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Revision 184 - (hide annotations) (download)
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Fix non-UTF-8 mode Unicode properties bug; prepare for 7.2 release.

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 ph10 184 escape sequences that match characters with specific properties are available.
380     When not in UTF-8 mode, these sequences are of course limited to testing
381     characters whose codepoints are less than 256, but they do work in this mode.
382     The extra escape sequences are:
383 nigel 75 .sp
384 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
385     \eP{\fIxx\fP} a character without the \fIxx\fP property
386     \eX an extended Unicode sequence
387 nigel 75 .sp
388 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
389     script names, the general category properties, and "Any", which matches any
390     character (including newline). Other properties such as "InMusicalSymbols" are
391     not currently supported by PCRE. Note that \eP{Any} does not match any
392     characters, so always causes a match failure.
393 nigel 75 .P
394 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
395     character from one of these sets can be matched using a script name. For
396     example:
397 nigel 75 .sp
398 nigel 87 \ep{Greek}
399     \eP{Han}
400     .sp
401     Those that are not part of an identified script are lumped together as
402     "Common". The current list of scripts is:
403     .P
404     Arabic,
405     Armenian,
406 nigel 93 Balinese,
407 nigel 87 Bengali,
408     Bopomofo,
409     Braille,
410     Buginese,
411     Buhid,
412     Canadian_Aboriginal,
413     Cherokee,
414     Common,
415     Coptic,
416 nigel 93 Cuneiform,
417 nigel 87 Cypriot,
418     Cyrillic,
419     Deseret,
420     Devanagari,
421     Ethiopic,
422     Georgian,
423     Glagolitic,
424     Gothic,
425     Greek,
426     Gujarati,
427     Gurmukhi,
428     Han,
429     Hangul,
430     Hanunoo,
431     Hebrew,
432     Hiragana,
433     Inherited,
434     Kannada,
435     Katakana,
436     Kharoshthi,
437     Khmer,
438     Lao,
439     Latin,
440     Limbu,
441     Linear_B,
442     Malayalam,
443     Mongolian,
444     Myanmar,
445     New_Tai_Lue,
446 nigel 93 Nko,
447 nigel 87 Ogham,
448     Old_Italic,
449     Old_Persian,
450     Oriya,
451     Osmanya,
452 nigel 93 Phags_Pa,
453     Phoenician,
454 nigel 87 Runic,
455     Shavian,
456     Sinhala,
457     Syloti_Nagri,
458     Syriac,
459     Tagalog,
460     Tagbanwa,
461     Tai_Le,
462     Tamil,
463     Telugu,
464     Thaana,
465     Thai,
466     Tibetan,
467     Tifinagh,
468     Ugaritic,
469     Yi.
470     .P
471     Each character has exactly one general category property, specified by a
472     two-letter abbreviation. For compatibility with Perl, negation can be specified
473     by including a circumflex between the opening brace and the property name. For
474     example, \ep{^Lu} is the same as \eP{Lu}.
475     .P
476     If only one letter is specified with \ep or \eP, it includes all the general
477     category properties that start with that letter. In this case, in the absence
478     of negation, the curly brackets in the escape sequence are optional; these two
479     examples have the same effect:
480     .sp
481 nigel 75 \ep{L}
482     \epL
483     .sp
484 nigel 87 The following general category property codes are supported:
485 nigel 75 .sp
486     C Other
487     Cc Control
488     Cf Format
489     Cn Unassigned
490     Co Private use
491     Cs Surrogate
492     .sp
493     L Letter
494     Ll Lower case letter
495     Lm Modifier letter
496     Lo Other letter
497     Lt Title case letter
498     Lu Upper case letter
499     .sp
500     M Mark
501     Mc Spacing mark
502     Me Enclosing mark
503     Mn Non-spacing mark
504     .sp
505     N Number
506     Nd Decimal number
507     Nl Letter number
508     No Other number
509     .sp
510     P Punctuation
511     Pc Connector punctuation
512     Pd Dash punctuation
513     Pe Close punctuation
514     Pf Final punctuation
515     Pi Initial punctuation
516     Po Other punctuation
517     Ps Open punctuation
518     .sp
519     S Symbol
520     Sc Currency symbol
521     Sk Modifier symbol
522     Sm Mathematical symbol
523     So Other symbol
524     .sp
525     Z Separator
526     Zl Line separator
527     Zp Paragraph separator
528     Zs Space separator
529     .sp
530 nigel 87 The special property L& is also supported: it matches a character that has
531     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
532     a modifier or "other".
533 nigel 75 .P
534 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter})
535 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
536 nigel 87 properties with "Is".
537     .P
538     No character that is in the Unicode table has the Cn (unassigned) property.
539     Instead, this property is assumed for any code point that is not in the
540     Unicode table.
541     .P
542 nigel 75 Specifying caseless matching does not affect these escape sequences. For
543     example, \ep{Lu} always matches only upper case letters.
544     .P
545     The \eX escape matches any number of Unicode characters that form an extended
546     Unicode sequence. \eX is equivalent to
547     .sp
548     (?>\ePM\epM*)
549     .sp
550     That is, it matches a character without the "mark" property, followed by zero
551     or more characters with the "mark" property, and treats the sequence as an
552     atomic group
553     .\" HTML <a href="#atomicgroup">
554     .\" </a>
555     (see below).
556     .\"
557     Characters with the "mark" property are typically accents that affect the
558 ph10 184 preceding character. None of them have codepoints less than 256, so in
559     non-UTF-8 mode \eX matches any one character.
560 nigel 75 .P
561     Matching characters by Unicode property is not fast, because PCRE has to search
562     a structure that contains data for over fifteen thousand characters. That is
563     why the traditional escape sequences such as \ed and \ew do not use Unicode
564     properties in PCRE.
565     .
566     .
567 ph10 168 .\" HTML <a name="resetmatchstart"></a>
568     .SS "Resetting the match start"
569     .rs
570     .sp
571 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
572     matched characters not to be included in the final matched sequence. For
573 ph10 168 example, the pattern:
574     .sp
575     foo\eKbar
576     .sp
577 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
578 ph10 168 similar to a lookbehind assertion
579     .\" HTML <a href="#lookbehind">
580     .\" </a>
581     (described below).
582     .\"
583 ph10 172 However, in this case, the part of the subject before the real match does not
584     have to be of fixed length, as lookbehind assertions do. The use of \eK does
585 ph10 168 not interfere with the setting of
586     .\" HTML <a href="#subpattern">
587     .\" </a>
588     captured substrings.
589 ph10 172 .\"
590 ph10 168 For example, when the pattern
591     .sp
592     (foo)\eKbar
593     .sp
594 ph10 172 matches "foobar", the first substring is still set to "foo".
595 ph10 168 .
596     .
597 nigel 75 .\" HTML <a name="smallassertions"></a>
598     .SS "Simple assertions"
599     .rs
600     .sp
601 nigel 93 The final use of backslash is for certain simple assertions. An assertion
602 nigel 63 specifies a condition that has to be met at a particular point in a match,
603     without consuming any characters from the subject string. The use of
604 nigel 75 subpatterns for more complicated assertions is described
605     .\" HTML <a href="#bigassertions">
606     .\" </a>
607     below.
