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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "05 April 2013" "PCRE 8.33"
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,  there is now also support for UTF-8 strings in the original library, an
25  PCRE must be built to include UTF-8 support, and you must call  extra library that supports 16-bit and UTF-16 character strings, and a
26  \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There  third library that supports 32-bit and UTF-32 character strings. To use these
27  is also a special sequence that can be given at the start of a pattern:  features, PCRE must be built to include appropriate support. When using UTF
28    strings you must either call the compiling function with the PCRE_UTF8,
29    PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
30    these special sequences:
31  .sp  .sp
32    (*UTF8)    (*UTF8)
33  .sp    (*UTF16)
34  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8    (*UTF32)
35  option. This feature is not Perl-compatible. How setting UTF-8 mode affects    (*UTF)
36    .sp
37    (*UTF) is a generic sequence that can be used with any of the libraries.
38    Starting a pattern with such a sequence is equivalent to setting the relevant
39    option. This feature is not Perl-compatible. How setting a UTF mode affects
40  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
41  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  
42  .\" HREF  .\" HREF
43  \fBpcre\fP  \fBpcreunicode\fP
44  .\"  .\"
45  page.  page.
46  .P  .P
47    Another special sequence that may appear at the start of a pattern or in
48    combination with (*UTF8), (*UTF16), (*UTF32) or (*UTF) is:
49    .sp
50      (*UCP)
51    .sp
52    This has the same effect as setting the PCRE_UCP option: it causes sequences
53    such as \ed and \ew to use Unicode properties to determine character types,
54    instead of recognizing only characters with codes less than 128 via a lookup
55    table.
56    .P
57    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
58    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
59    also some more of these special sequences that are concerned with the handling
60    of newlines; they are described below.
61    .P
62  The remainder of this document discusses the patterns that are supported by  The remainder of this document discusses the patterns that are supported by
63  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  PCRE when one its main matching functions, \fBpcre_exec()\fP (8-bit) or
64  From release 6.0, PCRE offers a second matching function,  \fBpcre[16|32]_exec()\fP (16- or 32-bit), is used. PCRE also has alternative
65  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  matching functions, \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP,
66  Perl-compatible. Some of the features discussed below are not available when  which match using a different algorithm that is not Perl-compatible. Some of
67  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  the features discussed below are not available when DFA matching is used. The
68  alternative function, and how it differs from the normal function, are  advantages and disadvantages of the alternative functions, and how they differ
69  discussed in the  from the normal functions, are discussed in the
70  .\" HREF  .\" HREF
71  \fBpcrematching\fP  \fBpcrematching\fP
72  .\"  .\"
73  page.  page.
74  .  .
75  .  .
76    .SH "EBCDIC CHARACTER CODES"
77    .rs
78    .sp
79    PCRE can be compiled to run in an environment that uses EBCDIC as its character
80    code rather than ASCII or Unicode (typically a mainframe system). In the
81    sections below, character code values are ASCII or Unicode; in an EBCDIC
82    environment these characters may have different code values, and there are no
83    code points greater than 255.
84    .
85    .
86    .\" HTML <a name="newlines"></a>
87  .SH "NEWLINE CONVENTIONS"  .SH "NEWLINE CONVENTIONS"
88  .rs  .rs
89  .sp  .sp
# Line 83  string with one of the following five se Line 111  string with one of the following five se
111    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
112    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
113  .sp  .sp
114  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function. For
115  \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default  example, on a Unix system where LF is the default newline sequence, the pattern
 newline sequence, the pattern  
116  .sp  .sp
117    (*CR)a.b    (*CR)a.b
118  .sp  .sp
# Line 95  Perl-compatible, are recognized only at Line 122  Perl-compatible, are recognized only at
122  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
123  is used.  is used.
124  .P  .P
125  The newline convention does not affect what the \eR escape sequence matches. By  The newline convention affects where the circumflex and dollar assertions are
126  default, this is any Unicode newline sequence, for Perl compatibility. However,  true. It also affects the interpretation of the dot metacharacter when
127  this can be changed; see the description of \eR in the section entitled  PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
128    what the \eR escape sequence matches. By default, this is any Unicode newline
129    sequence, for Perl compatibility. However, this can be changed; see the
130    description of \eR in the section entitled
131  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
132  .\" </a>  .\" </a>
133  "Newline sequences"  "Newline sequences"
# Line 117  corresponding characters in the subject. Line 147  corresponding characters in the subject.
147  .sp  .sp
148  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
149  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
150  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
151  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
152  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
153  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
154  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
155  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
156  UTF-8 support.  UTF support.
157  .P  .P
158  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
159  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 169  The following sections describe the use Line 199  The following sections describe the use
199  .rs  .rs
200  .sp  .sp
201  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
202  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
203  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
204  outside character classes.  both inside and outside character classes.
205  .P  .P
206  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.
207  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 179  otherwise be interpreted as a metacharac Line 209  otherwise be interpreted as a metacharac
209  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
210  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
211  .P  .P
212  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
213    backslash. All other characters (in particular, those whose codepoints are
214    greater than 127) are treated as literals.
215    .P
216    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
217  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
218  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
219  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.
220  .P  .P
221  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
222  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 198  Perl, $ and @ cause variable interpolati Line 232  Perl, $ and @ cause variable interpolati
232    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
233  .sp  .sp
234  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
235    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
236    by \eE later in the pattern, the literal interpretation continues to the end of
237    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
238    a character class, this causes an error, because the character class is not
239    terminated.
240  .  .
241  .  .
242  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 211  but when a pattern is being prepared by Line 250  but when a pattern is being prepared by
250  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
251  .sp  .sp
252    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
253    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
254    \ee        escape (hex 1B)    \ee        escape (hex 1B)
255    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
256    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
257    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
258    \et        tab (hex 09)    \et        tab (hex 09)
259    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
260    \exhh      character with hex code hh    \exhh      character with hex code hh
261    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
262      \euhhhh    character with hex code hhhh (JavaScript mode only)
263    .sp
264    The precise effect of \ecx on ASCII characters is as follows: if x is a lower
265    case letter, it is converted to upper case. Then bit 6 of the character (hex
266    40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
267    but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
268    data item (byte or 16-bit value) following \ec has a value greater than 127, a
269    compile-time error occurs. This locks out non-ASCII characters in all modes.
270    .P
271    The \ec facility was designed for use with ASCII characters, but with the
272    extension to Unicode it is even less useful than it once was. It is, however,
273    recognized when PCRE is compiled in EBCDIC mode, where data items are always
274    bytes. In this mode, all values are valid after \ec. If the next character is a
275    lower case letter, it is converted to upper case. Then the 0xc0 bits of the
276    byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
277    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
278    characters also generate different values.
279    .P
280    By default, after \ex, from zero to two hexadecimal digits are read (letters
281    can be in upper or lower case). Any number of hexadecimal digits may appear
282    between \ex{ and }, but the character code is constrained as follows:
283    .sp
284      8-bit non-UTF mode    less than 0x100
285      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
286      16-bit non-UTF mode   less than 0x10000
287      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
288      32-bit non-UTF mode   less than 0x80000000
289      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
290  .sp  .sp
291  The precise effect of \ecx is as follows: if x is a lower case letter, it  Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
292  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  "surrogate" codepoints), and 0xffef.
 Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  
 7B.  
 .P  
 After \ex, from zero to two hexadecimal digits are read (letters can be in  
 upper or lower case). Any number of hexadecimal digits may appear between \ex{  
 and }, but the value of the character code must be less than 256 in non-UTF-8  
 mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  
 hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  
 point, which is 10FFFF.  
293  .P  .P
294  If characters other than hexadecimal digits appear between \ex{ and }, or if  If characters other than hexadecimal digits appear between \ex{ and }, or if
295  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
296  initial \ex will be interpreted as a basic hexadecimal escape, with no  initial \ex will be interpreted as a basic hexadecimal escape, with no
297  following digits, giving a character whose value is zero.  following digits, giving a character whose value is zero.
298  .P  .P
299    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
300    as just described only when it is followed by two hexadecimal digits.
301    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
302    code points greater than 256 is provided by \eu, which must be followed by
303    four hexadecimal digits; otherwise it matches a literal "u" character.
304    Character codes specified by \eu in JavaScript mode are constrained in the same
305    was as those specified by \ex in non-JavaScript mode.
306    .P
307  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
308  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
309  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
310    \eu00dc in JavaScript mode).
311  .P  .P
312  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
313  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 333  parenthesized subpatterns.
333  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
334  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
335  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
336  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
337  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.
338  to \e777 are permitted. For example:  For example:
339  .sp  .sp
340    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
341  .\" JOIN  .\" JOIN
342    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
343              previous capturing subpatterns              previous capturing subpatterns
# Line 285  to \e777 are permitted. For example: Line 352  to \e777 are permitted. For example:
352              character with octal code 113              character with octal code 113
353  .\" JOIN  .\" JOIN
354    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
355              the byte consisting entirely of 1 bits              the value 255 (decimal)
356  .\" JOIN  .\" JOIN
357    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or a binary zero
358              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 361  Note that octal values of 100 or greater
361  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
362  .P  .P
363  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
364  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
365  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
366  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
367  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
368  meanings  inside a character class. Like other unrecognized escape sequences, they are
369  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
370  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
371  (see below).  sequences have different meanings.
372  .\"  .
373    .
374    .SS "Unsupported escape sequences"
375    .rs
376    .sp
377    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
378    handler and used to modify the case of following characters. By default, PCRE
379    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
380    option is set, \eU matches a "U" character, and \eu can be used to define a
381    character by code point, as described in the previous section.
382  .  .
383  .  .
384  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 333  syntax for referencing a subpattern as a Line 409  syntax for referencing a subpattern as a
409  later.  later.
