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Fix #947, recursive back reference bug.

1 .TH PCREPATTERN 3
2 .SH NAME
3 PCRE - Perl-compatible regular expressions
4 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 .rs
6 .sp
7 The syntax and semantics of the regular expressions that are supported by PCRE
8 are described in detail below. There is a quick-reference syntax summary in the
9 .\" HREF
10 \fBpcresyntax\fP
11 .\"
12 page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13 also supports some alternative regular expression syntax (which does not
14 conflict with the Perl syntax) in order to provide some compatibility with
15 regular expressions in Python, .NET, and Oniguruma.
16 .P
17 Perl's regular expressions are described in its own documentation, and
18 regular expressions in general are covered in a number of books, some of which
19 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20 published by O'Reilly, covers regular expressions in great detail. This
21 description of PCRE's regular expressions is intended as reference material.
22 .P
23 The original operation of PCRE was on strings of one-byte characters. However,
24 there is now also support for UTF-8 character strings. To use this,
25 PCRE must be built to include UTF-8 support, and you must call
26 \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27 is also a special sequence that can be given at the start of a pattern:
28 .sp
29 (*UTF8)
30 .sp
31 Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32 option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33 pattern matching is mentioned in several places below. There is also a summary
34 of UTF-8 features in the
35 .\" HTML <a href="pcre.html#utf8support">
36 .\" </a>
37 section on UTF-8 support
38 .\"
39 in the main
40 .\" HREF
41 \fBpcre\fP
42 .\"
43 page.
44 .P
45 The remainder of this document discusses the patterns that are supported by
46 PCRE when its main matching function, \fBpcre_exec()\fP, is used.
47 From release 6.0, PCRE offers a second matching function,
48 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
49 Perl-compatible. Some of the features discussed below are not available when
50 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
51 alternative function, and how it differs from the normal function, are
52 discussed in the
53 .\" HREF
54 \fBpcrematching\fP
55 .\"
56 page.
57 .
58 .
59 .SH "NEWLINE CONVENTIONS"
60 .rs
61 .sp
62 PCRE supports five different conventions for indicating line breaks in
63 strings: a single CR (carriage return) character, a single LF (linefeed)
64 character, the two-character sequence CRLF, any of the three preceding, or any
65 Unicode newline sequence. The
66 .\" HREF
67 \fBpcreapi\fP
68 .\"
69 page has
70 .\" HTML <a href="pcreapi.html#newlines">
71 .\" </a>
72 further discussion
73 .\"
74 about newlines, and shows how to set the newline convention in the
75 \fIoptions\fP arguments for the compiling and matching functions.
76 .P
77 It is also possible to specify a newline convention by starting a pattern
78 string with one of the following five sequences:
79 .sp
80 (*CR) carriage return
81 (*LF) linefeed
82 (*CRLF) carriage return, followed by linefeed
83 (*ANYCRLF) any of the three above
84 (*ANY) all Unicode newline sequences
85 .sp
86 These override the default and the options given to \fBpcre_compile()\fP or
87 \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
88 newline sequence, the pattern
89 .sp
90 (*CR)a.b
91 .sp
92 changes the convention to CR. That pattern matches "a\enb" because LF is no
93 longer a newline. Note that these special settings, which are not
94 Perl-compatible, are recognized only at the very start of a pattern, and that
95 they must be in upper case. If more than one of them is present, the last one
96 is used.
97 .P
98 The newline convention does not affect what the \eR escape sequence matches. By
99 default, this is any Unicode newline sequence, for Perl compatibility. However,
100 this can be changed; see the description of \eR in the section entitled
101 .\" HTML <a href="#newlineseq">
102 .\" </a>
103 "Newline sequences"
104 .\"
105 below. A change of \eR setting can be combined with a change of newline
106 convention.
107 .
108 .
109 .SH "CHARACTERS AND METACHARACTERS"
110 .rs
111 .sp
112 A regular expression is a pattern that is matched against a subject string from
113 left to right. Most characters stand for themselves in a pattern, and match the
114 corresponding characters in the subject. As a trivial example, the pattern
115 .sp
116 The quick brown fox
117 .sp
118 matches a portion of a subject string that is identical to itself. When
119 caseless matching is specified (the PCRE_CASELESS option), letters are matched
120 independently of case. In UTF-8 mode, PCRE always understands the concept of
121 case for characters whose values are less than 128, so caseless matching is
122 always possible. For characters with higher values, the concept of case is
123 supported if PCRE is compiled with Unicode property support, but not otherwise.
124 If you want to use caseless matching for characters 128 and above, you must
125 ensure that PCRE is compiled with Unicode property support as well as with
126 UTF-8 support.
127 .P
128 The power of regular expressions comes from the ability to include alternatives
129 and repetitions in the pattern. These are encoded in the pattern by the use of
130 \fImetacharacters\fP, which do not stand for themselves but instead are
131 interpreted in some special way.
132 .P
133 There are two different sets of metacharacters: those that are recognized
134 anywhere in the pattern except within square brackets, and those that are
135 recognized within square brackets. Outside square brackets, the metacharacters
136 are as follows:
137 .sp
138 \e general escape character with several uses
139 ^ assert start of string (or line, in multiline mode)
140 $ assert end of string (or line, in multiline mode)
141 . match any character except newline (by default)
142 [ start character class definition
143 | start of alternative branch
144 ( start subpattern
145 ) end subpattern
146 ? extends the meaning of (
147 also 0 or 1 quantifier
148 also quantifier minimizer
149 * 0 or more quantifier
150 + 1 or more quantifier
151 also "possessive quantifier"
152 { start min/max quantifier
153 .sp
154 Part of a pattern that is in square brackets is called a "character class". In
155 a character class the only metacharacters are:
156 .sp
157 \e general escape character
158 ^ negate the class, but only if the first character
159 - indicates character range
160 .\" JOIN
161 [ POSIX character class (only if followed by POSIX
162 syntax)
163 ] terminates the character class
164 .sp
165 The following sections describe the use of each of the metacharacters.
166 .
167 .
168 .SH BACKSLASH
169 .rs
170 .sp
171 The backslash character has several uses. Firstly, if it is followed by a
172 non-alphanumeric character, it takes away any special meaning that character
173 may have. This use of backslash as an escape character applies both inside and
174 outside character classes.
175 .P
176 For example, if you want to match a * character, you write \e* in the pattern.
177 This escaping action applies whether or not the following character would
178 otherwise be interpreted as a metacharacter, so it is always safe to precede a
179 non-alphanumeric with backslash to specify that it stands for itself. In
180 particular, if you want to match a backslash, you write \e\e.
181 .P
182 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
183 pattern (other than in a character class) and characters between a # outside
184 a character class and the next newline are ignored. An escaping backslash can
185 be used to include a whitespace or # character as part of the pattern.
186 .P
187 If you want to remove the special meaning from a sequence of characters, you
188 can do so by putting them between \eQ and \eE. This is different from Perl in
189 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
190 Perl, $ and @ cause variable interpolation. Note the following examples:
191 .sp
192 Pattern PCRE matches Perl matches
193 .sp
194 .\" JOIN
195 \eQabc$xyz\eE abc$xyz abc followed by the
196 contents of $xyz
197 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
198 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
199 .sp
200 The \eQ...\eE sequence is recognized both inside and outside character classes.
201 .
202 .
203 .\" HTML <a name="digitsafterbackslash"></a>
204 .SS "Non-printing characters"
205 .rs
206 .sp
207 A second use of backslash provides a way of encoding non-printing characters
208 in patterns in a visible manner. There is no restriction on the appearance of
209 non-printing characters, apart from the binary zero that terminates a pattern,
210 but when a pattern is being prepared by text editing, it is often easier to use
211 one of the following escape sequences than the binary character it represents:
212 .sp
213 \ea alarm, that is, the BEL character (hex 07)
214 \ecx "control-x", where x is any character
215 \ee escape (hex 1B)
216 \ef formfeed (hex 0C)
217 \en linefeed (hex 0A)
218 \er carriage return (hex 0D)
219 \et tab (hex 09)
220 \eddd character with octal code ddd, or back reference
221 \exhh character with hex code hh
222 \ex{hhh..} character with hex code hhh..
223 .sp
224 The precise effect of \ecx is as follows: if x is a lower case letter, it
225 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
226 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
227 7B.
228 .P
229 After \ex, from zero to two hexadecimal digits are read (letters can be in
230 upper or lower case). Any number of hexadecimal digits may appear between \ex{
231 and }, but the value of the character code must be less than 256 in non-UTF-8
232 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
233 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
234 point, which is 10FFFF.
235 .P
236 If characters other than hexadecimal digits appear between \ex{ and }, or if
237 there is no terminating }, this form of escape is not recognized. Instead, the
238 initial \ex will be interpreted as a basic hexadecimal escape, with no
239 following digits, giving a character whose value is zero.
240 .P
241 Characters whose value is less than 256 can be defined by either of the two
242 syntaxes for \ex. There is no difference in the way they are handled. For
243 example, \exdc is exactly the same as \ex{dc}.
244 .P
245 After \e0 up to two further octal digits are read. If there are fewer than two
246 digits, just those that are present are used. Thus the sequence \e0\ex\e07
247 specifies two binary zeros followed by a BEL character (code value 7). Make
248 sure you supply two digits after the initial zero if the pattern character that
249 follows is itself an octal digit.
250 .P
251 The handling of a backslash followed by a digit other than 0 is complicated.
252 Outside a character class, PCRE reads it and any following digits as a decimal
253 number. If the number is less than 10, or if there have been at least that many
254 previous capturing left parentheses in the expression, the entire sequence is
255 taken as a \fIback reference\fP. A description of how this works is given
256 .\" HTML <a href="#backreferences">
257 .\" </a>
258 later,
259 .\"
260 following the discussion of
261 .\" HTML <a href="#subpattern">
262 .\" </a>
263 parenthesized subpatterns.
264 .\"
265 .P
266 Inside a character class, or if the decimal number is greater than 9 and there
267 have not been that many capturing subpatterns, PCRE re-reads up to three octal
268 digits following the backslash, and uses them to generate a data character. Any
269 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
270 character specified in octal must be less than \e400. In UTF-8 mode, values up
271 to \e777 are permitted. For example:
272 .sp
273 \e040 is another way of writing a space
274 .\" JOIN
275 \e40 is the same, provided there are fewer than 40
276 previous capturing subpatterns
277 \e7 is always a back reference
278 .\" JOIN
279 \e11 might be a back reference, or another way of
280 writing a tab
281 \e011 is always a tab
282 \e0113 is a tab followed by the character "3"
283 .\" JOIN
284 \e113 might be a back reference, otherwise the
285 character with octal code 113
286 .\" JOIN
287 \e377 might be a back reference, otherwise
288 the byte consisting entirely of 1 bits
289 .\" JOIN
290 \e81 is either a back reference, or a binary zero
291 followed by the two characters "8" and "1"
292 .sp
293 Note that octal values of 100 or greater must not be introduced by a leading
294 zero, because no more than three octal digits are ever read.
295 .P
296 All the sequences that define a single character value can be used both inside
297 and outside character classes. In addition, inside a character class, the
298 sequence \eb is interpreted as the backspace character (hex 08), and the
299 sequences \eR and \eX are interpreted as the characters "R" and "X",
300 respectively. Outside a character class, these sequences have different
301 meanings
302 .\" HTML <a href="#uniextseq">
303 .\" </a>
304 (see below).
