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Add facility to make \R match only CR, LF, or CRLF.
1 <html>
2 <head>
3 <title>pcrepattern specification</title>
4 </head>
5 <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6 <h1>pcrepattern man page</h1>
7 <p>
8 Return to the <a href="index.html">PCRE index page</a>.
9 </p>
10 <p>
11 This page is part of the PCRE HTML documentation. It was generated automatically
12 from the original man page. If there is any nonsense in it, please consult the
13 man page, in case the conversion went wrong.
14 <br>
15 <ul>
16 <li><a name="TOC1" href="#SEC1">PCRE REGULAR EXPRESSION DETAILS</a>
17 <li><a name="TOC2" href="#SEC2">NEWLINE CONVENTIONS</a>
18 <li><a name="TOC3" href="#SEC3">CHARACTERS AND METACHARACTERS</a>
19 <li><a name="TOC4" href="#SEC4">BACKSLASH</a>
20 <li><a name="TOC5" href="#SEC5">CIRCUMFLEX AND DOLLAR</a>
21 <li><a name="TOC6" href="#SEC6">FULL STOP (PERIOD, DOT)</a>
22 <li><a name="TOC7" href="#SEC7">MATCHING A SINGLE BYTE</a>
24 <li><a name="TOC9" href="#SEC9">POSIX CHARACTER CLASSES</a>
25 <li><a name="TOC10" href="#SEC10">VERTICAL BAR</a>
26 <li><a name="TOC11" href="#SEC11">INTERNAL OPTION SETTING</a>
27 <li><a name="TOC12" href="#SEC12">SUBPATTERNS</a>
28 <li><a name="TOC13" href="#SEC13">DUPLICATE SUBPATTERN NUMBERS</a>
29 <li><a name="TOC14" href="#SEC14">NAMED SUBPATTERNS</a>
30 <li><a name="TOC15" href="#SEC15">REPETITION</a>
32 <li><a name="TOC17" href="#SEC17">BACK REFERENCES</a>
33 <li><a name="TOC18" href="#SEC18">ASSERTIONS</a>
34 <li><a name="TOC19" href="#SEC19">CONDITIONAL SUBPATTERNS</a>
35 <li><a name="TOC20" href="#SEC20">COMMENTS</a>
36 <li><a name="TOC21" href="#SEC21">RECURSIVE PATTERNS</a>
37 <li><a name="TOC22" href="#SEC22">SUBPATTERNS AS SUBROUTINES</a>
38 <li><a name="TOC23" href="#SEC23">CALLOUTS</a>
39 <li><a name="TOC24" href="#SEC24">BACTRACKING CONTROL</a>
40 <li><a name="TOC25" href="#SEC25">SEE ALSO</a>
41 <li><a name="TOC26" href="#SEC26">AUTHOR</a>
42 <li><a name="TOC27" href="#SEC27">REVISION</a>
43 </ul>
44 <br><a name="SEC1" href="#TOC1">PCRE REGULAR EXPRESSION DETAILS</a><br>
45 <P>
46 The syntax and semantics of the regular expressions that are supported by PCRE
47 are described in detail below. There is a quick-reference syntax summary in the
48 <a href="pcresyntax.html"><b>pcresyntax</b></a>
49 page. Perl's regular expressions are described in its own documentation, and
50 regular expressions in general are covered in a number of books, some of which
51 have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
52 published by O'Reilly, covers regular expressions in great detail. This
53 description of PCRE's regular expressions is intended as reference material.
54 </P>
55 <P>
56 The original operation of PCRE was on strings of one-byte characters. However,
57 there is now also support for UTF-8 character strings. To use this, you must
58 build PCRE to include UTF-8 support, and then call <b>pcre_compile()</b> with
59 the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
60 places below. There is also a summary of UTF-8 features in the
61 <a href="pcre.html#utf8support">section on UTF-8 support</a>
62 in the main
63 <a href="pcre.html"><b>pcre</b></a>
64 page.
65 </P>
66 <P>
67 The remainder of this document discusses the patterns that are supported by
68 PCRE when its main matching function, <b>pcre_exec()</b>, is used.
69 From release 6.0, PCRE offers a second matching function,
70 <b>pcre_dfa_exec()</b>, which matches using a different algorithm that is not
71 Perl-compatible. Some of the features discussed below are not available when
72 <b>pcre_dfa_exec()</b> is used. The advantages and disadvantages of the
73 alternative function, and how it differs from the normal function, are
74 discussed in the
75 <a href="pcrematching.html"><b>pcrematching</b></a>
76 page.
77 </P>
78 <br><a name="SEC2" href="#TOC1">NEWLINE CONVENTIONS</a><br>
79 <P>
80 PCRE supports five different conventions for indicating line breaks in
81 strings: a single CR (carriage return) character, a single LF (linefeed)
82 character, the two-character sequence CRLF, any of the three preceding, or any
83 Unicode newline sequence. The
84 <a href="pcreapi.html"><b>pcreapi</b></a>
85 page has
86 <a href="pcreapi.html#newlines">further discussion</a>
87 about newlines, and shows how to set the newline convention in the
88 <i>options</i> arguments for the compiling and matching functions.
89 </P>
90 <P>
91 It is also possible to specify a newline convention by starting a pattern
92 string with one of the following five sequences:
93 <pre>
94 (*CR) carriage return
95 (*LF) linefeed
96 (*CRLF) carriage return, followed by linefeed
97 (*ANYCRLF) any of the three above
98 (*ANY) all Unicode newline sequences
99 </pre>
100 These override the default and the options given to <b>pcre_compile()</b>. For
101 example, on a Unix system where LF is the default newline sequence, the pattern
102 <pre>
103 (*CR)a.b
104 </pre>
105 changes the convention to CR. That pattern matches "a\nb" because LF is no
106 longer a newline. Note that these special settings, which are not
107 Perl-compatible, are recognized only at the very start of a pattern, and that
108 they must be in upper case. If more than one of them is present, the last one
109 is used.
110 </P>
111 <P>
112 The newline convention does not affect what the \R escape sequence matches. By
113 default, this is any Unicode newline sequence, for Perl compatibility. However,
114 this can be changed; see the description of \R in the section entitled
115 <a href="#newlineseq">"Newline sequences"</a>
116 below.
117 </P>
118 <br><a name="SEC3" href="#TOC1">CHARACTERS AND METACHARACTERS</a><br>
119 <P>
120 A regular expression is a pattern that is matched against a subject string from
121 left to right. Most characters stand for themselves in a pattern, and match the
122 corresponding characters in the subject. As a trivial example, the pattern
123 <pre>
124 The quick brown fox
125 </pre>
126 matches a portion of a subject string that is identical to itself. When
127 caseless matching is specified (the PCRE_CASELESS option), letters are matched
128 independently of case. In UTF-8 mode, PCRE always understands the concept of
129 case for characters whose values are less than 128, so caseless matching is
130 always possible. For characters with higher values, the concept of case is
131 supported if PCRE is compiled with Unicode property support, but not otherwise.
132 If you want to use caseless matching for characters 128 and above, you must
133 ensure that PCRE is compiled with Unicode property support as well as with
134 UTF-8 support.
135 </P>
136 <P>
137 The power of regular expressions comes from the ability to include alternatives
138 and repetitions in the pattern. These are encoded in the pattern by the use of
139 <i>metacharacters</i>, which do not stand for themselves but instead are
140 interpreted in some special way.
141 </P>
142 <P>
143 There are two different sets of metacharacters: those that are recognized
144 anywhere in the pattern except within square brackets, and those that are
145 recognized within square brackets. Outside square brackets, the metacharacters
146 are as follows:
147 <pre>
148 \ general escape character with several uses
149 ^ assert start of string (or line, in multiline mode)
150 $ assert end of string (or line, in multiline mode)
151 . match any character except newline (by default)
152 [ start character class definition
153 | start of alternative branch
154 ( start subpattern
155 ) end subpattern
156 ? extends the meaning of (
157 also 0 or 1 quantifier
158 also quantifier minimizer
159 * 0 or more quantifier
160 + 1 or more quantifier
161 also "possessive quantifier"
162 { start min/max quantifier
163 </pre>
164 Part of a pattern that is in square brackets is called a "character class". In
165 a character class the only metacharacters are:
166 <pre>
167 \ general escape character
168 ^ negate the class, but only if the first character
169 - indicates character range
170 [ POSIX character class (only if followed by POSIX syntax)
171 ] terminates the character class
172 </pre>
173 The following sections describe the use of each of the metacharacters.
174 </P>
175 <br><a name="SEC4" href="#TOC1">BACKSLASH</a><br>
176 <P>
177 The backslash character has several uses. Firstly, if it is followed by a
178 non-alphanumeric character, it takes away any special meaning that character
179 may have. This use of backslash as an escape character applies both inside and
180 outside character classes.
181 </P>
182 <P>
183 For example, if you want to match a * character, you write \* in the pattern.
184 This escaping action applies whether or not the following character would
185 otherwise be interpreted as a metacharacter, so it is always safe to precede a
186 non-alphanumeric with backslash to specify that it stands for itself. In
187 particular, if you want to match a backslash, you write \\.
188 </P>
189 <P>
190 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
191 pattern (other than in a character class) and characters between a # outside
192 a character class and the next newline are ignored. An escaping backslash can
193 be used to include a whitespace or # character as part of the pattern.
194 </P>
195 <P>
196 If you want to remove the special meaning from a sequence of characters, you
197 can do so by putting them between \Q and \E. This is different from Perl in
198 that $ and @ are handled as literals in \Q...\E sequences in PCRE, whereas in
199 Perl, $ and @ cause variable interpolation. Note the following examples:
200 <pre>
201 Pattern PCRE matches Perl matches
203 \Qabc$xyz\E abc$xyz abc followed by the contents of $xyz
204 \Qabc\$xyz\E abc\$xyz abc\$xyz
205 \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
206 </pre>
207 The \Q...\E sequence is recognized both inside and outside character classes.
208 <a name="digitsafterbackslash"></a></P>
209 <br><b>
210 Non-printing characters
211 </b><br>
212 <P>
213 A second use of backslash provides a way of encoding non-printing characters
214 in patterns in a visible manner. There is no restriction on the appearance of
215 non-printing characters, apart from the binary zero that terminates a pattern,
216 but when a pattern is being prepared by text editing, it is usually easier to
217 use one of the following escape sequences than the binary character it
218 represents:
219 <pre>
220 \a alarm, that is, the BEL character (hex 07)
221 \cx "control-x", where x is any character
222 \e escape (hex 1B)
223 \f formfeed (hex 0C)
224 \n linefeed (hex 0A)
225 \r carriage return (hex 0D)
226 \t tab (hex 09)
227 \ddd character with octal code ddd, or backreference
228 \xhh character with hex code hh
229 \x{hhh..} character with hex code hhh..
230 </pre>
231 The precise effect of \cx is as follows: if x is a lower case letter, it
232 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
233 Thus \cz becomes hex 1A, but \c{ becomes hex 3B, while \c; becomes hex
234 7B.
235 </P>
236 <P>
237 After \x, from zero to two hexadecimal digits are read (letters can be in
238 upper or lower case). Any number of hexadecimal digits may appear between \x{
239 and }, but the value of the character code must be less than 256 in non-UTF-8
240 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
241 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
242 point, which is 10FFFF.