608     .\"
609 nigel 91 The backslashed assertions are:
610 nigel 75 .sp
611     \eb matches at a word boundary
612     \eB matches when not at a word boundary
613 nigel 93 \eA matches at the start of the subject
614     \eZ matches at the end of the subject
615     also matches before a newline at the end of the subject
616     \ez matches only at the end of the subject
617     \eG matches at the first matching position in the subject
618 nigel 75 .sp
619     These assertions may not appear in character classes (but note that \eb has a
620 nigel 63 different meaning, namely the backspace character, inside a character class).
621 nigel 75 .P
622 nigel 63 A word boundary is a position in the subject string where the current character
623 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
624     \ew and the other matches \eW), or the start or end of the string if the
625     first or last character matches \ew, respectively.
626     .P
627     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
628     dollar (described in the next section) in that they only ever match at the very
629     start and end of the subject string, whatever options are set. Thus, they are
630     independent of multiline mode. These three assertions are not affected by the
631     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
632     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
633     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
634     at a point other than the beginning of the subject, \eA can never match. The
635 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
636     of the string as well as at the very end, whereas \ez matches only at the end.
637 nigel 75 .P
638     The \eG assertion is true only when the current matching position is at the
639     start point of the match, as specified by the \fIstartoffset\fP argument of
640     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
641     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
642 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
643 nigel 75 implementation where \eG can be useful.
644     .P
645     Note, however, that PCRE's interpretation of \eG, as the start of the current
646 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
647     previous match. In Perl, these can be different when the previously matched
648     string was empty. Because PCRE does just one match at a time, it cannot
649     reproduce this behaviour.
650 nigel 75 .P
651     If all the alternatives of a pattern begin with \eG, the expression is anchored
652 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
653     regular expression.
654 nigel 75 .
655     .
656     .SH "CIRCUMFLEX AND DOLLAR"
657 nigel 63 .rs
658     .sp
659     Outside a character class, in the default matching mode, the circumflex
660 nigel 75 character is an assertion that is true only if the current matching point is
661     at the start of the subject string. If the \fIstartoffset\fP argument of
662     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
663 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
664 nigel 75 meaning
665     .\" HTML <a href="#characterclass">
666     .\" </a>
667     (see below).
668     .\"
669     .P
670 nigel 63 Circumflex need not be the first character of the pattern if a number of
671     alternatives are involved, but it should be the first thing in each alternative
672     in which it appears if the pattern is ever to match that branch. If all
673     possible alternatives start with a circumflex, that is, if the pattern is
674     constrained to match only at the start of the subject, it is said to be an
675     "anchored" pattern. (There are also other constructs that can cause a pattern
676     to be anchored.)
677 nigel 75 .P
678     A dollar character is an assertion that is true only if the current matching
679 nigel 63 point is at the end of the subject string, or immediately before a newline
680 nigel 91 at the end of the string (by default). Dollar need not be the last character of
681     the pattern if a number of alternatives are involved, but it should be the last
682     item in any branch in which it appears. Dollar has no special meaning in a
683     character class.
684 nigel 75 .P
685 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
686     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
687 nigel 75 does not affect the \eZ assertion.
688     .P
689 nigel 63 The meanings of the circumflex and dollar characters are changed if the
690 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
691     immediately after internal newlines as well as at the start of the subject
692     string. It does not match after a newline that ends the string. A dollar
693     matches before any newlines in the string, as well as at the very end, when
694     PCRE_MULTILINE is set. When newline is specified as the two-character
695     sequence CRLF, isolated CR and LF characters do not indicate newlines.
696 nigel 75 .P
697 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
698     \en represents a newline) in multiline mode, but not otherwise. Consequently,
699     patterns that are anchored in single line mode because all branches start with
700     ^ are not anchored in multiline mode, and a match for circumflex is possible
701     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
702     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
703     .P
704 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
705 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
706 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
707 nigel 75 .
708     .
709     .SH "FULL STOP (PERIOD, DOT)"
710 nigel 63 .rs
711     .sp
712     Outside a character class, a dot in the pattern matches any one character in
713 nigel 91 the subject string except (by default) a character that signifies the end of a
714 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
715 nigel 91 .P
716 nigel 93 When a line ending is defined as a single character, dot never matches that
717     character; when the two-character sequence CRLF is used, dot does not match CR
718     if it is immediately followed by LF, but otherwise it matches all characters
719     (including isolated CRs and LFs). When any Unicode line endings are being
720     recognized, dot does not match CR or LF or any of the other line ending
721     characters.
722     .P
723 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
724 nigel 93 option is set, a dot matches any one character, without exception. If the
725     two-character sequence CRLF is present in the subject string, it takes two dots
726     to match it.
727 nigel 91 .P
728     The handling of dot is entirely independent of the handling of circumflex and
729     dollar, the only relationship being that they both involve newlines. Dot has no
730     special meaning in a character class.
731 nigel 75 .
732     .
733     .SH "MATCHING A SINGLE BYTE"
734 nigel 63 .rs
735     .sp
736 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
737 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
738     characters. The feature is provided in Perl in order to match individual bytes
739     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
740     what remains in the string may be a malformed UTF-8 string. For this reason,
741     the \eC escape sequence is best avoided.
742 nigel 75 .P
743     PCRE does not allow \eC to appear in lookbehind assertions
744     .\" HTML <a href="#lookbehind">
745     .\" </a>
746     (described below),
747     .\"
748     because in UTF-8 mode this would make it impossible to calculate the length of
749     the lookbehind.
750     .
751     .
752     .\" HTML <a name="characterclass"></a>
753     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
754 nigel 63 .rs
755     .sp
756     An opening square bracket introduces a character class, terminated by a closing
757     square bracket. A closing square bracket on its own is not special. If a
758     closing square bracket is required as a member of the class, it should be the
759     first data character in the class (after an initial circumflex, if present) or
760     escaped with a backslash.
761 nigel 75 .P
762 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
763     character may occupy more than one byte. A matched character must be in the set
764     of characters defined by the class, unless the first character in the class
765     definition is a circumflex, in which case the subject character must not be in
766     the set defined by the class. If a circumflex is actually required as a member
767     of the class, ensure it is not the first character, or escape it with a
768     backslash.
769 nigel 75 .P
770 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
771     [^aeiou] matches any character that is not a lower case vowel. Note that a
772 nigel 75 circumflex is just a convenient notation for specifying the characters that
773     are in the class by enumerating those that are not. A class that starts with a
774     circumflex is not an assertion: it still consumes a character from the subject
775     string, and therefore it fails if the current pointer is at the end of the
776     string.
777     .P
778 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
779 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
780     .P
781 nigel 63 When caseless matching is set, any letters in a class represent both their
782     upper case and lower case versions, so for example, a caseless [aeiou] matches
783     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
784 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
785     case for characters whose values are less than 128, so caseless matching is
786     always possible. For characters with higher values, the concept of case is
787     supported if PCRE is compiled with Unicode property support, but not otherwise.
788     If you want to use caseless matching for characters 128 and above, you must
789     ensure that PCRE is compiled with Unicode property support as well as with
790     UTF-8 support.
791 nigel 75 .P
792 nigel 93 Characters that might indicate line breaks are never treated in any special way
793     when matching character classes, whatever line-ending sequence is in use, and
794     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
795     such as [^a] always matches one of these characters.
796 nigel 75 .P
797 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
798     character class. For example, [d-m] matches any letter between d and m,
799     inclusive. If a minus character is required in a class, it must be escaped with
800     a backslash or appear in a position where it cannot be interpreted as
801     indicating a range, typically as the first or last character in the class.
