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1 nigel 79 .TH PCREPATTERN 3
2 nigel 63 .SH NAME
3     PCRE - Perl-compatible regular expressions
4 nigel 75 .SH "PCRE REGULAR EXPRESSION DETAILS"
5 nigel 63 .rs
6     .sp
7     The syntax and semantics of the regular expressions supported by PCRE are
8     described below. Regular expressions are also described in the Perl
9 nigel 75 documentation and in a number of books, some of which have copious examples.
10     Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
11     regular expressions in great detail. This description of PCRE's regular
12     expressions is intended as reference material.
13     .P
14     The original operation of PCRE was on strings of one-byte characters. However,
15     there is now also support for UTF-8 character strings. To use this, you must
16     build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
17     the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
18     places below. There is also a summary of UTF-8 features in the
19 nigel 63 .\" HTML <a href="pcre.html#utf8support">
20     .\" </a>
21     section on UTF-8 support
22     .\"
23     in the main
24     .\" HREF
25 nigel 75 \fBpcre\fP
26 nigel 63 .\"
27     page.
28 nigel 75 .P
29 nigel 77 The remainder of this document discusses the patterns that are supported by
30     PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31     From release 6.0, PCRE offers a second matching function,
32     \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33     Perl-compatible. The advantages and disadvantages of the alternative function,
34     and how it differs from the normal function, are discussed in the
35     .\" HREF
36     \fBpcrematching\fP
37     .\"
38     page.
39     .P
40 nigel 63 A regular expression is a pattern that is matched against a subject string from
41     left to right. Most characters stand for themselves in a pattern, and match the
42     corresponding characters in the subject. As a trivial example, the pattern
43 nigel 75 .sp
44 nigel 63 The quick brown fox
45 nigel 75 .sp
46 nigel 77 matches a portion of a subject string that is identical to itself. When
47     caseless matching is specified (the PCRE_CASELESS option), letters are matched
48     independently of case. In UTF-8 mode, PCRE always understands the concept of
49     case for characters whose values are less than 128, so caseless matching is
50     always possible. For characters with higher values, the concept of case is
51     supported if PCRE is compiled with Unicode property support, but not otherwise.
52     If you want to use caseless matching for characters 128 and above, you must
53     ensure that PCRE is compiled with Unicode property support as well as with
54     UTF-8 support.
55     .P
56     The power of regular expressions comes from the ability to include alternatives
57     and repetitions in the pattern. These are encoded in the pattern by the use of
58 nigel 75 \fImetacharacters\fP, which do not stand for themselves but instead are
59 nigel 63 interpreted in some special way.
60 nigel 75 .P
61     There are two different sets of metacharacters: those that are recognized
62 nigel 63 anywhere in the pattern except within square brackets, and those that are
63 nigel 75 recognized in square brackets. Outside square brackets, the metacharacters are
64 nigel 63 as follows:
65 nigel 75 .sp
66     \e general escape character with several uses
67 nigel 63 ^ assert start of string (or line, in multiline mode)
68     $ assert end of string (or line, in multiline mode)
69     . match any character except newline (by default)
70     [ start character class definition
71     | start of alternative branch
72     ( start subpattern
73     ) end subpattern
74     ? extends the meaning of (
75     also 0 or 1 quantifier
76     also quantifier minimizer
77     * 0 or more quantifier
78     + 1 or more quantifier
79     also "possessive quantifier"
80     { start min/max quantifier
81 nigel 75 .sp
82 nigel 63 Part of a pattern that is in square brackets is called a "character class". In
83 nigel 75 a character class the only metacharacters are:
84     .sp
85     \e general escape character
86 nigel 63 ^ negate the class, but only if the first character
87     - indicates character range
88 nigel 75 .\" JOIN
89 nigel 63 [ POSIX character class (only if followed by POSIX
90     syntax)
91     ] terminates the character class
92 nigel 75 .sp
93     The following sections describe the use of each of the metacharacters.
94     .
95 nigel 63 .SH BACKSLASH
96     .rs
97     .sp
98     The backslash character has several uses. Firstly, if it is followed by a
99 nigel 91 non-alphanumeric character, it takes away any special meaning that character
100     may have. This use of backslash as an escape character applies both inside and
101 nigel 63 outside character classes.
102 nigel 75 .P
103     For example, if you want to match a * character, you write \e* in the pattern.
104 nigel 63 This escaping action applies whether or not the following character would
105 nigel 75 otherwise be interpreted as a metacharacter, so it is always safe to precede a
106     non-alphanumeric with backslash to specify that it stands for itself. In
107     particular, if you want to match a backslash, you write \e\e.
108     .P
109 nigel 63 If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
110     pattern (other than in a character class) and characters between a # outside
111 nigel 91 a character class and the next newline are ignored. An escaping backslash can
112     be used to include a whitespace or # character as part of the pattern.
113 nigel 75 .P
114 nigel 63 If you want to remove the special meaning from a sequence of characters, you
115 nigel 75 can do so by putting them between \eQ and \eE. This is different from Perl in
116     that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
117 nigel 63 Perl, $ and @ cause variable interpolation. Note the following examples:
118 nigel 75 .sp
119 nigel 63 Pattern PCRE matches Perl matches
120 nigel 75 .sp
121     .\" JOIN
122     \eQabc$xyz\eE abc$xyz abc followed by the
123 nigel 63 contents of $xyz
124 nigel 75 \eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
125     \eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
126     .sp
127     The \eQ...\eE sequence is recognized both inside and outside character classes.
128     .
129     .
130     .\" HTML <a name="digitsafterbackslash"></a>
131     .SS "Non-printing characters"
132     .rs
133     .sp
134 nigel 63 A second use of backslash provides a way of encoding non-printing characters
135     in patterns in a visible manner. There is no restriction on the appearance of
136     non-printing characters, apart from the binary zero that terminates a pattern,
137     but when a pattern is being prepared by text editing, it is usually easier to
138     use one of the following escape sequences than the binary character it
139     represents:
140 nigel 75 .sp
141     \ea alarm, that is, the BEL character (hex 07)
142     \ecx "control-x", where x is any character
143     \ee escape (hex 1B)
144     \ef formfeed (hex 0C)
145     \en newline (hex 0A)
146     \er carriage return (hex 0D)
147     \et tab (hex 09)
148     \eddd character with octal code ddd, or backreference
149     \exhh character with hex code hh
150 nigel 87 \ex{hhh..} character with hex code hhh..
151 nigel 75 .sp
152     The precise effect of \ecx is as follows: if x is a lower case letter, it
153 nigel 63 is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
154 nigel 75 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
155 nigel 63 7B.
156 nigel 75 .P
157     After \ex, from zero to two hexadecimal digits are read (letters can be in
158 nigel 87 upper or lower case). Any number of hexadecimal digits may appear between \ex{
159     and }, but the value of the character code must be less than 256 in non-UTF-8
160     mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
161     is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
162     and }, or if there is no terminating }, this form of escape is not recognized.
163     Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
164     with no following digits, giving a character whose value is zero.
165 nigel 75 .P
166 nigel 63 Characters whose value is less than 256 can be defined by either of the two
167 nigel 87 syntaxes for \ex. There is no difference in the way they are handled. For
168     example, \exdc is exactly the same as \ex{dc}.
169 nigel 75 .P
170 nigel 91 After \e0 up to two further octal digits are read. If there are fewer than two
171     digits, just those that are present are used. Thus the sequence \e0\ex\e07
172     specifies two binary zeros followed by a BEL character (code value 7). Make
173     sure you supply two digits after the initial zero if the pattern character that
174     follows is itself an octal digit.
175 nigel 75 .P
176 nigel 63 The handling of a backslash followed by a digit other than 0 is complicated.
177     Outside a character class, PCRE reads it and any following digits as a decimal
178     number. If the number is less than 10, or if there have been at least that many
179     previous capturing left parentheses in the expression, the entire sequence is
180 nigel 75 taken as a \fIback reference\fP. A description of how this works is given
181     .\" HTML <a href="#backreferences">
182     .\" </a>
183     later,
184     .\"
185     following the discussion of
186     .\" HTML <a href="#subpattern">
187     .\" </a>
188     parenthesized subpatterns.
189     .\"
190     .P
191 nigel 63 Inside a character class, or if the decimal number is greater than 9 and there
192     have not been that many capturing subpatterns, PCRE re-reads up to three octal
193 nigel 91 digits following the backslash, ane uses them to generate a data character. Any
194     subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
195     character specified in octal must be less than \e400. In UTF-8 mode, values up
196     to \e777 are permitted. For example:
197 nigel 75 .sp
198     \e040 is another way of writing a space
199     .\" JOIN
200     \e40 is the same, provided there are fewer than 40
201 nigel 63 previous capturing subpatterns
202 nigel 75 \e7 is always a back reference
203     .\" JOIN
204     \e11 might be a back reference, or another way of
205 nigel 63 writing a tab
206 nigel 75 \e011 is always a tab
207     \e0113 is a tab followed by the character "3"
208     .\" JOIN
209     \e113 might be a back reference, otherwise the
210 nigel 63 character with octal code 113
211 nigel 75 .\" JOIN
212     \e377 might be a back reference, otherwise
213 nigel 63 the byte consisting entirely of 1 bits
214 nigel 75 .\" JOIN
215     \e81 is either a back reference, or a binary zero
216 nigel 63 followed by the two characters "8" and "1"
217 nigel 75 .sp
218 nigel 63 Note that octal values of 100 or greater must not be introduced by a leading
219     zero, because no more than three octal digits are ever read.
