/[pcre]/code/trunk/doc/pcrepattern.3
ViewVC logotype

Diff of /code/trunk/doc/pcrepattern.3

Parent Directory Parent Directory | Revision Log Revision Log | View Patch Patch

revision 451 by ph10, Wed Sep 16 11:05:51 2009 UTC revision 972 by ph10, Fri Jun 1 18:03:18 2012 UTC
# Line 1  Line 1 
1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "04 May 2012" "PCRE 8.31"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH "PCRE REGULAR EXPRESSION DETAILS"  .SH "PCRE REGULAR EXPRESSION DETAILS"
# Line 21  published by O'Reilly, covers regular ex Line 21  published by O'Reilly, covers regular ex
21  description of PCRE's regular expressions is intended as reference material.  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. There is also a special sequence that can be given at the  features, PCRE must be built to include appropriate support. When using UTF
27  start of a pattern:  strings you must either call the compiling function with the PCRE_UTF8 or
28    PCRE_UTF16 option, or the pattern must start with one of these special
29    sequences:
30  .sp  .sp
31    (*UTF8)    (*UTF8)
32      (*UTF16)
33  .sp  .sp
34  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8  Starting a pattern with such a sequence is equivalent to setting the relevant
35  option. This feature is not Perl-compatible. How setting UTF-8 mode affects  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  pattern matching is mentioned in several places below. There is also a summary
37  of UTF-8 features in the  of features in the
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
 .\"  
 in the main  
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. Some of the features discussed below are not available when  a different algorithm that is not Perl-compatible. Some of the features
63  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  discussed below are not available when DFA matching is used. The advantages and
64  alternative function, and how it differs from the normal function, are  disadvantages of the alternative functions, and how they differ from the normal
65  discussed in the  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"  .SH "NEWLINE CONVENTIONS"
74  .rs  .rs
75  .sp  .sp
# Line 83  string with one of the following five se Line 97  string with one of the following five se
97    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
98    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
99  .sp  .sp
100  These override the default and the options given to \fBpcre_compile()\fP. For  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  example, on a Unix system where LF is the default newline sequence, the pattern
102  .sp  .sp
103    (*CR)a.b    (*CR)a.b
# Line 94  Perl-compatible, are recognized only at Line 108  Perl-compatible, are recognized only at
108  they must be in upper case. If more than one of them is present, the last one  they must be in upper case. If more than one of them is present, the last one
109  is used.  is used.
110  .P  .P
111  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects the interpretation of the dot metacharacter when
112  default, this is any Unicode newline sequence, for Perl compatibility. However,  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
113  this can be changed; see the description of \eR in the section entitled  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">  .\" HTML <a href="#newlineseq">
117  .\" </a>  .\" </a>
118  "Newline sequences"  "Newline sequences"
# Line 116  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 168  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 178  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  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  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, white space 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
204  be used to include a whitespace or # character as part of the pattern.  be used to include a white space or # character as part of the pattern.
205  .P  .P
206  If you want to remove the special meaning from a sequence of characters, you  If you want to remove the special meaning from a sequence of characters, you
207  can do so by putting them between \eQ and \eE. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
# Line 197  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 206  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        form feed (hex 0C)
241    \en        linefeed (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 value in  .P
258  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  By default, after \ex, from zero to two hexadecimal digits are read (letters
259  point, which is 10FFFF.  can be in upper or lower case). Any number of hexadecimal digits may appear
260    between \ex{ and }, but the character code is constrained as follows:
261    .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  .P
270  If characters other than hexadecimal digits appear between \ex{ and }, or if  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  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  initial \ex will be interpreted as a basic hexadecimal escape, with no
273  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
274  .P  .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
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 266  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 285  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 294  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"
# Line 333  syntax for referencing a subpattern as a Line 383  syntax for referencing a subpattern as a
383  later.  later.
384  .\"  .\"
385  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
386  synonymous. The former is a back reference; the latter is a subroutine call.  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    \eh     any horizontal white space character
403    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
404    \es     any whitespace character    \es     any white space character
405    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
406    \ev     any vertical whitespace character    \ev     any vertical white space character
407    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space 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  .P  .\" HTML <a href="#fullstopdot">
414  These character type sequences can appear both inside and outside character  .\" </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
420    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).
# Line 367  are HT (9), LF (10), FF (12), CR (13), a Line 430  are HT (9), LF (10), FF (12), CR (13), a
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  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  A "word" character is an underscore or any character that is a letter or digit.
434  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  By default, the definition of letters and digits is controlled by PCRE's
435  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
436  meanings from before UTF-8 support was available, mainly for efficiency  place (see
437  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
438  and \eW.  .\" </a>
439  .P  "Locale support"
440  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
441  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
442  The horizontal space characters are:  .\" HREF
443    \fBpcreapi\fP
444    .\"
445    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
447    accented letters, and these are then matched by \ew. The use of locales with
448    Unicode is discouraged.
449    .P
450    By default, in a UTF mode, characters with values greater than 128 never match
451    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
452    their original meanings from before UTF support was available, mainly for
453    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  .sp
472    U+0009     Horizontal tab    U+0009     Horizontal tab
473    U+0020     Space    U+0020     Space
# Line 402  The vertical space characters are: Line 493  The vertical space characters are:
493  .sp  .sp
494    U+000A     Linefeed    U+000A     Linefeed
495    U+000B     Vertical tab    U+000B     Vertical tab
496    U+000C     Formfeed    U+000C     Form feed
497    U+000D     Carriage return    U+000D     Carriage return
498    U+0085     Next line    U+0085     Next line
499    U+2028     Line separator    U+2028     Line separator
500    U+2029     Paragraph separator    U+2029     Paragraph separator
501  .P  .sp
502  A "word" character is an underscore or any character less than 256 that is a  In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
503  letter or digit. The definition of letters and digits is controlled by PCRE's  relevant.
 low-valued character tables, and may vary if locale-specific matching is taking  
 place (see  
 .\" HTML <a href="pcreapi.html#localesupport">  
 .\" </a>  
 "Locale support"  
 .\"  
 in the  
 .\" HREF  
 \fBpcreapi\fP  
 .\"  
 page). For example, in a French locale such as "fr_FR" in Unix-like systems,  
 or "french" in Windows, some character codes greater than 128 are used for  
 accented letters, and these are matched by \ew. The use of locales with Unicode  
 is discouraged.  
504  .  .
505  .  .
506  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 431  is discouraged. Line 508  is discouraged.
508  .rs  .rs
509  .sp  .sp
510  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
511  Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8 mode \eR is  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
512  equivalent to 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 443  below. Line 520  below.
520  .\"  .\"
521  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
522  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,  LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
523  U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next  U+000B), FF (form feed, U+000C), CR (carriage return, U+000D), or NEL (next
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.
# Line 463  one of the following sequences: Line 540  one of the following sequences:
540    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
541    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
542  .sp  .sp
543  These override the default and the options given to \fBpcre_compile()\fP, but  These override the default and the options given to the compiling function, but
544  they can be overridden by options given to \fBpcre_exec()\fP. Note that these  they can themselves be overridden by options given to a matching function. Note
545  special settings, which are not Perl-compatible, are recognized only at the  that these special settings, which are not Perl-compatible, are recognized only
546  very start of a pattern, and that they must be in upper case. If more than one  at the very start of a pattern, and that they must be in upper case. If more
547  of them is present, the last one is used. They can be combined with a change of  than one of them is present, the last one is used. They can be combined with a
548  newline convention, for example, a pattern can start with:  change of newline convention; for example, a pattern can start with:
549  .sp  .sp
550    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
551  .sp  .sp
552  Inside a character class, \eR matches the letter "R".  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 481  Inside a character class, \eR matches th Line 561  Inside a character class, \eR matches th
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 that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
564  When not in UTF-8 mode, these sequences are of course limited to testing  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.  characters whose codepoints are less than 256, but they do work in this mode.