802 nigel 75 .P
803 nigel 63 It is not possible to have the literal character "]" as the end character of a
804     range. A pattern such as [W-]46] is interpreted as a class of two characters
805     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
806     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
807 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
808     followed by two other characters. The octal or hexadecimal representation of
809     "]" can also be used to end a range.
810     .P
811 nigel 63 Ranges operate in the collating sequence of character values. They can also be
812 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
813 nigel 63 mode, ranges can include characters whose values are greater than 255, for
814 nigel 75 example [\ex{100}-\ex{2ff}].
815     .P
816 nigel 63 If a range that includes letters is used when caseless matching is set, it
817     matches the letters in either case. For example, [W-c] is equivalent to
818 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
819 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
820 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
821     characters with values greater than 128 only when it is compiled with Unicode
822     property support.
823     .P
824     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
825     in a character class, and add the characters that they match to the class. For
826     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
827 nigel 63 conveniently be used with the upper case character types to specify a more
828     restricted set of characters than the matching lower case type. For example,
829 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
830     .P
831     The only metacharacters that are recognized in character classes are backslash,
832     hyphen (only where it can be interpreted as specifying a range), circumflex
833     (only at the start), opening square bracket (only when it can be interpreted as
834     introducing a POSIX class name - see the next section), and the terminating
835     closing square bracket. However, escaping other non-alphanumeric characters
836     does no harm.
837     .
838     .
839     .SH "POSIX CHARACTER CLASSES"
840 nigel 63 .rs
841     .sp
842 nigel 75 Perl supports the POSIX notation for character classes. This uses names
843 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
844     this notation. For example,
845 nigel 75 .sp
846 nigel 63 [01[:alpha:]%]
847 nigel 75 .sp
848 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
849     are
850 nigel 75 .sp
851 nigel 63 alnum letters and digits
852     alpha letters
853     ascii character codes 0 - 127
854     blank space or tab only
855     cntrl control characters
856 nigel 75 digit decimal digits (same as \ed)
857 nigel 63 graph printing characters, excluding space
858     lower lower case letters
859     print printing characters, including space
860     punct printing characters, excluding letters and digits
861 nigel 75 space white space (not quite the same as \es)
862 nigel 63 upper upper case letters
863 nigel 75 word "word" characters (same as \ew)
864 nigel 63 xdigit hexadecimal digits
865 nigel 75 .sp
866 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
867     space (32). Notice that this list includes the VT character (code 11). This
868 nigel 75 makes "space" different to \es, which does not include VT (for Perl
869 nigel 63 compatibility).
870 nigel 75 .P
871 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
872     5.8. Another Perl extension is negation, which is indicated by a ^ character
873     after the colon. For example,
874 nigel 75 .sp
875 nigel 63 [12[:^digit:]]
876 nigel 75 .sp
877 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
878     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
879     supported, and an error is given if they are encountered.
880 nigel 75 .P
881     In UTF-8 mode, characters with values greater than 128 do not match any of
882 nigel 63 the POSIX character classes.
883 nigel 75 .
884     .
885     .SH "VERTICAL BAR"
886 nigel 63 .rs
887     .sp
888     Vertical bar characters are used to separate alternative patterns. For example,
889     the pattern
890 nigel 75 .sp
891 nigel 63 gilbert|sullivan
892 nigel 75 .sp
893 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
894 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
895     process tries each alternative in turn, from left to right, and the first one
896     that succeeds is used. If the alternatives are within a subpattern
897 nigel 75 .\" HTML <a href="#subpattern">
898     .\" </a>
899     (defined below),
900     .\"
901     "succeeds" means matching the rest of the main pattern as well as the
902     alternative in the subpattern.
903     .
904     .
905     .SH "INTERNAL OPTION SETTING"
906 nigel 63 .rs
907     .sp
908     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
909     PCRE_EXTENDED options can be changed from within the pattern by a sequence of
910     Perl option letters enclosed between "(?" and ")". The option letters are
911 nigel 75 .sp
912 nigel 63 i for PCRE_CASELESS
913     m for PCRE_MULTILINE
914     s for PCRE_DOTALL
915     x for PCRE_EXTENDED
916 nigel 75 .sp
917 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
918     unset these options by preceding the letter with a hyphen, and a combined
919     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
920     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
921     permitted. If a letter appears both before and after the hyphen, the option is
922     unset.
923 nigel 75 .P
924 nigel 63 When an option change occurs at top level (that is, not inside subpattern
925     parentheses), the change applies to the remainder of the pattern that follows.
926     If the change is placed right at the start of a pattern, PCRE extracts it into
927     the global options (and it will therefore show up in data extracted by the
928 nigel 75 \fBpcre_fullinfo()\fP function).
929     .P
930 nigel 93 An option change within a subpattern (see below for a description of
931     subpatterns) affects only that part of the current pattern that follows it, so
932 nigel 75 .sp
933 nigel 63 (a(?i)b)c
934 nigel 75 .sp
935 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
936     By this means, options can be made to have different settings in different
937     parts of the pattern. Any changes made in one alternative do carry on
938     into subsequent branches within the same subpattern. For example,
939 nigel 75 .sp
940 nigel 63 (a(?i)b|c)
941 nigel 75 .sp
942 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
943     branch is abandoned before the option setting. This is because the effects of
944     option settings happen at compile time. There would be some very weird
945     behaviour otherwise.
946 nigel 75 .P
947 nigel 91 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
948     changed in the same way as the Perl-compatible options by using the characters
949     J, U and X respectively.
950 nigel 75 .
951     .
952     .\" HTML <a name="subpattern"></a>
953 nigel 63 .SH SUBPATTERNS
954     .rs
955     .sp
956     Subpatterns are delimited by parentheses (round brackets), which can be nested.
957 nigel 75 Turning part of a pattern into a subpattern does two things:
958     .sp
959 nigel 63 1. It localizes a set of alternatives. For example, the pattern
960 nigel 75 .sp
961 nigel 63 cat(aract|erpillar|)
962 nigel 75 .sp
963 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
964 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
965 nigel 75 .sp
966     2. It sets up the subpattern as a capturing subpattern. This means that, when
967     the whole pattern matches, that portion of the subject string that matched the
968     subpattern is passed back to the caller via the \fIovector\fP argument of
969     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
970     from 1) to obtain numbers for the capturing subpatterns.
971     .P
972 nigel 63 For example, if the string "the red king" is matched against the pattern
973 nigel 75 .sp
974 nigel 63 the ((red|white) (king|queen))
975 nigel 75 .sp
976 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
977     2, and 3, respectively.
978 nigel 75 .P
979 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
980     There are often times when a grouping subpattern is required without a
981     capturing requirement. If an opening parenthesis is followed by a question mark
982     and a colon, the subpattern does not do any capturing, and is not counted when
983     computing the number of any subsequent capturing subpatterns. For example, if
984     the string "the white queen" is matched against the pattern
985 nigel 75 .sp
986 nigel 63 the ((?:red|white) (king|queen))
987 nigel 75 .sp
988 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
989 nigel 93 2. The maximum number of capturing subpatterns is 65535.
990 nigel 75 .P
991 nigel 63 As a convenient shorthand, if any option settings are required at the start of
992     a non-capturing subpattern, the option letters may appear between the "?" and
993     the ":". Thus the two patterns
994 nigel 75 .sp
995 nigel 63 (?i:saturday|sunday)
996     (?:(?i)saturday|sunday)
997 nigel 75 .sp
998 nigel 63 match exactly the same set of strings. Because alternative branches are tried
999     from left to right, and options are not reset until the end of the subpattern
1000     is reached, an option setting in one branch does affect subsequent branches, so
1001     the above patterns match "SUNDAY" as well as "Saturday".
