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# 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 strings in the original library, and a
25  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  second library that supports 16-bit and UTF-16 character strings. To use these
26  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  features, PCRE must be built to include appropriate support. When using UTF
27  places below. There is also a summary of UTF-8 features in the  strings you must either call the compiling function with the PCRE_UTF8 or
28  .\" HTML <a href="pcre.html#utf8support">  PCRE_UTF16 option, or the pattern must start with one of these special
29  .\" </a>  sequences:
30  section on UTF-8 support  .sp
31  .\"    (*UTF8)
32  in the main    (*UTF16)
33    .sp
34    Starting a pattern with such a sequence is equivalent to setting the relevant
35    option. This feature is not Perl-compatible. How setting a UTF mode affects
36    pattern matching is mentioned in several places below. There is also a summary
37    of features in the
38  .\" HREF  .\" HREF
39  \fBpcre\fP  \fBpcreunicode\fP
40  .\"  .\"
41  page.  page.
42  .P  .P
43    Another special sequence that may appear at the start of a pattern or in
44    combination with (*UTF8) or (*UTF16) is:
45    .sp
46      (*UCP)
47    .sp
48    This has the same effect as setting the PCRE_UCP option: it causes sequences
49    such as \ed and \ew to use Unicode properties to determine character types,
50    instead of recognizing only characters with codes less than 128 via a lookup
51    table.
52    .P
53    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
54    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
55    also some more of these special sequences that are concerned with the handling
56    of newlines; they are described below.
57    .P
58  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
59  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
60  From release 6.0, PCRE offers a second matching function,  \fBpcre16_exec()\fP (16-bit), is used. PCRE also has alternative matching
61  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  functions, \fBpcre_dfa_exec()\fP and \fBpcre16_dfa_exec()\fP, which match using
62  Perl-compatible. The advantages and disadvantages of the alternative function,  a different algorithm that is not Perl-compatible. Some of the features
63  and how it differs from the normal function, are discussed in the  discussed below are not available when DFA matching is used. The advantages and
64    disadvantages of the alternative functions, and how they differ from the normal
65    functions, are discussed in the
66  .\" HREF  .\" HREF
67  \fBpcrematching\fP  \fBpcrematching\fP
68  .\"  .\"
69  page.  page.
70  .  .
71  .  .
72    .\" HTML <a name="newlines"></a>
73    .SH "NEWLINE CONVENTIONS"
74    .rs
75    .sp
76    PCRE supports five different conventions for indicating line breaks in
77    strings: a single CR (carriage return) character, a single LF (linefeed)
78    character, the two-character sequence CRLF, any of the three preceding, or any
79    Unicode newline sequence. The
80    .\" HREF
81    \fBpcreapi\fP
82    .\"
83    page has
84    .\" HTML <a href="pcreapi.html#newlines">
85    .\" </a>
86    further discussion
87    .\"
88    about newlines, and shows how to set the newline convention in the
89    \fIoptions\fP arguments for the compiling and matching functions.
90    .P
91    It is also possible to specify a newline convention by starting a pattern
92    string with one of the following five sequences:
93    .sp
94      (*CR)        carriage return
95      (*LF)        linefeed
96      (*CRLF)      carriage return, followed by linefeed
97      (*ANYCRLF)   any of the three above
98      (*ANY)       all Unicode newline sequences
99    .sp
100    These override the default and the options given to the compiling function. For
101    example, on a Unix system where LF is the default newline sequence, the pattern
102    .sp
103      (*CR)a.b
104    .sp
105    changes the convention to CR. That pattern matches "a\enb" because LF is no
106    longer a newline. Note that these special settings, which are not
107    Perl-compatible, are recognized only at the very start of a pattern, and that
108    they must be in upper case. If more than one of them is present, the last one
109    is used.
110    .P
111    The newline convention affects the interpretation of the dot metacharacter when
112    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
113    affect what the \eR escape sequence matches. By default, this is any Unicode
114    newline sequence, for Perl compatibility. However, this can be changed; see the
115    description of \eR in the section entitled
116    .\" HTML <a href="#newlineseq">
117    .\" </a>
118    "Newline sequences"
119    .\"
120    below. A change of \eR setting can be combined with a change of newline
121    convention.
122    .
123    .
124  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
125  .rs  .rs
126  .sp  .sp
# Line 49  corresponding characters in the subject. Line 132  corresponding characters in the subject.
132  .sp  .sp
133  matches a portion of a subject string that is identical to itself. When  matches a portion of a subject string that is identical to itself. When
134  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
135  independently of case. In UTF-8 mode, PCRE always understands the concept of  independently of case. In a UTF mode, PCRE always understands the concept of
136  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
137  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
138  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
139  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching for characters 128 and above, you must
140  ensure that PCRE is compiled with Unicode property support as well as with  ensure that PCRE is compiled with Unicode property support as well as with
141  UTF-8 support.  UTF support.
142  .P  .P
143  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
144  and repetitions in the pattern. These are encoded in the pattern by the use of  and repetitions in the pattern. These are encoded in the pattern by the use of
# Line 101  The following sections describe the use Line 184  The following sections describe the use
184  .rs  .rs
185  .sp  .sp
186  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
187  non-alphanumeric character, it takes away any special meaning that character  character that is not a number or a letter, it takes away any special meaning
188  may have. This use of backslash as an escape character applies both inside and  that character may have. This use of backslash as an escape character applies
189  outside character classes.  both inside and outside character classes.
190  .P  .P
191  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.
192  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 111  otherwise be interpreted as a metacharac Line 194  otherwise be interpreted as a metacharac
194  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
195  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
196  .P  .P
197    In a UTF mode, only ASCII numbers and letters have any special meaning after a
198    backslash. All other characters (in particular, those whose codepoints are
199    greater than 127) are treated as literals.
200    .P
201  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
202  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
203  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
# Line 130  Perl, $ and @ cause variable interpolati Line 217  Perl, $ and @ cause variable interpolati
217    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
218  .sp  .sp
219  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
220    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
221    by \eE later in the pattern, the literal interpretation continues to the end of
222    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
223    a character class, this causes an error, because the character class is not
224    terminated.
225  .  .
226  .  .
227  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 139  The \eQ...\eE sequence is recognized bot Line 231  The \eQ...\eE sequence is recognized bot
231  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
232  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
233  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
234  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
235  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:  
236  .sp  .sp
237    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
238    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
239    \ee        escape (hex 1B)    \ee        escape (hex 1B)
240    \ef        formfeed (hex 0C)    \ef        formfeed (hex 0C)
241    \en        newline (hex 0A)    \en        linefeed (hex 0A)
242    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
243    \et        tab (hex 09)    \et        tab (hex 09)
244    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
245    \exhh      character with hex code hh    \exhh      character with hex code hh
246    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
247      \euhhhh    character with hex code hhhh (JavaScript mode only)
248  .sp  .sp
249  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
250  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.
251  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
252  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
253  .P  than 127, a compile-time error occurs. This locks out non-ASCII characters in
254  After \ex, from zero to two hexadecimal digits are read (letters can be in  all modes. (When PCRE is compiled in EBCDIC mode, all byte values are valid. A
255  upper or lower case). Any number of hexadecimal digits may appear between \ex{  lower case letter is converted to upper case, and then the 0xc0 bits are
256  and }, but the value of the character code must be less than 256 in non-UTF-8  flipped.)
257  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  .P
258  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  By default, after \ex, from zero to two hexadecimal digits are read (letters
259  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
260  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  between \ex{ and }, but the character code is constrained as follows:
261  with no following digits, giving a character whose value is zero.  .sp
262      8-bit non-UTF mode    less than 0x100
263      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
264      16-bit non-UTF mode   less than 0x10000
265      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
266    .sp
267    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
268    "surrogate" codepoints).
269    .P
270    If characters other than hexadecimal digits appear between \ex{ and }, or if
271    there is no terminating }, this form of escape is not recognized. Instead, the
272    initial \ex will be interpreted as a basic hexadecimal escape, with no
273    following digits, giving a character whose value is zero.
274    .P
275    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
276    as just described only when it is followed by two hexadecimal digits.
277    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
278    code points greater than 256 is provided by \eu, which must be followed by
279    four hexadecimal digits; otherwise it matches a literal "u" character.
280  .P  .P
281  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
282  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
283  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
284    \eu00dc in JavaScript mode).
285  .P  .P
286  After \e0 up to two further octal digits are read. If there are fewer than two  After \e0 up to two further octal digits are read. If there are fewer than two
287  digits, just those that are present are used. Thus the sequence \e0\ex\e07  digits, just those that are present are used. Thus the sequence \e0\ex\e07
# Line 196  parenthesized subpatterns. Line 307  parenthesized subpatterns.
307  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
308  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
309  digits following the backslash, and uses them to generate a data character. Any  digits following the backslash, and uses them to generate a data character. Any
310  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  subsequent digits stand for themselves. The value of the character is
311  character specified in octal must be less than \e400. In UTF-8 mode, values up  constrained in the same way as characters specified in hexadecimal.