220 nigel 75 .P
221 nigel 91 All the sequences that define a single character value can be used both inside
222     and outside character classes. In addition, inside a character class, the
223     sequence \eb is interpreted as the backspace character (hex 08), and the
224     sequence \eX is interpreted as the character "X". Outside a character class,
225     these sequences have different meanings
226 nigel 75 .\" HTML <a href="#uniextseq">
227     .\" </a>
228     (see below).
229     .\"
230     .
231     .
232     .SS "Generic character types"
233     .rs
234     .sp
235     The third use of backslash is for specifying generic character types. The
236     following are always recognized:
237     .sp
238     \ed any decimal digit
239     \eD any character that is not a decimal digit
240     \es any whitespace character
241     \eS any character that is not a whitespace character
242     \ew any "word" character
243     \eW any "non-word" character
244     .sp
245 nigel 63 Each pair of escape sequences partitions the complete set of characters into
246     two disjoint sets. Any given character matches one, and only one, of each pair.
247 nigel 75 .P
248     These character type sequences can appear both inside and outside character
249     classes. They each match one character of the appropriate type. If the current
250     matching point is at the end of the subject string, all of them fail, since
251     there is no character to match.
252     .P
253     For compatibility with Perl, \es does not match the VT character (code 11).
254     This makes it different from the the POSIX "space" class. The \es characters
255 nigel 91 are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is
256     included in a Perl script, \es may match the VT character. In PCRE, it never
257     does.)
258 nigel 75 .P
259     A "word" character is an underscore or any character less than 256 that is a
260     letter or digit. The definition of letters and digits is controlled by PCRE's
261     low-valued character tables, and may vary if locale-specific matching is taking
262     place (see
263 nigel 63 .\" HTML <a href="pcreapi.html#localesupport">
264     .\" </a>
265     "Locale support"
266     .\"
267     in the
268     .\" HREF
269 nigel 75 \fBpcreapi\fP
270 nigel 63 .\"
271 nigel 75 page). For example, in the "fr_FR" (French) locale, some character codes
272     greater than 128 are used for accented letters, and these are matched by \ew.
273     .P
274     In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
275     \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
276 nigel 87 character property support is available. The use of locales with Unicode is
277     discouraged.
278 nigel 75 .
279     .
280     .\" HTML <a name="uniextseq"></a>
281     .SS Unicode character properties
282     .rs
283     .sp
284     When PCRE is built with Unicode character property support, three additional
285 nigel 87 escape sequences to match character properties are available when UTF-8 mode
286 nigel 75 is selected. They are:
287     .sp
288 nigel 87 \ep{\fIxx\fP} a character with the \fIxx\fP property
289     \eP{\fIxx\fP} a character without the \fIxx\fP property
290     \eX an extended Unicode sequence
291 nigel 75 .sp
292 nigel 87 The property names represented by \fIxx\fP above are limited to the Unicode
293     script names, the general category properties, and "Any", which matches any
294     character (including newline). Other properties such as "InMusicalSymbols" are
295     not currently supported by PCRE. Note that \eP{Any} does not match any
296     characters, so always causes a match failure.
297 nigel 75 .P
298 nigel 87 Sets of Unicode characters are defined as belonging to certain scripts. A
299     character from one of these sets can be matched using a script name. For
300     example:
301 nigel 75 .sp
302 nigel 87 \ep{Greek}
303     \eP{Han}
304     .sp
305     Those that are not part of an identified script are lumped together as
306     "Common". The current list of scripts is:
307     .P
308     Arabic,
309     Armenian,
310     Bengali,
311     Bopomofo,
312     Braille,
313     Buginese,
314     Buhid,
315     Canadian_Aboriginal,
316     Cherokee,
317     Common,
318     Coptic,
319     Cypriot,
320     Cyrillic,
321     Deseret,
322     Devanagari,
323     Ethiopic,
324     Georgian,
325     Glagolitic,
326     Gothic,
327     Greek,
328     Gujarati,
329     Gurmukhi,
330     Han,
331     Hangul,
332     Hanunoo,
333     Hebrew,
334     Hiragana,
335     Inherited,
336     Kannada,
337     Katakana,
338     Kharoshthi,
339     Khmer,
340     Lao,
341     Latin,
342     Limbu,
343     Linear_B,
344     Malayalam,
345     Mongolian,
346     Myanmar,
347     New_Tai_Lue,
348     Ogham,
349     Old_Italic,
350     Old_Persian,
351     Oriya,
352     Osmanya,
353     Runic,
354     Shavian,
355     Sinhala,
356     Syloti_Nagri,
357     Syriac,
358     Tagalog,
359     Tagbanwa,
360     Tai_Le,
361     Tamil,
362     Telugu,
363     Thaana,
364     Thai,
365     Tibetan,
366     Tifinagh,
367     Ugaritic,
368     Yi.
369     .P
370     Each character has exactly one general category property, specified by a
371     two-letter abbreviation. For compatibility with Perl, negation can be specified
372     by including a circumflex between the opening brace and the property name. For
373     example, \ep{^Lu} is the same as \eP{Lu}.
374     .P
375     If only one letter is specified with \ep or \eP, it includes all the general
376     category properties that start with that letter. In this case, in the absence
377     of negation, the curly brackets in the escape sequence are optional; these two
378     examples have the same effect:
379     .sp
380 nigel 75 \ep{L}
381     \epL
382     .sp
383 nigel 87 The following general category property codes are supported:
384 nigel 75 .sp
385     C Other
386     Cc Control
387     Cf Format
388     Cn Unassigned
389     Co Private use
390     Cs Surrogate
391     .sp
392     L Letter
393     Ll Lower case letter
394     Lm Modifier letter
395     Lo Other letter
396     Lt Title case letter
397     Lu Upper case letter
398     .sp
399     M Mark
400     Mc Spacing mark
401     Me Enclosing mark
402     Mn Non-spacing mark
403     .sp
404     N Number
405     Nd Decimal number
406     Nl Letter number
407     No Other number
408     .sp
409     P Punctuation
410     Pc Connector punctuation
411     Pd Dash punctuation
412     Pe Close punctuation
413     Pf Final punctuation
414     Pi Initial punctuation
415     Po Other punctuation
416     Ps Open punctuation
417     .sp
418     S Symbol
419     Sc Currency symbol
420     Sk Modifier symbol
421     Sm Mathematical symbol
422     So Other symbol
423     .sp
424     Z Separator
425     Zl Line separator
426     Zp Paragraph separator
427     Zs Space separator
428     .sp
429 nigel 87 The special property L& is also supported: it matches a character that has
430     the Lu, Ll, or Lt property, in other words, a letter that is not classified as
431     a modifier or "other".
432 nigel 75 .P
433 nigel 87 The long synonyms for these properties that Perl supports (such as \ep{Letter})
434 nigel 91 are not supported by PCRE, nor is it permitted to prefix any of these
435 nigel 87 properties with "Is".
436     .P
437     No character that is in the Unicode table has the Cn (unassigned) property.
438     Instead, this property is assumed for any code point that is not in the
439     Unicode table.
440     .P
441 nigel 75 Specifying caseless matching does not affect these escape sequences. For
442     example, \ep{Lu} always matches only upper case letters.
443     .P
444     The \eX escape matches any number of Unicode characters that form an extended
445     Unicode sequence. \eX is equivalent to
446     .sp
447     (?>\ePM\epM*)
448     .sp
449     That is, it matches a character without the "mark" property, followed by zero
450     or more characters with the "mark" property, and treats the sequence as an
451     atomic group
452     .\" HTML <a href="#atomicgroup">
453     .\" </a>
454     (see below).
455     .\"
456     Characters with the "mark" property are typically accents that affect the
457     preceding character.
458     .P
459     Matching characters by Unicode property is not fast, because PCRE has to search
460     a structure that contains data for over fifteen thousand characters. That is
461     why the traditional escape sequences such as \ed and \ew do not use Unicode
462     properties in PCRE.
463     .
464     .
465     .\" HTML <a name="smallassertions"></a>
466     .SS "Simple assertions"
467     .rs
468     .sp
469 nigel 63 The fourth use of backslash is for certain simple assertions. An assertion
470     specifies a condition that has to be met at a particular point in a match,
471     without consuming any characters from the subject string. The use of
472 nigel 75 subpatterns for more complicated assertions is described
473     .\" HTML <a href="#bigassertions">
474     .\" </a>
475     below.
476     .\"
477 nigel 91 The backslashed assertions are:
478 nigel 75 .sp
479     \eb matches at a word boundary
480     \eB matches when not at a word boundary
481     \eA matches at start of subject
482     \eZ matches at end of subject or before newline at end
483     \ez matches at end of subject
484     \eG matches at first matching position in subject
485     .sp
486     These assertions may not appear in character classes (but note that \eb has a
487 nigel 63 different meaning, namely the backspace character, inside a character class).