1002 nigel 75 .
1003     .
1004 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1005     .rs
1006     .sp
1007 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1008     the same numbers for its capturing parentheses. Such a subpattern starts with
1009     (?| and is itself a non-capturing subpattern. For example, consider this
1010 ph10 175 pattern:
1011     .sp
1012     (?|(Sat)ur|(Sun))day
1013 ph10 182 .sp
1014     Because the two alternatives are inside a (?| group, both sets of capturing
1015     parentheses are numbered one. Thus, when the pattern matches, you can look
1016     at captured substring number one, whichever alternative matched. This construct
1017     is useful when you want to capture part, but not all, of one of a number of
1018     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1019 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1020 ph10 182 buffers that follow the subpattern start after the highest number used in any
1021     branch. The following example is taken from the Perl documentation.
1022 ph10 175 The numbers underneath show in which buffer the captured content will be
1023     stored.
1024     .sp
1025     # before ---------------branch-reset----------- after
1026     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1027     # 1 2 2 3 2 3 4
1028 ph10 182 .sp
1029 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to
1030     the first one in the pattern with the given number.
1031     .P
1032     An alternative approach to using this "branch reset" feature is to use
1033     duplicate named subpatterns, as described in the next section.
1034     .
1035     .
1036 nigel 75 .SH "NAMED SUBPATTERNS"
1037 nigel 63 .rs
1038     .sp
1039     Identifying capturing parentheses by number is simple, but it can be very hard
1040     to keep track of the numbers in complicated regular expressions. Furthermore,
1041 nigel 75 if an expression is modified, the numbers may change. To help with this
1042 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1043     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1044     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1045     the Perl and the Python syntax.
1046     .P
1047     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1048     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1049 nigel 91 parentheses from other parts of the pattern, such as
1050     .\" HTML <a href="#backreferences">
1051     .\" </a>
1052     backreferences,
1053     .\"
1054     .\" HTML <a href="#recursion">
1055     .\" </a>
1056     recursion,
1057     .\"
1058     and
1059     .\" HTML <a href="#conditions">
1060     .\" </a>
1061     conditions,
1062     .\"
1063     can be made by name as well as by number.
1064 nigel 75 .P
1065 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1066 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1067     if the names were not present. The PCRE API provides function calls for
1068     extracting the name-to-number translation table from a compiled pattern. There
1069     is also a convenience function for extracting a captured substring by name.
1070 nigel 91 .P
1071     By default, a name must be unique within a pattern, but it is possible to relax
1072     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1073     be useful for patterns where only one instance of the named parentheses can
1074     match. Suppose you want to match the name of a weekday, either as a 3-letter
1075     abbreviation or as the full name, and in both cases you want to extract the
1076     abbreviation. This pattern (ignoring the line breaks) does the job:
1077     .sp
1078 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1079     (?<DN>Tue)(?:sday)?|
1080     (?<DN>Wed)(?:nesday)?|
1081     (?<DN>Thu)(?:rsday)?|
1082     (?<DN>Sat)(?:urday)?
1083 nigel 91 .sp
1084     There are five capturing substrings, but only one is ever set after a match.
1085 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1086 ph10 175 subpattern, as described in the previous section.)
1087     .P
1088 nigel 91 The convenience function for extracting the data by name returns the substring
1089 nigel 93 for the first (and in this example, the only) subpattern of that name that
1090 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1091     make a reference to a non-unique named subpattern from elsewhere in the
1092     pattern, the one that corresponds to the lowest number is used. For further
1093     details of the interfaces for handling named subpatterns, see the
1094 nigel 63 .\" HREF
1095 nigel 75 \fBpcreapi\fP
1096 nigel 63 .\"
1097     documentation.
1098 nigel 75 .
1099     .
1100 nigel 63 .SH REPETITION
1101     .rs
1102     .sp
1103     Repetition is specified by quantifiers, which can follow any of the following
1104     items:
1105 nigel 75 .sp
1106 nigel 63 a literal data character
1107 nigel 93 the dot metacharacter
1108 nigel 75 the \eC escape sequence
1109     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1110 nigel 93 the \eR escape sequence
1111 nigel 75 an escape such as \ed that matches a single character
1112 nigel 63 a character class
1113     a back reference (see next section)
1114     a parenthesized subpattern (unless it is an assertion)
1115 nigel 75 .sp
1116 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1117     permitted matches, by giving the two numbers in curly brackets (braces),
1118     separated by a comma. The numbers must be less than 65536, and the first must
1119     be less than or equal to the second. For example:
1120 nigel 75 .sp
1121 nigel 63 z{2,4}
1122 nigel 75 .sp
1123 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1124     character. If the second number is omitted, but the comma is present, there is
1125     no upper limit; if the second number and the comma are both omitted, the
1126     quantifier specifies an exact number of required matches. Thus
1127 nigel 75 .sp
1128 nigel 63 [aeiou]{3,}
1129 nigel 75 .sp
1130 nigel 63 matches at least 3 successive vowels, but may match many more, while
1131 nigel 75 .sp
1132     \ed{8}
1133     .sp
1134 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1135     where a quantifier is not allowed, or one that does not match the syntax of a
1136     quantifier, is taken as a literal character. For example, {,6} is not a
1137     quantifier, but a literal string of four characters.
1138 nigel 75 .P
1139 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1140 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1141     which is represented by a two-byte sequence. Similarly, when Unicode property
1142     support is available, \eX{3} matches three Unicode extended sequences, each of
1143     which may be several bytes long (and they may be of different lengths).
1144     .P
1145 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1146     previous item and the quantifier were not present.
1147 nigel 75 .P
1148 nigel 93 For convenience, the three most common quantifiers have single-character
1149     abbreviations:
1150 nigel 75 .sp
1151 nigel 63 * is equivalent to {0,}
1152     + is equivalent to {1,}
1153     ? is equivalent to {0,1}
1154 nigel 75 .sp
1155 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1156     match no characters with a quantifier that has no upper limit, for example:
1157 nigel 75 .sp
1158 nigel 63 (a?)*
1159 nigel 75 .sp
1160 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1161     such patterns. However, because there are cases where this can be useful, such
1162     patterns are now accepted, but if any repetition of the subpattern does in fact
1163     match no characters, the loop is forcibly broken.
1164 nigel 75 .P
1165 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1166     possible (up to the maximum number of permitted times), without causing the
1167     rest of the pattern to fail. The classic example of where this gives problems
1168 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1169     and within the comment, individual * and / characters may appear. An attempt to
1170     match C comments by applying the pattern
1171     .sp
1172     /\e*.*\e*/
1173     .sp
1174 nigel 63 to the string
1175 nigel 75 .sp
1176     /* first comment */ not comment /* second comment */
1177     .sp
1178 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1179     item.
1180 nigel 75 .P
1181 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1182     greedy, and instead matches the minimum number of times possible, so the
1183     pattern
1184 nigel 75 .sp
1185     /\e*.*?\e*/
1186     .sp
1187 nigel 63 does the right thing with the C comments. The meaning of the various
1188     quantifiers is not otherwise changed, just the preferred number of matches.
