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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "03 December 2013" "PCRE 8.34"
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 20  have copious examples. Jeffrey Friedl's Line 20  have copious examples. Jeffrey Friedl's
20  published by O'Reilly, covers regular expressions in great detail. This  published by O'Reilly, covers regular expressions in great detail. This
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    This document discusses the patterns that are supported by PCRE when one its
24    main matching functions, \fBpcre_exec()\fP (8-bit) or \fBpcre[16|32]_exec()\fP
25    (16- or 32-bit), is used. PCRE also has alternative matching functions,
26    \fBpcre_dfa_exec()\fP and \fBpcre[16|32_dfa_exec()\fP, which match using a
27    different algorithm that is not Perl-compatible. Some of the features discussed
28    below are not available when DFA matching is used. The advantages and
29    disadvantages of the alternative functions, and how they differ from the normal
30    functions, are discussed in the
31    .\" HREF
32    \fBpcrematching\fP
33    .\"
34    page.
35    .
36    .
37    .SH "SPECIAL START-OF-PATTERN ITEMS"
38    .rs
39    .sp
40    A number of options that can be passed to \fBpcre_compile()\fP can also be set
41    by special items at the start of a pattern. These are not Perl-compatible, but
42    are provided to make these options accessible to pattern writers who are not
43    able to change the program that processes the pattern. Any number of these
44    items may appear, but they must all be together right at the start of the
45    pattern string, and the letters must be in upper case.
46    .
47    .
48    .SS "UTF support"
49    .rs
50    .sp
51  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,
52  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
53  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
54  \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
55  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
56    strings you must either call the compiling function with the PCRE_UTF8,
57    PCRE_UTF16, or PCRE_UTF32 option, or the pattern must start with one of
58    these special sequences:
59  .sp  .sp
60    (*UTF8)    (*UTF8)
61  .sp    (*UTF16)
62  Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8    (*UTF32)
63  option. This feature is not Perl-compatible. How setting UTF-8 mode affects    (*UTF)
64  pattern matching is mentioned in several places below. There is also a summary  .sp
65  of UTF-8 features in the  (*UTF) is a generic sequence that can be used with any of the libraries.
66  .\" HTML <a href="pcre.html#utf8support">  Starting a pattern with such a sequence is equivalent to setting the relevant
67  .\" </a>  option. How setting a UTF mode affects pattern matching is mentioned in several
68  section on UTF-8 support  places below. There is also a summary of features in the
 .\"  
 in the main  
69  .\" HREF  .\" HREF
70  \fBpcre\fP  \fBpcreunicode\fP
71  .\"  .\"
72  page.  page.
73  .P  .P
74  Another special sequence that may appear at the start of a pattern or in  Some applications that allow their users to supply patterns may wish to
75  combination with (*UTF8) is:  restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76  .sp  option is set at compile time, (*UTF) etc. are not allowed, and their
77    (*UCP)  appearance causes an error.
78    .
79    .
80    .SS "Unicode property support"
81    .rs
82  .sp  .sp
83    Another special sequence that may appear at the start of a pattern is (*UCP).
84  This has the same effect as setting the PCRE_UCP option: it causes sequences  This has the same effect as setting the PCRE_UCP option: it causes sequences
85  such as \ed and \ew to use Unicode properties to determine character types,  such as \ed and \ew to use Unicode properties to determine character types,
86  instead of recognizing only characters with codes less than 128 via a lookup  instead of recognizing only characters with codes less than 128 via a lookup
87  table.  table.
88  .P  .
89  The remainder of this document discusses the patterns that are supported by  .
90  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  .SS "Disabling auto-possessification"
91  From release 6.0, PCRE offers a second matching function,  .rs
92  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  .sp
93  Perl-compatible. Some of the features discussed below are not available when  If a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as setting
94  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  the PCRE_NO_AUTO_POSSESS option at compile time. This stops PCRE from making
95  alternative function, and how it differs from the normal function, are  quantifiers possessive when what follows cannot match the repeated item. For
96  discussed in the  example, by default a+b is treated as a++b. For more details, see the
97  .\" HREF  .\" HREF
98  \fBpcrematching\fP  \fBpcreapi\fP
99  .\"  .\"
100  page.  documentation.
101    .
102    .
103    .SS "Disabling start-up optimizations"
104    .rs
105    .sp
106    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
107    PCRE_NO_START_OPTIMIZE option either at compile or matching time. This disables
108    several optimizations for quickly reaching "no match" results. For more
109    details, see the
110    .\" HREF
111    \fBpcreapi\fP
112    .\"
113    documentation.
114  .  .
115  .  .
116  .SH "NEWLINE CONVENTIONS"  .\" HTML <a name="newlines"></a>
117    .SS "Newline conventions"
118  .rs  .rs
119  .sp  .sp
120  PCRE supports five different conventions for indicating line breaks in  PCRE supports five different conventions for indicating line breaks in
# Line 93  string with one of the following five se Line 141  string with one of the following five se
141    (*ANYCRLF)   any of the three above    (*ANYCRLF)   any of the three above
142    (*ANY)       all Unicode newline sequences    (*ANY)       all Unicode newline sequences
143  .sp  .sp
144  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
145  \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  
146  .sp  .sp
147    (*CR)a.b    (*CR)a.b
148  .sp  .sp
149  changes the convention to CR. That pattern matches "a\enb" because LF is no  changes the convention to CR. That pattern matches "a\enb" because LF is no
150  longer a newline. Note that these special settings, which are not  longer a newline. If more than one of these settings is present, the last one
 Perl-compatible, are recognized only at the very start of a pattern, and that  
 they must be in upper case. If more than one of them is present, the last one  
151  is used.  is used.
152  .P  .P
153  The newline convention affects the interpretation of the dot metacharacter when  The newline convention affects where the circumflex and dollar assertions are
154  PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not  true. It also affects the interpretation of the dot metacharacter when
155  affect what the \eR escape sequence matches. By default, this is any Unicode  PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
156  newline sequence, for Perl compatibility. However, this can be changed; see the  what the \eR escape sequence matches. By default, this is any Unicode newline
157    sequence, for Perl compatibility. However, this can be changed; see the
158  description of \eR in the section entitled  description of \eR in the section entitled
159  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
160  .\" </a>  .\" </a>
# Line 118  below. A change of \eR setting can be co Line 164  below. A change of \eR setting can be co
164  convention.  convention.
165  .  .
166  .  .
167    .SS "Setting match and recursion limits"
168    .rs
169    .sp
170    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
171    internal \fBmatch()\fP function is called and on the maximum depth of
172    recursive calls. These facilities are provided to catch runaway matches that
173    are provoked by patterns with huge matching trees (a typical example is a
174    pattern with nested unlimited repeats) and to avoid running out of system stack
175    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
176    gives an error return. The limits can also be set by items at the start of the
177    pattern of the form
178    .sp
179      (*LIMIT_MATCH=d)
180      (*LIMIT_RECURSION=d)
181    .sp
182    where d is any number of decimal digits. However, the value of the setting must
183    be less than the value set (or defaulted) by the caller of \fBpcre_exec()\fP
184    for it to have any effect. In other words, the pattern writer can lower the
185    limits set by the programmer, but not raise them. If there is more than one
186    setting of one of these limits, the lower value is used.
187    .
188    .
189    .SH "EBCDIC CHARACTER CODES"
190    .rs
191    .sp
192    PCRE can be compiled to run in an environment that uses EBCDIC as its character
193    code rather than ASCII or Unicode (typically a mainframe system). In the
194    sections below, character code values are ASCII or Unicode; in an EBCDIC
195    environment these characters may have different code values, and there are no
196    code points greater than 255.
197    .
198    .
199  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
200  .rs  .rs
201  .sp  .sp
# Line 129  corresponding characters in the subject. Line 207  corresponding characters in the subject.
207  .sp  .sp
208  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
209  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
210  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
211  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
212  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
213  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
214  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
215  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
216  UTF-8 support.  UTF support.
217  .P  .P
218  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
219  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 181  The following sections describe the use Line 259  The following sections describe the use
259  .rs  .rs
260  .sp  .sp
261  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
262  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
263  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
264  outside character classes.  both inside and outside character classes.
265  .P  .P
266  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.
267  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 191  otherwise be interpreted as a metacharac Line 269  otherwise be interpreted as a metacharac
269  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
270  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
271  .P  .P
272  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
273  pattern (other than in a character class) and characters between a # outside  backslash. All other characters (in particular, those whose codepoints are
274  a character class and the next newline are ignored. An escaping backslash can  greater than 127) are treated as literals.
275  be used to include a whitespace or # character as part of the pattern.  .P
276    If a pattern is compiled with the PCRE_EXTENDED option, most white space in the
277    pattern (other than in a character class), and characters between a # outside a
278    character class and the next newline, inclusive, are ignored. An escaping
279    backslash can be used to include a white space or # character as part of the
280    pattern.
281  .P  .P
282  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
283  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 210  Perl, $ and @ cause variable interpolati Line 293  Perl, $ and @ cause variable interpolati
293    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
294  .sp  .sp
295  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
296  An isolated \eE that is not preceded by \eQ is ignored.  An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
297    by \eE later in the pattern, the literal interpretation continues to the end of
298    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
299    a character class, this causes an error, because the character class is not
300    terminated.
301  .  .
302  .  .
303  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 224  but when a pattern is being prepared by Line 311  but when a pattern is being prepared by
311  one of the following escape sequences than the binary character it represents:  one of the following escape sequences than the binary character it represents:
312  .sp  .sp
313    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
314    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
315    \ee        escape (hex 1B)    \ee        escape (hex 1B)
316    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
317    \en        linefeed (hex 0A)    \en        linefeed (hex 0A)
318    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
319    \et        tab (hex 09)    \et        tab (hex 09)
320      \e0dd      character with octal code 0dd
321    \eddd      character with octal code ddd, or back reference    \eddd      character with octal code ddd, or back reference
322      \eo{ddd..} character with octal code ddd..
323    \exhh      character with hex code hh    \exhh      character with hex code hh
324    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
325      \euhhhh    character with hex code hhhh (JavaScript mode only)
326  .sp  .sp
327  The precise effect of \ecx is as follows: if x is a lower case letter, it  The precise effect of \ecx on ASCII characters is as follows: if x is a lower
328  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  case letter, it is converted to upper case. Then bit 6 of the character (hex
329  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex  40) is inverted. Thus \ecA to \ecZ become hex 01 to hex 1A (A is 41, Z is 5A),
330  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
331  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
332  After \ex, from zero to two hexadecimal digits are read (letters can be in  compile-time error occurs. This locks out non-ASCII characters in all modes.