243 </P>
244 <P>
245 If characters other than hexadecimal digits appear between \x{ and }, or if
246 there is no terminating }, this form of escape is not recognized. Instead, the
247 initial \x will be interpreted as a basic hexadecimal escape, with no
248 following digits, giving a character whose value is zero.
249 </P>
250 <P>
251 Characters whose value is less than 256 can be defined by either of the two
252 syntaxes for \x. There is no difference in the way they are handled. For
253 example, \xdc is exactly the same as \x{dc}.
254 </P>
255 <P>
256 After \0 up to two further octal digits are read. If there are fewer than two
257 digits, just those that are present are used. Thus the sequence \0\x\07
258 specifies two binary zeros followed by a BEL character (code value 7). Make
259 sure you supply two digits after the initial zero if the pattern character that
260 follows is itself an octal digit.
261 </P>
262 <P>
263 The handling of a backslash followed by a digit other than 0 is complicated.
264 Outside a character class, PCRE reads it and any following digits as a decimal
265 number. If the number is less than 10, or if there have been at least that many
266 previous capturing left parentheses in the expression, the entire sequence is
267 taken as a <i>back reference</i>. A description of how this works is given
268 <a href="#backreferences">later,</a>
269 following the discussion of
270 <a href="#subpattern">parenthesized subpatterns.</a>
271 </P>
272 <P>
273 Inside a character class, or if the decimal number is greater than 9 and there
274 have not been that many capturing subpatterns, PCRE re-reads up to three octal
275 digits following the backslash, and uses them to generate a data character. Any
276 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
277 character specified in octal must be less than \400. In UTF-8 mode, values up
278 to \777 are permitted. For example:
279 <pre>
280 \040 is another way of writing a space
281 \40 is the same, provided there are fewer than 40 previous capturing subpatterns
282 \7 is always a back reference
283 \11 might be a back reference, or another way of writing a tab
284 \011 is always a tab
285 \0113 is a tab followed by the character "3"
286 \113 might be a back reference, otherwise the character with octal code 113
287 \377 might be a back reference, otherwise the byte consisting entirely of 1 bits
288 \81 is either a back reference, or a binary zero followed by the two characters "8" and "1"
289 </pre>
290 Note that octal values of 100 or greater must not be introduced by a leading
291 zero, because no more than three octal digits are ever read.
292 </P>
293 <P>
294 All the sequences that define a single character value can be used both inside
295 and outside character classes. In addition, inside a character class, the
296 sequence \b is interpreted as the backspace character (hex 08), and the
297 sequences \R and \X are interpreted as the characters "R" and "X",
298 respectively. Outside a character class, these sequences have different
299 meanings
300 <a href="#uniextseq">(see below).</a>
301 </P>
302 <br><b>
303 Absolute and relative back references
304 </b><br>
305 <P>
306 The sequence \g followed by an unsigned or a negative number, optionally
307 enclosed in braces, is an absolute or relative back reference. A named back
308 reference can be coded as \g{name}. Back references are discussed
309 <a href="#backreferences">later,</a>
310 following the discussion of
311 <a href="#subpattern">parenthesized subpatterns.</a>
312 </P>
313 <br><b>
314 Generic character types
315 </b><br>
316 <P>
317 Another use of backslash is for specifying generic character types. The
318 following are always recognized:
319 <pre>
320 \d any decimal digit
321 \D any character that is not a decimal digit
322 \h any horizontal whitespace character
323 \H any character that is not a horizontal whitespace character
324 \s any whitespace character
325 \S any character that is not a whitespace character
326 \v any vertical whitespace character
327 \V any character that is not a vertical whitespace character
328 \w any "word" character
329 \W any "non-word" character
330 </pre>
331 Each pair of escape sequences partitions the complete set of characters into
332 two disjoint sets. Any given character matches one, and only one, of each pair.
333 </P>
334 <P>
335 These character type sequences can appear both inside and outside character
336 classes. They each match one character of the appropriate type. If the current
337 matching point is at the end of the subject string, all of them fail, since
338 there is no character to match.
339 </P>
340 <P>
341 For compatibility with Perl, \s does not match the VT character (code 11).
342 This makes it different from the the POSIX "space" class. The \s characters
343 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
344 included in a Perl script, \s may match the VT character. In PCRE, it never
345 does.
346 </P>
347 <P>
348 In UTF-8 mode, characters with values greater than 128 never match \d, \s, or
349 \w, and always match \D, \S, and \W. This is true even when Unicode
350 character property support is available. These sequences retain their original
351 meanings from before UTF-8 support was available, mainly for efficiency
352 reasons.
353 </P>
354 <P>
355 The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to the
356 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
357 The horizontal space characters are:
358 <pre>
359 U+0009 Horizontal tab
360 U+0020 Space
361 U+00A0 Non-break space
362 U+1680 Ogham space mark
363 U+180E Mongolian vowel separator
364 U+2000 En quad
365 U+2001 Em quad
366 U+2002 En space
367 U+2003 Em space
368 U+2004 Three-per-em space
369 U+2005 Four-per-em space
370 U+2006 Six-per-em space
371 U+2007 Figure space
372 U+2008 Punctuation space
373 U+2009 Thin space
374 U+200A Hair space
375 U+202F Narrow no-break space
376 U+205F Medium mathematical space
377 U+3000 Ideographic space
378 </pre>
379 The vertical space characters are:
380 <pre>
381 U+000A Linefeed
382 U+000B Vertical tab
383 U+000C Formfeed
384 U+000D Carriage return
385 U+0085 Next line
386 U+2028 Line separator
387 U+2029 Paragraph separator
388 </PRE>
389 </P>
390 <P>
391 A "word" character is an underscore or any character less than 256 that is a
392 letter or digit. The definition of letters and digits is controlled by PCRE's
393 low-valued character tables, and may vary if locale-specific matching is taking
394 place (see
395 <a href="pcreapi.html#localesupport">"Locale support"</a>
396 in the
397 <a href="pcreapi.html"><b>pcreapi</b></a>
398 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
399 or "french" in Windows, some character codes greater than 128 are used for
400 accented letters, and these are matched by \w. The use of locales with Unicode
401 is discouraged.
402 <a name="newlineseq"></a></P>
403 <br><b>
404 Newline sequences
405 </b><br>
406 <P>
407 Outside a character class, by default, the escape sequence \R matches any
408 Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \R is
409 equivalent to the following:
410 <pre>
411 (?&#62;\r\n|\n|\x0b|\f|\r|\x85)
412 </pre>
413 This is an example of an "atomic group", details of which are given
414 <a href="#atomicgroup">below.</a>
415 This particular group matches either the two-character sequence CR followed by
416 LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
417 U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
418 line, U+0085). The two-character sequence is treated as a single unit that
419 cannot be split.
420 </P>
421 <P>
422 In UTF-8 mode, two additional characters whose codepoints are greater than 255
423 are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
424 Unicode character property support is not needed for these characters to be
425 recognized.
426 </P>
427 <P>
428 It is possible to restrict \R to match only CR, LF, or CRLF (instead of the
429 complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
430 either at compile time or when the pattern is matched. This can be made the
431 default when PCRE is built; if this is the case, the other behaviour can be
432 requested via the PCRE_BSR_UNICODE option. It is also possible to specify these
433 settings by starting a pattern string with one of the following sequences:
434 <pre>
435 (*BSR_ANYCRLF) CR, LF, or CRLF only
436 (*BSR_UNICODE) any Unicode newline sequence
437 </pre>
438 These override the default and the options given to <b>pcre_compile()</b>, but
439 they can be overridden by options given to <b>pcre_exec()</b>. Note that these
440 special settings, which are not Perl-compatible, are recognized only at the
441 very start of a pattern, and that they must be in upper case. If more than one
442 of them is present, the last one is used.
443 </P>
444 <P>
445 Inside a character class, \R matches the letter "R".
446 <a name="uniextseq"></a></P>
447 <br><b>
448 Unicode character properties
449 </b><br>
450 <P>
451 When PCRE is built with Unicode character property support, three additional
452 escape sequences that match characters with specific properties are available.
453 When not in UTF-8 mode, these sequences are of course limited to testing
454 characters whose codepoints are less than 256, but they do work in this mode.
455 The extra escape sequences are:
456 <pre>
457 \p{<i>xx</i>} a character with the <i>xx</i> property
458 \P{<i>xx</i>} a character without the <i>xx</i> property
459 \X an extended Unicode sequence
460 </pre>
461 The property names represented by <i>xx</i> above are limited to the Unicode
462 script names, the general category properties, and "Any", which matches any
463 character (including newline). Other properties such as "InMusicalSymbols" are
464 not currently supported by PCRE. Note that \P{Any} does not match any
465 characters, so always causes a match failure.
466 </P>
467 <P>
468 Sets of Unicode characters are defined as belonging to certain scripts. A
469 character from one of these sets can be matched using a script name. For
470 example:
471 <pre>
472 \p{Greek}
473 \P{Han}
474 </pre>
475 Those that are not part of an identified script are lumped together as
476 "Common". The current list of scripts is:
477 </P>
478 <P>
479 Arabic,
480 Armenian,
481 Balinese,
482 Bengali,
483 Bopomofo,
484 Braille,
485 Buginese,
486 Buhid,
487 Canadian_Aboriginal,
488 Cherokee,
489 Common,
490 Coptic,
491 Cuneiform,
492 Cypriot,
493 Cyrillic,
494 Deseret,
495 Devanagari,
496 Ethiopic,
497 Georgian,
498 Glagolitic,
499 Gothic,
500 Greek,
501 Gujarati,
502 Gurmukhi,
503 Han,
504 Hangul,
505 Hanunoo,
506 Hebrew,
507 Hiragana,
508 Inherited,
509 Kannada,
510 Katakana,
511 Kharoshthi,
512 Khmer,
513 Lao,
514 Latin,
515 Limbu,
516 Linear_B,
517 Malayalam,
518 Mongolian,
519 Myanmar,
520 New_Tai_Lue,
521 Nko,
522 Ogham,
523 Old_Italic,
524 Old_Persian,
525 Oriya,
526 Osmanya,
527 Phags_Pa,
528 Phoenician,
529 Runic,
530 Shavian,
531 Sinhala,
532 Syloti_Nagri,
533 Syriac,
534 Tagalog,
535 Tagbanwa,
536 Tai_Le,
537 Tamil,
538 Telugu,
539 Thaana,
540 Thai,
541 Tibetan,
542 Tifinagh,
543 Ugaritic,
544 Yi.
545 </P>
546 <P>
547 Each character has exactly one general category property, specified by a
548 two-letter abbreviation. For compatibility with Perl, negation can be specified
549 by including a circumflex between the opening brace and the property name. For
550 example, \p{^Lu} is the same as \P{Lu}.
