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revision 91 by nigel, Sat Feb 24 21:41:34 2007 UTC revision 745 by ph10, Mon Nov 14 11:41:03 2011 UTC
# Line 4  PCRE - Perl-compatible regular expressio Line 4  PCRE - Perl-compatible regular expressio
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 books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
11  regular expressions in great detail. This description of PCRE's regular  .\"
12  expressions is intended as reference material.  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22  .P  .P
23  The original operation of PCRE was on strings of one-byte characters. However,  The original operation of PCRE was on strings of one-byte characters. However,
24  there is now also support for UTF-8 character strings. To use this, you must  there is now also support for UTF-8 character strings. To use this,
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  PCRE must be built to include UTF-8 support, and you must call
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27  places below. There is also a summary of UTF-8 features in the  is also a special sequence that can be given at the start of a pattern:
28  .\" HTML <a href="pcre.html#utf8support">  .sp
29  .\" </a>    (*UTF8)
30  section on UTF-8 support  .sp
31  .\"  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32  in the main  option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35  .\" HREF  .\" HREF
36  \fBpcre\fP  \fBpcreunicode\fP
37  .\"  .\"
38  page.  page.
39  .P  .P
40    Another special sequence that may appear at the start of a pattern or in
41    combination with (*UTF8) is:
42    .sp
43      (*UCP)
44    .sp
45    This has the same effect as setting the PCRE_UCP option: it causes sequences
46    such as \ed and \ew to use Unicode properties to determine character types,
47    instead of recognizing only characters with codes less than 128 via a lookup
48    table.
49    .P
50    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52    also some more of these special sequences that are concerned with the handling
53    of newlines; they are described below.
54    .P
55  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
56  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57  From release 6.0, PCRE offers a second matching function,  From release 6.0, PCRE offers a second matching function,
58  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
59  Perl-compatible. The advantages and disadvantages of the alternative function,  Perl-compatible. Some of the features discussed below are not available when
60  and how it differs from the normal function, are discussed in the  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61    alternative function, and how it differs from the normal function, are
62    discussed in the
63  .\" HREF  .\" HREF
64  \fBpcrematching\fP  \fBpcrematching\fP
65  .\"  .\"
66  page.  page.
67    .
68    .
69    .\" HTML <a name="newlines"></a>
70    .SH "NEWLINE CONVENTIONS"
71    .rs
72    .sp
73    PCRE supports five different conventions for indicating line breaks in
74    strings: a single CR (carriage return) character, a single LF (linefeed)
75    character, the two-character sequence CRLF, any of the three preceding, or any
76    Unicode newline sequence. The
77    .\" HREF
78    \fBpcreapi\fP
79    .\"
80    page has
81    .\" HTML <a href="pcreapi.html#newlines">
82    .\" </a>
83    further discussion
84    .\"
85    about newlines, and shows how to set the newline convention in the
86    \fIoptions\fP arguments for the compiling and matching functions.
87  .P  .P
88    It is also possible to specify a newline convention by starting a pattern
89    string with one of the following five sequences:
90    .sp
91      (*CR)        carriage return
92      (*LF)        linefeed
93      (*CRLF)      carriage return, followed by linefeed
94      (*ANYCRLF)   any of the three above
95      (*ANY)       all Unicode newline sequences
96    .sp
97    These override the default and the options given to \fBpcre_compile()\fP or
98    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99    newline sequence, the pattern
100    .sp
101      (*CR)a.b
102    .sp
103    changes the convention to CR. That pattern matches "a\enb" because LF is no
104    longer a newline. Note that these special settings, which are not
105    Perl-compatible, are recognized only at the very start of a pattern, and that
106    they must be in upper case. If more than one of them is present, the last one
107    is used.
108    .P
109    The newline convention affects the interpretation of the dot metacharacter when
110    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111    affect what the \eR escape sequence matches. By default, this is any Unicode
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114    .\" HTML <a href="#newlineseq">
115    .\" </a>
116    "Newline sequences"
117    .\"
118    below. A change of \eR setting can be combined with a change of newline
119    convention.
120    .
121    .
122    .SH "CHARACTERS AND METACHARACTERS"
123    .rs
124    .sp
125  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
126  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
127  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
# Line 60  interpreted in some special way. Line 145  interpreted in some special way.
145  .P  .P
146  There are two different sets of metacharacters: those that are recognized  There are two different sets of metacharacters: those that are recognized
147  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
148  recognized in square brackets. Outside square brackets, the metacharacters are  recognized within square brackets. Outside square brackets, the metacharacters
149  as follows:  are as follows:
150  .sp  .sp
151    \e      general escape character with several uses    \e      general escape character with several uses
152    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
# Line 92  a character class the only metacharacter Line 177  a character class the only metacharacter
177  .sp  .sp
178  The following sections describe the use of each of the metacharacters.  The following sections describe the use of each of the metacharacters.
179  .  .
180    .
181  .SH BACKSLASH  .SH BACKSLASH
182  .rs  .rs
183  .sp  .sp
184  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
185  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
186  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
187  outside character classes.  both inside and outside character classes.
188  .P  .P
189  For example, if you want to match a * character, you write \e* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 106  otherwise be interpreted as a metacharac Line 192  otherwise be interpreted as a metacharac
192  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
194  .P  .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  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
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 125  Perl, $ and @ cause variable interpolati Line 215  Perl, $ and @ cause variable interpolati
215    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216  .sp  .sp
217  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219    by \eE later in the pattern, the literal interpretation continues to the end of
220    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221    a character class, this causes an error, because the character class is not
222    terminated.
223  .  .
224  .  .
225  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 134  The \eQ...\eE sequence is recognized bot Line 229  The \eQ...\eE sequence is recognized bot
229  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
230  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
231  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
232  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 often easier to use
233  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
234  .sp  .sp
235    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
236    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
237    \ee        escape (hex 1B)    \ee        escape (hex 1B)
238    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
239    \en        newline (hex 0A)    \en        linefeed (hex 0A)
240    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
241    \et        tab (hex 09)    \et        tab (hex 09)
242    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
243    \exhh      character with hex code hh    \exhh      character with hex code hh
244    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
245      \euhhhh    character with hex code hhhh (JavaScript mode only)
246  .sp  .sp
247  The precise effect of \ecx 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
248  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.
249  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
250  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
251  .P  than 127, a compile-time error occurs. This locks out non-ASCII characters in
252  After \ex, from zero to two hexadecimal digits are read (letters can be in  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
253  upper or lower case). Any number of hexadecimal digits may appear between \ex{  values are valid. A lower case letter is converted to upper case, and then the
254  and }, but the value of the character code must be less than 256 in non-UTF-8  0xc0 bits are flipped.)
255  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  .P
256  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  By default, after \ex, from zero to two hexadecimal digits are read (letters
257  and }, or if there is no terminating }, this form of escape is not recognized.  can be in upper or lower case). Any number of hexadecimal digits may appear
258  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  between \ex{ and }, but the value of the character code must be less than 256
259  with no following digits, giving a character whose value is zero.  in non-UTF-8 mode, and less than 2**31 in UTF-8 mode. That is, the maximum
260    value in hexadecimal is 7FFFFFFF. Note that this is bigger than the largest
261    Unicode code point, which is 10FFFF.
262    .P
263    If characters other than hexadecimal digits appear between \ex{ and }, or if
264    there is no terminating }, this form of escape is not recognized. Instead, the
265    initial \ex will be interpreted as a basic hexadecimal escape, with no
266    following digits, giving a character whose value is zero.
267    .P
268    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
269    as just described only when it is followed by two hexadecimal digits.
270    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
271    code points greater than 256 is provided by \eu, which must be followed by
272    four hexadecimal digits; otherwise it matches a literal "u" character.
273  .P  .P
274  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
275  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
276  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
277    \eu00dc in JavaScript mode).
278  .P  .P
279  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
280  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 190  parenthesized subpatterns. Line 299  parenthesized subpatterns.
299  .P  .P
300  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
301  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
302  digits following the backslash, ane uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
303  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
304  character specified in octal must be less than \e400. In UTF-8 mode, values up  character specified in octal must be less than \e400. In UTF-8 mode, values up
305  to \e777 are permitted. For example:  to \e777 are permitted. For example:
# Line 220  zero, because no more than three octal d Line 329  zero, because no more than three octal d
329  .P  .P
330  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
331  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, the
332  sequence \eb is interpreted as the backspace character (hex 08), and the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
333  sequence \eX is interpreted as the character "X". Outside a character class,  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
334  these sequences have different meanings  unrecognized escape sequences, they are treated as the literal characters "B",
335  .\" HTML <a href="#uniextseq">  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
336    set. Outside a character class, these sequences have different meanings.
337    .
338    .
339    .SS "Unsupported escape sequences"
340    .rs
341    .sp
342    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
343    handler and used to modify the case of following characters. By default, PCRE
344    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
345    option is set, \eU matches a "U" character, and \eu can be used to define a
346    character by code point, as described in the previous section.
347    .
348    .
349    .SS "Absolute and relative back references"
350    .rs
351    .sp
352    The sequence \eg followed by an unsigned or a negative number, optionally
353    enclosed in braces, is an absolute or relative back reference. A named back
354    reference can be coded as \eg{name}. Back references are discussed
355    .\" HTML <a href="#backreferences">
356  .\" </a>  .\" </a>
357  (see below).  later,
358    .\"
359    following the discussion of
360    .\" HTML <a href="#subpattern">
361    .\" </a>
362    parenthesized subpatterns.
363  .\"  .\"
364  .  .
365  .  .
366    .SS "Absolute and relative subroutine calls"
367    .rs
368    .sp
369    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
370    a number enclosed either in angle brackets or single quotes, is an alternative
371    syntax for referencing a subpattern as a "subroutine". Details are discussed
372    .\" HTML <a href="#onigurumasubroutines">
373    .\" </a>
374    later.
375    .\"
376    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
377    synonymous. The former is a back reference; the latter is a
378    .\" HTML <a href="#subpatternsassubroutines">
379    .\" </a>
380    subroutine
381    .\"
382    call.
383    .
384    .