1189     Do not confuse this use of question mark with its use as a quantifier in its
1190     own right. Because it has two uses, it can sometimes appear doubled, as in
1191 nigel 75 .sp
1192     \ed??\ed
1193     .sp
1194 nigel 63 which matches one digit by preference, but can match two if that is the only
1195     way the rest of the pattern matches.
1196 nigel 75 .P
1197 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1198 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1199     greedy by following them with a question mark. In other words, it inverts the
1200     default behaviour.
1201 nigel 75 .P
1202 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1203 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1204 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1205 nigel 75 .P
1206 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1207 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1208 nigel 63 implicitly anchored, because whatever follows will be tried against every
1209     character position in the subject string, so there is no point in retrying the
1210     overall match at any position after the first. PCRE normally treats such a
1211 nigel 75 pattern as though it were preceded by \eA.
1212     .P
1213 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1214     worth setting PCRE_DOTALL in order to obtain this optimization, or
1215     alternatively using ^ to indicate anchoring explicitly.
1216 nigel 75 .P
1217 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1218     is inside capturing parentheses that are the subject of a backreference
1219 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1220     succeeds. Consider, for example:
1221 nigel 75 .sp
1222     (.*)abc\e1
1223     .sp
1224 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1225     this reason, such a pattern is not implicitly anchored.
1226 nigel 75 .P
1227 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1228     that matched the final iteration. For example, after
1229 nigel 75 .sp
1230     (tweedle[dume]{3}\es*)+
1231     .sp
1232 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1233     "tweedledee". However, if there are nested capturing subpatterns, the
1234     corresponding captured values may have been set in previous iterations. For
1235     example, after
1236 nigel 75 .sp
1237 nigel 63 /(a|(b))+/
1238 nigel 75 .sp
1239 nigel 63 matches "aba" the value of the second captured substring is "b".
1240 nigel 75 .
1241     .
1242     .\" HTML <a name="atomicgroup"></a>
1243     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1244 nigel 63 .rs
1245     .sp
1246 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1247     repetition, failure of what follows normally causes the repeated item to be
1248     re-evaluated to see if a different number of repeats allows the rest of the
1249     pattern to match. Sometimes it is useful to prevent this, either to change the
1250     nature of the match, or to cause it fail earlier than it otherwise might, when
1251     the author of the pattern knows there is no point in carrying on.
1252 nigel 75 .P
1253     Consider, for example, the pattern \ed+foo when applied to the subject line
1254     .sp
1255 nigel 63 123456bar
1256 nigel 75 .sp
1257 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1258 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1259 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1260     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1261     that once a subpattern has matched, it is not to be re-evaluated in this way.
1262 nigel 75 .P
1263 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1264 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1265     special parenthesis, starting with (?> as in this example:
1266 nigel 75 .sp
1267     (?>\ed+)foo
1268     .sp
1269 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1270     it has matched, and a failure further into the pattern is prevented from
1271     backtracking into it. Backtracking past it to previous items, however, works as
1272     normal.
1273 nigel 75 .P
1274 nigel 63 An alternative description is that a subpattern of this type matches the string
1275     of characters that an identical standalone pattern would match, if anchored at
1276     the current point in the subject string.
1277 nigel 75 .P
1278 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1279     the above example can be thought of as a maximizing repeat that must swallow
1280 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1281 nigel 63 number of digits they match in order to make the rest of the pattern match,
1282 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1283     .P
1284 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1285     subpatterns, and can be nested. However, when the subpattern for an atomic
1286     group is just a single repeated item, as in the example above, a simpler
1287     notation, called a "possessive quantifier" can be used. This consists of an
1288     additional + character following a quantifier. Using this notation, the
1289     previous example can be rewritten as
1290 nigel 75 .sp
1291     \ed++foo
1292     .sp
1293 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1294     option is ignored. They are a convenient notation for the simpler forms of
1295 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1296     quantifier and the equivalent atomic group, though there may be a performance
1297     difference; possessive quantifiers should be slightly faster.
1298 nigel 75 .P
1299 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1300     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1301     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1302     package, and PCRE copied it from there. It ultimately found its way into Perl
1303     at release 5.10.
1304 nigel 75 .P
1305 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1306     pattern constructs. For example, the sequence A+B is treated as A++B because
1307     there is no point in backtracking into a sequence of A's when B must follow.
1308     .P
1309 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1310     be repeated an unlimited number of times, the use of an atomic group is the
1311     only way to avoid some failing matches taking a very long time indeed. The
1312     pattern
1313 nigel 75 .sp
1314     (\eD+|<\ed+>)*[!?]
1315     .sp
1316 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1317     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1318     quickly. However, if it is applied to
1319 nigel 75 .sp
1320 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1321 nigel 75 .sp
1322 nigel 63 it takes a long time before reporting failure. This is because the string can
1323 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1324     large number of ways, and all have to be tried. (The example uses [!?] rather
1325     than a single character at the end, because both PCRE and Perl have an
1326     optimization that allows for fast failure when a single character is used. They
1327     remember the last single character that is required for a match, and fail early
1328     if it is not present in the string.) If the pattern is changed so that it uses
1329     an atomic group, like this:
1330     .sp
1331     ((?>\eD+)|<\ed+>)*[!?]
1332     .sp
1333 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1334 nigel 75 .
1335     .
1336     .\" HTML <a name="backreferences"></a>
1337     .SH "BACK REFERENCES"
1338 nigel 63 .rs
1339     .sp
1340     Outside a character class, a backslash followed by a digit greater than 0 (and
1341     possibly further digits) is a back reference to a capturing subpattern earlier
1342     (that is, to its left) in the pattern, provided there have been that many
1343     previous capturing left parentheses.
1344 nigel 75 .P
1345 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1346     always taken as a back reference, and causes an error only if there are not
1347     that many capturing left parentheses in the entire pattern. In other words, the
1348     parentheses that are referenced need not be to the left of the reference for
1349 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1350     when a repetition is involved and the subpattern to the right has participated
1351     in an earlier iteration.
1352     .P
1353 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1354     whose number is 10 or more using this syntax because a sequence such as \e50 is
1355     interpreted as a character defined in octal. See the subsection entitled
1356 nigel 91 "Non-printing characters"
1357 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1358     .\" </a>
1359     above
1360     .\"
1361 nigel 93 for further details of the handling of digits following a backslash. There is
1362     no such problem when named parentheses are used. A back reference to any
1363     subpattern is possible using named parentheses (see below).
1364 nigel 75 .P
1365 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1366     backslash is to use the \eg escape sequence, which is a feature introduced in
1367     Perl 5.10. This escape must be followed by a positive or a negative number,
1368     optionally enclosed in braces. These examples are all identical:
1369     .sp
1370     (ring), \e1
1371     (ring), \eg1
1372     (ring), \eg{1}
1373     .sp
1374     A positive number specifies an absolute reference without the ambiguity that is
1375     present in the older syntax. It is also useful when literal digits follow the
1376     reference. A negative number is a relative reference. Consider this example:
1377     .sp
1378     (abc(def)ghi)\eg{-1}
1379     .sp
1380     The sequence \eg{-1} is a reference to the most recently started capturing
1381     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1382     would be equivalent to \e1. The use of relative references can be helpful in
1383     long patterns, and also in patterns that are created by joining together
1384     fragments that contain references within themselves.
