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1  .TH PCREPATTERN 3  .TH PCREPATTERN 3 "08 October 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"
5  .rs  .rs
6  .sp  .sp
7  The syntax and semantics of the regular expressions supported by PCRE are  The syntax and semantics of the regular expressions that are supported by PCRE
8  described below. Regular expressions are also described in the Perl  are described in detail below. There is a quick-reference syntax summary in the
9  documentation and in a number of books, some of which have copious examples.  .\" HREF
10  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers  \fBpcresyntax\fP
 regular expressions in great detail. This description of PCRE's regular  
 expressions is intended as reference material.  
 .P  
 The original operation of PCRE was on strings of one-byte characters. However,  
 there is now also support for UTF-8 character strings. To use this, you must  
 build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with  
 the PCRE_UTF8 option. How this affects pattern matching is mentioned in several  
 places below. There is also a summary of UTF-8 features in the  
 .\" HTML <a href="pcre.html#utf8support">  
 .\" </a>  
 section on UTF-8 support  
11  .\"  .\"
12  in the main  page. PCRE tries to match Perl syntax and semantics as closely as it can. PCRE
13    also supports some alternative regular expression syntax (which does not
14    conflict with the Perl syntax) in order to provide some compatibility with
15    regular expressions in Python, .NET, and Oniguruma.
16    .P
17    Perl's regular expressions are described in its own documentation, and
18    regular expressions in general are covered in a number of books, some of which
19    have copious examples. Jeffrey Friedl's "Mastering Regular Expressions",
20    published by O'Reilly, covers regular expressions in great detail. This
21    description of PCRE's regular expressions is intended as reference material.
22    .P
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  .\" HREF
32  \fBpcre\fP  \fBpcrematching\fP
33  .\"  .\"
34  page.  page.
35  .P  .
36  The remainder of this document discusses the patterns that are supported by  .
37  PCRE when its main matching function, \fBpcre_exec()\fP, is used.  .SH "SPECIAL START-OF-PATTERN ITEMS"
38  From release 6.0, PCRE offers a second matching function,  .rs
39  \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not  .sp
40  Perl-compatible. Some of the features discussed below are not available when  A number of options that can be passed to \fBpcre_compile()\fP can also be set
41  \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the  by special items at the start of a pattern. These are not Perl-compatible, but
42  alternative function, and how it differs from the normal function, are  are provided to make these options accessible to pattern writers who are not
43  discussed in the  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,
52    there is now also support for UTF-8 strings in the original library, an
53    extra library that supports 16-bit and UTF-16 character strings, and a
54    third library that supports 32-bit and UTF-32 character strings. To use these
55    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
60      (*UTF8)
61      (*UTF16)
62      (*UTF32)
63      (*UTF)
64    .sp
65    (*UTF) is a generic sequence that can be used with any of the libraries.
66    Starting a pattern with such a sequence is equivalent to setting the relevant
67    option. How setting a UTF mode affects pattern matching is mentioned in several
68    places below. There is also a summary of features in the
69  .\" HREF  .\" HREF
70  \fBpcrematching\fP  \fBpcreunicode\fP
71  .\"  .\"
72  page.  page.
73    .P
74    Some applications that allow their users to supply patterns may wish to
75    restrict them to non-UTF data for security reasons. If the PCRE_NEVER_UTF
76    option is set at compile time, (*UTF) etc. are not allowed, and their
77    appearance causes an error.
78    .
79    .
80    .SS "Unicode property support"
81    .rs
82    .sp
83    Another special sequence that may appear at the start of a pattern is
84    .sp
85      (*UCP)
86    .sp
87    This has the same effect as setting the PCRE_UCP option: it causes sequences
88    such as \ed and \ew to use Unicode properties to determine character types,
89    instead of recognizing only characters with codes less than 128 via a lookup
90    table.
91    .
92    .
93    .SS "Disabling start-up optimizations"
94    .rs
95    .sp
96    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
97    PCRE_NO_START_OPTIMIZE option either at compile or matching time.
98    .
99    .
100    .\" HTML <a name="newlines"></a>
101    .SS "Newline conventions"
102    .rs
103    .sp
104    PCRE supports five different conventions for indicating line breaks in
105    strings: a single CR (carriage return) character, a single LF (linefeed)
106    character, the two-character sequence CRLF, any of the three preceding, or any
107    Unicode newline sequence. The
108    .\" HREF
109    \fBpcreapi\fP
110    .\"
111    page has
112    .\" HTML <a href="pcreapi.html#newlines">
113    .\" </a>
114    further discussion
115    .\"
116    about newlines, and shows how to set the newline convention in the
117    \fIoptions\fP arguments for the compiling and matching functions.
118    .P
119    It is also possible to specify a newline convention by starting a pattern
120    string with one of the following five sequences:
121    .sp
122      (*CR)        carriage return
123      (*LF)        linefeed
124      (*CRLF)      carriage return, followed by linefeed
125      (*ANYCRLF)   any of the three above
126      (*ANY)       all Unicode newline sequences
127    .sp
128    These override the default and the options given to the compiling function. For
129    example, on a Unix system where LF is the default newline sequence, the pattern
130    .sp
131      (*CR)a.b
132    .sp
133    changes the convention to CR. That pattern matches "a\enb" because LF is no
134    longer a newline. If more than one of these settings is present, the last one
135    is used.
136    .P
137    The newline convention affects where the circumflex and dollar assertions are
138    true. It also affects the interpretation of the dot metacharacter when
139    PCRE_DOTALL is not set, and the behaviour of \eN. However, it does not affect
140    what the \eR escape sequence matches. By default, this is any Unicode newline
141    sequence, for Perl compatibility. However, this can be changed; see the
142    description of \eR in the section entitled
143    .\" HTML <a href="#newlineseq">
144    .\" </a>
145    "Newline sequences"
146    .\"
147    below. A change of \eR setting can be combined with a change of newline
148    convention.
149    .
150    .
151    .SS "Setting match and recursion limits"
152    .rs
153    .sp
154    The caller of \fBpcre_exec()\fP can set a limit on the number of times the
155    internal \fBmatch()\fP function is called and on the maximum depth of
156    recursive calls. These facilities are provided to catch runaway matches that
157    are provoked by patterns with huge matching trees (a typical example is a
158    pattern with nested unlimited repeats) and to avoid running out of system stack
159    by too much recursion. When one of these limits is reached, \fBpcre_exec()\fP
160    gives an error return. The limits can also be set by items at the start of the
161    pattern of the form
162    .sp
163      (*LIMIT_MATCH=d)
164      (*LIMIT_RECURSION=d)
165    .sp
166    where d is any number of decimal digits. However, the value of the setting must
167    be less than the value set by the caller of \fBpcre_exec()\fP for it to have
168    any effect. In other words, the pattern writer can lower the limit set by the
169    programmer, but not raise it. If there is more than one setting of one of these
170    limits, the lower value is used.
171    .
172    .
173    .SH "EBCDIC CHARACTER CODES"
174    .rs
175    .sp
176    PCRE can be compiled to run in an environment that uses EBCDIC as its character
177    code rather than ASCII or Unicode (typically a mainframe system). In the
178    sections below, character code values are ASCII or Unicode; in an EBCDIC
179    environment these characters may have different code values, and there are no
180    code points greater than 255.
181  .  .
182  .  .
183  .SH "CHARACTERS AND METACHARACTERS"  .SH "CHARACTERS AND METACHARACTERS"
# Line 51  corresponding characters in the subject. Line 191  corresponding characters in the subject.
191  .sp  .sp
192  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
193  caseless matching is specified (the PCRE_CASELESS option), letters are matched  caseless matching is specified (the PCRE_CASELESS option), letters are matched
194  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
195  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
196  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
197  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
198  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
199  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
200  UTF-8 support.  UTF support.
201  .P  .P
202  The power of regular expressions comes from the ability to include alternatives  The power of regular expressions comes from the ability to include alternatives
203  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 103  The following sections describe the use Line 243  The following sections describe the use
243  .rs  .rs
244  .sp  .sp
245  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
246  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
247  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
248  outside character classes.  both inside and outside character classes.
249  .P  .P
250  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.
251  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
# Line 113  otherwise be interpreted as a metacharac Line 253  otherwise be interpreted as a metacharac
253  non-alphanumeric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
254  particular, if you want to match a backslash, you write \e\e.  particular, if you want to match a backslash, you write \e\e.
255  .P  .P
256  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
257    backslash. All other characters (in particular, those whose codepoints are
258    greater than 127) are treated as literals.
259    .P
260    If a pattern is compiled with the PCRE_EXTENDED option, white space in the
261  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
262  a character class and the next newline are ignored. An escaping backslash can  a character class and the next newline are ignored. An escaping backslash can
263  be used to include a whitespace or # character as part of the pattern.  be used to include a white space or # character as part of the pattern.
264  .P  .P
265  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
266  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 132  Perl, $ and @ cause variable interpolati Line 276  Perl, $ and @ cause variable interpolati
276    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
277  .sp  .sp
278  The \eQ...\eE sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
279    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
280    by \eE later in the pattern, the literal interpretation continues to the end of
281    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
282    a character class, this causes an error, because the character class is not
283    terminated.
284  .  .
285  .  .
286  .\" HTML <a name="digitsafterbackslash"></a>  .\" HTML <a name="digitsafterbackslash"></a>
# Line 141  The \eQ...\eE sequence is recognized bot Line 290  The \eQ...\eE sequence is recognized bot
290  A second use of backslash provides a way of encoding non-printing characters  A second use of backslash provides a way of encoding non-printing characters
291  in patterns in a visible manner. There is no restriction on the appearance of  in patterns in a visible manner. There is no restriction on the appearance of
292  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
293  but when a pattern is being prepared by text editing, it is usually easier to  but when a pattern is being prepared by text editing, it is often easier to use
294  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
 represents:  
295  .sp  .sp
296    \ea        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
297    \ecx       "control-x", where x is any character    \ecx       "control-x", where x is any ASCII character
298    \ee        escape (hex 1B)    \ee        escape (hex 1B)
299    \ef        formfeed (hex 0C)    \ef        form feed (hex 0C)
300    \en        newline (hex 0A)    \en        linefeed (hex 0A)
301    \er        carriage return (hex 0D)    \er        carriage return (hex 0D)
302    \et        tab (hex 09)    \et        tab (hex 09)
303    \eddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or back reference
304    \exhh      character with hex code hh    \exhh      character with hex code hh
305    \ex{hhh..} character with hex code hhh..    \ex{hhh..} character with hex code hhh.. (non-JavaScript mode)
306      \euhhhh    character with hex code hhhh (JavaScript mode only)
307  .sp  .sp
308  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
309  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
310  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),
311  7B.  but \ec{ becomes hex 3B ({ is 7B), and \ec; becomes hex 7B (; is 3B). If the
312  .P  data item (byte or 16-bit value) following \ec has a value greater than 127, a
313  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.
314  upper or lower case). Any number of hexadecimal digits may appear between \ex{  .P
315  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
316  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value  extension to Unicode it is even less useful than it once was. It is, however,
317  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{  recognized when PCRE is compiled in EBCDIC mode, where data items are always
318  and }, or if there is no terminating }, this form of escape is not recognized.  bytes. In this mode, all values are valid after \ec. If the next character is a
319  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,  lower case letter, it is converted to upper case. Then the 0xc0 bits of the
320  with no following digits, giving a character whose value is zero.  byte are inverted. Thus \ecA becomes hex 01, as in ASCII (A is C1), but because
321    the EBCDIC letters are disjoint, \ecZ becomes hex 29 (Z is E9), and other
322    characters also generate different values.