312  to \e777 are permitted. For example:  For example:
313  .sp  .sp
314    \e040   is another way of writing a space    \e040   is another way of writing a space
315  .\" JOIN  .\" JOIN
# Line 215  to \e777 are permitted. For example: Line 326  to \e777 are permitted. For example:
326              character with octal code 113              character with octal code 113
327  .\" JOIN  .\" JOIN
328    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
329              the byte consisting entirely of 1 bits              the value 255 (decimal)
330  .\" JOIN  .\" JOIN
331    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
332              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
# Line 224  Note that octal values of 100 or greater Line 335  Note that octal values of 100 or greater
335  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
336  .P  .P
337  All the sequences that define a single character value can be used both inside  All the sequences that define a single character value can be used both inside
338  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
339  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
340  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
341  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
342  meanings  inside a character class. Like other unrecognized escape sequences, they are
343  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
344  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
345  (see below).  sequences have different meanings.
346  .\"  .
347    .
348    .SS "Unsupported escape sequences"
349    .rs
350    .sp
351    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
352    handler and used to modify the case of following characters. By default, PCRE
353    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
354    option is set, \eU matches a "U" character, and \eu can be used to define a
355    character by code point, as described in the previous section.
356  .  .
357  .  .
358  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
359  .rs  .rs
360  .sp  .sp
361  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
362  in braces, is an absolute or relative back reference. Back references are  enclosed in braces, is an absolute or relative back reference. A named back
363  discussed  reference can be coded as \eg{name}. Back references are discussed
364  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
365  .\" </a>  .\" </a>
366  later,  later,
# Line 252  parenthesized subpatterns. Line 372  parenthesized subpatterns.
372  .\"  .\"
373  .  .
374  .  .
375    .SS "Absolute and relative subroutine calls"
376    .rs
377    .sp
378    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
379    a number enclosed either in angle brackets or single quotes, is an alternative
380    syntax for referencing a subpattern as a "subroutine". Details are discussed
381    .\" HTML <a href="#onigurumasubroutines">
382    .\" </a>
383    later.
384    .\"
385    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
386    synonymous. The former is a back reference; the latter is a
387    .\" HTML <a href="#subpatternsassubroutines">
388    .\" </a>
389    subroutine
390    .\"
391    call.
392    .
393    .
394    .\" HTML <a name="genericchartypes"></a>
395  .SS "Generic character types"  .SS "Generic character types"
396  .rs  .rs
397  .sp  .sp
398  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
399  .sp  .sp
400    \ed     any decimal digit    \ed     any decimal digit
401    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
402      \eh     any horizontal whitespace character
403      \eH     any character that is not a horizontal whitespace character
404    \es     any whitespace character    \es     any whitespace character
405    \eS     any character that is not a whitespace character    \eS     any character that is not a whitespace character
406      \ev     any vertical whitespace character
407      \eV     any character that is not a vertical whitespace character
408    \ew     any "word" character    \ew     any "word" character
409    \eW     any "non-word" character    \eW     any "non-word" character
410  .sp  .sp
411  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.
412  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
413    .\" HTML <a href="#fullstopdot">
414    .\" </a>
415    the "." metacharacter
416    .\"
417    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
418    PCRE does not support this.
419  .P  .P
420  These character type sequences can appear both inside and outside character  Each pair of lower and upper case escape sequences partitions the complete set
421    of characters into two disjoint sets. Any given character matches one, and only
422    one, of each pair. The sequences can appear both inside and outside character
423  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
424  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
425  there is no character to match.  there is no character to match.
426  .P  .P
427  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).
428  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
429  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
430  included in a Perl script, \es may match the VT character. In PCRE, it never  included in a Perl script, \es may match the VT character. In PCRE, it never
431  does.)  does.
432  .P  .P
433  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.
434  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
435  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
436  place (see  place (see
437  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 293  in the Line 444  in the
444  .\"  .\"
445  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
446  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
447  accented letters, and these are matched by \ew.  accented letters, and these are then matched by \ew. The use of locales with
448    Unicode is discouraged.
449  .P  .P
450  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  By default, in a UTF mode, characters with values greater than 128 never match
451  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
452  character property support is available. The use of locales with Unicode is  their original meanings from before UTF support was available, mainly for
453  discouraged.  efficiency reasons. However, if PCRE is compiled with Unicode property support,
454    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
455    properties are used to determine character types, as follows:
456    .sp
457      \ed  any character that \ep{Nd} matches (decimal digit)
458      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
459      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
460    .sp
461    The upper case escapes match the inverse sets of characters. Note that \ed
462    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
463    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
464    \eB because they are defined in terms of \ew and \eW. Matching these sequences
465    is noticeably slower when PCRE_UCP is set.
466    .P
467    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
468    release 5.10. In contrast to the other sequences, which match only ASCII
469    characters by default, these always match certain high-valued codepoints,
470    whether or not PCRE_UCP is set. The horizontal space characters are:
471    .sp
472      U+0009     Horizontal tab
473      U+0020     Space
474      U+00A0     Non-break space
475      U+1680     Ogham space mark
476      U+180E     Mongolian vowel separator
477      U+2000     En quad
478      U+2001     Em quad
479      U+2002     En space
480      U+2003     Em space
481      U+2004     Three-per-em space
482      U+2005     Four-per-em space
483      U+2006     Six-per-em space
484      U+2007     Figure space
485      U+2008     Punctuation space
486      U+2009     Thin space
487      U+200A     Hair space
488      U+202F     Narrow no-break space
489      U+205F     Medium mathematical space
490      U+3000     Ideographic space
491    .sp
492    The vertical space characters are:
493    .sp
494      U+000A     Linefeed
495      U+000B     Vertical tab
496      U+000C     Formfeed
497      U+000D     Carriage return
498      U+0085     Next line
499      U+2028     Line separator
500      U+2029     Paragraph separator
501    .sp
502    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
503    relevant.
504  .  .
505  .  .
506    .\" HTML <a name="newlineseq"></a>
507  .SS "Newline sequences"  .SS "Newline sequences"
508  .rs  .rs
509  .sp  .sp
510  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
511  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
512  the following:  following:
513  .sp  .sp
514    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
515  .sp  .sp
# Line 321  U+000B), FF (formfeed, U+000C), CR (carr Line 524  U+000B), FF (formfeed, U+000C), CR (carr
524  line, U+0085). The two-character sequence is treated as a single unit that  line, U+0085). The two-character sequence is treated as a single unit that
525  cannot be split.  cannot be split.
526  .P  .P
527  In UTF-8 mode, two additional characters whose codepoints are greater than 255  In other modes, two additional characters whose codepoints are greater than 255
528  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).  are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
529  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
530  recognized.  recognized.
531  .P  .P
532  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
533    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
534    either at compile time or when the pattern is matched. (BSR is an abbrevation
535    for "backslash R".) This can be made the default when PCRE is built; if this is
536    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
537    It is also possible to specify these settings by starting a pattern string with
538    one of the following sequences:
539    .sp
540      (*BSR_ANYCRLF)   CR, LF, or CRLF only
541      (*BSR_UNICODE)   any Unicode newline sequence
542    .sp
543    These override the default and the options given to the compiling function, but
544    they can themselves be overridden by options given to a matching function. Note
545    that these special settings, which are not Perl-compatible, are recognized only
546    at the very start of a pattern, and that they must be in upper case. If more
547    than one of them is present, the last one is used. They can be combined with a
548    change of newline convention; for example, a pattern can start with:
549    .sp
550      (*ANY)(*BSR_ANYCRLF)
551    .sp
552    They can also be combined with the (*UTF8), (*UTF16), or (*UCP) special
553    sequences. Inside a character class, \eR is treated as an unrecognized escape
554    sequence, and so matches the letter "R" by default, but causes an error if
555    PCRE_EXTRA is set.
556  .  .
557  .  .
558  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 334  Inside a character class, \eR matches th Line 560  Inside a character class, \eR matches th
560  .rs  .rs
561  .sp  .sp
562  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
563  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
564  is selected. They are:  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
565    characters whose codepoints are less than 256, but they do work in this mode.
566    The extra escape sequences are:
567  .sp  .sp
568    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
569    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
570    \eX       an extended Unicode sequence    \eX       an extended Unicode sequence
571  .sp  .sp
572  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
573  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
574  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
575  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
576  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
577    .\" </a>
578    next section).
579    .\"
580    Other Perl properties such as "InMusicalSymbols" are not currently supported by
581    PCRE. Note that \eP{Any} does not match any characters, so always causes a
582    match failure.
