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1 nigel 79 .TH PCREPATTERN 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 nigel 63 .rs
6     .sp
7     The syntax and semantics of the regular expressions supported by PCRE are
8     described below. Regular expressions are also described in the Perl
9 nigel 75 documentation and in a number of books, some of which have copious examples.
10     Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
11     regular expressions in great detail. This description of PCRE's regular
12     expressions is intended as reference material.
13     .P
14     The original operation of PCRE was on strings of one-byte characters. However,
15     there is now also support for UTF-8 character strings. To use this, you must
16     build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
17     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
18     places below. There is also a summary of UTF-8 features in the
19 nigel 63 .\" HTML <a href="pcre.html#utf8support">
20     .\" </a>
21     section on UTF-8 support
22     .\"
23     in the main
24     .\" HREF
25 nigel 75 \fBpcre\fP
26 nigel 63 .\"
27     page.
28 nigel 75 .P
29 nigel 77 The remainder of this document discusses the patterns that are supported by
30     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31     From release 6.0, PCRE offers a second matching function,
32     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33 ph10 172 Perl-compatible. Some of the features discussed below are not available when
34 ph10 168 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
35     alternative function, and how it differs from the normal function, are
36     discussed in the
37 nigel 77 .\" HREF
38     \fBpcrematching\fP
39     .\"
40     page.
41 nigel 93 .
42     .
43     .SH "CHARACTERS AND METACHARACTERS"
44     .rs
45     .sp
46 nigel 63 A regular expression is a pattern that is matched against a subject string from
47     left to right. Most characters stand for themselves in a pattern, and match the
48     corresponding characters in the subject. As a trivial example, the pattern
49 nigel 75 .sp
50 nigel 63 The quick brown fox
51 nigel 75 .sp
52 nigel 77 matches a portion of a subject string that is identical to itself. When
53     caseless matching is specified (the PCRE_CASELESS option), letters are matched
54     independently of case. In UTF-8 mode, PCRE always understands the concept of
55     case for characters whose values are less than 128, so caseless matching is
56     always possible. For characters with higher values, the concept of case is
57     supported if PCRE is compiled with Unicode property support, but not otherwise.
58     If you want to use caseless matching for characters 128 and above, you must
59     ensure that PCRE is compiled with Unicode property support as well as with
60     UTF-8 support.
61     .P
62     The power of regular expressions comes from the ability to include alternatives
63     and repetitions in the pattern. These are encoded in the pattern by the use of
64 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
65 nigel 63 interpreted in some special way.
66 nigel 75 .P
67     There are two different sets of metacharacters: those that are recognized
68 nigel 63 anywhere in the pattern except within square brackets, and those that are
69 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
70     are as follows:
71 nigel 75 .sp
72     \e general escape character with several uses
73 nigel 63 ^ assert start of string (or line, in multiline mode)
74     $ assert end of string (or line, in multiline mode)
75     . match any character except newline (by default)
76     [ start character class definition
77     | start of alternative branch
78     ( start subpattern
79     ) end subpattern
80     ? extends the meaning of (
81     also 0 or 1 quantifier
82     also quantifier minimizer
83     * 0 or more quantifier
84     + 1 or more quantifier
85     also "possessive quantifier"
86     { start min/max quantifier
87 nigel 75 .sp
88 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
89 nigel 75 a character class the only metacharacters are:
90     .sp
91     \e general escape character
92 nigel 63 ^ negate the class, but only if the first character
93     - indicates character range
94 nigel 75 .\" JOIN
95 nigel 63 [ POSIX character class (only if followed by POSIX
96     syntax)
97     ] terminates the character class
98 nigel 75 .sp
99     The following sections describe the use of each of the metacharacters.
100     .
101 nigel 93 .
102 nigel 63 .SH BACKSLASH
103     .rs
104     .sp
105     The backslash character has several uses. Firstly, if it is followed by a
106 nigel 91 non-alphanumeric character, it takes away any special meaning that character
107     may have. This use of backslash as an escape character applies both inside and
108 nigel 63 outside character classes.
109 nigel 75 .P
110     For example, if you want to match a * character, you write \e* in the pattern.
111 nigel 63 This escaping action applies whether or not the following character would
112 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
113     non-alphanumeric with backslash to specify that it stands for itself. In
114     particular, if you want to match a backslash, you write \e\e.
115     .P
116 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
117     pattern (other than in a character class) and characters between a # outside
118 nigel 91 a character class and the next newline are ignored. An escaping backslash can
119     be used to include a whitespace or # character as part of the pattern.
120 nigel 75 .P
121 nigel 63 If you want to remove the special meaning from a sequence of characters, you
122 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
123     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
124 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
125 nigel 75 .sp
126 nigel 63 Pattern PCRE matches Perl matches
127 nigel 75 .sp
128     .\" JOIN
129     \eQabc$xyz\eE abc$xyz abc followed by the
130 nigel 63 contents of $xyz
131 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
132     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
133     .sp
134     The \eQ...\eE sequence is recognized both inside and outside character classes.
135     .
136     .
137     .\" HTML <a name="digitsafterbackslash"></a>
138     .SS "Non-printing characters"
139     .rs
140     .sp
141 nigel 63 A second use of backslash provides a way of encoding non-printing characters
142     in patterns in a visible manner. There is no restriction on the appearance of
143     non-printing characters, apart from the binary zero that terminates a pattern,
144     but when a pattern is being prepared by text editing, it is usually easier to
145     use one of the following escape sequences than the binary character it
146     represents:
147 nigel 75 .sp
148     \ea alarm, that is, the BEL character (hex 07)
149     \ecx "control-x", where x is any character
150     \ee escape (hex 1B)
151     \ef formfeed (hex 0C)
152     \en newline (hex 0A)
153     \er carriage return (hex 0D)
154     \et tab (hex 09)
155     \eddd character with octal code ddd, or backreference
156     \exhh character with hex code hh
157 nigel 87 \ex{hhh..} character with hex code hhh..
158 nigel 75 .sp
159     The precise effect of \ecx is as follows: if x is a lower case letter, it
160 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
161 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
162 nigel 63 7B.
163 nigel 75 .P
164     After \ex, from zero to two hexadecimal digits are read (letters can be in
165 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
166     and }, but the value of the character code must be less than 256 in non-UTF-8
167     mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
168     is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
169     and }, or if there is no terminating }, this form of escape is not recognized.
170     Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
171     with no following digits, giving a character whose value is zero.
172 nigel 75 .P
173 nigel 63 Characters whose value is less than 256 can be defined by either of the two
174 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
175     example, \exdc is exactly the same as \ex{dc}.
176 nigel 75 .P
177 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
178     digits, just those that are present are used. Thus the sequence \e0\ex\e07
179     specifies two binary zeros followed by a BEL character (code value 7). Make
180     sure you supply two digits after the initial zero if the pattern character that
181     follows is itself an octal digit.
182 nigel 75 .P
183 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
184     Outside a character class, PCRE reads it and any following digits as a decimal
185     number. If the number is less than 10, or if there have been at least that many
186     previous capturing left parentheses in the expression, the entire sequence is
187 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
188     .\" HTML <a href="#backreferences">
189     .\" </a>
190     later,
191     .\"
192     following the discussion of
193     .\" HTML <a href="#subpattern">
194     .\" </a>
195     parenthesized subpatterns.
196     .\"
197     .P
198 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
199     have not been that many capturing subpatterns, PCRE re-reads up to three octal
200 nigel 93 digits following the backslash, and uses them to generate a data character. Any
201 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
202     character specified in octal must be less than \e400. In UTF-8 mode, values up
203     to \e777 are permitted. For example:
204 nigel 75 .sp
205     \e040 is another way of writing a space
206     .\" JOIN
207     \e40 is the same, provided there are fewer than 40
208 nigel 63 previous capturing subpatterns
209 nigel 75 \e7 is always a back reference
210     .\" JOIN
211     \e11 might be a back reference, or another way of
212 nigel 63 writing a tab
213 nigel 75 \e011 is always a tab
214     \e0113 is a tab followed by the character "3"
215     .\" JOIN
216     \e113 might be a back reference, otherwise the
217 nigel 63 character with octal code 113
218 nigel 75 .\" JOIN
219     \e377 might be a back reference, otherwise
220 nigel 63 the byte consisting entirely of 1 bits
221 nigel 75 .\" JOIN
222     \e81 is either a back reference, or a binary zero
223 nigel 63 followed by the two characters "8" and "1"
224 nigel 75 .sp
225 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
226     zero, because no more than three octal digits are ever read.
227 nigel 75 .P
228 nigel 91 All the sequences that define a single character value can be used both inside
229     and outside character classes. In addition, inside a character class, the
230     sequence \eb is interpreted as the backspace character (hex 08), and the
231 nigel 93 sequences \eR and \eX are interpreted as the characters "R" and "X",
232     respectively. Outside a character class, these sequences have different
233     meanings
234 nigel 75 .\" HTML <a href="#uniextseq">
235     .\" </a>
236     (see below).
237     .\"
238     .
239     .