551 </P>
552 <P>
553 If only one letter is specified with \p or \P, it includes all the general
554 category properties that start with that letter. In this case, in the absence
555 of negation, the curly brackets in the escape sequence are optional; these two
556 examples have the same effect:
557 <pre>
558 \p{L}
559 \pL
560 </pre>
561 The following general category property codes are supported:
562 <pre>
563 C Other
564 Cc Control
565 Cf Format
566 Cn Unassigned
567 Co Private use
568 Cs Surrogate
570 L Letter
571 Ll Lower case letter
572 Lm Modifier letter
573 Lo Other letter
574 Lt Title case letter
575 Lu Upper case letter
577 M Mark
578 Mc Spacing mark
579 Me Enclosing mark
580 Mn Non-spacing mark
582 N Number
583 Nd Decimal number
584 Nl Letter number
585 No Other number
587 P Punctuation
588 Pc Connector punctuation
589 Pd Dash punctuation
590 Pe Close punctuation
591 Pf Final punctuation
592 Pi Initial punctuation
593 Po Other punctuation
594 Ps Open punctuation
596 S Symbol
597 Sc Currency symbol
598 Sk Modifier symbol
599 Sm Mathematical symbol
600 So Other symbol
602 Z Separator
603 Zl Line separator
604 Zp Paragraph separator
605 Zs Space separator
606 </pre>
607 The special property L& is also supported: it matches a character that has
608 the Lu, Ll, or Lt property, in other words, a letter that is not classified as
609 a modifier or "other".
610 </P>
611 <P>
612 The Cs (Surrogate) property applies only to characters in the range U+D800 to
613 U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
614 cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
615 (see the discussion of PCRE_NO_UTF8_CHECK in the
616 <a href="pcreapi.html"><b>pcreapi</b></a>
617 page).
618 </P>
619 <P>
620 The long synonyms for these properties that Perl supports (such as \p{Letter})
621 are not supported by PCRE, nor is it permitted to prefix any of these
622 properties with "Is".
623 </P>
624 <P>
625 No character that is in the Unicode table has the Cn (unassigned) property.
626 Instead, this property is assumed for any code point that is not in the
627 Unicode table.
628 </P>
629 <P>
630 Specifying caseless matching does not affect these escape sequences. For
631 example, \p{Lu} always matches only upper case letters.
632 </P>
633 <P>
634 The \X escape matches any number of Unicode characters that form an extended
635 Unicode sequence. \X is equivalent to
636 <pre>
637 (?&#62;\PM\pM*)
638 </pre>
639 That is, it matches a character without the "mark" property, followed by zero
640 or more characters with the "mark" property, and treats the sequence as an
641 atomic group
642 <a href="#atomicgroup">(see below).</a>
643 Characters with the "mark" property are typically accents that affect the
644 preceding character. None of them have codepoints less than 256, so in
645 non-UTF-8 mode \X matches any one character.
646 </P>
647 <P>
648 Matching characters by Unicode property is not fast, because PCRE has to search
649 a structure that contains data for over fifteen thousand characters. That is
650 why the traditional escape sequences such as \d and \w do not use Unicode
651 properties in PCRE.
652 <a name="resetmatchstart"></a></P>
653 <br><b>
654 Resetting the match start
655 </b><br>
656 <P>
657 The escape sequence \K, which is a Perl 5.10 feature, causes any previously
658 matched characters not to be included in the final matched sequence. For
659 example, the pattern:
660 <pre>
661 foo\Kbar
662 </pre>
663 matches "foobar", but reports that it has matched "bar". This feature is
664 similar to a lookbehind assertion
665 <a href="#lookbehind">(described below).</a>
666 However, in this case, the part of the subject before the real match does not
667 have to be of fixed length, as lookbehind assertions do. The use of \K does
668 not interfere with the setting of
669 <a href="#subpattern">captured substrings.</a>
670 For example, when the pattern
671 <pre>
672 (foo)\Kbar
673 </pre>
674 matches "foobar", the first substring is still set to "foo".
675 <a name="smallassertions"></a></P>
676 <br><b>
677 Simple assertions
678 </b><br>
679 <P>
680 The final use of backslash is for certain simple assertions. An assertion
681 specifies a condition that has to be met at a particular point in a match,
682 without consuming any characters from the subject string. The use of
683 subpatterns for more complicated assertions is described
684 <a href="#bigassertions">below.</a>
685 The backslashed assertions are:
686 <pre>
687 \b matches at a word boundary
688 \B matches when not at a word boundary
689 \A matches at the start of the subject
690 \Z matches at the end of the subject
691 also matches before a newline at the end of the subject
692 \z matches only at the end of the subject
693 \G matches at the first matching position in the subject
694 </pre>
695 These assertions may not appear in character classes (but note that \b has a
696 different meaning, namely the backspace character, inside a character class).
697 </P>
698 <P>
699 A word boundary is a position in the subject string where the current character
700 and the previous character do not both match \w or \W (i.e. one matches
701 \w and the other matches \W), or the start or end of the string if the
702 first or last character matches \w, respectively.
703 </P>
704 <P>
705 The \A, \Z, and \z assertions differ from the traditional circumflex and
706 dollar (described in the next section) in that they only ever match at the very
707 start and end of the subject string, whatever options are set. Thus, they are
708 independent of multiline mode. These three assertions are not affected by the
709 PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
710 circumflex and dollar metacharacters. However, if the <i>startoffset</i>
711 argument of <b>pcre_exec()</b> is non-zero, indicating that matching is to start
712 at a point other than the beginning of the subject, \A can never match. The
713 difference between \Z and \z is that \Z matches before a newline at the end
714 of the string as well as at the very end, whereas \z matches only at the end.
715 </P>
716 <P>
717 The \G assertion is true only when the current matching position is at the
718 start point of the match, as specified by the <i>startoffset</i> argument of
719 <b>pcre_exec()</b>. It differs from \A when the value of <i>startoffset</i> is
720 non-zero. By calling <b>pcre_exec()</b> multiple times with appropriate
721 arguments, you can mimic Perl's /g option, and it is in this kind of
722 implementation where \G can be useful.
723 </P>
724 <P>
725 Note, however, that PCRE's interpretation of \G, as the start of the current
726 match, is subtly different from Perl's, which defines it as the end of the
727 previous match. In Perl, these can be different when the previously matched
728 string was empty. Because PCRE does just one match at a time, it cannot
729 reproduce this behaviour.
730 </P>
731 <P>
732 If all the alternatives of a pattern begin with \G, the expression is anchored
733 to the starting match position, and the "anchored" flag is set in the compiled
734 regular expression.
735 </P>
736 <br><a name="SEC5" href="#TOC1">CIRCUMFLEX AND DOLLAR</a><br>
737 <P>
738 Outside a character class, in the default matching mode, the circumflex
739 character is an assertion that is true only if the current matching point is
740 at the start of the subject string. If the <i>startoffset</i> argument of
741 <b>pcre_exec()</b> is non-zero, circumflex can never match if the PCRE_MULTILINE
742 option is unset. Inside a character class, circumflex has an entirely different
743 meaning
744 <a href="#characterclass">(see below).</a>
745 </P>
746 <P>
747 Circumflex need not be the first character of the pattern if a number of
748 alternatives are involved, but it should be the first thing in each alternative
749 in which it appears if the pattern is ever to match that branch. If all
750 possible alternatives start with a circumflex, that is, if the pattern is
751 constrained to match only at the start of the subject, it is said to be an
752 "anchored" pattern. (There are also other constructs that can cause a pattern
753 to be anchored.)
754 </P>
755 <P>
756 A dollar character is an assertion that is true only if the current matching
757 point is at the end of the subject string, or immediately before a newline
758 at the end of the string (by default). Dollar need not be the last character of
759 the pattern if a number of alternatives are involved, but it should be the last
760 item in any branch in which it appears. Dollar has no special meaning in a
761 character class.
762 </P>
763 <P>
764 The meaning of dollar can be changed so that it matches only at the very end of
765 the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
766 does not affect the \Z assertion.
767 </P>
768 <P>
769 The meanings of the circumflex and dollar characters are changed if the
770 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
771 immediately after internal newlines as well as at the start of the subject
772 string. It does not match after a newline that ends the string. A dollar
773 matches before any newlines in the string, as well as at the very end, when
774 PCRE_MULTILINE is set. When newline is specified as the two-character
775 sequence CRLF, isolated CR and LF characters do not indicate newlines.
776 </P>
777 <P>
778 For example, the pattern /^abc$/ matches the subject string "def\nabc" (where
779 \n represents a newline) in multiline mode, but not otherwise. Consequently,
780 patterns that are anchored in single line mode because all branches start with
781 ^ are not anchored in multiline mode, and a match for circumflex is possible
782 when the <i>startoffset</i> argument of <b>pcre_exec()</b> is non-zero. The
783 PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
784 </P>
785 <P>
786 Note that the sequences \A, \Z, and \z can be used to match the start and
787 end of the subject in both modes, and if all branches of a pattern start with
788 \A it is always anchored, whether or not PCRE_MULTILINE is set.
789 </P>
790 <br><a name="SEC6" href="#TOC1">FULL STOP (PERIOD, DOT)</a><br>
791 <P>
792 Outside a character class, a dot in the pattern matches any one character in
793 the subject string except (by default) a character that signifies the end of a
794 line. In UTF-8 mode, the matched character may be more than one byte long.
795 </P>
796 <P>
797 When a line ending is defined as a single character, dot never matches that
798 character; when the two-character sequence CRLF is used, dot does not match CR
799 if it is immediately followed by LF, but otherwise it matches all characters
800 (including isolated CRs and LFs). When any Unicode line endings are being
801 recognized, dot does not match CR or LF or any of the other line ending
802 characters.
803 </P>
804 <P>
805 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
806 option is set, a dot matches any one character, without exception. If the
807 two-character sequence CRLF is present in the subject string, it takes two dots
808 to match it.
809 </P>
810 <P>
811 The handling of dot is entirely independent of the handling of circumflex and
812 dollar, the only relationship being that they both involve newlines. Dot has no
813 special meaning in a character class.
814 </P>
815 <br><a name="SEC7" href="#TOC1">MATCHING A SINGLE BYTE</a><br>
816 <P>
817 Outside a character class, the escape sequence \C matches any one byte, both
818 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
819 characters. The feature is provided in Perl in order to match individual bytes
820 in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
821 what remains in the string may be a malformed UTF-8 string. For this reason,
822 the \C escape sequence is best avoided.
823 </P>
824 <P>
825 PCRE does not allow \C to appear in lookbehind assertions
826 <a href="#lookbehind">(described below),</a>
827 because in UTF-8 mode this would make it impossible to calculate the length of
828 the lookbehind.
829 <a name="characterclass"></a></P>
830 <br><a name="SEC8" href="#TOC1">SQUARE BRACKETS AND CHARACTER CLASSES</a><br>
831 <P>
832 An opening square bracket introduces a character class, terminated by a closing
833 square bracket. A closing square bracket on its own is not special. If a
834 closing square bracket is required as a member of the class, it should be the
835 first data character in the class (after an initial circumflex, if present) or
836 escaped with a backslash.
837 </P>
838 <P>
839 A character class matches a single character in the subject. In UTF-8 mode, the
840 character may occupy more than one byte. A matched character must be in the set
841 of characters defined by the class, unless the first character in the class
842 definition is a circumflex, in which case the subject character must not be in
843 the set defined by the class. If a circumflex is actually required as a member
844 of the class, ensure it is not the first character, or escape it with a
845 backslash.
846 </P>
847 <P>
848 For example, the character class [aeiou] matches any lower case vowel, while
849 [^aeiou] matches any character that is not a lower case vowel. Note that a
850 circumflex is just a convenient notation for specifying the characters that
851 are in the class by enumerating those that are not. A class that starts with a
852 circumflex is not an assertion: it still consumes a character from the subject
853 string, and therefore it fails if the current pointer is at the end of the
854 string.