385    .\" HTML <a name="genericchartypes"></a>
386  .SS "Generic character types"  .SS "Generic character types"
387  .rs  .rs
388  .sp  .sp
389  The third use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
390  .sp  .sp
391    \ed     any decimal digit    \ed     any decimal digit
392    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
393      \eh     any horizontal whitespace character
394      \eH     any character that is not a horizontal whitespace character
395    \es     any whitespace character    \es     any whitespace character
396    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
397      \ev     any vertical whitespace character
398      \eV     any character that is not a vertical whitespace character
399    \ew     any "word" character    \ew     any "word" character
400    \eW     any "non-word" character    \eW     any "non-word" character
401  .sp  .sp
402  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
403  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
404    .\" HTML <a href="#fullstopdot">
405    .\" </a>
406    the "." metacharacter
407    .\"
408    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
409    PCRE does not support this.
410  .P  .P
411  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
412    of characters into two disjoint sets. Any given character matches one, and only
413    one, of each pair. The sequences can appear both inside and outside character
414  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
415  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
416  there is no character to match.  there is no character to match.
417  .P  .P
418  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es does not match the VT character (code 11).
419  This makes it different from the the POSIX "space" class. The \es characters  This makes it different from the the POSIX "space" class. The \es characters
420  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
421  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
422  does.)  does.
423  .P  .P
424  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
425  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
426  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
427  place (see  place (see
428  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 268  in the Line 433  in the
433  .\" HREF  .\" HREF
434  \fBpcreapi\fP  \fBpcreapi\fP
435  .\"  .\"
436  page). For example, in the "fr_FR" (French) locale, some character codes  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
437  greater than 128 are used for accented letters, and these are matched by \ew.  or "french" in Windows, some character codes greater than 128 are used for
438  .P  accented letters, and these are then matched by \ew. The use of locales with
439  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Unicode is discouraged.
440  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .P
441  character property support is available. The use of locales with Unicode is  By default, in UTF-8 mode, characters with values greater than 128 never match
442  discouraged.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
443    their original meanings from before UTF-8 support was available, mainly for
444    efficiency reasons. However, if PCRE is compiled with Unicode property support,
445    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
446    properties are used to determine character types, as follows:
447    .sp
448      \ed  any character that \ep{Nd} matches (decimal digit)
449      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
450      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
451    .sp
452    The upper case escapes match the inverse sets of characters. Note that \ed
453    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
454    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
455    \eB because they are defined in terms of \ew and \eW. Matching these sequences
456    is noticeably slower when PCRE_UCP is set.
457    .P
458    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
459    release 5.10. In contrast to the other sequences, which match only ASCII
460    characters by default, these always match certain high-valued codepoints in
461    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
462    are:
463    .sp
464      U+0009     Horizontal tab
465      U+0020     Space
466      U+00A0     Non-break space
467      U+1680     Ogham space mark
468      U+180E     Mongolian vowel separator
469      U+2000     En quad
470      U+2001     Em quad
471      U+2002     En space
472      U+2003     Em space
473      U+2004     Three-per-em space
474      U+2005     Four-per-em space
475      U+2006     Six-per-em space
476      U+2007     Figure space
477      U+2008     Punctuation space
478      U+2009     Thin space
479      U+200A     Hair space
480      U+202F     Narrow no-break space
481      U+205F     Medium mathematical space
482      U+3000     Ideographic space
483    .sp
484    The vertical space characters are:
485    .sp
486      U+000A     Linefeed
487      U+000B     Vertical tab
488      U+000C     Formfeed
489      U+000D     Carriage return
490      U+0085     Next line
491      U+2028     Line separator
492      U+2029     Paragraph separator
493    .
494    .
495    .\" HTML <a name="newlineseq"></a>
496    .SS "Newline sequences"
497    .rs
498    .sp
499    Outside a character class, by default, the escape sequence \eR matches any
500    Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
501    .sp
502      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
503    .sp
504    This is an example of an "atomic group", details of which are given
505    .\" HTML <a href="#atomicgroup">
506    .\" </a>
507    below.
508    .\"
509    This particular group matches either the two-character sequence CR followed by
510    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
511    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
512    line, U+0085). The two-character sequence is treated as a single unit that
513    cannot be split.
514    .P
515    In UTF-8 mode, two additional characters whose codepoints are greater than 255
516    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
517    Unicode character property support is not needed for these characters to be
518    recognized.
519    .P
520    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
521    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
522    either at compile time or when the pattern is matched. (BSR is an abbrevation
523    for "backslash R".) This can be made the default when PCRE is built; if this is
524    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
525    It is also possible to specify these settings by starting a pattern string with
526    one of the following sequences:
527    .sp
528      (*BSR_ANYCRLF)   CR, LF, or CRLF only
529      (*BSR_UNICODE)   any Unicode newline sequence
530    .sp
531    These override the default and the options given to \fBpcre_compile()\fP or
532    \fBpcre_compile2()\fP, but they can be overridden by options given to
533    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
534    which are not Perl-compatible, are recognized only at the very start of a
535    pattern, and that they must be in upper case. If more than one of them is
536    present, the last one is used. They can be combined with a change of newline
537    convention; for example, a pattern can start with:
538    .sp
539      (*ANY)(*BSR_ANYCRLF)
540    .sp
541    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
542    a character class, \eR is treated as an unrecognized escape sequence, and so
543    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
544  .  .
545  .  .
546  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 282  discouraged. Line 548  discouraged.
548  .rs  .rs
549  .sp  .sp
550  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
551  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
552  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
553    characters whose codepoints are less than 256, but they do work in this mode.
554    The extra escape sequences are:
555  .sp  .sp
556    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
557    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
558    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
559  .sp  .sp
560  The property names represented by \fIxx\fP above are limited to the Unicode  The property names represented by \fIxx\fP above are limited to the Unicode
561  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
562  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
563  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
564  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
565    .\" </a>
566    next section).
567    .\"
568    Other Perl properties such as "InMusicalSymbols" are not currently supported by
569    PCRE. Note that \eP{Any} does not match any characters, so always causes a
570    match failure.
571  .P  .P
572  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
573  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 307  Those that are not part of an identified Line 581  Those that are not part of an identified
581  .P  .P
582  Arabic,  Arabic,
583  Armenian,  Armenian,
584    Avestan,
585    Balinese,
586    Bamum,
587  Bengali,  Bengali,
588  Bopomofo,  Bopomofo,
589  Braille,  Braille,
590  Buginese,  Buginese,
591  Buhid,  Buhid,
592  Canadian_Aboriginal,  Canadian_Aboriginal,
593    Carian,
594    Cham,
595  Cherokee,  Cherokee,
596  Common,  Common,
597  Coptic,  Coptic,
598    Cuneiform,
599  Cypriot,  Cypriot,
600  Cyrillic,  Cyrillic,
601  Deseret,  Deseret,
602  Devanagari,  Devanagari,
603    Egyptian_Hieroglyphs,
604  Ethiopic,  Ethiopic,
605  Georgian,  Georgian,
606  Glagolitic,  Glagolitic,
# Line 332  Hangul, Line 613  Hangul,
613  Hanunoo,  Hanunoo,
614  Hebrew,  Hebrew,
615  Hiragana,  Hiragana,
616    Imperial_Aramaic,
617  Inherited,  Inherited,
618    Inscriptional_Pahlavi,
619    Inscriptional_Parthian,
620    Javanese,
621    Kaithi,
622  Kannada,  Kannada,
623  Katakana,  Katakana,
624    Kayah_Li,
625  Kharoshthi,  Kharoshthi,
626  Khmer,  Khmer,
627  Lao,  Lao,
628  Latin,  Latin,
629    Lepcha,
630  Limbu,  Limbu,
631  Linear_B,  Linear_B,
632    Lisu,
633    Lycian,
634    Lydian,
635  Malayalam,  Malayalam,
636    Meetei_Mayek,
637  Mongolian,  Mongolian,
638  Myanmar,  Myanmar,
639  New_Tai_Lue,  New_Tai_Lue,
640    Nko,
641  Ogham,  Ogham,
642  Old_Italic,  Old_Italic,
643  Old_Persian,  Old_Persian,
644    Old_South_Arabian,
645    Old_Turkic,
646    Ol_Chiki,
647  Oriya,  Oriya,
648  Osmanya,  Osmanya,
649    Phags_Pa,
650    Phoenician,
651    Rejang,
652  Runic,  Runic,
653    Samaritan,
654    Saurashtra,
655  Shavian,  Shavian,
656  Sinhala,  Sinhala,
657    Sundanese,
658  Syloti_Nagri,  Syloti_Nagri,
659  Syriac,  Syriac,
660  Tagalog,  Tagalog,
661  Tagbanwa,  Tagbanwa,
662  Tai_Le,  Tai_Le,
663    Tai_Tham,
664    Tai_Viet,
665  Tamil,  Tamil,
666  Telugu,  Telugu,
667  Thaana,  Thaana,
# Line 365  Thai, Line 669  Thai,
669  Tibetan,  Tibetan,
670  Tifinagh,  Tifinagh,
671  Ugaritic,  Ugaritic,
672    Vai,
673  Yi.  Yi.
674  .P  .P
675  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
676  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
677  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
678  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
679  .P  .P
680  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
681  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 430  The special property L& is also supporte Line 735  The special property L& is also supporte
735  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
736  a modifier or "other".  a modifier or "other".
737  .P  .P
738  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
739    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
740    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
741    (see the discussion of PCRE_NO_UTF8_CHECK in the
742    .\" HREF
743    \fBpcreapi\fP
744    .\"
745    page). Perl does not support the Cs property.
746    .P
747    The long synonyms for property names that Perl supports (such as \ep{Letter})
748  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
749  properties with "Is".  properties with "Is".
750  .P  .P
# Line 454  atomic group Line 768  atomic group
768  (see below).  (see below).
769  .\"  .\"
770  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
771  preceding character.  preceding character. None of them have codepoints less than 256, so in
772    non-UTF-8 mode \eX matches any one character.
773    .P
774    Note that recent versions of Perl have changed \eX to match what Unicode calls
775    an "extended grapheme cluster", which has a more complicated definition.