305 .\"
306 .
307 .
308 .SS "Absolute and relative back references"
309 .rs
310 .sp
311 The sequence \eg followed by an unsigned or a negative number, optionally
312 enclosed in braces, is an absolute or relative back reference. A named back
313 reference can be coded as \eg{name}. Back references are discussed
314 .\" HTML <a href="#backreferences">
315 .\" </a>
316 later,
317 .\"
318 following the discussion of
319 .\" HTML <a href="#subpattern">
320 .\" </a>
321 parenthesized subpatterns.
322 .\"
323 .
324 .
325 .SS "Absolute and relative subroutine calls"
326 .rs
327 .sp
328 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
329 a number enclosed either in angle brackets or single quotes, is an alternative
330 syntax for referencing a subpattern as a "subroutine". Details are discussed
331 .\" HTML <a href="#onigurumasubroutines">
332 .\" </a>
333 later.
334 .\"
335 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
336 synonymous. The former is a back reference; the latter is a
337 .\" HTML <a href="#subpatternsassubroutines">
338 .\" </a>
339 subroutine
340 .\"
341 call.
342 .
343 .
344 .SS "Generic character types"
345 .rs
346 .sp
347 Another use of backslash is for specifying generic character types. The
348 following are always recognized:
349 .sp
350 \ed any decimal digit
351 \eD any character that is not a decimal digit
352 \eh any horizontal whitespace character
353 \eH any character that is not a horizontal whitespace character
354 \es any whitespace character
355 \eS any character that is not a whitespace character
356 \ev any vertical whitespace character
357 \eV any character that is not a vertical whitespace character
358 \ew any "word" character
359 \eW any "non-word" character
360 .sp
361 Each pair of escape sequences partitions the complete set of characters into
362 two disjoint sets. Any given character matches one, and only one, of each pair.
363 .P
364 These character type sequences can appear both inside and outside character
365 classes. They each match one character of the appropriate type. If the current
366 matching point is at the end of the subject string, all of them fail, since
367 there is no character to match.
368 .P
369 For compatibility with Perl, \es does not match the VT character (code 11).
370 This makes it different from the the POSIX "space" class. The \es characters
371 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
372 included in a Perl script, \es may match the VT character. In PCRE, it never
373 does.
374 .P
375 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
376 \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
377 character property support is available. These sequences retain their original
378 meanings from before UTF-8 support was available, mainly for efficiency
379 reasons. Note that this also affects \eb, because it is defined in terms of \ew
380 and \eW.
381 .P
382 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
383 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
384 The horizontal space characters are:
385 .sp
386 U+0009 Horizontal tab
387 U+0020 Space
388 U+00A0 Non-break space
389 U+1680 Ogham space mark
390 U+180E Mongolian vowel separator
391 U+2000 En quad
392 U+2001 Em quad
393 U+2002 En space
394 U+2003 Em space
395 U+2004 Three-per-em space
396 U+2005 Four-per-em space
397 U+2006 Six-per-em space
398 U+2007 Figure space
399 U+2008 Punctuation space
400 U+2009 Thin space
401 U+200A Hair space
402 U+202F Narrow no-break space
403 U+205F Medium mathematical space
404 U+3000 Ideographic space
405 .sp
406 The vertical space characters are:
407 .sp
408 U+000A Linefeed
409 U+000B Vertical tab
410 U+000C Formfeed
411 U+000D Carriage return
412 U+0085 Next line
413 U+2028 Line separator
414 U+2029 Paragraph separator
415 .P
416 A "word" character is an underscore or any character less than 256 that is a
417 letter or digit. The definition of letters and digits is controlled by PCRE's
418 low-valued character tables, and may vary if locale-specific matching is taking
419 place (see
420 .\" HTML <a href="pcreapi.html#localesupport">
421 .\" </a>
422 "Locale support"
423 .\"
424 in the
425 .\" HREF
426 \fBpcreapi\fP
427 .\"
428 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
429 or "french" in Windows, some character codes greater than 128 are used for
430 accented letters, and these are matched by \ew. The use of locales with Unicode
431 is discouraged.
432 .
433 .
434 .\" HTML <a name="newlineseq"></a>
435 .SS "Newline sequences"
436 .rs
437 .sp
438 Outside a character class, by default, the escape sequence \eR matches any
439 Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
440 equivalent to the following:
441 .sp
442 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
443 .sp
444 This is an example of an "atomic group", details of which are given
445 .\" HTML <a href="#atomicgroup">
446 .\" </a>
447 below.
448 .\"
449 This particular group matches either the two-character sequence CR followed by
450 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
451 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
452 line, U+0085). The two-character sequence is treated as a single unit that
453 cannot be split.
454 .P
455 In UTF-8 mode, two additional characters whose codepoints are greater than 255
456 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
457 Unicode character property support is not needed for these characters to be
458 recognized.
459 .P
460 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
461 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
462 either at compile time or when the pattern is matched. (BSR is an abbrevation
463 for "backslash R".) This can be made the default when PCRE is built; if this is
464 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
465 It is also possible to specify these settings by starting a pattern string with
466 one of the following sequences:
467 .sp
468 (*BSR_ANYCRLF) CR, LF, or CRLF only
469 (*BSR_UNICODE) any Unicode newline sequence
470 .sp
471 These override the default and the options given to \fBpcre_compile()\fP or
472 \fBpcre_compile2()\fP, but they can be overridden by options given to
473 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
474 which are not Perl-compatible, are recognized only at the very start of a
475 pattern, and that they must be in upper case. If more than one of them is
476 present, the last one is used. They can be combined with a change of newline
477 convention, for example, a pattern can start with:
478 .sp
479 (*ANY)(*BSR_ANYCRLF)
480 .sp
481 Inside a character class, \eR matches the letter "R".
482 .
483 .
484 .\" HTML <a name="uniextseq"></a>
485 .SS Unicode character properties
486 .rs
487 .sp
488 When PCRE is built with Unicode character property support, three additional
489 escape sequences that match characters with specific properties are available.
490 When not in UTF-8 mode, these sequences are of course limited to testing
491 characters whose codepoints are less than 256, but they do work in this mode.
492 The extra escape sequences are:
493 .sp
494 \ep{\fIxx\fP} a character with the \fIxx\fP property
495 \eP{\fIxx\fP} a character without the \fIxx\fP property
496 \eX an extended Unicode sequence
497 .sp
498 The property names represented by \fIxx\fP above are limited to the Unicode
499 script names, the general category properties, and "Any", which matches any
500 character (including newline). Other properties such as "InMusicalSymbols" are
501 not currently supported by PCRE. Note that \eP{Any} does not match any
502 characters, so always causes a match failure.
503 .P
504 Sets of Unicode characters are defined as belonging to certain scripts. A
505 character from one of these sets can be matched using a script name. For
506 example:
507 .sp
508 \ep{Greek}
509 \eP{Han}
510 .sp
511 Those that are not part of an identified script are lumped together as
512 "Common". The current list of scripts is:
513 .P
514 Arabic,
515 Armenian,
516 Balinese,
517 Bengali,
518 Bopomofo,
519 Braille,
520 Buginese,
521 Buhid,
522 Canadian_Aboriginal,
523 Cherokee,
524 Common,
525 Coptic,
526 Cuneiform,
527 Cypriot,
528 Cyrillic,
529 Deseret,
530 Devanagari,
531 Ethiopic,
532 Georgian,
533 Glagolitic,
534 Gothic,
535 Greek,
536 Gujarati,
537 Gurmukhi,
538 Han,
539 Hangul,
540 Hanunoo,
541 Hebrew,
542 Hiragana,
543 Inherited,
544 Kannada,
545 Katakana,
546 Kharoshthi,
547 Khmer,
548 Lao,
549 Latin,
550 Limbu,
551 Linear_B,
552 Malayalam,
553 Mongolian,
554 Myanmar,
555 New_Tai_Lue,
556 Nko,
557 Ogham,
558 Old_Italic,
559 Old_Persian,
560 Oriya,
561 Osmanya,
562 Phags_Pa,
563 Phoenician,
564 Runic,
565 Shavian,
566 Sinhala,
567 Syloti_Nagri,
568 Syriac,
569 Tagalog,
570 Tagbanwa,
571 Tai_Le,
572 Tamil,
573 Telugu,
574 Thaana,
575 Thai,
576 Tibetan,
577 Tifinagh,
578 Ugaritic,
579 Yi.
580 .P
581 Each character has exactly one general category property, specified by a
582 two-letter abbreviation. For compatibility with Perl, negation can be specified
583 by including a circumflex between the opening brace and the property name. For
584 example, \ep{^Lu} is the same as \eP{Lu}.
585 .P
586 If only one letter is specified with \ep or \eP, it includes all the general
587 category properties that start with that letter. In this case, in the absence
588 of negation, the curly brackets in the escape sequence are optional; these two
589 examples have the same effect:
590 .sp
591 \ep{L}
592 \epL
593 .sp
594 The following general category property codes are supported:
595 .sp
596 C Other
597 Cc Control
598 Cf Format
599 Cn Unassigned
600 Co Private use
601 Cs Surrogate
602 .sp
603 L Letter
604 Ll Lower case letter
605 Lm Modifier letter
606 Lo Other letter
607 Lt Title case letter
608 Lu Upper case letter
609 .sp
610 M Mark
611 Mc Spacing mark
612 Me Enclosing mark
613 Mn Non-spacing mark
614 .sp
615 N Number
616 Nd Decimal number
617 Nl Letter number
618 No Other number
619 .sp
620 P Punctuation
621 Pc Connector punctuation
622 Pd Dash punctuation
623 Pe Close punctuation
624 Pf Final punctuation
625 Pi Initial punctuation
626 Po Other punctuation
627 Ps Open punctuation
628 .sp
629 S Symbol
630 Sc Currency symbol
631 Sk Modifier symbol
632 Sm Mathematical symbol
633 So Other symbol
634 .sp
635 Z Separator
636 Zl Line separator
637 Zp Paragraph separator
638 Zs Space separator
639 .sp
640 The special property L& is also supported: it matches a character that has
641 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
642 a modifier or "other".
643 .P
644 The Cs (Surrogate) property applies only to characters in the range U+D800 to
645 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
646 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
647 (see the discussion of PCRE_NO_UTF8_CHECK in the
648 .\" HREF
649 \fBpcreapi\fP
650 .\"
651 page). Perl does not support the Cs property.
652 .P
653 The long synonyms for property names that Perl supports (such as \ep{Letter})
654 are not supported by PCRE, nor is it permitted to prefix any of these
655 properties with "Is".
656 .P
657 No character that is in the Unicode table has the Cn (unassigned) property.
658 Instead, this property is assumed for any code point that is not in the
659 Unicode table.
660 .P
661 Specifying caseless matching does not affect these escape sequences. For
662 example, \ep{Lu} always matches only upper case letters.
663 .P
664 The \eX escape matches any number of Unicode characters that form an extended
665 Unicode sequence. \eX is equivalent to
666 .sp
667 (?>\ePM\epM*)
668 .sp
669 That is, it matches a character without the "mark" property, followed by zero
670 or more characters with the "mark" property, and treats the sequence as an
671 atomic group
672 .\" HTML <a href="#atomicgroup">
673 .\" </a>
674 (see below).
