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1  .TH PCRE 3  .TH PCREPATTERN 3
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH PCRE REGULAR EXPRESSION DETAILS  .SH "PCRE REGULAR EXPRESSION DETAILS"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of other books, some of which have copious  .\" HREF
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  \fBpcresyntax\fP
11  O'Reilly, covers them in great detail. The description here is intended as  .\"
12  reference documentation.  page. Perl's regular expressions are described in its own documentation, and
13    regular expressions in general are covered in a number of books, some of which
14  The basic operation of PCRE is on strings of bytes. However, there is also  have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
15  support for UTF-8 character strings. To use this support you must build PCRE to  published by O'Reilly, covers regular expressions in great detail. This
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  description of PCRE's regular expressions is intended as reference material.
17  option. How this affects the pattern matching is mentioned in several places  .P
18  below. There is also a summary of UTF-8 features in the  The original operation of PCRE was on strings of one-byte characters. However,
19    there is now also support for UTF-8 character strings. To use this, you must
20    build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
21    the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
22    places below. There is also a summary of UTF-8 features in the
23  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
24  .\" </a>  .\" </a>
25  section on UTF-8 support  section on UTF-8 support
26  .\"  .\"
27  in the main  in the main
28  .\" HREF  .\" HREF
29  \fBpcre\fR  \fBpcre\fP
30    .\"
31    page.
32    .P
33    The remainder of this document discusses the patterns that are supported by
34    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
35    From release 6.0, PCRE offers a second matching function,
36    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
37    Perl-compatible. Some of the features discussed below are not available when
38    \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
39    alternative function, and how it differs from the normal function, are
40    discussed in the
41    .\" HREF
42    \fBpcrematching\fP
43  .\"  .\"
44  page.  page.
45    .
46    .
47    .SH "CHARACTERS AND METACHARACTERS"
48    .rs
49    .sp
50  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
51  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
52  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
53    .sp
54    The quick brown fox    The quick brown fox
55    .sp
56  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
57  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
58  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In UTF-8 mode, PCRE always understands the concept of
59  \fImeta-characters\fR, which do not stand for themselves but instead are  case for characters whose values are less than 128, so caseless matching is
60    always possible. For characters with higher values, the concept of case is
61    supported if PCRE is compiled with Unicode property support, but not otherwise.
62    If you want to use caseless matching for characters 128 and above, you must
63    ensure that PCRE is compiled with Unicode property support as well as with
64    UTF-8 support.
65    .P
66    The power of regular expressions comes from the ability to include alternatives
67    and repetitions in the pattern. These are encoded in the pattern by the use of
68    \fImetacharacters\fP, which do not stand for themselves but instead are
69  interpreted in some special way.  interpreted in some special way.
70    .P
71  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
72  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
73  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
74  as follows:  are as follows:
75    .sp
76    \\      general escape character with several uses    \e      general escape character with several uses
77    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
78    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
79    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 88  as follows:
88    +      1 or more quantifier    +      1 or more quantifier
89           also "possessive quantifier"           also "possessive quantifier"
90    {      start min/max quantifier    {      start min/max quantifier
91    .sp
92  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
93  a character class the only meta-characters are:  a character class the only metacharacters are:
94    .sp
95    \\      general escape character    \e      general escape character
96    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
97    -      indicates character range    -      indicates character range
98    .\" JOIN
99    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
100             syntax)             syntax)
101    ]      terminates the character class    ]      terminates the character class
102    .sp
103  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
104    .
105    .
106  .SH BACKSLASH  .SH BACKSLASH
107  .rs  .rs
108  .sp  .sp
109  The backslash character has several uses. Firstly, if it is followed by a  The backslash character has several uses. Firstly, if it is followed by a
110  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
111  have. This use of backslash as an escape character applies both inside and  may have. This use of backslash as an escape character applies both inside and
112  outside character classes.  outside character classes.
113    .P
114  For example, if you want to match a * character, you write \\* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
115  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
116  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
117  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
118  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
119    .P
120  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
121  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
122  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
123  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
124  pattern.  .P
   
125  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
126  can do so by putting them between \\Q and \\E. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
127  that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
128  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
129    .sp
130    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
131    .sp
132    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
133      \eQabc$xyz\eE        abc$xyz        abc followed by the
134                                        contents of $xyz                                        contents of $xyz
135    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
136    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
137    .sp
138  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
139    .
140    .
141    .\" HTML <a name="digitsafterbackslash"></a>
142    .SS "Non-printing characters"
143    .rs
144    .sp
145  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
146  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
147  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
148  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is usually easier to
149  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
150  represents:  represents:
151    .sp
152    \\a        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
153    \\cx       "control-x", where x is any character    \ecx       "control-x", where x is any character
154    \\e        escape (hex 1B)    \ee        escape (hex 1B)
155    \\f        formfeed (hex 0C)    \ef        formfeed (hex 0C)
156    \\n        newline (hex 0A)    \en        newline (hex 0A)
157    \\r        carriage return (hex 0D)    \er        carriage return (hex 0D)
158    \\t        tab (hex 09)    \et        tab (hex 09)
159    \\ddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
160    \\xhh      character with hex code hh    \exhh      character with hex code hh
161    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
162    .sp
163  The precise effect of \\cx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
164  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
165  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
166  7B.  7B.
167    .P
168  After \\x, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
169  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
170  appear between \\x{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
171  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
172  other than hexadecimal digits appear between \\x{ and }, or if there is no  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
173  terminating }, this form of escape is not recognized. Instead, the initial  point, which is 10FFFF.
174  \\x will be interpreted as a basic hexadecimal escape, with no following  .P
175  digits, giving a byte whose value is zero.  If characters other than hexadecimal digits appear between \ex{ and }, or if
176    there is no terminating }, this form of escape is not recognized. Instead, the
177    initial \ex will be interpreted as a basic hexadecimal escape, with no
178    following digits, giving a character whose value is zero.
179    .P
180  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
181  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
182  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
183    .P
184  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
185  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
186  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
187  (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
188  character that follows is itself an octal digit.  follows is itself an octal digit.
189    .P
190  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.
191  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
192  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
193  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
194  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
195  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
196    .\" </a>
197    later,
198    .\"
199    following the discussion of
200    .\" HTML <a href="#subpattern">
201    .\" </a>
202    parenthesized subpatterns.
203    .\"
204    .P
205  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
206  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
207  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
208  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
209  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
210    to \e777 are permitted. For example:
211    \\040   is another way of writing a space  .sp
212    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
213    .\" JOIN
214      \e40    is the same, provided there are fewer than 40
215              previous capturing subpatterns              previous capturing subpatterns
216    \\7     is always a back reference    \e7     is always a back reference
217    \\11    might be a back reference, or another way of  .\" JOIN
218      \e11    might be a back reference, or another way of
219              writing a tab              writing a tab
220    \\011   is always a tab    \e011   is always a tab
221    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
222    \\113   might be a back reference, otherwise the  .\" JOIN
223      \e113   might be a back reference, otherwise the
224              character with octal code 113              character with octal code 113
225    \\377   might be a back reference, otherwise  .\" JOIN
226      \e377   might be a back reference, otherwise
227              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
228    \\81    is either a back reference, or a binary zero  .\" JOIN
229      \e81    is either a back reference, or a binary zero
230              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
231    .sp
232  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
233  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
234    .P
235  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
236  (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
237  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
238  backspace character (hex 08). Outside a character class it has a different  sequences \eR and \eX are interpreted as the characters "R" and "X",
239  meaning (see below).  respectively. Outside a character class, these sequences have different
240    meanings
241  The third use of backslash is for specifying generic character types:  .\" HTML <a href="#uniextseq">
242    .\" </a>
243    \\d     any decimal digit  (see below).
