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1  .TH PCRE 3  .TH PCREPATTERN 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH PCRE REGULAR EXPRESSION DETAILS  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of other books, some of which have copious  .\" HREF
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  \fBpcresyntax\fP
11  O'Reilly, covers them in great detail. The description here is intended as  .\"
12  reference documentation.  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  The basic operation of PCRE is on strings of bytes. However, there is also  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15  support for UTF-8 character strings. To use this support you must build PCRE to  published by O'Reilly, covers regular expressions in great detail. This
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  description of PCRE's regular expressions is intended as reference material.
17  option. How this affects the pattern matching is mentioned in several places  .P
18  below. There is also a summary of UTF-8 features in the  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  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
24  .\" </a>  .\" </a>
25  section on UTF-8 support  section on UTF-8 support
26  .\"  .\"
27  in the main  in the main
28  .\" HREF  .\" HREF
29  \fBpcre\fR  \fBpcre\fP
30    .\"
31    page.
32    .P
33    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    Perl-compatible. Some of the features discussed below are not available when
38    \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    .\" HREF
42    \fBpcrematching\fP
43  .\"  .\"
44  page.  page.
45    .
46    .
47    .SH "CHARACTERS AND METACHARACTERS"
48    .rs
49    .sp
50  A regular expression is a pattern that is matched against a subject string from  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  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  corresponding characters in the subject. As a trivial example, the pattern
53    .sp
54    The quick brown fox    The quick brown fox
55    .sp
56  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
57  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
58  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In UTF-8 mode, PCRE always understands the concept of
59  \fImeta-characters\fR, which do not stand for themselves but instead are  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    \fImetacharacters\fP, which do not stand for themselves but instead are
69  interpreted in some special way.  interpreted in some special way.
70    .P
71  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
72  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
73  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
74  as follows:  are as follows:
75    .sp
76    \\      general escape character with several uses    \e      general escape character with several uses
77    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
78    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
79    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 88  as follows:
88    +      1 or more quantifier    +      1 or more quantifier
89           also "possessive quantifier"           also "possessive quantifier"
90    {      start min/max quantifier    {      start min/max quantifier
91    .sp
92  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
93  a character class the only meta-characters are:  a character class the only metacharacters are:
94    .sp
95    \\      general escape character    \e      general escape character
96    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
97    -      indicates character range    -      indicates character range
98    .\" JOIN
99    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
100             syntax)             syntax)
101    ]      terminates the character class    ]      terminates the character class
102    .sp
103  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
104    .
105    .
106  .SH BACKSLASH  .SH BACKSLASH
107  .rs  .rs
108  .sp  .sp
109  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
110  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
111  have. This use of backslash as an escape character applies both inside and  may have. This use of backslash as an escape character applies both inside and
112  outside character classes.  outside character classes.
113    .P
114  For example, if you want to match a * character, you write \\* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
115  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
116  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
117  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
118  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
119    .P
120  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  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  pattern (other than in a character class) and characters between a # outside
122  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
123  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
124  pattern.  .P
   
125  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
126  can do so by putting them between \\Q and \\E. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
127  that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
128  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
129    .sp
130    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
131    .sp
132    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
133      \eQabc$xyz\eE        abc$xyz        abc followed by the
134                                        contents of $xyz                                        contents of $xyz
135    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
136    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
137    .sp
138  The \\Q...\\E sequence is recognized both inside and outside character classes.  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  A second use of backslash provides a way of encoding non-printing characters  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  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,  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  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  use one of the following escape sequences than the binary character it
150  represents:  represents:
151    .sp
152    \\a        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
153    \\cx       "control-x", where x is any character    \ecx       "control-x", where x is any character
154    \\e        escape (hex 1B)    \ee        escape (hex 1B)
155    \\f        formfeed (hex 0C)    \ef        formfeed (hex 0C)
156    \\n        newline (hex 0A)    \en        newline (hex 0A)
157    \\r        carriage return (hex 0D)    \er        carriage return (hex 0D)
158    \\t        tab (hex 09)    \et        tab (hex 09)
159    \\ddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
160    \\xhh      character with hex code hh    \exhh      character with hex code hh
161    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
162    .sp
163  The precise effect of \\cx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
164  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
165  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
166  7B.  7B.
167    .P
168  After \\x, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
169  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
170  appear between \\x{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
171  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
172  other than hexadecimal digits appear between \\x{ and }, or if there is no  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
173  terminating }, this form of escape is not recognized. Instead, the initial  and }, or if there is no terminating }, this form of escape is not recognized.
174  \\x will be interpreted as a basic hexadecimal escape, with no following  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
175  digits, giving a byte whose value is zero.  with no following digits, giving a character whose value is zero.
176    .P
177  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
178  syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
179  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
180    .P
181  After \\0 up to two further octal digits are read. In both cases, if there  After \e0 up to two further octal digits are read. If there are fewer than two
182  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \e0\ex\e07
183  sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
184  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
185  character that follows is itself an octal digit.  follows is itself an octal digit.
186    .P
187  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
188  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
189  number. If the number is less than 10, or if there have been at least that many  number. If the number is less than 10, or if there have been at least that many
190  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
191  taken as a \fIback reference\fR. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
192  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
193    .\" </a>
194    later,
195    .\"
196    following the discussion of
197    .\" HTML <a href="#subpattern">
198    .\" </a>
199    parenthesized subpatterns.
200    .\"
201    .P
202  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
203  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
204  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
205  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
206  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
207    to \e777 are permitted. For example:
208    \\040   is another way of writing a space  .sp
209    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
210    .\" JOIN
211      \e40    is the same, provided there are fewer than 40
212              previous capturing subpatterns              previous capturing subpatterns
213    \\7     is always a back reference    \e7     is always a back reference
214    \\11    might be a back reference, or another way of  .\" JOIN
215      \e11    might be a back reference, or another way of
216              writing a tab              writing a tab
217    \\011   is always a tab    \e011   is always a tab
218    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
219    \\113   might be a back reference, otherwise the  .\" JOIN
220      \e113   might be a back reference, otherwise the
221              character with octal code 113              character with octal code 113
222    \\377   might be a back reference, otherwise  .\" JOIN
223      \e377   might be a back reference, otherwise
224              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
225    \\81    is either a back reference, or a binary zero  .\" JOIN
226      \e81    is either a back reference, or a binary zero
227              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
228    .sp
229  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
230  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
231    .P
232  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
233  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
234  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
235  backspace character (hex 08). Outside a character class it has a different  sequences \eR and \eX are interpreted as the characters "R" and "X",
236  meaning (see below).  respectively. Outside a character class, these sequences have different
237    meanings
238  The third use of backslash is for specifying generic character types:  .\" HTML <a href="#uniextseq">
239    .\" </a>
240    \\d     any decimal digit  (see below).
241    \\D     any character that is not a decimal digit  .\"
242    \\s     any whitespace character  .
243    \\S     any character that is not a whitespace character  .
244    \\w     any "word" character  .SS "Absolute and relative back references"
245    \\W     any "non-word" character  .rs
246    .sp
247    The sequence \eg followed by an unsigned or a negative number, optionally
248    enclosed in braces, is an absolute or relative back reference. A named back
249    reference can be coded as \eg{name}. Back references are discussed
250    .\" HTML <a href="#backreferences">
251    .\" </a>
252    later,
253    .\"
254    following the discussion of
255    .\" HTML <a href="#subpattern">
256    .\" </a>
257    parenthesized subpatterns.
258    .\"
259    .
260    .
261    .SS "Generic character types"
262    .rs
263    .sp
264    Another use of backslash is for specifying generic character types. The
265    following are always recognized:
266    .sp
267      \ed     any decimal digit
268      \eD     any character that is not a decimal digit
269      \eh     any horizontal whitespace character
270      \eH     any character that is not a horizontal whitespace character
271      \es     any whitespace character
272      \eS     any character that is not a whitespace character
273      \ev     any vertical whitespace character
274      \eV     any character that is not a vertical whitespace character
275      \ew     any "word" character
276      \eW     any "non-word" character
277    .sp
278  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
279  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
280    .P
281  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  These character type sequences can appear both inside and outside character
282  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
283    matching point is at the end of the subject string, all of them fail, since
284  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
285  This makes it different from the the POSIX "space" class. The \\s characters  .P
286  are HT (9), LF (10), FF (12), CR (13), and space (32).  For compatibility with Perl, \es does not match the VT character (code 11).