488 nigel 75 .P
489 nigel 63 A word boundary is a position in the subject string where the current character
490 nigel 75 and the previous character do not both match \ew or \eW (i.e. one matches
491     \ew and the other matches \eW), or the start or end of the string if the
492     first or last character matches \ew, respectively.
493     .P
494     The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
495     dollar (described in the next section) in that they only ever match at the very
496     start and end of the subject string, whatever options are set. Thus, they are
497     independent of multiline mode. These three assertions are not affected by the
498     PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
499     circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
500     argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
501     at a point other than the beginning of the subject, \eA can never match. The
502 nigel 91 difference between \eZ and \ez is that \eZ matches before a newline at the end
503     of the string as well as at the very end, whereas \ez matches only at the end.
504 nigel 75 .P
505     The \eG assertion is true only when the current matching position is at the
506     start point of the match, as specified by the \fIstartoffset\fP argument of
507     \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
508     non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
509 nigel 63 arguments, you can mimic Perl's /g option, and it is in this kind of
510 nigel 75 implementation where \eG can be useful.
511     .P
512     Note, however, that PCRE's interpretation of \eG, as the start of the current
513 nigel 63 match, is subtly different from Perl's, which defines it as the end of the
514     previous match. In Perl, these can be different when the previously matched
515     string was empty. Because PCRE does just one match at a time, it cannot
516     reproduce this behaviour.
517 nigel 75 .P
518     If all the alternatives of a pattern begin with \eG, the expression is anchored
519 nigel 63 to the starting match position, and the "anchored" flag is set in the compiled
520     regular expression.
521 nigel 75 .
522     .
523     .SH "CIRCUMFLEX AND DOLLAR"
524 nigel 63 .rs
525     .sp
526     Outside a character class, in the default matching mode, the circumflex
527 nigel 75 character is an assertion that is true only if the current matching point is
528     at the start of the subject string. If the \fIstartoffset\fP argument of
529     \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
530 nigel 63 option is unset. Inside a character class, circumflex has an entirely different
531 nigel 75 meaning
532     .\" HTML <a href="#characterclass">
533     .\" </a>
534     (see below).
535     .\"
536     .P
537 nigel 63 Circumflex need not be the first character of the pattern if a number of
538     alternatives are involved, but it should be the first thing in each alternative
539     in which it appears if the pattern is ever to match that branch. If all
540     possible alternatives start with a circumflex, that is, if the pattern is
541     constrained to match only at the start of the subject, it is said to be an
542     "anchored" pattern. (There are also other constructs that can cause a pattern
543     to be anchored.)
544 nigel 75 .P
545     A dollar character is an assertion that is true only if the current matching
546 nigel 63 point is at the end of the subject string, or immediately before a newline
547 nigel 91 at the end of the string (by default). Dollar need not be the last character of
548     the pattern if a number of alternatives are involved, but it should be the last
549     item in any branch in which it appears. Dollar has no special meaning in a
550     character class.
551 nigel 75 .P
552 nigel 63 The meaning of dollar can be changed so that it matches only at the very end of
553     the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
554 nigel 75 does not affect the \eZ assertion.
555     .P
556 nigel 63 The meanings of the circumflex and dollar characters are changed if the
557 nigel 91 PCRE_MULTILINE option is set. When this is the case, a circumflex matches
558     immediately after internal newlines as well as at the start of the subject
559     string. It does not match after a newline that ends the string. A dollar
560     matches before any newlines in the string, as well as at the very end, when
561     PCRE_MULTILINE is set. When newline is specified as the two-character
562     sequence CRLF, isolated CR and LF characters do not indicate newlines.
563 nigel 75 .P
564 nigel 91 For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
565     \en represents a newline) in multiline mode, but not otherwise. Consequently,
566     patterns that are anchored in single line mode because all branches start with
567     ^ are not anchored in multiline mode, and a match for circumflex is possible
568     when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
569     PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
570     .P
571 nigel 75 Note that the sequences \eA, \eZ, and \ez can be used to match the start and
572 nigel 63 end of the subject in both modes, and if all branches of a pattern start with
573 nigel 91 \eA it is always anchored, whether or not PCRE_MULTILINE is set.
574 nigel 75 .
575     .
576     .SH "FULL STOP (PERIOD, DOT)"
577 nigel 63 .rs
578     .sp
579     Outside a character class, a dot in the pattern matches any one character in
580 nigel 91 the subject string except (by default) a character that signifies the end of a
581     line. In UTF-8 mode, the matched character may be more than one byte long. When
582     a line ending is defined as a single character (CR or LF), dot never matches
583     that character; when the two-character sequence CRLF is used, dot does not
584     match CR if it is immediately followed by LF, but otherwise it matches all
585     characters (including isolated CRs and LFs).
586     .P
587     The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
588     option is set, a dot matches any one character, without exception. If newline
589     is defined as the two-character sequence CRLF, it takes two dots to match it.
590     .P
591     The handling of dot is entirely independent of the handling of circumflex and
592     dollar, the only relationship being that they both involve newlines. Dot has no
593     special meaning in a character class.
594 nigel 75 .
595     .
596     .SH "MATCHING A SINGLE BYTE"
597 nigel 63 .rs
598     .sp
599 nigel 75 Outside a character class, the escape sequence \eC matches any one byte, both
600 nigel 91 in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The
601     feature is provided in Perl in order to match individual bytes in UTF-8 mode.
602     Because it breaks up UTF-8 characters into individual bytes, what remains in
603     the string may be a malformed UTF-8 string. For this reason, the \eC escape
604     sequence is best avoided.
605 nigel 75 .P
606     PCRE does not allow \eC to appear in lookbehind assertions
607     .\" HTML <a href="#lookbehind">
608     .\" </a>
609     (described below),
610     .\"
611     because in UTF-8 mode this would make it impossible to calculate the length of
612     the lookbehind.
613     .
614     .
615     .\" HTML <a name="characterclass"></a>
616     .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
617 nigel 63 .rs
618     .sp
619     An opening square bracket introduces a character class, terminated by a closing
620     square bracket. A closing square bracket on its own is not special. If a
621     closing square bracket is required as a member of the class, it should be the
622     first data character in the class (after an initial circumflex, if present) or
623     escaped with a backslash.
624 nigel 75 .P
625 nigel 63 A character class matches a single character in the subject. In UTF-8 mode, the
626     character may occupy more than one byte. A matched character must be in the set
627     of characters defined by the class, unless the first character in the class
628     definition is a circumflex, in which case the subject character must not be in
629     the set defined by the class. If a circumflex is actually required as a member
630     of the class, ensure it is not the first character, or escape it with a
631     backslash.
632 nigel 75 .P
633 nigel 63 For example, the character class [aeiou] matches any lower case vowel, while
634     [^aeiou] matches any character that is not a lower case vowel. Note that a
635 nigel 75 circumflex is just a convenient notation for specifying the characters that
636     are in the class by enumerating those that are not. A class that starts with a
637     circumflex is not an assertion: it still consumes a character from the subject
638     string, and therefore it fails if the current pointer is at the end of the
639     string.
640     .P
641 nigel 63 In UTF-8 mode, characters with values greater than 255 can be included in a
642 nigel 75 class as a literal string of bytes, or by using the \ex{ escaping mechanism.
643     .P
644 nigel 63 When caseless matching is set, any letters in a class represent both their
645     upper case and lower case versions, so for example, a caseless [aeiou] matches
646     "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
647 nigel 77 caseful version would. In UTF-8 mode, PCRE always understands the concept of
648     case for characters whose values are less than 128, so caseless matching is
649     always possible. For characters with higher values, the concept of case is
650     supported if PCRE is compiled with Unicode property support, but not otherwise.
651     If you want to use caseless matching for characters 128 and above, you must
652     ensure that PCRE is compiled with Unicode property support as well as with
653     UTF-8 support.
654 nigel 75 .P
655 nigel 91 Characters that might indicate line breaks (CR and LF) are never treated in any
656     special way when matching character classes, whatever line-ending sequence is
657     in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is
658     used. A class such as [^a] always matches one of these characters.
659 nigel 75 .P
660 nigel 63 The minus (hyphen) character can be used to specify a range of characters in a
661     character class. For example, [d-m] matches any letter between d and m,
662     inclusive. If a minus character is required in a class, it must be escaped with
663     a backslash or appear in a position where it cannot be interpreted as
664     indicating a range, typically as the first or last character in the class.
665 nigel 75 .P
666 nigel 63 It is not possible to have the literal character "]" as the end character of a
667     range. A pattern such as [W-]46] is interpreted as a class of two characters
668     ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
669     "-46]". However, if the "]" is escaped with a backslash it is interpreted as
670 nigel 75 the end of range, so [W-\e]46] is interpreted as a class containing a range
671     followed by two other characters. The octal or hexadecimal representation of
672     "]" can also be used to end a range.
