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Document that an isolated \E is ignored.

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 Another special sequence that may appear at the start of a pattern or in
46 combination with (*UTF8) is:
47 .sp
48 (*UCP)
49 .sp
50 This has the same effect as setting the PCRE_UCP option: it causes sequences
51 such as \ed and \ew to use Unicode properties to determine character types,
52 instead of recognizing only characters with codes less than 128 via a lookup
53 table.
54 .P
55 The remainder of this document discusses the patterns that are supported by
56 PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57 From release 6.0, PCRE offers a second matching function,
58 \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
59 Perl-compatible. Some of the features discussed below are not available when
60 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61 alternative function, and how it differs from the normal function, are
62 discussed in the
63 .\" HREF
64 \fBpcrematching\fP
65 .\"
66 page.
67 .
68 .
69 .SH "NEWLINE CONVENTIONS"
70 .rs
71 .sp
72 PCRE supports five different conventions for indicating line breaks in
73 strings: a single CR (carriage return) character, a single LF (linefeed)
74 character, the two-character sequence CRLF, any of the three preceding, or any
75 Unicode newline sequence. The
76 .\" HREF
77 \fBpcreapi\fP
78 .\"
79 page has
80 .\" HTML <a href="pcreapi.html#newlines">
81 .\" </a>
82 further discussion
83 .\"
84 about newlines, and shows how to set the newline convention in the
85 \fIoptions\fP arguments for the compiling and matching functions.
86 .P
87 It is also possible to specify a newline convention by starting a pattern
88 string with one of the following five sequences:
89 .sp
90 (*CR) carriage return
91 (*LF) linefeed
92 (*CRLF) carriage return, followed by linefeed
93 (*ANYCRLF) any of the three above
94 (*ANY) all Unicode newline sequences
95 .sp
96 These override the default and the options given to \fBpcre_compile()\fP or
97 \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
98 newline sequence, the pattern
99 .sp
100 (*CR)a.b
101 .sp
102 changes the convention to CR. That pattern matches "a\enb" because LF is no
103 longer a newline. Note that these special settings, which are not
104 Perl-compatible, are recognized only at the very start of a pattern, and that
105 they must be in upper case. If more than one of them is present, the last one
106 is used.
107 .P
108 The newline convention affects the interpretation of the dot metacharacter when
109 PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
110 affect what the \eR escape sequence matches. By default, this is any Unicode
111 newline sequence, for Perl compatibility. However, this can be changed; see the
112 description of \eR in the section entitled
113 .\" HTML <a href="#newlineseq">
114 .\" </a>
115 "Newline sequences"
116 .\"
117 below. A change of \eR setting can be combined with a change of newline
118 convention.
119 .
120 .
121 .SH "CHARACTERS AND METACHARACTERS"
122 .rs
123 .sp
124 A regular expression is a pattern that is matched against a subject string from
125 left to right. Most characters stand for themselves in a pattern, and match the
126 corresponding characters in the subject. As a trivial example, the pattern
127 .sp
128 The quick brown fox
129 .sp
130 matches a portion of a subject string that is identical to itself. When
131 caseless matching is specified (the PCRE_CASELESS option), letters are matched
132 independently of case. In UTF-8 mode, PCRE always understands the concept of
133 case for characters whose values are less than 128, so caseless matching is
134 always possible. For characters with higher values, the concept of case is
135 supported if PCRE is compiled with Unicode property support, but not otherwise.
136 If you want to use caseless matching for characters 128 and above, you must
137 ensure that PCRE is compiled with Unicode property support as well as with
138 UTF-8 support.
139 .P
140 The power of regular expressions comes from the ability to include alternatives
141 and repetitions in the pattern. These are encoded in the pattern by the use of
142 \fImetacharacters\fP, which do not stand for themselves but instead are
143 interpreted in some special way.
144 .P
145 There are two different sets of metacharacters: those that are recognized
146 anywhere in the pattern except within square brackets, and those that are
147 recognized within square brackets. Outside square brackets, the metacharacters
148 are as follows:
149 .sp
150 \e general escape character with several uses
151 ^ assert start of string (or line, in multiline mode)
152 $ assert end of string (or line, in multiline mode)
153 . match any character except newline (by default)
154 [ start character class definition
155 | start of alternative branch
156 ( start subpattern
157 ) end subpattern
158 ? extends the meaning of (
159 also 0 or 1 quantifier
160 also quantifier minimizer
161 * 0 or more quantifier
162 + 1 or more quantifier
163 also "possessive quantifier"
164 { start min/max quantifier
165 .sp
166 Part of a pattern that is in square brackets is called a "character class". In
167 a character class the only metacharacters are:
168 .sp
169 \e general escape character
170 ^ negate the class, but only if the first character
171 - indicates character range
172 .\" JOIN
173 [ POSIX character class (only if followed by POSIX
174 syntax)
175 ] terminates the character class
176 .sp
177 The following sections describe the use of each of the metacharacters.
178 .
179 .
180 .SH BACKSLASH
181 .rs
182 .sp
183 The backslash character has several uses. Firstly, if it is followed by a
184 non-alphanumeric character, it takes away any special meaning that character
185 may have. This use of backslash as an escape character applies both inside and
186 outside character classes.
187 .P
188 For example, if you want to match a * character, you write \e* in the pattern.
189 This escaping action applies whether or not the following character would
190 otherwise be interpreted as a metacharacter, so it is always safe to precede a
191 non-alphanumeric with backslash to specify that it stands for itself. In
192 particular, if you want to match a backslash, you write \e\e.
193 .P
194 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
195 pattern (other than in a character class) and characters between a # outside
196 a character class and the next newline are ignored. An escaping backslash can
197 be used to include a whitespace or # character as part of the pattern.
198 .P
199 If you want to remove the special meaning from a sequence of characters, you
200 can do so by putting them between \eQ and \eE. This is different from Perl in
201 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
202 Perl, $ and @ cause variable interpolation. Note the following examples:
203 .sp
204 Pattern PCRE matches Perl matches
205 .sp
206 .\" JOIN
207 \eQabc$xyz\eE abc$xyz abc followed by the
208 contents of $xyz
209 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
210 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
211 .sp
212 The \eQ...\eE sequence is recognized both inside and outside character classes.
213 An isolated \eE that is not preceded by \eQ is ignored.
214 .
215 .
216 .\" HTML <a name="digitsafterbackslash"></a>
217 .SS "Non-printing characters"
218 .rs
219 .sp
220 A second use of backslash provides a way of encoding non-printing characters
221 in patterns in a visible manner. There is no restriction on the appearance of
222 non-printing characters, apart from the binary zero that terminates a pattern,
223 but when a pattern is being prepared by text editing, it is often easier to use
224 one of the following escape sequences than the binary character it represents:
225 .sp
226 \ea alarm, that is, the BEL character (hex 07)
227 \ecx "control-x", where x is any character
228 \ee escape (hex 1B)
229 \ef formfeed (hex 0C)
230 \en linefeed (hex 0A)
231 \er carriage return (hex 0D)
232 \et tab (hex 09)
233 \eddd character with octal code ddd, or back reference
234 \exhh character with hex code hh
235 \ex{hhh..} character with hex code hhh..
236 .sp
237 The precise effect of \ecx is as follows: if x is a lower case letter, it
238 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
239 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
240 7B.
241 .P
242 After \ex, from zero to two hexadecimal digits are read (letters can be in
243 upper or lower case). Any number of hexadecimal digits may appear between \ex{
244 and }, but the value of the character code must be less than 256 in non-UTF-8
245 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
246 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
247 point, which is 10FFFF.
248 .P
249 If characters other than hexadecimal digits appear between \ex{ and }, or if
250 there is no terminating }, this form of escape is not recognized. Instead, the
251 initial \ex will be interpreted as a basic hexadecimal escape, with no
252 following digits, giving a character whose value is zero.
253 .P
254 Characters whose value is less than 256 can be defined by either of the two
255 syntaxes for \ex. There is no difference in the way they are handled. For
256 example, \exdc is exactly the same as \ex{dc}.
257 .P
258 After \e0 up to two further octal digits are read. If there are fewer than two
259 digits, just those that are present are used. Thus the sequence \e0\ex\e07
260 specifies two binary zeros followed by a BEL character (code value 7). Make
261 sure you supply two digits after the initial zero if the pattern character that
262 follows is itself an octal digit.
263 .P
264 The handling of a backslash followed by a digit other than 0 is complicated.
265 Outside a character class, PCRE reads it and any following digits as a decimal
266 number. If the number is less than 10, or if there have been at least that many
267 previous capturing left parentheses in the expression, the entire sequence is
268 taken as a \fIback reference\fP. A description of how this works is given
269 .\" HTML <a href="#backreferences">
270 .\" </a>
271 later,
272 .\"
273 following the discussion of
274 .\" HTML <a href="#subpattern">
275 .\" </a>
276 parenthesized subpatterns.
277 .\"
278 .P
279 Inside a character class, or if the decimal number is greater than 9 and there
280 have not been that many capturing subpatterns, PCRE re-reads up to three octal
281 digits following the backslash, and uses them to generate a data character. Any
282 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
283 character specified in octal must be less than \e400. In UTF-8 mode, values up
284 to \e777 are permitted. For example:
285 .sp
286 \e040 is another way of writing a space
287 .\" JOIN
288 \e40 is the same, provided there are fewer than 40
289 previous capturing subpatterns
290 \e7 is always a back reference
291 .\" JOIN
292 \e11 might be a back reference, or another way of
293 writing a tab
294 \e011 is always a tab
295 \e0113 is a tab followed by the character "3"
296 .\" JOIN
297 \e113 might be a back reference, otherwise the
298 character with octal code 113
299 .\" JOIN
300 \e377 might be a back reference, otherwise
301 the byte consisting entirely of 1 bits
302 .\" JOIN
303 \e81 is either a back reference, or a binary zero
304 followed by the two characters "8" and "1"
305 .sp
306 Note that octal values of 100 or greater must not be introduced by a leading
307 zero, because no more than three octal digits are ever read.
308 .P
309 All the sequences that define a single character value can be used both inside
310 and outside character classes. In addition, inside a character class, the
311 sequence \eb is interpreted as the backspace character (hex 08). The sequences
312 \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
313 unrecognized escape sequences, they are treated as the literal characters "B",
314 "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
315 set. Outside a character class, these sequences have different meanings.
316 .
317 .
318 .SS "Absolute and relative back references"
319 .rs
320 .sp
321 The sequence \eg followed by an unsigned or a negative number, optionally
322 enclosed in braces, is an absolute or relative back reference. A named back
323 reference can be coded as \eg{name}. Back references are discussed
324 .\" HTML <a href="#backreferences">
325 .\" </a>
326 later,
327 .\"
328 following the discussion of
329 .\" HTML <a href="#subpattern">
330 .\" </a>
331 parenthesized subpatterns.
332 .\"
333 .
334 .
335 .SS "Absolute and relative subroutine calls"
336 .rs
337 .sp
338 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
339 a number enclosed either in angle brackets or single quotes, is an alternative
340 syntax for referencing a subpattern as a "subroutine". Details are discussed
341 .\" HTML <a href="#onigurumasubroutines">
342 .\" </a>
343 later.
344 .\"
345 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
346 synonymous. The former is a back reference; the latter is a
347 .\" HTML <a href="#subpatternsassubroutines">
348 .\" </a>
349 subroutine
350 .\"
351 call.
352 .
353 .
354 .\" HTML <a name="genericchartypes"></a>
355 .SS "Generic character types"
356 .rs
357 .sp
358 Another use of backslash is for specifying generic character types:
359 .sp
360 \ed any decimal digit
361 \eD any character that is not a decimal digit
362 \eh any horizontal whitespace character
363 \eH any character that is not a horizontal whitespace character
364 \es any whitespace character
365 \eS any character that is not a whitespace character
366 \ev any vertical whitespace character
367 \eV any character that is not a vertical whitespace character
368 \ew any "word" character
369 \eW any "non-word" character
370 .sp
371 There is also the single sequence \eN, which matches a non-newline character.
372 This is the same as
373 .\" HTML <a href="#fullstopdot">
374 .\" </a>
375 the "." metacharacter
376 .\"
377 when PCRE_DOTALL is not set.
378 .P
379 Each pair of lower and upper case escape sequences partitions the complete set
380 of characters into two disjoint sets. Any given character matches one, and only
381 one, of each pair. The sequences can appear both inside and outside character
382 classes. They each match one character of the appropriate type. If the current
383 matching point is at the end of the subject string, all of them fail, because
384 there is no character to match.
