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revision 87 by nigel, Sat Feb 24 21:41:21 2007 UTC revision 758 by ph10, Mon Nov 21 12:05:36 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 may  character that is not a number or a letter, it takes away any special meaning
186  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 character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
202  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
 pattern.  
203  .P  .P
204  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
205  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 126  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 135  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  .P  \eu00dc in JavaScript mode).
278  After \e0 up to two further octal digits are read. In both cases, if there  .P
279  are fewer than two digits, just those that are present are used. Thus the  After \e0 up to two further octal digits are read. If there are fewer than two
280  sequence \e0\ex\e07 specifies two binary zeros followed by a BEL character  digits, just those that are present are used. Thus the sequence \e0\ex\e07
281  (code value 7). Make sure you supply two digits after the initial zero if the  specifies two binary zeros followed by a BEL character (code value 7). Make
282  pattern character that follows is itself an octal digit.  sure you supply two digits after the initial zero if the pattern character that
283    follows is itself an octal digit.
284  .P  .P
285  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated.
286  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
# Line 191  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, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
303  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
304  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
305    to \e777 are permitted. For example:
306  .sp  .sp
307    \e040   is another way of writing a space    \e040   is another way of writing a space
308  .\" JOIN  .\" JOIN
# Line 218  For example: Line 327  For example:
327  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
328  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
329  .P  .P
330  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
331  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, \eb is
332  addition, inside a character class, the sequence \eb is interpreted as the  interpreted as the backspace character (hex 08).
333  backspace character (hex 08), and the sequence \eX is interpreted as the  .P
334  character "X". Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
335  meanings  inside a character class. Like other unrecognized escape sequences, they are
336  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
337    error if the PCRE_EXTRA option is set. Outside a character class, these
338    sequences have different meanings.
339    .
340    .
341    .SS "Unsupported escape sequences"
342    .rs
343    .sp
344    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
345    handler and used to modify the case of following characters. By default, PCRE
346    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
347    option is set, \eU matches a "U" character, and \eu can be used to define a
348    character by code point, as described in the previous section.
349    .
350    .
351    .SS "Absolute and relative back references"
352    .rs
353    .sp
354    The sequence \eg followed by an unsigned or a negative number, optionally
355    enclosed in braces, is an absolute or relative back reference. A named back
356    reference can be coded as \eg{name}. Back references are discussed
357    .\" HTML <a href="#backreferences">
358  .\" </a>  .\" </a>
359  (see below).  later,
360    .\"
361    following the discussion of
362    .\" HTML <a href="#subpattern">
363    .\" </a>
364    parenthesized subpatterns.
365    .\"
366    .
367    .
368    .SS "Absolute and relative subroutine calls"
369    .rs
370    .sp
371    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
372    a number enclosed either in angle brackets or single quotes, is an alternative
373    syntax for referencing a subpattern as a "subroutine". Details are discussed
374    .\" HTML <a href="#onigurumasubroutines">
375    .\" </a>
376    later.
377    .\"
378    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
379    synonymous. The former is a back reference; the latter is a
380    .\" HTML <a href="#subpatternsassubroutines">
381    .\" </a>
382    subroutine
383  .\"  .\"
384    call.
385  .  .
386  .  .
387    .\" HTML <a name="genericchartypes"></a>
388  .SS "Generic character types"  .SS "Generic character types"
389  .rs  .rs
390  .sp  .sp
391  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:  
392  .sp  .sp
393    \ed     any decimal digit    \ed     any decimal digit
394    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
395      \eh     any horizontal whitespace character
396      \eH     any character that is not a horizontal whitespace character
397    \es     any whitespace character    \es     any whitespace character
398    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
399      \ev     any vertical whitespace character
400      \eV     any character that is not a vertical whitespace character
401    \ew     any "word" character    \ew     any "word" character
402    \eW     any "non-word" character    \eW     any "non-word" character
403  .sp  .sp
404  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.
405  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
406    .\" HTML <a href="#fullstopdot">
407    .\" </a>
408    the "." metacharacter
409    .\"
410    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
411    PCRE does not support this.
412  .P  .P
413  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
414    of characters into two disjoint sets. Any given character matches one, and only
415    one, of each pair. The sequences can appear both inside and outside character
416  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
417  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
418  there is no character to match.  there is no character to match.
419  .P  .P
420  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).
421  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
422  are HT (9), LF (10), FF (12), CR (13), and space (32).  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
423    included in a Perl script, \es may match the VT character. In PCRE, it never
424    does.
425  .P  .P
426  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.
427  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
428  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
429  place (see  place (see
430  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 267  in the Line 435  in the
435  .\" HREF  .\" HREF
436  \fBpcreapi\fP  \fBpcreapi\fP
437  .\"  .\"
438  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,
439  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
440  .P  accented letters, and these are then matched by \ew. The use of locales with
441  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  Unicode is discouraged.
442  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  .P
443  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
444  discouraged.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
445    their original meanings from before UTF-8 support was available, mainly for
446    efficiency reasons. However, if PCRE is compiled with Unicode property support,
447    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
448    properties are used to determine character types, as follows:
449    .sp
450      \ed  any character that \ep{Nd} matches (decimal digit)
451      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
452      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
453    .sp
454    The upper case escapes match the inverse sets of characters. Note that \ed
455    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
456    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
457    \eB because they are defined in terms of \ew and \eW. Matching these sequences
458    is noticeably slower when PCRE_UCP is set.
459    .P
460    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
461    release 5.10. In contrast to the other sequences, which match only ASCII
462    characters by default, these always match certain high-valued codepoints in
463    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
464    are:
465    .sp
466      U+0009     Horizontal tab
467      U+0020     Space
468      U+00A0     Non-break space
469      U+1680     Ogham space mark
470      U+180E     Mongolian vowel separator
471      U+2000     En quad
472      U+2001     Em quad
473      U+2002     En space
474      U+2003     Em space
475      U+2004     Three-per-em space
476      U+2005     Four-per-em space
477      U+2006     Six-per-em space
478      U+2007     Figure space
479      U+2008     Punctuation space
480      U+2009     Thin space
481      U+200A     Hair space
482      U+202F     Narrow no-break space
483      U+205F     Medium mathematical space
484      U+3000     Ideographic space
485    .sp
486    The vertical space characters are:
487    .sp
488      U+000A     Linefeed
489      U+000B     Vertical tab
490      U+000C     Formfeed
491      U+000D     Carriage return
492      U+0085     Next line
493      U+2028     Line separator
494      U+2029     Paragraph separator
495    .
496    .
497    .\" HTML <a name="newlineseq"></a>
498    .SS "Newline sequences"
499    .rs
500    .sp
501    Outside a character class, by default, the escape sequence \eR matches any
502    Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
503    .sp
504      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
505    .sp
506    This is an example of an "atomic group", details of which are given
507    .\" HTML <a href="#atomicgroup">
508    .\" </a>
509    below.
510    .\"
511    This particular group matches either the two-character sequence CR followed by
512    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
513    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
514    line, U+0085). The two-character sequence is treated as a single unit that
515    cannot be split.
516    .P
517    In UTF-8 mode, two additional characters whose codepoints are greater than 255
518    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
519    Unicode character property support is not needed for these characters to be
520    recognized.
521    .P
522    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
523    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
524    either at compile time or when the pattern is matched. (BSR is an abbrevation
525    for "backslash R".) This can be made the default when PCRE is built; if this is
526    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
527    It is also possible to specify these settings by starting a pattern string with
528    one of the following sequences:
529    .sp
530      (*BSR_ANYCRLF)   CR, LF, or CRLF only
531      (*BSR_UNICODE)   any Unicode newline sequence
532    .sp
533    These override the default and the options given to \fBpcre_compile()\fP or
534    \fBpcre_compile2()\fP, but they can be overridden by options given to
535    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
536    which are not Perl-compatible, are recognized only at the very start of a
537    pattern, and that they must be in upper case. If more than one of them is
538    present, the last one is used. They can be combined with a change of newline
539    convention; for example, a pattern can start with:
540    .sp
541      (*ANY)(*BSR_ANYCRLF)
542    .sp
543    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
544    a character class, \eR is treated as an unrecognized escape sequence, and so
545    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
546  .  .
547  .  .
548  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 281  discouraged. Line 550  discouraged.
550  .rs  .rs
551  .sp  .sp
552  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
553  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
554  is selected. They are:  When not in UTF-8 mode, these sequences are of course limited to testing
555    characters whose codepoints are less than 256, but they do work in this mode.
556    The extra escape sequences are:
557  .sp  .sp
558    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
559    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
560    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
561  .sp  .sp
562  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
563  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
564  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
565  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
566  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
567    .\" </a>
568    next section).
569    .\"
570    Other Perl properties such as "InMusicalSymbols" are not currently supported by
571    PCRE. Note that \eP{Any} does not match any characters, so always causes a
572    match failure.
