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3 PCRE - Perl-compatible regular expressions
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 .\" HTML <a name="newlines"></a>
71 .rs
72 .sp
73 PCRE supports five different conventions for indicating line breaks in
74 strings: a single CR (carriage return) character, a single LF (linefeed)
75 character, the two-character sequence CRLF, any of the three preceding, or any
76 Unicode newline sequence. The
77 .\" HREF
78 \fBpcreapi\fP
79 .\"
80 page has
81 .\" HTML <a href="pcreapi.html#newlines">
82 .\" </a>
83 further discussion
84 .\"
85 about newlines, and shows how to set the newline convention in the
86 \fIoptions\fP arguments for the compiling and matching functions.
87 .P
88 It is also possible to specify a newline convention by starting a pattern
89 string with one of the following five sequences:
90 .sp
91 (*CR) carriage return
92 (*LF) linefeed
93 (*CRLF) carriage return, followed by linefeed
94 (*ANYCRLF) any of the three above
95 (*ANY) all Unicode newline sequences
96 .sp
97 These override the default and the options given to \fBpcre_compile()\fP or
98 \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99 newline sequence, the pattern
100 .sp
101 (*CR)a.b
102 .sp
103 changes the convention to CR. That pattern matches "a\enb" because LF is no
104 longer a newline. Note that these special settings, which are not
105 Perl-compatible, are recognized only at the very start of a pattern, and that
106 they must be in upper case. If more than one of them is present, the last one
107 is used.
108 .P
109 The newline convention affects the interpretation of the dot metacharacter when
110 PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111 affect what the \eR escape sequence matches. By default, this is any Unicode
112 newline sequence, for Perl compatibility. However, this can be changed; see the
113 description of \eR in the section entitled
114 .\" HTML <a href="#newlineseq">
115 .\" </a>
116 "Newline sequences"
117 .\"
118 below. A change of \eR setting can be combined with a change of newline
119 convention.
120 .
121 .
123 .rs
124 .sp
125 A regular expression is a pattern that is matched against a subject string from
126 left to right. Most characters stand for themselves in a pattern, and match the
127 corresponding characters in the subject. As a trivial example, the pattern
128 .sp
129 The quick brown fox
130 .sp
131 matches a portion of a subject string that is identical to itself. When
132 caseless matching is specified (the PCRE_CASELESS option), letters are matched
133 independently of case. In UTF-8 mode, PCRE always understands the concept of
134 case for characters whose values are less than 128, so caseless matching is
135 always possible. For characters with higher values, the concept of case is
136 supported if PCRE is compiled with Unicode property support, but not otherwise.
137 If you want to use caseless matching for characters 128 and above, you must
138 ensure that PCRE is compiled with Unicode property support as well as with
139 UTF-8 support.
140 .P
141 The power of regular expressions comes from the ability to include alternatives
142 and repetitions in the pattern. These are encoded in the pattern by the use of
143 \fImetacharacters\fP, which do not stand for themselves but instead are
144 interpreted in some special way.
145 .P
146 There are two different sets of metacharacters: those that are recognized
147 anywhere in the pattern except within square brackets, and those that are
148 recognized within square brackets. Outside square brackets, the metacharacters
149 are as follows:
150 .sp
151 \e general escape character with several uses
152 ^ assert start of string (or line, in multiline mode)
153 $ assert end of string (or line, in multiline mode)
154 . match any character except newline (by default)
155 [ start character class definition
156 | start of alternative branch
157 ( start subpattern
158 ) end subpattern
159 ? extends the meaning of (
160 also 0 or 1 quantifier
161 also quantifier minimizer
162 * 0 or more quantifier
163 + 1 or more quantifier
164 also "possessive quantifier"
165 { start min/max quantifier
166 .sp
167 Part of a pattern that is in square brackets is called a "character class". In
168 a character class the only metacharacters are:
169 .sp
170 \e general escape character
171 ^ negate the class, but only if the first character
172 - indicates character range
173 .\" JOIN
174 [ POSIX character class (only if followed by POSIX
175 syntax)
176 ] terminates the character class
177 .sp
178 The following sections describe the use of each of the metacharacters.
179 .
180 .
182 .rs
183 .sp
184 The backslash character has several uses. Firstly, if it is followed by a
185 character that is not a number or a letter, it takes away any special meaning
186 that character may have. This use of backslash as an escape character applies
187 both inside and outside character classes.
188 .P
189 For example, if you want to match a * character, you write \e* in the pattern.
190 This escaping action applies whether or not the following character would
191 otherwise be interpreted as a metacharacter, so it is always safe to precede a
192 non-alphanumeric with backslash to specify that it stands for itself. In
193 particular, if you want to match a backslash, you write \e\e.
194 .P
195 In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196 backslash. All other characters (in particular, those whose codepoints are
197 greater than 127) are treated as literals.
198 .P
199 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200 pattern (other than in a character class) and characters between a # outside
201 a character class and the next newline are ignored. An escaping backslash can
202 be used to include a whitespace or # character as part of the pattern.
203 .P
204 If you want to remove the special meaning from a sequence of characters, you
205 can do so by putting them between \eQ and \eE. This is different from Perl in
206 that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
207 Perl, $ and @ cause variable interpolation. Note the following examples:
208 .sp
209 Pattern PCRE matches Perl matches
210 .sp
211 .\" JOIN
212 \eQabc$xyz\eE abc$xyz abc followed by the
213 contents of $xyz
214 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
215 \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
216 .sp
217 The \eQ...\eE sequence is recognized both inside and outside character classes.
218 An isolated \eE that is not preceded by \eQ is ignored.
219 .
220 .
221 .\" HTML <a name="digitsafterbackslash"></a>
222 .SS "Non-printing characters"
223 .rs
224 .sp
225 A second use of backslash provides a way of encoding non-printing characters
226 in patterns in a visible manner. There is no restriction on the appearance of
227 non-printing characters, apart from the binary zero that terminates a pattern,
228 but when a pattern is being prepared by text editing, it is often easier to use
229 one of the following escape sequences than the binary character it represents:
230 .sp
231 \ea alarm, that is, the BEL character (hex 07)
232 \ecx "control-x", where x is any ASCII character
233 \ee escape (hex 1B)
234 \ef formfeed (hex 0C)
235 \en linefeed (hex 0A)
236 \er carriage return (hex 0D)
237 \et tab (hex 09)
238 \eddd character with octal code ddd, or back reference
239 \exhh character with hex code hh
240 \ex{hhh..} character with hex code hhh..
241 .sp
242 The precise effect of \ecx is as follows: if x is a lower case letter, it
243 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
244 Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
245 \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
246 than 127, a compile-time error occurs. This locks out non-ASCII characters in
247 both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
248 values are valid. A lower case letter is converted to upper case, and then the
249 0xc0 bits are flipped.)
250 .P
251 After \ex, from zero to two hexadecimal digits are read (letters can be in
252 upper or lower case). Any number of hexadecimal digits may appear between \ex{
253 and }, but the value of the character code must be less than 256 in non-UTF-8
254 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
255 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
256 point, which is 10FFFF.
257 .P
258 If characters other than hexadecimal digits appear between \ex{ and }, or if
259 there is no terminating }, this form of escape is not recognized. Instead, the
260 initial \ex will be interpreted as a basic hexadecimal escape, with no
261 following digits, giving a character whose value is zero.
262 .P
263 Characters whose value is less than 256 can be defined by either of the two
264 syntaxes for \ex. There is no difference in the way they are handled. For
265 example, \exdc is exactly the same as \ex{dc}.
266 .P
267 After \e0 up to two further octal digits are read. If there are fewer than two
268 digits, just those that are present are used. Thus the sequence \e0\ex\e07
269 specifies two binary zeros followed by a BEL character (code value 7). Make
270 sure you supply two digits after the initial zero if the pattern character that
271 follows is itself an octal digit.
272 .P
273 The handling of a backslash followed by a digit other than 0 is complicated.
274 Outside a character class, PCRE reads it and any following digits as a decimal
275 number. If the number is less than 10, or if there have been at least that many
276 previous capturing left parentheses in the expression, the entire sequence is
277 taken as a \fIback reference\fP. A description of how this works is given
278 .\" HTML <a href="#backreferences">
279 .\" </a>
280 later,
281 .\"
282 following the discussion of
283 .\" HTML <a href="#subpattern">
284 .\" </a>
285 parenthesized subpatterns.
286 .\"
287 .P
288 Inside a character class, or if the decimal number is greater than 9 and there
289 have not been that many capturing subpatterns, PCRE re-reads up to three octal
290 digits following the backslash, and uses them to generate a data character. Any
291 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
292 character specified in octal must be less than \e400. In UTF-8 mode, values up
293 to \e777 are permitted. For example:
294 .sp
295 \e040 is another way of writing a space
296 .\" JOIN
297 \e40 is the same, provided there are fewer than 40
298 previous capturing subpatterns
299 \e7 is always a back reference
300 .\" JOIN
301 \e11 might be a back reference, or another way of
302 writing a tab
303 \e011 is always a tab
304 \e0113 is a tab followed by the character "3"
305 .\" JOIN
306 \e113 might be a back reference, otherwise the
307 character with octal code 113
308 .\" JOIN
309 \e377 might be a back reference, otherwise
310 the byte consisting entirely of 1 bits
311 .\" JOIN
312 \e81 is either a back reference, or a binary zero
313 followed by the two characters "8" and "1"
314 .sp
315 Note that octal values of 100 or greater must not be introduced by a leading
316 zero, because no more than three octal digits are ever read.
317 .P
318 All the sequences that define a single character value can be used both inside
319 and outside character classes. In addition, inside a character class, the
320 sequence \eb is interpreted as the backspace character (hex 08). The sequences
321 \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
322 unrecognized escape sequences, they are treated as the literal characters "B",
323 "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
324 set. Outside a character class, these sequences have different meanings.
325 .
326 .
327 .SS "Absolute and relative back references"
328 .rs
329 .sp
330 The sequence \eg followed by an unsigned or a negative number, optionally
331 enclosed in braces, is an absolute or relative back reference. A named back
332 reference can be coded as \eg{name}. Back references are discussed
333 .\" HTML <a href="#backreferences">
334 .\" </a>
335 later,
336 .\"
337 following the discussion of
338 .\" HTML <a href="#subpattern">
339 .\" </a>
340 parenthesized subpatterns.
341 .\"
342 .
343 .
344 .SS "Absolute and relative subroutine calls"
345 .rs
346 .sp
347 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
348 a number enclosed either in angle brackets or single quotes, is an alternative
349 syntax for referencing a subpattern as a "subroutine". Details are discussed
350 .\" HTML <a href="#onigurumasubroutines">
351 .\" </a>
352 later.
353 .\"
354 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
355 synonymous. The former is a back reference; the latter is a
356 .\" HTML <a href="#subpatternsassubroutines">
357 .\" </a>
358 subroutine
359 .\"
360 call.
361 .
362 .
363 .\" HTML <a name="genericchartypes"></a>
364 .SS "Generic character types"
365 .rs
366 .sp
367 Another use of backslash is for specifying generic character types:
368 .sp
369 \ed any decimal digit
370 \eD any character that is not a decimal digit
371 \eh any horizontal whitespace character
372 \eH any character that is not a horizontal whitespace character
373 \es any whitespace character
374 \eS any character that is not a whitespace character
375 \ev any vertical whitespace character
376 \eV any character that is not a vertical whitespace character
377 \ew any "word" character
378 \eW any "non-word" character
379 .sp
380 There is also the single sequence \eN, which matches a non-newline character.
381 This is the same as
382 .\" HTML <a href="#fullstopdot">
383 .\" </a>
384 the "." metacharacter
385 .\"
386 when PCRE_DOTALL is not set.
387 .P
388 Each pair of lower and upper case escape sequences partitions the complete set
389 of characters into two disjoint sets. Any given character matches one, and only
390 one, of each pair. The sequences can appear both inside and outside character
391 classes. They each match one character of the appropriate type. If the current
392 matching point is at the end of the subject string, all of them fail, because
393 there is no character to match.
394 .P
395 For compatibility with Perl, \es does not match the VT character (code 11).
396 This makes it different from the the POSIX "space" class. The \es characters
397 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
398 included in a Perl script, \es may match the VT character. In PCRE, it never
399 does.