855 </P>
856 <P>
857 In UTF-8 mode, characters with values greater than 255 can be included in a
858 class as a literal string of bytes, or by using the \x{ escaping mechanism.
859 </P>
860 <P>
861 When caseless matching is set, any letters in a class represent both their
862 upper case and lower case versions, so for example, a caseless [aeiou] matches
863 "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
864 caseful version would. In UTF-8 mode, PCRE always understands the concept of
865 case for characters whose values are less than 128, so caseless matching is
866 always possible. For characters with higher values, the concept of case is
867 supported if PCRE is compiled with Unicode property support, but not otherwise.
868 If you want to use caseless matching for characters 128 and above, you must
869 ensure that PCRE is compiled with Unicode property support as well as with
870 UTF-8 support.
871 </P>
872 <P>
873 Characters that might indicate line breaks are never treated in any special way
874 when matching character classes, whatever line-ending sequence is in use, and
875 whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
876 such as [^a] always matches one of these characters.
877 </P>
878 <P>
879 The minus (hyphen) character can be used to specify a range of characters in a
880 character class. For example, [d-m] matches any letter between d and m,
881 inclusive. If a minus character is required in a class, it must be escaped with
882 a backslash or appear in a position where it cannot be interpreted as
883 indicating a range, typically as the first or last character in the class.
884 </P>
885 <P>
886 It is not possible to have the literal character "]" as the end character of a
887 range. A pattern such as [W-]46] is interpreted as a class of two characters
888 ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
889 "-46]". However, if the "]" is escaped with a backslash it is interpreted as
890 the end of range, so [W-\]46] is interpreted as a class containing a range
891 followed by two other characters. The octal or hexadecimal representation of
892 "]" can also be used to end a range.
893 </P>
894 <P>
895 Ranges operate in the collating sequence of character values. They can also be
896 used for characters specified numerically, for example [\000-\037]. In UTF-8
897 mode, ranges can include characters whose values are greater than 255, for
898 example [\x{100}-\x{2ff}].
899 </P>
900 <P>
901 If a range that includes letters is used when caseless matching is set, it
902 matches the letters in either case. For example, [W-c] is equivalent to
903 [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
904 tables for a French locale are in use, [\xc8-\xcb] matches accented E
905 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
906 characters with values greater than 128 only when it is compiled with Unicode
907 property support.
908 </P>
909 <P>
910 The character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear
911 in a character class, and add the characters that they match to the class. For
912 example, [\dABCDEF] matches any hexadecimal digit. A circumflex can
913 conveniently be used with the upper case character types to specify a more
914 restricted set of characters than the matching lower case type. For example,
915 the class [^\W_] matches any letter or digit, but not underscore.
916 </P>
917 <P>
918 The only metacharacters that are recognized in character classes are backslash,
919 hyphen (only where it can be interpreted as specifying a range), circumflex
920 (only at the start), opening square bracket (only when it can be interpreted as
921 introducing a POSIX class name - see the next section), and the terminating
922 closing square bracket. However, escaping other non-alphanumeric characters
923 does no harm.
924 </P>
925 <br><a name="SEC9" href="#TOC1">POSIX CHARACTER CLASSES</a><br>
926 <P>
927 Perl supports the POSIX notation for character classes. This uses names
928 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
929 this notation. For example,
930 <pre>
931 [01[:alpha:]%]
932 </pre>
933 matches "0", "1", any alphabetic character, or "%". The supported class names
934 are
935 <pre>
936 alnum letters and digits
937 alpha letters
938 ascii character codes 0 - 127
939 blank space or tab only
940 cntrl control characters
941 digit decimal digits (same as \d)
942 graph printing characters, excluding space
943 lower lower case letters
944 print printing characters, including space
945 punct printing characters, excluding letters and digits
946 space white space (not quite the same as \s)
947 upper upper case letters
948 word "word" characters (same as \w)
949 xdigit hexadecimal digits
950 </pre>
951 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
952 space (32). Notice that this list includes the VT character (code 11). This
953 makes "space" different to \s, which does not include VT (for Perl
954 compatibility).
955 </P>
956 <P>
957 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
958 5.8. Another Perl extension is negation, which is indicated by a ^ character
959 after the colon. For example,
960 <pre>
961 [12[:^digit:]]
962 </pre>
963 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
964 syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
965 supported, and an error is given if they are encountered.
966 </P>
967 <P>
968 In UTF-8 mode, characters with values greater than 128 do not match any of
969 the POSIX character classes.
970 </P>
971 <br><a name="SEC10" href="#TOC1">VERTICAL BAR</a><br>
972 <P>
973 Vertical bar characters are used to separate alternative patterns. For example,
974 the pattern
975 <pre>
976 gilbert|sullivan
977 </pre>
978 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
979 and an empty alternative is permitted (matching the empty string). The matching
980 process tries each alternative in turn, from left to right, and the first one
981 that succeeds is used. If the alternatives are within a subpattern
982 <a href="#subpattern">(defined below),</a>
983 "succeeds" means matching the rest of the main pattern as well as the
984 alternative in the subpattern.
985 </P>
986 <br><a name="SEC11" href="#TOC1">INTERNAL OPTION SETTING</a><br>
987 <P>
989 PCRE_EXTENDED options can be changed from within the pattern by a sequence of
990 Perl option letters enclosed between "(?" and ")". The option letters are
991 <pre>
994 s for PCRE_DOTALL
996 </pre>
997 For example, (?im) sets caseless, multiline matching. It is also possible to
998 unset these options by preceding the letter with a hyphen, and a combined
999 setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1000 PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1001 permitted. If a letter appears both before and after the hyphen, the option is
1002 unset.
1003 </P>
1004 <P>
1005 When an option change occurs at top level (that is, not inside subpattern
1006 parentheses), the change applies to the remainder of the pattern that follows.
1007 If the change is placed right at the start of a pattern, PCRE extracts it into
1008 the global options (and it will therefore show up in data extracted by the
1009 <b>pcre_fullinfo()</b> function).
1010 </P>
1011 <P>
1012 An option change within a subpattern (see below for a description of
1013 subpatterns) affects only that part of the current pattern that follows it, so
1014 <pre>
1015 (a(?i)b)c
1016 </pre>
1017 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1018 By this means, options can be made to have different settings in different
1019 parts of the pattern. Any changes made in one alternative do carry on
1020 into subsequent branches within the same subpattern. For example,
1021 <pre>
1022 (a(?i)b|c)
1023 </pre>
1024 matches "ab", "aB", "c", and "C", even though when matching "C" the first
1025 branch is abandoned before the option setting. This is because the effects of
1026 option settings happen at compile time. There would be some very weird
1027 behaviour otherwise.
1028 </P>
1029 <P>
1030 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1031 changed in the same way as the Perl-compatible options by using the characters
1032 J, U and X respectively.
1033 <a name="subpattern"></a></P>
1034 <br><a name="SEC12" href="#TOC1">SUBPATTERNS</a><br>
1035 <P>
1036 Subpatterns are delimited by parentheses (round brackets), which can be nested.
1037 Turning part of a pattern into a subpattern does two things:
1038 <br>
1039 <br>
1040 1. It localizes a set of alternatives. For example, the pattern
1041 <pre>
1042 cat(aract|erpillar|)
1043 </pre>
1044 matches one of the words "cat", "cataract", or "caterpillar". Without the
1045 parentheses, it would match "cataract", "erpillar" or an empty string.
1046 <br>
1047 <br>
1048 2. It sets up the subpattern as a capturing subpattern. This means that, when
1049 the whole pattern matches, that portion of the subject string that matched the
1050 subpattern is passed back to the caller via the <i>ovector</i> argument of
1051 <b>pcre_exec()</b>. Opening parentheses are counted from left to right (starting
1052 from 1) to obtain numbers for the capturing subpatterns.
1053 </P>
1054 <P>
1055 For example, if the string "the red king" is matched against the pattern
1056 <pre>
1057 the ((red|white) (king|queen))
1058 </pre>
1059 the captured substrings are "red king", "red", and "king", and are numbered 1,
1060 2, and 3, respectively.
1061 </P>
1062 <P>
1063 The fact that plain parentheses fulfil two functions is not always helpful.
1064 There are often times when a grouping subpattern is required without a
1065 capturing requirement. If an opening parenthesis is followed by a question mark
1066 and a colon, the subpattern does not do any capturing, and is not counted when
1067 computing the number of any subsequent capturing subpatterns. For example, if
1068 the string "the white queen" is matched against the pattern
1069 <pre>
1070 the ((?:red|white) (king|queen))
1071 </pre>
1072 the captured substrings are "white queen" and "queen", and are numbered 1 and
1073 2. The maximum number of capturing subpatterns is 65535.
1074 </P>
1075 <P>
1076 As a convenient shorthand, if any option settings are required at the start of
1077 a non-capturing subpattern, the option letters may appear between the "?" and
1078 the ":". Thus the two patterns
1079 <pre>
1080 (?i:saturday|sunday)
1081 (?:(?i)saturday|sunday)
1082 </pre>
1083 match exactly the same set of strings. Because alternative branches are tried
1084 from left to right, and options are not reset until the end of the subpattern
1085 is reached, an option setting in one branch does affect subsequent branches, so
1086 the above patterns match "SUNDAY" as well as "Saturday".
1087 </P>
1088 <br><a name="SEC13" href="#TOC1">DUPLICATE SUBPATTERN NUMBERS</a><br>
1089 <P>
1090 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1091 the same numbers for its capturing parentheses. Such a subpattern starts with
1092 (?| and is itself a non-capturing subpattern. For example, consider this
1093 pattern:
1094 <pre>
1095 (?|(Sat)ur|(Sun))day
1096 </pre>
1097 Because the two alternatives are inside a (?| group, both sets of capturing
1098 parentheses are numbered one. Thus, when the pattern matches, you can look
1099 at captured substring number one, whichever alternative matched. This construct
1100 is useful when you want to capture part, but not all, of one of a number of
1101 alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1102 number is reset at the start of each branch. The numbers of any capturing
1103 buffers that follow the subpattern start after the highest number used in any
1104 branch. The following example is taken from the Perl documentation.
1105 The numbers underneath show in which buffer the captured content will be
1106 stored.
1107 <pre>
1108 # before ---------------branch-reset----------- after
1109 / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1110 # 1 2 2 3 2 3 4
1111 </pre>
1112 A backreference or a recursive call to a numbered subpattern always refers to
1113 the first one in the pattern with the given number.
1114 </P>
1115 <P>
1116 An alternative approach to using this "branch reset" feature is to use
1117 duplicate named subpatterns, as described in the next section.
1118 </P>
1119 <br><a name="SEC14" href="#TOC1">NAMED SUBPATTERNS</a><br>
1120 <P>
1121 Identifying capturing parentheses by number is simple, but it can be very hard
1122 to keep track of the numbers in complicated regular expressions. Furthermore,
1123 if an expression is modified, the numbers may change. To help with this
1124 difficulty, PCRE supports the naming of subpatterns. This feature was not
1125 added to Perl until release 5.10. Python had the feature earlier, and PCRE
1126 introduced it at release 4.0, using the Python syntax. PCRE now supports both
1127 the Perl and the Python syntax.