573  .P  .P
574  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
575  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 306  Those that are not part of an identified Line 583  Those that are not part of an identified
583  .P  .P
584  Arabic,  Arabic,
585  Armenian,  Armenian,
586    Avestan,
587    Balinese,
588    Bamum,
589  Bengali,  Bengali,
590  Bopomofo,  Bopomofo,
591  Braille,  Braille,
592  Buginese,  Buginese,
593  Buhid,  Buhid,
594  Canadian_Aboriginal,  Canadian_Aboriginal,
595    Carian,
596    Cham,
597  Cherokee,  Cherokee,
598  Common,  Common,
599  Coptic,  Coptic,
600    Cuneiform,
601  Cypriot,  Cypriot,
602  Cyrillic,  Cyrillic,
603  Deseret,  Deseret,
604  Devanagari,  Devanagari,
605    Egyptian_Hieroglyphs,
606  Ethiopic,  Ethiopic,
607  Georgian,  Georgian,
608  Glagolitic,  Glagolitic,
# Line 331  Hangul, Line 615  Hangul,
615  Hanunoo,  Hanunoo,
616  Hebrew,  Hebrew,
617  Hiragana,  Hiragana,
618    Imperial_Aramaic,
619  Inherited,  Inherited,
620    Inscriptional_Pahlavi,
621    Inscriptional_Parthian,
622    Javanese,
623    Kaithi,
624  Kannada,  Kannada,
625  Katakana,  Katakana,
626    Kayah_Li,
627  Kharoshthi,  Kharoshthi,
628  Khmer,  Khmer,
629  Lao,  Lao,
630  Latin,  Latin,
631    Lepcha,
632  Limbu,  Limbu,
633  Linear_B,  Linear_B,
634    Lisu,
635    Lycian,
636    Lydian,
637  Malayalam,  Malayalam,
638    Meetei_Mayek,
639  Mongolian,  Mongolian,
640  Myanmar,  Myanmar,
641  New_Tai_Lue,  New_Tai_Lue,
642    Nko,
643  Ogham,  Ogham,
644  Old_Italic,  Old_Italic,
645  Old_Persian,  Old_Persian,
646    Old_South_Arabian,
647    Old_Turkic,
648    Ol_Chiki,
649  Oriya,  Oriya,
650  Osmanya,  Osmanya,
651    Phags_Pa,
652    Phoenician,
653    Rejang,
654  Runic,  Runic,
655    Samaritan,
656    Saurashtra,
657  Shavian,  Shavian,
658  Sinhala,  Sinhala,
659    Sundanese,
660  Syloti_Nagri,  Syloti_Nagri,
661  Syriac,  Syriac,
662  Tagalog,  Tagalog,
663  Tagbanwa,  Tagbanwa,
664  Tai_Le,  Tai_Le,
665    Tai_Tham,
666    Tai_Viet,
667  Tamil,  Tamil,
668  Telugu,  Telugu,
669  Thaana,  Thaana,
# Line 364  Thai, Line 671  Thai,
671  Tibetan,  Tibetan,
672  Tifinagh,  Tifinagh,
673  Ugaritic,  Ugaritic,
674    Vai,
675  Yi.  Yi.
676  .P  .P
677  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
678  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
679  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
680  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
681  .P  .P
682  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
683  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 429  The special property L& is also supporte Line 737  The special property L& is also supporte
737  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
738  a modifier or "other".  a modifier or "other".
739  .P  .P
740  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
741  are not supported by PCRE. Nor is is permitted to prefix any of these  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
742    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
743    (see the discussion of PCRE_NO_UTF8_CHECK in the
744    .\" HREF
745    \fBpcreapi\fP
746    .\"
747    page). Perl does not support the Cs property.
748    .P
749    The long synonyms for property names that Perl supports (such as \ep{Letter})
750    are not supported by PCRE, nor is it permitted to prefix any of these
751  properties with "Is".  properties with "Is".
752  .P  .P
753  No character that is in the Unicode table has the Cn (unassigned) property.  No character that is in the Unicode table has the Cn (unassigned) property.
# Line 453  atomic group Line 770  atomic group
770  (see below).  (see below).
771  .\"  .\"
772  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
773  preceding character.  preceding character. None of them have codepoints less than 256, so in
774    non-UTF-8 mode \eX matches any one character.
775    .P
776    Note that recent versions of Perl have changed \eX to match what Unicode calls
777    an "extended grapheme cluster", which has a more complicated definition.
778  .P  .P
779  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
780  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
781  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
782  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
783    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
784    (*UCP).
785    .
786    .
787    .\" HTML <a name="extraprops"></a>
788    .SS PCRE's additional properties
789    .rs
790    .sp
791    As well as the standard Unicode properties described in the previous
792    section, PCRE supports four more that make it possible to convert traditional
793    escape sequences such as \ew and \es and POSIX character classes to use Unicode
794    properties. PCRE uses these non-standard, non-Perl properties internally when
795    PCRE_UCP is set. They are:
796    .sp
797      Xan   Any alphanumeric character
798      Xps   Any POSIX space character
799      Xsp   Any Perl space character
800      Xwd   Any Perl "word" character
801    .sp
802    Xan matches characters that have either the L (letter) or the N (number)
803    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
804    carriage return, and any other character that has the Z (separator) property.
805    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
806    same characters as Xan, plus underscore.
807    .
808    .
809    .\" HTML <a name="resetmatchstart"></a>
810    .SS "Resetting the match start"
811    .rs
812    .sp
813    The escape sequence \eK causes any previously matched characters not to be
814    included in the final matched sequence. For example, the pattern:
815    .sp
816      foo\eKbar
817    .sp
818    matches "foobar", but reports that it has matched "bar". This feature is
819    similar to a lookbehind assertion
820    .\" HTML <a href="#lookbehind">
821    .\" </a>
822    (described below).
823    .\"
824    However, in this case, the part of the subject before the real match does not
825    have to be of fixed length, as lookbehind assertions do. The use of \eK does
826    not interfere with the setting of
827    .\" HTML <a href="#subpattern">
828    .\" </a>
829    captured substrings.
830    .\"
831    For example, when the pattern
832    .sp
833      (foo)\eKbar
834    .sp
835    matches "foobar", the first substring is still set to "foo".
836    .P
837    Perl documents that the use of \eK within assertions is "not well defined". In
838    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
839    ignored in negative assertions.
840  .  .
841  .  .
842  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
843  .SS "Simple assertions"  .SS "Simple assertions"
844  .rs  .rs
845  .sp  .sp
846  The fourth use of backslash is for certain simple assertions. An assertion  The final use of backslash is for certain simple assertions. An assertion
847  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,
848  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
849  subpatterns for more complicated assertions is described  subpatterns for more complicated assertions is described
# Line 473  subpatterns for more complicated asserti Line 851  subpatterns for more complicated asserti
851  .\" </a>  .\" </a>
852  below.  below.
853  .\"  .\"
854  The backslashed  The backslashed assertions are:
 assertions are:  
855  .sp  .sp
856    \eb     matches at a word boundary    \eb     matches at a word boundary
857    \eB     matches when not at a word boundary    \eB     matches when not at a word boundary
858    \eA     matches at start of subject    \eA     matches at the start of the subject
859    \eZ     matches at end of subject or before newline at end    \eZ     matches at the end of the subject
860    \ez     matches at end of subject            also matches before a newline at the end of the subject
861    \eG     matches at first matching position in subject    \ez     matches only at the end of the subject
862  .sp    \eG     matches at the first matching position in the subject
863  These assertions may not appear in character classes (but note that \eb has a  .sp
864  different meaning, namely the backspace character, inside a character class).  Inside a character class, \eb has a different meaning; it matches the backspace
865    character. If any other of these assertions appears in a character class, by
866    default it matches the corresponding literal character (for example, \eB
867    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
868    escape sequence" error is generated instead.
869  .P  .P
870  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
871  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
872  \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
873  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
874    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
875    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
876    of word" or "end of word" metasequence. However, whatever follows \eb normally
877    determines which it is. For example, the fragment \eba matches "a" at the start
878    of a word.
879  .P  .P
880  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
881  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 499  PCRE_NOTBOL or PCRE_NOTEOL options, whic Line 885  PCRE_NOTBOL or PCRE_NOTEOL options, whic
885  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
886  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
887  at a point other than the beginning of the subject, \eA can never match. The  at a point other than the beginning of the subject, \eA can never match. The
888  difference between \eZ and \ez is that \eZ matches before a newline that is the  difference between \eZ and \ez is that \eZ matches before a newline at the end
889  last character of the string as well as at the end of the string, whereas \ez  of the string as well as at the very end, whereas \ez matches only at the end.
 matches only at the end.  
890  .P  .P
891  The \eG assertion is true only when the current matching position is at the  The \eG assertion is true only when the current matching position is at the
892  start point of the match, as specified by the \fIstartoffset\fP argument of  start point of the match, as specified by the \fIstartoffset\fP argument of
# Line 545  to be anchored.) Line 930  to be anchored.)
930  .P  .P
931  A dollar character is an assertion that is true only if the current matching  A dollar character is an assertion that is true only if the current matching
932  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline
933  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
934  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
935  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
936  Dollar has no special meaning in a character class.  character class.
937  .P  .P
938  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
939  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
940  does not affect the \eZ assertion.  does not affect the \eZ assertion.
941  .P  .P
942  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
943  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
944  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
945  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
946  the pattern /^abc$/ matches the subject string "def\enabc" (where \en  matches before any newlines in the string, as well as at the very end, when
947  represents a newline character) in multiline mode, but not otherwise.  PCRE_MULTILINE is set. When newline is specified as the two-character
948  Consequently, patterns that are anchored in single line mode because all  sequence CRLF, isolated CR and LF characters do not indicate newlines.