400 .P
401 A "word" character is an underscore or any character that is a letter or digit.
402 By default, the definition of letters and digits is controlled by PCRE's
403 low-valued character tables, and may vary if locale-specific matching is taking
404 place (see
405 .\" HTML <a href="pcreapi.html#localesupport">
406 .\" </a>
407 "Locale support"
408 .\"
409 in the
410 .\" HREF
411 \fBpcreapi\fP
412 .\"
413 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
414 or "french" in Windows, some character codes greater than 128 are used for
415 accented letters, and these are then matched by \ew. The use of locales with
416 Unicode is discouraged.
417 .P
418 By default, in UTF-8 mode, characters with values greater than 128 never match
419 \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
420 their original meanings from before UTF-8 support was available, mainly for
421 efficiency reasons. However, if PCRE is compiled with Unicode property support,
422 and the PCRE_UCP option is set, the behaviour is changed so that Unicode
423 properties are used to determine character types, as follows:
424 .sp
425 \ed any character that \ep{Nd} matches (decimal digit)
426 \es any character that \ep{Z} matches, plus HT, LF, FF, CR
427 \ew any character that \ep{L} or \ep{N} matches, plus underscore
428 .sp
429 The upper case escapes match the inverse sets of characters. Note that \ed
430 matches only decimal digits, whereas \ew matches any Unicode digit, as well as
431 any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
432 \eB because they are defined in terms of \ew and \eW. Matching these sequences
433 is noticeably slower when PCRE_UCP is set.
434 .P
435 The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
436 release 5.10. In contrast to the other sequences, which match only ASCII
437 characters by default, these always match certain high-valued codepoints in
438 UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
439 are:
440 .sp
441 U+0009 Horizontal tab
442 U+0020 Space
443 U+00A0 Non-break space
444 U+1680 Ogham space mark
445 U+180E Mongolian vowel separator
446 U+2000 En quad
447 U+2001 Em quad
448 U+2002 En space
449 U+2003 Em space
450 U+2004 Three-per-em space
451 U+2005 Four-per-em space
452 U+2006 Six-per-em space
453 U+2007 Figure space
454 U+2008 Punctuation space
455 U+2009 Thin space
456 U+200A Hair space
457 U+202F Narrow no-break space
458 U+205F Medium mathematical space
459 U+3000 Ideographic space
460 .sp
461 The vertical space characters are:
462 .sp
463 U+000A Linefeed
464 U+000B Vertical tab
465 U+000C Formfeed
466 U+000D Carriage return
467 U+0085 Next line
468 U+2028 Line separator
469 U+2029 Paragraph separator
470 .
471 .
472 .\" HTML <a name="newlineseq"></a>
473 .SS "Newline sequences"
474 .rs
475 .sp
476 Outside a character class, by default, the escape sequence \eR matches any
477 Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
478 .sp
479 (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
480 .sp
481 This is an example of an "atomic group", details of which are given
482 .\" HTML <a href="#atomicgroup">
483 .\" </a>
484 below.
485 .\"
486 This particular group matches either the two-character sequence CR followed by
487 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
488 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
489 line, U+0085). The two-character sequence is treated as a single unit that
490 cannot be split.
491 .P
492 In UTF-8 mode, two additional characters whose codepoints are greater than 255
493 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
494 Unicode character property support is not needed for these characters to be
495 recognized.
496 .P
497 It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
498 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
499 either at compile time or when the pattern is matched. (BSR is an abbrevation
500 for "backslash R".) This can be made the default when PCRE is built; if this is
501 the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
502 It is also possible to specify these settings by starting a pattern string with
503 one of the following sequences:
504 .sp
505 (*BSR_ANYCRLF) CR, LF, or CRLF only
506 (*BSR_UNICODE) any Unicode newline sequence
507 .sp
508 These override the default and the options given to \fBpcre_compile()\fP or
509 \fBpcre_compile2()\fP, but they can be overridden by options given to
510 \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
511 which are not Perl-compatible, are recognized only at the very start of a
512 pattern, and that they must be in upper case. If more than one of them is
513 present, the last one is used. They can be combined with a change of newline
514 convention; for example, a pattern can start with:
515 .sp
517 .sp
518 They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
519 a character class, \eR is treated as an unrecognized escape sequence, and so
520 matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
521 .
522 .
523 .\" HTML <a name="uniextseq"></a>
524 .SS Unicode character properties
525 .rs
526 .sp
527 When PCRE is built with Unicode character property support, three additional
528 escape sequences that match characters with specific properties are available.
529 When not in UTF-8 mode, these sequences are of course limited to testing
530 characters whose codepoints are less than 256, but they do work in this mode.
531 The extra escape sequences are:
532 .sp
533 \ep{\fIxx\fP} a character with the \fIxx\fP property
534 \eP{\fIxx\fP} a character without the \fIxx\fP property
535 \eX an extended Unicode sequence
536 .sp
537 The property names represented by \fIxx\fP above are limited to the Unicode
538 script names, the general category properties, "Any", which matches any
539 character (including newline), and some special PCRE properties (described
540 in the
541 .\" HTML <a href="#extraprops">
542 .\" </a>
543 next section).
544 .\"
545 Other Perl properties such as "InMusicalSymbols" are not currently supported by
546 PCRE. Note that \eP{Any} does not match any characters, so always causes a
547 match failure.
548 .P
549 Sets of Unicode characters are defined as belonging to certain scripts. A
550 character from one of these sets can be matched using a script name. For
551 example:
552 .sp
553 \ep{Greek}
554 \eP{Han}
555 .sp
556 Those that are not part of an identified script are lumped together as
557 "Common". The current list of scripts is:
558 .P
559 Arabic,
560 Armenian,
561 Avestan,
562 Balinese,
563 Bamum,
564 Bengali,
565 Bopomofo,
566 Braille,
567 Buginese,
568 Buhid,
569 Canadian_Aboriginal,
570 Carian,
571 Cham,
572 Cherokee,
573 Common,
574 Coptic,
575 Cuneiform,
576 Cypriot,
577 Cyrillic,
578 Deseret,
579 Devanagari,
580 Egyptian_Hieroglyphs,
581 Ethiopic,
582 Georgian,
583 Glagolitic,
584 Gothic,
585 Greek,
586 Gujarati,
587 Gurmukhi,
588 Han,
589 Hangul,
590 Hanunoo,
591 Hebrew,
592 Hiragana,
593 Imperial_Aramaic,
594 Inherited,
595 Inscriptional_Pahlavi,
596 Inscriptional_Parthian,
597 Javanese,
598 Kaithi,
599 Kannada,
600 Katakana,
601 Kayah_Li,
602 Kharoshthi,
603 Khmer,
604 Lao,
605 Latin,
606 Lepcha,
607 Limbu,
608 Linear_B,
609 Lisu,
610 Lycian,
611 Lydian,
612 Malayalam,
613 Meetei_Mayek,
614 Mongolian,
615 Myanmar,
616 New_Tai_Lue,
617 Nko,
618 Ogham,
619 Old_Italic,
620 Old_Persian,
621 Old_South_Arabian,
622 Old_Turkic,
623 Ol_Chiki,
624 Oriya,
625 Osmanya,
626 Phags_Pa,
627 Phoenician,
628 Rejang,
629 Runic,
630 Samaritan,
631 Saurashtra,
632 Shavian,
633 Sinhala,
634 Sundanese,
635 Syloti_Nagri,
636 Syriac,
637 Tagalog,
638 Tagbanwa,
639 Tai_Le,
640 Tai_Tham,
641 Tai_Viet,
642 Tamil,
643 Telugu,
644 Thaana,
645 Thai,
646 Tibetan,
647 Tifinagh,
648 Ugaritic,
649 Vai,
650 Yi.
651 .P
652 Each character has exactly one Unicode general category property, specified by
653 a two-letter abbreviation. For compatibility with Perl, negation can be
654 specified by including a circumflex between the opening brace and the property
655 name. For example, \ep{^Lu} is the same as \eP{Lu}.
656 .P
657 If only one letter is specified with \ep or \eP, it includes all the general
658 category properties that start with that letter. In this case, in the absence
659 of negation, the curly brackets in the escape sequence are optional; these two
660 examples have the same effect:
661 .sp
662 \ep{L}
663 \epL
664 .sp
665 The following general category property codes are supported:
666 .sp
667 C Other
668 Cc Control
669 Cf Format
670 Cn Unassigned
671 Co Private use
672 Cs Surrogate
673 .sp
674 L Letter
675 Ll Lower case letter
676 Lm Modifier letter
677 Lo Other letter
678 Lt Title case letter
679 Lu Upper case letter
680 .sp
681 M Mark
682 Mc Spacing mark
683 Me Enclosing mark
684 Mn Non-spacing mark
685 .sp
686 N Number
687 Nd Decimal number
688 Nl Letter number
689 No Other number
690 .sp
691 P Punctuation
692 Pc Connector punctuation
693 Pd Dash punctuation
694 Pe Close punctuation
695 Pf Final punctuation
696 Pi Initial punctuation
697 Po Other punctuation
698 Ps Open punctuation
699 .sp
700 S Symbol
701 Sc Currency symbol
702 Sk Modifier symbol
703 Sm Mathematical symbol
704 So Other symbol
705 .sp
706 Z Separator
707 Zl Line separator
708 Zp Paragraph separator
709 Zs Space separator
710 .sp
711 The special property L& is also supported: it matches a character that has
712 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
713 a modifier or "other".
714 .P
715 The Cs (Surrogate) property applies only to characters in the range U+D800 to
716 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
717 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
718 (see the discussion of PCRE_NO_UTF8_CHECK in the
719 .\" HREF
720 \fBpcreapi\fP
721 .\"
722 page). Perl does not support the Cs property.
723 .P
724 The long synonyms for property names that Perl supports (such as \ep{Letter})
725 are not supported by PCRE, nor is it permitted to prefix any of these
726 properties with "Is".
727 .P
728 No character that is in the Unicode table has the Cn (unassigned) property.
729 Instead, this property is assumed for any code point that is not in the
730 Unicode table.
731 .P
732 Specifying caseless matching does not affect these escape sequences. For
733 example, \ep{Lu} always matches only upper case letters.
734 .P
735 The \eX escape matches any number of Unicode characters that form an extended
736 Unicode sequence. \eX is equivalent to
737 .sp
738 (?>\ePM\epM*)
739 .sp
740 That is, it matches a character without the "mark" property, followed by zero
741 or more characters with the "mark" property, and treats the sequence as an
742 atomic group
743 .\" HTML <a href="#atomicgroup">
744 .\" </a>
745 (see below).
746 .\"
747 Characters with the "mark" property are typically accents that affect the
748 preceding character. None of them have codepoints less than 256, so in
749 non-UTF-8 mode \eX matches any one character.
750 .P
751 Matching characters by Unicode property is not fast, because PCRE has to search
752 a structure that contains data for over fifteen thousand characters. That is
753 why the traditional escape sequences such as \ed and \ew do not use Unicode
754 properties in PCRE by default, though you can make them do so by setting the
755 PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
756 (*UCP).
757 .
758 .
759 .\" HTML <a name="extraprops"></a>
760 .SS PCRE's additional properties
761 .rs
762 .sp
763 As well as the standard Unicode properties described in the previous
764 section, PCRE supports four more that make it possible to convert traditional
765 escape sequences such as \ew and \es and POSIX character classes to use Unicode
766 properties. PCRE uses these non-standard, non-Perl properties internally when
767 PCRE_UCP is set. They are:
768 .sp
769 Xan Any alphanumeric character
770 Xps Any POSIX space character
771 Xsp Any Perl space character
772 Xwd Any Perl "word" character
773 .sp
774 Xan matches characters that have either the L (letter) or the N (number)
775 property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
776 carriage return, and any other character that has the Z (separator) property.
777 Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
778 same characters as Xan, plus underscore.
779 .
780 .
781 .\" HTML <a name="resetmatchstart"></a>
782 .SS "Resetting the match start"
783 .rs
784 .sp
785 The escape sequence \eK causes any previously matched characters not to be
786 included in the final matched sequence. For example, the pattern:
787 .sp
788 foo\eKbar
789 .sp
790 matches "foobar", but reports that it has matched "bar". This feature is
791 similar to a lookbehind assertion
792 .\" HTML <a href="#lookbehind">
793 .\" </a>
794 (described below).
795 .\"
796 However, in this case, the part of the subject before the real match does not
797 have to be of fixed length, as lookbehind assertions do. The use of \eK does
798 not interfere with the setting of
799 .\" HTML <a href="#subpattern">
800 .\" </a>
801 captured substrings.
802 .\"
803 For example, when the pattern
804 .sp
805 (foo)\eKbar
806 .sp
807 matches "foobar", the first substring is still set to "foo".