385 .P
386 For compatibility with Perl, \es does not match the VT character (code 11).
387 This makes it different from the the POSIX "space" class. The \es characters
388 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
389 included in a Perl script, \es may match the VT character. In PCRE, it never
390 does.
391 .P
392 A "word" character is an underscore or any character that is a letter or digit.
393 By default, the definition of letters and digits is controlled by PCRE's
394 low-valued character tables, and may vary if locale-specific matching is taking
395 place (see
396 .\" HTML <a href="pcreapi.html#localesupport">
397 .\" </a>
398 "Locale support"
399 .\"
400 in the
401 .\" HREF
402 \fBpcreapi\fP
403 .\"
404 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
405 or "french" in Windows, some character codes greater than 128 are used for
406 accented letters, and these are then matched by \ew. The use of locales with
407 Unicode is discouraged.
408 .P
409 By default, in UTF-8 mode, characters with values greater than 128 never match
410 \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
411 their original meanings from before UTF-8 support was available, mainly for
412 efficiency reasons. However, if PCRE is compiled with Unicode property support,
413 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
414 properties are used to determine character types, as follows:
415 .sp
416 \ed any character that \ep{Nd} matches (decimal digit)
417 \es any character that \ep{Z} matches, plus HT, LF, FF, CR
418 \ew any character that \ep{L} or \ep{N} matches, plus underscore
419 .sp
420 The upper case escapes match the inverse sets of characters. Note that \ed
421 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
422 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
423 \eB because they are defined in terms of \ew and \eW. Matching these sequences
424 is noticeably slower when PCRE_UCP is set.
425 .P
426 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
427 other sequences, which match only ASCII characters by default, these always
428 match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is
429 set. The horizontal space characters are:
430 .sp
431 U+0009 Horizontal tab
432 U+0020 Space
433 U+00A0 Non-break space
434 U+1680 Ogham space mark
435 U+180E Mongolian vowel separator
436 U+2000 En quad
437 U+2001 Em quad
438 U+2002 En space
439 U+2003 Em space
440 U+2004 Three-per-em space
441 U+2005 Four-per-em space
442 U+2006 Six-per-em space
443 U+2007 Figure space
444 U+2008 Punctuation space
445 U+2009 Thin space
446 U+200A Hair space
447 U+202F Narrow no-break space
448 U+205F Medium mathematical space
449 U+3000 Ideographic space
450 .sp
451 The vertical space characters are:
452 .sp
453 U+000A Linefeed
454 U+000B Vertical tab
455 U+000C Formfeed
456 U+000D Carriage return
457 U+0085 Next line
458 U+2028 Line separator
459 U+2029 Paragraph separator
460 .
461 .
462 .\" HTML <a name="newlineseq"></a>
463 .SS "Newline sequences"
464 .rs
465 .sp
466 Outside a character class, by default, the escape sequence \eR matches any
467 Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is
468 equivalent to the following:
469 .sp
470 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
471 .sp
472 This is an example of an "atomic group", details of which are given
473 .\" HTML <a href="#atomicgroup">
474 .\" </a>
475 below.
476 .\"
477 This particular group matches either the two-character sequence CR followed by
478 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
479 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
480 line, U+0085). The two-character sequence is treated as a single unit that
481 cannot be split.
482 .P
483 In UTF-8 mode, two additional characters whose codepoints are greater than 255
484 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
485 Unicode character property support is not needed for these characters to be
486 recognized.
487 .P
488 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
489 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
490 either at compile time or when the pattern is matched. (BSR is an abbrevation
491 for "backslash R".) This can be made the default when PCRE is built; if this is
492 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
493 It is also possible to specify these settings by starting a pattern string with
494 one of the following sequences:
495 .sp
496 (*BSR_ANYCRLF) CR, LF, or CRLF only
497 (*BSR_UNICODE) any Unicode newline sequence
498 .sp
499 These override the default and the options given to \fBpcre_compile()\fP or
500 \fBpcre_compile2()\fP, but they can be overridden by options given to
501 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
502 which are not Perl-compatible, are recognized only at the very start of a
503 pattern, and that they must be in upper case. If more than one of them is
504 present, the last one is used. They can be combined with a change of newline
505 convention; for example, a pattern can start with:
506 .sp
507 (*ANY)(*BSR_ANYCRLF)
508 .sp
509 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
510 a character class, \eR is treated as an unrecognized escape sequence, and so
511 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
512 .
513 .
514 .\" HTML <a name="uniextseq"></a>
515 .SS Unicode character properties
516 .rs
517 .sp
518 When PCRE is built with Unicode character property support, three additional
519 escape sequences that match characters with specific properties are available.
520 When not in UTF-8 mode, these sequences are of course limited to testing
521 characters whose codepoints are less than 256, but they do work in this mode.
522 The extra escape sequences are:
523 .sp
524 \ep{\fIxx\fP} a character with the \fIxx\fP property
525 \eP{\fIxx\fP} a character without the \fIxx\fP property
526 \eX an extended Unicode sequence
527 .sp
528 The property names represented by \fIxx\fP above are limited to the Unicode
529 script names, the general category properties, "Any", which matches any
530 character (including newline), and some special PCRE properties (described
531 in the
532 .\" HTML <a href="#extraprops">
533 .\" </a>
534 next section).
535 .\"
536 Other Perl properties such as "InMusicalSymbols" are not currently supported by
537 PCRE. Note that \eP{Any} does not match any characters, so always causes a
538 match failure.
539 .P
540 Sets of Unicode characters are defined as belonging to certain scripts. A
541 character from one of these sets can be matched using a script name. For
542 example:
543 .sp
544 \ep{Greek}
545 \eP{Han}
546 .sp
547 Those that are not part of an identified script are lumped together as
548 "Common". The current list of scripts is:
549 .P
550 Arabic,
551 Armenian,
552 Avestan,
553 Balinese,
554 Bamum,
555 Bengali,
556 Bopomofo,
557 Braille,
558 Buginese,
559 Buhid,
560 Canadian_Aboriginal,
561 Carian,
562 Cham,
563 Cherokee,
564 Common,
565 Coptic,
566 Cuneiform,
567 Cypriot,
568 Cyrillic,
569 Deseret,
570 Devanagari,
571 Egyptian_Hieroglyphs,
572 Ethiopic,
573 Georgian,
574 Glagolitic,
575 Gothic,
576 Greek,
577 Gujarati,
578 Gurmukhi,
579 Han,
580 Hangul,
581 Hanunoo,
582 Hebrew,
583 Hiragana,
584 Imperial_Aramaic,
585 Inherited,
586 Inscriptional_Pahlavi,
587 Inscriptional_Parthian,
588 Javanese,
589 Kaithi,
590 Kannada,
591 Katakana,
592 Kayah_Li,
593 Kharoshthi,
594 Khmer,
595 Lao,
596 Latin,
597 Lepcha,
598 Limbu,
599 Linear_B,
600 Lisu,
601 Lycian,
602 Lydian,
603 Malayalam,
604 Meetei_Mayek,
605 Mongolian,
606 Myanmar,
607 New_Tai_Lue,
608 Nko,
609 Ogham,
610 Old_Italic,
611 Old_Persian,
612 Old_South_Arabian,
613 Old_Turkic,
614 Ol_Chiki,
615 Oriya,
616 Osmanya,
617 Phags_Pa,
618 Phoenician,
619 Rejang,
620 Runic,
621 Samaritan,
622 Saurashtra,
623 Shavian,
624 Sinhala,
625 Sundanese,
626 Syloti_Nagri,
627 Syriac,
628 Tagalog,
629 Tagbanwa,
630 Tai_Le,
631 Tai_Tham,
632 Tai_Viet,
633 Tamil,
634 Telugu,
635 Thaana,
636 Thai,
637 Tibetan,
638 Tifinagh,
639 Ugaritic,
640 Vai,
641 Yi.
642 .P
643 Each character has exactly one Unicode general category property, specified by
644 a two-letter abbreviation. For compatibility with Perl, negation can be
645 specified by including a circumflex between the opening brace and the property
646 name. For example, \ep{^Lu} is the same as \eP{Lu}.
647 .P
648 If only one letter is specified with \ep or \eP, it includes all the general
649 category properties that start with that letter. In this case, in the absence
650 of negation, the curly brackets in the escape sequence are optional; these two
651 examples have the same effect:
652 .sp
653 \ep{L}
654 \epL
655 .sp
656 The following general category property codes are supported:
657 .sp
658 C Other
659 Cc Control
660 Cf Format
661 Cn Unassigned
662 Co Private use
663 Cs Surrogate
664 .sp
665 L Letter
666 Ll Lower case letter
667 Lm Modifier letter
668 Lo Other letter
669 Lt Title case letter
670 Lu Upper case letter
671 .sp
672 M Mark
673 Mc Spacing mark
674 Me Enclosing mark
675 Mn Non-spacing mark
676 .sp
677 N Number
678 Nd Decimal number
679 Nl Letter number
680 No Other number
681 .sp
682 P Punctuation
683 Pc Connector punctuation
684 Pd Dash punctuation
685 Pe Close punctuation
686 Pf Final punctuation
687 Pi Initial punctuation
688 Po Other punctuation
689 Ps Open punctuation
690 .sp
691 S Symbol
692 Sc Currency symbol
693 Sk Modifier symbol
694 Sm Mathematical symbol
695 So Other symbol
696 .sp
697 Z Separator
698 Zl Line separator
699 Zp Paragraph separator
700 Zs Space separator
701 .sp
702 The special property L& is also supported: it matches a character that has
703 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
704 a modifier or "other".
705 .P
706 The Cs (Surrogate) property applies only to characters in the range U+D800 to
707 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
708 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
709 (see the discussion of PCRE_NO_UTF8_CHECK in the
710 .\" HREF
711 \fBpcreapi\fP
712 .\"
713 page). Perl does not support the Cs property.
714 .P
715 The long synonyms for property names that Perl supports (such as \ep{Letter})
716 are not supported by PCRE, nor is it permitted to prefix any of these
717 properties with "Is".
718 .P
719 No character that is in the Unicode table has the Cn (unassigned) property.
720 Instead, this property is assumed for any code point that is not in the
721 Unicode table.
722 .P
723 Specifying caseless matching does not affect these escape sequences. For
724 example, \ep{Lu} always matches only upper case letters.
725 .P
726 The \eX escape matches any number of Unicode characters that form an extended
727 Unicode sequence. \eX is equivalent to
728 .sp
729 (?>\ePM\epM*)
730 .sp
731 That is, it matches a character without the "mark" property, followed by zero
732 or more characters with the "mark" property, and treats the sequence as an
733 atomic group
734 .\" HTML <a href="#atomicgroup">
735 .\" </a>
736 (see below).
737 .\"
738 Characters with the "mark" property are typically accents that affect the
739 preceding character. None of them have codepoints less than 256, so in
740 non-UTF-8 mode \eX matches any one character.
741 .P
742 Matching characters by Unicode property is not fast, because PCRE has to search
743 a structure that contains data for over fifteen thousand characters. That is
744 why the traditional escape sequences such as \ed and \ew do not use Unicode
745 properties in PCRE by default, though you can make them do so by setting the
746 PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
747 (*UCP).
748 .
749 .
750 .\" HTML <a name="extraprops"></a>
751 .SS PCRE's additional properties
752 .rs
753 .sp
754 As well as the standard Unicode properties described in the previous
755 section, PCRE supports four more that make it possible to convert traditional
756 escape sequences such as \ew and \es and POSIX character classes to use Unicode
757 properties. PCRE uses these non-standard, non-Perl properties internally when
758 PCRE_UCP is set. They are:
759 .sp
760 Xan Any alphanumeric character
761 Xps Any POSIX space character
762 Xsp Any Perl space character
763 Xwd Any Perl "word" character
764 .sp
765 Xan matches characters that have either the L (letter) or the N (number)
766 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
767 carriage return, and any other character that has the Z (separator) property.
768 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
769 same characters as Xan, plus underscore.
770 .
771 .
772 .\" HTML <a name="resetmatchstart"></a>
773 .SS "Resetting the match start"
774 .rs
775 .sp
776 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
777 matched characters not to be included in the final matched sequence. For
778 example, the pattern:
779 .sp
780 foo\eKbar
781 .sp
782 matches "foobar", but reports that it has matched "bar". This feature is
783 similar to a lookbehind assertion
784 .\" HTML <a href="#lookbehind">
785 .\" </a>
786 (described below).