673     .P
674 nigel 63 Ranges operate in the collating sequence of character values. They can also be
675 nigel 75 used for characters specified numerically, for example [\e000-\e037]. In UTF-8
676 nigel 63 mode, ranges can include characters whose values are greater than 255, for
677 nigel 75 example [\ex{100}-\ex{2ff}].
678     .P
679 nigel 63 If a range that includes letters is used when caseless matching is set, it
680     matches the letters in either case. For example, [W-c] is equivalent to
681 nigel 75 [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
682     tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E
683     characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
684     characters with values greater than 128 only when it is compiled with Unicode
685     property support.
686     .P
687     The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
688     in a character class, and add the characters that they match to the class. For
689     example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
690 nigel 63 conveniently be used with the upper case character types to specify a more
691     restricted set of characters than the matching lower case type. For example,
692 nigel 75 the class [^\eW_] matches any letter or digit, but not underscore.
693     .P
694     The only metacharacters that are recognized in character classes are backslash,
695     hyphen (only where it can be interpreted as specifying a range), circumflex
696     (only at the start), opening square bracket (only when it can be interpreted as
697     introducing a POSIX class name - see the next section), and the terminating
698     closing square bracket. However, escaping other non-alphanumeric characters
699     does no harm.
700     .
701     .
702     .SH "POSIX CHARACTER CLASSES"
703 nigel 63 .rs
704     .sp
705 nigel 75 Perl supports the POSIX notation for character classes. This uses names
706 nigel 63 enclosed by [: and :] within the enclosing square brackets. PCRE also supports
707     this notation. For example,
708 nigel 75 .sp
709 nigel 63 [01[:alpha:]%]
710 nigel 75 .sp
711 nigel 63 matches "0", "1", any alphabetic character, or "%". The supported class names
712     are
713 nigel 75 .sp
714 nigel 63 alnum letters and digits
715     alpha letters
716     ascii character codes 0 - 127
717     blank space or tab only
718     cntrl control characters
719 nigel 75 digit decimal digits (same as \ed)
720 nigel 63 graph printing characters, excluding space
721     lower lower case letters
722     print printing characters, including space
723     punct printing characters, excluding letters and digits
724 nigel 75 space white space (not quite the same as \es)
725 nigel 63 upper upper case letters
726 nigel 75 word "word" characters (same as \ew)
727 nigel 63 xdigit hexadecimal digits
728 nigel 75 .sp
729 nigel 63 The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
730     space (32). Notice that this list includes the VT character (code 11). This
731 nigel 75 makes "space" different to \es, which does not include VT (for Perl
732 nigel 63 compatibility).
733 nigel 75 .P
734 nigel 63 The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
735     5.8. Another Perl extension is negation, which is indicated by a ^ character
736     after the colon. For example,
737 nigel 75 .sp
738 nigel 63 [12[:^digit:]]
739 nigel 75 .sp
740 nigel 63 matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
741     syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
742     supported, and an error is given if they are encountered.
743 nigel 75 .P
744     In UTF-8 mode, characters with values greater than 128 do not match any of
745 nigel 63 the POSIX character classes.
746 nigel 75 .
747     .
748     .SH "VERTICAL BAR"
749 nigel 63 .rs
750     .sp
751     Vertical bar characters are used to separate alternative patterns. For example,
752     the pattern
753 nigel 75 .sp
754 nigel 63 gilbert|sullivan
755 nigel 75 .sp
756 nigel 63 matches either "gilbert" or "sullivan". Any number of alternatives may appear,
757 nigel 91 and an empty alternative is permitted (matching the empty string). The matching
758     process tries each alternative in turn, from left to right, and the first one
759     that succeeds is used. If the alternatives are within a subpattern
760 nigel 75 .\" HTML <a href="#subpattern">
761     .\" </a>
762     (defined below),
763     .\"
764     "succeeds" means matching the rest of the main pattern as well as the
765     alternative in the subpattern.
766     .
767     .
768     .SH "INTERNAL OPTION SETTING"
769 nigel 63 .rs
770     .sp
771     The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
772     PCRE_EXTENDED options can be changed from within the pattern by a sequence of
773     Perl option letters enclosed between "(?" and ")". The option letters are
774 nigel 75 .sp
775 nigel 63 i for PCRE_CASELESS
776     m for PCRE_MULTILINE
777     s for PCRE_DOTALL
778     x for PCRE_EXTENDED
779 nigel 75 .sp
780 nigel 63 For example, (?im) sets caseless, multiline matching. It is also possible to
781     unset these options by preceding the letter with a hyphen, and a combined
782     setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
783     PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
784     permitted. If a letter appears both before and after the hyphen, the option is
785     unset.
786 nigel 75 .P
787 nigel 63 When an option change occurs at top level (that is, not inside subpattern
788     parentheses), the change applies to the remainder of the pattern that follows.
789     If the change is placed right at the start of a pattern, PCRE extracts it into
790     the global options (and it will therefore show up in data extracted by the
791 nigel 75 \fBpcre_fullinfo()\fP function).
792     .P
793 nigel 63 An option change within a subpattern affects only that part of the current
794     pattern that follows it, so
795 nigel 75 .sp
796 nigel 63 (a(?i)b)c
797 nigel 75 .sp
798 nigel 63 matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
799     By this means, options can be made to have different settings in different
800     parts of the pattern. Any changes made in one alternative do carry on
801     into subsequent branches within the same subpattern. For example,
802 nigel 75 .sp
803 nigel 63 (a(?i)b|c)
804 nigel 75 .sp
805 nigel 63 matches "ab", "aB", "c", and "C", even though when matching "C" the first
806     branch is abandoned before the option setting. This is because the effects of
807     option settings happen at compile time. There would be some very weird
808     behaviour otherwise.
809 nigel 75 .P
810 nigel 91 The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
811     changed in the same way as the Perl-compatible options by using the characters
812     J, U and X respectively.
813 nigel 75 .
814     .
815     .\" HTML <a name="subpattern"></a>
816 nigel 63 .SH SUBPATTERNS
817     .rs
818     .sp
819     Subpatterns are delimited by parentheses (round brackets), which can be nested.
820 nigel 75 Turning part of a pattern into a subpattern does two things:
821     .sp
822 nigel 63 1. It localizes a set of alternatives. For example, the pattern
823 nigel 75 .sp
824 nigel 63 cat(aract|erpillar|)
825 nigel 75 .sp
826 nigel 63 matches one of the words "cat", "cataract", or "caterpillar". Without the
827     parentheses, it would match "cataract", "erpillar" or the empty string.
828 nigel 75 .sp
829     2. It sets up the subpattern as a capturing subpattern. This means that, when
830     the whole pattern matches, that portion of the subject string that matched the
831     subpattern is passed back to the caller via the \fIovector\fP argument of
832     \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
833     from 1) to obtain numbers for the capturing subpatterns.
834     .P
835 nigel 63 For example, if the string "the red king" is matched against the pattern
836 nigel 75 .sp
837 nigel 63 the ((red|white) (king|queen))
838 nigel 75 .sp
839 nigel 63 the captured substrings are "red king", "red", and "king", and are numbered 1,
840     2, and 3, respectively.
841 nigel 75 .P
842 nigel 63 The fact that plain parentheses fulfil two functions is not always helpful.
843     There are often times when a grouping subpattern is required without a
844     capturing requirement. If an opening parenthesis is followed by a question mark
845     and a colon, the subpattern does not do any capturing, and is not counted when
846     computing the number of any subsequent capturing subpatterns. For example, if
847     the string "the white queen" is matched against the pattern
848 nigel 75 .sp
849 nigel 63 the ((?:red|white) (king|queen))
850 nigel 75 .sp
851 nigel 63 the captured substrings are "white queen" and "queen", and are numbered 1 and
852     2. The maximum number of capturing subpatterns is 65535, and the maximum depth
853     of nesting of all subpatterns, both capturing and non-capturing, is 200.
854 nigel 75 .P
855 nigel 63 As a convenient shorthand, if any option settings are required at the start of
856     a non-capturing subpattern, the option letters may appear between the "?" and
857     the ":". Thus the two patterns
858 nigel 75 .sp
859 nigel 63 (?i:saturday|sunday)
860     (?:(?i)saturday|sunday)
861 nigel 75 .sp
862 nigel 63 match exactly the same set of strings. Because alternative branches are tried
863     from left to right, and options are not reset until the end of the subpattern
864     is reached, an option setting in one branch does affect subsequent branches, so
865     the above patterns match "SUNDAY" as well as "Saturday".
866 nigel 75 .
867     .
868     .SH "NAMED SUBPATTERNS"
869 nigel 63 .rs
870     .sp
871     Identifying capturing parentheses by number is simple, but it can be very hard
872     to keep track of the numbers in complicated regular expressions. Furthermore,
873 nigel 75 if an expression is modified, the numbers may change. To help with this
874 nigel 63 difficulty, PCRE supports the naming of subpatterns, something that Perl does
875 nigel 91 not provide. The Python syntax (?P<name>...) is used. References to capturing
876     parentheses from other parts of the pattern, such as
877     .\" HTML <a href="#backreferences">
878     .\" </a>
879     backreferences,
880     .\"
881     .\" HTML <a href="#recursion">
882     .\" </a>
883     recursion,
884     .\"
885     and
886     .\" HTML <a href="#conditions">
887     .\" </a>
888     conditions,
889     .\"
890     can be made by name as well as by number.