808 .P
809 Perl documents that the use of \eK within assertions is "not well defined". In
810 PCRE, \eK is acted upon when it occurs inside positive assertions, but is
811 ignored in negative assertions.
812 .
813 .
814 .\" HTML <a name="smallassertions"></a>
815 .SS "Simple assertions"
816 .rs
817 .sp
818 The final use of backslash is for certain simple assertions. An assertion
819 specifies a condition that has to be met at a particular point in a match,
820 without consuming any characters from the subject string. The use of
821 subpatterns for more complicated assertions is described
822 .\" HTML <a href="#bigassertions">
823 .\" </a>
824 below.
825 .\"
826 The backslashed assertions are:
827 .sp
828 \eb matches at a word boundary
829 \eB matches when not at a word boundary
830 \eA matches at the start of the subject
831 \eZ matches at the end of the subject
832 also matches before a newline at the end of the subject
833 \ez matches only at the end of the subject
834 \eG matches at the first matching position in the subject
835 .sp
836 Inside a character class, \eb has a different meaning; it matches the backspace
837 character. If any other of these assertions appears in a character class, by
838 default it matches the corresponding literal character (for example, \eB
839 matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
840 escape sequence" error is generated instead.
841 .P
842 A word boundary is a position in the subject string where the current character
843 and the previous character do not both match \ew or \eW (i.e. one matches
844 \ew and the other matches \eW), or the start or end of the string if the
845 first or last character matches \ew, respectively. In UTF-8 mode, the meanings
846 of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
847 done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
848 of word" or "end of word" metasequence. However, whatever follows \eb normally
849 determines which it is. For example, the fragment \eba matches "a" at the start
850 of a word.
851 .P
852 The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
853 dollar (described in the next section) in that they only ever match at the very
854 start and end of the subject string, whatever options are set. Thus, they are
855 independent of multiline mode. These three assertions are not affected by the
856 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
857 circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
858 argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
859 at a point other than the beginning of the subject, \eA can never match. The
860 difference between \eZ and \ez is that \eZ matches before a newline at the end
861 of the string as well as at the very end, whereas \ez matches only at the end.
862 .P
863 The \eG assertion is true only when the current matching position is at the
864 start point of the match, as specified by the \fIstartoffset\fP argument of
865 \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
866 non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
867 arguments, you can mimic Perl's /g option, and it is in this kind of
868 implementation where \eG can be useful.
869 .P
870 Note, however, that PCRE's interpretation of \eG, as the start of the current
871 match, is subtly different from Perl's, which defines it as the end of the
872 previous match. In Perl, these can be different when the previously matched
873 string was empty. Because PCRE does just one match at a time, it cannot
874 reproduce this behaviour.
875 .P
876 If all the alternatives of a pattern begin with \eG, the expression is anchored
877 to the starting match position, and the "anchored" flag is set in the compiled
878 regular expression.
879 .
880 .
882 .rs
883 .sp
884 Outside a character class, in the default matching mode, the circumflex
885 character is an assertion that is true only if the current matching point is
886 at the start of the subject string. If the \fIstartoffset\fP argument of
887 \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
888 option is unset. Inside a character class, circumflex has an entirely different
889 meaning
890 .\" HTML <a href="#characterclass">
891 .\" </a>
892 (see below).
893 .\"
894 .P
895 Circumflex need not be the first character of the pattern if a number of
896 alternatives are involved, but it should be the first thing in each alternative
897 in which it appears if the pattern is ever to match that branch. If all
898 possible alternatives start with a circumflex, that is, if the pattern is
899 constrained to match only at the start of the subject, it is said to be an
900 "anchored" pattern. (There are also other constructs that can cause a pattern
901 to be anchored.)
902 .P
903 A dollar character is an assertion that is true only if the current matching
904 point is at the end of the subject string, or immediately before a newline
905 at the end of the string (by default). Dollar need not be the last character of
906 the pattern if a number of alternatives are involved, but it should be the last
907 item in any branch in which it appears. Dollar has no special meaning in a
908 character class.
909 .P
910 The meaning of dollar can be changed so that it matches only at the very end of
911 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
912 does not affect the \eZ assertion.
913 .P
914 The meanings of the circumflex and dollar characters are changed if the
915 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
916 immediately after internal newlines as well as at the start of the subject
917 string. It does not match after a newline that ends the string. A dollar
918 matches before any newlines in the string, as well as at the very end, when
919 PCRE_MULTILINE is set. When newline is specified as the two-character
920 sequence CRLF, isolated CR and LF characters do not indicate newlines.
921 .P
922 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
923 \en represents a newline) in multiline mode, but not otherwise. Consequently,
924 patterns that are anchored in single line mode because all branches start with
925 ^ are not anchored in multiline mode, and a match for circumflex is possible
926 when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
927 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
928 .P
929 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
930 end of the subject in both modes, and if all branches of a pattern start with
931 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
932 .
933 .
934 .\" HTML <a name="fullstopdot"></a>
936 .rs
937 .sp
938 Outside a character class, a dot in the pattern matches any one character in
939 the subject string except (by default) a character that signifies the end of a
940 line. In UTF-8 mode, the matched character may be more than one byte long.
941 .P
942 When a line ending is defined as a single character, dot never matches that
943 character; when the two-character sequence CRLF is used, dot does not match CR
944 if it is immediately followed by LF, but otherwise it matches all characters
945 (including isolated CRs and LFs). When any Unicode line endings are being
946 recognized, dot does not match CR or LF or any of the other line ending
947 characters.
948 .P
949 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
950 option is set, a dot matches any one character, without exception. If the
951 two-character sequence CRLF is present in the subject string, it takes two dots
952 to match it.
953 .P
954 The handling of dot is entirely independent of the handling of circumflex and
955 dollar, the only relationship being that they both involve newlines. Dot has no
956 special meaning in a character class.
957 .P
958 The escape sequence \eN behaves like a dot, except that it is not affected by
959 the PCRE_DOTALL option. In other words, it matches any character except one
960 that signifies the end of a line.
961 .
962 .
964 .rs
965 .sp
966 Outside a character class, the escape sequence \eC matches any one byte, both
967 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
968 characters. The feature is provided in Perl in order to match individual bytes
969 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
970 rest of the string may start with a malformed UTF-8 character. For this reason,
971 the \eC escape sequence is best avoided.
972 .P
973 PCRE does not allow \eC to appear in lookbehind assertions
974 .\" HTML <a href="#lookbehind">
975 .\" </a>
976 (described below),
977 .\"
978 because in UTF-8 mode this would make it impossible to calculate the length of
979 the lookbehind.
980 .
981 .
982 .\" HTML <a name="characterclass"></a>
984 .rs
985 .sp
986 An opening square bracket introduces a character class, terminated by a closing
987 square bracket. A closing square bracket on its own is not special by default.
988 However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
989 bracket causes a compile-time error. If a closing square bracket is required as
990 a member of the class, it should be the first data character in the class
991 (after an initial circumflex, if present) or escaped with a backslash.
992 .P
993 A character class matches a single character in the subject. In UTF-8 mode, the
994 character may be more than one byte long. A matched character must be in the
995 set of characters defined by the class, unless the first character in the class
996 definition is a circumflex, in which case the subject character must not be in
997 the set defined by the class. If a circumflex is actually required as a member
998 of the class, ensure it is not the first character, or escape it with a
999 backslash.
1000 .P
1001 For example, the character class [aeiou] matches any lower case vowel, while
1002 [^aeiou] matches any character that is not a lower case vowel. Note that a
1003 circumflex is just a convenient notation for specifying the characters that
1004 are in the class by enumerating those that are not. A class that starts with a
1005 circumflex is not an assertion; it still consumes a character from the subject
1006 string, and therefore it fails if the current pointer is at the end of the
1007 string.
1008 .P
1009 In UTF-8 mode, characters with values greater than 255 can be included in a
1010 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
1011 .P
1012 When caseless matching is set, any letters in a class represent both their
1013 upper case and lower case versions, so for example, a caseless [aeiou] matches
1014 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1015 caseful version would. In UTF-8 mode, PCRE always understands the concept of
1016 case for characters whose values are less than 128, so caseless matching is
1017 always possible. For characters with higher values, the concept of case is
1018 supported if PCRE is compiled with Unicode property support, but not otherwise.
1019 If you want to use caseless matching in UTF8-mode for characters 128 and above,
1020 you must ensure that PCRE is compiled with Unicode property support as well as
1021 with UTF-8 support.
1022 .P
1023 Characters that might indicate line breaks are never treated in any special way
1024 when matching character classes, whatever line-ending sequence is in use, and
1025 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1026 such as [^a] always matches one of these characters.
1027 .P
1028 The minus (hyphen) character can be used to specify a range of characters in a
1029 character class. For example, [d-m] matches any letter between d and m,
1030 inclusive. If a minus character is required in a class, it must be escaped with
1031 a backslash or appear in a position where it cannot be interpreted as
1032 indicating a range, typically as the first or last character in the class.
1033 .P
1034 It is not possible to have the literal character "]" as the end character of a
1035 range. A pattern such as [W-]46] is interpreted as a class of two characters
1036 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1037 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1038 the end of range, so [W-\e]46] is interpreted as a class containing a range
1039 followed by two other characters. The octal or hexadecimal representation of
1040 "]" can also be used to end a range.
1041 .P
1042 Ranges operate in the collating sequence of character values. They can also be
1043 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
1044 mode, ranges can include characters whose values are greater than 255, for
1045 example [\ex{100}-\ex{2ff}].
1046 .P
1047 If a range that includes letters is used when caseless matching is set, it
1048 matches the letters in either case. For example, [W-c] is equivalent to
1049 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1050 tables for a French locale are in use, [\exc8-\excb] matches accented E
1051 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1052 characters with values greater than 128 only when it is compiled with Unicode
1053 property support.
1054 .P
1055 The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and
1056 \eW may also appear in a character class, and add the characters that they
1057 match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A
1058 circumflex can conveniently be used with the upper case character types to
1059 specify a more restricted set of characters than the matching lower case type.
1060 For example, the class [^\eW_] matches any letter or digit, but not underscore.
1061 .P
1062 The only metacharacters that are recognized in character classes are backslash,
1063 hyphen (only where it can be interpreted as specifying a range), circumflex
1064 (only at the start), opening square bracket (only when it can be interpreted as
1065 introducing a POSIX class name - see the next section), and the terminating
1066 closing square bracket. However, escaping other non-alphanumeric characters
1067 does no harm.
1068 .
1069 .
1071 .rs
1072 .sp
1073 Perl supports the POSIX notation for character classes. This uses names
1074 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1075 this notation. For example,
1076 .sp
1077 [01[:alpha:]%]
1078 .sp
1079 matches "0", "1", any alphabetic character, or "%". The supported class names
1080 are:
1081 .sp
1082 alnum letters and digits
1083 alpha letters
1084 ascii character codes 0 - 127
1085 blank space or tab only
1086 cntrl control characters
1087 digit decimal digits (same as \ed)
1088 graph printing characters, excluding space
1089 lower lower case letters
1090 print printing characters, including space
1091 punct printing characters, excluding letters and digits and space
1092 space white space (not quite the same as \es)
1093 upper upper case letters
1094 word "word" characters (same as \ew)
1095 xdigit hexadecimal digits
1096 .sp
1097 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1098 space (32). Notice that this list includes the VT character (code 11). This
1099 makes "space" different to \es, which does not include VT (for Perl
1100 compatibility).
1101 .P
1102 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1103 5.8. Another Perl extension is negation, which is indicated by a ^ character
1104 after the colon. For example,
1105 .sp
1106 [12[:^digit:]]
1107 .sp
1108 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1109 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
1110 supported, and an error is given if they are encountered.
1111 .P
1112 By default, in UTF-8 mode, characters with values greater than 128 do not match
1113 any of the POSIX character classes. However, if the PCRE_UCP option is passed
1114 to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1115 character properties are used. This is achieved by replacing the POSIX classes
1116 by other sequences, as follows:
1117 .sp
1118 [:alnum:] becomes \ep{Xan}
1119 [:alpha:] becomes \ep{L}
1120 [:blank:] becomes \eh
1121 [:digit:] becomes \ep{Nd}
1122 [:lower:] becomes \ep{Ll}
1123 [:space:] becomes \ep{Xps}
1124 [:upper:] becomes \ep{Lu}
1125 [:word:] becomes \ep{Xwd}
1126 .sp
1127 Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1128 classes are unchanged, and match only characters with code points less than
1129 128.
1130 .
1131 .
