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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 supported by PCRE are
8  described below. Regular expressions are also described in the Perl  described below. Regular expressions are also described in the Perl
9  documentation and in a number of other books, some of which have copious  documentation and in a number of books, some of which have copious examples.
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
11  O'Reilly, covers them in great detail. The description here is intended as  regular expressions in great detail. This description of PCRE's regular
12  reference documentation.  expressions is intended as reference material.
13    .P
14  The basic operation of PCRE is on strings of bytes. However, there is also  The original operation of PCRE was on strings of one-byte characters. However,
15  support for UTF-8 character strings. To use this support you must build PCRE to  there is now also support for UTF-8 character strings. To use this, you must
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
17  option. How this affects the pattern matching is mentioned in several places  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
18  below. There is also a summary of UTF-8 features in the  places below. There is also a summary of UTF-8 features in the
19  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
20  .\" </a>  .\" </a>
21  section on UTF-8 support  section on UTF-8 support
22  .\"  .\"
23  in the main  in the main
24  .\" HREF  .\" HREF
25  \fBpcre\fR  \fBpcre\fP
26  .\"  .\"
27  page.  page.
28    .P
29    The remainder of this document discusses the patterns that are supported by
30    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31    From release 6.0, PCRE offers a second matching function,
32    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33    Perl-compatible. Some of the features discussed below are not available when
34    \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
35    alternative function, and how it differs from the normal function, are
36    discussed in the
37    .\" HREF
38    \fBpcrematching\fP
39    .\"
40    page.
41    .
42    .
43    .SH "CHARACTERS AND METACHARACTERS"
44    .rs
45    .sp
46  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
47  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
48  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
49    .sp
50    The quick brown fox    The quick brown fox
51    .sp
52  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
53  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
54  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
55  \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
56    always possible. For characters with higher values, the concept of case is
57    supported if PCRE is compiled with Unicode property support, but not otherwise.
58    If you want to use caseless matching for characters 128 and above, you must
59    ensure that PCRE is compiled with Unicode property support as well as with
60    UTF-8 support.
61    .P
62    The power of regular expressions comes from the ability to include alternatives
63    and repetitions in the pattern. These are encoded in the pattern by the use of
64    \fImetacharacters\fP, which do not stand for themselves but instead are
65  interpreted in some special way.  interpreted in some special way.
66    .P
67  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
68  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
69  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
70  as follows:  are as follows:
71    .sp
72    \\      general escape character with several uses    \e      general escape character with several uses
73    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
74    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
75    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 84  as follows:
84    +      1 or more quantifier    +      1 or more quantifier
85           also "possessive quantifier"           also "possessive quantifier"
86    {      start min/max quantifier    {      start min/max quantifier
87    .sp
88  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
89  a character class the only meta-characters are:  a character class the only metacharacters are:
90    .sp
91    \\      general escape character    \e      general escape character
92    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
93    -      indicates character range    -      indicates character range
94    .\" JOIN
95    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
96             syntax)             syntax)
97    ]      terminates the character class    ]      terminates the character class
98    .sp
99  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
100    .
101    .
102  .SH BACKSLASH  .SH BACKSLASH
103  .rs  .rs
104  .sp  .sp
105  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
106  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
107  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
108  outside character classes.  outside character classes.
109    .P
110  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.
111  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
112  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
113  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
114  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
115    .P
116  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
117  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
118  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
119  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.
120  pattern.  .P
   
121  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
122  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
123  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
124  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
125    .sp
126    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
127    .sp
128    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
129      \eQabc$xyz\eE        abc$xyz        abc followed by the
130                                        contents of $xyz                                        contents of $xyz
131    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
132    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
133    .sp
134  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
135    .
136    .
137    .\" HTML <a name="digitsafterbackslash"></a>
138    .SS "Non-printing characters"
139    .rs
140    .sp
141  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
142  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
143  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
144  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is usually easier to
145  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
146  represents:  represents:
147    .sp
148    \\a        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
149    \\cx       "control-x", where x is any character    \ecx       "control-x", where x is any character
150    \\e        escape (hex 1B)    \ee        escape (hex 1B)
151    \\f        formfeed (hex 0C)    \ef        formfeed (hex 0C)
152    \\n        newline (hex 0A)    \en        newline (hex 0A)
153    \\r        carriage return (hex 0D)    \er        carriage return (hex 0D)
154    \\t        tab (hex 09)    \et        tab (hex 09)
155    \\ddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
156    \\xhh      character with hex code hh    \exhh      character with hex code hh
157    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
158    .sp
159  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
160  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.
161  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
162  7B.  7B.
163    .P
164  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
165  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{
166  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
167  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
168  other than hexadecimal digits appear between \\x{ and }, or if there is no  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
169  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.
170  \\x will be interpreted as a basic hexadecimal escape, with no following  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
171  digits, giving a byte whose value is zero.  with no following digits, giving a character whose value is zero.
172    .P
173  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
174  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
175  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
176    .P
177  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
178  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
179  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
180  (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
181  character that follows is itself an octal digit.  follows is itself an octal digit.
182    .P
183  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.
184  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
185  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
186  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
187  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
188  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
189    .\" </a>
190    later,
191    .\"
192    following the discussion of
193    .\" HTML <a href="#subpattern">
194    .\" </a>
195    parenthesized subpatterns.
196    .\"
197    .P
198  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
199  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
200  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
201  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
202  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
203    to \e777 are permitted. For example:
204    \\040   is another way of writing a space  .sp
205    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
206    .\" JOIN
207      \e40    is the same, provided there are fewer than 40
208              previous capturing subpatterns              previous capturing subpatterns
209    \\7     is always a back reference    \e7     is always a back reference
210    \\11    might be a back reference, or another way of  .\" JOIN
211      \e11    might be a back reference, or another way of
212              writing a tab              writing a tab
213    \\011   is always a tab    \e011   is always a tab
214    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
215    \\113   might be a back reference, otherwise the  .\" JOIN
216      \e113   might be a back reference, otherwise the
217              character with octal code 113              character with octal code 113
218    \\377   might be a back reference, otherwise  .\" JOIN
219      \e377   might be a back reference, otherwise
220              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
221    \\81    is either a back reference, or a binary zero  .\" JOIN
222      \e81    is either a back reference, or a binary zero
223              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
224    .sp
225  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
226  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
227    .P
228  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
229  (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
230  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
231  backspace character (hex 08). Outside a character class it has a different  sequences \eR and \eX are interpreted as the characters "R" and "X",
232  meaning (see below).  respectively. Outside a character class, these sequences have different
233    meanings
234  The third use of backslash is for specifying generic character types:  .\" HTML <a href="#uniextseq">
235    .\" </a>
236    \\d     any decimal digit  (see below).