949  branches start with ^ are not anchored in multiline mode, and a match for  .P
950  circumflex is possible when the \fIstartoffset\fP argument of \fBpcre_exec()\fP  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
951  is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is  \en represents a newline) in multiline mode, but not otherwise. Consequently,
952  set.  patterns that are anchored in single line mode because all branches start with
953    ^ are not anchored in multiline mode, and a match for circumflex is possible
954    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
955    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
956  .P  .P
957  Note that the sequences \eA, \eZ, and \ez can be used to match the start and  Note that the sequences \eA, \eZ, and \ez can be used to match the start and
958  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
959  \eA it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
960  .  .
961  .  .
962  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
963    .SH "FULL STOP (PERIOD, DOT) AND \eN"
964  .rs  .rs
965  .sp  .sp
966  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
967  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
968  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
969  byte long, except (by default) newline. If the PCRE_DOTALL option is set,  .P
970  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
971  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
972  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
973    (including isolated CRs and LFs). When any Unicode line endings are being
974    recognized, dot does not match CR or LF or any of the other line ending
975    characters.
976    .P
977    The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
978    option is set, a dot matches any one character, without exception. If the
979    two-character sequence CRLF is present in the subject string, it takes two dots
980    to match it.
981    .P
982    The handling of dot is entirely independent of the handling of circumflex and
983    dollar, the only relationship being that they both involve newlines. Dot has no
984    special meaning in a character class.
985    .P
986    The escape sequence \eN behaves like a dot, except that it is not affected by
987    the PCRE_DOTALL option. In other words, it matches any character except one
988    that signifies the end of a line. Perl also uses \eN to match characters by
989    name; PCRE does not support this.
990  .  .
991  .  .
992  .SH "MATCHING A SINGLE BYTE"  .SH "MATCHING A SINGLE BYTE"
993  .rs  .rs
994  .sp  .sp
995  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
996  in and out of UTF-8 mode. Unlike a dot, it can match a newline. The feature is  in and out of UTF-8 mode. Unlike a dot, it always matches line-ending
997  provided in Perl in order to match individual bytes in UTF-8 mode. Because it  characters. The feature is provided in Perl in order to match individual bytes
998  breaks up UTF-8 characters into individual bytes, what remains in the string  in UTF-8 mode, but it is unclear how it can usefully be used. Because \eC
999  may be a malformed UTF-8 string. For this reason, the \eC escape sequence is  breaks up characters into individual bytes, matching one byte with \eC in UTF-8
1000  best avoided.  mode means that the rest of the string may start with a malformed UTF-8
1001    character. This has undefined results, because PCRE assumes that it is dealing
1002    with valid UTF-8 strings (and by default it checks this at the start of
1003    processing unless the PCRE_NO_UTF8_CHECK option is used).
1004  .P  .P
1005  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1006  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1007  .\" </a>  .\" </a>
1008  (described below),  (described below)
1009  .\"  .\"
1010  because in UTF-8 mode this would make it impossible to calculate the length of  in UTF-8 mode, because this would make it impossible to calculate the length of
1011  the lookbehind.  the lookbehind.
1012    .P
1013    In general, the \eC escape sequence is best avoided in UTF-8 mode. However, one
1014    way of using it that avoids the problem of malformed UTF-8 characters is to
1015    use a lookahead to check the length of the next character, as in this pattern
1016    (ignore white space and line breaks):
1017    .sp
1018      (?| (?=[\ex00-\ex7f])(\eC) |
1019          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1020          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1021          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1022    .sp
1023    A group that starts with (?| resets the capturing parentheses numbers in each
1024    alternative (see
1025    .\" HTML <a href="#dupsubpatternnumber">
1026    .\" </a>
1027    "Duplicate Subpattern Numbers"
1028    .\"
1029    below). The assertions at the start of each branch check the next UTF-8
1030    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1031    character's individual bytes are then captured by the appropriate number of
1032    groups.
1033  .  .
1034  .  .
1035  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 607  the lookbehind. Line 1037  the lookbehind.
1037  .rs  .rs
1038  .sp  .sp
1039  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1040  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.
1041  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
1042  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
1043  escaped with a backslash.  a member of the class, it should be the first data character in the class
1044    (after an initial circumflex, if present) or escaped with a backslash.
1045  .P  .P
1046  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
1047  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
1048  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
1049  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
1050  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
1051  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 624  For example, the character class [aeiou] Line 1055  For example, the character class [aeiou]
1055  [^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
1056  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1057  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
1058  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
1059  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
1060  string.  string.
1061  .P  .P
# Line 638  caseful version would. In UTF-8 mode, PC Line 1069  caseful version would. In UTF-8 mode, PC
1069  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
1070  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1071  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1072  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,
1073  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
1074  UTF-8 support.  with UTF-8 support.
1075  .P  .P
1076  The newline character is never treated in any special way in character classes,  Characters that might indicate line breaks are never treated in any special way
1077  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  when matching character classes, whatever line-ending sequence is in use, and
1078  such as [^a] will always match a newline.  whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1079    such as [^a] always matches one of these characters.
1080  .P  .P
1081  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
1082  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 668  example [\ex{100}-\ex{2ff}]. Line 1100  example [\ex{100}-\ex{2ff}].
1100  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
1101  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
1102  [][\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
1103  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
1104  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
1105  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
1106  property support.  property support.
1107  .P  .P
1108  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,
1109  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
1110  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1111  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
1112  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
1113  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1114    .\" HTML <a href="#genericchartypes">
1115    .\" </a>
1116    "Generic character types"
1117    .\"
1118    above. The escape sequence \eb has a different meaning inside a character
1119    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1120    are not special inside a character class. Like any other unrecognized escape
1121    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1122    default, but cause an error if the PCRE_EXTRA option is set.
1123    .P
1124    A circumflex can conveniently be used with the upper case character types to
1125    specify a more restricted set of characters than the matching lower case type.
1126    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1127    whereas [\ew] includes underscore. A positive character class should be read as
1128    "something OR something OR ..." and a negative class as "NOT something AND NOT
1129    something AND NOT ...".
1130  .P  .P
1131  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1132  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 698  this notation. For example, Line 1146  this notation. For example,
1146    [01[:alpha:]%]    [01[:alpha:]%]
1147  .sp  .sp
1148  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1149  are  are:
1150  .sp  .sp
1151    alnum    letters and digits    alnum    letters and digits
1152    alpha    letters    alpha    letters
# Line 709  are Line 1157  are
1157    graph    printing characters, excluding space    graph    printing characters, excluding space
1158    lower    lower case letters    lower    lower case letters
1159    print    printing characters, including space    print    printing characters, including space
1160    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1161    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1162    upper    upper case letters    upper    upper case letters
1163    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 730  matches "1", "2", or any non-digit. PCRE Line 1178  matches "1", "2", or any non-digit. PCRE
1178  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
1179  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1180  .P  .P
1181  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
1182  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1183    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1184    character properties are used. This is achieved by replacing the POSIX classes
1185    by other sequences, as follows:
1186    .sp
1187      [:alnum:]  becomes  \ep{Xan}
1188      [:alpha:]  becomes  \ep{L}
1189      [:blank:]  becomes  \eh
1190      [:digit:]  becomes  \ep{Nd}
1191      [:lower:]  becomes  \ep{Ll}
1192      [:space:]  becomes  \ep{Xps}
1193      [:upper:]  becomes  \ep{Lu}
1194      [:word:]   becomes  \ep{Xwd}
1195    .sp
1196    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1197    classes are unchanged, and match only characters with code points less than
1198    128.
1199  .  .
1200  .  .
1201  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 743  the pattern Line 1207  the pattern
1207    gilbert|sullivan    gilbert|sullivan
1208  .sp  .sp
1209  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1210  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1211  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
1212  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
 subpattern  
1213  .\" HTML <a href="#subpattern">  .\" HTML <a href="#subpattern">
1214  .\" </a>  .\" </a>
1215  (defined below),  (defined below),
# Line 759  alternative in the subpattern. Line 1222  alternative in the subpattern.
1222  .rs  .rs
1223  .sp  .sp
1224  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1225  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
1226  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1227    The option letters are
1228  .sp  .sp
1229    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1230    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 774  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1238  PCRE_MULTILINE while unsetting PCRE_DOTA
1238  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
1239  unset.  unset.
1240  .P  .P
1241  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
1242  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
1243  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1244  the global options (and it will therefore show up in data extracted by the  .P
1245  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1246    subpattern parentheses), the change applies to the remainder of the pattern
1247    that follows. If the change is placed right at the start of a pattern, PCRE
1248    extracts it into the global options (and it will therefore show up in data
1249    extracted by the \fBpcre_fullinfo()\fP function).
1250  .P  .P
1251  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
1252  pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1253  .sp  .sp
1254    (a(?i)b)c    (a(?i)b)c
1255  .sp  .sp
# Line 797  branch is abandoned before the option se Line 1265  branch is abandoned before the option se
1265  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1266  behaviour otherwise.  behaviour otherwise.
1267  .P  .P
1268  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1269  same way as the Perl-compatible options by using the characters U and X  application when the compile or match functions are called. In some cases the
1270  respectively. The (?X) flag setting is special in that it must always occur  pattern can contain special leading sequences such as (*CRLF) to override what
1271  earlier in the pattern than any of the additional features it turns on, even  the application has set or what has been defaulted. Details are given in the
1272  when it is at top level. It is best to put it at the start.  section entitled
1273    .\" HTML <a href="#newlineseq">
1274    .\" </a>
1275    "Newline sequences"
1276    .\"
1277    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1278    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1279    PCRE_UTF8 and the PCRE_UCP options, respectively.
1280  .  .
1281  .  .