583  .P  .P
584  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
585  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 359  Those that are not part of an identified Line 593  Those that are not part of an identified
593  .P  .P
594  Arabic,  Arabic,
595  Armenian,  Armenian,
596    Avestan,
597  Balinese,  Balinese,
598    Bamum,
599  Bengali,  Bengali,
600  Bopomofo,  Bopomofo,
601  Braille,  Braille,
602  Buginese,  Buginese,
603  Buhid,  Buhid,
604  Canadian_Aboriginal,  Canadian_Aboriginal,
605    Carian,
606    Cham,
607  Cherokee,  Cherokee,
608  Common,  Common,
609  Coptic,  Coptic,
# Line 374  Cypriot, Line 612  Cypriot,
612  Cyrillic,  Cyrillic,
613  Deseret,  Deseret,
614  Devanagari,  Devanagari,
615    Egyptian_Hieroglyphs,
616  Ethiopic,  Ethiopic,
617  Georgian,  Georgian,
618  Glagolitic,  Glagolitic,
# Line 386  Hangul, Line 625  Hangul,
625  Hanunoo,  Hanunoo,
626  Hebrew,  Hebrew,
627  Hiragana,  Hiragana,
628    Imperial_Aramaic,
629  Inherited,  Inherited,
630    Inscriptional_Pahlavi,
631    Inscriptional_Parthian,
632    Javanese,
633    Kaithi,
634  Kannada,  Kannada,
635  Katakana,  Katakana,
636    Kayah_Li,
637  Kharoshthi,  Kharoshthi,
638  Khmer,  Khmer,
639  Lao,  Lao,
640  Latin,  Latin,
641    Lepcha,
642  Limbu,  Limbu,
643  Linear_B,  Linear_B,
644    Lisu,
645    Lycian,
646    Lydian,
647  Malayalam,  Malayalam,
648    Meetei_Mayek,
649  Mongolian,  Mongolian,
650  Myanmar,  Myanmar,
651  New_Tai_Lue,  New_Tai_Lue,
# Line 403  Nko, Line 653  Nko,
653  Ogham,  Ogham,
654  Old_Italic,  Old_Italic,
655  Old_Persian,  Old_Persian,
656    Old_South_Arabian,
657    Old_Turkic,
658    Ol_Chiki,
659  Oriya,  Oriya,
660  Osmanya,  Osmanya,
661  Phags_Pa,  Phags_Pa,
662  Phoenician,  Phoenician,
663    Rejang,
664  Runic,  Runic,
665    Samaritan,
666    Saurashtra,
667  Shavian,  Shavian,
668  Sinhala,  Sinhala,
669    Sundanese,
670  Syloti_Nagri,  Syloti_Nagri,
671  Syriac,  Syriac,
672  Tagalog,  Tagalog,
673  Tagbanwa,  Tagbanwa,
674  Tai_Le,  Tai_Le,
675    Tai_Tham,
676    Tai_Viet,
677  Tamil,  Tamil,
678  Telugu,  Telugu,
679  Thaana,  Thaana,
# Line 422  Thai, Line 681  Thai,
681  Tibetan,  Tibetan,
682  Tifinagh,  Tifinagh,
683  Ugaritic,  Ugaritic,
684    Vai,
685  Yi.  Yi.
686  .P  .P
687  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
688  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
689  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
690  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
691  .P  .P
692  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
693  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 487  The special property L& is also supporte Line 747  The special property L& is also supporte
747  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
748  a modifier or "other".  a modifier or "other".
749  .P  .P
750  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
751    U+DFFF. Such characters are not valid in Unicode strings and so
752    cannot be tested by PCRE, unless UTF validity checking has been turned off
753    (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
754    .\" HREF
755    \fBpcreapi\fP
756    .\"
757    page). Perl does not support the Cs property.
758    .P
759    The long synonyms for property names that Perl supports (such as \ep{Letter})
760  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
761  properties with "Is".  properties with "Is".
762  .P  .P
# Line 511  atomic group Line 780  atomic group
780  (see below).  (see below).
781  .\"  .\"
782  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
783  preceding character.  preceding character. None of them have codepoints less than 256, so in
784    8-bit non-UTF-8 mode \eX matches any one character.
785    .P
786    Note that recent versions of Perl have changed \eX to match what Unicode calls
787    an "extended grapheme cluster", which has a more complicated definition.
788  .P  .P
789  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
790  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
791  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
792  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
793    PCRE_UCP option or by starting the pattern with (*UCP).
794    .
795    .
796    .\" HTML <a name="extraprops"></a>
797    .SS PCRE's additional properties
798    .rs
799    .sp
800    As well as the standard Unicode properties described in the previous
801    section, PCRE supports four more that make it possible to convert traditional
802    escape sequences such as \ew and \es and POSIX character classes to use Unicode
803    properties. PCRE uses these non-standard, non-Perl properties internally when
804    PCRE_UCP is set. They are:
805    .sp
806      Xan   Any alphanumeric character
807      Xps   Any POSIX space character
808      Xsp   Any Perl space character
809      Xwd   Any Perl "word" character
810    .sp
811    Xan matches characters that have either the L (letter) or the N (number)
812    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
813    carriage return, and any other character that has the Z (separator) property.
814    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
815    same characters as Xan, plus underscore.
816    .
817    .
818    .\" HTML <a name="resetmatchstart"></a>
819    .SS "Resetting the match start"
820    .rs
821    .sp
822    The escape sequence \eK causes any previously matched characters not to be
823    included in the final matched sequence. For example, the pattern:
824    .sp
825      foo\eKbar
826    .sp
827    matches "foobar", but reports that it has matched "bar". This feature is
828    similar to a lookbehind assertion
829    .\" HTML <a href="#lookbehind">
830    .\" </a>
831    (described below).
832    .\"
833    However, in this case, the part of the subject before the real match does not
834    have to be of fixed length, as lookbehind assertions do. The use of \eK does
835    not interfere with the setting of
836    .\" HTML <a href="#subpattern">
837    .\" </a>
838    captured substrings.
839    .\"
840    For example, when the pattern
841    .sp
842      (foo)\eKbar
843    .sp
844    matches "foobar", the first substring is still set to "foo".
845    .P
846    Perl documents that the use of \eK within assertions is "not well defined". In
847    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
848    ignored in negative assertions.
849  .  .
850  .  .
851  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 541  The backslashed assertions are: Line 870  The backslashed assertions are:
870    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
871    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
872  .sp  .sp
873  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
874  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
875    default it matches the corresponding literal character (for example, \eB
876    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
877    escape sequence" error is generated instead.
878  .P  .P
879  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
880  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
881  \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
882  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
883    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
884    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
885    of word" or "end of word" metasequence. However, whatever follows \eb normally
886    determines which it is. For example, the fragment \eba matches "a" at the start
887    of a word.
888  .P  .P
889  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
890  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 631  end of the subject in both modes, and if Line 968  end of the subject in both modes, and if
968  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
969  .  .
970  .  .
971  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
972    .SH "FULL STOP (PERIOD, DOT) AND \eN"
973  .rs  .rs
974  .sp  .sp
975  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
976  the subject string except (by default) a character that signifies the end of a  the subject string except (by default) a character that signifies the end of a
977  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
978  .P  .P
979  When a line ending is defined as a single character, dot never matches that  When a line ending is defined as a single character, dot never matches that
980  character; when the two-character sequence CRLF is used, dot does not match CR  character; when the two-character sequence CRLF is used, dot does not match CR
# Line 653  to match it. Line 991  to match it.
991  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
992  dollar, the only relationship being that they both involve newlines. Dot has no  dollar, the only relationship being that they both involve newlines. Dot has no
993  special meaning in a character class.  special meaning in a character class.
994  .  .P
995  .  The escape sequence \eN behaves like a dot, except that it is not affected by
996  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
997  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
998  .sp  name; PCRE does not support this.
999  Outside a character class, the escape sequence \eC matches any one byte, both  .
1000  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1001  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1002  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1003  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1004  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1005    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1006    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1007    matches line-ending characters. The feature is provided in Perl in order to
1008    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1009    used. Because \eC breaks up characters into individual data units, matching one
1010    unit with \eC in a UTF mode means that the rest of the string may start with a
1011    malformed UTF character. This has undefined results, because PCRE assumes that
1012    it is dealing with valid UTF strings (and by default it checks this at the
1013    start of processing unless the PCRE_NO_UTF8_CHECK option is used).
1014  .P  .P
1015  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1016  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1017  .\" </a>  .\" </a>
1018  (described below),  (described below)
1019  .\"  .\"
1020  because in UTF-8 mode this would make it impossible to calculate the length of  in a UTF mode, because this would make it impossible to calculate the length of
1021  the lookbehind.  the lookbehind.
1022    .P
1023    In general, the \eC escape sequence is best avoided. However, one
1024    way of using it that avoids the problem of malformed UTF characters is to use a
1025    lookahead to check the length of the next character, as in this pattern, which
1026    could be used with a UTF-8 string (ignore white space and line breaks):
1027    .sp
1028      (?| (?=[\ex00-\ex7f])(\eC) |
1029          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1030          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1031          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1032    .sp
1033    A group that starts with (?| resets the capturing parentheses numbers in each
1034    alternative (see
1035    .\" HTML <a href="#dupsubpatternnumber">
1036    .\" </a>
1037    "Duplicate Subpattern Numbers"
1038    .\"
1039    below). The assertions at the start of each branch check the next UTF-8
1040    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1041    character's individual bytes are then captured by the appropriate number of
1042    groups.
1043  .  .
1044  .  .
1045  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 679  the lookbehind. Line 1047  the lookbehind.
1047  .rs  .rs
1048  .sp  .sp
1049  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1050  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.
1051  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
1052  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
1053  escaped with a backslash.  a member of the class, it should be the first data character in the class
1054  .P  (after an initial circumflex, if present) or escaped with a backslash.