1133 .rs
1134 .sp
1135 Vertical bar characters are used to separate alternative patterns. For example,
1136 the pattern
1137 .sp
1138 gilbert|sullivan
1139 .sp
1140 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1141 and an empty alternative is permitted (matching the empty string). The matching
1142 process tries each alternative in turn, from left to right, and the first one
1143 that succeeds is used. If the alternatives are within a subpattern
1144 .\" HTML <a href="#subpattern">
1145 .\" </a>
1146 (defined below),
1147 .\"
1148 "succeeds" means matching the rest of the main pattern as well as the
1149 alternative in the subpattern.
1150 .
1151 .
1153 .rs
1154 .sp
1155 The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1156 PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1157 the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1158 The option letters are
1159 .sp
1160 i for PCRE_CASELESS
1162 s for PCRE_DOTALL
1163 x for PCRE_EXTENDED
1164 .sp
1165 For example, (?im) sets caseless, multiline matching. It is also possible to
1166 unset these options by preceding the letter with a hyphen, and a combined
1167 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1168 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1169 permitted. If a letter appears both before and after the hyphen, the option is
1170 unset.
1171 .P
1172 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1173 changed in the same way as the Perl-compatible options by using the characters
1174 J, U and X respectively.
1175 .P
1176 When one of these option changes occurs at top level (that is, not inside
1177 subpattern parentheses), the change applies to the remainder of the pattern
1178 that follows. If the change is placed right at the start of a pattern, PCRE
1179 extracts it into the global options (and it will therefore show up in data
1180 extracted by the \fBpcre_fullinfo()\fP function).
1181 .P
1182 An option change within a subpattern (see below for a description of
1183 subpatterns) affects only that part of the subpattern that follows it, so
1184 .sp
1185 (a(?i)b)c
1186 .sp
1187 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1188 By this means, options can be made to have different settings in different
1189 parts of the pattern. Any changes made in one alternative do carry on
1190 into subsequent branches within the same subpattern. For example,
1191 .sp
1192 (a(?i)b|c)
1193 .sp
1194 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1195 branch is abandoned before the option setting. This is because the effects of
1196 option settings happen at compile time. There would be some very weird
1197 behaviour otherwise.
1198 .P
1199 \fBNote:\fP There are other PCRE-specific options that can be set by the
1200 application when the compile or match functions are called. In some cases the
1201 pattern can contain special leading sequences such as (*CRLF) to override what
1202 the application has set or what has been defaulted. Details are given in the
1203 section entitled
1204 .\" HTML <a href="#newlineseq">
1205 .\" </a>
1206 "Newline sequences"
1207 .\"
1208 above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1209 to set UTF-8 and Unicode property modes; they are equivalent to setting the
1210 PCRE_UTF8 and the PCRE_UCP options, respectively.
1211 .
1212 .
1213 .\" HTML <a name="subpattern"></a>
1215 .rs
1216 .sp
1217 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1218 Turning part of a pattern into a subpattern does two things:
1219 .sp
1220 1. It localizes a set of alternatives. For example, the pattern
1221 .sp
1222 cat(aract|erpillar|)
1223 .sp
1224 matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1225 match "cataract", "erpillar" or an empty string.
1226 .sp
1227 2. It sets up the subpattern as a capturing subpattern. This means that, when
1228 the whole pattern matches, that portion of the subject string that matched the
1229 subpattern is passed back to the caller via the \fIovector\fP argument of
1230 \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1231 from 1) to obtain numbers for the capturing subpatterns. For example, if the
1232 string "the red king" is matched against the pattern
1233 .sp
1234 the ((red|white) (king|queen))
1235 .sp
1236 the captured substrings are "red king", "red", and "king", and are numbered 1,
1237 2, and 3, respectively.
1238 .P
1239 The fact that plain parentheses fulfil two functions is not always helpful.
1240 There are often times when a grouping subpattern is required without a
1241 capturing requirement. If an opening parenthesis is followed by a question mark
1242 and a colon, the subpattern does not do any capturing, and is not counted when
1243 computing the number of any subsequent capturing subpatterns. For example, if
1244 the string "the white queen" is matched against the pattern
1245 .sp
1246 the ((?:red|white) (king|queen))
1247 .sp
1248 the captured substrings are "white queen" and "queen", and are numbered 1 and
1249 2. The maximum number of capturing subpatterns is 65535.
1250 .P
1251 As a convenient shorthand, if any option settings are required at the start of
1252 a non-capturing subpattern, the option letters may appear between the "?" and
1253 the ":". Thus the two patterns
1254 .sp
1255 (?i:saturday|sunday)
1256 (?:(?i)saturday|sunday)
1257 .sp
1258 match exactly the same set of strings. Because alternative branches are tried
1259 from left to right, and options are not reset until the end of the subpattern
1260 is reached, an option setting in one branch does affect subsequent branches, so
1261 the above patterns match "SUNDAY" as well as "Saturday".
1262 .
1263 .
1264 .\" HTML <a name="dupsubpatternnumber"></a>
1266 .rs
1267 .sp
1268 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1269 the same numbers for its capturing parentheses. Such a subpattern starts with
1270 (?| and is itself a non-capturing subpattern. For example, consider this
1271 pattern:
1272 .sp
1273 (?|(Sat)ur|(Sun))day
1274 .sp
1275 Because the two alternatives are inside a (?| group, both sets of capturing
1276 parentheses are numbered one. Thus, when the pattern matches, you can look
1277 at captured substring number one, whichever alternative matched. This construct
1278 is useful when you want to capture part, but not all, of one of a number of
1279 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1280 number is reset at the start of each branch. The numbers of any capturing
1281 parentheses that follow the subpattern start after the highest number used in
1282 any branch. The following example is taken from the Perl documentation. The
1283 numbers underneath show in which buffer the captured content will be stored.
1284 .sp
1285 # before ---------------branch-reset----------- after
1286 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1287 # 1 2 2 3 2 3 4
1288 .sp
1289 A back reference to a numbered subpattern uses the most recent value that is
1290 set for that number by any subpattern. The following pattern matches "abcabc"
1291 or "defdef":
1292 .sp
1293 /(?|(abc)|(def))\e1/
1294 .sp
1295 In contrast, a recursive or "subroutine" call to a numbered subpattern always
1296 refers to the first one in the pattern with the given number. The following
1297 pattern matches "abcabc" or "defabc":
1298 .sp
1299 /(?|(abc)|(def))(?1)/
1300 .sp
1301 If a
1302 .\" HTML <a href="#conditions">
1303 .\" </a>
1304 condition test
1305 .\"
1306 for a subpattern's having matched refers to a non-unique number, the test is
1307 true if any of the subpatterns of that number have matched.
1308 .P
1309 An alternative approach to using this "branch reset" feature is to use
1310 duplicate named subpatterns, as described in the next section.
1311 .
1312 .
1314 .rs
1315 .sp
1316 Identifying capturing parentheses by number is simple, but it can be very hard
1317 to keep track of the numbers in complicated regular expressions. Furthermore,
1318 if an expression is modified, the numbers may change. To help with this
1319 difficulty, PCRE supports the naming of subpatterns. This feature was not
1320 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1321 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1322 the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1323 have different names, but PCRE does not.
1324 .P
1325 In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1326 (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1327 parentheses from other parts of the pattern, such as
1328 .\" HTML <a href="#backreferences">
1329 .\" </a>
1330 back references,
1331 .\"
1332 .\" HTML <a href="#recursion">
1333 .\" </a>
1334 recursion,
1335 .\"
1336 and
1337 .\" HTML <a href="#conditions">
1338 .\" </a>
1339 conditions,
1340 .\"
1341 can be made by name as well as by number.
1342 .P
1343 Names consist of up to 32 alphanumeric characters and underscores. Named
1344 capturing parentheses are still allocated numbers as well as names, exactly as
1345 if the names were not present. The PCRE API provides function calls for
1346 extracting the name-to-number translation table from a compiled pattern. There
1347 is also a convenience function for extracting a captured substring by name.
1348 .P
1349 By default, a name must be unique within a pattern, but it is possible to relax
1350 this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1351 names are also always permitted for subpatterns with the same number, set up as
1352 described in the previous section.) Duplicate names can be useful for patterns
1353 where only one instance of the named parentheses can match. Suppose you want to
1354 match the name of a weekday, either as a 3-letter abbreviation or as the full
1355 name, and in both cases you want to extract the abbreviation. This pattern
1356 (ignoring the line breaks) does the job:
1357 .sp
1358 (?<DN>Mon|Fri|Sun)(?:day)?|
1359 (?<DN>Tue)(?:sday)?|
1360 (?<DN>Wed)(?:nesday)?|
1361 (?<DN>Thu)(?:rsday)?|
1362 (?<DN>Sat)(?:urday)?
1363 .sp
1364 There are five capturing substrings, but only one is ever set after a match.
1365 (An alternative way of solving this problem is to use a "branch reset"
1366 subpattern, as described in the previous section.)
1367 .P
1368 The convenience function for extracting the data by name returns the substring
1369 for the first (and in this example, the only) subpattern of that name that
1370 matched. This saves searching to find which numbered subpattern it was.
1371 .P
1372 If you make a back reference to a non-unique named subpattern from elsewhere in
1373 the pattern, the one that corresponds to the first occurrence of the name is
1374 used. In the absence of duplicate numbers (see the previous section) this is
1375 the one with the lowest number. If you use a named reference in a condition
1376 test (see the
1377 .\"
1378 .\" HTML <a href="#conditions">
1379 .\" </a>
1380 section about conditions
1381 .\"
1382 below), either to check whether a subpattern has matched, or to check for
1383 recursion, all subpatterns with the same name are tested. If the condition is
1384 true for any one of them, the overall condition is true. This is the same
1385 behaviour as testing by number. For further details of the interfaces for
1386 handling named subpatterns, see the
1387 .\" HREF
1388 \fBpcreapi\fP
1389 .\"
1390 documentation.
1391 .P
1392 \fBWarning:\fP You cannot use different names to distinguish between two
1393 subpatterns with the same number because PCRE uses only the numbers when
1394 matching. For this reason, an error is given at compile time if different names
1395 are given to subpatterns with the same number. However, you can give the same
1396 name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1397 .
1398 .
1400 .rs
1401 .sp
1402 Repetition is specified by quantifiers, which can follow any of the following
1403 items:
1404 .sp
1405 a literal data character
1406 the dot metacharacter
1407 the \eC escape sequence
1408 the \eX escape sequence (in UTF-8 mode with Unicode properties)
1409 the \eR escape sequence
1410 an escape such as \ed or \epL that matches a single character
1411 a character class
1412 a back reference (see next section)
1413 a parenthesized subpattern (unless it is an assertion)
1414 a recursive or "subroutine" call to a subpattern
1415 .sp
1416 The general repetition quantifier specifies a minimum and maximum number of
1417 permitted matches, by giving the two numbers in curly brackets (braces),
1418 separated by a comma. The numbers must be less than 65536, and the first must
1419 be less than or equal to the second. For example:
1420 .sp
1421 z{2,4}
1422 .sp
1423 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1424 character. If the second number is omitted, but the comma is present, there is
1425 no upper limit; if the second number and the comma are both omitted, the
1426 quantifier specifies an exact number of required matches. Thus
1427 .sp
1428 [aeiou]{3,}
1429 .sp
1430 matches at least 3 successive vowels, but may match many more, while
1431 .sp
1432 \ed{8}
1433 .sp
1434 matches exactly 8 digits. An opening curly bracket that appears in a position
1435 where a quantifier is not allowed, or one that does not match the syntax of a
1436 quantifier, is taken as a literal character. For example, {,6} is not a
1437 quantifier, but a literal string of four characters.
1438 .P
1439 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1440 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1441 which is represented by a two-byte sequence. Similarly, when Unicode property
1442 support is available, \eX{3} matches three Unicode extended sequences, each of
1443 which may be several bytes long (and they may be of different lengths).
1444 .P
1445 The quantifier {0} is permitted, causing the expression to behave as if the
1446 previous item and the quantifier were not present. This may be useful for
1447 subpatterns that are referenced as
1448 .\" HTML <a href="#subpatternsassubroutines">
1449 .\" </a>
1450 subroutines
1451 .\"
1452 from elsewhere in the pattern (but see also the section entitled
1453 .\" HTML <a href="#subdefine">
1454 .\" </a>
1455 "Defining subpatterns for use by reference only"
1456 .\"
1457 below). Items other than subpatterns that have a {0} quantifier are omitted
1458 from the compiled pattern.