1282  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 815  Turning part of a pattern into a subpatt Line 1290  Turning part of a pattern into a subpatt
1290  .sp  .sp
1291    cat(aract|erpillar|)    cat(aract|erpillar|)
1292  .sp  .sp
1293  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1294  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1295  .sp  .sp
1296  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
1297  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
1298  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
1299  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1300  from 1) to obtain numbers for the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1301  .P  string "the red king" is matched against the pattern
 For example, if the string "the red king" is matched against the pattern  
1302  .sp  .sp
1303    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1304  .sp  .sp
# Line 841  the string "the white queen" is matched Line 1315  the string "the white queen" is matched
1315    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1316  .sp  .sp
1317  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
1318  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.  
1319  .P  .P
1320  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
1321  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 857  is reached, an option setting in one bra Line 1330  is reached, an option setting in one bra
1330  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1331  .  .
1332  .  .
1333    .\" HTML <a name="dupsubpatternnumber"></a>
1334    .SH "DUPLICATE SUBPATTERN NUMBERS"
1335    .rs
1336    .sp
1337    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1338    the same numbers for its capturing parentheses. Such a subpattern starts with
1339    (?| and is itself a non-capturing subpattern. For example, consider this
1340    pattern:
1341    .sp
1342      (?|(Sat)ur|(Sun))day
1343    .sp
1344    Because the two alternatives are inside a (?| group, both sets of capturing
1345    parentheses are numbered one. Thus, when the pattern matches, you can look
1346    at captured substring number one, whichever alternative matched. This construct
1347    is useful when you want to capture part, but not all, of one of a number of
1348    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1349    number is reset at the start of each branch. The numbers of any capturing
1350    parentheses that follow the subpattern start after the highest number used in
1351    any branch. The following example is taken from the Perl documentation. The
1352    numbers underneath show in which buffer the captured content will be stored.
1353    .sp
1354      # before  ---------------branch-reset----------- after
1355      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1356      # 1            2         2  3        2     3     4
1357    .sp
1358    A back reference to a numbered subpattern uses the most recent value that is
1359    set for that number by any subpattern. The following pattern matches "abcabc"
1360    or "defdef":
1361    .sp
1362      /(?|(abc)|(def))\e1/
1363    .sp
1364    In contrast, a subroutine call to a numbered subpattern always refers to the
1365    first one in the pattern with the given number. The following pattern matches
1366    "abcabc" or "defabc":
1367    .sp
1368      /(?|(abc)|(def))(?1)/
1369    .sp
1370    If a
1371    .\" HTML <a href="#conditions">
1372    .\" </a>
1373    condition test
1374    .\"
1375    for a subpattern's having matched refers to a non-unique number, the test is
1376    true if any of the subpatterns of that number have matched.
1377    .P
1378    An alternative approach to using this "branch reset" feature is to use
1379    duplicate named subpatterns, as described in the next section.
1380    .
1381    .
1382  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1383  .rs  .rs
1384  .sp  .sp
1385  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
1386  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1387  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
1388  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1389  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1390  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1391  .P  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1392  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1393  PCRE API provides function calls for extracting the name-to-number translation  .P
1394  table from a compiled pattern. There is also a convenience function for  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1395  extracting a captured substring by name. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1396    parentheses from other parts of the pattern, such as
1397    .\" HTML <a href="#backreferences">
1398    .\" </a>
1399    back references,
1400    .\"
1401    .\" HTML <a href="#recursion">
1402    .\" </a>
1403    recursion,
1404    .\"
1405    and
1406    .\" HTML <a href="#conditions">
1407    .\" </a>
1408    conditions,
1409    .\"
1410    can be made by name as well as by number.
1411    .P
1412    Names consist of up to 32 alphanumeric characters and underscores. Named
1413    capturing parentheses are still allocated numbers as well as names, exactly as
1414    if the names were not present. The PCRE API provides function calls for
1415    extracting the name-to-number translation table from a compiled pattern. There
1416    is also a convenience function for extracting a captured substring by name.
1417    .P
1418    By default, a name must be unique within a pattern, but it is possible to relax
1419    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1420    names are also always permitted for subpatterns with the same number, set up as
1421    described in the previous section.) Duplicate names can be useful for patterns
1422    where only one instance of the named parentheses can match. Suppose you want to
1423    match the name of a weekday, either as a 3-letter abbreviation or as the full
1424    name, and in both cases you want to extract the abbreviation. This pattern
1425    (ignoring the line breaks) does the job:
1426    .sp
1427      (?<DN>Mon|Fri|Sun)(?:day)?|
1428      (?<DN>Tue)(?:sday)?|
1429      (?<DN>Wed)(?:nesday)?|
1430      (?<DN>Thu)(?:rsday)?|
1431      (?<DN>Sat)(?:urday)?
1432    .sp
1433    There are five capturing substrings, but only one is ever set after a match.
1434    (An alternative way of solving this problem is to use a "branch reset"
1435    subpattern, as described in the previous section.)
1436    .P
1437    The convenience function for extracting the data by name returns the substring
1438    for the first (and in this example, the only) subpattern of that name that
1439    matched. This saves searching to find which numbered subpattern it was.
1440    .P
1441    If you make a back reference to a non-unique named subpattern from elsewhere in
1442    the pattern, the one that corresponds to the first occurrence of the name is
1443    used. In the absence of duplicate numbers (see the previous section) this is
1444    the one with the lowest number. If you use a named reference in a condition
1445    test (see the
1446    .\"
1447    .\" HTML <a href="#conditions">
1448    .\" </a>
1449    section about conditions
1450    .\"
1451    below), either to check whether a subpattern has matched, or to check for
1452    recursion, all subpatterns with the same name are tested. If the condition is
1453    true for any one of them, the overall condition is true. This is the same
1454    behaviour as testing by number. For further details of the interfaces for
1455    handling named subpatterns, see the
1456  .\" HREF  .\" HREF
1457  \fBpcreapi\fP  \fBpcreapi\fP
1458  .\"  .\"
1459  documentation.  documentation.
1460    .P
1461    \fBWarning:\fP You cannot use different names to distinguish between two
1462    subpatterns with the same number because PCRE uses only the numbers when
1463    matching. For this reason, an error is given at compile time if different names
1464    are given to subpatterns with the same number. However, you can give the same
1465    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1466  .  .
1467  .  .
1468  .SH REPETITION  .SH REPETITION
# Line 884  Repetition is specified by quantifiers, Line 1472  Repetition is specified by quantifiers,
1472  items:  items:
1473  .sp  .sp
1474    a literal data character    a literal data character
1475    the . metacharacter    the dot metacharacter
1476    the \eC escape sequence    the \eC escape sequence
1477    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1478    an escape such as \ed that matches a single character    the \eR escape sequence
1479      an escape such as \ed or \epL that matches a single character
1480    a character class    a character class
1481    a back reference (see next section)    a back reference (see next section)
1482    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1483      a subroutine call to a subpattern (recursive or otherwise)
1484  .sp  .sp
1485  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1486  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 922  support is available, \eX{3} matches thr Line 1512  support is available, \eX{3} matches thr
1512  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).
1513  .P  .P
1514  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
1515  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1516    subpatterns that are referenced as
1517    .\" HTML <a href="#subpatternsassubroutines">
1518    .\" </a>
1519    subroutines
1520    .\"
1521    from elsewhere in the pattern (but see also the section entitled
1522    .\" HTML <a href="#subdefine">
1523    .\" </a>
1524    "Defining subpatterns for use by reference only"
1525    .\"
1526    below). Items other than subpatterns that have a {0} quantifier are omitted
1527    from the compiled pattern.
1528  .P  .P
1529  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1530  quantifiers have single-character abbreviations:  abbreviations:
1531  .sp  .sp
1532    *    is equivalent to {0,}    *    is equivalent to {0,}
1533    +    is equivalent to {1,}    +    is equivalent to {1,}
# Line 973  own right. Because it has two uses, it c Line 1575  own right. Because it has two uses, it c
1575  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
1576  way the rest of the pattern matches.  way the rest of the pattern matches.
1577  .P  .P
1578  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),
1579  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
1580  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
1581  default behaviour.  default behaviour.
# Line 983  is greater than 1 or with a limited maxi Line 1585  is greater than 1 or with a limited maxi
1585  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1586  .P  .P
1587  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
1588  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
1589  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1590  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
1591  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 994  worth setting PCRE_DOTALL in order to ob Line 1596  worth setting PCRE_DOTALL in order to ob
1596  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1597  .P  .P
1598  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1599  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1600  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
1601  succeed. Consider, for example:  succeeds. Consider, for example:
1602  .sp  .sp
1603    (.*)abc\e1    (.*)abc\e1
1604  .sp  .sp
# Line 1022  matches "aba" the value of the second ca Line 1624  matches "aba" the value of the second ca
1624  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"  .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1625  .rs  .rs
1626  .sp  .sp
1627  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1628  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
1629  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
1630  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
1631  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
1632  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1633  .P  .P
1634  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
1635  .sp  .sp
# Line 1039  item, and then with 4, and so on, before Line 1641  item, and then with 4, and so on, before
1641  (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
1642  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.
1643  .P  .P
1644  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
1645  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
1646  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1647  .sp  .sp
# Line 1069  previous example can be rewritten as Line 1671  previous example can be rewritten as
1671  .sp  .sp
1672    \ed++foo    \ed++foo
1673  .sp  .sp
1674    Note that a possessive quantifier can be used with an entire group, for
1675    example:
1676    .sp
1677      (abc|xyz){2,3}+
1678    .sp
1679  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1680  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
1681  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
1682  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1683  .P  difference; possessive quantifiers should be slightly faster.