410  .\"  .\"
411  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP  Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
412  synonymous. The former is a back reference; the latter is a  synonymous. The former is a back reference; the latter is a
413  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
414  .\" </a>  .\" </a>
415  subroutine  subroutine
# Line 341  subroutine Line 417  subroutine
417  call.  call.
418  .  .
419  .  .
420    .\" HTML <a name="genericchartypes"></a>
421  .SS "Generic character types"  .SS "Generic character types"
422  .rs  .rs
423  .sp  .sp
424  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:  
425  .sp  .sp
426    \ed     any decimal digit    \ed     any decimal digit
427    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
428    \eh     any horizontal whitespace character    \eh     any horizontal white space character
429    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
430    \es     any whitespace character    \es     any white space character
431    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
432    \ev     any vertical whitespace character    \ev     any vertical white space character
433    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
434    \ew     any "word" character    \ew     any "word" character
435    \eW     any "non-word" character    \eW     any "non-word" character
436  .sp  .sp
437  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.
438  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
439  .P  .\" HTML <a href="#fullstopdot">
440  These character type sequences can appear both inside and outside character  .\" </a>
441    the "." metacharacter
442    .\"
443    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
444    PCRE does not support this.
445    .P
446    Each pair of lower and upper case escape sequences partitions the complete set
447    of characters into two disjoint sets. Any given character matches one, and only
448    one, of each pair. The sequences can appear both inside and outside character
449  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
450  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
451  there is no character to match.  there is no character to match.
452  .P  .P
453  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 372  are HT (9), LF (10), FF (12), CR (13), a Line 456  are HT (9), LF (10), FF (12), CR (13), a
456  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
457  does.  does.
458  .P  .P
459  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.
460  \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
461  character property support is available. These sequences retain their original  low-valued character tables, and may vary if locale-specific matching is taking
462  meanings from before UTF-8 support was available, mainly for efficiency  place (see
463  reasons. Note that this also affects \eb, because it is defined in terms of \ew  .\" HTML <a href="pcreapi.html#localesupport">
464  and \eW.  .\" </a>
465  .P  "Locale support"
466  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  .\"
467  other sequences, these do match certain high-valued codepoints in UTF-8 mode.  in the
468  The horizontal space characters are:  .\" HREF
469    \fBpcreapi\fP
470    .\"
471    page). For example, in a French locale such as "fr_FR" in Unix-like systems,
472    or "french" in Windows, some character codes greater than 128 are used for
473    accented letters, and these are then matched by \ew. The use of locales with
474    Unicode is discouraged.
475    .P
476    By default, in a UTF mode, characters with values greater than 128 never match
477    \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
478    their original meanings from before UTF support was available, mainly for
479    efficiency reasons. However, if PCRE is compiled with Unicode property support,
480    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
481    properties are used to determine character types, as follows:
482    .sp
483      \ed  any character that \ep{Nd} matches (decimal digit)
484      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
485      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
486    .sp
487    The upper case escapes match the inverse sets of characters. Note that \ed
488    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
489    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
490    \eB because they are defined in terms of \ew and \eW. Matching these sequences
491    is noticeably slower when PCRE_UCP is set.
492    .P
493    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
494    release 5.10. In contrast to the other sequences, which match only ASCII
495    characters by default, these always match certain high-valued codepoints,
496    whether or not PCRE_UCP is set. The horizontal space characters are:
497  .sp  .sp
498    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
499    U+0020     Space    U+0020     Space
500    U+00A0     Non-break space    U+00A0     Non-break space
501    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 405  The horizontal space characters are: Line 517  The horizontal space characters are:
517  .sp  .sp
518  The vertical space characters are:  The vertical space characters are:
519  .sp  .sp
520    U+000A     Linefeed    U+000A     Linefeed (LF)
521    U+000B     Vertical tab    U+000B     Vertical tab (VT)
522    U+000C     Formfeed    U+000C     Form feed (FF)
523    U+000D     Carriage return    U+000D     Carriage return (CR)
524    U+0085     Next line    U+0085     Next line (NEL)
525    U+2028     Line separator    U+2028     Line separator
526    U+2029     Paragraph separator    U+2029     Paragraph separator
527  .P  .sp
528  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
529  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.  
530  .  .
531  .  .
532  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 436  is discouraged. Line 534  is discouraged.
534  .rs  .rs
535  .sp  .sp
536  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
537  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
538  equivalent to the following:  following:
539  .sp  .sp
540    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
541  .sp  .sp
# Line 448  below. Line 546  below.
546  .\"  .\"
547  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
548  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,
549  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
550  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
551  cannot be split.  cannot be split.
552  .P  .P
553  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
554  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).
555  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
556  recognized.  recognized.
# Line 468  one of the following sequences: Line 566  one of the following sequences:
566    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
567    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
568  .sp  .sp
569  These override the default and the options given to \fBpcre_compile()\fP or  These override the default and the options given to the compiling function, but
570  \fBpcre_compile2()\fP, but they can be overridden by options given to  they can themselves be overridden by options given to a matching function. Note
571  \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,  that these special settings, which are not Perl-compatible, are recognized only
572  which are not Perl-compatible, are recognized only at the very start of a  at the very start of a pattern, and that they must be in upper case. If more
573  pattern, and that they must be in upper case. If more than one of them is  than one of them is present, the last one is used. They can be combined with a
574  present, the last one is used. They can be combined with a change of newline  change of newline convention; for example, a pattern can start with:
 convention, for example, a pattern can start with:  
575  .sp  .sp
576    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
577  .sp  .sp
578  Inside a character class, \eR matches the letter "R".  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
579    (*UCP) special sequences. Inside a character class, \eR is treated as an
580    unrecognized escape sequence, and so matches the letter "R" by default, but
581    causes an error if PCRE_EXTRA is set.
582  .  .
583  .  .
584  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 487  Inside a character class, \eR matches th Line 587  Inside a character class, \eR matches th
587  .sp  .sp
588  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
589  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
590  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
591  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.
592  The extra escape sequences are:  The extra escape sequences are:
593  .sp  .sp
594    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
595    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
596    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
597  .sp  .sp
598  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
599  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
600  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
601  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
602  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
603    .\" </a>
604    next section).
605    .\"
606    Other Perl properties such as "InMusicalSymbols" are not currently supported by
607    PCRE. Note that \eP{Any} does not match any characters, so always causes a
608    match failure.
609  .P  .P
610  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
611  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 513  Those that are not part of an identified Line 619  Those that are not part of an identified
619  .P  .P
620  Arabic,  Arabic,
621  Armenian,  Armenian,
622    Avestan,
623  Balinese,  Balinese,
624    Bamum,
625    Batak,
626  Bengali,  Bengali,
627  Bopomofo,  Bopomofo,
628    Brahmi,
629  Braille,  Braille,
630  Buginese,  Buginese,
631  Buhid,  Buhid,
632  Canadian_Aboriginal,  Canadian_Aboriginal,
633    Carian,
634    Chakma,
635    Cham,
636  Cherokee,  Cherokee,
637  Common,  Common,
638  Coptic,  Coptic,
# Line 528  Cypriot, Line 641  Cypriot,
641  Cyrillic,  Cyrillic,
642  Deseret,  Deseret,
643  Devanagari,  Devanagari,
644    Egyptian_Hieroglyphs,
645  Ethiopic,  Ethiopic,
646  Georgian,  Georgian,
647  Glagolitic,  Glagolitic,
# Line 540  Hangul, Line 654  Hangul,
654  Hanunoo,  Hanunoo,
655  Hebrew,  Hebrew,
656  Hiragana,  Hiragana,
657    Imperial_Aramaic,
658  Inherited,  Inherited,
659    Inscriptional_Pahlavi,
660    Inscriptional_Parthian,
661    Javanese,
662    Kaithi,
663  Kannada,  Kannada,
664  Katakana,  Katakana,
665    Kayah_Li,
666  Kharoshthi,  Kharoshthi,
667  Khmer,  Khmer,
668  Lao,  Lao,
669  Latin,  Latin,
670    Lepcha,
671  Limbu,  Limbu,
672  Linear_B,  Linear_B,
673    Lisu,
674    Lycian,
675    Lydian,
676  Malayalam,  Malayalam,
677    Mandaic,
678    Meetei_Mayek,
679    Meroitic_Cursive,
680    Meroitic_Hieroglyphs,
681    Miao,
682  Mongolian,  Mongolian,
683  Myanmar,  Myanmar,
684  New_Tai_Lue,  New_Tai_Lue,
# Line 557  Nko, Line 686  Nko,
686  Ogham,  Ogham,
687  Old_Italic,  Old_Italic,
688  Old_Persian,  Old_Persian,
689    Old_South_Arabian,
690    Old_Turkic,
691    Ol_Chiki,
692  Oriya,  Oriya,
693  Osmanya,  Osmanya,
694  Phags_Pa,  Phags_Pa,
695  Phoenician,  Phoenician,
696    Rejang,
697  Runic,  Runic,
698    Samaritan,
699    Saurashtra,
700    Sharada,
701  Shavian,  Shavian,
702  Sinhala,  Sinhala,
703    Sora_Sompeng,
704    Sundanese,
705  Syloti_Nagri,  Syloti_Nagri,
706  Syriac,  Syriac,
707  Tagalog,  Tagalog,
708  Tagbanwa,  Tagbanwa,
709  Tai_Le,  Tai_Le,
710    Tai_Tham,
711    Tai_Viet,
712    Takri,
713  Tamil,  Tamil,
714  Telugu,  Telugu,
715  Thaana,  Thaana,
# Line 576  Thai, Line 717  Thai,
717  Tibetan,  Tibetan,
718  Tifinagh,  Tifinagh,
719  Ugaritic,  Ugaritic,
720    Vai,
721  Yi.  Yi.
722  .P  .P
723  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
724  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
725  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
726  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
727  .P  .P
728  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
729  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 642  the Lu, Ll, or Lt property, in other wor Line 784  the Lu, Ll, or Lt property, in other wor
784  a modifier or "other".  a modifier or "other".