333  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
334  and }, but the value of the character code must be less than 256 in non-UTF-8  The \ec facility was designed for use with ASCII characters, but with the
335  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in  extension to Unicode it is even less useful than it once was. It is, however,
336  hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code  recognized when PCRE is compiled in EBCDIC mode, where data items are always
337  point, which is 10FFFF.  bytes. In this mode, all values are valid after \ec. If the next character is a
338  .P  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
339  If characters other than hexadecimal digits appear between \ex{ and }, or if  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
340  there is no terminating }, this form of escape is not recognized. Instead, the  the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
341  initial \ex will be interpreted as a basic hexadecimal escape, with no  characters also generate different values.
 following digits, giving a character whose value is zero.  
 .P  
 Characters whose value is less than 256 can be defined by either of the two  
 syntaxes for \ex. There is no difference in the way they are handled. For  
 example, \exdc is exactly the same as \ex{dc}.  
342  .P  .P
343  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
344  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 261  specifies two binary zeros followed by a Line 346  specifies two binary zeros followed by a
346  sure you supply two digits after the initial zero if the pattern character that  sure you supply two digits after the initial zero if the pattern character that
347  follows is itself an octal digit.  follows is itself an octal digit.
348  .P  .P
349  The handling of a backslash followed by a digit other than 0 is complicated.  The escape \eo must be followed by a sequence of octal digits, enclosed in
350  Outside a character class, PCRE reads it and any following digits as a decimal  braces. An error occurs if this is not the case. This escape is a recent
351  number. If the number is less than 10, or if there have been at least that many  addition to Perl; it provides way of specifying character code points as octal
352    numbers greater than 0777, and it also allows octal numbers and back references
353    to be unambiguously specified.
354    .P
355    For greater clarity and unambiguity, it is best to avoid following \e by a
356    digit greater than zero. Instead, use \eo{} or \ex{} to specify character
357    numbers, and \eg{} to specify back references. The following paragraphs
358    describe the old, ambiguous syntax.
359    .P
360    The handling of a backslash followed by a digit other than 0 is complicated,
361    and Perl has changed in recent releases, causing PCRE also to change. Outside a
362    character class, PCRE reads the digit and any following digits as a decimal
363    number. If the number is less than 8, or if there have been at least that many
364  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
365  taken as a \fIback reference\fP. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
366  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
# Line 276  following the discussion of Line 373  following the discussion of
373  parenthesized subpatterns.  parenthesized subpatterns.
374  .\"  .\"
375  .P  .P
376  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number following \e is greater than
377  have not been that many capturing subpatterns, PCRE re-reads up to three octal  7 and there have not been that many capturing subpatterns, PCRE handles \e8 and
378  digits following the backslash, and uses them to generate a data character. Any  \e9 as the literal characters "8" and "9", and otherwise re-reads up to three
379  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a  octal digits following the backslash, using them to generate a data character.
380  character specified in octal must be less than \e400. In UTF-8 mode, values up  Any subsequent digits stand for themselves. For example:
 to \e777 are permitted. For example:  
381  .sp  .sp
382    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
383  .\" JOIN  .\" JOIN
384    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
385              previous capturing subpatterns              previous capturing subpatterns
# Line 298  to \e777 are permitted. For example: Line 394  to \e777 are permitted. For example:
394              character with octal code 113              character with octal code 113
395  .\" JOIN  .\" JOIN
396    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
397              the byte consisting entirely of 1 bits              the value 255 (decimal)
398  .\" JOIN  .\" JOIN
399    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or the two
400              followed by the two characters "8" and "1"              characters "8" and "1"
401  .sp  .sp
402  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater that are specified using this syntax
403  zero, because no more than three octal digits are ever read.  must not be introduced by a leading zero, because no more than three octal
404    digits are ever read.
405    .P
406    By default, after \ex that is not followed by {, from zero to two hexadecimal
407    digits are read (letters can be in upper or lower case). Any number of
408    hexadecimal digits may appear between \ex{ and }. If a character other than
409    a hexadecimal digit appears between \ex{ and }, or if there is no terminating
410    }, an error occurs.
411    .P
412    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
413    as just described only when it is followed by two hexadecimal digits.
414    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
415    code points greater than 256 is provided by \eu, which must be followed by
416    four hexadecimal digits; otherwise it matches a literal "u" character.
417  .P  .P
418    Characters whose value is less than 256 can be defined by either of the two
419    syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
420    way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
421    \eu00dc in JavaScript mode).
422    .
423    .
424    .SS "Constraints on character values"
425    .rs
426    .sp
427    Characters that are specified using octal or hexadecimal numbers are
428    limited to certain values, as follows:
429    .sp
430      8-bit non-UTF mode    less than 0x100
431      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
432      16-bit non-UTF mode   less than 0x10000
433      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
434      32-bit non-UTF mode   less than 0x100000000
435      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
436    .sp
437    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
438    "surrogate" codepoints), and 0xffef.
439    .
440    .
441    .SS "Escape sequences in character classes"
442    .rs
443    .sp
444  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
445  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
446  sequence \eb is interpreted as the backspace character (hex 08). The sequences  interpreted as the backspace character (hex 08).
447  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other  .P
448  unrecognized escape sequences, they are treated as the literal characters "B",  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
449  "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is  inside a character class. Like other unrecognized escape sequences, they are
450  set. Outside a character class, these sequences have different meanings.  treated as the literal characters "B", "R", and "X" by default, but cause an
451    error if the PCRE_EXTRA option is set. Outside a character class, these
452    sequences have different meanings.
453    .
454    .
455    .SS "Unsupported escape sequences"
456    .rs
457    .sp
458    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
459    handler and used to modify the case of following characters. By default, PCRE
460    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
461    option is set, \eU matches a "U" character, and \eu can be used to define a
462    character by code point, as described in the previous section.
463  .  .
464  .  .
465  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
# Line 359  Another use of backslash is for specifyi Line 506  Another use of backslash is for specifyi
506  .sp  .sp
507    \ed     any decimal digit    \ed     any decimal digit
508    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
509    \eh     any horizontal whitespace character    \eh     any horizontal white space character
510    \eH     any character that is not a horizontal whitespace character    \eH     any character that is not a horizontal white space character
511    \es     any whitespace character    \es     any white space character
512    \eS     any character that is not a whitespace character    \eS     any character that is not a white space character
513    \ev     any vertical whitespace character    \ev     any vertical white space character
514    \eV     any character that is not a vertical whitespace character    \eV     any character that is not a vertical white space character
515    \ew     any "word" character    \ew     any "word" character
516    \eW     any "non-word" character    \eW     any "non-word" character
517  .sp  .sp
# Line 374  This is the same as Line 521  This is the same as
521  .\" </a>  .\" </a>
522  the "." metacharacter  the "." metacharacter
523  .\"  .\"
524  when PCRE_DOTALL is not set.  when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
525    PCRE does not support this.
526  .P  .P
527  Each pair of lower and upper case escape sequences partitions the complete set  Each pair of lower and upper case escape sequences partitions the complete set
528  of characters into two disjoint sets. Any given character matches one, and only  of characters into two disjoint sets. Any given character matches one, and only
# Line 383  classes. They each match one character o Line 531  classes. They each match one character o
531  matching point is at the end of the subject string, all of them fail, because  matching point is at the end of the subject string, all of them fail, because
532  there is no character to match.  there is no character to match.
533  .P  .P
534  For compatibility with Perl, \es does not match the VT character (code 11).  For compatibility with Perl, \es did not used to match the VT character (code
535  This makes it different from the the POSIX "space" class. The \es characters  11), which made it different from the the POSIX "space" class. However, Perl
536  are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is  added VT at release 5.18, and PCRE followed suit at release 8.34. The default
537  included in a Perl script, \es may match the VT character. In PCRE, it never  \es characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space
538  does.  (32), which are defined as white space in the "C" locale. This list may vary if
539    locale-specific matching is taking place. For example, in some locales the
540    "non-breaking space" character (\exA0) is recognized as white space, and in
541    others the VT character is not.
542  .P  .P
543  A "word" character is an underscore or any character that is a letter or digit.  A "word" character is an underscore or any character that is a letter or digit.
544  By default, the definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
# Line 402  in the Line 553  in the
553  \fBpcreapi\fP  \fBpcreapi\fP
554  .\"  .\"
555  page). For example, in a French locale such as "fr_FR" in Unix-like systems,  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
556  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 127 are used for
557  accented letters, and these are then matched by \ew. The use of locales with  accented letters, and these are then matched by \ew. The use of locales with
558  Unicode is discouraged.  Unicode is discouraged.
559  .P  .P
560  By default, in UTF-8 mode, characters with values greater than 128 never match  By default, characters whose code points are greater than 127 never match \ed,
561  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain  \es, or \ew, and always match \eD, \eS, and \eW, although this may vary for
562  their original meanings from before UTF-8 support was available, mainly for  characters in the range 128-255 when locale-specific matching is happening.
563  efficiency reasons. However, if PCRE is compiled with Unicode property support,  These escape sequences retain their original meanings from before Unicode
564  and the PCRE_UCP option is set, the behaviour is changed so that Unicode  support was available, mainly for efficiency reasons. If PCRE is compiled with
565  properties are used to determine character types, as follows:  Unicode property support, and the PCRE_UCP option is set, the behaviour is
566  .sp  changed so that Unicode properties are used to determine character types, as
567    \ed  any character that \ep{Nd} matches (decimal digit)  follows:
568    \es  any character that \ep{Z} matches, plus HT, LF, FF, CR  .sp
569    \ew  any character that \ep{L} or \ep{N} matches, plus underscore    \ed  any character that matches \ep{Nd} (decimal digit)
570      \es  any character that matches \ep{Z} or \eh or \ev
571      \ew  any character that matches \ep{L} or \ep{N}, plus underscore
572  .sp  .sp
573  The upper case escapes match the inverse sets of characters. Note that \ed  The upper case escapes match the inverse sets of characters. Note that \ed
574  matches only decimal digits, whereas \ew matches any Unicode digit, as well as  matches only decimal digits, whereas \ew matches any Unicode digit, as well as
# Line 423  any Unicode letter, and underscore. Note Line 576  any Unicode letter, and underscore. Note
576  \eB because they are defined in terms of \ew and \eW. Matching these sequences  \eB because they are defined in terms of \ew and \eW. Matching these sequences
577  is noticeably slower when PCRE_UCP is set.  is noticeably slower when PCRE_UCP is set.