1684  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1685  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1686    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1687    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1688    package, and PCRE copied it from there. It ultimately found its way into Perl
1689    at release 5.10.
1690    .P
1691    PCRE has an optimization that automatically "possessifies" certain simple
1692    pattern constructs. For example, the sequence A+B is treated as A++B because
1693    there is no point in backtracking into a sequence of A's when B must follow.
1694  .P  .P
1695  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
1696  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 1117  However, if the decimal number following Line 1732  However, if the decimal number following
1732  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1733  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1734  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1735  numbers less than 10. See the subsection entitled "Non-printing characters"  numbers less than 10. A "forward back reference" of this type can make sense
1736    when a repetition is involved and the subpattern to the right has participated
1737    in an earlier iteration.
1738    .P
1739    It is not possible to have a numerical "forward back reference" to a subpattern
1740    whose number is 10 or more using this syntax because a sequence such as \e50 is
1741    interpreted as a character defined in octal. See the subsection entitled
1742    "Non-printing characters"
1743  .\" HTML <a href="#digitsafterbackslash">  .\" HTML <a href="#digitsafterbackslash">
1744  .\" </a>  .\" </a>
1745  above  above
1746  .\"  .\"
1747  for further details of the handling of digits following a backslash.  for further details of the handling of digits following a backslash. There is
1748    no such problem when named parentheses are used. A back reference to any
1749    subpattern is possible using named parentheses (see below).
1750    .P
1751    Another way of avoiding the ambiguity inherent in the use of digits following a
1752    backslash is to use the \eg escape sequence. This escape must be followed by an
1753    unsigned number or a negative number, optionally enclosed in braces. These
1754    examples are all identical:
1755    .sp
1756      (ring), \e1
1757      (ring), \eg1
1758      (ring), \eg{1}
1759    .sp
1760    An unsigned number specifies an absolute reference without the ambiguity that
1761    is present in the older syntax. It is also useful when literal digits follow
1762    the reference. A negative number is a relative reference. Consider this
1763    example:
1764    .sp
1765      (abc(def)ghi)\eg{-1}
1766    .sp
1767    The sequence \eg{-1} is a reference to the most recently started capturing
1768    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1769    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1770    can be helpful in long patterns, and also in patterns that are created by
1771    joining together fragments that contain references within themselves.
1772  .P  .P
1773  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1774  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1144  back reference, the case of letters is r Line 1790  back reference, the case of letters is r
1790  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
1791  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1792  .P  .P
1793  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1794  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1795  .sp  \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1796    (?<p1>(?i)rah)\es+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1797    references, is also supported. We could rewrite the above example in any of
1798    the following ways:
1799    .sp
1800      (?<p1>(?i)rah)\es+\ek<p1>
1801      (?'p1'(?i)rah)\es+\ek{p1}
1802      (?P<p1>(?i)rah)\es+(?P=p1)
1803      (?<p1>(?i)rah)\es+\eg{p1}
1804  .sp  .sp
1805    A subpattern that is referenced by name may appear in the pattern before or
1806    after the reference.
1807    .P
1808  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
1809  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1810  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1811  .sp  .sp
1812    (a|(bc))\e2    (a|(bc))\e2
1813  .sp  .sp
1814  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
1815  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
1816  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1817  with a digit character, some delimiter must be used to terminate the back  .P
1818  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1819  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1820    If the pattern continues with a digit character, some delimiter must be used to
1821    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1822    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1823  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1824  .\" </a>  .\" </a>
1825  "Comments"  "Comments"
1826  .\"  .\"
1827  below) can be used.  below) can be used.
1828  .P  .
1829    .SS "Recursive back references"
1830    .rs
1831    .sp
1832  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
1833  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.
1834  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1180  to the previous iteration. In order for Line 1842  to the previous iteration. In order for
1842  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
1843  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
1844  minimum of zero.  minimum of zero.
1845    .P
1846    Back references of this type cause the group that they reference to be treated
1847    as an
1848    .\" HTML <a href="#atomicgroup">
1849    .\" </a>
1850    atomic group.
1851    .\"
1852    Once the whole group has been matched, a subsequent matching failure cannot
1853    cause backtracking into the middle of the group.
1854  .  .
1855  .  .
1856  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1199  those that look ahead of the current pos Line 1870  those that look ahead of the current pos
1870  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,
1871  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.
1872  .P  .P
1873  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1874  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
1875  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1876  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1877  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1878  because it does not make sense for negative assertions.  .P
1879    For compatibility with Perl, assertion subpatterns may be repeated; though
1880    it makes no sense to assert the same thing several times, the side effect of
1881    capturing parentheses may occasionally be useful. In practice, there only three
1882    cases:
1883    .sp
1884    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1885    However, it may contain internal capturing parenthesized groups that are called
1886    from elsewhere via the
1887    .\" HTML <a href="#subpatternsassubroutines">
1888    .\" </a>
1889    subroutine mechanism.
1890    .\"
1891    .sp
1892    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1893    were {0,1}. At run time, the rest of the pattern match is tried with and
1894    without the assertion, the order depending on the greediness of the quantifier.
1895    .sp
1896    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1897    The assertion is obeyed just once when encountered during matching.
1898  .  .
1899  .  .
1900  .SS "Lookahead assertions"  .SS "Lookahead assertions"
1901  .rs  .rs
1902  .sp  .sp
1903  Lookahead assertions start  Lookahead assertions start with (?= for positive assertions and (?! for
1904  with (?= for positive assertions and (?! for negative assertions. For example,  negative assertions. For example,
1905  .sp  .sp
1906    \ew+(?=;)    \ew+(?=;)
1907  .sp  .sp
# Line 1233  lookbehind assertion is needed to achiev Line 1923  lookbehind assertion is needed to achiev
1923  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
1924  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
1925  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.
1926    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1927  .  .
1928  .  .
1929  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1246  negative assertions. For example, Line 1937  negative assertions. For example,
1937  .sp  .sp
1938  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1939  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1940  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1941  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1942  .sp  .sp
1943    (?<=bullock|donkey)    (?<=bullock|donkey)
1944  .sp  .sp
# Line 1257  is permitted, but Line 1948  is permitted, but
1948  .sp  .sp
1949  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1950  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
1951  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
1952  match the same length of string. An assertion such as  length of string. An assertion such as
1953  .sp  .sp
1954    (?<=ab(c|de))    (?<=ab(c|de))
1955  .sp  .sp
1956  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
1957  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
1958    branches:
1959  .sp  .sp
1960    (?<=abc|abde)    (?<=abc|abde)
1961  .sp  .sp
1962    In some cases, the escape sequence \eK
1963    .\" HTML <a href="#resetmatchstart">
1964    .\" </a>
1965    (see above)
1966    .\"
1967    can be used instead of a lookbehind assertion to get round the fixed-length
1968    restriction.
1969    .P
1970  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1971  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
1972  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1973  match is deemed to fail.  assertion fails.
1974    .P
1975    In UTF-8 mode, PCRE does not allow the \eC escape (which matches a single byte,
1976    even in UTF-8 mode) to appear in lookbehind assertions, because it makes it
1977    impossible to calculate the length of the lookbehind. The \eX and \eR escapes,
1978    which can match different numbers of bytes, are also not permitted.
1979    .P
1980    .\" HTML <a href="#subpatternsassubroutines">
1981    .\" </a>
1982    "Subroutine"
1983    .\"
1984    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1985    as the subpattern matches a fixed-length string.
1986    .\" HTML <a href="#recursion">
1987    .\" </a>
1988    Recursion,
1989    .\"
1990    however, is not supported.
1991  .P  .P
1992  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1993  to appear in lookbehind assertions, because it makes it impossible to calculate  specify efficient matching of fixed-length strings at the end of subject
1994  the length of the lookbehind. The \eX escape, which can match different numbers  strings. Consider a simple pattern such as
 of bytes, is also not permitted.  
 .P  
 Atomic groups can be used in conjunction with lookbehind assertions to specify  
 efficient matching at the end of the subject string. Consider a simple pattern  
 such as  
1995  .sp  .sp
1996    abcd$    abcd$
1997  .sp  .sp
# Line 1295  then all but the last two characters, an Line 2007  then all but the last two characters, an
2007  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,
2008  if the pattern is written as  if the pattern is written as
2009  .sp  .sp
   ^(?>.*)(?<=abcd)  
 .sp  
 or, equivalently, using the possessive quantifier syntax,  
 .sp  
2010    ^.*+(?<=abcd)    ^.*+(?<=abcd)
2011  .sp  .sp
2012  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
2013  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
2014  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
2015  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
# Line 1341  is another pattern that matches "foo" pr Line 2049  is another pattern that matches "foo" pr
2049  characters that are not "999".  characters that are not "999".
2050  .  .
2051  .  .
2052    .\" HTML <a name="conditions"></a>
2053  .SH "CONDITIONAL SUBPATTERNS"  .SH "CONDITIONAL SUBPATTERNS"
2054  .rs  .rs
2055  .sp  .sp
2056  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2057  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2058  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2059  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2060  .sp  .sp
2061    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2062    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2063  .sp  .sp
2064  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2065  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
2066  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2067    itself contain nested subpatterns of any form, including conditional
2068    subpatterns; the restriction to two alternatives applies only at the level of
2069    the condition. This pattern fragment is an example where the alternatives are
2070    complex:
2071    .sp
2072      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2073    .sp
2074  .P  .P
2075  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
2076  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
2077  subpattern of that number has previously matched. The number must be greater  .
