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1 nigel 79 .TH PCREPATTERN 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 nigel 63 .rs
6     .sp
7 ph10 208 The syntax and semantics of the regular expressions that are supported by PCRE
8     are described in detail below. There is a quick-reference syntax summary in the
9     .\" HREF
10     \fBpcresyntax\fP
11     .\"
12     page. Perl's regular expressions are described in its own documentation, and
13     regular expressions in general are covered in a number of books, some of which
14     have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15     published by O'Reilly, covers regular expressions in great detail. This
16     description of PCRE's regular expressions is intended as reference material.
17 nigel 75 .P
18     The original operation of PCRE was on strings of one-byte characters. However,
19     there is now also support for UTF-8 character strings. To use this, you must
20     build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
21     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
22     places below. There is also a summary of UTF-8 features in the
23 nigel 63 .\" HTML <a href="pcre.html#utf8support">
24     .\" </a>
25     section on UTF-8 support
26     .\"
27     in the main
28     .\" HREF
29 nigel 75 \fBpcre\fP
30 nigel 63 .\"
31     page.
32 nigel 75 .P
33 nigel 77 The remainder of this document discusses the patterns that are supported by
34     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
35     From release 6.0, PCRE offers a second matching function,
36     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
37 ph10 172 Perl-compatible. Some of the features discussed below are not available when
38 ph10 168 \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
39     alternative function, and how it differs from the normal function, are
40     discussed in the
41 nigel 77 .\" HREF
42     \fBpcrematching\fP
43     .\"
44     page.
45 nigel 93 .
46     .
47     .SH "CHARACTERS AND METACHARACTERS"
48     .rs
49     .sp
50 nigel 63 A regular expression is a pattern that is matched against a subject string from
51     left to right. Most characters stand for themselves in a pattern, and match the
52     corresponding characters in the subject. As a trivial example, the pattern
53 nigel 75 .sp
54 nigel 63 The quick brown fox
55 nigel 75 .sp
56 nigel 77 matches a portion of a subject string that is identical to itself. When
57     caseless matching is specified (the PCRE_CASELESS option), letters are matched
58     independently of case. In UTF-8 mode, PCRE always understands the concept of
59     case for characters whose values are less than 128, so caseless matching is
60     always possible. For characters with higher values, the concept of case is
61     supported if PCRE is compiled with Unicode property support, but not otherwise.
62     If you want to use caseless matching for characters 128 and above, you must
63     ensure that PCRE is compiled with Unicode property support as well as with
64     UTF-8 support.
65     .P
66     The power of regular expressions comes from the ability to include alternatives
67     and repetitions in the pattern. These are encoded in the pattern by the use of
68 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
69 nigel 63 interpreted in some special way.
70 nigel 75 .P
71     There are two different sets of metacharacters: those that are recognized
72 nigel 63 anywhere in the pattern except within square brackets, and those that are
73 nigel 93 recognized within square brackets. Outside square brackets, the metacharacters
74     are as follows:
75 nigel 75 .sp
76     \e general escape character with several uses
77 nigel 63 ^ assert start of string (or line, in multiline mode)
78     $ assert end of string (or line, in multiline mode)
79     . match any character except newline (by default)
80     [ start character class definition
81     | start of alternative branch
82     ( start subpattern
83     ) end subpattern
84     ? extends the meaning of (
85     also 0 or 1 quantifier
86     also quantifier minimizer
87     * 0 or more quantifier
88     + 1 or more quantifier
89     also "possessive quantifier"
90     { start min/max quantifier
91 nigel 75 .sp
92 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
93 nigel 75 a character class the only metacharacters are:
94     .sp
95     \e general escape character
96 nigel 63 ^ negate the class, but only if the first character
97     - indicates character range
98 nigel 75 .\" JOIN
99 nigel 63 [ POSIX character class (only if followed by POSIX
100     syntax)
101     ] terminates the character class
102 nigel 75 .sp
103     The following sections describe the use of each of the metacharacters.
104     .
105 nigel 93 .
106 nigel 63 .SH BACKSLASH
107     .rs
108     .sp
109     The backslash character has several uses. Firstly, if it is followed by a
110 nigel 91 non-alphanumeric character, it takes away any special meaning that character
111     may have. This use of backslash as an escape character applies both inside and
112 nigel 63 outside character classes.
113 nigel 75 .P
114     For example, if you want to match a * character, you write \e* in the pattern.
115 nigel 63 This escaping action applies whether or not the following character would
116 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
117     non-alphanumeric with backslash to specify that it stands for itself. In
118     particular, if you want to match a backslash, you write \e\e.
119     .P
120 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
121     pattern (other than in a character class) and characters between a # outside
122 nigel 91 a character class and the next newline are ignored. An escaping backslash can
123     be used to include a whitespace or # character as part of the pattern.
124 nigel 75 .P
125 nigel 63 If you want to remove the special meaning from a sequence of characters, you
126 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
127     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
128 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
129 nigel 75 .sp
130 nigel 63 Pattern PCRE matches Perl matches
131 nigel 75 .sp
132     .\" JOIN
133     \eQabc$xyz\eE abc$xyz abc followed by the
134 nigel 63 contents of $xyz
135 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
136     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
137     .sp
138     The \eQ...\eE sequence is recognized both inside and outside character classes.
139     .
140     .
141     .\" HTML <a name="digitsafterbackslash"></a>
142     .SS "Non-printing characters"
143     .rs
144     .sp
145 nigel 63 A second use of backslash provides a way of encoding non-printing characters
146     in patterns in a visible manner. There is no restriction on the appearance of
147     non-printing characters, apart from the binary zero that terminates a pattern,
148     but when a pattern is being prepared by text editing, it is usually easier to
149     use one of the following escape sequences than the binary character it
150     represents:
151 nigel 75 .sp
152     \ea alarm, that is, the BEL character (hex 07)
153     \ecx "control-x", where x is any character
154     \ee escape (hex 1B)
155     \ef formfeed (hex 0C)
156     \en newline (hex 0A)
157     \er carriage return (hex 0D)
158     \et tab (hex 09)
159     \eddd character with octal code ddd, or backreference
160     \exhh character with hex code hh
161 nigel 87 \ex{hhh..} character with hex code hhh..
162 nigel 75 .sp
163     The precise effect of \ecx is as follows: if x is a lower case letter, it
164 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
165 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
166 nigel 63 7B.
167 nigel 75 .P
168     After \ex, from zero to two hexadecimal digits are read (letters can be in
169 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
170     and }, but the value of the character code must be less than 256 in non-UTF-8
171 ph10 211 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
172     hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
173     point, which is 10FFFF.
174 nigel 75 .P
175 ph10 211 If characters other than hexadecimal digits appear between \ex{ and }, or if
176     there is no terminating }, this form of escape is not recognized. Instead, the
177     initial \ex will be interpreted as a basic hexadecimal escape, with no
178     following digits, giving a character whose value is zero.
179     .P
180 nigel 63 Characters whose value is less than 256 can be defined by either of the two
181 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
182     example, \exdc is exactly the same as \ex{dc}.
183 nigel 75 .P
184 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
185     digits, just those that are present are used. Thus the sequence \e0\ex\e07
186     specifies two binary zeros followed by a BEL character (code value 7). Make
187     sure you supply two digits after the initial zero if the pattern character that
188     follows is itself an octal digit.
189 nigel 75 .P
190 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
191     Outside a character class, PCRE reads it and any following digits as a decimal
192     number. If the number is less than 10, or if there have been at least that many
193     previous capturing left parentheses in the expression, the entire sequence is
194 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
195     .\" HTML <a href="#backreferences">
196     .\" </a>
197     later,
198     .\"
199     following the discussion of
200     .\" HTML <a href="#subpattern">
201     .\" </a>
202     parenthesized subpatterns.
203     .\"
204     .P
205 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
206     have not been that many capturing subpatterns, PCRE re-reads up to three octal
207 nigel 93 digits following the backslash, and uses them to generate a data character. Any
208 nigel 91 subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
209     character specified in octal must be less than \e400. In UTF-8 mode, values up
210     to \e777 are permitted. For example:
211 nigel 75 .sp
212     \e040 is another way of writing a space
213     .\" JOIN
214     \e40 is the same, provided there are fewer than 40
215 nigel 63 previous capturing subpatterns
216 nigel 75 \e7 is always a back reference
217     .\" JOIN
218     \e11 might be a back reference, or another way of
219 nigel 63 writing a tab
220 nigel 75 \e011 is always a tab
221     \e0113 is a tab followed by the character "3"
222     .\" JOIN
223     \e113 might be a back reference, otherwise the
224 nigel 63 character with octal code 113
225 nigel 75 .\" JOIN
226     \e377 might be a back reference, otherwise
227 nigel 63 the byte consisting entirely of 1 bits
228 nigel 75 .\" JOIN
229     \e81 is either a back reference, or a binary zero
230 nigel 63 followed by the two characters "8" and "1"
231 nigel 75 .sp
232 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
233     zero, because no more than three octal digits are ever read.
234 nigel 75 .P
235 nigel 91 All the sequences that define a single character value can be used both inside
236     and outside character classes. In addition, inside a character class, the
237     sequence \eb is interpreted as the backspace character (hex 08), and the
238 nigel 93 sequences \eR and \eX are interpreted as the characters "R" and "X",
239     respectively. Outside a character class, these sequences have different
240     meanings
241 nigel 75 .\" HTML <a href="#uniextseq">
242     .\" </a>
243     (see below).
244     .\"
245     .
246     .
247 nigel 93 .SS "Absolute and relative back references"
248     .rs
249     .sp
250 ph10 208 The sequence \eg followed by an unsigned or a negative number, optionally
251     enclosed in braces, is an absolute or relative back reference. A named back
252     reference can be coded as \eg{name}. Back references are discussed
253 nigel 93 .\" HTML <a href="#backreferences">
254     .\" </a>
255     later,
256     .\"
257     following the discussion of
258     .\" HTML <a href="#subpattern">
259     .\" </a>
260     parenthesized subpatterns.
261     .\"
262     .
263     .