1385     .P
1386 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1387     the current subject string, rather than anything matching the subpattern
1388     itself (see
1389     .\" HTML <a href="#subpatternsassubroutines">
1390     .\" </a>
1391     "Subpatterns as subroutines"
1392     .\"
1393     below for a way of doing that). So the pattern
1394 nigel 75 .sp
1395     (sens|respons)e and \e1ibility
1396     .sp
1397 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1398     "sense and responsibility". If caseful matching is in force at the time of the
1399     back reference, the case of letters is relevant. For example,
1400 nigel 75 .sp
1401     ((?i)rah)\es+\e1
1402     .sp
1403 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1404     capturing subpattern is matched caselessly.
1405 nigel 75 .P
1406 ph10 171 There are several different ways of writing back references to named
1407     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1408     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1409     back reference syntax, in which \eg can be used for both numeric and named
1410     references, is also supported. We could rewrite the above example in any of
1411 nigel 93 the following ways:
1412 nigel 75 .sp
1413 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1414 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1415 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1416 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1417 nigel 75 .sp
1418 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1419     after the reference.
1420     .P
1421 nigel 63 There may be more than one back reference to the same subpattern. If a
1422     subpattern has not actually been used in a particular match, any back
1423     references to it always fail. For example, the pattern
1424 nigel 75 .sp
1425     (a|(bc))\e2
1426     .sp
1427 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1428     many capturing parentheses in a pattern, all digits following the backslash are
1429     taken as part of a potential back reference number. If the pattern continues
1430     with a digit character, some delimiter must be used to terminate the back
1431     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1432 nigel 75 Otherwise an empty comment (see
1433     .\" HTML <a href="#comments">
1434     .\" </a>
1435     "Comments"
1436     .\"
1437     below) can be used.
1438     .P
1439 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1440 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1441 nigel 63 However, such references can be useful inside repeated subpatterns. For
1442     example, the pattern
1443 nigel 75 .sp
1444     (a|b\e1)+
1445     .sp
1446 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1447     the subpattern, the back reference matches the character string corresponding
1448     to the previous iteration. In order for this to work, the pattern must be such
1449     that the first iteration does not need to match the back reference. This can be
1450     done using alternation, as in the example above, or by a quantifier with a
1451     minimum of zero.
1452 nigel 75 .
1453     .
1454     .\" HTML <a name="bigassertions"></a>
1455 nigel 63 .SH ASSERTIONS
1456     .rs
1457     .sp
1458     An assertion is a test on the characters following or preceding the current
1459     matching point that does not actually consume any characters. The simple
1460 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1461     .\" HTML <a href="#smallassertions">
1462     .\" </a>
1463     above.
1464     .\"
1465     .P
1466 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1467     those that look ahead of the current position in the subject string, and those
1468 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1469     except that it does not cause the current matching position to be changed.
1470     .P
1471     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1472     because it makes no sense to assert the same thing several times. If any kind
1473     of assertion contains capturing subpatterns within it, these are counted for
1474     the purposes of numbering the capturing subpatterns in the whole pattern.
1475     However, substring capturing is carried out only for positive assertions,
1476     because it does not make sense for negative assertions.
1477     .
1478     .
1479     .SS "Lookahead assertions"
1480     .rs
1481     .sp
1482 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1483     negative assertions. For example,
1484 nigel 75 .sp
1485     \ew+(?=;)
1486     .sp
1487 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1488     the match, and
1489 nigel 75 .sp
1490 nigel 63 foo(?!bar)
1491 nigel 75 .sp
1492 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1493     apparently similar pattern
1494 nigel 75 .sp
1495 nigel 63 (?!foo)bar
1496 nigel 75 .sp
1497 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1498     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1499     (?!foo) is always true when the next three characters are "bar". A
1500 nigel 75 lookbehind assertion is needed to achieve the other effect.
1501     .P
1502 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1503     convenient way to do it is with (?!) because an empty string always matches, so
1504     an assertion that requires there not to be an empty string must always fail.
1505 nigel 75 .
1506     .
1507     .\" HTML <a name="lookbehind"></a>
1508     .SS "Lookbehind assertions"
1509     .rs
1510     .sp
1511 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1512     negative assertions. For example,
1513 nigel 75 .sp
1514 nigel 63 (?<!foo)bar
1515 nigel 75 .sp
1516 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1517     a lookbehind assertion are restricted such that all the strings it matches must
1518 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1519     do not all have to have the same fixed length. Thus
1520 nigel 75 .sp
1521 nigel 63 (?<=bullock|donkey)
1522 nigel 75 .sp
1523 nigel 63 is permitted, but
1524 nigel 75 .sp
1525 nigel 63 (?<!dogs?|cats?)
1526 nigel 75 .sp
1527 nigel 63 causes an error at compile time. Branches that match different length strings
1528     are permitted only at the top level of a lookbehind assertion. This is an
1529     extension compared with Perl (at least for 5.8), which requires all branches to
1530     match the same length of string. An assertion such as
1531 nigel 75 .sp
1532 nigel 63 (?<=ab(c|de))
1533 nigel 75 .sp
1534 nigel 63 is not permitted, because its single top-level branch can match two different
1535     lengths, but it is acceptable if rewritten to use two top-level branches:
1536 nigel 75 .sp
1537 nigel 63 (?<=abc|abde)
1538 nigel 75 .sp
1539 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1540 ph10 168 .\" HTML <a href="#resetmatchstart">
1541     .\" </a>
1542     (see above)
1543     .\"
1544     can be used instead of a lookbehind assertion; this is not restricted to a
1545     fixed-length.
1546     .P
1547 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1548 nigel 93 temporarily move the current position back by the fixed length and then try to
1549 nigel 63 match. If there are insufficient characters before the current position, the
1550 nigel 93 assertion fails.
1551 nigel 75 .P
1552     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1553 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1554 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1555     different numbers of bytes, are also not permitted.
1556 nigel 75 .P
1557 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1558     specify efficient matching at the end of the subject string. Consider a simple
1559     pattern such as
1560 nigel 75 .sp
1561 nigel 63 abcd$
1562 nigel 75 .sp
1563 nigel 63 when applied to a long string that does not match. Because matching proceeds
1564     from left to right, PCRE will look for each "a" in the subject and then see if
1565     what follows matches the rest of the pattern. If the pattern is specified as
1566 nigel 75 .sp
1567 nigel 63 ^.*abcd$
1568 nigel 75 .sp
1569 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1570     there is no following "a"), it backtracks to match all but the last character,
1571     then all but the last two characters, and so on. Once again the search for "a"
1572     covers the entire string, from right to left, so we are no better off. However,
1573     if the pattern is written as
1574 nigel 75 .sp
1575 nigel 63 ^.*+(?<=abcd)
1576 nigel 75 .sp
1577 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1578 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1579     characters. If it fails, the match fails immediately. For long strings, this
1580     approach makes a significant difference to the processing time.
1581 nigel 75 .
1582     .
1583     .SS "Using multiple assertions"
1584     .rs
1585     .sp
1586 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1587 nigel 75 .sp
1588     (?<=\ed{3})(?<!999)foo
1589     .sp
1590 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1591     the assertions is applied independently at the same point in the subject
1592     string. First there is a check that the previous three characters are all
1593     digits, and then there is a check that the same three characters are not "999".