2078  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
2079  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
2080  it into three parts for ease of discussion:  .sp
2081    If the text between the parentheses consists of a sequence of digits, the
2082    condition is true if a capturing subpattern of that number has previously
2083    matched. If there is more than one capturing subpattern with the same number
2084    (see the earlier
2085    .\"
2086    .\" HTML <a href="#recursion">
2087    .\" </a>
2088    section about duplicate subpattern numbers),
2089    .\"
2090    the condition is true if any of them have matched. An alternative notation is
2091    to precede the digits with a plus or minus sign. In this case, the subpattern
2092    number is relative rather than absolute. The most recently opened parentheses
2093    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2094    loops it can also make sense to refer to subsequent groups. The next
2095    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2096    zero in any of these forms is not used; it provokes a compile-time error.)
2097    .P
2098    Consider the following pattern, which contains non-significant white space to
2099    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2100    three parts for ease of discussion:
2101  .sp  .sp
2102    ( \e( )?    [^()]+    (?(1) \e) )    ( \e( )?    [^()]+    (?(1) \e) )
2103  .sp  .sp
2104  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2105  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
2106  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
2107  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2108  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,
2109  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
2110  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2111  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2112  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2113  .P  .P
2114  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
2115  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2116  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
2117      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2118    .sp
2119    This makes the fragment independent of the parentheses in the larger pattern.
2120    .
2121    .SS "Checking for a used subpattern by name"
2122    .rs
2123    .sp
2124    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2125    subpattern by name. For compatibility with earlier versions of PCRE, which had
2126    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2127    there is a possible ambiguity with this syntax, because subpattern names may
2128    consist entirely of digits. PCRE looks first for a named subpattern; if it
2129    cannot find one and the name consists entirely of digits, PCRE looks for a
2130    subpattern of that number, which must be greater than zero. Using subpattern
2131    names that consist entirely of digits is not recommended.
2132    .P
2133    Rewriting the above example to use a named subpattern gives this:
2134    .sp
2135      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2136    .sp
2137    If the name used in a condition of this kind is a duplicate, the test is
2138    applied to all subpatterns of the same name, and is true if any one of them has
2139    matched.
2140    .
2141    .SS "Checking for pattern recursion"
2142    .rs
2143    .sp
2144    If the condition is the string (R), and there is no subpattern with the name R,
2145    the condition is true if a recursive call to the whole pattern or any
2146    subpattern has been made. If digits or a name preceded by ampersand follow the
2147    letter R, for example:
2148    .sp
2149      (?(R3)...) or (?(R&name)...)
2150    .sp
2151    the condition is true if the most recent recursion is into a subpattern whose
2152    number or name is given. This condition does not check the entire recursion
2153    stack. If the name used in a condition of this kind is a duplicate, the test is
2154    applied to all subpatterns of the same name, and is true if any one of them is
2155    the most recent recursion.
2156  .P  .P
2157  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.
2158    .\" HTML <a href="#recursion">
2159    .\" </a>
2160    The syntax for recursive patterns
2161    .\"
2162    is described below.
2163    .
2164    .\" HTML <a name="subdefine"></a>
2165    .SS "Defining subpatterns for use by reference only"
2166    .rs
2167    .sp
2168    If the condition is the string (DEFINE), and there is no subpattern with the
2169    name DEFINE, the condition is always false. In this case, there may be only one
2170    alternative in the subpattern. It is always skipped if control reaches this
2171    point in the pattern; the idea of DEFINE is that it can be used to define
2172    subroutines that can be referenced from elsewhere. (The use of
2173    .\" HTML <a href="#subpatternsassubroutines">
2174    .\" </a>
2175    subroutines
2176    .\"
2177    is described below.) For example, a pattern to match an IPv4 address such as
2178    "192.168.23.245" could be written like this (ignore whitespace and line
2179    breaks):
2180    .sp
2181      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2182      \eb (?&byte) (\e.(?&byte)){3} \eb
2183    .sp
2184    The first part of the pattern is a DEFINE group inside which a another group
2185    named "byte" is defined. This matches an individual component of an IPv4
2186    address (a number less than 256). When matching takes place, this part of the
2187    pattern is skipped because DEFINE acts like a false condition. The rest of the
2188    pattern uses references to the named group to match the four dot-separated
2189    components of an IPv4 address, insisting on a word boundary at each end.
2190    .
2191    .SS "Assertion conditions"
2192    .rs
2193    .sp
2194    If the condition is not in any of the above formats, it must be an assertion.
2195  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2196  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2197  alternatives on the second line:  alternatives on the second line:
# Line 1399  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2211  dd-aaa-dd or dd-dd-dd, where aaa are let
2211  .SH COMMENTS  .SH COMMENTS
2212  .rs  .rs
2213  .sp  .sp
2214  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
2215  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,
2216  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
2217    subpattern name or number. The characters that make up a comment play no part
2218    in the pattern matching.
2219  .P  .P
2220  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
2221  character class introduces a comment that continues up to the next newline  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2222  character in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2223    this case continues to immediately after the next newline character or
2224    character sequence in the pattern. Which characters are interpreted as newlines
2225    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2226    sequence at the start of the pattern, as described in the section entitled
2227    .\" HTML <a href="#newlines">
2228    .\" </a>
2229    "Newline conventions"
2230    .\"
2231    above. Note that the end of this type of comment is a literal newline sequence
2232    in the pattern; escape sequences that happen to represent a newline do not
2233    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2234    default newline convention is in force:
2235    .sp
2236      abc #comment \en still comment
2237    .sp
2238    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2239    a newline in the pattern. The sequence \en is still literal at this stage, so
2240    it does not terminate the comment. Only an actual character with the code value
2241    0x0a (the default newline) does so.
2242  .  .
2243  .  .
2244    .\" HTML <a name="recursion"></a>
2245  .SH "RECURSIVE PATTERNS"  .SH "RECURSIVE PATTERNS"
2246  .rs  .rs
2247  .sp  .sp
2248  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2249  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2250  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
2251  is not possible to handle an arbitrary nesting depth. Perl provides a facility  is not possible to handle an arbitrary nesting depth.
2252  that allows regular expressions to recurse (amongst other things). It does this  .P
2253  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
2254  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
2255  can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2256    pattern using code interpolation to solve the parentheses problem can be
2257    created like this:
2258  .sp  .sp
2259    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;    $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2260  .sp  .sp
2261  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
2262  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2263  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2264  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2265  .P  supports special syntax for recursion of the entire pattern, and also for
2266  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2267  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.
2268  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2269  "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
2270  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive subroutine call of the subpattern of the
2271  .P  given number, provided that it occurs inside that subpattern. (If not, it is a
2272  A recursive subpattern call is always treated as an atomic group. That is, once  .\" HTML <a href="#subpatternsassubroutines">
2273  it has matched some of the subject string, it is never re-entered, even if  .\" </a>
2274  it contains untried alternatives and there is a subsequent matching failure.  non-recursive subroutine
2275    .\"
2276    call, which is described in the next section.) The special item (?R) or (?0) is
2277    a recursive call of the entire regular expression.
2278  .P  .P
2279  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2280  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2281  .sp  .sp
2282    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2283  .sp  .sp
2284  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2285  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
2286  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2287  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2288    to avoid backtracking into sequences of non-parentheses.
2289  .P  .P
2290  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
2291  pattern, so instead you could use this:  pattern, so instead you could use this:
2292  .sp  .sp
2293    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2294  .sp  .sp
2295  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
2296  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2297  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2298  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
2299  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
2300  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2301  .sp  parentheses preceding the recursion. In other words, a negative number counts
2302    (?P<pn> \e( ( (?>[^()]+) | (?P>pn) )* \e) )  capturing parentheses leftwards from the point at which it is encountered.
2303  .sp  .P
2304  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2305  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2306  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
2307  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2308    .\" </a>
2309    non-recursive subroutine
2310    .\"
2311    calls, as described in the next section.
2312    .P
2313    An alternative approach is to use named parentheses instead. The Perl syntax
2314    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2315    could rewrite the above example as follows:
2316    .sp
2317      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2318    .sp
2319    If there is more than one subpattern with the same name, the earliest one is
2320    used.
2321    .P
2322    This particular example pattern that we have been looking at contains nested
2323    unlimited repeats, and so the use of a possessive quantifier for matching
2324    strings of non-parentheses is important when applying the pattern to strings
2325    that do not match. For example, when this pattern is applied to
2326  .sp  .sp
2327    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2328  .sp  .sp
2329  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,
2330  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
2331  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
2332  before failure can be reported.  before failure can be reported.
2333  .P  .P
2334  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
2335  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
2336  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  
2337  .\" HREF  .\" HREF
2338  \fBpcrecallout\fP  \fBpcrecallout\fP
2339  .\"  .\"
# Line 1484  documentation). If the pattern above is Line 2341  documentation). If the pattern above is
2341  .sp  .sp
2342    (ab(cd)ef)    (ab(cd)ef)
2343  .sp  .sp
2344  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
2345  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
2346  .sp  matched at the top level, its final captured value is unset, even if it was
2347    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2348       ^                        ^  .P
2349       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2350  .sp  obtain extra memory to store data during a recursion, which it does by using
2351  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
2352  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.  
2353  .P  .P
2354  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.