787 .\"
788 However, in this case, the part of the subject before the real match does not
789 have to be of fixed length, as lookbehind assertions do. The use of \eK does
790 not interfere with the setting of
791 .\" HTML <a href="#subpattern">
792 .\" </a>
793 captured substrings.
794 .\"
795 For example, when the pattern
796 .sp
797 (foo)\eKbar
798 .sp
799 matches "foobar", the first substring is still set to "foo".
800 .P
801 Perl documents that the use of \eK within assertions is "not well defined". In
802 PCRE, \eK is acted upon when it occurs inside positive assertions, but is
803 ignored in negative assertions.
804 .
805 .
806 .\" HTML <a name="smallassertions"></a>
807 .SS "Simple assertions"
808 .rs
809 .sp
810 The final use of backslash is for certain simple assertions. An assertion
811 specifies a condition that has to be met at a particular point in a match,
812 without consuming any characters from the subject string. The use of
813 subpatterns for more complicated assertions is described
814 .\" HTML <a href="#bigassertions">
815 .\" </a>
816 below.
817 .\"
818 The backslashed assertions are:
819 .sp
820 \eb matches at a word boundary
821 \eB matches when not at a word boundary
822 \eA matches at the start of the subject
823 \eZ matches at the end of the subject
824 also matches before a newline at the end of the subject
825 \ez matches only at the end of the subject
826 \eG matches at the first matching position in the subject
827 .sp
828 Inside a character class, \eb has a different meaning; it matches the backspace
829 character. If any other of these assertions appears in a character class, by
830 default it matches the corresponding literal character (for example, \eB
831 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
832 escape sequence" error is generated instead.
833 .P
834 A word boundary is a position in the subject string where the current character
835 and the previous character do not both match \ew or \eW (i.e. one matches
836 \ew and the other matches \eW), or the start or end of the string if the
837 first or last character matches \ew, respectively. In UTF-8 mode, the meanings
838 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
839 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
840 of word" or "end of word" metasequence. However, whatever follows \eb normally
841 determines which it is. For example, the fragment \eba matches "a" at the start
842 of a word.
843 .P
844 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
845 dollar (described in the next section) in that they only ever match at the very
846 start and end of the subject string, whatever options are set. Thus, they are
847 independent of multiline mode. These three assertions are not affected by the
848 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
849 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
850 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
851 at a point other than the beginning of the subject, \eA can never match. The
852 difference between \eZ and \ez is that \eZ matches before a newline at the end
853 of the string as well as at the very end, whereas \ez matches only at the end.
854 .P
855 The \eG assertion is true only when the current matching position is at the
856 start point of the match, as specified by the \fIstartoffset\fP argument of
857 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
858 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
859 arguments, you can mimic Perl's /g option, and it is in this kind of
860 implementation where \eG can be useful.
861 .P
862 Note, however, that PCRE's interpretation of \eG, as the start of the current
863 match, is subtly different from Perl's, which defines it as the end of the
864 previous match. In Perl, these can be different when the previously matched
865 string was empty. Because PCRE does just one match at a time, it cannot
866 reproduce this behaviour.
867 .P
868 If all the alternatives of a pattern begin with \eG, the expression is anchored
869 to the starting match position, and the "anchored" flag is set in the compiled
870 regular expression.
871 .
872 .
873 .SH "CIRCUMFLEX AND DOLLAR"
874 .rs
875 .sp
876 Outside a character class, in the default matching mode, the circumflex
877 character is an assertion that is true only if the current matching point is
878 at the start of the subject string. If the \fIstartoffset\fP argument of
879 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
880 option is unset. Inside a character class, circumflex has an entirely different
881 meaning
882 .\" HTML <a href="#characterclass">
883 .\" </a>
884 (see below).
885 .\"
886 .P
887 Circumflex need not be the first character of the pattern if a number of
888 alternatives are involved, but it should be the first thing in each alternative
889 in which it appears if the pattern is ever to match that branch. If all
890 possible alternatives start with a circumflex, that is, if the pattern is
891 constrained to match only at the start of the subject, it is said to be an
892 "anchored" pattern. (There are also other constructs that can cause a pattern
893 to be anchored.)
894 .P
895 A dollar character is an assertion that is true only if the current matching
896 point is at the end of the subject string, or immediately before a newline
897 at the end of the string (by default). Dollar need not be the last character of
898 the pattern if a number of alternatives are involved, but it should be the last
899 item in any branch in which it appears. Dollar has no special meaning in a
900 character class.
901 .P
902 The meaning of dollar can be changed so that it matches only at the very end of
903 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
904 does not affect the \eZ assertion.
905 .P
906 The meanings of the circumflex and dollar characters are changed if the
907 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
908 immediately after internal newlines as well as at the start of the subject
909 string. It does not match after a newline that ends the string. A dollar
910 matches before any newlines in the string, as well as at the very end, when
911 PCRE_MULTILINE is set. When newline is specified as the two-character
912 sequence CRLF, isolated CR and LF characters do not indicate newlines.
913 .P
914 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
915 \en represents a newline) in multiline mode, but not otherwise. Consequently,
916 patterns that are anchored in single line mode because all branches start with
917 ^ are not anchored in multiline mode, and a match for circumflex is possible
918 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
919 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
920 .P
921 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
922 end of the subject in both modes, and if all branches of a pattern start with
923 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
924 .
925 .
926 .\" HTML <a name="fullstopdot"></a>
927 .SH "FULL STOP (PERIOD, DOT) AND \eN"
928 .rs
929 .sp
930 Outside a character class, a dot in the pattern matches any one character in
931 the subject string except (by default) a character that signifies the end of a
932 line. In UTF-8 mode, the matched character may be more than one byte long.
933 .P
934 When a line ending is defined as a single character, dot never matches that
935 character; when the two-character sequence CRLF is used, dot does not match CR
936 if it is immediately followed by LF, but otherwise it matches all characters
937 (including isolated CRs and LFs). When any Unicode line endings are being
938 recognized, dot does not match CR or LF or any of the other line ending
939 characters.
940 .P
941 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
942 option is set, a dot matches any one character, without exception. If the
943 two-character sequence CRLF is present in the subject string, it takes two dots
944 to match it.
945 .P
946 The handling of dot is entirely independent of the handling of circumflex and
947 dollar, the only relationship being that they both involve newlines. Dot has no
948 special meaning in a character class.
949 .P
950 The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not
951 set. In other words, it matches any one character except one that signifies the
952 end of a line.
953 .
954 .
955 .SH "MATCHING A SINGLE BYTE"
956 .rs
957 .sp
958 Outside a character class, the escape sequence \eC matches any one byte, both
959 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
960 characters. The feature is provided in Perl in order to match individual bytes
961 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
962 what remains in the string may be a malformed UTF-8 string. For this reason,
963 the \eC escape sequence is best avoided.
964 .P
965 PCRE does not allow \eC to appear in lookbehind assertions
966 .\" HTML <a href="#lookbehind">
967 .\" </a>
968 (described below),
969 .\"
970 because in UTF-8 mode this would make it impossible to calculate the length of
971 the lookbehind.
972 .
973 .
974 .\" HTML <a name="characterclass"></a>
975 .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
976 .rs
977 .sp
978 An opening square bracket introduces a character class, terminated by a closing
979 square bracket. A closing square bracket on its own is not special by default.
980 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
981 bracket causes a compile-time error. If a closing square bracket is required as
982 a member of the class, it should be the first data character in the class
983 (after an initial circumflex, if present) or escaped with a backslash.
984 .P
985 A character class matches a single character in the subject. In UTF-8 mode, the
986 character may be more than one byte long. A matched character must be in the
987 set of characters defined by the class, unless the first character in the class
988 definition is a circumflex, in which case the subject character must not be in
989 the set defined by the class. If a circumflex is actually required as a member
990 of the class, ensure it is not the first character, or escape it with a
991 backslash.
992 .P
993 For example, the character class [aeiou] matches any lower case vowel, while
994 [^aeiou] matches any character that is not a lower case vowel. Note that a
995 circumflex is just a convenient notation for specifying the characters that
996 are in the class by enumerating those that are not. A class that starts with a
997 circumflex is not an assertion; it still consumes a character from the subject
998 string, and therefore it fails if the current pointer is at the end of the
999 string.
1000 .P
1001 In UTF-8 mode, characters with values greater than 255 can be included in a
1002 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
1003 .P
1004 When caseless matching is set, any letters in a class represent both their
1005 upper case and lower case versions, so for example, a caseless [aeiou] matches
1006 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1007 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1008 case for characters whose values are less than 128, so caseless matching is
1009 always possible. For characters with higher values, the concept of case is
1010 supported if PCRE is compiled with Unicode property support, but not otherwise.
1011 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1012 you must ensure that PCRE is compiled with Unicode property support as well as
1013 with UTF-8 support.
1014 .P
1015 Characters that might indicate line breaks are never treated in any special way
1016 when matching character classes, whatever line-ending sequence is in use, and
1017 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1018 such as [^a] always matches one of these characters.
1019 .P
1020 The minus (hyphen) character can be used to specify a range of characters in a
1021 character class. For example, [d-m] matches any letter between d and m,
1022 inclusive. If a minus character is required in a class, it must be escaped with
1023 a backslash or appear in a position where it cannot be interpreted as
1024 indicating a range, typically as the first or last character in the class.
1025 .P
1026 It is not possible to have the literal character "]" as the end character of a
1027 range. A pattern such as [W-]46] is interpreted as a class of two characters
1028 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1029 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1030 the end of range, so [W-\e]46] is interpreted as a class containing a range
1031 followed by two other characters. The octal or hexadecimal representation of
1032 "]" can also be used to end a range.
1033 .P
1034 Ranges operate in the collating sequence of character values. They can also be
1035 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
1036 mode, ranges can include characters whose values are greater than 255, for
1037 example [\ex{100}-\ex{2ff}].
1038 .P
1039 If a range that includes letters is used when caseless matching is set, it
1040 matches the letters in either case. For example, [W-c] is equivalent to
1041 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1042 tables for a French locale are in use, [\exc8-\excb] matches accented E
1043 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1044 characters with values greater than 128 only when it is compiled with Unicode
1045 property support.
1046 .P
1047 The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1048 \eW may also appear in a character class, and add the characters that they
1049 match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1050 circumflex can conveniently be used with the upper case character types to
1051 specify a more restricted set of characters than the matching lower case type.
1052 For example, the class [^\eW_] matches any letter or digit, but not underscore.
1053 .P
1054 The only metacharacters that are recognized in character classes are backslash,
1055 hyphen (only where it can be interpreted as specifying a range), circumflex
1056 (only at the start), opening square bracket (only when it can be interpreted as
1057 introducing a POSIX class name - see the next section), and the terminating
1058 closing square bracket. However, escaping other non-alphanumeric characters
1059 does no harm.
1060 .
1061 .
1062 .SH "POSIX CHARACTER CLASSES"
1063 .rs
1064 .sp
1065 Perl supports the POSIX notation for character classes. This uses names
1066 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1067 this notation. For example,
1068 .sp
1069 [01[:alpha:]%]
1070 .sp
1071 matches "0", "1", any alphabetic character, or "%". The supported class names
1072 are:
1073 .sp
1074 alnum letters and digits
1075 alpha letters
1076 ascii character codes 0 - 127
1077 blank space or tab only
1078 cntrl control characters
1079 digit decimal digits (same as \ed)
1080 graph printing characters, excluding space
1081 lower lower case letters
1082 print printing characters, including space
1083 punct printing characters, excluding letters and digits and space
1084 space white space (not quite the same as \es)
1085 upper upper case letters
1086 word "word" characters (same as \ew)
1087 xdigit hexadecimal digits
1088 .sp
1089 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1090 space (32). Notice that this list includes the VT character (code 11). This
1091 makes "space" different to \es, which does not include VT (for Perl
1092 compatibility).
1093 .P
1094 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1095 5.8. Another Perl extension is negation, which is indicated by a ^ character
1096 after the colon. For example,
1097 .sp
1098 [12[:^digit:]]
1099 .sp
1100 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1101 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1102 supported, and an error is given if they are encountered.