244    \\D     any character that is not a decimal digit  .\"
245    \\s     any whitespace character  .
246    \\S     any character that is not a whitespace character  .
247    \\w     any "word" character  .SS "Absolute and relative back references"
248    \\W     any "non-word" character  .rs
249    .sp
250    The sequence \eg followed by an unsigned or a negative number, optionally
251    enclosed in braces, is an absolute or relative back reference. A named back
252    reference can be coded as \eg{name}. Back references are discussed
253    .\" HTML <a href="#backreferences">
254    .\" </a>
255    later,
256    .\"
257    following the discussion of
258    .\" HTML <a href="#subpattern">
259    .\" </a>
260    parenthesized subpatterns.
261    .\"
262    .
263    .
264    .SS "Generic character types"
265    .rs
266    .sp
267    Another use of backslash is for specifying generic character types. The
268    following are always recognized:
269    .sp
270      \ed     any decimal digit
271      \eD     any character that is not a decimal digit
272      \eh     any horizontal whitespace character
273      \eH     any character that is not a horizontal whitespace character
274      \es     any whitespace character
275      \eS     any character that is not a whitespace character
276      \ev     any vertical whitespace character
277      \eV     any character that is not a vertical whitespace character
278      \ew     any "word" character
279      \eW     any "non-word" character
280    .sp
281  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
282  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
283    .P
284  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
285  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
286    matching point is at the end of the subject string, all of them fail, since
287  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
288  This makes it different from the the POSIX "space" class. The \\s characters  .P
289  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).
290    This makes it different from the the POSIX "space" class. The \es characters
291  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
292  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
293  digits is controlled by PCRE's character tables, and may vary if locale-  does.
294  specific matching is taking place (see  .P
295    In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
296    \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
297    character property support is available. These sequences retain their original
298    meanings from before UTF-8 support was available, mainly for efficiency
299    reasons.
300    .P
301    The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the
302    other sequences, these do match certain high-valued codepoints in UTF-8 mode.
303    The horizontal space characters are:
304    .sp
305      U+0009     Horizontal tab
306      U+0020     Space
307      U+00A0     Non-break space
308      U+1680     Ogham space mark
309      U+180E     Mongolian vowel separator
310      U+2000     En quad
311      U+2001     Em quad
312      U+2002     En space
313      U+2003     Em space
314      U+2004     Three-per-em space
315      U+2005     Four-per-em space
316      U+2006     Six-per-em space
317      U+2007     Figure space
318      U+2008     Punctuation space
319      U+2009     Thin space
320      U+200A     Hair space
321      U+202F     Narrow no-break space
322      U+205F     Medium mathematical space
323      U+3000     Ideographic space
324    .sp
325    The vertical space characters are:
326    .sp
327      U+000A     Linefeed
328      U+000B     Vertical tab
329      U+000C     Formfeed
330      U+000D     Carriage return
331      U+0085     Next line
332      U+2028     Line separator
333      U+2029     Paragraph separator
334    .P
335    A "word" character is an underscore or any character less than 256 that is a
336    letter or digit. The definition of letters and digits is controlled by PCRE's
337    low-valued character tables, and may vary if locale-specific matching is taking
338    place (see
339  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
340  .\" </a>  .\" </a>
341  "Locale support"  "Locale support"
342  .\"  .\"
343  in the  in the
344  .\" HREF  .\" HREF
345  \fBpcreapi\fR  \fBpcreapi\fP
346  .\"  .\"
347  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,
348  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
349    accented letters, and these are matched by \ew. The use of locales with Unicode
350  These character type sequences can appear both inside and outside character  is discouraged.
351  classes. They each match one character of the appropriate type. If the current  .
352  matching point is at the end of the subject string, all of them fail, since  .
353  there is no character to match.  .SS "Newline sequences"
354    .rs
355  The fourth use of backslash is for certain simple assertions. An assertion  .sp
356    Outside a character class, the escape sequence \eR matches any Unicode newline
357    sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is equivalent to
358    the following:
359    .sp
360      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
361    .sp
362    This is an example of an "atomic group", details of which are given
363    .\" HTML <a href="#atomicgroup">
364    .\" </a>
365    below.
366    .\"
367    This particular group matches either the two-character sequence CR followed by
368    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
369    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
370    line, U+0085). The two-character sequence is treated as a single unit that
371    cannot be split.
372    .P
373    In UTF-8 mode, two additional characters whose codepoints are greater than 255
374    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
375    Unicode character property support is not needed for these characters to be
376    recognized.
377    .P
378    Inside a character class, \eR matches the letter "R".
379    .
380    .
381    .\" HTML <a name="uniextseq"></a>
382    .SS Unicode character properties
383    .rs
384    .sp
385    When PCRE is built with Unicode character property support, three additional
386    escape sequences that match characters with specific properties are available.
387    When not in UTF-8 mode, these sequences are of course limited to testing
388    characters whose codepoints are less than 256, but they do work in this mode.
389    The extra escape sequences are:
390    .sp
391      \ep{\fIxx\fP}   a character with the \fIxx\fP property
392      \eP{\fIxx\fP}   a character without the \fIxx\fP property
393      \eX       an extended Unicode sequence
394    .sp
395    The property names represented by \fIxx\fP above are limited to the Unicode
396    script names, the general category properties, and "Any", which matches any
397    character (including newline). Other properties such as "InMusicalSymbols" are
398    not currently supported by PCRE. Note that \eP{Any} does not match any
399    characters, so always causes a match failure.
400    .P
401    Sets of Unicode characters are defined as belonging to certain scripts. A
402    character from one of these sets can be matched using a script name. For
403    example:
404    .sp
405      \ep{Greek}
406      \eP{Han}
407    .sp
408    Those that are not part of an identified script are lumped together as
409    "Common". The current list of scripts is:
410    .P
411    Arabic,
412    Armenian,
413    Balinese,
414    Bengali,
415    Bopomofo,
416    Braille,
417    Buginese,
418    Buhid,
419    Canadian_Aboriginal,
420    Cherokee,
421    Common,
422    Coptic,
423    Cuneiform,
424    Cypriot,
425    Cyrillic,
426    Deseret,
427    Devanagari,
428    Ethiopic,
429    Georgian,
430    Glagolitic,
431    Gothic,
432    Greek,
433    Gujarati,
434    Gurmukhi,
435    Han,
436    Hangul,
437    Hanunoo,
438    Hebrew,
439    Hiragana,
440    Inherited,
441    Kannada,
442    Katakana,
443    Kharoshthi,
444    Khmer,
445    Lao,
446    Latin,
447    Limbu,
448    Linear_B,
449    Malayalam,
450    Mongolian,
451    Myanmar,
452    New_Tai_Lue,
453    Nko,
454    Ogham,
455    Old_Italic,
456    Old_Persian,
457    Oriya,
458    Osmanya,
459    Phags_Pa,
460    Phoenician,
461    Runic,
462    Shavian,
463    Sinhala,
464    Syloti_Nagri,
465    Syriac,
466    Tagalog,
467    Tagbanwa,
468    Tai_Le,
469    Tamil,
470    Telugu,
471    Thaana,
472    Thai,
473    Tibetan,
474    Tifinagh,
475    Ugaritic,
476    Yi.
477    .P
478    Each character has exactly one general category property, specified by a
479    two-letter abbreviation. For compatibility with Perl, negation can be specified
480    by including a circumflex between the opening brace and the property name. For
481    example, \ep{^Lu} is the same as \eP{Lu}.