323    .P
324    By default, after \ex, from zero to two hexadecimal digits are read (letters
325    can be in upper or lower case). Any number of hexadecimal digits may appear
326    between \ex{ and }, but the character code is constrained as follows:
327    .sp
328      8-bit non-UTF mode    less than 0x100
329      8-bit UTF-8 mode      less than 0x10ffff and a valid codepoint
330      16-bit non-UTF mode   less than 0x10000
331      16-bit UTF-16 mode    less than 0x10ffff and a valid codepoint
332      32-bit non-UTF mode   less than 0x80000000
333      32-bit UTF-32 mode    less than 0x10ffff and a valid codepoint
334    .sp
335    Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-called
336    "surrogate" codepoints), and 0xffef.
337    .P
338    If characters other than hexadecimal digits appear between \ex{ and }, or if
339    there is no terminating }, this form of escape is not recognized. Instead, the
340    initial \ex will be interpreted as a basic hexadecimal escape, with no
341    following digits, giving a character whose value is zero.
342    .P
343    If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \ex is
344    as just described only when it is followed by two hexadecimal digits.
345    Otherwise, it matches a literal "x" character. In JavaScript mode, support for
346    code points greater than 256 is provided by \eu, which must be followed by
347    four hexadecimal digits; otherwise it matches a literal "u" character.
348    Character codes specified by \eu in JavaScript mode are constrained in the same
349    was as those specified by \ex in non-JavaScript mode.
350  .P  .P
351  Characters whose value is less than 256 can be defined by either of the two  Characters whose value is less than 256 can be defined by either of the two
352  syntaxes for \ex. There is no difference in the way they are handled. For  syntaxes for \ex (or by \eu in JavaScript mode). There is no difference in the
353  example, \exdc is exactly the same as \ex{dc}.  way they are handled. For example, \exdc is exactly the same as \ex{dc} (or
354    \eu00dc in JavaScript mode).
355  .P  .P
356  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
357  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 180  specifies two binary zeros followed by a Line 359  specifies two binary zeros followed by a
359  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
360  follows is itself an octal digit.  follows is itself an octal digit.
361  .P  .P
362  The handling of a backslash followed by a digit other than 0 is complicated.  The handling of a backslash followed by a digit other than 0 is complicated,
363  Outside a character class, PCRE reads it and any following digits as a decimal  and Perl has changed in recent releases, causing PCRE also to change. Outside a
364  number. If the number is less than 10, or if there have been at least that many  character class, PCRE reads the digit and any following digits as a decimal
365    number. If the number is less than 8, or if there have been at least that many
366  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
367  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
368  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
# Line 195  following the discussion of Line 375  following the discussion of
375  parenthesized subpatterns.  parenthesized subpatterns.
376  .\"  .\"
377  .P  .P
378  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
379  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
380  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
381  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.
382  character specified in octal must be less than \e400. In UTF-8 mode, values up  Any subsequent digits stand for themselves. The value of the character is
383  to \e777 are permitted. For example:  constrained in the same way as characters specified in hexadecimal. For
384    example:
385  .sp  .sp
386    \e040   is another way of writing a space    \e040   is another way of writing an ASCII space
387  .\" JOIN  .\" JOIN
388    \e40    is the same, provided there are fewer than 40    \e40    is the same, provided there are fewer than 40
389              previous capturing subpatterns              previous capturing subpatterns
# Line 217  to \e777 are permitted. For example: Line 398  to \e777 are permitted. For example:
398              character with octal code 113              character with octal code 113
399  .\" JOIN  .\" JOIN
400    \e377   might be a back reference, otherwise    \e377   might be a back reference, otherwise
401              the byte consisting entirely of 1 bits              the value 255 (decimal)
402  .\" JOIN  .\" JOIN
403    \e81    is either a back reference, or a binary zero    \e81    is either a back reference, or the two
404              followed by the two characters "8" and "1"              characters "8" and "1"
405  .sp  .sp
406  Note that octal values of 100 or greater must not be introduced by a leading  Note that octal values of 100 or greater must not be introduced by a leading
407  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
408  .P  .P
409  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
410  and outside character classes. In addition, inside a character class, the  and outside character classes. In addition, inside a character class, \eb is
411  sequence \eb is interpreted as the backspace character (hex 08), and the  interpreted as the backspace character (hex 08).
412  sequences \eR and \eX are interpreted as the characters "R" and "X",  .P
413  respectively. Outside a character class, these sequences have different  \eN is not allowed in a character class. \eB, \eR, and \eX are not special
414  meanings  inside a character class. Like other unrecognized escape sequences, they are
415  .\" HTML <a href="#uniextseq">  treated as the literal characters "B", "R", and "X" by default, but cause an
416  .\" </a>  error if the PCRE_EXTRA option is set. Outside a character class, these
417  (see below).  sequences have different meanings.
418  .\"  .
419    .
420    .SS "Unsupported escape sequences"
421    .rs
422    .sp
423    In Perl, the sequences \el, \eL, \eu, and \eU are recognized by its string
424    handler and used to modify the case of following characters. By default, PCRE
425    does not support these escape sequences. However, if the PCRE_JAVASCRIPT_COMPAT
426    option is set, \eU matches a "U" character, and \eu can be used to define a
427    character by code point, as described in the previous section.
428  .  .
429  .  .
430  .SS "Absolute and relative back references"  .SS "Absolute and relative back references"
431  .rs  .rs
432  .sp  .sp
433  The sequence \eg followed by a positive or negative number, optionally enclosed  The sequence \eg followed by an unsigned or a negative number, optionally
434  in braces, is an absolute or relative back reference. A named back reference  enclosed in braces, is an absolute or relative back reference. A named back
435  can be coded as \eg{name}. Back references are discussed  reference can be coded as \eg{name}. Back references are discussed
436  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
437  .\" </a>  .\" </a>
438  later,  later,
# Line 254  parenthesized subpatterns. Line 444  parenthesized subpatterns.
444  .\"  .\"
445  .  .
446  .  .
447    .SS "Absolute and relative subroutine calls"
448    .rs
449    .sp
450    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
451    a number enclosed either in angle brackets or single quotes, is an alternative
452    syntax for referencing a subpattern as a "subroutine". Details are discussed
453    .\" HTML <a href="#onigurumasubroutines">
454    .\" </a>
455    later.
456    .\"
457    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
458    synonymous. The former is a back reference; the latter is a
459    .\" HTML <a href="#subpatternsassubroutines">
460    .\" </a>
461    subroutine
462    .\"
463    call.
464    .
465    .
466    .\" HTML <a name="genericchartypes"></a>
467  .SS "Generic character types"  .SS "Generic character types"
468  .rs  .rs
469  .sp  .sp
470  Another use of backslash is for specifying generic character types. The  Another use of backslash is for specifying generic character types:
 following are always recognized:  
471  .sp  .sp
472    \ed     any decimal digit    \ed     any decimal digit
473    \eD     any character that is not a decimal digit    \eD     any character that is not a decimal digit
474    \es     any whitespace character    \eh     any horizontal white space character
475    \eS     any character that is not a whitespace character    \eH     any character that is not a horizontal white space character
476      \es     any white space character
477      \eS     any character that is not a white space character
478      \ev     any vertical white space character
479      \eV     any character that is not a vertical white space character
480    \ew     any "word" character    \ew     any "word" character
481    \eW     any "non-word" character    \eW     any "non-word" character
482  .sp  .sp
483  Each pair of escape sequences partitions the complete set of characters into  There is also the single sequence \eN, which matches a non-newline character.
484  two disjoint sets. Any given character matches one, and only one, of each pair.  This is the same as
485  .P  .\" HTML <a href="#fullstopdot">
486  These character type sequences can appear both inside and outside character  .\" </a>
487    the "." metacharacter
488    .\"
489    when PCRE_DOTALL is not set. Perl also uses \eN to match characters by name;
490    PCRE does not support this.
491    .P
492    Each pair of lower and upper case escape sequences partitions the complete set
493    of characters into two disjoint sets. Any given character matches one, and only
494    one, of each pair. The sequences can appear both inside and outside character
495  classes. They each match one character of the appropriate type. If the current  classes. They each match one character of the appropriate type. If the current
496  matching point is at the end of the subject string, all of them fail, since  matching point is at the end of the subject string, all of them fail, because
497  there is no character to match.  there is no character to match.
498  .P  .P
499  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
500  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
501  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 \es
502  included in a Perl script, \es may match the VT character. In PCRE, it never  characters are now HT (9), LF (10), VT (11), FF (12), CR (13), and space (32).
 does.)  
503  .P  .P
504  A "word" character is an underscore or any character less than 256 that is a  A "word" character is an underscore or any character that is a letter or digit.
505  letter or digit. The definition of letters and digits is controlled by PCRE's  By default, the definition of letters and digits is controlled by PCRE's
506  low-valued character tables, and may vary if locale-specific matching is taking  low-valued character tables, and may vary if locale-specific matching is taking
507  place (see  place (see
508  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
# Line 295  in the Line 515  in the
515  .\"  .\"
516  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,
517  or "french" in Windows, some character codes greater than 128 are used for  or "french" in Windows, some character codes greater than 128 are used for
518  accented letters, and these are matched by \ew.  accented letters, and these are then matched by \ew. The use of locales with
519    Unicode is discouraged.
520  .P  .P
521  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or  By default, in a UTF mode, characters with values greater than 128 never match
522  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
523  character property support is available. The use of locales with Unicode is  their original meanings from before UTF support was available, mainly for
524  discouraged.  efficiency reasons. However, if PCRE is compiled with Unicode property support,
525    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
526    properties are used to determine character types, as follows:
527    .sp
528      \ed  any character that \ep{Nd} matches (decimal digit)
529      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
530      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
531    .sp
532    The upper case escapes match the inverse sets of characters. Note that \ed
533    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
534    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
535    \eB because they are defined in terms of \ew and \eW. Matching these sequences
536    is noticeably slower when PCRE_UCP is set.
537    .P
538    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
539    release 5.10. In contrast to the other sequences, which match only ASCII
540    characters by default, these always match certain high-valued codepoints,
541    whether or not PCRE_UCP is set. The horizontal space characters are:
542    .sp
543      U+0009     Horizontal tab (HT)
544      U+0020     Space
545      U+00A0     Non-break space
546      U+1680     Ogham space mark
547      U+180E     Mongolian vowel separator
548      U+2000     En quad
549      U+2001     Em quad
550      U+2002     En space
551      U+2003     Em space
552      U+2004     Three-per-em space
553      U+2005     Four-per-em space
554      U+2006     Six-per-em space
555      U+2007     Figure space
556      U+2008     Punctuation space
557      U+2009     Thin space
558      U+200A     Hair space
559      U+202F     Narrow no-break space
560      U+205F     Medium mathematical space
561      U+3000     Ideographic space
562    .sp
563    The vertical space characters are:
564    .sp
565      U+000A     Linefeed (LF)
566      U+000B     Vertical tab (VT)
567      U+000C     Form feed (FF)
568      U+000D     Carriage return (CR)
569      U+0085     Next line (NEL)
570      U+2028     Line separator
571      U+2029     Paragraph separator
572    .sp
573    In 8-bit, non-UTF-8 mode, only the characters with codepoints less than 256 are
574    relevant.