1055  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1056  character may occupy more than one byte. A matched character must be in the set  A character class matches a single character in the subject. In a UTF mode, the
1057  of characters defined by the class, unless the first character in the class  character may be more than one data unit long. A matched character must be in
1058  definition is a circumflex, in which case the subject character must not be in  the set of characters defined by the class, unless the first character in the
1059  the set defined by the class. If a circumflex is actually required as a member  class definition is a circumflex, in which case the subject character must not
1060  of the class, ensure it is not the first character, or escape it with a  be in the set defined by the class. If a circumflex is actually required as a
1061    member of the class, ensure it is not the first character, or escape it with a
1062  backslash.  backslash.
1063  .P  .P
1064  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1065  [^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
1066  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1067  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
1068  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
1069  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
1070  string.  string.
1071  .P  .P
1072  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16) mode, characters with values greater than 255 (0xffff) can be
1073  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  included in a class as a literal string of data units, or by using the \ex{
1074    escaping mechanism.
1075  .P  .P
1076  When caseless matching is set, any letters in a class represent both their  When caseless matching is set, any letters in a class represent both their
1077  upper case and lower case versions, so for example, a caseless [aeiou] matches  upper case and lower case versions, so for example, a caseless [aeiou] matches
1078  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a  "A" as well as "a", and a caseless [^aeiou] does not match "A", whereas a
1079  caseful version would. In UTF-8 mode, PCRE always understands the concept of  caseful version would. In a UTF mode, PCRE always understands the concept of
1080  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
1081  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1082  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1083  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in a UTF mode for characters 128 and
1084  ensure that PCRE is compiled with Unicode property support as well as with  above, you must ensure that PCRE is compiled with Unicode property support as
1085  UTF-8 support.  well as with UTF support.
1086  .P  .P
1087  Characters that might indicate line breaks are never treated in any special way  Characters that might indicate line breaks are never treated in any special way
1088  when matching character classes, whatever line-ending sequence is in use, and  when matching character classes, whatever line-ending sequence is in use, and
# Line 734  followed by two other characters. The oc Line 1104  followed by two other characters. The oc
1104  "]" can also be used to end a range.  "]" can also be used to end a range.
1105  .P  .P
1106  Ranges operate in the collating sequence of character values. They can also be  Ranges operate in the collating sequence of character values. They can also be
1107  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1108  mode, ranges can include characters whose values are greater than 255, for  can include any characters that are valid for the current mode.
 example [\ex{100}-\ex{2ff}].  
1109  .P  .P
1110  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
1111  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
1112  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character  [][\e\e^_`wxyzabc], matched caselessly, and in a non-UTF mode, if character
1113  tables for a French locale are in use, [\exc8-\excb] matches accented E  tables for a French locale are in use, [\exc8-\excb] matches accented E
1114  characters in both cases. In UTF-8 mode, PCRE supports the concept of case for  characters in both cases. In UTF modes, PCRE supports the concept of case for
1115  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
1116  property support.  property support.
1117  .P  .P
1118  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,
1119  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
1120  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1121  conveniently be used with the upper case character types to specify a more  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1122  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
1123  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1124    .\" HTML <a href="#genericchartypes">
1125    .\" </a>
1126    "Generic character types"
1127    .\"
1128    above. The escape sequence \eb has a different meaning inside a character
1129    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1130    are not special inside a character class. Like any other unrecognized escape
1131    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1132    default, but cause an error if the PCRE_EXTRA option is set.
1133    .P
1134    A circumflex can conveniently be used with the upper case character types to
1135    specify a more restricted set of characters than the matching lower case type.
1136    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1137    whereas [\ew] includes underscore. A positive character class should be read as
1138    "something OR something OR ..." and a negative class as "NOT something AND NOT
1139    something AND NOT ...".
1140  .P  .P
1141  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1142  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 771  this notation. For example, Line 1156  this notation. For example,
1156    [01[:alpha:]%]    [01[:alpha:]%]
1157  .sp  .sp
1158  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1159  are  are:
1160  .sp  .sp
1161    alnum    letters and digits    alnum    letters and digits
1162    alpha    letters    alpha    letters
# Line 782  are Line 1167  are
1167    graph    printing characters, excluding space    graph    printing characters, excluding space
1168    lower    lower case letters    lower    lower case letters
1169    print    printing characters, including space    print    printing characters, including space
1170    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1171    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1172    upper    upper case letters    upper    upper case letters
1173    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 803  matches "1", "2", or any non-digit. PCRE Line 1188  matches "1", "2", or any non-digit. PCRE
1188  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
1189  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1190  .P  .P
1191  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1192  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1193    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1194    character properties are used. This is achieved by replacing the POSIX classes
1195    by other sequences, as follows:
1196    .sp
1197      [:alnum:]  becomes  \ep{Xan}
1198      [:alpha:]  becomes  \ep{L}
1199      [:blank:]  becomes  \eh
1200      [:digit:]  becomes  \ep{Nd}
1201      [:lower:]  becomes  \ep{Ll}
1202      [:space:]  becomes  \ep{Xps}
1203      [:upper:]  becomes  \ep{Lu}
1204      [:word:]   becomes  \ep{Xwd}
1205    .sp
1206    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1207    classes are unchanged, and match only characters with code points less than
1208    128.
1209  .  .
1210  .  .
1211  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 831  alternative in the subpattern. Line 1232  alternative in the subpattern.
1232  .rs  .rs
1233  .sp  .sp
1234  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1235  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
1236  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1237    The option letters are
1238  .sp  .sp
1239    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1240    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 846  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1248  PCRE_MULTILINE while unsetting PCRE_DOTA
1248  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
1249  unset.  unset.
1250  .P  .P
1251  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
1252  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
1253  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1254  the global options (and it will therefore show up in data extracted by the  .P
1255  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1256    subpattern parentheses), the change applies to the remainder of the pattern
1257    that follows. If the change is placed right at the start of a pattern, PCRE
1258    extracts it into the global options (and it will therefore show up in data
1259    extracted by the \fBpcre_fullinfo()\fP function).
1260  .P  .P
1261  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1262  subpatterns) affects only that part of the current pattern that follows it, so  subpatterns) affects only that part of the subpattern that follows it, so
1263  .sp  .sp
1264    (a(?i)b)c    (a(?i)b)c
1265  .sp  .sp
# Line 869  branch is abandoned before the option se Line 1275  branch is abandoned before the option se
1275  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1276  behaviour otherwise.  behaviour otherwise.
1277  .P  .P
1278  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1279  changed in the same way as the Perl-compatible options by using the characters  application when the compiling or matching functions are called. In some cases
1280  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1281    what the application has set or what has been defaulted. Details are given in
1282    the section entitled
1283    .\" HTML <a href="#newlineseq">
1284    .\" </a>
1285    "Newline sequences"
1286    .\"
1287    above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1288    can be used to set UTF and Unicode property modes; they are equivalent to
1289    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1290  .  .
1291  .  .
1292  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 885  Turning part of a pattern into a subpatt Line 1300  Turning part of a pattern into a subpatt
1300  .sp  .sp
1301    cat(aract|erpillar|)    cat(aract|erpillar|)
1302  .sp  .sp
1303  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1304  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1305  .sp  .sp
1306  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
1307  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
1308  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 the
1309  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1310  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1311  .P  .P
1312  For example, if the string "the red king" is matched against the pattern  Opening parentheses are counted from left to right (starting from 1) to obtain
1313    numbers for the capturing subpatterns. For example, if the string "the red
1314    king" is matched against the pattern
1315  .sp  .sp
1316    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1317  .sp  .sp
# Line 926  is reached, an option setting in one bra Line 1343  is reached, an option setting in one bra
1343  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1344  .  .
1345  .  .
1346    .\" HTML <a name="dupsubpatternnumber"></a>
1347    .SH "DUPLICATE SUBPATTERN NUMBERS"
1348    .rs
1349    .sp
1350    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1351    the same numbers for its capturing parentheses. Such a subpattern starts with
1352    (?| and is itself a non-capturing subpattern. For example, consider this
1353    pattern:
1354    .sp
1355      (?|(Sat)ur|(Sun))day
1356    .sp
1357    Because the two alternatives are inside a (?| group, both sets of capturing
1358    parentheses are numbered one. Thus, when the pattern matches, you can look
1359    at captured substring number one, whichever alternative matched. This construct
1360    is useful when you want to capture part, but not all, of one of a number of
1361    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1362    number is reset at the start of each branch. The numbers of any capturing
1363    parentheses that follow the subpattern start after the highest number used in
1364    any branch. The following example is taken from the Perl documentation. The
1365    numbers underneath show in which buffer the captured content will be stored.
1366    .sp
1367      # before  ---------------branch-reset----------- after
1368      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1369      # 1            2         2  3        2     3     4
1370    .sp
1371    A back reference to a numbered subpattern uses the most recent value that is
1372    set for that number by any subpattern. The following pattern matches "abcabc"
1373    or "defdef":
1374    .sp
1375      /(?|(abc)|(def))\e1/
1376    .sp
1377    In contrast, a subroutine call to a numbered subpattern always refers to the
1378    first one in the pattern with the given number. The following pattern matches
1379    "abcabc" or "defabc":
1380    .sp
1381      /(?|(abc)|(def))(?1)/
1382    .sp
1383    If a
1384    .\" HTML <a href="#conditions">
1385    .\" </a>
1386    condition test
1387    .\"
1388    for a subpattern's having matched refers to a non-unique number, the test is
1389    true if any of the subpatterns of that number have matched.