264 nigel 75 .SS "Generic character types"
265     .rs
266     .sp
267 nigel 93 Another use of backslash is for specifying generic character types. The
268 nigel 75 following are always recognized:
269     .sp
270 ph10 182 \ed any decimal digit
271 nigel 75 \eD any character that is not a decimal digit
272 ph10 178 \eh any horizontal whitespace character
273 ph10 182 \eH any character that is not a horizontal whitespace character
274 nigel 75 \es any whitespace character
275     \eS any character that is not a whitespace character
276 ph10 178 \ev any vertical whitespace character
277 ph10 182 \eV any character that is not a vertical whitespace character
278 nigel 75 \ew any "word" character
279     \eW any "non-word" character
280     .sp
281 nigel 63 Each pair of escape sequences partitions the complete set of characters into
282     two disjoint sets. Any given character matches one, and only one, of each pair.
283 nigel 75 .P
284     These character type sequences can appear both inside and outside character
285     classes. They each match one character of the appropriate type. If the current
286     matching point is at the end of the subject string, all of them fail, since
287     there is no character to match.
288     .P
289     For compatibility with Perl, \es does not match the VT character (code 11).
290     This makes it different from the the POSIX "space" class. The \es characters
291 ph10 178 are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
292 nigel 91 included in a Perl script, \es may match the VT character. In PCRE, it never
293 ph10 178 does.
294 nigel 75 .P
295 ph10 178 In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
296     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
297 ph10 182 character property support is available. These sequences retain their original
298     meanings from before UTF-8 support was available, mainly for efficiency
299 ph10 178 reasons.
300     .P
301 ph10 182 The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
302 ph10 178 other sequences, these do match certain high-valued codepoints in UTF-8 mode.
303     The horizontal space characters are:
304     .sp
305     U+0009 Horizontal tab
306     U+0020 Space
307     U+00A0 Non-break space
308     U+1680 Ogham space mark
309     U+180E Mongolian vowel separator
310     U+2000 En quad
311     U+2001 Em quad
312     U+2002 En space
313     U+2003 Em space
314     U+2004 Three-per-em space
315     U+2005 Four-per-em space
316     U+2006 Six-per-em space
317     U+2007 Figure space
318     U+2008 Punctuation space
319     U+2009 Thin space
320     U+200A Hair space
321     U+202F Narrow no-break space
322     U+205F Medium mathematical space
323     U+3000 Ideographic space
324     .sp
325     The vertical space characters are:
326     .sp
327     U+000A Linefeed
328     U+000B Vertical tab
329     U+000C Formfeed
330     U+000D Carriage return
331     U+0085 Next line
332     U+2028 Line separator
333     U+2029 Paragraph separator
334     .P
335 nigel 75 A "word" character is an underscore or any character less than 256 that is a
336     letter or digit. The definition of letters and digits is controlled by PCRE's
337     low-valued character tables, and may vary if locale-specific matching is taking
338     place (see
339 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
340     .\" </a>
341     "Locale support"
342     .\"
343     in the
344     .\" HREF
345 nigel 75 \fBpcreapi\fP
346 nigel 63 .\"
347 ph10 139 page). For example, in a French locale such as "fr_FR" in Unix-like systems,
348     or "french" in Windows, some character codes greater than 128 are used for
349 ph10 178 accented letters, and these are matched by \ew. The use of locales with Unicode
350     is discouraged.
351 nigel 75 .
352     .
353 nigel 93 .SS "Newline sequences"
354     .rs
355     .sp
356     Outside a character class, the escape sequence \eR matches any Unicode newline
357 ph10 178 sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
358 nigel 93 the following:
359     .sp
360     (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
361     .sp
362     This is an example of an "atomic group", details of which are given
363     .\" HTML <a href="#atomicgroup">
364     .\" </a>
365     below.
366     .\"
367     This particular group matches either the two-character sequence CR followed by
368     LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
369     U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
370     line, U+0085). The two-character sequence is treated as a single unit that
371     cannot be split.
372     .P
373     In UTF-8 mode, two additional characters whose codepoints are greater than 255
374     are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
375     Unicode character property support is not needed for these characters to be
376     recognized.
377     .P
378     Inside a character class, \eR matches the letter "R".
379     .
380     .
381 nigel 75 .\" HTML <a name="uniextseq"></a>
382     .SS Unicode character properties
383     .rs
384     .sp
385     When PCRE is built with Unicode character property support, three additional
386 ph10 184 escape sequences that match characters with specific properties are available.
387     When not in UTF-8 mode, these sequences are of course limited to testing
388     characters whose codepoints are less than 256, but they do work in this mode.
389     The extra escape sequences are:
390 nigel 75 .sp
391 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
392     \eP{\fIxx\fP} a character without the \fIxx\fP property
393     \eX an extended Unicode sequence
394 nigel 75 .sp
395 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
396     script names, the general category properties, and "Any", which matches any
397     character (including newline). Other properties such as "InMusicalSymbols" are
398     not currently supported by PCRE. Note that \eP{Any} does not match any
399     characters, so always causes a match failure.
400 nigel 75 .P
401 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
402     character from one of these sets can be matched using a script name. For
403     example:
404 nigel 75 .sp
405 nigel 87 \ep{Greek}
406     \eP{Han}
407     .sp
408     Those that are not part of an identified script are lumped together as
409     "Common". The current list of scripts is:
410     .P
411     Arabic,
412     Armenian,
413 nigel 93 Balinese,
414 nigel 87 Bengali,
415     Bopomofo,
416     Braille,
417     Buginese,
418     Buhid,
419     Canadian_Aboriginal,
420     Cherokee,
421     Common,
422     Coptic,
423 nigel 93 Cuneiform,
424 nigel 87 Cypriot,
425     Cyrillic,
426     Deseret,
427     Devanagari,
428     Ethiopic,
429     Georgian,
430     Glagolitic,
431     Gothic,
432     Greek,
433     Gujarati,
434     Gurmukhi,
435     Han,
436     Hangul,
437     Hanunoo,
438     Hebrew,
439     Hiragana,
440     Inherited,
441     Kannada,
442     Katakana,
443     Kharoshthi,
444     Khmer,
445     Lao,
446     Latin,
447     Limbu,
448     Linear_B,
449     Malayalam,
450     Mongolian,
451     Myanmar,
452     New_Tai_Lue,
453 nigel 93 Nko,
454 nigel 87 Ogham,
455     Old_Italic,
456     Old_Persian,
457     Oriya,
458     Osmanya,
459 nigel 93 Phags_Pa,
460     Phoenician,
461 nigel 87 Runic,
462     Shavian,
463     Sinhala,
464     Syloti_Nagri,
465     Syriac,
466     Tagalog,
467     Tagbanwa,
468     Tai_Le,
469     Tamil,
470     Telugu,
471     Thaana,
472     Thai,
473     Tibetan,
474     Tifinagh,
475     Ugaritic,
476     Yi.
477     .P
478     Each character has exactly one general category property, specified by a
479     two-letter abbreviation. For compatibility with Perl, negation can be specified
480     by including a circumflex between the opening brace and the property name. For
481     example, \ep{^Lu} is the same as \eP{Lu}.
482     .P
483     If only one letter is specified with \ep or \eP, it includes all the general
484     category properties that start with that letter. In this case, in the absence
485     of negation, the curly brackets in the escape sequence are optional; these two
486     examples have the same effect:
487     .sp
488 nigel 75 \ep{L}
489     \epL
490     .sp
491 nigel 87 The following general category property codes are supported:
492 nigel 75 .sp
493     C Other
494     Cc Control
495     Cf Format
496     Cn Unassigned
497     Co Private use
498     Cs Surrogate
499     .sp
500     L Letter
501     Ll Lower case letter
502     Lm Modifier letter
503     Lo Other letter
504     Lt Title case letter
505     Lu Upper case letter
506     .sp
507     M Mark
508     Mc Spacing mark
509     Me Enclosing mark
510     Mn Non-spacing mark
511     .sp
512     N Number
513     Nd Decimal number
514     Nl Letter number
515     No Other number
516     .sp
517     P Punctuation
518     Pc Connector punctuation
519     Pd Dash punctuation
520     Pe Close punctuation
521     Pf Final punctuation
522     Pi Initial punctuation
523     Po Other punctuation
524     Ps Open punctuation
525     .sp
526     S Symbol
527     Sc Currency symbol
528     Sk Modifier symbol
529     Sm Mathematical symbol
530     So Other symbol
531     .sp
532     Z Separator
533     Zl Line separator
534     Zp Paragraph separator
535     Zs Space separator
536     .sp
537 nigel 87 The special property L& is also supported: it matches a character that has
538     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
539     a modifier or "other".
540 nigel 75 .P
541 ph10 211 The Cs (Surrogate) property applies only to characters in the range U+D800 to
542     U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
543     cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
544     (see the discussion of PCRE_NO_UTF8_CHECK in the
545     .\" HREF
546     \fBpcreapi\fP
547     .\"
548     page).
549     .P
550 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter})
551 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
552 nigel 87 properties with "Is".
553     .P
554     No character that is in the Unicode table has the Cn (unassigned) property.
555     Instead, this property is assumed for any code point that is not in the
556     Unicode table.
557     .P
558 nigel 75 Specifying caseless matching does not affect these escape sequences. For
559     example, \ep{Lu} always matches only upper case letters.
560     .P
561     The \eX escape matches any number of Unicode characters that form an extended
562     Unicode sequence. \eX is equivalent to
563     .sp
564     (?>\ePM\epM*)
565     .sp
566     That is, it matches a character without the "mark" property, followed by zero
567     or more characters with the "mark" property, and treats the sequence as an
568     atomic group
569     .\" HTML <a href="#atomicgroup">
570     .\" </a>
571     (see below).
572     .\"
573     Characters with the "mark" property are typically accents that affect the
574 ph10 185 preceding character. None of them have codepoints less than 256, so in
575 ph10 184 non-UTF-8 mode \eX matches any one character.
576 nigel 75 .P
577     Matching characters by Unicode property is not fast, because PCRE has to search
578     a structure that contains data for over fifteen thousand characters. That is
579     why the traditional escape sequences such as \ed and \ew do not use Unicode
580     properties in PCRE.
581     .
582     .
583 ph10 168 .\" HTML <a name="resetmatchstart"></a>
584     .SS "Resetting the match start"
585     .rs
586     .sp
587 ph10 172 The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
588     matched characters not to be included in the final matched sequence. For
589 ph10 168 example, the pattern:
590     .sp
591     foo\eKbar
592     .sp
593 ph10 172 matches "foobar", but reports that it has matched "bar". This feature is
594 ph10 168 similar to a lookbehind assertion
595     .\" HTML <a href="#lookbehind">
596     .\" </a>
597     (described below).