675 .\"
676 Characters with the "mark" property are typically accents that affect the
677 preceding character. None of them have codepoints less than 256, so in
678 non-UTF-8 mode \eX matches any one character.
679 .P
680 Matching characters by Unicode property is not fast, because PCRE has to search
681 a structure that contains data for over fifteen thousand characters. That is
682 why the traditional escape sequences such as \ed and \ew do not use Unicode
683 properties in PCRE.
684 .
685 .
686 .\" HTML <a name="resetmatchstart"></a>
687 .SS "Resetting the match start"
688 .rs
689 .sp
690 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
691 matched characters not to be included in the final matched sequence. For
692 example, the pattern:
693 .sp
694 foo\eKbar
695 .sp
696 matches "foobar", but reports that it has matched "bar". This feature is
697 similar to a lookbehind assertion
698 .\" HTML <a href="#lookbehind">
699 .\" </a>
700 (described below).
701 .\"
702 However, in this case, the part of the subject before the real match does not
703 have to be of fixed length, as lookbehind assertions do. The use of \eK does
704 not interfere with the setting of
705 .\" HTML <a href="#subpattern">
706 .\" </a>
707 captured substrings.
708 .\"
709 For example, when the pattern
710 .sp
711 (foo)\eKbar
712 .sp
713 matches "foobar", the first substring is still set to "foo".
714 .
715 .
716 .\" HTML <a name="smallassertions"></a>
717 .SS "Simple assertions"
718 .rs
719 .sp
720 The final use of backslash is for certain simple assertions. An assertion
721 specifies a condition that has to be met at a particular point in a match,
722 without consuming any characters from the subject string. The use of
723 subpatterns for more complicated assertions is described
724 .\" HTML <a href="#bigassertions">
725 .\" </a>
726 below.
727 .\"
728 The backslashed assertions are:
729 .sp
730 \eb matches at a word boundary
731 \eB matches when not at a word boundary
732 \eA matches at the start of the subject
733 \eZ matches at the end of the subject
734 also matches before a newline at the end of the subject
735 \ez matches only at the end of the subject
736 \eG matches at the first matching position in the subject
737 .sp
738 These assertions may not appear in character classes (but note that \eb has a
739 different meaning, namely the backspace character, inside a character class).
740 .P
741 A word boundary is a position in the subject string where the current character
742 and the previous character do not both match \ew or \eW (i.e. one matches
743 \ew and the other matches \eW), or the start or end of the string if the
744 first or last character matches \ew, respectively. Neither PCRE nor Perl has a
745 separte "start of word" or "end of word" metasequence. However, whatever
746 follows \eb normally determines which it is. For example, the fragment
747 \eba matches "a" at the start of a word.
748 .P
749 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
750 dollar (described in the next section) in that they only ever match at the very
751 start and end of the subject string, whatever options are set. Thus, they are
752 independent of multiline mode. These three assertions are not affected by the
753 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
754 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
755 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
756 at a point other than the beginning of the subject, \eA can never match. The
757 difference between \eZ and \ez is that \eZ matches before a newline at the end
758 of the string as well as at the very end, whereas \ez matches only at the end.
759 .P
760 The \eG assertion is true only when the current matching position is at the
761 start point of the match, as specified by the \fIstartoffset\fP argument of
762 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
763 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
764 arguments, you can mimic Perl's /g option, and it is in this kind of
765 implementation where \eG can be useful.
766 .P
767 Note, however, that PCRE's interpretation of \eG, as the start of the current
768 match, is subtly different from Perl's, which defines it as the end of the
769 previous match. In Perl, these can be different when the previously matched
770 string was empty. Because PCRE does just one match at a time, it cannot
771 reproduce this behaviour.
772 .P
773 If all the alternatives of a pattern begin with \eG, the expression is anchored
774 to the starting match position, and the "anchored" flag is set in the compiled
775 regular expression.
776 .
777 .
778 .SH "CIRCUMFLEX AND DOLLAR"
779 .rs
780 .sp
781 Outside a character class, in the default matching mode, the circumflex
782 character is an assertion that is true only if the current matching point is
783 at the start of the subject string. If the \fIstartoffset\fP argument of
784 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
785 option is unset. Inside a character class, circumflex has an entirely different
786 meaning
787 .\" HTML <a href="#characterclass">
788 .\" </a>
789 (see below).
790 .\"
791 .P
792 Circumflex need not be the first character of the pattern if a number of
793 alternatives are involved, but it should be the first thing in each alternative
794 in which it appears if the pattern is ever to match that branch. If all
795 possible alternatives start with a circumflex, that is, if the pattern is
796 constrained to match only at the start of the subject, it is said to be an
797 "anchored" pattern. (There are also other constructs that can cause a pattern
798 to be anchored.)
799 .P
800 A dollar character is an assertion that is true only if the current matching
801 point is at the end of the subject string, or immediately before a newline
802 at the end of the string (by default). Dollar need not be the last character of
803 the pattern if a number of alternatives are involved, but it should be the last
804 item in any branch in which it appears. Dollar has no special meaning in a
805 character class.
806 .P
807 The meaning of dollar can be changed so that it matches only at the very end of
808 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
809 does not affect the \eZ assertion.
810 .P
811 The meanings of the circumflex and dollar characters are changed if the
812 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
813 immediately after internal newlines as well as at the start of the subject
814 string. It does not match after a newline that ends the string. A dollar
815 matches before any newlines in the string, as well as at the very end, when
816 PCRE_MULTILINE is set. When newline is specified as the two-character
817 sequence CRLF, isolated CR and LF characters do not indicate newlines.
818 .P
819 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
820 \en represents a newline) in multiline mode, but not otherwise. Consequently,
821 patterns that are anchored in single line mode because all branches start with
822 ^ are not anchored in multiline mode, and a match for circumflex is possible
823 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
824 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
825 .P
826 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
827 end of the subject in both modes, and if all branches of a pattern start with
828 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
829 .
830 .
831 .SH "FULL STOP (PERIOD, DOT)"
832 .rs
833 .sp
834 Outside a character class, a dot in the pattern matches any one character in
835 the subject string except (by default) a character that signifies the end of a
836 line. In UTF-8 mode, the matched character may be more than one byte long.
837 .P
838 When a line ending is defined as a single character, dot never matches that
839 character; when the two-character sequence CRLF is used, dot does not match CR
840 if it is immediately followed by LF, but otherwise it matches all characters
841 (including isolated CRs and LFs). When any Unicode line endings are being
842 recognized, dot does not match CR or LF or any of the other line ending
843 characters.
844 .P
845 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
846 option is set, a dot matches any one character, without exception. If the
847 two-character sequence CRLF is present in the subject string, it takes two dots
848 to match it.
849 .P
850 The handling of dot is entirely independent of the handling of circumflex and
851 dollar, the only relationship being that they both involve newlines. Dot has no
852 special meaning in a character class.
853 .
854 .
855 .SH "MATCHING A SINGLE BYTE"
856 .rs
857 .sp
858 Outside a character class, the escape sequence \eC matches any one byte, both
859 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
860 characters. The feature is provided in Perl in order to match individual bytes
861 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
862 what remains in the string may be a malformed UTF-8 string. For this reason,
863 the \eC escape sequence is best avoided.
864 .P
865 PCRE does not allow \eC to appear in lookbehind assertions
866 .\" HTML <a href="#lookbehind">
867 .\" </a>
868 (described below),
869 .\"
870 because in UTF-8 mode this would make it impossible to calculate the length of
871 the lookbehind.
872 .
873 .
874 .\" HTML <a name="characterclass"></a>
875 .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
876 .rs
877 .sp
878 An opening square bracket introduces a character class, terminated by a closing
879 square bracket. A closing square bracket on its own is not special by default.
880 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
881 bracket causes a compile-time error. If a closing square bracket is required as
882 a member of the class, it should be the first data character in the class
883 (after an initial circumflex, if present) or escaped with a backslash.
884 .P
885 A character class matches a single character in the subject. In UTF-8 mode, the
886 character may be more than one byte long. A matched character must be in the
887 set of characters defined by the class, unless the first character in the class
888 definition is a circumflex, in which case the subject character must not be in
889 the set defined by the class. If a circumflex is actually required as a member
890 of the class, ensure it is not the first character, or escape it with a
891 backslash.
892 .P
893 For example, the character class [aeiou] matches any lower case vowel, while
894 [^aeiou] matches any character that is not a lower case vowel. Note that a
895 circumflex is just a convenient notation for specifying the characters that
896 are in the class by enumerating those that are not. A class that starts with a
897 circumflex is not an assertion; it still consumes a character from the subject
898 string, and therefore it fails if the current pointer is at the end of the
899 string.
900 .P
901 In UTF-8 mode, characters with values greater than 255 can be included in a
902 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
903 .P
904 When caseless matching is set, any letters in a class represent both their
905 upper case and lower case versions, so for example, a caseless [aeiou] matches
906 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
907 caseful version would. In UTF-8 mode, PCRE always understands the concept of
908 case for characters whose values are less than 128, so caseless matching is
909 always possible. For characters with higher values, the concept of case is
910 supported if PCRE is compiled with Unicode property support, but not otherwise.
911 If you want to use caseless matching in UTF8-mode for characters 128 and above,
912 you must ensure that PCRE is compiled with Unicode property support as well as
913 with UTF-8 support.
914 .P
915 Characters that might indicate line breaks are never treated in any special way
916 when matching character classes, whatever line-ending sequence is in use, and
917 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
918 such as [^a] always matches one of these characters.
919 .P
920 The minus (hyphen) character can be used to specify a range of characters in a
921 character class. For example, [d-m] matches any letter between d and m,
922 inclusive. If a minus character is required in a class, it must be escaped with
923 a backslash or appear in a position where it cannot be interpreted as
924 indicating a range, typically as the first or last character in the class.
925 .P
926 It is not possible to have the literal character "]" as the end character of a
927 range. A pattern such as [W-]46] is interpreted as a class of two characters
928 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
929 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
930 the end of range, so [W-\e]46] is interpreted as a class containing a range
931 followed by two other characters. The octal or hexadecimal representation of
932 "]" can also be used to end a range.
933 .P
934 Ranges operate in the collating sequence of character values. They can also be
935 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
936 mode, ranges can include characters whose values are greater than 255, for
937 example [\ex{100}-\ex{2ff}].
938 .P
939 If a range that includes letters is used when caseless matching is set, it
940 matches the letters in either case. For example, [W-c] is equivalent to
941 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
942 tables for a French locale are in use, [\exc8-\excb] matches accented E
943 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
944 characters with values greater than 128 only when it is compiled with Unicode
945 property support.
946 .P
947 The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
948 in a character class, and add the characters that they match to the class. For
949 example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
950 conveniently be used with the upper case character types to specify a more
951 restricted set of characters than the matching lower case type. For example,
952 the class [^\eW_] matches any letter or digit, but not underscore.
953 .P
954 The only metacharacters that are recognized in character classes are backslash,
955 hyphen (only where it can be interpreted as specifying a range), circumflex
956 (only at the start), opening square bracket (only when it can be interpreted as
957 introducing a POSIX class name - see the next section), and the terminating
958 closing square bracket. However, escaping other non-alphanumeric characters
959 does no harm.
960 .
961 .