237    \\D     any character that is not a decimal digit  .\"
238    \\s     any whitespace character  .
239    \\S     any character that is not a whitespace character  .
240    \\w     any "word" character  .SS "Absolute and relative back references"
241    \\W     any "non-word" character  .rs
242    .sp
243    The sequence \eg followed by a positive or negative number, optionally enclosed
244    in braces, is an absolute or relative back reference. A named back reference
245    can be coded as \eg{name}. Back references are discussed
246    .\" HTML <a href="#backreferences">
247    .\" </a>
248    later,
249    .\"
250    following the discussion of
251    .\" HTML <a href="#subpattern">
252    .\" </a>
253    parenthesized subpatterns.
254    .\"
255    .
256    .
257    .SS "Generic character types"
258    .rs
259    .sp
260    Another use of backslash is for specifying generic character types. The
261    following are always recognized:
262    .sp
263      \ed     any decimal digit
264      \eD     any character that is not a decimal digit
265      \eh     any horizontal whitespace character
266      \eH     any character that is not a horizontal whitespace character
267      \es     any whitespace character
268      \eS     any character that is not a whitespace character
269      \ev     any vertical whitespace character
270      \eV     any character that is not a vertical whitespace character
271      \ew     any "word" character
272      \eW     any "non-word" character
273    .sp
274  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
275  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
276    .P
277  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
278  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
279    matching point is at the end of the subject string, all of them fail, since
280  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
281  This makes it different from the the POSIX "space" class. The \\s characters  .P
282  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).
283    This makes it different from the the POSIX "space" class. The \es characters
284  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
285  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
286  digits is controlled by PCRE's character tables, and may vary if locale-  does.
287  specific matching is taking place (see  .P
288    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
289    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
290    character property support is available. These sequences retain their original
291    meanings from before UTF-8 support was available, mainly for efficiency
292    reasons.
293    .P
294    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
295    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
296    The horizontal space characters are:
297    .sp
298      U+0009     Horizontal tab
299      U+0020     Space
300      U+00A0     Non-break space
301      U+1680     Ogham space mark
302      U+180E     Mongolian vowel separator
303      U+2000     En quad
304      U+2001     Em quad
305      U+2002     En space
306      U+2003     Em space
307      U+2004     Three-per-em space
308      U+2005     Four-per-em space
309      U+2006     Six-per-em space
310      U+2007     Figure space
311      U+2008     Punctuation space
312      U+2009     Thin space
313      U+200A     Hair space
314      U+202F     Narrow no-break space
315      U+205F     Medium mathematical space
316      U+3000     Ideographic space
317    .sp
318    The vertical space characters are:
319    .sp
320      U+000A     Linefeed
321      U+000B     Vertical tab
322      U+000C     Formfeed
323      U+000D     Carriage return
324      U+0085     Next line
325      U+2028     Line separator
326      U+2029     Paragraph separator
327    .P
328    A "word" character is an underscore or any character less than 256 that is a
329    letter or digit. The definition of letters and digits is controlled by PCRE's
330    low-valued character tables, and may vary if locale-specific matching is taking
331    place (see
332  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
333  .\" </a>  .\" </a>
334  "Locale support"  "Locale support"
335  .\"  .\"
336  in the  in the
337  .\" HREF  .\" HREF
338  \fBpcreapi\fR  \fBpcreapi\fP
339  .\"  .\"
340  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,
341  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
342    accented letters, and these are matched by \ew. The use of locales with Unicode
343  These character type sequences can appear both inside and outside character  is discouraged.
344  classes. They each match one character of the appropriate type. If the current  .
345  matching point is at the end of the subject string, all of them fail, since  .
346  there is no character to match.  .SS "Newline sequences"
347    .rs
348  The fourth use of backslash is for certain simple assertions. An assertion  .sp
349    Outside a character class, the escape sequence \eR matches any Unicode newline
350    sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
351    the following:
352    .sp
353      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
354    .sp
355    This is an example of an "atomic group", details of which are given
356    .\" HTML <a href="#atomicgroup">
357    .\" </a>
358    below.
359    .\"
360    This particular group matches either the two-character sequence CR followed by
361    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
362    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
363    line, U+0085). The two-character sequence is treated as a single unit that
364    cannot be split.
365    .P
366    In UTF-8 mode, two additional characters whose codepoints are greater than 255
367    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
368    Unicode character property support is not needed for these characters to be
369    recognized.
370    .P
371    Inside a character class, \eR matches the letter "R".
372    .
373    .
374    .\" HTML <a name="uniextseq"></a>
375    .SS Unicode character properties
376    .rs
377    .sp
378    When PCRE is built with Unicode character property support, three additional
379    escape sequences that match characters with specific properties are available.
380    When not in UTF-8 mode, these sequences are of course limited to testing
381    characters whose codepoints are less than 256, but they do work in this mode.
382    The extra escape sequences are:
383    .sp
384      \ep{\fIxx\fP}   a character with the \fIxx\fP property
385      \eP{\fIxx\fP}   a character without the \fIxx\fP property
386      \eX       an extended Unicode sequence
387    .sp
388    The property names represented by \fIxx\fP above are limited to the Unicode
389    script names, the general category properties, and "Any", which matches any
390    character (including newline). Other properties such as "InMusicalSymbols" are
391    not currently supported by PCRE. Note that \eP{Any} does not match any
392    characters, so always causes a match failure.
393    .P
394    Sets of Unicode characters are defined as belonging to certain scripts. A
395    character from one of these sets can be matched using a script name. For
396    example:
397    .sp
398      \ep{Greek}
399      \eP{Han}
400    .sp
401    Those that are not part of an identified script are lumped together as
402    "Common". The current list of scripts is:
403    .P
404    Arabic,
405    Armenian,
406    Balinese,
407    Bengali,
408    Bopomofo,
409    Braille,
410    Buginese,
411    Buhid,
412    Canadian_Aboriginal,
413    Cherokee,
414    Common,
415    Coptic,
416    Cuneiform,
417    Cypriot,
418    Cyrillic,
419    Deseret,
420    Devanagari,
421    Ethiopic,
422    Georgian,
423    Glagolitic,
424    Gothic,
425    Greek,
426    Gujarati,
427    Gurmukhi,
428    Han,
429    Hangul,
430    Hanunoo,
431    Hebrew,
432    Hiragana,
433    Inherited,
434    Kannada,
435    Katakana,
436    Kharoshthi,
437    Khmer,
438    Lao,
439    Latin,
440    Limbu,
441    Linear_B,
442    Malayalam,
443    Mongolian,
444    Myanmar,
445    New_Tai_Lue,
446    Nko,
447    Ogham,
448    Old_Italic,
449    Old_Persian,
450    Oriya,
451    Osmanya,
452    Phags_Pa,
453    Phoenician,
454    Runic,
455    Shavian,
456    Sinhala,
457    Syloti_Nagri,
458    Syriac,
459    Tagalog,
460    Tagbanwa,
461    Tai_Le,
462    Tamil,
463    Telugu,
464    Thaana,
465    Thai,
466    Tibetan,
467    Tifinagh,
468    Ugaritic,
469    Yi.
470    .P
471    Each character has exactly one general category property, specified by a
472    two-letter abbreviation. For compatibility with Perl, negation can be specified
473    by including a circumflex between the opening brace and the property name. For
474    example, \ep{^Lu} is the same as \eP{Lu}.