1459 .P
1460 For convenience, the three most common quantifiers have single-character
1461 abbreviations:
1462 .sp
1463 * is equivalent to {0,}
1464 + is equivalent to {1,}
1465 ? is equivalent to {0,1}
1466 .sp
1467 It is possible to construct infinite loops by following a subpattern that can
1468 match no characters with a quantifier that has no upper limit, for example:
1469 .sp
1470 (a?)*
1471 .sp
1472 Earlier versions of Perl and PCRE used to give an error at compile time for
1473 such patterns. However, because there are cases where this can be useful, such
1474 patterns are now accepted, but if any repetition of the subpattern does in fact
1475 match no characters, the loop is forcibly broken.
1476 .P
1477 By default, the quantifiers are "greedy", that is, they match as much as
1478 possible (up to the maximum number of permitted times), without causing the
1479 rest of the pattern to fail. The classic example of where this gives problems
1480 is in trying to match comments in C programs. These appear between /* and */
1481 and within the comment, individual * and / characters may appear. An attempt to
1482 match C comments by applying the pattern
1483 .sp
1484 /\e*.*\e*/
1485 .sp
1486 to the string
1487 .sp
1488 /* first comment */ not comment /* second comment */
1489 .sp
1490 fails, because it matches the entire string owing to the greediness of the .*
1491 item.
1492 .P
1493 However, if a quantifier is followed by a question mark, it ceases to be
1494 greedy, and instead matches the minimum number of times possible, so the
1495 pattern
1496 .sp
1497 /\e*.*?\e*/
1498 .sp
1499 does the right thing with the C comments. The meaning of the various
1500 quantifiers is not otherwise changed, just the preferred number of matches.
1501 Do not confuse this use of question mark with its use as a quantifier in its
1502 own right. Because it has two uses, it can sometimes appear doubled, as in
1503 .sp
1504 \ed??\ed
1505 .sp
1506 which matches one digit by preference, but can match two if that is the only
1507 way the rest of the pattern matches.
1508 .P
1509 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1510 the quantifiers are not greedy by default, but individual ones can be made
1511 greedy by following them with a question mark. In other words, it inverts the
1512 default behaviour.
1513 .P
1514 When a parenthesized subpattern is quantified with a minimum repeat count that
1515 is greater than 1 or with a limited maximum, more memory is required for the
1516 compiled pattern, in proportion to the size of the minimum or maximum.
1517 .P
1518 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1519 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1520 implicitly anchored, because whatever follows will be tried against every
1521 character position in the subject string, so there is no point in retrying the
1522 overall match at any position after the first. PCRE normally treats such a
1523 pattern as though it were preceded by \eA.
1524 .P
1525 In cases where it is known that the subject string contains no newlines, it is
1526 worth setting PCRE_DOTALL in order to obtain this optimization, or
1527 alternatively using ^ to indicate anchoring explicitly.
1528 .P
1529 However, there is one situation where the optimization cannot be used. When .*
1530 is inside capturing parentheses that are the subject of a back reference
1531 elsewhere in the pattern, a match at the start may fail where a later one
1532 succeeds. Consider, for example:
1533 .sp
1534 (.*)abc\e1
1535 .sp
1536 If the subject is "xyz123abc123" the match point is the fourth character. For
1537 this reason, such a pattern is not implicitly anchored.
1538 .P
1539 When a capturing subpattern is repeated, the value captured is the substring
1540 that matched the final iteration. For example, after
1541 .sp
1542 (tweedle[dume]{3}\es*)+
1543 .sp
1544 has matched "tweedledum tweedledee" the value of the captured substring is
1545 "tweedledee". However, if there are nested capturing subpatterns, the
1546 corresponding captured values may have been set in previous iterations. For
1547 example, after
1548 .sp
1549 /(a|(b))+/
1550 .sp
1551 matches "aba" the value of the second captured substring is "b".
1552 .
1553 .
1554 .\" HTML <a name="atomicgroup"></a>
1556 .rs
1557 .sp
1558 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1559 repetition, failure of what follows normally causes the repeated item to be
1560 re-evaluated to see if a different number of repeats allows the rest of the
1561 pattern to match. Sometimes it is useful to prevent this, either to change the
1562 nature of the match, or to cause it fail earlier than it otherwise might, when
1563 the author of the pattern knows there is no point in carrying on.
1564 .P
1565 Consider, for example, the pattern \ed+foo when applied to the subject line
1566 .sp
1567 123456bar
1568 .sp
1569 After matching all 6 digits and then failing to match "foo", the normal
1570 action of the matcher is to try again with only 5 digits matching the \ed+
1571 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1572 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1573 that once a subpattern has matched, it is not to be re-evaluated in this way.
1574 .P
1575 If we use atomic grouping for the previous example, the matcher gives up
1576 immediately on failing to match "foo" the first time. The notation is a kind of
1577 special parenthesis, starting with (?> as in this example:
1578 .sp
1579 (?>\ed+)foo
1580 .sp
1581 This kind of parenthesis "locks up" the part of the pattern it contains once
1582 it has matched, and a failure further into the pattern is prevented from
1583 backtracking into it. Backtracking past it to previous items, however, works as
1584 normal.
1585 .P
1586 An alternative description is that a subpattern of this type matches the string
1587 of characters that an identical standalone pattern would match, if anchored at
1588 the current point in the subject string.
1589 .P
1590 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1591 the above example can be thought of as a maximizing repeat that must swallow
1592 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1593 number of digits they match in order to make the rest of the pattern match,
1594 (?>\ed+) can only match an entire sequence of digits.
1595 .P
1596 Atomic groups in general can of course contain arbitrarily complicated
1597 subpatterns, and can be nested. However, when the subpattern for an atomic
1598 group is just a single repeated item, as in the example above, a simpler
1599 notation, called a "possessive quantifier" can be used. This consists of an
1600 additional + character following a quantifier. Using this notation, the
1601 previous example can be rewritten as
1602 .sp
1603 \ed++foo
1604 .sp
1605 Note that a possessive quantifier can be used with an entire group, for
1606 example:
1607 .sp
1608 (abc|xyz){2,3}+
1609 .sp
1610 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1611 option is ignored. They are a convenient notation for the simpler forms of
1612 atomic group. However, there is no difference in the meaning of a possessive
1613 quantifier and the equivalent atomic group, though there may be a performance
1614 difference; possessive quantifiers should be slightly faster.
1615 .P
1616 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1617 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1618 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1619 package, and PCRE copied it from there. It ultimately found its way into Perl
1620 at release 5.10.
1621 .P
1622 PCRE has an optimization that automatically "possessifies" certain simple
1623 pattern constructs. For example, the sequence A+B is treated as A++B because
1624 there is no point in backtracking into a sequence of A's when B must follow.
1625 .P
1626 When a pattern contains an unlimited repeat inside a subpattern that can itself
1627 be repeated an unlimited number of times, the use of an atomic group is the
1628 only way to avoid some failing matches taking a very long time indeed. The
1629 pattern
1630 .sp
1631 (\eD+|<\ed+>)*[!?]
1632 .sp
1633 matches an unlimited number of substrings that either consist of non-digits, or
1634 digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1635 quickly. However, if it is applied to
1636 .sp
1637 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1638 .sp
1639 it takes a long time before reporting failure. This is because the string can
1640 be divided between the internal \eD+ repeat and the external * repeat in a
1641 large number of ways, and all have to be tried. (The example uses [!?] rather
1642 than a single character at the end, because both PCRE and Perl have an
1643 optimization that allows for fast failure when a single character is used. They
1644 remember the last single character that is required for a match, and fail early
1645 if it is not present in the string.) If the pattern is changed so that it uses
1646 an atomic group, like this:
1647 .sp
1648 ((?>\eD+)|<\ed+>)*[!?]
1649 .sp
1650 sequences of non-digits cannot be broken, and failure happens quickly.
1651 .
1652 .
1653 .\" HTML <a name="backreferences"></a>
1655 .rs
1656 .sp
1657 Outside a character class, a backslash followed by a digit greater than 0 (and
1658 possibly further digits) is a back reference to a capturing subpattern earlier
1659 (that is, to its left) in the pattern, provided there have been that many
1660 previous capturing left parentheses.
1661 .P
1662 However, if the decimal number following the backslash is less than 10, it is
1663 always taken as a back reference, and causes an error only if there are not
1664 that many capturing left parentheses in the entire pattern. In other words, the
1665 parentheses that are referenced need not be to the left of the reference for
1666 numbers less than 10. A "forward back reference" of this type can make sense
1667 when a repetition is involved and the subpattern to the right has participated
1668 in an earlier iteration.
1669 .P
1670 It is not possible to have a numerical "forward back reference" to a subpattern
1671 whose number is 10 or more using this syntax because a sequence such as \e50 is
1672 interpreted as a character defined in octal. See the subsection entitled
1673 "Non-printing characters"
1674 .\" HTML <a href="#digitsafterbackslash">
1675 .\" </a>
1676 above
1677 .\"
1678 for further details of the handling of digits following a backslash. There is
1679 no such problem when named parentheses are used. A back reference to any
1680 subpattern is possible using named parentheses (see below).
1681 .P
1682 Another way of avoiding the ambiguity inherent in the use of digits following a
1683 backslash is to use the \eg escape sequence. This escape must be followed by an
1684 unsigned number or a negative number, optionally enclosed in braces. These
1685 examples are all identical:
1686 .sp
1687 (ring), \e1
1688 (ring), \eg1
1689 (ring), \eg{1}
1690 .sp
1691 An unsigned number specifies an absolute reference without the ambiguity that
1692 is present in the older syntax. It is also useful when literal digits follow
1693 the reference. A negative number is a relative reference. Consider this
1694 example:
1695 .sp
1696 (abc(def)ghi)\eg{-1}
1697 .sp
1698 The sequence \eg{-1} is a reference to the most recently started capturing
1699 subpattern before \eg, that is, is it equivalent to \e2 in this example.
1700 Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1701 can be helpful in long patterns, and also in patterns that are created by
1702 joining together fragments that contain references within themselves.
1703 .P
1704 A back reference matches whatever actually matched the capturing subpattern in
1705 the current subject string, rather than anything matching the subpattern
1706 itself (see
1707 .\" HTML <a href="#subpatternsassubroutines">
1708 .\" </a>
1709 "Subpatterns as subroutines"
1710 .\"
1711 below for a way of doing that). So the pattern
1712 .sp
1713 (sens|respons)e and \e1ibility
1714 .sp
1715 matches "sense and sensibility" and "response and responsibility", but not
1716 "sense and responsibility". If caseful matching is in force at the time of the
1717 back reference, the case of letters is relevant. For example,
1718 .sp
1719 ((?i)rah)\es+\e1
1720 .sp
1721 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1722 capturing subpattern is matched caselessly.
1723 .P
1724 There are several different ways of writing back references to named
1725 subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1726 \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1727 back reference syntax, in which \eg can be used for both numeric and named
1728 references, is also supported. We could rewrite the above example in any of
1729 the following ways:
1730 .sp
1731 (?<p1>(?i)rah)\es+\ek<p1>
1732 (?'p1'(?i)rah)\es+\ek{p1}
1733 (?P<p1>(?i)rah)\es+(?P=p1)
1734 (?<p1>(?i)rah)\es+\eg{p1}
1735 .sp
1736 A subpattern that is referenced by name may appear in the pattern before or
1737 after the reference.
1738 .P
1739 There may be more than one back reference to the same subpattern. If a
1740 subpattern has not actually been used in a particular match, any back
1741 references to it always fail by default. For example, the pattern
1742 .sp
1743 (a|(bc))\e2
1744 .sp
1745 always fails if it starts to match "a" rather than "bc". However, if the
1746 PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1747 unset value matches an empty string.
1748 .P
1749 Because there may be many capturing parentheses in a pattern, all digits
1750 following a backslash are taken as part of a potential back reference number.
1751 If the pattern continues with a digit character, some delimiter must be used to
1752 terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1753 whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1754 .\" HTML <a href="#comments">
1755 .\" </a>
1756 "Comments"
1757 .\"
1758 below) can be used.
1759 .
1760 .SS "Recursive back references"
1761 .rs
1762 .sp
1763 A back reference that occurs inside the parentheses to which it refers fails
1764 when the subpattern is first used, so, for example, (a\e1) never matches.
1765 However, such references can be useful inside repeated subpatterns. For
1766 example, the pattern
1767 .sp
1768 (a|b\e1)+
1769 .sp
1770 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1771 the subpattern, the back reference matches the character string corresponding
1772 to the previous iteration. In order for this to work, the pattern must be such
1773 that the first iteration does not need to match the back reference. This can be
1774 done using alternation, as in the example above, or by a quantifier with a
1775 minimum of zero.