287    This makes it different from the the POSIX "space" class. The \es characters
288  A "word" character is any letter or digit or the underscore character, that is,  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
289  any character which can be part of a Perl "word". The definition of letters and  included in a Perl script, \es may match the VT character. In PCRE, it never
290  digits is controlled by PCRE's character tables, and may vary if locale-  does.
291  specific matching is taking place (see  .P
292    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
293    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
294    character property support is available. These sequences retain their original
295    meanings from before UTF-8 support was available, mainly for efficiency
296    reasons.
297    .P
298    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
299    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
300    The horizontal space characters are:
301    .sp
302      U+0009     Horizontal tab
303      U+0020     Space
304      U+00A0     Non-break space
305      U+1680     Ogham space mark
306      U+180E     Mongolian vowel separator
307      U+2000     En quad
308      U+2001     Em quad
309      U+2002     En space
310      U+2003     Em space
311      U+2004     Three-per-em space
312      U+2005     Four-per-em space
313      U+2006     Six-per-em space
314      U+2007     Figure space
315      U+2008     Punctuation space
316      U+2009     Thin space
317      U+200A     Hair space
318      U+202F     Narrow no-break space
319      U+205F     Medium mathematical space
320      U+3000     Ideographic space
321    .sp
322    The vertical space characters are:
323    .sp
324      U+000A     Linefeed
325      U+000B     Vertical tab
326      U+000C     Formfeed
327      U+000D     Carriage return
328      U+0085     Next line
329      U+2028     Line separator
330      U+2029     Paragraph separator
331    .P
332    A "word" character is an underscore or any character less than 256 that is a
333    letter or digit. The definition of letters and digits is controlled by PCRE's
334    low-valued character tables, and may vary if locale-specific matching is taking
335    place (see
336  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
337  .\" </a>  .\" </a>
338  "Locale support"  "Locale support"
339  .\"  .\"
340  in the  in the
341  .\" HREF  .\" HREF
342  \fBpcreapi\fR  \fBpcreapi\fP
343  .\"  .\"
344  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
345  than 128 are used for accented letters, and these are matched by \\w.  or "french" in Windows, some character codes greater than 128 are used for
346    accented letters, and these are matched by \ew. The use of locales with Unicode
347  These character type sequences can appear both inside and outside character  is discouraged.
348  classes. They each match one character of the appropriate type. If the current  .
349  matching point is at the end of the subject string, all of them fail, since  .
350  there is no character to match.  .SS "Newline sequences"
351    .rs
352  The fourth use of backslash is for certain simple assertions. An assertion  .sp
353    Outside a character class, the escape sequence \eR matches any Unicode newline
354    sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
355    the following:
356    .sp
357      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
358    .sp
359    This is an example of an "atomic group", details of which are given
360    .\" HTML <a href="#atomicgroup">
361    .\" </a>
362    below.
363    .\"
364    This particular group matches either the two-character sequence CR followed by
365    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
366    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
367    line, U+0085). The two-character sequence is treated as a single unit that
368    cannot be split.
369    .P
370    In UTF-8 mode, two additional characters whose codepoints are greater than 255
371    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
372    Unicode character property support is not needed for these characters to be
373    recognized.
374    .P
375    Inside a character class, \eR matches the letter "R".
376    .
377    .
378    .\" HTML <a name="uniextseq"></a>
379    .SS Unicode character properties
380    .rs
381    .sp
382    When PCRE is built with Unicode character property support, three additional
383    escape sequences that match characters with specific properties are available.
384    When not in UTF-8 mode, these sequences are of course limited to testing
385    characters whose codepoints are less than 256, but they do work in this mode.
386    The extra escape sequences are:
387    .sp
388      \ep{\fIxx\fP}   a character with the \fIxx\fP property
389      \eP{\fIxx\fP}   a character without the \fIxx\fP property
390      \eX       an extended Unicode sequence
391    .sp
392    The property names represented by \fIxx\fP above are limited to the Unicode
393    script names, the general category properties, and "Any", which matches any
394    character (including newline). Other properties such as "InMusicalSymbols" are
395    not currently supported by PCRE. Note that \eP{Any} does not match any
396    characters, so always causes a match failure.
397    .P
398    Sets of Unicode characters are defined as belonging to certain scripts. A
399    character from one of these sets can be matched using a script name. For
400    example:
401    .sp
402      \ep{Greek}
403      \eP{Han}
404    .sp
405    Those that are not part of an identified script are lumped together as
406    "Common". The current list of scripts is:
407    .P
408    Arabic,
409    Armenian,
410    Balinese,
411    Bengali,
412    Bopomofo,
413    Braille,
414    Buginese,
415    Buhid,
416    Canadian_Aboriginal,
417    Cherokee,
418    Common,
419    Coptic,
420    Cuneiform,
421    Cypriot,
422    Cyrillic,
423    Deseret,
424    Devanagari,
425    Ethiopic,
426    Georgian,
427    Glagolitic,
428    Gothic,
429    Greek,
430    Gujarati,
431    Gurmukhi,
432    Han,
433    Hangul,
434    Hanunoo,
435    Hebrew,
436    Hiragana,
437    Inherited,
438    Kannada,
439    Katakana,
440    Kharoshthi,
441    Khmer,
442    Lao,
443    Latin,
444    Limbu,
445    Linear_B,
446    Malayalam,
447    Mongolian,
448    Myanmar,
449    New_Tai_Lue,
450    Nko,
451    Ogham,
452    Old_Italic,
453    Old_Persian,
454    Oriya,
455    Osmanya,
456    Phags_Pa,
457    Phoenician,
458    Runic,
459    Shavian,
460    Sinhala,
461    Syloti_Nagri,
462    Syriac,
463    Tagalog,
464    Tagbanwa,
465    Tai_Le,
466    Tamil,
467    Telugu,
468    Thaana,
469    Thai,
470    Tibetan,
471    Tifinagh,
472    Ugaritic,
473    Yi.
474    .P
475    Each character has exactly one general category property, specified by a
476    two-letter abbreviation. For compatibility with Perl, negation can be specified
477    by including a circumflex between the opening brace and the property name. For
478    example, \ep{^Lu} is the same as \eP{Lu}.
479    .P
480    If only one letter is specified with \ep or \eP, it includes all the general
481    category properties that start with that letter. In this case, in the absence
482    of negation, the curly brackets in the escape sequence are optional; these two
483    examples have the same effect:
484    .sp
485      \ep{L}
486      \epL
487    .sp
488    The following general category property codes are supported:
489    .sp
490      C     Other
491      Cc    Control
492      Cf    Format
493      Cn    Unassigned
494      Co    Private use
495      Cs    Surrogate
496    .sp
497      L     Letter
498      Ll    Lower case letter
499      Lm    Modifier letter
500      Lo    Other letter
501      Lt    Title case letter
502      Lu    Upper case letter
503    .sp
504      M     Mark
505      Mc    Spacing mark
506      Me    Enclosing mark
507      Mn    Non-spacing mark
508    .sp
509      N     Number
510      Nd    Decimal number
511      Nl    Letter number
512      No    Other number
513    .sp
514      P     Punctuation
515      Pc    Connector punctuation
516      Pd    Dash punctuation
517      Pe    Close punctuation
518      Pf    Final punctuation
519      Pi    Initial punctuation
520      Po    Other punctuation
521      Ps    Open punctuation
522    .sp
523      S     Symbol
524      Sc    Currency symbol
525      Sk    Modifier symbol
526      Sm    Mathematical symbol
527      So    Other symbol
528    .sp
529      Z     Separator
530      Zl    Line separator
531      Zp    Paragraph separator
532      Zs    Space separator
533    .sp
534    The special property L& is also supported: it matches a character that has
535    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
536    a modifier or "other".
537    .P
538    The long synonyms for these properties that Perl supports (such as \ep{Letter})
539    are not supported by PCRE, nor is it permitted to prefix any of these
540    properties with "Is".
541    .P
542    No character that is in the Unicode table has the Cn (unassigned) property.
543    Instead, this property is assumed for any code point that is not in the
544    Unicode table.
545    .P
546    Specifying caseless matching does not affect these escape sequences. For
547    example, \ep{Lu} always matches only upper case letters.
548    .P
549    The \eX escape matches any number of Unicode characters that form an extended
550    Unicode sequence. \eX is equivalent to
551    .sp
552      (?>\ePM\epM*)
553    .sp
554    That is, it matches a character without the "mark" property, followed by zero
555    or more characters with the "mark" property, and treats the sequence as an
556    atomic group
557    .\" HTML <a href="#atomicgroup">
558    .\" </a>
559    (see below).