785  .P  .P
786  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
787  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
788  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
789  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
790    PCRE_NO_UTF32_CHECK in the
791  .\" HREF  .\" HREF
792  \fBpcreapi\fP  \fBpcreapi\fP
793  .\"  .\"
# Line 659  Instead, this property is assumed for an Line 802  Instead, this property is assumed for an
802  Unicode table.  Unicode table.
803  .P  .P
804  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
805  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
806    the behaviour of current versions of Perl.
807  .P  .P
808  The \eX escape matches any number of Unicode characters that form an extended  Matching characters by Unicode property is not fast, because PCRE has to do a
809  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
810  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
811    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
812    PCRE_UCP option or by starting the pattern with (*UCP).
813    .
814    .
815    .SS Extended grapheme clusters
816    .rs
817  .sp  .sp
818  That is, it matches a character without the "mark" property, followed by zero  The \eX escape matches any number of Unicode characters that form an "extended
819  or more characters with the "mark" property, and treats the sequence as an  grapheme cluster", and treats the sequence as an atomic group
 atomic group  
820  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
821  .\" </a>  .\" </a>
822  (see below).  (see below).
823  .\"  .\"
824  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
825  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
826  non-UTF-8 mode \eX matches any one character.  .sp
827  .P    (?>\ePM\epM*)
828  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
829  a structure that contains data for over fifteen thousand characters. That is  That is, it matched a character without the "mark" property, followed by zero
830  why the traditional escape sequences such as \ed and \ew do not use Unicode  or more characters with the "mark" property. Characters with the "mark"
831  properties in PCRE.  property are typically non-spacing accents that affect the preceding character.
832    .P
833    This simple definition was extended in Unicode to include more complicated
834    kinds of composite character by giving each character a grapheme breaking
835    property, and creating rules that use these properties to define the boundaries
836    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
837    one of these clusters.
838    .P
839    \eX always matches at least one character. Then it decides whether to add
840    additional characters according to the following rules for ending a cluster:
841    .P
842    1. End at the end of the subject string.
843    .P
844    2. Do not end between CR and LF; otherwise end after any control character.
845    .P
846    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
847    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
848    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
849    character; an LVT or T character may be follwed only by a T character.
850    .P
851    4. Do not end before extending characters or spacing marks. Characters with
852    the "mark" property always have the "extend" grapheme breaking property.
853    .P
854    5. Do not end after prepend characters.
855    .P
856    6. Otherwise, end the cluster.
857    .
858    .
859    .\" HTML <a name="extraprops"></a>
860    .SS PCRE's additional properties
861    .rs
862    .sp
863    As well as the standard Unicode properties described above, PCRE supports four
864    more that make it possible to convert traditional escape sequences such as \ew
865    and \es and POSIX character classes to use Unicode properties. PCRE uses these
866    non-standard, non-Perl properties internally when PCRE_UCP is set. However,
867    they may also be used explicitly. These properties are:
868    .sp
869      Xan   Any alphanumeric character
870      Xps   Any POSIX space character
871      Xsp   Any Perl space character
872      Xwd   Any Perl "word" character
873    .sp
874    Xan matches characters that have either the L (letter) or the N (number)
875    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
876    carriage return, and any other character that has the Z (separator) property.
877    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
878    same characters as Xan, plus underscore.
879    .P
880    There is another non-standard property, Xuc, which matches any character that
881    can be represented by a Universal Character Name in C++ and other programming
882    languages. These are the characters $, @, ` (grave accent), and all characters
883    with Unicode code points greater than or equal to U+00A0, except for the
884    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
885    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
886    where H is a hexadecimal digit. Note that the Xuc property does not match these
887    sequences but the characters that they represent.)
888  .  .
889  .  .
890  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
891  .SS "Resetting the match start"  .SS "Resetting the match start"
892  .rs  .rs
893  .sp  .sp
894  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
895  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:  
896  .sp  .sp
897    foo\eKbar    foo\eKbar
898  .sp  .sp
# Line 711  For example, when the pattern Line 914  For example, when the pattern
914    (foo)\eKbar    (foo)\eKbar
915  .sp  .sp
916  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
917    .P
918    Perl documents that the use of \eK within assertions is "not well defined". In
919    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
920    ignored in negative assertions.
921  .  .
922  .  .
923  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 735  The backslashed assertions are: Line 942  The backslashed assertions are:
942    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
943    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
944  .sp  .sp
945  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
946  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
947    default it matches the corresponding literal character (for example, \eB
948    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
949    escape sequence" error is generated instead.
950  .P  .P
951  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
952  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
953  \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
954  first or last character matches \ew, respectively. Neither PCRE nor Perl has a  first or last character matches \ew, respectively. In a UTF mode, the meanings
955  separte "start of word" or "end of word" metasequence. However, whatever  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
956  follows \eb normally determines which it is. For example, the fragment  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
957  \eba matches "a" at the start of a word.  of word" or "end of word" metasequence. However, whatever follows \eb normally
958    determines which it is. For example, the fragment \eba matches "a" at the start
959    of a word.
960  .P  .P
961  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
962  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 778  regular expression. Line 990  regular expression.
990  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
991  .rs  .rs
992  .sp  .sp
993    The circumflex and dollar metacharacters are zero-width assertions. That is,
994    they test for a particular condition being true without consuming any
995    characters from the subject string.
996    .P
997  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
998  character is an assertion that is true only if the current matching point is  character is an assertion that is true only if the current matching point is at
999  at the start of the subject string. If the \fIstartoffset\fP argument of  the start of the subject string. If the \fIstartoffset\fP argument of
1000  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE  \fBpcre_exec()\fP is non-zero, circumflex can never match if the PCRE_MULTILINE
1001  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1002  meaning  meaning
# Line 797  constrained to match only at the start o Line 1013  constrained to match only at the start o
1013  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1014  to be anchored.)  to be anchored.)
1015  .P  .P
1016  A dollar character is an assertion that is true only if the current matching  The dollar character is an assertion that is true only if the current matching
1017  point is at the end of the subject string, or immediately before a newline  point is at the end of the subject string, or immediately before a newline at
1018  at the end of the string (by default). Dollar need not be the last character of  the end of the string (by default). Note, however, that it does not actually
1019  the pattern if a number of alternatives are involved, but it should be the last  match the newline. Dollar need not be the last character of the pattern if a
1020  item in any branch in which it appears. Dollar has no special meaning in a  number of alternatives are involved, but it should be the last item in any
1021  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1022  .P  .P
1023  The meaning of dollar can be changed so that it matches only at the very end of  The meaning of dollar can be changed so that it matches only at the very end of
1024  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This  the string, by setting the PCRE_DOLLAR_ENDONLY option at compile time. This
# Line 828  end of the subject in both modes, and if Line 1044  end of the subject in both modes, and if
1044  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1045  .  .
1046  .  .
1047  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1048    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1049  .rs  .rs
1050  .sp  .sp
1051  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
1052  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
1053  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1054  .P  .P
1055  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
1056  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 850  to match it. Line 1067  to match it.
1067  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
1068  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
1069  special meaning in a character class.  special meaning in a character class.
1070  .  .P
1071  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1072  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1073  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1074  .sp  name; PCRE does not support this.
1075  Outside a character class, the escape sequence \eC matches any one byte, both  .
1076  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1077  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1078  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1079  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1080  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1081    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1082    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1083    a 32-bit unit. Unlike a dot, \eC always
1084    matches line-ending characters. The feature is provided in Perl in order to
1085    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1086    used. Because \eC breaks up characters into individual data units, matching one
1087    unit with \eC in a UTF mode means that the rest of the string may start with a
1088    malformed UTF character. This has undefined results, because PCRE assumes that
1089    it is dealing with valid UTF strings (and by default it checks this at the
1090    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1091    PCRE_NO_UTF32_CHECK option is used).
1092  .P  .P
1093  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1094  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1095  .\" </a>  .\" </a>
1096  (described below),  (described below)
1097  .\"  .\"
1098  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
1099  the lookbehind.  the lookbehind.
1100    .P
1101    In general, the \eC escape sequence is best avoided. However, one
1102    way of using it that avoids the problem of malformed UTF characters is to use a
1103    lookahead to check the length of the next character, as in this pattern, which
1104    could be used with a UTF-8 string (ignore white space and line breaks):
1105    .sp
1106      (?| (?=[\ex00-\ex7f])(\eC) |
1107          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1108          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1109          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1110    .sp
1111    A group that starts with (?| resets the capturing parentheses numbers in each
1112    alternative (see
1113    .\" HTML <a href="#dupsubpatternnumber">
1114    .\" </a>
1115    "Duplicate Subpattern Numbers"
1116    .\"
1117    below). The assertions at the start of each branch check the next UTF-8
1118    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1119    character's individual bytes are then captured by the appropriate number of
1120    groups.
1121  .  .
1122  .  .
1123  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 876  the lookbehind. Line 1125  the lookbehind.
1125  .rs  .rs
1126  .sp  .sp
1127  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1128  square bracket. A closing square bracket on its own is not special by default.  square bracket. A closing square bracket on its own is not special by default.