1103 .P
1104 By default, in UTF-8 mode, characters with values greater than 128 do not match
1105 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1106 to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1107 character properties are used. This is achieved by replacing the POSIX classes
1108 by other sequences, as follows:
1109 .sp
1110 [:alnum:] becomes \ep{Xan}
1111 [:alpha:] becomes \ep{L}
1112 [:blank:] becomes \eh
1113 [:digit:] becomes \ep{Nd}
1114 [:lower:] becomes \ep{Ll}
1115 [:space:] becomes \ep{Xps}
1116 [:upper:] becomes \ep{Lu}
1117 [:word:] becomes \ep{Xwd}
1118 .sp
1119 Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1120 classes are unchanged, and match only characters with code points less than
1121 128.
1122 .
1123 .
1124 .SH "VERTICAL BAR"
1125 .rs
1126 .sp
1127 Vertical bar characters are used to separate alternative patterns. For example,
1128 the pattern
1129 .sp
1130 gilbert|sullivan
1131 .sp
1132 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1133 and an empty alternative is permitted (matching the empty string). The matching
1134 process tries each alternative in turn, from left to right, and the first one
1135 that succeeds is used. If the alternatives are within a subpattern
1136 .\" HTML <a href="#subpattern">
1137 .\" </a>
1138 (defined below),
1139 .\"
1140 "succeeds" means matching the rest of the main pattern as well as the
1141 alternative in the subpattern.
1142 .
1143 .
1144 .SH "INTERNAL OPTION SETTING"
1145 .rs
1146 .sp
1147 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1148 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1149 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1150 The option letters are
1151 .sp
1152 i for PCRE_CASELESS
1153 m for PCRE_MULTILINE
1154 s for PCRE_DOTALL
1155 x for PCRE_EXTENDED
1156 .sp
1157 For example, (?im) sets caseless, multiline matching. It is also possible to
1158 unset these options by preceding the letter with a hyphen, and a combined
1159 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1160 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1161 permitted. If a letter appears both before and after the hyphen, the option is
1162 unset.
1163 .P
1164 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1165 changed in the same way as the Perl-compatible options by using the characters
1166 J, U and X respectively.
1167 .P
1168 When one of these option changes occurs at top level (that is, not inside
1169 subpattern parentheses), the change applies to the remainder of the pattern
1170 that follows. If the change is placed right at the start of a pattern, PCRE
1171 extracts it into the global options (and it will therefore show up in data
1172 extracted by the \fBpcre_fullinfo()\fP function).
1173 .P
1174 An option change within a subpattern (see below for a description of
1175 subpatterns) affects only that part of the current pattern that follows it, so
1176 .sp
1177 (a(?i)b)c
1178 .sp
1179 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1180 By this means, options can be made to have different settings in different
1181 parts of the pattern. Any changes made in one alternative do carry on
1182 into subsequent branches within the same subpattern. For example,
1183 .sp
1184 (a(?i)b|c)
1185 .sp
1186 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1187 branch is abandoned before the option setting. This is because the effects of
1188 option settings happen at compile time. There would be some very weird
1189 behaviour otherwise.
1190 .P
1191 \fBNote:\fP There are other PCRE-specific options that can be set by the
1192 application when the compile or match functions are called. In some cases the
1193 pattern can contain special leading sequences such as (*CRLF) to override what
1194 the application has set or what has been defaulted. Details are given in the
1195 section entitled
1196 .\" HTML <a href="#newlineseq">
1197 .\" </a>
1198 "Newline sequences"
1199 .\"
1200 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1201 to set UTF-8 and Unicode property modes; they are equivalent to setting the
1202 PCRE_UTF8 and the PCRE_UCP options, respectively.
1203 .
1204 .
1205 .\" HTML <a name="subpattern"></a>
1206 .SH SUBPATTERNS
1207 .rs
1208 .sp
1209 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1210 Turning part of a pattern into a subpattern does two things:
1211 .sp
1212 1. It localizes a set of alternatives. For example, the pattern
1213 .sp
1214 cat(aract|erpillar|)
1215 .sp
1216 matches one of the words "cat", "cataract", or "caterpillar". Without the
1217 parentheses, it would match "cataract", "erpillar" or an empty string.
1218 .sp
1219 2. It sets up the subpattern as a capturing subpattern. This means that, when
1220 the whole pattern matches, that portion of the subject string that matched the
1221 subpattern is passed back to the caller via the \fIovector\fP argument of
1222 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1223 from 1) to obtain numbers for the capturing subpatterns.
1224 .P
1225 For example, if the string "the red king" is matched against the pattern
1226 .sp
1227 the ((red|white) (king|queen))
1228 .sp
1229 the captured substrings are "red king", "red", and "king", and are numbered 1,
1230 2, and 3, respectively.
1231 .P
1232 The fact that plain parentheses fulfil two functions is not always helpful.
1233 There are often times when a grouping subpattern is required without a
1234 capturing requirement. If an opening parenthesis is followed by a question mark
1235 and a colon, the subpattern does not do any capturing, and is not counted when
1236 computing the number of any subsequent capturing subpatterns. For example, if
1237 the string "the white queen" is matched against the pattern
1238 .sp
1239 the ((?:red|white) (king|queen))
1240 .sp
1241 the captured substrings are "white queen" and "queen", and are numbered 1 and
1242 2. The maximum number of capturing subpatterns is 65535.
1243 .P
1244 As a convenient shorthand, if any option settings are required at the start of
1245 a non-capturing subpattern, the option letters may appear between the "?" and
1246 the ":". Thus the two patterns
1247 .sp
1248 (?i:saturday|sunday)
1249 (?:(?i)saturday|sunday)
1250 .sp
1251 match exactly the same set of strings. Because alternative branches are tried
1252 from left to right, and options are not reset until the end of the subpattern
1253 is reached, an option setting in one branch does affect subsequent branches, so
1254 the above patterns match "SUNDAY" as well as "Saturday".
1255 .
1256 .
1257 .\" HTML <a name="dupsubpatternnumber"></a>
1258 .SH "DUPLICATE SUBPATTERN NUMBERS"
1259 .rs
1260 .sp
1261 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1262 the same numbers for its capturing parentheses. Such a subpattern starts with
1263 (?| and is itself a non-capturing subpattern. For example, consider this
1264 pattern:
1265 .sp
1266 (?|(Sat)ur|(Sun))day
1267 .sp
1268 Because the two alternatives are inside a (?| group, both sets of capturing
1269 parentheses are numbered one. Thus, when the pattern matches, you can look
1270 at captured substring number one, whichever alternative matched. This construct
1271 is useful when you want to capture part, but not all, of one of a number of
1272 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1273 number is reset at the start of each branch. The numbers of any capturing
1274 buffers that follow the subpattern start after the highest number used in any
1275 branch. The following example is taken from the Perl documentation.
1276 The numbers underneath show in which buffer the captured content will be
1277 stored.
1278 .sp
1279 # before ---------------branch-reset----------- after
1280 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1281 # 1 2 2 3 2 3 4
1282 .sp
1283 A back reference to a numbered subpattern uses the most recent value that is
1284 set for that number by any subpattern. The following pattern matches "abcabc"
1285 or "defdef":
1286 .sp
1287 /(?|(abc)|(def))\e1/
1288 .sp
1289 In contrast, a recursive or "subroutine" call to a numbered subpattern always
1290 refers to the first one in the pattern with the given number. The following
1291 pattern matches "abcabc" or "defabc":
1292 .sp
1293 /(?|(abc)|(def))(?1)/
1294 .sp
1295 If a
1296 .\" HTML <a href="#conditions">
1297 .\" </a>
1298 condition test
1299 .\"
1300 for a subpattern's having matched refers to a non-unique number, the test is
1301 true if any of the subpatterns of that number have matched.
1302 .P
1303 An alternative approach to using this "branch reset" feature is to use
1304 duplicate named subpatterns, as described in the next section.
1305 .
1306 .
1307 .SH "NAMED SUBPATTERNS"
1308 .rs
1309 .sp
1310 Identifying capturing parentheses by number is simple, but it can be very hard
1311 to keep track of the numbers in complicated regular expressions. Furthermore,
1312 if an expression is modified, the numbers may change. To help with this
1313 difficulty, PCRE supports the naming of subpatterns. This feature was not
1314 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1315 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1316 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1317 have different names, but PCRE does not.
1318 .P
1319 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1320 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1321 parentheses from other parts of the pattern, such as
1322 .\" HTML <a href="#backreferences">
1323 .\" </a>
1324 back references,
1325 .\"
1326 .\" HTML <a href="#recursion">
1327 .\" </a>
1328 recursion,
1329 .\"
1330 and
1331 .\" HTML <a href="#conditions">
1332 .\" </a>
1333 conditions,
1334 .\"
1335 can be made by name as well as by number.
1336 .P
1337 Names consist of up to 32 alphanumeric characters and underscores. Named
1338 capturing parentheses are still allocated numbers as well as names, exactly as
1339 if the names were not present. The PCRE API provides function calls for
1340 extracting the name-to-number translation table from a compiled pattern. There
1341 is also a convenience function for extracting a captured substring by name.
1342 .P
1343 By default, a name must be unique within a pattern, but it is possible to relax
1344 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1345 names are also always permitted for subpatterns with the same number, set up as
1346 described in the previous section.) Duplicate names can be useful for patterns
1347 where only one instance of the named parentheses can match. Suppose you want to
1348 match the name of a weekday, either as a 3-letter abbreviation or as the full
1349 name, and in both cases you want to extract the abbreviation. This pattern
1350 (ignoring the line breaks) does the job:
1351 .sp
1352 (?<DN>Mon|Fri|Sun)(?:day)?|
1353 (?<DN>Tue)(?:sday)?|
1354 (?<DN>Wed)(?:nesday)?|
1355 (?<DN>Thu)(?:rsday)?|
1356 (?<DN>Sat)(?:urday)?
1357 .sp
1358 There are five capturing substrings, but only one is ever set after a match.
1359 (An alternative way of solving this problem is to use a "branch reset"
1360 subpattern, as described in the previous section.)
1361 .P
1362 The convenience function for extracting the data by name returns the substring
1363 for the first (and in this example, the only) subpattern of that name that
1364 matched. This saves searching to find which numbered subpattern it was.
1365 .P
1366 If you make a back reference to a non-unique named subpattern from elsewhere in
1367 the pattern, the one that corresponds to the first occurrence of the name is
1368 used. In the absence of duplicate numbers (see the previous section) this is
1369 the one with the lowest number. If you use a named reference in a condition
1370 test (see the
1371 .\"
1372 .\" HTML <a href="#conditions">
1373 .\" </a>
1374 section about conditions
1375 .\"
1376 below), either to check whether a subpattern has matched, or to check for
1377 recursion, all subpatterns with the same name are tested. If the condition is
1378 true for any one of them, the overall condition is true. This is the same
1379 behaviour as testing by number. For further details of the interfaces for
1380 handling named subpatterns, see the
1381 .\" HREF
1382 \fBpcreapi\fP
1383 .\"
1384 documentation.
1385 .P
1386 \fBWarning:\fP You cannot use different names to distinguish between two
1387 subpatterns with the same number because PCRE uses only the numbers when
1388 matching. For this reason, an error is given at compile time if different names
1389 are given to subpatterns with the same number. However, you can give the same
1390 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1391 .
1392 .
1393 .SH REPETITION
1394 .rs
1395 .sp
1396 Repetition is specified by quantifiers, which can follow any of the following
1397 items:
1398 .sp
1399 a literal data character
1400 the dot metacharacter
1401 the \eC escape sequence
1402 the \eX escape sequence (in UTF-8 mode with Unicode properties)
1403 the \eR escape sequence
1404 an escape such as \ed that matches a single character
1405 a character class
1406 a back reference (see next section)
1407 a parenthesized subpattern (unless it is an assertion)
1408 a recursive or "subroutine" call to a subpattern
1409 .sp
1410 The general repetition quantifier specifies a minimum and maximum number of
1411 permitted matches, by giving the two numbers in curly brackets (braces),
1412 separated by a comma. The numbers must be less than 65536, and the first must
1413 be less than or equal to the second. For example:
1414 .sp
1415 z{2,4}
1416 .sp
1417 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1418 character. If the second number is omitted, but the comma is present, there is
1419 no upper limit; if the second number and the comma are both omitted, the
1420 quantifier specifies an exact number of required matches. Thus
1421 .sp
1422 [aeiou]{3,}
1423 .sp
1424 matches at least 3 successive vowels, but may match many more, while
1425 .sp
1426 \ed{8}
1427 .sp
1428 matches exactly 8 digits. An opening curly bracket that appears in a position
1429 where a quantifier is not allowed, or one that does not match the syntax of a
1430 quantifier, is taken as a literal character. For example, {,6} is not a
1431 quantifier, but a literal string of four characters.