891 nigel 75 .P
892 nigel 91 Names consist of up to 32 alphanumeric characters and underscores. Named
893     capturing parentheses are still allocated numbers as well as names. The PCRE
894     API provides function calls for extracting the name-to-number translation table
895     from a compiled pattern. There is also a convenience function for extracting a
896     captured substring by name.
897     .P
898     By default, a name must be unique within a pattern, but it is possible to relax
899     this constraint by setting the PCRE_DUPNAMES option at compile time. This can
900     be useful for patterns where only one instance of the named parentheses can
901     match. Suppose you want to match the name of a weekday, either as a 3-letter
902     abbreviation or as the full name, and in both cases you want to extract the
903     abbreviation. This pattern (ignoring the line breaks) does the job:
904     .sp
905     (?P<DN>Mon|Fri|Sun)(?:day)?|
906     (?P<DN>Tue)(?:sday)?|
907     (?P<DN>Wed)(?:nesday)?|
908     (?P<DN>Thu)(?:rsday)?|
909     (?P<DN>Sat)(?:urday)?
910     .sp
911     There are five capturing substrings, but only one is ever set after a match.
912     The convenience function for extracting the data by name returns the substring
913     for the first, and in this example, the only, subpattern of that name that
914     matched. This saves searching to find which numbered subpattern it was. If you
915     make a reference to a non-unique named subpattern from elsewhere in the
916     pattern, the one that corresponds to the lowest number is used. For further
917     details of the interfaces for handling named subpatterns, see the
918 nigel 63 .\" HREF
919 nigel 75 \fBpcreapi\fP
920 nigel 63 .\"
921     documentation.
922 nigel 75 .
923     .
924 nigel 63 .SH REPETITION
925     .rs
926     .sp
927     Repetition is specified by quantifiers, which can follow any of the following
928     items:
929 nigel 75 .sp
930 nigel 63 a literal data character
931     the . metacharacter
932 nigel 75 the \eC escape sequence
933     the \eX escape sequence (in UTF-8 mode with Unicode properties)
934     an escape such as \ed that matches a single character
935 nigel 63 a character class
936     a back reference (see next section)
937     a parenthesized subpattern (unless it is an assertion)
938 nigel 75 .sp
939 nigel 63 The general repetition quantifier specifies a minimum and maximum number of
940     permitted matches, by giving the two numbers in curly brackets (braces),
941     separated by a comma. The numbers must be less than 65536, and the first must
942     be less than or equal to the second. For example:
943 nigel 75 .sp
944 nigel 63 z{2,4}
945 nigel 75 .sp
946 nigel 63 matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
947     character. If the second number is omitted, but the comma is present, there is
948     no upper limit; if the second number and the comma are both omitted, the
949     quantifier specifies an exact number of required matches. Thus
950 nigel 75 .sp
951 nigel 63 [aeiou]{3,}
952 nigel 75 .sp
953 nigel 63 matches at least 3 successive vowels, but may match many more, while
954 nigel 75 .sp
955     \ed{8}
956     .sp
957 nigel 63 matches exactly 8 digits. An opening curly bracket that appears in a position
958     where a quantifier is not allowed, or one that does not match the syntax of a
959     quantifier, is taken as a literal character. For example, {,6} is not a
960     quantifier, but a literal string of four characters.
961 nigel 75 .P
962 nigel 63 In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
963 nigel 75 bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
964     which is represented by a two-byte sequence. Similarly, when Unicode property
965     support is available, \eX{3} matches three Unicode extended sequences, each of
966     which may be several bytes long (and they may be of different lengths).
967     .P
968 nigel 63 The quantifier {0} is permitted, causing the expression to behave as if the
969     previous item and the quantifier were not present.
970 nigel 75 .P
971 nigel 63 For convenience (and historical compatibility) the three most common
972     quantifiers have single-character abbreviations:
973 nigel 75 .sp
974 nigel 63 * is equivalent to {0,}
975     + is equivalent to {1,}
976     ? is equivalent to {0,1}
977 nigel 75 .sp
978 nigel 63 It is possible to construct infinite loops by following a subpattern that can
979     match no characters with a quantifier that has no upper limit, for example:
980 nigel 75 .sp
981 nigel 63 (a?)*
982 nigel 75 .sp
983 nigel 63 Earlier versions of Perl and PCRE used to give an error at compile time for
984     such patterns. However, because there are cases where this can be useful, such
985     patterns are now accepted, but if any repetition of the subpattern does in fact
986     match no characters, the loop is forcibly broken.
987 nigel 75 .P
988 nigel 63 By default, the quantifiers are "greedy", that is, they match as much as
989     possible (up to the maximum number of permitted times), without causing the
990     rest of the pattern to fail. The classic example of where this gives problems
991 nigel 75 is in trying to match comments in C programs. These appear between /* and */
992     and within the comment, individual * and / characters may appear. An attempt to
993     match C comments by applying the pattern
994     .sp
995     /\e*.*\e*/
996     .sp
997 nigel 63 to the string
998 nigel 75 .sp
999     /* first comment */ not comment /* second comment */
1000     .sp
1001 nigel 63 fails, because it matches the entire string owing to the greediness of the .*
1002     item.
1003 nigel 75 .P
1004 nigel 63 However, if a quantifier is followed by a question mark, it ceases to be
1005     greedy, and instead matches the minimum number of times possible, so the
1006     pattern
1007 nigel 75 .sp
1008     /\e*.*?\e*/
1009     .sp
1010 nigel 63 does the right thing with the C comments. The meaning of the various
1011     quantifiers is not otherwise changed, just the preferred number of matches.
1012     Do not confuse this use of question mark with its use as a quantifier in its
1013     own right. Because it has two uses, it can sometimes appear doubled, as in
1014 nigel 75 .sp
1015     \ed??\ed
1016     .sp
1017 nigel 63 which matches one digit by preference, but can match two if that is the only
1018     way the rest of the pattern matches.
1019 nigel 75 .P
1020 nigel 63 If the PCRE_UNGREEDY option is set (an option which is not available in Perl),
1021     the quantifiers are not greedy by default, but individual ones can be made
1022     greedy by following them with a question mark. In other words, it inverts the
1023     default behaviour.
1024 nigel 75 .P
1025 nigel 63 When a parenthesized subpattern is quantified with a minimum repeat count that
1026 nigel 75 is greater than 1 or with a limited maximum, more memory is required for the
1027 nigel 63 compiled pattern, in proportion to the size of the minimum or maximum.
1028 nigel 75 .P
1029 nigel 63 If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1030     to Perl's /s) is set, thus allowing the . to match newlines, the pattern is
1031     implicitly anchored, because whatever follows will be tried against every
1032     character position in the subject string, so there is no point in retrying the
1033     overall match at any position after the first. PCRE normally treats such a
1034 nigel 75 pattern as though it were preceded by \eA.
1035     .P
1036 nigel 63 In cases where it is known that the subject string contains no newlines, it is
1037     worth setting PCRE_DOTALL in order to obtain this optimization, or
1038     alternatively using ^ to indicate anchoring explicitly.
1039 nigel 75 .P
1040 nigel 63 However, there is one situation where the optimization cannot be used. When .*
1041     is inside capturing parentheses that are the subject of a backreference
1042     elsewhere in the pattern, a match at the start may fail, and a later one
1043     succeed. Consider, for example:
1044 nigel 75 .sp
1045     (.*)abc\e1
1046     .sp
1047 nigel 63 If the subject is "xyz123abc123" the match point is the fourth character. For
1048     this reason, such a pattern is not implicitly anchored.
1049 nigel 75 .P
1050 nigel 63 When a capturing subpattern is repeated, the value captured is the substring
1051     that matched the final iteration. For example, after
1052 nigel 75 .sp
1053     (tweedle[dume]{3}\es*)+
1054     .sp
1055 nigel 63 has matched "tweedledum tweedledee" the value of the captured substring is
1056     "tweedledee". However, if there are nested capturing subpatterns, the
1057     corresponding captured values may have been set in previous iterations. For
1058     example, after
1059 nigel 75 .sp
1060 nigel 63 /(a|(b))+/
1061 nigel 75 .sp
1062 nigel 63 matches "aba" the value of the second captured substring is "b".
1063 nigel 75 .
1064     .
1065     .\" HTML <a name="atomicgroup"></a>
1066     .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1067 nigel 63 .rs
1068     .sp
1069     With both maximizing and minimizing repetition, failure of what follows
1070     normally causes the repeated item to be re-evaluated to see if a different
1071     number of repeats allows the rest of the pattern to match. Sometimes it is
1072     useful to prevent this, either to change the nature of the match, or to cause
1073     it fail earlier than it otherwise might, when the author of the pattern knows
1074     there is no point in carrying on.