1776 .P
1777 Back references of this type cause the group that they reference to be treated
1778 as an
1779 .\" HTML <a href="#atomicgroup">
1780 .\" </a>
1781 atomic group.
1782 .\"
1783 Once the whole group has been matched, a subsequent matching failure cannot
1784 cause backtracking into the middle of the group.
1785 .
1786 .
1787 .\" HTML <a name="bigassertions"></a>
1789 .rs
1790 .sp
1791 An assertion is a test on the characters following or preceding the current
1792 matching point that does not actually consume any characters. The simple
1793 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1794 .\" HTML <a href="#smallassertions">
1795 .\" </a>
1796 above.
1797 .\"
1798 .P
1799 More complicated assertions are coded as subpatterns. There are two kinds:
1800 those that look ahead of the current position in the subject string, and those
1801 that look behind it. An assertion subpattern is matched in the normal way,
1802 except that it does not cause the current matching position to be changed.
1803 .P
1804 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1805 because it makes no sense to assert the same thing several times. If any kind
1806 of assertion contains capturing subpatterns within it, these are counted for
1807 the purposes of numbering the capturing subpatterns in the whole pattern.
1808 However, substring capturing is carried out only for positive assertions,
1809 because it does not make sense for negative assertions.
1810 .
1811 .
1812 .SS "Lookahead assertions"
1813 .rs
1814 .sp
1815 Lookahead assertions start with (?= for positive assertions and (?! for
1816 negative assertions. For example,
1817 .sp
1818 \ew+(?=;)
1819 .sp
1820 matches a word followed by a semicolon, but does not include the semicolon in
1821 the match, and
1822 .sp
1823 foo(?!bar)
1824 .sp
1825 matches any occurrence of "foo" that is not followed by "bar". Note that the
1826 apparently similar pattern
1827 .sp
1828 (?!foo)bar
1829 .sp
1830 does not find an occurrence of "bar" that is preceded by something other than
1831 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1832 (?!foo) is always true when the next three characters are "bar". A
1833 lookbehind assertion is needed to achieve the other effect.
1834 .P
1835 If you want to force a matching failure at some point in a pattern, the most
1836 convenient way to do it is with (?!) because an empty string always matches, so
1837 an assertion that requires there not to be an empty string must always fail.
1838 The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1839 .
1840 .
1841 .\" HTML <a name="lookbehind"></a>
1842 .SS "Lookbehind assertions"
1843 .rs
1844 .sp
1845 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1846 negative assertions. For example,
1847 .sp
1848 (?<!foo)bar
1849 .sp
1850 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1851 a lookbehind assertion are restricted such that all the strings it matches must
1852 have a fixed length. However, if there are several top-level alternatives, they
1853 do not all have to have the same fixed length. Thus
1854 .sp
1855 (?<=bullock|donkey)
1856 .sp
1857 is permitted, but
1858 .sp
1859 (?<!dogs?|cats?)
1860 .sp
1861 causes an error at compile time. Branches that match different length strings
1862 are permitted only at the top level of a lookbehind assertion. This is an
1863 extension compared with Perl, which requires all branches to match the same
1864 length of string. An assertion such as
1865 .sp
1866 (?<=ab(c|de))
1867 .sp
1868 is not permitted, because its single top-level branch can match two different
1869 lengths, but it is acceptable to PCRE if rewritten to use two top-level
1870 branches:
1871 .sp
1872 (?<=abc|abde)
1873 .sp
1874 In some cases, the escape sequence \eK
1875 .\" HTML <a href="#resetmatchstart">
1876 .\" </a>
1877 (see above)
1878 .\"
1879 can be used instead of a lookbehind assertion to get round the fixed-length
1880 restriction.
1881 .P
1882 The implementation of lookbehind assertions is, for each alternative, to
1883 temporarily move the current position back by the fixed length and then try to
1884 match. If there are insufficient characters before the current position, the
1885 assertion fails.
1886 .P
1887 PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1888 to appear in lookbehind assertions, because it makes it impossible to calculate
1889 the length of the lookbehind. The \eX and \eR escapes, which can match
1890 different numbers of bytes, are also not permitted.
1891 .P
1892 .\" HTML <a href="#subpatternsassubroutines">
1893 .\" </a>
1894 "Subroutine"
1895 .\"
1896 calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1897 as the subpattern matches a fixed-length string.
1898 .\" HTML <a href="#recursion">
1899 .\" </a>
1900 Recursion,
1901 .\"
1902 however, is not supported.
1903 .P
1904 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1905 specify efficient matching of fixed-length strings at the end of subject
1906 strings. Consider a simple pattern such as
1907 .sp
1908 abcd$
1909 .sp
1910 when applied to a long string that does not match. Because matching proceeds
1911 from left to right, PCRE will look for each "a" in the subject and then see if
1912 what follows matches the rest of the pattern. If the pattern is specified as
1913 .sp
1914 ^.*abcd$
1915 .sp
1916 the initial .* matches the entire string at first, but when this fails (because
1917 there is no following "a"), it backtracks to match all but the last character,
1918 then all but the last two characters, and so on. Once again the search for "a"
1919 covers the entire string, from right to left, so we are no better off. However,
1920 if the pattern is written as
1921 .sp
1922 ^.*+(?<=abcd)
1923 .sp
1924 there can be no backtracking for the .*+ item; it can match only the entire
1925 string. The subsequent lookbehind assertion does a single test on the last four
1926 characters. If it fails, the match fails immediately. For long strings, this
1927 approach makes a significant difference to the processing time.
1928 .
1929 .
1930 .SS "Using multiple assertions"
1931 .rs
1932 .sp
1933 Several assertions (of any sort) may occur in succession. For example,
1934 .sp
1935 (?<=\ed{3})(?<!999)foo
1936 .sp
1937 matches "foo" preceded by three digits that are not "999". Notice that each of
1938 the assertions is applied independently at the same point in the subject
1939 string. First there is a check that the previous three characters are all
1940 digits, and then there is a check that the same three characters are not "999".
1941 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1942 of which are digits and the last three of which are not "999". For example, it
1943 doesn't match "123abcfoo". A pattern to do that is
1944 .sp
1945 (?<=\ed{3}...)(?<!999)foo
1946 .sp
1947 This time the first assertion looks at the preceding six characters, checking
1948 that the first three are digits, and then the second assertion checks that the
1949 preceding three characters are not "999".
1950 .P
1951 Assertions can be nested in any combination. For example,
1952 .sp
1953 (?<=(?<!foo)bar)baz
1954 .sp
1955 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1956 preceded by "foo", while
1957 .sp
1958 (?<=\ed{3}(?!999)...)foo
1959 .sp
1960 is another pattern that matches "foo" preceded by three digits and any three
1961 characters that are not "999".
1962 .
1963 .
1964 .\" HTML <a name="conditions"></a>
1966 .rs
1967 .sp
1968 It is possible to cause the matching process to obey a subpattern
1969 conditionally or to choose between two alternative subpatterns, depending on
1970 the result of an assertion, or whether a specific capturing subpattern has
1971 already been matched. The two possible forms of conditional subpattern are:
1972 .sp
1973 (?(condition)yes-pattern)
1974 (?(condition)yes-pattern|no-pattern)
1975 .sp
1976 If the condition is satisfied, the yes-pattern is used; otherwise the
1977 no-pattern (if present) is used. If there are more than two alternatives in the
1978 subpattern, a compile-time error occurs. Each of the two alternatives may
1979 itself contain nested subpatterns of any form, including conditional
1980 subpatterns; the restriction to two alternatives applies only at the level of
1981 the condition. This pattern fragment is an example where the alternatives are
1982 complex:
1983 .sp
1984 (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
1985 .sp
1986 .P
1987 There are four kinds of condition: references to subpatterns, references to
1988 recursion, a pseudo-condition called DEFINE, and assertions.
1989 .
1990 .SS "Checking for a used subpattern by number"
1991 .rs
1992 .sp
1993 If the text between the parentheses consists of a sequence of digits, the
1994 condition is true if a capturing subpattern of that number has previously
1995 matched. If there is more than one capturing subpattern with the same number
1996 (see the earlier
1997 .\"
1998 .\" HTML <a href="#recursion">
1999 .\" </a>
2000 section about duplicate subpattern numbers),
2001 .\"
2002 the condition is true if any of them have matched. An alternative notation is
2003 to precede the digits with a plus or minus sign. In this case, the subpattern
2004 number is relative rather than absolute. The most recently opened parentheses
2005 can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2006 loops it can also make sense to refer to subsequent groups. The next
2007 parentheses to be opened can be referenced as (?(+1), and so on. (The value
2008 zero in any of these forms is not used; it provokes a compile-time error.)
2009 .P
2010 Consider the following pattern, which contains non-significant white space to
2011 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2012 three parts for ease of discussion:
2013 .sp
2014 ( \e( )? [^()]+ (?(1) \e) )
2015 .sp
2016 The first part matches an optional opening parenthesis, and if that
2017 character is present, sets it as the first captured substring. The second part
2018 matches one or more characters that are not parentheses. The third part is a
2019 conditional subpattern that tests whether or not the first set of parentheses
2020 matched. If they did, that is, if subject started with an opening parenthesis,
2021 the condition is true, and so the yes-pattern is executed and a closing
2022 parenthesis is required. Otherwise, since no-pattern is not present, the
2023 subpattern matches nothing. In other words, this pattern matches a sequence of
2024 non-parentheses, optionally enclosed in parentheses.
2025 .P
2026 If you were embedding this pattern in a larger one, you could use a relative
2027 reference:
2028 .sp
2029 ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
2030 .sp
2031 This makes the fragment independent of the parentheses in the larger pattern.
2032 .
2033 .SS "Checking for a used subpattern by name"
2034 .rs
2035 .sp
2036 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2037 subpattern by name. For compatibility with earlier versions of PCRE, which had
2038 this facility before Perl, the syntax (?(name)...) is also recognized. However,
2039 there is a possible ambiguity with this syntax, because subpattern names may
2040 consist entirely of digits. PCRE looks first for a named subpattern; if it
2041 cannot find one and the name consists entirely of digits, PCRE looks for a
2042 subpattern of that number, which must be greater than zero. Using subpattern
2043 names that consist entirely of digits is not recommended.
2044 .P
2045 Rewriting the above example to use a named subpattern gives this:
2046 .sp
2047 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
2048 .sp
2049 If the name used in a condition of this kind is a duplicate, the test is
2050 applied to all subpatterns of the same name, and is true if any one of them has
2051 matched.
2052 .
2053 .SS "Checking for pattern recursion"
2054 .rs
2055 .sp
2056 If the condition is the string (R), and there is no subpattern with the name R,
2057 the condition is true if a recursive call to the whole pattern or any
2058 subpattern has been made. If digits or a name preceded by ampersand follow the
2059 letter R, for example:
2060 .sp
2061 (?(R3)...) or (?(R&name)...)
2062 .sp
2063 the condition is true if the most recent recursion is into a subpattern whose
2064 number or name is given. This condition does not check the entire recursion
2065 stack. If the name used in a condition of this kind is a duplicate, the test is
2066 applied to all subpatterns of the same name, and is true if any one of them is
2067 the most recent recursion.
2068 .P
2069 At "top level", all these recursion test conditions are false.
2070 .\" HTML <a href="#recursion">
2071 .\" </a>
2072 The syntax for recursive patterns
2073 .\"
2074 is described below.
2075 .
2076 .\" HTML <a name="subdefine"></a>
2077 .SS "Defining subpatterns for use by reference only"
2078 .rs
2079 .sp
2080 If the condition is the string (DEFINE), and there is no subpattern with the
2081 name DEFINE, the condition is always false. In this case, there may be only one
2082 alternative in the subpattern. It is always skipped if control reaches this
2083 point in the pattern; the idea of DEFINE is that it can be used to define
2084 "subroutines" that can be referenced from elsewhere. (The use of
2085 .\" HTML <a href="#subpatternsassubroutines">
2086 .\" </a>
2087 "subroutines"
2088 .\"
2089 is described below.) For example, a pattern to match an IPv4 address such as
2090 "" could be written like this (ignore whitespace and line
2091 breaks):
2092 .sp
2093 (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2094 \eb (?&byte) (\e.(?&byte)){3} \eb
2095 .sp
2096 The first part of the pattern is a DEFINE group inside which a another group
2097 named "byte" is defined. This matches an individual component of an IPv4
2098 address (a number less than 256). When matching takes place, this part of the
2099 pattern is skipped because DEFINE acts like a false condition. The rest of the
2100 pattern uses references to the named group to match the four dot-separated
2101 components of an IPv4 address, insisting on a word boundary at each end.