560    .\"
561    Characters with the "mark" property are typically accents that affect the
562    preceding character. None of them have codepoints less than 256, so in
563    non-UTF-8 mode \eX matches any one character.
564    .P
565    Matching characters by Unicode property is not fast, because PCRE has to search
566    a structure that contains data for over fifteen thousand characters. That is
567    why the traditional escape sequences such as \ed and \ew do not use Unicode
568    properties in PCRE.
569    .
570    .
571    .\" HTML <a name="resetmatchstart"></a>
572    .SS "Resetting the match start"
573    .rs
574    .sp
575    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
576    matched characters not to be included in the final matched sequence. For
577    example, the pattern:
578    .sp
579      foo\eKbar
580    .sp
581    matches "foobar", but reports that it has matched "bar". This feature is
582    similar to a lookbehind assertion
583    .\" HTML <a href="#lookbehind">
584    .\" </a>
585    (described below).
586    .\"
587    However, in this case, the part of the subject before the real match does not
588    have to be of fixed length, as lookbehind assertions do. The use of \eK does
589    not interfere with the setting of
590    .\" HTML <a href="#subpattern">
591    .\" </a>
592    captured substrings.
593    .\"
594    For example, when the pattern
595    .sp
596      (foo)\eKbar
597    .sp
598    matches "foobar", the first substring is still set to "foo".
599    .
600    .
601    .\" HTML <a name="smallassertions"></a>
602    .SS "Simple assertions"
603    .rs
604    .sp
605    The final use of backslash is for certain simple assertions. An assertion
606  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
607  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
608  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
609  assertions are  .\" HTML <a href="#bigassertions">
610    .\" </a>
611    \\b     matches at a word boundary  below.
612    \\B     matches when not at a word boundary  .\"
613    \\A     matches at start of subject  The backslashed assertions are:
614    \\Z     matches at end of subject or before newline at end  .sp
615    \\z     matches at end of subject    \eb     matches at a word boundary
616    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
617      \eA     matches at the start of the subject
618  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
619              also matches before a newline at the end of the subject
620      \ez     matches only at the end of the subject
621      \eG     matches at the first matching position in the subject
622    .sp
623    These assertions may not appear in character classes (but note that \eb has a
624  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
625    .P
626  A word boundary is a position in the subject string where the current character  A word boundary is a position in the subject string where the current character
627  and the previous character do not both match \\w or \\W (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
628  \\w and the other matches \\W), or the start or end of the string if the  \ew and the other matches \eW), or the start or end of the string if the
629  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
630    .P
631  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
632  dollar (described below) in that they only ever match at the very start and end  dollar (described in the next section) in that they only ever match at the very
633  of the subject string, whatever options are set. Thus, they are independent of  start and end of the subject string, whatever options are set. Thus, they are
634  multiline mode.  independent of multiline mode. These three assertions are not affected by the
635    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
636  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
637  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
638  matching is to start at a point other than the beginning of the subject, \\A  at a point other than the beginning of the subject, \eA can never match. The
639  can never match. The difference between \\Z and \\z is that \\Z matches before  difference between \eZ and \ez is that \eZ matches before a newline at the end
640  a newline that is the last character of the string as well as at the end of the  of the string as well as at the very end, whereas \ez matches only at the end.
641  string, whereas \\z matches only at the end.  .P
642    The \eG assertion is true only when the current matching position is at the
643  The \\G assertion is true only when the current matching position is at the  start point of the match, as specified by the \fIstartoffset\fP argument of
644  start point of the match, as specified by the \fIstartoffset\fR argument of  \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
645  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
 non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  
646  arguments, you can mimic Perl's /g option, and it is in this kind of  arguments, you can mimic Perl's /g option, and it is in this kind of
647  implementation where \\G can be useful.  implementation where \eG can be useful.
648    .P
649  Note, however, that PCRE's interpretation of \\G, as the start of the current  Note, however, that PCRE's interpretation of \eG, as the start of the current
650  match, is subtly different from Perl's, which defines it as the end of the  match, is subtly different from Perl's, which defines it as the end of the
651  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
652  string was empty. Because PCRE does just one match at a time, it cannot  string was empty. Because PCRE does just one match at a time, it cannot
653  reproduce this behaviour.  reproduce this behaviour.
654    .P
655  If all the alternatives of a pattern begin with \\G, the expression is anchored  If all the alternatives of a pattern begin with \eG, the expression is anchored
656  to the starting match position, and the "anchored" flag is set in the compiled  to the starting match position, and the "anchored" flag is set in the compiled
657  regular expression.  regular expression.
658    .
659  .SH CIRCUMFLEX AND DOLLAR  .
660    .SH "CIRCUMFLEX AND DOLLAR"
661  .rs  .rs
662  .sp  .sp
663  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
664  character is an assertion which is true only if the current matching point is  character is an assertion that is true only if the current matching point is
665  at the start of the subject string. If the \fIstartoffset\fR argument of  at the start of the subject string. If the \fIstartoffset\fP argument of
666  \fBpcre_exec()\fR is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
667  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
668  meaning (see below).  meaning
669    .\" HTML <a href="#characterclass">
670    .\" </a>
671    (see below).
672    .\"
673    .P
674  Circumflex need not be the first character of the pattern if a number of  Circumflex need not be the first character of the pattern if a number of
675  alternatives are involved, but it should be the first thing in each alternative  alternatives are involved, but it should be the first thing in each alternative
676  in which it appears if the pattern is ever to match that branch. If all  in which it appears if the pattern is ever to match that branch. If all
# Line 290  possible alternatives start with a circu Line 678  possible alternatives start with a circu
678  constrained to match only at the start of the subject, it is said to be an  constrained to match only at the start of the subject, it is said to be an
679  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
680  to be anchored.)  to be anchored.)
681    .P
682  A dollar character is an assertion which is true only if the current matching  A dollar character is an assertion that is true only if the current matching
683  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline
684  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
685  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
686  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
687  Dollar has no special meaning in a character class.  character class.
688    .P
689  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
690  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
691  does not affect the \\Z assertion.  does not affect the \eZ assertion.
692    .P
693  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
694  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
695  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
696  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
697  the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,  matches before any newlines in the string, as well as at the very end, when
698  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
699  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
700  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
701  \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
702  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
703    patterns that are anchored in single line mode because all branches start with
704  Note that the sequences \\A, \\Z, and \\z can be used to match the start and  ^ are not anchored in multiline mode, and a match for circumflex is possible
705    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
706    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
707    .P
708    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
709  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
710  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
711    .
712  .SH FULL STOP (PERIOD, DOT)  .
713    .SH "FULL STOP (PERIOD, DOT)"
714  .rs  .rs
715  .sp  .sp
716  Outside a character class, a dot in the pattern matches any one character in  Outside a character class, a dot in the pattern matches any one character in
717  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
718  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
719  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
720  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
721  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
722  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
723    (including isolated CRs and LFs). When any Unicode line endings are being
724  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
725  .rs  characters.
726  .sp  .P
727  Outside a character class, the escape sequence \\C matches any one byte, both  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
728  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  option is set, a dot matches any one character, without exception. If the
729  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  two-character sequence CRLF is present in the subject string, it takes two dots
730  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
731  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
732    The handling of dot is entirely independent of the handling of circumflex and
733  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  dollar, the only relationship being that they both involve newlines. Dot has no
734  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
735    .
736  .SH SQUARE BRACKETS  .
737    .SH "MATCHING A SINGLE BYTE"
738    .rs
739    .sp
740    Outside a character class, the escape sequence \eC matches any one byte, both
741    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
742    characters. The feature is provided in Perl in order to match individual bytes
743    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
744    what remains in the string may be a malformed UTF-8 string. For this reason,
745    the \eC escape sequence is best avoided.
746    .P
747    PCRE does not allow \eC to appear in lookbehind assertions
748    .\" HTML <a href="#lookbehind">
749    .\" </a>
750    (described below),
751    .\"
752    because in UTF-8 mode this would make it impossible to calculate the length of
753    the lookbehind.
754    .
755    .
756    .\" HTML <a name="characterclass"></a>
757    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
758  .rs  .rs
759  .sp  .sp
760  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
# Line 348  square bracket. A closing square bracket Line 762  square bracket. A closing square bracket
762  closing square bracket is required as a member of the class, it should be the  closing square bracket is required as a member of the class, it should be the
763  first data character in the class (after an initial circumflex, if present) or  first data character in the class (after an initial circumflex, if present) or
764  escaped with a backslash.  escaped with a backslash.