962 .SH "POSIX CHARACTER CLASSES"
963 .rs
964 .sp
965 Perl supports the POSIX notation for character classes. This uses names
966 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
967 this notation. For example,
968 .sp
969 [01[:alpha:]%]
970 .sp
971 matches "0", "1", any alphabetic character, or "%". The supported class names
972 are
973 .sp
974 alnum letters and digits
975 alpha letters
976 ascii character codes 0 - 127
977 blank space or tab only
978 cntrl control characters
979 digit decimal digits (same as \ed)
980 graph printing characters, excluding space
981 lower lower case letters
982 print printing characters, including space
983 punct printing characters, excluding letters and digits
984 space white space (not quite the same as \es)
985 upper upper case letters
986 word "word" characters (same as \ew)
987 xdigit hexadecimal digits
988 .sp
989 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
990 space (32). Notice that this list includes the VT character (code 11). This
991 makes "space" different to \es, which does not include VT (for Perl
992 compatibility).
993 .P
994 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
995 5.8. Another Perl extension is negation, which is indicated by a ^ character
996 after the colon. For example,
997 .sp
998 [12[:^digit:]]
999 .sp
1000 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1001 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1002 supported, and an error is given if they are encountered.
1003 .P
1004 In UTF-8 mode, characters with values greater than 128 do not match any of
1005 the POSIX character classes.
1006 .
1007 .
1008 .SH "VERTICAL BAR"
1009 .rs
1010 .sp
1011 Vertical bar characters are used to separate alternative patterns. For example,
1012 the pattern
1013 .sp
1014 gilbert|sullivan
1015 .sp
1016 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1017 and an empty alternative is permitted (matching the empty string). The matching
1018 process tries each alternative in turn, from left to right, and the first one
1019 that succeeds is used. If the alternatives are within a subpattern
1020 .\" HTML <a href="#subpattern">
1021 .\" </a>
1022 (defined below),
1023 .\"
1024 "succeeds" means matching the rest of the main pattern as well as the
1025 alternative in the subpattern.
1026 .
1027 .
1028 .SH "INTERNAL OPTION SETTING"
1029 .rs
1030 .sp
1031 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1032 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1033 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1034 The option letters are
1035 .sp
1036 i for PCRE_CASELESS
1037 m for PCRE_MULTILINE
1038 s for PCRE_DOTALL
1039 x for PCRE_EXTENDED
1040 .sp
1041 For example, (?im) sets caseless, multiline matching. It is also possible to
1042 unset these options by preceding the letter with a hyphen, and a combined
1043 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1044 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1045 permitted. If a letter appears both before and after the hyphen, the option is
1046 unset.
1047 .P
1048 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1049 changed in the same way as the Perl-compatible options by using the characters
1050 J, U and X respectively.
1051 .P
1052 When one of these option changes occurs at top level (that is, not inside
1053 subpattern parentheses), the change applies to the remainder of the pattern
1054 that follows. If the change is placed right at the start of a pattern, PCRE
1055 extracts it into the global options (and it will therefore show up in data
1056 extracted by the \fBpcre_fullinfo()\fP function).
1057 .P
1058 An option change within a subpattern (see below for a description of
1059 subpatterns) affects only that part of the current pattern that follows it, so
1060 .sp
1061 (a(?i)b)c
1062 .sp
1063 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1064 By this means, options can be made to have different settings in different
1065 parts of the pattern. Any changes made in one alternative do carry on
1066 into subsequent branches within the same subpattern. For example,
1067 .sp
1068 (a(?i)b|c)
1069 .sp
1070 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1071 branch is abandoned before the option setting. This is because the effects of
1072 option settings happen at compile time. There would be some very weird
1073 behaviour otherwise.
1074 .P
1075 \fBNote:\fP There are other PCRE-specific options that can be set by the
1076 application when the compile or match functions are called. In some cases the
1077 pattern can contain special leading sequences such as (*CRLF) to override what
1078 the application has set or what has been defaulted. Details are given in the
1079 section entitled
1080 .\" HTML <a href="#newlineseq">
1081 .\" </a>
1082 "Newline sequences"
1083 .\"
1084 above. There is also the (*UTF8) leading sequence that can be used to set UTF-8
1085 mode; this is equivalent to setting the PCRE_UTF8 option.
1086 .
1087 .
1088 .\" HTML <a name="subpattern"></a>
1089 .SH SUBPATTERNS
1090 .rs
1091 .sp
1092 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1093 Turning part of a pattern into a subpattern does two things:
1094 .sp
1095 1. It localizes a set of alternatives. For example, the pattern
1096 .sp
1097 cat(aract|erpillar|)
1098 .sp
1099 matches one of the words "cat", "cataract", or "caterpillar". Without the
1100 parentheses, it would match "cataract", "erpillar" or an empty string.
1101 .sp
1102 2. It sets up the subpattern as a capturing subpattern. This means that, when
1103 the whole pattern matches, that portion of the subject string that matched the
1104 subpattern is passed back to the caller via the \fIovector\fP argument of
1105 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1106 from 1) to obtain numbers for the capturing subpatterns.
1107 .P
1108 For example, if the string "the red king" is matched against the pattern
1109 .sp
1110 the ((red|white) (king|queen))
1111 .sp
1112 the captured substrings are "red king", "red", and "king", and are numbered 1,
1113 2, and 3, respectively.
1114 .P
1115 The fact that plain parentheses fulfil two functions is not always helpful.
1116 There are often times when a grouping subpattern is required without a
1117 capturing requirement. If an opening parenthesis is followed by a question mark
1118 and a colon, the subpattern does not do any capturing, and is not counted when
1119 computing the number of any subsequent capturing subpatterns. For example, if
1120 the string "the white queen" is matched against the pattern
1121 .sp
1122 the ((?:red|white) (king|queen))
1123 .sp
1124 the captured substrings are "white queen" and "queen", and are numbered 1 and
1125 2. The maximum number of capturing subpatterns is 65535.
1126 .P
1127 As a convenient shorthand, if any option settings are required at the start of
1128 a non-capturing subpattern, the option letters may appear between the "?" and
1129 the ":". Thus the two patterns
1130 .sp
1131 (?i:saturday|sunday)
1132 (?:(?i)saturday|sunday)
1133 .sp
1134 match exactly the same set of strings. Because alternative branches are tried
1135 from left to right, and options are not reset until the end of the subpattern
1136 is reached, an option setting in one branch does affect subsequent branches, so
1137 the above patterns match "SUNDAY" as well as "Saturday".
1138 .
1139 .
1140 .\" HTML <a name="dupsubpatternnumber"></a>
1141 .SH "DUPLICATE SUBPATTERN NUMBERS"
1142 .rs
1143 .sp
1144 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1145 the same numbers for its capturing parentheses. Such a subpattern starts with
1146 (?| and is itself a non-capturing subpattern. For example, consider this
1147 pattern:
1148 .sp
1149 (?|(Sat)ur|(Sun))day
1150 .sp
1151 Because the two alternatives are inside a (?| group, both sets of capturing
1152 parentheses are numbered one. Thus, when the pattern matches, you can look
1153 at captured substring number one, whichever alternative matched. This construct
1154 is useful when you want to capture part, but not all, of one of a number of
1155 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1156 number is reset at the start of each branch. The numbers of any capturing
1157 buffers that follow the subpattern start after the highest number used in any
1158 branch. The following example is taken from the Perl documentation.
1159 The numbers underneath show in which buffer the captured content will be
1160 stored.
1161 .sp
1162 # before ---------------branch-reset----------- after
1163 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1164 # 1 2 2 3 2 3 4
1165 .sp
1166 A back reference to a numbered subpattern uses the most recent value that is
1167 set for that number by any subpattern. The following pattern matches "abcabc"
1168 or "defdef":
1169 .sp
1170 /(?|(abc)|(def))\e1/
1171 .sp
1172 In contrast, a recursive or "subroutine" call to a numbered subpattern always
1173 refers to the first one in the pattern with the given number. The following
1174 pattern matches "abcabc" or "defabc":
1175 .sp
1176 /(?|(abc)|(def))(?1)/
1177 .sp
1178 If a
1179 .\" HTML <a href="#conditions">
1180 .\" </a>
1181 condition test
1182 .\"
1183 for a subpattern's having matched refers to a non-unique number, the test is
1184 true if any of the subpatterns of that number have matched.
1185 .P
1186 An alternative approach to using this "branch reset" feature is to use
1187 duplicate named subpatterns, as described in the next section.
1188 .
1189 .
1190 .SH "NAMED SUBPATTERNS"
1191 .rs
1192 .sp
1193 Identifying capturing parentheses by number is simple, but it can be very hard
1194 to keep track of the numbers in complicated regular expressions. Furthermore,
1195 if an expression is modified, the numbers may change. To help with this
1196 difficulty, PCRE supports the naming of subpatterns. This feature was not
1197 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1198 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1199 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1200 have different names, but PCRE does not.
1201 .P
1202 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1203 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1204 parentheses from other parts of the pattern, such as
1205 .\" HTML <a href="#backreferences">
1206 .\" </a>
1207 back references,
1208 .\"
1209 .\" HTML <a href="#recursion">
1210 .\" </a>
1211 recursion,
1212 .\"
1213 and
1214 .\" HTML <a href="#conditions">
1215 .\" </a>
1216 conditions,
1217 .\"
1218 can be made by name as well as by number.
1219 .P
1220 Names consist of up to 32 alphanumeric characters and underscores. Named
1221 capturing parentheses are still allocated numbers as well as names, exactly as
1222 if the names were not present. The PCRE API provides function calls for
1223 extracting the name-to-number translation table from a compiled pattern. There
1224 is also a convenience function for extracting a captured substring by name.
1225 .P
1226 By default, a name must be unique within a pattern, but it is possible to relax
1227 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1228 names are also always permitted for subpatterns with the same number, set up as
1229 described in the previous section.) Duplicate names can be useful for patterns
1230 where only one instance of the named parentheses can match. Suppose you want to
1231 match the name of a weekday, either as a 3-letter abbreviation or as the full
1232 name, and in both cases you want to extract the abbreviation. This pattern
1233 (ignoring the line breaks) does the job:
1234 .sp
1235 (?<DN>Mon|Fri|Sun)(?:day)?|
1236 (?<DN>Tue)(?:sday)?|
1237 (?<DN>Wed)(?:nesday)?|
1238 (?<DN>Thu)(?:rsday)?|
1239 (?<DN>Sat)(?:urday)?
1240 .sp
1241 There are five capturing substrings, but only one is ever set after a match.
1242 (An alternative way of solving this problem is to use a "branch reset"
1243 subpattern, as described in the previous section.)
1244 .P
1245 The convenience function for extracting the data by name returns the substring
1246 for the first (and in this example, the only) subpattern of that name that
1247 matched. This saves searching to find which numbered subpattern it was.
1248 .P
1249 If you make a back reference to a non-unique named subpattern from elsewhere in
1250 the pattern, the one that corresponds to the first occurrence of the name is
1251 used. In the absence of duplicate numbers (see the previous section) this is
1252 the one with the lowest number. If you use a named reference in a condition
1253 test (see the
1254 .\"
1255 .\" HTML <a href="#conditions">
1256 .\" </a>
1257 section about conditions
1258 .\"
1259 below), either to check whether a subpattern has matched, or to check for
1260 recursion, all subpatterns with the same name are tested. If the condition is
1261 true for any one of them, the overall condition is true. This is the same
1262 behaviour as testing by number. For further details of the interfaces for
1263 handling named subpatterns, see the
1264 .\" HREF
1265 \fBpcreapi\fP
1266 .\"
1267 documentation.