482    .P
483    If only one letter is specified with \ep or \eP, it includes all the general
484    category properties that start with that letter. In this case, in the absence
485    of negation, the curly brackets in the escape sequence are optional; these two
486    examples have the same effect:
487    .sp
488      \ep{L}
489      \epL
490    .sp
491    The following general category property codes are supported:
492    .sp
493      C     Other
494      Cc    Control
495      Cf    Format
496      Cn    Unassigned
497      Co    Private use
498      Cs    Surrogate
499    .sp
500      L     Letter
501      Ll    Lower case letter
502      Lm    Modifier letter
503      Lo    Other letter
504      Lt    Title case letter
505      Lu    Upper case letter
506    .sp
507      M     Mark
508      Mc    Spacing mark
509      Me    Enclosing mark
510      Mn    Non-spacing mark
511    .sp
512      N     Number
513      Nd    Decimal number
514      Nl    Letter number
515      No    Other number
516    .sp
517      P     Punctuation
518      Pc    Connector punctuation
519      Pd    Dash punctuation
520      Pe    Close punctuation
521      Pf    Final punctuation
522      Pi    Initial punctuation
523      Po    Other punctuation
524      Ps    Open punctuation
525    .sp
526      S     Symbol
527      Sc    Currency symbol
528      Sk    Modifier symbol
529      Sm    Mathematical symbol
530      So    Other symbol
531    .sp
532      Z     Separator
533      Zl    Line separator
534      Zp    Paragraph separator
535      Zs    Space separator
536    .sp
537    The special property L& is also supported: it matches a character that has
538    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
539    a modifier or "other".
540    .P
541    The Cs (Surrogate) property applies only to characters in the range U+D800 to
542    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
543    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
544    (see the discussion of PCRE_NO_UTF8_CHECK in the
545    .\" HREF
546    \fBpcreapi\fP
547    .\"
548    page).
549    .P
550    The long synonyms for these properties that Perl supports (such as \ep{Letter})
551    are not supported by PCRE, nor is it permitted to prefix any of these
552    properties with "Is".
553    .P
554    No character that is in the Unicode table has the Cn (unassigned) property.
555    Instead, this property is assumed for any code point that is not in the
556    Unicode table.
557    .P
558    Specifying caseless matching does not affect these escape sequences. For
559    example, \ep{Lu} always matches only upper case letters.
560    .P
561    The \eX escape matches any number of Unicode characters that form an extended
562    Unicode sequence. \eX is equivalent to
563    .sp
564      (?>\ePM\epM*)
565    .sp
566    That is, it matches a character without the "mark" property, followed by zero
567    or more characters with the "mark" property, and treats the sequence as an
568    atomic group
569    .\" HTML <a href="#atomicgroup">
570    .\" </a>
571    (see below).
572    .\"
573    Characters with the "mark" property are typically accents that affect the
574    preceding character. None of them have codepoints less than 256, so in
575    non-UTF-8 mode \eX matches any one character.
576    .P
577    Matching characters by Unicode property is not fast, because PCRE has to search
578    a structure that contains data for over fifteen thousand characters. That is
579    why the traditional escape sequences such as \ed and \ew do not use Unicode
580    properties in PCRE.
581    .
582    .
583    .\" HTML <a name="resetmatchstart"></a>
584    .SS "Resetting the match start"
585    .rs
586    .sp
587    The escape sequence \eK, which is a Perl 5.10 feature, causes any previously
588    matched characters not to be included in the final matched sequence. For
589    example, the pattern:
590    .sp
591      foo\eKbar
592    .sp
593    matches "foobar", but reports that it has matched "bar". This feature is
594    similar to a lookbehind assertion
595    .\" HTML <a href="#lookbehind">
596    .\" </a>
597    (described below).
598    .\"
599    However, in this case, the part of the subject before the real match does not
600    have to be of fixed length, as lookbehind assertions do. The use of \eK does
601    not interfere with the setting of
602    .\" HTML <a href="#subpattern">
603    .\" </a>
604    captured substrings.
605    .\"
606    For example, when the pattern
607    .sp
608      (foo)\eKbar
609    .sp
610    matches "foobar", the first substring is still set to "foo".
611    .
612    .
613    .\" HTML <a name="smallassertions"></a>
614    .SS "Simple assertions"
615    .rs
616    .sp
617    The final use of backslash is for certain simple assertions. An assertion
618  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,
619  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
620  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
621  assertions are  .\" HTML <a href="#bigassertions">
622    .\" </a>
623    \\b     matches at a word boundary  below.
624    \\B     matches when not at a word boundary  .\"
625    \\A     matches at start of subject  The backslashed assertions are:
626    \\Z     matches at end of subject or before newline at end  .sp
627    \\z     matches at end of subject    \eb     matches at a word boundary
628    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
629      \eA     matches at the start of the subject
630  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
631              also matches before a newline at the end of the subject
632      \ez     matches only at the end of the subject
633      \eG     matches at the first matching position in the subject
634    .sp
635    These assertions may not appear in character classes (but note that \eb has a
636  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
637    .P
638  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
639  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
640  \\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
641  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
642    .P
643  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
644  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
645  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
646  multiline mode.  independent of multiline mode. These three assertions are not affected by the
647    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
648  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
649  \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
650  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
651  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
652  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.
653  string, whereas \\z matches only at the end.  .P
654    The \eG assertion is true only when the current matching position is at the
655  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
656  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
657  \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  
658  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
659  implementation where \\G can be useful.  implementation where \eG can be useful.
660    .P
661  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
662  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
663  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
664  string was empty. Because PCRE does just one match at a time, it cannot  string was empty. Because PCRE does just one match at a time, it cannot
665  reproduce this behaviour.  reproduce this behaviour.
666    .P
667  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
668  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
669  regular expression.  regular expression.
670    .
671  .SH CIRCUMFLEX AND DOLLAR  .
672    .SH "CIRCUMFLEX AND DOLLAR"
673  .rs  .rs
674  .sp  .sp
675  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
676  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
677  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
678  \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
679  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
680  meaning (see below).  meaning
681    .\" HTML <a href="#characterclass">
682    .\" </a>
683    (see below).
684    .\"
685    .P
686  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
687  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
688  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 690  possible alternatives start with a circu
690  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
691  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
692  to be anchored.)  to be anchored.)
693    .P
694  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
695  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
696  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
697  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
698  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
699  Dollar has no special meaning in a character class.  character class.
700    .P
701  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
702  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
703  does not affect the \\Z assertion.  does not affect the \eZ assertion.
704    .P
705  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
706  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
707  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
708  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
709  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
710  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
711  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
712  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
713  \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
714  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
715    patterns that are anchored in single line mode because all branches start with
716  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
717    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
718    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
719    .P
720    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
721  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
722  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
723    .
724  .SH FULL STOP (PERIOD, DOT)  .
725    .SH "FULL STOP (PERIOD, DOT)"
726  .rs  .rs
727  .sp  .sp
728  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
729  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
730  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.
731  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
732  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
733  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
734  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
735    (including isolated CRs and LFs). When any Unicode line endings are being
736  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
737  .rs  characters.
738  .sp  .P
739  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
740  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
741  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
742  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
743  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
744    The handling of dot is entirely independent of the handling of circumflex and
745  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
746  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
747    .
748  .SH SQUARE BRACKETS  .
749    .SH "MATCHING A SINGLE BYTE"
750    .rs
751    .sp
752    Outside a character class, the escape sequence \eC matches any one byte, both
753    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
754    characters. The feature is provided in Perl in order to match individual bytes
755    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
756    what remains in the string may be a malformed UTF-8 string. For this reason,
757    the \eC escape sequence is best avoided.
