
5734
5735 (*COMMIT)
5736
5737 This verb, which may not be followed by a name, causes the whole match
5738 to fail outright if the rest of the pattern does not match. Even if the
5739 pattern is unanchored, no further attempts to find a match by advancing
5740 the starting point take place. Once (*COMMIT) has been passed,
5741 pcre_exec() is committed to finding a match at the current starting
5742 point, or not at all. For example:
5743
5744 a+(*COMMIT)b
5745
5746 This matches "xxaab" but not "aacaab". It can be thought of as a kind
5747 of dynamic anchor, or "I've started, so I must finish." The name of the
5748 most recently passed (*MARK) in the path is passed back when (*COMMIT)
5749 forces a match failure.
5750
5751 Note that (*COMMIT) at the start of a pattern is not the same as an
5752 anchor, unless PCRE's start-of-match optimizations are turned off, as
5753 shown in this pcretest example:
5754
5755 /(*COMMIT)abc/
5756 xyzabc
5757 0: abc
5758 xyzabc\Y
5759 No match
5760
5761 PCRE knows that any match must start with "a", so the optimization
5762 skips along the subject to "a" before running the first match attempt,
5763 which succeeds. When the optimization is disabled by the \Y escape in
5764 the second subject, the match starts at "x" and so the (*COMMIT) causes
5765 it to fail without trying any other starting points.
5766
5767 (*PRUNE) or (*PRUNE:NAME)
5768
5769 This verb causes the match to fail at the current starting position in
5770 the subject if the rest of the pattern does not match. If the pattern
5771 is unanchored, the normal "bumpalong" advance to the next starting
5772 character then happens. Backtracking can occur as usual to the left of
5773 (*PRUNE), before it is reached, or when matching to the right of
5774 (*PRUNE), but if there is no match to the right, backtracking cannot
5775 cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter-
5776 native to an atomic group or possessive quantifier, but there are some
5777 uses of (*PRUNE) that cannot be expressed in any other way. The behav-
5778 iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
5779 match fails completely; the name is passed back if this is the final
5780 attempt. (*PRUNE:NAME) does not pass back a name if the match suc-
5781 ceeds. In an anchored pattern (*PRUNE) has the same effect as (*COM-
5782 MIT).
5783
5784 (*SKIP)
5785
5786 This verb, when given without a name, is like (*PRUNE), except that if
5787 the pattern is unanchored, the "bumpalong" advance is not to the next
5788 character, but to the position in the subject where (*SKIP) was encoun-
5789 tered. (*SKIP) signifies that whatever text was matched leading up to
5790 it cannot be part of a successful match. Consider:
5791
5792 a+(*SKIP)b
5793
5794 If the subject is "aaaac...", after the first match attempt fails
5795 (starting at the first character in the string), the starting point
5796 skips on to start the next attempt at "c". Note that a possessive quan-
5797 tifer does not have the same effect as this example; although it would
5798 suppress backtracking during the first match attempt, the second
5799 attempt would start at the second character instead of skipping on to
5800 "c".
5801
5802 (*SKIP:NAME)
5803
5804 When (*SKIP) has an associated name, its behaviour is modified. If the
5805 following pattern fails to match, the previous path through the pattern
5806 is searched for the most recent (*MARK) that has the same name. If one
5807 is found, the "bumpalong" advance is to the subject position that cor-
5808 responds to that (*MARK) instead of to where (*SKIP) was encountered.
5809 If no (*MARK) with a matching name is found, normal "bumpalong" of one
5810 character happens (the (*SKIP) is ignored).
5811
5812 (*THEN) or (*THEN:NAME)
5813
5814 This verb causes a skip to the next alternation in the innermost
5815 enclosing group if the rest of the pattern does not match. That is, it
5816 cancels pending backtracking, but only within the current alternation.
5817 Its name comes from the observation that it can be used for a pattern-
5818 based if-then-else block:
5819
5820 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
5821
5822 If the COND1 pattern matches, FOO is tried (and possibly further items
5823 after the end of the group if FOO succeeds); on failure the matcher
5824 skips to the second alternative and tries COND2, without backtracking
5825 into COND1. The behaviour of (*THEN:NAME) is exactly the same as
5826 (*MARK:NAME)(*THEN) if the overall match fails. If (*THEN) is not
5827 directly inside an alternation, it acts like (*PRUNE).
5828
5829 The above verbs provide four different "strengths" of control when sub-
5830 sequent matching fails. (*THEN) is the weakest, carrying on the match
5831 at the next alternation. (*PRUNE) comes next, failing the match at the
5832 current starting position, but allowing an advance to the next charac-
5833 ter (for an unanchored pattern). (*SKIP) is similar, except that the
5834 advance may be more than one character. (*COMMIT) is the strongest,
5835 causing the entire match to fail.
5836
5837 If more than one is present in a pattern, the "stongest" one wins. For
5838 example, consider this pattern, where A, B, etc. are complex pattern
5839 fragments:
5840
5841 (A(*COMMIT)B(*THEN)C|D)
5842
5843 Once A has matched, PCRE is committed to this match, at the current
5844 starting position. If subsequently B matches, but C does not, the nor-
5845 mal (*THEN) action of trying the next alternation (that is, D) does not
5846 happen because (*COMMIT) overrides.