1129  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1130  bracket causes a compile-time error. If a closing square bracket is required as  bracket causes a compile-time error. If a closing square bracket is required as
1131  a member of the class, it should be the first data character in the class  a member of the class, it should be the first data character in the class
1132  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
1133  .P  .P
1134  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In a UTF mode, the
1135  character may be more than one byte long. A matched character must be in the  character may be more than one data unit long. A matched character must be in
1136  set of characters defined by the class, unless the first character in the class  the set of characters defined by the class, unless the first character in the
1137  definition is a circumflex, in which case the subject character must not be in  class definition is a circumflex, in which case the subject character must not
1138  the set defined by the class. If a circumflex is actually required as a member  be in the set defined by the class. If a circumflex is actually required as a
1139  of the class, ensure it is not the first character, or escape it with a  member of the class, ensure it is not the first character, or escape it with a
1140  backslash.  backslash.
1141  .P  .P
1142  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
# Line 898  circumflex is not an assertion; it still Line 1147  circumflex is not an assertion; it still
1147  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
1148  string.  string.
1149  .P  .P
1150  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 (UTF-16, UTF-32) mode, characters with values greater than 255 (0xffff)
1151  class as a literal string of bytes, or by using the \ex{ escaping mechanism.  can be included in a class as a literal string of data units, or by using the
1152    \ex{ escaping mechanism.
1153  .P  .P
1154  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
1155  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
1156  "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
1157  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
1158  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
1159  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1160  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1161  If you want to use caseless matching in UTF8-mode for characters 128 and above,  If you want to use caseless matching in a UTF mode for characters 128 and
1162  you must ensure that PCRE is compiled with Unicode property support as well as  above, you must ensure that PCRE is compiled with Unicode property support as
1163  with UTF-8 support.  well as with UTF support.
1164  .P  .P
1165  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
1166  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 932  followed by two other characters. The oc Line 1182  followed by two other characters. The oc
1182  "]" can also be used to end a range.  "]" can also be used to end a range.
1183  .P  .P
1184  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
1185  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1186  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}].  
1187  .P  .P
1188  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
1189  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
1190  [][\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
1191  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
1192  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
1193  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
1194  property support.  property support.
1195  .P  .P
1196  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,
1197  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
1198  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1199  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
1200  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
1201  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1202    .\" HTML <a href="#genericchartypes">
1203    .\" </a>
1204    "Generic character types"
1205    .\"
1206    above. The escape sequence \eb has a different meaning inside a character
1207    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1208    are not special inside a character class. Like any other unrecognized escape
1209    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1210    default, but cause an error if the PCRE_EXTRA option is set.
1211    .P
1212    A circumflex can conveniently be used with the upper case character types to
1213    specify a more restricted set of characters than the matching lower case type.
1214    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1215    whereas [\ew] includes underscore. A positive character class should be read as
1216    "something OR something OR ..." and a negative class as "NOT something AND NOT
1217    something AND NOT ...".
1218  .P  .P
1219  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1220  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 969  this notation. For example, Line 1234  this notation. For example,
1234    [01[:alpha:]%]    [01[:alpha:]%]
1235  .sp  .sp
1236  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1237  are  are:
1238  .sp  .sp
1239    alnum    letters and digits    alnum    letters and digits
1240    alpha    letters    alpha    letters
# Line 980  are Line 1245  are
1245    graph    printing characters, excluding space    graph    printing characters, excluding space
1246    lower    lower case letters    lower    lower case letters
1247    print    printing characters, including space    print    printing characters, including space
1248    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1249    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1250    upper    upper case letters    upper    upper case letters
1251    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
# Line 1001  matches "1", "2", or any non-digit. PCRE Line 1266  matches "1", "2", or any non-digit. PCRE
1266  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
1267  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1268  .P  .P
1269  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
1270  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1271    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1272    character properties are used. This is achieved by replacing the POSIX classes
1273    by other sequences, as follows:
1274    .sp
1275      [:alnum:]  becomes  \ep{Xan}
1276      [:alpha:]  becomes  \ep{L}
1277      [:blank:]  becomes  \eh
1278      [:digit:]  becomes  \ep{Nd}
1279      [:lower:]  becomes  \ep{Ll}
1280      [:space:]  becomes  \ep{Xps}
1281      [:upper:]  becomes  \ep{Lu}
1282      [:word:]   becomes  \ep{Xwd}
1283    .sp
1284    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1285    classes are unchanged, and match only characters with code points less than
1286    128.
1287  .  .
1288  .  .
1289  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1056  extracts it into the global options (and Line 1337  extracts it into the global options (and
1337  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1338  .P  .P
1339  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1340  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
1341  .sp  .sp
1342    (a(?i)b)c    (a(?i)b)c
1343  .sp  .sp
# Line 1073  option settings happen at compile time. Line 1354  option settings happen at compile time.
1354  behaviour otherwise.  behaviour otherwise.
1355  .P  .P
1356  \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
1357  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
1358  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1359  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
1360  section entitled  the section entitled
1361  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1362  .\" </a>  .\" </a>
1363  "Newline sequences"  "Newline sequences"
1364  .\"  .\"
1365  above. There is also the (*UTF8) leading sequence that can be used to set UTF-8  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1366  mode; this is equivalent to setting the PCRE_UTF8 option.  sequences that can be used to set UTF and Unicode property modes; they are
1367    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1368    options, respectively. The (*UTF) sequence is a generic version that can be
1369    used with any of the libraries. However, the application can set the
1370    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1371  .  .
1372  .  .
1373  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1096  Turning part of a pattern into a subpatt Line 1381  Turning part of a pattern into a subpatt
1381  .sp  .sp
1382    cat(aract|erpillar|)    cat(aract|erpillar|)
1383  .sp  .sp
1384  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1385  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1386  .sp  .sp
1387  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
1388  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
1389  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
1390  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1391  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1392  .P  .P
1393  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
1394    numbers for the capturing subpatterns. For example, if the string "the red
1395    king" is matched against the pattern
1396  .sp  .sp
1397    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1398  .sp  .sp
# Line 1154  at captured substring number one, whiche Line 1441  at captured substring number one, whiche
1441  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
1442  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1443  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
1444  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
1445  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1446  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.  
1447  .sp  .sp
1448    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1449    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1450    # 1            2         2  3        2     3     4    # 1            2         2  3        2     3     4
1451  .sp  .sp
1452  A backreference to a numbered subpattern uses the most recent value that is set  A back reference to a numbered subpattern uses the most recent value that is
1453  for that number by any subpattern. The following pattern matches "abcabc" or  set for that number by any subpattern. The following pattern matches "abcabc"
1454  "defdef":  or "defdef":
1455  .sp  .sp
1456    /(?|(abc)|(def))\1/    /(?|(abc)|(def))\e1/
1457  .sp  .sp
1458  In contrast, a recursive or "subroutine" call to a numbered subpattern always  In contrast, a subroutine call to a numbered subpattern always refers to the
1459  refers to the first one in the pattern with the given number. The following  first one in the pattern with the given number. The following pattern matches
1460  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1461  .sp  .sp
1462    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1463  .sp  .sp
1464    If a
1465    .\" HTML <a href="#conditions">
1466    .\" </a>
1467    condition test
1468    .\"
1469    for a subpattern's having matched refers to a non-unique number, the test is
1470    true if any of the subpatterns of that number have matched.
1471  .P  .P
1472  An alternative approach to using the "branch reset" feature is to use  An alternative approach to using this "branch reset" feature is to use
1473  duplicate named subpatterns, as described in the next section.  duplicate named subpatterns, as described in the next section.
1474  .  .
1475  .  .
# Line 1189  if an expression is modified, the number Line 1482  if an expression is modified, the number
1482  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1483  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
1484  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
1485  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1486    have different names, but PCRE does not.
1487  .P  .P
1488  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
1489  (?'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
1490  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1491  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1492  .\" </a>  .\" </a>
1493  backreferences,  back references,
1494  .\"  .\"
1495  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1496  .\" </a>  .\" </a>
# Line 1216  extracting the name-to-number translatio Line 1510  extracting the name-to-number translatio
1510  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1511  .P  .P
1512  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
1513  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
1514  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
1515  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
1516  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
1517  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
1518    name, and in both cases you want to extract the abbreviation. This pattern
1519    (ignoring the line breaks) does the job:
1520  .sp  .sp
1521    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1522    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1234  subpattern, as described in the previous Line 1530  subpattern, as described in the previous
1530  .P  .P
1531  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1532  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
1533  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1534  make a reference to a non-unique named subpattern from elsewhere in the  .P
1535  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
1536  details of the interfaces for handling named subpatterns, see the  the pattern, the one that corresponds to the first occurrence of the name is
1537    used. In the absence of duplicate numbers (see the previous section) this is
1538    the one with the lowest number. If you use a named reference in a condition
1539    test (see the
1540    .\"
1541    .\" HTML <a href="#conditions">
1542    .\" </a>
1543    section about conditions
1544    .\"
1545    below), either to check whether a subpattern has matched, or to check for
1546    recursion, all subpatterns with the same name are tested. If the condition is
1547    true for any one of them, the overall condition is true. This is the same
1548    behaviour as testing by number. For further details of the interfaces for
1549    handling named subpatterns, see the
1550  .\" HREF  .\" HREF
1551  \fBpcreapi\fP  \fBpcreapi\fP
1552  .\"  .\"
1553  documentation.  documentation.
1554  .P  .P
1555  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1556  subpatterns with the same number (see the previous section) because PCRE uses  subpatterns with the same number because PCRE uses only the numbers when
1557  only the numbers when matching.  matching. For this reason, an error is given at compile time if different names
1558    are given to subpatterns with the same number. However, you can give the same
1559    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1560  .  .
1561  .  .