240 nigel 93 .SS "Absolute and relative back references"
241     .rs
242     .sp
243     The sequence \eg followed by a positive or negative number, optionally enclosed
244 ph10 172 in braces, is an absolute or relative back reference. A named back reference
245 ph10 171 can be coded as \eg{name}. Back references are discussed
246 nigel 93 .\" HTML <a href="#backreferences">
247     .\" </a>
248     later,
249     .\"
250     following the discussion of
251     .\" HTML <a href="#subpattern">
252     .\" </a>
253     parenthesized subpatterns.
254     .\"
255     .
256     .
257 nigel 75 .SS "Generic character types"
258     .rs
259     .sp
260 nigel 93 Another use of backslash is for specifying generic character types. The
261 nigel 75 following are always recognized:
262     .sp
263 ph10 182 \ed any decimal digit
264 nigel 75 \eD any character that is not a decimal digit
265 ph10 178 \eh any horizontal whitespace character
266 ph10 182 \eH any character that is not a horizontal whitespace character
267 nigel 75 \es any whitespace character
268     \eS any character that is not a whitespace character
269 ph10 178 \ev any vertical whitespace character
270 ph10 182 \eV any character that is not a vertical whitespace character
271 nigel 75 \ew any "word" character
272     \eW any "non-word" character
273     .sp
274 nigel 63 Each pair of escape sequences partitions the complete set of characters into
275     two disjoint sets. Any given character matches one, and only one, of each pair.
276 nigel 75 .P
277     These character type sequences can appear both inside and outside character
278     classes. They each match one character of the appropriate type. If the current
279     matching point is at the end of the subject string, all of them fail, since
280     there is no character to match.
281     .P
282     For compatibility with Perl, \es does not match the VT character (code 11).
283     This makes it different from the the POSIX "space" class. The \es characters
284 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
285 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never
286 ph10 178 does.
287 nigel 75 .P
288 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
289     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
290 ph10 182 character property support is available. These sequences retain their original
291     meanings from before UTF-8 support was available, mainly for efficiency
292 ph10 178 reasons.
293     .P
294 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
295 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
296     The horizontal space characters are:
297     .sp
298     U+0009 Horizontal tab
299     U+0020 Space
300     U+00A0 Non-break space
301     U+1680 Ogham space mark
302     U+180E Mongolian vowel separator
303     U+2000 En quad
304     U+2001 Em quad
305     U+2002 En space
306     U+2003 Em space
307     U+2004 Three-per-em space
308     U+2005 Four-per-em space
309     U+2006 Six-per-em space
310     U+2007 Figure space
311     U+2008 Punctuation space
312     U+2009 Thin space
313     U+200A Hair space
314     U+202F Narrow no-break space
315     U+205F Medium mathematical space
316     U+3000 Ideographic space
317     .sp
318     The vertical space characters are:
319     .sp
320     U+000A Linefeed
321     U+000B Vertical tab
322     U+000C Formfeed
323     U+000D Carriage return
324     U+0085 Next line
325     U+2028 Line separator
326     U+2029 Paragraph separator
327     .P
328 nigel 75 A "word" character is an underscore or any character less than 256 that is a
329     letter or digit. The definition of letters and digits is controlled by PCRE's
330     low-valued character tables, and may vary if locale-specific matching is taking
331     place (see
332 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
333     .\" </a>
334     "Locale support"
335     .\"
336     in the
337     .\" HREF
338 nigel 75 \fBpcreapi\fP
339 nigel 63 .\"
340 ph10 139 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
341     or "french" in Windows, some character codes greater than 128 are used for
342 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode
343     is discouraged.
344 nigel 75 .
345     .
346 nigel 93 .SS "Newline sequences"
347     .rs
348     .sp
349     Outside a character class, the escape sequence \eR matches any Unicode newline
350 ph10 178 sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
351 nigel 93 the following:
352     .sp
353     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
354     .sp
355     This is an example of an "atomic group", details of which are given
356     .\" HTML <a href="#atomicgroup">
357     .\" </a>
358     below.
359     .\"
360     This particular group matches either the two-character sequence CR followed by
361     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
362     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
363     line, U+0085). The two-character sequence is treated as a single unit that
364     cannot be split.
365     .P
366     In UTF-8 mode, two additional characters whose codepoints are greater than 255
367     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
368     Unicode character property support is not needed for these characters to be
369     recognized.
370     .P
371     Inside a character class, \eR matches the letter "R".
372     .
373     .
374 nigel 75 .\" HTML <a name="uniextseq"></a>
375     .SS Unicode character properties
376     .rs
377     .sp
378     When PCRE is built with Unicode character property support, three additional
379 nigel 87 escape sequences to match character properties are available when UTF-8 mode
380 nigel 75 is selected. They are:
381     .sp
382 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
383     \eP{\fIxx\fP} a character without the \fIxx\fP property
384     \eX an extended Unicode sequence
385 nigel 75 .sp
386 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
387     script names, the general category properties, and "Any", which matches any
388     character (including newline). Other properties such as "InMusicalSymbols" are
389     not currently supported by PCRE. Note that \eP{Any} does not match any
390     characters, so always causes a match failure.
391 nigel 75 .P
392 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
393     character from one of these sets can be matched using a script name. For
394     example:
395 nigel 75 .sp
396 nigel 87 \ep{Greek}
397     \eP{Han}
398     .sp
399     Those that are not part of an identified script are lumped together as
400     "Common". The current list of scripts is:
401     .P
402     Arabic,
403     Armenian,
404 nigel 93 Balinese,
405 nigel 87 Bengali,
406     Bopomofo,
407     Braille,
408     Buginese,
409     Buhid,
410     Canadian_Aboriginal,
411     Cherokee,
412     Common,
413     Coptic,
414 nigel 93 Cuneiform,
415 nigel 87 Cypriot,
416     Cyrillic,
417     Deseret,
418     Devanagari,
419     Ethiopic,
420     Georgian,
421     Glagolitic,
422     Gothic,
423     Greek,
424     Gujarati,
425     Gurmukhi,
426     Han,
427     Hangul,
428     Hanunoo,
429     Hebrew,
430     Hiragana,
431     Inherited,
432     Kannada,
433     Katakana,
434     Kharoshthi,
435     Khmer,
436     Lao,
437     Latin,
438     Limbu,
439     Linear_B,
440     Malayalam,
441     Mongolian,
442     Myanmar,
443     New_Tai_Lue,
444 nigel 93 Nko,
445 nigel 87 Ogham,
446     Old_Italic,
447     Old_Persian,
448     Oriya,
449     Osmanya,
450 nigel 93 Phags_Pa,
451     Phoenician,
452 nigel 87 Runic,
453     Shavian,
454     Sinhala,
455     Syloti_Nagri,
456     Syriac,
457     Tagalog,
458     Tagbanwa,
459     Tai_Le,
460     Tamil,
461     Telugu,
462     Thaana,
463     Thai,
464     Tibetan,
465     Tifinagh,
466     Ugaritic,
467     Yi.
468     .P
469     Each character has exactly one general category property, specified by a
470     two-letter abbreviation. For compatibility with Perl, negation can be specified
471     by including a circumflex between the opening brace and the property name. For
472     example, \ep{^Lu} is the same as \eP{Lu}.
473     .P
474     If only one letter is specified with \ep or \eP, it includes all the general
475     category properties that start with that letter. In this case, in the absence
476     of negation, the curly brackets in the escape sequence are optional; these two
477     examples have the same effect:
478     .sp
479 nigel 75 \ep{L}
480     \epL
481     .sp
482 nigel 87 The following general category property codes are supported:
483 nigel 75 .sp
484     C Other
485     Cc Control
486     Cf Format
487     Cn Unassigned
488     Co Private use
489     Cs Surrogate
490     .sp
491     L Letter
492     Ll Lower case letter
493     Lm Modifier letter
494     Lo Other letter
495     Lt Title case letter
496     Lu Upper case letter
497     .sp
498     M Mark
499     Mc Spacing mark
500     Me Enclosing mark
501     Mn Non-spacing mark
502     .sp
503     N Number
504     Nd Decimal number
505     Nl Letter number
506     No Other number
507     .sp
508     P Punctuation
509     Pc Connector punctuation
510     Pd Dash punctuation
511     Pe Close punctuation
512     Pf Final punctuation
513     Pi Initial punctuation
514     Po Other punctuation
515     Ps Open punctuation
516     .sp
517     S Symbol
518     Sc Currency symbol
519     Sk Modifier symbol
520     Sm Mathematical symbol
521     So Other symbol
522     .sp
523     Z Separator
524     Zl Line separator
525     Zp Paragraph separator
526     Zs Space separator
527     .sp
528 nigel 87 The special property L& is also supported: it matches a character that has
529     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
530     a modifier or "other".
531 nigel 75 .P
532 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter})
533 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
534 nigel 87 properties with "Is".
535     .P
536     No character that is in the Unicode table has the Cn (unassigned) property.
537     Instead, this property is assumed for any code point that is not in the
538     Unicode table.
539     .P
540 nigel 75 Specifying caseless matching does not affect these escape sequences. For
541     example, \ep{Lu} always matches only upper case letters.