475    .P
476    If only one letter is specified with \ep or \eP, it includes all the general
477    category properties that start with that letter. In this case, in the absence
478    of negation, the curly brackets in the escape sequence are optional; these two
479    examples have the same effect:
480    .sp
481      \ep{L}
482      \epL
483    .sp
484    The following general category property codes are supported:
485    .sp
486      C     Other
487      Cc    Control
488      Cf    Format
489      Cn    Unassigned
490      Co    Private use
491      Cs    Surrogate
492    .sp
493      L     Letter
494      Ll    Lower case letter
495      Lm    Modifier letter
496      Lo    Other letter
497      Lt    Title case letter
498      Lu    Upper case letter
499    .sp
500      M     Mark
501      Mc    Spacing mark
502      Me    Enclosing mark
503      Mn    Non-spacing mark
504    .sp
505      N     Number
506      Nd    Decimal number
507      Nl    Letter number
508      No    Other number
509    .sp
510      P     Punctuation
511      Pc    Connector punctuation
512      Pd    Dash punctuation
513      Pe    Close punctuation
514      Pf    Final punctuation
515      Pi    Initial punctuation
516      Po    Other punctuation
517      Ps    Open punctuation
518    .sp
519      S     Symbol
520      Sc    Currency symbol
521      Sk    Modifier symbol
522      Sm    Mathematical symbol
523      So    Other symbol
524    .sp
525      Z     Separator
526      Zl    Line separator
527      Zp    Paragraph separator
528      Zs    Space separator
529    .sp
530    The special property L& is also supported: it matches a character that has
531    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
532    a modifier or "other".
533    .P
534    The long synonyms for these properties that Perl supports (such as \ep{Letter})
535    are not supported by PCRE, nor is it permitted to prefix any of these
536    properties with "Is".
537    .P
538    No character that is in the Unicode table has the Cn (unassigned) property.
539    Instead, this property is assumed for any code point that is not in the
540    Unicode table.
541    .P
542    Specifying caseless matching does not affect these escape sequences. For
543    example, \ep{Lu} always matches only upper case letters.
544    .P
545    The \eX escape matches any number of Unicode characters that form an extended
546    Unicode sequence. \eX is equivalent to
547    .sp
548      (?>\ePM\epM*)
549    .sp
550    That is, it matches a character without the "mark" property, followed by zero
551    or more characters with the "mark" property, and treats the sequence as an
552    atomic group
553    .\" HTML <a href="#atomicgroup">
554    .\" </a>
555    (see below).
556    .\"
557    Characters with the "mark" property are typically accents that affect the
558    preceding character. None of them have codepoints less than 256, so in
559    non-UTF-8 mode \eX matches any one character.
560    .P
561    Matching characters by Unicode property is not fast, because PCRE has to search
562    a structure that contains data for over fifteen thousand characters. That is
563    why the traditional escape sequences such as \ed and \ew do not use Unicode
564    properties in PCRE.
565    .
566    .
567    .\" HTML <a name="resetmatchstart"></a>
568    .SS "Resetting the match start"
569    .rs
570    .sp
571    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
572    matched characters not to be included in the final matched sequence. For
573    example, the pattern:
574    .sp
575      foo\eKbar
576    .sp
577    matches "foobar", but reports that it has matched "bar". This feature is
578    similar to a lookbehind assertion
579    .\" HTML <a href="#lookbehind">
580    .\" </a>
581    (described below).
582    .\"
583    However, in this case, the part of the subject before the real match does not
584    have to be of fixed length, as lookbehind assertions do. The use of \eK does
585    not interfere with the setting of
586    .\" HTML <a href="#subpattern">
587    .\" </a>
588    captured substrings.
589    .\"
590    For example, when the pattern
591    .sp
592      (foo)\eKbar
593    .sp
594    matches "foobar", the first substring is still set to "foo".
595    .
596    .
597    .\" HTML <a name="smallassertions"></a>
598    .SS "Simple assertions"
599    .rs
600    .sp
601    The final use of backslash is for certain simple assertions. An assertion
602  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,
603  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
604  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
605  assertions are  .\" HTML <a href="#bigassertions">
606    .\" </a>
607    \\b     matches at a word boundary  below.
608    \\B     matches when not at a word boundary  .\"
609    \\A     matches at start of subject  The backslashed assertions are:
610    \\Z     matches at end of subject or before newline at end  .sp
611    \\z     matches at end of subject    \eb     matches at a word boundary
612    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
613      \eA     matches at the start of the subject
614  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
615              also matches before a newline at the end of the subject
616      \ez     matches only at the end of the subject
617      \eG     matches at the first matching position in the subject
618    .sp
619    These assertions may not appear in character classes (but note that \eb has a
620  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
621    .P
622  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
623  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
624  \\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
625  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
626    .P
627  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
628  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
629  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
630  multiline mode.  independent of multiline mode. These three assertions are not affected by the
631    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
632  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
633  \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
634  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
635  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
636  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.
637  string, whereas \\z matches only at the end.  .P
638    The \eG assertion is true only when the current matching position is at the
639  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
640  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
641  \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  
642  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
643  implementation where \\G can be useful.  implementation where \eG can be useful.
644    .P
645  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
646  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
647  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
648  string was empty. Because PCRE does just one match at a time, it cannot  string was empty. Because PCRE does just one match at a time, it cannot
649  reproduce this behaviour.  reproduce this behaviour.
650    .P
651  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
652  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
653  regular expression.  regular expression.
654    .
655  .SH CIRCUMFLEX AND DOLLAR  .
656    .SH "CIRCUMFLEX AND DOLLAR"
657  .rs  .rs
658  .sp  .sp
659  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
660  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
661  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
662  \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
663  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
664  meaning (see below).  meaning
665    .\" HTML <a href="#characterclass">
666    .\" </a>
667    (see below).
668    .\"
669    .P
670  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
671  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
672  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 674  possible alternatives start with a circu
674  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
675  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
676  to be anchored.)  to be anchored.)
677    .P
678  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
679  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
680  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
681  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
682  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
683  Dollar has no special meaning in a character class.  character class.
684    .P
685  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
686  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
687  does not affect the \\Z assertion.  does not affect the \eZ assertion.