1562  .SH REPETITION  .SH REPETITION
# Line 1257  items: Line 1568  items:
1568    a literal data character    a literal data character
1569    the dot metacharacter    the dot metacharacter
1570    the \eC escape sequence    the \eC escape sequence
1571    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1572    the \eR escape sequence    the \eR escape sequence
1573    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1574    a character class    a character class
1575    a back reference (see next section)    a back reference (see next section)
1576    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1577    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1578  .sp  .sp
1579  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1580  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1288  where a quantifier is not allowed, or on Line 1599  where a quantifier is not allowed, or on
1599  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
1600  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1601  .P  .P
1602  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
1603  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
1604  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,
1605  support is available, \eX{3} matches three Unicode extended sequences, each of  \eX{3} matches three Unicode extended grapheme clusters, each of which may be
1606  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1607  .P  .P
1608  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
1609  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 1301  subpatterns that are referenced as Line 1612  subpatterns that are referenced as
1612  .\" </a>  .\" </a>
1613  subroutines  subroutines
1614  .\"  .\"
1615  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1616  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1617    .\" </a>
1618    "Defining subpatterns for use by reference only"
1619    .\"
1620    below). Items other than subpatterns that have a {0} quantifier are omitted
1621    from the compiled pattern.
1622  .P  .P
1623  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1624  abbreviations:  abbreviations:
# Line 1373  In cases where it is known that the subj Line 1689  In cases where it is known that the subj
1689  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1690  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1691  .P  .P
1692  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1693  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1694  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
1695  succeeds. Consider, for example:  succeeds. Consider, for example:
1696  .sp  .sp
# Line 1383  succeeds. Consider, for example: Line 1699  succeeds. Consider, for example:
1699  If the subject is "xyz123abc123" the match point is the fourth character. For  If the subject is "xyz123abc123" the match point is the fourth character. For
1700  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1701  .P  .P
1702    Another case where implicit anchoring is not applied is when the leading .* is
1703    inside an atomic group. Once again, a match at the start may fail where a later
1704    one succeeds. Consider this pattern:
1705    .sp
1706      (?>.*?a)b
1707    .sp
1708    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1709    (*PRUNE) and (*SKIP) also disable this optimization.
1710    .P
1711  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1712  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1713  .sp  .sp
# Line 1527  no such problem when named parentheses a Line 1852  no such problem when named parentheses a
1852  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1853  .P  .P
1854  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
1855  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
1856  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
1857  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
1858  .sp  .sp
1859    (ring), \e1    (ring), \e1
1860    (ring), \eg1    (ring), \eg1
# Line 1543  example: Line 1868  example:
1868    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1869  .sp  .sp
1870  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
1871  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.
1872  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
1873  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
1874  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1875  .P  .P
1876  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1877  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1589  references to it always fail by default. Line 1914  references to it always fail by default.
1914  .sp  .sp
1915    (a|(bc))\e2    (a|(bc))\e2
1916  .sp  .sp
1917  always fails if it starts to match "a" rather than "bc". However, if the  always fails if it starts to match "a" rather than "bc". However, if the
1918  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1919  unset value matches an empty string.  unset value matches an empty string.
1920  .P  .P
1921  Because there may be many capturing parentheses in a pattern, all digits  Because there may be many capturing parentheses in a pattern, all digits
1922  following a backslash are taken as part of a potential back reference number.  following a backslash are taken as part of a potential back reference number.
1923  If the pattern continues with a digit character, some delimiter must be used to  If the pattern continues with a digit character, some delimiter must be used to
1924  terminate the back reference. If the PCRE_EXTENDED option is set, this can be  terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1925  whitespace. Otherwise, the \eg{ syntax or an empty comment (see  white space. Otherwise, the \eg{ syntax or an empty comment (see
1926  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1927  .\" </a>  .\" </a>
1928  "Comments"  "Comments"
1929  .\"  .\"
1930  below) can be used.  below) can be used.
1931  .P  .
1932    .SS "Recursive back references"
1933    .rs
1934    .sp
1935  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
1936  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.
1937  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1617  to the previous iteration. In order for Line 1945  to the previous iteration. In order for
1945  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
1946  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
1947  minimum of zero.  minimum of zero.
1948    .P
1949    Back references of this type cause the group that they reference to be treated
1950    as an
1951    .\" HTML <a href="#atomicgroup">
1952    .\" </a>
1953    atomic group.
1954    .\"
1955    Once the whole group has been matched, a subsequent matching failure cannot
1956    cause backtracking into the middle of the group.
1957  .  .
1958  .  .
1959  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1636  those that look ahead of the current pos Line 1973  those that look ahead of the current pos
1973  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,
1974  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.
1975  .P  .P
1976  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
1977  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
1978  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
1979  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
1980  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
1981  because it does not make sense for negative assertions.  .P
1982    For compatibility with Perl, assertion subpatterns may be repeated; though
1983    it makes no sense to assert the same thing several times, the side effect of
1984    capturing parentheses may occasionally be useful. In practice, there only three
1985    cases:
1986    .sp
1987    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1988    However, it may contain internal capturing parenthesized groups that are called
1989    from elsewhere via the
1990    .\" HTML <a href="#subpatternsassubroutines">
1991    .\" </a>
1992    subroutine mechanism.
1993    .\"
1994    .sp
1995    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1996    were {0,1}. At run time, the rest of the pattern match is tried with and
1997    without the assertion, the order depending on the greediness of the quantifier.
1998    .sp
1999    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2000    The assertion is obeyed just once when encountered during matching.
2001  .  .
2002  .  .
2003  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1670  lookbehind assertion is needed to achiev Line 2026  lookbehind assertion is needed to achiev
2026  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
2027  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
2028  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.
2029  The Perl 5.10 backtracking control verb (*FAIL) or (*F) is essentially a  The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
 synonym for (?!).  
2030  .  .
2031  .  .
2032  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1696  is permitted, but Line 2051  is permitted, but
2051  .sp  .sp
2052  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2053  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
2054  extension compared with Perl (5.8 and 5.10), which requires all branches to  extension compared with Perl, which requires all branches to match the same
2055  match the same length of string. An assertion such as  length of string. An assertion such as
2056  .sp  .sp
2057    (?<=ab(c|de))    (?<=ab(c|de))
2058  .sp  .sp
# Line 1707  branches: Line 2062  branches:
2062  .sp  .sp
2063    (?<=abc|abde)    (?<=abc|abde)
2064  .sp  .sp
2065  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2066  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2067  .\" </a>  .\" </a>
2068  (see above)  (see above)
2069  .\"  .\"
2070  can be used instead of a lookbehind assertion to get round the fixed-length  can be used instead of a lookbehind assertion to get round the fixed-length
2071  restriction.  restriction.
2072  .P  .P
2073  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
# Line 1720  temporarily move the current position ba Line 2075  temporarily move the current position ba
2075  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2076  assertion fails.  assertion fails.
2077  .P  .P
2078  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
2079  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
2080  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
2081  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2082    permitted.
2083  .P  .P
2084  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2085  .\" </a>  .\" </a>
2086  "Subroutine"  "Subroutine"
2087  .\"  .\"
2088  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long  calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2089  as the subpattern matches a fixed-length string.  as the subpattern matches a fixed-length string.
2090  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2091  .\" </a>  .\" </a>
2092  Recursion,  Recursion,
# Line 1803  characters that are not "999". Line 2159  characters that are not "999".
2159  .sp  .sp
2160  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2161  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2162  the result of an assertion, or whether a specific capturing subpattern has  the result of an assertion, or whether a specific capturing subpattern has
2163  already been matched. The two possible forms of conditional subpattern are:  already been matched. The two possible forms of conditional subpattern are:
2164  .sp  .sp
2165    (?(condition)yes-pattern)    (?(condition)yes-pattern)
# Line 1811  already been matched. The two possible f Line 2167  already been matched. The two possible f
2167  .sp  .sp
2168  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2169  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
2170  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2171    itself contain nested subpatterns of any form, including conditional
2172    subpatterns; the restriction to two alternatives applies only at the level of
2173    the condition. This pattern fragment is an example where the alternatives are
2174    complex:
2175    .sp
2176      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2177    .sp
2178  .P  .P
2179  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2180  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1821  recursion, a pseudo-condition called DEF Line 2184  recursion, a pseudo-condition called DEF
2184  .sp  .sp
2185  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
2186  condition is true if a capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2187  matched. If there is more than one capturing subpattern with the same number  matched. If there is more than one capturing subpattern with the same number
2188  (see the earlier  (see the earlier
2189  .\"  .\"
2190  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2191  .\" </a>  .\" </a>
2192  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2193  .\"  .\"
2194  the condition is true if any of them have been set. An alternative notation is  the condition is true if any of them have matched. An alternative notation is
2195  to precede the digits with a plus or minus sign. In this case, the subpattern  to precede the digits with a plus or minus sign. In this case, the subpattern
2196  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2197  can be referenced by (?(-1), the next most recent by (?(-2), and so on. In  can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2198  looping constructs it can also make sense to refer to subsequent groups with  loops it can also make sense to refer to subsequent groups. The next
2199  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2200    zero in any of these forms is not used; it provokes a compile-time error.)
2201  .P  .P
2202  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2203  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 1844  three parts for ease of discussion: Line 2208  three parts for ease of discussion:
2208  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2209  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
2210  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
2211  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2212  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,
2213  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
2214  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2215  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 1874  Rewriting the above example to use a nam Line 2238  Rewriting the above example to use a nam
2238  .sp  .sp
2239    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2240  .sp  .sp
2241    If the name used in a condition of this kind is a duplicate, the test is
2242    applied to all subpatterns of the same name, and is true if any one of them has
2243    matched.
2244  .  .
2245  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2246  .rs  .rs
# Line 1887  letter R, for example: Line 2254  letter R, for example:
2254  .sp  .sp
2255  the condition is true if the most recent recursion is into a subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2256  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
2257  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2258    applied to all subpatterns of the same name, and is true if any one of them is
2259    the most recent recursion.
2260  .P  .P
2261  At "top level", all these recursion test conditions are false.  At "top level", all these recursion test conditions are false.