765    .P
766  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
767  character may occupy more than one byte. A matched character must be in the set  character may occupy more than one byte. A matched character must be in the set
768  of characters defined by the class, unless the first character in the class  of characters defined by the class, unless the first character in the class
# Line 356  definition is a circumflex, in which cas Line 770  definition is a circumflex, in which cas
770  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
771  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
772  backslash.  backslash.
773    .P
774  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
775  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
776  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
777  are in the class by enumerating those that are not. It is not an assertion: it  are in the class by enumerating those that are not. A class that starts with a
778  still consumes a character from the subject string, and fails if the current  circumflex is not an assertion: it still consumes a character from the subject
779  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
780    string.
781    .P
782  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 mode, characters with values greater than 255 can be included in a
783  class as a literal string of bytes, or by using the \\x{ escaping mechanism.  class as a literal string of bytes, or by using the \ex{ escaping mechanism.
784    .P
785  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
786  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
787  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
788  caseful version would. PCRE does not support the concept of case for characters  caseful version would. In UTF-8 mode, PCRE always understands the concept of
789  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
790    always possible. For characters with higher values, the concept of case is
791  The newline character is never treated in any special way in character classes,  supported if PCRE is compiled with Unicode property support, but not otherwise.
792  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  If you want to use caseless matching for characters 128 and above, you must
793  such as [^a] will always match a newline.  ensure that PCRE is compiled with Unicode property support as well as with
794    UTF-8 support.
795    .P
796    Characters that might indicate line breaks are never treated in any special way
797    when matching character classes, whatever line-ending sequence is in use, and
798    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
799    such as [^a] always matches one of these characters.
800    .P
801  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
802  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
803  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
804  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
805  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class.
806    .P
807  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
808  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
809  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
810  "-46]". However, if the "]" is escaped with a backslash it is interpreted as  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
811  the end of range, so [W-\\]46] is interpreted as a single class containing a  the end of range, so [W-\e]46] is interpreted as a class containing a range
812  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
813  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
814    .P
815  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
816  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
817  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
818  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
819    .P
820  If a range that includes letters is used when caseless matching is set, it  If a range that includes letters is used when caseless matching is set, it
821  matches the letters in either case. For example, [W-c] is equivalent to  matches the letters in either case. For example, [W-c] is equivalent to
822  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
823  locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.  tables for a French locale are in use, [\exc8-\excb] matches accented E
824    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
825  The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a  characters with values greater than 128 only when it is compiled with Unicode
826  character class, and add the characters that they match to the class. For  property support.
827  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
828    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
829    in a character class, and add the characters that they match to the class. For
830    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
831  conveniently be used with the upper case character types to specify a more  conveniently be used with the upper case character types to specify a more
832  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
833  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
834    .P
835  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
836  terminating ] are non-special in character classes, but it does no harm if they  hyphen (only where it can be interpreted as specifying a range), circumflex
837  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
838    introducing a POSIX class name - see the next section), and the terminating
839  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
840    does no harm.
841    .
842    .
843    .SH "POSIX CHARACTER CLASSES"
844  .rs  .rs
845  .sp  .sp
846  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
847  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
848  this notation. For example,  this notation. For example,
849    .sp
850    [01[:alpha:]%]    [01[:alpha:]%]
851    .sp
852  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
853  are  are
854    .sp
855    alnum    letters and digits    alnum    letters and digits
856    alpha    letters    alpha    letters
857    ascii    character codes 0 - 127    ascii    character codes 0 - 127
858    blank    space or tab only    blank    space or tab only
859    cntrl    control characters    cntrl    control characters
860    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
861    graph    printing characters, excluding space    graph    printing characters, excluding space
862    lower    lower case letters    lower    lower case letters
863    print    printing characters, including space    print    printing characters, including space
864    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
865    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
866    upper    upper case letters    upper    upper case letters
867    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
868    xdigit   hexadecimal digits    xdigit   hexadecimal digits
869    .sp
870  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
871  space (32). Notice that this list includes the VT character (code 11). This  space (32). Notice that this list includes the VT character (code 11). This
872  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
873  compatibility).  compatibility).
874    .P
875  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
876  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
877  after the colon. For example,  after the colon. For example,
878    .sp
879    [12[:^digit:]]    [12[:^digit:]]
880    .sp
881  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
882  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not  syntax [.ch.] and [=ch=] where "ch" is a "collating element", but these are not
883  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
884    .P
885  In UTF-8 mode, characters with values greater than 255 do not match any of  In UTF-8 mode, characters with values greater than 128 do not match any of
886  the POSIX character classes.  the POSIX character classes.
887    .
888  .SH VERTICAL BAR  .
889    .SH "VERTICAL BAR"
890  .rs  .rs
891  .sp  .sp
892  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
893  the pattern  the pattern
894    .sp
895    gilbert|sullivan    gilbert|sullivan
896    .sp
897  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
898  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
899  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
900  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
901  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
902  pattern as well as the alternative in the subpattern.  .\" </a>
903    (defined below),
904  .SH INTERNAL OPTION SETTING  .\"
905    "succeeds" means matching the rest of the main pattern as well as the
906    alternative in the subpattern.
907    .
908    .
909    .SH "INTERNAL OPTION SETTING"
910  .rs  .rs
911  .sp  .sp
912  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
913  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
914  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
915    .sp
916    i  for PCRE_CASELESS    i  for PCRE_CASELESS
917    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
918    s  for PCRE_DOTALL    s  for PCRE_DOTALL
919    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
920    .sp
921  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
922  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
923  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
924  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
925  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
926  unset.  unset.
927    .P
928  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
929  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
930  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
931  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
932  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
933    .P
934  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
935  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
936    .sp
937    (a(?i)b)c    (a(?i)b)c
938    .sp
939  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
940  By this means, options can be made to have different settings in different  By this means, options can be made to have different settings in different
941  parts of the pattern. Any changes made in one alternative do carry on  parts of the pattern. Any changes made in one alternative do carry on
942  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
943    .sp
944    (a(?i)b|c)    (a(?i)b|c)
945    .sp
946  matches "ab", "aB", "c", and "C", even though when matching "C" the first  matches "ab", "aB", "c", and "C", even though when matching "C" the first
947  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
948  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
949  behaviour otherwise.  behaviour otherwise.
950    .P
951  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
952  same way as the Perl-compatible options by using the characters U and X  changed in the same way as the Perl-compatible options by using the characters
953  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
954  earlier in the pattern than any of the additional features it turns on, even  .
955  when it is at top level. It is best put at the start.  .
956    .\" HTML <a name="subpattern"></a>
957  .SH SUBPATTERNS  .SH SUBPATTERNS
958  .rs  .rs
959  .sp  .sp
960  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
961  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
962    .sp
963  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
964    .sp
965    cat(aract|erpillar|)    cat(aract|erpillar|)
966    .sp
967  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
968  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
969    .sp
970  2. It sets up the subpattern as a capturing subpattern (as defined above).  2. It sets up the subpattern as a capturing subpattern. This means that, when
971  When the whole pattern matches, that portion of the subject string that matched  the whole pattern matches, that portion of the subject string that matched the
972  the subpattern is passed back to the caller via the \fIovector\fR argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
973  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
974  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
975    .P
976  For example, if the string "the red king" is matched against the pattern  For example, if the string "the red king" is matched against the pattern
977    .sp
978    the ((red|white) (king|queen))    the ((red|white) (king|queen))
979    .sp
980  the captured substrings are "red king", "red", and "king", and are numbered 1,  the captured substrings are "red king", "red", and "king", and are numbered 1,
981  2, and 3, respectively.  2, and 3, respectively.
982    .P
983  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
984  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
985  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
986  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
987  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
988  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
989    .sp
990    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
991    .sp
992  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
993  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
994  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
995  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
996  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
997  the ":". Thus the two patterns  the ":". Thus the two patterns
998    .sp
999    (?i:saturday|sunday)    (?i:saturday|sunday)
1000    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1001    .sp
1002  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1003  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
1004  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
1005  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1006    .
1007  .SH NAMED SUBPATTERNS  .
1008    .SH "DUPLICATE SUBPATTERN NUMBERS"
1009    .rs
1010    .sp
1011    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1012    the same numbers for its capturing parentheses. Such a subpattern starts with
1013    (?| and is itself a non-capturing subpattern. For example, consider this
1014    pattern:
1015    .sp
1016      (?|(Sat)ur|(Sun))day
1017    .sp
1018    Because the two alternatives are inside a (?| group, both sets of capturing
1019    parentheses are numbered one. Thus, when the pattern matches, you can look
1020    at captured substring number one, whichever alternative matched. This construct
1021    is useful when you want to capture part, but not all, of one of a number of
1022    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1023    number is reset at the start of each branch. The numbers of any capturing
1024    buffers that follow the subpattern start after the highest number used in any
1025    branch. The following example is taken from the Perl documentation.