758    .P
759    PCRE does not allow \eC to appear in lookbehind assertions
760    .\" HTML <a href="#lookbehind">
761    .\" </a>
762    (described below),
763    .\"
764    because in UTF-8 mode this would make it impossible to calculate the length of
765    the lookbehind.
766    .
767    .
768    .\" HTML <a name="characterclass"></a>
769    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
770  .rs  .rs
771  .sp  .sp
772  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 774  square bracket. A closing square bracket
774  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
775  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
776  escaped with a backslash.  escaped with a backslash.
777    .P
778  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
779  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
780  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 782  definition is a circumflex, in which cas
782  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
783  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
784  backslash.  backslash.
785    .P
786  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
787  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
788  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
789  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
790  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
791  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
792    string.
793    .P
794  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
795  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.
796    .P
797  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
798  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
799  "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
800  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
801  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
802    always possible. For characters with higher values, the concept of case is
803  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.
804  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
805  such as [^a] will always match a newline.  ensure that PCRE is compiled with Unicode property support as well as with
806    UTF-8 support.
807    .P
808    Characters that might indicate line breaks are never treated in any special way
809    when matching character classes, whatever line-ending sequence is in use, and
810    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
811    such as [^a] always matches one of these characters.
812    .P
813  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
814  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,
815  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
816  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
817  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.
818    .P
819  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
820  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
821  ("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
822  "-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
823  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
824  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
825  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
826    .P
827  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
828  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
829  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
830  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
831    .P
832  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
833  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
834  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
835  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
836    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
837  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
838  character class, and add the characters that they match to the class. For  property support.
839  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
840    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
841    in a character class, and add the characters that they match to the class. For
842    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
843  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
844  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
845  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
846    .P
847  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
848  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
849  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
850    introducing a POSIX class name - see the next section), and the terminating
851  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
852    does no harm.
853    .
854    .
855    .SH "POSIX CHARACTER CLASSES"
856  .rs  .rs
857  .sp  .sp
858  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
859  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
860  this notation. For example,  this notation. For example,
861    .sp
862    [01[:alpha:]%]    [01[:alpha:]%]
863    .sp
864  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
865  are  are
866    .sp
867    alnum    letters and digits    alnum    letters and digits
868    alpha    letters    alpha    letters
869    ascii    character codes 0 - 127    ascii    character codes 0 - 127
870    blank    space or tab only    blank    space or tab only
871    cntrl    control characters    cntrl    control characters
872    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
873    graph    printing characters, excluding space    graph    printing characters, excluding space
874    lower    lower case letters    lower    lower case letters
875    print    printing characters, including space    print    printing characters, including space
876    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
877    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
878    upper    upper case letters    upper    upper case letters
879    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
880    xdigit   hexadecimal digits    xdigit   hexadecimal digits
881    .sp
882  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
883  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
884  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
885  compatibility).  compatibility).
886    .P
887  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
888  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
889  after the colon. For example,  after the colon. For example,
890    .sp
891    [12[:^digit:]]    [12[:^digit:]]
892    .sp
893  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
894  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
895  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
896    .P
897  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
898  the POSIX character classes.  the POSIX character classes.
899    .
900  .SH VERTICAL BAR  .
901    .SH "VERTICAL BAR"
902  .rs  .rs
903  .sp  .sp
904  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
905  the pattern  the pattern
906    .sp
907    gilbert|sullivan    gilbert|sullivan
908    .sp
909  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
910  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
911  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
912  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
913  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
914  pattern as well as the alternative in the subpattern.  .\" </a>
915    (defined below),
916  .SH INTERNAL OPTION SETTING  .\"
917    "succeeds" means matching the rest of the main pattern as well as the
918    alternative in the subpattern.
919    .
920    .
921    .SH "INTERNAL OPTION SETTING"
922  .rs  .rs
923  .sp  .sp
924  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
925  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
926  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
927    .sp
928    i  for PCRE_CASELESS    i  for PCRE_CASELESS
929    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
930    s  for PCRE_DOTALL    s  for PCRE_DOTALL
931    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
932    .sp
933  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
934  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
935  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
936  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
937  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
938  unset.  unset.
939    .P
940  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
941  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
942  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
943  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
944  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
945    .P
946  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
947  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
948    .sp
949    (a(?i)b)c    (a(?i)b)c
950    .sp
951  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).
952  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
953  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
954  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
955    .sp
956    (a(?i)b|c)    (a(?i)b|c)
957    .sp
958  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
959  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
960  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
961  behaviour otherwise.  behaviour otherwise.
962    .P
963  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
964  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
965  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
966  earlier in the pattern than any of the additional features it turns on, even  .
967  when it is at top level. It is best put at the start.  .
968    .\" HTML <a name="subpattern"></a>
969  .SH SUBPATTERNS  .SH SUBPATTERNS
970  .rs  .rs
971  .sp  .sp
972  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
973  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
974    .sp
975  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
976    .sp
977    cat(aract|erpillar|)    cat(aract|erpillar|)
978    .sp
979  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
980  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
981    .sp
982  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
983  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
984  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
985  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
986  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
987    .P
988  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
989    .sp
990    the ((red|white) (king|queen))    the ((red|white) (king|queen))
991    .sp
992  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,
993  2, and 3, respectively.  2, and 3, respectively.
994    .P
995  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
996  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
997  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
998  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
999  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
1000  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
1001    .sp
1002    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1003    .sp
1004  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
1005  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
1006  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
1007  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
1008  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
1009  the ":". Thus the two patterns  the ":". Thus the two patterns
1010    .sp
1011    (?i:saturday|sunday)    (?i:saturday|sunday)
1012    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1013    .sp
1014  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1015  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
1016  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
1017  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1018    .
1019  .SH NAMED SUBPATTERNS  .
1020    .SH "DUPLICATE SUBPATTERN NUMBERS"
1021    .rs
1022    .sp
1023    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1024    the same numbers for its capturing parentheses. Such a subpattern starts with
1025    (?| and is itself a non-capturing subpattern. For example, consider this
1026    pattern:
1027    .sp
1028      (?|(Sat)ur|(Sun))day
1029    .sp
1030    Because the two alternatives are inside a (?| group, both sets of capturing
1031    parentheses are numbered one. Thus, when the pattern matches, you can look
1032    at captured substring number one, whichever alternative matched. This construct
1033    is useful when you want to capture part, but not all, of one of a number of
1034    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1035    number is reset at the start of each branch. The numbers of any capturing
1036    buffers that follow the subpattern start after the highest number used in any
1037    branch. The following example is taken from the Perl documentation.
1038    The numbers underneath show in which buffer the captured content will be
1039    stored.
1040    .sp
1041      # before  ---------------branch-reset----------- after
1042      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1043      # 1            2         2  3        2     3     4
1044    .sp
1045    A backreference or a recursive call to a numbered subpattern always refers to
1046    the first one in the pattern with the given number.
1047    .P
1048    An alternative approach to using this "branch reset" feature is to use
1049    duplicate named subpatterns, as described in the next section.
1050    .
1051    .
1052    .SH "NAMED SUBPATTERNS"
1053  .rs  .rs
1054  .sp  .sp
1055  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
1056  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1057  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
1058  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1059  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
1060  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
1061    the Perl and the Python syntax.