542     .P
543     The \eX escape matches any number of Unicode characters that form an extended
544     Unicode sequence. \eX is equivalent to
545     .sp
546     (?>\ePM\epM*)
547     .sp
548     That is, it matches a character without the "mark" property, followed by zero
549     or more characters with the "mark" property, and treats the sequence as an
550     atomic group
551     .\" HTML <a href="#atomicgroup">
552     .\" </a>
553     (see below).
554     .\"
555     Characters with the "mark" property are typically accents that affect the
556     preceding character.
557     .P
558     Matching characters by Unicode property is not fast, because PCRE has to search
559     a structure that contains data for over fifteen thousand characters. That is
560     why the traditional escape sequences such as \ed and \ew do not use Unicode
561     properties in PCRE.
562     .
563     .
564 ph10 168 .\" HTML <a name="resetmatchstart"></a>
565     .SS "Resetting the match start"
566     .rs
567     .sp
568 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
569     matched characters not to be included in the final matched sequence. For
570 ph10 168 example, the pattern:
571     .sp
572     foo\eKbar
573     .sp
574 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
575 ph10 168 similar to a lookbehind assertion
576     .\" HTML <a href="#lookbehind">
577     .\" </a>
578     (described below).
579     .\"
580 ph10 172 However, in this case, the part of the subject before the real match does not
581     have to be of fixed length, as lookbehind assertions do. The use of \eK does
582 ph10 168 not interfere with the setting of
583     .\" HTML <a href="#subpattern">
584     .\" </a>
585     captured substrings.
586 ph10 172 .\"
587 ph10 168 For example, when the pattern
588     .sp
589     (foo)\eKbar
590     .sp
591 ph10 172 matches "foobar", the first substring is still set to "foo".
592 ph10 168 .
593     .
594 nigel 75 .\" HTML <a name="smallassertions"></a>
595     .SS "Simple assertions"
596     .rs
597     .sp
598 nigel 93 The final use of backslash is for certain simple assertions. An assertion
599 nigel 63 specifies a condition that has to be met at a particular point in a match,
600     without consuming any characters from the subject string. The use of
601 nigel 75 subpatterns for more complicated assertions is described
602     .\" HTML <a href="#bigassertions">
603     .\" </a>
604     below.
605     .\"
606 nigel 91 The backslashed assertions are:
607 nigel 75 .sp
608     \eb matches at a word boundary
609     \eB matches when not at a word boundary
610 nigel 93 \eA matches at the start of the subject
611     \eZ matches at the end of the subject
612     also matches before a newline at the end of the subject
613     \ez matches only at the end of the subject
614     \eG matches at the first matching position in the subject
615 nigel 75 .sp
616     These assertions may not appear in character classes (but note that \eb has a
617 nigel 63 different meaning, namely the backspace character, inside a character class).
618 nigel 75 .P
619 nigel 63 A word boundary is a position in the subject string where the current character
620 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
621     \ew and the other matches \eW), or the start or end of the string if the
622     first or last character matches \ew, respectively.
623     .P
624     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
625     dollar (described in the next section) in that they only ever match at the very
626     start and end of the subject string, whatever options are set. Thus, they are
627     independent of multiline mode. These three assertions are not affected by the
628     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
629     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
630     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
631     at a point other than the beginning of the subject, \eA can never match. The
632 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
633     of the string as well as at the very end, whereas \ez matches only at the end.
634 nigel 75 .P
635     The \eG assertion is true only when the current matching position is at the
636     start point of the match, as specified by the \fIstartoffset\fP argument of
637     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
638     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
639 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
640 nigel 75 implementation where \eG can be useful.
641     .P
642     Note, however, that PCRE's interpretation of \eG, as the start of the current
643 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
644     previous match. In Perl, these can be different when the previously matched
645     string was empty. Because PCRE does just one match at a time, it cannot
646     reproduce this behaviour.
647 nigel 75 .P
648     If all the alternatives of a pattern begin with \eG, the expression is anchored
649 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
650     regular expression.
651 nigel 75 .
652     .
653     .SH "CIRCUMFLEX AND DOLLAR"
654 nigel 63 .rs
655     .sp
656     Outside a character class, in the default matching mode, the circumflex
657 nigel 75 character is an assertion that is true only if the current matching point is
658     at the start of the subject string. If the \fIstartoffset\fP argument of
659     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
660 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
661 nigel 75 meaning
662     .\" HTML <a href="#characterclass">
663     .\" </a>
664     (see below).
665     .\"
666     .P
667 nigel 63 Circumflex need not be the first character of the pattern if a number of
668     alternatives are involved, but it should be the first thing in each alternative
669     in which it appears if the pattern is ever to match that branch. If all
670     possible alternatives start with a circumflex, that is, if the pattern is
671     constrained to match only at the start of the subject, it is said to be an
672     "anchored" pattern. (There are also other constructs that can cause a pattern
673     to be anchored.)
674 nigel 75 .P
675     A dollar character is an assertion that is true only if the current matching
676 nigel 63 point is at the end of the subject string, or immediately before a newline
677 nigel 91 at the end of the string (by default). Dollar need not be the last character of
678     the pattern if a number of alternatives are involved, but it should be the last
679     item in any branch in which it appears. Dollar has no special meaning in a
680     character class.
681 nigel 75 .P
682 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
683     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
684 nigel 75 does not affect the \eZ assertion.
685     .P
686 nigel 63 The meanings of the circumflex and dollar characters are changed if the
687 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
688     immediately after internal newlines as well as at the start of the subject
689     string. It does not match after a newline that ends the string. A dollar
690     matches before any newlines in the string, as well as at the very end, when
691     PCRE_MULTILINE is set. When newline is specified as the two-character
692     sequence CRLF, isolated CR and LF characters do not indicate newlines.
693 nigel 75 .P
694 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
695     \en represents a newline) in multiline mode, but not otherwise. Consequently,
696     patterns that are anchored in single line mode because all branches start with
697     ^ are not anchored in multiline mode, and a match for circumflex is possible
698     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
699     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
700     .P
701 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
702 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
703 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
704 nigel 75 .
705     .
706     .SH "FULL STOP (PERIOD, DOT)"
707 nigel 63 .rs
708     .sp
709     Outside a character class, a dot in the pattern matches any one character in
710 nigel 91 the subject string except (by default) a character that signifies the end of a
711 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
712 nigel 91 .P
713 nigel 93 When a line ending is defined as a single character, dot never matches that
714     character; when the two-character sequence CRLF is used, dot does not match CR
715     if it is immediately followed by LF, but otherwise it matches all characters
716     (including isolated CRs and LFs). When any Unicode line endings are being
717     recognized, dot does not match CR or LF or any of the other line ending
718     characters.
719     .P
720 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
721 nigel 93 option is set, a dot matches any one character, without exception. If the
722     two-character sequence CRLF is present in the subject string, it takes two dots
723     to match it.
724 nigel 91 .P
725     The handling of dot is entirely independent of the handling of circumflex and
726     dollar, the only relationship being that they both involve newlines. Dot has no
727     special meaning in a character class.
728 nigel 75 .
729     .
730     .SH "MATCHING A SINGLE BYTE"
731 nigel 63 .rs
732     .sp
733 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
734 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
735     characters. The feature is provided in Perl in order to match individual bytes
736     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
737     what remains in the string may be a malformed UTF-8 string. For this reason,
738     the \eC escape sequence is best avoided.
739 nigel 75 .P
740     PCRE does not allow \eC to appear in lookbehind assertions
741     .\" HTML <a href="#lookbehind">
742     .\" </a>
743     (described below),
744     .\"
745     because in UTF-8 mode this would make it impossible to calculate the length of
746     the lookbehind.
747     .
748     .
749     .\" HTML <a name="characterclass"></a>
750     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
751 nigel 63 .rs
752     .sp
753     An opening square bracket introduces a character class, terminated by a closing
754     square bracket. A closing square bracket on its own is not special. If a
755     closing square bracket is required as a member of the class, it should be the
756     first data character in the class (after an initial circumflex, if present) or
757     escaped with a backslash.
758 nigel 75 .P
759 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
760     character may occupy more than one byte. A matched character must be in the set
761     of characters defined by the class, unless the first character in the class
762     definition is a circumflex, in which case the subject character must not be in
763     the set defined by the class. If a circumflex is actually required as a member
764     of the class, ensure it is not the first character, or escape it with a
765     backslash.
766 nigel 75 .P
767 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
768     [^aeiou] matches any character that is not a lower case vowel. Note that a
769 nigel 75 circumflex is just a convenient notation for specifying the characters that
770     are in the class by enumerating those that are not. A class that starts with a
771     circumflex is not an assertion: it still consumes a character from the subject
772     string, and therefore it fails if the current pointer is at the end of the
773     string.