598     .\"
599 ph10 172 However, in this case, the part of the subject before the real match does not
600     have to be of fixed length, as lookbehind assertions do. The use of \eK does
601 ph10 168 not interfere with the setting of
602     .\" HTML <a href="#subpattern">
603     .\" </a>
604     captured substrings.
605 ph10 172 .\"
606 ph10 168 For example, when the pattern
607     .sp
608     (foo)\eKbar
609     .sp
610 ph10 172 matches "foobar", the first substring is still set to "foo".
611 ph10 168 .
612     .
613 nigel 75 .\" HTML <a name="smallassertions"></a>
614     .SS "Simple assertions"
615     .rs
616     .sp
617 nigel 93 The final use of backslash is for certain simple assertions. An assertion
618 nigel 63 specifies a condition that has to be met at a particular point in a match,
619     without consuming any characters from the subject string. The use of
620 nigel 75 subpatterns for more complicated assertions is described
621     .\" HTML <a href="#bigassertions">
622     .\" </a>
623     below.
624     .\"
625 nigel 91 The backslashed assertions are:
626 nigel 75 .sp
627     \eb matches at a word boundary
628     \eB matches when not at a word boundary
629 nigel 93 \eA matches at the start of the subject
630     \eZ matches at the end of the subject
631     also matches before a newline at the end of the subject
632     \ez matches only at the end of the subject
633     \eG matches at the first matching position in the subject
634 nigel 75 .sp
635     These assertions may not appear in character classes (but note that \eb has a
636 nigel 63 different meaning, namely the backspace character, inside a character class).
637 nigel 75 .P
638 nigel 63 A word boundary is a position in the subject string where the current character
639 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
640     \ew and the other matches \eW), or the start or end of the string if the
641     first or last character matches \ew, respectively.
642     .P
643     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
644     dollar (described in the next section) in that they only ever match at the very
645     start and end of the subject string, whatever options are set. Thus, they are
646     independent of multiline mode. These three assertions are not affected by the
647     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
648     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
649     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
650     at a point other than the beginning of the subject, \eA can never match. The
651 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
652     of the string as well as at the very end, whereas \ez matches only at the end.
653 nigel 75 .P
654     The \eG assertion is true only when the current matching position is at the
655     start point of the match, as specified by the \fIstartoffset\fP argument of
656     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
657     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
658 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
659 nigel 75 implementation where \eG can be useful.
660     .P
661     Note, however, that PCRE's interpretation of \eG, as the start of the current
662 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
663     previous match. In Perl, these can be different when the previously matched
664     string was empty. Because PCRE does just one match at a time, it cannot
665     reproduce this behaviour.
666 nigel 75 .P
667     If all the alternatives of a pattern begin with \eG, the expression is anchored
668 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
669     regular expression.
670 nigel 75 .
671     .
672     .SH "CIRCUMFLEX AND DOLLAR"
673 nigel 63 .rs
674     .sp
675     Outside a character class, in the default matching mode, the circumflex
676 nigel 75 character is an assertion that is true only if the current matching point is
677     at the start of the subject string. If the \fIstartoffset\fP argument of
678     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
679 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
680 nigel 75 meaning
681     .\" HTML <a href="#characterclass">
682     .\" </a>
683     (see below).
684     .\"
685     .P
686 nigel 63 Circumflex need not be the first character of the pattern if a number of
687     alternatives are involved, but it should be the first thing in each alternative
688     in which it appears if the pattern is ever to match that branch. If all
689     possible alternatives start with a circumflex, that is, if the pattern is
690     constrained to match only at the start of the subject, it is said to be an
691     "anchored" pattern. (There are also other constructs that can cause a pattern
692     to be anchored.)
693 nigel 75 .P
694     A dollar character is an assertion that is true only if the current matching
695 nigel 63 point is at the end of the subject string, or immediately before a newline
696 nigel 91 at the end of the string (by default). Dollar need not be the last character of
697     the pattern if a number of alternatives are involved, but it should be the last
698     item in any branch in which it appears. Dollar has no special meaning in a
699     character class.
700 nigel 75 .P
701 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
702     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
703 nigel 75 does not affect the \eZ assertion.
704     .P
705 nigel 63 The meanings of the circumflex and dollar characters are changed if the
706 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
707     immediately after internal newlines as well as at the start of the subject
708     string. It does not match after a newline that ends the string. A dollar
709     matches before any newlines in the string, as well as at the very end, when
710     PCRE_MULTILINE is set. When newline is specified as the two-character
711     sequence CRLF, isolated CR and LF characters do not indicate newlines.
712 nigel 75 .P
713 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
714     \en represents a newline) in multiline mode, but not otherwise. Consequently,
715     patterns that are anchored in single line mode because all branches start with
716     ^ are not anchored in multiline mode, and a match for circumflex is possible
717     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
718     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
719     .P
720 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
721 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
722 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
723 nigel 75 .
724     .
725     .SH "FULL STOP (PERIOD, DOT)"
726 nigel 63 .rs
727     .sp
728     Outside a character class, a dot in the pattern matches any one character in
729 nigel 91 the subject string except (by default) a character that signifies the end of a
730 nigel 93 line. In UTF-8 mode, the matched character may be more than one byte long.
731 nigel 91 .P
732 nigel 93 When a line ending is defined as a single character, dot never matches that
733     character; when the two-character sequence CRLF is used, dot does not match CR
734     if it is immediately followed by LF, but otherwise it matches all characters
735     (including isolated CRs and LFs). When any Unicode line endings are being
736     recognized, dot does not match CR or LF or any of the other line ending
737     characters.
738     .P
739 nigel 91 The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
740 nigel 93 option is set, a dot matches any one character, without exception. If the
741     two-character sequence CRLF is present in the subject string, it takes two dots
742     to match it.
743 nigel 91 .P
744     The handling of dot is entirely independent of the handling of circumflex and
745     dollar, the only relationship being that they both involve newlines. Dot has no
746     special meaning in a character class.
747 nigel 75 .
748     .
749     .SH "MATCHING A SINGLE BYTE"
750 nigel 63 .rs
751     .sp
752 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
753 nigel 93 in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
754     characters. The feature is provided in Perl in order to match individual bytes
755     in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
756     what remains in the string may be a malformed UTF-8 string. For this reason,
757     the \eC escape sequence is best avoided.
758 nigel 75 .P
759     PCRE does not allow \eC to appear in lookbehind assertions
760     .\" HTML <a href="#lookbehind">
761     .\" </a>
762     (described below),
763     .\"
764     because in UTF-8 mode this would make it impossible to calculate the length of
765     the lookbehind.
766     .
767     .
768     .\" HTML <a name="characterclass"></a>
769     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
770 nigel 63 .rs
771     .sp
772     An opening square bracket introduces a character class, terminated by a closing
773     square bracket. A closing square bracket on its own is not special. If a
774     closing square bracket is required as a member of the class, it should be the
775     first data character in the class (after an initial circumflex, if present) or
776     escaped with a backslash.
777 nigel 75 .P
778 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
779     character may occupy more than one byte. A matched character must be in the set
780     of characters defined by the class, unless the first character in the class
781     definition is a circumflex, in which case the subject character must not be in
782     the set defined by the class. If a circumflex is actually required as a member
783     of the class, ensure it is not the first character, or escape it with a
784     backslash.
785 nigel 75 .P
786 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
787     [^aeiou] matches any character that is not a lower case vowel. Note that a
788 nigel 75 circumflex is just a convenient notation for specifying the characters that
789     are in the class by enumerating those that are not. A class that starts with a
790     circumflex is not an assertion: it still consumes a character from the subject
791     string, and therefore it fails if the current pointer is at the end of the
792     string.
793     .P
794 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
795 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
796     .P
797 nigel 63 When caseless matching is set, any letters in a class represent both their
798     upper case and lower case versions, so for example, a caseless [aeiou] matches
799     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
800 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
801     case for characters whose values are less than 128, so caseless matching is
802     always possible. For characters with higher values, the concept of case is
803     supported if PCRE is compiled with Unicode property support, but not otherwise.
804     If you want to use caseless matching for characters 128 and above, you must
805     ensure that PCRE is compiled with Unicode property support as well as with
806     UTF-8 support.
807 nigel 75 .P
808 nigel 93 Characters that might indicate line breaks are never treated in any special way
809     when matching character classes, whatever line-ending sequence is in use, and
810     whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
811     such as [^a] always matches one of these characters.
812 nigel 75 .P
813 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
814     character class. For example, [d-m] matches any letter between d and m,
815     inclusive. If a minus character is required in a class, it must be escaped with
816     a backslash or appear in a position where it cannot be interpreted as
817     indicating a range, typically as the first or last character in the class.
818 nigel 75 .P
819 nigel 63 It is not possible to have the literal character "]" as the end character of a
820     range. A pattern such as [W-]46] is interpreted as a class of two characters
821     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
822     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
823 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
824     followed by two other characters. The octal or hexadecimal representation of
825     "]" can also be used to end a range.
826     .P
827 nigel 63 Ranges operate in the collating sequence of character values. They can also be
828 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
829 nigel 63 mode, ranges can include characters whose values are greater than 255, for
830 nigel 75 example [\ex{100}-\ex{2ff}].
831     .P
832 nigel 63 If a range that includes letters is used when caseless matching is set, it
833     matches the letters in either case. For example, [W-c] is equivalent to
834 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
835 ph10 139 tables for a French locale are in use, [\exc8-\excb] matches accented E
836 nigel 75 characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
837     characters with values greater than 128 only when it is compiled with Unicode
838     property support.
839     .P
840     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
841     in a character class, and add the characters that they match to the class. For
842     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
843 nigel 63 conveniently be used with the upper case character types to specify a more
844     restricted set of characters than the matching lower case type. For example,
845 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
846     .P
847     The only metacharacters that are recognized in character classes are backslash,
848     hyphen (only where it can be interpreted as specifying a range), circumflex
849     (only at the start), opening square bracket (only when it can be interpreted as
850     introducing a POSIX class name - see the next section), and the terminating
851     closing square bracket. However, escaping other non-alphanumeric characters
852     does no harm.