1432 .P
1433 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1434 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1435 which is represented by a two-byte sequence. Similarly, when Unicode property
1436 support is available, \eX{3} matches three Unicode extended sequences, each of
1437 which may be several bytes long (and they may be of different lengths).
1438 .P
1439 The quantifier {0} is permitted, causing the expression to behave as if the
1440 previous item and the quantifier were not present. This may be useful for
1441 subpatterns that are referenced as
1442 .\" HTML <a href="#subpatternsassubroutines">
1443 .\" </a>
1444 subroutines
1445 .\"
1446 from elsewhere in the pattern. Items other than subpatterns that have a {0}
1447 quantifier are omitted from the compiled pattern.
1448 .P
1449 For convenience, the three most common quantifiers have single-character
1450 abbreviations:
1451 .sp
1452 * is equivalent to {0,}
1453 + is equivalent to {1,}
1454 ? is equivalent to {0,1}
1455 .sp
1456 It is possible to construct infinite loops by following a subpattern that can
1457 match no characters with a quantifier that has no upper limit, for example:
1458 .sp
1459 (a?)*
1460 .sp
1461 Earlier versions of Perl and PCRE used to give an error at compile time for
1462 such patterns. However, because there are cases where this can be useful, such
1463 patterns are now accepted, but if any repetition of the subpattern does in fact
1464 match no characters, the loop is forcibly broken.
1465 .P
1466 By default, the quantifiers are "greedy", that is, they match as much as
1467 possible (up to the maximum number of permitted times), without causing the
1468 rest of the pattern to fail. The classic example of where this gives problems
1469 is in trying to match comments in C programs. These appear between /* and */
1470 and within the comment, individual * and / characters may appear. An attempt to
1471 match C comments by applying the pattern
1472 .sp
1473 /\e*.*\e*/
1474 .sp
1475 to the string
1476 .sp
1477 /* first comment */ not comment /* second comment */
1478 .sp
1479 fails, because it matches the entire string owing to the greediness of the .*
1480 item.
1481 .P
1482 However, if a quantifier is followed by a question mark, it ceases to be
1483 greedy, and instead matches the minimum number of times possible, so the
1484 pattern
1485 .sp
1486 /\e*.*?\e*/
1487 .sp
1488 does the right thing with the C comments. The meaning of the various
1489 quantifiers is not otherwise changed, just the preferred number of matches.
1490 Do not confuse this use of question mark with its use as a quantifier in its
1491 own right. Because it has two uses, it can sometimes appear doubled, as in
1492 .sp
1493 \ed??\ed
1494 .sp
1495 which matches one digit by preference, but can match two if that is the only
1496 way the rest of the pattern matches.
1497 .P
1498 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1499 the quantifiers are not greedy by default, but individual ones can be made
1500 greedy by following them with a question mark. In other words, it inverts the
1501 default behaviour.
1502 .P
1503 When a parenthesized subpattern is quantified with a minimum repeat count that
1504 is greater than 1 or with a limited maximum, more memory is required for the
1505 compiled pattern, in proportion to the size of the minimum or maximum.
1506 .P
1507 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1508 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1509 implicitly anchored, because whatever follows will be tried against every
1510 character position in the subject string, so there is no point in retrying the
1511 overall match at any position after the first. PCRE normally treats such a
1512 pattern as though it were preceded by \eA.
1513 .P
1514 In cases where it is known that the subject string contains no newlines, it is
1515 worth setting PCRE_DOTALL in order to obtain this optimization, or
1516 alternatively using ^ to indicate anchoring explicitly.
1517 .P
1518 However, there is one situation where the optimization cannot be used. When .*
1519 is inside capturing parentheses that are the subject of a back reference
1520 elsewhere in the pattern, a match at the start may fail where a later one
1521 succeeds. Consider, for example:
1522 .sp
1523 (.*)abc\e1
1524 .sp
1525 If the subject is "xyz123abc123" the match point is the fourth character. For
1526 this reason, such a pattern is not implicitly anchored.
1527 .P
1528 When a capturing subpattern is repeated, the value captured is the substring
1529 that matched the final iteration. For example, after
1530 .sp
1531 (tweedle[dume]{3}\es*)+
1532 .sp
1533 has matched "tweedledum tweedledee" the value of the captured substring is
1534 "tweedledee". However, if there are nested capturing subpatterns, the
1535 corresponding captured values may have been set in previous iterations. For
1536 example, after
1537 .sp
1538 /(a|(b))+/
1539 .sp
1540 matches "aba" the value of the second captured substring is "b".
1541 .
1542 .
1543 .\" HTML <a name="atomicgroup"></a>
1544 .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1545 .rs
1546 .sp
1547 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1548 repetition, failure of what follows normally causes the repeated item to be
1549 re-evaluated to see if a different number of repeats allows the rest of the
1550 pattern to match. Sometimes it is useful to prevent this, either to change the
1551 nature of the match, or to cause it fail earlier than it otherwise might, when
1552 the author of the pattern knows there is no point in carrying on.
1553 .P
1554 Consider, for example, the pattern \ed+foo when applied to the subject line
1555 .sp
1556 123456bar
1557 .sp
1558 After matching all 6 digits and then failing to match "foo", the normal
1559 action of the matcher is to try again with only 5 digits matching the \ed+
1560 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1561 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1562 that once a subpattern has matched, it is not to be re-evaluated in this way.
1563 .P
1564 If we use atomic grouping for the previous example, the matcher gives up
1565 immediately on failing to match "foo" the first time. The notation is a kind of
1566 special parenthesis, starting with (?> as in this example:
1567 .sp
1568 (?>\ed+)foo
1569 .sp
1570 This kind of parenthesis "locks up" the part of the pattern it contains once
1571 it has matched, and a failure further into the pattern is prevented from
1572 backtracking into it. Backtracking past it to previous items, however, works as
1573 normal.
1574 .P
1575 An alternative description is that a subpattern of this type matches the string
1576 of characters that an identical standalone pattern would match, if anchored at
1577 the current point in the subject string.
1578 .P
1579 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1580 the above example can be thought of as a maximizing repeat that must swallow
1581 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1582 number of digits they match in order to make the rest of the pattern match,
1583 (?>\ed+) can only match an entire sequence of digits.
1584 .P
1585 Atomic groups in general can of course contain arbitrarily complicated
1586 subpatterns, and can be nested. However, when the subpattern for an atomic
1587 group is just a single repeated item, as in the example above, a simpler
1588 notation, called a "possessive quantifier" can be used. This consists of an
1589 additional + character following a quantifier. Using this notation, the
1590 previous example can be rewritten as
1591 .sp
1592 \ed++foo
1593 .sp
1594 Note that a possessive quantifier can be used with an entire group, for
1595 example:
1596 .sp
1597 (abc|xyz){2,3}+
1598 .sp
1599 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1600 option is ignored. They are a convenient notation for the simpler forms of
1601 atomic group. However, there is no difference in the meaning of a possessive
1602 quantifier and the equivalent atomic group, though there may be a performance
1603 difference; possessive quantifiers should be slightly faster.
1604 .P
1605 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1606 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1607 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1608 package, and PCRE copied it from there. It ultimately found its way into Perl
1609 at release 5.10.
1610 .P
1611 PCRE has an optimization that automatically "possessifies" certain simple
1612 pattern constructs. For example, the sequence A+B is treated as A++B because
1613 there is no point in backtracking into a sequence of A's when B must follow.
1614 .P
1615 When a pattern contains an unlimited repeat inside a subpattern that can itself
1616 be repeated an unlimited number of times, the use of an atomic group is the
1617 only way to avoid some failing matches taking a very long time indeed. The
1618 pattern
1619 .sp
1620 (\eD+|<\ed+>)*[!?]
1621 .sp
1622 matches an unlimited number of substrings that either consist of non-digits, or
1623 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1624 quickly. However, if it is applied to
1625 .sp
1626 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1627 .sp
1628 it takes a long time before reporting failure. This is because the string can
1629 be divided between the internal \eD+ repeat and the external * repeat in a
1630 large number of ways, and all have to be tried. (The example uses [!?] rather
1631 than a single character at the end, because both PCRE and Perl have an
1632 optimization that allows for fast failure when a single character is used. They
1633 remember the last single character that is required for a match, and fail early
1634 if it is not present in the string.) If the pattern is changed so that it uses
1635 an atomic group, like this:
1636 .sp
1637 ((?>\eD+)|<\ed+>)*[!?]
1638 .sp
1639 sequences of non-digits cannot be broken, and failure happens quickly.
1640 .
1641 .
1642 .\" HTML <a name="backreferences"></a>
1643 .SH "BACK REFERENCES"
1644 .rs
1645 .sp
1646 Outside a character class, a backslash followed by a digit greater than 0 (and
1647 possibly further digits) is a back reference to a capturing subpattern earlier
1648 (that is, to its left) in the pattern, provided there have been that many
1649 previous capturing left parentheses.
1650 .P
1651 However, if the decimal number following the backslash is less than 10, it is
1652 always taken as a back reference, and causes an error only if there are not
1653 that many capturing left parentheses in the entire pattern. In other words, the
1654 parentheses that are referenced need not be to the left of the reference for
1655 numbers less than 10. A "forward back reference" of this type can make sense
1656 when a repetition is involved and the subpattern to the right has participated
1657 in an earlier iteration.
1658 .P
1659 It is not possible to have a numerical "forward back reference" to a subpattern
1660 whose number is 10 or more using this syntax because a sequence such as \e50 is
1661 interpreted as a character defined in octal. See the subsection entitled
1662 "Non-printing characters"
1663 .\" HTML <a href="#digitsafterbackslash">
1664 .\" </a>
1665 above
1666 .\"
1667 for further details of the handling of digits following a backslash. There is
1668 no such problem when named parentheses are used. A back reference to any
1669 subpattern is possible using named parentheses (see below).
1670 .P
1671 Another way of avoiding the ambiguity inherent in the use of digits following a
1672 backslash is to use the \eg escape sequence, which is a feature introduced in
1673 Perl 5.10. This escape must be followed by an unsigned number or a negative
1674 number, optionally enclosed in braces. These examples are all identical:
1675 .sp
1676 (ring), \e1
1677 (ring), \eg1
1678 (ring), \eg{1}
1679 .sp
1680 An unsigned number specifies an absolute reference without the ambiguity that
1681 is present in the older syntax. It is also useful when literal digits follow
1682 the reference. A negative number is a relative reference. Consider this
1683 example:
1684 .sp
1685 (abc(def)ghi)\eg{-1}
1686 .sp
1687 The sequence \eg{-1} is a reference to the most recently started capturing
1688 subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1689 would be equivalent to \e1. The use of relative references can be helpful in
1690 long patterns, and also in patterns that are created by joining together
1691 fragments that contain references within themselves.
1692 .P
1693 A back reference matches whatever actually matched the capturing subpattern in
1694 the current subject string, rather than anything matching the subpattern
1695 itself (see
1696 .\" HTML <a href="#subpatternsassubroutines">
1697 .\" </a>
1698 "Subpatterns as subroutines"
1699 .\"
1700 below for a way of doing that). So the pattern
1701 .sp
1702 (sens|respons)e and \e1ibility
1703 .sp
1704 matches "sense and sensibility" and "response and responsibility", but not
1705 "sense and responsibility". If caseful matching is in force at the time of the
1706 back reference, the case of letters is relevant. For example,
1707 .sp
1708 ((?i)rah)\es+\e1
1709 .sp
1710 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1711 capturing subpattern is matched caselessly.
1712 .P
1713 There are several different ways of writing back references to named
1714 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1715 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1716 back reference syntax, in which \eg can be used for both numeric and named
1717 references, is also supported. We could rewrite the above example in any of
1718 the following ways:
1719 .sp
1720 (?<p1>(?i)rah)\es+\ek<p1>
1721 (?'p1'(?i)rah)\es+\ek{p1}
1722 (?P<p1>(?i)rah)\es+(?P=p1)
1723 (?<p1>(?i)rah)\es+\eg{p1}
1724 .sp
1725 A subpattern that is referenced by name may appear in the pattern before or
1726 after the reference.