1390    .P
1391    An alternative approach to using this "branch reset" feature is to use
1392    duplicate named subpatterns, as described in the next section.
1393    .
1394    .
1395  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1396  .rs  .rs
1397  .sp  .sp
# Line 935  if an expression is modified, the number Line 1401  if an expression is modified, the number
1401  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1402  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1403  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1404  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1405    have different names, but PCRE does not.
1406  .P  .P
1407  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1408  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1409  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1410  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1411  .\" </a>  .\" </a>
1412  backreferences,  back references,
1413  .\"  .\"
1414  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1415  .\" </a>  .\" </a>
# Line 962  extracting the name-to-number translatio Line 1429  extracting the name-to-number translatio
1429  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1430  .P  .P
1431  By default, a name must be unique within a pattern, but it is possible to relax  By default, a name must be unique within a pattern, but it is possible to relax
1432  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1433  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1434  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1435  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1436  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1437    name, and in both cases you want to extract the abbreviation. This pattern
1438    (ignoring the line breaks) does the job:
1439  .sp  .sp
1440    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1441    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 975  abbreviation. This pattern (ignoring the Line 1444  abbreviation. This pattern (ignoring the
1444    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1445  .sp  .sp
1446  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1447    (An alternative way of solving this problem is to use a "branch reset"
1448    subpattern, as described in the previous section.)
1449    .P
1450  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1451  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1452  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1453  make a reference to a non-unique named subpattern from elsewhere in the  .P
1454  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1455  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1456    used. In the absence of duplicate numbers (see the previous section) this is
1457    the one with the lowest number. If you use a named reference in a condition
1458    test (see the
1459    .\"
1460    .\" HTML <a href="#conditions">
1461    .\" </a>
1462    section about conditions
1463    .\"
1464    below), either to check whether a subpattern has matched, or to check for
1465    recursion, all subpatterns with the same name are tested. If the condition is
1466    true for any one of them, the overall condition is true. This is the same
1467    behaviour as testing by number. For further details of the interfaces for
1468    handling named subpatterns, see the
1469  .\" HREF  .\" HREF
1470  \fBpcreapi\fP  \fBpcreapi\fP
1471  .\"  .\"
1472  documentation.  documentation.
1473    .P
1474    \fBWarning:\fP You cannot use different names to distinguish between two
1475    subpatterns with the same number because PCRE uses only the numbers when
1476    matching. For this reason, an error is given at compile time if different names
1477    are given to subpatterns with the same number. However, you can give the same
1478    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1479  .  .
1480  .  .
1481  .SH REPETITION  .SH REPETITION
# Line 996  items: Line 1487  items:
1487    a literal data character    a literal data character
1488    the dot metacharacter    the dot metacharacter
1489    the \eC escape sequence    the \eC escape sequence
1490    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1491    the \eR escape sequence    the \eR escape sequence
1492    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1493    a character class    a character class
1494    a back reference (see next section)    a back reference (see next section)
1495    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1496      a subroutine call to a subpattern (recursive or otherwise)
1497  .sp  .sp
1498  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1499  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1026  where a quantifier is not allowed, or on Line 1518  where a quantifier is not allowed, or on
1518  quantifier, is taken as a literal character. For example, {,6} is not a  quantifier, is taken as a literal character. For example, {,6} is not a
1519  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1520  .P  .P
1521  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF modes, quantifiers apply to characters rather than to individual data
1522  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of  units. Thus, for example, \ex{100}{2} matches two characters, each of
1523  which is represented by a two-byte sequence. Similarly, when Unicode property  which is represented by a two-byte sequence in a UTF-8 string. Similarly,
1524  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended sequences, each of which may be several
1525  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1526  .P  .P
1527  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
1528  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1529    subpatterns that are referenced as
1530    .\" HTML <a href="#subpatternsassubroutines">
1531    .\" </a>
1532    subroutines
1533    .\"
1534    from elsewhere in the pattern (but see also the section entitled
1535    .\" HTML <a href="#subdefine">
1536    .\" </a>
1537    "Defining subpatterns for use by reference only"
1538    .\"
1539    below). Items other than subpatterns that have a {0} quantifier are omitted
1540    from the compiled pattern.
1541  .P  .P
1542  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1543  abbreviations:  abbreviations:
# Line 1105  worth setting PCRE_DOTALL in order to ob Line 1609  worth setting PCRE_DOTALL in order to ob
1609  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1610  .P  .P
1611  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1612  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1613  elsewhere in the pattern, a match at the start may fail where a later one  elsewhere in the pattern, a match at the start may fail where a later one
1614  succeeds. Consider, for example:  succeeds. Consider, for example:
1615  .sp  .sp
# Line 1180  previous example can be rewritten as Line 1684  previous example can be rewritten as
1684  .sp  .sp
1685    \ed++foo    \ed++foo
1686  .sp  .sp
1687    Note that a possessive quantifier can be used with an entire group, for
1688    example:
1689    .sp
1690      (abc|xyz){2,3}+
1691    .sp
1692  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1693  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
1694  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1253  no such problem when named parentheses a Line 1762  no such problem when named parentheses a
1762  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1763  .P  .P
1764  Another way of avoiding the ambiguity inherent in the use of digits following a  Another way of avoiding the ambiguity inherent in the use of digits following a
1765  backslash is to use the \eg escape sequence, which is a feature introduced in  backslash is to use the \eg escape sequence. This escape must be followed by an
1766  Perl 5.10. This escape must be followed by a positive or a negative number,  unsigned number or a negative number, optionally enclosed in braces. These
1767  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1768  .sp  .sp
1769    (ring), \e1    (ring), \e1
1770    (ring), \eg1    (ring), \eg1
1771    (ring), \eg{1}    (ring), \eg{1}
1772  .sp  .sp
1773  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1774  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1775  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1776    example:
1777  .sp  .sp
1778    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1779  .sp  .sp
1780  The sequence \eg{-1} is a reference to the most recently started capturing  The sequence \eg{-1} is a reference to the most recently started capturing
1781  subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}  subpattern before \eg, that is, is it equivalent to \e2 in this example.
1782  would be equivalent to \e1. The use of relative references can be helpful in  Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1783  long patterns, and also in patterns that are created by joining together  can be helpful in long patterns, and also in patterns that are created by
1784  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1785  .P  .P
1786  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1787  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1293  back reference, the case of letters is r Line 1803  back reference, the case of letters is r
1803  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
1804  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1805  .P  .P
1806  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'  There are several different ways of writing back references to named
1807  or the Python syntax (?P=name). We could rewrite the above example in either of  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1808    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1809    back reference syntax, in which \eg can be used for both numeric and named
1810    references, is also supported. We could rewrite the above example in any of
1811  the following ways:  the following ways:
1812  .sp  .sp
1813    (?<p1>(?i)rah)\es+\ek<p1>    (?<p1>(?i)rah)\es+\ek<p1>
1814      (?'p1'(?i)rah)\es+\ek{p1}
1815    (?P<p1>(?i)rah)\es+(?P=p1)    (?P<p1>(?i)rah)\es+(?P=p1)
1816      (?<p1>(?i)rah)\es+\eg{p1}
1817  .sp  .sp
1818  A subpattern that is referenced by name may appear in the pattern before or  A subpattern that is referenced by name may appear in the pattern before or
1819  after the reference.  after the reference.
1820  .P  .P
1821  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
1822  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1823  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1824  .sp  .sp
1825    (a|(bc))\e2    (a|(bc))\e2
1826  .sp  .sp
1827  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
1828  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
1829  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1830  with a digit character, some delimiter must be used to terminate the back  .P
1831  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1832  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1833    If the pattern continues with a digit character, some delimiter must be used to
1834    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1835    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1836  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1837  .\" </a>  .\" </a>
1838  "Comments"  "Comments"
1839  .\"  .\"
1840  below) can be used.  below) can be used.
1841  .P  .
1842    .SS "Recursive back references"
1843    .rs
1844    .sp
1845  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
1846  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.
1847  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1334  to the previous iteration. In order for Line 1855  to the previous iteration. In order for
1855  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
1856  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
1857  minimum of zero.  minimum of zero.
1858    .P
1859    Back references of this type cause the group that they reference to be treated
1860    as an
1861    .\" HTML <a href="#atomicgroup">
1862    .\" </a>
1863    atomic group.
1864    .\"
1865    Once the whole group has been matched, a subsequent matching failure cannot
1866    cause backtracking into the middle of the group.
1867  .  .
1868  .  .
1869  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1353  those that look ahead of the current pos Line 1883  those that look ahead of the current pos
1883  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,
1884  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.
1885  .P  .P
1886  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1887  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
1888  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1889  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1890  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1891  because it does not make sense for negative assertions.  .P
1892    For compatibility with Perl, assertion subpatterns may be repeated; though
1893    it makes no sense to assert the same thing several times, the side effect of
1894    capturing parentheses may occasionally be useful. In practice, there only three
1895    cases:
1896    .sp
1897    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1898    However, it may contain internal capturing parenthesized groups that are called
1899    from elsewhere via the
1900    .\" HTML <a href="#subpatternsassubroutines">
1901    .\" </a>
1902    subroutine mechanism.