1268 .P
1269 \fBWarning:\fP You cannot use different names to distinguish between two
1270 subpatterns with the same number because PCRE uses only the numbers when
1271 matching. For this reason, an error is given at compile time if different names
1272 are given to subpatterns with the same number. However, you can give the same
1273 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1274 .
1275 .
1276 .SH REPETITION
1277 .rs
1278 .sp
1279 Repetition is specified by quantifiers, which can follow any of the following
1280 items:
1281 .sp
1282 a literal data character
1283 the dot metacharacter
1284 the \eC escape sequence
1285 the \eX escape sequence (in UTF-8 mode with Unicode properties)
1286 the \eR escape sequence
1287 an escape such as \ed that matches a single character
1288 a character class
1289 a back reference (see next section)
1290 a parenthesized subpattern (unless it is an assertion)
1291 a recursive or "subroutine" call to a subpattern
1292 .sp
1293 The general repetition quantifier specifies a minimum and maximum number of
1294 permitted matches, by giving the two numbers in curly brackets (braces),
1295 separated by a comma. The numbers must be less than 65536, and the first must
1296 be less than or equal to the second. For example:
1297 .sp
1298 z{2,4}
1299 .sp
1300 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1301 character. If the second number is omitted, but the comma is present, there is
1302 no upper limit; if the second number and the comma are both omitted, the
1303 quantifier specifies an exact number of required matches. Thus
1304 .sp
1305 [aeiou]{3,}
1306 .sp
1307 matches at least 3 successive vowels, but may match many more, while
1308 .sp
1309 \ed{8}
1310 .sp
1311 matches exactly 8 digits. An opening curly bracket that appears in a position
1312 where a quantifier is not allowed, or one that does not match the syntax of a
1313 quantifier, is taken as a literal character. For example, {,6} is not a
1314 quantifier, but a literal string of four characters.
1315 .P
1316 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1317 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1318 which is represented by a two-byte sequence. Similarly, when Unicode property
1319 support is available, \eX{3} matches three Unicode extended sequences, each of
1320 which may be several bytes long (and they may be of different lengths).
1321 .P
1322 The quantifier {0} is permitted, causing the expression to behave as if the
1323 previous item and the quantifier were not present. This may be useful for
1324 subpatterns that are referenced as
1325 .\" HTML <a href="#subpatternsassubroutines">
1326 .\" </a>
1327 subroutines
1328 .\"
1329 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1330 quantifier are omitted from the compiled pattern.
1331 .P
1332 For convenience, the three most common quantifiers have single-character
1333 abbreviations:
1334 .sp
1335 * is equivalent to {0,}
1336 + is equivalent to {1,}
1337 ? is equivalent to {0,1}
1338 .sp
1339 It is possible to construct infinite loops by following a subpattern that can
1340 match no characters with a quantifier that has no upper limit, for example:
1341 .sp
1342 (a?)*
1343 .sp
1344 Earlier versions of Perl and PCRE used to give an error at compile time for
1345 such patterns. However, because there are cases where this can be useful, such
1346 patterns are now accepted, but if any repetition of the subpattern does in fact
1347 match no characters, the loop is forcibly broken.
1348 .P
1349 By default, the quantifiers are "greedy", that is, they match as much as
1350 possible (up to the maximum number of permitted times), without causing the
1351 rest of the pattern to fail. The classic example of where this gives problems
1352 is in trying to match comments in C programs. These appear between /* and */
1353 and within the comment, individual * and / characters may appear. An attempt to
1354 match C comments by applying the pattern
1355 .sp
1356 /\e*.*\e*/
1357 .sp
1358 to the string
1359 .sp
1360 /* first comment */ not comment /* second comment */
1361 .sp
1362 fails, because it matches the entire string owing to the greediness of the .*
1363 item.
1364 .P
1365 However, if a quantifier is followed by a question mark, it ceases to be
1366 greedy, and instead matches the minimum number of times possible, so the
1367 pattern
1368 .sp
1369 /\e*.*?\e*/
1370 .sp
1371 does the right thing with the C comments. The meaning of the various
1372 quantifiers is not otherwise changed, just the preferred number of matches.
1373 Do not confuse this use of question mark with its use as a quantifier in its
1374 own right. Because it has two uses, it can sometimes appear doubled, as in
1375 .sp
1376 \ed??\ed
1377 .sp
1378 which matches one digit by preference, but can match two if that is the only
1379 way the rest of the pattern matches.
1380 .P
1381 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1382 the quantifiers are not greedy by default, but individual ones can be made
1383 greedy by following them with a question mark. In other words, it inverts the
1384 default behaviour.
1385 .P
1386 When a parenthesized subpattern is quantified with a minimum repeat count that
1387 is greater than 1 or with a limited maximum, more memory is required for the
1388 compiled pattern, in proportion to the size of the minimum or maximum.
1389 .P
1390 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1391 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1392 implicitly anchored, because whatever follows will be tried against every
1393 character position in the subject string, so there is no point in retrying the
1394 overall match at any position after the first. PCRE normally treats such a
1395 pattern as though it were preceded by \eA.
1396 .P
1397 In cases where it is known that the subject string contains no newlines, it is
1398 worth setting PCRE_DOTALL in order to obtain this optimization, or
1399 alternatively using ^ to indicate anchoring explicitly.
1400 .P
1401 However, there is one situation where the optimization cannot be used. When .*
1402 is inside capturing parentheses that are the subject of a back reference
1403 elsewhere in the pattern, a match at the start may fail where a later one
1404 succeeds. Consider, for example:
1405 .sp
1406 (.*)abc\e1
1407 .sp
1408 If the subject is "xyz123abc123" the match point is the fourth character. For
1409 this reason, such a pattern is not implicitly anchored.
1410 .P
1411 When a capturing subpattern is repeated, the value captured is the substring
1412 that matched the final iteration. For example, after
1413 .sp
1414 (tweedle[dume]{3}\es*)+
1415 .sp
1416 has matched "tweedledum tweedledee" the value of the captured substring is
1417 "tweedledee". However, if there are nested capturing subpatterns, the
1418 corresponding captured values may have been set in previous iterations. For
1419 example, after
1420 .sp
1421 /(a|(b))+/
1422 .sp
1423 matches "aba" the value of the second captured substring is "b".
1424 .
1425 .
1426 .\" HTML <a name="atomicgroup"></a>
1427 .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1428 .rs
1429 .sp
1430 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1431 repetition, failure of what follows normally causes the repeated item to be
1432 re-evaluated to see if a different number of repeats allows the rest of the
1433 pattern to match. Sometimes it is useful to prevent this, either to change the
1434 nature of the match, or to cause it fail earlier than it otherwise might, when
1435 the author of the pattern knows there is no point in carrying on.
1436 .P
1437 Consider, for example, the pattern \ed+foo when applied to the subject line
1438 .sp
1439 123456bar
1440 .sp
1441 After matching all 6 digits and then failing to match "foo", the normal
1442 action of the matcher is to try again with only 5 digits matching the \ed+
1443 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1444 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1445 that once a subpattern has matched, it is not to be re-evaluated in this way.
1446 .P
1447 If we use atomic grouping for the previous example, the matcher gives up
1448 immediately on failing to match "foo" the first time. The notation is a kind of
1449 special parenthesis, starting with (?> as in this example:
1450 .sp
1451 (?>\ed+)foo
1452 .sp
1453 This kind of parenthesis "locks up" the part of the pattern it contains once
1454 it has matched, and a failure further into the pattern is prevented from
1455 backtracking into it. Backtracking past it to previous items, however, works as
1456 normal.
1457 .P
1458 An alternative description is that a subpattern of this type matches the string
1459 of characters that an identical standalone pattern would match, if anchored at
1460 the current point in the subject string.
1461 .P
1462 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1463 the above example can be thought of as a maximizing repeat that must swallow
1464 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1465 number of digits they match in order to make the rest of the pattern match,
1466 (?>\ed+) can only match an entire sequence of digits.
1467 .P
1468 Atomic groups in general can of course contain arbitrarily complicated
1469 subpatterns, and can be nested. However, when the subpattern for an atomic
1470 group is just a single repeated item, as in the example above, a simpler
1471 notation, called a "possessive quantifier" can be used. This consists of an
1472 additional + character following a quantifier. Using this notation, the
1473 previous example can be rewritten as
1474 .sp
1475 \ed++foo
1476 .sp
1477 Note that a possessive quantifier can be used with an entire group, for
1478 example:
1479 .sp
1480 (abc|xyz){2,3}+
1481 .sp
1482 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1483 option is ignored. They are a convenient notation for the simpler forms of
1484 atomic group. However, there is no difference in the meaning of a possessive
1485 quantifier and the equivalent atomic group, though there may be a performance
1486 difference; possessive quantifiers should be slightly faster.
1487 .P
1488 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1489 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1490 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1491 package, and PCRE copied it from there. It ultimately found its way into Perl
1492 at release 5.10.
1493 .P
1494 PCRE has an optimization that automatically "possessifies" certain simple
1495 pattern constructs. For example, the sequence A+B is treated as A++B because
1496 there is no point in backtracking into a sequence of A's when B must follow.
1497 .P
1498 When a pattern contains an unlimited repeat inside a subpattern that can itself
1499 be repeated an unlimited number of times, the use of an atomic group is the
1500 only way to avoid some failing matches taking a very long time indeed. The
1501 pattern
1502 .sp
1503 (\eD+|<\ed+>)*[!?]
1504 .sp
1505 matches an unlimited number of substrings that either consist of non-digits, or
1506 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1507 quickly. However, if it is applied to
1508 .sp
1509 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1510 .sp
1511 it takes a long time before reporting failure. This is because the string can
1512 be divided between the internal \eD+ repeat and the external * repeat in a
1513 large number of ways, and all have to be tried. (The example uses [!?] rather
1514 than a single character at the end, because both PCRE and Perl have an
1515 optimization that allows for fast failure when a single character is used. They
1516 remember the last single character that is required for a match, and fail early
1517 if it is not present in the string.) If the pattern is changed so that it uses
1518 an atomic group, like this:
1519 .sp
1520 ((?>\eD+)|<\ed+>)*[!?]
1521 .sp
1522 sequences of non-digits cannot be broken, and failure happens quickly.
1523 .
1524 .
1525 .\" HTML <a name="backreferences"></a>
1526 .SH "BACK REFERENCES"
1527 .rs
1528 .sp
1529 Outside a character class, a backslash followed by a digit greater than 0 (and
1530 possibly further digits) is a back reference to a capturing subpattern earlier
1531 (that is, to its left) in the pattern, provided there have been that many
1532 previous capturing left parentheses.
1533 .P
1534 However, if the decimal number following the backslash is less than 10, it is
1535 always taken as a back reference, and causes an error only if there are not
1536 that many capturing left parentheses in the entire pattern. In other words, the
1537 parentheses that are referenced need not be to the left of the reference for
1538 numbers less than 10. A "forward back reference" of this type can make sense
1539 when a repetition is involved and the subpattern to the right has participated
1540 in an earlier iteration.