776  .P  .P
777  Matching characters by Unicode property is not fast, because PCRE has to search  Matching characters by Unicode property is not fast, because PCRE has to search
778  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
779  why the traditional escape sequences such as \ed and \ew do not use Unicode  why the traditional escape sequences such as \ed and \ew do not use Unicode
780  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
781    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
782    (*UCP).
783    .
784    .
785    .\" HTML <a name="extraprops"></a>
786    .SS PCRE's additional properties
787    .rs
788    .sp
789    As well as the standard Unicode properties described in the previous
790    section, PCRE supports four more that make it possible to convert traditional
791    escape sequences such as \ew and \es and POSIX character classes to use Unicode
792    properties. PCRE uses these non-standard, non-Perl properties internally when
793    PCRE_UCP is set. They are:
794    .sp
795      Xan   Any alphanumeric character
796      Xps   Any POSIX space character
797      Xsp   Any Perl space character
798      Xwd   Any Perl "word" character
799    .sp
800    Xan matches characters that have either the L (letter) or the N (number)
801    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
802    carriage return, and any other character that has the Z (separator) property.
803    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
804    same characters as Xan, plus underscore.
805    .
806    .
807    .\" HTML <a name="resetmatchstart"></a>
808    .SS "Resetting the match start"
809    .rs
810    .sp
811    The escape sequence \eK causes any previously matched characters not to be
812    included in the final matched sequence. For example, the pattern:
813    .sp
814      foo\eKbar
815    .sp
816    matches "foobar", but reports that it has matched "bar". This feature is
817    similar to a lookbehind assertion
818    .\" HTML <a href="#lookbehind">
819    .\" </a>
820    (described below).
821    .\"
822    However, in this case, the part of the subject before the real match does not
823    have to be of fixed length, as lookbehind assertions do. The use of \eK does
824    not interfere with the setting of
825    .\" HTML <a href="#subpattern">
826    .\" </a>
827    captured substrings.
828    .\"
829    For example, when the pattern
830    .sp
831      (foo)\eKbar
832    .sp
833    matches "foobar", the first substring is still set to "foo".
834    .P
835    Perl documents that the use of \eK within assertions is "not well defined". In
836    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
837    ignored in negative assertions.
838  .  .
839  .  .
840  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
841  .SS "Simple assertions"  .SS "Simple assertions"
842  .rs  .rs
843  .sp  .sp
844  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
845  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,
846  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
847  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 478  The backslashed assertions are: Line 853  The backslashed assertions are:
853  .sp  .sp
854    \eb     matches at a word boundary    \eb     matches at a word boundary
855    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
856    \eA     matches at start of subject    \eA     matches at the start of the subject
857    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
858    \ez     matches at end of subject            also matches before a newline at the end of the subject
859    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
860  .sp    \eG     matches at the first matching position in the subject
861  These assertions may not appear in character classes (but note that \eb has a  .sp
862  different meaning, namely the backspace character, inside a character class).  Inside a character class, \eb has a different meaning; it matches the backspace
863    character. If any other of these assertions appears in a character class, by
864    default it matches the corresponding literal character (for example, \eB
865    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
866    escape sequence" error is generated instead.
867  .P  .P
868  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
869  and the previous character do not both match \ew or \eW (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
870  \ew and the other matches \eW), 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
871  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
872    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
873    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
874    of word" or "end of word" metasequence. However, whatever follows \eb normally
875    determines which it is. For example, the fragment \eba matches "a" at the start
876    of a word.
877  .P  .P
878  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
879  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 573  end of the subject in both modes, and if Line 957  end of the subject in both modes, and if
957  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
958  .  .
959  .  .
960  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
961    .SH "FULL STOP (PERIOD, DOT) AND \eN"
962  .rs  .rs
963  .sp  .sp
964  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
965  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
966  line. In UTF-8 mode, the matched character may be more than one byte long. When  line. In UTF-8 mode, the matched character may be more than one byte long.
967  a line ending is defined as a single character (CR or LF), dot never matches  .P
968  that character; when the two-character sequence CRLF is used, dot does not  When a line ending is defined as a single character, dot never matches that
969  match CR if it is immediately followed by LF, but otherwise it matches all  character; when the two-character sequence CRLF is used, dot does not match CR
970  characters (including isolated CRs and LFs).  if it is immediately followed by LF, but otherwise it matches all characters
971    (including isolated CRs and LFs). When any Unicode line endings are being
972    recognized, dot does not match CR or LF or any of the other line ending
973    characters.
974  .P  .P
975  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
976  option is set, a dot matches any one character, without exception. If newline  option is set, a dot matches any one character, without exception. If the
977  is defined as the two-character sequence CRLF, it takes two dots to match it.  two-character sequence CRLF is present in the subject string, it takes two dots
978    to match it.
979  .P  .P
980  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
981  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
982  special meaning in a character class.  special meaning in a character class.
983    .P
984    The escape sequence \eN behaves like a dot, except that it is not affected by
985    the PCRE_DOTALL option. In other words, it matches any character except one
986    that signifies the end of a line. Perl also uses \eN to match characters by
987    name; PCRE does not support this.
988  .  .
989  .  .
990  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
991  .rs  .rs
992  .sp  .sp
993  Outside a character class, the escape sequence \eC matches any one byte, both  Outside a character class, the escape sequence \eC matches any one byte, both
994  in and out of UTF-8 mode. Unlike a dot, it always matches CR and LF. The  in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
995  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  characters. The feature is provided in Perl in order to match individual bytes
996  Because it breaks up UTF-8 characters into individual bytes, what remains in  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
997  the string may be a malformed UTF-8 string. For this reason, the \eC escape  breaks up characters into individual bytes, matching one byte with \eC in UTF-8
998  sequence is best avoided.  mode means that the rest of the string may start with a malformed UTF-8
999    character. This has undefined results, because PCRE assumes that it is dealing
1000    with valid UTF-8 strings (and by default it checks this at the start of
1001    processing unless the PCRE_NO_UTF8_CHECK option is used).
1002  .P  .P
1003  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1004  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
# Line 610  PCRE does not allow \eC to appear in loo Line 1007  PCRE does not allow \eC to appear in loo
1007  .\"  .\"
1008  because in UTF-8 mode this would make it impossible to calculate the length of  because in UTF-8 mode this would make it impossible to calculate the length of
1009  the lookbehind.  the lookbehind.
1010    .P
1011    In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
1012    way of using it that avoids the problem of malformed UTF-8 characters is to
1013    use a lookahead to check the length of the next character, as in this pattern
1014    (ignore white space and line breaks):
1015    .sp
1016      (?| (?=[\ex00-\ex7f])(\eC) |
1017          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1018          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1019          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1020    .sp
1021    A group that starts with (?| resets the capturing parentheses numbers in each
1022    alternative (see
1023    .\" HTML <a href="#dupsubpatternnumber">
1024    .\" </a>
1025    "Duplicate Subpattern Numbers"
1026    .\"
1027    below). The assertions at the start of each branch check the next UTF-8
1028    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1029    character's individual bytes are then captured by the appropriate number of
1030    groups.
1031  .  .
1032  .  .
1033  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 617  the lookbehind. Line 1035  the lookbehind.
1035  .rs  .rs
1036  .sp  .sp
1037  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1038  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
1039  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1040  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
1041  escaped with a backslash.  a member of the class, it should be the first data character in the class
1042    (after an initial circumflex, if present) or escaped with a backslash.
1043  .P  .P
1044  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
1045  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
1046  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
1047  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
1048  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
1049  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
# Line 634  For example, the character class [aeiou] Line 1053  For example, the character class [aeiou]
1053  [^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
1054  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1055  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1056  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1057  string, and therefore it fails if the current pointer is at the end of the  string, and therefore it fails if the current pointer is at the end of the
1058  string.  string.
1059  .P  .P
# Line 648  caseful version would. In UTF-8 mode, PC Line 1067  caseful version would. In UTF-8 mode, PC
1067  case for characters whose values are less than 128, so caseless matching is  case for characters whose values are less than 128, so caseless matching is
1068  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1069  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1070  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in UTF8-mode for characters 128 and above,
1071  ensure that PCRE is compiled with Unicode property support as well as with  you must ensure that PCRE is compiled with Unicode property support as well as
1072  UTF-8 support.  with UTF-8 support.
1073  .P  .P
1074  Characters that might indicate line breaks (CR and LF) are never treated in any  Characters that might indicate line breaks are never treated in any special way
1075  special way when matching character classes, whatever line-ending sequence is  when matching character classes, whatever line-ending sequence is in use, and
1076  in use, and whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1077  used. A class such as [^a] always matches one of these characters.  such as [^a] always matches one of these characters.
1078  .P  .P
1079  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
1080  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,
# Line 679  example [\ex{100}-\ex{2ff}]. Line 1098  example [\ex{100}-\ex{2ff}].
1098  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
1099  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
1100  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1101  tables for the "fr_FR" locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1102  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1103  characters with values greater than 128 only when it is compiled with Unicode  characters with values greater than 128 only when it is compiled with Unicode
1104  property support.  property support.
1105  .P  .P
1106  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1107  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1108  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1109  conveniently be used with the upper case character types to specify a more  digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1110  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1111  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1112    .\" HTML <a href="#genericchartypes">
1113    .\" </a>
1114    "Generic character types"
1115    .\"
1116    above. The escape sequence \eb has a different meaning inside a character
1117    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1118    are not special inside a character class. Like any other unrecognized escape
1119    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1120    default, but cause an error if the PCRE_EXTRA option is set.
1121    .P
1122    A circumflex can conveniently be used with the upper case character types to
1123    specify a more restricted set of characters than the matching lower case type.
1124    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1125    whereas [\ew] includes underscore. A positive character class should be read as
1126    "something OR something OR ..." and a negative class as "NOT something AND NOT
1127    something AND NOT ...".