853     .
854     .
855     .SH "POSIX CHARACTER CLASSES"
856 nigel 63 .rs
857     .sp
858 nigel 75 Perl supports the POSIX notation for character classes. This uses names
859 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
860     this notation. For example,
861 nigel 75 .sp
862 nigel 63 [01[:alpha:]%]
863 nigel 75 .sp
864 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
865     are
866 nigel 75 .sp
867 nigel 63 alnum letters and digits
868     alpha letters
869     ascii character codes 0 - 127
870     blank space or tab only
871     cntrl control characters
872 nigel 75 digit decimal digits (same as \ed)
873 nigel 63 graph printing characters, excluding space
874     lower lower case letters
875     print printing characters, including space
876     punct printing characters, excluding letters and digits
877 nigel 75 space white space (not quite the same as \es)
878 nigel 63 upper upper case letters
879 nigel 75 word "word" characters (same as \ew)
880 nigel 63 xdigit hexadecimal digits
881 nigel 75 .sp
882 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
883     space (32). Notice that this list includes the VT character (code 11). This
884 nigel 75 makes "space" different to \es, which does not include VT (for Perl
885 nigel 63 compatibility).
886 nigel 75 .P
887 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
888     5.8. Another Perl extension is negation, which is indicated by a ^ character
889     after the colon. For example,
890 nigel 75 .sp
891 nigel 63 [12[:^digit:]]
892 nigel 75 .sp
893 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
894     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
895     supported, and an error is given if they are encountered.
896 nigel 75 .P
897     In UTF-8 mode, characters with values greater than 128 do not match any of
898 nigel 63 the POSIX character classes.
899 nigel 75 .
900     .
901     .SH "VERTICAL BAR"
902 nigel 63 .rs
903     .sp
904     Vertical bar characters are used to separate alternative patterns. For example,
905     the pattern
906 nigel 75 .sp
907 nigel 63 gilbert|sullivan
908 nigel 75 .sp
909 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
910 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
911     process tries each alternative in turn, from left to right, and the first one
912     that succeeds is used. If the alternatives are within a subpattern
913 nigel 75 .\" HTML <a href="#subpattern">
914     .\" </a>
915     (defined below),
916     .\"
917     "succeeds" means matching the rest of the main pattern as well as the
918     alternative in the subpattern.
919     .
920     .
921     .SH "INTERNAL OPTION SETTING"
922 nigel 63 .rs
923     .sp
924     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
925     PCRE_EXTENDED options can be changed from within the pattern by a sequence of
926     Perl option letters enclosed between "(?" and ")". The option letters are
927 nigel 75 .sp
928 nigel 63 i for PCRE_CASELESS
929     m for PCRE_MULTILINE
930     s for PCRE_DOTALL
931     x for PCRE_EXTENDED
932 nigel 75 .sp
933 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
934     unset these options by preceding the letter with a hyphen, and a combined
935     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
936     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
937     permitted. If a letter appears both before and after the hyphen, the option is
938     unset.
939 nigel 75 .P
940 nigel 63 When an option change occurs at top level (that is, not inside subpattern
941     parentheses), the change applies to the remainder of the pattern that follows.
942     If the change is placed right at the start of a pattern, PCRE extracts it into
943     the global options (and it will therefore show up in data extracted by the
944 nigel 75 \fBpcre_fullinfo()\fP function).
945     .P
946 nigel 93 An option change within a subpattern (see below for a description of
947     subpatterns) affects only that part of the current pattern that follows it, so
948 nigel 75 .sp
949 nigel 63 (a(?i)b)c
950 nigel 75 .sp
951 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
952     By this means, options can be made to have different settings in different
953     parts of the pattern. Any changes made in one alternative do carry on
954     into subsequent branches within the same subpattern. For example,
955 nigel 75 .sp
956 nigel 63 (a(?i)b|c)
957 nigel 75 .sp
958 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
959     branch is abandoned before the option setting. This is because the effects of
960     option settings happen at compile time. There would be some very weird
961     behaviour otherwise.
962 nigel 75 .P
963 nigel 91 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
964     changed in the same way as the Perl-compatible options by using the characters
965     J, U and X respectively.
966 nigel 75 .
967     .
968     .\" HTML <a name="subpattern"></a>
969 nigel 63 .SH SUBPATTERNS
970     .rs
971     .sp
972     Subpatterns are delimited by parentheses (round brackets), which can be nested.
973 nigel 75 Turning part of a pattern into a subpattern does two things:
974     .sp
975 nigel 63 1. It localizes a set of alternatives. For example, the pattern
976 nigel 75 .sp
977 nigel 63 cat(aract|erpillar|)
978 nigel 75 .sp
979 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
980 nigel 93 parentheses, it would match "cataract", "erpillar" or an empty string.
981 nigel 75 .sp
982     2. It sets up the subpattern as a capturing subpattern. This means that, when
983     the whole pattern matches, that portion of the subject string that matched the
984     subpattern is passed back to the caller via the \fIovector\fP argument of
985     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
986     from 1) to obtain numbers for the capturing subpatterns.
987     .P
988 nigel 63 For example, if the string "the red king" is matched against the pattern
989 nigel 75 .sp
990 nigel 63 the ((red|white) (king|queen))
991 nigel 75 .sp
992 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
993     2, and 3, respectively.
994 nigel 75 .P
995 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
996     There are often times when a grouping subpattern is required without a
997     capturing requirement. If an opening parenthesis is followed by a question mark
998     and a colon, the subpattern does not do any capturing, and is not counted when
999     computing the number of any subsequent capturing subpatterns. For example, if
1000     the string "the white queen" is matched against the pattern
1001 nigel 75 .sp
1002 nigel 63 the ((?:red|white) (king|queen))
1003 nigel 75 .sp
1004 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
1005 nigel 93 2. The maximum number of capturing subpatterns is 65535.
1006 nigel 75 .P
1007 nigel 63 As a convenient shorthand, if any option settings are required at the start of
1008     a non-capturing subpattern, the option letters may appear between the "?" and
1009     the ":". Thus the two patterns
1010 nigel 75 .sp
1011 nigel 63 (?i:saturday|sunday)
1012     (?:(?i)saturday|sunday)
1013 nigel 75 .sp
1014 nigel 63 match exactly the same set of strings. Because alternative branches are tried
1015     from left to right, and options are not reset until the end of the subpattern
1016     is reached, an option setting in one branch does affect subsequent branches, so
1017     the above patterns match "SUNDAY" as well as "Saturday".
1018 nigel 75 .
1019     .
1020 ph10 175 .SH "DUPLICATE SUBPATTERN NUMBERS"
1021     .rs
1022     .sp
1023 ph10 182 Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1024     the same numbers for its capturing parentheses. Such a subpattern starts with
1025     (?| and is itself a non-capturing subpattern. For example, consider this
1026 ph10 175 pattern:
1027     .sp
1028     (?|(Sat)ur|(Sun))day
1029 ph10 182 .sp
1030     Because the two alternatives are inside a (?| group, both sets of capturing
1031     parentheses are numbered one. Thus, when the pattern matches, you can look
1032     at captured substring number one, whichever alternative matched. This construct
1033     is useful when you want to capture part, but not all, of one of a number of
1034     alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1035 ph10 175 number is reset at the start of each branch. The numbers of any capturing
1036 ph10 182 buffers that follow the subpattern start after the highest number used in any
1037     branch. The following example is taken from the Perl documentation.
1038 ph10 175 The numbers underneath show in which buffer the captured content will be
1039     stored.
1040     .sp
1041     # before ---------------branch-reset----------- after
1042     / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1043     # 1 2 2 3 2 3 4
1044 ph10 182 .sp
1045 ph10 175 A backreference or a recursive call to a numbered subpattern always refers to
1046     the first one in the pattern with the given number.
1047     .P
1048     An alternative approach to using this "branch reset" feature is to use
1049     duplicate named subpatterns, as described in the next section.
1050     .
1051     .
1052 nigel 75 .SH "NAMED SUBPATTERNS"
1053 nigel 63 .rs
1054     .sp
1055     Identifying capturing parentheses by number is simple, but it can be very hard
1056     to keep track of the numbers in complicated regular expressions. Furthermore,
1057 nigel 75 if an expression is modified, the numbers may change. To help with this
1058 nigel 93 difficulty, PCRE supports the naming of subpatterns. This feature was not
1059     added to Perl until release 5.10. Python had the feature earlier, and PCRE
1060     introduced it at release 4.0, using the Python syntax. PCRE now supports both
1061     the Perl and the Python syntax.
1062     .P
1063     In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1064     (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1065 nigel 91 parentheses from other parts of the pattern, such as
1066     .\" HTML <a href="#backreferences">
1067     .\" </a>
1068     backreferences,
1069     .\"
1070     .\" HTML <a href="#recursion">
1071     .\" </a>
1072     recursion,
1073     .\"
1074     and
1075     .\" HTML <a href="#conditions">
1076     .\" </a>
1077     conditions,
1078     .\"
1079     can be made by name as well as by number.
1080 nigel 75 .P
1081 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
1082 nigel 93 capturing parentheses are still allocated numbers as well as names, exactly as
1083     if the names were not present. The PCRE API provides function calls for
1084     extracting the name-to-number translation table from a compiled pattern. There
1085     is also a convenience function for extracting a captured substring by name.
1086 nigel 91 .P
1087     By default, a name must be unique within a pattern, but it is possible to relax
1088     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1089     be useful for patterns where only one instance of the named parentheses can
1090     match. Suppose you want to match the name of a weekday, either as a 3-letter
1091     abbreviation or as the full name, and in both cases you want to extract the
1092     abbreviation. This pattern (ignoring the line breaks) does the job:
1093     .sp
1094 nigel 93 (?<DN>Mon|Fri|Sun)(?:day)?|
1095     (?<DN>Tue)(?:sday)?|
1096     (?<DN>Wed)(?:nesday)?|
1097     (?<DN>Thu)(?:rsday)?|
1098     (?<DN>Sat)(?:urday)?
1099 nigel 91 .sp
1100     There are five capturing substrings, but only one is ever set after a match.
1101 ph10 182 (An alternative way of solving this problem is to use a "branch reset"
1102 ph10 175 subpattern, as described in the previous section.)