575  .  .
576  .  .
577    .\" HTML <a name="newlineseq"></a>
578  .SS "Newline sequences"  .SS "Newline sequences"
579  .rs  .rs
580  .sp  .sp
581  Outside a character class, the escape sequence \eR matches any Unicode newline  Outside a character class, by default, the escape sequence \eR matches any
582  sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to  Unicode newline sequence. In 8-bit non-UTF-8 mode \eR is equivalent to the
583  the following:  following:
584  .sp  .sp
585    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)    (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
586  .sp  .sp
# Line 319  below. Line 591  below.
591  .\"  .\"
592  This particular group matches either the two-character sequence CR followed by  This particular group matches either the two-character sequence CR followed by
593  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,
594  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
595  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
596  cannot be split.  cannot be split.
597  .P  .P
598  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
599  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).
600  Unicode character property support is not needed for these characters to be  Unicode character property support is not needed for these characters to be
601  recognized.  recognized.
602  .P  .P
603  Inside a character class, \eR matches the letter "R".  It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
604    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
605    either at compile time or when the pattern is matched. (BSR is an abbrevation
606    for "backslash R".) This can be made the default when PCRE is built; if this is
607    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
608    It is also possible to specify these settings by starting a pattern string with
609    one of the following sequences:
610    .sp
611      (*BSR_ANYCRLF)   CR, LF, or CRLF only
612      (*BSR_UNICODE)   any Unicode newline sequence
613    .sp
614    These override the default and the options given to the compiling function, but
615    they can themselves be overridden by options given to a matching function. Note
616    that these special settings, which are not Perl-compatible, are recognized only
617    at the very start of a pattern, and that they must be in upper case. If more
618    than one of them is present, the last one is used. They can be combined with a
619    change of newline convention; for example, a pattern can start with:
620    .sp
621      (*ANY)(*BSR_ANYCRLF)
622    .sp
623    They can also be combined with the (*UTF8), (*UTF16), (*UTF32), (*UTF) or
624    (*UCP) special sequences. Inside a character class, \eR is treated as an
625    unrecognized escape sequence, and so matches the letter "R" by default, but
626    causes an error if PCRE_EXTRA is set.
627  .  .
628  .  .
629  .\" HTML <a name="uniextseq"></a>  .\" HTML <a name="uniextseq"></a>
# Line 336  Inside a character class, \eR matches th Line 631  Inside a character class, \eR matches th
631  .rs  .rs
632  .sp  .sp
633  When PCRE is built with Unicode character property support, three additional  When PCRE is built with Unicode character property support, three additional
634  escape sequences to match character properties are available when UTF-8 mode  escape sequences that match characters with specific properties are available.
635  is selected. They are:  When in 8-bit non-UTF-8 mode, these sequences are of course limited to testing
636    characters whose codepoints are less than 256, but they do work in this mode.
637    The extra escape sequences are:
638  .sp  .sp
639    \ep{\fIxx\fP}   a character with the \fIxx\fP property    \ep{\fIxx\fP}   a character with the \fIxx\fP property
640    \eP{\fIxx\fP}   a character without the \fIxx\fP property    \eP{\fIxx\fP}   a character without the \fIxx\fP property
641    \eX       an extended Unicode sequence    \eX       a Unicode extended grapheme cluster
642  .sp  .sp
643  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
644  script names, the general category properties, and "Any", which matches any  script names, the general category properties, "Any", which matches any
645  character (including newline). Other properties such as "InMusicalSymbols" are  character (including newline), and some special PCRE properties (described
646  not currently supported by PCRE. Note that \eP{Any} does not match any  in the
647  characters, so always causes a match failure.  .\" HTML <a href="#extraprops">
648    .\" </a>
649    next section).
650    .\"
651    Other Perl properties such as "InMusicalSymbols" are not currently supported by
652    PCRE. Note that \eP{Any} does not match any characters, so always causes a
653    match failure.
654  .P  .P
655  Sets of Unicode characters are defined as belonging to certain scripts. A  Sets of Unicode characters are defined as belonging to certain scripts. A
656  character from one of these sets can be matched using a script name. For  character from one of these sets can be matched using a script name. For
# Line 361  Those that are not part of an identified Line 664  Those that are not part of an identified
664  .P  .P
665  Arabic,  Arabic,
666  Armenian,  Armenian,
667    Avestan,
668  Balinese,  Balinese,
669    Bamum,
670    Batak,
671  Bengali,  Bengali,
672  Bopomofo,  Bopomofo,
673    Brahmi,
674  Braille,  Braille,
675  Buginese,  Buginese,
676  Buhid,  Buhid,
677  Canadian_Aboriginal,  Canadian_Aboriginal,
678    Carian,
679    Chakma,
680    Cham,
681  Cherokee,  Cherokee,
682  Common,  Common,
683  Coptic,  Coptic,
# Line 376  Cypriot, Line 686  Cypriot,
686  Cyrillic,  Cyrillic,
687  Deseret,  Deseret,
688  Devanagari,  Devanagari,
689    Egyptian_Hieroglyphs,
690  Ethiopic,  Ethiopic,
691  Georgian,  Georgian,
692  Glagolitic,  Glagolitic,
# Line 388  Hangul, Line 699  Hangul,
699  Hanunoo,  Hanunoo,
700  Hebrew,  Hebrew,
701  Hiragana,  Hiragana,
702    Imperial_Aramaic,
703  Inherited,  Inherited,
704    Inscriptional_Pahlavi,
705    Inscriptional_Parthian,
706    Javanese,
707    Kaithi,
708  Kannada,  Kannada,
709  Katakana,  Katakana,
710    Kayah_Li,
711  Kharoshthi,  Kharoshthi,
712  Khmer,  Khmer,
713  Lao,  Lao,
714  Latin,  Latin,
715    Lepcha,
716  Limbu,  Limbu,
717  Linear_B,  Linear_B,
718    Lisu,
719    Lycian,
720    Lydian,
721  Malayalam,  Malayalam,
722    Mandaic,
723    Meetei_Mayek,
724    Meroitic_Cursive,
725    Meroitic_Hieroglyphs,
726    Miao,
727  Mongolian,  Mongolian,
728  Myanmar,  Myanmar,
729  New_Tai_Lue,  New_Tai_Lue,
# Line 405  Nko, Line 731  Nko,
731  Ogham,  Ogham,
732  Old_Italic,  Old_Italic,
733  Old_Persian,  Old_Persian,
734    Old_South_Arabian,
735    Old_Turkic,
736    Ol_Chiki,
737  Oriya,  Oriya,
738  Osmanya,  Osmanya,
739  Phags_Pa,  Phags_Pa,
740  Phoenician,  Phoenician,
741    Rejang,
742  Runic,  Runic,
743    Samaritan,
744    Saurashtra,
745    Sharada,
746  Shavian,  Shavian,
747  Sinhala,  Sinhala,
748    Sora_Sompeng,
749    Sundanese,
750  Syloti_Nagri,  Syloti_Nagri,
751  Syriac,  Syriac,
752  Tagalog,  Tagalog,
753  Tagbanwa,  Tagbanwa,
754  Tai_Le,  Tai_Le,
755    Tai_Tham,
756    Tai_Viet,
757    Takri,
758  Tamil,  Tamil,
759  Telugu,  Telugu,
760  Thaana,  Thaana,
# Line 424  Thai, Line 762  Thai,
762  Tibetan,  Tibetan,
763  Tifinagh,  Tifinagh,
764  Ugaritic,  Ugaritic,
765    Vai,
766  Yi.  Yi.
767  .P  .P
768  Each character has exactly one general category property, specified by a  Each character has exactly one Unicode general category property, specified by
769  two-letter abbreviation. For compatibility with Perl, negation can be specified  a two-letter abbreviation. For compatibility with Perl, negation can be
770  by including a circumflex between the opening brace and the property name. For  specified by including a circumflex between the opening brace and the property
771  example, \ep{^Lu} is the same as \eP{Lu}.  name. For example, \ep{^Lu} is the same as \eP{Lu}.
772  .P  .P
773  If only one letter is specified with \ep or \eP, it includes all the general  If only one letter is specified with \ep or \eP, it includes all the general
774  category properties that start with that letter. In this case, in the absence  category properties that start with that letter. In this case, in the absence
# Line 489  The special property L& is also supporte Line 828  The special property L& is also supporte
828  the Lu, Ll, or Lt property, in other words, a letter that is not classified as  the Lu, Ll, or Lt property, in other words, a letter that is not classified as
829  a modifier or "other".  a modifier or "other".
830  .P  .P
831  The long synonyms for these properties that Perl supports (such as \ep{Letter})  The Cs (Surrogate) property applies only to characters in the range U+D800 to
832    U+DFFF. Such characters are not valid in Unicode strings and so
833    cannot be tested by PCRE, unless UTF validity checking has been turned off
834    (see the discussion of PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK and
835    PCRE_NO_UTF32_CHECK in the
836    .\" HREF
837    \fBpcreapi\fP
838    .\"
839    page). Perl does not support the Cs property.
840    .P
841    The long synonyms for property names that Perl supports (such as \ep{Letter})
842  are not supported by PCRE, nor is it permitted to prefix any of these  are not supported by PCRE, nor is it permitted to prefix any of these
843  properties with "Is".  properties with "Is".
844  .P  .P
# Line 498  Instead, this property is assumed for an Line 847  Instead, this property is assumed for an
847  Unicode table.  Unicode table.
848  .P  .P
849  Specifying caseless matching does not affect these escape sequences. For  Specifying caseless matching does not affect these escape sequences. For
850  example, \ep{Lu} always matches only upper case letters.  example, \ep{Lu} always matches only upper case letters. This is different from
851    the behaviour of current versions of Perl.
852  .P  .P
853  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
854  Unicode sequence. \eX is equivalent to  multistage table lookup in order to find a character's property. That is why
855  .sp  the traditional escape sequences such as \ed and \ew do not use Unicode
856    (?>\ePM\epM*)  properties in PCRE by default, though you can make them do so by setting the
857    PCRE_UCP option or by starting the pattern with (*UCP).
858    .
859    .
860    .SS Extended grapheme clusters
861    .rs
862  .sp  .sp
863  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
864  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  
865  .\" HTML <a href="#atomicgroup">  .\" HTML <a href="#atomicgroup">
866  .\" </a>  .\" </a>
867  (see below).  (see below).
868  .\"  .\"
869  Characters with the "mark" property are typically accents that affect the  Up to and including release 8.31, PCRE matched an earlier, simpler definition
870  preceding character.  that was equivalent to
871    .sp
872      (?>\ePM\epM*)
873    .sp
874    That is, it matched a character without the "mark" property, followed by zero
875    or more characters with the "mark" property. Characters with the "mark"
876    property are typically non-spacing accents that affect the preceding character.
877    .P
878    This simple definition was extended in Unicode to include more complicated
879    kinds of composite character by giving each character a grapheme breaking
880    property, and creating rules that use these properties to define the boundaries
881    of extended grapheme clusters. In releases of PCRE later than 8.31, \eX matches
882    one of these clusters.