578  .P  .P
579  The sequences \eh, \eH, \ev, and \eV are Perl 5.10 features. In contrast to the  The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
580  other sequences, which match only ASCII characters by default, these always  release 5.10. In contrast to the other sequences, which match only ASCII
581  match certain high-valued codepoints in UTF-8 mode, whether or not PCRE_UCP is  characters by default, these always match certain high-valued code points,
582  set. The horizontal space characters are:  whether or not PCRE_UCP is set. The horizontal space characters are:
583  .sp  .sp
584    U+0009     Horizontal tab    U+0009     Horizontal tab (HT)
585    U+0020     Space    U+0020     Space
586    U+00A0     Non-break space    U+00A0     Non-break space
587    U+1680     Ogham space mark    U+1680     Ogham space mark
# Line 450  set. The horizontal space characters are Line 603  set. The horizontal space characters are
603  .sp  .sp
604  The vertical space characters are:  The vertical space characters are:
605  .sp  .sp
606    U+000A     Linefeed    U+000A     Linefeed (LF)
607    U+000B     Vertical tab    U+000B     Vertical tab (VT)
608    U+000C     Formfeed    U+000C     Form feed (FF)
609    U+000D     Carriage return    U+000D     Carriage return (CR)
610    U+0085     Next line    U+0085     Next line (NEL)
611    U+2028     Line separator    U+2028     Line separator
612    U+2029     Paragraph separator    U+2029     Paragraph separator
613    .sp
614    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
615    relevant.
616  .  .
617  .  .
618  .\" HTML <a name="newlineseq"></a>  .\" HTML <a name="newlineseq"></a>
# Line 464  The vertical space characters are: Line 620  The vertical space characters are:
620  .rs  .rs
621  .sp  .sp
622  Outside a character class, by default, the escape sequence \eR matches any  Outside a character class, by default, the escape sequence \eR matches any
623  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
624  equivalent to the following:  following:
625  .sp  .sp
626    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
627  .sp  .sp
# Line 476  below. Line 632  below.
632  .\"  .\"
633  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
634  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,
635  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
636  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
637  cannot be split.  cannot be split.
638  .P  .P
639  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
640  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).
641  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
642  recognized.  recognized.
# Line 496  one of the following sequences: Line 652  one of the following sequences:
652    (*BSR_ANYCRLF)   CR, LF, or CRLF only    (*BSR_ANYCRLF)   CR, LF, or CRLF only
653    (*BSR_UNICODE)   any Unicode newline sequence    (*BSR_UNICODE)   any Unicode newline sequence
654  .sp  .sp
655  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
656  \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
657  \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
658  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
659  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
660  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:  
661  .sp  .sp
662    (*ANY)(*BSR_ANYCRLF)    (*ANY)(*BSR_ANYCRLF)
663  .sp  .sp
664  They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside  They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
665  a character class, \eR is treated as an unrecognized escape sequence, and so  (*UCP) special sequences. Inside a character class, \eR is treated as an
666  matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.  unrecognized escape sequence, and so matches the letter "R" by default, but
667    causes an error if PCRE_EXTRA is set.
668  .  .
669  .  .
670  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 517  matches the letter "R" by default, but c Line 673  matches the letter "R" by default, but c
673  .sp  .sp
674  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
675  escape sequences that match characters with specific properties are available.  escape sequences that match characters with specific properties are available.
676  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
677  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.
678  The extra escape sequences are:  The extra escape sequences are:
679  .sp  .sp
680    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
681    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
682    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
683  .sp  .sp
684  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
685  script names, the general category properties, "Any", which matches any  script names, the general category properties, "Any", which matches any
# Line 552  Armenian, Line 708  Armenian,
708  Avestan,  Avestan,
709  Balinese,  Balinese,
710  Bamum,  Bamum,
711    Batak,
712  Bengali,  Bengali,
713  Bopomofo,  Bopomofo,
714    Brahmi,
715  Braille,  Braille,
716  Buginese,  Buginese,
717  Buhid,  Buhid,
718  Canadian_Aboriginal,  Canadian_Aboriginal,
719  Carian,  Carian,
720    Chakma,
721  Cham,  Cham,
722  Cherokee,  Cherokee,
723  Common,  Common,
# Line 601  Lisu, Line 760  Lisu,
760  Lycian,  Lycian,
761  Lydian,  Lydian,
762  Malayalam,  Malayalam,
763    Mandaic,
764  Meetei_Mayek,  Meetei_Mayek,
765    Meroitic_Cursive,
766    Meroitic_Hieroglyphs,
767    Miao,
768  Mongolian,  Mongolian,
769  Myanmar,  Myanmar,
770  New_Tai_Lue,  New_Tai_Lue,
# Line 620  Rejang, Line 783  Rejang,
783  Runic,  Runic,
784  Samaritan,  Samaritan,
785  Saurashtra,  Saurashtra,
786    Sharada,
787  Shavian,  Shavian,
788  Sinhala,  Sinhala,
789    Sora_Sompeng,
790  Sundanese,  Sundanese,
791  Syloti_Nagri,  Syloti_Nagri,
792  Syriac,  Syriac,
# Line 630  Tagbanwa, Line 795  Tagbanwa,
795  Tai_Le,  Tai_Le,
796  Tai_Tham,  Tai_Tham,
797  Tai_Viet,  Tai_Viet,
798    Takri,
799  Tamil,  Tamil,
800  Telugu,  Telugu,
801  Thaana,  Thaana,
# Line 704  the Lu, Ll, or Lt property, in other wor Line 870  the Lu, Ll, or Lt property, in other wor
870  a modifier or "other".  a modifier or "other".
871  .P  .P
872  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
873  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
874  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
875  (see the discussion of PCRE_NO_UTF8_CHECK in the  (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
876    PCRE_NO_UTF32_CHECK in the
877  .\" HREF  .\" HREF
878  \fBpcreapi\fP  \fBpcreapi\fP
879  .\"  .\"
# Line 721  Instead, this property is assumed for an Line 888  Instead, this property is assumed for an
888  Unicode table.  Unicode table.
889  .P  .P
890  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
891  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
892    the behaviour of current versions of Perl.
893  .P  .P
894  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
895  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
896  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
897    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
898    PCRE_UCP option or by starting the pattern with (*UCP).
899    .
900    .
901    .SS Extended grapheme clusters
902    .rs
903  .sp  .sp
904  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
905  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  
906  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
907  .\" </a>  .\" </a>
908  (see below).  (see below).
909  .\"  .\"
910  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
911  preceding character. None of them have codepoints less than 256, so in  that was equivalent to
912  non-UTF-8 mode \eX matches any one character.  .sp
913  .P    (?>\ePM\epM*)
914  Matching characters by Unicode property is not fast, because PCRE has to search  .sp
915  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
916  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"
917  properties in PCRE by default, though you can make them do so by setting the  property are typically non-spacing accents that affect the preceding character.
918  PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with  .P
919  (*UCP).  This simple definition was extended in Unicode to include more complicated
920    kinds of composite character by giving each character a grapheme breaking
921    property, and creating rules that use these properties to define the boundaries
922    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
923    one of these clusters.
924    .P
925    \eX always matches at least one character. Then it decides whether to add
926    additional characters according to the following rules for ending a cluster:
927    .P
928    1. End at the end of the subject string.
929    .P
930    2. Do not end between CR and LF; otherwise end after any control character.
931    .P
932    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
933    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
934    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
935    character; an LVT or T character may be follwed only by a T character.
936    .P
937    4. Do not end before extending characters or spacing marks. Characters with
938    the "mark" property always have the "extend" grapheme breaking property.
939    .P
940    5. Do not end after prepend characters.
941    .P
942    6. Otherwise, end the cluster.
943  .  .
944  .  .
945  .\" HTML <a name="extraprops"></a>  .\" HTML <a name="extraprops"></a>
946  .SS PCRE's additional properties  .SS PCRE's additional properties
947  .rs  .rs
948  .sp  .sp
949  As well as the standard Unicode properties described in the previous  As well as the standard Unicode properties described above, PCRE supports four
950  section, PCRE supports four more that make it possible to convert traditional  more that make it possible to convert traditional escape sequences such as \ew
951  escape sequences such as \ew and \es and POSIX character classes to use Unicode  and \es to use Unicode properties. PCRE uses these non-standard, non-Perl
952  properties. PCRE uses these non-standard, non-Perl properties internally when  properties internally when PCRE_UCP is set. However, they may also be used
953  PCRE_UCP is set. They are:  explicitly. These properties are:
954  .sp  .sp
955    Xan   Any alphanumeric character    Xan   Any alphanumeric character
956    Xps   Any POSIX space character    Xps   Any POSIX space character
# Line 763  PCRE_UCP is set. They are: Line 958  PCRE_UCP is set. They are:
958    Xwd   Any Perl "word" character    Xwd   Any Perl "word" character
959  .sp  .sp
960  Xan matches characters that have either the L (letter) or the N (number)  Xan matches characters that have either the L (letter) or the N (number)
961  property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or  property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
962  carriage return, and any other character that has the Z (separator) property.  carriage return, and any other character that has the Z (separator) property.
963  Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the  Xsp is the same as Xps; it used to exclude vertical tab, for Perl
964  same characters as Xan, plus underscore.  compatibility, but Perl changed, and so PCRE followed at release 8.34. Xwd
965    matches the same characters as Xan, plus underscore.
966    .P
967    There is another non-standard property, Xuc, which matches any character that
968    can be represented by a Universal Character Name in C++ and other programming
969    languages. These are the characters $, @, ` (grave accent), and all characters
970    with Unicode code points greater than or equal to U+00A0, except for the
971    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
972    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
973    where H is a hexadecimal digit. Note that the Xuc property does not match these
974    sequences but the characters that they represent.)
975  .  .
976  .  .
977  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
978  .SS "Resetting the match start"  .SS "Resetting the match start"
979  .rs  .rs
980  .sp  .sp
981  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
982  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:  
983  .sp  .sp
984    foo\eKbar    foo\eKbar
985  .sp  .sp
# Line 834  escape sequence" error is generated inst Line 1038  escape sequence" error is generated inst
1038  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
1039  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
1040  \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
1041  first or last character matches \ew, respectively. In UTF-8 mode, the meanings  first or last character matches \ew, respectively. In a UTF mode, the meanings
1042  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is  of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1043  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1044  of word" or "end of word" metasequence. However, whatever follows \eb normally  of word" or "end of word" metasequence. However, whatever follows \eb normally
# Line 873  regular expression. Line 1077  regular expression.
1077  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1078  .rs  .rs
1079  .sp  .sp
1080    The circumflex and dollar metacharacters are zero-width assertions. That is,
1081    they test for a particular condition being true without consuming any
1082    characters from the subject string.
1083    .P
1084  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1085  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
1086  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
1087  \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
1088  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1089  meaning  meaning
# Line 892  constrained to match only at the start o Line 1100  constrained to match only at the start o
1100  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1101  to be anchored.)  to be anchored.)
1102  .P  .P
1103  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
1104  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
1105  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
1106  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
1107  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
1108  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1109  .P  .P
1110  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
1111  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 929  end of the subject in both modes, and if Line 1137  end of the subject in both modes, and if
1137  .sp  .sp
1138  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
1139  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
1140  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1141  .P  .P
1142  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
1143  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 947  The handling of dot is entirely independ Line 1155  The handling of dot is entirely independ
1155  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
1156  special meaning in a character class.  special meaning in a character class.