1727 .P
1728 There may be more than one back reference to the same subpattern. If a
1729 subpattern has not actually been used in a particular match, any back
1730 references to it always fail by default. For example, the pattern
1731 .sp
1732 (a|(bc))\e2
1733 .sp
1734 always fails if it starts to match "a" rather than "bc". However, if the
1735 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1736 unset value matches an empty string.
1737 .P
1738 Because there may be many capturing parentheses in a pattern, all digits
1739 following a backslash are taken as part of a potential back reference number.
1740 If the pattern continues with a digit character, some delimiter must be used to
1741 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1742 whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1743 .\" HTML <a href="#comments">
1744 .\" </a>
1745 "Comments"
1746 .\"
1747 below) can be used.
1748 .
1749 .SS "Recursive back references"
1750 .rs
1751 .sp
1752 A back reference that occurs inside the parentheses to which it refers fails
1753 when the subpattern is first used, so, for example, (a\e1) never matches.
1754 However, such references can be useful inside repeated subpatterns. For
1755 example, the pattern
1756 .sp
1757 (a|b\e1)+
1758 .sp
1759 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1760 the subpattern, the back reference matches the character string corresponding
1761 to the previous iteration. In order for this to work, the pattern must be such
1762 that the first iteration does not need to match the back reference. This can be
1763 done using alternation, as in the example above, or by a quantifier with a
1764 minimum of zero.
1765 .P
1766 Back references of this type cause the group that they reference to be treated
1767 as an
1768 .\" HTML <a href="#atomicgroup">
1769 .\" </a>
1770 atomic group.
1771 .\"
1772 Once the whole group has been matched, a subsequent matching failure cannot
1773 cause backtracking into the middle of the group.
1774 .
1775 .
1776 .\" HTML <a name="bigassertions"></a>
1777 .SH ASSERTIONS
1778 .rs
1779 .sp
1780 An assertion is a test on the characters following or preceding the current
1781 matching point that does not actually consume any characters. The simple
1782 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1783 .\" HTML <a href="#smallassertions">
1784 .\" </a>
1785 above.
1786 .\"
1787 .P
1788 More complicated assertions are coded as subpatterns. There are two kinds:
1789 those that look ahead of the current position in the subject string, and those
1790 that look behind it. An assertion subpattern is matched in the normal way,
1791 except that it does not cause the current matching position to be changed.
1792 .P
1793 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1794 because it makes no sense to assert the same thing several times. If any kind
1795 of assertion contains capturing subpatterns within it, these are counted for
1796 the purposes of numbering the capturing subpatterns in the whole pattern.
1797 However, substring capturing is carried out only for positive assertions,
1798 because it does not make sense for negative assertions.
1799 .
1800 .
1801 .SS "Lookahead assertions"
1802 .rs
1803 .sp
1804 Lookahead assertions start with (?= for positive assertions and (?! for
1805 negative assertions. For example,
1806 .sp
1807 \ew+(?=;)
1808 .sp
1809 matches a word followed by a semicolon, but does not include the semicolon in
1810 the match, and
1811 .sp
1812 foo(?!bar)
1813 .sp
1814 matches any occurrence of "foo" that is not followed by "bar". Note that the
1815 apparently similar pattern
1816 .sp
1817 (?!foo)bar
1818 .sp
1819 does not find an occurrence of "bar" that is preceded by something other than
1820 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1821 (?!foo) is always true when the next three characters are "bar". A
1822 lookbehind assertion is needed to achieve the other effect.
1823 .P
1824 If you want to force a matching failure at some point in a pattern, the most
1825 convenient way to do it is with (?!) because an empty string always matches, so
1826 an assertion that requires there not to be an empty string must always fail.
1827 The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a
1828 synonym for (?!).
1829 .
1830 .
1831 .\" HTML <a name="lookbehind"></a>
1832 .SS "Lookbehind assertions"
1833 .rs
1834 .sp
1835 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1836 negative assertions. For example,
1837 .sp
1838 (?<!foo)bar
1839 .sp
1840 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1841 a lookbehind assertion are restricted such that all the strings it matches must
1842 have a fixed length. However, if there are several top-level alternatives, they
1843 do not all have to have the same fixed length. Thus
1844 .sp
1845 (?<=bullock|donkey)
1846 .sp
1847 is permitted, but
1848 .sp
1849 (?<!dogs?|cats?)
1850 .sp
1851 causes an error at compile time. Branches that match different length strings
1852 are permitted only at the top level of a lookbehind assertion. This is an
1853 extension compared with Perl (5.8 and 5.10), which requires all branches to
1854 match the same length of string. An assertion such as
1855 .sp
1856 (?<=ab(c|de))
1857 .sp
1858 is not permitted, because its single top-level branch can match two different
1859 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1860 branches:
1861 .sp
1862 (?<=abc|abde)
1863 .sp
1864 In some cases, the Perl 5.10 escape sequence \eK
1865 .\" HTML <a href="#resetmatchstart">
1866 .\" </a>
1867 (see above)
1868 .\"
1869 can be used instead of a lookbehind assertion to get round the fixed-length
1870 restriction.
1871 .P
1872 The implementation of lookbehind assertions is, for each alternative, to
1873 temporarily move the current position back by the fixed length and then try to
1874 match. If there are insufficient characters before the current position, the
1875 assertion fails.
1876 .P
1877 PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1878 to appear in lookbehind assertions, because it makes it impossible to calculate
1879 the length of the lookbehind. The \eX and \eR escapes, which can match
1880 different numbers of bytes, are also not permitted.
1881 .P
1882 .\" HTML <a href="#subpatternsassubroutines">
1883 .\" </a>
1884 "Subroutine"
1885 .\"
1886 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1887 as the subpattern matches a fixed-length string.
1888 .\" HTML <a href="#recursion">
1889 .\" </a>
1890 Recursion,
1891 .\"
1892 however, is not supported.
1893 .P
1894 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1895 specify efficient matching of fixed-length strings at the end of subject
1896 strings. Consider a simple pattern such as
1897 .sp
1898 abcd$
1899 .sp
1900 when applied to a long string that does not match. Because matching proceeds
1901 from left to right, PCRE will look for each "a" in the subject and then see if
1902 what follows matches the rest of the pattern. If the pattern is specified as
1903 .sp
1904 ^.*abcd$
1905 .sp
1906 the initial .* matches the entire string at first, but when this fails (because
1907 there is no following "a"), it backtracks to match all but the last character,
1908 then all but the last two characters, and so on. Once again the search for "a"
1909 covers the entire string, from right to left, so we are no better off. However,
1910 if the pattern is written as
1911 .sp
1912 ^.*+(?<=abcd)
1913 .sp
1914 there can be no backtracking for the .*+ item; it can match only the entire
1915 string. The subsequent lookbehind assertion does a single test on the last four
1916 characters. If it fails, the match fails immediately. For long strings, this
1917 approach makes a significant difference to the processing time.
1918 .
1919 .
1920 .SS "Using multiple assertions"
1921 .rs
1922 .sp
1923 Several assertions (of any sort) may occur in succession. For example,
1924 .sp
1925 (?<=\ed{3})(?<!999)foo
1926 .sp
1927 matches "foo" preceded by three digits that are not "999". Notice that each of
1928 the assertions is applied independently at the same point in the subject
1929 string. First there is a check that the previous three characters are all
1930 digits, and then there is a check that the same three characters are not "999".
1931 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1932 of which are digits and the last three of which are not "999". For example, it
1933 doesn't match "123abcfoo". A pattern to do that is
1934 .sp
1935 (?<=\ed{3}...)(?<!999)foo
1936 .sp
1937 This time the first assertion looks at the preceding six characters, checking
1938 that the first three are digits, and then the second assertion checks that the
1939 preceding three characters are not "999".
1940 .P
1941 Assertions can be nested in any combination. For example,
1942 .sp
1943 (?<=(?<!foo)bar)baz
1944 .sp
1945 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1946 preceded by "foo", while
1947 .sp
1948 (?<=\ed{3}(?!999)...)foo
1949 .sp
1950 is another pattern that matches "foo" preceded by three digits and any three
1951 characters that are not "999".
1952 .
1953 .
1954 .\" HTML <a name="conditions"></a>
1955 .SH "CONDITIONAL SUBPATTERNS"
1956 .rs
1957 .sp
1958 It is possible to cause the matching process to obey a subpattern
1959 conditionally or to choose between two alternative subpatterns, depending on
1960 the result of an assertion, or whether a specific capturing subpattern has
1961 already been matched. The two possible forms of conditional subpattern are:
1962 .sp
1963 (?(condition)yes-pattern)
1964 (?(condition)yes-pattern|no-pattern)
1965 .sp
1966 If the condition is satisfied, the yes-pattern is used; otherwise the
1967 no-pattern (if present) is used. If there are more than two alternatives in the
1968 subpattern, a compile-time error occurs.
1969 .P
1970 There are four kinds of condition: references to subpatterns, references to
1971 recursion, a pseudo-condition called DEFINE, and assertions.
1972 .
1973 .SS "Checking for a used subpattern by number"
1974 .rs
1975 .sp
1976 If the text between the parentheses consists of a sequence of digits, the
1977 condition is true if a capturing subpattern of that number has previously
1978 matched. If there is more than one capturing subpattern with the same number
1979 (see the earlier
1980 .\"
1981 .\" HTML <a href="#recursion">
1982 .\" </a>
1983 section about duplicate subpattern numbers),
1984 .\"
1985 the condition is true if any of them have been set. An alternative notation is
1986 to precede the digits with a plus or minus sign. In this case, the subpattern
1987 number is relative rather than absolute. The most recently opened parentheses
1988 can be referenced by (?(-1), the next most recent by (?(-2), and so on. In
1989 looping constructs it can also make sense to refer to subsequent groups with
1990 constructs such as (?(+2).
1991 .P
1992 Consider the following pattern, which contains non-significant white space to
1993 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1994 three parts for ease of discussion:
1995 .sp
1996 ( \e( )? [^()]+ (?(1) \e) )
1997 .sp
1998 The first part matches an optional opening parenthesis, and if that
1999 character is present, sets it as the first captured substring. The second part
2000 matches one or more characters that are not parentheses. The third part is a
2001 conditional subpattern that tests whether the first set of parentheses matched
2002 or not. If they did, that is, if subject started with an opening parenthesis,
2003 the condition is true, and so the yes-pattern is executed and a closing
2004 parenthesis is required. Otherwise, since no-pattern is not present, the
2005 subpattern matches nothing. In other words, this pattern matches a sequence of
2006 non-parentheses, optionally enclosed in parentheses.
2007 .P
2008 If you were embedding this pattern in a larger one, you could use a relative
2009 reference:
2010 .sp
2011 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2012 .sp
2013 This makes the fragment independent of the parentheses in the larger pattern.
2014 .
2015 .SS "Checking for a used subpattern by name"
2016 .rs
2017 .sp
2018 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2019 subpattern by name. For compatibility with earlier versions of PCRE, which had
2020 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2021 there is a possible ambiguity with this syntax, because subpattern names may
2022 consist entirely of digits. PCRE looks first for a named subpattern; if it
2023 cannot find one and the name consists entirely of digits, PCRE looks for a
2024 subpattern of that number, which must be greater than zero. Using subpattern
2025 names that consist entirely of digits is not recommended.
2026 .P
2027 Rewriting the above example to use a named subpattern gives this:
2028 .sp
2029 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2030 .sp
2031 If the name used in a condition of this kind is a duplicate, the test is
2032 applied to all subpatterns of the same name, and is true if any one of them has
2033 matched.
2034 .
2035 .SS "Checking for pattern recursion"
2036 .rs
2037 .sp
2038 If the condition is the string (R), and there is no subpattern with the name R,
2039 the condition is true if a recursive call to the whole pattern or any
2040 subpattern has been made. If digits or a name preceded by ampersand follow the
2041 letter R, for example:
2042 .sp
2043 (?(R3)...) or (?(R&name)...)
2044 .sp
2045 the condition is true if the most recent recursion is into a subpattern whose
2046 number or name is given. This condition does not check the entire recursion
2047 stack. If the name used in a condition of this kind is a duplicate, the test is
2048 applied to all subpatterns of the same name, and is true if any one of them is
2049 the most recent recursion.