1594 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1595 nigel 63 of which are digits and the last three of which are not "999". For example, it
1596     doesn't match "123abcfoo". A pattern to do that is
1597 nigel 75 .sp
1598     (?<=\ed{3}...)(?<!999)foo
1599     .sp
1600 nigel 63 This time the first assertion looks at the preceding six characters, checking
1601     that the first three are digits, and then the second assertion checks that the
1602     preceding three characters are not "999".
1603 nigel 75 .P
1604 nigel 63 Assertions can be nested in any combination. For example,
1605 nigel 75 .sp
1606 nigel 63 (?<=(?<!foo)bar)baz
1607 nigel 75 .sp
1608 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1609     preceded by "foo", while
1610 nigel 75 .sp
1611     (?<=\ed{3}(?!999)...)foo
1612     .sp
1613     is another pattern that matches "foo" preceded by three digits and any three
1614 nigel 63 characters that are not "999".
1615 nigel 75 .
1616     .
1617 nigel 91 .\" HTML <a name="conditions"></a>
1618 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1619 nigel 63 .rs
1620     .sp
1621     It is possible to cause the matching process to obey a subpattern
1622     conditionally or to choose between two alternative subpatterns, depending on
1623     the result of an assertion, or whether a previous capturing subpattern matched
1624     or not. The two possible forms of conditional subpattern are
1625 nigel 75 .sp
1626 nigel 63 (?(condition)yes-pattern)
1627     (?(condition)yes-pattern|no-pattern)
1628 nigel 75 .sp
1629 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1630     no-pattern (if present) is used. If there are more than two alternatives in the
1631     subpattern, a compile-time error occurs.
1632 nigel 75 .P
1633 nigel 93 There are four kinds of condition: references to subpatterns, references to
1634     recursion, a pseudo-condition called DEFINE, and assertions.
1635     .
1636     .SS "Checking for a used subpattern by number"
1637     .rs
1638     .sp
1639     If the text between the parentheses consists of a sequence of digits, the
1640     condition is true if the capturing subpattern of that number has previously
1641 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1642 ph10 167 sign. In this case, the subpattern number is relative rather than absolute.
1643 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most
1644 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to
1645     refer to subsequent groups with constructs such as (?(+2).
1646 nigel 91 .P
1647     Consider the following pattern, which contains non-significant white space to
1648     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1649     three parts for ease of discussion:
1650 nigel 75 .sp
1651     ( \e( )? [^()]+ (?(1) \e) )
1652     .sp
1653 nigel 63 The first part matches an optional opening parenthesis, and if that
1654     character is present, sets it as the first captured substring. The second part
1655     matches one or more characters that are not parentheses. The third part is a
1656     conditional subpattern that tests whether the first set of parentheses matched
1657     or not. If they did, that is, if subject started with an opening parenthesis,
1658     the condition is true, and so the yes-pattern is executed and a closing
1659     parenthesis is required. Otherwise, since no-pattern is not present, the
1660     subpattern matches nothing. In other words, this pattern matches a sequence of
1661 nigel 93 non-parentheses, optionally enclosed in parentheses.
1662 ph10 167 .P
1663 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1664 ph10 167 reference:
1665     .sp
1666     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1667     .sp
1668     This makes the fragment independent of the parentheses in the larger pattern.
1669 nigel 93 .
1670     .SS "Checking for a used subpattern by name"
1671     .rs
1672 nigel 91 .sp
1673 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1674     subpattern by name. For compatibility with earlier versions of PCRE, which had
1675     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1676     there is a possible ambiguity with this syntax, because subpattern names may
1677     consist entirely of digits. PCRE looks first for a named subpattern; if it
1678     cannot find one and the name consists entirely of digits, PCRE looks for a
1679     subpattern of that number, which must be greater than zero. Using subpattern
1680     names that consist entirely of digits is not recommended.
1681     .P
1682     Rewriting the above example to use a named subpattern gives this:
1683 nigel 91 .sp
1684 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1685     .sp
1686     .
1687     .SS "Checking for pattern recursion"
1688     .rs
1689     .sp
1690 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1691 nigel 93 the condition is true if a recursive call to the whole pattern or any
1692     subpattern has been made. If digits or a name preceded by ampersand follow the
1693     letter R, for example:
1694     .sp
1695     (?(R3)...) or (?(R&name)...)
1696     .sp
1697     the condition is true if the most recent recursion is into the subpattern whose
1698     number or name is given. This condition does not check the entire recursion
1699     stack.
1700 nigel 75 .P
1701 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1702     patterns are described below.
1703     .
1704     .SS "Defining subpatterns for use by reference only"
1705     .rs
1706     .sp
1707     If the condition is the string (DEFINE), and there is no subpattern with the
1708     name DEFINE, the condition is always false. In this case, there may be only one
1709     alternative in the subpattern. It is always skipped if control reaches this
1710     point in the pattern; the idea of DEFINE is that it can be used to define
1711     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1712     is described below.) For example, a pattern to match an IPv4 address could be
1713     written like this (ignore whitespace and line breaks):
1714     .sp
1715     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1716     \eb (?&byte) (\e.(?&byte)){3} \eb
1717     .sp
1718     The first part of the pattern is a DEFINE group inside which a another group
1719     named "byte" is defined. This matches an individual component of an IPv4
1720     address (a number less than 256). When matching takes place, this part of the
1721     pattern is skipped because DEFINE acts like a false condition.
1722     .P
1723     The rest of the pattern uses references to the named group to match the four
1724     dot-separated components of an IPv4 address, insisting on a word boundary at
1725     each end.
1726     .
1727     .SS "Assertion conditions"
1728     .rs
1729     .sp
1730     If the condition is not in any of the above formats, it must be an assertion.
1731 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1732     this pattern, again containing non-significant white space, and with the two
1733     alternatives on the second line:
1734 nigel 75 .sp
1735 nigel 63 (?(?=[^a-z]*[a-z])
1736 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1737     .sp
1738 nigel 63 The condition is a positive lookahead assertion that matches an optional
1739     sequence of non-letters followed by a letter. In other words, it tests for the
1740     presence of at least one letter in the subject. If a letter is found, the
1741     subject is matched against the first alternative; otherwise it is matched
1742     against the second. This pattern matches strings in one of the two forms
1743     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1744 nigel 75 .
1745     .
1746     .\" HTML <a name="comments"></a>
1747 nigel 63 .SH COMMENTS
1748     .rs
1749     .sp
1750 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1751 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1752     that make up a comment play no part in the pattern matching at all.
1753 nigel 75 .P
1754 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1755 nigel 91 character class introduces a comment that continues to immediately after the
1756     next newline in the pattern.
1757 nigel 75 .
1758     .
1759 nigel 91 .\" HTML <a name="recursion"></a>
1760 nigel 75 .SH "RECURSIVE PATTERNS"
1761 nigel 63 .rs
1762     .sp
1763     Consider the problem of matching a string in parentheses, allowing for
1764     unlimited nested parentheses. Without the use of recursion, the best that can
1765     be done is to use a pattern that matches up to some fixed depth of nesting. It
1766 nigel 93 is not possible to handle an arbitrary nesting depth.
1767     .P
1768     For some time, Perl has provided a facility that allows regular expressions to
1769     recurse (amongst other things). It does this by interpolating Perl code in the
1770     expression at run time, and the code can refer to the expression itself. A Perl
1771     pattern using code interpolation to solve the parentheses problem can be
1772     created like this:
1773 nigel 75 .sp
1774     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1775     .sp
1776 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1777 nigel 93 recursively to the pattern in which it appears.