1157  .P  .P
1158  The escape sequence \eN always behaves as a dot does when PCRE_DOTALL is not  The escape sequence \eN behaves like a dot, except that it is not affected by
1159  set. In other words, it matches any one character except one that signifies the  the PCRE_DOTALL option. In other words, it matches any character except one
1160  end of a line.  that signifies the end of a line. Perl also uses \eN to match characters by
1161  .  name; PCRE does not support this.
1162  .  .
1163  .SH "MATCHING A SINGLE BYTE"  .
1164  .rs  .SH "MATCHING A SINGLE DATA UNIT"
1165  .sp  .rs
1166  Outside a character class, the escape sequence \eC matches any one byte, both  .sp
1167  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  Outside a character class, the escape sequence \eC matches any one data unit,
1168  characters. The feature is provided in Perl in order to match individual bytes  whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1169  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1170  what remains in the string may be a malformed UTF-8 string. For this reason,  a 32-bit unit. Unlike a dot, \eC always
1171  the \eC escape sequence is best avoided.  matches line-ending characters. The feature is provided in Perl in order to
1172    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1173    used. Because \eC breaks up characters into individual data units, matching one
1174    unit with \eC in a UTF mode means that the rest of the string may start with a
1175    malformed UTF character. This has undefined results, because PCRE assumes that
1176    it is dealing with valid UTF strings (and by default it checks this at the
1177    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1178    PCRE_NO_UTF32_CHECK option is used).
1179  .P  .P
1180  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1181  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1182  .\" </a>  .\" </a>
1183  (described below),  (described below)
1184  .\"  .\"
1185  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
1186  the lookbehind.  the lookbehind.
1187    .P
1188    In general, the \eC escape sequence is best avoided. However, one
1189    way of using it that avoids the problem of malformed UTF characters is to use a
1190    lookahead to check the length of the next character, as in this pattern, which
1191    could be used with a UTF-8 string (ignore white space and line breaks):
1192    .sp
1193      (?| (?=[\ex00-\ex7f])(\eC) |
1194          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1195          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1196          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1197    .sp
1198    A group that starts with (?| resets the capturing parentheses numbers in each
1199    alternative (see
1200    .\" HTML <a href="#dupsubpatternnumber">
1201    .\" </a>
1202    "Duplicate Subpattern Numbers"
1203    .\"
1204    below). The assertions at the start of each branch check the next UTF-8
1205    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1206    character's individual bytes are then captured by the appropriate number of
1207    groups.
1208  .  .
1209  .  .
1210  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 982  bracket causes a compile-time error. If Line 1218  bracket causes a compile-time error. If
1218  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
1219  (after an initial circumflex, if present) or escaped with a backslash.  (after an initial circumflex, if present) or escaped with a backslash.
1220  .P  .P
1221  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
1222  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
1223  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
1224  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
1225  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
1226  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
1227  backslash.  backslash.
1228  .P  .P
1229  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 998  circumflex is not an assertion; it still Line 1234  circumflex is not an assertion; it still
1234  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
1235  string.  string.
1236  .P  .P
1237  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)
1238  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
1239    \ex{ escaping mechanism.
1240  .P  .P
1241  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
1242  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
1243  "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
1244  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
1245  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
1246  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1247  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1248  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
1249  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
1250  with UTF-8 support.  well as with UTF support.
1251  .P  .P
1252  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
1253  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 1021  The minus (hyphen) character can be used Line 1258  The minus (hyphen) character can be used
1258  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
1259  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
1260  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
1261  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class, or
1262    immediately after a range. For example, [b-d-z] matches letters in the range b
1263    to d, a hyphen character, or z.
1264  .P  .P
1265  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
1266  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
# Line 1031  the end of range, so [W-\e]46] is interp Line 1270  the end of range, so [W-\e]46] is interp
1270  followed by two other characters. The octal or hexadecimal representation of  followed by two other characters. The octal or hexadecimal representation of
1271  "]" can also be used to end a range.  "]" can also be used to end a range.
1272  .P  .P
1273    An error is generated if a POSIX character class (see below) or an escape
1274    sequence other than one that defines a single character appears at a point
1275    where a range ending character is expected. For example, [z-\exff] is valid,
1276    but [A-\ed] and [A-[:digit:]] are not.
1277    .P
1278  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
1279  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1280  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}].  
1281  .P  .P
1282  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
1283  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
1284  [][\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
1285  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
1286  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
1287  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
1288  property support.  property support.
1289  .P  .P
1290  The character types \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev, \eV, \ew, and  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1291  \eW may also appear in a character class, and add the characters that they  \eV, \ew, and \eW may appear in a character class, and add the characters that
1292  match to the class. For example, [\edABCDEF] matches any hexadecimal digit. A  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1293  circumflex can conveniently be used with the upper case character types to  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1294    and their upper case partners, just as it does when they appear outside a
1295    character class, as described in the section entitled
1296    .\" HTML <a href="#genericchartypes">
1297    .\" </a>
1298    "Generic character types"
1299    .\"
1300    above. The escape sequence \eb has a different meaning inside a character
1301    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1302    are not special inside a character class. Like any other unrecognized escape
1303    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1304    default, but cause an error if the PCRE_EXTRA option is set.
1305    .P
1306    A circumflex can conveniently be used with the upper case character types to
1307  specify a more restricted set of characters than the matching lower case type.  specify a more restricted set of characters than the matching lower case type.
1308  For example, the class [^\eW_] matches any letter or digit, but not underscore.  For example, the class [^\eW_] matches any letter or digit, but not underscore,
1309    whereas [\ew] includes underscore. A positive character class should be read as
1310    "something OR something OR ..." and a negative class as "NOT something AND NOT
1311    something AND NOT ...".
1312  .P  .P
1313  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1314  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
1315  (only at the start), opening square bracket (only when it can be interpreted as  (only at the start), opening square bracket (only when it can be interpreted as
1316  introducing a POSIX class name - see the next section), and the terminating  introducing a POSIX class name, or for a special compatibility feature - see
1317  closing square bracket. However, escaping other non-alphanumeric characters  the next two sections), and the terminating closing square bracket. However,
1318  does no harm.  escaping other non-alphanumeric characters does no harm.
1319  .  .
1320  .  .
1321  .SH "POSIX CHARACTER CLASSES"  .SH "POSIX CHARACTER CLASSES"
# Line 1081  are: Line 1340  are:
1340    lower    lower case letters    lower    lower case letters
1341    print    printing characters, including space    print    printing characters, including space
1342    punct    printing characters, excluding letters and digits and space    punct    printing characters, excluding letters and digits and space
1343    space    white space (not quite the same as \es)    space    white space (the same as \es from PCRE 8.34)
1344    upper    upper case letters    upper    upper case letters
1345    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
1346    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1347  .sp  .sp
1348  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The default "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
1349  space (32). Notice that this list includes the VT character (code 11). This  and space (32). If locale-specific matching is taking place, the list of space
1350  makes "space" different to \es, which does not include VT (for Perl  characters may be different; there may be fewer or more of them. "Space" used
1351  compatibility).  to be different to \es, which did not include VT, for Perl compatibility.
1352    However, Perl changed at release 5.18, and PCRE followed at release 8.34.
1353    "Space" and \es now match the same set of characters.
1354  .P  .P
1355  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl  The name "word" is a Perl extension, and "blank" is a GNU extension from Perl
1356  5.8. Another Perl extension is negation, which is indicated by a ^ character  5.8. Another Perl extension is negation, which is indicated by a ^ character
# Line 1101  matches "1", "2", or any non-digit. PCRE Line 1362  matches "1", "2", or any non-digit. PCRE
1362  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
1363  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1364  .P  .P
1365  By default, in UTF-8 mode, characters with values greater than 128 do not match  By default, characters with values greater than 128 do not match any of the
1366  any of the POSIX character classes. However, if the PCRE_UCP option is passed  POSIX character classes. However, if the PCRE_UCP option is passed to
1367  to \fBpcre_compile()\fP, some of the classes are changed so that Unicode  \fBpcre_compile()\fP, some of the classes are changed so that Unicode character
1368  character properties are used. This is achieved by replacing the POSIX classes  properties are used. This is achieved by replacing certain POSIX classes by
1369  by other sequences, as follows:  other sequences, as follows:
1370  .sp  .sp
1371    [:alnum:]  becomes  \ep{Xan}    [:alnum:]  becomes  \ep{Xan}
1372    [:alpha:]  becomes  \ep{L}    [:alpha:]  becomes  \ep{L}
# Line 1116  by other sequences, as follows: Line 1377  by other sequences, as follows:
1377    [:upper:]  becomes  \ep{Lu}    [:upper:]  becomes  \ep{Lu}
1378    [:word:]   becomes  \ep{Xwd}    [:word:]   becomes  \ep{Xwd}
1379  .sp  .sp
1380  Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX  Negated versions, such as [:^alpha:] use \eP instead of \ep. Three other POSIX
1381  classes are unchanged, and match only characters with code points less than  classes are handled specially in UCP mode:
1382  128.  .TP 10
1383    [:graph:]
1384    This matches characters that have glyphs that mark the page when printed. In
1385    Unicode property terms, it matches all characters with the L, M, N, P, S, or Cf
1386    properties, except for:
1387    .sp
1388      U+061C           Arabic Letter Mark
1389      U+180E           Mongolian Vowel Separator
1390      U+2066 - U+2069  Various "isolate"s
1391    .sp
1392    .TP 10
1393    [:print:]
1394    This matches the same characters as [:graph:] plus space characters that are
1395    not controls, that is, characters with the Zs property.
1396    .TP 10
1397    [:punct:]
1398    This matches all characters that have the Unicode P (punctuation) property,
1399    plus those characters whose code points are less than 128 that have the S
1400    (Symbol) property.
1401    .P
1402    The other POSIX classes are unchanged, and match only characters with code
1403    points less than 128.
1404    .
1405    .
1406    .SH "COMPATIBILITY FEATURE FOR WORD BOUNDARIES"
1407    .rs
1408    .sp
1409    In the POSIX.2 compliant library that was included in 4.4BSD Unix, the ugly
1410    syntax [[:<:]] and [[:>:]] is used for matching "start of word" and "end of
1411    word". PCRE treats these items as follows:
1412    .sp
1413      [[:<:]]  is converted to  \eb(?=\ew)
1414      [[:>:]]  is converted to  \eb(?<=\ew)
1415    .sp
1416    Only these exact character sequences are recognized. A sequence such as
1417    [a[:<:]b] provokes error for an unrecognized POSIX class name. This support is
1418    not compatible with Perl. It is provided to help migrations from other
1419    environments, and is best not used in any new patterns. Note that \eb matches
1420    at the start and the end of a word (see
1421    .\" HTML <a href="#smallassertions">
1422    .\" </a>
1423    "Simple assertions"
1424    .\"
1425    above), and in a Perl-style pattern the preceding or following character
1426    normally shows which is wanted, without the need for the assertions that are
1427    used above in order to give exactly the POSIX behaviour.