1903    .\"
1904    .sp
1905    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1906    were {0,1}. At run time, the rest of the pattern match is tried with and
1907    without the assertion, the order depending on the greediness of the quantifier.
1908    .sp
1909    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1910    The assertion is obeyed just once when encountered during matching.
1911  .  .
1912  .  .
1913  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1387  lookbehind assertion is needed to achiev Line 1936  lookbehind assertion is needed to achiev
1936  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
1937  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
1938  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.
1939    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1940  .  .
1941  .  .
1942  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1411  is permitted, but Line 1961  is permitted, but
1961  .sp  .sp
1962  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1963  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
1964  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
1965  match the same length of string. An assertion such as  length of string. An assertion such as
1966  .sp  .sp
1967    (?<=ab(c|de))    (?<=ab(c|de))
1968  .sp  .sp
1969  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
1970  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
1971    branches:
1972  .sp  .sp
1973    (?<=abc|abde)    (?<=abc|abde)
1974  .sp  .sp
1975    In some cases, the escape sequence \eK
1976    .\" HTML <a href="#resetmatchstart">
1977    .\" </a>
1978    (see above)
1979    .\"
1980    can be used instead of a lookbehind assertion to get round the fixed-length
1981    restriction.
1982    .P
1983  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1984  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
1985  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1986  assertion fails.  assertion fails.
1987  .P  .P
1988  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)  In a UTF mode, PCRE does not allow the \eC escape (which matches a single data
1989  to appear in lookbehind assertions, because it makes it impossible to calculate  unit even in a UTF mode) to appear in lookbehind assertions, because it makes
1990  the length of the lookbehind. The \eX and \eR escapes, which can match  it impossible to calculate the length of the lookbehind. The \eX and \eR
1991  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
1992    permitted.
1993    .P
1994    .\" HTML <a href="#subpatternsassubroutines">
1995    .\" </a>
1996    "Subroutine"
1997    .\"
1998    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1999    as the subpattern matches a fixed-length string.
2000    .\" HTML <a href="#recursion">
2001    .\" </a>
2002    Recursion,
2003    .\"
2004    however, is not supported.
2005  .P  .P
2006  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2007  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
2008  pattern such as  strings. Consider a simple pattern such as
2009  .sp  .sp
2010    abcd$    abcd$
2011  .sp  .sp
# Line 1497  characters that are not "999". Line 2069  characters that are not "999".
2069  .sp  .sp
2070  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2071  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2072  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2073  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2074  .sp  .sp
2075    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2076    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2077  .sp  .sp
2078  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2079  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
2080  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2081    itself contain nested subpatterns of any form, including conditional
2082    subpatterns; the restriction to two alternatives applies only at the level of
2083    the condition. This pattern fragment is an example where the alternatives are
2084    complex:
2085    .sp
2086      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2087    .sp
2088  .P  .P
2089  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2090  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1514  recursion, a pseudo-condition called DEF Line 2093  recursion, a pseudo-condition called DEF
2093  .rs  .rs
2094  .sp  .sp
2095  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
2096  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2097  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
2098  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2099  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2100  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2101  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2102    section about duplicate subpattern numbers),
2103    .\"
2104    the condition is true if any of them have matched. An alternative notation is
2105    to precede the digits with a plus or minus sign. In this case, the subpattern
2106    number is relative rather than absolute. The most recently opened parentheses
2107    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2108    loops it can also make sense to refer to subsequent groups. The next
2109    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2110    zero in any of these forms is not used; it provokes a compile-time error.)
2111  .P  .P
2112  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2113  make it more readable (assume the PCRE_EXTENDED option) and to divide it into  make it more readable (assume the PCRE_EXTENDED option) and to divide it into
# Line 1530  three parts for ease of discussion: Line 2118  three parts for ease of discussion:
2118  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2119  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
2120  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
2121  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2122  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,
2123  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
2124  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2125  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2126  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2127  .P  .P
2128  If you were embedding this pattern in a larger one, you could use a relative  If you were embedding this pattern in a larger one, you could use a relative
2129  reference:  reference:
2130  .sp  .sp
2131    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...    ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
# Line 1560  Rewriting the above example to use a nam Line 2148  Rewriting the above example to use a nam
2148  .sp  .sp
2149    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2150  .sp  .sp
2151    If the name used in a condition of this kind is a duplicate, the test is
2152    applied to all subpatterns of the same name, and is true if any one of them has
2153    matched.
2154  .  .
2155  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2156  .rs  .rs
# Line 1571  letter R, for example: Line 2162  letter R, for example:
2162  .sp  .sp
2163    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2164  .sp  .sp
2165  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2166  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2167  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2168    applied to all subpatterns of the same name, and is true if any one of them is
2169    the most recent recursion.
2170  .P  .P
2171  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2172  patterns are described below.  .\" HTML <a href="#recursion">
2173    .\" </a>
2174    The syntax for recursive patterns
2175    .\"
2176    is described below.
2177  .  .
2178    .\" HTML <a name="subdefine"></a>
2179  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2180  .rs  .rs
2181  .sp  .sp
# Line 1585  If the condition is the string (DEFINE), Line 2183  If the condition is the string (DEFINE),
2183  name DEFINE, the condition is always false. In this case, there may be only one  name DEFINE, the condition is always false. In this case, there may be only one
2184  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2185  point in the pattern; the idea of DEFINE is that it can be used to define  point in the pattern; the idea of DEFINE is that it can be used to define
2186  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2187  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2188  written like this (ignore whitespace and line breaks):  .\" </a>
2189    subroutines
2190    .\"
2191    is described below.) For example, a pattern to match an IPv4 address such as
2192    "192.168.23.245" could be written like this (ignore whitespace and line
2193    breaks):
2194  .sp  .sp
2195    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )    (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2196    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1595  written like this (ignore whitespace and Line 2198  written like this (ignore whitespace and
2198  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2199  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2200  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2201  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2202  .P  pattern uses references to the named group to match the four dot-separated
2203  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2204  .  .
2205  .SS "Assertion conditions"  .SS "Assertion conditions"
2206  .rs  .rs
# Line 1624  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2225  dd-aaa-dd or dd-dd-dd, where aaa are let
2225  .SH COMMENTS  .SH COMMENTS
2226  .rs  .rs
2227  .sp  .sp
2228  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
2229  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,
2230  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
2231    subpattern name or number. The characters that make up a comment play no part
2232    in the pattern matching.
2233  .P  .P
2234  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
2235  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2236  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2237    this case continues to immediately after the next newline character or
2238    character sequence in the pattern. Which characters are interpreted as newlines
2239    is controlled by the options passed to a compiling function or by a special
2240    sequence at the start of the pattern, as described in the section entitled
2241    .\" HTML <a href="#newlines">
2242    .\" </a>
2243    "Newline conventions"
2244    .\"
2245    above. Note that the end of this type of comment is a literal newline sequence
2246    in the pattern; escape sequences that happen to represent a newline do not
2247    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2248    default newline convention is in force:
2249    .sp
2250      abc #comment \en still comment
2251    .sp
2252    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2253    a newline in the pattern. The sequence \en is still literal at this stage, so
2254    it does not terminate the comment. Only an actual character with the code value
2255    0x0a (the default newline) does so.
2256  .  .
2257  .  .
2258  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1656  recursively to the pattern in which it a Line 2278  recursively to the pattern in which it a
2278  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2279  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2280  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2281  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2282  .P  .P
2283  A special item that consists of (? followed by a number greater than zero and a  A special item that consists of (? followed by a number greater than zero and a
2284  closing parenthesis is a recursive call of the subpattern of the given number,  closing parenthesis is a recursive subroutine call of the subpattern of the
2285  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  given number, provided that it occurs inside that subpattern. (If not, it is a
2286    .\" HTML <a href="#subpatternsassubroutines">
2287    .\" </a>
2288    non-recursive subroutine
2289    .\"
2290  call, which is described in the next section.) The special item (?R) or (?0) is  call, which is described in the next section.) The special item (?R) or (?0) is
2291  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2292  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2293  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2294  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2295  .sp  .sp
2296    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2297  .sp  .sp
2298  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2299  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
2300  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2301  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2302    to avoid backtracking into sequences of non-parentheses.
2303  .P  .P
2304  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
2305  pattern, so instead you could use this:  pattern, so instead you could use this:
2306  .sp  .sp
2307    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2308  .sp  .sp
2309  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
2310  them instead of the whole pattern.  them instead of the whole pattern.
2311  .P  .P
2312  In a larger pattern, keeping track of parenthesis numbers can be tricky. This  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2313  is made easier by the use of relative references. (A Perl 5.10 feature.)  is made easier by the use of relative references. Instead of (?1) in the
2314  Instead of (?1) in the pattern above you can write (?-2) to refer to the second  pattern above you can write (?-2) to refer to the second most recently opened
2315  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2316  negative number counts capturing parentheses leftwards from the point at which  capturing parentheses leftwards from the point at which it is encountered.
 it is encountered.  
2317  .P  .P
2318  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2319  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2320  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2321  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2322    .\" </a>
2323    non-recursive subroutine
2324    .\"
2325    calls, as described in the next section.