883    .P
884    \eX always matches at least one character. Then it decides whether to add
885    additional characters according to the following rules for ending a cluster:
886    .P
887    1. End at the end of the subject string.
888    .P
889    2. Do not end between CR and LF; otherwise end after any control character.
890    .P
891    3. Do not break Hangul (a Korean script) syllable sequences. Hangul characters
892    are of five types: L, V, T, LV, and LVT. An L character may be followed by an
893    L, V, LV, or LVT character; an LV or V character may be followed by a V or T
894    character; an LVT or T character may be follwed only by a T character.
895  .P  .P
896  Matching characters by Unicode property is not fast, because PCRE has to search  4. Do not end before extending characters or spacing marks. Characters with
897  a structure that contains data for over fifteen thousand characters. That is  the "mark" property always have the "extend" grapheme breaking property.
898  why the traditional escape sequences such as \ed and \ew do not use Unicode  .P
899  properties in PCRE.  5. Do not end after prepend characters.
900    .P
901    6. Otherwise, end the cluster.
902    .
903    .
904    .\" HTML <a name="extraprops"></a>
905    .SS PCRE's additional properties
906    .rs
907    .sp
908    As well as the standard Unicode properties described above, PCRE supports four
909    more that make it possible to convert traditional escape sequences such as \ew
910    and \es and POSIX character classes to use Unicode properties. PCRE uses these
911    non-standard, non-Perl properties internally when PCRE_UCP is set. However,
912    they may also be used explicitly. These properties are:
913    .sp
914      Xan   Any alphanumeric character
915      Xps   Any POSIX space character
916      Xsp   Any Perl space character
917      Xwd   Any Perl "word" character
918    .sp
919    Xan matches characters that have either the L (letter) or the N (number)
920    property. Xps matches the characters tab, linefeed, vertical tab, form feed, or
921    carriage return, and any other character that has the Z (separator) property.
922    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
923    same characters as Xan, plus underscore.
924    .P
925    There is another non-standard property, Xuc, which matches any character that
926    can be represented by a Universal Character Name in C++ and other programming
927    languages. These are the characters $, @, ` (grave accent), and all characters
928    with Unicode code points greater than or equal to U+00A0, except for the
929    surrogates U+D800 to U+DFFF. Note that most base (ASCII) characters are
930    excluded. (Universal Character Names are of the form \euHHHH or \eUHHHHHHHH
931    where H is a hexadecimal digit. Note that the Xuc property does not match these
932    sequences but the characters that they represent.)
933  .  .
934  .  .
935  .\" HTML <a name="resetmatchstart"></a>  .\" HTML <a name="resetmatchstart"></a>
936  .SS "Resetting the match start"  .SS "Resetting the match start"
937  .rs  .rs
938  .sp  .sp
939  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
940  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:  
941  .sp  .sp
942    foo\eKbar    foo\eKbar
943  .sp  .sp
# Line 549  For example, when the pattern Line 959  For example, when the pattern
959    (foo)\eKbar    (foo)\eKbar
960  .sp  .sp
961  matches "foobar", the first substring is still set to "foo".  matches "foobar", the first substring is still set to "foo".
962    .P
963    Perl documents that the use of \eK within assertions is "not well defined". In
964    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
965    ignored in negative assertions.
966  .  .
967  .  .
968  .\" HTML <a name="smallassertions"></a>  .\" HTML <a name="smallassertions"></a>
# Line 573  The backslashed assertions are: Line 987  The backslashed assertions are:
987    \ez     matches only at the end of the subject    \ez     matches only at the end of the subject
988    \eG     matches at the first matching position in the subject    \eG     matches at the first matching position in the subject
989  .sp  .sp
990  These assertions may not appear in character classes (but note that \eb has a  Inside a character class, \eb has a different meaning; it matches the backspace
991  different meaning, namely the backspace character, inside a character class).  character. If any other of these assertions appears in a character class, by
992    default it matches the corresponding literal character (for example, \eB
993    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
994    escape sequence" error is generated instead.
995  .P  .P
996  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
997  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
998  \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
999  first or last character matches \ew, respectively.  first or last character matches \ew, respectively. In a UTF mode, the meanings
1000    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
1001    done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
1002    of word" or "end of word" metasequence. However, whatever follows \eb normally
1003    determines which it is. For example, the fragment \eba matches "a" at the start
1004    of a word.
1005  .P  .P
1006  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
1007  dollar (described in the next section) in that they only ever match at the very  dollar (described in the next section) in that they only ever match at the very
# Line 613  regular expression. Line 1035  regular expression.
1035  .SH "CIRCUMFLEX AND DOLLAR"  .SH "CIRCUMFLEX AND DOLLAR"
1036  .rs  .rs
1037  .sp  .sp
1038    The circumflex and dollar metacharacters are zero-width assertions. That is,
1039    they test for a particular condition being true without consuming any
1040    characters from the subject string.
1041    .P
1042  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
1043  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
1044  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
1045  \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
1046  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
1047  meaning  meaning
# Line 632  constrained to match only at the start o Line 1058  constrained to match only at the start o
1058  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
1059  to be anchored.)  to be anchored.)
1060  .P  .P
1061  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
1062  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
1063  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
1064  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
1065  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
1066  character class.  branch in which it appears. Dollar has no special meaning in a character class.
1067  .P  .P
1068  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
1069  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 663  end of the subject in both modes, and if Line 1089  end of the subject in both modes, and if
1089  \eA it is always anchored, whether or not PCRE_MULTILINE is set.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
1090  .  .
1091  .  .
1092  .SH "FULL STOP (PERIOD, DOT)"  .\" HTML <a name="fullstopdot"></a>
1093    .SH "FULL STOP (PERIOD, DOT) AND \eN"
1094  .rs  .rs
1095  .sp  .sp
1096  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
1097  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
1098  line. In UTF-8 mode, the matched character may be more than one byte long.  line.
1099  .P  .P
1100  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
1101  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 685  to match it. Line 1112  to match it.
1112  The handling of dot is entirely independent of the handling of circumflex and  The handling of dot is entirely independent of the handling of circumflex and
1113  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
1114  special meaning in a character class.  special meaning in a character class.
1115  .  .P
1116  .  The escape sequence \eN behaves like a dot, except that it is not affected by
1117  .SH "MATCHING A SINGLE BYTE"  the PCRE_DOTALL option. In other words, it matches any character except one
1118  .rs  that signifies the end of a line. Perl also uses \eN to match characters by
1119  .sp  name; PCRE does not support this.
1120  Outside a character class, the escape sequence \eC matches any one byte, both  .
1121  in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending  .
1122  characters. The feature is provided in Perl in order to match individual bytes  .SH "MATCHING A SINGLE DATA UNIT"
1123  in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,  .rs
1124  what remains in the string may be a malformed UTF-8 string. For this reason,  .sp
1125  the \eC escape sequence is best avoided.  Outside a character class, the escape sequence \eC matches any one data unit,
1126    whether or not a UTF mode is set. In the 8-bit library, one data unit is one
1127    byte; in the 16-bit library it is a 16-bit unit; in the 32-bit library it is
1128    a 32-bit unit. Unlike a dot, \eC always
1129    matches line-ending characters. The feature is provided in Perl in order to
1130    match individual bytes in UTF-8 mode, but it is unclear how it can usefully be
1131    used. Because \eC breaks up characters into individual data units, matching one
1132    unit with \eC in a UTF mode means that the rest of the string may start with a
1133    malformed UTF character. This has undefined results, because PCRE assumes that
1134    it is dealing with valid UTF strings (and by default it checks this at the
1135    start of processing unless the PCRE_NO_UTF8_CHECK, PCRE_NO_UTF16_CHECK or
1136    PCRE_NO_UTF32_CHECK option is used).
1137  .P  .P
1138  PCRE does not allow \eC to appear in lookbehind assertions  PCRE does not allow \eC to appear in lookbehind assertions
1139  .\" HTML <a href="#lookbehind">  .\" HTML <a href="#lookbehind">
1140  .\" </a>  .\" </a>
1141  (described below),  (described below)
1142  .\"  .\"
1143  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
1144  the lookbehind.  the lookbehind.
1145    .P
1146    In general, the \eC escape sequence is best avoided. However, one
1147    way of using it that avoids the problem of malformed UTF characters is to use a
1148    lookahead to check the length of the next character, as in this pattern, which
1149    could be used with a UTF-8 string (ignore white space and line breaks):
1150    .sp
1151      (?| (?=[\ex00-\ex7f])(\eC) |
1152          (?=[\ex80-\ex{7ff}])(\eC)(\eC) |
1153          (?=[\ex{800}-\ex{ffff}])(\eC)(\eC)(\eC) |
1154          (?=[\ex{10000}-\ex{1fffff}])(\eC)(\eC)(\eC)(\eC))
1155    .sp
1156    A group that starts with (?| resets the capturing parentheses numbers in each
1157    alternative (see
1158    .\" HTML <a href="#dupsubpatternnumber">
1159    .\" </a>
1160    "Duplicate Subpattern Numbers"
1161    .\"
1162    below). The assertions at the start of each branch check the next UTF-8
1163    character for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
1164    character's individual bytes are then captured by the appropriate number of
1165    groups.
1166  .  .
1167  .  .
1168  .\" HTML <a name="characterclass"></a>  .\" HTML <a name="characterclass"></a>
# Line 711  the lookbehind. Line 1170  the lookbehind.
1170  .rs  .rs
1171  .sp  .sp
1172  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
1173  square bracket. A closing square bracket on its own is not special. If a  square bracket. A closing square bracket on its own is not special by default.
1174  closing square bracket is required as a member of the class, it should be the  However, if the PCRE_JAVASCRIPT_COMPAT option is set, a lone closing square
1175  first data character in the class (after an initial circumflex, if present) or  bracket causes a compile-time error. If a closing square bracket is required as
1176  escaped with a backslash.  a member of the class, it should be the first data character in the class
1177  .P  (after an initial circumflex, if present) or escaped with a backslash.
1178  A character class matches a single character in the subject. In UTF-8 mode, the  .P
1179  character may occupy more than one byte. A matched character must be in the set  A character class matches a single character in the subject. In a UTF mode, the
1180  of characters defined by the class, unless the first character in the class  character may be more than one data unit long. A matched character must be in
1181  definition is a circumflex, in which case the subject character must not be in  the set of characters defined by the class, unless the first character in the
1182  the set defined by the class. If a circumflex is actually required as a member  class definition is a circumflex, in which case the subject character must not
1183  of the class, ensure it is not the first character, or escape it with a  be in the set defined by the class. If a circumflex is actually required as a
1184    member of the class, ensure it is not the first character, or escape it with a
1185  backslash.  backslash.
1186  .P  .P
1187  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1188  [^aeiou] matches any character that is not a lower case vowel. Note that a  [^aeiou] matches any character that is not a lower case vowel. Note that a
1189  circumflex is just a convenient notation for specifying the characters that  circumflex is just a convenient notation for specifying the characters that
1190  are in the class by enumerating those that are not. A class that starts with a  are in the class by enumerating those that are not. A class that starts with a
1191  circumflex is not an assertion: it still consumes a character from the subject  circumflex is not an assertion; it still consumes a character from the subject
1192  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
1193  string.  string.