566  The extra escape sequences are:  The extra escape sequences are:
567  .sp  .sp
# Line 490  The extra escape sequences are: Line 570  The extra escape sequences are:
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 507  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    Batak,
600  Bengali,  Bengali,
601  Bopomofo,  Bopomofo,
602    Brahmi,
603  Braille,  Braille,
604  Buginese,  Buginese,
605  Buhid,  Buhid,
606  Canadian_Aboriginal,  Canadian_Aboriginal,
607    Carian,
608    Chakma,
609    Cham,
610  Cherokee,  Cherokee,
611  Common,  Common,
612  Coptic,  Coptic,
# Line 522  Cypriot, Line 615  Cypriot,
615  Cyrillic,  Cyrillic,
616  Deseret,  Deseret,
617  Devanagari,  Devanagari,
618    Egyptian_Hieroglyphs,
619  Ethiopic,  Ethiopic,
620  Georgian,  Georgian,
621  Glagolitic,  Glagolitic,
# Line 534  Hangul, Line 628  Hangul,
628  Hanunoo,  Hanunoo,
629  Hebrew,  Hebrew,
630  Hiragana,  Hiragana,
631    Imperial_Aramaic,
632  Inherited,  Inherited,
633    Inscriptional_Pahlavi,
634    Inscriptional_Parthian,
635    Javanese,
636    Kaithi,
637  Kannada,  Kannada,
638  Katakana,  Katakana,
639    Kayah_Li,
640  Kharoshthi,  Kharoshthi,
641  Khmer,  Khmer,
642  Lao,  Lao,
643  Latin,  Latin,
644    Lepcha,
645  Limbu,  Limbu,
646  Linear_B,  Linear_B,
647    Lisu,
648    Lycian,
649    Lydian,
650  Malayalam,  Malayalam,
651    Mandaic,
652    Meetei_Mayek,
653    Meroitic_Cursive,
654    Meroitic_Hieroglyphs,
655    Miao,
656  Mongolian,  Mongolian,
657  Myanmar,  Myanmar,
658  New_Tai_Lue,  New_Tai_Lue,
# Line 551  Nko, Line 660  Nko,
660  Ogham,  Ogham,
661  Old_Italic,  Old_Italic,
662  Old_Persian,  Old_Persian,
663    Old_South_Arabian,
664    Old_Turkic,
665    Ol_Chiki,
666  Oriya,  Oriya,
667  Osmanya,  Osmanya,
668  Phags_Pa,  Phags_Pa,
669  Phoenician,  Phoenician,
670    Rejang,
671  Runic,  Runic,
672    Samaritan,
673    Saurashtra,
674    Sharada,
675  Shavian,  Shavian,
676  Sinhala,  Sinhala,
677    Sora_Sompeng,
678    Sundanese,
679  Syloti_Nagri,  Syloti_Nagri,
680  Syriac,  Syriac,
681  Tagalog,  Tagalog,
682  Tagbanwa,  Tagbanwa,
683  Tai_Le,  Tai_Le,
684    Tai_Tham,
685    Tai_Viet,
686    Takri,
687  Tamil,  Tamil,
688  Telugu,  Telugu,
689  Thaana,  Thaana,
# Line 570  Thai, Line 691  Thai,
691  Tibetan,  Tibetan,
692  Tifinagh,  Tifinagh,
693  Ugaritic,  Ugaritic,
694    Vai,
695  Yi.  Yi.
696  .P  .P
697  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
698  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
699  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
700  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
701  .P  .P
702  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
703  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 636  the Lu, Ll, or Lt property, in other wor Line 758  the Lu, Ll, or Lt property, in other wor
758  a modifier or "other".  a modifier or "other".
759  .P  .P
760  The Cs (Surrogate) property applies only to characters in the range U+D800 to  The Cs (Surrogate) property applies only to characters in the range U+D800 to
761  U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so  U+DFFF. Such characters are not valid in Unicode strings and so
762  cannot be tested by PCRE, unless UTF-8 validity checking has been turned off  cannot be tested by PCRE, unless UTF validity checking has been turned off
763  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK and PCRE_NO_UTF16_CHECK in the
764  .\" HREF  .\" HREF
765  \fBpcreapi\fP  \fBpcreapi\fP
766  .\"  .\"
# Line 669  atomic group Line 791  atomic group
791  .\"  .\"
792  Characters with the "mark" property are typically accents that affect the  Characters with the "mark" property are typically accents that affect the
793  preceding character. None of them have codepoints less than 256, so in  preceding character. None of them have codepoints less than 256, so in
794  non-UTF-8 mode \eX matches any one character.  8-bit non-UTF-8 mode \eX matches any one character.
795    .P
796    Note that recent versions of Perl have changed \eX to match what Unicode calls
797    an "extended grapheme cluster", which has a more complicated definition.
798  .P  .P
799  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
800  a structure that contains data for over fifteen thousand characters. That is  a structure that contains data for over fifteen thousand characters. That is
801  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
802  properties in PCRE.  properties in PCRE by default, though you can make them do so by setting the
803    PCRE_UCP option or by starting the pattern with (*UCP).
804    .
805    .
806    .\" HTML <a name="extraprops"></a>
807    .SS PCRE's additional properties
808    .rs
809    .sp
810    As well as the standard Unicode properties described in the previous
811    section, PCRE supports four more that make it possible to convert traditional
812    escape sequences such as \ew and \es and POSIX character classes to use Unicode
813    properties. PCRE uses these non-standard, non-Perl properties internally when
814    PCRE_UCP is set. They are:
815    .sp
816      Xan   Any alphanumeric character
817      Xps   Any POSIX space character
818      Xsp   Any Perl space character
819      Xwd   Any Perl "word" character
820    .sp
821    Xan matches characters that have either the L (letter) or the N (number)
822    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
823    carriage return, and any other character that has the Z (separator) property.
824    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
825    same characters as Xan, plus underscore.
826  .  .
827  .  .
828  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
829  .SS "Resetting the match start"  .SS "Resetting the match start"
830  .rs  .rs
831  .sp  .sp
832  The escape sequence \eK, which is a Perl 5.10 feature, causes any previously  The escape sequence \eK causes any previously matched characters not to be
833  matched characters not to be included in the final matched sequence. For  included in the final matched sequence. For example, the pattern:
 example, the pattern:  
834  .sp  .sp
835    foo\eKbar    foo\eKbar
836  .sp  .sp
# Line 705  For example, when the pattern Line 852  For example, when the pattern
852    (foo)\eKbar    (foo)\eKbar
853  .sp  .sp
854  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
855    .P
856    Perl documents that the use of \eK within assertions is "not well defined". In
857    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
858    ignored in negative assertions.
859  .  .
860  .  .
861  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 729  The backslashed assertions are: Line 880  The backslashed assertions are:
880    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
881    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
882  .sp  .sp
883  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
884  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
885    default it matches the corresponding literal character (for example, \eB
886    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
887    escape sequence" error is generated instead.
888  .P  .P
889  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
890  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
891  \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
892  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
893    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
894    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
895    of word" or "end of word" metasequence. However, whatever follows \eb normally
896    determines which it is. For example, the fragment \eba matches "a" at the start
897    of a word.