2326  .P  .P
2327  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2328  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2329  could rewrite the above example as follows:  could rewrite the above example as follows:
2330  .sp  .sp
2331    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2332  .sp  .sp
2333  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2334  used.  used.
2335  .P  .P
2336  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2337  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2338  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2339  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2340  .sp  .sp
2341    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2342  .sp  .sp
2343  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,
2344  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
2345  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
2346  before failure can be reported.  before failure can be reported.
2347  .P  .P
2348  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
2349  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
2350  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2351  .\" HREF  .\" HREF
2352  \fBpcrecallout\fP  \fBpcrecallout\fP
2353  .\"  .\"
# Line 1731  documentation). If the pattern above is Line 2355  documentation). If the pattern above is
2355  .sp  .sp
2356    (ab(cd)ef)    (ab(cd)ef)
2357  .sp  .sp
2358  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
2359  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
2360  .sp  matched at the top level, its final captured value is unset, even if it was
2361    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2362       ^                        ^  .P
2363       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2364  .sp  obtain extra memory to store data during a recursion, which it does by using
2365  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
2366  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.  
2367  .P  .P
2368  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.
2369  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1756  different alternatives for the recursive Line 2377  different alternatives for the recursive
2377  is the actual recursive call.  is the actual recursive call.
2378  .  .
2379  .  .
2380    .\" HTML <a name="recursiondifference"></a>
2381    .SS "Differences in recursion processing between PCRE and Perl"
2382    .rs
2383    .sp
2384    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2385    (like Python, but unlike Perl), a recursive subpattern call is always treated
2386    as an atomic group. That is, once it has matched some of the subject string, it
2387    is never re-entered, even if it contains untried alternatives and there is a
2388    subsequent matching failure. This can be illustrated by the following pattern,
2389    which purports to match a palindromic string that contains an odd number of
2390    characters (for example, "a", "aba", "abcba", "abcdcba"):
2391    .sp
2392      ^(.|(.)(?1)\e2)$
2393    .sp
2394    The idea is that it either matches a single character, or two identical
2395    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2396    it does not if the pattern is longer than three characters. Consider the
2397    subject string "abcba":
2398    .P
2399    At the top level, the first character is matched, but as it is not at the end
2400    of the string, the first alternative fails; the second alternative is taken
2401    and the recursion kicks in. The recursive call to subpattern 1 successfully
2402    matches the next character ("b"). (Note that the beginning and end of line
2403    tests are not part of the recursion).
2404    .P
2405    Back at the top level, the next character ("c") is compared with what
2406    subpattern 2 matched, which was "a". This fails. Because the recursion is
2407    treated as an atomic group, there are now no backtracking points, and so the
2408    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2409    try the second alternative.) However, if the pattern is written with the
2410    alternatives in the other order, things are different:
2411    .sp
2412      ^((.)(?1)\e2|.)$
2413    .sp
2414    This time, the recursing alternative is tried first, and continues to recurse
2415    until it runs out of characters, at which point the recursion fails. But this
2416    time we do have another alternative to try at the higher level. That is the big
2417    difference: in the previous case the remaining alternative is at a deeper
2418    recursion level, which PCRE cannot use.
2419    .P
2420    To change the pattern so that it matches all palindromic strings, not just
2421    those with an odd number of characters, it is tempting to change the pattern to
2422    this:
2423    .sp
2424      ^((.)(?1)\e2|.?)$
2425    .sp
2426    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2427    deeper recursion has matched a single character, it cannot be entered again in
2428    order to match an empty string. The solution is to separate the two cases, and
2429    write out the odd and even cases as alternatives at the higher level:
2430    .sp
2431      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2432    .sp
2433    If you want to match typical palindromic phrases, the pattern has to ignore all
2434    non-word characters, which can be done like this:
2435    .sp
2436      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2437    .sp
2438    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2439    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2440    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2441    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2442    more) to match typical phrases, and Perl takes so long that you think it has
2443    gone into a loop.
2444    .P
2445    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2446    string does not start with a palindrome that is shorter than the entire string.
2447    For example, although "abcba" is correctly matched, if the subject is "ababa",
2448    PCRE finds the palindrome "aba" at the start, then fails at top level because
2449    the end of the string does not follow. Once again, it cannot jump back into the
2450    recursion to try other alternatives, so the entire match fails.
2451    .P
2452    The second way in which PCRE and Perl differ in their recursion processing is
2453    in the handling of captured values. In Perl, when a subpattern is called
2454    recursively or as a subpattern (see the next section), it has no access to any
2455    values that were captured outside the recursion, whereas in PCRE these values
2456    can be referenced. Consider this pattern:
2457    .sp
2458      ^(.)(\e1|a(?2))
2459    .sp
2460    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2461    then in the second group, when the back reference \e1 fails to match "b", the
2462    second alternative matches "a" and then recurses. In the recursion, \e1 does
2463    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2464    match because inside the recursive call \e1 cannot access the externally set
2465    value.
2466    .
2467    .
2468  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2469  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2470  .rs  .rs
2471  .sp  .sp
2472  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
2473  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
2474  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2475  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2476  relative, as in these examples:  relative, as in these examples:
2477  .sp  .sp
2478    (...(absolute)...)...(?2)...    (...(absolute)...)...(?2)...
2479    (...(relative)...)...(?-1)...    (...(relative)...)...(?-1)...
2480    (...(?+1)...(relative)...    (...(?+1)...(relative)...
2481  .sp  .sp
2482  An earlier example pointed out that the pattern  An earlier example pointed out that the pattern
2483  .sp  .sp
# Line 1782  matches "sense and sensibility" and "res Line 2491  matches "sense and sensibility" and "res
2491  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
2492  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2493  .P  .P
2494  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2495  group. That is, once it has matched some of the subject string, it is never  groups. That is, once a subroutine has matched some of the subject string, it
2496  re-entered, even if it contains untried alternatives and there is a subsequent  is never re-entered, even if it contains untried alternatives and there is a
2497  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2498  .P  subroutine call revert to their previous values afterwards.
2499  When a subpattern is used as a subroutine, processing options such as  .P
2500  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2501  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2502    different calls. For example, consider this pattern:
2503  .sp  .sp
2504    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2505  .sp  .sp
# Line 1797  It matches "abcabc". It does not match " Line 2507  It matches "abcabc". It does not match "
2507  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2508  .  .
2509  .  .
2510    .\" HTML <a name="onigurumasubroutines"></a>
2511    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2512    .rs
2513    .sp
2514    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2515    a number enclosed either in angle brackets or single quotes, is an alternative
2516    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2517    are two of the examples used above, rewritten using this syntax:
2518    .sp
2519      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2520      (sens|respons)e and \eg'1'ibility
2521    .sp
2522    PCRE supports an extension to Oniguruma: if a number is preceded by a
2523    plus or a minus sign it is taken as a relative reference. For example:
2524    .sp
2525      (abc)(?i:\eg<-1>)
2526    .sp
2527    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2528    synonymous. The former is a back reference; the latter is a subroutine call.
2529    .
2530    .
2531  .SH CALLOUTS  .SH CALLOUTS
2532  .rs  .rs
2533  .sp  .sp
# Line 1807  same pair of parentheses when there is a Line 2538  same pair of parentheses when there is a
2538  .P  .P
2539  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl  PCRE provides a similar feature, but of course it cannot obey arbitrary Perl
2540  code. The feature is called "callout". The caller of PCRE provides an external  code. The feature is called "callout". The caller of PCRE provides an external
2541  function by putting its entry point in the global variable \fIpcre_callout\fP.  function by putting its entry point in the global variable \fIpcre_callout\fP
2542  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2543    variable contains NULL, which disables all calling out.
2544  .P  .P
2545  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2546  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
# Line 1817  For example, this pattern has two callou Line 2549  For example, this pattern has two callou
2549  .sp  .sp
2550    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2551  .sp  .sp
2552  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are  If the PCRE_AUTO_CALLOUT flag is passed to a compiling function, callouts are
2553  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2554  255.  255.
2555  .P  .P
2556  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  During matching, when PCRE reaches a callout point, the external function is
2557  set), the external function is called. It is provided with the number of the  called. It is provided with the number of the callout, the position in the
2558  callout, the position in the pattern, and, optionally, one item of data  pattern, and, optionally, one item of data originally supplied by the caller of
2559  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2560  may cause matching to proceed, to backtrack, or to fail altogether. A complete  backtrack, or to fail altogether. A complete description of the interface to
2561  description of the interface to the callout function is given in the  the callout function is given in the
2562  .\" HREF  .\" HREF
2563  \fBpcrecallout\fP  \fBpcrecallout\fP
2564  .\"  .\"
2565  documentation.  documentation.
2566  .  .
2567  .  .
2568    .\" HTML <a name="backtrackcontrol"></a>
2569    .SH "BACKTRACKING CONTROL"
2570    .rs
2571    .sp
2572    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2573    are described in the Perl documentation as "experimental and subject to change
2574    or removal in a future version of Perl". It goes on to say: "Their usage in
2575    production code should be noted to avoid problems during upgrades." The same
2576    remarks apply to the PCRE features described in this section.