774     .P
775 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
776 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
777     .P
778 nigel 63 When caseless matching is set, any letters in a class represent both their
779     upper case and lower case versions, so for example, a caseless [aeiou] matches
780     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
781 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
782     case for characters whose values are less than 128, so caseless matching is
783     always possible. For characters with higher values, the concept of case is
784     supported if PCRE is compiled with Unicode property support, but not otherwise.
785     If you want to use caseless matching for characters 128 and above, you must
786     ensure that PCRE is compiled with Unicode property support as well as with
787     UTF-8 support.
788 nigel 75 .P
789 nigel 93 Characters that might indicate line breaks are never treated in any special way
790     when matching character classes, whatever line-ending sequence is in use, and
791     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
792     such as [^a] always matches one of these characters.
793 nigel 75 .P
794 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
795     character class. For example, [d-m] matches any letter between d and m,
796     inclusive. If a minus character is required in a class, it must be escaped with
797     a backslash or appear in a position where it cannot be interpreted as
798     indicating a range, typically as the first or last character in the class.
799 nigel 75 .P
800 nigel 63 It is not possible to have the literal character "]" as the end character of a
801     range. A pattern such as [W-]46] is interpreted as a class of two characters
802     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
803     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
804 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
805     followed by two other characters. The octal or hexadecimal representation of
806     "]" can also be used to end a range.
807     .P
808 nigel 63 Ranges operate in the collating sequence of character values. They can also be
809 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
810 nigel 63 mode, ranges can include characters whose values are greater than 255, for
811 nigel 75 example [\ex{100}-\ex{2ff}].
812     .P
813 nigel 63 If a range that includes letters is used when caseless matching is set, it
814     matches the letters in either case. For example, [W-c] is equivalent to
815 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
816 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
817 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
818     characters with values greater than 128 only when it is compiled with Unicode
819     property support.
820     .P
821     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
822     in a character class, and add the characters that they match to the class. For
823     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
824 nigel 63 conveniently be used with the upper case character types to specify a more
825     restricted set of characters than the matching lower case type. For example,
826 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
827     .P
828     The only metacharacters that are recognized in character classes are backslash,
829     hyphen (only where it can be interpreted as specifying a range), circumflex
830     (only at the start), opening square bracket (only when it can be interpreted as
831     introducing a POSIX class name - see the next section), and the terminating
832     closing square bracket. However, escaping other non-alphanumeric characters
833     does no harm.
834     .
835     .
836     .SH "POSIX CHARACTER CLASSES"
837 nigel 63 .rs
838     .sp
839 nigel 75 Perl supports the POSIX notation for character classes. This uses names
840 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
841     this notation. For example,
842 nigel 75 .sp
843 nigel 63 [01[:alpha:]%]
844 nigel 75 .sp
845 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
846     are
847 nigel 75 .sp
848 nigel 63 alnum letters and digits
849     alpha letters
850     ascii character codes 0 - 127
851     blank space or tab only
852     cntrl control characters
853 nigel 75 digit decimal digits (same as \ed)
854 nigel 63 graph printing characters, excluding space
855     lower lower case letters
856     print printing characters, including space
857     punct printing characters, excluding letters and digits
858 nigel 75 space white space (not quite the same as \es)
859 nigel 63 upper upper case letters
860 nigel 75 word "word" characters (same as \ew)
861 nigel 63 xdigit hexadecimal digits
862 nigel 75 .sp
863 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
864     space (32). Notice that this list includes the VT character (code 11). This
865 nigel 75 makes "space" different to \es, which does not include VT (for Perl
866 nigel 63 compatibility).
867 nigel 75 .P
868 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
869     5.8. Another Perl extension is negation, which is indicated by a ^ character
870     after the colon. For example,
871 nigel 75 .sp
872 nigel 63 [12[:^digit:]]
873 nigel 75 .sp
874 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
875     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
876     supported, and an error is given if they are encountered.
877 nigel 75 .P
878     In UTF-8 mode, characters with values greater than 128 do not match any of
879 nigel 63 the POSIX character classes.
880 nigel 75 .
881     .
882     .SH "VERTICAL BAR"
883 nigel 63 .rs
884     .sp
885     Vertical bar characters are used to separate alternative patterns. For example,
886     the pattern
887 nigel 75 .sp
888 nigel 63 gilbert|sullivan
889 nigel 75 .sp
890 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
891 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
892     process tries each alternative in turn, from left to right, and the first one
893     that succeeds is used. If the alternatives are within a subpattern
894 nigel 75 .\" HTML <a href="#subpattern">
895     .\" </a>
896     (defined below),
897     .\"
898     "succeeds" means matching the rest of the main pattern as well as the
899     alternative in the subpattern.
900     .
901     .
902     .SH "INTERNAL OPTION SETTING"
903 nigel 63 .rs
904     .sp
905     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
906     PCRE_EXTENDED options can be changed from within the pattern by a sequence of
907     Perl option letters enclosed between "(?" and ")". The option letters are
908 nigel 75 .sp
909 nigel 63 i for PCRE_CASELESS
910     m for PCRE_MULTILINE
911     s for PCRE_DOTALL
912     x for PCRE_EXTENDED
913 nigel 75 .sp
914 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
915     unset these options by preceding the letter with a hyphen, and a combined
916     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
917     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
918     permitted. If a letter appears both before and after the hyphen, the option is
919     unset.
920 nigel 75 .P
921 nigel 63 When an option change occurs at top level (that is, not inside subpattern
922     parentheses), the change applies to the remainder of the pattern that follows.
923     If the change is placed right at the start of a pattern, PCRE extracts it into
924     the global options (and it will therefore show up in data extracted by the
925 nigel 75 \fBpcre_fullinfo()\fP function).
926     .P
927 nigel 93 An option change within a subpattern (see below for a description of
928     subpatterns) affects only that part of the current pattern that follows it, so
929 nigel 75 .sp
930 nigel 63 (a(?i)b)c
931 nigel 75 .sp
932 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
933     By this means, options can be made to have different settings in different
934     parts of the pattern. Any changes made in one alternative do carry on
935     into subsequent branches within the same subpattern. For example,
936 nigel 75 .sp
937 nigel 63 (a(?i)b|c)
938 nigel 75 .sp
939 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
940     branch is abandoned before the option setting. This is because the effects of
941     option settings happen at compile time. There would be some very weird
942     behaviour otherwise.
943 nigel 75 .P
944 nigel 91 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
945     changed in the same way as the Perl-compatible options by using the characters
946     J, U and X respectively.
947 nigel 75 .
948     .
949     .\" HTML <a name="subpattern"></a>
950 nigel 63 .SH SUBPATTERNS
951     .rs
952     .sp
953     Subpatterns are delimited by parentheses (round brackets), which can be nested.
954 nigel 75 Turning part of a pattern into a subpattern does two things:
955     .sp
956 nigel 63 1. It localizes a set of alternatives. For example, the pattern
957 nigel 75 .sp
958 nigel 63 cat(aract|erpillar|)
959 nigel 75 .sp
960 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
961 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
962 nigel 75 .sp
963     2. It sets up the subpattern as a capturing subpattern. This means that, when
964     the whole pattern matches, that portion of the subject string that matched the
965     subpattern is passed back to the caller via the \fIovector\fP argument of
966     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
967     from 1) to obtain numbers for the capturing subpatterns.
968     .P
969 nigel 63 For example, if the string "the red king" is matched against the pattern
970 nigel 75 .sp
971 nigel 63 the ((red|white) (king|queen))
972 nigel 75 .sp
973 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
974     2, and 3, respectively.
975 nigel 75 .P
976 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
977     There are often times when a grouping subpattern is required without a
978     capturing requirement. If an opening parenthesis is followed by a question mark
979     and a colon, the subpattern does not do any capturing, and is not counted when
980     computing the number of any subsequent capturing subpatterns. For example, if
981     the string "the white queen" is matched against the pattern
982 nigel 75 .sp
983 nigel 63 the ((?:red|white) (king|queen))
984 nigel 75 .sp
985 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
986 nigel 93 2. The maximum number of capturing subpatterns is 65535.
987 nigel 75 .P
988 nigel 63 As a convenient shorthand, if any option settings are required at the start of
989     a non-capturing subpattern, the option letters may appear between the "?" and
990     the ":". Thus the two patterns
991 nigel 75 .sp
992 nigel 63 (?i:saturday|sunday)
993     (?:(?i)saturday|sunday)
994 nigel 75 .sp
995 nigel 63 match exactly the same set of strings. Because alternative branches are tried
996     from left to right, and options are not reset until the end of the subpattern
997     is reached, an option setting in one branch does affect subsequent branches, so
998     the above patterns match "SUNDAY" as well as "Saturday".
999 nigel 75 .
1000     .
1001 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1002     .rs
1003     .sp
1004 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1005     the same numbers for its capturing parentheses. Such a subpattern starts with
1006     (?| and is itself a non-capturing subpattern. For example, consider this
1007 ph10 175 pattern:
1008     .sp
1009     (?|(Sat)ur|(Sun))day
1010 ph10 182 .sp
1011     Because the two alternatives are inside a (?| group, both sets of capturing
1012     parentheses are numbered one. Thus, when the pattern matches, you can look
1013     at captured substring number one, whichever alternative matched. This construct
1014     is useful when you want to capture part, but not all, of one of a number of
1015     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1016 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1017 ph10 182 buffers that follow the subpattern start after the highest number used in any
1018     branch. The following example is taken from the Perl documentation.