1541 .P
1542 It is not possible to have a numerical "forward back reference" to a subpattern
1543 whose number is 10 or more using this syntax because a sequence such as \e50 is
1544 interpreted as a character defined in octal. See the subsection entitled
1545 "Non-printing characters"
1546 .\" HTML <a href="#digitsafterbackslash">
1547 .\" </a>
1548 above
1549 .\"
1550 for further details of the handling of digits following a backslash. There is
1551 no such problem when named parentheses are used. A back reference to any
1552 subpattern is possible using named parentheses (see below).
1553 .P
1554 Another way of avoiding the ambiguity inherent in the use of digits following a
1555 backslash is to use the \eg escape sequence, which is a feature introduced in
1556 Perl 5.10. This escape must be followed by an unsigned number or a negative
1557 number, optionally enclosed in braces. These examples are all identical:
1558 .sp
1559 (ring), \e1
1560 (ring), \eg1
1561 (ring), \eg{1}
1562 .sp
1563 An unsigned number specifies an absolute reference without the ambiguity that
1564 is present in the older syntax. It is also useful when literal digits follow
1565 the reference. A negative number is a relative reference. Consider this
1566 example:
1567 .sp
1568 (abc(def)ghi)\eg{-1}
1569 .sp
1570 The sequence \eg{-1} is a reference to the most recently started capturing
1571 subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1572 would be equivalent to \e1. The use of relative references can be helpful in
1573 long patterns, and also in patterns that are created by joining together
1574 fragments that contain references within themselves.
1575 .P
1576 A back reference matches whatever actually matched the capturing subpattern in
1577 the current subject string, rather than anything matching the subpattern
1578 itself (see
1579 .\" HTML <a href="#subpatternsassubroutines">
1580 .\" </a>
1581 "Subpatterns as subroutines"
1582 .\"
1583 below for a way of doing that). So the pattern
1584 .sp
1585 (sens|respons)e and \e1ibility
1586 .sp
1587 matches "sense and sensibility" and "response and responsibility", but not
1588 "sense and responsibility". If caseful matching is in force at the time of the
1589 back reference, the case of letters is relevant. For example,
1590 .sp
1591 ((?i)rah)\es+\e1
1592 .sp
1593 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1594 capturing subpattern is matched caselessly.
1595 .P
1596 There are several different ways of writing back references to named
1597 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1598 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1599 back reference syntax, in which \eg can be used for both numeric and named
1600 references, is also supported. We could rewrite the above example in any of
1601 the following ways:
1602 .sp
1603 (?<p1>(?i)rah)\es+\ek<p1>
1604 (?'p1'(?i)rah)\es+\ek{p1}
1605 (?P<p1>(?i)rah)\es+(?P=p1)
1606 (?<p1>(?i)rah)\es+\eg{p1}
1607 .sp
1608 A subpattern that is referenced by name may appear in the pattern before or
1609 after the reference.
1610 .P
1611 There may be more than one back reference to the same subpattern. If a
1612 subpattern has not actually been used in a particular match, any back
1613 references to it always fail by default. For example, the pattern
1614 .sp
1615 (a|(bc))\e2
1616 .sp
1617 always fails if it starts to match "a" rather than "bc". However, if the
1618 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1619 unset value matches an empty string.
1620 .P
1621 Because there may be many capturing parentheses in a pattern, all digits
1622 following a backslash are taken as part of a potential back reference number.
1623 If the pattern continues with a digit character, some delimiter must be used to
1624 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1625 whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1626 .\" HTML <a href="#comments">
1627 .\" </a>
1628 "Comments"
1629 .\"
1630 below) can be used.
1631 .
1632 .SS "Recursive back references"
1633 .rs
1634 .sp
1635 A back reference that occurs inside the parentheses to which it refers fails
1636 when the subpattern is first used, so, for example, (a\e1) never matches.
1637 However, such references can be useful inside repeated subpatterns. For
1638 example, the pattern
1639 .sp
1640 (a|b\e1)+
1641 .sp
1642 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1643 the subpattern, the back reference matches the character string corresponding
1644 to the previous iteration. In order for this to work, the pattern must be such
1645 that the first iteration does not need to match the back reference. This can be
1646 done using alternation, as in the example above, or by a quantifier with a
1647 minimum of zero.
1648 .P
1649 Back references of this type cause the group that they reference to be treated
1650 as an
1651 .\" HTML <a href="#atomicgroup">
1652 .\" </a>
1653 atomic group.
1654 .\"
1655 Once the whole group has been matched, a subsequent matching failure cannot
1656 cause backtracking into the middle of the group.
1657 .
1658 .
1659 .\" HTML <a name="bigassertions"></a>
1660 .SH ASSERTIONS
1661 .rs
1662 .sp
1663 An assertion is a test on the characters following or preceding the current
1664 matching point that does not actually consume any characters. The simple
1665 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1666 .\" HTML <a href="#smallassertions">
1667 .\" </a>
1668 above.
1669 .\"
1670 .P
1671 More complicated assertions are coded as subpatterns. There are two kinds:
1672 those that look ahead of the current position in the subject string, and those
1673 that look behind it. An assertion subpattern is matched in the normal way,
1674 except that it does not cause the current matching position to be changed.
1675 .P
1676 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1677 because it makes no sense to assert the same thing several times. If any kind
1678 of assertion contains capturing subpatterns within it, these are counted for
1679 the purposes of numbering the capturing subpatterns in the whole pattern.
1680 However, substring capturing is carried out only for positive assertions,
1681 because it does not make sense for negative assertions.
1682 .
1683 .
1684 .SS "Lookahead assertions"
1685 .rs
1686 .sp
1687 Lookahead assertions start with (?= for positive assertions and (?! for
1688 negative assertions. For example,
1689 .sp
1690 \ew+(?=;)
1691 .sp
1692 matches a word followed by a semicolon, but does not include the semicolon in
1693 the match, and
1694 .sp
1695 foo(?!bar)
1696 .sp
1697 matches any occurrence of "foo" that is not followed by "bar". Note that the
1698 apparently similar pattern
1699 .sp
1700 (?!foo)bar
1701 .sp
1702 does not find an occurrence of "bar" that is preceded by something other than
1703 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1704 (?!foo) is always true when the next three characters are "bar". A
1705 lookbehind assertion is needed to achieve the other effect.
1706 .P
1707 If you want to force a matching failure at some point in a pattern, the most
1708 convenient way to do it is with (?!) because an empty string always matches, so
1709 an assertion that requires there not to be an empty string must always fail.
1710 The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1711 synonym for (?!).
1712 .
1713 .
1714 .\" HTML <a name="lookbehind"></a>
1715 .SS "Lookbehind assertions"
1716 .rs
1717 .sp
1718 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1719 negative assertions. For example,
1720 .sp
1721 (?<!foo)bar
1722 .sp
1723 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1724 a lookbehind assertion are restricted such that all the strings it matches must
1725 have a fixed length. However, if there are several top-level alternatives, they
1726 do not all have to have the same fixed length. Thus
1727 .sp
1728 (?<=bullock|donkey)
1729 .sp
1730 is permitted, but
1731 .sp
1732 (?<!dogs?|cats?)
1733 .sp
1734 causes an error at compile time. Branches that match different length strings
1735 are permitted only at the top level of a lookbehind assertion. This is an
1736 extension compared with Perl (5.8 and 5.10), which requires all branches to
1737 match the same length of string. An assertion such as
1738 .sp
1739 (?<=ab(c|de))
1740 .sp
1741 is not permitted, because its single top-level branch can match two different
1742 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1743 branches:
1744 .sp
1745 (?<=abc|abde)
1746 .sp
1747 In some cases, the Perl 5.10 escape sequence \eK
1748 .\" HTML <a href="#resetmatchstart">
1749 .\" </a>
1750 (see above)
1751 .\"
1752 can be used instead of a lookbehind assertion to get round the fixed-length
1753 restriction.
1754 .P
1755 The implementation of lookbehind assertions is, for each alternative, to
1756 temporarily move the current position back by the fixed length and then try to
1757 match. If there are insufficient characters before the current position, the
1758 assertion fails.
1759 .P
1760 PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1761 to appear in lookbehind assertions, because it makes it impossible to calculate
1762 the length of the lookbehind. The \eX and \eR escapes, which can match
1763 different numbers of bytes, are also not permitted.
1764 .P
1765 .\" HTML <a href="#subpatternsassubroutines">
1766 .\" </a>
1767 "Subroutine"
1768 .\"
1769 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1770 as the subpattern matches a fixed-length string.
1771 .\" HTML <a href="#recursion">
1772 .\" </a>
1773 Recursion,
1774 .\"
1775 however, is not supported.
1776 .P
1777 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1778 specify efficient matching of fixed-length strings at the end of subject
1779 strings. Consider a simple pattern such as
1780 .sp
1781 abcd$
1782 .sp
1783 when applied to a long string that does not match. Because matching proceeds
1784 from left to right, PCRE will look for each "a" in the subject and then see if
1785 what follows matches the rest of the pattern. If the pattern is specified as
1786 .sp
1787 ^.*abcd$
1788 .sp
1789 the initial .* matches the entire string at first, but when this fails (because
1790 there is no following "a"), it backtracks to match all but the last character,
1791 then all but the last two characters, and so on. Once again the search for "a"
1792 covers the entire string, from right to left, so we are no better off. However,
1793 if the pattern is written as
1794 .sp
1795 ^.*+(?<=abcd)
1796 .sp
1797 there can be no backtracking for the .*+ item; it can match only the entire
1798 string. The subsequent lookbehind assertion does a single test on the last four
1799 characters. If it fails, the match fails immediately. For long strings, this
1800 approach makes a significant difference to the processing time.
1801 .
1802 .
1803 .SS "Using multiple assertions"
1804 .rs
1805 .sp
1806 Several assertions (of any sort) may occur in succession. For example,
1807 .sp
1808 (?<=\ed{3})(?<!999)foo
1809 .sp
1810 matches "foo" preceded by three digits that are not "999". Notice that each of
1811 the assertions is applied independently at the same point in the subject
1812 string. First there is a check that the previous three characters are all
1813 digits, and then there is a check that the same three characters are not "999".
1814 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1815 of which are digits and the last three of which are not "999". For example, it
1816 doesn't match "123abcfoo". A pattern to do that is
1817 .sp
1818 (?<=\ed{3}...)(?<!999)foo
1819 .sp
1820 This time the first assertion looks at the preceding six characters, checking
1821 that the first three are digits, and then the second assertion checks that the
1822 preceding three characters are not "999".
1823 .P
1824 Assertions can be nested in any combination. For example,
1825 .sp
1826 (?<=(?<!foo)bar)baz
1827 .sp
1828 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1829 preceded by "foo", while
1830 .sp
1831 (?<=\ed{3}(?!999)...)foo
1832 .sp
1833 is another pattern that matches "foo" preceded by three digits and any three
1834 characters that are not "999".
1835 .
1836 .
1837 .\" HTML <a name="conditions"></a>
1838 .SH "CONDITIONAL SUBPATTERNS"
1839 .rs
1840 .sp
1841 It is possible to cause the matching process to obey a subpattern
1842 conditionally or to choose between two alternative subpatterns, depending on
1843 the result of an assertion, or whether a specific capturing subpattern has
1844 already been matched. The two possible forms of conditional subpattern are:
1845 .sp
1846 (?(condition)yes-pattern)
1847 (?(condition)yes-pattern|no-pattern)
1848 .sp
1849 If the condition is satisfied, the yes-pattern is used; otherwise the
1850 no-pattern (if present) is used. If there are more than two alternatives in the
1851 subpattern, a compile-time error occurs.