2577    .P
2578    Since these verbs are specifically related to backtracking, most of them can be
2579    used only when the pattern is to be matched using one of the traditional
2580    matching functions, which use a backtracking algorithm. With the exception of
2581    (*FAIL), which behaves like a failing negative assertion, they cause an error
2582    if encountered by a DFA matching function.
2583    .P
2584    If any of these verbs are used in an assertion or in a subpattern that is
2585    called as a subroutine (whether or not recursively), their effect is confined
2586    to that subpattern; it does not extend to the surrounding pattern, with one
2587    exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2588    a successful positive assertion \fIis\fP passed back when a match succeeds
2589    (compare capturing parentheses in assertions). Note that such subpatterns are
2590    processed as anchored at the point where they are tested. Note also that Perl's
2591    treatment of subroutines is different in some cases.
2592    .P
2593    The new verbs make use of what was previously invalid syntax: an opening
2594    parenthesis followed by an asterisk. They are generally of the form
2595    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2596    depending on whether or not an argument is present. A name is any sequence of
2597    characters that does not include a closing parenthesis. If the name is empty,
2598    that is, if the closing parenthesis immediately follows the colon, the effect
2599    is as if the colon were not there. Any number of these verbs may occur in a
2600    pattern.
2601    .P
2602    PCRE contains some optimizations that are used to speed up matching by running
2603    some checks at the start of each match attempt. For example, it may know the
2604    minimum length of matching subject, or that a particular character must be
2605    present. When one of these optimizations suppresses the running of a match, any
2606    included backtracking verbs will not, of course, be processed. You can suppress
2607    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2608    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2609    pattern with (*NO_START_OPT).
2610    .P
2611    Experiments with Perl suggest that it too has similar optimizations, sometimes
2612    leading to anomalous results.
2613    .
2614    .
2615    .SS "Verbs that act immediately"
2616    .rs
2617    .sp
2618    The following verbs act as soon as they are encountered. They may not be
2619    followed by a name.
2620    .sp
2621       (*ACCEPT)
2622    .sp
2623    This verb causes the match to end successfully, skipping the remainder of the
2624    pattern. However, when it is inside a subpattern that is called as a
2625    subroutine, only that subpattern is ended successfully. Matching then continues
2626    at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2627    far is captured. For example:
2628    .sp
2629      A((?:A|B(*ACCEPT)|C)D)
2630    .sp
2631    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2632    the outer parentheses.
2633    .sp
2634      (*FAIL) or (*F)
2635    .sp
2636    This verb causes a matching failure, forcing backtracking to occur. It is
2637    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2638    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2639    Perl features that are not present in PCRE. The nearest equivalent is the
2640    callout feature, as for example in this pattern:
2641    .sp
2642      a+(?C)(*FAIL)
2643    .sp
2644    A match with the string "aaaa" always fails, but the callout is taken before
2645    each backtrack happens (in this example, 10 times).
2646    .
2647    .
2648    .SS "Recording which path was taken"
2649    .rs
2650    .sp
2651    There is one verb whose main purpose is to track how a match was arrived at,
2652    though it also has a secondary use in conjunction with advancing the match
2653    starting point (see (*SKIP) below).
2654    .sp
2655      (*MARK:NAME) or (*:NAME)
2656    .sp
2657    A name is always required with this verb. There may be as many instances of
2658    (*MARK) as you like in a pattern, and their names do not have to be unique.
2659    .P
2660    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2661    path is passed back to the caller as described in the section entitled
2662    .\" HTML <a href="pcreapi.html#extradata">
2663    .\" </a>
2664    "Extra data for \fBpcre_exec()\fP"
2665    .\"
2666    in the
2667    .\" HREF
2668    \fBpcreapi\fP
2669    .\"
2670    documentation. Here is an example of \fBpcretest\fP output, where the /K
2671    modifier requests the retrieval and outputting of (*MARK) data:
2672    .sp
2673        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2674      data> XY
2675       0: XY
2676      MK: A
2677      XZ
2678       0: XZ
2679      MK: B
2680    .sp
2681    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2682    indicates which of the two alternatives matched. This is a more efficient way
2683    of obtaining this information than putting each alternative in its own
2684    capturing parentheses.
2685    .P
2686    If (*MARK) is encountered in a positive assertion, its name is recorded and
2687    passed back if it is the last-encountered. This does not happen for negative
2688    assertions.
2689    .P
2690    After a partial match or a failed match, the name of the last encountered
2691    (*MARK) in the entire match process is returned. For example:
2692    .sp
2693        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2694      data> XP
2695      No match, mark = B
2696    .sp
2697    Note that in this unanchored example the mark is retained from the match
2698    attempt that started at the letter "X". Subsequent match attempts starting at
2699    "P" and then with an empty string do not get as far as the (*MARK) item, but
2700    nevertheless do not reset it.
2701    .
2702    .
2703    .SS "Verbs that act after backtracking"
2704    .rs
2705    .sp
2706    The following verbs do nothing when they are encountered. Matching continues
2707    with what follows, but if there is no subsequent match, causing a backtrack to
2708    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2709    the verb. However, when one of these verbs appears inside an atomic group, its
2710    effect is confined to that group, because once the group has been matched,
2711    there is never any backtracking into it. In this situation, backtracking can
2712    "jump back" to the left of the entire atomic group. (Remember also, as stated
2713    above, that this localization also applies in subroutine calls and assertions.)
2714    .P
2715    These verbs differ in exactly what kind of failure occurs when backtracking
2716    reaches them.
2717    .sp
2718      (*COMMIT)
2719    .sp
2720    This verb, which may not be followed by a name, causes the whole match to fail
2721    outright if the rest of the pattern does not match. Even if the pattern is
2722    unanchored, no further attempts to find a match by advancing the starting point
2723    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2724    finding a match at the current starting point, or not at all. For example:
2725    .sp
2726      a+(*COMMIT)b
2727    .sp
2728    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2729    dynamic anchor, or "I've started, so I must finish." The name of the most
2730    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2731    match failure.
2732    .P
2733    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2734    unless PCRE's start-of-match optimizations are turned off, as shown in this
2735    \fBpcretest\fP example:
2736    .sp
2737        re> /(*COMMIT)abc/
2738      data> xyzabc
2739       0: abc
2740      xyzabc\eY
2741      No match
2742    .sp
2743    PCRE knows that any match must start with "a", so the optimization skips along
2744    the subject to "a" before running the first match attempt, which succeeds. When
2745    the optimization is disabled by the \eY escape in the second subject, the match
2746    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2747    starting points.
2748    .sp
2749      (*PRUNE) or (*PRUNE:NAME)
2750    .sp
2751    This verb causes the match to fail at the current starting position in the
2752    subject if the rest of the pattern does not match. If the pattern is
2753    unanchored, the normal "bumpalong" advance to the next starting character then
2754    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2755    reached, or when matching to the right of (*PRUNE), but if there is no match to
2756    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2757    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2758    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2759    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2760    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2761    .sp
2762      (*SKIP)
2763    .sp
2764    This verb, when given without a name, is like (*PRUNE), except that if the
2765    pattern is unanchored, the "bumpalong" advance is not to the next character,
2766    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2767    signifies that whatever text was matched leading up to it cannot be part of a
2768    successful match. Consider:
2769    .sp
2770      a+(*SKIP)b
2771    .sp
2772    If the subject is "aaaac...", after the first match attempt fails (starting at
2773    the first character in the string), the starting point skips on to start the
2774    next attempt at "c". Note that a possessive quantifer does not have the same
2775    effect as this example; although it would suppress backtracking during the
2776    first match attempt, the second attempt would start at the second character
2777    instead of skipping on to "c".
2778    .sp
2779      (*SKIP:NAME)
2780    .sp
2781    When (*SKIP) has an associated name, its behaviour is modified. If the
2782    following pattern fails to match, the previous path through the pattern is
2783    searched for the most recent (*MARK) that has the same name. If one is found,
2784    the "bumpalong" advance is to the subject position that corresponds to that
2785    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2786    matching name is found, the (*SKIP) is ignored.
2787    .sp
2788      (*THEN) or (*THEN:NAME)
2789    .sp
2790    This verb causes a skip to the next innermost alternative if the rest of the
2791    pattern does not match. That is, it cancels pending backtracking, but only
2792    within the current alternative. Its name comes from the observation that it can
2793    be used for a pattern-based if-then-else block:
2794    .sp
2795      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2796    .sp
2797    If the COND1 pattern matches, FOO is tried (and possibly further items after
2798    the end of the group if FOO succeeds); on failure, the matcher skips to the
2799    second alternative and tries COND2, without backtracking into COND1. The
2800    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2801    If (*THEN) is not inside an alternation, it acts like (*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"  .SH "SEE ALSO"
2859  .rs  .rs
2860  .sp  .sp
2861  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2862    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2863  .  .
2864  .  .
2865  .SH AUTHOR  .SH AUTHOR
# Line 1853  Cambridge CB2 3QH, England. Line 2876  Cambridge CB2 3QH, England.
2876  .rs  .rs
2877  .sp  .sp
2878  .nf  .nf
2879  Last updated: 09 May 2007  Last updated: 09 January 2012
2880  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2881  .fi  .fi

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