1128  .P  .P
1129  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1130  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 709  this notation. For example, Line 1144  this notation. For example,
1144    [01[:alpha:]%]    [01[:alpha:]%]
1145  .sp  .sp
1146  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1147  are  are:
1148  .sp  .sp
1149    alnum    letters and digits    alnum    letters and digits
1150    alpha    letters    alpha    letters
# Line 720  are Line 1155  are
1155    graph    printing characters, excluding space    graph    printing characters, excluding space
1156    lower    lower case letters    lower    lower case letters
1157    print    printing characters, including space    print    printing characters, including space
1158    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1159    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1160    upper    upper case letters    upper    upper case letters
1161    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 741  matches "1", "2", or any non-digit. PCRE Line 1176  matches "1", "2", or any non-digit. PCRE
1176  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
1177  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1178  .P  .P
1179  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1180  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1181    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1182    character properties are used. This is achieved by replacing the POSIX classes
1183    by other sequences, as follows:
1184    .sp
1185      [:alnum:]  becomes  \ep{Xan}
1186      [:alpha:]  becomes  \ep{L}
1187      [:blank:]  becomes  \eh
1188      [:digit:]  becomes  \ep{Nd}
1189      [:lower:]  becomes  \ep{Ll}
1190      [:space:]  becomes  \ep{Xps}
1191      [:upper:]  becomes  \ep{Lu}
1192      [:word:]   becomes  \ep{Xwd}
1193    .sp
1194    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1195    classes are unchanged, and match only characters with code points less than
1196    128.
1197  .  .
1198  .  .
1199  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 769  alternative in the subpattern. Line 1220  alternative in the subpattern.
1220  .rs  .rs
1221  .sp  .sp
1222  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1223  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1224  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1225    The option letters are
1226  .sp  .sp
1227    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1228    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 784  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1236  PCRE_MULTILINE while unsetting PCRE_DOTA
1236  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
1237  unset.  unset.
1238  .P  .P
1239  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1240  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1241  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
 the global options (and it will therefore show up in data extracted by the  
 \fBpcre_fullinfo()\fP function).  
1242  .P  .P
1243  An option change within a subpattern affects only that part of the current  When one of these option changes occurs at top level (that is, not inside
1244  pattern that follows it, so  subpattern parentheses), the change applies to the remainder of the pattern
1245    that follows. If the change is placed right at the start of a pattern, PCRE
1246    extracts it into the global options (and it will therefore show up in data
1247    extracted by the \fBpcre_fullinfo()\fP function).
1248    .P
1249    An option change within a subpattern (see below for a description of
1250    subpatterns) affects only that part of the subpattern that follows it, so
1251  .sp  .sp
1252    (a(?i)b)c    (a(?i)b)c
1253  .sp  .sp
# Line 807  branch is abandoned before the option se Line 1263  branch is abandoned before the option se
1263  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1264  behaviour otherwise.  behaviour otherwise.
1265  .P  .P
1266  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1267  changed in the same way as the Perl-compatible options by using the characters  application when the compile or match functions are called. In some cases the
1268  J, U and X respectively.  pattern can contain special leading sequences such as (*CRLF) to override what
1269    the application has set or what has been defaulted. Details are given in the
1270    section entitled
1271    .\" HTML <a href="#newlineseq">
1272    .\" </a>
1273    "Newline sequences"
1274    .\"
1275    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1276    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1277    PCRE_UTF8 and the PCRE_UCP options, respectively.
1278  .  .
1279  .  .
1280  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 823  Turning part of a pattern into a subpatt Line 1288  Turning part of a pattern into a subpatt
1288  .sp  .sp
1289    cat(aract|erpillar|)    cat(aract|erpillar|)
1290  .sp  .sp
1291  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1292  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1293  .sp  .sp
1294  2. It sets up the subpattern as a capturing subpattern. This means that, when  2. It sets up the subpattern as a capturing subpattern. This means that, when
1295  the whole pattern matches, that portion of the subject string that matched the  the whole pattern matches, that portion of the subject string that matched the
1296  subpattern is passed back to the caller via the \fIovector\fP argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1297  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1298  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1299  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1300  .sp  .sp
1301    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1302  .sp  .sp
# Line 849  the string "the white queen" is matched Line 1313  the string "the white queen" is matched
1313    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1314  .sp  .sp
1315  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
1316  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
 of nesting of all subpatterns, both capturing and non-capturing, is 200.  
1317  .P  .P
1318  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
1319  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
# Line 865  is reached, an option setting in one bra Line 1328  is reached, an option setting in one bra
1328  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1329  .  .
1330  .  .
1331    .\" HTML <a name="dupsubpatternnumber"></a>
1332    .SH "DUPLICATE SUBPATTERN NUMBERS"
1333    .rs
1334    .sp
1335    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1336    the same numbers for its capturing parentheses. Such a subpattern starts with
1337    (?| and is itself a non-capturing subpattern. For example, consider this
1338    pattern:
1339    .sp
1340      (?|(Sat)ur|(Sun))day
1341    .sp
1342    Because the two alternatives are inside a (?| group, both sets of capturing
1343    parentheses are numbered one. Thus, when the pattern matches, you can look
1344    at captured substring number one, whichever alternative matched. This construct
1345    is useful when you want to capture part, but not all, of one of a number of
1346    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1347    number is reset at the start of each branch. The numbers of any capturing
1348    parentheses that follow the subpattern start after the highest number used in
1349    any branch. The following example is taken from the Perl documentation. The
1350    numbers underneath show in which buffer the captured content will be stored.
1351    .sp
1352      # before  ---------------branch-reset----------- after
1353      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1354      # 1            2         2  3        2     3     4
1355    .sp
1356    A back reference to a numbered subpattern uses the most recent value that is
1357    set for that number by any subpattern. The following pattern matches "abcabc"
1358    or "defdef":
1359    .sp
1360      /(?|(abc)|(def))\e1/
1361    .sp
1362    In contrast, a subroutine call to a numbered subpattern always refers to the
1363    first one in the pattern with the given number. The following pattern matches
1364    "abcabc" or "defabc":
1365    .sp
1366      /(?|(abc)|(def))(?1)/
1367    .sp
1368    If a
1369    .\" HTML <a href="#conditions">
1370    .\" </a>
1371    condition test
1372    .\"
1373    for a subpattern's having matched refers to a non-unique number, the test is
1374    true if any of the subpatterns of that number have matched.
1375    .P
1376    An alternative approach to using this "branch reset" feature is to use
1377    duplicate named subpatterns, as described in the next section.
1378    .
1379    .
1380  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1381  .rs  .rs
1382  .sp  .sp
1383  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
1384  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1385  if an expression is modified, the numbers may change. To help with this  if an expression is modified, the numbers may change. To help with this
1386  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1387  not provide. The Python syntax (?P<name>...) is used. References to capturing  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1388    introduced it at release 4.0, using the Python syntax. PCRE now supports both
1389    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1390    have different names, but PCRE does not.
1391    .P
1392    In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1393    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1394  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1395  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1396  .\" </a>  .\" </a>
1397  backreferences,  back references,
1398  .\"  .\"
1399  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1400  .\" </a>  .\" </a>
# Line 890  conditions, Line 1408  conditions,
1408  can be made by name as well as by number.  can be made by name as well as by number.
1409  .P  .P
1410  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores. Named
1411  capturing parentheses are still allocated numbers as well as names. The PCRE  capturing parentheses are still allocated numbers as well as names, exactly as
1412  API provides function calls for extracting the name-to-number translation table  if the names were not present. The PCRE API provides function calls for
1413  from a compiled pattern. There is also a convenience function for extracting a  extracting the name-to-number translation table from a compiled pattern. There
1414  captured substring by name.  is also a convenience function for extracting a captured substring by name.
1415  .P  .P
1416  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1417  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1418  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1419  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1420  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1421  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1422  .sp  name, and in both cases you want to extract the abbreviation. This pattern
1423    (?P<DN>Mon|Fri|Sun)(?:day)?|  (ignoring the line breaks) does the job:
1424    (?P<DN>Tue)(?:sday)?|  .sp
1425    (?P<DN>Wed)(?:nesday)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1426    (?P<DN>Thu)(?:rsday)?|    (?<DN>Tue)(?:sday)?|
1427    (?P<DN>Sat)(?:urday)?    (?<DN>Wed)(?:nesday)?|
1428      (?<DN>Thu)(?:rsday)?|
1429      (?<DN>Sat)(?:urday)?
1430  .sp  .sp
1431  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1432    (An alternative way of solving this problem is to use a "branch reset"
1433    subpattern, as described in the previous section.)
1434    .P
1435  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1436  for the first, and in this example, the only, subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1437  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1438  make a reference to a non-unique named subpattern from elsewhere in the  .P
1439  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1440  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1441    used. In the absence of duplicate numbers (see the previous section) this is
1442    the one with the lowest number. If you use a named reference in a condition
1443    test (see the
1444    .\"
1445    .\" HTML <a href="#conditions">
1446    .\" </a>
1447    section about conditions
1448    .\"
1449    below), either to check whether a subpattern has matched, or to check for
1450    recursion, all subpatterns with the same name are tested. If the condition is
1451    true for any one of them, the overall condition is true. This is the same
1452    behaviour as testing by number. For further details of the interfaces for
1453    handling named subpatterns, see the
1454  .\" HREF  .\" HREF
1455  \fBpcreapi\fP  \fBpcreapi\fP
1456  .\"  .\"
1457  documentation.  documentation.
1458    .P
1459    \fBWarning:\fP You cannot use different names to distinguish between two
1460    subpatterns with the same number because PCRE uses only the numbers when
1461    matching. For this reason, an error is given at compile time if different names
1462    are given to subpatterns with the same number. However, you can give the same
1463    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1464  .  .
1465  .  .
1466  .SH REPETITION  .SH REPETITION
# Line 928  Repetition is specified by quantifiers, Line 1470  Repetition is specified by quantifiers,
1470  items:  items:
1471  .sp  .sp
1472    a literal data character    a literal data character
1473    the . metacharacter    the dot metacharacter
1474    the \eC escape sequence    the \eC escape sequence
1475    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1476    an escape such as \ed that matches a single character    the \eR escape sequence
1477      an escape such as \ed or \epL that matches a single character
1478    a character class    a character class
1479    a back reference (see next section)    a back reference (see next section)
1480    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1481      a subroutine call to a subpattern (recursive or otherwise)
1482  .sp  .sp
1483  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1484  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 966  support is available, \eX{3} matches thr Line 1510  support is available, \eX{3} matches thr
1510  which may be several bytes long (and they may be of different lengths).  which may be several bytes long (and they may be of different lengths).