1075 nigel 75 .P
1076     Consider, for example, the pattern \ed+foo when applied to the subject line
1077     .sp
1078 nigel 63 123456bar
1079 nigel 75 .sp
1080 nigel 63 After matching all 6 digits and then failing to match "foo", the normal
1081 nigel 75 action of the matcher is to try again with only 5 digits matching the \ed+
1082 nigel 63 item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1083     (a term taken from Jeffrey Friedl's book) provides the means for specifying
1084     that once a subpattern has matched, it is not to be re-evaluated in this way.
1085 nigel 75 .P
1086 nigel 63 If we use atomic grouping for the previous example, the matcher would give up
1087     immediately on failing to match "foo" the first time. The notation is a kind of
1088     special parenthesis, starting with (?> as in this example:
1089 nigel 75 .sp
1090     (?>\ed+)foo
1091     .sp
1092 nigel 63 This kind of parenthesis "locks up" the part of the pattern it contains once
1093     it has matched, and a failure further into the pattern is prevented from
1094     backtracking into it. Backtracking past it to previous items, however, works as
1095     normal.
1096 nigel 75 .P
1097 nigel 63 An alternative description is that a subpattern of this type matches the string
1098     of characters that an identical standalone pattern would match, if anchored at
1099     the current point in the subject string.
1100 nigel 75 .P
1101 nigel 63 Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1102     the above example can be thought of as a maximizing repeat that must swallow
1103 nigel 75 everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1104 nigel 63 number of digits they match in order to make the rest of the pattern match,
1105 nigel 75 (?>\ed+) can only match an entire sequence of digits.
1106     .P
1107 nigel 63 Atomic groups in general can of course contain arbitrarily complicated
1108     subpatterns, and can be nested. However, when the subpattern for an atomic
1109     group is just a single repeated item, as in the example above, a simpler
1110     notation, called a "possessive quantifier" can be used. This consists of an
1111     additional + character following a quantifier. Using this notation, the
1112     previous example can be rewritten as
1113 nigel 75 .sp
1114     \ed++foo
1115     .sp
1116 nigel 63 Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1117     option is ignored. They are a convenient notation for the simpler forms of
1118     atomic group. However, there is no difference in the meaning or processing of a
1119     possessive quantifier and the equivalent atomic group.
1120 nigel 75 .P
1121 nigel 91 The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey
1122     Friedl originated the idea (and the name) in the first edition of his book.
1123     Mike McCloskey liked it, so implemented it when he built Sun's Java package,
1124     and PCRE copied it from there.
1125 nigel 75 .P
1126 nigel 63 When a pattern contains an unlimited repeat inside a subpattern that can itself
1127     be repeated an unlimited number of times, the use of an atomic group is the
1128     only way to avoid some failing matches taking a very long time indeed. The
1129     pattern
1130 nigel 75 .sp
1131     (\eD+|<\ed+>)*[!?]
1132     .sp
1133 nigel 63 matches an unlimited number of substrings that either consist of non-digits, or
1134     digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1135     quickly. However, if it is applied to
1136 nigel 75 .sp
1137 nigel 63 aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1138 nigel 75 .sp
1139 nigel 63 it takes a long time before reporting failure. This is because the string can
1140 nigel 75 be divided between the internal \eD+ repeat and the external * repeat in a
1141     large number of ways, and all have to be tried. (The example uses [!?] rather
1142     than a single character at the end, because both PCRE and Perl have an
1143     optimization that allows for fast failure when a single character is used. They
1144     remember the last single character that is required for a match, and fail early
1145     if it is not present in the string.) If the pattern is changed so that it uses
1146     an atomic group, like this:
1147     .sp
1148     ((?>\eD+)|<\ed+>)*[!?]
1149     .sp
1150 nigel 63 sequences of non-digits cannot be broken, and failure happens quickly.
1151 nigel 75 .
1152     .
1153     .\" HTML <a name="backreferences"></a>
1154     .SH "BACK REFERENCES"
1155 nigel 63 .rs
1156     .sp
1157     Outside a character class, a backslash followed by a digit greater than 0 (and
1158     possibly further digits) is a back reference to a capturing subpattern earlier
1159     (that is, to its left) in the pattern, provided there have been that many
1160     previous capturing left parentheses.
1161 nigel 75 .P
1162 nigel 63 However, if the decimal number following the backslash is less than 10, it is
1163     always taken as a back reference, and causes an error only if there are not
1164     that many capturing left parentheses in the entire pattern. In other words, the
1165     parentheses that are referenced need not be to the left of the reference for
1166 nigel 91 numbers less than 10. A "forward back reference" of this type can make sense
1167     when a repetition is involved and the subpattern to the right has participated
1168     in an earlier iteration.
1169     .P
1170     It is not possible to have a numerical "forward back reference" to subpattern
1171     whose number is 10 or more. However, a back reference to any subpattern is
1172     possible using named parentheses (see below). See also the subsection entitled
1173     "Non-printing characters"
1174 nigel 75 .\" HTML <a href="#digitsafterbackslash">
1175     .\" </a>
1176     above
1177     .\"
1178     for further details of the handling of digits following a backslash.
1179     .P
1180 nigel 63 A back reference matches whatever actually matched the capturing subpattern in
1181     the current subject string, rather than anything matching the subpattern
1182     itself (see
1183     .\" HTML <a href="#subpatternsassubroutines">
1184     .\" </a>
1185     "Subpatterns as subroutines"
1186     .\"
1187     below for a way of doing that). So the pattern
1188 nigel 75 .sp
1189     (sens|respons)e and \e1ibility
1190     .sp
1191 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1192     "sense and responsibility". If caseful matching is in force at the time of the
1193     back reference, the case of letters is relevant. For example,
1194 nigel 75 .sp
1195     ((?i)rah)\es+\e1
1196     .sp
1197 nigel 63 matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1198     capturing subpattern is matched caselessly.
1199 nigel 75 .P
1200 nigel 63 Back references to named subpatterns use the Python syntax (?P=name). We could
1201     rewrite the above example as follows:
1202 nigel 75 .sp
1203 nigel 91 (?P<p1>(?i)rah)\es+(?P=p1)
1204 nigel 75 .sp
1205 nigel 91 A subpattern that is referenced by name may appear in the pattern before or
1206     after the reference.
1207     .P
1208 nigel 63 There may be more than one back reference to the same subpattern. If a
1209     subpattern has not actually been used in a particular match, any back
1210     references to it always fail. For example, the pattern
1211 nigel 75 .sp
1212     (a|(bc))\e2
1213     .sp
1214 nigel 63 always fails if it starts to match "a" rather than "bc". Because there may be
1215     many capturing parentheses in a pattern, all digits following the backslash are
1216     taken as part of a potential back reference number. If the pattern continues
1217     with a digit character, some delimiter must be used to terminate the back
1218     reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1219 nigel 75 Otherwise an empty comment (see
1220     .\" HTML <a href="#comments">
1221     .\" </a>
1222     "Comments"
1223     .\"
1224     below) can be used.
1225     .P
1226 nigel 63 A back reference that occurs inside the parentheses to which it refers fails
1227 nigel 75 when the subpattern is first used, so, for example, (a\e1) never matches.
1228 nigel 63 However, such references can be useful inside repeated subpatterns. For
1229     example, the pattern
1230 nigel 75 .sp
1231     (a|b\e1)+
1232     .sp
1233 nigel 63 matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1234     the subpattern, the back reference matches the character string corresponding
1235     to the previous iteration. In order for this to work, the pattern must be such
1236     that the first iteration does not need to match the back reference. This can be
1237     done using alternation, as in the example above, or by a quantifier with a
1238     minimum of zero.
1239 nigel 75 .
1240     .
1241     .\" HTML <a name="bigassertions"></a>
1242 nigel 63 .SH ASSERTIONS
1243     .rs
1244     .sp
1245     An assertion is a test on the characters following or preceding the current
1246     matching point that does not actually consume any characters. The simple
1247 nigel 75 assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1248     .\" HTML <a href="#smallassertions">
1249     .\" </a>
1250     above.
1251     .\"
1252     .P
1253 nigel 63 More complicated assertions are coded as subpatterns. There are two kinds:
1254     those that look ahead of the current position in the subject string, and those
1255 nigel 75 that look behind it. An assertion subpattern is matched in the normal way,
1256     except that it does not cause the current matching position to be changed.
1257     .P
1258     Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1259     because it makes no sense to assert the same thing several times. If any kind
1260     of assertion contains capturing subpatterns within it, these are counted for
1261     the purposes of numbering the capturing subpatterns in the whole pattern.
1262     However, substring capturing is carried out only for positive assertions,
1263     because it does not make sense for negative assertions.
1264     .
1265     .
1266     .SS "Lookahead assertions"
1267     .rs
1268     .sp
1269 nigel 91 Lookahead assertions start with (?= for positive assertions and (?! for
1270     negative assertions. For example,
1271 nigel 75 .sp
1272     \ew+(?=;)
1273     .sp
1274 nigel 63 matches a word followed by a semicolon, but does not include the semicolon in
1275     the match, and
1276 nigel 75 .sp
1277 nigel 63 foo(?!bar)
1278 nigel 75 .sp
1279 nigel 63 matches any occurrence of "foo" that is not followed by "bar". Note that the
1280     apparently similar pattern
1281 nigel 75 .sp
1282 nigel 63 (?!foo)bar
1283 nigel 75 .sp
1284 nigel 63 does not find an occurrence of "bar" that is preceded by something other than
1285     "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1286     (?!foo) is always true when the next three characters are "bar". A
1287 nigel 75 lookbehind assertion is needed to achieve the other effect.