2355  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1509  different alternatives for the recursive Line 2363  different alternatives for the recursive
2363  is the actual recursive call.  is the actual recursive call.
2364  .  .
2365  .  .
2366    .\" HTML <a name="recursiondifference"></a>
2367    .SS "Differences in recursion processing between PCRE and Perl"
2368    .rs
2369    .sp
2370    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2371    (like Python, but unlike Perl), a recursive subpattern call is always treated
2372    as an atomic group. That is, once it has matched some of the subject string, it
2373    is never re-entered, even if it contains untried alternatives and there is a
2374    subsequent matching failure. This can be illustrated by the following pattern,
2375    which purports to match a palindromic string that contains an odd number of
2376    characters (for example, "a", "aba", "abcba", "abcdcba"):
2377    .sp
2378      ^(.|(.)(?1)\e2)$
2379    .sp
2380    The idea is that it either matches a single character, or two identical
2381    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2382    it does not if the pattern is longer than three characters. Consider the
2383    subject string "abcba":
2384    .P
2385    At the top level, the first character is matched, but as it is not at the end
2386    of the string, the first alternative fails; the second alternative is taken
2387    and the recursion kicks in. The recursive call to subpattern 1 successfully
2388    matches the next character ("b"). (Note that the beginning and end of line
2389    tests are not part of the recursion).
2390    .P
2391    Back at the top level, the next character ("c") is compared with what
2392    subpattern 2 matched, which was "a". This fails. Because the recursion is
2393    treated as an atomic group, there are now no backtracking points, and so the
2394    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2395    try the second alternative.) However, if the pattern is written with the
2396    alternatives in the other order, things are different:
2397    .sp
2398      ^((.)(?1)\e2|.)$
2399    .sp
2400    This time, the recursing alternative is tried first, and continues to recurse
2401    until it runs out of characters, at which point the recursion fails. But this
2402    time we do have another alternative to try at the higher level. That is the big
2403    difference: in the previous case the remaining alternative is at a deeper
2404    recursion level, which PCRE cannot use.
2405    .P
2406    To change the pattern so that it matches all palindromic strings, not just
2407    those with an odd number of characters, it is tempting to change the pattern to
2408    this:
2409    .sp
2410      ^((.)(?1)\e2|.?)$
2411    .sp
2412    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2413    deeper recursion has matched a single character, it cannot be entered again in
2414    order to match an empty string. The solution is to separate the two cases, and
2415    write out the odd and even cases as alternatives at the higher level:
2416    .sp
2417      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2418    .sp
2419    If you want to match typical palindromic phrases, the pattern has to ignore all
2420    non-word characters, which can be done like this:
2421    .sp
2422      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2423    .sp
2424    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2425    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2426    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2427    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2428    more) to match typical phrases, and Perl takes so long that you think it has
2429    gone into a loop.
2430    .P
2431    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2432    string does not start with a palindrome that is shorter than the entire string.
2433    For example, although "abcba" is correctly matched, if the subject is "ababa",
2434    PCRE finds the palindrome "aba" at the start, then fails at top level because
2435    the end of the string does not follow. Once again, it cannot jump back into the
2436    recursion to try other alternatives, so the entire match fails.
2437    .P
2438    The second way in which PCRE and Perl differ in their recursion processing is
2439    in the handling of captured values. In Perl, when a subpattern is called
2440    recursively or as a subpattern (see the next section), it has no access to any
2441    values that were captured outside the recursion, whereas in PCRE these values
2442    can be referenced. Consider this pattern:
2443    .sp
2444      ^(.)(\e1|a(?2))
2445    .sp
2446    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2447    then in the second group, when the back reference \e1 fails to match "b", the
2448    second alternative matches "a" and then recurses. In the recursion, \e1 does
2449    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2450    match because inside the recursive call \e1 cannot access the externally set
2451    value.
2452    .
2453    .
2454  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2455  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2456  .rs  .rs
2457  .sp  .sp
2458  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
2459  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
2460  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The called subpattern may be defined
2461  pattern  before or after the reference. A numbered reference can be absolute or
2462    relative, as in these examples:
2463    .sp
2464      (...(absolute)...)...(?2)...
2465      (...(relative)...)...(?-1)...
2466      (...(?+1)...(relative)...
2467    .sp
2468    An earlier example pointed out that the pattern
2469  .sp  .sp
2470    (sens|respons)e and \e1ibility    (sens|respons)e and \e1ibility
2471  .sp  .sp
# Line 1526  matches "sense and sensibility" and "res Line 2475  matches "sense and sensibility" and "res
2475    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2476  .sp  .sp
2477  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
2478  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2479  refer.  .P
2480    All subroutine calls, whether recursive or not, are always treated as atomic
2481    groups. That is, once a subroutine has matched some of the subject string, it
2482    is never re-entered, even if it contains untried alternatives and there is a
2483    subsequent matching failure. Any capturing parentheses that are set during the
2484    subroutine call revert to their previous values afterwards.
2485  .P  .P
2486  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  Processing options such as case-independence are fixed when a subpattern is
2487  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
2488  re-entered, even if it contains untried alternatives and there is a subsequent  different calls. For example, consider this pattern:
2489  matching failure.  .sp
2490      (abc)(?i:(?-1))
2491    .sp
2492    It matches "abcabc". It does not match "abcABC" because the change of
2493    processing option does not affect the called subpattern.
2494    .
2495    .
2496    .\" HTML <a name="onigurumasubroutines"></a>
2497    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2498    .rs
2499    .sp
2500    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2501    a number enclosed either in angle brackets or single quotes, is an alternative
2502    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2503    are two of the examples used above, rewritten using this syntax:
2504    .sp
2505      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2506      (sens|respons)e and \eg'1'ibility
2507    .sp
2508    PCRE supports an extension to Oniguruma: if a number is preceded by a
2509    plus or a minus sign it is taken as a relative reference. For example:
2510    .sp
2511      (abc)(?i:\eg<-1>)
2512    .sp
2513    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2514    synonymous. The former is a back reference; the latter is a subroutine call.
2515  .  .
2516  .  .
2517  .SH CALLOUTS  .SH CALLOUTS
# Line 1553  function is to be called. If you want to Line 2532  function is to be called. If you want to
2532  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.
2533  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2534  .sp  .sp
2535    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2536  .sp  .sp
2537  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
2538  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 1569  description of the interface to the call Line 2548  description of the interface to the call
2548  \fBpcrecallout\fP  \fBpcrecallout\fP
2549  .\"  .\"
2550  documentation.  documentation.
2551    .
2552    .
2553    .\" HTML <a name="backtrackcontrol"></a>
2554    .SH "BACKTRACKING CONTROL"
2555    .rs
2556    .sp
2557    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2558    are described in the Perl documentation as "experimental and subject to change
2559    or removal in a future version of Perl". It goes on to say: "Their usage in
2560    production code should be noted to avoid problems during upgrades." The same
2561    remarks apply to the PCRE features described in this section.
2562    .P
2563    Since these verbs are specifically related to backtracking, most of them can be
2564    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2565    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2566    failing negative assertion, they cause an error if encountered by
2567    \fBpcre_dfa_exec()\fP.
2568    .P
2569    If any of these verbs are used in an assertion or in a subpattern that is
2570    called as a subroutine (whether or not recursively), their effect is confined
2571    to that subpattern; it does not extend to the surrounding pattern, with one
2572    exception: a *MARK that is encountered in a positive assertion \fIis\fP passed
2573    back (compare capturing parentheses in assertions). Note that such subpatterns
2574    are processed as anchored at the point where they are tested. Note also that
2575    Perl's treatment of subroutines is different in some cases.
2576    .P
2577    The new verbs make use of what was previously invalid syntax: an opening
2578    parenthesis followed by an asterisk. They are generally of the form
2579    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2580    depending on whether or not an argument is present. A name is any sequence of
2581    characters that does not include a closing parenthesis. If the name is empty,
2582    that is, if the closing parenthesis immediately follows the colon, the effect
2583    is as if the colon were not there. Any number of these verbs may occur in a
2584    pattern.
2585    .P
2586    PCRE contains some optimizations that are used to speed up matching by running
2587    some checks at the start of each match attempt. For example, it may know the
2588    minimum length of matching subject, or that a particular character must be
2589    present. When one of these optimizations suppresses the running of a match, any
2590    included backtracking verbs will not, of course, be processed. You can suppress
2591    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2592    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2593    pattern with (*NO_START_OPT).
2594    .
2595    .
2596    .SS "Verbs that act immediately"
2597    .rs
2598    .sp
2599    The following verbs act as soon as they are encountered. They may not be
2600    followed by a name.
2601    .sp
2602       (*ACCEPT)
2603    .sp
2604    This verb causes the match to end successfully, skipping the remainder of the
2605    pattern. However, when it is inside a subpattern that is called as a
2606    subroutine, only that subpattern is ended successfully. Matching then continues
2607    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2608    far is captured. For example:
2609    .sp
2610      A((?:A|B(*ACCEPT)|C)D)
2611    .sp
2612    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2613    the outer parentheses.
2614    .sp
2615      (*FAIL) or (*F)
2616    .sp
2617    This verb causes a matching failure, forcing backtracking to occur. It is
2618    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2619    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2620    Perl features that are not present in PCRE. The nearest equivalent is the
2621    callout feature, as for example in this pattern:
2622    .sp
2623      a+(?C)(*FAIL)
2624    .sp
2625    A match with the string "aaaa" always fails, but the callout is taken before
2626    each backtrack happens (in this example, 10 times).