1062  Named capturing parentheses are still allocated numbers as well as names. The  .P
1063  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
1064  table from a compiled pattern. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1065    parentheses from other parts of the pattern, such as
1066    .\" HTML <a href="#backreferences">
1067    .\" </a>
1068    backreferences,
1069    .\"
1070    .\" HTML <a href="#recursion">
1071    .\" </a>
1072    recursion,
1073    .\"
1074    and
1075    .\" HTML <a href="#conditions">
1076    .\" </a>
1077    conditions,
1078    .\"
1079    can be made by name as well as by number.
1080    .P
1081    Names consist of up to 32 alphanumeric characters and underscores. Named
1082    capturing parentheses are still allocated numbers as well as names, exactly as
1083    if the names were not present. The PCRE API provides function calls for
1084    extracting the name-to-number translation table from a compiled pattern. There
1085    is also a convenience function for extracting a captured substring by name.
1086    .P
1087    By default, a name must be unique within a pattern, but it is possible to relax
1088    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
1089    be useful for patterns where only one instance of the named parentheses can
1090    match. Suppose you want to match the name of a weekday, either as a 3-letter
1091    abbreviation or as the full name, and in both cases you want to extract the
1092    abbreviation. This pattern (ignoring the line breaks) does the job:
1093    .sp
1094      (?<DN>Mon|Fri|Sun)(?:day)?|
1095      (?<DN>Tue)(?:sday)?|
1096      (?<DN>Wed)(?:nesday)?|
1097      (?<DN>Thu)(?:rsday)?|
1098      (?<DN>Sat)(?:urday)?
1099    .sp
1100    There are five capturing substrings, but only one is ever set after a match.
1101    (An alternative way of solving this problem is to use a "branch reset"
1102    subpattern, as described in the previous section.)
1103    .P
1104    The convenience function for extracting the data by name returns the substring
1105    for the first (and in this example, the only) subpattern of that name that
1106    matched. This saves searching to find which numbered subpattern it was. If you
1107    make a reference to a non-unique named subpattern from elsewhere in the
1108    pattern, the one that corresponds to the lowest number is used. For further
1109    details of the interfaces for handling named subpatterns, see the
1110  .\" HREF  .\" HREF
1111  \fBpcreapi\fR  \fBpcreapi\fP
1112  .\"  .\"
1113  documentation.  documentation.
1114    .
1115    .
1116  .SH REPETITION  .SH REPETITION
1117  .rs  .rs
1118  .sp  .sp
1119  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1120  items:  items:
1121    .sp
1122    a literal data character    a literal data character
1123    the . metacharacter    the dot metacharacter
1124    the \\C escape sequence    the \eC escape sequence
1125    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1126      the \eR escape sequence
1127      an escape such as \ed that matches a single character
1128    a character class    a character class
1129    a back reference (see next section)    a back reference (see next section)
1130    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1131    .sp
1132  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1133  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1134  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
1135  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1136    .sp
1137    z{2,4}    z{2,4}
1138    .sp
1139  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
1140  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
1141  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
1142  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1143    .sp
1144    [aeiou]{3,}    [aeiou]{3,}
1145    .sp
1146  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1147    .sp
1148    \\d{8}    \ed{8}
1149    .sp
1150  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
1151  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
1152  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
1153  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1154    .P
1155  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
1156  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
1157  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1158    support is available, \eX{3} matches three Unicode extended sequences, each of
1159    which may be several bytes long (and they may be of different lengths).
1160    .P
1161  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
1162  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1163    .P
1164  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1165  quantifiers have single-character abbreviations:  abbreviations:
1166    .sp
1167    *    is equivalent to {0,}    *    is equivalent to {0,}
1168    +    is equivalent to {1,}    +    is equivalent to {1,}
1169    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1170    .sp
1171  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
1172  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:
1173    .sp
1174    (a?)*    (a?)*
1175    .sp
1176  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
1177  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
1178  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
1179  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1180    .P
1181  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
1182  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1183  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
1184  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 */
1185  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1186  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1187    .sp
1188    /\\*.*\\*/    /\e*.*\e*/
1189    .sp
1190  to the string  to the string
1191    .sp
1192    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1193    .sp
1194  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 .*
1195  item.  item.
1196    .P
1197  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
1198  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1199  pattern  pattern
1200    .sp
1201    /\\*.*?\\*/    /\e*.*?\e*/
1202    .sp
1203  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
1204  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1205  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
1206  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
1207    .sp
1208    \\d??\\d    \ed??\ed
1209    .sp
1210  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
1211  way the rest of the pattern matches.  way the rest of the pattern matches.
1212    .P
1213  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),
1214  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
1215  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
1216  default behaviour.  default behaviour.
1217    .P
1218  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1219  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
1220  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1221    .P
1222  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
1223  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
1224  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1225  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1226  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
1227  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1228    .P
1229  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
1230  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1231  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1232    .P
1233  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1234  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1235  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
1236  succeed. Consider, for example:  succeeds. Consider, for example:
1237    .sp
1238    (.*)abc\\1    (.*)abc\e1
1239    .sp
1240  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
1241  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1242    .P
1243  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1244  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1245    .sp
1246    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1247    .sp
1248  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1249  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1250  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1251  example, after  example, after
1252    .sp
1253    /(a|(b))+/    /(a|(b))+/
1254    .sp
1255  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1256    .
1257  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1258    .\" HTML <a name="atomicgroup"></a>
1259    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1260  .rs  .rs
1261  .sp  .sp
1262  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1263  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
1264  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
1265  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
1266  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
1267  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1268    .P
1269  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
1270    .sp
1271    123456bar    123456bar
1272    .sp
1273  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
1274  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+
1275  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"
1276  (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
1277  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.
1278    .P
1279  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
1280  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
1281  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1282    .sp
1283    (?>\\d+)foo    (?>\ed+)foo
1284    .sp
1285  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
1286  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
1287  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1288  normal.  normal.
1289    .P
1290  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
1291  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1292  the current point in the subject string.  the current point in the subject string.
1293    .P
1294  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1295  the above example can be thought of as a maximizing repeat that must swallow  the above example can be thought of as a maximizing repeat that must swallow
1296  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
1297  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,
1298  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1299    .P
1300  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1301  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1302  group is just a single repeated item, as in the example above, a simpler  group is just a single repeated item, as in the example above, a simpler
1303  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1304  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1305  previous example can be rewritten as  previous example can be rewritten as
1306    .sp
1307    \\d++bar    \ed++foo
1308    .sp
1309    Note that a possessive quantifier can be used with an entire group, for
1310    example:
1311    .sp
1312      (abc|xyz){2,3}+
1313    .sp
1314  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1315  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1316  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
1317  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1318    difference; possessive quantifiers should be slightly faster.
1319  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1320  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1321    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1322    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1323    package, and PCRE copied it from there. It ultimately found its way into Perl
1324    at release 5.10.
1325    .P
1326    PCRE has an optimization that automatically "possessifies" certain simple
1327    pattern constructs. For example, the sequence A+B is treated as A++B because
1328    there is no point in backtracking into a sequence of A's when B must follow.
1329    .P
1330  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
1331  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
1332  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
1333  pattern  pattern
1334    .sp
1335    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1336    .sp
1337  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
1338  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1339  quickly. However, if it is applied to  quickly. However, if it is applied to
1340    .sp
1341    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1342    .sp
1343  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
1344  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
1345  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
1346  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
1347  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
1348  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
1349  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1350    an atomic group, like this:
1351    ((?>\\D+)|<\\d+>)*[!?]  .sp
1352      ((?>\eD+)|<\ed+>)*[!?]
1353    .sp
1354  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1355    .
1356  .SH BACK REFERENCES  .