1128 </P>
1129 <P>
1130 In PCRE, a subpattern can be named in one of three ways: (?&#60;name&#62;...) or
1131 (?'name'...) as in Perl, or (?P&#60;name&#62;...) as in Python. References to capturing
1132 parentheses from other parts of the pattern, such as
1133 <a href="#backreferences">backreferences,</a>
1134 <a href="#recursion">recursion,</a>
1135 and
1136 <a href="#conditions">conditions,</a>
1137 can be made by name as well as by number.
1138 </P>
1139 <P>
1140 Names consist of up to 32 alphanumeric characters and underscores. Named
1141 capturing parentheses are still allocated numbers as well as names, exactly as
1142 if the names were not present. The PCRE API provides function calls for
1143 extracting the name-to-number translation table from a compiled pattern. There
1144 is also a convenience function for extracting a captured substring by name.
1145 </P>
1146 <P>
1147 By default, a name must be unique within a pattern, but it is possible to relax
1148 this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1149 be useful for patterns where only one instance of the named parentheses can
1150 match. Suppose you want to match the name of a weekday, either as a 3-letter
1151 abbreviation or as the full name, and in both cases you want to extract the
1152 abbreviation. This pattern (ignoring the line breaks) does the job:
1153 <pre>
1154 (?&#60;DN&#62;Mon|Fri|Sun)(?:day)?|
1155 (?&#60;DN&#62;Tue)(?:sday)?|
1156 (?&#60;DN&#62;Wed)(?:nesday)?|
1157 (?&#60;DN&#62;Thu)(?:rsday)?|
1158 (?&#60;DN&#62;Sat)(?:urday)?
1159 </pre>
1160 There are five capturing substrings, but only one is ever set after a match.
1161 (An alternative way of solving this problem is to use a "branch reset"
1162 subpattern, as described in the previous section.)
1163 </P>
1164 <P>
1165 The convenience function for extracting the data by name returns the substring
1166 for the first (and in this example, the only) subpattern of that name that
1167 matched. This saves searching to find which numbered subpattern it was. If you
1168 make a reference to a non-unique named subpattern from elsewhere in the
1169 pattern, the one that corresponds to the lowest number is used. For further
1170 details of the interfaces for handling named subpatterns, see the
1171 <a href="pcreapi.html"><b>pcreapi</b></a>
1172 documentation.
1173 </P>
1174 <br><a name="SEC15" href="#TOC1">REPETITION</a><br>
1175 <P>
1176 Repetition is specified by quantifiers, which can follow any of the following
1177 items:
1178 <pre>
1179 a literal data character
1180 the dot metacharacter
1181 the \C escape sequence
1182 the \X escape sequence (in UTF-8 mode with Unicode properties)
1183 the \R escape sequence
1184 an escape such as \d that matches a single character
1185 a character class
1186 a back reference (see next section)
1187 a parenthesized subpattern (unless it is an assertion)
1188 </pre>
1189 The general repetition quantifier specifies a minimum and maximum number of
1190 permitted matches, by giving the two numbers in curly brackets (braces),
1191 separated by a comma. The numbers must be less than 65536, and the first must
1192 be less than or equal to the second. For example:
1193 <pre>
1194 z{2,4}
1195 </pre>
1196 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1197 character. If the second number is omitted, but the comma is present, there is
1198 no upper limit; if the second number and the comma are both omitted, the
1199 quantifier specifies an exact number of required matches. Thus
1200 <pre>
1201 [aeiou]{3,}
1202 </pre>
1203 matches at least 3 successive vowels, but may match many more, while
1204 <pre>
1205 \d{8}
1206 </pre>
1207 matches exactly 8 digits. An opening curly bracket that appears in a position
1208 where a quantifier is not allowed, or one that does not match the syntax of a
1209 quantifier, is taken as a literal character. For example, {,6} is not a
1210 quantifier, but a literal string of four characters.
1211 </P>
1212 <P>
1213 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1214 bytes. Thus, for example, \x{100}{2} matches two UTF-8 characters, each of
1215 which is represented by a two-byte sequence. Similarly, when Unicode property
1216 support is available, \X{3} matches three Unicode extended sequences, each of
1217 which may be several bytes long (and they may be of different lengths).
1218 </P>
1219 <P>
1220 The quantifier {0} is permitted, causing the expression to behave as if the
1221 previous item and the quantifier were not present.
1222 </P>
1223 <P>
1224 For convenience, the three most common quantifiers have single-character
1225 abbreviations:
1226 <pre>
1227 * is equivalent to {0,}
1228 + is equivalent to {1,}
1229 ? is equivalent to {0,1}
1230 </pre>
1231 It is possible to construct infinite loops by following a subpattern that can
1232 match no characters with a quantifier that has no upper limit, for example:
1233 <pre>
1234 (a?)*
1235 </pre>
1236 Earlier versions of Perl and PCRE used to give an error at compile time for
1237 such patterns. However, because there are cases where this can be useful, such
1238 patterns are now accepted, but if any repetition of the subpattern does in fact
1239 match no characters, the loop is forcibly broken.
1240 </P>
1241 <P>
1242 By default, the quantifiers are "greedy", that is, they match as much as
1243 possible (up to the maximum number of permitted times), without causing the
1244 rest of the pattern to fail. The classic example of where this gives problems
1245 is in trying to match comments in C programs. These appear between /* and */
1246 and within the comment, individual * and / characters may appear. An attempt to
1247 match C comments by applying the pattern
1248 <pre>
1249 /\*.*\*/
1250 </pre>
1251 to the string
1252 <pre>
1253 /* first comment */ not comment /* second comment */
1254 </pre>
1255 fails, because it matches the entire string owing to the greediness of the .*
1256 item.
1257 </P>
1258 <P>
1259 However, if a quantifier is followed by a question mark, it ceases to be
1260 greedy, and instead matches the minimum number of times possible, so the
1261 pattern
1262 <pre>
1263 /\*.*?\*/
1264 </pre>
1265 does the right thing with the C comments. The meaning of the various
1266 quantifiers is not otherwise changed, just the preferred number of matches.
1267 Do not confuse this use of question mark with its use as a quantifier in its
1268 own right. Because it has two uses, it can sometimes appear doubled, as in
1269 <pre>
1270 \d??\d
1271 </pre>
1272 which matches one digit by preference, but can match two if that is the only
1273 way the rest of the pattern matches.
1274 </P>
1275 <P>
1276 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1277 the quantifiers are not greedy by default, but individual ones can be made
1278 greedy by following them with a question mark. In other words, it inverts the
1279 default behaviour.
1280 </P>
1281 <P>
1282 When a parenthesized subpattern is quantified with a minimum repeat count that
1283 is greater than 1 or with a limited maximum, more memory is required for the
1284 compiled pattern, in proportion to the size of the minimum or maximum.
1285 </P>
1286 <P>
1287 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1288 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1289 implicitly anchored, because whatever follows will be tried against every
1290 character position in the subject string, so there is no point in retrying the
1291 overall match at any position after the first. PCRE normally treats such a
1292 pattern as though it were preceded by \A.
1293 </P>
1294 <P>
1295 In cases where it is known that the subject string contains no newlines, it is
1296 worth setting PCRE_DOTALL in order to obtain this optimization, or
1297 alternatively using ^ to indicate anchoring explicitly.
1298 </P>
1299 <P>
1300 However, there is one situation where the optimization cannot be used. When .*
1301 is inside capturing parentheses that are the subject of a backreference
1302 elsewhere in the pattern, a match at the start may fail where a later one
1303 succeeds. Consider, for example:
1304 <pre>
1305 (.*)abc\1
1306 </pre>
1307 If the subject is "xyz123abc123" the match point is the fourth character. For
1308 this reason, such a pattern is not implicitly anchored.
1309 </P>
1310 <P>
1311 When a capturing subpattern is repeated, the value captured is the substring
1312 that matched the final iteration. For example, after
1313 <pre>
1314 (tweedle[dume]{3}\s*)+
1315 </pre>
1316 has matched "tweedledum tweedledee" the value of the captured substring is
1317 "tweedledee". However, if there are nested capturing subpatterns, the
1318 corresponding captured values may have been set in previous iterations. For
1319 example, after
1320 <pre>
1321 /(a|(b))+/
1322 </pre>
1323 matches "aba" the value of the second captured substring is "b".
1324 <a name="atomicgroup"></a></P>
1325 <br><a name="SEC16" href="#TOC1">ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS</a><br>
1326 <P>
1327 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1328 repetition, failure of what follows normally causes the repeated item to be
1329 re-evaluated to see if a different number of repeats allows the rest of the
1330 pattern to match. Sometimes it is useful to prevent this, either to change the
1331 nature of the match, or to cause it fail earlier than it otherwise might, when
1332 the author of the pattern knows there is no point in carrying on.
1333 </P>
1334 <P>
1335 Consider, for example, the pattern \d+foo when applied to the subject line
1336 <pre>
1337 123456bar
1338 </pre>
1339 After matching all 6 digits and then failing to match "foo", the normal
1340 action of the matcher is to try again with only 5 digits matching the \d+
1341 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1342 (a term taken from Jeffrey Friedl's book) provides the means for specifying
1343 that once a subpattern has matched, it is not to be re-evaluated in this way.
1344 </P>
1345 <P>
1346 If we use atomic grouping for the previous example, the matcher gives up
1347 immediately on failing to match "foo" the first time. The notation is a kind of
1348 special parenthesis, starting with (?&#62; as in this example:
1349 <pre>
1350 (?&#62;\d+)foo
1351 </pre>
1352 This kind of parenthesis "locks up" the part of the pattern it contains once
1353 it has matched, and a failure further into the pattern is prevented from
1354 backtracking into it. Backtracking past it to previous items, however, works as
1355 normal.
1356 </P>
1357 <P>
1358 An alternative description is that a subpattern of this type matches the string
1359 of characters that an identical standalone pattern would match, if anchored at
1360 the current point in the subject string.
1361 </P>
1362 <P>
1363 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1364 the above example can be thought of as a maximizing repeat that must swallow
1365 everything it can. So, while both \d+ and \d+? are prepared to adjust the
1366 number of digits they match in order to make the rest of the pattern match,
1367 (?&#62;\d+) can only match an entire sequence of digits.
1368 </P>
1369 <P>
1370 Atomic groups in general can of course contain arbitrarily complicated
1371 subpatterns, and can be nested. However, when the subpattern for an atomic
1372 group is just a single repeated item, as in the example above, a simpler
1373 notation, called a "possessive quantifier" can be used. This consists of an
1374 additional + character following a quantifier. Using this notation, the
1375 previous example can be rewritten as
1376 <pre>
1377 \d++foo
1378 </pre>
1379 Note that a possessive quantifier can be used with an entire group, for
1380 example:
1381 <pre>
1382 (abc|xyz){2,3}+
1383 </pre>
1384 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1385 option is ignored. They are a convenient notation for the simpler forms of
1386 atomic group. However, there is no difference in the meaning of a possessive
1387 quantifier and the equivalent atomic group, though there may be a performance
1388 difference; possessive quantifiers should be slightly faster.
1389 </P>
1390 <P>
1391 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1392 Jeffrey Friedl originated the idea (and the name) in the first edition of his
1393 book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1394 package, and PCRE copied it from there. It ultimately found its way into Perl
1395 at release 5.10.