688    .P
689  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
690  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
691  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
692  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
693  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
694  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
695  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
696  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
697  \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
698  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
699    patterns that are anchored in single line mode because all branches start with
700  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
701    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
702    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
703    .P
704    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
705  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
706  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
707    .
708  .SH FULL STOP (PERIOD, DOT)  .
709    .SH "FULL STOP (PERIOD, DOT)"
710  .rs  .rs
711  .sp  .sp
712  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
713  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
714  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.
715  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
716  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
717  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
718  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
719    (including isolated CRs and LFs). When any Unicode line endings are being
720  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
721  .rs  characters.
722  .sp  .P
723  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
724  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
725  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
726  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
727  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
728    The handling of dot is entirely independent of the handling of circumflex and
729  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
730  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
731    .
732  .SH SQUARE BRACKETS  .
733    .SH "MATCHING A SINGLE BYTE"
734    .rs
735    .sp
736    Outside a character class, the escape sequence \eC matches any one byte, both
737    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
738    characters. The feature is provided in Perl in order to match individual bytes
739    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
740    what remains in the string may be a malformed UTF-8 string. For this reason,
741    the \eC escape sequence is best avoided.
742    .P
743    PCRE does not allow \eC to appear in lookbehind assertions
744    .\" HTML <a href="#lookbehind">
745    .\" </a>
746    (described below),
747    .\"
748    because in UTF-8 mode this would make it impossible to calculate the length of
749    the lookbehind.
750    .
751    .
752    .\" HTML <a name="characterclass"></a>
753    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
754  .rs  .rs
755  .sp  .sp
756  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 758  square bracket. A closing square bracket
758  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
759  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
760  escaped with a backslash.  escaped with a backslash.
761    .P
762  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
763  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
764  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 766  definition is a circumflex, in which cas
766  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
767  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
768  backslash.  backslash.
769    .P
770  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
771  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
772  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
773  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
774  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
775  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
776    string.
777    .P
778  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
779  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.
780    .P
781  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
782  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
783  "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
784  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
785  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
786    always possible. For characters with higher values, the concept of case is
787  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.
788  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
789  such as [^a] will always match a newline.  ensure that PCRE is compiled with Unicode property support as well as with
790    UTF-8 support.
791    .P
792    Characters that might indicate line breaks are never treated in any special way
793    when matching character classes, whatever line-ending sequence is in use, and
794    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
795    such as [^a] always matches one of these characters.
796    .P
797  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
798  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,
799  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
800  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
801  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.
802    .P
803  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
804  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
805  ("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
806  "-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
807  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
808  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
809  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
810    .P
811  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
812  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
813  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
814  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
815    .P
816  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
817  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
818  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
819  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
820    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
821  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
822  character class, and add the characters that they match to the class. For  property support.
823  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
824    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
825    in a character class, and add the characters that they match to the class. For
826    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
827  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
828  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
829  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
830    .P
831  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
832  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
833  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
834    introducing a POSIX class name - see the next section), and the terminating
835  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
836    does no harm.
837    .
838    .
839    .SH "POSIX CHARACTER CLASSES"
840  .rs  .rs
841  .sp  .sp
842  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
843  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
844  this notation. For example,  this notation. For example,
845    .sp
846    [01[:alpha:]%]    [01[:alpha:]%]
847    .sp
848  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
849  are  are
850    .sp
851    alnum    letters and digits    alnum    letters and digits
852    alpha    letters    alpha    letters
853    ascii    character codes 0 - 127    ascii    character codes 0 - 127
854    blank    space or tab only    blank    space or tab only
855    cntrl    control characters    cntrl    control characters
856    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
857    graph    printing characters, excluding space    graph    printing characters, excluding space
858    lower    lower case letters    lower    lower case letters
859    print    printing characters, including space    print    printing characters, including space
860    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
861    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
862    upper    upper case letters    upper    upper case letters
863    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
864    xdigit   hexadecimal digits    xdigit   hexadecimal digits
865    .sp
866  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
867  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
868  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
869  compatibility).  compatibility).
870    .P
871  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
872  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
873  after the colon. For example,  after the colon. For example,
874    .sp
875    [12[:^digit:]]    [12[:^digit:]]
876    .sp
877  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
878  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
879  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
880    .P
881  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
882  the POSIX character classes.  the POSIX character classes.
883    .
884  .SH VERTICAL BAR  .
885    .SH "VERTICAL BAR"
886  .rs  .rs
887  .sp  .sp
888  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
889  the pattern  the pattern
890    .sp
891    gilbert|sullivan    gilbert|sullivan
892    .sp
893  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
894  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
895  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
896  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
897  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
898  pattern as well as the alternative in the subpattern.  .\" </a>
899    (defined below),
900  .SH INTERNAL OPTION SETTING  .\"
901    "succeeds" means matching the rest of the main pattern as well as the
902    alternative in the subpattern.
903    .
904    .
905    .SH "INTERNAL OPTION SETTING"
906  .rs  .rs
907  .sp  .sp
908  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
909  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
910  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
911    .sp
912    i  for PCRE_CASELESS    i  for PCRE_CASELESS
913    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
914    s  for PCRE_DOTALL    s  for PCRE_DOTALL
915    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
916    .sp
917  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
918  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
919  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
920  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
921  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
922  unset.  unset.
923    .P
924  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
925  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
926  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
927  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
928  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
929    .P
930  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
931  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
932    .sp
933    (a(?i)b)c    (a(?i)b)c
934    .sp
935  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).
936  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
937  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
938  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
939    .sp
940    (a(?i)b|c)    (a(?i)b|c)
941    .sp
942  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
943  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
944  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
945  behaviour otherwise.  behaviour otherwise.
946    .P
947  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
948  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
949  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
950  earlier in the pattern than any of the additional features it turns on, even  .
951  when it is at top level. It is best put at the start.  .
952    .\" HTML <a name="subpattern"></a>
953  .SH SUBPATTERNS  .SH SUBPATTERNS
954  .rs  .rs
955  .sp  .sp
956  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
957  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
958    .sp
959  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
960    .sp
961    cat(aract|erpillar|)    cat(aract|erpillar|)
962    .sp
963  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
964  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
965    .sp
966  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
967  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
968  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
969  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
970  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
971    .P
972  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
973    .sp
974    the ((red|white) (king|queen))    the ((red|white) (king|queen))
975    .sp
976  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,
977  2, and 3, respectively.  2, and 3, respectively.
978    .P
979  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
980  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
981  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
982  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
983  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
984  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
985    .sp
986    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
987    .sp
988  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
989  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
990  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
991  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
992  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
993  the ":". Thus the two patterns  the ":". Thus the two patterns
994    .sp
995    (?i:saturday|sunday)    (?i:saturday|sunday)
996    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
997    .sp
998  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
999  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
1000  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
1001  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1002    .