1194  .P  .P
1195  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)
1196  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
1197    \ex{ escaping mechanism.
1198  .P  .P
1199  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
1200  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
1201  "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
1202  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
1203  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
1204  always possible. For characters with higher values, the concept of case is  always possible. For characters with higher values, the concept of case is
1205  supported if PCRE is compiled with Unicode property support, but not otherwise.  supported if PCRE is compiled with Unicode property support, but not otherwise.
1206  If you want to use caseless matching for characters 128 and above, you must  If you want to use caseless matching in a UTF mode for characters 128 and
1207  ensure that PCRE is compiled with Unicode property support as well as with  above, you must ensure that PCRE is compiled with Unicode property support as
1208  UTF-8 support.  well as with UTF support.
1209  .P  .P
1210  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
1211  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 766  followed by two other characters. The oc Line 1227  followed by two other characters. The oc
1227  "]" can also be used to end a range.  "]" can also be used to end a range.
1228  .P  .P
1229  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
1230  used for characters specified numerically, for example [\e000-\e037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. Ranges
1231  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}].  
1232  .P  .P
1233  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
1234  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
1235  [][\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
1236  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
1237  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
1238  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
1239  property support.  property support.
1240  .P  .P
1241  The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1242  in a character class, and add the characters that they match to the class. For  \eV, \ew, and \eW may appear in a character class, and add the characters that
1243  example, [\edABCDEF] matches any hexadecimal digit. A circumflex can  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1244  conveniently be used with the upper case character types to specify a more  digit. In UTF modes, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1245  restricted set of characters than the matching lower case type. For example,  and their upper case partners, just as it does when they appear outside a
1246  the class [^\eW_] matches any letter or digit, but not underscore.  character class, as described in the section entitled
1247    .\" HTML <a href="#genericchartypes">
1248    .\" </a>
1249    "Generic character types"
1250    .\"
1251    above. The escape sequence \eb has a different meaning inside a character
1252    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1253    are not special inside a character class. Like any other unrecognized escape
1254    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1255    default, but cause an error if the PCRE_EXTRA option is set.
1256    .P
1257    A circumflex can conveniently be used with the upper case character types to
1258    specify a more restricted set of characters than the matching lower case type.
1259    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1260    whereas [\ew] includes underscore. A positive character class should be read as
1261    "something OR something OR ..." and a negative class as "NOT something AND NOT
1262    something AND NOT ...".
1263  .P  .P
1264  The only metacharacters that are recognized in character classes are backslash,  The only metacharacters that are recognized in character classes are backslash,
1265  hyphen (only where it can be interpreted as specifying a range), circumflex  hyphen (only where it can be interpreted as specifying a range), circumflex
# Line 803  this notation. For example, Line 1279  this notation. For example,
1279    [01[:alpha:]%]    [01[:alpha:]%]
1280  .sp  .sp
1281  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1282  are  are:
1283  .sp  .sp
1284    alnum    letters and digits    alnum    letters and digits
1285    alpha    letters    alpha    letters
# Line 814  are Line 1290  are
1290    graph    printing characters, excluding space    graph    printing characters, excluding space
1291    lower    lower case letters    lower    lower case letters
1292    print    printing characters, including space    print    printing characters, including space
1293    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1294    space    white space (not quite the same as \es)    space    white space (not quite the same as \es)
1295    upper    upper case letters    upper    upper case letters
1296    word     "word" characters (same as \ew)    word     "word" characters (same as \ew)
1297    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1298  .sp  .sp
1299  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and  The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13), and
1300  space (32). Notice that this list includes the VT character (code 11). This  space (32). "Space" used to be different to \es, which did not include VT, for
1301  makes "space" different to \es, which does not include VT (for Perl  Perl compatibility. However, Perl changed at release 5.18, and PCRE followed at
1302  compatibility).  release 8.34. "Space" and \es now match the same set of characters.
1303  .P  .P
1304  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
1305  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 835  matches "1", "2", or any non-digit. PCRE Line 1311  matches "1", "2", or any non-digit. PCRE
1311  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
1312  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1313  .P  .P
1314  In UTF-8 mode, characters with values greater than 128 do not match any of  By default, in UTF modes, characters with values greater than 128 do not match
1315  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1316    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1317    character properties are used. This is achieved by replacing the POSIX classes
1318    by other sequences, as follows:
1319    .sp
1320      [:alnum:]  becomes  \ep{Xan}
1321      [:alpha:]  becomes  \ep{L}
1322      [:blank:]  becomes  \eh
1323      [:digit:]  becomes  \ep{Nd}
1324      [:lower:]  becomes  \ep{Ll}
1325      [:space:]  becomes  \ep{Xps}
1326      [:upper:]  becomes  \ep{Lu}
1327      [:word:]   becomes  \ep{Xwd}
1328    .sp
1329    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1330    classes are unchanged, and match only characters with code points less than
1331    128.
1332  .  .
1333  .  .
1334  .SH "VERTICAL BAR"  .SH "VERTICAL BAR"
# Line 863  alternative in the subpattern. Line 1355  alternative in the subpattern.
1355  .rs  .rs
1356  .sp  .sp
1357  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1358  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options (which are Perl-compatible) can be changed from within
1359  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1360    The option letters are
1361  .sp  .sp
1362    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1363    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
# Line 878  PCRE_MULTILINE while unsetting PCRE_DOTA Line 1371  PCRE_MULTILINE while unsetting PCRE_DOTA
1371  permitted. If a letter appears both before and after the hyphen, the option is  permitted. If a letter appears both before and after the hyphen, the option is
1372  unset.  unset.
1373  .P  .P
1374  When an option change occurs at top level (that is, not inside subpattern  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
1375  parentheses), the change applies to the remainder of the pattern that follows.  changed in the same way as the Perl-compatible options by using the characters
1376  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1377  the global options (and it will therefore show up in data extracted by the  .P
1378  \fBpcre_fullinfo()\fP function).  When one of these option changes occurs at top level (that is, not inside
1379    subpattern parentheses), the change applies to the remainder of the pattern
1380    that follows. If the change is placed right at the start of a pattern, PCRE
1381    extracts it into the global options (and it will therefore show up in data
1382    extracted by the \fBpcre_fullinfo()\fP function).
1383  .P  .P
1384  An option change within a subpattern (see below for a description of  An option change within a subpattern (see below for a description of
1385  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
1386  .sp  .sp
1387    (a(?i)b)c    (a(?i)b)c
1388  .sp  .sp
# Line 901  branch is abandoned before the option se Line 1398  branch is abandoned before the option se
1398  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1399  behaviour otherwise.  behaviour otherwise.
1400  .P  .P
1401  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be  \fBNote:\fP There are other PCRE-specific options that can be set by the
1402  changed in the same way as the Perl-compatible options by using the characters  application when the compiling or matching functions are called. In some cases
1403  J, U and X respectively.  the pattern can contain special leading sequences such as (*CRLF) to override
1404    what the application has set or what has been defaulted. Details are given in
1405    the section entitled
1406    .\" HTML <a href="#newlineseq">
1407    .\" </a>
1408    "Newline sequences"
1409    .\"
1410    above. There are also the (*UTF8), (*UTF16),(*UTF32), and (*UCP) leading
1411    sequences that can be used to set UTF and Unicode property modes; they are
1412    equivalent to setting the PCRE_UTF8, PCRE_UTF16, PCRE_UTF32 and the PCRE_UCP
1413    options, respectively. The (*UTF) sequence is a generic version that can be
1414    used with any of the libraries. However, the application can set the
1415    PCRE_NEVER_UTF option, which locks out the use of the (*UTF) sequences.
1416  .  .
1417  .  .
1418  .\" HTML <a name="subpattern"></a>  .\" HTML <a name="subpattern"></a>
# Line 917  Turning part of a pattern into a subpatt Line 1426  Turning part of a pattern into a subpatt
1426  .sp  .sp
1427    cat(aract|erpillar|)    cat(aract|erpillar|)
1428  .sp  .sp
1429  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1430  parentheses, it would match "cataract", "erpillar" or an empty string.  match "cataract", "erpillar" or an empty string.
1431  .sp  .sp
1432  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
1433  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
1434  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
1435  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting  matching function. (This applies only to the traditional matching functions;
1436  from 1) to obtain numbers for the capturing subpatterns.  the DFA matching functions do not support capturing.)
1437  .P  .P
1438  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
1439    numbers for the capturing subpatterns. For example, if the string "the red
1440    king" is matched against the pattern
1441  .sp  .sp
1442    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1443  .sp  .sp
# Line 958  is reached, an option setting in one bra Line 1469  is reached, an option setting in one bra
1469  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1470  .  .
1471  .  .
1472    .\" HTML <a name="dupsubpatternnumber"></a>
1473    .SH "DUPLICATE SUBPATTERN NUMBERS"
1474    .rs
1475    .sp
1476    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1477    the same numbers for its capturing parentheses. Such a subpattern starts with
1478    (?| and is itself a non-capturing subpattern. For example, consider this
1479    pattern:
1480    .sp
1481      (?|(Sat)ur|(Sun))day
1482    .sp
1483    Because the two alternatives are inside a (?| group, both sets of capturing
1484    parentheses are numbered one. Thus, when the pattern matches, you can look
1485    at captured substring number one, whichever alternative matched. This construct
1486    is useful when you want to capture part, but not all, of one of a number of
1487    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1488    number is reset at the start of each branch. The numbers of any capturing
1489    parentheses that follow the subpattern start after the highest number used in
1490    any branch. The following example is taken from the Perl documentation. The
1491    numbers underneath show in which buffer the captured content will be stored.
1492    .sp
1493      # before  ---------------branch-reset----------- after
1494      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1495      # 1            2         2  3        2     3     4
1496    .sp
1497    A back reference to a numbered subpattern uses the most recent value that is
1498    set for that number by any subpattern. The following pattern matches "abcabc"
1499    or "defdef":
1500    .sp
1501      /(?|(abc)|(def))\e1/
1502    .sp
1503    In contrast, a subroutine call to a numbered subpattern always refers to the
1504    first one in the pattern with the given number. The following pattern matches
1505    "abcabc" or "defabc":
1506    .sp
1507      /(?|(abc)|(def))(?1)/
1508    .sp
1509    If a
1510    .\" HTML <a href="#conditions">
1511    .\" </a>
1512    condition test
1513    .\"
1514    for a subpattern's having matched refers to a non-unique number, the test is
1515    true if any of the subpatterns of that number have matched.
1516    .P
1517    An alternative approach to using this "branch reset" feature is to use
1518    duplicate named subpatterns, as described in the next section.
1519    .
1520    .
1521  .SH "NAMED SUBPATTERNS"  .SH "NAMED SUBPATTERNS"
1522  .rs  .rs
1523  .sp  .sp
# Line 967  if an expression is modified, the number Line 1527  if an expression is modified, the number
1527  difficulty, PCRE supports the naming of subpatterns. This feature was not  difficulty, PCRE supports the naming of subpatterns. This feature was not
1528  added to Perl until release 5.10. Python had the feature earlier, and PCRE  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1529  introduced it at release 4.0, using the Python syntax. PCRE now supports both  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1530  the Perl and the Python syntax.  the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1531    have different names, but PCRE does not.