898  .P  .P
899  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
900  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 819  end of the subject in both modes, and if Line 978  end of the subject in both modes, and if
978  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
979  .  .
980  .  .
981  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
982    .SH "FULL STOP (PERIOD, DOT) AND \eN"
983  .rs  .rs
984  .sp  .sp
985  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
986  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
987  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
988  .P  .P
989  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
990  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 841  to match it. Line 1001  to match it.
1001  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
1002  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
1003  special meaning in a character class.  special meaning in a character class.
1004  .  .P
1005  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1006  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1007  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1008  .sp  name; PCRE does not support this.
1009  Outside a character class, the escape sequence \eC matches any one byte, both  .
1010  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1011  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1012  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1013  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1014  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1015    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1016    byte; in the 16-bit library it is a 16-bit unit. Unlike a dot, \eC always
1017    matches line-ending characters. The feature is provided in Perl in order to
1018    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1019    used. Because \eC breaks up characters into individual data units, matching one
1020    unit with \eC in a UTF mode means that the rest of the string may start with a
1021    malformed UTF character. This has undefined results, because PCRE assumes that
1022    it is dealing with valid UTF strings (and by default it checks this at the
1023    start of processing unless the PCRE_NO_UTF8_CHECK or PCRE_NO_UTF16_CHECK option
1024    is used).
1025  .P  .P
1026  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1027  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1028  .\" </a>  .\" </a>
1029  (described below),  (described below)
1030  .\"  .\"
1031  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
1032  the lookbehind.  the lookbehind.
1033    .P
1034    In general, the \eC escape sequence is best avoided. However, one
1035    way of using it that avoids the problem of malformed UTF characters is to use a
1036    lookahead to check the length of the next character, as in this pattern, which
1037    could be used with a UTF-8 string (ignore white space and line breaks):
1038    .sp
1039      (?| (?=[\ex00-\ex7f])(\eC) |
1040          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1041          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1042          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1043    .sp
1044    A group that starts with (?| resets the capturing parentheses numbers in each
1045    alternative (see
1046    .\" HTML <a href="#dupsubpatternnumber">
1047    .\" </a>
1048    "Duplicate Subpattern Numbers"
1049    .\"
1050    below). The assertions at the start of each branch check the next UTF-8
1051    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1052    character's individual bytes are then captured by the appropriate number of
1053    groups.
1054  .  .
1055  .  .
1056  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 867  the lookbehind. Line 1058  the lookbehind.
1058  .rs  .rs
1059  .sp  .sp
1060  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1061  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.
1062  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
1063  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
1064  escaped with a backslash.  a member of the class, it should be the first data character in the class
1065  .P  (after an initial circumflex, if present) or escaped with a backslash.
1066  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1067  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
1068  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
1069  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
1070  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
1071  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
1072    member of the class, ensure it is not the first character, or escape it with a
1073  backslash.  backslash.
1074  .P  .P
1075  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1076  [^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
1077  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1078  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
1079  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
1080  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
1081  string.  string.
1082  .P  .P
1083  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
1084  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{
1085    escaping mechanism.
1086  .P  .P
1087  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
1088  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
1089  "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
1090  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
1091  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
1092  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1093  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1094  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
1095  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
1096  UTF-8 support.  well as with UTF support.
1097  .P  .P
1098  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
1099  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 922  followed by two other characters. The oc Line 1115  followed by two other characters. The oc
1115  "]" can also be used to end a range.  "]" can also be used to end a range.
1116  .P  .P
1117  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
1118  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1119  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}].  
1120  .P  .P
1121  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
1122  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
1123  [][\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
1124  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
1125  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
1126  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
1127  property support.  property support.
1128  .P  .P
1129  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,
1130  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
1131  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1132  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
1133  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
1134  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1135    .\" HTML <a href="#genericchartypes">
1136    .\" </a>
1137    "Generic character types"
1138    .\"
1139    above. The escape sequence \eb has a different meaning inside a character
1140    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1141    are not special inside a character class. Like any other unrecognized escape
1142    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1143    default, but cause an error if the PCRE_EXTRA option is set.
1144    .P
1145    A circumflex can conveniently be used with the upper case character types to
1146    specify a more restricted set of characters than the matching lower case type.
1147    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1148    whereas [\ew] includes underscore. A positive character class should be read as
1149    "something OR something OR ..." and a negative class as "NOT something AND NOT
1150    something AND NOT ...".
1151  .P  .P
1152  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1153  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 959  this notation. For example, Line 1167  this notation. For example,
1167    [01[:alpha:]%]    [01[:alpha:]%]
1168  .sp  .sp
1169  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1170  are  are:
1171  .sp  .sp
1172    alnum    letters and digits    alnum    letters and digits
1173    alpha    letters    alpha    letters
# Line 970  are Line 1178  are
1178    graph    printing characters, excluding space    graph    printing characters, excluding space
1179    lower    lower case letters    lower    lower case letters
1180    print    printing characters, including space    print    printing characters, including space
1181    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1182    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1183    upper    upper case letters    upper    upper case letters
1184    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 991  matches "1", "2", or any non-digit. PCRE Line 1199  matches "1", "2", or any non-digit. PCRE
1199  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
1200  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1201  .P  .P
1202  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
1203  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1204    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1205    character properties are used. This is achieved by replacing the POSIX classes
1206    by other sequences, as follows:
1207    .sp
1208      [:alnum:]  becomes  \ep{Xan}
1209      [:alpha:]  becomes  \ep{L}
1210      [:blank:]  becomes  \eh
1211      [:digit:]  becomes  \ep{Nd}
1212      [:lower:]  becomes  \ep{Ll}
1213      [:space:]  becomes  \ep{Xps}
1214      [:upper:]  becomes  \ep{Lu}
1215      [:word:]   becomes  \ep{Xwd}
1216    .sp
1217    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1218    classes are unchanged, and match only characters with code points less than
1219    128.
1220  .  .
1221  .  .
1222  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1046  extracts it into the global options (and Line 1270  extracts it into the global options (and
1270  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1271  .P  .P
1272  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1273  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
1274  .sp  .sp
1275    (a(?i)b)c    (a(?i)b)c
1276  .sp  .sp
# Line 1063  option settings happen at compile time. Line 1287  option settings happen at compile time.
1287  behaviour otherwise.  behaviour otherwise.
1288  .P  .P
1289  \fBNote:\fP There are other PCRE-specific options that can be set by the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1290  application when the compile or match functions are called. In some cases the  application when the compiling or matching functions are called. In some cases
1291  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1292  the application has set or what has been defaulted. Details are given in the  what the application has set or what has been defaulted. Details are given in
1293  section entitled  the section entitled
1294  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1295  .\" </a>  .\" </a>
1296  "Newline sequences"  "Newline sequences"
1297  .\"  .\"
1298  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8), (*UTF16), and (*UCP) leading sequences that
1299  mode; this is equivalent to setting the PCRE_UTF8 option.  can be used to set UTF and Unicode property modes; they are equivalent to
1300    setting the PCRE_UTF8, PCRE_UTF16, and the PCRE_UCP options, respectively.
1301  .  .
1302  .  .
1303  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1086  Turning part of a pattern into a subpatt Line 1311  Turning part of a pattern into a subpatt
1311  .sp  .sp
1312    cat(aract|erpillar|)    cat(aract|erpillar|)
1313  .sp  .sp
1314  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1315  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1316  .sp  .sp
1317  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
1318  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
1319  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
1320  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1321  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1322  .P  .P
1323  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
1324    numbers for the capturing subpatterns. For example, if the string "the red
1325    king" is matched against the pattern
1326  .sp  .sp
1327    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1328  .sp  .sp
# Line 1127  is reached, an option setting in one bra Line 1354  is reached, an option setting in one bra
1354  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1355  .  .