1019 ph10 175 The numbers underneath show in which buffer the captured content will be
1020     stored.
1021     .sp
1022     # before ---------------branch-reset----------- after
1023     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1024     # 1 2 2 3 2 3 4
1025 ph10 182 .sp
1026 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to
1027     the first one in the pattern with the given number.
1028     .P
1029     An alternative approach to using this "branch reset" feature is to use
1030     duplicate named subpatterns, as described in the next section.
1031     .
1032     .
1033 nigel 75 .SH "NAMED SUBPATTERNS"
1034 nigel 63 .rs
1035     .sp
1036     Identifying capturing parentheses by number is simple, but it can be very hard
1037     to keep track of the numbers in complicated regular expressions. Furthermore,
1038 nigel 75 if an expression is modified, the numbers may change. To help with this
1039 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1040     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1041     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1042     the Perl and the Python syntax.
1043     .P
1044     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1045     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1046 nigel 91 parentheses from other parts of the pattern, such as
1047     .\" HTML <a href="#backreferences">
1048     .\" </a>
1049     backreferences,
1050     .\"
1051     .\" HTML <a href="#recursion">
1052     .\" </a>
1053     recursion,
1054     .\"
1055     and
1056     .\" HTML <a href="#conditions">
1057     .\" </a>
1058     conditions,
1059     .\"
1060     can be made by name as well as by number.
1061 nigel 75 .P
1062 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1063 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1064     if the names were not present. The PCRE API provides function calls for
1065     extracting the name-to-number translation table from a compiled pattern. There
1066     is also a convenience function for extracting a captured substring by name.
1067 nigel 91 .P
1068     By default, a name must be unique within a pattern, but it is possible to relax
1069     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1070     be useful for patterns where only one instance of the named parentheses can
1071     match. Suppose you want to match the name of a weekday, either as a 3-letter
1072     abbreviation or as the full name, and in both cases you want to extract the
1073     abbreviation. This pattern (ignoring the line breaks) does the job:
1074     .sp
1075 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1076     (?<DN>Tue)(?:sday)?|
1077     (?<DN>Wed)(?:nesday)?|
1078     (?<DN>Thu)(?:rsday)?|
1079     (?<DN>Sat)(?:urday)?
1080 nigel 91 .sp
1081     There are five capturing substrings, but only one is ever set after a match.
1082 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1083 ph10 175 subpattern, as described in the previous section.)
1084     .P
1085 nigel 91 The convenience function for extracting the data by name returns the substring
1086 nigel 93 for the first (and in this example, the only) subpattern of that name that
1087 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1088     make a reference to a non-unique named subpattern from elsewhere in the
1089     pattern, the one that corresponds to the lowest number is used. For further
1090     details of the interfaces for handling named subpatterns, see the
1091 nigel 63 .\" HREF
1092 nigel 75 \fBpcreapi\fP
1093 nigel 63 .\"
1094     documentation.
1095 nigel 75 .
1096     .
1097 nigel 63 .SH REPETITION
1098     .rs
1099     .sp
1100     Repetition is specified by quantifiers, which can follow any of the following
1101     items:
1102 nigel 75 .sp
1103 nigel 63 a literal data character
1104 nigel 93 the dot metacharacter
1105 nigel 75 the \eC escape sequence
1106     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1107 nigel 93 the \eR escape sequence
1108 nigel 75 an escape such as \ed that matches a single character
1109 nigel 63 a character class
1110     a back reference (see next section)
1111     a parenthesized subpattern (unless it is an assertion)
1112 nigel 75 .sp
1113 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1114     permitted matches, by giving the two numbers in curly brackets (braces),
1115     separated by a comma. The numbers must be less than 65536, and the first must
1116     be less than or equal to the second. For example:
1117 nigel 75 .sp
1118 nigel 63 z{2,4}
1119 nigel 75 .sp
1120 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1121     character. If the second number is omitted, but the comma is present, there is
1122     no upper limit; if the second number and the comma are both omitted, the
1123     quantifier specifies an exact number of required matches. Thus
1124 nigel 75 .sp
1125 nigel 63 [aeiou]{3,}
1126 nigel 75 .sp
1127 nigel 63 matches at least 3 successive vowels, but may match many more, while
1128 nigel 75 .sp
1129     \ed{8}
1130     .sp
1131 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1132     where a quantifier is not allowed, or one that does not match the syntax of a
1133     quantifier, is taken as a literal character. For example, {,6} is not a
1134     quantifier, but a literal string of four characters.
1135 nigel 75 .P
1136 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1137 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1138     which is represented by a two-byte sequence. Similarly, when Unicode property
1139     support is available, \eX{3} matches three Unicode extended sequences, each of
1140     which may be several bytes long (and they may be of different lengths).
1141     .P
1142 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1143     previous item and the quantifier were not present.
1144 nigel 75 .P
1145 nigel 93 For convenience, the three most common quantifiers have single-character
1146     abbreviations:
1147 nigel 75 .sp
1148 nigel 63 * is equivalent to {0,}
1149     + is equivalent to {1,}
1150     ? is equivalent to {0,1}
1151 nigel 75 .sp
1152 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1153     match no characters with a quantifier that has no upper limit, for example:
1154 nigel 75 .sp
1155 nigel 63 (a?)*
1156 nigel 75 .sp
1157 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1158     such patterns. However, because there are cases where this can be useful, such
1159     patterns are now accepted, but if any repetition of the subpattern does in fact
1160     match no characters, the loop is forcibly broken.
1161 nigel 75 .P
1162 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1163     possible (up to the maximum number of permitted times), without causing the
1164     rest of the pattern to fail. The classic example of where this gives problems
1165 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1166     and within the comment, individual * and / characters may appear. An attempt to
1167     match C comments by applying the pattern
1168     .sp
1169     /\e*.*\e*/
1170     .sp
1171 nigel 63 to the string
1172 nigel 75 .sp
1173     /* first comment */ not comment /* second comment */
1174     .sp
1175 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1176     item.
1177 nigel 75 .P
1178 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1179     greedy, and instead matches the minimum number of times possible, so the
1180     pattern
1181 nigel 75 .sp
1182     /\e*.*?\e*/
1183     .sp
1184 nigel 63 does the right thing with the C comments. The meaning of the various
1185     quantifiers is not otherwise changed, just the preferred number of matches.
1186     Do not confuse this use of question mark with its use as a quantifier in its
1187     own right. Because it has two uses, it can sometimes appear doubled, as in
1188 nigel 75 .sp
1189     \ed??\ed
1190     .sp
1191 nigel 63 which matches one digit by preference, but can match two if that is the only
1192     way the rest of the pattern matches.
1193 nigel 75 .P
1194 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1195 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1196     greedy by following them with a question mark. In other words, it inverts the
1197     default behaviour.
1198 nigel 75 .P
1199 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1200 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1201 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1202 nigel 75 .P
1203 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1204 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1205 nigel 63 implicitly anchored, because whatever follows will be tried against every
1206     character position in the subject string, so there is no point in retrying the
1207     overall match at any position after the first. PCRE normally treats such a
1208 nigel 75 pattern as though it were preceded by \eA.
1209     .P
1210 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1211     worth setting PCRE_DOTALL in order to obtain this optimization, or
1212     alternatively using ^ to indicate anchoring explicitly.
1213 nigel 75 .P
1214 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1215     is inside capturing parentheses that are the subject of a backreference
1216 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1217     succeeds. Consider, for example:
1218 nigel 75 .sp
1219     (.*)abc\e1
1220     .sp
1221 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1222     this reason, such a pattern is not implicitly anchored.
1223 nigel 75 .P
1224 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1225     that matched the final iteration. For example, after
1226 nigel 75 .sp
1227     (tweedle[dume]{3}\es*)+
1228     .sp
1229 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1230     "tweedledee". However, if there are nested capturing subpatterns, the
1231     corresponding captured values may have been set in previous iterations. For
1232     example, after
1233 nigel 75 .sp
1234 nigel 63 /(a|(b))+/
1235 nigel 75 .sp
1236 nigel 63 matches "aba" the value of the second captured substring is "b".
1237 nigel 75 .
1238     .
1239     .\" HTML <a name="atomicgroup"></a>
1240     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1241 nigel 63 .rs
1242     .sp
1243 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1244     repetition, failure of what follows normally causes the repeated item to be
1245     re-evaluated to see if a different number of repeats allows the rest of the
1246     pattern to match. Sometimes it is useful to prevent this, either to change the
1247     nature of the match, or to cause it fail earlier than it otherwise might, when
1248     the author of the pattern knows there is no point in carrying on.