1026    The numbers underneath show in which buffer the captured content will be
1027    stored.
1028    .sp
1029      # before  ---------------branch-reset----------- after
1030      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1031      # 1            2         2  3        2     3     4
1032    .sp
1033    A backreference or a recursive call to a numbered subpattern always refers to
1034    the first one in the pattern with the given number.
1035    .P
1036    An alternative approach to using this "branch reset" feature is to use
1037    duplicate named subpatterns, as described in the next section.
1038    .
1039    .
1040    .SH "NAMED SUBPATTERNS"
1041  .rs  .rs
1042  .sp  .sp
1043  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1044  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1045  if an expression is modified, the numbers may change. To help with the  if an expression is modified, the numbers may change. To help with this
1046  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1047  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1048  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1049    the Perl and the Python syntax.
1050  Named capturing parentheses are still allocated numbers as well as names. The  .P
1051  PCRE API provides function calls for extracting the name-to-number translation  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1052  table from a compiled pattern. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1053    parentheses from other parts of the pattern, such as
1054    .\" HTML <a href="#backreferences">
1055    .\" </a>
1056    backreferences,
1057    .\"
1058    .\" HTML <a href="#recursion">
1059    .\" </a>
1060    recursion,
1061    .\"
1062    and
1063    .\" HTML <a href="#conditions">
1064    .\" </a>
1065    conditions,
1066    .\"
1067    can be made by name as well as by number.
1068    .P
1069    Names consist of up to 32 alphanumeric characters and underscores. Named
1070    capturing parentheses are still allocated numbers as well as names, exactly as
1071    if the names were not present. The PCRE API provides function calls for
1072    extracting the name-to-number translation table from a compiled pattern. There
1073    is also a convenience function for extracting a captured substring by name.
1074    .P
1075    By default, a name must be unique within a pattern, but it is possible to relax
1076    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1077    be useful for patterns where only one instance of the named parentheses can
1078    match. Suppose you want to match the name of a weekday, either as a 3-letter
1079    abbreviation or as the full name, and in both cases you want to extract the
1080    abbreviation. This pattern (ignoring the line breaks) does the job:
1081    .sp
1082      (?<DN>Mon|Fri|Sun)(?:day)?|
1083      (?<DN>Tue)(?:sday)?|
1084      (?<DN>Wed)(?:nesday)?|
1085      (?<DN>Thu)(?:rsday)?|
1086      (?<DN>Sat)(?:urday)?
1087    .sp
1088    There are five capturing substrings, but only one is ever set after a match.
1089    (An alternative way of solving this problem is to use a "branch reset"
1090    subpattern, as described in the previous section.)
1091    .P
1092    The convenience function for extracting the data by name returns the substring
1093    for the first (and in this example, the only) subpattern of that name that
1094    matched. This saves searching to find which numbered subpattern it was. If you
1095    make a reference to a non-unique named subpattern from elsewhere in the
1096    pattern, the one that corresponds to the lowest number is used. For further
1097    details of the interfaces for handling named subpatterns, see the
1098  .\" HREF  .\" HREF
1099  \fBpcreapi\fR  \fBpcreapi\fP
1100  .\"  .\"
1101  documentation.  documentation.
1102    .
1103    .
1104  .SH REPETITION  .SH REPETITION
1105  .rs  .rs
1106  .sp  .sp
1107  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1108  items:  items:
1109    .sp
1110    a literal data character    a literal data character
1111    the . metacharacter    the dot metacharacter
1112    the \\C escape sequence    the \eC escape sequence
1113    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1114      the \eR escape sequence
1115      an escape such as \ed that matches a single character
1116    a character class    a character class
1117    a back reference (see next section)    a back reference (see next section)
1118    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1119    .sp
1120  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1121  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1122  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
1123  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1124    .sp
1125    z{2,4}    z{2,4}
1126    .sp
1127  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1128  character. If the second number is omitted, but the comma is present, there is  character. If the second number is omitted, but the comma is present, there is
1129  no upper limit; if the second number and the comma are both omitted, the  no upper limit; if the second number and the comma are both omitted, the
1130  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1131    .sp
1132    [aeiou]{3,}    [aeiou]{3,}
1133    .sp
1134  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1135    .sp
1136    \\d{8}    \ed{8}
1137    .sp
1138  matches exactly 8 digits. An opening curly bracket that appears in a position  matches exactly 8 digits. An opening curly bracket that appears in a position
1139  where a quantifier is not allowed, or one that does not match the syntax of a  where a quantifier is not allowed, or one that does not match the syntax of a
1140  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
1141  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1142    .P
1143  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1144  bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1145  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1146    support is available, \eX{3} matches three Unicode extended sequences, each of
1147    which may be several bytes long (and they may be of different lengths).
1148    .P
1149  The quantifier {0} is permitted, causing the expression to behave as if the  The quantifier {0} is permitted, causing the expression to behave as if the
1150  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1151    .P
1152  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1153  quantifiers have single-character abbreviations:  abbreviations:
1154    .sp
1155    *    is equivalent to {0,}    *    is equivalent to {0,}
1156    +    is equivalent to {1,}    +    is equivalent to {1,}
1157    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1158    .sp
1159  It is possible to construct infinite loops by following a subpattern that can  It is possible to construct infinite loops by following a subpattern that can
1160  match no characters with a quantifier that has no upper limit, for example:  match no characters with a quantifier that has no upper limit, for example:
1161    .sp
1162    (a?)*    (a?)*
1163    .sp
1164  Earlier versions of Perl and PCRE used to give an error at compile time for  Earlier versions of Perl and PCRE used to give an error at compile time for
1165  such patterns. However, because there are cases where this can be useful, such  such patterns. However, because there are cases where this can be useful, such
1166  patterns are now accepted, but if any repetition of the subpattern does in fact  patterns are now accepted, but if any repetition of the subpattern does in fact
1167  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1168    .P
1169  By default, the quantifiers are "greedy", that is, they match as much as  By default, the quantifiers are "greedy", that is, they match as much as
1170  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1171  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
1172  is in trying to match comments in C programs. These appear between the  is in trying to match comments in C programs. These appear between /* and */
1173  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1174  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1175    .sp
1176    /\\*.*\\*/    /\e*.*\e*/
1177    .sp
1178  to the string  to the string
1179    .sp
1180    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1181    .sp
1182  fails, because it matches the entire string owing to the greediness of the .*  fails, because it matches the entire string owing to the greediness of the .*
1183  item.  item.
1184    .P
1185  However, if a quantifier is followed by a question mark, it ceases to be  However, if a quantifier is followed by a question mark, it ceases to be
1186  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1187  pattern  pattern
1188    .sp
1189    /\\*.*?\\*/    /\e*.*?\e*/
1190    .sp
1191  does the right thing with the C comments. The meaning of the various  does the right thing with the C comments. The meaning of the various
1192  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1193  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
1194  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
1195    .sp
1196    \\d??\\d    \ed??\ed
1197    .sp
1198  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1199  way the rest of the pattern matches.  way the rest of the pattern matches.
1200    .P
1201  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1202  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1203  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1204  default behaviour.  default behaviour.
1205    .P
1206  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1207  is greater than 1 or with a limited maximum, more store is required for the  is greater than 1 or with a limited maximum, more memory is required for the
1208  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1209    .P
1210  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1211  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1212  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1213  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1214  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
1215  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1216    .P
1217  In cases where it is known that the subject string contains no newlines, it is  In cases where it is known that the subject string contains no newlines, it is
1218  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1219  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1220    .P
1221  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1222  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1223  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1224  succeed. Consider, for example:  succeeds. Consider, for example:
1225    .sp
1226    (.*)abc\\1    (.*)abc\e1
1227    .sp
1228  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1229  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1230    .P
1231  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1232  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1233    .sp
1234    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1235    .sp
1236  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1237  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1238  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1239  example, after  example, after
1240    .sp
1241    /(a|(b))+/    /(a|(b))+/
1242    .sp
1243  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1244    .