1778 nigel 75 .P
1779 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1780     supports special syntax for recursion of the entire pattern, and also for
1781     individual subpattern recursion. After its introduction in PCRE and Python,
1782     this kind of recursion was introduced into Perl at release 5.10.
1783 nigel 75 .P
1784 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1785     closing parenthesis is a recursive call of the subpattern of the given number,
1786     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1787     call, which is described in the next section.) The special item (?R) or (?0) is
1788     a recursive call of the entire regular expression.
1789 nigel 87 .P
1790 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1791     treated as an atomic group. That is, once it has matched some of the subject
1792     string, it is never re-entered, even if it contains untried alternatives and
1793     there is a subsequent matching failure.
1794     .P
1795 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1796     PCRE_EXTENDED option is set so that white space is ignored):
1797 nigel 75 .sp
1798     \e( ( (?>[^()]+) | (?R) )* \e)
1799     .sp
1800 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1801     substrings which can either be a sequence of non-parentheses, or a recursive
1802 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1803 nigel 63 Finally there is a closing parenthesis.
1804 nigel 75 .P
1805 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1806     pattern, so instead you could use this:
1807 nigel 75 .sp
1808     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1809     .sp
1810 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1811 ph10 172 them instead of the whole pattern.
1812 ph10 166 .P
1813     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1814     is made easier by the use of relative references. (A Perl 5.10 feature.)
1815     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1816     most recently opened parentheses preceding the recursion. In other words, a
1817     negative number counts capturing parentheses leftwards from the point at which
1818     it is encountered.
1819     .P
1820     It is also possible to refer to subsequently opened parentheses, by writing
1821     references such as (?+2). However, these cannot be recursive because the
1822     reference is not inside the parentheses that are referenced. They are always
1823     "subroutine" calls, as described in the next section.
1824     .P
1825     An alternative approach is to use named parentheses instead. The Perl syntax
1826     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1827     could rewrite the above example as follows:
1828 nigel 75 .sp
1829 nigel 93 (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1830 nigel 75 .sp
1831 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1832 ph10 172 used.
1833 ph10 166 .P
1834     This particular example pattern that we have been looking at contains nested
1835     unlimited repeats, and so the use of atomic grouping for matching strings of
1836     non-parentheses is important when applying the pattern to strings that do not
1837     match. For example, when this pattern is applied to
1838 nigel 75 .sp
1839 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1840 nigel 75 .sp
1841 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1842     the match runs for a very long time indeed because there are so many different
1843     ways the + and * repeats can carve up the subject, and all have to be tested
1844     before failure can be reported.
1845 nigel 75 .P
1846 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1847     from the outermost level of the recursion at which the subpattern value is set.
1848     If you want to obtain intermediate values, a callout function can be used (see
1849 nigel 93 below and the
1850 nigel 63 .\" HREF
1851 nigel 75 \fBpcrecallout\fP
1852 nigel 63 .\"
1853     documentation). If the pattern above is matched against
1854 nigel 75 .sp
1855 nigel 63 (ab(cd)ef)
1856 nigel 75 .sp
1857 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1858     on at the top level. If additional parentheses are added, giving
1859 nigel 75 .sp
1860     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1861 nigel 63 ^ ^
1862     ^ ^
1863 nigel 75 .sp
1864 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1865     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1866     has to obtain extra memory to store data during a recursion, which it does by
1867 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1868 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1869 nigel 75 .P
1870 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
1871     Consider this pattern, which matches text in angle brackets, allowing for
1872     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1873     recursing), whereas any characters are permitted at the outer level.
1874 nigel 75 .sp
1875     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
1876     .sp
1877 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1878     different alternatives for the recursive and non-recursive cases. The (?R) item
1879     is the actual recursive call.
1880 nigel 75 .
1881     .
1882 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
1883 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
1884 nigel 63 .rs
1885     .sp
1886     If the syntax for a recursive subpattern reference (either by number or by
1887     name) is used outside the parentheses to which it refers, it operates like a
1888 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
1889 ph10 166 before or after the reference. A numbered reference can be absolute or
1890     relative, as in these examples:
1891 nigel 75 .sp
1892 ph10 166 (...(absolute)...)...(?2)...
1893     (...(relative)...)...(?-1)...
1894 ph10 172 (...(?+1)...(relative)...
1895 ph10 166 .sp
1896     An earlier example pointed out that the pattern
1897     .sp
1898 nigel 75 (sens|respons)e and \e1ibility
1899     .sp
1900 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1901     "sense and responsibility". If instead the pattern
1902 nigel 75 .sp
1903 nigel 63 (sens|respons)e and (?1)ibility
1904 nigel 75 .sp
1905 nigel 63 is used, it does match "sense and responsibility" as well as the other two
1906 nigel 93 strings. Another example is given in the discussion of DEFINE above.
1907 nigel 87 .P
1908     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1909     group. That is, once it has matched some of the subject string, it is never
1910     re-entered, even if it contains untried alternatives and there is a subsequent
1911     matching failure.
1912 nigel 93 .P
1913     When a subpattern is used as a subroutine, processing options such as
1914     case-independence are fixed when the subpattern is defined. They cannot be
1915     changed for different calls. For example, consider this pattern:
1916     .sp
1917 ph10 166 (abc)(?i:(?-1))
1918 nigel 93 .sp
1919     It matches "abcabc". It does not match "abcABC" because the change of
1920     processing option does not affect the called subpattern.
1921 nigel 75 .
1922     .
1923 nigel 63 .SH CALLOUTS
1924     .rs
1925     .sp
1926     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
1927     code to be obeyed in the middle of matching a regular expression. This makes it
1928     possible, amongst other things, to extract different substrings that match the
1929     same pair of parentheses when there is a repetition.
1930 nigel 75 .P
1931 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1932     code. The feature is called "callout". The caller of PCRE provides an external
1933 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
1934 nigel 63 By default, this variable contains NULL, which disables all calling out.
1935 nigel 75 .P
1936 nigel 63 Within a regular expression, (?C) indicates the points at which the external
1937     function is to be called. If you want to identify different callout points, you
1938     can put a number less than 256 after the letter C. The default value is zero.
1939     For example, this pattern has two callout points:
1940 nigel 75 .sp
1941 ph10 155 (?C1)abc(?C2)def
1942 nigel 75 .sp
1943     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1944     automatically installed before each item in the pattern. They are all numbered
1945     255.
1946     .P
1947     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1948 nigel 63 set), the external function is called. It is provided with the number of the
1949 nigel 75 callout, the position in the pattern, and, optionally, one item of data
1950     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
1951     may cause matching to proceed, to backtrack, or to fail altogether. A complete
1952     description of the interface to the callout function is given in the
1953 nigel 63 .\" HREF
1954 nigel 75 \fBpcrecallout\fP
1955 nigel 63 .\"
1956     documentation.
1957 nigel 93 .
1958     .
1959     .SH "SEE ALSO"
1960     .rs
1961     .sp
1962     \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1963 ph10 99 .
1964     .
1965     .SH AUTHOR
1966     .rs
1967     .sp
1968     .nf
1969     Philip Hazel
1970     University Computing Service
1971     Cambridge CB2 3QH, England.
1972     .fi
1973     .
1974     .
1975     .SH REVISION
1976     .rs
1977     .sp
1978     .nf
1979 ph10 184 Last updated: 19 June 2007
1980 ph10 99 Copyright (c) 1997-2007 University of Cambridge.
1981     .fi

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