1103     .P
1104 nigel 91 The convenience function for extracting the data by name returns the substring
1105 nigel 93 for the first (and in this example, the only) subpattern of that name that
1106 nigel 91 matched. This saves searching to find which numbered subpattern it was. If you
1107     make a reference to a non-unique named subpattern from elsewhere in the
1108     pattern, the one that corresponds to the lowest number is used. For further
1109     details of the interfaces for handling named subpatterns, see the
1110 nigel 63 .\" HREF
1111 nigel 75 \fBpcreapi\fP
1112 nigel 63 .\"
1113     documentation.
1114 nigel 75 .
1115     .
1116 nigel 63 .SH REPETITION
1117     .rs
1118     .sp
1119     Repetition is specified by quantifiers, which can follow any of the following
1120     items:
1121 nigel 75 .sp
1122 nigel 63 a literal data character
1123 nigel 93 the dot metacharacter
1124 nigel 75 the \eC escape sequence
1125     the \eX escape sequence (in UTF-8 mode with Unicode properties)
1126 nigel 93 the \eR escape sequence
1127 nigel 75 an escape such as \ed that matches a single character
1128 nigel 63 a character class
1129     a back reference (see next section)
1130     a parenthesized subpattern (unless it is an assertion)
1131 nigel 75 .sp
1132 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
1133     permitted matches, by giving the two numbers in curly brackets (braces),
1134     separated by a comma. The numbers must be less than 65536, and the first must
1135     be less than or equal to the second. For example:
1136 nigel 75 .sp
1137 nigel 63 z{2,4}
1138 nigel 75 .sp
1139 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1140     character. If the second number is omitted, but the comma is present, there is
1141     no upper limit; if the second number and the comma are both omitted, the
1142     quantifier specifies an exact number of required matches. Thus
1143 nigel 75 .sp
1144 nigel 63 [aeiou]{3,}
1145 nigel 75 .sp
1146 nigel 63 matches at least 3 successive vowels, but may match many more, while
1147 nigel 75 .sp
1148     \ed{8}
1149     .sp
1150 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
1151     where a quantifier is not allowed, or one that does not match the syntax of a
1152     quantifier, is taken as a literal character. For example, {,6} is not a
1153     quantifier, but a literal string of four characters.
1154 nigel 75 .P
1155 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1156 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1157     which is represented by a two-byte sequence. Similarly, when Unicode property
1158     support is available, \eX{3} matches three Unicode extended sequences, each of
1159     which may be several bytes long (and they may be of different lengths).
1160     .P
1161 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
1162     previous item and the quantifier were not present.
1163 nigel 75 .P
1164 nigel 93 For convenience, the three most common quantifiers have single-character
1165     abbreviations:
1166 nigel 75 .sp
1167 nigel 63 * is equivalent to {0,}
1168     + is equivalent to {1,}
1169     ? is equivalent to {0,1}
1170 nigel 75 .sp
1171 nigel 63 It is possible to construct infinite loops by following a subpattern that can
1172     match no characters with a quantifier that has no upper limit, for example:
1173 nigel 75 .sp
1174 nigel 63 (a?)*
1175 nigel 75 .sp
1176 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
1177     such patterns. However, because there are cases where this can be useful, such
1178     patterns are now accepted, but if any repetition of the subpattern does in fact
1179     match no characters, the loop is forcibly broken.
1180 nigel 75 .P
1181 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
1182     possible (up to the maximum number of permitted times), without causing the
1183     rest of the pattern to fail. The classic example of where this gives problems
1184 nigel 75 is in trying to match comments in C programs. These appear between /* and */
1185     and within the comment, individual * and / characters may appear. An attempt to
1186     match C comments by applying the pattern
1187     .sp
1188     /\e*.*\e*/
1189     .sp
1190 nigel 63 to the string
1191 nigel 75 .sp
1192     /* first comment */ not comment /* second comment */
1193     .sp
1194 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1195     item.
1196 nigel 75 .P
1197 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1198     greedy, and instead matches the minimum number of times possible, so the
1199     pattern
1200 nigel 75 .sp
1201     /\e*.*?\e*/
1202     .sp
1203 nigel 63 does the right thing with the C comments. The meaning of the various
1204     quantifiers is not otherwise changed, just the preferred number of matches.
1205     Do not confuse this use of question mark with its use as a quantifier in its
1206     own right. Because it has two uses, it can sometimes appear doubled, as in
1207 nigel 75 .sp
1208     \ed??\ed
1209     .sp
1210 nigel 63 which matches one digit by preference, but can match two if that is the only
1211     way the rest of the pattern matches.
1212 nigel 75 .P
1213 nigel 93 If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1214 nigel 63 the quantifiers are not greedy by default, but individual ones can be made
1215     greedy by following them with a question mark. In other words, it inverts the
1216     default behaviour.
1217 nigel 75 .P
1218 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1219 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1220 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1221 nigel 75 .P
1222 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1223 nigel 93 to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1224 nigel 63 implicitly anchored, because whatever follows will be tried against every
1225     character position in the subject string, so there is no point in retrying the
1226     overall match at any position after the first. PCRE normally treats such a
1227 nigel 75 pattern as though it were preceded by \eA.
1228     .P
1229 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1230     worth setting PCRE_DOTALL in order to obtain this optimization, or
1231     alternatively using ^ to indicate anchoring explicitly.
1232 nigel 75 .P
1233 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1234     is inside capturing parentheses that are the subject of a backreference
1235 nigel 93 elsewhere in the pattern, a match at the start may fail where a later one
1236     succeeds. Consider, for example:
1237 nigel 75 .sp
1238     (.*)abc\e1
1239     .sp
1240 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1241     this reason, such a pattern is not implicitly anchored.
1242 nigel 75 .P
1243 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1244     that matched the final iteration. For example, after
1245 nigel 75 .sp
1246     (tweedle[dume]{3}\es*)+
1247     .sp
1248 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1249     "tweedledee". However, if there are nested capturing subpatterns, the
1250     corresponding captured values may have been set in previous iterations. For
1251     example, after
1252 nigel 75 .sp
1253 nigel 63 /(a|(b))+/
1254 nigel 75 .sp
1255 nigel 63 matches "aba" the value of the second captured substring is "b".
1256 nigel 75 .
1257     .
1258     .\" HTML <a name="atomicgroup"></a>
1259     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1260 nigel 63 .rs
1261     .sp
1262 nigel 93 With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1263     repetition, failure of what follows normally causes the repeated item to be
1264     re-evaluated to see if a different number of repeats allows the rest of the
1265     pattern to match. Sometimes it is useful to prevent this, either to change the
1266     nature of the match, or to cause it fail earlier than it otherwise might, when
1267     the author of the pattern knows there is no point in carrying on.
1268 nigel 75 .P
1269     Consider, for example, the pattern \ed+foo when applied to the subject line
1270     .sp
1271 nigel 63 123456bar
1272 nigel 75 .sp
1273 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1274 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1275 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1276     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1277     that once a subpattern has matched, it is not to be re-evaluated in this way.
1278 nigel 75 .P
1279 nigel 93 If we use atomic grouping for the previous example, the matcher gives up
1280 nigel 63 immediately on failing to match "foo" the first time. The notation is a kind of
1281     special parenthesis, starting with (?> as in this example:
1282 nigel 75 .sp
1283     (?>\ed+)foo
1284     .sp
1285 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1286     it has matched, and a failure further into the pattern is prevented from
1287     backtracking into it. Backtracking past it to previous items, however, works as
1288     normal.
1289 nigel 75 .P
1290 nigel 63 An alternative description is that a subpattern of this type matches the string
1291     of characters that an identical standalone pattern would match, if anchored at
1292     the current point in the subject string.
1293 nigel 75 .P
1294 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1295     the above example can be thought of as a maximizing repeat that must swallow
1296 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1297 nigel 63 number of digits they match in order to make the rest of the pattern match,
1298 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1299     .P
1300 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1301     subpatterns, and can be nested. However, when the subpattern for an atomic
1302     group is just a single repeated item, as in the example above, a simpler
1303     notation, called a "possessive quantifier" can be used. This consists of an
1304     additional + character following a quantifier. Using this notation, the
1305     previous example can be rewritten as
1306 nigel 75 .sp
1307     \ed++foo
1308     .sp
1309 ph10 208 Note that a possessive quantifier can be used with an entire group, for
1310     example:
1311     .sp
1312     (abc|xyz){2,3}+
1313     .sp
1314 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1315     option is ignored. They are a convenient notation for the simpler forms of
1316 nigel 93 atomic group. However, there is no difference in the meaning of a possessive
1317     quantifier and the equivalent atomic group, though there may be a performance
1318     difference; possessive quantifiers should be slightly faster.
1319 nigel 75 .P
1320 nigel 93 The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1321     Jeffrey Friedl originated the idea (and the name) in the first edition of his
1322     book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1323     package, and PCRE copied it from there. It ultimately found its way into Perl
1324     at release 5.10.
1325 nigel 75 .P
1326 nigel 93 PCRE has an optimization that automatically "possessifies" certain simple
1327     pattern constructs. For example, the sequence A+B is treated as A++B because
1328     there is no point in backtracking into a sequence of A's when B must follow.
1329     .P
1330 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1331     be repeated an unlimited number of times, the use of an atomic group is the
1332     only way to avoid some failing matches taking a very long time indeed. The
1333     pattern
1334 nigel 75 .sp
1335     (\eD+|<\ed+>)*[!?]
1336     .sp
1337 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1338     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1339     quickly. However, if it is applied to
1340 nigel 75 .sp
1341 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1342 nigel 75 .sp
1343 nigel 63 it takes a long time before reporting failure. This is because the string can
1344 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1345     large number of ways, and all have to be tried. (The example uses [!?] rather
1346     than a single character at the end, because both PCRE and Perl have an
1347     optimization that allows for fast failure when a single character is used. They
1348     remember the last single character that is required for a match, and fail early
1349     if it is not present in the string.) If the pattern is changed so that it uses
1350     an atomic group, like this:
1351     .sp
1352     ((?>\eD+)|<\ed+>)*[!?]
1353     .sp
1354 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1355 nigel 75 .
1356     .
1357     .\" HTML <a name="backreferences"></a>
1358     .SH "BACK REFERENCES"
1359 nigel 63 .rs
1360     .sp
1361     Outside a character class, a backslash followed by a digit greater than 0 (and
1362     possibly further digits) is a back reference to a capturing subpattern earlier
1363     (that is, to its left) in the pattern, provided there have been that many
1364     previous capturing left parentheses.