1003  .SH NAMED SUBPATTERNS  .
1004    .SH "DUPLICATE SUBPATTERN NUMBERS"
1005    .rs
1006    .sp
1007    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1008    the same numbers for its capturing parentheses. Such a subpattern starts with
1009    (?| and is itself a non-capturing subpattern. For example, consider this
1010    pattern:
1011    .sp
1012      (?|(Sat)ur|(Sun))day
1013    .sp
1014    Because the two alternatives are inside a (?| group, both sets of capturing
1015    parentheses are numbered one. Thus, when the pattern matches, you can look
1016    at captured substring number one, whichever alternative matched. This construct
1017    is useful when you want to capture part, but not all, of one of a number of
1018    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1019    number is reset at the start of each branch. The numbers of any capturing
1020    buffers that follow the subpattern start after the highest number used in any
1021    branch. The following example is taken from the Perl documentation.
1022    The numbers underneath show in which buffer the captured content will be
1023    stored.
1024    .sp
1025      # before  ---------------branch-reset----------- after
1026      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1027      # 1            2         2  3        2     3     4
1028    .sp
1029    A backreference or a recursive call to a numbered subpattern always refers to
1030    the first one in the pattern with the given number.
1031    .P
1032    An alternative approach to using this "branch reset" feature is to use
1033    duplicate named subpatterns, as described in the next section.
1034    .
1035    .
1036    .SH "NAMED SUBPATTERNS"
1037  .rs  .rs
1038  .sp  .sp
1039  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
1040  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1041  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
1042  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1043  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
1044  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
1045    the Perl and the Python syntax.
1046  Named capturing parentheses are still allocated numbers as well as names. The  .P
1047  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
1048  table from a compiled pattern. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1049    parentheses from other parts of the pattern, such as
1050    .\" HTML <a href="#backreferences">
1051    .\" </a>
1052    backreferences,
1053    .\"
1054    .\" HTML <a href="#recursion">
1055    .\" </a>
1056    recursion,
1057    .\"
1058    and
1059    .\" HTML <a href="#conditions">
1060    .\" </a>
1061    conditions,
1062    .\"
1063    can be made by name as well as by number.
1064    .P
1065    Names consist of up to 32 alphanumeric characters and underscores. Named
1066    capturing parentheses are still allocated numbers as well as names, exactly as
1067    if the names were not present. The PCRE API provides function calls for
1068    extracting the name-to-number translation table from a compiled pattern. There
1069    is also a convenience function for extracting a captured substring by name.
1070    .P
1071    By default, a name must be unique within a pattern, but it is possible to relax
1072    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1073    be useful for patterns where only one instance of the named parentheses can
1074    match. Suppose you want to match the name of a weekday, either as a 3-letter
1075    abbreviation or as the full name, and in both cases you want to extract the
1076    abbreviation. This pattern (ignoring the line breaks) does the job:
1077    .sp
1078      (?<DN>Mon|Fri|Sun)(?:day)?|
1079      (?<DN>Tue)(?:sday)?|
1080      (?<DN>Wed)(?:nesday)?|
1081      (?<DN>Thu)(?:rsday)?|
1082      (?<DN>Sat)(?:urday)?
1083    .sp
1084    There are five capturing substrings, but only one is ever set after a match.
1085    (An alternative way of solving this problem is to use a "branch reset"
1086    subpattern, as described in the previous section.)
1087    .P
1088    The convenience function for extracting the data by name returns the substring
1089    for the first (and in this example, the only) subpattern of that name that
1090    matched. This saves searching to find which numbered subpattern it was. If you
1091    make a reference to a non-unique named subpattern from elsewhere in the
1092    pattern, the one that corresponds to the lowest number is used. For further
1093    details of the interfaces for handling named subpatterns, see the
1094  .\" HREF  .\" HREF
1095  \fBpcreapi\fR  \fBpcreapi\fP
1096  .\"  .\"
1097  documentation.  documentation.
1098    .
1099    .
1100  .SH REPETITION  .SH REPETITION
1101  .rs  .rs
1102  .sp  .sp
1103  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1104  items:  items:
1105    .sp
1106    a literal data character    a literal data character
1107    the . metacharacter    the dot metacharacter
1108    the \\C escape sequence    the \eC escape sequence
1109    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1110      the \eR escape sequence
1111      an escape such as \ed that matches a single character
1112    a character class    a character class
1113    a back reference (see next section)    a back reference (see next section)
1114    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1115    .sp
1116  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1117  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1118  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
1119  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1120    .sp
1121    z{2,4}    z{2,4}
1122    .sp
1123  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
1124  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
1125  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
1126  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1127    .sp
1128    [aeiou]{3,}    [aeiou]{3,}
1129    .sp
1130  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1131    .sp
1132    \\d{8}    \ed{8}
1133    .sp
1134  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
1135  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
1136  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
1137  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1138    .P
1139  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
1140  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
1141  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1142    support is available, \eX{3} matches three Unicode extended sequences, each of
1143    which may be several bytes long (and they may be of different lengths).
1144    .P
1145  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
1146  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1147    .P
1148  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1149  quantifiers have single-character abbreviations:  abbreviations:
1150    .sp
1151    *    is equivalent to {0,}    *    is equivalent to {0,}
1152    +    is equivalent to {1,}    +    is equivalent to {1,}
1153    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1154    .sp
1155  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
1156  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:
1157    .sp
1158    (a?)*    (a?)*
1159    .sp
1160  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
1161  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
1162  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
1163  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1164    .P
1165  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
1166  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1167  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
1168  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 */
1169  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1170  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1171    .sp
1172    /\\*.*\\*/    /\e*.*\e*/
1173    .sp
1174  to the string  to the string
1175    .sp
1176    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1177    .sp
1178  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 .*
1179  item.  item.
1180    .P
1181  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
1182  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1183  pattern  pattern
1184    .sp
1185    /\\*.*?\\*/    /\e*.*?\e*/
1186    .sp
1187  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
1188  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1189  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
1190  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
1191    .sp
1192    \\d??\\d    \ed??\ed
1193    .sp
1194  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
1195  way the rest of the pattern matches.  way the rest of the pattern matches.
1196    .P
1197  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),
1198  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
1199  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
1200  default behaviour.  default behaviour.
1201    .P
1202  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1203  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
1204  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1205    .P
1206  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
1207  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
1208  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1209  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1210  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
1211  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1212    .P
1213  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
1214  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1215  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1216    .P
1217  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1218  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1219  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
1220  succeed. Consider, for example:  succeeds. Consider, for example:
1221    .sp
1222    (.*)abc\\1    (.*)abc\e1
1223    .sp
1224  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
1225  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1226    .P
1227  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1228  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1229    .sp
1230    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1231    .sp
1232  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1233  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1234  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1235  example, after  example, after
1236    .sp
1237    /(a|(b))+/    /(a|(b))+/
1238    .sp
1239  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1240    .