1396 </P>
1397 <P>
1398 PCRE has an optimization that automatically "possessifies" certain simple
1399 pattern constructs. For example, the sequence A+B is treated as A++B because
1400 there is no point in backtracking into a sequence of A's when B must follow.
1401 </P>
1402 <P>
1403 When a pattern contains an unlimited repeat inside a subpattern that can itself
1404 be repeated an unlimited number of times, the use of an atomic group is the
1405 only way to avoid some failing matches taking a very long time indeed. The
1406 pattern
1407 <pre>
1408 (\D+|&#60;\d+&#62;)*[!?]
1409 </pre>
1410 matches an unlimited number of substrings that either consist of non-digits, or
1411 digits enclosed in &#60;&#62;, followed by either ! or ?. When it matches, it runs
1412 quickly. However, if it is applied to
1413 <pre>
1414 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1415 </pre>
1416 it takes a long time before reporting failure. This is because the string can
1417 be divided between the internal \D+ repeat and the external * repeat in a
1418 large number of ways, and all have to be tried. (The example uses [!?] rather
1419 than a single character at the end, because both PCRE and Perl have an
1420 optimization that allows for fast failure when a single character is used. They
1421 remember the last single character that is required for a match, and fail early
1422 if it is not present in the string.) If the pattern is changed so that it uses
1423 an atomic group, like this:
1424 <pre>
1425 ((?&#62;\D+)|&#60;\d+&#62;)*[!?]
1426 </pre>
1427 sequences of non-digits cannot be broken, and failure happens quickly.
1428 <a name="backreferences"></a></P>
1429 <br><a name="SEC17" href="#TOC1">BACK REFERENCES</a><br>
1430 <P>
1431 Outside a character class, a backslash followed by a digit greater than 0 (and
1432 possibly further digits) is a back reference to a capturing subpattern earlier
1433 (that is, to its left) in the pattern, provided there have been that many
1434 previous capturing left parentheses.
1435 </P>
1436 <P>
1437 However, if the decimal number following the backslash is less than 10, it is
1438 always taken as a back reference, and causes an error only if there are not
1439 that many capturing left parentheses in the entire pattern. In other words, the
1440 parentheses that are referenced need not be to the left of the reference for
1441 numbers less than 10. A "forward back reference" of this type can make sense
1442 when a repetition is involved and the subpattern to the right has participated
1443 in an earlier iteration.
1444 </P>
1445 <P>
1446 It is not possible to have a numerical "forward back reference" to a subpattern
1447 whose number is 10 or more using this syntax because a sequence such as \50 is
1448 interpreted as a character defined in octal. See the subsection entitled
1449 "Non-printing characters"
1450 <a href="#digitsafterbackslash">above</a>
1451 for further details of the handling of digits following a backslash. There is
1452 no such problem when named parentheses are used. A back reference to any
1453 subpattern is possible using named parentheses (see below).
1454 </P>
1455 <P>
1456 Another way of avoiding the ambiguity inherent in the use of digits following a
1457 backslash is to use the \g escape sequence, which is a feature introduced in
1458 Perl 5.10. This escape must be followed by an unsigned number or a negative
1459 number, optionally enclosed in braces. These examples are all identical:
1460 <pre>
1461 (ring), \1
1462 (ring), \g1
1463 (ring), \g{1}
1464 </pre>
1465 An unsigned number specifies an absolute reference without the ambiguity that
1466 is present in the older syntax. It is also useful when literal digits follow
1467 the reference. A negative number is a relative reference. Consider this
1468 example:
1469 <pre>
1470 (abc(def)ghi)\g{-1}
1471 </pre>
1472 The sequence \g{-1} is a reference to the most recently started capturing
1473 subpattern before \g, that is, is it equivalent to \2. Similarly, \g{-2}
1474 would be equivalent to \1. The use of relative references can be helpful in
1475 long patterns, and also in patterns that are created by joining together
1476 fragments that contain references within themselves.
1477 </P>
1478 <P>
1479 A back reference matches whatever actually matched the capturing subpattern in
1480 the current subject string, rather than anything matching the subpattern
1481 itself (see
1482 <a href="#subpatternsassubroutines">"Subpatterns as subroutines"</a>
1483 below for a way of doing that). So the pattern
1484 <pre>
1485 (sens|respons)e and \1ibility
1486 </pre>
1487 matches "sense and sensibility" and "response and responsibility", but not
1488 "sense and responsibility". If caseful matching is in force at the time of the
1489 back reference, the case of letters is relevant. For example,
1490 <pre>
1491 ((?i)rah)\s+\1
1492 </pre>
1493 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1494 capturing subpattern is matched caselessly.
1495 </P>
1496 <P>
1497 There are several different ways of writing back references to named
1498 subpatterns. The .NET syntax \k{name} and the Perl syntax \k&#60;name&#62; or
1499 \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1500 back reference syntax, in which \g can be used for both numeric and named
1501 references, is also supported. We could rewrite the above example in any of
1502 the following ways:
1503 <pre>
1504 (?&#60;p1&#62;(?i)rah)\s+\k&#60;p1&#62;
1505 (?'p1'(?i)rah)\s+\k{p1}
1506 (?P&#60;p1&#62;(?i)rah)\s+(?P=p1)
1507 (?&#60;p1&#62;(?i)rah)\s+\g{p1}
1508 </pre>
1509 A subpattern that is referenced by name may appear in the pattern before or
1510 after the reference.
1511 </P>
1512 <P>
1513 There may be more than one back reference to the same subpattern. If a
1514 subpattern has not actually been used in a particular match, any back
1515 references to it always fail. For example, the pattern
1516 <pre>
1517 (a|(bc))\2
1518 </pre>
1519 always fails if it starts to match "a" rather than "bc". Because there may be
1520 many capturing parentheses in a pattern, all digits following the backslash are
1521 taken as part of a potential back reference number. If the pattern continues
1522 with a digit character, some delimiter must be used to terminate the back
1523 reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1524 Otherwise an empty comment (see
1525 <a href="#comments">"Comments"</a>
1526 below) can be used.
1527 </P>
1528 <P>
1529 A back reference that occurs inside the parentheses to which it refers fails
1530 when the subpattern is first used, so, for example, (a\1) never matches.
1531 However, such references can be useful inside repeated subpatterns. For
1532 example, the pattern
1533 <pre>
1534 (a|b\1)+
1535 </pre>
1536 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1537 the subpattern, the back reference matches the character string corresponding
1538 to the previous iteration. In order for this to work, the pattern must be such
1539 that the first iteration does not need to match the back reference. This can be
1540 done using alternation, as in the example above, or by a quantifier with a
1541 minimum of zero.
1542 <a name="bigassertions"></a></P>
1543 <br><a name="SEC18" href="#TOC1">ASSERTIONS</a><br>
1544 <P>
1545 An assertion is a test on the characters following or preceding the current
1546 matching point that does not actually consume any characters. The simple
1547 assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are described
1548 <a href="#smallassertions">above.</a>
1549 </P>
1550 <P>
1551 More complicated assertions are coded as subpatterns. There are two kinds:
1552 those that look ahead of the current position in the subject string, and those
1553 that look behind it. An assertion subpattern is matched in the normal way,
1554 except that it does not cause the current matching position to be changed.
1555 </P>
1556 <P>
1557 Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1558 because it makes no sense to assert the same thing several times. If any kind
1559 of assertion contains capturing subpatterns within it, these are counted for
1560 the purposes of numbering the capturing subpatterns in the whole pattern.
1561 However, substring capturing is carried out only for positive assertions,
1562 because it does not make sense for negative assertions.
1563 </P>
1564 <br><b>
1565 Lookahead assertions
1566 </b><br>
1567 <P>
1568 Lookahead assertions start with (?= for positive assertions and (?! for
1569 negative assertions. For example,
1570 <pre>
1571 \w+(?=;)
1572 </pre>
1573 matches a word followed by a semicolon, but does not include the semicolon in
1574 the match, and
1575 <pre>
1576 foo(?!bar)
1577 </pre>
1578 matches any occurrence of "foo" that is not followed by "bar". Note that the
1579 apparently similar pattern
1580 <pre>
1581 (?!foo)bar
1582 </pre>
1583 does not find an occurrence of "bar" that is preceded by something other than
1584 "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1585 (?!foo) is always true when the next three characters are "bar". A
1586 lookbehind assertion is needed to achieve the other effect.
1587 </P>
1588 <P>
1589 If you want to force a matching failure at some point in a pattern, the most
1590 convenient way to do it is with (?!) because an empty string always matches, so
1591 an assertion that requires there not to be an empty string must always fail.
1592 <a name="lookbehind"></a></P>
1593 <br><b>
1594 Lookbehind assertions
1595 </b><br>
1596 <P>
1597 Lookbehind assertions start with (?&#60;= for positive assertions and (?&#60;! for
1598 negative assertions. For example,
1599 <pre>
1600 (?&#60;!foo)bar
1601 </pre>
1602 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1603 a lookbehind assertion are restricted such that all the strings it matches must
1604 have a fixed length. However, if there are several top-level alternatives, they
1605 do not all have to have the same fixed length. Thus
1606 <pre>
1607 (?&#60;=bullock|donkey)
1608 </pre>
1609 is permitted, but
1610 <pre>
1611 (?&#60;!dogs?|cats?)
1612 </pre>
1613 causes an error at compile time. Branches that match different length strings
1614 are permitted only at the top level of a lookbehind assertion. This is an
1615 extension compared with Perl (at least for 5.8), which requires all branches to
1616 match the same length of string. An assertion such as
1617 <pre>
1618 (?&#60;=ab(c|de))
1619 </pre>
1620 is not permitted, because its single top-level branch can match two different
1621 lengths, but it is acceptable if rewritten to use two top-level branches:
1622 <pre>
1623 (?&#60;=abc|abde)
1624 </pre>
1625 In some cases, the Perl 5.10 escape sequence \K
1626 <a href="#resetmatchstart">(see above)</a>
1627 can be used instead of a lookbehind assertion; this is not restricted to a
1628 fixed-length.
1629 </P>
1630 <P>
1631 The implementation of lookbehind assertions is, for each alternative, to
1632 temporarily move the current position back by the fixed length and then try to
1633 match. If there are insufficient characters before the current position, the
1634 assertion fails.
1635 </P>
1636 <P>
1637 PCRE does not allow the \C escape (which matches a single byte in UTF-8 mode)
1638 to appear in lookbehind assertions, because it makes it impossible to calculate
1639 the length of the lookbehind. The \X and \R escapes, which can match
1640 different numbers of bytes, are also not permitted.
1641 </P>
1642 <P>
1643 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1644 specify efficient matching at the end of the subject string. Consider a simple
1645 pattern such as
1646 <pre>
1647 abcd$
1648 </pre>
1649 when applied to a long string that does not match. Because matching proceeds
1650 from left to right, PCRE will look for each "a" in the subject and then see if
1651 what follows matches the rest of the pattern. If the pattern is specified as
1652 <pre>
1653 ^.*abcd$
1654 </pre>
1655 the initial .* matches the entire string at first, but when this fails (because
1656 there is no following "a"), it backtracks to match all but the last character,
1657 then all but the last two characters, and so on. Once again the search for "a"
1658 covers the entire string, from right to left, so we are no better off. However,
1659 if the pattern is written as
1660 <pre>
1661 ^.*+(?&#60;=abcd)
1662 </pre>
1663 there can be no backtracking for the .*+ item; it can match only the entire
1664 string. The subsequent lookbehind assertion does a single test on the last four
1665 characters. If it fails, the match fails immediately. For long strings, this
1666 approach makes a significant difference to the processing time.