1511  .P  .P
1512  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
1513  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1514    subpatterns that are referenced as
1515    .\" HTML <a href="#subpatternsassubroutines">
1516    .\" </a>
1517    subroutines
1518    .\"
1519    from elsewhere in the pattern (but see also the section entitled
1520    .\" HTML <a href="#subdefine">
1521    .\" </a>
1522    "Defining subpatterns for use by reference only"
1523    .\"
1524    below). Items other than subpatterns that have a {0} quantifier are omitted
1525    from the compiled pattern.
1526  .P  .P
1527  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1528  quantifiers have single-character abbreviations:  abbreviations:
1529  .sp  .sp
1530    *    is equivalent to {0,}    *    is equivalent to {0,}
1531    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 1017  own right. Because it has two uses, it c Line 1573  own right. Because it has two uses, it c
1573  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
1574  way the rest of the pattern matches.  way the rest of the pattern matches.
1575  .P  .P
1576  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),
1577  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
1578  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
1579  default behaviour.  default behaviour.
# Line 1027  is greater than 1 or with a limited maxi Line 1583  is greater than 1 or with a limited maxi
1583  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1584  .P  .P
1585  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
1586  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
1587  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1588  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
1589  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
# Line 1038  worth setting PCRE_DOTALL in order to ob Line 1594  worth setting PCRE_DOTALL in order to ob
1594  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1595  .P  .P
1596  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1597  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1598  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
1599  succeed. Consider, for example:  succeeds. Consider, for example:
1600  .sp  .sp
1601    (.*)abc\e1    (.*)abc\e1
1602  .sp  .sp
# Line 1066  matches "aba" the value of the second ca Line 1622  matches "aba" the value of the second ca
1622  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1623  .rs  .rs
1624  .sp  .sp
1625  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1626  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
1627  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
1628  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
1629  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
1630  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1631  .P  .P
1632  Consider, for example, the pattern \ed+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1633  .sp  .sp
# Line 1083  item, and then with 4, and so on, before Line 1639  item, and then with 4, and so on, before
1639  (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
1640  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.
1641  .P  .P
1642  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
1643  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
1644  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1645  .sp  .sp
# Line 1113  previous example can be rewritten as Line 1669  previous example can be rewritten as
1669  .sp  .sp
1670    \ed++foo    \ed++foo
1671  .sp  .sp
1672    Note that a possessive quantifier can be used with an entire group, for
1673    example:
1674    .sp
1675      (abc|xyz){2,3}+
1676    .sp
1677  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1678  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
1679  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
1680  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1681  .P  difference; possessive quantifiers should be slightly faster.
1682  The possessive quantifier syntax is an extension to the Perl syntax. Jeffrey  .P
1683  Friedl originated the idea (and the name) in the first edition of his book.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1684  Mike McCloskey liked it, so implemented it when he built Sun's Java package,  Jeffrey Friedl originated the idea (and the name) in the first edition of his
1685  and PCRE copied it from there.  book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1686    package, and PCRE copied it from there. It ultimately found its way into Perl
1687    at release 5.10.
1688    .P
1689    PCRE has an optimization that automatically "possessifies" certain simple
1690    pattern constructs. For example, the sequence A+B is treated as A++B because
1691    there is no point in backtracking into a sequence of A's when B must follow.
1692  .P  .P
1693  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
1694  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
# Line 1167  numbers less than 10. A "forward back re Line 1734  numbers less than 10. A "forward back re
1734  when a repetition is involved and the subpattern to the right has participated  when a repetition is involved and the subpattern to the right has participated
1735  in an earlier iteration.  in an earlier iteration.
1736  .P  .P
1737  It is not possible to have a numerical "forward back reference" to subpattern  It is not possible to have a numerical "forward back reference" to a subpattern
1738  whose number is 10 or more. However, a back reference to any subpattern is  whose number is 10 or more using this syntax because a sequence such as \e50 is
1739  possible using named parentheses (see below). See also the subsection entitled  interpreted as a character defined in octal. See the subsection entitled
1740  "Non-printing characters"  "Non-printing characters"
1741  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1742  .\" </a>  .\" </a>
1743  above  above
1744  .\"  .\"
1745  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1746    no such problem when named parentheses are used. A back reference to any
1747    subpattern is possible using named parentheses (see below).
1748    .P
1749    Another way of avoiding the ambiguity inherent in the use of digits following a
1750    backslash is to use the \eg escape sequence. This escape must be followed by an
1751    unsigned number or a negative number, optionally enclosed in braces. These
1752    examples are all identical:
1753    .sp
1754      (ring), \e1
1755      (ring), \eg1
1756      (ring), \eg{1}
1757    .sp
1758    An unsigned number specifies an absolute reference without the ambiguity that
1759    is present in the older syntax. It is also useful when literal digits follow
1760    the reference. A negative number is a relative reference. Consider this
1761    example:
1762    .sp
1763      (abc(def)ghi)\eg{-1}
1764    .sp
1765    The sequence \eg{-1} is a reference to the most recently started capturing
1766    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1767    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1768    can be helpful in long patterns, and also in patterns that are created by
1769    joining together fragments that contain references within themselves.
1770  .P  .P
1771  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1772  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1197  back reference, the case of letters is r Line 1788  back reference, the case of letters is r
1788  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
1789  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1790  .P  .P
1791  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1792  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1793    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1794    back reference syntax, in which \eg can be used for both numeric and named
1795    references, is also supported. We could rewrite the above example in any of
1796    the following ways:
1797  .sp  .sp
1798      (?<p1>(?i)rah)\es+\ek<p1>
1799      (?'p1'(?i)rah)\es+\ek{p1}
1800    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1801      (?<p1>(?i)rah)\es+\eg{p1}
1802  .sp  .sp
1803  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1804  after the reference.  after the reference.
1805  .P  .P
1806  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
1807  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1808  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1809  .sp  .sp
1810    (a|(bc))\e2    (a|(bc))\e2
1811  .sp  .sp
1812  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". However, if the
1813  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1814  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1815  with a digit character, some delimiter must be used to terminate the back  .P
1816  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1817  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1818    If the pattern continues with a digit character, some delimiter must be used to
1819    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1820    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1821  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1822  .\" </a>  .\" </a>
1823  "Comments"  "Comments"
1824  .\"  .\"
1825  below) can be used.  below) can be used.
1826  .P  .
1827    .SS "Recursive back references"
1828    .rs
1829    .sp
1830  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
1831  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1832  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1236  to the previous iteration. In order for Line 1840  to the previous iteration. In order for
1840  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
1841  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
1842  minimum of zero.  minimum of zero.
1843    .P
1844    Back references of this type cause the group that they reference to be treated
1845    as an
1846    .\" HTML <a href="#atomicgroup">
1847    .\" </a>
1848    atomic group.
1849    .\"
1850    Once the whole group has been matched, a subsequent matching failure cannot
1851    cause backtracking into the middle of the group.
1852  .  .
1853  .  .
1854  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1255  those that look ahead of the current pos Line 1868  those that look ahead of the current pos
1868  that look behind it. An assertion subpattern is matched in the normal way,  that look behind it. An assertion subpattern is matched in the normal way,
1869  except that it does not cause the current matching position to be changed.  except that it does not cause the current matching position to be changed.
1870  .P  .P
1871  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1872  because it makes no sense to assert the same thing several times. If any kind  contains capturing subpatterns within it, these are counted for the purposes of
1873  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1874  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1875  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1876  because it does not make sense for negative assertions.  .P
1877    For compatibility with Perl, assertion subpatterns may be repeated; though
1878    it makes no sense to assert the same thing several times, the side effect of
1879    capturing parentheses may occasionally be useful. In practice, there only three
1880    cases:
1881    .sp
1882    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1883    However, it may contain internal capturing parenthesized groups that are called
1884    from elsewhere via the
1885    .\" HTML <a href="#subpatternsassubroutines">
1886    .\" </a>
1887    subroutine mechanism.
1888    .\"
1889    .sp
1890    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1891    were {0,1}. At run time, the rest of the pattern match is tried with and
1892    without the assertion, the order depending on the greediness of the quantifier.
1893    .sp
1894    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1895    The assertion is obeyed just once when encountered during matching.
1896  .  .
1897  .  .
1898  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1289  lookbehind assertion is needed to achiev Line 1921  lookbehind assertion is needed to achiev
1921  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
1922  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
1923  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.
1924    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1925  .  .
1926  .  .
1927  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1313  is permitted, but Line 1946  is permitted, but
1946  .sp  .sp
1947  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1948  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
1949  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
1950  match the same length of string. An assertion such as  length of string. An assertion such as
1951  .sp  .sp
1952    (?<=ab(c|de))    (?<=ab(c|de))
1953  .sp  .sp
1954  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
1955  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
1956    branches:
1957  .sp  .sp
1958    (?<=abc|abde)    (?<=abc|abde)
1959  .sp  .sp
1960    In some cases, the escape sequence \eK
1961    .\" HTML <a href="#resetmatchstart">
1962    .\" </a>
1963    (see above)
1964    .\"
1965    can be used instead of a lookbehind assertion to get round the fixed-length
1966    restriction.
1967    .P
1968  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1969  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
1970  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1971  match is deemed to fail.  assertion fails.
1972  .P  .P
1973  PCRE does not allow the \eC 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)
1974  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1975  the length of the lookbehind. The \eX escape, which can match different numbers  the length of the lookbehind. The \eX and \eR escapes, which can match
1976  of bytes, is also not permitted.  different numbers of bytes, are also not permitted.
1977    .P
1978    .\" HTML <a href="#subpatternsassubroutines">
1979    .\" </a>
1980    "Subroutine"
1981    .\"
1982    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1983    as the subpattern matches a fixed-length string.
1984    .\" HTML <a href="#recursion">
1985    .\" </a>
1986    Recursion,
1987    .\"
1988    however, is not supported.