1241  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1242    .\" HTML <a name="atomicgroup"></a>
1243    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1244  .rs  .rs
1245  .sp  .sp
1246  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1247  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
1248  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
1249  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
1250  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
1251  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1252    .P
1253  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
1254    .sp
1255    123456bar    123456bar
1256    .sp
1257  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
1258  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+
1259  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"
1260  (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
1261  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.
1262    .P
1263  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
1264  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
1265  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1266    .sp
1267    (?>\\d+)foo    (?>\ed+)foo
1268    .sp
1269  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
1270  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
1271  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1272  normal.  normal.
1273    .P
1274  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
1275  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1276  the current point in the subject string.  the current point in the subject string.
1277    .P
1278  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1279  the above example can be thought of as a maximizing repeat that must swallow  the above example can be thought of as a maximizing repeat that must swallow
1280  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
1281  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,
1282  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1283    .P
1284  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1285  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1286  group is just a single repeated item, as in the example above, a simpler  group is just a single repeated item, as in the example above, a simpler
1287  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1288  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1289  previous example can be rewritten as  previous example can be rewritten as
1290    .sp
1291    \\d++bar    \ed++foo
1292    .sp
1293  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1294  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1295  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
1296  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1297    difference; possessive quantifiers should be slightly faster.
1298  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1299  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1300    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1301    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1302    package, and PCRE copied it from there. It ultimately found its way into Perl
1303    at release 5.10.
1304    .P
1305    PCRE has an optimization that automatically "possessifies" certain simple
1306    pattern constructs. For example, the sequence A+B is treated as A++B because
1307    there is no point in backtracking into a sequence of A's when B must follow.
1308    .P
1309  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
1310  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
1311  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
1312  pattern  pattern
1313    .sp
1314    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1315    .sp
1316  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
1317  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1318  quickly. However, if it is applied to  quickly. However, if it is applied to
1319    .sp
1320    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1321    .sp
1322  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
1323  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
1324  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
1325  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
1326  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
1327  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
1328  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1329    an atomic group, like this:
1330    ((?>\\D+)|<\\d+>)*[!?]  .sp
1331      ((?>\eD+)|<\ed+>)*[!?]
1332    .sp
1333  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1334    .
1335  .SH BACK REFERENCES  .
1336    .\" HTML <a name="backreferences"></a>
1337    .SH "BACK REFERENCES"
1338  .rs  .rs
1339  .sp  .sp
1340  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
1341  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1342  (that is, to its left) in the pattern, provided there have been that many  (that is, to its left) in the pattern, provided there have been that many
1343  previous capturing left parentheses.  previous capturing left parentheses.
1344    .P
1345  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
1346  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
1347  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
1348  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
1349  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
1350  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1351    in an earlier iteration.
1352    .P
1353    It is not possible to have a numerical "forward back reference" to a subpattern
1354    whose number is 10 or more using this syntax because a sequence such as \e50 is
1355    interpreted as a character defined in octal. See the subsection entitled
1356    "Non-printing characters"
1357    .\" HTML <a href="#digitsafterbackslash">
1358    .\" </a>
1359    above
1360    .\"
1361    for further details of the handling of digits following a backslash. There is
1362    no such problem when named parentheses are used. A back reference to any
1363    subpattern is possible using named parentheses (see below).
1364    .P
1365    Another way of avoiding the ambiguity inherent in the use of digits following a
1366    backslash is to use the \eg escape sequence, which is a feature introduced in
1367    Perl 5.10. This escape must be followed by a positive or a negative number,
1368    optionally enclosed in braces. These examples are all identical:
1369    .sp
1370      (ring), \e1
1371      (ring), \eg1
1372      (ring), \eg{1}
1373    .sp
1374    A positive number specifies an absolute reference without the ambiguity that is
1375    present in the older syntax. It is also useful when literal digits follow the
1376    reference. A negative number is a relative reference. Consider this example:
1377    .sp
1378      (abc(def)ghi)\eg{-1}
1379    .sp
1380    The sequence \eg{-1} is a reference to the most recently started capturing
1381    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1382    would be equivalent to \e1. The use of relative references can be helpful in
1383    long patterns, and also in patterns that are created by joining together
1384    fragments that contain references within themselves.
1385    .P
1386  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1387  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1388  itself (see  itself (see
# Line 832  itself (see Line 1391  itself (see
1391  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1392  .\"  .\"
1393  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1394    .sp
1395    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1396    .sp
1397  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1398  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1399  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1400    .sp
1401    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1402    .sp
1403  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
1404  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1405    .P
1406  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1407  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1408    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1409    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1410    references, is also supported. We could rewrite the above example in any of
1411    the following ways:
1412    .sp
1413      (?<p1>(?i)rah)\es+\ek<p1>
1414      (?'p1'(?i)rah)\es+\ek{p1}
1415      (?P<p1>(?i)rah)\es+(?P=p1)
1416      (?<p1>(?i)rah)\es+\eg{p1}
1417    .sp
1418    A subpattern that is referenced by name may appear in the pattern before or
1419    after the reference.
1420    .P
1421  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
1422  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1423  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1424    .sp
1425    (a|(bc))\\2    (a|(bc))\e2
1426    .sp
1427  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
1428  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1429  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1430  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1431  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1432  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1433    .\" HTML <a href="#comments">
1434    .\" </a>
1435    "Comments"
1436    .\"
1437    below) can be used.
1438    .P
1439  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
1440  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.
1441  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1442  example, the pattern  example, the pattern
1443    .sp
1444    (a|b\\1)+    (a|b\e1)+
1445    .sp
1446  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
1447  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1448  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1449  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1450  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1451  minimum of zero.  minimum of zero.
1452    .
1453    .
1454    .\" HTML <a name="bigassertions"></a>
1455  .SH ASSERTIONS  .SH ASSERTIONS
1456  .rs  .rs
1457  .sp  .sp
1458  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
1459  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1460  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
1461    .\" HTML <a href="#smallassertions">
1462    .\" </a>
1463    above.
1464    .\"
1465    .P
1466  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1467  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1468  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1469    except that it does not cause the current matching position to be changed.
1470  An assertion subpattern is matched in the normal way, except that it does not  .P
1471  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1472  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
1473    of assertion contains capturing subpatterns within it, these are counted for
1474    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1475    However, substring capturing is carried out only for positive assertions,
1476    because it does not make sense for negative assertions.
1477    .
1478    .