2262  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
2263  .\" </a>  .\" </a>
2264  Recursive patterns  The syntax for recursive patterns
2265  .\"  .\"
2266  are described below.  is described below.
2267  .  .
2268    .\" HTML <a name="subdefine"></a>
2269  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2270  .rs  .rs
2271  .sp  .sp
# Line 1903  If the condition is the string (DEFINE), Line 2273  If the condition is the string (DEFINE),
2273  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
2274  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2275  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
2276  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2277  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2278  .\" </a>  .\" </a>
2279  "subroutines"  subroutines
2280  .\"  .\"
2281  is described below.) For example, a pattern to match an IPv4 address could be  is described below.) For example, a pattern to match an IPv4 address such as
2282  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore white space and line
2283    breaks):
2284  .sp  .sp
2285    (?(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) )
2286    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1944  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2315  dd-aaa-dd or dd-dd-dd, where aaa are let
2315  .SH COMMENTS  .SH COMMENTS
2316  .rs  .rs
2317  .sp  .sp
2318  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
2319  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,
2320  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
2321    subpattern name or number. The characters that make up a comment play no part
2322    in the pattern matching.
2323  .P  .P
2324  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
2325  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2326  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2327    this case continues to immediately after the next newline character or
2328    character sequence in the pattern. Which characters are interpreted as newlines
2329    is controlled by the options passed to a compiling function or by a special
2330    sequence at the start of the pattern, as described in the section entitled
2331    .\" HTML <a href="#newlines">
2332    .\" </a>
2333    "Newline conventions"
2334    .\"
2335    above. Note that the end of this type of comment is a literal newline sequence
2336    in the pattern; escape sequences that happen to represent a newline do not
2337    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2338    default newline convention is in force:
2339    .sp
2340      abc #comment \en still comment
2341    .sp
2342    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2343    a newline in the pattern. The sequence \en is still literal at this stage, so
2344    it does not terminate the comment. Only an actual character with the code value
2345    0x0a (the default newline) does so.
2346  .  .
2347  .  .
2348  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1979  individual subpattern recursion. After i Line 2371  individual subpattern recursion. After i
2371  this kind of recursion was subsequently introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2372  .P  .P
2373  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
2374  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
2375  provided that it occurs inside that subpattern. (If not, it is a  given number, provided that it occurs inside that subpattern. (If not, it is a
2376  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2377  .\" </a>  .\" </a>
2378  "subroutine"  non-recursive subroutine
2379  .\"  .\"
2380  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
2381  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 1996  PCRE_EXTENDED option is set so that whit Line 2388  PCRE_EXTENDED option is set so that whit
2388  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2389  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
2390  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2391  Finally there is a closing parenthesis. Note the use of a possessive quantifier  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2392  to avoid backtracking into sequences of non-parentheses.  to avoid backtracking into sequences of non-parentheses.
2393  .P  .P
2394  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
# Line 2008  We have put the pattern into parentheses Line 2400  We have put the pattern into parentheses
2400  them instead of the whole pattern.  them instead of the whole pattern.
2401  .P  .P
2402  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
2403  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
2404  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
2405  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2406  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.  
2407  .P  .P
2408  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2409  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2410  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2411  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2412  .\" </a>  .\" </a>
2413  "subroutine"  non-recursive subroutine
2414  .\"  .\"
2415  calls, as described in the next section.  calls, as described in the next section.
2416  .P  .P
# Line 2044  the match runs for a very long time inde Line 2435  the match runs for a very long time inde
2435  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
2436  before failure can be reported.  before failure can be reported.
2437  .P  .P
2438  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
2439  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
2440  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  
2441  .\" HREF  .\" HREF
2442  \fBpcrecallout\fP  \fBpcrecallout\fP
2443  .\"  .\"
# Line 2055  documentation). If the pattern above is Line 2445  documentation). If the pattern above is
2445  .sp  .sp
2446    (ab(cd)ef)    (ab(cd)ef)
2447  .sp  .sp
2448  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
2449  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
2450  .sp  matched at the top level, its final captured value is unset, even if it was
2451    \e( ( ( [^()]++ | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2452       ^                        ^  .P
2453       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2454  .sp  obtain extra memory to store data during a recursion, which it does by using
2455  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
2456  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.  
2457  .P  .P
2458  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.
2459  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 2081  is the actual recursive call. Line 2468  is the actual recursive call.
2468  .  .
2469  .  .
2470  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2471  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2472  .rs  .rs
2473  .sp  .sp
2474  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2475  treated as an atomic group. That is, once it has matched some of the subject  (like Python, but unlike Perl), a recursive subpattern call is always treated
2476  string, it is never re-entered, even if it contains untried alternatives and  as an atomic group. That is, once it has matched some of the subject string, it
2477  there is a subsequent matching failure. This can be illustrated by the  is never re-entered, even if it contains untried alternatives and there is a
2478  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2479  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2480    characters (for example, "a", "aba", "abcba", "abcdcba"):
2481  .sp  .sp
2482    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2483  .sp  .sp
2484  The idea is that it either matches a single character, or two identical  The idea is that it either matches a single character, or two identical
2485  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE  characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2486  it does not if the pattern is longer than three characters. Consider the  it does not if the pattern is longer than three characters. Consider the
2487  subject string "abcba":  subject string "abcba":
2488  .P  .P
2489  At the top level, the first character is matched, but as it is not at the end  At the top level, the first character is matched, but as it is not at the end
2490  of the string, the first alternative fails; the second alternative is taken  of the string, the first alternative fails; the second alternative is taken
2491  and the recursion kicks in. The recursive call to subpattern 1 successfully  and the recursion kicks in. The recursive call to subpattern 1 successfully
2492  matches the next character ("b"). (Note that the beginning and end of line  matches the next character ("b"). (Note that the beginning and end of line
2493  tests are not part of the recursion).  tests are not part of the recursion).
2494  .P  .P
2495  Back at the top level, the next character ("c") is compared with what  Back at the top level, the next character ("c") is compared with what
2496  subpattern 2 matched, which was "a". This fails. Because the recursion is  subpattern 2 matched, which was "a". This fails. Because the recursion is
2497  treated as an atomic group, there are now no backtracking points, and so the  treated as an atomic group, there are now no backtracking points, and so the
2498  entire match fails. (Perl is able, at this point, to re-enter the recursion and  entire match fails. (Perl is able, at this point, to re-enter the recursion and
2499  try the second alternative.) However, if the pattern is written with the  try the second alternative.) However, if the pattern is written with the
# Line 2113  alternatives in the other order, things Line 2501  alternatives in the other order, things
2501  .sp  .sp
2502    ^((.)(?1)\e2|.)$    ^((.)(?1)\e2|.)$
2503  .sp  .sp
2504  This time, the recursing alternative is tried first, and continues to recurse  This time, the recursing alternative is tried first, and continues to recurse
2505  until it runs out of characters, at which point the recursion fails. But this  until it runs out of characters, at which point the recursion fails. But this
2506  time we do have another alternative to try at the higher level. That is the big  time we do have another alternative to try at the higher level. That is the big
2507  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2508  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2509  .P  .P
2510  To change the pattern so that matches all palindromic strings, not just those  To change the pattern so that it matches all palindromic strings, not just
2511  with an odd number of characters, it is tempting to change the pattern to this:  those with an odd number of characters, it is tempting to change the pattern to
2512    this:
2513  .sp  .sp
2514    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2515  .sp  .sp
2516  Again, this works in Perl, but not in PCRE, and for the same reason. When a  Again, this works in Perl, but not in PCRE, and for the same reason. When a
2517  deeper recursion has matched a single character, it cannot be entered again in  deeper recursion has matched a single character, it cannot be entered again in
2518  order to match an empty string. The solution is to separate the two cases, and  order to match an empty string. The solution is to separate the two cases, and
2519  write out the odd and even cases as alternatives at the higher level:  write out the odd and even cases as alternatives at the higher level:
2520  .sp  .sp
2521    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))    ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2522  .sp  .sp
2523  If you want to match typical palindromic phrases, the pattern has to ignore all  If you want to match typical palindromic phrases, the pattern has to ignore all
2524  non-word characters, which can be done like this:  non-word characters, which can be done like this:
2525  .sp  .sp
2526    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\4|\eW*+.\eW*+))\eW*+$    ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2527  .sp  .sp
2528  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A  If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2529  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note  man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2530  the use of the possessive quantifier *+ to avoid backtracking into sequences of  the use of the possessive quantifier *+ to avoid backtracking into sequences of
2531  non-word characters. Without this, PCRE takes a great deal longer (ten times or  non-word characters. Without this, PCRE takes a great deal longer (ten times or
2532  more) to match typical phrases, and Perl takes so long that you think it has  more) to match typical phrases, and Perl takes so long that you think it has
2533  gone into a loop.  gone into a loop.
# Line 2149  For example, although "abcba" is correct Line 2538  For example, although "abcba" is correct
2538  PCRE finds the palindrome "aba" at the start, then fails at top level because  PCRE finds the palindrome "aba" at the start, then fails at top level because
2539  the end of the string does not follow. Once again, it cannot jump back into the  the end of the string does not follow. Once again, it cannot jump back into the
2540  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2541    .P
2542    The second way in which PCRE and Perl differ in their recursion processing is
2543    in the handling of captured values. In Perl, when a subpattern is called
2544    recursively or as a subpattern (see the next section), it has no access to any
2545    values that were captured outside the recursion, whereas in PCRE these values
2546    can be referenced. Consider this pattern:
2547    .sp
2548      ^(.)(\e1|a(?2))
2549    .sp
2550    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2551    then in the second group, when the back reference \e1 fails to match "b", the
2552    second alternative matches "a" and then recurses. In the recursion, \e1 does
2553    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2554    match because inside the recursive call \e1 cannot access the externally set
2555    value.