1989  .P  .P
1990  Atomic groups can be used in conjunction with lookbehind assertions to specify  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1991  efficient matching at the end of the subject string. Consider a simple pattern  specify efficient matching of fixed-length strings at the end of subject
1992  such as  strings. Consider a simple pattern such as
1993  .sp  .sp
1994    abcd$    abcd$
1995  .sp  .sp
# Line 1351  then all but the last two characters, an Line 2005  then all but the last two characters, an
2005  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,
2006  if the pattern is written as  if the pattern is written as
2007  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
2008    ^.*+(?<=abcd)    ^.*+(?<=abcd)
2009  .sp  .sp
2010  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
2011  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
2012  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
2013  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1403  characters that are not "999". Line 2053  characters that are not "999".
2053  .sp  .sp
2054  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2055  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2056  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2057  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2058  .sp  .sp
2059    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2060    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2061  .sp  .sp
2062  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2063  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
2064  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2065    itself contain nested subpatterns of any form, including conditional
2066    subpatterns; the restriction to two alternatives applies only at the level of
2067    the condition. This pattern fragment is an example where the alternatives are
2068    complex:
2069    .sp
2070      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2071    .sp
2072  .P  .P
2073  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
2074  consists of a sequence of digits, or a sequence of alphanumeric characters and  recursion, a pseudo-condition called DEFINE, and assertions.
2075  underscores, the condition is satisfied if the capturing subpattern of that  .
2076  number or name has previously matched. There is a possible ambiguity here,  .SS "Checking for a used subpattern by number"
2077  because subpattern names may consist entirely of digits. PCRE looks first for a  .rs
2078  named subpattern; if it cannot find one and the text consists entirely of  .sp
2079  digits, it looks for a subpattern of that number, which must be greater than  If the text between the parentheses consists of a sequence of digits, the
2080  zero. Using subpattern names that consist entirely of digits is not  condition is true if a capturing subpattern of that number has previously
2081  recommended.  matched. If there is more than one capturing subpattern with the same number
2082    (see the earlier
2083    .\"
2084    .\" HTML <a href="#recursion">
2085    .\" </a>
2086    section about duplicate subpattern numbers),
2087    .\"
2088    the condition is true if any of them have matched. An alternative notation is
2089    to precede the digits with a plus or minus sign. In this case, the subpattern
2090    number is relative rather than absolute. The most recently opened parentheses
2091    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2092    loops it can also make sense to refer to subsequent groups. The next
2093    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2094    zero in any of these forms is not used; it provokes a compile-time error.)
2095  .P  .P
2096  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2097  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1432  three parts for ease of discussion: Line 2102  three parts for ease of discussion:
2102  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2103  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
2104  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
2105  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2106  or not. If they did, that is, if subject started with an opening parenthesis,  matched. If they did, that is, if subject started with an opening parenthesis,
2107  the condition is true, and so the yes-pattern is executed and a closing  the condition is true, and so the yes-pattern is executed and a closing
2108  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2109  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2110  non-parentheses, optionally enclosed in parentheses. Rewriting it to use a  non-parentheses, optionally enclosed in parentheses.
2111  named subpattern gives this:  .P
2112    If you were embedding this pattern in a larger one, you could use a relative
2113    reference:
2114    .sp
2115      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2116    .sp
2117    This makes the fragment independent of the parentheses in the larger pattern.
2118    .
2119    .SS "Checking for a used subpattern by name"
2120    .rs
2121    .sp
2122    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2123    subpattern by name. For compatibility with earlier versions of PCRE, which had
2124    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2125    there is a possible ambiguity with this syntax, because subpattern names may
2126    consist entirely of digits. PCRE looks first for a named subpattern; if it
2127    cannot find one and the name consists entirely of digits, PCRE looks for a
2128    subpattern of that number, which must be greater than zero. Using subpattern
2129    names that consist entirely of digits is not recommended.
2130    .P
2131    Rewriting the above example to use a named subpattern gives this:
2132  .sp  .sp
2133    (?P<OPEN> \e( )?    [^()]+    (?(OPEN) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2134    .sp
2135    If the name used in a condition of this kind is a duplicate, the test is
2136    applied to all subpatterns of the same name, and is true if any one of them has
2137    matched.
2138    .
2139    .SS "Checking for pattern recursion"
2140    .rs
2141  .sp  .sp
2142  If the condition is the string (R), and there is no subpattern with the name R,  If the condition is the string (R), and there is no subpattern with the name R,
2143  the condition is satisfied if a recursive call to the pattern or subpattern has  the condition is true if a recursive call to the whole pattern or any
2144  been made. At "top level", the condition is false. This is a PCRE extension.  subpattern has been made. If digits or a name preceded by ampersand follow the
2145  Recursive patterns are described in the next section.  letter R, for example:
2146    .sp
2147      (?(R3)...) or (?(R&name)...)
2148    .sp
2149    the condition is true if the most recent recursion is into a subpattern whose
2150    number or name is given. This condition does not check the entire recursion
2151    stack. If the name used in a condition of this kind is a duplicate, the test is
2152    applied to all subpatterns of the same name, and is true if any one of them is
2153    the most recent recursion.
2154  .P  .P
2155  If the condition is not a sequence of digits or (R), it must be an assertion.  At "top level", all these recursion test conditions are false.
2156    .\" HTML <a href="#recursion">
2157    .\" </a>
2158    The syntax for recursive patterns
2159    .\"
2160    is described below.
2161    .
2162    .\" HTML <a name="subdefine"></a>
2163    .SS "Defining subpatterns for use by reference only"
2164    .rs
2165    .sp
2166    If the condition is the string (DEFINE), and there is no subpattern with the
2167    name DEFINE, the condition is always false. In this case, there may be only one
2168    alternative in the subpattern. It is always skipped if control reaches this
2169    point in the pattern; the idea of DEFINE is that it can be used to define
2170    subroutines that can be referenced from elsewhere. (The use of
2171    .\" HTML <a href="#subpatternsassubroutines">
2172    .\" </a>
2173    subroutines
2174    .\"
2175    is described below.) For example, a pattern to match an IPv4 address such as
2176    "192.168.23.245" could be written like this (ignore whitespace and line
2177    breaks):
2178    .sp
2179      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2180      \eb (?&byte) (\e.(?&byte)){3} \eb
2181    .sp
2182    The first part of the pattern is a DEFINE group inside which a another group
2183    named "byte" is defined. This matches an individual component of an IPv4
2184    address (a number less than 256). When matching takes place, this part of the
2185    pattern is skipped because DEFINE acts like a false condition. The rest of the
2186    pattern uses references to the named group to match the four dot-separated
2187    components of an IPv4 address, insisting on a word boundary at each end.
2188    .
2189    .SS "Assertion conditions"
2190    .rs
2191    .sp
2192    If the condition is not in any of the above formats, it must be an assertion.
2193  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2194  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2195  alternatives on the second line:  alternatives on the second line:
# Line 1467  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2209  dd-aaa-dd or dd-dd-dd, where aaa are let
2209  .SH COMMENTS  .SH COMMENTS
2210  .rs  .rs
2211  .sp  .sp
2212  The sequence (?# marks the start of a comment that continues up to the next  There are two ways of including comments in patterns that are processed by
2213  closing parenthesis. Nested parentheses are not permitted. The characters  PCRE. In both cases, the start of the comment must not be in a character class,
2214  that make up a comment play no part in the pattern matching at all.  nor in the middle of any other sequence of related characters such as (?: or a
2215    subpattern name or number. The characters that make up a comment play no part
2216    in the pattern matching.
2217  .P  .P
2218  If the PCRE_EXTENDED option is set, an unescaped # character outside a  The sequence (?# marks the start of a comment that continues up to the next
2219  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2220  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2221    this case continues to immediately after the next newline character or
2222    character sequence in the pattern. Which characters are interpreted as newlines
2223    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2224    sequence at the start of the pattern, as described in the section entitled
2225    .\" HTML <a href="#newlines">
2226    .\" </a>
2227    "Newline conventions"
2228    .\"
2229    above. Note that the end of this type of comment is a literal newline sequence
2230    in the pattern; escape sequences that happen to represent a newline do not
2231    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2232    default newline convention is in force:
2233    .sp
2234      abc #comment \en still comment
2235    .sp
2236    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2237    a newline in the pattern. The sequence \en is still literal at this stage, so
2238    it does not terminate the comment. Only an actual character with the code value
2239    0x0a (the default newline) does so.
2240  .  .
2241  .  .
2242  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1483  next newline in the pattern. Line 2246  next newline in the pattern.
2246  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2247  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2248  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
2249  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2250  that allows regular expressions to recurse (amongst other things). It does this  .P
2251  by interpolating Perl code in the expression at run time, and the code can  For some time, Perl has provided a facility that allows regular expressions to
2252  refer to the expression itself. A Perl pattern to solve the parentheses problem  recurse (amongst other things). It does this by interpolating Perl code in the
2253  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2254    pattern using code interpolation to solve the parentheses problem can be
2255    created like this:
2256  .sp  .sp
2257    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2258  .sp  .sp
2259  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
2260  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2261  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2262  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2263  .P  supports special syntax for recursion of the entire pattern, and also for
2264  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2265  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2266  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2267  "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
2268  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive subroutine call of the subpattern of the
2269  .P  given number, provided that it occurs inside that subpattern. (If not, it is a
2270  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
2271  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
2272  it contains untried alternatives and there is a subsequent matching failure.  non-recursive subroutine
2273    .\"
2274    call, which is described in the next section.) The special item (?R) or (?0) is
2275    a recursive call of the entire regular expression.
2276  .P  .P
2277  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2278  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2279  .sp  .sp
2280    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2281  .sp  .sp
2282  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2283  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
2284  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2285  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2286    to avoid backtracking into sequences of non-parentheses.
2287  .P  .P
2288  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
2289  pattern, so instead you could use this:  pattern, so instead you could use this:
2290  .sp  .sp
2291    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2292  .sp  .sp
2293  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
2294  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2295  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2296  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
2297  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. Instead of (?1) in the
2298  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2299  .sp  parentheses preceding the recursion. In other words, a negative number counts
2300    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  capturing parentheses leftwards from the point at which it is encountered.
2301  .sp  .P
2302  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2303  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2304  when applying the pattern to strings that do not match. For example, when this  reference is not inside the parentheses that are referenced. They are always
2305  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2306    .\" </a>
2307    non-recursive subroutine
2308    .\"
2309    calls, as described in the next section.