1356  .  .
1357    .\" HTML <a name="dupsubpatternnumber"></a>
1358  .SH "DUPLICATE SUBPATTERN NUMBERS"  .SH "DUPLICATE SUBPATTERN NUMBERS"
1359  .rs  .rs
1360  .sp  .sp
# Line 1143  at captured substring number one, whiche Line 1371  at captured substring number one, whiche
1371  is useful when you want to capture part, but not all, of one of a number of  is useful when you want to capture part, but not all, of one of a number of
1372  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1373  number is reset at the start of each branch. The numbers of any capturing  number is reset at the start of each branch. The numbers of any capturing
1374  buffers that follow the subpattern start after the highest number used in any  parentheses that follow the subpattern start after the highest number used in
1375  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1376  The numbers underneath show in which buffer the captured content will be  numbers underneath show in which buffer the captured content will be stored.
 stored.  
1377  .sp  .sp
1378    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1379    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1380    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1381  .sp  .sp
1382  A backreference or a recursive call to a numbered subpattern always refers to  A back reference to a numbered subpattern uses the most recent value that is
1383  the first one in the pattern with the given number.  set for that number by any subpattern. The following pattern matches "abcabc"
1384    or "defdef":
1385    .sp
1386      /(?|(abc)|(def))\e1/
1387    .sp
1388    In contrast, a subroutine call to a numbered subpattern always refers to the
1389    first one in the pattern with the given number. The following pattern matches
1390    "abcabc" or "defabc":
1391    .sp
1392      /(?|(abc)|(def))(?1)/
1393    .sp
1394    If a
1395    .\" HTML <a href="#conditions">
1396    .\" </a>
1397    condition test
1398    .\"
1399    for a subpattern's having matched refers to a non-unique number, the test is
1400    true if any of the subpatterns of that number have matched.
1401  .P  .P
1402  An alternative approach to using this "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1403  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
# Line 1168  if an expression is modified, the number Line 1412  if an expression is modified, the number
1412  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1413  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
1414  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
1415  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1416    have different names, but PCRE does not.
1417  .P  .P
1418  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
1419  (?'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
1420  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1421  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1422  .\" </a>  .\" </a>
1423  backreferences,  back references,
1424  .\"  .\"
1425  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1426  .\" </a>  .\" </a>
# Line 1195  extracting the name-to-number translatio Line 1440  extracting the name-to-number translatio
1440  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1441  .P  .P
1442  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
1443  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
1444  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
1445  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
1446  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
1447  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
1448    name, and in both cases you want to extract the abbreviation. This pattern
1449    (ignoring the line breaks) does the job:
1450  .sp  .sp
1451    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1452    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1213  subpattern, as described in the previous Line 1460  subpattern, as described in the previous
1460  .P  .P
1461  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1462  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
1463  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1464  make a reference to a non-unique named subpattern from elsewhere in the  .P
1465  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
1466  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1467    used. In the absence of duplicate numbers (see the previous section) this is
1468    the one with the lowest number. If you use a named reference in a condition
1469    test (see the
1470    .\"
1471    .\" HTML <a href="#conditions">
1472    .\" </a>
1473    section about conditions
1474    .\"
1475    below), either to check whether a subpattern has matched, or to check for
1476    recursion, all subpatterns with the same name are tested. If the condition is
1477    true for any one of them, the overall condition is true. This is the same
1478    behaviour as testing by number. For further details of the interfaces for
1479    handling named subpatterns, see the
1480  .\" HREF  .\" HREF
1481  \fBpcreapi\fP  \fBpcreapi\fP
1482  .\"  .\"
1483  documentation.  documentation.
1484  .P  .P
1485  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1486  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1487  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1488    are given to subpatterns with the same number. However, you can give the same
1489    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1490  .  .
1491  .  .
1492  .SH REPETITION  .SH REPETITION
# Line 1236  items: Line 1498  items:
1498    a literal data character    a literal data character
1499    the dot metacharacter    the dot metacharacter
1500    the \eC escape sequence    the \eC escape sequence
1501    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1502    the \eR escape sequence    the \eR escape sequence
1503    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1504    a character class    a character class
1505    a back reference (see next section)    a back reference (see next section)
1506    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1507      a subroutine call to a subpattern (recursive or otherwise)
1508  .sp  .sp
1509  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1510  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1266  where a quantifier is not allowed, or on Line 1529  where a quantifier is not allowed, or on
1529  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
1530  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1531  .P  .P
1532  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
1533  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
1534  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,
1535  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
1536  which may be several bytes long (and they may be of different lengths).  data units long (and they may be of different lengths).
1537  .P  .P
1538  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
1539  previous item and the quantifier were not present. This may be useful for  previous item and the quantifier were not present. This may be useful for
# Line 1279  subpatterns that are referenced as Line 1542  subpatterns that are referenced as
1542  .\" </a>  .\" </a>
1543  subroutines  subroutines
1544  .\"  .\"
1545  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1546  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1547    .\" </a>
1548    "Defining subpatterns for use by reference only"
1549    .\"
1550    below). Items other than subpatterns that have a {0} quantifier are omitted
1551    from the compiled pattern.
1552  .P  .P
1553  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1554  abbreviations:  abbreviations:
# Line 1352  worth setting PCRE_DOTALL in order to ob Line 1620  worth setting PCRE_DOTALL in order to ob
1620  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1621  .P  .P
1622  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1623  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1624  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
1625  succeeds. Consider, for example:  succeeds. Consider, for example:
1626  .sp  .sp
# Line 1505  no such problem when named parentheses a Line 1773  no such problem when named parentheses a
1773  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1774  .P  .P
1775  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
1776  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
1777  Perl 5.10. This escape must be followed by an unsigned number or a negative  unsigned number or a negative number, optionally enclosed in braces. These
1778  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1779  .sp  .sp
1780    (ring), \e1    (ring), \e1
1781    (ring), \eg1    (ring), \eg1
# Line 1521  example: Line 1789  example:
1789    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1790  .sp  .sp
1791  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
1792  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.
1793  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
1794  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
1795  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1796  .P  .P
1797  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1798  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1563  after the reference. Line 1831  after the reference.
1831  .P  .P
1832  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
1833  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1834  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1835  .sp  .sp
1836    (a|(bc))\e2    (a|(bc))\e2
1837  .sp  .sp
1838  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
1839  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
1840  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1841  with a digit character, some delimiter must be used to terminate the back  .P
1842  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1843  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1844    If the pattern continues with a digit character, some delimiter must be used to
1845    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1846    white space. Otherwise, the \eg{ syntax or an empty comment (see
1847  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1848  .\" </a>  .\" </a>
1849  "Comments"  "Comments"
1850  .\"  .\"
1851  below) can be used.  below) can be used.
1852  .P  .
1853    .SS "Recursive back references"
1854    .rs
1855    .sp
1856  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
1857  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.
1858  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1592  to the previous iteration. In order for Line 1866  to the previous iteration. In order for
1866  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
1867  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
1868  minimum of zero.  minimum of zero.
1869    .P
1870    Back references of this type cause the group that they reference to be treated
1871    as an
1872    .\" HTML <a href="#atomicgroup">
1873    .\" </a>
1874    atomic group.
1875    .\"
1876    Once the whole group has been matched, a subsequent matching failure cannot
1877    cause backtracking into the middle of the group.
1878  .  .
1879  .  .
1880  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1611  those that look ahead of the current pos Line 1894  those that look ahead of the current pos
1894  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,
1895  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.