1245  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1246    .\" HTML <a name="atomicgroup"></a>
1247    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1248  .rs  .rs
1249  .sp  .sp
1250  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1251  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1252  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1253  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1254  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1255  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1256    .P
1257  Consider, for example, the pattern \\d+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1258    .sp
1259    123456bar    123456bar
1260    .sp
1261  After matching all 6 digits and then failing to match "foo", the normal  After matching all 6 digits and then failing to match "foo", the normal
1262  action of the matcher is to try again with only 5 digits matching the \\d+  action of the matcher is to try again with only 5 digits matching the \ed+
1263  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1264  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1265  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1266    .P
1267  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1268  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1269  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1270    .sp
1271    (?>\\d+)bar    (?>\ed+)foo
1272    .sp
1273  This kind of parenthesis "locks up" the  part of the pattern it contains once  This kind of parenthesis "locks up" the  part of the pattern it contains once
1274  it has matched, and a failure further into the pattern is prevented from  it has matched, and a failure further into the pattern is prevented from
1275  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1276  normal.  normal.
1277    .P
1278  An alternative description is that a subpattern of this type matches the string  An alternative description is that a subpattern of this type matches the string
1279  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1280  the current point in the subject string.  the current point in the subject string.
1281    .P
1282  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1283  the above example can be thought of as a maximizing repeat that must swallow  the above example can be thought of as a maximizing repeat that must swallow
1284  everything it can. So, while both \\d+ and \\d+? are prepared to adjust the  everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1285  number of digits they match in order to make the rest of the pattern match,  number of digits they match in order to make the rest of the pattern match,
1286  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1287    .P
1288  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1289  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1290  group is just a single repeated item, as in the example above, a simpler  group is just a single repeated item, as in the example above, a simpler
1291  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1292  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1293  previous example can be rewritten as  previous example can be rewritten as
1294    .sp
1295    \\d++bar    \ed++foo
1296    .sp
1297    Note that a possessive quantifier can be used with an entire group, for
1298    example:
1299    .sp
1300      (abc|xyz){2,3}+
1301    .sp
1302  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1303  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1304  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1305  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1306    difference; possessive quantifiers should be slightly faster.
1307  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1308  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1309    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1310    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1311    package, and PCRE copied it from there. It ultimately found its way into Perl
1312    at release 5.10.
1313    .P
1314    PCRE has an optimization that automatically "possessifies" certain simple
1315    pattern constructs. For example, the sequence A+B is treated as A++B because
1316    there is no point in backtracking into a sequence of A's when B must follow.
1317    .P
1318  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1319  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
1320  only way to avoid some failing matches taking a very long time indeed. The  only way to avoid some failing matches taking a very long time indeed. The
1321  pattern  pattern
1322    .sp
1323    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1324    .sp
1325  matches an unlimited number of substrings that either consist of non-digits, or  matches an unlimited number of substrings that either consist of non-digits, or
1326  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1327  quickly. However, if it is applied to  quickly. However, if it is applied to
1328    .sp
1329    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1330    .sp
1331  it takes a long time before reporting failure. This is because the string can  it takes a long time before reporting failure. This is because the string can
1332  be divided between the two repeats in a large number of ways, and all have to  be divided between the internal \eD+ repeat and the external * repeat in a
1333  be tried. (The example used [!?] rather than a single character at the end,  large number of ways, and all have to be tried. (The example uses [!?] rather
1334  because both PCRE and Perl have an optimization that allows for fast failure  than a single character at the end, because both PCRE and Perl have an
1335  when a single character is used. They remember the last single character that  optimization that allows for fast failure when a single character is used. They
1336  is required for a match, and fail early if it is not present in the string.)  remember the last single character that is required for a match, and fail early
1337  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1338    an atomic group, like this:
1339    ((?>\\D+)|<\\d+>)*[!?]  .sp
1340      ((?>\eD+)|<\ed+>)*[!?]
1341    .sp
1342  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1343    .
1344  .SH BACK REFERENCES  .
1345    .\" HTML <a name="backreferences"></a>
1346    .SH "BACK REFERENCES"
1347  .rs  .rs
1348  .sp  .sp
1349  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
1350  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1351  (that is, to its left) in the pattern, provided there have been that many  (that is, to its left) in the pattern, provided there have been that many
1352  previous capturing left parentheses.  previous capturing left parentheses.
1353    .P
1354  However, if the decimal number following the backslash is less than 10, it is  However, if the decimal number following the backslash is less than 10, it is
1355  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1356  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1357  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1358  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. A "forward back reference" of this type can make sense
1359  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1360    in an earlier iteration.
1361    .P
1362    It is not possible to have a numerical "forward back reference" to a subpattern
1363    whose number is 10 or more using this syntax because a sequence such as \e50 is
1364    interpreted as a character defined in octal. See the subsection entitled
1365    "Non-printing characters"
1366    .\" HTML <a href="#digitsafterbackslash">
1367    .\" </a>
1368    above
1369    .\"
1370    for further details of the handling of digits following a backslash. There is
1371    no such problem when named parentheses are used. A back reference to any
1372    subpattern is possible using named parentheses (see below).
1373    .P
1374    Another way of avoiding the ambiguity inherent in the use of digits following a
1375    backslash is to use the \eg escape sequence, which is a feature introduced in
1376    Perl 5.10. This escape must be followed by an unsigned number or a negative
1377    number, optionally enclosed in braces. These examples are all identical:
1378    .sp
1379      (ring), \e1
1380      (ring), \eg1
1381      (ring), \eg{1}
1382    .sp
1383    An unsigned number specifies an absolute reference without the ambiguity that
1384    is present in the older syntax. It is also useful when literal digits follow
1385    the reference. A negative number is a relative reference. Consider this
1386    example:
1387    .sp
1388      (abc(def)ghi)\eg{-1}
1389    .sp
1390    The sequence \eg{-1} is a reference to the most recently started capturing
1391    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1392    would be equivalent to \e1. The use of relative references can be helpful in
1393    long patterns, and also in patterns that are created by joining together
1394    fragments that contain references within themselves.
1395    .P
1396  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1397  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1398  itself (see  itself (see
# Line 832  itself (see Line 1401  itself (see
1401  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1402  .\"  .\"
1403  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1404    .sp
1405    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1406    .sp
1407  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1408  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1409  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1410    .sp
1411    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1412    .sp
1413  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1414  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1415    .P
1416  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1417  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1418    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1419    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1420    references, is also supported. We could rewrite the above example in any of
1421    the following ways:
1422    .sp
1423      (?<p1>(?i)rah)\es+\ek<p1>
1424      (?'p1'(?i)rah)\es+\ek{p1}
1425      (?P<p1>(?i)rah)\es+(?P=p1)
1426      (?<p1>(?i)rah)\es+\eg{p1}
1427    .sp
1428    A subpattern that is referenced by name may appear in the pattern before or
1429    after the reference.
1430    .P
1431  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1432  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1433  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1434    .sp
1435    (a|(bc))\\2    (a|(bc))\e2
1436    .sp
1437  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". Because there may be
1438  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1439  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1440  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1441  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1442  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1443    .\" HTML <a href="#comments">
1444    .\" </a>
1445    "Comments"
1446    .\"
1447    below) can be used.
1448    .P
1449  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1450  when the subpattern is first used, so, for example, (a\\1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1451  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1452  example, the pattern  example, the pattern
1453    .sp
1454    (a|b\\1)+    (a|b\e1)+
1455    .sp
1456  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1457  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1458  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1459  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1460  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1461  minimum of zero.  minimum of zero.
1462    .
1463    .
1464    .\" HTML <a name="bigassertions"></a>
1465  .SH ASSERTIONS  .SH ASSERTIONS
1466  .rs  .rs
1467  .sp  .sp
1468  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
1469  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1470  assertions coded as \\b, \\B, \\A, \\G, \\Z, \\z, ^ and $ are described above.  assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1471    .\" HTML <a href="#smallassertions">
1472    .\" </a>
1473    above.
1474    .\"
1475    .P
1476  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1477  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1478  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1479    except that it does not cause the current matching position to be changed.
1480  An assertion subpattern is matched in the normal way, except that it does not  .P
1481  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1482  with (?= for positive assertions and (?! for negative assertions. For example,  because it makes no sense to assert the same thing several times. If any kind
1483    of assertion contains capturing subpatterns within it, these are counted for
1484    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1485    However, substring capturing is carried out only for positive assertions,
1486    because it does not make sense for negative assertions.
1487    .
1488    .