1365 nigel 75 .P
1366 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1367     always taken as a back reference, and causes an error only if there are not
1368     that many capturing left parentheses in the entire pattern. In other words, the
1369     parentheses that are referenced need not be to the left of the reference for
1370 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1371     when a repetition is involved and the subpattern to the right has participated
1372     in an earlier iteration.
1373     .P
1374 nigel 93 It is not possible to have a numerical "forward back reference" to a subpattern
1375     whose number is 10 or more using this syntax because a sequence such as \e50 is
1376     interpreted as a character defined in octal. See the subsection entitled
1377 nigel 91 "Non-printing characters"
1378 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1379     .\" </a>
1380     above
1381     .\"
1382 nigel 93 for further details of the handling of digits following a backslash. There is
1383     no such problem when named parentheses are used. A back reference to any
1384     subpattern is possible using named parentheses (see below).
1385 nigel 75 .P
1386 nigel 93 Another way of avoiding the ambiguity inherent in the use of digits following a
1387     backslash is to use the \eg escape sequence, which is a feature introduced in
1388 ph10 208 Perl 5.10. This escape must be followed by an unsigned number or a negative
1389     number, optionally enclosed in braces. These examples are all identical:
1390 nigel 93 .sp
1391     (ring), \e1
1392     (ring), \eg1
1393     (ring), \eg{1}
1394     .sp
1395 ph10 208 An unsigned number specifies an absolute reference without the ambiguity that
1396     is present in the older syntax. It is also useful when literal digits follow
1397     the reference. A negative number is a relative reference. Consider this
1398     example:
1399 nigel 93 .sp
1400     (abc(def)ghi)\eg{-1}
1401     .sp
1402     The sequence \eg{-1} is a reference to the most recently started capturing
1403     subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1404     would be equivalent to \e1. The use of relative references can be helpful in
1405     long patterns, and also in patterns that are created by joining together
1406     fragments that contain references within themselves.
1407     .P
1408 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1409     the current subject string, rather than anything matching the subpattern
1410     itself (see
1411     .\" HTML <a href="#subpatternsassubroutines">
1412     .\" </a>
1413     "Subpatterns as subroutines"
1414     .\"
1415     below for a way of doing that). So the pattern
1416 nigel 75 .sp
1417     (sens|respons)e and \e1ibility
1418     .sp
1419 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1420     "sense and responsibility". If caseful matching is in force at the time of the
1421     back reference, the case of letters is relevant. For example,
1422 nigel 75 .sp
1423     ((?i)rah)\es+\e1
1424     .sp
1425 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1426     capturing subpattern is matched caselessly.
1427 nigel 75 .P
1428 ph10 171 There are several different ways of writing back references to named
1429     subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1430     \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1431     back reference syntax, in which \eg can be used for both numeric and named
1432     references, is also supported. We could rewrite the above example in any of
1433 nigel 93 the following ways:
1434 nigel 75 .sp
1435 nigel 93 (?<p1>(?i)rah)\es+\ek<p1>
1436 ph10 171 (?'p1'(?i)rah)\es+\ek{p1}
1437 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1438 ph10 171 (?<p1>(?i)rah)\es+\eg{p1}
1439 nigel 75 .sp
1440 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1441     after the reference.
1442     .P
1443 nigel 63 There may be more than one back reference to the same subpattern. If a
1444     subpattern has not actually been used in a particular match, any back
1445     references to it always fail. For example, the pattern
1446 nigel 75 .sp
1447     (a|(bc))\e2
1448     .sp
1449 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1450     many capturing parentheses in a pattern, all digits following the backslash are
1451     taken as part of a potential back reference number. If the pattern continues
1452     with a digit character, some delimiter must be used to terminate the back
1453     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1454 nigel 75 Otherwise an empty comment (see
1455     .\" HTML <a href="#comments">
1456     .\" </a>
1457     "Comments"
1458     .\"
1459     below) can be used.
1460     .P
1461 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1462 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1463 nigel 63 However, such references can be useful inside repeated subpatterns. For
1464     example, the pattern
1465 nigel 75 .sp
1466     (a|b\e1)+
1467     .sp
1468 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1469     the subpattern, the back reference matches the character string corresponding
1470     to the previous iteration. In order for this to work, the pattern must be such
1471     that the first iteration does not need to match the back reference. This can be
1472     done using alternation, as in the example above, or by a quantifier with a
1473     minimum of zero.
1474 nigel 75 .
1475     .
1476     .\" HTML <a name="bigassertions"></a>
1477 nigel 63 .SH ASSERTIONS
1478     .rs
1479     .sp
1480     An assertion is a test on the characters following or preceding the current
1481     matching point that does not actually consume any characters. The simple
1482 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1483     .\" HTML <a href="#smallassertions">
1484     .\" </a>
1485     above.
1486     .\"
1487     .P
1488 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1489     those that look ahead of the current position in the subject string, and those
1490 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1491     except that it does not cause the current matching position to be changed.
1492     .P
1493     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1494     because it makes no sense to assert the same thing several times. If any kind
1495     of assertion contains capturing subpatterns within it, these are counted for
1496     the purposes of numbering the capturing subpatterns in the whole pattern.
1497     However, substring capturing is carried out only for positive assertions,
1498     because it does not make sense for negative assertions.
1499     .
1500     .
1501     .SS "Lookahead assertions"
1502     .rs
1503     .sp
1504 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1505     negative assertions. For example,
1506 nigel 75 .sp
1507     \ew+(?=;)
1508     .sp
1509 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1510     the match, and
1511 nigel 75 .sp
1512 nigel 63 foo(?!bar)
1513 nigel 75 .sp
1514 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1515     apparently similar pattern
1516 nigel 75 .sp
1517 nigel 63 (?!foo)bar
1518 nigel 75 .sp
1519 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1520     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1521     (?!foo) is always true when the next three characters are "bar". A
1522 nigel 75 lookbehind assertion is needed to achieve the other effect.
1523     .P
1524 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1525     convenient way to do it is with (?!) because an empty string always matches, so
1526     an assertion that requires there not to be an empty string must always fail.
1527 nigel 75 .
1528     .
1529     .\" HTML <a name="lookbehind"></a>
1530     .SS "Lookbehind assertions"
1531     .rs
1532     .sp
1533 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1534     negative assertions. For example,
1535 nigel 75 .sp
1536 nigel 63 (?<!foo)bar
1537 nigel 75 .sp
1538 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1539     a lookbehind assertion are restricted such that all the strings it matches must
1540 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1541     do not all have to have the same fixed length. Thus
1542 nigel 75 .sp
1543 nigel 63 (?<=bullock|donkey)
1544 nigel 75 .sp
1545 nigel 63 is permitted, but
1546 nigel 75 .sp
1547 nigel 63 (?<!dogs?|cats?)
1548 nigel 75 .sp
1549 nigel 63 causes an error at compile time. Branches that match different length strings
1550     are permitted only at the top level of a lookbehind assertion. This is an
1551     extension compared with Perl (at least for 5.8), which requires all branches to
1552     match the same length of string. An assertion such as
1553 nigel 75 .sp
1554 nigel 63 (?<=ab(c|de))
1555 nigel 75 .sp
1556 nigel 63 is not permitted, because its single top-level branch can match two different
1557     lengths, but it is acceptable if rewritten to use two top-level branches:
1558 nigel 75 .sp
1559 nigel 63 (?<=abc|abde)
1560 nigel 75 .sp
1561 ph10 172 In some cases, the Perl 5.10 escape sequence \eK
1562 ph10 168 .\" HTML <a href="#resetmatchstart">
1563     .\" </a>
1564     (see above)
1565     .\"
1566     can be used instead of a lookbehind assertion; this is not restricted to a
1567     fixed-length.
1568     .P
1569 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1570 nigel 93 temporarily move the current position back by the fixed length and then try to
1571 nigel 63 match. If there are insufficient characters before the current position, the
1572 nigel 93 assertion fails.
1573 nigel 75 .P
1574     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1575 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1576 nigel 93 the length of the lookbehind. The \eX and \eR escapes, which can match
1577     different numbers of bytes, are also not permitted.
1578 nigel 75 .P
1579 nigel 93 Possessive quantifiers can be used in conjunction with lookbehind assertions to
1580     specify efficient matching at the end of the subject string. Consider a simple
1581     pattern such as
1582 nigel 75 .sp
1583 nigel 63 abcd$
1584 nigel 75 .sp
1585 nigel 63 when applied to a long string that does not match. Because matching proceeds
1586     from left to right, PCRE will look for each "a" in the subject and then see if
1587     what follows matches the rest of the pattern. If the pattern is specified as
1588 nigel 75 .sp
1589 nigel 63 ^.*abcd$
1590 nigel 75 .sp
1591 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1592     there is no following "a"), it backtracks to match all but the last character,
1593     then all but the last two characters, and so on. Once again the search for "a"
1594     covers the entire string, from right to left, so we are no better off. However,
1595     if the pattern is written as
1596 nigel 75 .sp
1597 nigel 63 ^.*+(?<=abcd)
1598 nigel 75 .sp
1599 nigel 93 there can be no backtracking for the .*+ item; it can match only the entire
1600 nigel 63 string. The subsequent lookbehind assertion does a single test on the last four
1601     characters. If it fails, the match fails immediately. For long strings, this
1602     approach makes a significant difference to the processing time.
1603 nigel 75 .
1604     .
1605     .SS "Using multiple assertions"
1606     .rs
1607     .sp
1608 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1609 nigel 75 .sp
1610     (?<=\ed{3})(?<!999)foo
1611     .sp
1612 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1613     the assertions is applied independently at the same point in the subject
1614     string. First there is a check that the previous three characters are all
1615     digits, and then there is a check that the same three characters are not "999".
1616 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1617 nigel 63 of which are digits and the last three of which are not "999". For example, it
1618     doesn't match "123abcfoo". A pattern to do that is
1619 nigel 75 .sp
1620     (?<=\ed{3}...)(?<!999)foo
1621     .sp
1622 nigel 63 This time the first assertion looks at the preceding six characters, checking
1623     that the first three are digits, and then the second assertion checks that the
1624     preceding three characters are not "999".