1288     .P
1289 nigel 63 If you want to force a matching failure at some point in a pattern, the most
1290     convenient way to do it is with (?!) because an empty string always matches, so
1291     an assertion that requires there not to be an empty string must always fail.
1292 nigel 75 .
1293     .
1294     .\" HTML <a name="lookbehind"></a>
1295     .SS "Lookbehind assertions"
1296     .rs
1297     .sp
1298 nigel 63 Lookbehind assertions start with (?<= for positive assertions and (?<! for
1299     negative assertions. For example,
1300 nigel 75 .sp
1301 nigel 63 (?<!foo)bar
1302 nigel 75 .sp
1303 nigel 63 does find an occurrence of "bar" that is not preceded by "foo". The contents of
1304     a lookbehind assertion are restricted such that all the strings it matches must
1305 nigel 91 have a fixed length. However, if there are several top-level alternatives, they
1306     do not all have to have the same fixed length. Thus
1307 nigel 75 .sp
1308 nigel 63 (?<=bullock|donkey)
1309 nigel 75 .sp
1310 nigel 63 is permitted, but
1311 nigel 75 .sp
1312 nigel 63 (?<!dogs?|cats?)
1313 nigel 75 .sp
1314 nigel 63 causes an error at compile time. Branches that match different length strings
1315     are permitted only at the top level of a lookbehind assertion. This is an
1316     extension compared with Perl (at least for 5.8), which requires all branches to
1317     match the same length of string. An assertion such as
1318 nigel 75 .sp
1319 nigel 63 (?<=ab(c|de))
1320 nigel 75 .sp
1321 nigel 63 is not permitted, because its single top-level branch can match two different
1322     lengths, but it is acceptable if rewritten to use two top-level branches:
1323 nigel 75 .sp
1324 nigel 63 (?<=abc|abde)
1325 nigel 75 .sp
1326 nigel 63 The implementation of lookbehind assertions is, for each alternative, to
1327     temporarily move the current position back by the fixed width and then try to
1328     match. If there are insufficient characters before the current position, the
1329     match is deemed to fail.
1330 nigel 75 .P
1331     PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1332 nigel 63 to appear in lookbehind assertions, because it makes it impossible to calculate
1333 nigel 75 the length of the lookbehind. The \eX escape, which can match different numbers
1334     of bytes, is also not permitted.
1335     .P
1336 nigel 63 Atomic groups can be used in conjunction with lookbehind assertions to specify
1337     efficient matching at the end of the subject string. Consider a simple pattern
1338     such as
1339 nigel 75 .sp
1340 nigel 63 abcd$
1341 nigel 75 .sp
1342 nigel 63 when applied to a long string that does not match. Because matching proceeds
1343     from left to right, PCRE will look for each "a" in the subject and then see if
1344     what follows matches the rest of the pattern. If the pattern is specified as
1345 nigel 75 .sp
1346 nigel 63 ^.*abcd$
1347 nigel 75 .sp
1348 nigel 63 the initial .* matches the entire string at first, but when this fails (because
1349     there is no following "a"), it backtracks to match all but the last character,
1350     then all but the last two characters, and so on. Once again the search for "a"
1351     covers the entire string, from right to left, so we are no better off. However,
1352     if the pattern is written as
1353 nigel 75 .sp
1354 nigel 63 ^(?>.*)(?<=abcd)
1355 nigel 75 .sp
1356     or, equivalently, using the possessive quantifier syntax,
1357     .sp
1358 nigel 63 ^.*+(?<=abcd)
1359 nigel 75 .sp
1360 nigel 63 there can be no backtracking for the .* item; it can match only the entire
1361     string. The subsequent lookbehind assertion does a single test on the last four
1362     characters. If it fails, the match fails immediately. For long strings, this
1363     approach makes a significant difference to the processing time.
1364 nigel 75 .
1365     .
1366     .SS "Using multiple assertions"
1367     .rs
1368     .sp
1369 nigel 63 Several assertions (of any sort) may occur in succession. For example,
1370 nigel 75 .sp
1371     (?<=\ed{3})(?<!999)foo
1372     .sp
1373 nigel 63 matches "foo" preceded by three digits that are not "999". Notice that each of
1374     the assertions is applied independently at the same point in the subject
1375     string. First there is a check that the previous three characters are all
1376     digits, and then there is a check that the same three characters are not "999".
1377 nigel 75 This pattern does \fInot\fP match "foo" preceded by six characters, the first
1378 nigel 63 of which are digits and the last three of which are not "999". For example, it
1379     doesn't match "123abcfoo". A pattern to do that is
1380 nigel 75 .sp
1381     (?<=\ed{3}...)(?<!999)foo
1382     .sp
1383 nigel 63 This time the first assertion looks at the preceding six characters, checking
1384     that the first three are digits, and then the second assertion checks that the
1385     preceding three characters are not "999".
1386 nigel 75 .P
1387 nigel 63 Assertions can be nested in any combination. For example,
1388 nigel 75 .sp
1389 nigel 63 (?<=(?<!foo)bar)baz
1390 nigel 75 .sp
1391 nigel 63 matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1392     preceded by "foo", while
1393 nigel 75 .sp
1394     (?<=\ed{3}(?!999)...)foo
1395     .sp
1396     is another pattern that matches "foo" preceded by three digits and any three
1397 nigel 63 characters that are not "999".
1398 nigel 75 .
1399     .
1400 nigel 91 .\" HTML <a name="conditions"></a>
1401 nigel 75 .SH "CONDITIONAL SUBPATTERNS"
1402 nigel 63 .rs
1403     .sp
1404     It is possible to cause the matching process to obey a subpattern
1405     conditionally or to choose between two alternative subpatterns, depending on
1406     the result of an assertion, or whether a previous capturing subpattern matched
1407     or not. The two possible forms of conditional subpattern are
1408 nigel 75 .sp
1409 nigel 63 (?(condition)yes-pattern)
1410     (?(condition)yes-pattern|no-pattern)
1411 nigel 75 .sp
1412 nigel 63 If the condition is satisfied, the yes-pattern is used; otherwise the
1413     no-pattern (if present) is used. If there are more than two alternatives in the
1414     subpattern, a compile-time error occurs.
1415 nigel 75 .P
1416 nigel 63 There are three kinds of condition. If the text between the parentheses
1417 nigel 91 consists of a sequence of digits, or a sequence of alphanumeric characters and
1418     underscores, the condition is satisfied if the capturing subpattern of that
1419     number or name has previously matched. There is a possible ambiguity here,
1420     because subpattern names may consist entirely of digits. PCRE looks first for a
1421     named subpattern; if it cannot find one and the text consists entirely of
1422     digits, it looks for a subpattern of that number, which must be greater than
1423     zero. Using subpattern names that consist entirely of digits is not
1424     recommended.
1425     .P
1426     Consider the following pattern, which contains non-significant white space to
1427     make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1428     three parts for ease of discussion:
1429 nigel 75 .sp
1430     ( \e( )? [^()]+ (?(1) \e) )
1431     .sp
1432 nigel 63 The first part matches an optional opening parenthesis, and if that
1433     character is present, sets it as the first captured substring. The second part
1434     matches one or more characters that are not parentheses. The third part is a
1435     conditional subpattern that tests whether the first set of parentheses matched
1436     or not. If they did, that is, if subject started with an opening parenthesis,
1437     the condition is true, and so the yes-pattern is executed and a closing
1438     parenthesis is required. Otherwise, since no-pattern is not present, the
1439     subpattern matches nothing. In other words, this pattern matches a sequence of
1440 nigel 91 non-parentheses, optionally enclosed in parentheses. Rewriting it to use a
1441     named subpattern gives this:
1442     .sp
1443     (?P<OPEN> \e( )? [^()]+ (?(OPEN) \e) )
1444     .sp
1445     If the condition is the string (R), and there is no subpattern with the name R,
1446     the condition is satisfied if a recursive call to the pattern or subpattern has
1447     been made. At "top level", the condition is false. This is a PCRE extension.
1448     Recursive patterns are described in the next section.
1449 nigel 75 .P
1450 nigel 63 If the condition is not a sequence of digits or (R), it must be an assertion.
1451     This may be a positive or negative lookahead or lookbehind assertion. Consider
1452     this pattern, again containing non-significant white space, and with the two
1453     alternatives on the second line:
1454 nigel 75 .sp
1455 nigel 63 (?(?=[^a-z]*[a-z])
1456 nigel 75 \ed{2}-[a-z]{3}-\ed{2} | \ed{2}-\ed{2}-\ed{2} )
1457     .sp
1458 nigel 63 The condition is a positive lookahead assertion that matches an optional
1459     sequence of non-letters followed by a letter. In other words, it tests for the
1460     presence of at least one letter in the subject. If a letter is found, the
1461     subject is matched against the first alternative; otherwise it is matched
1462     against the second. This pattern matches strings in one of the two forms
1463     dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1464 nigel 75 .