1532  .P  .P
1533  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1534  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1535  parentheses from other parts of the pattern, such as  parentheses from other parts of the pattern, such as
1536  .\" HTML <a href="#backreferences">  .\" HTML <a href="#backreferences">
1537  .\" </a>  .\" </a>
1538  backreferences,  back references,
1539  .\"  .\"
1540  .\" HTML <a href="#recursion">  .\" HTML <a href="#recursion">
1541  .\" </a>  .\" </a>
# Line 994  extracting the name-to-number translatio Line 1555  extracting the name-to-number translatio
1555  is also a convenience function for extracting a captured substring by name.  is also a convenience function for extracting a captured substring by name.
1556  .P  .P
1557  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
1558  this constraint by setting the PCRE_DUPNAMES option at compile time. This can  this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1559  be useful for patterns where only one instance of the named parentheses can  names are also always permitted for subpatterns with the same number, set up as
1560  match. Suppose you want to match the name of a weekday, either as a 3-letter  described in the previous section.) Duplicate names can be useful for patterns
1561  abbreviation or as the full name, and in both cases you want to extract the  where only one instance of the named parentheses can match. Suppose you want to
1562  abbreviation. This pattern (ignoring the line breaks) does the job:  match the name of a weekday, either as a 3-letter abbreviation or as the full
1563    name, and in both cases you want to extract the abbreviation. This pattern
1564    (ignoring the line breaks) does the job:
1565  .sp  .sp
1566    (?<DN>Mon|Fri|Sun)(?:day)?|    (?<DN>Mon|Fri|Sun)(?:day)?|
1567    (?<DN>Tue)(?:sday)?|    (?<DN>Tue)(?:sday)?|
# Line 1007  abbreviation. This pattern (ignoring the Line 1570  abbreviation. This pattern (ignoring the
1570    (?<DN>Sat)(?:urday)?    (?<DN>Sat)(?:urday)?
1571  .sp  .sp
1572  There are five capturing substrings, but only one is ever set after a match.  There are five capturing substrings, but only one is ever set after a match.
1573    (An alternative way of solving this problem is to use a "branch reset"
1574    subpattern, as described in the previous section.)
1575    .P
1576  The convenience function for extracting the data by name returns the substring  The convenience function for extracting the data by name returns the substring
1577  for the first (and in this example, the only) subpattern of that name that  for the first (and in this example, the only) subpattern of that name that
1578  matched. This saves searching to find which numbered subpattern it was. If you  matched. This saves searching to find which numbered subpattern it was.
1579  make a reference to a non-unique named subpattern from elsewhere in the  .P
1580  pattern, the one that corresponds to the lowest number is used. For further  If you make a back reference to a non-unique named subpattern from elsewhere in
1581  details of the interfaces for handling named subpatterns, see the  the pattern, the subpatterns to which the name refers are checked in the order
1582    in which they appear in the overall pattern. The first one that is set is used
1583    for the reference. For example, this pattern matches both "foofoo" and
1584    "barbar" but not "foobar" or "barfoo":
1585    .sp
1586      (?:(?<n>foo)|(?<n>bar))\k<n>
1587    .sp
1588    .P
1589    If you make a subroutine call to a non-unique named subpattern, the one that
1590    corresponds to the first occurrence of the name is used. In the absence of
1591    duplicate numbers (see the previous section) this is the one with the lowest
1592    number.
1593    .P
1594    If you use a named reference in a condition
1595    test (see the
1596    .\"
1597    .\" HTML <a href="#conditions">
1598    .\" </a>
1599    section about conditions
1600    .\"
1601    below), either to check whether a subpattern has matched, or to check for
1602    recursion, all subpatterns with the same name are tested. If the condition is
1603    true for any one of them, the overall condition is true. This is the same
1604    behaviour as testing by number. For further details of the interfaces for
1605    handling named subpatterns, see the
1606  .\" HREF  .\" HREF
1607  \fBpcreapi\fP  \fBpcreapi\fP
1608  .\"  .\"
1609  documentation.  documentation.
1610    .P
1611    \fBWarning:\fP You cannot use different names to distinguish between two
1612    subpatterns with the same number because PCRE uses only the numbers when
1613    matching. For this reason, an error is given at compile time if different names
1614    are given to subpatterns with the same number. However, you can always give the
1615    same name to subpatterns with the same number, even when PCRE_DUPNAMES is not
1616    set.
1617  .  .
1618  .  .
1619  .SH REPETITION  .SH REPETITION
# Line 1028  items: Line 1625  items:
1625    a literal data character    a literal data character
1626    the dot metacharacter    the dot metacharacter
1627    the \eC escape sequence    the \eC escape sequence
1628    the \eX escape sequence (in UTF-8 mode with Unicode properties)    the \eX escape sequence
1629    the \eR escape sequence    the \eR escape sequence
1630    an escape such as \ed that matches a single character    an escape such as \ed or \epL that matches a single character
1631    a character class    a character class
1632    a back reference (see next section)    a back reference (see next section)
1633    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1634      a subroutine call to a subpattern (recursive or otherwise)
1635  .sp  .sp
1636  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1637  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
# Line 1058  where a quantifier is not allowed, or on Line 1656  where a quantifier is not allowed, or on
1656  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
1657  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1658  .P  .P
1659  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
1660  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
1661  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,
1662  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
1663  which may be several bytes long (and they may be of different lengths).  several data units long (and they may be of different lengths).
1664  .P  .P
1665  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
1666  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1667    subpatterns that are referenced as
1668    .\" HTML <a href="#subpatternsassubroutines">
1669    .\" </a>
1670    subroutines
1671    .\"
1672    from elsewhere in the pattern (but see also the section entitled
1673    .\" HTML <a href="#subdefine">
1674    .\" </a>
1675    "Defining subpatterns for use by reference only"
1676    .\"
1677    below). Items other than subpatterns that have a {0} quantifier are omitted
1678    from the compiled pattern.
1679  .P  .P
1680  For convenience, the three most common quantifiers have single-character  For convenience, the three most common quantifiers have single-character
1681  abbreviations:  abbreviations:
# Line 1136  In cases where it is known that the subj Line 1746  In cases where it is known that the subj
1746  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1747  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1748  .P  .P
1749  However, there is one situation where the optimization cannot be used. When .*  However, there are some cases where the optimization cannot be used. When .*
1750  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1751  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
1752  succeeds. Consider, for example:  succeeds. Consider, for example:
1753  .sp  .sp
# Line 1146  succeeds. Consider, for example: Line 1756  succeeds. Consider, for example:
1756  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
1757  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1758  .P  .P
1759    Another case where implicit anchoring is not applied is when the leading .* is
1760    inside an atomic group. Once again, a match at the start may fail where a later
1761    one succeeds. Consider this pattern:
1762    .sp
1763      (?>.*?a)b
1764    .sp
1765    It matches "ab" in the subject "aab". The use of the backtracking control verbs
1766    (*PRUNE) and (*SKIP) also disable this optimization.
1767    .P
1768  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1769  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1770  .sp  .sp
# Line 1212  previous example can be rewritten as Line 1831  previous example can be rewritten as
1831  .sp  .sp
1832    \ed++foo    \ed++foo
1833  .sp  .sp
1834    Note that a possessive quantifier can be used with an entire group, for
1835    example:
1836    .sp
1837      (abc|xyz){2,3}+
1838    .sp
1839  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1840  option is ignored. They are a convenient notation for the simpler forms of  option is ignored. They are a convenient notation for the simpler forms of
1841  atomic group. However, there is no difference in the meaning of a possessive  atomic group. However, there is no difference in the meaning of a possessive
# Line 1285  no such problem when named parentheses a Line 1909  no such problem when named parentheses a
1909  subpattern is possible using named parentheses (see below).  subpattern is possible using named parentheses (see below).
1910  .P  .P
1911  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
1912  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
1913  Perl 5.10. This escape must be followed by a positive or a negative number,  unsigned number or a negative number, optionally enclosed in braces. These
1914  optionally enclosed in braces. These examples are all identical:  examples are all identical:
1915  .sp  .sp
1916    (ring), \e1    (ring), \e1
1917    (ring), \eg1    (ring), \eg1
1918    (ring), \eg{1}    (ring), \eg{1}
1919  .sp  .sp
1920  A positive number specifies an absolute reference without the ambiguity that is  An unsigned number specifies an absolute reference without the ambiguity that
1921  present in the older syntax. It is also useful when literal digits follow the  is present in the older syntax. It is also useful when literal digits follow
1922  reference. A negative number is a relative reference. Consider this example:  the reference. A negative number is a relative reference. Consider this
1923    example:
1924  .sp  .sp
1925    (abc(def)ghi)\eg{-1}    (abc(def)ghi)\eg{-1}
1926  .sp  .sp
1927  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
1928  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.
1929  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
1930  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
1931  fragments that contain references within themselves.  joining together fragments that contain references within themselves.
1932  .P  .P
1933  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1934  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
# Line 1342  after the reference. Line 1967  after the reference.
1967  .P  .P
1968  There may be more than one back reference to the same subpattern. If a  There may be more than one back reference to the same subpattern. If a
1969  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1970  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1971  .sp  .sp
1972    (a|(bc))\e2    (a|(bc))\e2
1973  .sp  .sp
1974  always fails if it starts to match "a" rather than "bc". Because there may be  always fails if it starts to match "a" rather than "bc". However, if the
1975  many capturing parentheses in a pattern, all digits following the backslash are  PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back reference to an
1976  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1977  with a digit character, some delimiter must be used to terminate the back  .P
1978  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1979  Otherwise an empty comment (see  following a backslash are taken as part of a potential back reference number.
1980    If the pattern continues with a digit character, some delimiter must be used to
1981    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1982    white space. Otherwise, the \eg{ syntax or an empty comment (see
1983  .\" HTML <a href="#comments">  .\" HTML <a href="#comments">
1984  .\" </a>  .\" </a>
1985  "Comments"  "Comments"
1986  .\"  .\"
1987  below) can be used.  below) can be used.
1988  .P  .
1989    .SS "Recursive back references"
1990    .rs
1991    .sp
1992  A back reference that occurs inside the parentheses to which it refers fails  A back reference that occurs inside the parentheses to which it refers fails
1993  when the subpattern is first used, so, for example, (a\e1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1994  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
# Line 1371  to the previous iteration. In order for Line 2002  to the previous iteration. In order for
2002  that the first iteration does not need to match the back reference. This can be  that the first iteration does not need to match the back reference. This can be
2003  done using alternation, as in the example above, or by a quantifier with a  done using alternation, as in the example above, or by a quantifier with a
2004  minimum of zero.  minimum of zero.
2005    .P
2006    Back references of this type cause the group that they reference to be treated
2007    as an
2008    .\" HTML <a href="#atomicgroup">
2009    .\" </a>
2010    atomic group.
2011    .\"
2012    Once the whole group has been matched, a subsequent matching failure cannot
2013    cause backtracking into the middle of the group.
2014  .  .
2015  .  .
2016  .\" HTML <a name="bigassertions"></a>  .\" HTML <a name="bigassertions"></a>
# Line 1390  those that look ahead of the current pos Line 2030  those that look ahead of the current pos
2030  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,
2031  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.