1428  .  .
1429  .  .
1430  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 1172  extracts it into the global options (and Line 1478  extracts it into the global options (and
1478  extracted by the \fBpcre_fullinfo()\fP function).  extracted by the \fBpcre_fullinfo()\fP function).
1479  .P  .P
1480  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1481  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
1482  .sp  .sp
1483    (a(?i)b)c    (a(?i)b)c
1484  .sp  .sp
# Line 1189  option settings happen at compile time. Line 1495  option settings happen at compile time.
1495  behaviour otherwise.  behaviour otherwise.
1496  .P  .P
1497  \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
1498  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
1499  pattern can contain special leading sequences such as (*CRLF) to override what  the pattern can contain special leading sequences such as (*CRLF) to override
1500  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
1501  section entitled  the section entitled
1502  .\" HTML <a href="#newlineseq">  .\" HTML <a href="#newlineseq">
1503  .\" </a>  .\" </a>
1504  "Newline sequences"  "Newline sequences"
1505  .\"  .\"
1506  above. There are also the (*UTF8) and (*UCP) leading sequences that can be used  above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1507  to set UTF-8 and Unicode property modes; they are equivalent to setting the  sequences that can be used to set UTF and Unicode property modes; they are
1508  PCRE_UTF8 and the PCRE_UCP options, respectively.  equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1509    options, respectively. The (*UTF) sequence is a generic version that can be
1510    used with any of the libraries. However, the application can set the
1511    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1512  .  .
1513  .  .
1514  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 1213  Turning part of a pattern into a subpatt Line 1522  Turning part of a pattern into a subpatt
1522  .sp  .sp
1523    cat(aract|erpillar|)    cat(aract|erpillar|)
1524  .sp  .sp
1525  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1526  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1527  .sp  .sp
1528  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
1529  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
1530  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
1531  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1532  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1533  .P  .P
1534  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
1535    numbers for the capturing subpatterns. For example, if the string "the red
1536    king" is matched against the pattern
1537  .sp  .sp
1538    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1539  .sp  .sp
# Line 1271  at captured substring number one, whiche Line 1582  at captured substring number one, whiche
1582  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
1583  alternatives. Inside a (?| group, parentheses are numbered as usual, but the  alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1584  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
1585  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
1586  branch. The following example is taken from the Perl documentation.  any branch. The following example is taken from the Perl documentation. The
1587  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.  
1588  .sp  .sp
1589    # before  ---------------branch-reset----------- after    # before  ---------------branch-reset----------- after
1590    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x    / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
# Line 1286  or "defdef": Line 1596  or "defdef":
1596  .sp  .sp
1597    /(?|(abc)|(def))\e1/    /(?|(abc)|(def))\e1/
1598  .sp  .sp
1599  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
1600  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
1601  pattern matches "abcabc" or "defabc":  "abcabc" or "defabc":
1602  .sp  .sp
1603    /(?|(abc)|(def))(?1)/    /(?|(abc)|(def))(?1)/
1604  .sp  .sp
# Line 1334  conditions, Line 1644  conditions,
1644  .\"  .\"
1645  can be made by name as well as by number.  can be made by name as well as by number.
1646  .P  .P
1647  Names consist of up to 32 alphanumeric characters and underscores. Named  Names consist of up to 32 alphanumeric characters and underscores, but must
1648  capturing parentheses are still allocated numbers as well as names, exactly as  start with a non-digit. Named capturing parentheses are still allocated numbers
1649  if the names were not present. The PCRE API provides function calls for  as well as names, exactly as if the names were not present. The PCRE API
1650  extracting the name-to-number translation table from a compiled pattern. There  provides function calls for extracting the name-to-number translation table
1651  is also a convenience function for extracting a captured substring by name.  from a compiled pattern. There is also a convenience function for extracting a
1652    captured substring by name.
1653  .P  .P
1654  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
1655  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
# Line 1364  for the first (and in this example, the Line 1675  for the first (and in this example, the
1675  matched. This saves searching to find which numbered subpattern it was.  matched. This saves searching to find which numbered subpattern it was.
1676  .P  .P
1677  If you make a back reference to a non-unique named subpattern from elsewhere in  If you make a back reference to a non-unique named subpattern from elsewhere in
1678  the pattern, the one that corresponds to the first occurrence of the name is  the pattern, the subpatterns to which the name refers are checked in the order
1679  used. In the absence of duplicate numbers (see the previous section) this is  in which they appear in the overall pattern. The first one that is set is used
1680  the one with the lowest number. If you use a named reference in a condition  for the reference. For example, this pattern matches both "foofoo" and
1681    "barbar" but not "foobar" or "barfoo":
1682    .sp
1683      (?:(?<n>foo)|(?<n>bar))\ek<n>
1684    .sp
1685    .P
1686    If you make a subroutine call to a non-unique named subpattern, the one that
1687    corresponds to the first occurrence of the name is used. In the absence of
1688    duplicate numbers (see the previous section) this is the one with the lowest
1689    number.
1690    .P
1691    If you use a named reference in a condition
1692  test (see the  test (see the
1693  .\"  .\"
1694  .\" HTML <a href="#conditions">  .\" HTML <a href="#conditions">
# Line 1386  documentation. Line 1708  documentation.
1708  \fBWarning:\fP You cannot use different names to distinguish between two  \fBWarning:\fP You cannot use different names to distinguish between two
1709  subpatterns with the same number because PCRE uses only the numbers when  subpatterns with the same number because PCRE uses only the numbers when
1710  matching. For this reason, an error is given at compile time if different names  matching. For this reason, an error is given at compile time if different names
1711  are given to subpatterns with the same number. However, you can give the same  are given to subpatterns with the same number. However, you can always give the
1712  name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.  same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1713    set.
1714  .  .
1715  .  .
1716  .SH REPETITION  .SH REPETITION
# Line 1399  items: Line 1722  items:
1722    a literal data character    a literal data character
1723    the dot metacharacter    the dot metacharacter
1724    the \eC escape sequence    the \eC escape sequence
1725    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1726    the \eR escape sequence    the \eR escape sequence
1727    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1728    a character class    a character class
1729    a back reference (see next section)    a back reference (see next section)
1730    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1731    a recursive or "subroutine" call to a subpattern    a subroutine call to a subpattern (recursive or otherwise)
1732  .sp  .sp
1733  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1734  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1430  where a quantifier is not allowed, or on Line 1753  where a quantifier is not allowed, or on
1753  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
1754  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1755  .P  .P
1756  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
1757  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
1758  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,
1759  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
1760  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1761  .P  .P
1762  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
1763  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 1443  subpatterns that are referenced as Line 1766  subpatterns that are referenced as
1766  .\" </a>  .\" </a>
1767  subroutines  subroutines
1768  .\"  .\"
1769  from elsewhere in the pattern. Items other than subpatterns that have a {0}  from elsewhere in the pattern (but see also the section entitled
1770  quantifier are omitted from the compiled pattern.  .\" HTML <a href="#subdefine">
1771    .\" </a>
1772    "Defining subpatterns for use by reference only"
1773    .\"
1774    below). Items other than subpatterns that have a {0} quantifier are omitted
1775    from the compiled pattern.
1776  .P  .P
1777  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1778  abbreviations:  abbreviations:
# Line 1515  In cases where it is known that the subj Line 1843  In cases where it is known that the subj
1843  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1844  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1845  .P  .P
1846  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1847  is inside capturing parentheses that are the subject of a back reference  is inside capturing parentheses that are the subject of a back reference
1848  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
1849  succeeds. Consider, for example:  succeeds. Consider, for example:
# Line 1525  succeeds. Consider, for example: Line 1853  succeeds. Consider, for example:
1853  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
1854  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1855  .P  .P
1856    Another case where implicit anchoring is not applied is when the leading .* is
1857    inside an atomic group. Once again, a match at the start may fail where a later
1858    one succeeds. Consider this pattern:
1859    .sp
1860      (?>.*?a)b
1861    .sp
1862    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1863    (*PRUNE) and (*SKIP) also disable this optimization.
1864    .P
1865  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1866  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1867  .sp  .sp
# Line 1669  no such problem when named parentheses a Line 2006  no such problem when named parentheses a
2006  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
2007  .P  .P
2008  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
2009  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
2010  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
2011  number, optionally enclosed in braces. These examples are all identical:  examples are all identical:
2012  .sp  .sp
2013    (ring), \e1    (ring), \e1
2014    (ring), \eg1    (ring), \eg1
# Line 1685  example: Line 2022  example:
2022    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
2023  .sp  .sp
2024  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
2025  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.
2026  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
2027  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
2028  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
2029  .P  .P
2030  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
2031  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1739  Because there may be many capturing pare Line 2076  Because there may be many capturing pare
2076  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.
2077  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
2078  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
2079  whitespace. Otherwise, the \eg{ syntax or an empty comment (see  white space. Otherwise, the \eg{ syntax or an empty comment (see
2080  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
2081  .\" </a>  .\" </a>
2082  "Comments"  "Comments"
# Line 1790  those that look ahead of the current pos Line 2127  those that look ahead of the current pos
2127  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,
2128  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.
2129  .P  .P
2130  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
2131  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
2132  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
2133  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
2134  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
2135  because it does not make sense for negative assertions.  .P
2136    For compatibility with Perl, assertion subpatterns may be repeated; though
2137    it makes no sense to assert the same thing several times, the side effect of
2138    capturing parentheses may occasionally be useful. In practice, there only three
2139    cases:
2140    .sp
2141    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2142    However, it may contain internal capturing parenthesized groups that are called
2143    from elsewhere via the
2144    .\" HTML <a href="#subpatternsassubroutines">
2145    .\" </a>
2146    subroutine mechanism.
2147    .\"
2148    .sp
2149    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2150    were {0,1}. At run time, the rest of the pattern match is tried with and
2151    without the assertion, the order depending on the greediness of the quantifier.
2152    .sp
2153    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2154    The assertion is obeyed just once when encountered during matching.
2155  .  .
2156  .  .
2157  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1824  lookbehind assertion is needed to achiev Line 2180  lookbehind assertion is needed to achiev
2180  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
2181  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
2182  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.
2183  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 (?!).  
2184  .  .
2185  .  .
2186  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1850  is permitted, but Line 2205  is permitted, but
2205  .sp  .sp
2206  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2207  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
2208  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
2209  match the same length of string. An assertion such as  length of string. An assertion such as
2210  .sp  .sp
2211    (?<=ab(c|de))    (?<=ab(c|de))
2212  .sp  .sp
# Line 1861  branches: Line 2216  branches:
2216  .sp  .sp
2217    (?<=abc|abde)    (?<=abc|abde)
2218  .sp  .sp
2219  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2220  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2221  .\" </a>  .\" </a>
2222  (see above)  (see above)
# Line 1874  temporarily move the current position ba Line 2229  temporarily move the current position ba
2229  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2230  assertion fails.  assertion fails.