1896  .P  .P
1897  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1898  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
1899  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1900  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions, because it does not make
1901  However, substring capturing is carried out only for positive assertions,  sense for negative assertions.
1902  because it does not make sense for negative assertions.  .P
1903    For compatibility with Perl, assertion subpatterns may be repeated; though
1904    it makes no sense to assert the same thing several times, the side effect of
1905    capturing parentheses may occasionally be useful. In practice, there only three
1906    cases:
1907    .sp
1908    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1909    However, it may contain internal capturing parenthesized groups that are called
1910    from elsewhere via the
1911    .\" HTML <a href="#subpatternsassubroutines">
1912    .\" </a>
1913    subroutine mechanism.
1914    .\"
1915    .sp
1916    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1917    were {0,1}. At run time, the rest of the pattern match is tried with and
1918    without the assertion, the order depending on the greediness of the quantifier.
1919    .sp
1920    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1921    The assertion is obeyed just once when encountered during matching.
1922  .  .
1923  .  .
1924  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1645  lookbehind assertion is needed to achiev Line 1947  lookbehind assertion is needed to achiev
1947  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
1948  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
1949  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.
1950    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1951  .  .
1952  .  .
1953  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1669  is permitted, but Line 1972  is permitted, but
1972  .sp  .sp
1973  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1974  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
1975  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
1976  match the same length of string. An assertion such as  length of string. An assertion such as
1977  .sp  .sp
1978    (?<=ab(c|de))    (?<=ab(c|de))
1979  .sp  .sp
1980  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
1981  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
1982    branches:
1983  .sp  .sp
1984    (?<=abc|abde)    (?<=abc|abde)
1985  .sp  .sp
1986  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
1987  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
1988  .\" </a>  .\" </a>
1989  (see above)  (see above)
1990  .\"  .\"
1991  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
1992  fixed-length.  restriction.
1993  .P  .P
1994  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1995  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
1996  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1997  assertion fails.  assertion fails.
1998  .P  .P
1999  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
2000  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
2001  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
2002  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2003    permitted.
2004    .P
2005    .\" HTML <a href="#subpatternsassubroutines">
2006    .\" </a>
2007    "Subroutine"
2008    .\"
2009    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2010    as the subpattern matches a fixed-length string.
2011    .\" HTML <a href="#recursion">
2012    .\" </a>
2013    Recursion,
2014    .\"
2015    however, is not supported.
2016  .P  .P
2017  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2018  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
2019  pattern such as  strings. Consider a simple pattern such as
2020  .sp  .sp
2021    abcd$    abcd$
2022  .sp  .sp
# Line 1763  characters that are not "999". Line 2080  characters that are not "999".
2080  .sp  .sp
2081  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2082  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2083  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2084  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2085  .sp  .sp
2086    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2087    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2088  .sp  .sp
2089  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2090  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
2091  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2092    itself contain nested subpatterns of any form, including conditional
2093    subpatterns; the restriction to two alternatives applies only at the level of
2094    the condition. This pattern fragment is an example where the alternatives are
2095    complex:
2096    .sp
2097      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2098    .sp
2099  .P  .P
2100  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2101  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1780  recursion, a pseudo-condition called DEF Line 2104  recursion, a pseudo-condition called DEF
2104  .rs  .rs
2105  .sp  .sp
2106  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
2107  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2108  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
2109  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2110  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2111  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2112  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2113    section about duplicate subpattern numbers),
2114    .\"
2115    the condition is true if any of them have matched. An alternative notation is
2116    to precede the digits with a plus or minus sign. In this case, the subpattern
2117    number is relative rather than absolute. The most recently opened parentheses
2118    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2119    loops it can also make sense to refer to subsequent groups. The next
2120    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2121    zero in any of these forms is not used; it provokes a compile-time error.)
2122  .P  .P
2123  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2124  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 1796  three parts for ease of discussion: Line 2129  three parts for ease of discussion:
2129  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2130  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
2131  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
2132  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2133  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,
2134  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
2135  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2136  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
# Line 1826  Rewriting the above example to use a nam Line 2159  Rewriting the above example to use a nam
2159  .sp  .sp
2160    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2161  .sp  .sp
2162    If the name used in a condition of this kind is a duplicate, the test is
2163    applied to all subpatterns of the same name, and is true if any one of them has
2164    matched.
2165  .  .
2166  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2167  .rs  .rs
# Line 1837  letter R, for example: Line 2173  letter R, for example:
2173  .sp  .sp
2174    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2175  .sp  .sp
2176  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
2177  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
2178  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2179    applied to all subpatterns of the same name, and is true if any one of them is
2180    the most recent recursion.
2181  .P  .P
2182  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2183  patterns are described below.  .\" HTML <a href="#recursion">
2184    .\" </a>
2185    The syntax for recursive patterns
2186    .\"
2187    is described below.
2188  .  .
2189    .\" HTML <a name="subdefine"></a>
2190  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2191  .rs  .rs
2192  .sp  .sp
# Line 1851  If the condition is the string (DEFINE), Line 2194  If the condition is the string (DEFINE),
2194  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
2195  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2196  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
2197  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2198  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2199  written like this (ignore whitespace and line breaks):  .\" </a>
2200    subroutines
2201    .\"
2202    is described below.) For example, a pattern to match an IPv4 address such as
2203    "192.168.23.245" could be written like this (ignore white space and line
2204    breaks):
2205  .sp  .sp
2206    (?(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) )
2207    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1861  written like this (ignore whitespace and Line 2209  written like this (ignore whitespace and
2209  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
2210  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2211  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
2212  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2213  .P  pattern uses references to the named group to match the four dot-separated
2214  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.  
2215  .  .
2216  .SS "Assertion conditions"  .SS "Assertion conditions"
2217  .rs  .rs
# Line 1890  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2236  dd-aaa-dd or dd-dd-dd, where aaa are let
2236  .SH COMMENTS  .SH COMMENTS
2237  .rs  .rs
2238  .sp  .sp
2239  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
2240  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,
2241  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
2242    subpattern name or number. The characters that make up a comment play no part
2243    in the pattern matching.
2244  .P  .P
2245  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
2246  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2247  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2248    this case continues to immediately after the next newline character or
2249    character sequence in the pattern. Which characters are interpreted as newlines
2250    is controlled by the options passed to a compiling function or by a special
2251    sequence at the start of the pattern, as described in the section entitled
2252    .\" HTML <a href="#newlines">
2253    .\" </a>
2254    "Newline conventions"
2255    .\"
2256    above. Note that the end of this type of comment is a literal newline sequence
2257    in the pattern; escape sequences that happen to represent a newline do not
2258    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2259    default newline convention is in force:
2260    .sp
2261      abc #comment \en still comment
2262    .sp
2263    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2264    a newline in the pattern. The sequence \en is still literal at this stage, so
2265    it does not terminate the comment. Only an actual character with the code value
2266    0x0a (the default newline) does so.
2267  .  .
2268  .  .
2269  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1922  recursively to the pattern in which it a Line 2289  recursively to the pattern in which it a
2289  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2290  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2291  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2292  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.
2293  .P  .P
2294  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
2295  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
2296  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
2297    .\" HTML <a href="#subpatternsassubroutines">
2298    .\" </a>
2299    non-recursive subroutine
2300    .\"
2301  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
2302  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2303  .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  
2304  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2305  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2306  .sp  .sp
2307    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2308  .sp  .sp
2309  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2310  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
2311  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2312  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2313    to avoid backtracking into sequences of non-parentheses.