2032  .P  .P
2033  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  Assertion subpatterns are not capturing subpatterns. If such an assertion
2034  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
2035  of assertion contains capturing subpatterns within it, these are counted for  numbering the capturing subpatterns in the whole pattern. However, substring
2036  the purposes of numbering the capturing subpatterns in the whole pattern.  capturing is carried out only for positive assertions. (Perl sometimes, but not
2037  However, substring capturing is carried out only for positive assertions,  always, does do capturing in negative assertions.)
2038  because it does not make sense for negative assertions.  .P
2039    For compatibility with Perl, assertion subpatterns may be repeated; though
2040    it makes no sense to assert the same thing several times, the side effect of
2041    capturing parentheses may occasionally be useful. In practice, there only three
2042    cases:
2043    .sp
2044    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
2045    However, it may contain internal capturing parenthesized groups that are called
2046    from elsewhere via the
2047    .\" HTML <a href="#subpatternsassubroutines">
2048    .\" </a>
2049    subroutine mechanism.
2050    .\"
2051    .sp
2052    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
2053    were {0,1}. At run time, the rest of the pattern match is tried with and
2054    without the assertion, the order depending on the greediness of the quantifier.
2055    .sp
2056    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
2057    The assertion is obeyed just once when encountered during matching.
2058  .  .
2059  .  .
2060  .SS "Lookahead assertions"  .SS "Lookahead assertions"
# Line 1424  lookbehind assertion is needed to achiev Line 2083  lookbehind assertion is needed to achiev
2083  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
2084  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
2085  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.
2086    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
2087  .  .
2088  .  .
2089  .\" HTML <a name="lookbehind"></a>  .\" HTML <a name="lookbehind"></a>
# Line 1448  is permitted, but Line 2108  is permitted, but
2108  .sp  .sp
2109  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
2110  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
2111  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl, which requires all branches to match the same
2112  match the same length of string. An assertion such as  length of string. An assertion such as
2113  .sp  .sp
2114    (?<=ab(c|de))    (?<=ab(c|de))
2115  .sp  .sp
2116  is not permitted, because its single top-level branch can match two different  is not permitted, because its single top-level branch can match two different
2117  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable to PCRE if rewritten to use two top-level
2118    branches:
2119  .sp  .sp
2120    (?<=abc|abde)    (?<=abc|abde)
2121  .sp  .sp
2122  In some cases, the Perl 5.10 escape sequence \eK  In some cases, the escape sequence \eK
2123  .\" HTML <a href="#resetmatchstart">  .\" HTML <a href="#resetmatchstart">
2124  .\" </a>  .\" </a>
2125  (see above)  (see above)
2126  .\"  .\"
2127  can be used instead of a lookbehind assertion; this is not restricted to a  can be used instead of a lookbehind assertion to get round the fixed-length
2128  fixed-length.  restriction.
2129  .P  .P
2130  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
2131  temporarily move the current position back by the fixed length and then try to  temporarily move the current position back by the fixed length and then try to
2132  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
2133  assertion fails.  assertion fails.
2134  .P  .P
2135  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
2136  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
2137  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
2138  different numbers of bytes, are also not permitted.  escapes, which can match different numbers of data units, are also not
2139    permitted.
2140    .P
2141    .\" HTML <a href="#subpatternsassubroutines">
2142    .\" </a>
2143    "Subroutine"
2144    .\"
2145    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
2146    as the subpattern matches a fixed-length string.
2147    .\" HTML <a href="#recursion">
2148    .\" </a>
2149    Recursion,
2150    .\"
2151    however, is not supported.
2152  .P  .P
2153  Possessive quantifiers can be used in conjunction with lookbehind assertions to  Possessive quantifiers can be used in conjunction with lookbehind assertions to
2154  specify efficient matching at the end of the subject string. Consider a simple  specify efficient matching of fixed-length strings at the end of subject
2155  pattern such as  strings. Consider a simple pattern such as
2156  .sp  .sp
2157    abcd$    abcd$
2158  .sp  .sp
# Line 1542  characters that are not "999". Line 2216  characters that are not "999".
2216  .sp  .sp
2217  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2218  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2219  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2220  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2221  .sp  .sp
2222    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2223    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2224  .sp  .sp
2225  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2226  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
2227  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2228    itself contain nested subpatterns of any form, including conditional
2229    subpatterns; the restriction to two alternatives applies only at the level of
2230    the condition. This pattern fragment is an example where the alternatives are
2231    complex:
2232    .sp
2233      (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2234    .sp
2235  .P  .P
2236  There are four kinds of condition: references to subpatterns, references to  There are four kinds of condition: references to subpatterns, references to
2237  recursion, a pseudo-condition called DEFINE, and assertions.  recursion, a pseudo-condition called DEFINE, and assertions.
# Line 1559  recursion, a pseudo-condition called DEF Line 2240  recursion, a pseudo-condition called DEF
2240  .rs  .rs
2241  .sp  .sp
2242  If the text between the parentheses consists of a sequence of digits, the  If the text between the parentheses consists of a sequence of digits, the
2243  condition is true if the capturing subpattern of that number has previously  condition is true if a capturing subpattern of that number has previously
2244  matched. An alternative notation is to precede the digits with a plus or minus  matched. If there is more than one capturing subpattern with the same number
2245  sign. In this case, the subpattern number is relative rather than absolute.  (see the earlier
2246  The most recently opened parentheses can be referenced by (?(-1), the next most  .\"
2247  recent by (?(-2), and so on. In looping constructs it can also make sense to  .\" HTML <a href="#recursion">
2248  refer to subsequent groups with constructs such as (?(+2).  .\" </a>
2249    section about duplicate subpattern numbers),
2250    .\"
2251    the condition is true if any of them have matched. An alternative notation is
2252    to precede the digits with a plus or minus sign. In this case, the subpattern
2253    number is relative rather than absolute. The most recently opened parentheses
2254    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2255    loops it can also make sense to refer to subsequent groups. The next
2256    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2257    zero in any of these forms is not used; it provokes a compile-time error.)
2258  .P  .P
2259  Consider the following pattern, which contains non-significant white space to  Consider the following pattern, which contains non-significant white space to
2260  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 1575  three parts for ease of discussion: Line 2265  three parts for ease of discussion:
2265  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2266  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
2267  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
2268  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2269  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,
2270  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
2271  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2272  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 1605  Rewriting the above example to use a nam Line 2295  Rewriting the above example to use a nam
2295  .sp  .sp
2296    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )    (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2297  .sp  .sp
2298    If the name used in a condition of this kind is a duplicate, the test is
2299    applied to all subpatterns of the same name, and is true if any one of them has
2300    matched.
2301  .  .
2302  .SS "Checking for pattern recursion"  .SS "Checking for pattern recursion"
2303  .rs  .rs
# Line 1616  letter R, for example: Line 2309  letter R, for example:
2309  .sp  .sp
2310    (?(R3)...) or (?(R&name)...)    (?(R3)...) or (?(R&name)...)
2311  .sp  .sp
2312  the condition is true if the most recent recursion is into the subpattern whose  the condition is true if the most recent recursion is into a subpattern whose
2313  number or name is given. This condition does not check the entire recursion  number or name is given. This condition does not check the entire recursion
2314  stack.  stack. If the name used in a condition of this kind is a duplicate, the test is
2315    applied to all subpatterns of the same name, and is true if any one of them is
2316    the most recent recursion.
2317  .P  .P
2318  At "top level", all these recursion test conditions are false. Recursive  At "top level", all these recursion test conditions are false.
2319  patterns are described below.  .\" HTML <a href="#recursion">
2320    .\" </a>
2321    The syntax for recursive patterns
2322    .\"
2323    is described below.
2324  .  .
2325    .\" HTML <a name="subdefine"></a>
2326  .SS "Defining subpatterns for use by reference only"  .SS "Defining subpatterns for use by reference only"
2327  .rs  .rs
2328  .sp  .sp
# Line 1630  If the condition is the string (DEFINE), Line 2330  If the condition is the string (DEFINE),
2330  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
2331  alternative in the subpattern. It is always skipped if control reaches this  alternative in the subpattern. It is always skipped if control reaches this
2332  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
2333  "subroutines" that can be referenced from elsewhere. (The use of "subroutines"  subroutines that can be referenced from elsewhere. (The use of
2334  is described below.) For example, a pattern to match an IPv4 address could be  .\" HTML <a href="#subpatternsassubroutines">
2335  written like this (ignore whitespace and line breaks):  .\" </a>
2336    subroutines
2337    .\"
2338    is described below.) For example, a pattern to match an IPv4 address such as
2339    "192.168.23.245" could be written like this (ignore white space and line
2340    breaks):
2341  .sp  .sp
2342    (?(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) )
2343    \eb (?&byte) (\e.(?&byte)){3} \eb    \eb (?&byte) (\e.(?&byte)){3} \eb
# Line 1640  written like this (ignore whitespace and Line 2345  written like this (ignore whitespace and
2345  The first part of the pattern is a DEFINE group inside which a another group  The first part of the pattern is a DEFINE group inside which a another group
2346  named "byte" is defined. This matches an individual component of an IPv4  named "byte" is defined. This matches an individual component of an IPv4
2347  address (a number less than 256). When matching takes place, this part of the  address (a number less than 256). When matching takes place, this part of the
2348  pattern is skipped because DEFINE acts like a false condition.  pattern is skipped because DEFINE acts like a false condition. The rest of the
2349  .P  pattern uses references to the named group to match the four dot-separated
2350  The rest of the pattern uses references to the named group to match the four  components of an IPv4 address, insisting on a word boundary at each end.
 dot-separated components of an IPv4 address, insisting on a word boundary at  
 each end.  
2351  .  .
2352  .SS "Assertion conditions"  .SS "Assertion conditions"
2353  .rs  .rs
# Line 1669  dd-aaa-dd or dd-dd-dd, where aaa are let Line 2372  dd-aaa-dd or dd-dd-dd, where aaa are let
2372  .SH COMMENTS  .SH COMMENTS
2373  .rs  .rs
2374  .sp  .sp
2375  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
2376  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,
2377  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
2378    subpattern name or number. The characters that make up a comment play no part
2379    in the pattern matching.
2380  .P  .P
2381  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
2382  character class introduces a comment that continues to immediately after the  closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2383  next newline in the pattern.  option is set, an unescaped # character also introduces a comment, which in
2384    this case continues to immediately after the next newline character or
2385    character sequence in the pattern. Which characters are interpreted as newlines
2386    is controlled by the options passed to a compiling function or by a special
2387    sequence at the start of the pattern, as described in the section entitled
2388    .\" HTML <a href="#newlines">
2389    .\" </a>
2390    "Newline conventions"
2391    .\"
2392    above. Note that the end of this type of comment is a literal newline sequence
2393    in the pattern; escape sequences that happen to represent a newline do not
2394    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2395    default newline convention is in force:
2396    .sp
2397      abc #comment \en still comment
2398    .sp
2399    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2400    a newline in the pattern. The sequence \en is still literal at this stage, so
2401    it does not terminate the comment. Only an actual character with the code value
2402    0x0a (the default newline) does so.
2403  .  .
2404  .  .