1249 nigel 75 .P
1250     Consider, for example, the pattern \ed+foo when applied to the subject line
1251     .sp
1252 nigel 63 123456bar
1253 nigel 75 .sp
1254 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1255 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1256 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1257     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1258     that once a subpattern has matched, it is not to be re-evaluated in this way.
1259 nigel 75 .P
1260 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1261 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1262     special parenthesis, starting with (?> as in this example:
1263 nigel 75 .sp
1264     (?>\ed+)foo
1265     .sp
1266 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1267     it has matched, and a failure further into the pattern is prevented from
1268     backtracking into it. Backtracking past it to previous items, however, works as
1269     normal.
1270 nigel 75 .P
1271 nigel 63 An alternative description is that a subpattern of this type matches the string
1272     of characters that an identical standalone pattern would match, if anchored at
1273     the current point in the subject string.
1274 nigel 75 .P
1275 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1276     the above example can be thought of as a maximizing repeat that must swallow
1277 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1278 nigel 63 number of digits they match in order to make the rest of the pattern match,
1279 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1280     .P
1281 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1282     subpatterns, and can be nested. However, when the subpattern for an atomic
1283     group is just a single repeated item, as in the example above, a simpler
1284     notation, called a "possessive quantifier" can be used. This consists of an
1285     additional + character following a quantifier. Using this notation, the
1286     previous example can be rewritten as
1287 nigel 75 .sp
1288     \ed++foo
1289     .sp
1290 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1291     option is ignored. They are a convenient notation for the simpler forms of
1292 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1293     quantifier and the equivalent atomic group, though there may be a performance
1294     difference; possessive quantifiers should be slightly faster.
1295 nigel 75 .P
1296 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1297     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1298     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1299     package, and PCRE copied it from there. It ultimately found its way into Perl
1300     at release 5.10.
1301 nigel 75 .P
1302 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1303     pattern constructs. For example, the sequence A+B is treated as A++B because
1304     there is no point in backtracking into a sequence of A's when B must follow.
1305     .P
1306 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1307     be repeated an unlimited number of times, the use of an atomic group is the
1308     only way to avoid some failing matches taking a very long time indeed. The
1309     pattern
1310 nigel 75 .sp
1311     (\eD+|<\ed+>)*[!?]
1312     .sp
1313 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1314     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1315     quickly. However, if it is applied to
1316 nigel 75 .sp
1317 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1318 nigel 75 .sp
1319 nigel 63 it takes a long time before reporting failure. This is because the string can
1320 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1321     large number of ways, and all have to be tried. (The example uses [!?] rather
1322     than a single character at the end, because both PCRE and Perl have an
1323     optimization that allows for fast failure when a single character is used. They
1324     remember the last single character that is required for a match, and fail early
1325     if it is not present in the string.) If the pattern is changed so that it uses
1326     an atomic group, like this:
1327     .sp
1328     ((?>\eD+)|<\ed+>)*[!?]
1329     .sp
1330 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1331 nigel 75 .
1332     .
1333     .\" HTML <a name="backreferences"></a>
1334     .SH "BACK REFERENCES"
1335 nigel 63 .rs
1336     .sp
1337     Outside a character class, a backslash followed by a digit greater than 0 (and
1338     possibly further digits) is a back reference to a capturing subpattern earlier
1339     (that is, to its left) in the pattern, provided there have been that many
1340     previous capturing left parentheses.
1341 nigel 75 .P
1342 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1343     always taken as a back reference, and causes an error only if there are not
1344     that many capturing left parentheses in the entire pattern. In other words, the
1345     parentheses that are referenced need not be to the left of the reference for
1346 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1347     when a repetition is involved and the subpattern to the right has participated
1348     in an earlier iteration.
1349     .P
1350 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1351     whose number is 10 or more using this syntax because a sequence such as \e50 is
1352     interpreted as a character defined in octal. See the subsection entitled
1353 nigel 91 "Non-printing characters"
1354 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1355     .\" </a>
1356     above
1357     .\"
1358 nigel 93 for further details of the handling of digits following a backslash. There is
1359     no such problem when named parentheses are used. A back reference to any
1360     subpattern is possible using named parentheses (see below).
1361 nigel 75 .P
1362 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1363     backslash is to use the \eg escape sequence, which is a feature introduced in
1364     Perl 5.10. This escape must be followed by a positive or a negative number,
1365     optionally enclosed in braces. These examples are all identical:
1366     .sp
1367     (ring), \e1
1368     (ring), \eg1
1369     (ring), \eg{1}
1370     .sp
1371     A positive number specifies an absolute reference without the ambiguity that is
1372     present in the older syntax. It is also useful when literal digits follow the
1373     reference. A negative number is a relative reference. Consider this example:
1374     .sp
1375     (abc(def)ghi)\eg{-1}
1376     .sp
1377     The sequence \eg{-1} is a reference to the most recently started capturing
1378     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1379     would be equivalent to \e1. The use of relative references can be helpful in
1380     long patterns, and also in patterns that are created by joining together
1381     fragments that contain references within themselves.
1382     .P
1383 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1384     the current subject string, rather than anything matching the subpattern
1385     itself (see
1386     .\" HTML <a href="#subpatternsassubroutines">
1387     .\" </a>
1388     "Subpatterns as subroutines"
1389     .\"
1390     below for a way of doing that). So the pattern
1391 nigel 75 .sp
1392     (sens|respons)e and \e1ibility
1393     .sp
1394 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1395     "sense and responsibility". If caseful matching is in force at the time of the
1396     back reference, the case of letters is relevant. For example,
1397 nigel 75 .sp
1398     ((?i)rah)\es+\e1
1399     .sp
1400 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1401     capturing subpattern is matched caselessly.
1402 nigel 75 .P
1403 ph10 171 There are several different ways of writing back references to named
1404     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1405     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1406     back reference syntax, in which \eg can be used for both numeric and named
1407     references, is also supported. We could rewrite the above example in any of
1408 nigel 93 the following ways:
1409 nigel 75 .sp
1410 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1411 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1412 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1413 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1414 nigel 75 .sp
1415 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1416     after the reference.
1417     .P
1418 nigel 63 There may be more than one back reference to the same subpattern. If a
1419     subpattern has not actually been used in a particular match, any back
1420     references to it always fail. For example, the pattern
1421 nigel 75 .sp
1422     (a|(bc))\e2
1423     .sp
1424 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1425     many capturing parentheses in a pattern, all digits following the backslash are
1426     taken as part of a potential back reference number. If the pattern continues
1427     with a digit character, some delimiter must be used to terminate the back
1428     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1429 nigel 75 Otherwise an empty comment (see
1430     .\" HTML <a href="#comments">
1431     .\" </a>
1432     "Comments"
1433     .\"
1434     below) can be used.
1435     .P
1436 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1437 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1438 nigel 63 However, such references can be useful inside repeated subpatterns. For
1439     example, the pattern
1440 nigel 75 .sp
1441     (a|b\e1)+
1442     .sp
1443 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1444     the subpattern, the back reference matches the character string corresponding
1445     to the previous iteration. In order for this to work, the pattern must be such
1446     that the first iteration does not need to match the back reference. This can be
1447     done using alternation, as in the example above, or by a quantifier with a
1448     minimum of zero.
1449 nigel 75 .
1450     .
1451     .\" HTML <a name="bigassertions"></a>
1452 nigel 63 .SH ASSERTIONS
1453     .rs
1454     .sp
1455     An assertion is a test on the characters following or preceding the current
1456     matching point that does not actually consume any characters. The simple
1457 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1458     .\" HTML <a href="#smallassertions">
1459     .\" </a>
1460     above.
1461     .\"
1462     .P
1463 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1464     those that look ahead of the current position in the subject string, and those
1465 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1466     except that it does not cause the current matching position to be changed.
1467     .P
1468     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1469     because it makes no sense to assert the same thing several times. If any kind
1470     of assertion contains capturing subpatterns within it, these are counted for
1471     the purposes of numbering the capturing subpatterns in the whole pattern.
1472     However, substring capturing is carried out only for positive assertions,
1473     because it does not make sense for negative assertions.
1474     .
1475     .
1476     .SS "Lookahead assertions"
1477     .rs
1478     .sp
1479 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1480     negative assertions. For example,
1481 nigel 75 .sp
1482     \ew+(?=;)
1483     .sp
1484 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1485     the match, and
1486 nigel 75 .sp
1487 nigel 63 foo(?!bar)
1488 nigel 75 .sp
1489 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1490     apparently similar pattern
1491 nigel 75 .sp
1492 nigel 63 (?!foo)bar
1493 nigel 75 .sp
1494 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1495     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1496     (?!foo) is always true when the next three characters are "bar". A
1497 nigel 75 lookbehind assertion is needed to achieve the other effect.
1498     .P
1499 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1500     convenient way to do it is with (?!) because an empty string always matches, so
1501     an assertion that requires there not to be an empty string must always fail.
1502 nigel 75 .
1503     .