1479    .SS "Lookahead assertions"
1480    .rs
1481    .sp
1482    Lookahead assertions start with (?= for positive assertions and (?! for
1483    negative assertions. For example,
1484    .sp
1485      \ew+(?=;)
1486    .sp
1487  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
1488  the match, and  the match, and
1489    .sp
1490    foo(?!bar)    foo(?!bar)
1491    .sp
1492  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
1493  apparently similar pattern  apparently similar pattern
1494    .sp
1495    (?!foo)bar    (?!foo)bar
1496    .sp
1497  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
1498  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1499  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1500  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1501    .P
1502  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
1503  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
1504  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.
1505    .
1506    .
1507    .\" HTML <a name="lookbehind"></a>
1508    .SS "Lookbehind assertions"
1509    .rs
1510    .sp
1511  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1512  negative assertions. For example,  negative assertions. For example,
1513    .sp
1514    (?<!foo)bar    (?<!foo)bar
1515    .sp
1516  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
1517  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
1518  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
1519  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1520    .sp
1521    (?<=bullock|donkey)    (?<=bullock|donkey)
1522    .sp
1523  is permitted, but  is permitted, but
1524    .sp
1525    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1526    .sp
1527  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1528  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1529  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1530  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1531    .sp
1532    (?<=ab(c|de))    (?<=ab(c|de))
1533    .sp
1534  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
1535  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:
1536    .sp
1537    (?<=abc|abde)    (?<=abc|abde)
1538    .sp
1539    In some cases, the Perl 5.10 escape sequence \eK
1540    .\" HTML <a href="#resetmatchstart">
1541    .\" </a>
1542    (see above)
1543    .\"
1544    can be used instead of a lookbehind assertion; this is not restricted to a
1545    fixed-length.
1546    .P
1547  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1548  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
1549  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1550  match is deemed to fail.  assertion fails.
1551    .P
1552  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)
1553  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1554  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1555    different numbers of bytes, are also not permitted.
1556  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1557  efficient matching at the end of the subject string. Consider a simple pattern  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1558  such as  specify efficient matching at the end of the subject string. Consider a simple
1559    pattern such as
1560    .sp
1561    abcd$    abcd$
1562    .sp
1563  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
1564  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
1565  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
1566    .sp
1567    ^.*abcd$    ^.*abcd$
1568    .sp
1569  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
1570  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,
1571  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"
1572  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,
1573  if the pattern is written as  if the pattern is written as
1574    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1575    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1576    .sp
1577  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
1578  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
1579  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1580  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1581    .
1582    .
1583    .SS "Using multiple assertions"
1584    .rs
1585    .sp
1586  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1587    .sp
1588    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1589    .sp
1590  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
1591  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1592  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
1593  digits, and then there is a check that the same three characters are not "999".  digits, and then there is a check that the same three characters are not "999".
1594  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
1595  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
1596  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1597    .sp
1598    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1599    .sp
1600  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1601  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1602  preceding three characters are not "999".  preceding three characters are not "999".
1603    .P
1604  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1605    .sp
1606    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1607    .sp
1608  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
1609  preceded by "foo", while  preceded by "foo", while
1610    .sp
1611    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1612    .sp
1613  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
1614  characters that are not "999".  characters that are not "999".
1615    .
1616  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1617  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1618  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  
1619  .rs  .rs
1620  .sp  .sp
1621  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1622  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1623  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1624  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1625    .sp
1626    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1627    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1628    .sp
1629  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1630  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1631  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1632    .P
1633  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1634  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1635  subpattern of that number has previously matched. The number must be greater  .
1636  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1637  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1638  it into three parts for ease of discussion:  .sp
1639    If the text between the parentheses consists of a sequence of digits, the
1640    ( \\( )?    [^()]+    (?(1) \\) )  condition is true if the capturing subpattern of that number has previously
1641    matched. An alternative notation is to precede the digits with a plus or minus
1642    sign. In this case, the subpattern number is relative rather than absolute.
1643    The most recently opened parentheses can be referenced by (?(-1), the next most
1644    recent by (?(-2), and so on. In looping constructs it can also make sense to
1645    refer to subsequent groups with constructs such as (?(+2).
1646    .P
1647    Consider the following pattern, which contains non-significant white space to
1648    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1649    three parts for ease of discussion:
1650    .sp
1651      ( \e( )?    [^()]+    (?(1) \e) )
1652    .sp
1653  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1654  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
1655  matches one or more characters that are not parentheses. The third part is a  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 1659  the condition is true, and so the yes-pa
1659  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1660  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1661  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1662    .P
1663  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
1664  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1665  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1666      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1667  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
1668    This makes the fragment independent of the parentheses in the larger pattern.
1669    .
1670    .SS "Checking for a used subpattern by name"
1671    .rs
1672    .sp
1673    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1674    subpattern by name. For compatibility with earlier versions of PCRE, which had
1675    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1676    there is a possible ambiguity with this syntax, because subpattern names may
1677    consist entirely of digits. PCRE looks first for a named subpattern; if it
1678    cannot find one and the name consists entirely of digits, PCRE looks for a
1679    subpattern of that number, which must be greater than zero. Using subpattern
1680    names that consist entirely of digits is not recommended.
1681    .P
1682    Rewriting the above example to use a named subpattern gives this:
1683    .sp
1684      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1685    .sp
1686    .
1687    .SS "Checking for pattern recursion"
1688    .rs
1689    .sp
1690    If the condition is the string (R), and there is no subpattern with the name R,
1691    the condition is true if a recursive call to the whole pattern or any
1692    subpattern has been made. If digits or a name preceded by ampersand follow the
1693    letter R, for example:
1694    .sp
1695      (?(R3)...) or (?(R&name)...)
1696    .sp
1697    the condition is true if the most recent recursion is into the subpattern whose
1698    number or name is given. This condition does not check the entire recursion
1699    stack.
1700    .P
1701    At "top level", all these recursion test conditions are false. Recursive
1702    patterns are described below.
1703    .
1704    .SS "Defining subpatterns for use by reference only"
1705    .rs
1706    .sp
1707    If the condition is the string (DEFINE), and there is no subpattern with the
1708    name DEFINE, the condition is always false. In this case, there may be only one
1709    alternative in the subpattern. It is always skipped if control reaches this
1710    point in the pattern; the idea of DEFINE is that it can be used to define
1711    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1712    is described below.) For example, a pattern to match an IPv4 address could be
1713    written like this (ignore whitespace and line breaks):
1714    .sp
1715      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1716      \eb (?&byte) (\e.(?&byte)){3} \eb
1717    .sp
1718    The first part of the pattern is a DEFINE group inside which a another group
1719    named "byte" is defined. This matches an individual component of an IPv4
1720    address (a number less than 256). When matching takes place, this part of the
1721    pattern is skipped because DEFINE acts like a false condition.