1625 nigel 75 .P
1626 nigel 63 Assertions can be nested in any combination. For example,
1627 nigel 75 .sp
1628 nigel 63 (?<=(?<!foo)bar)baz
1629 nigel 75 .sp
1630 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1631     preceded by "foo", while
1632 nigel 75 .sp
1633     (?<=\ed{3}(?!999)...)foo
1634     .sp
1635     is another pattern that matches "foo" preceded by three digits and any three
1636 nigel 63 characters that are not "999".
1637 nigel 75 .
1638     .
1639 nigel 91 .\" HTML <a name="conditions"></a>
1640 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1641 nigel 63 .rs
1642     .sp
1643     It is possible to cause the matching process to obey a subpattern
1644     conditionally or to choose between two alternative subpatterns, depending on
1645     the result of an assertion, or whether a previous capturing subpattern matched
1646     or not. The two possible forms of conditional subpattern are
1647 nigel 75 .sp
1648 nigel 63 (?(condition)yes-pattern)
1649     (?(condition)yes-pattern|no-pattern)
1650 nigel 75 .sp
1651 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1652     no-pattern (if present) is used. If there are more than two alternatives in the
1653     subpattern, a compile-time error occurs.
1654 nigel 75 .P
1655 nigel 93 There are four kinds of condition: references to subpatterns, references to
1656     recursion, a pseudo-condition called DEFINE, and assertions.
1657     .
1658     .SS "Checking for a used subpattern by number"
1659     .rs
1660     .sp
1661     If the text between the parentheses consists of a sequence of digits, the
1662     condition is true if the capturing subpattern of that number has previously
1663 ph10 172 matched. An alternative notation is to precede the digits with a plus or minus
1664 ph10 167 sign. In this case, the subpattern number is relative rather than absolute.
1665 ph10 172 The most recently opened parentheses can be referenced by (?(-1), the next most
1666 ph10 167 recent by (?(-2), and so on. In looping constructs it can also make sense to
1667     refer to subsequent groups with constructs such as (?(+2).
1668 nigel 91 .P
1669     Consider the following pattern, which contains non-significant white space to
1670     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1671     three parts for ease of discussion:
1672 nigel 75 .sp
1673     ( \e( )? [^()]+ (?(1) \e) )
1674     .sp
1675 nigel 63 The first part matches an optional opening parenthesis, and if that
1676     character is present, sets it as the first captured substring. The second part
1677     matches one or more characters that are not parentheses. The third part is a
1678     conditional subpattern that tests whether the first set of parentheses matched
1679     or not. If they did, that is, if subject started with an opening parenthesis,
1680     the condition is true, and so the yes-pattern is executed and a closing
1681     parenthesis is required. Otherwise, since no-pattern is not present, the
1682     subpattern matches nothing. In other words, this pattern matches a sequence of
1683 nigel 93 non-parentheses, optionally enclosed in parentheses.
1684 ph10 167 .P
1685 ph10 172 If you were embedding this pattern in a larger one, you could use a relative
1686 ph10 167 reference:
1687     .sp
1688     ...other stuff... ( \e( )? [^()]+ (?(-1) \e) ) ...
1689     .sp
1690     This makes the fragment independent of the parentheses in the larger pattern.
1691 nigel 93 .
1692     .SS "Checking for a used subpattern by name"
1693     .rs
1694 nigel 91 .sp
1695 nigel 93 Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1696     subpattern by name. For compatibility with earlier versions of PCRE, which had
1697     this facility before Perl, the syntax (?(name)...) is also recognized. However,
1698     there is a possible ambiguity with this syntax, because subpattern names may
1699     consist entirely of digits. PCRE looks first for a named subpattern; if it
1700     cannot find one and the name consists entirely of digits, PCRE looks for a
1701     subpattern of that number, which must be greater than zero. Using subpattern
1702     names that consist entirely of digits is not recommended.
1703     .P
1704     Rewriting the above example to use a named subpattern gives this:
1705 nigel 91 .sp
1706 nigel 93 (?<OPEN> \e( )? [^()]+ (?(<OPEN>) \e) )
1707     .sp
1708     .
1709     .SS "Checking for pattern recursion"
1710     .rs
1711     .sp
1712 nigel 91 If the condition is the string (R), and there is no subpattern with the name R,
1713 nigel 93 the condition is true if a recursive call to the whole pattern or any
1714     subpattern has been made. If digits or a name preceded by ampersand follow the
1715     letter R, for example:
1716     .sp
1717     (?(R3)...) or (?(R&name)...)
1718     .sp
1719     the condition is true if the most recent recursion is into the subpattern whose
1720     number or name is given. This condition does not check the entire recursion
1721     stack.
1722 nigel 75 .P
1723 nigel 93 At "top level", all these recursion test conditions are false. Recursive
1724     patterns are described below.
1725     .
1726     .SS "Defining subpatterns for use by reference only"
1727     .rs
1728     .sp
1729     If the condition is the string (DEFINE), and there is no subpattern with the
1730     name DEFINE, the condition is always false. In this case, there may be only one
1731     alternative in the subpattern. It is always skipped if control reaches this
1732     point in the pattern; the idea of DEFINE is that it can be used to define
1733     "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1734     is described below.) For example, a pattern to match an IPv4 address could be
1735     written like this (ignore whitespace and line breaks):
1736     .sp
1737     (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1738     \eb (?&byte) (\e.(?&byte)){3} \eb
1739     .sp
1740     The first part of the pattern is a DEFINE group inside which a another group
1741     named "byte" is defined. This matches an individual component of an IPv4
1742     address (a number less than 256). When matching takes place, this part of the
1743     pattern is skipped because DEFINE acts like a false condition.
1744     .P
1745     The rest of the pattern uses references to the named group to match the four
1746     dot-separated components of an IPv4 address, insisting on a word boundary at
1747     each end.
1748     .
1749     .SS "Assertion conditions"
1750     .rs
1751     .sp
1752     If the condition is not in any of the above formats, it must be an assertion.
1753 nigel 63 This may be a positive or negative lookahead or lookbehind assertion. Consider
1754     this pattern, again containing non-significant white space, and with the two
1755     alternatives on the second line:
1756 nigel 75 .sp
1757 nigel 63 (?(?=[^a-z]*[a-z])
1758 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1759     .sp
1760 nigel 63 The condition is a positive lookahead assertion that matches an optional
1761     sequence of non-letters followed by a letter. In other words, it tests for the
1762     presence of at least one letter in the subject. If a letter is found, the
1763     subject is matched against the first alternative; otherwise it is matched
1764     against the second. This pattern matches strings in one of the two forms
1765     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1766 nigel 75 .
1767     .
1768     .\" HTML <a name="comments"></a>
1769 nigel 63 .SH COMMENTS
1770     .rs
1771     .sp
1772 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1773 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1774     that make up a comment play no part in the pattern matching at all.
1775 nigel 75 .P
1776 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1777 nigel 91 character class introduces a comment that continues to immediately after the
1778     next newline in the pattern.
1779 nigel 75 .
1780     .
1781 nigel 91 .\" HTML <a name="recursion"></a>
1782 nigel 75 .SH "RECURSIVE PATTERNS"
1783 nigel 63 .rs
1784     .sp
1785     Consider the problem of matching a string in parentheses, allowing for
1786     unlimited nested parentheses. Without the use of recursion, the best that can
1787     be done is to use a pattern that matches up to some fixed depth of nesting. It
1788 nigel 93 is not possible to handle an arbitrary nesting depth.
1789     .P
1790     For some time, Perl has provided a facility that allows regular expressions to
1791     recurse (amongst other things). It does this by interpolating Perl code in the
1792     expression at run time, and the code can refer to the expression itself. A Perl
1793     pattern using code interpolation to solve the parentheses problem can be
1794     created like this:
1795 nigel 75 .sp
1796     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1797     .sp
1798 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1799 nigel 93 recursively to the pattern in which it appears.
1800 nigel 75 .P
1801 nigel 93 Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1802     supports special syntax for recursion of the entire pattern, and also for
1803     individual subpattern recursion. After its introduction in PCRE and Python,
1804     this kind of recursion was introduced into Perl at release 5.10.
1805 nigel 75 .P
1806 nigel 93 A special item that consists of (? followed by a number greater than zero and a
1807     closing parenthesis is a recursive call of the subpattern of the given number,
1808     provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1809     call, which is described in the next section.) The special item (?R) or (?0) is
1810     a recursive call of the entire regular expression.
1811 nigel 87 .P
1812 nigel 93 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1813     treated as an atomic group. That is, once it has matched some of the subject
1814     string, it is never re-entered, even if it contains untried alternatives and
1815     there is a subsequent matching failure.
1816     .P
1817 nigel 87 This PCRE pattern solves the nested parentheses problem (assume the
1818     PCRE_EXTENDED option is set so that white space is ignored):
1819 nigel 75 .sp
1820     \e( ( (?>[^()]+) | (?R) )* \e)
1821     .sp
1822 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1823     substrings which can either be a sequence of non-parentheses, or a recursive
1824 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1825 nigel 63 Finally there is a closing parenthesis.
1826 nigel 75 .P
1827 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1828     pattern, so instead you could use this:
1829 nigel 75 .sp
1830     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1831     .sp
1832 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1833 ph10 172 them instead of the whole pattern.
1834 ph10 166 .P
1835     In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1836     is made easier by the use of relative references. (A Perl 5.10 feature.)
1837     Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1838     most recently opened parentheses preceding the recursion. In other words, a
1839     negative number counts capturing parentheses leftwards from the point at which
1840     it is encountered.
1841     .P
1842     It is also possible to refer to subsequently opened parentheses, by writing
1843     references such as (?+2). However, these cannot be recursive because the
1844     reference is not inside the parentheses that are referenced. They are always
1845     "subroutine" calls, as described in the next section.
1846     .P
1847     An alternative approach is to use named parentheses instead. The Perl syntax
1848     for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1849     could rewrite the above example as follows:
1850 nigel 75 .sp
1851 nigel 93 (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1852 nigel 75 .sp
1853 nigel 93 If there is more than one subpattern with the same name, the earliest one is
1854 ph10 172 used.