2556  .  .
2557  .  .
2558  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2559  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2560  .rs  .rs
2561  .sp  .sp
2562  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
2563  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
2564  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2565  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2566  relative, as in these examples:  relative, as in these examples:
2567  .sp  .sp
# Line 2177  matches "sense and sensibility" and "res Line 2581  matches "sense and sensibility" and "res
2581  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
2582  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2583  .P  .P
2584  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2585  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
2586  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
2587  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2588  .P  subroutine call revert to their previous values afterwards.
2589  When a subpattern is used as a subroutine, processing options such as  .P
2590  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2591  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2592    different calls. For example, consider this pattern:
2593  .sp  .sp
2594    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2595  .sp  .sp
# Line 2223  same pair of parentheses when there is a Line 2628  same pair of parentheses when there is a
2628  .P  .P
2629  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
2630  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
2631  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
2632    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2633  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2634  .P  .P
2635  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
# Line 2233  For example, this pattern has two callou Line 2639  For example, this pattern has two callou
2639  .sp  .sp
2640    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2641  .sp  .sp
2642  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
2643  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2644  255.  255. If there is a conditional group in the pattern whose condition is an
2645  .P  assertion, an additional callout is inserted just before the condition. An
2646  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is  explicit callout may also be set at this position, as in this example:
2647  set), the external function is called. It is provided with the number of the  .sp
2648  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2649  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2650  may cause matching to proceed, to backtrack, or to fail altogether. A complete  Note that this applies only to assertion conditions, not to other types of
2651  description of the interface to the callout function is given in the  condition.
2652    .P
2653    During matching, when PCRE reaches a callout point, the external function is
2654    called. It is provided with the number of the callout, the position in the
2655    pattern, and, optionally, one item of data originally supplied by the caller of
2656    the matching function. The callout function may cause matching to proceed, to
2657    backtrack, or to fail altogether. A complete description of the interface to
2658    the callout function is given in the
2659  .\" HREF  .\" HREF
2660  \fBpcrecallout\fP  \fBpcrecallout\fP
2661  .\"  .\"
2662  documentation.  documentation.
2663  .  .
2664  .  .
2665    .\" HTML <a name="backtrackcontrol"></a>
2666  .SH "BACKTRACKING CONTROL"  .SH "BACKTRACKING CONTROL"
2667  .rs  .rs
2668  .sp  .sp
2669  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2670  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2671  or removal in a future version of Perl". It goes on to say: "Their usage in  change or removal in a future version of Perl". It goes on to say: "Their usage
2672  production code should be noted to avoid problems during upgrades." The same  in production code should be noted to avoid problems during upgrades." The same
2673  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2674  .P  .P
2675    The new verbs make use of what was previously invalid syntax: an opening
2676    parenthesis followed by an asterisk. They are generally of the form
2677    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2678    differently depending on whether or not a name is present. A name is any
2679    sequence of characters that does not include a closing parenthesis. The maximum
2680    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2681    libraries. If the name is empty, that is, if the closing parenthesis
2682    immediately follows the colon, the effect is as if the colon were not there.
2683    Any number of these verbs may occur in a pattern.
2684    .P
2685  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
2686  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
2687  a backtracking algorithm. With the exception of (*FAIL), which behaves like a  matching functions, because these use a backtracking algorithm. With the
2688  failing negative assertion, they cause an error if encountered by  exception of (*FAIL), which behaves like a failing negative assertion, the
2689  \fBpcre_dfa_exec()\fP.  backtracking control verbs cause an error if encountered by a DFA matching
2690  .P  function.
 If any of these verbs are used in an assertion subpattern, their effect is  
 confined to that subpattern; it does not extend to the surrounding pattern.  
 Note that assertion subpatterns are processed as anchored at the point where  
 they are tested.  
2691  .P  .P
2692  The new verbs make use of what was previously invalid syntax: an opening  The behaviour of these verbs in
2693  parenthesis followed by an asterisk. In Perl, they are generally of the form  .\" HTML <a href="#btrepeat">
2694  (*VERB:ARG) but PCRE does not support the use of arguments, so its general  .\" </a>
2695  form is just (*VERB). Any number of these verbs may occur in a pattern. There  repeated groups,
2696  are two kinds:  .\"
2697    .\" HTML <a href="#btassert">
2698    .\" </a>
2699    assertions,
2700    .\"
2701    and in
2702    .\" HTML <a href="#btsub">
2703    .\" </a>
2704    subpatterns called as subroutines
2705    .\"
2706    (whether or not recursively) is documented below.
2707    .
2708    .
2709    .\" HTML <a name="nooptimize"></a>
2710    .SS "Optimizations that affect backtracking verbs"
2711    .rs
2712    .sp
2713    PCRE contains some optimizations that are used to speed up matching by running
2714    some checks at the start of each match attempt. For example, it may know the
2715    minimum length of matching subject, or that a particular character must be
2716    present. When one of these optimizations bypasses the running of a match, any
2717    included backtracking verbs will not, of course, be processed. You can suppress
2718    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2719    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2720    pattern with (*NO_START_OPT). There is more discussion of this option in the
2721    section entitled
2722    .\" HTML <a href="pcreapi.html#execoptions">
2723    .\" </a>
2724    "Option bits for \fBpcre_exec()\fP"
2725    .\"
2726    in the
2727    .\" HREF
2728    \fBpcreapi\fP
2729    .\"
2730    documentation.
2731    .P
2732    Experiments with Perl suggest that it too has similar optimizations, sometimes
2733    leading to anomalous results.
2734    .
2735  .  .
2736  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
2737  .rs  .rs
2738  .sp  .sp
2739  The following verbs act as soon as they are encountered:  The following verbs act as soon as they are encountered. They may not be
2740    followed by a name.
2741  .sp  .sp
2742     (*ACCEPT)     (*ACCEPT)
2743  .sp  .sp
2744  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
2745  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2746  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2747  captured. (This feature was added to PCRE at release 8.00.) For example:  at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2748    assertion succeeds; in a negative assertion, the assertion fails.
2749    .P
2750    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2751    example:
2752  .sp  .sp
2753    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2754  .sp  .sp
2755  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
2756  the outer parentheses.  the outer parentheses.
2757  .sp  .sp
2758    (*FAIL) or (*F)    (*FAIL) or (*F)
2759  .sp  .sp
2760  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
2761  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
2762  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2763  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 2305  callout feature, as for example in this Line 2768  callout feature, as for example in this
2768  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
2769  each backtrack happens (in this example, 10 times).  each backtrack happens (in this example, 10 times).
2770  .  .
2771    .
2772    .SS "Recording which path was taken"
2773    .rs
2774    .sp
2775    There is one verb whose main purpose is to track how a match was arrived at,
2776    though it also has a secondary use in conjunction with advancing the match
2777    starting point (see (*SKIP) below).
2778    .sp
2779      (*MARK:NAME) or (*:NAME)
2780    .sp
2781    A name is always required with this verb. There may be as many instances of
2782    (*MARK) as you like in a pattern, and their names do not have to be unique.
2783    .P
2784    When a match succeeds, the name of the last-encountered (*MARK:NAME),
2785    (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2786    caller as described in the section entitled
2787    .\" HTML <a href="pcreapi.html#extradata">
2788    .\" </a>
2789    "Extra data for \fBpcre_exec()\fP"
2790    .\"
2791    in the
2792    .\" HREF
2793    \fBpcreapi\fP
2794    .\"
2795    documentation. Here is an example of \fBpcretest\fP output, where the /K
2796    modifier requests the retrieval and outputting of (*MARK) data:
2797    .sp
2798        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2799      data> XY
2800       0: XY
2801      MK: A
2802      XZ
2803       0: XZ
2804      MK: B
2805    .sp
2806    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2807    indicates which of the two alternatives matched. This is a more efficient way
2808    of obtaining this information than putting each alternative in its own
2809    capturing parentheses.
2810    .P
2811    If a verb with a name is encountered in a positive assertion that is true, the
2812    name is recorded and passed back if it is the last-encountered. This does not
2813    happen for negative assertions or failing positive assertions.
2814    .P
2815    After a partial match or a failed match, the last encountered name in the
2816    entire match process is returned. For example:
2817    .sp
2818        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2819      data> XP
2820      No match, mark = B
2821    .sp
2822    Note that in this unanchored example the mark is retained from the match
2823    attempt that started at the letter "X" in the subject. Subsequent match
2824    attempts starting at "P" and then with an empty string do not get as far as the
2825    (*MARK) item, but nevertheless do not reset it.
2826    .P
2827    If you are interested in (*MARK) values after failed matches, you should
2828    probably set the PCRE_NO_START_OPTIMIZE option
2829    .\" HTML <a href="#nooptimize">
2830    .\" </a>
2831    (see above)
2832    .\"
2833    to ensure that the match is always attempted.
2834    .
2835    .
2836  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
2837  .rs  .rs
2838  .sp  .sp
2839  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2840  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
2841  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
2842    the verb. However, when one of these verbs appears inside an atomic group or an
2843    assertion that is true, its effect is confined to that group, because once the
2844    group has been matched, there is never any backtracking into it. In this
2845    situation, backtracking can "jump back" to the left of the entire atomic group
2846    or assertion. (Remember also, as stated above, that this localization also
2847    applies in subroutine calls.)
2848    .P
2849    These verbs differ in exactly what kind of failure occurs when backtracking
2850    reaches them. The behaviour described below is what happens when the verb is
2851    not in a subroutine or an assertion. Subsequent sections cover these special
2852    cases.