2314  .P  .P
2315  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
2316  pattern, so instead you could use this:  pattern, so instead you could use this:
2317  .sp  .sp
2318    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2319  .sp  .sp
2320  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
2321  them instead of the whole pattern.  them instead of the whole pattern.
2322  .P  .P
2323  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
2324  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
2325  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
2326  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2327  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.  
2328  .P  .P
2329  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2330  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2331  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2332  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2333    .\" </a>
2334    non-recursive subroutine
2335    .\"
2336    calls, as described in the next section.
2337  .P  .P
2338  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2339  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
2340  could rewrite the above example as follows:  could rewrite the above example as follows:
2341  .sp  .sp
2342    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2343  .sp  .sp
2344  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
2345  used.  used.
2346  .P  .P
2347  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2348  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
2349  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
2350  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2351  .sp  .sp
2352    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2353  .sp  .sp
2354  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,
2355  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
2356  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
2357  before failure can be reported.  before failure can be reported.
2358  .P  .P
2359  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
2360  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
2361  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  
2362  .\" HREF  .\" HREF
2363  \fBpcrecallout\fP  \fBpcrecallout\fP
2364  .\"  .\"
# Line 1997  documentation). If the pattern above is Line 2366  documentation). If the pattern above is
2366  .sp  .sp
2367    (ab(cd)ef)    (ab(cd)ef)
2368  .sp  .sp
2369  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
2370  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
2371  .sp  matched at the top level, its final captured value is unset, even if it was
2372    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2373       ^                        ^  .P
2374       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2375  .sp  obtain extra memory to store data during a recursion, which it does by using
2376  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
2377  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.  
2378  .P  .P
2379  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.
2380  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2022  different alternatives for the recursive Line 2388  different alternatives for the recursive
2388  is the actual recursive call.  is the actual recursive call.
2389  .  .
2390  .  .
2391    .\" HTML <a name="recursiondifference"></a>
2392    .SS "Differences in recursion processing between PCRE and Perl"
2393    .rs
2394    .sp
2395    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2396    (like Python, but unlike Perl), a recursive subpattern call is always treated
2397    as an atomic group. That is, once it has matched some of the subject string, it
2398    is never re-entered, even if it contains untried alternatives and there is a
2399    subsequent matching failure. This can be illustrated by the following pattern,
2400    which purports to match a palindromic string that contains an odd number of
2401    characters (for example, "a", "aba", "abcba", "abcdcba"):
2402    .sp
2403      ^(.|(.)(?1)\e2)$
2404    .sp
2405    The idea is that it either matches a single character, or two identical
2406    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2407    it does not if the pattern is longer than three characters. Consider the
2408    subject string "abcba":
2409    .P
2410    At the top level, the first character is matched, but as it is not at the end
2411    of the string, the first alternative fails; the second alternative is taken
2412    and the recursion kicks in. The recursive call to subpattern 1 successfully
2413    matches the next character ("b"). (Note that the beginning and end of line
2414    tests are not part of the recursion).
2415    .P
2416    Back at the top level, the next character ("c") is compared with what
2417    subpattern 2 matched, which was "a". This fails. Because the recursion is
2418    treated as an atomic group, there are now no backtracking points, and so the
2419    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2420    try the second alternative.) However, if the pattern is written with the
2421    alternatives in the other order, things are different:
2422    .sp
2423      ^((.)(?1)\e2|.)$
2424    .sp
2425    This time, the recursing alternative is tried first, and continues to recurse
2426    until it runs out of characters, at which point the recursion fails. But this
2427    time we do have another alternative to try at the higher level. That is the big
2428    difference: in the previous case the remaining alternative is at a deeper
2429    recursion level, which PCRE cannot use.
2430    .P
2431    To change the pattern so that it matches all palindromic strings, not just
2432    those with an odd number of characters, it is tempting to change the pattern to
2433    this:
2434    .sp
2435      ^((.)(?1)\e2|.?)$
2436    .sp
2437    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2438    deeper recursion has matched a single character, it cannot be entered again in
2439    order to match an empty string. The solution is to separate the two cases, and
2440    write out the odd and even cases as alternatives at the higher level:
2441    .sp
2442      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2443    .sp
2444    If you want to match typical palindromic phrases, the pattern has to ignore all
2445    non-word characters, which can be done like this:
2446    .sp
2447      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2448    .sp
2449    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2450    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2451    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2452    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2453    more) to match typical phrases, and Perl takes so long that you think it has
2454    gone into a loop.
2455    .P
2456    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2457    string does not start with a palindrome that is shorter than the entire string.
2458    For example, although "abcba" is correctly matched, if the subject is "ababa",
2459    PCRE finds the palindrome "aba" at the start, then fails at top level because
2460    the end of the string does not follow. Once again, it cannot jump back into the
2461    recursion to try other alternatives, so the entire match fails.
2462    .P
2463    The second way in which PCRE and Perl differ in their recursion processing is
2464    in the handling of captured values. In Perl, when a subpattern is called
2465    recursively or as a subpattern (see the next section), it has no access to any
2466    values that were captured outside the recursion, whereas in PCRE these values
2467    can be referenced. Consider this pattern:
2468    .sp
2469      ^(.)(\e1|a(?2))
2470    .sp
2471    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2472    then in the second group, when the back reference \e1 fails to match "b", the
2473    second alternative matches "a" and then recurses. In the recursion, \e1 does
2474    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2475    match because inside the recursive call \e1 cannot access the externally set
2476    value.
2477    .
2478    .
2479  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2480  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2481  .rs  .rs
2482  .sp  .sp
2483  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
2484  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
2485  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2486  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2487  relative, as in these examples:  relative, as in these examples:
2488  .sp  .sp
# Line 2048  matches "sense and sensibility" and "res Line 2502  matches "sense and sensibility" and "res
2502  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
2503  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2504  .P  .P
2505  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2506  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
2507  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
2508  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2509  .P  subroutine call revert to their previous values afterwards.
2510  When a subpattern is used as a subroutine, processing options such as  .P
2511  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2512  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2513    different calls. For example, consider this pattern:
2514  .sp  .sp
2515    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2516  .sp  .sp
# Line 2094  same pair of parentheses when there is a Line 2549  same pair of parentheses when there is a
2549  .P  .P
2550  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
2551  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
2552  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
2553  By default, this variable contains NULL, which disables all calling out.  (8-bit library) or \fIpcre16_callout\fP (16-bit library). By default, this
2554    variable contains NULL, which disables all calling out.
2555  .P  .P
2556  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2557  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 2104  For example, this pattern has two callou Line 2560  For example, this pattern has two callou
2560  .sp  .sp
2561    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2562  .sp  .sp
2563  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
2564  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2565  255.  255.
2566  .P  .P
2567  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
2568  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
2569  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
2570  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  the matching function. The callout function may cause matching to proceed, to
2571  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
2572  description of the interface to the callout function is given in the  the callout function is given in the
2573  .\" HREF  .\" HREF
2574  \fBpcrecallout\fP  \fBpcrecallout\fP
2575  .\"  .\"
2576  documentation.  documentation.
2577  .  .
2578  .  .
2579    .\" HTML <a name="backtrackcontrol"></a>
2580  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2581  .rs  .rs
2582  .sp  .sp
# Line 2130  production code should be noted to avoid Line 2587  production code should be noted to avoid
2587  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2588  .P  .P
2589  Since these verbs are specifically related to backtracking, most of them can be  Since these verbs are specifically related to backtracking, most of them can be
2590  used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses  used only when the pattern is to be matched using one of the traditional
2591  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, which use a backtracking algorithm. With the exception of
2592  failing negative assertion, they cause an error if encountered by  (*FAIL), which behaves like a failing negative assertion, they cause an error
2593  \fBpcre_dfa_exec()\fP.  if encountered by a DFA matching function.