2627    .
2628    .
2629    .SS "Recording which path was taken"
2630    .rs
2631    .sp
2632    There is one verb whose main purpose is to track how a match was arrived at,
2633    though it also has a secondary use in conjunction with advancing the match
2634    starting point (see (*SKIP) below).
2635    .sp
2636      (*MARK:NAME) or (*:NAME)
2637    .sp
2638    A name is always required with this verb. There may be as many instances of
2639    (*MARK) as you like in a pattern, and their names do not have to be unique.
2640  .P  .P
2641  .in 0  When a match succeeds, the name of the last-encountered (*MARK) is passed back
2642  Last updated: 24 January 2006  to the caller via the \fIpcre_extra\fP data structure, as described in the
2643  .br  .\" HTML <a href="pcreapi.html#extradata">
2644  Copyright (c) 1997-2006 University of Cambridge.  .\" </a>
2645    section on \fIpcre_extra\fP
2646    .\"
2647    in the
2648    .\" HREF
2649    \fBpcreapi\fP
2650    .\"
2651    documentation. No data is returned for a partial match. Here is an example of
2652    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2653    outputting of (*MARK) data:
2654    .sp
2655      /X(*MARK:A)Y|X(*MARK:B)Z/K
2656      XY
2657       0: XY
2658      MK: A
2659      XZ
2660       0: XZ
2661      MK: B
2662    .sp
2663    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2664    indicates which of the two alternatives matched. This is a more efficient way
2665    of obtaining this information than putting each alternative in its own
2666    capturing parentheses.
2667    .P
2668    If (*MARK) is encountered in a positive assertion, its name is recorded and
2669    passed back if it is the last-encountered. This does not happen for negative
2670    assertions.
2671    .P
2672    A name may also be returned after a failed match if the final path through the
2673    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2674    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2675    starting point for matching is advanced, the final check is often with an empty
2676    string, causing a failure before (*MARK) is reached. For example:
2677    .sp
2678      /X(*MARK:A)Y|X(*MARK:B)Z/K
2679      XP
2680      No match
2681    .sp
2682    There are three potential starting points for this match (starting with X,
2683    starting with P, and with an empty string). If the pattern is anchored, the
2684    result is different:
2685    .sp
2686      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2687      XP
2688      No match, mark = B
2689    .sp
2690    PCRE's start-of-match optimizations can also interfere with this. For example,
2691    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2692    subject length for a match, a shorter subject will not be scanned at all.
2693    .P
2694    Note that similar anomalies (though different in detail) exist in Perl, no
2695    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2696    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2697    .
2698    .
2699    .SS "Verbs that act after backtracking"
2700    .rs
2701    .sp
2702    The following verbs do nothing when they are encountered. Matching continues
2703    with what follows, but if there is no subsequent match, causing a backtrack to
2704    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2705    the verb. However, when one of these verbs appears inside an atomic group, its
2706    effect is confined to that group, because once the group has been matched,
2707    there is never any backtracking into it. In this situation, backtracking can
2708    "jump back" to the left of the entire atomic group. (Remember also, as stated
2709    above, that this localization also applies in subroutine calls and assertions.)
2710    .P
2711    These verbs differ in exactly what kind of failure occurs when backtracking
2712    reaches them.
2713    .sp
2714      (*COMMIT)
2715    .sp
2716    This verb, which may not be followed by a name, causes the whole match to fail
2717    outright if the rest of the pattern does not match. Even if the pattern is
2718    unanchored, no further attempts to find a match by advancing the starting point
2719    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2720    finding a match at the current starting point, or not at all. For example:
2721    .sp
2722      a+(*COMMIT)b
2723    .sp
2724    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2725    dynamic anchor, or "I've started, so I must finish." The name of the most
2726    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2727    match failure.
2728    .P
2729    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2730    unless PCRE's start-of-match optimizations are turned off, as shown in this
2731    \fBpcretest\fP example:
2732    .sp
2733      /(*COMMIT)abc/
2734      xyzabc
2735       0: abc
2736      xyzabc\eY
2737      No match
2738    .sp
2739    PCRE knows that any match must start with "a", so the optimization skips along
2740    the subject to "a" before running the first match attempt, which succeeds. When
2741    the optimization is disabled by the \eY escape in the second subject, the match
2742    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2743    starting points.
2744    .sp
2745      (*PRUNE) or (*PRUNE:NAME)
2746    .sp
2747    This verb causes the match to fail at the current starting position in the
2748    subject if the rest of the pattern does not match. If the pattern is
2749    unanchored, the normal "bumpalong" advance to the next starting character then
2750    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2751    reached, or when matching to the right of (*PRUNE), but if there is no match to
2752    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2753    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2754    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2755    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2756    match fails completely; the name is passed back if this is the final attempt.
2757    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2758    pattern (*PRUNE) has the same effect as (*COMMIT).
2759    .sp
2760      (*SKIP)
2761    .sp
2762    This verb, when given without a name, is like (*PRUNE), except that if the
2763    pattern is unanchored, the "bumpalong" advance is not to the next character,
2764    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2765    signifies that whatever text was matched leading up to it cannot be part of a
2766    successful match. Consider:
2767    .sp
2768      a+(*SKIP)b
2769    .sp
2770    If the subject is "aaaac...", after the first match attempt fails (starting at
2771    the first character in the string), the starting point skips on to start the
2772    next attempt at "c". Note that a possessive quantifer does not have the same
2773    effect as this example; although it would suppress backtracking during the
2774    first match attempt, the second attempt would start at the second character
2775    instead of skipping on to "c".
2776    .sp
2777      (*SKIP:NAME)
2778    .sp
2779    When (*SKIP) has an associated name, its behaviour is modified. If the
2780    following pattern fails to match, the previous path through the pattern is
2781    searched for the most recent (*MARK) that has the same name. If one is found,
2782    the "bumpalong" advance is to the subject position that corresponds to that
2783    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2784    matching name is found, normal "bumpalong" of one character happens (that is,
2785    the (*SKIP) is ignored).
2786    .sp
2787      (*THEN) or (*THEN:NAME)
2788    .sp
2789    This verb causes a skip to the next innermost alternative if the rest of the
2790    pattern does not match. That is, it cancels pending backtracking, but only
2791    within the current alternative. Its name comes from the observation that it can
2792    be used for a pattern-based if-then-else block:
2793    .sp
2794      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2795    .sp
2796    If the COND1 pattern matches, FOO is tried (and possibly further items after
2797    the end of the group if FOO succeeds); on failure, the matcher skips to the
2798    second alternative and tries COND2, without backtracking into COND1. The
2799    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2800    overall match fails. If (*THEN) is not inside an alternation, it acts like
2801    (*PRUNE).
2802    .P
2803    Note that a subpattern that does not contain a | character is just a part of
2804    the enclosing alternative; it is not a nested alternation with only one
2805    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2806    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2807    pattern fragments that do not contain any | characters at this level:
2808    .sp
2809      A (B(*THEN)C) | D
2810    .sp
2811    If A and B are matched, but there is a failure in C, matching does not
2812    backtrack into A; instead it moves to the next alternative, that is, D.
2813    However, if the subpattern containing (*THEN) is given an alternative, it
2814    behaves differently:
2815    .sp
2816      A (B(*THEN)C | (*FAIL)) | D
2817    .sp
2818    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2819    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2820    because there are no more alternatives to try. In this case, matching does now
2821    backtrack into A.
2822    .P
2823    Note also that a conditional subpattern is not considered as having two
2824    alternatives, because only one is ever used. In other words, the | character in
2825    a conditional subpattern has a different meaning. Ignoring white space,
2826    consider:
2827    .sp
2828      ^.*? (?(?=a) a | b(*THEN)c )
2829    .sp
2830    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2831    it initially matches zero characters. The condition (?=a) then fails, the
2832    character "b" is matched, but "c" is not. At this point, matching does not
2833    backtrack to .*? as might perhaps be expected from the presence of the |
2834    character. The conditional subpattern is part of the single alternative that
2835    comprises the whole pattern, and so the match fails. (If there was a backtrack
2836    into .*?, allowing it to match "b", the match would succeed.)
2837    .P
2838    The verbs just described provide four different "strengths" of control when
2839    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2840    next alternative. (*PRUNE) comes next, failing the match at the current
2841    starting position, but allowing an advance to the next character (for an
2842    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2843    than one character. (*COMMIT) is the strongest, causing the entire match to
2844    fail.
2845    .P
2846    If more than one such verb is present in a pattern, the "strongest" one wins.
2847    For example, consider this pattern, where A, B, etc. are complex pattern
2848    fragments:
2849    .sp
2850      (A(*COMMIT)B(*THEN)C|D)
2851    .sp
2852    Once A has matched, PCRE is committed to this match, at the current starting
2853    position. If subsequently B matches, but C does not, the normal (*THEN) action
2854    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2855    overrides.
2856    .
2857    .
2858    .SH "SEE ALSO"
2859    .rs
2860    .sp
2861    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2862    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2863    .
2864    .
2865    .SH AUTHOR
2866    .rs
2867    .sp
2868    .nf
2869    Philip Hazel
2870    University Computing Service
2871    Cambridge CB2 3QH, England.
2872    .fi
2873    .
2874    .
2875    .SH REVISION
2876    .rs
2877    .sp
2878    .nf
2879    Last updated: 19 November 2011
2880    Copyright (c) 1997-2011 University of Cambridge.
2881    .fi

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