2853  .sp  .sp
2854    (*COMMIT)    (*COMMIT)
2855  .sp  .sp
2856  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
2857  does not match. Even if the pattern is unanchored, no further attempts to find  outright if there is a later matching failure that causes backtracking to reach
2858  a match by advancing the starting point take place. Once (*COMMIT) has been  it. Even if the pattern is unanchored, no further attempts to find a match by
2859  passed, \fBpcre_exec()\fP is committed to finding a match at the current  advancing the starting point take place. If (*COMMIT) is the only backtracking
2860  starting point, or not at all. For example:  verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2861    committed to finding a match at the current starting point, or not at all. For
2862    example:
2863  .sp  .sp
2864    a+(*COMMIT)b    a+(*COMMIT)b
2865  .sp  .sp
2866  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
2867  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
2868  .sp  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2869    (*PRUNE)  match failure.
2870  .sp  .P
2871  This verb causes the match to fail at the current position if the rest of the  If there is more than one backtracking verb in a pattern, a different one that
2872  pattern does not match. If the pattern is unanchored, the normal "bumpalong"  follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2873  advance to the next starting character then happens. Backtracking can occur as  match does not always guarantee that a match must be at this starting point.
2874  usual to the left of (*PRUNE), or when matching to the right of (*PRUNE), but  .P
2875  if there is no match to the right, backtracking cannot cross (*PRUNE).  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2876  In simple cases, the use of (*PRUNE) is just an alternative to an atomic  unless PCRE's start-of-match optimizations are turned off, as shown in this
2877  group or possessive quantifier, but there are some uses of (*PRUNE) that cannot  \fBpcretest\fP example:
2878  be expressed in any other way.  .sp
2879        re> /(*COMMIT)abc/
2880      data> xyzabc
2881       0: abc
2882      xyzabc\eY
2883      No match
2884    .sp
2885    PCRE knows that any match must start with "a", so the optimization skips along
2886    the subject to "a" before running the first match attempt, which succeeds. When
2887    the optimization is disabled by the \eY escape in the second subject, the match
2888    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2889    starting points.
2890    .sp
2891      (*PRUNE) or (*PRUNE:NAME)
2892    .sp
2893    This verb causes the match to fail at the current starting position in the
2894    subject if there is a later matching failure that causes backtracking to reach
2895    it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2896    starting character then happens. Backtracking can occur as usual to the left of
2897    (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
2898    if there is no match to the right, backtracking cannot cross (*PRUNE). In
2899    simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
2900    possessive quantifier, but there are some uses of (*PRUNE) that cannot be
2901    expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2902    as (*COMMIT).
2903    .P
2904    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
2905    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2906    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2907  .sp  .sp
2908    (*SKIP)    (*SKIP)
2909  .sp  .sp
2910  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
2911  "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,
2912  subject where (*SKIP) was encountered. (*SKIP) signifies that whatever text  but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2913  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
2914    successful match. Consider:
2915  .sp  .sp
2916    a+(*SKIP)b    a+(*SKIP)b
2917  .sp  .sp
# Line 2352  effect as this example; although it woul Line 2922  effect as this example; although it woul
2922  first match attempt, the second attempt would start at the second character  first match attempt, the second attempt would start at the second character
2923  instead of skipping on to "c".  instead of skipping on to "c".
2924  .sp  .sp
2925    (*THEN)    (*SKIP:NAME)
2926  .sp  .sp
2927  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. When it is
2928  not match. That is, it cancels pending backtracking, but only within the  triggered, the previous path through the pattern is searched for the most
2929  current alternation. Its name comes from the observation that it can be used  recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
2930  for a pattern-based if-then-else block:  is to the subject position that corresponds to that (*MARK) instead of to where
2931    (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
2932    (*SKIP) is ignored.
2933    .P
2934    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
2935    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
2936    .sp
2937      (*THEN) or (*THEN:NAME)
2938    .sp
2939    This verb causes a skip to the next innermost alternative when backtracking
2940    reaches it. That is, it cancels any further backtracking within the current
2941    alternative. Its name comes from the observation that it can be used for a
2942    pattern-based if-then-else block:
2943  .sp  .sp
2944    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2945  .sp  .sp
2946  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
2947  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
2948  second alternative and tries COND2, without backtracking into COND1. If (*THEN)  second alternative and tries COND2, without backtracking into COND1. If that
2949  is used outside of any alternation, it acts exactly like (*PRUNE).  succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
2950    more alternatives, so there is a backtrack to whatever came before the entire
2951    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
2952    .P
2953    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
2954    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2955    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2956    .P
2957    A subpattern that does not contain a | character is just a part of the
2958    enclosing alternative; it is not a nested alternation with only one
2959    alternative. The effect of (*THEN) extends beyond such a subpattern to the
2960    enclosing alternative. Consider this pattern, where A, B, etc. are complex
2961    pattern fragments that do not contain any | characters at this level:
2962    .sp
2963      A (B(*THEN)C) | D
2964    .sp
2965    If A and B are matched, but there is a failure in C, matching does not
2966    backtrack into A; instead it moves to the next alternative, that is, D.
2967    However, if the subpattern containing (*THEN) is given an alternative, it
2968    behaves differently:
2969    .sp
2970      A (B(*THEN)C | (*FAIL)) | D
2971    .sp
2972    The effect of (*THEN) is now confined to the inner subpattern. After a failure
2973    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
2974    because there are no more alternatives to try. In this case, matching does now
2975    backtrack into A.
2976    .P
2977    Note that a conditional subpattern is not considered as having two
2978    alternatives, because only one is ever used. In other words, the | character in
2979    a conditional subpattern has a different meaning. Ignoring white space,
2980    consider:
2981    .sp
2982      ^.*? (?(?=a) a | b(*THEN)c )
2983    .sp
2984    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
2985    it initially matches zero characters. The condition (?=a) then fails, the
2986    character "b" is matched, but "c" is not. At this point, matching does not
2987    backtrack to .*? as might perhaps be expected from the presence of the |
2988    character. The conditional subpattern is part of the single alternative that
2989    comprises the whole pattern, and so the match fails. (If there was a backtrack
2990    into .*?, allowing it to match "b", the match would succeed.)
2991    .P
2992    The verbs just described provide four different "strengths" of control when
2993    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
2994    next alternative. (*PRUNE) comes next, failing the match at the current
2995    starting position, but allowing an advance to the next character (for an
2996    unanchored pattern). (*SKIP) is similar, except that the advance may be more
2997    than one character. (*COMMIT) is the strongest, causing the entire match to
2998    fail.
2999    .
3000    .
3001    .SS "More than one backtracking verb"
3002    .rs
3003    .sp
3004    If more than one backtracking verb is present in a pattern, the one that is
3005    backtracked onto first acts. For example, consider this pattern, where A, B,
3006    etc. are complex pattern fragments:
3007    .sp
3008      (A(*COMMIT)B(*THEN)C|ABD)
3009    .sp
3010    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3011    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3012    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3013    not always the same as Perl's. It means that if two or more backtracking verbs
3014    appear in succession, all the the last of them has no effect. Consider this
3015    example:
3016    .sp
3017      ...(*COMMIT)(*PRUNE)...
3018    .sp
3019    If there is a matching failure to the right, backtracking onto (*PRUNE) cases
3020    it to be triggered, and its action is taken. There can never be a backtrack
3021    onto (*COMMIT).
3022    .
3023    .
3024    .\" HTML <a name="btrepeat"></a>
3025    .SS "Backtracking verbs in repeated groups"
3026    .rs
3027    .sp
3028    PCRE differs from Perl in its handling of backtracking verbs in repeated
3029    groups. For example, consider:
3030    .sp
3031      /(a(*COMMIT)b)+ac/
3032    .sp
3033    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3034    the second repeat of the group acts.
3035    .
3036    .
3037    .\" HTML <a name="btassert"></a>
3038    .SS "Backtracking verbs in assertions"
3039    .rs
3040    .sp
3041    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3042    .P
3043    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3044    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3045    fail without any further processing.
3046    .P
3047    The other backtracking verbs are not treated specially if they appear in a
3048    positive assertion. In particular, (*THEN) skips to the next alternative in the
3049    innermost enclosing group that has alternations, whether or not this is within
3050    the assertion.
3051    .P
3052    Negative assertions are, however, different, in order to ensure that changing a
3053    positive assertion into a negative assertion changes its result. Backtracking
3054    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3055    without considering any further alternative branches in the assertion.
3056    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3057    within the assertion (the normal behaviour), but if the assertion does not have
3058    such an alternative, (*THEN) behaves like (*PRUNE).
3059    .
3060    .
3061    .\" HTML <a name="btsub"></a>
3062    .SS "Backtracking verbs in subroutines"
3063    .rs
3064    .sp
3065    These behaviours occur whether or not the subpattern is called recursively.
3066    Perl's treatment of subroutines is different in some cases.
3067    .P
3068    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3069    an immediate backtrack.
3070    .P
3071    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3072    succeed without any further processing. Matching then continues after the
3073    subroutine call.
3074    .P
3075    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3076    the subroutine match to fail.
3077    .P
3078    (*THEN) skips to the next alternative in the innermost enclosing group within
3079    the subpattern that has alternatives. If there is no such group within the
3080    subpattern, (*THEN) causes the subroutine match to fail.
3081  .  .
3082  .  .
3083  .SH "SEE ALSO"  .SH "SEE ALSO"
3084  .rs  .rs
3085  .sp  .sp
3086  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3087  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3088  .  .
3089  .  .
3090  .SH AUTHOR  .SH AUTHOR
# Line 2388  Cambridge CB2 3QH, England. Line 3101  Cambridge CB2 3QH, England.
3101  .rs  .rs
3102  .sp  .sp
3103  .nf  .nf
3104  Last updated: 30 September 2009  Last updated: 05 April 2013
3105  Copyright (c) 1997-2009 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3106  .fi  .fi

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