1852 .P
1853 There are four kinds of condition: references to subpatterns, references to
1854 recursion, a pseudo-condition called DEFINE, and assertions.
1855 .
1856 .SS "Checking for a used subpattern by number"
1857 .rs
1858 .sp
1859 If the text between the parentheses consists of a sequence of digits, the
1860 condition is true if a capturing subpattern of that number has previously
1861 matched. If there is more than one capturing subpattern with the same number
1862 (see the earlier
1863 .\"
1864 .\" HTML <a href="#recursion">
1865 .\" </a>
1866 section about duplicate subpattern numbers),
1867 .\"
1868 the condition is true if any of them have been set. An alternative notation is
1869 to precede the digits with a plus or minus sign. In this case, the subpattern
1870 number is relative rather than absolute. The most recently opened parentheses
1871 can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1872 looping constructs it can also make sense to refer to subsequent groups with
1873 constructs such as (?(+2).
1874 .P
1875 Consider the following pattern, which contains non-significant white space to
1876 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1877 three parts for ease of discussion:
1878 .sp
1879 ( \e( )? [^()]+ (?(1) \e) )
1880 .sp
1881 The first part matches an optional opening parenthesis, and if that
1882 character is present, sets it as the first captured substring. The second part
1883 matches one or more characters that are not parentheses. The third part is a
1884 conditional subpattern that tests whether the first set of parentheses matched
1885 or not. If they did, that is, if subject started with an opening parenthesis,
1886 the condition is true, and so the yes-pattern is executed and a closing
1887 parenthesis is required. Otherwise, since no-pattern is not present, the
1888 subpattern matches nothing. In other words, this pattern matches a sequence of
1889 non-parentheses, optionally enclosed in parentheses.
1890 .P
1891 If you were embedding this pattern in a larger one, you could use a relative
1892 reference:
1893 .sp
1894 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1895 .sp
1896 This makes the fragment independent of the parentheses in the larger pattern.
1897 .
1898 .SS "Checking for a used subpattern by name"
1899 .rs
1900 .sp
1901 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1902 subpattern by name. For compatibility with earlier versions of PCRE, which had
1903 this facility before Perl, the syntax (?(name)...) is also recognized. However,
1904 there is a possible ambiguity with this syntax, because subpattern names may
1905 consist entirely of digits. PCRE looks first for a named subpattern; if it
1906 cannot find one and the name consists entirely of digits, PCRE looks for a
1907 subpattern of that number, which must be greater than zero. Using subpattern
1908 names that consist entirely of digits is not recommended.
1909 .P
1910 Rewriting the above example to use a named subpattern gives this:
1911 .sp
1912 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1913 .sp
1914 If the name used in a condition of this kind is a duplicate, the test is
1915 applied to all subpatterns of the same name, and is true if any one of them has
1916 matched.
1917 .
1918 .SS "Checking for pattern recursion"
1919 .rs
1920 .sp
1921 If the condition is the string (R), and there is no subpattern with the name R,
1922 the condition is true if a recursive call to the whole pattern or any
1923 subpattern has been made. If digits or a name preceded by ampersand follow the
1924 letter R, for example:
1925 .sp
1926 (?(R3)...) or (?(R&name)...)
1927 .sp
1928 the condition is true if the most recent recursion is into a subpattern whose
1929 number or name is given. This condition does not check the entire recursion
1930 stack. If the name used in a condition of this kind is a duplicate, the test is
1931 applied to all subpatterns of the same name, and is true if any one of them is
1932 the most recent recursion.
1933 .P
1934 At "top level", all these recursion test conditions are false.
1935 .\" HTML <a href="#recursion">
1936 .\" </a>
1937 The syntax for recursive patterns
1938 .\"
1939 is described below.
1940 .
1941 .SS "Defining subpatterns for use by reference only"
1942 .rs
1943 .sp
1944 If the condition is the string (DEFINE), and there is no subpattern with the
1945 name DEFINE, the condition is always false. In this case, there may be only one
1946 alternative in the subpattern. It is always skipped if control reaches this
1947 point in the pattern; the idea of DEFINE is that it can be used to define
1948 "subroutines" that can be referenced from elsewhere. (The use of
1949 .\" HTML <a href="#subpatternsassubroutines">
1950 .\" </a>
1951 "subroutines"
1952 .\"
1953 is described below.) For example, a pattern to match an IPv4 address could be
1954 written like this (ignore whitespace and line breaks):
1955 .sp
1956 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1957 \eb (?&byte) (\e.(?&byte)){3} \eb
1958 .sp
1959 The first part of the pattern is a DEFINE group inside which a another group
1960 named "byte" is defined. This matches an individual component of an IPv4
1961 address (a number less than 256). When matching takes place, this part of the
1962 pattern is skipped because DEFINE acts like a false condition. The rest of the
1963 pattern uses references to the named group to match the four dot-separated
1964 components of an IPv4 address, insisting on a word boundary at each end.
1965 .
1966 .SS "Assertion conditions"
1967 .rs
1968 .sp
1969 If the condition is not in any of the above formats, it must be an assertion.
1970 This may be a positive or negative lookahead or lookbehind assertion. Consider
1971 this pattern, again containing non-significant white space, and with the two
1972 alternatives on the second line:
1973 .sp
1974 (?(?=[^a-z]*[a-z])
1975 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1976 .sp
1977 The condition is a positive lookahead assertion that matches an optional
1978 sequence of non-letters followed by a letter. In other words, it tests for the
1979 presence of at least one letter in the subject. If a letter is found, the
1980 subject is matched against the first alternative; otherwise it is matched
1981 against the second. This pattern matches strings in one of the two forms
1982 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1983 .
1984 .
1985 .\" HTML <a name="comments"></a>
1986 .SH COMMENTS
1987 .rs
1988 .sp
1989 The sequence (?# marks the start of a comment that continues up to the next
1990 closing parenthesis. Nested parentheses are not permitted. The characters
1991 that make up a comment play no part in the pattern matching at all.
1992 .P
1993 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1994 character class introduces a comment that continues to immediately after the
1995 next newline in the pattern.
1996 .
1997 .
1998 .\" HTML <a name="recursion"></a>
1999 .SH "RECURSIVE PATTERNS"
2000 .rs
2001 .sp
2002 Consider the problem of matching a string in parentheses, allowing for
2003 unlimited nested parentheses. Without the use of recursion, the best that can
2004 be done is to use a pattern that matches up to some fixed depth of nesting. It
2005 is not possible to handle an arbitrary nesting depth.
2006 .P
2007 For some time, Perl has provided a facility that allows regular expressions to
2008 recurse (amongst other things). It does this by interpolating Perl code in the
2009 expression at run time, and the code can refer to the expression itself. A Perl
2010 pattern using code interpolation to solve the parentheses problem can be
2011 created like this:
2012 .sp
2013 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2014 .sp
2015 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2016 recursively to the pattern in which it appears.
2017 .P
2018 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2019 supports special syntax for recursion of the entire pattern, and also for
2020 individual subpattern recursion. After its introduction in PCRE and Python,
2021 this kind of recursion was subsequently introduced into Perl at release 5.10.
2022 .P
2023 A special item that consists of (? followed by a number greater than zero and a
2024 closing parenthesis is a recursive call of the subpattern of the given number,
2025 provided that it occurs inside that subpattern. (If not, it is a
2026 .\" HTML <a href="#subpatternsassubroutines">
2027 .\" </a>
2028 "subroutine"
2029 .\"
2030 call, which is described in the next section.) The special item (?R) or (?0) is
2031 a recursive call of the entire regular expression.
2032 .P
2033 This PCRE pattern solves the nested parentheses problem (assume the
2034 PCRE_EXTENDED option is set so that white space is ignored):
2035 .sp
2036 \e( ( [^()]++ | (?R) )* \e)
2037 .sp
2038 First it matches an opening parenthesis. Then it matches any number of
2039 substrings which can either be a sequence of non-parentheses, or a recursive
2040 match of the pattern itself (that is, a correctly parenthesized substring).
2041 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2042 to avoid backtracking into sequences of non-parentheses.
2043 .P
2044 If this were part of a larger pattern, you would not want to recurse the entire
2045 pattern, so instead you could use this:
2046 .sp
2047 ( \e( ( [^()]++ | (?1) )* \e) )
2048 .sp
2049 We have put the pattern into parentheses, and caused the recursion to refer to
2050 them instead of the whole pattern.
2051 .P
2052 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2053 is made easier by the use of relative references (a Perl 5.10 feature).
2054 Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2055 most recently opened parentheses preceding the recursion. In other words, a
2056 negative number counts capturing parentheses leftwards from the point at which
2057 it is encountered.
2058 .P
2059 It is also possible to refer to subsequently opened parentheses, by writing
2060 references such as (?+2). However, these cannot be recursive because the
2061 reference is not inside the parentheses that are referenced. They are always
2062 .\" HTML <a href="#subpatternsassubroutines">
2063 .\" </a>
2064 "subroutine"
2065 .\"
2066 calls, as described in the next section.
2067 .P
2068 An alternative approach is to use named parentheses instead. The Perl syntax
2069 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2070 could rewrite the above example as follows:
2071 .sp
2072 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2073 .sp
2074 If there is more than one subpattern with the same name, the earliest one is
2075 used.
2076 .P
2077 This particular example pattern that we have been looking at contains nested
2078 unlimited repeats, and so the use of a possessive quantifier for matching
2079 strings of non-parentheses is important when applying the pattern to strings
2080 that do not match. For example, when this pattern is applied to
2081 .sp
2082 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2083 .sp
2084 it yields "no match" quickly. However, if a possessive quantifier is not used,
2085 the match runs for a very long time indeed because there are so many different
2086 ways the + and * repeats can carve up the subject, and all have to be tested
2087 before failure can be reported.
2088 .P
2089 At the end of a match, the values of capturing parentheses are those from
2090 the outermost level. If you want to obtain intermediate values, a callout
2091 function can be used (see below and the
2092 .\" HREF
2093 \fBpcrecallout\fP
2094 .\"
2095 documentation). If the pattern above is matched against
2096 .sp
2097 (ab(cd)ef)
2098 .sp
2099 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2100 the last value taken on at the top level. If a capturing subpattern is not
2101 matched at the top level, its final value is unset, even if it is (temporarily)
2102 set at a deeper level.
2103 .P
2104 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2105 obtain extra memory to store data during a recursion, which it does by using
2106 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2107 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2108 .P
2109 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2110 Consider this pattern, which matches text in angle brackets, allowing for
2111 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2112 recursing), whereas any characters are permitted at the outer level.
2113 .sp
2114 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2115 .sp
2116 In this pattern, (?(R) is the start of a conditional subpattern, with two
2117 different alternatives for the recursive and non-recursive cases. The (?R) item
2118 is the actual recursive call.
2119 .
2120 .