1357    .\" HTML <a name="backreferences"></a>
1358    .SH "BACK REFERENCES"
1359  .rs  .rs
1360  .sp  .sp
1361  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
1362  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1363  (that is, to its left) in the pattern, provided there have been that many  (that is, to its left) in the pattern, provided there have been that many
1364  previous capturing left parentheses.  previous capturing left parentheses.
1365    .P
1366  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
1367  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
1368  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
1369  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
1370  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
1371  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1372    in an earlier iteration.
1373    .P
1374    It is not possible to have a numerical "forward back reference" to a subpattern
1375    whose number is 10 or more using this syntax because a sequence such as \e50 is
1376    interpreted as a character defined in octal. See the subsection entitled
1377    "Non-printing characters"
1378    .\" HTML <a href="#digitsafterbackslash">
1379    .\" </a>
1380    above
1381    .\"
1382    for further details of the handling of digits following a backslash. There is
1383    no such problem when named parentheses are used. A back reference to any
1384    subpattern is possible using named parentheses (see below).
1385    .P
1386    Another way of avoiding the ambiguity inherent in the use of digits following a
1387    backslash is to use the \eg escape sequence, which is a feature introduced in
1388    Perl 5.10. This escape must be followed by an unsigned number or a negative
1389    number, optionally enclosed in braces. These examples are all identical:
1390    .sp
1391      (ring), \e1
1392      (ring), \eg1
1393      (ring), \eg{1}
1394    .sp
1395    An unsigned number specifies an absolute reference without the ambiguity that
1396    is present in the older syntax. It is also useful when literal digits follow
1397    the reference. A negative number is a relative reference. Consider this
1398    example:
1399    .sp
1400      (abc(def)ghi)\eg{-1}
1401    .sp
1402    The sequence \eg{-1} is a reference to the most recently started capturing
1403    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1404    would be equivalent to \e1. The use of relative references can be helpful in
1405    long patterns, and also in patterns that are created by joining together
1406    fragments that contain references within themselves.
1407    .P
1408  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1409  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1410  itself (see  itself (see
# Line 832  itself (see Line 1413  itself (see
1413  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1414  .\"  .\"
1415  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1416    .sp
1417    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1418    .sp
1419  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1420  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1421  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1422    .sp
1423    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1424    .sp
1425  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
1426  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1427    .P
1428  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1429  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1430    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1431    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1432    references, is also supported. We could rewrite the above example in any of
1433    the following ways:
1434    .sp
1435      (?<p1>(?i)rah)\es+\ek<p1>
1436      (?'p1'(?i)rah)\es+\ek{p1}
1437      (?P<p1>(?i)rah)\es+(?P=p1)
1438      (?<p1>(?i)rah)\es+\eg{p1}
1439    .sp
1440    A subpattern that is referenced by name may appear in the pattern before or
1441    after the reference.
1442    .P
1443  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
1444  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1445  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1446    .sp
1447    (a|(bc))\\2    (a|(bc))\e2
1448    .sp
1449  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
1450  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1451  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1452  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1453  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1454  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1455    .\" HTML <a href="#comments">
1456    .\" </a>
1457    "Comments"
1458    .\"
1459    below) can be used.
1460    .P
1461  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
1462  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.
1463  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1464  example, the pattern  example, the pattern
1465    .sp
1466    (a|b\\1)+    (a|b\e1)+
1467    .sp
1468  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
1469  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1470  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1471  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
1472  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
1473  minimum of zero.  minimum of zero.
1474    .
1475    .
1476    .\" HTML <a name="bigassertions"></a>
1477  .SH ASSERTIONS  .SH ASSERTIONS
1478  .rs  .rs
1479  .sp  .sp
1480  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
1481  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1482  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
1483    .\" HTML <a href="#smallassertions">
1484    .\" </a>
1485    above.
1486    .\"
1487    .P
1488  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1489  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1490  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1491    except that it does not cause the current matching position to be changed.
1492  An assertion subpattern is matched in the normal way, except that it does not  .P
1493  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1494  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
1495    of assertion contains capturing subpatterns within it, these are counted for
1496    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1497    However, substring capturing is carried out only for positive assertions,
1498    because it does not make sense for negative assertions.
1499    .
1500    .
1501    .SS "Lookahead assertions"
1502    .rs
1503    .sp
1504    Lookahead assertions start with (?= for positive assertions and (?! for
1505    negative assertions. For example,
1506    .sp
1507      \ew+(?=;)
1508    .sp
1509  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
1510  the match, and  the match, and
1511    .sp
1512    foo(?!bar)    foo(?!bar)
1513    .sp
1514  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
1515  apparently similar pattern  apparently similar pattern
1516    .sp
1517    (?!foo)bar    (?!foo)bar
1518    .sp
1519  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
1520  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1521  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1522  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1523    .P
1524  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
1525  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
1526  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.
1527    .
1528    .
1529    .\" HTML <a name="lookbehind"></a>
1530    .SS "Lookbehind assertions"
1531    .rs
1532    .sp
1533  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1534  negative assertions. For example,  negative assertions. For example,
1535    .sp
1536    (?<!foo)bar    (?<!foo)bar
1537    .sp
1538  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
1539  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
1540  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
1541  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1542    .sp
1543    (?<=bullock|donkey)    (?<=bullock|donkey)
1544    .sp
1545  is permitted, but  is permitted, but
1546    .sp
1547    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1548    .sp
1549  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1550  are permitted only at the top level of a lookbehind assertion. This is an  are permitted only at the top level of a lookbehind assertion. This is an
1551  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1552  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1553    .sp
1554    (?<=ab(c|de))    (?<=ab(c|de))
1555    .sp
1556  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
1557  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:
1558    .sp
1559    (?<=abc|abde)    (?<=abc|abde)
1560    .sp
1561    In some cases, the Perl 5.10 escape sequence \eK
1562    .\" HTML <a href="#resetmatchstart">
1563    .\" </a>
1564    (see above)
1565    .\"
1566    can be used instead of a lookbehind assertion; this is not restricted to a
1567    fixed-length.
1568    .P
1569  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1570  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
1571  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1572  match is deemed to fail.  assertion fails.
1573    .P
1574  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)
1575  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1576  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1577    different numbers of bytes, are also not permitted.
1578  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1579  efficient matching at the end of the subject string. Consider a simple pattern  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1580  such as  specify efficient matching at the end of the subject string. Consider a simple
1581    pattern such as
1582    .sp
1583    abcd$    abcd$
1584    .sp
1585  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
1586  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
1587  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
1588    .sp
1589    ^.*abcd$    ^.*abcd$
1590    .sp
1591  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
1592  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,
1593  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"
1594  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,
1595  if the pattern is written as  if the pattern is written as
1596    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1597    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1598    .sp
1599  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
1600  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
1601  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1602  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1603    .
1604    .
1605    .SS "Using multiple assertions"
1606    .rs
1607    .sp
1608  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1609    .sp
1610    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1611    .sp
1612  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
1613  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1614  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
1615  digits, and then there is a check that the same three characters are not "999".  digits, and then there is a check that the same three characters are not "999".
1616  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
1617  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
1618  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1619    .sp
1620    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1621    .sp
1622  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1623  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1624  preceding three characters are not "999".  preceding three characters are not "999".
1625    .P
1626  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1627    .sp
1628    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1629    .sp
1630  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
1631  preceded by "foo", while  preceded by "foo", while
1632    .sp
1633    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1634    .sp
1635  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
1636  characters that are not "999".  characters that are not "999".
1637    .
1638  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1639  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1640  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  
1641  .rs  .rs
1642  .sp  .sp
1643  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1644  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1645  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1646  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1647    .sp
1648    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1649    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1650    .sp
1651  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1652  no-pattern (if present) is used. If there are more than two alternatives in the  no-pattern (if present) is used. If there are more than two alternatives in the
1653  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1654    .P
1655  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1656  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1657  subpattern of that number has previously matched. The number must be greater  .