2231  .P  .P
2232  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
2233  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
2234  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
2235  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2236    permitted.
2237  .P  .P
2238  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2239  .\" </a>  .\" </a>
# Line 1965  already been matched. The two possible f Line 2321  already been matched. The two possible f
2321  .sp  .sp
2322  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2323  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
2324  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2325    itself contain nested subpatterns of any form, including conditional
2326    subpatterns; the restriction to two alternatives applies only at the level of
2327    the condition. This pattern fragment is an example where the alternatives are
2328    complex:
2329    .sp
2330      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2331    .sp
2332  .P  .P
2333  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2334  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1982  matched. If there is more than one captu Line 2345  matched. If there is more than one captu
2345  .\" </a>  .\" </a>
2346  section about duplicate subpattern numbers),  section about duplicate subpattern numbers),
2347  .\"  .\"
2348  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
2349  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
2350  number is relative rather than absolute. The most recently opened parentheses  number is relative rather than absolute. The most recently opened parentheses
2351  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
2352  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
2353  constructs such as (?(+2).  parentheses to be opened can be referenced as (?(+1), and so on. (The value
2354    zero in any of these forms is not used; it provokes a compile-time error.)
2355  .P  .P
2356  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2357  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 1998  three parts for ease of discussion: Line 2362  three parts for ease of discussion:
2362  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2363  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
2364  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
2365  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2366  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,
2367  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
2368  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2369  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 2017  This makes the fragment independent of t Line 2381  This makes the fragment independent of t
2381  .sp  .sp
2382  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used  Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2383  subpattern by name. For compatibility with earlier versions of PCRE, which had  subpattern by name. For compatibility with earlier versions of PCRE, which had
2384  this facility before Perl, the syntax (?(name)...) is also recognized. However,  this facility before Perl, the syntax (?(name)...) is also recognized.
 there is a possible ambiguity with this syntax, because subpattern names may  
 consist entirely of digits. PCRE looks first for a named subpattern; if it  
 cannot find one and the name consists entirely of digits, PCRE looks for a  
 subpattern of that number, which must be greater than zero. Using subpattern  
 names that consist entirely of digits is not recommended.  
2385  .P  .P
2386  Rewriting the above example to use a named subpattern gives this:  Rewriting the above example to use a named subpattern gives this:
2387  .sp  .sp
# Line 2055  The syntax for recursive patterns Line 2414  The syntax for recursive patterns
2414  .\"  .\"
2415  is described below.  is described below.
2416  .  .
2417    .\" HTML <a name="subdefine"></a>
2418  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2419  .rs  .rs
2420  .sp  .sp
# Line 2062  If the condition is the string (DEFINE), Line 2422  If the condition is the string (DEFINE),
2422  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
2423  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2424  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
2425  "subroutines" that can be referenced from elsewhere. (The use of  subroutines that can be referenced from elsewhere. (The use of
2426  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2427  .\" </a>  .\" </a>
2428  "subroutines"  subroutines
2429  .\"  .\"
2430  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
2431  written like this (ignore whitespace and line breaks):  "192.168.23.245" could be written like this (ignore white space and line
2432    breaks):
2433  .sp  .sp
2434    (?(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) )
2435    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 2103  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2464  dd-aaa-dd or dd-dd-dd, where aaa are let
2464  .SH COMMENTS  .SH COMMENTS
2465  .rs  .rs
2466  .sp  .sp
2467  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
2468  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,
2469  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
2470    subpattern name or number. The characters that make up a comment play no part
2471    in the pattern matching.
2472  .P  .P
2473  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
2474  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2475  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2476    this case continues to immediately after the next newline character or
2477    character sequence in the pattern. Which characters are interpreted as newlines
2478    is controlled by the options passed to a compiling function or by a special
2479    sequence at the start of the pattern, as described in the section entitled
2480    .\" HTML <a href="#newlines">
2481    .\" </a>
2482    "Newline conventions"
2483    .\"
2484    above. Note that the end of this type of comment is a literal newline sequence
2485    in the pattern; escape sequences that happen to represent a newline do not
2486    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2487    default newline convention is in force:
2488    .sp
2489      abc #comment \en still comment
2490    .sp
2491    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2492    a newline in the pattern. The sequence \en is still literal at this stage, so
2493    it does not terminate the comment. Only an actual character with the code value
2494    0x0a (the default newline) does so.
2495  .  .
2496  .  .
2497  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 2138  individual subpattern recursion. After i Line 2520  individual subpattern recursion. After i
2520  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.
2521  .P  .P
2522  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
2523  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
2524  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
2525  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2526  .\" </a>  .\" </a>
2527  "subroutine"  non-recursive subroutine
2528  .\"  .\"
2529  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
2530  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
# Line 2167  We have put the pattern into parentheses Line 2549  We have put the pattern into parentheses
2549  them instead of the whole pattern.  them instead of the whole pattern.
2550  .P  .P
2551  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
2552  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
2553  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
2554  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2555  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.  
2556  .P  .P
2557  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2558  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2559  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2560  .\" HTML <a href="#subpatternsassubroutines">  .\" HTML <a href="#subpatternsassubroutines">
2561  .\" </a>  .\" </a>
2562  "subroutine"  non-recursive subroutine
2563  .\"  .\"
2564  calls, as described in the next section.  calls, as described in the next section.
2565  .P  .P
# Line 2215  documentation). If the pattern above is Line 2596  documentation). If the pattern above is
2596  .sp  .sp
2597  the value for the inner capturing parentheses (numbered 2) is "ef", which is  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2598  the last value taken on at the top level. If a capturing subpattern is not  the last value taken on at the top level. If a capturing subpattern is not
2599  matched at the top level, its final value is unset, even if it is (temporarily)  matched at the top level, its final captured value is unset, even if it was
2600  set at a deeper level.  (temporarily) set at a deeper level during the matching process.
2601  .P  .P
2602  If there are more than 15 capturing parentheses in a pattern, PCRE has to  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2603  obtain extra memory to store data during a recursion, which it does by using  obtain extra memory to store data during a recursion, which it does by using
# Line 2236  is the actual recursive call. Line 2617  is the actual recursive call.
2617  .  .
2618  .  .
2619  .\" HTML <a name="recursiondifference"></a>  .\" HTML <a name="recursiondifference"></a>
2620  .SS "Recursion difference from Perl"  .SS "Differences in recursion processing between PCRE and Perl"
2621  .rs  .rs
2622  .sp  .sp
2623  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
2624  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
2625  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
2626  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
2627  following pattern, which purports to match a palindromic string that contains  subsequent matching failure. This can be illustrated by the following pattern,
2628  an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):  which purports to match a palindromic string that contains an odd number of
2629    characters (for example, "a", "aba", "abcba", "abcdcba"):
2630  .sp  .sp
2631    ^(.|(.)(?1)\e2)$    ^(.|(.)(?1)\e2)$
2632  .sp  .sp
# Line 2274  time we do have another alternative to t Line 2656  time we do have another alternative to t
2656  difference: in the previous case the remaining alternative is at a deeper  difference: in the previous case the remaining alternative is at a deeper
2657  recursion level, which PCRE cannot use.  recursion level, which PCRE cannot use.
2658  .P  .P
2659  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
2660  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
2661    this:
2662  .sp  .sp
2663    ^((.)(?1)\e2|.?)$    ^((.)(?1)\e2|.?)$
2664  .sp  .sp
# Line 2304  For example, although "abcba" is correct Line 2687  For example, although "abcba" is correct
2687  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
2688  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
2689  recursion to try other alternatives, so the entire match fails.  recursion to try other alternatives, so the entire match fails.
2690    .P
2691    The second way in which PCRE and Perl differ in their recursion processing is
2692    in the handling of captured values. In Perl, when a subpattern is called
2693    recursively or as a subpattern (see the next section), it has no access to any
2694    values that were captured outside the recursion, whereas in PCRE these values
2695    can be referenced. Consider this pattern:
2696    .sp
2697      ^(.)(\e1|a(?2))
2698    .sp
2699    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2700    then in the second group, when the back reference \e1 fails to match "b", the
2701    second alternative matches "a" and then recurses. In the recursion, \e1 does
2702    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2703    match because inside the recursive call \e1 cannot access the externally set
2704    value.
2705  .  .
2706  .  .
2707  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2708  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2709  .rs  .rs
2710  .sp  .sp
2711  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
2712  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
2713  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2714  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2715  relative, as in these examples:  relative, as in these examples:
2716  .sp  .sp
# Line 2332  matches "sense and sensibility" and "res Line 2730  matches "sense and sensibility" and "res
2730  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
2731  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2732  .P  .P
2733  Like recursive subpatterns, a subroutine call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2734  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
2735  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
2736  matching failure. Any capturing parentheses that are set during the subroutine  subsequent matching failure. Any capturing parentheses that are set during the
2737  call revert to their previous values afterwards.  subroutine call revert to their previous values afterwards.
2738  .P  .P
2739  When a subpattern is used as a subroutine, processing options such as  Processing options such as case-independence are fixed when a subpattern is
2740  case-independence are fixed when the subpattern is defined. They cannot be  defined, so if it is used as a subroutine, such options cannot be changed for
2741  changed for different calls. For example, consider this pattern:  different calls. For example, consider this pattern:
2742  .sp  .sp
2743    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2744  .sp  .sp
# Line 2379  same pair of parentheses when there is a Line 2777  same pair of parentheses when there is a
2777  .P  .P
2778  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
2779  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
2780  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
2781    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2782  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2783  .P  .P
2784  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 2389  For example, this pattern has two callou Line 2788  For example, this pattern has two callou
2788  .sp  .sp
2789    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2790  .sp  .sp
2791  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
2792  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2793  255.  255. If there is a conditional group in the pattern whose condition is an
2794  .P  assertion, an additional callout is inserted just before the condition. An
2795  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:
2796  set), the external function is called. It is provided with the number of the  .sp
2797  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2798  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2799  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
2800  description of the interface to the callout function is given in the  condition.
2801    .P
2802    During matching, when PCRE reaches a callout point, the external function is
2803    called. It is provided with the number of the callout, the position in the
2804    pattern, and, optionally, one item of data originally supplied by the caller of
2805    the matching function. The callout function may cause matching to proceed, to
2806    backtrack, or to fail altogether.