1489    .SS "Lookahead assertions"
1490    .rs
1491    .sp
1492    Lookahead assertions start with (?= for positive assertions and (?! for
1493    negative assertions. For example,
1494    .sp
1495      \ew+(?=;)
1496    .sp
1497  matches a word followed by a semicolon, but does not include the semicolon in  matches a word followed by a semicolon, but does not include the semicolon in
1498  the match, and  the match, and
1499    .sp
1500    foo(?!bar)    foo(?!bar)
1501    .sp
1502  matches any occurrence of "foo" that is not followed by "bar". Note that the  matches any occurrence of "foo" that is not followed by "bar". Note that the
1503  apparently similar pattern  apparently similar pattern
1504    .sp
1505    (?!foo)bar    (?!foo)bar
1506    .sp
1507  does not find an occurrence of "bar" that is preceded by something other than  does not find an occurrence of "bar" that is preceded by something other than
1508  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1509  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1510  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1511    .P
1512  If you want to force a matching failure at some point in a pattern, the most  If you want to force a matching failure at some point in a pattern, the most
1513  convenient way to do it is with (?!) because an empty string always matches, so  convenient way to do it is with (?!) because an empty string always matches, so
1514  an assertion that requires there not to be an empty string must always fail.  an assertion that requires there not to be an empty string must always fail.
1515    .
1516    .
1517    .\" HTML <a name="lookbehind"></a>
1518    .SS "Lookbehind assertions"
1519    .rs
1520    .sp
1521  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1522  negative assertions. For example,  negative assertions. For example,
1523    .sp
1524    (?<!foo)bar    (?<!foo)bar
1525    .sp
1526  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1527  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1528  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1529  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1530    .sp
1531    (?<=bullock|donkey)    (?<=bullock|donkey)
1532    .sp
1533  is permitted, but  is permitted, but
1534    .sp
1535    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1536    .sp
1537  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1538  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1539  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1540  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1541    .sp
1542    (?<=ab(c|de))    (?<=ab(c|de))
1543    .sp
1544  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
1545  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable if rewritten to use two top-level branches:
1546    .sp
1547    (?<=abc|abde)    (?<=abc|abde)
1548    .sp
1549    In some cases, the Perl 5.10 escape sequence \eK
1550    .\" HTML <a href="#resetmatchstart">
1551    .\" </a>
1552    (see above)
1553    .\"
1554    can be used instead of a lookbehind assertion; this is not restricted to a
1555    fixed-length.
1556    .P
1557  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1558  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1559  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1560  match is deemed to fail.  assertion fails.
1561    .P
1562  PCRE does not allow the \\C escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1563  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1564  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1565    different numbers of bytes, are also not permitted.
1566  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1567  efficient matching at the end of the subject string. Consider a simple pattern  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1568  such as  specify efficient matching at the end of the subject string. Consider a simple
1569    pattern such as
1570    .sp
1571    abcd$    abcd$
1572    .sp
1573  when applied to a long string that does not match. Because matching proceeds  when applied to a long string that does not match. Because matching proceeds
1574  from left to right, PCRE will look for each "a" in the subject and then see if  from left to right, PCRE will look for each "a" in the subject and then see if
1575  what follows matches the rest of the pattern. If the pattern is specified as  what follows matches the rest of the pattern. If the pattern is specified as
1576    .sp
1577    ^.*abcd$    ^.*abcd$
1578    .sp
1579  the initial .* matches the entire string at first, but when this fails (because  the initial .* matches the entire string at first, but when this fails (because
1580  there is no following "a"), it backtracks to match all but the last character,  there is no following "a"), it backtracks to match all but the last character,
1581  then all but the last two characters, and so on. Once again the search for "a"  then all but the last two characters, and so on. Once again the search for "a"
1582  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1583  if the pattern is written as  if the pattern is written as
1584    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1585    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1586    .sp
1587  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1588  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1589  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1590  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1591    .
1592    .
1593    .SS "Using multiple assertions"
1594    .rs
1595    .sp
1596  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1597    .sp
1598    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1599    .sp
1600  matches "foo" preceded by three digits that are not "999". Notice that each of  matches "foo" preceded by three digits that are not "999". Notice that each of
1601  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1602  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
1603  digits, and then there is a check that the same three characters are not "999".  digits, and then there is a check that the same three characters are not "999".
1604  This pattern does \fInot\fR match "foo" preceded by six characters, the first  This pattern does \fInot\fP match "foo" preceded by six characters, the first
1605  of which are digits and the last three of which are not "999". For example, it  of which are digits and the last three of which are not "999". For example, it
1606  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1607    .sp
1608    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1609    .sp
1610  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1611  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1612  preceding three characters are not "999".  preceding three characters are not "999".
1613    .P
1614  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1615    .sp
1616    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1617    .sp
1618  matches an occurrence of "baz" that is preceded by "bar" which in turn is not  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1619  preceded by "foo", while  preceded by "foo", while
1620    .sp
1621    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1622    .sp
1623  is another pattern which matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
1624  characters that are not "999".  characters that are not "999".
1625    .
1626  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1627  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1628  of assertion contains capturing subpatterns within it, these are counted for  .SH "CONDITIONAL SUBPATTERNS"
 the purposes of numbering the capturing subpatterns in the whole pattern.  
 However, substring capturing is carried out only for positive assertions,  
 because it does not make sense for negative assertions.  
   
 .SH CONDITIONAL SUBPATTERNS  
1629  .rs  .rs
1630  .sp  .sp
1631  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1632  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1633  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1634  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1635    .sp
1636    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1637    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1638    .sp
1639  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1640  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1641  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1642    .P
1643  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1644  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1645  subpattern of that number has previously matched. The number must be greater  .
1646  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1647  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1648  it into three parts for ease of discussion:  .sp
1649    If the text between the parentheses consists of a sequence of digits, the
1650    ( \\( )?    [^()]+    (?(1) \\) )  condition is true if the capturing subpattern of that number has previously
1651    matched. An alternative notation is to precede the digits with a plus or minus
1652    sign. In this case, the subpattern number is relative rather than absolute.
1653    The most recently opened parentheses can be referenced by (?(-1), the next most
1654    recent by (?(-2), and so on. In looping constructs it can also make sense to
1655    refer to subsequent groups with constructs such as (?(+2).
1656    .P
1657    Consider the following pattern, which contains non-significant white space to
1658    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1659    three parts for ease of discussion:
1660    .sp
1661      ( \e( )?    [^()]+    (?(1) \e) )
1662    .sp
1663  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1664  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
1665  matches one or more characters that are not parentheses. The third part is a  matches one or more characters that are not parentheses. The third part is a
# Line 1047  the condition is true, and so the yes-pa Line 1669  the condition is true, and so the yes-pa
1669  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1670  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1671  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1672    .P
1673  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
1674  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1675  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1676      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1677  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
1678    This makes the fragment independent of the parentheses in the larger pattern.
1679    .
1680    .SS "Checking for a used subpattern by name"
1681    .rs
1682    .sp
1683    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1684    subpattern by name. For compatibility with earlier versions of PCRE, which had
1685    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1686    there is a possible ambiguity with this syntax, because subpattern names may
1687    consist entirely of digits. PCRE looks first for a named subpattern; if it
1688    cannot find one and the name consists entirely of digits, PCRE looks for a
1689    subpattern of that number, which must be greater than zero. Using subpattern
1690    names that consist entirely of digits is not recommended.
1691    .P
1692    Rewriting the above example to use a named subpattern gives this:
1693    .sp
1694      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1695    .sp
1696    .
1697    .SS "Checking for pattern recursion"
1698    .rs
1699    .sp
1700    If the condition is the string (R), and there is no subpattern with the name R,
1701    the condition is true if a recursive call to the whole pattern or any
1702    subpattern has been made. If digits or a name preceded by ampersand follow the
1703    letter R, for example:
1704    .sp
1705      (?(R3)...) or (?(R&name)...)
1706    .sp
1707    the condition is true if the most recent recursion is into the subpattern whose
1708    number or name is given. This condition does not check the entire recursion
1709    stack.
1710    .P
1711    At "top level", all these recursion test conditions are false. Recursive
1712    patterns are described below.
1713    .
1714    .SS "Defining subpatterns for use by reference only"
1715    .rs
1716    .sp
1717    If the condition is the string (DEFINE), and there is no subpattern with the
1718    name DEFINE, the condition is always false. In this case, there may be only one
1719    alternative in the subpattern. It is always skipped if control reaches this
1720    point in the pattern; the idea of DEFINE is that it can be used to define
1721    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1722    is described below.) For example, a pattern to match an IPv4 address could be
1723    written like this (ignore whitespace and line breaks):
1724    .sp
1725      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1726      \eb (?&byte) (\e.(?&byte)){3} \eb
1727    .sp
1728    The first part of the pattern is a DEFINE group inside which a another group
1729    named "byte" is defined. This matches an individual component of an IPv4
1730    address (a number less than 256). When matching takes place, this part of the
1731    pattern is skipped because DEFINE acts like a false condition.