2121 .\" HTML <a name="recursiondifference"></a>
2122 .SS "Recursion difference from Perl"
2123 .rs
2124 .sp
2125 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2126 treated as an atomic group. That is, once it has matched some of the subject
2127 string, it is never re-entered, even if it contains untried alternatives and
2128 there is a subsequent matching failure. This can be illustrated by the
2129 following pattern, which purports to match a palindromic string that contains
2130 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2131 .sp
2132 ^(.|(.)(?1)\e2)$
2133 .sp
2134 The idea is that it either matches a single character, or two identical
2135 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2136 it does not if the pattern is longer than three characters. Consider the
2137 subject string "abcba":
2138 .P
2139 At the top level, the first character is matched, but as it is not at the end
2140 of the string, the first alternative fails; the second alternative is taken
2141 and the recursion kicks in. The recursive call to subpattern 1 successfully
2142 matches the next character ("b"). (Note that the beginning and end of line
2143 tests are not part of the recursion).
2144 .P
2145 Back at the top level, the next character ("c") is compared with what
2146 subpattern 2 matched, which was "a". This fails. Because the recursion is
2147 treated as an atomic group, there are now no backtracking points, and so the
2148 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2149 try the second alternative.) However, if the pattern is written with the
2150 alternatives in the other order, things are different:
2151 .sp
2152 ^((.)(?1)\e2|.)$
2153 .sp
2154 This time, the recursing alternative is tried first, and continues to recurse
2155 until it runs out of characters, at which point the recursion fails. But this
2156 time we do have another alternative to try at the higher level. That is the big
2157 difference: in the previous case the remaining alternative is at a deeper
2158 recursion level, which PCRE cannot use.
2159 .P
2160 To change the pattern so that matches all palindromic strings, not just those
2161 with an odd number of characters, it is tempting to change the pattern to this:
2162 .sp
2163 ^((.)(?1)\e2|.?)$
2164 .sp
2165 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2166 deeper recursion has matched a single character, it cannot be entered again in
2167 order to match an empty string. The solution is to separate the two cases, and
2168 write out the odd and even cases as alternatives at the higher level:
2169 .sp
2170 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2171 .sp
2172 If you want to match typical palindromic phrases, the pattern has to ignore all
2173 non-word characters, which can be done like this:
2174 .sp
2175 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2176 .sp
2177 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2178 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2179 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2180 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2181 more) to match typical phrases, and Perl takes so long that you think it has
2182 gone into a loop.
2183 .P
2184 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2185 string does not start with a palindrome that is shorter than the entire string.
2186 For example, although "abcba" is correctly matched, if the subject is "ababa",
2187 PCRE finds the palindrome "aba" at the start, then fails at top level because
2188 the end of the string does not follow. Once again, it cannot jump back into the
2189 recursion to try other alternatives, so the entire match fails.
2190 .
2191 .
2192 .\" HTML <a name="subpatternsassubroutines"></a>
2193 .SH "SUBPATTERNS AS SUBROUTINES"
2194 .rs
2195 .sp
2196 If the syntax for a recursive subpattern reference (either by number or by
2197 name) is used outside the parentheses to which it refers, it operates like a
2198 subroutine in a programming language. The "called" subpattern may be defined
2199 before or after the reference. A numbered reference can be absolute or
2200 relative, as in these examples:
2201 .sp
2202 (...(absolute)...)...(?2)...
2203 (...(relative)...)...(?-1)...
2204 (...(?+1)...(relative)...
2205 .sp
2206 An earlier example pointed out that the pattern
2207 .sp
2208 (sens|respons)e and \e1ibility
2209 .sp
2210 matches "sense and sensibility" and "response and responsibility", but not
2211 "sense and responsibility". If instead the pattern
2212 .sp
2213 (sens|respons)e and (?1)ibility
2214 .sp
2215 is used, it does match "sense and responsibility" as well as the other two
2216 strings. Another example is given in the discussion of DEFINE above.
2217 .P
2218 Like recursive subpatterns, a subroutine call is always treated as an atomic
2219 group. That is, once it has matched some of the subject string, it is never
2220 re-entered, even if it contains untried alternatives and there is a subsequent
2221 matching failure. Any capturing parentheses that are set during the subroutine
2222 call revert to their previous values afterwards.
2223 .P
2224 When a subpattern is used as a subroutine, processing options such as
2225 case-independence are fixed when the subpattern is defined. They cannot be
2226 changed for different calls. For example, consider this pattern:
2227 .sp
2228 (abc)(?i:(?-1))
2229 .sp
2230 It matches "abcabc". It does not match "abcABC" because the change of
2231 processing option does not affect the called subpattern.
2232 .
2233 .
2234 .\" HTML <a name="onigurumasubroutines"></a>
2235 .SH "ONIGURUMA SUBROUTINE SYNTAX"
2236 .rs
2237 .sp
2238 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2239 a number enclosed either in angle brackets or single quotes, is an alternative
2240 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2241 are two of the examples used above, rewritten using this syntax:
2242 .sp
2243 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2244 (sens|respons)e and \eg'1'ibility
2245 .sp
2246 PCRE supports an extension to Oniguruma: if a number is preceded by a
2247 plus or a minus sign it is taken as a relative reference. For example:
2248 .sp
2249 (abc)(?i:\eg<-1>)
2250 .sp
2251 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2252 synonymous. The former is a back reference; the latter is a subroutine call.
2253 .
2254 .
2255 .SH CALLOUTS
2256 .rs
2257 .sp
2258 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2259 code to be obeyed in the middle of matching a regular expression. This makes it
2260 possible, amongst other things, to extract different substrings that match the
2261 same pair of parentheses when there is a repetition.
2262 .P
2263 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2264 code. The feature is called "callout". The caller of PCRE provides an external
2265 function by putting its entry point in the global variable \fIpcre_callout\fP.
2266 By default, this variable contains NULL, which disables all calling out.
2267 .P
2268 Within a regular expression, (?C) indicates the points at which the external
2269 function is to be called. If you want to identify different callout points, you
2270 can put a number less than 256 after the letter C. The default value is zero.
2271 For example, this pattern has two callout points:
2272 .sp
2273 (?C1)abc(?C2)def
2274 .sp
2275 If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2276 automatically installed before each item in the pattern. They are all numbered
2277 255.
2278 .P
2279 During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2280 set), the external function is called. It is provided with the number of the
2281 callout, the position in the pattern, and, optionally, one item of data
2282 originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2283 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2284 description of the interface to the callout function is given in the
2285 .\" HREF
2286 \fBpcrecallout\fP
2287 .\"
2288 documentation.
2289 .
2290 .
2291 .SH "BACKTRACKING CONTROL"
2292 .rs
2293 .sp
2294 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2295 are described in the Perl documentation as "experimental and subject to change
2296 or removal in a future version of Perl". It goes on to say: "Their usage in
2297 production code should be noted to avoid problems during upgrades." The same
2298 remarks apply to the PCRE features described in this section.
2299 .P
2300 Since these verbs are specifically related to backtracking, most of them can be
2301 used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2302 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2303 failing negative assertion, they cause an error if encountered by
2304 \fBpcre_dfa_exec()\fP.
2305 .P
2306 If any of these verbs are used in an assertion or subroutine subpattern
2307 (including recursive subpatterns), their effect is confined to that subpattern;
2308 it does not extend to the surrounding pattern. Note that such subpatterns are
2309 processed as anchored at the point where they are tested.
2310 .P
2311 The new verbs make use of what was previously invalid syntax: an opening
2312 parenthesis followed by an asterisk. In Perl, they are generally of the form
2313 (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2314 form is just (*VERB). Any number of these verbs may occur in a pattern. There
2315 are two kinds:
2316 .
2317 .SS "Verbs that act immediately"
2318 .rs
2319 .sp
2320 The following verbs act as soon as they are encountered:
2321 .sp
2322 (*ACCEPT)
2323 .sp
2324 This verb causes the match to end successfully, skipping the remainder of the
2325 pattern. When inside a recursion, only the innermost pattern is ended
2326 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2327 captured. (This feature was added to PCRE at release 8.00.) For example:
2328 .sp
2329 A((?:A|B(*ACCEPT)|C)D)
2330 .sp
2331 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2332 the outer parentheses.
2333 .sp
2334 (*FAIL) or (*F)
2335 .sp
2336 This verb causes the match to fail, forcing backtracking to occur. It is
2337 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2338 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2339 Perl features that are not present in PCRE. The nearest equivalent is the
2340 callout feature, as for example in this pattern:
2341 .sp
2342 a+(?C)(*FAIL)
2343 .sp
2344 A match with the string "aaaa" always fails, but the callout is taken before
2345 each backtrack happens (in this example, 10 times).
2346 .
2347 .SS "Verbs that act after backtracking"
2348 .rs
2349 .sp
2350 The following verbs do nothing when they are encountered. Matching continues
2351 with what follows, but if there is no subsequent match, a failure is forced.
2352 The verbs differ in exactly what kind of failure occurs.
2353 .sp
2354 (*COMMIT)
2355 .sp
2356 This verb causes the whole match to fail outright if the rest of the pattern
2357 does not match. Even if the pattern is unanchored, no further attempts to find
2358 a match by advancing the starting point take place. Once (*COMMIT) has been
2359 passed, \fBpcre_exec()\fP is committed to finding a match at the current
2360 starting point, or not at all. For example:
2361 .sp
2362 a+(*COMMIT)b
2363 .sp
2364 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2365 dynamic anchor, or "I've started, so I must finish."
2366 .sp
2367 (*PRUNE)
2368 .sp
2369 This verb causes the match to fail at the current position if the rest of the
2370 pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2371 advance to the next starting character then happens. Backtracking can occur as
2372 usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2373 if there is no match to the right, backtracking cannot cross (*PRUNE).
2374 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2375 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2376 be expressed in any other way.
2377 .sp
2378 (*SKIP)
2379 .sp
2380 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2381 "bumpalong" advance is not to the next character, but to the position in the
2382 subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2383 was matched leading up to it cannot be part of a successful match. Consider:
2384 .sp
2385 a+(*SKIP)b
2386 .sp
2387 If the subject is "aaaac...", after the first match attempt fails (starting at
2388 the first character in the string), the starting point skips on to start the
2389 next attempt at "c". Note that a possessive quantifer does not have the same
2390 effect as this example; although it would suppress backtracking during the
2391 first match attempt, the second attempt would start at the second character
2392 instead of skipping on to "c".
2393 .sp
2394 (*THEN)
2395 .sp
2396 This verb causes a skip to the next alternation if the rest of the pattern does
2397 not match. That is, it cancels pending backtracking, but only within the
2398 current alternation. Its name comes from the observation that it can be used
2399 for a pattern-based if-then-else block:
2400 .sp
2401 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2402 .sp
2403 If the COND1 pattern matches, FOO is tried (and possibly further items after
2404 the end of the group if FOO succeeds); on failure the matcher skips to the
2405 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2406 is used outside of any alternation, it acts exactly like (*PRUNE).
2407 .
2408 .
2409 .SH "SEE ALSO"
2410 .rs
2411 .sp
2412 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2413 \fBpcresyntax\fP(3), \fBpcre\fP(3).
2414 .
2415 .
2416 .SH AUTHOR
2417 .rs
2418 .sp
2419 .nf
2420 Philip Hazel
2421 University Computing Service
2422 Cambridge CB2 3QH, England.
2423 .fi
2424 .
2425 .
2426 .SH REVISION
2427 .rs
2428 .sp
2429 .nf
2430 Last updated: 11 January 2010
2431 Copyright (c) 1997-2010 University of Cambridge.
2432 .fi

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