1667 </P>
1668 <br><b>
1669 Using multiple assertions
1670 </b><br>
1671 <P>
1672 Several assertions (of any sort) may occur in succession. For example,
1673 <pre>
1674 (?&#60;=\d{3})(?&#60;!999)foo
1675 </pre>
1676 matches "foo" preceded by three digits that are not "999". Notice that each of
1677 the assertions is applied independently at the same point in the subject
1678 string. First there is a check that the previous three characters are all
1679 digits, and then there is a check that the same three characters are not "999".
1680 This pattern does <i>not</i> match "foo" preceded by six characters, the first
1681 of which are digits and the last three of which are not "999". For example, it
1682 doesn't match "123abcfoo". A pattern to do that is
1683 <pre>
1684 (?&#60;=\d{3}...)(?&#60;!999)foo
1685 </pre>
1686 This time the first assertion looks at the preceding six characters, checking
1687 that the first three are digits, and then the second assertion checks that the
1688 preceding three characters are not "999".
1689 </P>
1690 <P>
1691 Assertions can be nested in any combination. For example,
1692 <pre>
1693 (?&#60;=(?&#60;!foo)bar)baz
1694 </pre>
1695 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1696 preceded by "foo", while
1697 <pre>
1698 (?&#60;=\d{3}(?!999)...)foo
1699 </pre>
1700 is another pattern that matches "foo" preceded by three digits and any three
1701 characters that are not "999".
1702 <a name="conditions"></a></P>
1703 <br><a name="SEC19" href="#TOC1">CONDITIONAL SUBPATTERNS</a><br>
1704 <P>
1705 It is possible to cause the matching process to obey a subpattern
1706 conditionally or to choose between two alternative subpatterns, depending on
1707 the result of an assertion, or whether a previous capturing subpattern matched
1708 or not. The two possible forms of conditional subpattern are
1709 <pre>
1710 (?(condition)yes-pattern)
1711 (?(condition)yes-pattern|no-pattern)
1712 </pre>
1713 If the condition is satisfied, the yes-pattern is used; otherwise the
1714 no-pattern (if present) is used. If there are more than two alternatives in the
1715 subpattern, a compile-time error occurs.
1716 </P>
1717 <P>
1718 There are four kinds of condition: references to subpatterns, references to
1719 recursion, a pseudo-condition called DEFINE, and assertions.
1720 </P>
1721 <br><b>
1722 Checking for a used subpattern by number
1723 </b><br>
1724 <P>
1725 If the text between the parentheses consists of a sequence of digits, the
1726 condition is true if the capturing subpattern of that number has previously
1727 matched. An alternative notation is to precede the digits with a plus or minus
1728 sign. In this case, the subpattern number is relative rather than absolute.
1729 The most recently opened parentheses can be referenced by (?(-1), the next most
1730 recent by (?(-2), and so on. In looping constructs it can also make sense to
1731 refer to subsequent groups with constructs such as (?(+2).
1732 </P>
1733 <P>
1734 Consider the following pattern, which contains non-significant white space to
1735 make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1736 three parts for ease of discussion:
1737 <pre>
1738 ( \( )? [^()]+ (?(1) \) )
1739 </pre>
1740 The first part matches an optional opening parenthesis, and if that
1741 character is present, sets it as the first captured substring. The second part
1742 matches one or more characters that are not parentheses. The third part is a
1743 conditional subpattern that tests whether the first set of parentheses matched
1744 or not. If they did, that is, if subject started with an opening parenthesis,
1745 the condition is true, and so the yes-pattern is executed and a closing
1746 parenthesis is required. Otherwise, since no-pattern is not present, the
1747 subpattern matches nothing. In other words, this pattern matches a sequence of
1748 non-parentheses, optionally enclosed in parentheses.
1749 </P>
1750 <P>
1751 If you were embedding this pattern in a larger one, you could use a relative
1752 reference:
1753 <pre>
1754 ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
1755 </pre>
1756 This makes the fragment independent of the parentheses in the larger pattern.
1757 </P>
1758 <br><b>
1759 Checking for a used subpattern by name
1760 </b><br>
1761 <P>
1762 Perl uses the syntax (?(&#60;name&#62;)...) or (?('name')...) to test for a used
1763 subpattern by name. For compatibility with earlier versions of PCRE, which had
1764 this facility before Perl, the syntax (?(name)...) is also recognized. However,
1765 there is a possible ambiguity with this syntax, because subpattern names may
1766 consist entirely of digits. PCRE looks first for a named subpattern; if it
1767 cannot find one and the name consists entirely of digits, PCRE looks for a
1768 subpattern of that number, which must be greater than zero. Using subpattern
1769 names that consist entirely of digits is not recommended.
1770 </P>
1771 <P>
1772 Rewriting the above example to use a named subpattern gives this:
1773 <pre>
1774 (?&#60;OPEN&#62; \( )? [^()]+ (?(&#60;OPEN&#62;) \) )
1776 </PRE>
1777 </P>
1778 <br><b>
1779 Checking for pattern recursion
1780 </b><br>
1781 <P>
1782 If the condition is the string (R), and there is no subpattern with the name R,
1783 the condition is true if a recursive call to the whole pattern or any
1784 subpattern has been made. If digits or a name preceded by ampersand follow the
1785 letter R, for example:
1786 <pre>
1787 (?(R3)...) or (?(R&name)...)
1788 </pre>
1789 the condition is true if the most recent recursion is into the subpattern whose
1790 number or name is given. This condition does not check the entire recursion
1791 stack.
1792 </P>
1793 <P>
1794 At "top level", all these recursion test conditions are false. Recursive
1795 patterns are described below.
1796 </P>
1797 <br><b>
1798 Defining subpatterns for use by reference only
1799 </b><br>
1800 <P>
1801 If the condition is the string (DEFINE), and there is no subpattern with the
1802 name DEFINE, the condition is always false. In this case, there may be only one
1803 alternative in the subpattern. It is always skipped if control reaches this
1804 point in the pattern; the idea of DEFINE is that it can be used to define
1805 "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1806 is described below.) For example, a pattern to match an IPv4 address could be
1807 written like this (ignore whitespace and line breaks):
1808 <pre>
1809 (?(DEFINE) (?&#60;byte&#62; 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
1810 \b (?&byte) (\.(?&byte)){3} \b
1811 </pre>
1812 The first part of the pattern is a DEFINE group inside which a another group
1813 named "byte" is defined. This matches an individual component of an IPv4
1814 address (a number less than 256). When matching takes place, this part of the
1815 pattern is skipped because DEFINE acts like a false condition.
1816 </P>
1817 <P>
1818 The rest of the pattern uses references to the named group to match the four
1819 dot-separated components of an IPv4 address, insisting on a word boundary at
1820 each end.
1821 </P>
1822 <br><b>
1823 Assertion conditions
1824 </b><br>
1825 <P>
1826 If the condition is not in any of the above formats, it must be an assertion.
1827 This may be a positive or negative lookahead or lookbehind assertion. Consider
1828 this pattern, again containing non-significant white space, and with the two
1829 alternatives on the second line:
1830 <pre>
1831 (?(?=[^a-z]*[a-z])
1832 \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
1833 </pre>
1834 The condition is a positive lookahead assertion that matches an optional
1835 sequence of non-letters followed by a letter. In other words, it tests for the
1836 presence of at least one letter in the subject. If a letter is found, the
1837 subject is matched against the first alternative; otherwise it is matched
1838 against the second. This pattern matches strings in one of the two forms
1839 dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1840 <a name="comments"></a></P>
1841 <br><a name="SEC20" href="#TOC1">COMMENTS</a><br>
1842 <P>
1843 The sequence (?# marks the start of a comment that continues up to the next
1844 closing parenthesis. Nested parentheses are not permitted. The characters
1845 that make up a comment play no part in the pattern matching at all.
1846 </P>
1847 <P>
1848 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1849 character class introduces a comment that continues to immediately after the
1850 next newline in the pattern.
1851 <a name="recursion"></a></P>
1852 <br><a name="SEC21" href="#TOC1">RECURSIVE PATTERNS</a><br>
1853 <P>
1854 Consider the problem of matching a string in parentheses, allowing for
1855 unlimited nested parentheses. Without the use of recursion, the best that can
1856 be done is to use a pattern that matches up to some fixed depth of nesting. It
1857 is not possible to handle an arbitrary nesting depth.
1858 </P>
1859 <P>
1860 For some time, Perl has provided a facility that allows regular expressions to
1861 recurse (amongst other things). It does this by interpolating Perl code in the
1862 expression at run time, and the code can refer to the expression itself. A Perl
1863 pattern using code interpolation to solve the parentheses problem can be
1864 created like this:
1865 <pre>
1866 $re = qr{\( (?: (?&#62;[^()]+) | (?p{$re}) )* \)}x;
1867 </pre>
1868 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1869 recursively to the pattern in which it appears.
1870 </P>
1871 <P>
1872 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1873 supports special syntax for recursion of the entire pattern, and also for
1874 individual subpattern recursion. After its introduction in PCRE and Python,
1875 this kind of recursion was introduced into Perl at release 5.10.
1876 </P>
1877 <P>
1878 A special item that consists of (? followed by a number greater than zero and a
1879 closing parenthesis is a recursive call of the subpattern of the given number,
1880 provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1881 call, which is described in the next section.) The special item (?R) or (?0) is
1882 a recursive call of the entire regular expression.
1883 </P>
1884 <P>
1885 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1886 treated as an atomic group. That is, once it has matched some of the subject
1887 string, it is never re-entered, even if it contains untried alternatives and
1888 there is a subsequent matching failure.
1889 </P>
1890 <P>
1891 This PCRE pattern solves the nested parentheses problem (assume the
1892 PCRE_EXTENDED option is set so that white space is ignored):
1893 <pre>
1894 \( ( (?&#62;[^()]+) | (?R) )* \)
1895 </pre>
1896 First it matches an opening parenthesis. Then it matches any number of
1897 substrings which can either be a sequence of non-parentheses, or a recursive
1898 match of the pattern itself (that is, a correctly parenthesized substring).
1899 Finally there is a closing parenthesis.
1900 </P>
1901 <P>
1902 If this were part of a larger pattern, you would not want to recurse the entire
1903 pattern, so instead you could use this:
1904 <pre>
1905 ( \( ( (?&#62;[^()]+) | (?1) )* \) )
1906 </pre>
1907 We have put the pattern into parentheses, and caused the recursion to refer to
1908 them instead of the whole pattern.
1909 </P>
1910 <P>
1911 In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1912 is made easier by the use of relative references. (A Perl 5.10 feature.)
1913 Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1914 most recently opened parentheses preceding the recursion. In other words, a
1915 negative number counts capturing parentheses leftwards from the point at which
1916 it is encountered.
1917 </P>
1918 <P>
1919 It is also possible to refer to subsequently opened parentheses, by writing
1920 references such as (?+2). However, these cannot be recursive because the
1921 reference is not inside the parentheses that are referenced. They are always
1922 "subroutine" calls, as described in the next section.