1504     .\" HTML <a name="lookbehind"></a>
1505     .SS "Lookbehind assertions"
1506     .rs
1507     .sp
1508 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1509     negative assertions. For example,
1510 nigel 75 .sp
1511 nigel 63 (?<!foo)bar
1512 nigel 75 .sp
1513 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1514     a lookbehind assertion are restricted such that all the strings it matches must
1515 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1516     do not all have to have the same fixed length. Thus
1517 nigel 75 .sp
1518 nigel 63 (?<=bullock|donkey)
1519 nigel 75 .sp
1520 nigel 63 is permitted, but
1521 nigel 75 .sp
1522 nigel 63 (?<!dogs?|cats?)
1523 nigel 75 .sp
1524 nigel 63 causes an error at compile time. Branches that match different length strings
1525     are permitted only at the top level of a lookbehind assertion. This is an
1526     extension compared with Perl (at least for 5.8), which requires all branches to
1527     match the same length of string. An assertion such as
1528 nigel 75 .sp
1529 nigel 63 (?<=ab(c|de))
1530 nigel 75 .sp
1531 nigel 63 is not permitted, because its single top-level branch can match two different
1532     lengths, but it is acceptable if rewritten to use two top-level branches:
1533 nigel 75 .sp
1534 nigel 63 (?<=abc|abde)
1535 nigel 75 .sp
1536 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1537 ph10 168 .\" HTML <a href="#resetmatchstart">
1538     .\" </a>
1539     (see above)
1540     .\"
1541     can be used instead of a lookbehind assertion; this is not restricted to a
1542     fixed-length.
1543     .P
1544 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1545 nigel 93 temporarily move the current position back by the fixed length and then try to
1546 nigel 63 match. If there are insufficient characters before the current position, the
1547 nigel 93 assertion fails.
1548 nigel 75 .P
1549     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1550 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1551 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1552     different numbers of bytes, are also not permitted.
1553 nigel 75 .P
1554 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1555     specify efficient matching at the end of the subject string. Consider a simple
1556     pattern such as
1557 nigel 75 .sp
1558 nigel 63 abcd$
1559 nigel 75 .sp
1560 nigel 63 when applied to a long string that does not match. Because matching proceeds
1561     from left to right, PCRE will look for each "a" in the subject and then see if
1562     what follows matches the rest of the pattern. If the pattern is specified as
1563 nigel 75 .sp
1564 nigel 63 ^.*abcd$
1565 nigel 75 .sp
1566 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1567     there is no following "a"), it backtracks to match all but the last character,
1568     then all but the last two characters, and so on. Once again the search for "a"
1569     covers the entire string, from right to left, so we are no better off. However,
1570     if the pattern is written as
1571 nigel 75 .sp
1572 nigel 63 ^.*+(?<=abcd)
1573 nigel 75 .sp
1574 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1575 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1576     characters. If it fails, the match fails immediately. For long strings, this
1577     approach makes a significant difference to the processing time.
1578 nigel 75 .
1579     .
1580     .SS "Using multiple assertions"
1581     .rs
1582     .sp
1583 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1584 nigel 75 .sp
1585     (?<=\ed{3})(?<!999)foo
1586     .sp
1587 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1588     the assertions is applied independently at the same point in the subject
1589     string. First there is a check that the previous three characters are all
1590     digits, and then there is a check that the same three characters are not "999".
1591 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1592 nigel 63 of which are digits and the last three of which are not "999". For example, it
1593     doesn't match "123abcfoo". A pattern to do that is
1594 nigel 75 .sp
1595     (?<=\ed{3}...)(?<!999)foo
1596     .sp
1597 nigel 63 This time the first assertion looks at the preceding six characters, checking
1598     that the first three are digits, and then the second assertion checks that the
1599     preceding three characters are not "999".
1600 nigel 75 .P
1601 nigel 63 Assertions can be nested in any combination. For example,
1602 nigel 75 .sp
1603 nigel 63 (?<=(?<!foo)bar)baz
1604 nigel 75 .sp
1605 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1606     preceded by "foo", while
1607 nigel 75 .sp
1608     (?<=\ed{3}(?!999)...)foo
1609     .sp
1610     is another pattern that matches "foo" preceded by three digits and any three
1611 nigel 63 characters that are not "999".
1612 nigel 75 .
1613     .
1614 nigel 91 .\" HTML <a name="conditions"></a>
1615 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1616 nigel 63 .rs
1617     .sp
1618     It is possible to cause the matching process to obey a subpattern
1619     conditionally or to choose between two alternative subpatterns, depending on
1620     the result of an assertion, or whether a previous capturing subpattern matched
1621     or not. The two possible forms of conditional subpattern are
1622 nigel 75 .sp
1623 nigel 63 (?(condition)yes-pattern)
1624     (?(condition)yes-pattern|no-pattern)
1625 nigel 75 .sp
1626 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1627     no-pattern (if present) is used. If there are more than two alternatives in the
1628     subpattern, a compile-time error occurs.
1629 nigel 75 .P
1630 nigel 93 There are four kinds of condition: references to subpatterns, references to
1631     recursion, a pseudo-condition called DEFINE, and assertions.
1632     .
1633     .SS "Checking for a used subpattern by number"
1634     .rs
1635     .sp
1636     If the text between the parentheses consists of a sequence of digits, the
1637     condition is true if the capturing subpattern of that number has previously
1638 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1639 ph10 167 sign. In this case, the subpattern number is relative rather than absolute.
1640 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most
1641 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to
1642     refer to subsequent groups with constructs such as (?(+2).
1643 nigel 91 .P
1644     Consider the following pattern, which contains non-significant white space to
1645     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1646     three parts for ease of discussion:
1647 nigel 75 .sp
1648     ( \e( )? [^()]+ (?(1) \e) )
1649     .sp
1650 nigel 63 The first part matches an optional opening parenthesis, and if that
1651     character is present, sets it as the first captured substring. The second part
1652     matches one or more characters that are not parentheses. The third part is a
1653     conditional subpattern that tests whether the first set of parentheses matched
1654     or not. If they did, that is, if subject started with an opening parenthesis,
1655     the condition is true, and so the yes-pattern is executed and a closing
1656     parenthesis is required. Otherwise, since no-pattern is not present, the
1657     subpattern matches nothing. In other words, this pattern matches a sequence of
1658 nigel 93 non-parentheses, optionally enclosed in parentheses.
1659 ph10 167 .P
1660 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1661 ph10 167 reference:
1662     .sp
1663     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1664     .sp
1665     This makes the fragment independent of the parentheses in the larger pattern.
1666 nigel 93 .
1667     .SS "Checking for a used subpattern by name"
1668     .rs
1669 nigel 91 .sp
1670 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1671     subpattern by name. For compatibility with earlier versions of PCRE, which had
1672     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1673     there is a possible ambiguity with this syntax, because subpattern names may
1674     consist entirely of digits. PCRE looks first for a named subpattern; if it
1675     cannot find one and the name consists entirely of digits, PCRE looks for a
1676     subpattern of that number, which must be greater than zero. Using subpattern
1677     names that consist entirely of digits is not recommended.
1678     .P
1679     Rewriting the above example to use a named subpattern gives this:
1680 nigel 91 .sp
1681 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1682     .sp
1683     .
1684     .SS "Checking for pattern recursion"
1685     .rs
1686     .sp
1687 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1688 nigel 93 the condition is true if a recursive call to the whole pattern or any
1689     subpattern has been made. If digits or a name preceded by ampersand follow the
1690     letter R, for example:
1691     .sp
1692     (?(R3)...) or (?(R&name)...)
1693     .sp
1694     the condition is true if the most recent recursion is into the subpattern whose
1695     number or name is given. This condition does not check the entire recursion
1696     stack.
1697 nigel 75 .P
1698 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1699     patterns are described below.
1700     .
1701     .SS "Defining subpatterns for use by reference only"
1702     .rs
1703     .sp
1704     If the condition is the string (DEFINE), and there is no subpattern with the
1705     name DEFINE, the condition is always false. In this case, there may be only one
1706     alternative in the subpattern. It is always skipped if control reaches this
1707     point in the pattern; the idea of DEFINE is that it can be used to define
1708     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1709     is described below.) For example, a pattern to match an IPv4 address could be
1710     written like this (ignore whitespace and line breaks):
1711     .sp
1712     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1713     \eb (?&byte) (\e.(?&byte)){3} \eb
1714     .sp
1715     The first part of the pattern is a DEFINE group inside which a another group
1716     named "byte" is defined. This matches an individual component of an IPv4
1717     address (a number less than 256). When matching takes place, this part of the
1718     pattern is skipped because DEFINE acts like a false condition.
1719     .P
1720     The rest of the pattern uses references to the named group to match the four
1721     dot-separated components of an IPv4 address, insisting on a word boundary at
1722     each end.
1723     .
1724     .SS "Assertion conditions"
1725     .rs
1726     .sp
1727     If the condition is not in any of the above formats, it must be an assertion.
1728 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1729     this pattern, again containing non-significant white space, and with the two
1730     alternatives on the second line:
1731 nigel 75 .sp
1732 nigel 63 (?(?=[^a-z]*[a-z])
1733 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1734     .sp
1735 nigel 63 The condition is a positive lookahead assertion that matches an optional
1736     sequence of non-letters followed by a letter. In other words, it tests for the
1737     presence of at least one letter in the subject. If a letter is found, the
1738     subject is matched against the first alternative; otherwise it is matched
1739     against the second. This pattern matches strings in one of the two forms
1740     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1741 nigel 75 .