2050 .P
2051 At "top level", all these recursion test conditions are false.
2052 .\" HTML <a href="#recursion">
2053 .\" </a>
2054 The syntax for recursive patterns
2055 .\"
2056 is described below.
2057 .
2058 .SS "Defining subpatterns for use by reference only"
2059 .rs
2060 .sp
2061 If the condition is the string (DEFINE), and there is no subpattern with the
2062 name DEFINE, the condition is always false. In this case, there may be only one
2063 alternative in the subpattern. It is always skipped if control reaches this
2064 point in the pattern; the idea of DEFINE is that it can be used to define
2065 "subroutines" that can be referenced from elsewhere. (The use of
2066 .\" HTML <a href="#subpatternsassubroutines">
2067 .\" </a>
2068 "subroutines"
2069 .\"
2070 is described below.) For example, a pattern to match an IPv4 address could be
2071 written like this (ignore whitespace and line breaks):
2072 .sp
2073 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2074 \eb (?&byte) (\e.(?&byte)){3} \eb
2075 .sp
2076 The first part of the pattern is a DEFINE group inside which a another group
2077 named "byte" is defined. This matches an individual component of an IPv4
2078 address (a number less than 256). When matching takes place, this part of the
2079 pattern is skipped because DEFINE acts like a false condition. The rest of the
2080 pattern uses references to the named group to match the four dot-separated
2081 components of an IPv4 address, insisting on a word boundary at each end.
2082 .
2083 .SS "Assertion conditions"
2084 .rs
2085 .sp
2086 If the condition is not in any of the above formats, it must be an assertion.
2087 This may be a positive or negative lookahead or lookbehind assertion. Consider
2088 this pattern, again containing non-significant white space, and with the two
2089 alternatives on the second line:
2090 .sp
2091 (?(?=[^a-z]*[a-z])
2092 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2093 .sp
2094 The condition is a positive lookahead assertion that matches an optional
2095 sequence of non-letters followed by a letter. In other words, it tests for the
2096 presence of at least one letter in the subject. If a letter is found, the
2097 subject is matched against the first alternative; otherwise it is matched
2098 against the second. This pattern matches strings in one of the two forms
2099 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2100 .
2101 .
2102 .\" HTML <a name="comments"></a>
2103 .SH COMMENTS
2104 .rs
2105 .sp
2106 The sequence (?# marks the start of a comment that continues up to the next
2107 closing parenthesis. Nested parentheses are not permitted. The characters
2108 that make up a comment play no part in the pattern matching at all.
2109 .P
2110 If the PCRE_EXTENDED option is set, an unescaped # character outside a
2111 character class introduces a comment that continues to immediately after the
2112 next newline in the pattern.
2113 .
2114 .
2115 .\" HTML <a name="recursion"></a>
2116 .SH "RECURSIVE PATTERNS"
2117 .rs
2118 .sp
2119 Consider the problem of matching a string in parentheses, allowing for
2120 unlimited nested parentheses. Without the use of recursion, the best that can
2121 be done is to use a pattern that matches up to some fixed depth of nesting. It
2122 is not possible to handle an arbitrary nesting depth.
2123 .P
2124 For some time, Perl has provided a facility that allows regular expressions to
2125 recurse (amongst other things). It does this by interpolating Perl code in the
2126 expression at run time, and the code can refer to the expression itself. A Perl
2127 pattern using code interpolation to solve the parentheses problem can be
2128 created like this:
2129 .sp
2130 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2131 .sp
2132 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2133 recursively to the pattern in which it appears.
2134 .P
2135 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2136 supports special syntax for recursion of the entire pattern, and also for
2137 individual subpattern recursion. After its introduction in PCRE and Python,
2138 this kind of recursion was subsequently introduced into Perl at release 5.10.
2139 .P
2140 A special item that consists of (? followed by a number greater than zero and a
2141 closing parenthesis is a recursive call of the subpattern of the given number,
2142 provided that it occurs inside that subpattern. (If not, it is a
2143 .\" HTML <a href="#subpatternsassubroutines">
2144 .\" </a>
2145 "subroutine"
2146 .\"
2147 call, which is described in the next section.) The special item (?R) or (?0) is
2148 a recursive call of the entire regular expression.
2149 .P
2150 This PCRE pattern solves the nested parentheses problem (assume the
2151 PCRE_EXTENDED option is set so that white space is ignored):
2152 .sp
2153 \e( ( [^()]++ | (?R) )* \e)
2154 .sp
2155 First it matches an opening parenthesis. Then it matches any number of
2156 substrings which can either be a sequence of non-parentheses, or a recursive
2157 match of the pattern itself (that is, a correctly parenthesized substring).
2158 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2159 to avoid backtracking into sequences of non-parentheses.
2160 .P
2161 If this were part of a larger pattern, you would not want to recurse the entire
2162 pattern, so instead you could use this:
2163 .sp
2164 ( \e( ( [^()]++ | (?1) )* \e) )
2165 .sp
2166 We have put the pattern into parentheses, and caused the recursion to refer to
2167 them instead of the whole pattern.
2168 .P
2169 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2170 is made easier by the use of relative references (a Perl 5.10 feature).
2171 Instead of (?1) in the pattern above you can write (?-2) to refer to the second
2172 most recently opened parentheses preceding the recursion. In other words, a
2173 negative number counts capturing parentheses leftwards from the point at which
2174 it is encountered.
2175 .P
2176 It is also possible to refer to subsequently opened parentheses, by writing
2177 references such as (?+2). However, these cannot be recursive because the
2178 reference is not inside the parentheses that are referenced. They are always
2179 .\" HTML <a href="#subpatternsassubroutines">
2180 .\" </a>
2181 "subroutine"
2182 .\"
2183 calls, as described in the next section.
2184 .P
2185 An alternative approach is to use named parentheses instead. The Perl syntax
2186 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2187 could rewrite the above example as follows:
2188 .sp
2189 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2190 .sp
2191 If there is more than one subpattern with the same name, the earliest one is
2192 used.
2193 .P
2194 This particular example pattern that we have been looking at contains nested
2195 unlimited repeats, and so the use of a possessive quantifier for matching
2196 strings of non-parentheses is important when applying the pattern to strings
2197 that do not match. For example, when this pattern is applied to
2198 .sp
2199 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2200 .sp
2201 it yields "no match" quickly. However, if a possessive quantifier is not used,
2202 the match runs for a very long time indeed because there are so many different
2203 ways the + and * repeats can carve up the subject, and all have to be tested
2204 before failure can be reported.
2205 .P
2206 At the end of a match, the values of capturing parentheses are those from
2207 the outermost level. If you want to obtain intermediate values, a callout
2208 function can be used (see below and the
2209 .\" HREF
2210 \fBpcrecallout\fP
2211 .\"
2212 documentation). If the pattern above is matched against
2213 .sp
2214 (ab(cd)ef)
2215 .sp
2216 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2217 the last value taken on at the top level. If a capturing subpattern is not
2218 matched at the top level, its final value is unset, even if it is (temporarily)
2219 set at a deeper level.
2220 .P
2221 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2222 obtain extra memory to store data during a recursion, which it does by using
2223 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2224 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2225 .P
2226 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2227 Consider this pattern, which matches text in angle brackets, allowing for
2228 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2229 recursing), whereas any characters are permitted at the outer level.
2230 .sp
2231 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2232 .sp
2233 In this pattern, (?(R) is the start of a conditional subpattern, with two
2234 different alternatives for the recursive and non-recursive cases. The (?R) item
2235 is the actual recursive call.
2236 .
2237 .
2238 .\" HTML <a name="recursiondifference"></a>
2239 .SS "Recursion difference from Perl"
2240 .rs
2241 .sp
2242 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2243 treated as an atomic group. That is, once it has matched some of the subject
2244 string, it is never re-entered, even if it contains untried alternatives and
2245 there is a subsequent matching failure. This can be illustrated by the
2246 following pattern, which purports to match a palindromic string that contains
2247 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2248 .sp
2249 ^(.|(.)(?1)\e2)$
2250 .sp
2251 The idea is that it either matches a single character, or two identical
2252 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2253 it does not if the pattern is longer than three characters. Consider the
2254 subject string "abcba":
2255 .P
2256 At the top level, the first character is matched, but as it is not at the end
2257 of the string, the first alternative fails; the second alternative is taken
2258 and the recursion kicks in. The recursive call to subpattern 1 successfully
2259 matches the next character ("b"). (Note that the beginning and end of line
2260 tests are not part of the recursion).
2261 .P
2262 Back at the top level, the next character ("c") is compared with what
2263 subpattern 2 matched, which was "a". This fails. Because the recursion is
2264 treated as an atomic group, there are now no backtracking points, and so the
2265 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2266 try the second alternative.) However, if the pattern is written with the
2267 alternatives in the other order, things are different:
2268 .sp
2269 ^((.)(?1)\e2|.)$
2270 .sp
2271 This time, the recursing alternative is tried first, and continues to recurse
2272 until it runs out of characters, at which point the recursion fails. But this
2273 time we do have another alternative to try at the higher level. That is the big
2274 difference: in the previous case the remaining alternative is at a deeper
2275 recursion level, which PCRE cannot use.
2276 .P
2277 To change the pattern so that matches all palindromic strings, not just those
2278 with an odd number of characters, it is tempting to change the pattern to this:
2279 .sp
2280 ^((.)(?1)\e2|.?)$
2281 .sp
2282 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2283 deeper recursion has matched a single character, it cannot be entered again in
2284 order to match an empty string. The solution is to separate the two cases, and
2285 write out the odd and even cases as alternatives at the higher level:
2286 .sp
2287 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2288 .sp
2289 If you want to match typical palindromic phrases, the pattern has to ignore all
2290 non-word characters, which can be done like this:
2291 .sp
2292 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2293 .sp
2294 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2295 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2296 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2297 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2298 more) to match typical phrases, and Perl takes so long that you think it has
2299 gone into a loop.
2300 .P
2301 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2302 string does not start with a palindrome that is shorter than the entire string.
2303 For example, although "abcba" is correctly matched, if the subject is "ababa",
2304 PCRE finds the palindrome "aba" at the start, then fails at top level because
2305 the end of the string does not follow. Once again, it cannot jump back into the
2306 recursion to try other alternatives, so the entire match fails.
2307 .
2308 .
2309 .\" HTML <a name="subpatternsassubroutines"></a>
2310 .SH "SUBPATTERNS AS SUBROUTINES"
2311 .rs
2312 .sp
2313 If the syntax for a recursive subpattern reference (either by number or by
2314 name) is used outside the parentheses to which it refers, it operates like a
2315 subroutine in a programming language. The "called" subpattern may be defined
2316 before or after the reference. A numbered reference can be absolute or
2317 relative, as in these examples:
2318 .sp
2319 (...(absolute)...)...(?2)...
2320 (...(relative)...)...(?-1)...
2321 (...(?+1)...(relative)...
2322 .sp
2323 An earlier example pointed out that the pattern
2324 .sp
2325 (sens|respons)e and \e1ibility
2326 .sp
2327 matches "sense and sensibility" and "response and responsibility", but not
2328 "sense and responsibility". If instead the pattern
2329 .sp
2330 (sens|respons)e and (?1)ibility
2331 .sp
2332 is used, it does match "sense and responsibility" as well as the other two
2333 strings. Another example is given in the discussion of DEFINE above.
2334 .P
2335 Like recursive subpatterns, a subroutine call is always treated as an atomic
2336 group. That is, once it has matched some of the subject string, it is never
2337 re-entered, even if it contains untried alternatives and there is a subsequent
2338 matching failure. Any capturing parentheses that are set during the subroutine
2339 call revert to their previous values afterwards.
2340 .P
2341 When a subpattern is used as a subroutine, processing options such as
2342 case-independence are fixed when the subpattern is defined. They cannot be
2343 changed for different calls. For example, consider this pattern:
2344 .sp
2345 (abc)(?i:(?-1))
2346 .sp
2347 It matches "abcabc". It does not match "abcABC" because the change of
2348 processing option does not affect the called subpattern.
2349 .
2350 .
2351 .\" HTML <a name="onigurumasubroutines"></a>
2352 .SH "ONIGURUMA SUBROUTINE SYNTAX"
2353 .rs
2354 .sp
2355 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2356 a number enclosed either in angle brackets or single quotes, is an alternative
2357 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2358 are two of the examples used above, rewritten using this syntax:
2359 .sp
2360 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2361 (sens|respons)e and \eg'1'ibility
2362 .sp
2363 PCRE supports an extension to Oniguruma: if a number is preceded by a
2364 plus or a minus sign it is taken as a relative reference. For example:
2365 .sp
2366 (abc)(?i:\eg<-1>)
2367 .sp
2368 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2369 synonymous. The former is a back reference; the latter is a subroutine call.