1732    .P
1733    The rest of the pattern uses references to the named group to match the four
1734    dot-separated components of an IPv4 address, insisting on a word boundary at
1735    each end.
1736    .
1737    .SS "Assertion conditions"
1738    .rs
1739    .sp
1740    If the condition is not in any of the above formats, it must be an assertion.
1741  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1742  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1743  alternatives on the second line:  alternatives on the second line:
1744    .sp
1745    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1746    \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )    \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
1747    .sp
1748  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1749  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
1750  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
1751  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1752  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
1753  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1754    .
1755    .
1756    .\" HTML <a name="comments"></a>
1757  .SH COMMENTS  .SH COMMENTS
1758  .rs  .rs
1759  .sp  .sp
1760  The sequence (?# marks the start of a comment which continues up to the next  The sequence (?# marks the start of a comment that continues up to the next
1761  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1762  that make up a comment play no part in the pattern matching at all.  that make up a comment play no part in the pattern matching at all.
1763    .P
1764  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1765  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1766  character in the pattern.  next newline in the pattern.
1767    .
1768  .SH RECURSIVE PATTERNS  .
1769    .\" HTML <a name="recursion"></a>
1770    .SH "RECURSIVE PATTERNS"
1771  .rs  .rs
1772  .sp  .sp
1773  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1774  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1775  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
1776  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
1777  experimental facility that allows regular expressions to recurse (amongst other  .P
1778  things). It does this by interpolating Perl code in the expression at run time,  For some time, Perl has provided a facility that allows regular expressions to
1779  and the code can refer to the expression itself. A Perl pattern to solve the  recurse (amongst other things). It does this by interpolating Perl code in the
1780  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1781    pattern using code interpolation to solve the parentheses problem can be
1782    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
1783    .sp
1784      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1785    .sp
1786  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
1787  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1788  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1789  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1790    supports special syntax for recursion of the entire pattern, and also for
1791  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1792  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1793  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1794  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
1795  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1796    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1797  For example, this PCRE pattern solves the nested parentheses problem (assume  call, which is described in the next section.) The special item (?R) or (?0) is
1798  the PCRE_EXTENDED option is set so that white space is ignored):  a recursive call of the entire regular expression.
1799    .P
1800    \\( ( (?>[^()]+) | (?R) )* \\)  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1801    treated as an atomic group. That is, once it has matched some of the subject
1802    string, it is never re-entered, even if it contains untried alternatives and
1803    there is a subsequent matching failure.
1804    .P
1805    This PCRE pattern solves the nested parentheses problem (assume the
1806    PCRE_EXTENDED option is set so that white space is ignored):
1807    .sp
1808      \e( ( (?>[^()]+) | (?R) )* \e)
1809    .sp
1810  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1811  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
1812  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1813  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1814    .P
1815  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
1816  pattern, so instead you could use this:  pattern, so instead you could use this:
1817    .sp
1818    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1819    .sp
1820  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
1821  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1822  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1823  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
1824  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. (A Perl 5.10 feature.)
1825  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1826    most recently opened parentheses preceding the recursion. In other words, a
1827    (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  negative number counts capturing parentheses leftwards from the point at which
1828    it is encountered.
1829  This particular example pattern contains nested unlimited repeats, and so the  .P
1830  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1831  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
1832  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1833    "subroutine" calls, as described in the next section.
1834    .P
1835    An alternative approach is to use named parentheses instead. The Perl syntax
1836    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1837    could rewrite the above example as follows:
1838    .sp
1839      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1840    .sp
1841    If there is more than one subpattern with the same name, the earliest one is
1842    used.
1843    .P
1844    This particular example pattern that we have been looking at contains nested
1845    unlimited repeats, and so the use of atomic grouping for matching strings of
1846    non-parentheses is important when applying the pattern to strings that do not
1847    match. For example, when this pattern is applied to
1848    .sp
1849    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1850    .sp
1851  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1852  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
1853  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
1854  before failure can be reported.  before failure can be reported.
1855    .P
1856  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1857  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1858  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1859  below and the  below and the
1860  .\" HREF  .\" HREF
1861  \fBpcrecallout\fR  \fBpcrecallout\fP
1862  .\"  .\"
1863  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1864    .sp
1865    (ab(cd)ef)    (ab(cd)ef)
1866    .sp
1867  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
1868  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1869    .sp
1870    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1871       ^                        ^       ^                        ^
1872       ^                        ^       ^                        ^
1873    .sp
1874  the string they capture is "ab(cd)ef", the contents of the top level  the string they capture is "ab(cd)ef", the contents of the top level
1875  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1876  has to obtain extra memory to store data during a recursion, which it does by  has to obtain extra memory to store data during a recursion, which it does by
1877  using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1878  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1879    .P
1880  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
1881  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1882  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1883  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1884    .sp
1885    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1886    .sp
1887  In this pattern, (?(R) is the start of a conditional subpattern, with two  In this pattern, (?(R) is the start of a conditional subpattern, with two
1888  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1889  is the actual recursive call.  is the actual recursive call.
1890    .
1891    .
1892  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1893  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1894  .rs  .rs
1895  .sp  .sp
1896  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
1897  name) is used outside the parentheses to which it refers, it operates like a  name) is used outside the parentheses to which it refers, it operates like a
1898  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1899  pattern  before or after the reference. A numbered reference can be absolute or
1900    relative, as in these examples:
1901    (sens|respons)e and \\1ibility  .sp
1902      (...(absolute)...)...(?2)...
1903      (...(relative)...)...(?-1)...
1904      (...(?+1)...(relative)...
1905    .sp
1906    An earlier example pointed out that the pattern
1907    .sp
1908      (sens|respons)e and \e1ibility
1909    .sp
1910  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1911  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1912    .sp
1913    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1914    .sp
1915  is used, it does match "sense and responsibility" as well as the other two  is used, it does match "sense and responsibility" as well as the other two
1916  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
1917  refer.  .P
1918    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1919    group. That is, once it has matched some of the subject string, it is never
1920    re-entered, even if it contains untried alternatives and there is a subsequent
1921    matching failure.
1922    .P
1923    When a subpattern is used as a subroutine, processing options such as
1924    case-independence are fixed when the subpattern is defined. They cannot be
1925    changed for different calls. For example, consider this pattern:
1926    .sp
1927      (abc)(?i:(?-1))
1928    .sp
1929    It matches "abcabc". It does not match "abcABC" because the change of
1930    processing option does not affect the called subpattern.
1931    .
1932    .
1933  .SH CALLOUTS  .SH CALLOUTS
1934  .rs  .rs
1935  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1937  Perl has a feature whereby using the seq
1937  code to be obeyed in the middle of matching a regular expression. This makes it  code to be obeyed in the middle of matching a regular expression. This makes it
1938  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1939  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1940    .P
1941  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
1942  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
1943  function by putting its entry point in the global variable \fIpcre_callout\fR.  function by putting its entry point in the global variable \fIpcre_callout\fP.
1944  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1945    .P
1946  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1947  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
1948  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
1949  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1950    .sp
1951    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1952    .sp
1953  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
1954    automatically installed before each item in the pattern. They are all numbered
1955    255.
1956    .P
1957    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1958  set), the external function is called. It is provided with the number of the  set), the external function is called. It is provided with the number of the
1959  callout, and, optionally, one item of data originally supplied by the caller of  callout, the position in the pattern, and, optionally, one item of data
1960  \fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to  originally supplied by the caller of \fBpcre_exec()\fP. The callout function
1961  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1962  function is given in the  description of the interface to the callout function is given in the
1963  .\" HREF  .\" HREF
1964  \fBpcrecallout\fR  \fBpcrecallout\fP
1965  .\"  .\"
1966  documentation.  documentation.
1967    .
1968  .in 0  .
1969  Last updated: 03 February 2003  .SH "SEE ALSO"
1970  .br  .rs
1971  Copyright (c) 1997-2003 University of Cambridge.  .sp
1972    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1973    .
1974    .
1975    .SH AUTHOR
1976    .rs
1977    .sp
1978    .nf
1979    Philip Hazel
1980    University Computing Service
1981    Cambridge CB2 3QH, England.
1982    .fi
1983    .
1984    .
1985    .SH REVISION
1986    .rs
1987    .sp
1988    .nf
1989    Last updated: 06 August 2007
1990    Copyright (c) 1997-2007 University of Cambridge.
1991    .fi

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