2102 .
2103 .SS "Assertion conditions"
2104 .rs
2105 .sp
2106 If the condition is not in any of the above formats, it must be an assertion.
2107 This may be a positive or negative lookahead or lookbehind assertion. Consider
2108 this pattern, again containing non-significant white space, and with the two
2109 alternatives on the second line:
2110 .sp
2111 (?(?=[^a-z]*[a-z])
2112 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
2113 .sp
2114 The condition is a positive lookahead assertion that matches an optional
2115 sequence of non-letters followed by a letter. In other words, it tests for the
2116 presence of at least one letter in the subject. If a letter is found, the
2117 subject is matched against the first alternative; otherwise it is matched
2118 against the second. This pattern matches strings in one of the two forms
2119 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2120 .
2121 .
2122 .\" HTML <a name="comments"></a>
2124 .rs
2125 .sp
2126 There are two ways of including comments in patterns that are processed by
2127 PCRE. In both cases, the start of the comment must not be in a character class,
2128 nor in the middle of any other sequence of related characters such as (?: or a
2129 subpattern name or number. The characters that make up a comment play no part
2130 in the pattern matching.
2131 .P
2132 The sequence (?# marks the start of a comment that continues up to the next
2133 closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2134 option is set, an unescaped # character also introduces a comment, which in
2135 this case continues to immediately after the next newline character or
2136 character sequence in the pattern. Which characters are interpreted as newlines
2137 is controlled by the options passed to \fBpcre_compile()\fP or by a special
2138 sequence at the start of the pattern, as described in the section entitled
2139 .\" HTML <a href="#newlines">
2140 .\" </a>
2141 "Newline conventions"
2142 .\"
2143 above. Note that the end of this type of comment is a literal newline sequence
2144 in the pattern; escape sequences that happen to represent a newline do not
2145 count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2146 default newline convention is in force:
2147 .sp
2148 abc #comment \en still comment
2149 .sp
2150 On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2151 a newline in the pattern. The sequence \en is still literal at this stage, so
2152 it does not terminate the comment. Only an actual character with the code value
2153 0x0a (the default newline) does so.
2154 .
2155 .
2156 .\" HTML <a name="recursion"></a>
2158 .rs
2159 .sp
2160 Consider the problem of matching a string in parentheses, allowing for
2161 unlimited nested parentheses. Without the use of recursion, the best that can
2162 be done is to use a pattern that matches up to some fixed depth of nesting. It
2163 is not possible to handle an arbitrary nesting depth.
2164 .P
2165 For some time, Perl has provided a facility that allows regular expressions to
2166 recurse (amongst other things). It does this by interpolating Perl code in the
2167 expression at run time, and the code can refer to the expression itself. A Perl
2168 pattern using code interpolation to solve the parentheses problem can be
2169 created like this:
2170 .sp
2171 $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2172 .sp
2173 The (?p{...}) item interpolates Perl code at run time, and in this case refers
2174 recursively to the pattern in which it appears.
2175 .P
2176 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2177 supports special syntax for recursion of the entire pattern, and also for
2178 individual subpattern recursion. After its introduction in PCRE and Python,
2179 this kind of recursion was subsequently introduced into Perl at release 5.10.
2180 .P
2181 A special item that consists of (? followed by a number greater than zero and a
2182 closing parenthesis is a recursive call of the subpattern of the given number,
2183 provided that it occurs inside that subpattern. (If not, it is a
2184 .\" HTML <a href="#subpatternsassubroutines">
2185 .\" </a>
2186 "subroutine"
2187 .\"
2188 call, which is described in the next section.) The special item (?R) or (?0) is
2189 a recursive call of the entire regular expression.
2190 .P
2191 This PCRE pattern solves the nested parentheses problem (assume the
2192 PCRE_EXTENDED option is set so that white space is ignored):
2193 .sp
2194 \e( ( [^()]++ | (?R) )* \e)
2195 .sp
2196 First it matches an opening parenthesis. Then it matches any number of
2197 substrings which can either be a sequence of non-parentheses, or a recursive
2198 match of the pattern itself (that is, a correctly parenthesized substring).
2199 Finally there is a closing parenthesis. Note the use of a possessive quantifier
2200 to avoid backtracking into sequences of non-parentheses.
2201 .P
2202 If this were part of a larger pattern, you would not want to recurse the entire
2203 pattern, so instead you could use this:
2204 .sp
2205 ( \e( ( [^()]++ | (?1) )* \e) )
2206 .sp
2207 We have put the pattern into parentheses, and caused the recursion to refer to
2208 them instead of the whole pattern.
2209 .P
2210 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2211 is made easier by the use of relative references. Instead of (?1) in the
2212 pattern above you can write (?-2) to refer to the second most recently opened
2213 parentheses preceding the recursion. In other words, a negative number counts
2214 capturing parentheses leftwards from the point at which it is encountered.
2215 .P
2216 It is also possible to refer to subsequently opened parentheses, by writing
2217 references such as (?+2). However, these cannot be recursive because the
2218 reference is not inside the parentheses that are referenced. They are always
2219 .\" HTML <a href="#subpatternsassubroutines">
2220 .\" </a>
2221 "subroutine"
2222 .\"
2223 calls, as described in the next section.
2224 .P
2225 An alternative approach is to use named parentheses instead. The Perl syntax
2226 for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2227 could rewrite the above example as follows:
2228 .sp
2229 (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2230 .sp
2231 If there is more than one subpattern with the same name, the earliest one is
2232 used.
2233 .P
2234 This particular example pattern that we have been looking at contains nested
2235 unlimited repeats, and so the use of a possessive quantifier for matching
2236 strings of non-parentheses is important when applying the pattern to strings
2237 that do not match. For example, when this pattern is applied to
2238 .sp
2239 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2240 .sp
2241 it yields "no match" quickly. However, if a possessive quantifier is not used,
2242 the match runs for a very long time indeed because there are so many different
2243 ways the + and * repeats can carve up the subject, and all have to be tested
2244 before failure can be reported.
2245 .P
2246 At the end of a match, the values of capturing parentheses are those from
2247 the outermost level. If you want to obtain intermediate values, a callout
2248 function can be used (see below and the
2249 .\" HREF
2250 \fBpcrecallout\fP
2251 .\"
2252 documentation). If the pattern above is matched against
2253 .sp
2254 (ab(cd)ef)
2255 .sp
2256 the value for the inner capturing parentheses (numbered 2) is "ef", which is
2257 the last value taken on at the top level. If a capturing subpattern is not
2258 matched at the top level, its final value is unset, even if it is (temporarily)
2259 set at a deeper level.
2260 .P
2261 If there are more than 15 capturing parentheses in a pattern, PCRE has to
2262 obtain extra memory to store data during a recursion, which it does by using
2263 \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2264 be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2265 .P
2266 Do not confuse the (?R) item with the condition (R), which tests for recursion.
2267 Consider this pattern, which matches text in angle brackets, allowing for
2268 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2269 recursing), whereas any characters are permitted at the outer level.
2270 .sp
2271 < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
2272 .sp
2273 In this pattern, (?(R) is the start of a conditional subpattern, with two
2274 different alternatives for the recursive and non-recursive cases. The (?R) item
2275 is the actual recursive call.
2276 .
2277 .
2278 .\" HTML <a name="recursiondifference"></a>
2279 .SS "Recursion difference from Perl"
2280 .rs
2281 .sp
2282 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2283 treated as an atomic group. That is, once it has matched some of the subject
2284 string, it is never re-entered, even if it contains untried alternatives and
2285 there is a subsequent matching failure. This can be illustrated by the
2286 following pattern, which purports to match a palindromic string that contains
2287 an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2288 .sp
2289 ^(.|(.)(?1)\e2)$
2290 .sp
2291 The idea is that it either matches a single character, or two identical
2292 characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2293 it does not if the pattern is longer than three characters. Consider the
2294 subject string "abcba":
2295 .P
2296 At the top level, the first character is matched, but as it is not at the end
2297 of the string, the first alternative fails; the second alternative is taken
2298 and the recursion kicks in. The recursive call to subpattern 1 successfully
2299 matches the next character ("b"). (Note that the beginning and end of line
2300 tests are not part of the recursion).
2301 .P
2302 Back at the top level, the next character ("c") is compared with what
2303 subpattern 2 matched, which was "a". This fails. Because the recursion is
2304 treated as an atomic group, there are now no backtracking points, and so the
2305 entire match fails. (Perl is able, at this point, to re-enter the recursion and
2306 try the second alternative.) However, if the pattern is written with the
2307 alternatives in the other order, things are different:
2308 .sp
2309 ^((.)(?1)\e2|.)$
2310 .sp
2311 This time, the recursing alternative is tried first, and continues to recurse
2312 until it runs out of characters, at which point the recursion fails. But this
2313 time we do have another alternative to try at the higher level. That is the big
2314 difference: in the previous case the remaining alternative is at a deeper
2315 recursion level, which PCRE cannot use.
2316 .P
2317 To change the pattern so that it matches all palindromic strings, not just
2318 those with an odd number of characters, it is tempting to change the pattern to
2319 this:
2320 .sp
2321 ^((.)(?1)\e2|.?)$
2322 .sp
2323 Again, this works in Perl, but not in PCRE, and for the same reason. When a
2324 deeper recursion has matched a single character, it cannot be entered again in
2325 order to match an empty string. The solution is to separate the two cases, and
2326 write out the odd and even cases as alternatives at the higher level:
2327 .sp
2328 ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2329 .sp
2330 If you want to match typical palindromic phrases, the pattern has to ignore all
2331 non-word characters, which can be done like this:
2332 .sp
2333 ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2334 .sp
2335 If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2336 man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2337 the use of the possessive quantifier *+ to avoid backtracking into sequences of
2338 non-word characters. Without this, PCRE takes a great deal longer (ten times or
2339 more) to match typical phrases, and Perl takes so long that you think it has
2340 gone into a loop.
2341 .P
2342 \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2343 string does not start with a palindrome that is shorter than the entire string.
2344 For example, although "abcba" is correctly matched, if the subject is "ababa",
2345 PCRE finds the palindrome "aba" at the start, then fails at top level because
2346 the end of the string does not follow. Once again, it cannot jump back into the
2347 recursion to try other alternatives, so the entire match fails.
2348 .
2349 .
2350 .\" HTML <a name="subpatternsassubroutines"></a>
2352 .rs
2353 .sp
2354 If the syntax for a recursive subpattern reference (either by number or by
2355 name) is used outside the parentheses to which it refers, it operates like a
2356 subroutine in a programming language. The "called" subpattern may be defined
2357 before or after the reference. A numbered reference can be absolute or
2358 relative, as in these examples:
2359 .sp
2360 (...(absolute)...)...(?2)...
2361 (...(relative)...)...(?-1)...
2362 (...(?+1)...(relative)...
2363 .sp
2364 An earlier example pointed out that the pattern
2365 .sp
2366 (sens|respons)e and \e1ibility
2367 .sp
2368 matches "sense and sensibility" and "response and responsibility", but not
2369 "sense and responsibility". If instead the pattern
2370 .sp
2371 (sens|respons)e and (?1)ibility
2372 .sp
2373 is used, it does match "sense and responsibility" as well as the other two
2374 strings. Another example is given in the discussion of DEFINE above.
2375 .P
2376 Like recursive subpatterns, a subroutine call is always treated as an atomic
2377 group. That is, once it has matched some of the subject string, it is never
2378 re-entered, even if it contains untried alternatives and there is a subsequent
2379 matching failure. Any capturing parentheses that are set during the subroutine
2380 call revert to their previous values afterwards.
2381 .P
2382 When a subpattern is used as a subroutine, processing options such as
2383 case-independence are fixed when the subpattern is defined. They cannot be
2384 changed for different calls. For example, consider this pattern:
2385 .sp
2386 (abc)(?i:(?-1))
2387 .sp
2388 It matches "abcabc". It does not match "abcABC" because the change of
2389 processing option does not affect the called subpattern.
2390 .
2391 .