2370 .
2371 .
2372 .SH CALLOUTS
2373 .rs
2374 .sp
2375 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2376 code to be obeyed in the middle of matching a regular expression. This makes it
2377 possible, amongst other things, to extract different substrings that match the
2378 same pair of parentheses when there is a repetition.
2379 .P
2380 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2381 code. The feature is called "callout". The caller of PCRE provides an external
2382 function by putting its entry point in the global variable \fIpcre_callout\fP.
2383 By default, this variable contains NULL, which disables all calling out.
2384 .P
2385 Within a regular expression, (?C) indicates the points at which the external
2386 function is to be called. If you want to identify different callout points, you
2387 can put a number less than 256 after the letter C. The default value is zero.
2388 For example, this pattern has two callout points:
2389 .sp
2390 (?C1)abc(?C2)def
2391 .sp
2392 If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2393 automatically installed before each item in the pattern. They are all numbered
2394 255.
2395 .P
2396 During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2397 set), the external function is called. It is provided with the number of the
2398 callout, the position in the pattern, and, optionally, one item of data
2399 originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2400 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2401 description of the interface to the callout function is given in the
2402 .\" HREF
2403 \fBpcrecallout\fP
2404 .\"
2405 documentation.
2406 .
2407 .
2408 .\" HTML <a name="backtrackcontrol"></a>
2409 .SH "BACKTRACKING CONTROL"
2410 .rs
2411 .sp
2412 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2413 are described in the Perl documentation as "experimental and subject to change
2414 or removal in a future version of Perl". It goes on to say: "Their usage in
2415 production code should be noted to avoid problems during upgrades." The same
2416 remarks apply to the PCRE features described in this section.
2417 .P
2418 Since these verbs are specifically related to backtracking, most of them can be
2419 used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2420 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2421 failing negative assertion, they cause an error if encountered by
2422 \fBpcre_dfa_exec()\fP.
2423 .P
2424 If any of these verbs are used in an assertion or subroutine subpattern
2425 (including recursive subpatterns), their effect is confined to that subpattern;
2426 it does not extend to the surrounding pattern. Note that such subpatterns are
2427 processed as anchored at the point where they are tested.
2428 .P
2429 The new verbs make use of what was previously invalid syntax: an opening
2430 parenthesis followed by an asterisk. They are generally of the form
2431 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2432 depending on whether or not an argument is present. An name is a sequence of
2433 letters, digits, and underscores. If the name is empty, that is, if the closing
2434 parenthesis immediately follows the colon, the effect is as if the colon were
2435 not there. Any number of these verbs may occur in a pattern.
2436 .P
2437 PCRE contains some optimizations that are used to speed up matching by running
2438 some checks at the start of each match attempt. For example, it may know the
2439 minimum length of matching subject, or that a particular character must be
2440 present. When one of these optimizations suppresses the running of a match, any
2441 included backtracking verbs will not, of course, be processed. You can suppress
2442 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2443 when calling \fBpcre_exec()\fP.
2444 .
2445 .
2446 .SS "Verbs that act immediately"
2447 .rs
2448 .sp
2449 The following verbs act as soon as they are encountered. They may not be
2450 followed by a name.
2451 .sp
2452 (*ACCEPT)
2453 .sp
2454 This verb causes the match to end successfully, skipping the remainder of the
2455 pattern. When inside a recursion, only the innermost pattern is ended
2456 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2457 captured. (This feature was added to PCRE at release 8.00.) For example:
2458 .sp
2459 A((?:A|B(*ACCEPT)|C)D)
2460 .sp
2461 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2462 the outer parentheses.
2463 .sp
2464 (*FAIL) or (*F)
2465 .sp
2466 This verb causes the match to fail, forcing backtracking to occur. It is
2467 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2468 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2469 Perl features that are not present in PCRE. The nearest equivalent is the
2470 callout feature, as for example in this pattern:
2471 .sp
2472 a+(?C)(*FAIL)
2473 .sp
2474 A match with the string "aaaa" always fails, but the callout is taken before
2475 each backtrack happens (in this example, 10 times).
2476 .
2477 .
2478 .SS "Recording which path was taken"
2479 .rs
2480 .sp
2481 There is one verb whose main purpose is to track how a match was arrived at,
2482 though it also has a secondary use in conjunction with advancing the match
2483 starting point (see (*SKIP) below).
2484 .sp
2485 (*MARK:NAME) or (*:NAME)
2486 .sp
2487 A name is always required with this verb. There may be as many instances of
2488 (*MARK) as you like in a pattern, and their names do not have to be unique.
2489 .P
2490 When a match succeeds, the name of the last-encountered (*MARK) is passed back
2491 to the caller via the \fIpcre_extra\fP data structure, as described in the
2492 .\" HTML <a href="pcreapi.html#extradata">
2493 .\" </a>
2494 section on \fIpcre_extra\fP
2495 .\"
2496 in the
2497 .\" HREF
2498 \fBpcreapi\fP
2499 .\"
2500 documentation. No data is returned for a partial match. Here is an example of
2501 \fBpcretest\fP output, where the /K modifier requests the retrieval and
2502 outputting of (*MARK) data:
2503 .sp
2504 /X(*MARK:A)Y|X(*MARK:B)Z/K
2505 XY
2506 0: XY
2507 MK: A
2508 XZ
2509 0: XZ
2510 MK: B
2511 .sp
2512 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2513 indicates which of the two alternatives matched. This is a more efficient way
2514 of obtaining this information than putting each alternative in its own
2515 capturing parentheses.
2516 .P
2517 A name may also be returned after a failed match if the final path through the
2518 pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2519 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2520 starting point for matching is advanced, the final check is often with an empty
2521 string, causing a failure before (*MARK) is reached. For example:
2522 .sp
2523 /X(*MARK:A)Y|X(*MARK:B)Z/K
2524 XP
2525 No match
2526 .sp
2527 There are three potential starting points for this match (starting with X,
2528 starting with P, and with an empty string). If the pattern is anchored, the
2529 result is different:
2530 .sp
2531 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2532 XP
2533 No match, mark = B
2534 .sp
2535 PCRE's start-of-match optimizations can also interfere with this. For example,
2536 if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2537 subject length for a match, a shorter subject will not be scanned at all.
2538 .P
2539 Note that similar anomalies (though different in detail) exist in Perl, no
2540 doubt for the same reasons. The use of (*MARK) data after a failed match of an
2541 unanchored pattern is not recommended, unless (*COMMIT) is involved.
2542 .
2543 .
2544 .SS "Verbs that act after backtracking"
2545 .rs
2546 .sp
2547 The following verbs do nothing when they are encountered. Matching continues
2548 with what follows, but if there is no subsequent match, causing a backtrack to
2549 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2550 the verb. However, when one of these verbs appears inside an atomic group, its
2551 effect is confined to that group, because once the group has been matched,
2552 there is never any backtracking into it. In this situation, backtracking can
2553 "jump back" to the left of the entire atomic group. (Remember also, as stated
2554 above, that this localization also applies in subroutine calls and assertions.)
2555 .P
2556 These verbs differ in exactly what kind of failure occurs when backtracking
2557 reaches them.
2558 .sp
2559 (*COMMIT)
2560 .sp
2561 This verb, which may not be followed by a name, causes the whole match to fail
2562 outright if the rest of the pattern does not match. Even if the pattern is
2563 unanchored, no further attempts to find a match by advancing the starting point
2564 take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2565 finding a match at the current starting point, or not at all. For example:
2566 .sp
2567 a+(*COMMIT)b
2568 .sp
2569 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2570 dynamic anchor, or "I've started, so I must finish." The name of the most
2571 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2572 match failure.
2573 .P
2574 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2575 unless PCRE's start-of-match optimizations are turned off, as shown in this
2576 \fBpcretest\fP example:
2577 .sp
2578 /(*COMMIT)abc/
2579 xyzabc
2580 0: abc
2581 xyzabc\eY
2582 No match
2583 .sp
2584 PCRE knows that any match must start with "a", so the optimization skips along
2585 the subject to "a" before running the first match attempt, which succeeds. When
2586 the optimization is disabled by the \eY escape in the second subject, the match
2587 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2588 starting points.
2589 .sp
2590 (*PRUNE) or (*PRUNE:NAME)
2591 .sp
2592 This verb causes the match to fail at the current starting position in the
2593 subject if the rest of the pattern does not match. If the pattern is
2594 unanchored, the normal "bumpalong" advance to the next starting character then
2595 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2596 reached, or when matching to the right of (*PRUNE), but if there is no match to
2597 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2598 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2599 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2600 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2601 match fails completely; the name is passed back if this is the final attempt.
2602 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2603 pattern (*PRUNE) has the same effect as (*COMMIT).
2604 .sp
2605 (*SKIP)
2606 .sp
2607 This verb, when given without a name, is like (*PRUNE), except that if the
2608 pattern is unanchored, the "bumpalong" advance is not to the next character,
2609 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2610 signifies that whatever text was matched leading up to it cannot be part of a
2611 successful match. Consider:
2612 .sp
2613 a+(*SKIP)b
2614 .sp
2615 If the subject is "aaaac...", after the first match attempt fails (starting at
2616 the first character in the string), the starting point skips on to start the
2617 next attempt at "c". Note that a possessive quantifer does not have the same
2618 effect as this example; although it would suppress backtracking during the
2619 first match attempt, the second attempt would start at the second character
2620 instead of skipping on to "c".
2621 .sp
2622 (*SKIP:NAME)
2623 .sp
2624 When (*SKIP) has an associated name, its behaviour is modified. If the
2625 following pattern fails to match, the previous path through the pattern is
2626 searched for the most recent (*MARK) that has the same name. If one is found,
2627 the "bumpalong" advance is to the subject position that corresponds to that
2628 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2629 matching name is found, normal "bumpalong" of one character happens (the
2630 (*SKIP) is ignored).
2631 .sp
2632 (*THEN) or (*THEN:NAME)
2633 .sp
2634 This verb causes a skip to the next alternation in the innermost enclosing
2635 group if the rest of the pattern does not match. That is, it cancels pending
2636 backtracking, but only within the current alternation. Its name comes from the
2637 observation that it can be used for a pattern-based if-then-else block:
2638 .sp
2639 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2640 .sp
2641 If the COND1 pattern matches, FOO is tried (and possibly further items after
2642 the end of the group if FOO succeeds); on failure the matcher skips to the
2643 second alternative and tries COND2, without backtracking into COND1. The
2644 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2645 overall match fails. If (*THEN) is not directly inside an alternation, it acts
2646 like (*PRUNE).
2647 .
2648 .P
2649 The above verbs provide four different "strengths" of control when subsequent
2650 matching fails. (*THEN) is the weakest, carrying on the match at the next
2651 alternation. (*PRUNE) comes next, failing the match at the current starting
2652 position, but allowing an advance to the next character (for an unanchored
2653 pattern). (*SKIP) is similar, except that the advance may be more than one
2654 character. (*COMMIT) is the strongest, causing the entire match to fail.
2655 .P
2656 If more than one is present in a pattern, the "stongest" one wins. For example,
2657 consider this pattern, where A, B, etc. are complex pattern fragments:
2658 .sp
2659 (A(*COMMIT)B(*THEN)C|D)
2660 .sp
2661 Once A has matched, PCRE is committed to this match, at the current starting
2662 position. If subsequently B matches, but C does not, the normal (*THEN) action
2663 of trying the next alternation (that is, D) does not happen because (*COMMIT)
2664 overrides.
2665 .
2666 .
2667 .SH "SEE ALSO"
2668 .rs
2669 .sp
2670 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2671 \fBpcresyntax\fP(3), \fBpcre\fP(3).
2672 .
2673 .
2674 .SH AUTHOR
2675 .rs
2676 .sp
2677 .nf
2678 Philip Hazel
2679 University Computing Service
2680 Cambridge CB2 3QH, England.
2681 .fi
2682 .
2683 .
2684 .SH REVISION
2685 .rs
2686 .sp
2687 .nf
2688 Last updated: 26 October 2010
2689 Copyright (c) 1997-2010 University of Cambridge.
2690 .fi

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