1465     .
1466     .\" HTML <a name="comments"></a>
1467 nigel 63 .SH COMMENTS
1468     .rs
1469     .sp
1470 nigel 75 The sequence (?# marks the start of a comment that continues up to the next
1471 nigel 63 closing parenthesis. Nested parentheses are not permitted. The characters
1472     that make up a comment play no part in the pattern matching at all.
1473 nigel 75 .P
1474 nigel 63 If the PCRE_EXTENDED option is set, an unescaped # character outside a
1475 nigel 91 character class introduces a comment that continues to immediately after the
1476     next newline in the pattern.
1477 nigel 75 .
1478     .
1479 nigel 91 .\" HTML <a name="recursion"></a>
1480 nigel 75 .SH "RECURSIVE PATTERNS"
1481 nigel 63 .rs
1482     .sp
1483     Consider the problem of matching a string in parentheses, allowing for
1484     unlimited nested parentheses. Without the use of recursion, the best that can
1485     be done is to use a pattern that matches up to some fixed depth of nesting. It
1486 nigel 75 is not possible to handle an arbitrary nesting depth. Perl provides a facility
1487     that allows regular expressions to recurse (amongst other things). It does this
1488     by interpolating Perl code in the expression at run time, and the code can
1489     refer to the expression itself. A Perl pattern to solve the parentheses problem
1490     can be created like this:
1491     .sp
1492     $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1493     .sp
1494 nigel 63 The (?p{...}) item interpolates Perl code at run time, and in this case refers
1495     recursively to the pattern in which it appears. Obviously, PCRE cannot support
1496     the interpolation of Perl code. Instead, it supports some special syntax for
1497     recursion of the entire pattern, and also for individual subpattern recursion.
1498 nigel 75 .P
1499 nigel 63 The special item that consists of (? followed by a number greater than zero and
1500     a closing parenthesis is a recursive call of the subpattern of the given
1501     number, provided that it occurs inside that subpattern. (If not, it is a
1502     "subroutine" call, which is described in the next section.) The special item
1503     (?R) is a recursive call of the entire regular expression.
1504 nigel 75 .P
1505 nigel 87 A recursive subpattern call is always treated as an atomic group. That is, once
1506     it has matched some of the subject string, it is never re-entered, even if
1507     it contains untried alternatives and there is a subsequent matching failure.
1508     .P
1509     This PCRE pattern solves the nested parentheses problem (assume the
1510     PCRE_EXTENDED option is set so that white space is ignored):
1511 nigel 75 .sp
1512     \e( ( (?>[^()]+) | (?R) )* \e)
1513     .sp
1514 nigel 63 First it matches an opening parenthesis. Then it matches any number of
1515     substrings which can either be a sequence of non-parentheses, or a recursive
1516 nigel 87 match of the pattern itself (that is, a correctly parenthesized substring).
1517 nigel 63 Finally there is a closing parenthesis.
1518 nigel 75 .P
1519 nigel 63 If this were part of a larger pattern, you would not want to recurse the entire
1520     pattern, so instead you could use this:
1521 nigel 75 .sp
1522     ( \e( ( (?>[^()]+) | (?1) )* \e) )
1523     .sp
1524 nigel 63 We have put the pattern into parentheses, and caused the recursion to refer to
1525     them instead of the whole pattern. In a larger pattern, keeping track of
1526     parenthesis numbers can be tricky. It may be more convenient to use named
1527     parentheses instead. For this, PCRE uses (?P>name), which is an extension to
1528     the Python syntax that PCRE uses for named parentheses (Perl does not provide
1529     named parentheses). We could rewrite the above example as follows:
1530 nigel 75 .sp
1531     (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )
1532     .sp
1533 nigel 63 This particular example pattern contains nested unlimited repeats, and so the
1534     use of atomic grouping for matching strings of non-parentheses is important
1535     when applying the pattern to strings that do not match. For example, when this
1536     pattern is applied to
1537 nigel 75 .sp
1538 nigel 63 (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1539 nigel 75 .sp
1540 nigel 63 it yields "no match" quickly. However, if atomic grouping is not used,
1541     the match runs for a very long time indeed because there are so many different
1542     ways the + and * repeats can carve up the subject, and all have to be tested
1543     before failure can be reported.
1544 nigel 75 .P
1545 nigel 63 At the end of a match, the values set for any capturing subpatterns are those
1546     from the outermost level of the recursion at which the subpattern value is set.
1547     If you want to obtain intermediate values, a callout function can be used (see
1548 nigel 75 the next section and the
1549 nigel 63 .\" HREF
1550 nigel 75 \fBpcrecallout\fP
1551 nigel 63 .\"
1552     documentation). If the pattern above is matched against
1553 nigel 75 .sp
1554 nigel 63 (ab(cd)ef)
1555 nigel 75 .sp
1556 nigel 63 the value for the capturing parentheses is "ef", which is the last value taken
1557     on at the top level. If additional parentheses are added, giving
1558 nigel 75 .sp
1559     \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1560 nigel 63 ^ ^
1561     ^ ^
1562 nigel 75 .sp
1563 nigel 63 the string they capture is "ab(cd)ef", the contents of the top level
1564     parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1565     has to obtain extra memory to store data during a recursion, which it does by
1566 nigel 75 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1567 nigel 63 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1568 nigel 75 .P
1569 nigel 63 Do not confuse the (?R) item with the condition (R), which tests for recursion.
1570     Consider this pattern, which matches text in angle brackets, allowing for
1571     arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1572     recursing), whereas any characters are permitted at the outer level.
1573 nigel 75 .sp
1574     < (?: (?(R) \ed++ | [^<>]*+) | (?R)) * >
1575     .sp
1576 nigel 63 In this pattern, (?(R) is the start of a conditional subpattern, with two
1577     different alternatives for the recursive and non-recursive cases. The (?R) item
1578     is the actual recursive call.
1579 nigel 75 .
1580     .
1581 nigel 63 .\" HTML <a name="subpatternsassubroutines"></a>
1582 nigel 75 .SH "SUBPATTERNS AS SUBROUTINES"
1583 nigel 63 .rs
1584     .sp
1585     If the syntax for a recursive subpattern reference (either by number or by
1586     name) is used outside the parentheses to which it refers, it operates like a
1587     subroutine in a programming language. An earlier example pointed out that the
1588     pattern
1589 nigel 75 .sp
1590     (sens|respons)e and \e1ibility
1591     .sp
1592 nigel 63 matches "sense and sensibility" and "response and responsibility", but not
1593     "sense and responsibility". If instead the pattern
1594 nigel 75 .sp
1595 nigel 63 (sens|respons)e and (?1)ibility
1596 nigel 75 .sp
1597 nigel 63 is used, it does match "sense and responsibility" as well as the other two
1598 nigel 91 strings. Such references, if given numerically, must follow the subpattern to
1599     which they refer. However, named references can refer to later subpatterns.
1600 nigel 87 .P
1601     Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1602     group. That is, once it has matched some of the subject string, it is never
1603     re-entered, even if it contains untried alternatives and there is a subsequent
1604     matching failure.
1605 nigel 75 .
1606     .
1607 nigel 63 .SH CALLOUTS
1608     .rs
1609     .sp
1610     Perl has a feature whereby using the sequence (?{...}) causes arbitrary Perl
1611     code to be obeyed in the middle of matching a regular expression. This makes it
1612     possible, amongst other things, to extract different substrings that match the
1613     same pair of parentheses when there is a repetition.
1614 nigel 75 .P
1615 nigel 63 PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1616     code. The feature is called "callout". The caller of PCRE provides an external
1617 nigel 75 function by putting its entry point in the global variable \fIpcre_callout\fP.
1618 nigel 63 By default, this variable contains NULL, which disables all calling out.
1619 nigel 75 .P
1620 nigel 63 Within a regular expression, (?C) indicates the points at which the external
1621     function is to be called. If you want to identify different callout points, you
1622     can put a number less than 256 after the letter C. The default value is zero.
1623     For example, this pattern has two callout points:
1624 nigel 75 .sp
1625 nigel 63 (?C1)\dabc(?C2)def
1626 nigel 75 .sp
1627     If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1628     automatically installed before each item in the pattern. They are all numbered
1629     255.
1630     .P
1631     During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1632 nigel 63 set), the external function is called. It is provided with the number of the
1633 nigel 75 callout, the position in the pattern, and, optionally, one item of data
1634     originally supplied by the caller of \fBpcre_exec()\fP. The callout function
1635     may cause matching to proceed, to backtrack, or to fail altogether. A complete
1636     description of the interface to the callout function is given in the
1637 nigel 63 .\" HREF
1638 nigel 75 \fBpcrecallout\fP
1639 nigel 63 .\"
1640     documentation.
1641 nigel 75 .P
1642 nigel 63 .in 0
1643 nigel 91 Last updated: 06 June 2006
1644 nigel 63 .br
1645 nigel 87 Copyright (c) 1997-2006 University of Cambridge.

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