5847
5848
5849 SEE ALSO
5850
5851 pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3).
5852
5853
5854 AUTHOR
5855
5856 Philip Hazel
5857 University Computing Service
5858 Cambridge CB2 3QH, England.
5859
5860
5861 REVISION
5862
5863 Last updated: 24 July 2011
5864 Copyright (c) 1997-2011 University of Cambridge.
5865 ------------------------------------------------------------------------------
5866
5867
5868 PCRESYNTAX(3) PCRESYNTAX(3)
5869
5870
5871 NAME
5872 PCRE - Perl-compatible regular expressions
5873
5874
5875 PCRE REGULAR EXPRESSION SYNTAX SUMMARY
5876
5877 The full syntax and semantics of the regular expressions that are sup-
5878 ported by PCRE are described in the pcrepattern documentation. This
5879 document contains just a quick-reference summary of the syntax.
5880
5881
5882 QUOTING
5883
5884 \x where x is non-alphanumeric is a literal x
5885 \Q...\E treat enclosed characters as literal
5886
5887
5888 CHARACTERS
5889
5890 \a alarm, that is, the BEL character (hex 07)
5891 \cx "control-x", where x is any ASCII character
5892 \e escape (hex 1B)
5893 \f formfeed (hex 0C)
5894 \n newline (hex 0A)
5895 \r carriage return (hex 0D)
5896 \t tab (hex 09)
5897 \ddd character with octal code ddd, or backreference
5898 \xhh character with hex code hh
5899 \x{hhh..} character with hex code hhh..
5900
5901
5902 CHARACTER TYPES
5903
5904 . any character except newline;
5905 in dotall mode, any character whatsoever
5906 \C one byte, even in UTF-8 mode (best avoided)
5907 \d a decimal digit
5908 \D a character that is not a decimal digit
5909 \h a horizontal whitespace character
5910 \H a character that is not a horizontal whitespace character
5911 \N a character that is not a newline
5912 \p{xx} a character with the xx property
5913 \P{xx} a character without the xx property
5914 \R a newline sequence
5915 \s a whitespace character
5916 \S a character that is not a whitespace character
5917 \v a vertical whitespace character
5918 \V a character that is not a vertical whitespace character
5919 \w a "word" character
5920 \W a "non-word" character
5921 \X an extended Unicode sequence
5922
5923 In PCRE, by default, \d, \D, \s, \S, \w, and \W recognize only ASCII
5924 characters, even in UTF-8 mode. However, this can be changed by setting
5925 the PCRE_UCP option.
5926
5927
5928 GENERAL CATEGORY PROPERTIES FOR \p and \P
5929
5930 C Other
5931 Cc Control
5932 Cf Format
5933 Cn Unassigned
5934 Co Private use
5935 Cs Surrogate
5936
5937 L Letter
5938 Ll Lower case letter
5939 Lm Modifier letter
5940 Lo Other letter
5941 Lt Title case letter
5942 Lu Upper case letter
5943 L& Ll, Lu, or Lt
5944
5945 M Mark
5946 Mc Spacing mark
5947 Me Enclosing mark
5948 Mn Non-spacing mark
5949
5950 N Number
5951 Nd Decimal number
5952 Nl Letter number
5953 No Other number
5954
5955 P Punctuation
5956 Pc Connector punctuation
5957 Pd Dash punctuation
5958 Pe Close punctuation
5959 Pf Final punctuation
5960 Pi Initial punctuation
5961 Po Other punctuation
5962 Ps Open punctuation
5963
5964 S Symbol
5965 Sc Currency symbol
5966 Sk Modifier symbol
5967 Sm Mathematical symbol
5968 So Other symbol
5969
5970 Z Separator
5971 Zl Line separator
5972 Zp Paragraph separator
5973 Zs Space separator
5974
5975
5976 PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P
5977
5978 Xan Alphanumeric: union of properties L and N
5979 Xps POSIX space: property Z or tab, NL, VT, FF, CR
5980 Xsp Perl space: property Z or tab, NL, FF, CR
5981 Xwd Perl word: property Xan or underscore
5982
5983
5984 SCRIPT NAMES FOR \p AND \P
5985
5986 Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
5987 Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
5988 Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
5989 tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
5990 Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
5991 rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
5992 Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
5993 Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
5994 Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
5995 Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
5996 Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
5997 Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
5998 Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
5999 Ugaritic, Vai, Yi.
6000
6001
6002 CHARACTER CLASSES
6003
6004 [...] positive character class
6005 [^...] negative character class
6006 [x-y] range (can be used for hex characters)
6007 [[:xxx:]] positive POSIX named set
6008 [[:^xxx:]] negative POSIX named set
6009
6010 alnum alphanumeric
6011 alpha alphabetic
6012 ascii 0-127
6013 blank space or tab
6014 cntrl control character
6015 digit decimal digit
6016 graph printing, excluding space
6017