2310    .P
2311    An alternative approach is to use named parentheses instead. The Perl syntax
2312    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2313    could rewrite the above example as follows:
2314    .sp
2315      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2316    .sp
2317    If there is more than one subpattern with the same name, the earliest one is
2318    used.
2319    .P
2320    This particular example pattern that we have been looking at contains nested
2321    unlimited repeats, and so the use of a possessive quantifier for matching
2322    strings of non-parentheses is important when applying the pattern to strings
2323    that do not match. For example, when this pattern is applied to
2324  .sp  .sp
2325    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2326  .sp  .sp
2327  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2328  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
2329  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
2330  before failure can be reported.  before failure can be reported.
2331  .P  .P
2332  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2333  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2334  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 the next section and the  
2335  .\" HREF  .\" HREF
2336  \fBpcrecallout\fP  \fBpcrecallout\fP
2337  .\"  .\"
# Line 1553  documentation). If the pattern above is Line 2339  documentation). If the pattern above is
2339  .sp  .sp
2340    (ab(cd)ef)    (ab(cd)ef)
2341  .sp  .sp
2342  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2343  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2344  .sp  matched at the top level, its final captured value is unset, even if it was
2345    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2346       ^                        ^  .P
2347       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2348  .sp  obtain extra memory to store data during a recursion, which it does by using
2349  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2350  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2351  .P  .P
2352  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.
2353  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1578  different alternatives for the recursive Line 2361  different alternatives for the recursive
2361  is the actual recursive call.  is the actual recursive call.
2362  .  .
2363  .  .
2364    .\" HTML <a name="recursiondifference"></a>
2365    .SS "Differences in recursion processing between PCRE and Perl"
2366    .rs
2367    .sp
2368    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2369    (like Python, but unlike Perl), a recursive subpattern call is always treated
2370    as an atomic group. That is, once it has matched some of the subject string, it
2371    is never re-entered, even if it contains untried alternatives and there is a
2372    subsequent matching failure. This can be illustrated by the following pattern,
2373    which purports to match a palindromic string that contains an odd number of
2374    characters (for example, "a", "aba", "abcba", "abcdcba"):
2375    .sp
2376      ^(.|(.)(?1)\e2)$
2377    .sp
2378    The idea is that it either matches a single character, or two identical
2379    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2380    it does not if the pattern is longer than three characters. Consider the
2381    subject string "abcba":
2382    .P
2383    At the top level, the first character is matched, but as it is not at the end
2384    of the string, the first alternative fails; the second alternative is taken
2385    and the recursion kicks in. The recursive call to subpattern 1 successfully
2386    matches the next character ("b"). (Note that the beginning and end of line
2387    tests are not part of the recursion).
2388    .P
2389    Back at the top level, the next character ("c") is compared with what
2390    subpattern 2 matched, which was "a". This fails. Because the recursion is
2391    treated as an atomic group, there are now no backtracking points, and so the
2392    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2393    try the second alternative.) However, if the pattern is written with the
2394    alternatives in the other order, things are different:
2395    .sp
2396      ^((.)(?1)\e2|.)$
2397    .sp
2398    This time, the recursing alternative is tried first, and continues to recurse
2399    until it runs out of characters, at which point the recursion fails. But this
2400    time we do have another alternative to try at the higher level. That is the big
2401    difference: in the previous case the remaining alternative is at a deeper
2402    recursion level, which PCRE cannot use.
2403    .P
2404    To change the pattern so that it matches all palindromic strings, not just
2405    those with an odd number of characters, it is tempting to change the pattern to
2406    this:
2407    .sp
2408      ^((.)(?1)\e2|.?)$
2409    .sp
2410    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2411    deeper recursion has matched a single character, it cannot be entered again in
2412    order to match an empty string. The solution is to separate the two cases, and
2413    write out the odd and even cases as alternatives at the higher level:
2414    .sp
2415      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2416    .sp
2417    If you want to match typical palindromic phrases, the pattern has to ignore all
2418    non-word characters, which can be done like this:
2419    .sp
2420      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2421    .sp
2422    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2423    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2424    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2425    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2426    more) to match typical phrases, and Perl takes so long that you think it has
2427    gone into a loop.
2428    .P
2429    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2430    string does not start with a palindrome that is shorter than the entire string.
2431    For example, although "abcba" is correctly matched, if the subject is "ababa",
2432    PCRE finds the palindrome "aba" at the start, then fails at top level because
2433    the end of the string does not follow. Once again, it cannot jump back into the
2434    recursion to try other alternatives, so the entire match fails.
2435    .P
2436    The second way in which PCRE and Perl differ in their recursion processing is
2437    in the handling of captured values. In Perl, when a subpattern is called
2438    recursively or as a subpattern (see the next section), it has no access to any
2439    values that were captured outside the recursion, whereas in PCRE these values
2440    can be referenced. Consider this pattern:
2441    .sp
2442      ^(.)(\e1|a(?2))
2443    .sp
2444    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2445    then in the second group, when the back reference \e1 fails to match "b", the
2446    second alternative matches "a" and then recurses. In the recursion, \e1 does
2447    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2448    match because inside the recursive call \e1 cannot access the externally set
2449    value.
2450    .
2451    .
2452  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2453  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2454  .rs  .rs
2455  .sp  .sp
2456  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern call (either by number or by
2457  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
2458  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The called subpattern may be defined
2459  pattern  before or after the reference. A numbered reference can be absolute or
2460    relative, as in these examples:
2461    .sp
2462      (...(absolute)...)...(?2)...
2463      (...(relative)...)...(?-1)...
2464      (...(?+1)...(relative)...
2465    .sp
2466    An earlier example pointed out that the pattern
2467  .sp  .sp
2468    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2469  .sp  .sp
# Line 1595  matches "sense and sensibility" and "res Line 2473  matches "sense and sensibility" and "res
2473    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2474  .sp  .sp
2475  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
2476  strings. Such references, if given numerically, must follow the subpattern to  strings. Another example is given in the discussion of DEFINE above.
2477  which they refer. However, named references can refer to later subpatterns.  .P
2478    All subroutine calls, whether recursive or not, are always treated as atomic
2479    groups. That is, once a subroutine has matched some of the subject string, it
2480    is never re-entered, even if it contains untried alternatives and there is a
2481    subsequent matching failure. Any capturing parentheses that are set during the
2482    subroutine call revert to their previous values afterwards.
2483  .P  .P
2484  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Processing options such as case-independence are fixed when a subpattern is
2485  group. That is, once it has matched some of the subject string, it is never  defined, so if it is used as a subroutine, such options cannot be changed for
2486  re-entered, even if it contains untried alternatives and there is a subsequent  different calls. For example, consider this pattern:
2487  matching failure.  .sp
2488      (abc)(?i:(?-1))
2489    .sp
2490    It matches "abcabc". It does not match "abcABC" because the change of
2491    processing option does not affect the called subpattern.
2492    .
2493    .
2494    .\" HTML <a name="onigurumasubroutines"></a>
2495    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2496    .rs
2497    .sp
2498    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2499    a number enclosed either in angle brackets or single quotes, is an alternative
2500    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2501    are two of the examples used above, rewritten using this syntax:
2502    .sp
2503      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2504      (sens|respons)e and \eg'1'ibility
2505    .sp
2506    PCRE supports an extension to Oniguruma: if a number is preceded by a
2507    plus or a minus sign it is taken as a relative reference. For example:
2508    .sp
2509      (abc)(?i:\eg<-1>)
2510    .sp
2511    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2512    synonymous. The former is a back reference; the latter is a subroutine call.
2513  .  .
2514  .  .
2515  .SH CALLOUTS  .SH CALLOUTS
# Line 1622  function is to be called. If you want to Line 2530  function is to be called. If you want to
2530  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.
2531  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2532  .sp  .sp
2533    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2534  .sp  .sp
2535  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2536  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
# Line 1638  description of the interface to the call Line 2546  description of the interface to the call
2546  \fBpcrecallout\fP  \fBpcrecallout\fP
2547  .\"  .\"
2548  documentation.  documentation.
2549    .
2550    .
2551    .\" HTML <a name="backtrackcontrol"></a>
2552    .SH "BACKTRACKING CONTROL"
2553    .rs
2554    .sp
2555    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2556    are described in the Perl documentation as "experimental and subject to change
2557    or removal in a future version of Perl". It goes on to say: "Their usage in
2558    production code should be noted to avoid problems during upgrades." The same
2559    remarks apply to the PCRE features described in this section.
2560    .P
2561    Since these verbs are specifically related to backtracking, most of them can be
2562    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2563    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2564    failing negative assertion, they cause an error if encountered by
2565    \fBpcre_dfa_exec()\fP.
2566    .P
2567    If any of these verbs are used in an assertion or in a subpattern that is
2568    called as a subroutine (whether or not recursively), their effect is confined
2569    to that subpattern; it does not extend to the surrounding pattern, with one
2570    exception: a *MARK that is encountered in a positive assertion \fIis\fP passed
2571    back (compare capturing parentheses in assertions). Note that such subpatterns
2572    are processed as anchored at the point where they are tested. Note also that
2573    Perl's treatment of subroutines is different in some cases.
2574    .P
2575    The new verbs make use of what was previously invalid syntax: an opening
2576    parenthesis followed by an asterisk. They are generally of the form
2577    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2578    depending on whether or not an argument is present. A name is any sequence of
2579    characters that does not include a closing parenthesis. If the name is empty,
2580    that is, if the closing parenthesis immediately follows the colon, the effect
2581    is as if the colon were not there. Any number of these verbs may occur in a
2582    pattern.
2583    .P
2584    PCRE contains some optimizations that are used to speed up matching by running
2585    some checks at the start of each match attempt. For example, it may know the
2586    minimum length of matching subject, or that a particular character must be
2587    present. When one of these optimizations suppresses the running of a match, any
2588    included backtracking verbs will not, of course, be processed. You can suppress
2589    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2590    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2591    pattern with (*NO_START_OPT).
2592    .
2593    .
2594    .SS "Verbs that act immediately"
2595    .rs
2596    .sp
2597    The following verbs act as soon as they are encountered. They may not be
2598    followed by a name.