1658  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1659  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1660  it into three parts for ease of discussion:  .sp
1661    If the text between the parentheses consists of a sequence of digits, the
1662    ( \\( )?    [^()]+    (?(1) \\) )  condition is true if the capturing subpattern of that number has previously
1663    matched. An alternative notation is to precede the digits with a plus or minus
1664    sign. In this case, the subpattern number is relative rather than absolute.
1665    The most recently opened parentheses can be referenced by (?(-1), the next most
1666    recent by (?(-2), and so on. In looping constructs it can also make sense to
1667    refer to subsequent groups with constructs such as (?(+2).
1668    .P
1669    Consider the following pattern, which contains non-significant white space to
1670    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1671    three parts for ease of discussion:
1672    .sp
1673      ( \e( )?    [^()]+    (?(1) \e) )
1674    .sp
1675  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1676  character is present, sets it as the first captured substring. The second part  character is present, sets it as the first captured substring. The second part
1677  matches one or more characters that are not parentheses. The third part is a  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 1681  the condition is true, and so the yes-pa
1681  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1682  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1683  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1684    .P
1685  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
1686  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1687  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1688      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1689  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
1690    This makes the fragment independent of the parentheses in the larger pattern.
1691    .
1692    .SS "Checking for a used subpattern by name"
1693    .rs
1694    .sp
1695    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1696    subpattern by name. For compatibility with earlier versions of PCRE, which had
1697    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1698    there is a possible ambiguity with this syntax, because subpattern names may
1699    consist entirely of digits. PCRE looks first for a named subpattern; if it
1700    cannot find one and the name consists entirely of digits, PCRE looks for a
1701    subpattern of that number, which must be greater than zero. Using subpattern
1702    names that consist entirely of digits is not recommended.
1703    .P
1704    Rewriting the above example to use a named subpattern gives this:
1705    .sp
1706      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1707    .sp
1708    .
1709    .SS "Checking for pattern recursion"
1710    .rs
1711    .sp
1712    If the condition is the string (R), and there is no subpattern with the name R,
1713    the condition is true if a recursive call to the whole pattern or any
1714    subpattern has been made. If digits or a name preceded by ampersand follow the
1715    letter R, for example:
1716    .sp
1717      (?(R3)...) or (?(R&name)...)
1718    .sp
1719    the condition is true if the most recent recursion is into the subpattern whose
1720    number or name is given. This condition does not check the entire recursion
1721    stack.
1722    .P
1723    At "top level", all these recursion test conditions are false. Recursive
1724    patterns are described below.
1725    .
1726    .SS "Defining subpatterns for use by reference only"
1727    .rs
1728    .sp
1729    If the condition is the string (DEFINE), and there is no subpattern with the
1730    name DEFINE, the condition is always false. In this case, there may be only one
1731    alternative in the subpattern. It is always skipped if control reaches this
1732    point in the pattern; the idea of DEFINE is that it can be used to define
1733    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1734    is described below.) For example, a pattern to match an IPv4 address could be
1735    written like this (ignore whitespace and line breaks):
1736    .sp
1737      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1738      \eb (?&byte) (\e.(?&byte)){3} \eb
1739    .sp
1740    The first part of the pattern is a DEFINE group inside which a another group
1741    named "byte" is defined. This matches an individual component of an IPv4
1742    address (a number less than 256). When matching takes place, this part of the
1743    pattern is skipped because DEFINE acts like a false condition.
1744    .P
1745    The rest of the pattern uses references to the named group to match the four
1746    dot-separated components of an IPv4 address, insisting on a word boundary at
1747    each end.
1748    .
1749    .SS "Assertion conditions"
1750    .rs
1751    .sp
1752    If the condition is not in any of the above formats, it must be an assertion.
1753  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1754  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1755  alternatives on the second line:  alternatives on the second line:
1756    .sp
1757    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1758    \\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} )
1759    .sp
1760  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1761  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
1762  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
1763  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1764  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
1765  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
1766    .
1767    .
1768    .\" HTML <a name="comments"></a>
1769  .SH COMMENTS  .SH COMMENTS
1770  .rs  .rs
1771  .sp  .sp
1772  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
1773  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1774  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.
1775    .P
1776  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1777  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1778  character in the pattern.  next newline in the pattern.
1779    .
1780  .SH RECURSIVE PATTERNS  .
1781    .\" HTML <a name="recursion"></a>
1782    .SH "RECURSIVE PATTERNS"
1783  .rs  .rs
1784  .sp  .sp
1785  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1786  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1787  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
1788  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
1789  experimental facility that allows regular expressions to recurse (amongst other  .P
1790  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
1791  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
1792  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1793    pattern using code interpolation to solve the parentheses problem can be
1794    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
1795    .sp
1796      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1797    .sp
1798  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
1799  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1800  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1801  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1802    supports special syntax for recursion of the entire pattern, and also for
1803  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1804  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.
1805  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1806  "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
1807  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1808    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1809  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
1810  the PCRE_EXTENDED option is set so that white space is ignored):  a recursive call of the entire regular expression.
1811    .P
1812    \\( ( (?>[^()]+) | (?R) )* \\)  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1813    treated as an atomic group. That is, once it has matched some of the subject
1814    string, it is never re-entered, even if it contains untried alternatives and
1815    there is a subsequent matching failure.
1816    .P
1817    This PCRE pattern solves the nested parentheses problem (assume the
1818    PCRE_EXTENDED option is set so that white space is ignored):
1819    .sp
1820      \e( ( (?>[^()]+) | (?R) )* \e)
1821    .sp
1822  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1823  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
1824  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1825  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1826    .P
1827  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
1828  pattern, so instead you could use this:  pattern, so instead you could use this:
1829    .sp
1830    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1831    .sp
1832  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
1833  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1834  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1835  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
1836  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.)
1837  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
1838    most recently opened parentheses preceding the recursion. In other words, a
1839    (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  negative number counts capturing parentheses leftwards from the point at which
1840    it is encountered.
1841  This particular example pattern contains nested unlimited repeats, and so the  .P
1842  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1843  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
1844  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1845    "subroutine" calls, as described in the next section.
1846    .P
1847    An alternative approach is to use named parentheses instead. The Perl syntax
1848    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1849    could rewrite the above example as follows:
1850    .sp
1851      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1852    .sp
1853    If there is more than one subpattern with the same name, the earliest one is
1854    used.
1855    .P
1856    This particular example pattern that we have been looking at contains nested
1857    unlimited repeats, and so the use of atomic grouping for matching strings of
1858    non-parentheses is important when applying the pattern to strings that do not
1859    match. For example, when this pattern is applied to
1860    .sp
1861    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1862    .sp
1863  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1864  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
1865  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
1866  before failure can be reported.  before failure can be reported.
1867    .P
1868  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
1869  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.
1870  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
1871  below and the  below and the
1872  .\" HREF  .\" HREF
1873  \fBpcrecallout\fR  \fBpcrecallout\fP
1874  .\"  .\"
1875  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1876    .sp
1877    (ab(cd)ef)    (ab(cd)ef)
1878    .sp
1879  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
1880  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1881    .sp
1882    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1883       ^                        ^       ^                        ^
1884       ^                        ^       ^                        ^
1885    .sp
1886  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
1887  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
1888  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
1889  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
1890  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.
1891    .P
1892  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.
1893  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1894  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1895  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1896    .sp
1897    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1898    .sp
1899  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
1900  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1901  is the actual recursive call.  is the actual recursive call.