1923 </P>
1924 <P>
1925 An alternative approach is to use named parentheses instead. The Perl syntax
1926 for this is (?&name); PCRE's earlier syntax (?P&#62;name) is also supported. We
1927 could rewrite the above example as follows:
1928 <pre>
1929 (?&#60;pn&#62; \( ( (?&#62;[^()]+) | (?&pn) )* \) )
1930 </pre>
1931 If there is more than one subpattern with the same name, the earliest one is
1932 used.
1933 </P>
1934 <P>
1935 This particular example pattern that we have been looking at contains nested
1936 unlimited repeats, and so the use of atomic grouping for matching strings of
1937 non-parentheses is important when applying the pattern to strings that do not
1938 match. For example, when this pattern is applied to
1939 <pre>
1940 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1941 </pre>
1942 it yields "no match" quickly. However, if atomic grouping is not used,
1943 the match runs for a very long time indeed because there are so many different
1944 ways the + and * repeats can carve up the subject, and all have to be tested
1945 before failure can be reported.
1946 </P>
1947 <P>
1948 At the end of a match, the values set for any capturing subpatterns are those
1949 from the outermost level of the recursion at which the subpattern value is set.
1950 If you want to obtain intermediate values, a callout function can be used (see
1951 below and the
1952 <a href="pcrecallout.html"><b>pcrecallout</b></a>
1953 documentation). If the pattern above is matched against
1954 <pre>
1955 (ab(cd)ef)
1956 </pre>
1957 the value for the capturing parentheses is "ef", which is the last value taken
1958 on at the top level. If additional parentheses are added, giving
1959 <pre>
1960 \( ( ( (?&#62;[^()]+) | (?R) )* ) \)
1961 ^ ^
1962 ^ ^
1963 </pre>
1964 the string they capture is "ab(cd)ef", the contents of the top level
1965 parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1966 has to obtain extra memory to store data during a recursion, which it does by
1967 using <b>pcre_malloc</b>, freeing it via <b>pcre_free</b> afterwards. If no
1968 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1969 </P>
1970 <P>
1971 Do not confuse the (?R) item with the condition (R), which tests for recursion.
1972 Consider this pattern, which matches text in angle brackets, allowing for
1973 arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1974 recursing), whereas any characters are permitted at the outer level.
1975 <pre>
1976 &#60; (?: (?(R) \d++ | [^&#60;&#62;]*+) | (?R)) * &#62;
1977 </pre>
1978 In this pattern, (?(R) is the start of a conditional subpattern, with two
1979 different alternatives for the recursive and non-recursive cases. The (?R) item
1980 is the actual recursive call.
1981 <a name="subpatternsassubroutines"></a></P>
1982 <br><a name="SEC22" href="#TOC1">SUBPATTERNS AS SUBROUTINES</a><br>
1983 <P>
1984 If the syntax for a recursive subpattern reference (either by number or by
1985 name) is used outside the parentheses to which it refers, it operates like a
1986 subroutine in a programming language. The "called" subpattern may be defined
1987 before or after the reference. A numbered reference can be absolute or
1988 relative, as in these examples:
1989 <pre>
1990 (...(absolute)...)...(?2)...
1991 (...(relative)...)...(?-1)...
1992 (...(?+1)...(relative)...
1993 </pre>
1994 An earlier example pointed out that the pattern
1995 <pre>
1996 (sens|respons)e and \1ibility
1997 </pre>
1998 matches "sense and sensibility" and "response and responsibility", but not
1999 "sense and responsibility". If instead the pattern
2000 <pre>
2001 (sens|respons)e and (?1)ibility
2002 </pre>
2003 is used, it does match "sense and responsibility" as well as the other two
2004 strings. Another example is given in the discussion of DEFINE above.
2005 </P>
2006 <P>
2007 Like recursive subpatterns, a "subroutine" call is always treated as an atomic
2008 group. That is, once it has matched some of the subject string, it is never
2009 re-entered, even if it contains untried alternatives and there is a subsequent
2010 matching failure.
2011 </P>
2012 <P>
2013 When a subpattern is used as a subroutine, processing options such as
2014 case-independence are fixed when the subpattern is defined. They cannot be
2015 changed for different calls. For example, consider this pattern:
2016 <pre>
2017 (abc)(?i:(?-1))
2018 </pre>
2019 It matches "abcabc". It does not match "abcABC" because the change of
2020 processing option does not affect the called subpattern.
2021 </P>
2022 <br><a name="SEC23" href="#TOC1">CALLOUTS</a><br>
2023 <P>
2024 Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
2025 code to be obeyed in the middle of matching a regular expression. This makes it
2026 possible, amongst other things, to extract different substrings that match the
2027 same pair of parentheses when there is a repetition.
2028 </P>
2029 <P>
2030 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2031 code. The feature is called "callout". The caller of PCRE provides an external
2032 function by putting its entry point in the global variable <i>pcre_callout</i>.
2033 By default, this variable contains NULL, which disables all calling out.
2034 </P>
2035 <P>
2036 Within a regular expression, (?C) indicates the points at which the external
2037 function is to be called. If you want to identify different callout points, you
2038 can put a number less than 256 after the letter C. The default value is zero.
2039 For example, this pattern has two callout points:
2040 <pre>
2041 (?C1)abc(?C2)def
2042 </pre>
2043 If the PCRE_AUTO_CALLOUT flag is passed to <b>pcre_compile()</b>, callouts are
2044 automatically installed before each item in the pattern. They are all numbered
2045 255.
2046 </P>
2047 <P>
2048 During matching, when PCRE reaches a callout point (and <i>pcre_callout</i> is
2049 set), the external function is called. It is provided with the number of the
2050 callout, the position in the pattern, and, optionally, one item of data
2051 originally supplied by the caller of <b>pcre_exec()</b>. The callout function
2052 may cause matching to proceed, to backtrack, or to fail altogether. A complete
2053 description of the interface to the callout function is given in the
2054 <a href="pcrecallout.html"><b>pcrecallout</b></a>
2055 documentation.
2056 </P>
2057 <br><a name="SEC24" href="#TOC1">BACTRACKING CONTROL</a><br>
2058 <P>
2059 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2060 are described in the Perl documentation as "experimental and subject to change
2061 or removal in a future version of Perl". It goes on to say: "Their usage in
2062 production code should be noted to avoid problems during upgrades." The same
2063 remarks apply to the PCRE features described in this section.
2064 </P>
2065 <P>
2066 Since these verbs are specifically related to backtracking, they can be used
2067 only when the pattern is to be matched using <b>pcre_exec()</b>, which uses a
2068 backtracking algorithm. They cause an error if encountered by
2069 <b>pcre_dfa_exec()</b>.
2070 </P>
2071 <P>
2072 The new verbs make use of what was previously invalid syntax: an opening
2073 parenthesis followed by an asterisk. In Perl, they are generally of the form
2074 (*VERB:ARG) but PCRE does not support the use of arguments, so its general
2075 form is just (*VERB). Any number of these verbs may occur in a pattern. There
2076 are two kinds:
2077 </P>
2078 <br><b>
2079 Verbs that act immediately
2080 </b><br>
2081 <P>
2082 The following verbs act as soon as they are encountered:
2083 <pre>
2084 (*ACCEPT)
2085 </pre>
2086 This verb causes the match to end successfully, skipping the remainder of the
2087 pattern. When inside a recursion, only the innermost pattern is ended
2088 immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2089 capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2090 captured. For example:
2091 <pre>
2092 A(A|B(*ACCEPT)|C)D
2093 </pre>
2094 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2095 captured.
2096 <pre>
2097 (*FAIL) or (*F)
2098 </pre>
2099 This verb causes the match to fail, forcing backtracking to occur. It is
2100 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2101 probably useful only when combined with (?{}) or (??{}). Those are, of course,
2102 Perl features that are not present in PCRE. The nearest equivalent is the
2103 callout feature, as for example in this pattern:
2104 <pre>
2105 a+(?C)(*FAIL)
2106 </pre>
2107 A match with the string "aaaa" always fails, but the callout is taken before
2108 each backtrack happens (in this example, 10 times).
2109 </P>
2110 <br><b>
2111 Verbs that act after backtracking
2112 </b><br>
2113 <P>
2114 The following verbs do nothing when they are encountered. Matching continues
2115 with what follows, but if there is no subsequent match, a failure is forced.
2116 The verbs differ in exactly what kind of failure occurs.
2117 <pre>
2118 (*COMMIT)
2119 </pre>
2120 This verb causes the whole match to fail outright if the rest of the pattern
2121 does not match. Even if the pattern is unanchored, no further attempts to find
2122 a match by advancing the start point take place. Once (*COMMIT) has been
2123 passed, <b>pcre_exec()</b> is committed to finding a match at the current
2124 starting point, or not at all. For example:
2125 <pre>
2126 a+(*COMMIT)b
2127 </pre>
2128 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2129 dynamic anchor, or "I've started, so I must finish."
2130 <pre>
2131 (*PRUNE)
2132 </pre>
2133 This verb causes the match to fail at the current position if the rest of the
2134 pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2135 advance to the next starting character then happens. Backtracking can occur as
2136 usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2137 if there is no match to the right, backtracking cannot cross (*PRUNE).
2138 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2139 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2140 be expressed in any other way.
2141 <pre>
2142 (*SKIP)
2143 </pre>
2144 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2145 "bumpalong" advance is not to the next character, but to the position in the
2146 subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2147 was matched leading up to it cannot be part of a successful match. Consider:
2148 <pre>
2149 a+(*SKIP)b
2150 </pre>
2151 If the subject is "aaaac...", after the first match attempt fails (starting at
2152 the first character in the string), the starting point skips on to start the
2153 next attempt at "c". Note that a possessive quantifer does not have the same
2154 effect in this example; although it would suppress backtracking during the
2155 first match attempt, the second attempt would start at the second character
2156 instead of skipping on to "c".
2157 <pre>
2158 (*THEN)
2159 </pre>
2160 This verb causes a skip to the next alternation if the rest of the pattern does
2161 not match. That is, it cancels pending backtracking, but only within the
2162 current alternation. Its name comes from the observation that it can be used
2163 for a pattern-based if-then-else block:
2164 <pre>
2165 ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2166 </pre>
2167 If the COND1 pattern matches, FOO is tried (and possibly further items after
2168 the end of the group if FOO succeeds); on failure the matcher skips to the
2169 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2170 is used outside of any alternation, it acts exactly like (*PRUNE).
2171 </P>
2172 <br><a name="SEC25" href="#TOC1">SEE ALSO</a><br>
2173 <P>
2174 <b>pcreapi</b>(3), <b>pcrecallout</b>(3), <b>pcrematching</b>(3), <b>pcre</b>(3).
2175 </P>
2176 <br><a name="SEC26" href="#TOC1">AUTHOR</a><br>
2177 <P>
2178 Philip Hazel
2179 <br>
2180 University Computing Service
2181 <br>
2182 Cambridge CB2 3QH, England.
2183 <br>
2184 </P>
2185 <br><a name="SEC27" href="#TOC1">REVISION</a><br>
2186 <P>
2187 Last updated: 11 September 2007
2188 <br>
2189 Copyright &copy; 1997-2007 University of Cambridge.
2190 <br>
2191 <p>
2192 Return to the <a href="index.html">PCRE index page</a>.
2193 </p>


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