2594  .P  .P
2595  If any of these verbs are used in an assertion subpattern, their effect is  If any of these verbs are used in an assertion or in a subpattern that is
2596  confined to that subpattern; it does not extend to the surrounding pattern.  called as a subroutine (whether or not recursively), their effect is confined
2597  Note that assertion subpatterns are processed as anchored at the point where  to that subpattern; it does not extend to the surrounding pattern, with one
2598  they are tested.  exception: the name from a *(MARK), (*PRUNE), or (*THEN) that is encountered in
2599    a successful positive assertion \fIis\fP passed back when a match succeeds
2600    (compare capturing parentheses in assertions). Note that such subpatterns are
2601    processed as anchored at the point where they are tested. Note also that Perl's
2602    treatment of subroutines and assertions is different in some cases.
2603  .P  .P
2604  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2605  parenthesis followed by an asterisk. In Perl, they are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2606  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2607  form is just (*VERB). Any number of these verbs may occur in a pattern. There  depending on whether or not an argument is present. A name is any sequence of
2608  are two kinds:  characters that does not include a closing parenthesis. The maximum length of
2609    name is 255 in the 8-bit library and 65535 in the 16-bit library. If the name
2610    is empty, that is, if the closing parenthesis immediately follows the colon,
2611    the effect is as if the colon were not there. Any number of these verbs may
2612    occur in a pattern.
2613    .
2614    .
2615    .\" HTML <a name="nooptimize"></a>
2616    .SS "Optimizations that affect backtracking verbs"
2617    .rs
2618    .sp
2619    PCRE contains some optimizations that are used to speed up matching by running
2620    some checks at the start of each match attempt. For example, it may know the
2621    minimum length of matching subject, or that a particular character must be
2622    present. When one of these optimizations suppresses the running of a match, any
2623    included backtracking verbs will not, of course, be processed. You can suppress
2624    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2625    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2626    pattern with (*NO_START_OPT). There is more discussion of this option in the
2627    section entitled
2628    .\" HTML <a href="pcreapi.html#execoptions">
2629    .\" </a>
2630    "Option bits for \fBpcre_exec()\fP"
2631    .\"
2632    in the
2633    .\" HREF
2634    \fBpcreapi\fP
2635    .\"
2636    documentation.
2637    .P
2638    Experiments with Perl suggest that it too has similar optimizations, sometimes
2639    leading to anomalous results.
2640    .
2641  .  .
2642  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2643  .rs  .rs
2644  .sp  .sp
2645  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2646    followed by a name.
2647  .sp  .sp
2648     (*ACCEPT)     (*ACCEPT)
2649  .sp  .sp
2650  This verb causes the match to end successfully, skipping the remainder of the  This verb causes the match to end successfully, skipping the remainder of the
2651  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2652  immediately. If the (*ACCEPT) is inside capturing parentheses, the data so far  subroutine, only that subpattern is ended successfully. Matching then continues
2653  is captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) is inside capturing parentheses, the data so
2654    far is captured. For example:
2655  .sp  .sp
2656    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2657  .sp  .sp
2658  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by  This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2659  the outer parentheses.  the outer parentheses.
2660  .sp  .sp
2661    (*FAIL) or (*F)    (*FAIL) or (*F)
2662  .sp  .sp
2663  This verb causes the match to fail, forcing backtracking to occur. It is  This verb causes a matching failure, forcing backtracking to occur. It is
2664  equivalent to (?!) but easier to read. The Perl documentation notes that it is  equivalent to (?!) but easier to read. The Perl documentation notes that it is
2665  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2666  Perl features that are not present in PCRE. The nearest equivalent is the  Perl features that are not present in PCRE. The nearest equivalent is the
# Line 2176  callout feature, as for example in this Line 2671  callout feature, as for example in this
2671  A match with the string "aaaa" always fails, but the callout is taken before  A match with the string "aaaa" always fails, but the callout is taken before
2672  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2673  .  .
2674    .
2675    .SS "Recording which path was taken"
2676    .rs
2677    .sp
2678    There is one verb whose main purpose is to track how a match was arrived at,
2679    though it also has a secondary use in conjunction with advancing the match
2680    starting point (see (*SKIP) below).
2681    .sp
2682      (*MARK:NAME) or (*:NAME)
2683    .sp
2684    A name is always required with this verb. There may be as many instances of
2685    (*MARK) as you like in a pattern, and their names do not have to be unique.
2686    .P
2687    When a match succeeds, the name of the last-encountered (*MARK) on the matching
2688    path is passed back to the caller as described in the section entitled
2689    .\" HTML <a href="pcreapi.html#extradata">
2690    .\" </a>
2691    "Extra data for \fBpcre_exec()\fP"
2692    .\"
2693    in the
2694    .\" HREF
2695    \fBpcreapi\fP
2696    .\"
2697    documentation. Here is an example of \fBpcretest\fP output, where the /K
2698    modifier requests the retrieval and outputting of (*MARK) data:
2699    .sp
2700        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2701      data> XY
2702       0: XY
2703      MK: A
2704      XZ
2705       0: XZ
2706      MK: B
2707    .sp
2708    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2709    indicates which of the two alternatives matched. This is a more efficient way
2710    of obtaining this information than putting each alternative in its own
2711    capturing parentheses.
2712    .P
2713    If (*MARK) is encountered in a positive assertion, its name is recorded and
2714    passed back if it is the last-encountered. This does not happen for negative
2715    assertions.
2716    .P
2717    After a partial match or a failed match, the name of the last encountered
2718    (*MARK) in the entire match process is returned. For example:
2719    .sp
2720        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2721      data> XP
2722      No match, mark = B
2723    .sp
2724    Note that in this unanchored example the mark is retained from the match
2725    attempt that started at the letter "X" in the subject. Subsequent match
2726    attempts starting at "P" and then with an empty string do not get as far as the
2727    (*MARK) item, but nevertheless do not reset it.
2728    .P
2729    If you are interested in (*MARK) values after failed matches, you should
2730    probably set the PCRE_NO_START_OPTIMIZE option
2731    .\" HTML <a href="#nooptimize">
2732    .\" </a>
2733    (see above)
2734    .\"
2735    to ensure that the match is always attempted.
2736    .
2737    .
2738  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2739  .rs  .rs
2740  .sp  .sp
2741  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2742  with what follows, but if there is no subsequent match, a failure is forced.  with what follows, but if there is no subsequent match, causing a backtrack to
2743  The verbs differ in exactly what kind of failure occurs.  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2744    the verb. However, when one of these verbs appears inside an atomic group, its
2745    effect is confined to that group, because once the group has been matched,
2746    there is never any backtracking into it. In this situation, backtracking can
2747    "jump back" to the left of the entire atomic group. (Remember also, as stated
2748    above, that this localization also applies in subroutine calls and assertions.)
2749    .P
2750    These verbs differ in exactly what kind of failure occurs when backtracking
2751    reaches them.