2392 .\" HTML <a name="onigurumasubroutines"></a>
2394 .rs
2395 .sp
2396 For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2397 a number enclosed either in angle brackets or single quotes, is an alternative
2398 syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2399 are two of the examples used above, rewritten using this syntax:
2400 .sp
2401 (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2402 (sens|respons)e and \eg'1'ibility
2403 .sp
2404 PCRE supports an extension to Oniguruma: if a number is preceded by a
2405 plus or a minus sign it is taken as a relative reference. For example:
2406 .sp
2407 (abc)(?i:\eg<-1>)
2408 .sp
2409 Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2410 synonymous. The former is a back reference; the latter is a subroutine call.
2411 .
2412 .
2414 .rs
2415 .sp
2416 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2417 code to be obeyed in the middle of matching a regular expression. This makes it
2418 possible, amongst other things, to extract different substrings that match the
2419 same pair of parentheses when there is a repetition.
2420 .P
2421 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2422 code. The feature is called "callout". The caller of PCRE provides an external
2423 function by putting its entry point in the global variable \fIpcre_callout\fP.
2424 By default, this variable contains NULL, which disables all calling out.
2425 .P
2426 Within a regular expression, (?C) indicates the points at which the external
2427 function is to be called. If you want to identify different callout points, you
2428 can put a number less than 256 after the letter C. The default value is zero.
2429 For example, this pattern has two callout points:
2430 .sp
2431 (?C1)abc(?C2)def
2432 .sp
2433 If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2434 automatically installed before each item in the pattern. They are all numbered
2435 255.
2436 .P
2437 During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2438 set), the external function is called. It is provided with the number of the
2439 callout, the position in the pattern, and, optionally, one item of data
2440 originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2441 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2442 description of the interface to the callout function is given in the
2443 .\" HREF
2444 \fBpcrecallout\fP
2445 .\"
2446 documentation.
2447 .
2448 .
2449 .\" HTML <a name="backtrackcontrol"></a>
2451 .rs
2452 .sp
2453 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2454 are described in the Perl documentation as "experimental and subject to change
2455 or removal in a future version of Perl". It goes on to say: "Their usage in
2456 production code should be noted to avoid problems during upgrades." The same
2457 remarks apply to the PCRE features described in this section.
2458 .P
2459 Since these verbs are specifically related to backtracking, most of them can be
2460 used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2461 a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2462 failing negative assertion, they cause an error if encountered by
2463 \fBpcre_dfa_exec()\fP.
2464 .P
2465 If any of these verbs are used in an assertion or subroutine subpattern
2466 (including recursive subpatterns), their effect is confined to that subpattern;
2467 it does not extend to the surrounding pattern. Note that such subpatterns are
2468 processed as anchored at the point where they are tested.
2469 .P
2470 The new verbs make use of what was previously invalid syntax: an opening
2471 parenthesis followed by an asterisk. They are generally of the form
2472 (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2473 depending on whether or not an argument is present. An name is a sequence of
2474 letters, digits, and underscores. If the name is empty, that is, if the closing
2475 parenthesis immediately follows the colon, the effect is as if the colon were
2476 not there. Any number of these verbs may occur in a pattern.
2477 .P
2478 PCRE contains some optimizations that are used to speed up matching by running
2479 some checks at the start of each match attempt. For example, it may know the
2480 minimum length of matching subject, or that a particular character must be
2481 present. When one of these optimizations suppresses the running of a match, any
2482 included backtracking verbs will not, of course, be processed. You can suppress
2483 the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2484 when calling \fBpcre_exec()\fP.
2485 .
2486 .
2487 .SS "Verbs that act immediately"
2488 .rs
2489 .sp
2490 The following verbs act as soon as they are encountered. They may not be
2491 followed by a name.
2492 .sp
2493 (*ACCEPT)
2494 .sp
2495 This verb causes the match to end successfully, skipping the remainder of the
2496 pattern. When inside a recursion, only the innermost pattern is ended
2497 immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2498 captured. (This feature was added to PCRE at release 8.00.) For example:
2499 .sp
2500 A((?:A|B(*ACCEPT)|C)D)
2501 .sp
2502 This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2503 the outer parentheses.
2504 .sp
2505 (*FAIL) or (*F)
2506 .sp
2507 This verb causes the match to fail, forcing backtracking to occur. It is
2508 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2509 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2510 Perl features that are not present in PCRE. The nearest equivalent is the
2511 callout feature, as for example in this pattern:
2512 .sp
2513 a+(?C)(*FAIL)
2514 .sp
2515 A match with the string "aaaa" always fails, but the callout is taken before
2516 each backtrack happens (in this example, 10 times).
2517 .
2518 .
2519 .SS "Recording which path was taken"
2520 .rs
2521 .sp
2522 There is one verb whose main purpose is to track how a match was arrived at,
2523 though it also has a secondary use in conjunction with advancing the match
2524 starting point (see (*SKIP) below).
2525 .sp
2526 (*MARK:NAME) or (*:NAME)
2527 .sp
2528 A name is always required with this verb. There may be as many instances of
2529 (*MARK) as you like in a pattern, and their names do not have to be unique.
2530 .P
2531 When a match succeeds, the name of the last-encountered (*MARK) is passed back
2532 to the caller via the \fIpcre_extra\fP data structure, as described in the
2533 .\" HTML <a href="pcreapi.html#extradata">
2534 .\" </a>
2535 section on \fIpcre_extra\fP
2536 .\"
2537 in the
2538 .\" HREF
2539 \fBpcreapi\fP
2540 .\"
2541 documentation. No data is returned for a partial match. Here is an example of
2542 \fBpcretest\fP output, where the /K modifier requests the retrieval and
2543 outputting of (*MARK) data:
2544 .sp
2545 /X(*MARK:A)Y|X(*MARK:B)Z/K
2546 XY
2547 0: XY
2548 MK: A
2549 XZ
2550 0: XZ
2551 MK: B
2552 .sp
2553 The (*MARK) name is tagged with "MK:" in this output, and in this example it
2554 indicates which of the two alternatives matched. This is a more efficient way
2555 of obtaining this information than putting each alternative in its own
2556 capturing parentheses.
2557 .P
2558 A name may also be returned after a failed match if the final path through the
2559 pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2560 (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2561 starting point for matching is advanced, the final check is often with an empty
2562 string, causing a failure before (*MARK) is reached. For example:
2563 .sp
2564 /X(*MARK:A)Y|X(*MARK:B)Z/K
2565 XP
2566 No match
2567 .sp
2568 There are three potential starting points for this match (starting with X,
2569 starting with P, and with an empty string). If the pattern is anchored, the
2570 result is different:
2571 .sp
2572 /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2573 XP
2574 No match, mark = B
2575 .sp
2576 PCRE's start-of-match optimizations can also interfere with this. For example,
2577 if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2578 subject length for a match, a shorter subject will not be scanned at all.
2579 .P
2580 Note that similar anomalies (though different in detail) exist in Perl, no
2581 doubt for the same reasons. The use of (*MARK) data after a failed match of an
2582 unanchored pattern is not recommended, unless (*COMMIT) is involved.
2583 .
2584 .
2585 .SS "Verbs that act after backtracking"
2586 .rs
2587 .sp
2588 The following verbs do nothing when they are encountered. Matching continues
2589 with what follows, but if there is no subsequent match, causing a backtrack to
2590 the verb, a failure is forced. That is, backtracking cannot pass to the left of
2591 the verb. However, when one of these verbs appears inside an atomic group, its
2592 effect is confined to that group, because once the group has been matched,
2593 there is never any backtracking into it. In this situation, backtracking can
2594 "jump back" to the left of the entire atomic group. (Remember also, as stated
2595 above, that this localization also applies in subroutine calls and assertions.)
2596 .P
2597 These verbs differ in exactly what kind of failure occurs when backtracking
2598 reaches them.
2599 .sp
2600 (*COMMIT)
2601 .sp
2602 This verb, which may not be followed by a name, causes the whole match to fail
2603 outright if the rest of the pattern does not match. Even if the pattern is
2604 unanchored, no further attempts to find a match by advancing the starting point
2605 take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2606 finding a match at the current starting point, or not at all. For example:
2607 .sp
2608 a+(*COMMIT)b
2609 .sp
2610 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2611 dynamic anchor, or "I've started, so I must finish." The name of the most
2612 recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2613 match failure.
2614 .P
2615 Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2616 unless PCRE's start-of-match optimizations are turned off, as shown in this
2617 \fBpcretest\fP example:
2618 .sp
2619 /(*COMMIT)abc/
2620 xyzabc
2621 0: abc
2622 xyzabc\eY
2623 No match
2624 .sp
2625 PCRE knows that any match must start with "a", so the optimization skips along
2626 the subject to "a" before running the first match attempt, which succeeds. When
2627 the optimization is disabled by the \eY escape in the second subject, the match
2628 starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2629 starting points.
2630 .sp
2631 (*PRUNE) or (*PRUNE:NAME)
2632 .sp
2633 This verb causes the match to fail at the current starting position in the
2634 subject if the rest of the pattern does not match. If the pattern is
2635 unanchored, the normal "bumpalong" advance to the next starting character then
2636 happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2637 reached, or when matching to the right of (*PRUNE), but if there is no match to
2638 the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2639 (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2640 but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2641 The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2642 match fails completely; the name is passed back if this is the final attempt.
2643 (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2644 pattern (*PRUNE) has the same effect as (*COMMIT).
2645 .sp
2646 (*SKIP)
2647 .sp
2648 This verb, when given without a name, is like (*PRUNE), except that if the
2649 pattern is unanchored, the "bumpalong" advance is not to the next character,
2650 but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2651 signifies that whatever text was matched leading up to it cannot be part of a
2652 successful match. Consider:
2653 .sp
2654 a+(*SKIP)b
2655 .sp
2656 If the subject is "aaaac...", after the first match attempt fails (starting at
2657 the first character in the string), the starting point skips on to start the
2658 next attempt at "c". Note that a possessive quantifer does not have the same
2659 effect as this example; although it would suppress backtracking during the
2660 first match attempt, the second attempt would start at the second character
2661 instead of skipping on to "c".
2662 .sp
2663 (*SKIP:NAME)
2664 .sp
2665 When (*SKIP) has an associated name, its behaviour is modified. If the
2666 following pattern fails to match, the previous path through the pattern is
2667 searched for the most recent (*MARK) that has the same name. If one is found,
2668 the "bumpalong" advance is to the subject position that corresponds to that
2669 (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2670 matching name is found, normal "bumpalong" of one character happens (the
2671 (*SKIP) is ignored).
2672 .sp
2673 (*THEN) or (*THEN:NAME)
2674 .sp
2675 This verb causes a skip to the next alternation in the innermost enclosing
2676 group if the rest of the pattern does not match. That is, it cancels pending
2677 backtracking, but only within the current alternation. Its name comes from the
2678 observation that it can be used for a pattern-based if-then-else block:
2679 .sp
2680 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2681 .sp
2682 If the COND1 pattern matches, FOO is tried (and possibly further items after
2683 the end of the group if FOO succeeds); on failure the matcher skips to the
2684 second alternative and tries COND2, without backtracking into COND1. The
2685 behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2686 overall match fails. If (*THEN) is not directly inside an alternation, it acts
2687 like (*PRUNE).
2688 .
2689 .P
2690 The above verbs provide four different "strengths" of control when subsequent
2691 matching fails. (*THEN) is the weakest, carrying on the match at the next
2692 alternation. (*PRUNE) comes next, failing the match at the current starting
2693 position, but allowing an advance to the next character (for an unanchored
2694 pattern). (*SKIP) is similar, except that the advance may be more than one
2695 character. (*COMMIT) is the strongest, causing the entire match to fail.
2696 .P
2697 If more than one is present in a pattern, the "stongest" one wins. For example,
2698 consider this pattern, where A, B, etc. are complex pattern fragments:
2699 .sp
2700 (A(*COMMIT)B(*THEN)C|D)
2701 .sp
2702 Once A has matched, PCRE is committed to this match, at the current starting
2703 position. If subsequently B matches, but C does not, the normal (*THEN) action
2704 of trying the next alternation (that is, D) does not happen because (*COMMIT)
2705 overrides.
2706 .
2707 .
2708 .SH "SEE ALSO"
2709 .rs
2710 .sp
2711 \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2712 \fBpcresyntax\fP(3), \fBpcre\fP(3).
2713 .
2714 .
2716 .rs
2717 .sp
2718 .nf
2719 Philip Hazel
2720 University Computing Service
2721 Cambridge CB2 3QH, England.
2722 .fi
2723 .
2724 .
2726 .rs
2727 .sp
2728 .nf
2729 Last updated: 20 November 2010
2730 Copyright (c) 1997-2010 University of Cambridge.
2731 .fi


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