2807    .P
2808    By default, PCRE implements a number of optimizations at compile time and
2809    matching time, and one side-effect is that sometimes callouts are skipped. If
2810    you need all possible callouts to happen, you need to set options that disable
2811    the relevant optimizations. More details, and a complete description of the
2812    interface to the callout function, are given in the
2813  .\" HREF  .\" HREF
2814  \fBpcrecallout\fP  \fBpcrecallout\fP
2815  .\"  .\"
# Line 2410  documentation. Line 2821  documentation.
2821  .rs  .rs
2822  .sp  .sp
2823  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which  Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2824  are described in the Perl documentation as "experimental and subject to change  are still described in the Perl documentation as "experimental and subject to
2825  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
2826  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
2827  remarks apply to the PCRE features described in this section.  remarks apply to the PCRE features described in this section.
2828  .P  .P
 Since these verbs are specifically related to backtracking, most of them can be  
 used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses  
 a backtracking algorithm. With the exception of (*FAIL), which behaves like a  
 failing negative assertion, they cause an error if encountered by  
 \fBpcre_dfa_exec()\fP.  
 .P  
 If any of these verbs are used in an assertion or subroutine subpattern  
 (including recursive subpatterns), their effect is confined to that subpattern;  
 it does not extend to the surrounding pattern. Note that such subpatterns are  
 processed as anchored at the point where they are tested.  
 .P  
2829  The new verbs make use of what was previously invalid syntax: an opening  The new verbs make use of what was previously invalid syntax: an opening
2830  parenthesis followed by an asterisk. They are generally of the form  parenthesis followed by an asterisk. They are generally of the form
2831  (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,  (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2832  depending on whether or not an argument is present. An name is a sequence of  differently depending on whether or not a name is present. A name is any
2833  letters, digits, and underscores. If the name is empty, that is, if the closing  sequence of characters that does not include a closing parenthesis. The maximum
2834  parenthesis immediately follows the colon, the effect is as if the colon were  length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2835  not there. Any number of these verbs may occur in a pattern.  libraries. If the name is empty, that is, if the closing parenthesis
2836    immediately follows the colon, the effect is as if the colon were not there.
2837    Any number of these verbs may occur in a pattern.
2838    .P
2839    Since these verbs are specifically related to backtracking, most of them can be
2840    used only when the pattern is to be matched using one of the traditional
2841    matching functions, because these use a backtracking algorithm. With the
2842    exception of (*FAIL), which behaves like a failing negative assertion, the
2843    backtracking control verbs cause an error if encountered by a DFA matching
2844    function.
2845  .P  .P
2846    The behaviour of these verbs in
2847    .\" HTML <a href="#btrepeat">
2848    .\" </a>
2849    repeated groups,
2850    .\"
2851    .\" HTML <a href="#btassert">
2852    .\" </a>
2853    assertions,
2854    .\"
2855    and in
2856    .\" HTML <a href="#btsub">
2857    .\" </a>
2858    subpatterns called as subroutines
2859    .\"
2860    (whether or not recursively) is documented below.
2861    .
2862    .
2863    .\" HTML <a name="nooptimize"></a>
2864    .SS "Optimizations that affect backtracking verbs"
2865    .rs
2866    .sp
2867  PCRE contains some optimizations that are used to speed up matching by running  PCRE contains some optimizations that are used to speed up matching by running
2868  some checks at the start of each match attempt. For example, it may know the  some checks at the start of each match attempt. For example, it may know the
2869  minimum length of matching subject, or that a particular character must be  minimum length of matching subject, or that a particular character must be
2870  present. When one of these optimizations suppresses the running of a match, any  present. When one of these optimizations bypasses the running of a match, any
2871  included backtracking verbs will not, of course, be processed. You can suppress  included backtracking verbs will not, of course, be processed. You can suppress
2872  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option  the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2873  when calling \fBpcre_exec()\fP.  when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2874    pattern with (*NO_START_OPT). There is more discussion of this option in the
2875    section entitled
2876    .\" HTML <a href="pcreapi.html#execoptions">
2877    .\" </a>
2878    "Option bits for \fBpcre_exec()\fP"
2879    .\"
2880    in the
2881    .\" HREF
2882    \fBpcreapi\fP
2883    .\"
2884    documentation.
2885    .P
2886    Experiments with Perl suggest that it too has similar optimizations, sometimes
2887    leading to anomalous results.
2888  .  .
2889  .  .
2890  .SS "Verbs that act immediately"  .SS "Verbs that act immediately"
# Line 2452  followed by a name. Line 2896  followed by a name.
2896     (*ACCEPT)     (*ACCEPT)
2897  .sp  .sp
2898  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
2899  pattern. When inside a recursion, only the innermost pattern is ended  pattern. However, when it is inside a subpattern that is called as a
2900  immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is  subroutine, only that subpattern is ended successfully. Matching then continues
2901  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
2902    assertion succeeds; in a negative assertion, the assertion fails.
2903    .P
2904    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2905    example:
2906  .sp  .sp
2907    A((?:A|B(*ACCEPT)|C)D)    A((?:A|B(*ACCEPT)|C)D)
2908  .sp  .sp
# Line 2463  the outer parentheses. Line 2911  the outer parentheses.
2911  .sp  .sp
2912    (*FAIL) or (*F)    (*FAIL) or (*F)
2913  .sp  .sp
2914  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
2915  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
2916  probably useful only when combined with (?{}) or (??{}). Those are, of course,  probably useful only when combined with (?{}) or (??{}). Those are, of course,
2917  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 2487  starting point (see (*SKIP) below). Line 2935  starting point (see (*SKIP) below).
2935  A name is always required with this verb. There may be as many instances of  A name is always required with this verb. There may be as many instances of
2936  (*MARK) as you like in a pattern, and their names do not have to be unique.  (*MARK) as you like in a pattern, and their names do not have to be unique.
2937  .P  .P
2938  When a match succeeds, the name of the last-encountered (*MARK) is passed back  When a match succeeds, the name of the last-encountered (*MARK:NAME),
2939  to the caller via the \fIpcre_extra\fP data structure, as described in the  (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2940    caller as described in the section entitled
2941  .\" HTML <a href="pcreapi.html#extradata">  .\" HTML <a href="pcreapi.html#extradata">
2942  .\" </a>  .\" </a>
2943  section on \fIpcre_extra\fP  "Extra data for \fBpcre_exec()\fP"
2944  .\"  .\"
2945  in the  in the
2946  .\" HREF  .\" HREF
2947  \fBpcreapi\fP  \fBpcreapi\fP
2948  .\"  .\"
2949  documentation. No data is returned for a partial match. Here is an example of  documentation. Here is an example of \fBpcretest\fP output, where the /K
2950  \fBpcretest\fP output, where the /K modifier requests the retrieval and  modifier requests the retrieval and outputting of (*MARK) data:
 outputting of (*MARK) data:  
2951  .sp  .sp
2952    /X(*MARK:A)Y|X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2953    XY    data> XY
2954     0: XY     0: XY
2955    MK: A    MK: A
2956    XZ    XZ
# Line 2514  indicates which of the two alternatives Line 2962  indicates which of the two alternatives
2962  of obtaining this information than putting each alternative in its own  of obtaining this information than putting each alternative in its own
2963  capturing parentheses.  capturing parentheses.
2964  .P  .P
2965  A name may also be returned after a failed match if the final path through the  If a verb with a name is encountered in a positive assertion that is true, the
2966  pattern involves (*MARK). However, unless (*MARK) used in conjunction with  name is recorded and passed back if it is the last-encountered. This does not
2967  (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the  happen for negative assertions or failing positive assertions.
2968  starting point for matching is advanced, the final check is often with an empty  .P
2969  string, causing a failure before (*MARK) is reached. For example:  After a partial match or a failed match, the last encountered name in the
2970  .sp  entire match process is returned. For example:
   /X(*MARK:A)Y|X(*MARK:B)Z/K  
   XP  
   No match  
 .sp  
 There are three potential starting points for this match (starting with X,  
 starting with P, and with an empty string). If the pattern is anchored, the  
 result is different:  
2971  .sp  .sp
2972    /^X(*MARK:A)Y|^X(*MARK:B)Z/K      re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2973    XP    data> XP
2974    No match, mark = B    No match, mark = B
2975  .sp  .sp
2976  PCRE's start-of-match optimizations can also interfere with this. For example,  Note that in this unanchored example the mark is retained from the match
2977  if, as a result of a call to \fBpcre_study()\fP, it knows the minimum  attempt that started at the letter "X" in the subject. Subsequent match
2978  subject length for a match, a shorter subject will not be scanned at all.  attempts starting at "P" and then with an empty string do not get as far as the
2979  .P  (*MARK) item, but nevertheless do not reset it.
2980  Note that similar anomalies (though different in detail) exist in Perl, no  .P
2981  doubt for the same reasons. The use of (*MARK) data after a failed match of an  If you are interested in (*MARK) values after failed matches, you should
2982  unanchored pattern is not recommended, unless (*COMMIT) is involved.  probably set the PCRE_NO_START_OPTIMIZE option
2983    .\" HTML <a href="#nooptimize">
2984    .\" </a>
2985    (see above)
2986    .\"
2987    to ensure that the match is always attempted.
2988  .  .
2989  .  .
2990  .SS "Verbs that act after backtracking"  .SS "Verbs that act after backtracking"
# Line 2547  unanchored pattern is not recommended, u Line 2993  unanchored pattern is not recommended, u
2993  The following verbs do nothing when they are encountered. Matching continues  The following verbs do nothing when they are encountered. Matching continues
2994  with what follows, but if there is no subsequent match, causing a backtrack to  with what follows, but if there is no subsequent match, causing a backtrack to
2995  the verb, a failure is forced. That is, backtracking cannot pass to the left of  the verb, a failure is forced. That is, backtracking cannot pass to the left of
2996  the verb. However, when one of these verbs appears inside an atomic group, its  the verb. However, when one of these verbs appears inside an atomic group or an
2997  effect is confined to that group, because once the group has been matched,  assertion that is true, its effect is confined to that group, because once the
2998  there is never any backtracking into it. In this situation, backtracking can  group has been matched, there is never any backtracking into it. In this
2999  "jump back" to the left of the entire atomic group. (Remember also, as stated  situation, backtracking can "jump back" to the left of the entire atomic group
3000  above, that this localization also applies in subroutine calls and assertions.)  or assertion. (Remember also, as stated above, that this localization also
3001    applies in subroutine calls.)
3002  .P  .P
3003  These verbs differ in exactly what kind of failure occurs when backtracking  These verbs differ in exactly what kind of failure occurs when backtracking
3004  reaches them.  reaches them. The behaviour described below is what happens when the verb is
3005    not in a subroutine or an assertion. Subsequent sections cover these special
3006    cases.