2752  .sp  .sp
2753    (*COMMIT)    (*COMMIT)
2754  .sp  .sp
2755  This verb causes the whole match to fail outright if the rest of the pattern  This verb, which may not be followed by a name, causes the whole match to fail
2756  does not match. Even if the pattern is unanchored, no further attempts to find  outright if the rest of the pattern does not match. Even if the pattern is
2757  a match by advancing the start point take place. Once (*COMMIT) has been  unanchored, no further attempts to find a match by advancing the starting point
2758  passed, \fBpcre_exec()\fP is committed to finding a match at the current  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2759  starting point, or not at all. For example:  finding a match at the current starting point, or not at all. For example:
2760  .sp  .sp
2761    a+(*COMMIT)b    a+(*COMMIT)b
2762  .sp  .sp
2763  This matches "xxaab" but not "aacaab". It can be thought of as a kind of  This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2764  dynamic anchor, or "I've started, so I must finish."  dynamic anchor, or "I've started, so I must finish." The name of the most
2765  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2766    (*PRUNE)  match failure.
2767  .sp  .P
2768  This verb causes the match to fail at the current position if the rest of the  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2769  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  unless PCRE's start-of-match optimizations are turned off, as shown in this
2770  advance to the next starting character then happens. Backtracking can occur as  \fBpcretest\fP example:
2771  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .sp
2772  if there is no match to the right, backtracking cannot cross (*PRUNE).      re> /(*COMMIT)abc/
2773  In simple cases, the use of (*PRUNE) is just an alternative to an atomic    data> xyzabc
2774  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot     0: abc
2775  be expressed in any other way.    xyzabc\eY
2776      No match
2777    .sp
2778    PCRE knows that any match must start with "a", so the optimization skips along
2779    the subject to "a" before running the first match attempt, which succeeds. When
2780    the optimization is disabled by the \eY escape in the second subject, the match
2781    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2782    starting points.
2783    .sp
2784      (*PRUNE) or (*PRUNE:NAME)
2785    .sp
2786    This verb causes the match to fail at the current starting position in the
2787    subject if the rest of the pattern does not match. If the pattern is
2788    unanchored, the normal "bumpalong" advance to the next starting character then
2789    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2790    reached, or when matching to the right of (*PRUNE), but if there is no match to
2791    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2792    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2793    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2794    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
2795    anchored pattern (*PRUNE) has the same effect as (*COMMIT).
2796  .sp  .sp
2797    (*SKIP)    (*SKIP)
2798  .sp  .sp
2799  This verb is like (*PRUNE), except that if the pattern is unanchored, the  This verb, when given without a name, is like (*PRUNE), except that if the
2800  "bumpalong" advance is not to the next character, but to the position in the  pattern is unanchored, the "bumpalong" advance is not to the next character,
2801  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2802  was matched leading up to it cannot be part of a successful match. Consider:  signifies that whatever text was matched leading up to it cannot be part of a
2803    successful match. Consider:
2804  .sp  .sp
2805    a+(*SKIP)b    a+(*SKIP)b
2806  .sp  .sp
2807  If the subject is "aaaac...", after the first match attempt fails (starting at  If the subject is "aaaac...", after the first match attempt fails (starting at
2808  the first character in the string), the starting point skips on to start the  the first character in the string), the starting point skips on to start the
2809  next attempt at "c". Note that a possessive quantifer does not have the same  next attempt at "c". Note that a possessive quantifer does not have the same
2810  effect in this example; although it would suppress backtracking during the  effect as this example; although it would suppress backtracking during the
2811  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2812  instead of skipping on to "c".  instead of skipping on to "c".
2813  .sp  .sp
2814    (*THEN)    (*SKIP:NAME)
2815  .sp  .sp
2816  This verb causes a skip to the next alternation if the rest of the pattern does  When (*SKIP) has an associated name, its behaviour is modified. If the
2817  not match. That is, it cancels pending backtracking, but only within the  following pattern fails to match, the previous path through the pattern is
2818  current alternation. Its name comes from the observation that it can be used  searched for the most recent (*MARK) that has the same name. If one is found,
2819  for a pattern-based if-then-else block:  the "bumpalong" advance is to the subject position that corresponds to that
2820    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2821    matching name is found, the (*SKIP) is ignored.
2822    .sp
2823      (*THEN) or (*THEN:NAME)
2824    .sp
2825    This verb causes a skip to the next innermost alternative if the rest of the
2826    pattern does not match. That is, it cancels pending backtracking, but only
2827    within the current alternative. Its name comes from the observation that it can
2828    be used for a pattern-based if-then-else block:
2829  .sp  .sp
2830    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2831  .sp  .sp
2832  If the COND1 pattern matches, FOO is tried (and possibly further items after  If the COND1 pattern matches, FOO is tried (and possibly further items after
2833  the end of the group if FOO succeeds); on failure the matcher skips to the  the end of the group if FOO succeeds); on failure, the matcher skips to the
2834  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. The
2835  is used outside of any alternation, it acts exactly like (*PRUNE).  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN).
2836    If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2837    .P
2838    Note that a subpattern that does not contain a | character is just a part of
2839    the enclosing alternative; it is not a nested alternation with only one
2840    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2841    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2842    pattern fragments that do not contain any | characters at this level:
2843    .sp
2844      A (B(*THEN)C) | D
2845    .sp
2846    If A and B are matched, but there is a failure in C, matching does not
2847    backtrack into A; instead it moves to the next alternative, that is, D.
2848    However, if the subpattern containing (*THEN) is given an alternative, it
2849    behaves differently:
2850    .sp
2851      A (B(*THEN)C | (*FAIL)) | D
2852    .sp
2853    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2854    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2855    because there are no more alternatives to try. In this case, matching does now
2856    backtrack into A.
2857    .P
2858    Note also that a conditional subpattern is not considered as having two
2859    alternatives, because only one is ever used. In other words, the | character in
2860    a conditional subpattern has a different meaning. Ignoring white space,
2861    consider:
2862    .sp
2863      ^.*? (?(?=a) a | b(*THEN)c )
2864    .sp
2865    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2866    it initially matches zero characters. The condition (?=a) then fails, the
2867    character "b" is matched, but "c" is not. At this point, matching does not
2868    backtrack to .*? as might perhaps be expected from the presence of the |
2869    character. The conditional subpattern is part of the single alternative that
2870    comprises the whole pattern, and so the match fails. (If there was a backtrack
2871    into .*?, allowing it to match "b", the match would succeed.)
2872    .P
2873    The verbs just described provide four different "strengths" of control when
2874    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2875    next alternative. (*PRUNE) comes next, failing the match at the current
2876    starting position, but allowing an advance to the next character (for an
2877    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2878    than one character. (*COMMIT) is the strongest, causing the entire match to
2879    fail.
2880    .P
2881    If more than one such verb is present in a pattern, the "strongest" one wins.
2882    For example, consider this pattern, where A, B, etc. are complex pattern
2883    fragments:
2884    .sp
2885      (A(*COMMIT)B(*THEN)C|D)
2886    .sp
2887    Once A has matched, PCRE is committed to this match, at the current starting
2888    position. If subsequently B matches, but C does not, the normal (*THEN) action
2889    of trying the next alternative (that is, D) does not happen because (*COMMIT)
2890    overrides.
2891  .  .
2892  .  .
2893  .SH "SEE ALSO"  .SH "SEE ALSO"
2894  .rs  .rs
2895  .sp  .sp
2896  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2897    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP.
2898  .  .
2899  .  .
2900  .SH AUTHOR  .SH AUTHOR
# Line 2258  Cambridge CB2 3QH, England. Line 2911  Cambridge CB2 3QH, England.
2911  .rs  .rs
2912  .sp  .sp
2913  .nf  .nf
2914  Last updated: 16 September 2009  Last updated: 01 June 2012
2915  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2012 University of Cambridge.
2916  .fi  .fi

Legend:
Removed from v.451  
changed lines
  Added in v.972

  ViewVC Help
Powered by ViewVC 1.1.5