1855 ph10 166 .P
1856     This particular example pattern that we have been looking at contains nested
1857     unlimited repeats, and so the use of atomic grouping for matching strings of
1858     non-parentheses is important when applying the pattern to strings that do not
1859     match. For example, when this pattern is applied to
1860 nigel 75 .sp
1861 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1862 nigel 75 .sp
1863 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1864     the match runs for a very long time indeed because there are so many different
1865     ways the + and * repeats can carve up the subject, and all have to be tested
1866     before failure can be reported.
1867 nigel 75 .P
1868 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1869     from the outermost level of the recursion at which the subpattern value is set.
1870     If you want to obtain intermediate values, a callout function can be used (see
1871 nigel 93 below and the
1872 nigel 63 .\" HREF
1873 nigel 75 \fBpcrecallout\fP
1874 nigel 63 .\"
1875     documentation). If the pattern above is matched against
1876 nigel 75 .sp
1877 nigel 63 (ab(cd)ef)
1878 nigel 75 .sp
1879 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1880     on at the top level. If additional parentheses are added, giving
1881 nigel 75 .sp
1882     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1883 nigel 63 ^ ^
1884     ^ ^
1885 nigel 75 .sp
1886 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1887     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1888     has to obtain extra memory to store data during a recursion, which it does by
1889 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1890 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1891 nigel 75 .P
1892 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
1893     Consider this pattern, which matches text in angle brackets, allowing for
1894     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1895     recursing), whereas any characters are permitted at the outer level.
1896 nigel 75 .sp
1897     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
1898     .sp
1899 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1900     different alternatives for the recursive and non-recursive cases. The (?R) item
1901     is the actual recursive call.
1902 nigel 75 .
1903     .
1904 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
1905 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
1906 nigel 63 .rs
1907     .sp
1908     If the syntax for a recursive subpattern reference (either by number or by
1909     name) is used outside the parentheses to which it refers, it operates like a
1910 nigel 93 subroutine in a programming language. The "called" subpattern may be defined
1911 ph10 166 before or after the reference. A numbered reference can be absolute or
1912     relative, as in these examples:
1913 nigel 75 .sp
1914 ph10 166 (...(absolute)...)...(?2)...
1915     (...(relative)...)...(?-1)...
1916 ph10 172 (...(?+1)...(relative)...
1917 ph10 166 .sp
1918     An earlier example pointed out that the pattern
1919     .sp
1920 nigel 75 (sens|respons)e and \e1ibility
1921     .sp
1922 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1923     "sense and responsibility". If instead the pattern
1924 nigel 75 .sp
1925 nigel 63 (sens|respons)e and (?1)ibility
1926 nigel 75 .sp
1927 nigel 63 is used, it does match "sense and responsibility" as well as the other two
1928 nigel 93 strings. Another example is given in the discussion of DEFINE above.
1929 nigel 87 .P
1930     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1931     group. That is, once it has matched some of the subject string, it is never
1932     re-entered, even if it contains untried alternatives and there is a subsequent
1933     matching failure.
1934 nigel 93 .P
1935     When a subpattern is used as a subroutine, processing options such as
1936     case-independence are fixed when the subpattern is defined. They cannot be
1937     changed for different calls. For example, consider this pattern:
1938     .sp
1939 ph10 166 (abc)(?i:(?-1))
1940 nigel 93 .sp
1941     It matches "abcabc". It does not match "abcABC" because the change of
1942     processing option does not affect the called subpattern.
1943 nigel 75 .
1944     .
1945 nigel 63 .SH CALLOUTS
1946     .rs
1947     .sp
1948     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
1949     code to be obeyed in the middle of matching a regular expression. This makes it
1950     possible, amongst other things, to extract different substrings that match the
1951     same pair of parentheses when there is a repetition.
1952 nigel 75 .P
1953 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1954     code. The feature is called "callout". The caller of PCRE provides an external
1955 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
1956 nigel 63 By default, this variable contains NULL, which disables all calling out.
1957 nigel 75 .P
1958 nigel 63 Within a regular expression, (?C) indicates the points at which the external
1959     function is to be called. If you want to identify different callout points, you
1960     can put a number less than 256 after the letter C. The default value is zero.
1961     For example, this pattern has two callout points:
1962 nigel 75 .sp
1963 ph10 155 (?C1)abc(?C2)def
1964 nigel 75 .sp
1965     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1966     automatically installed before each item in the pattern. They are all numbered
1967     255.
1968     .P
1969     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1970 nigel 63 set), the external function is called. It is provided with the number of the
1971 nigel 75 callout, the position in the pattern, and, optionally, one item of data
1972     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
1973     may cause matching to proceed, to backtrack, or to fail altogether. A complete
1974     description of the interface to the callout function is given in the
1975 nigel 63 .\" HREF
1976 nigel 75 \fBpcrecallout\fP
1977 nigel 63 .\"
1978     documentation.
1979 nigel 93 .
1980     .
1981 ph10 210 .SH "BACTRACKING CONTROL"
1982     .rs
1983     .sp
1984 ph10 211 Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
1985 ph10 210 are described in the Perl documentation as "experimental and subject to change
1986 ph10 211 or removal in a future version of Perl". It goes on to say: "Their usage in
1987     production code should be noted to avoid problems during upgrades." The same
1988 ph10 210 remarks apply to the PCRE features described in this section.
1989     .P
1990 ph10 211 Since these verbs are specifically related to backtracking, they can be used
1991     only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
1992     backtracking algorithm. They cause an error if encountered by
1993 ph10 210 \fBpcre_dfa_exec()\fP.
1994     .P
1995 ph10 211 The new verbs make use of what was previously invalid syntax: an opening
1996 ph10 210 parenthesis followed by an asterisk. In Perl, they are generally of the form
1997     (*VERB:ARG) but PCRE does not support the use of arguments, so its general
1998     form is just (*VERB). Any number of these verbs may occur in a pattern. There
1999     are two kinds:
2000     .
2001     .SS "Verbs that act immediately"
2002     .rs
2003     .sp
2004     The following verbs act as soon as they are encountered:
2005     .sp
2006     (*ACCEPT)
2007     .sp
2008     This verb causes the match to end successfully, skipping the remainder of the
2009 ph10 211 pattern. When inside a recursion, only the innermost pattern is ended
2010     immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2011 ph10 210 capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2012     captured. For example:
2013     .sp
2014     A(A|B(*ACCEPT)|C)D
2015     .sp
2016 ph10 211 This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2017 ph10 210 captured.
2018     .sp
2019     (*FAIL) or (*F)
2020     .sp
2021 ph10 211 This verb causes the match to fail, forcing backtracking to occur. It is
2022 ph10 210 equivalent to (?!) but easier to read. The Perl documentation notes that it is
2023     probably useful only when combined with (?{}) or (??{}). Those are, of course,
2024     Perl features that are not present in PCRE. The nearest equivalent is the
2025     callout feature, as for example in this pattern:
2026     .sp
2027     a+(?C)(*FAIL)
2028     .sp
2029 ph10 211 A match with the string "aaaa" always fails, but the callout is taken before
2030     each backtrack happens (in this example, 10 times).
2031 ph10 210 .
2032     .SS "Verbs that act after backtracking"
2033     .rs
2034     .sp
2035 ph10 211 The following verbs do nothing when they are encountered. Matching continues
2036     with what follows, but if there is no subsequent match, a failure is forced.
2037 ph10 210 The verbs differ in exactly what kind of failure occurs.
2038     .sp
2039     (*COMMIT)
2040     .sp
2041 ph10 211 This verb causes the whole match to fail outright if the rest of the pattern
2042 ph10 210 does not match. Even if the pattern is unanchored, no further attempts to find
2043     a match by advancing the start point take place. Once (*COMMIT) has been
2044 ph10 211 passed, \fBpcre_exec()\fP is committed to finding a match at the current
2045 ph10 210 starting point, or not at all. For example:
2046     .sp
2047     a+(*COMMIT)b
2048     .sp
2049 ph10 211 This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2050 ph10 210 dynamic anchor, or "I've started, so I must finish."
2051     .sp
2052     (*PRUNE)
2053     .sp
2054 ph10 211 This verb causes the match to fail at the current position if the rest of the
2055     pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2056 ph10 210 advance to the next starting character then happens. Backtracking can occur as
2057     usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2058     if there is no match to the right, backtracking cannot cross (*PRUNE).
2059 ph10 211 In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2060 ph10 210 group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2061     be expressed in any other way.
2062     .sp
2063     (*SKIP)
2064     .sp
2065 ph10 211 This verb is like (*PRUNE), except that if the pattern is unanchored, the
2066 ph10 210 "bumpalong" advance is not to the next character, but to the position in the
2067     subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2068     was matched leading up to it cannot be part of a successful match. Consider:
2069     .sp
2070     a+(*SKIP)b
2071     .sp
2072 ph10 211 If the subject is "aaaac...", after the first match attempt fails (starting at
2073 ph10 210 the first character in the string), the starting point skips on to start the
2074 ph10 211 next attempt at "c". Note that a possessive quantifer does not have the same
2075 ph10 210 effect in this example; although it would suppress backtracking during the
2076     first match attempt, the second attempt would start at the second character
2077     instead of skipping on to "c".
2078     .sp
2079     (*THEN)
2080 ph10 211 .sp
2081 ph10 210 This verb causes a skip to the next alternation if the rest of the pattern does
2082     not match. That is, it cancels pending backtracking, but only within the
2083     current alternation. Its name comes from the observation that it can be used
2084     for a pattern-based if-then-else block:
2085     .sp
2086     ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2087     .sp
2088 ph10 211 If the COND1 pattern matches, FOO is tried (and possibly further items after
2089 ph10 210 the end of the group if FOO succeeds); on failure the matcher skips to the
2090 ph10 211 second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2091 ph10 210 is used outside of any alternation, it acts exactly like (*PRUNE).
2092     .
2093     .
2094 nigel 93 .SH "SEE ALSO"
2095     .rs
2096     .sp
2097     \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2098 ph10 99 .
2099     .
2100     .SH AUTHOR
2101     .rs
2102     .sp
2103     .nf
2104     Philip Hazel
2105     University Computing Service
2106     Cambridge CB2 3QH, England.
2107     .fi
2108     .
2109     .
2110     .SH REVISION
2111     .rs
2112     .sp
2113     .nf
2114 ph10 211 Last updated: 09 August 2007
2115 ph10 99 Copyright (c) 1997-2007 University of Cambridge.
2116     .fi

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