1722    .P
1723    The rest of the pattern uses references to the named group to match the four
1724    dot-separated components of an IPv4 address, insisting on a word boundary at
1725    each end.
1726    .
1727    .SS "Assertion conditions"
1728    .rs
1729    .sp
1730    If the condition is not in any of the above formats, it must be an assertion.
1731  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1732  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1733  alternatives on the second line:  alternatives on the second line:
1734    .sp
1735    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1736    \\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} )
1737    .sp
1738  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1739  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
1740  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
1741  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1742  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
1743  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1744    .
1745    .
1746    .\" HTML <a name="comments"></a>
1747  .SH COMMENTS  .SH COMMENTS
1748  .rs  .rs
1749  .sp  .sp
1750  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
1751  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1752  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.
1753    .P
1754  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1755  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1756  character in the pattern.  next newline in the pattern.
1757    .
1758  .SH RECURSIVE PATTERNS  .
1759    .\" HTML <a name="recursion"></a>
1760    .SH "RECURSIVE PATTERNS"
1761  .rs  .rs
1762  .sp  .sp
1763  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1764  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1765  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
1766  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
1767  experimental facility that allows regular expressions to recurse (amongst other  .P
1768  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
1769  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
1770  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1771    pattern using code interpolation to solve the parentheses problem can be
1772    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
1773    .sp
1774      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1775    .sp
1776  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
1777  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1778  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1779  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1780    supports special syntax for recursion of the entire pattern, and also for
1781  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1782  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.
1783  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1784  "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
1785  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1786    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1787  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
1788  the PCRE_EXTENDED option is set so that white space is ignored):  a recursive call of the entire regular expression.
1789    .P
1790    \\( ( (?>[^()]+) | (?R) )* \\)  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1791    treated as an atomic group. That is, once it has matched some of the subject
1792    string, it is never re-entered, even if it contains untried alternatives and
1793    there is a subsequent matching failure.
1794    .P
1795    This PCRE pattern solves the nested parentheses problem (assume the
1796    PCRE_EXTENDED option is set so that white space is ignored):
1797    .sp
1798      \e( ( (?>[^()]+) | (?R) )* \e)
1799    .sp
1800  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1801  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
1802  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1803  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1804    .P
1805  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
1806  pattern, so instead you could use this:  pattern, so instead you could use this:
1807    .sp
1808    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1809    .sp
1810  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
1811  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1812  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1813  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
1814  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.)
1815  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
1816    most recently opened parentheses preceding the recursion. In other words, a
1817    (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  negative number counts capturing parentheses leftwards from the point at which
1818    it is encountered.
1819  This particular example pattern contains nested unlimited repeats, and so the  .P
1820  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1821  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
1822  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1823    "subroutine" calls, as described in the next section.
1824    .P
1825    An alternative approach is to use named parentheses instead. The Perl syntax
1826    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1827    could rewrite the above example as follows:
1828    .sp
1829      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1830    .sp
1831    If there is more than one subpattern with the same name, the earliest one is
1832    used.
1833    .P
1834    This particular example pattern that we have been looking at contains nested
1835    unlimited repeats, and so the use of atomic grouping for matching strings of
1836    non-parentheses is important when applying the pattern to strings that do not
1837    match. For example, when this pattern is applied to
1838    .sp
1839    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1840    .sp
1841  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1842  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
1843  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
1844  before failure can be reported.  before failure can be reported.
1845    .P
1846  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
1847  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.
1848  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
1849  below and the  below and the
1850  .\" HREF  .\" HREF
1851  \fBpcrecallout\fR  \fBpcrecallout\fP
1852  .\"  .\"
1853  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1854    .sp
1855    (ab(cd)ef)    (ab(cd)ef)
1856    .sp
1857  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
1858  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1859    .sp
1860    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1861       ^                        ^       ^                        ^
1862       ^                        ^       ^                        ^
1863    .sp
1864  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
1865  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
1866  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
1867  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
1868  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.
1869    .P
1870  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.
1871  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1872  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1873  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1874    .sp
1875    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1876    .sp
1877  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
1878  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1879  is the actual recursive call.  is the actual recursive call.
1880    .
1881    .
1882  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1883  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1884  .rs  .rs
1885  .sp  .sp
1886  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
1887  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
1888  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1889  pattern  before or after the reference. A numbered reference can be absolute or
1890    relative, as in these examples:
1891    (sens|respons)e and \\1ibility  .sp
1892      (...(absolute)...)...(?2)...
1893      (...(relative)...)...(?-1)...
1894      (...(?+1)...(relative)...
1895    .sp
1896    An earlier example pointed out that the pattern
1897    .sp
1898      (sens|respons)e and \e1ibility
1899    .sp
1900  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1901  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1902    .sp
1903    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1904    .sp
1905  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
1906  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
1907  refer.  .P
1908    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1909    group. That is, once it has matched some of the subject string, it is never
1910    re-entered, even if it contains untried alternatives and there is a subsequent
1911    matching failure.
1912    .P
1913    When a subpattern is used as a subroutine, processing options such as
1914    case-independence are fixed when the subpattern is defined. They cannot be
1915    changed for different calls. For example, consider this pattern:
1916    .sp
1917      (abc)(?i:(?-1))
1918    .sp
1919    It matches "abcabc". It does not match "abcABC" because the change of
1920    processing option does not affect the called subpattern.
1921    .
1922    .
1923  .SH CALLOUTS  .SH CALLOUTS
1924  .rs  .rs
1925  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1927  Perl has a feature whereby using the seq
1927  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
1928  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1929  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1930    .P
1931  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
1932  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
1933  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.
1934  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1935    .P
1936  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1937  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
1938  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
1939  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1940    .sp
1941    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1942    .sp
1943  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
1944    automatically installed before each item in the pattern. They are all numbered
1945    255.
1946    .P
1947    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1948  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
1949  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
1950  \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
1951  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1952  function is given in the  description of the interface to the callout function is given in the
1953  .\" HREF  .\" HREF
1954  \fBpcrecallout\fR  \fBpcrecallout\fP
1955  .\"  .\"
1956  documentation.  documentation.
1957    .
1958  .in 0  .
1959  Last updated: 03 February 2003  .SH "SEE ALSO"
1960  .br  .rs
1961  Copyright (c) 1997-2003 University of Cambridge.  .sp
1962    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1963    .
1964    .
1965    .SH AUTHOR
1966    .rs
1967    .sp
1968    .nf
1969    Philip Hazel
1970    University Computing Service
1971    Cambridge CB2 3QH, England.
1972    .fi
1973    .
1974    .
1975    .SH REVISION
1976    .rs
1977    .sp
1978    .nf
1979    Last updated: 19 June 2007
1980    Copyright (c) 1997-2007 University of Cambridge.
1981    .fi

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