1742     .
1743     .\" HTML <a name="comments"></a>
1744 nigel 63 .SH COMMENTS
1745     .rs
1746     .sp
1747 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1748 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1749     that make up a comment play no part in the pattern matching at all.
1750 nigel 75 .P
1751 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1752 nigel 91 character class introduces a comment that continues to immediately after the
1753     next newline in the pattern.
1754 nigel 75 .
1755     .
1756 nigel 91 .\" HTML <a name="recursion"></a>
1757 nigel 75 .SH "RECURSIVE PATTERNS"
1758 nigel 63 .rs
1759     .sp
1760     Consider the problem of matching a string in parentheses, allowing for
1761     unlimited nested parentheses. Without the use of recursion, the best that can
1762     be done is to use a pattern that matches up to some fixed depth of nesting. It
1763 nigel 93 is not possible to handle an arbitrary nesting depth.
1764     .P
1765     For some time, Perl has provided a facility that allows regular expressions to
1766     recurse (amongst other things). It does this by interpolating Perl code in the
1767     expression at run time, and the code can refer to the expression itself. A Perl
1768     pattern using code interpolation to solve the parentheses problem can be
1769     created like this:
1770 nigel 75 .sp
1771     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1772     .sp
1773 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1774 nigel 93 recursively to the pattern in which it appears.
1775 nigel 75 .P
1776 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1777     supports special syntax for recursion of the entire pattern, and also for
1778     individual subpattern recursion. After its introduction in PCRE and Python,
1779     this kind of recursion was introduced into Perl at release 5.10.
1780 nigel 75 .P
1781 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1782     closing parenthesis is a recursive call of the subpattern of the given number,
1783     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1784     call, which is described in the next section.) The special item (?R) or (?0) is
1785     a recursive call of the entire regular expression.
1786 nigel 87 .P
1787 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1788     treated as an atomic group. That is, once it has matched some of the subject
1789     string, it is never re-entered, even if it contains untried alternatives and
1790     there is a subsequent matching failure.
1791     .P
1792 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1793     PCRE_EXTENDED option is set so that white space is ignored):
1794 nigel 75 .sp
1795     \e( ( (?>[^()]+) | (?R) )* \e)
1796     .sp
1797 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1798     substrings which can either be a sequence of non-parentheses, or a recursive
1799 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1800 nigel 63 Finally there is a closing parenthesis.
1801 nigel 75 .P
1802 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1803     pattern, so instead you could use this:
1804 nigel 75 .sp
1805     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1806     .sp
1807 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1808 ph10 172 them instead of the whole pattern.
1809 ph10 166 .P
1810     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1811     is made easier by the use of relative references. (A Perl 5.10 feature.)
1812     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1813     most recently opened parentheses preceding the recursion. In other words, a
1814     negative number counts capturing parentheses leftwards from the point at which
1815     it is encountered.
1816     .P
1817     It is also possible to refer to subsequently opened parentheses, by writing
1818     references such as (?+2). However, these cannot be recursive because the
1819     reference is not inside the parentheses that are referenced. They are always
1820     "subroutine" calls, as described in the next section.
1821     .P
1822     An alternative approach is to use named parentheses instead. The Perl syntax
1823     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1824     could rewrite the above example as follows:
1825 nigel 75 .sp
1826 nigel 93 (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1827 nigel 75 .sp
1828 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1829 ph10 172 used.
1830 ph10 166 .P
1831     This particular example pattern that we have been looking at contains nested
1832     unlimited repeats, and so the use of atomic grouping for matching strings of
1833     non-parentheses is important when applying the pattern to strings that do not
1834     match. For example, when this pattern is applied to
1835 nigel 75 .sp
1836 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1837 nigel 75 .sp
1838 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1839     the match runs for a very long time indeed because there are so many different
1840     ways the + and * repeats can carve up the subject, and all have to be tested
1841     before failure can be reported.
1842 nigel 75 .P
1843 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1844     from the outermost level of the recursion at which the subpattern value is set.
1845     If you want to obtain intermediate values, a callout function can be used (see
1846 nigel 93 below and the
1847 nigel 63 .\" HREF
1848 nigel 75 \fBpcrecallout\fP
1849 nigel 63 .\"
1850     documentation). If the pattern above is matched against
1851 nigel 75 .sp
1852 nigel 63 (ab(cd)ef)
1853 nigel 75 .sp
1854 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1855     on at the top level. If additional parentheses are added, giving
1856 nigel 75 .sp
1857     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1858 nigel 63 ^ ^
1859     ^ ^
1860 nigel 75 .sp
1861 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1862     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1863     has to obtain extra memory to store data during a recursion, which it does by
1864 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1865 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1866 nigel 75 .P
1867 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
1868     Consider this pattern, which matches text in angle brackets, allowing for
1869     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1870     recursing), whereas any characters are permitted at the outer level.
1871 nigel 75 .sp
1872     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
1873     .sp
1874 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1875     different alternatives for the recursive and non-recursive cases. The (?R) item
1876     is the actual recursive call.
1877 nigel 75 .
1878     .
1879 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
1880 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
1881 nigel 63 .rs
1882     .sp
1883     If the syntax for a recursive subpattern reference (either by number or by
1884     name) is used outside the parentheses to which it refers, it operates like a
1885 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
1886 ph10 166 before or after the reference. A numbered reference can be absolute or
1887     relative, as in these examples:
1888 nigel 75 .sp
1889 ph10 166 (...(absolute)...)...(?2)...
1890     (...(relative)...)...(?-1)...
1891 ph10 172 (...(?+1)...(relative)...
1892 ph10 166 .sp
1893     An earlier example pointed out that the pattern
1894     .sp
1895 nigel 75 (sens|respons)e and \e1ibility
1896     .sp
1897 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1898     "sense and responsibility". If instead the pattern
1899 nigel 75 .sp
1900 nigel 63 (sens|respons)e and (?1)ibility
1901 nigel 75 .sp
1902 nigel 63 is used, it does match "sense and responsibility" as well as the other two
1903 nigel 93 strings. Another example is given in the discussion of DEFINE above.
1904 nigel 87 .P
1905     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1906     group. That is, once it has matched some of the subject string, it is never
1907     re-entered, even if it contains untried alternatives and there is a subsequent
1908     matching failure.
1909 nigel 93 .P
1910     When a subpattern is used as a subroutine, processing options such as
1911     case-independence are fixed when the subpattern is defined. They cannot be
1912     changed for different calls. For example, consider this pattern:
1913     .sp
1914 ph10 166 (abc)(?i:(?-1))
1915 nigel 93 .sp
1916     It matches "abcabc". It does not match "abcABC" because the change of
1917     processing option does not affect the called subpattern.
1918 nigel 75 .
1919     .
1920 nigel 63 .SH CALLOUTS
1921     .rs
1922     .sp
1923     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
1924     code to be obeyed in the middle of matching a regular expression. This makes it
1925     possible, amongst other things, to extract different substrings that match the
1926     same pair of parentheses when there is a repetition.
1927 nigel 75 .P
1928 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1929     code. The feature is called "callout". The caller of PCRE provides an external
1930 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
1931 nigel 63 By default, this variable contains NULL, which disables all calling out.
1932 nigel 75 .P
1933 nigel 63 Within a regular expression, (?C) indicates the points at which the external
1934     function is to be called. If you want to identify different callout points, you
1935     can put a number less than 256 after the letter C. The default value is zero.
1936     For example, this pattern has two callout points:
1937 nigel 75 .sp
1938 ph10 155 (?C1)abc(?C2)def
1939 nigel 75 .sp
1940     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1941     automatically installed before each item in the pattern. They are all numbered
1942     255.
1943     .P
1944     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1945 nigel 63 set), the external function is called. It is provided with the number of the
1946 nigel 75 callout, the position in the pattern, and, optionally, one item of data
1947     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
1948     may cause matching to proceed, to backtrack, or to fail altogether. A complete
1949     description of the interface to the callout function is given in the
1950 nigel 63 .\" HREF
1951 nigel 75 \fBpcrecallout\fP
1952 nigel 63 .\"
1953     documentation.
1954 nigel 93 .
1955     .
1956     .SH "SEE ALSO"
1957     .rs
1958     .sp
1959     \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1960 ph10 99 .
1961     .
1962     .SH AUTHOR
1963     .rs
1964     .sp
1965     .nf
1966     Philip Hazel
1967     University Computing Service
1968     Cambridge CB2 3QH, England.
1969     .fi
1970     .
1971     .
1972     .SH REVISION
1973     .rs
1974     .sp
1975     .nf
1976 ph10 178 Last updated: 13 June 2007
1977 ph10 99 Copyright (c) 1997-2007 University of Cambridge.
1978     .fi

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