3007  .sp  .sp
3008    (*COMMIT)    (*COMMIT)
3009  .sp  .sp
3010  This verb, which may not be followed by a name, causes the whole match to fail  This verb, which may not be followed by a name, causes the whole match to fail
3011  outright if the rest of the pattern does not match. Even if the pattern is  outright if there is a later matching failure that causes backtracking to reach
3012  unanchored, no further attempts to find a match by advancing the starting point  it. Even if the pattern is unanchored, no further attempts to find a match by
3013  take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to  advancing the starting point take place. If (*COMMIT) is the only backtracking
3014  finding a match at the current starting point, or not at all. For example:  verb that is encountered, once it has been passed \fBpcre_exec()\fP is
3015    committed to finding a match at the current starting point, or not at all. For
3016    example:
3017  .sp  .sp
3018    a+(*COMMIT)b    a+(*COMMIT)b
3019  .sp  .sp
# Line 2571  dynamic anchor, or "I've started, so I m Line 3022  dynamic anchor, or "I've started, so I m
3022  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a  recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
3023  match failure.  match failure.
3024  .P  .P
3025    If there is more than one backtracking verb in a pattern, a different one that
3026    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
3027    match does not always guarantee that a match must be at this starting point.
3028    .P
3029  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,  Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
3030  unless PCRE's start-of-match optimizations are turned off, as shown in this  unless PCRE's start-of-match optimizations are turned off, as shown in this
3031  \fBpcretest\fP example:  \fBpcretest\fP example:
3032  .sp  .sp
3033    /(*COMMIT)abc/      re> /(*COMMIT)abc/
3034    xyzabc    data> xyzabc
3035     0: abc     0: abc
3036    xyzabc\eY    xyzabc\eY
3037    No match    No match
# Line 2590  starting points. Line 3045  starting points.
3045    (*PRUNE) or (*PRUNE:NAME)    (*PRUNE) or (*PRUNE:NAME)
3046  .sp  .sp
3047  This verb causes the match to fail at the current starting position in the  This verb causes the match to fail at the current starting position in the
3048  subject if the rest of the pattern does not match. If the pattern is  subject if there is a later matching failure that causes backtracking to reach
3049  unanchored, the normal "bumpalong" advance to the next starting character then  it. If the pattern is unanchored, the normal "bumpalong" advance to the next
3050  happens. Backtracking can occur as usual to the left of (*PRUNE), before it is  starting character then happens. Backtracking can occur as usual to the left of
3051  reached, or when matching to the right of (*PRUNE), but if there is no match to  (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
3052  the right, backtracking cannot cross (*PRUNE). In simple cases, the use of  if there is no match to the right, backtracking cannot cross (*PRUNE). In
3053  (*PRUNE) is just an alternative to an atomic group or possessive quantifier,  simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
3054  but there are some uses of (*PRUNE) that cannot be expressed in any other way.  possessive quantifier, but there are some uses of (*PRUNE) that cannot be
3055  The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the  expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
3056  match fails completely; the name is passed back if this is the final attempt.  as (*COMMIT).
3057  (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored  .P
3058  pattern (*PRUNE) has the same effect as (*COMMIT).  The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
3059    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3060    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3061  .sp  .sp
3062    (*SKIP)    (*SKIP)
3063  .sp  .sp
# Line 2621  instead of skipping on to "c". Line 3078  instead of skipping on to "c".
3078  .sp  .sp
3079    (*SKIP:NAME)    (*SKIP:NAME)
3080  .sp  .sp
3081  When (*SKIP) has an associated name, its behaviour is modified. If the  When (*SKIP) has an associated name, its behaviour is modified. When it is
3082  following pattern fails to match, the previous path through the pattern is  triggered, the previous path through the pattern is searched for the most
3083  searched for the most recent (*MARK) that has the same name. If one is found,  recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
3084  the "bumpalong" advance is to the subject position that corresponds to that  is to the subject position that corresponds to that (*MARK) instead of to where
3085  (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a  (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
3086  matching name is found, normal "bumpalong" of one character happens (the  (*SKIP) is ignored.
3087  (*SKIP) is ignored).  .P
3088    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
3089    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
3090  .sp  .sp
3091    (*THEN) or (*THEN:NAME)    (*THEN) or (*THEN:NAME)
3092  .sp  .sp
3093  This verb causes a skip to the next alternation in the innermost enclosing  This verb causes a skip to the next innermost alternative when backtracking
3094  group if the rest of the pattern does not match. That is, it cancels pending  reaches it. That is, it cancels any further backtracking within the current
3095  backtracking, but only within the current alternation. Its name comes from the  alternative. Its name comes from the observation that it can be used for a
3096  observation that it can be used for a pattern-based if-then-else block:  pattern-based if-then-else block:
3097  .sp  .sp
3098    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...    ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3099  .sp  .sp
3100  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
3101  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
3102  second alternative and tries COND2, without backtracking into COND1. The  second alternative and tries COND2, without backtracking into COND1. If that
3103  behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the  succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3104  overall match fails. If (*THEN) is not directly inside an alternation, it acts  more alternatives, so there is a backtrack to whatever came before the entire
3105  like (*PRUNE).  group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3106  .  .P
3107  .P  The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3108  The above verbs provide four different "strengths" of control when subsequent  It is like (*MARK:NAME) in that the name is remembered for passing back to the
3109  matching fails. (*THEN) is the weakest, carrying on the match at the next  caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3110  alternation. (*PRUNE) comes next, failing the match at the current starting  .P
3111  position, but allowing an advance to the next character (for an unanchored  A subpattern that does not contain a | character is just a part of the
3112  pattern). (*SKIP) is similar, except that the advance may be more than one  enclosing alternative; it is not a nested alternation with only one
3113  character. (*COMMIT) is the strongest, causing the entire match to fail.  alternative. The effect of (*THEN) extends beyond such a subpattern to the
3114  .P  enclosing alternative. Consider this pattern, where A, B, etc. are complex
3115  If more than one is present in a pattern, the "stongest" one wins. For example,  pattern fragments that do not contain any | characters at this level:
3116  consider this pattern, where A, B, etc. are complex pattern fragments:  .sp
3117  .sp    A (B(*THEN)C) | D
3118    (A(*COMMIT)B(*THEN)C|D)  .sp
3119  .sp  If A and B are matched, but there is a failure in C, matching does not
3120  Once A has matched, PCRE is committed to this match, at the current starting  backtrack into A; instead it moves to the next alternative, that is, D.
3121  position. If subsequently B matches, but C does not, the normal (*THEN) action  However, if the subpattern containing (*THEN) is given an alternative, it
3122  of trying the next alternation (that is, D) does not happen because (*COMMIT)  behaves differently:
3123  overrides.  .sp
3124      A (B(*THEN)C | (*FAIL)) | D
3125    .sp
3126    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3127    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3128    because there are no more alternatives to try. In this case, matching does now
3129    backtrack into A.
3130    .P
3131    Note that a conditional subpattern is not considered as having two
3132    alternatives, because only one is ever used. In other words, the | character in
3133    a conditional subpattern has a different meaning. Ignoring white space,
3134    consider:
3135    .sp
3136      ^.*? (?(?=a) a | b(*THEN)c )
3137    .sp
3138    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3139    it initially matches zero characters. The condition (?=a) then fails, the
3140    character "b" is matched, but "c" is not. At this point, matching does not
3141    backtrack to .*? as might perhaps be expected from the presence of the |
3142    character. The conditional subpattern is part of the single alternative that
3143    comprises the whole pattern, and so the match fails. (If there was a backtrack
3144    into .*?, allowing it to match "b", the match would succeed.)
3145    .P
3146    The verbs just described provide four different "strengths" of control when
3147    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3148    next alternative. (*PRUNE) comes next, failing the match at the current
3149    starting position, but allowing an advance to the next character (for an
3150    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3151    than one character. (*COMMIT) is the strongest, causing the entire match to
3152    fail.
3153    .
3154    .
3155    .SS "More than one backtracking verb"
3156    .rs
3157    .sp
3158    If more than one backtracking verb is present in a pattern, the one that is
3159    backtracked onto first acts. For example, consider this pattern, where A, B,
3160    etc. are complex pattern fragments:
3161    .sp
3162      (A(*COMMIT)B(*THEN)C|ABD)
3163    .sp
3164    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3165    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3166    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3167    not always the same as Perl's. It means that if two or more backtracking verbs
3168    appear in succession, all the the last of them has no effect. Consider this
3169    example:
3170    .sp
3171      ...(*COMMIT)(*PRUNE)...
3172    .sp
3173    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3174    it to be triggered, and its action is taken. There can never be a backtrack
3175    onto (*COMMIT).
3176    .
3177    .
3178    .\" HTML <a name="btrepeat"></a>
3179    .SS "Backtracking verbs in repeated groups"
3180    .rs
3181    .sp
3182    PCRE differs from Perl in its handling of backtracking verbs in repeated
3183    groups. For example, consider:
3184    .sp
3185      /(a(*COMMIT)b)+ac/
3186    .sp
3187    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3188    the second repeat of the group acts.
3189    .
3190    .
3191    .\" HTML <a name="btassert"></a>
3192    .SS "Backtracking verbs in assertions"
3193    .rs
3194    .sp
3195    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3196    .P
3197    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3198    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3199    fail without any further processing.
3200    .P
3201    The other backtracking verbs are not treated specially if they appear in a
3202    positive assertion. In particular, (*THEN) skips to the next alternative in the
3203    innermost enclosing group that has alternations, whether or not this is within
3204    the assertion.
3205    .P
3206    Negative assertions are, however, different, in order to ensure that changing a
3207    positive assertion into a negative assertion changes its result. Backtracking
3208    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3209    without considering any further alternative branches in the assertion.
3210    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3211    within the assertion (the normal behaviour), but if the assertion does not have
3212    such an alternative, (*THEN) behaves like (*PRUNE).
3213    .
3214    .
3215    .\" HTML <a name="btsub"></a>
3216    .SS "Backtracking verbs in subroutines"
3217    .rs
3218    .sp
3219    These behaviours occur whether or not the subpattern is called recursively.
3220    Perl's treatment of subroutines is different in some cases.
3221    .P
3222    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3223    an immediate backtrack.
3224    .P
3225    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3226    succeed without any further processing. Matching then continues after the
3227    subroutine call.
3228    .P
3229    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3230    the subroutine match to fail.
3231    .P
3232    (*THEN) skips to the next alternative in the innermost enclosing group within
3233    the subpattern that has alternatives. If there is no such group within the
3234    subpattern, (*THEN) causes the subroutine match to fail.
3235  .  .
3236  .  .
3237  .SH "SEE ALSO"  .SH "SEE ALSO"
3238  .rs  .rs
3239  .sp  .sp
3240  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3241  \fBpcresyntax\fP(3), \fBpcre\fP(3).  \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3242  .  .
3243  .  .
3244  .SH AUTHOR  .SH AUTHOR
# Line 2685  Cambridge CB2 3QH, England. Line 3255  Cambridge CB2 3QH, England.
3255  .rs  .rs
3256  .sp  .sp
3257  .nf  .nf
3258  Last updated: 26 October 2010  Last updated: 03 December 2013
3259  Copyright (c) 1997-2010 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3260  .fi  .fi

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