2599    .sp
2600       (*ACCEPT)
2601    .sp
2602    This verb causes the match to end successfully, skipping the remainder of the
2603    pattern. However, when it is inside a subpattern that is called as a
2604    subroutine, only that subpattern is ended successfully. Matching then continues
2605    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2606    far is captured. For example:
2607    .sp
2608      A((?:A|B(*ACCEPT)|C)D)
2609    .sp
2610    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2611    the outer parentheses.
2612    .sp
2613      (*FAIL) or (*F)
2614    .sp
2615    This verb causes a matching failure, forcing backtracking to occur. It is
2616    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2617    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2618    Perl features that are not present in PCRE. The nearest equivalent is the
2619    callout feature, as for example in this pattern:
2620    .sp
2621      a+(?C)(*FAIL)
2622    .sp
2623    A match with the string "aaaa" always fails, but the callout is taken before
2624    each backtrack happens (in this example, 10 times).
2625    .
2626    .
2627    .SS "Recording which path was taken"
2628    .rs
2629    .sp
2630    There is one verb whose main purpose is to track how a match was arrived at,
2631    though it also has a secondary use in conjunction with advancing the match
2632    starting point (see (*SKIP) below).
2633    .sp
2634      (*MARK:NAME) or (*:NAME)
2635    .sp
2636    A name is always required with this verb. There may be as many instances of
2637    (*MARK) as you like in a pattern, and their names do not have to be unique.
2638  .P  .P
2639  .in 0  When a match succeeds, the name of the last-encountered (*MARK) is passed back
2640  Last updated: 06 June 2006  to the caller via the \fIpcre_extra\fP data structure, as described in the
2641  .br  .\" HTML <a href="pcreapi.html#extradata">
2642  Copyright (c) 1997-2006 University of Cambridge.  .\" </a>
2643    section on \fIpcre_extra\fP
2644    .\"
2645    in the
2646    .\" HREF
2647    \fBpcreapi\fP
2648    .\"
2649    documentation. No data is returned for a partial match. Here is an example of
2650    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2651    outputting of (*MARK) data:
2652    .sp
2653      /X(*MARK:A)Y|X(*MARK:B)Z/K
2654      XY
2655       0: XY
2656      MK: A
2657      XZ
2658       0: XZ
2659      MK: B
2660    .sp
2661    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2662    indicates which of the two alternatives matched. This is a more efficient way
2663    of obtaining this information than putting each alternative in its own
2664    capturing parentheses.
2665    .P
2666    If (*MARK) is encountered in a positive assertion, its name is recorded and
2667    passed back if it is the last-encountered. This does not happen for negative
2668    assertions.
2669    .P
2670    A name may also be returned after a failed match if the final path through the
2671    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2672    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2673    starting point for matching is advanced, the final check is often with an empty
2674    string, causing a failure before (*MARK) is reached. For example:
2675    .sp
2676      /X(*MARK:A)Y|X(*MARK:B)Z/K
2677      XP
2678      No match
2679    .sp
2680    There are three potential starting points for this match (starting with X,
2681    starting with P, and with an empty string). If the pattern is anchored, the
2682    result is different:
2683    .sp
2684      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2685      XP
2686      No match, mark = B
2687    .sp
2688    PCRE's start-of-match optimizations can also interfere with this. For example,
2689    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2690    subject length for a match, a shorter subject will not be scanned at all.
2691    .P
2692    Note that similar anomalies (though different in detail) exist in Perl, no
2693    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2694    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2695    .
2696    .
2697    .SS "Verbs that act after backtracking"
2698    .rs
2699    .sp
2700    The following verbs do nothing when they are encountered. Matching continues
2701    with what follows, but if there is no subsequent match, causing a backtrack to
2702    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2703    the verb. However, when one of these verbs appears inside an atomic group, its
2704    effect is confined to that group, because once the group has been matched,
2705    there is never any backtracking into it. In this situation, backtracking can
2706    "jump back" to the left of the entire atomic group. (Remember also, as stated
2707    above, that this localization also applies in subroutine calls and assertions.)
2708    .P
2709    These verbs differ in exactly what kind of failure occurs when backtracking
2710    reaches them.
2711    .sp
2712      (*COMMIT)
2713    .sp
2714    This verb, which may not be followed by a name, causes the whole match to fail
2715    outright if the rest of the pattern does not match. Even if the pattern is
2716    unanchored, no further attempts to find a match by advancing the starting point
2717    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2718    finding a match at the current starting point, or not at all. For example:
2719    .sp
2720      a+(*COMMIT)b
2721    .sp
2722    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2723    dynamic anchor, or "I've started, so I must finish." The name of the most
2724    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2725    match failure.
2726    .P
2727    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2728    unless PCRE's start-of-match optimizations are turned off, as shown in this
2729    \fBpcretest\fP example:
2730    .sp
2731      /(*COMMIT)abc/
2732      xyzabc
2733       0: abc
2734      xyzabc\eY
2735      No match
2736    .sp
2737    PCRE knows that any match must start with "a", so the optimization skips along
2738    the subject to "a" before running the first match attempt, which succeeds. When
2739    the optimization is disabled by the \eY escape in the second subject, the match
2740    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2741    starting points.
2742    .sp
2743      (*PRUNE) or (*PRUNE:NAME)
2744    .sp
2745    This verb causes the match to fail at the current starting position in the
2746    subject if the rest of the pattern does not match. If the pattern is
2747    unanchored, the normal "bumpalong" advance to the next starting character then
2748    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2749    reached, or when matching to the right of (*PRUNE), but if there is no match to
2750    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2751    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2752    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2753    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2754    match fails completely; the name is passed back if this is the final attempt.
2755    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2756    pattern (*PRUNE) has the same effect as (*COMMIT).
2757    .sp
2758      (*SKIP)
2759    .sp
2760    This verb, when given without a name, is like (*PRUNE), except that if the
2761    pattern is unanchored, the "bumpalong" advance is not to the next character,
2762    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2763    signifies that whatever text was matched leading up to it cannot be part of a
2764    successful match. Consider:
2765    .sp
2766      a+(*SKIP)b
2767    .sp
2768    If the subject is "aaaac...", after the first match attempt fails (starting at
2769    the first character in the string), the starting point skips on to start the
2770    next attempt at "c". Note that a possessive quantifer does not have the same
2771    effect as this example; although it would suppress backtracking during the
2772    first match attempt, the second attempt would start at the second character
2773    instead of skipping on to "c".
2774    .sp
2775      (*SKIP:NAME)
2776    .sp
2777    When (*SKIP) has an associated name, its behaviour is modified. If the
2778    following pattern fails to match, the previous path through the pattern is
2779    searched for the most recent (*MARK) that has the same name. If one is found,
2780    the "bumpalong" advance is to the subject position that corresponds to that
2781    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2782    matching name is found, normal "bumpalong" of one character happens (that is,
2783    the (*SKIP) is ignored).
2784    .sp
2785      (*THEN) or (*THEN:NAME)
2786    .sp
2787    This verb causes a skip to the next innermost alternative if the rest of the
2788    pattern does not match. That is, it cancels pending backtracking, but only
2789    within the current alternative. Its name comes from the observation that it can
2790    be used for a pattern-based if-then-else block:
2791    .sp
2792      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2793    .sp
2794    If the COND1 pattern matches, FOO is tried (and possibly further items after
2795    the end of the group if FOO succeeds); on failure, the matcher skips to the
2796    second alternative and tries COND2, without backtracking into COND1. The
2797    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2798    overall match fails. If (*THEN) is not inside an alternation, it acts like
2799    (*PRUNE).
2800    .P
2801    Note that a subpattern that does not contain a | character is just a part of
2802    the enclosing alternative; it is not a nested alternation with only one
2803    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2804    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2805    pattern fragments that do not contain any | characters at this level:
2806    .sp
2807      A (B(*THEN)C) | D
2808    .sp
2809    If A and B are matched, but there is a failure in C, matching does not
2810    backtrack into A; instead it moves to the next alternative, that is, D.
2811    However, if the subpattern containing (*THEN) is given an alternative, it
2812    behaves differently:
2813    .sp
2814      A (B(*THEN)C | (*FAIL)) | D
2815    .sp
2816    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2817    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2818    because there are no more alternatives to try. In this case, matching does now
2819    backtrack into A.
2820    .P
2821    Note also that a conditional subpattern is not considered as having two
2822    alternatives, because only one is ever used. In other words, the | character in
2823    a conditional subpattern has a different meaning. Ignoring white space,
2824    consider:
2825    .sp
2826      ^.*? (?(?=a) a | b(*THEN)c )
2827    .sp
2828    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2829    it initially matches zero characters. The condition (?=a) then fails, the
2830    character "b" is matched, but "c" is not. At this point, matching does not
2831    backtrack to .*? as might perhaps be expected from the presence of the |
2832    character. The conditional subpattern is part of the single alternative that
2833    comprises the whole pattern, and so the match fails. (If there was a backtrack
2834    into .*?, allowing it to match "b", the match would succeed.)
2835    .P
2836    The verbs just described provide four different "strengths" of control when
2837    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2838    next alternative. (*PRUNE) comes next, failing the match at the current
2839    starting position, but allowing an advance to the next character (for an
2840    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2841    than one character. (*COMMIT) is the strongest, causing the entire match to
2842    fail.
2843    .P
2844    If more than one such verb is present in a pattern, the "strongest" one wins.
2845    For example, consider this pattern, where A, B, etc. are complex pattern
2846    fragments:
2847    .sp
2848      (A(*COMMIT)B(*THEN)C|D)
2849    .sp
2850    Once A has matched, PCRE is committed to this match, at the current starting
2851    position. If subsequently B matches, but C does not, the normal (*THEN) action
2852    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2853    overrides.
2854    .
2855    .
2856    .SH "SEE ALSO"
2857    .rs
2858    .sp
2859    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2860    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2861    .
2862    .
2863    .SH AUTHOR
2864    .rs
2865    .sp
2866    .nf
2867    Philip Hazel
2868    University Computing Service
2869    Cambridge CB2 3QH, England.
2870    .fi
2871    .
2872    .
2873    .SH REVISION
2874    .rs
2875    .sp
2876    .nf
2877    Last updated: 14 November 2011
2878    Copyright (c) 1997-2011 University of Cambridge.
2879    .fi

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