1902    .
1903    .
1904  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1905  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1906  .rs  .rs
1907  .sp  .sp
1908  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
1909  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
1910  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1911  pattern  before or after the reference. A numbered reference can be absolute or
1912    relative, as in these examples:
1913    (sens|respons)e and \\1ibility  .sp
1914      (...(absolute)...)...(?2)...
1915      (...(relative)...)...(?-1)...
1916      (...(?+1)...(relative)...
1917    .sp
1918    An earlier example pointed out that the pattern
1919    .sp
1920      (sens|respons)e and \e1ibility
1921    .sp
1922  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1923  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1924    .sp
1925    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1926    .sp
1927  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
1928  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
1929  refer.  .P
1930    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1931    group. That is, once it has matched some of the subject string, it is never
1932    re-entered, even if it contains untried alternatives and there is a subsequent
1933    matching failure.
1934    .P
1935    When a subpattern is used as a subroutine, processing options such as
1936    case-independence are fixed when the subpattern is defined. They cannot be
1937    changed for different calls. For example, consider this pattern:
1938    .sp
1939      (abc)(?i:(?-1))
1940    .sp
1941    It matches "abcabc". It does not match "abcABC" because the change of
1942    processing option does not affect the called subpattern.
1943    .
1944    .
1945  .SH CALLOUTS  .SH CALLOUTS
1946  .rs  .rs
1947  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1949  Perl has a feature whereby using the seq
1949  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
1950  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1951  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1952    .P
1953  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
1954  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
1955  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.
1956  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1957    .P
1958  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1959  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
1960  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
1961  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1962    .sp
1963    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1964    .sp
1965  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
1966    automatically installed before each item in the pattern. They are all numbered
1967    255.
1968    .P
1969    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1970  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
1971  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
1972  \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
1973  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1974  function is given in the  description of the interface to the callout function is given in the
1975  .\" HREF  .\" HREF
1976  \fBpcrecallout\fR  \fBpcrecallout\fP
1977  .\"  .\"
1978  documentation.  documentation.
1979    .
1980  .in 0  .
1981  Last updated: 03 February 2003  .SH "BACTRACKING CONTROL"
1982  .br  .rs
1983  Copyright (c) 1997-2003 University of Cambridge.  .sp
1984    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
1985    are described in the Perl documentation as "experimental and subject to change
1986    or removal in a future version of Perl". It goes on to say: "Their usage in
1987    production code should be noted to avoid problems during upgrades." The same
1988    remarks apply to the PCRE features described in this section.
1989    .P
1990    Since these verbs are specifically related to backtracking, they can be used
1991    only when the pattern is to be matched using \fBpcre_exec()\fP, which uses a
1992    backtracking algorithm. They cause an error if encountered by
1993    \fBpcre_dfa_exec()\fP.
1994    .P
1995    The new verbs make use of what was previously invalid syntax: an opening
1996    parenthesis followed by an asterisk. In Perl, they are generally of the form
1997    (*VERB:ARG) but PCRE does not support the use of arguments, so its general
1998    form is just (*VERB). Any number of these verbs may occur in a pattern. There
1999    are two kinds:
2000    .
2001    .SS "Verbs that act immediately"
2002    .rs
2003    .sp
2004    The following verbs act as soon as they are encountered:
2005    .sp
2006       (*ACCEPT)
2007    .sp
2008    This verb causes the match to end successfully, skipping the remainder of the
2009    pattern. When inside a recursion, only the innermost pattern is ended
2010    immediately. PCRE differs from Perl in what happens if the (*ACCEPT) is inside
2011    capturing parentheses. In Perl, the data so far is captured: in PCRE no data is
2012    captured. For example:
2013    .sp
2014      A(A|B(*ACCEPT)|C)D
2015    .sp
2016    This matches "AB", "AAD", or "ACD", but when it matches "AB", no data is
2017    captured.
2018    .sp
2019      (*FAIL) or (*F)
2020    .sp
2021    This verb causes the match to fail, forcing backtracking to occur. It is
2022    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2023    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2024    Perl features that are not present in PCRE. The nearest equivalent is the
2025    callout feature, as for example in this pattern:
2026    .sp
2027      a+(?C)(*FAIL)
2028    .sp
2029    A match with the string "aaaa" always fails, but the callout is taken before
2030    each backtrack happens (in this example, 10 times).
2031    .
2032    .SS "Verbs that act after backtracking"
2033    .rs
2034    .sp
2035    The following verbs do nothing when they are encountered. Matching continues
2036    with what follows, but if there is no subsequent match, a failure is forced.
2037    The verbs differ in exactly what kind of failure occurs.
2038    .sp
2039      (*COMMIT)
2040    .sp
2041    This verb causes the whole match to fail outright if the rest of the pattern
2042    does not match. Even if the pattern is unanchored, no further attempts to find
2043    a match by advancing the start point take place. Once (*COMMIT) has been
2044    passed, \fBpcre_exec()\fP is committed to finding a match at the current
2045    starting point, or not at all. For example:
2046    .sp
2047      a+(*COMMIT)b
2048    .sp
2049    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2050    dynamic anchor, or "I've started, so I must finish."
2051    .sp
2052      (*PRUNE)
2053    .sp
2054    This verb causes the match to fail at the current position if the rest of the
2055    pattern does not match. If the pattern is unanchored, the normal "bumpalong"
2056    advance to the next starting character then happens. Backtracking can occur as
2057    usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but
2058    if there is no match to the right, backtracking cannot cross (*PRUNE).
2059    In simple cases, the use of (*PRUNE) is just an alternative to an atomic
2060    group or possessive quantifier, but there are some uses of (*PRUNE) that cannot
2061    be expressed in any other way.
2062    .sp
2063      (*SKIP)
2064    .sp
2065    This verb is like (*PRUNE), except that if the pattern is unanchored, the
2066    "bumpalong" advance is not to the next character, but to the position in the
2067    subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text
2068    was matched leading up to it cannot be part of a successful match. Consider:
2069    .sp
2070      a+(*SKIP)b
2071    .sp
2072    If the subject is "aaaac...", after the first match attempt fails (starting at
2073    the first character in the string), the starting point skips on to start the
2074    next attempt at "c". Note that a possessive quantifer does not have the same
2075    effect in this example; although it would suppress backtracking during the
2076    first match attempt, the second attempt would start at the second character
2077    instead of skipping on to "c".
2078    .sp
2079      (*THEN)
2080    .sp
2081    This verb causes a skip to the next alternation if the rest of the pattern does
2082    not match. That is, it cancels pending backtracking, but only within the
2083    current alternation. Its name comes from the observation that it can be used
2084    for a pattern-based if-then-else block:
2085    .sp
2086      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2087    .sp
2088    If the COND1 pattern matches, FOO is tried (and possibly further items after
2089    the end of the group if FOO succeeds); on failure the matcher skips to the
2090    second alternative and tries COND2, without backtracking into COND1. If (*THEN)
2091    is used outside of any alternation, it acts exactly like (*PRUNE).
2092    .
2093    .
2094    .SH "SEE ALSO"
2095    .rs
2096    .sp
2097    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
2098    .
2099    .
2100    .SH AUTHOR
2101    .rs
2102    .sp
2103    .nf
2104    Philip Hazel
2105    University Computing Service
2106    Cambridge CB2 3QH, England.
2107    .fi
2108    .
2109    .
2110    .SH REVISION
2111    .rs
2112    .sp
2113    .nf
2114    Last updated: 09 August 2007
2115    Copyright (c) 1997-2007 University of Cambridge.
2116    .fi

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