2405  .\" HTML <a name="recursion"></a>  .\" HTML <a name="recursion"></a>
# Line 1701  recursively to the pattern in which it a Line 2425  recursively to the pattern in which it a
2425  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2426  supports special syntax for recursion of the entire pattern, and also for  supports special syntax for recursion of the entire pattern, and also for
2427  individual subpattern recursion. After its introduction in PCRE and Python,  individual subpattern recursion. After its introduction in PCRE and Python,
2428  this kind of recursion was introduced into Perl at release 5.10.  this kind of recursion was subsequently introduced into Perl at release 5.10.
2429  .P  .P
2430  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
2431  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
2432  provided that it occurs inside that subpattern. (If not, it is a "subroutine"  given number, provided that it occurs inside that subpattern. (If not, it is a
2433    .\" HTML <a href="#subpatternsassubroutines">
2434    .\" </a>
2435    non-recursive subroutine
2436    .\"
2437  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
2438  a recursive call of the entire regular expression.  a recursive call of the entire regular expression.
2439  .P  .P
 In PCRE (like Python, but unlike Perl), a recursive subpattern call is always  
 treated as an atomic group. That is, once it has matched some of the subject  
 string, it is never re-entered, even if it contains untried alternatives and  
 there is a subsequent matching failure.  
 .P  
2440  This PCRE pattern solves the nested parentheses problem (assume the  This PCRE pattern solves the nested parentheses problem (assume the
2441  PCRE_EXTENDED option is set so that white space is ignored):  PCRE_EXTENDED option is set so that white space is ignored):
2442  .sp  .sp
2443    \e( ( (?>[^()]+) | (?R) )* \e)    \e( ( [^()]++ | (?R) )* \e)
2444  .sp  .sp
2445  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2446  substrings which can either be a sequence of non-parentheses, or a recursive  substrings which can either be a sequence of non-parentheses, or a recursive
2447  match of the pattern itself (that is, a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2448  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2449    to avoid backtracking into sequences of non-parentheses.
2450  .P  .P
2451  If this were part of a larger pattern, you would not want to recurse the entire  If this were part of a larger pattern, you would not want to recurse the entire
2452  pattern, so instead you could use this:  pattern, so instead you could use this:
2453  .sp  .sp
2454    ( \e( ( (?>[^()]+) | (?1) )* \e) )    ( \e( ( [^()]++ | (?1) )* \e) )
2455  .sp  .sp
2456  We have put the pattern into parentheses, and caused the recursion to refer to  We have put the pattern into parentheses, and caused the recursion to refer to
2457  them instead of the whole pattern.  them instead of the whole pattern.
2458  .P  .P
2459  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
2460  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
2461  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
2462  most recently opened parentheses preceding the recursion. In other words, a  parentheses preceding the recursion. In other words, a negative number counts
2463  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.  
2464  .P  .P
2465  It is also possible to refer to subsequently opened parentheses, by writing  It is also possible to refer to subsequently opened parentheses, by writing
2466  references such as (?+2). However, these cannot be recursive because the  references such as (?+2). However, these cannot be recursive because the
2467  reference is not inside the parentheses that are referenced. They are always  reference is not inside the parentheses that are referenced. They are always
2468  "subroutine" calls, as described in the next section.  .\" HTML <a href="#subpatternsassubroutines">
2469    .\" </a>
2470    non-recursive subroutine
2471    .\"
2472    calls, as described in the next section.
2473  .P  .P
2474  An alternative approach is to use named parentheses instead. The Perl syntax  An alternative approach is to use named parentheses instead. The Perl syntax
2475  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We  for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2476  could rewrite the above example as follows:  could rewrite the above example as follows:
2477  .sp  .sp
2478    (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )    (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2479  .sp  .sp
2480  If there is more than one subpattern with the same name, the earliest one is  If there is more than one subpattern with the same name, the earliest one is
2481  used.  used.
2482  .P  .P
2483  This particular example pattern that we have been looking at contains nested  This particular example pattern that we have been looking at contains nested
2484  unlimited repeats, and so the use of atomic grouping for matching strings of  unlimited repeats, and so the use of a possessive quantifier for matching
2485  non-parentheses is important when applying the pattern to strings that do not  strings of non-parentheses is important when applying the pattern to strings
2486  match. For example, when this pattern is applied to  that do not match. For example, when this pattern is applied to
2487  .sp  .sp
2488    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2489  .sp  .sp
2490  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if a possessive quantifier is not used,
2491  the match runs for a very long time indeed because there are so many different  the match runs for a very long time indeed because there are so many different
2492  ways the + and * repeats can carve up the subject, and all have to be tested  ways the + and * repeats can carve up the subject, and all have to be tested
2493  before failure can be reported.  before failure can be reported.
2494  .P  .P
2495  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values of capturing parentheses are those from
2496  from the outermost level of the recursion at which the subpattern value is set.  the outermost level. If you want to obtain intermediate values, a callout
2497  If you want to obtain intermediate values, a callout function can be used (see  function can be used (see below and the
 below and the  
2498  .\" HREF  .\" HREF
2499  \fBpcrecallout\fP  \fBpcrecallout\fP
2500  .\"  .\"
# Line 1776  documentation). If the pattern above is Line 2502  documentation). If the pattern above is
2502  .sp  .sp
2503    (ab(cd)ef)    (ab(cd)ef)
2504  .sp  .sp
2505  the value for the capturing parentheses is "ef", which is the last value taken  the value for the inner capturing parentheses (numbered 2) is "ef", which is
2506  on at the top level. If additional parentheses are added, giving  the last value taken on at the top level. If a capturing subpattern is not
2507  .sp  matched at the top level, its final captured value is unset, even if it was
2508    \e( ( ( (?>[^()]+) | (?R) )* ) \e)  (temporarily) set at a deeper level during the matching process.
2509       ^                        ^  .P
2510       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2511  .sp  obtain extra memory to store data during a recursion, which it does by using
2512  the string they capture is "ab(cd)ef", the contents of the top level  \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no memory can
2513  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
 has to obtain extra memory to store data during a recursion, which it does by  
 using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
2514  .P  .P
2515  Do not confuse the (?R) item with the condition (R), which tests for recursion.  Do not confuse the (?R) item with the condition (R), which tests for recursion.
2516  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
# Line 1801  different alternatives for the recursive Line 2524  different alternatives for the recursive
2524  is the actual recursive call.  is the actual recursive call.
2525  .  .
2526  .  .
2527    .\" HTML <a name="recursiondifference"></a>
2528    .SS "Differences in recursion processing between PCRE and Perl"
2529    .rs
2530    .sp
2531    Recursion processing in PCRE differs from Perl in two important ways. In PCRE
2532    (like Python, but unlike Perl), a recursive subpattern call is always treated
2533    as an atomic group. That is, once it has matched some of the subject string, it
2534    is never re-entered, even if it contains untried alternatives and there is a
2535    subsequent matching failure. This can be illustrated by the following pattern,
2536    which purports to match a palindromic string that contains an odd number of
2537    characters (for example, "a", "aba", "abcba", "abcdcba"):
2538    .sp
2539      ^(.|(.)(?1)\e2)$
2540    .sp
2541    The idea is that it either matches a single character, or two identical
2542    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2543    it does not if the pattern is longer than three characters. Consider the
2544    subject string "abcba":
2545    .P
2546    At the top level, the first character is matched, but as it is not at the end
2547    of the string, the first alternative fails; the second alternative is taken
2548    and the recursion kicks in. The recursive call to subpattern 1 successfully
2549    matches the next character ("b"). (Note that the beginning and end of line
2550    tests are not part of the recursion).
2551    .P
2552    Back at the top level, the next character ("c") is compared with what
2553    subpattern 2 matched, which was "a". This fails. Because the recursion is
2554    treated as an atomic group, there are now no backtracking points, and so the
2555    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2556    try the second alternative.) However, if the pattern is written with the
2557    alternatives in the other order, things are different:
2558    .sp
2559      ^((.)(?1)\e2|.)$
2560    .sp
2561    This time, the recursing alternative is tried first, and continues to recurse
2562    until it runs out of characters, at which point the recursion fails. But this
2563    time we do have another alternative to try at the higher level. That is the big
2564    difference: in the previous case the remaining alternative is at a deeper
2565    recursion level, which PCRE cannot use.
2566    .P
2567    To change the pattern so that it matches all palindromic strings, not just
2568    those with an odd number of characters, it is tempting to change the pattern to
2569    this:
2570    .sp
2571      ^((.)(?1)\e2|.?)$
2572    .sp
2573    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2574    deeper recursion has matched a single character, it cannot be entered again in
2575    order to match an empty string. The solution is to separate the two cases, and
2576    write out the odd and even cases as alternatives at the higher level:
2577    .sp
2578      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2579    .sp
2580    If you want to match typical palindromic phrases, the pattern has to ignore all
2581    non-word characters, which can be done like this:
2582    .sp
2583      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2584    .sp
2585    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2586    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2587    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2588    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2589    more) to match typical phrases, and Perl takes so long that you think it has
2590    gone into a loop.
2591    .P
2592    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2593    string does not start with a palindrome that is shorter than the entire string.
2594    For example, although "abcba" is correctly matched, if the subject is "ababa",
2595    PCRE finds the palindrome "aba" at the start, then fails at top level because
2596    the end of the string does not follow. Once again, it cannot jump back into the
2597    recursion to try other alternatives, so the entire match fails.
2598    .P
2599    The second way in which PCRE and Perl differ in their recursion processing is
2600    in the handling of captured values. In Perl, when a subpattern is called
2601    recursively or as a subpattern (see the next section), it has no access to any
2602    values that were captured outside the recursion, whereas in PCRE these values
2603    can be referenced. Consider this pattern:
2604    .sp
2605      ^(.)(\e1|a(?2))
2606    .sp
2607    In PCRE, this pattern matches "bab". The first capturing parentheses match "b",
2608    then in the second group, when the back reference \e1 fails to match "b", the
2609    second alternative matches "a" and then recurses. In the recursion, \e1 does
2610    now match "b" and so the whole match succeeds. In Perl, the pattern fails to
2611    match because inside the recursive call \e1 cannot access the externally set
2612    value.
2613    .
2614    .
2615  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2616  .SH "SUBPATTERNS AS SUBROUTINES"  .SH "SUBPATTERNS AS SUBROUTINES"
2617  .rs  .rs
2618  .sp  .sp
2619  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
2620  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
2621  subroutine in a programming language. The "called" subpattern may be defined  subroutine in a programming language. The called subpattern may be defined
2622  before or after the reference. A numbered reference can be absolute or  before or after the reference. A numbered reference can be absolute or
2623  relative, as in these examples:  relative, as in these examples:
2624  .sp  .sp
# Line 1827  matches "sense and sensibility" and "res Line 2638  matches "sense and sensibility" and "res
2638  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
2639  strings. Another example is given in the discussion of DEFINE above.  strings. Another example is given in the discussion of DEFINE above.
2640  .P  .P
2641  Like recursive subpatterns, a "subroutine" call is always treated as an atomic  All subroutine calls, whether recursive or not, are always treated as atomic
2642  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
2643  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
2644  matching failure.  subsequent matching failure. Any capturing parentheses that are set during the
2645  .P  subroutine call revert to their previous values afterwards.
2646  When a subpattern is used as a subroutine, processing options such as  .P
2647  case-independence are fixed when the subpattern is defined. They cannot be  Processing options such as case-independence are fixed when a subpattern is
2648  changed for different calls. For example, consider this pattern:  defined, so if it is used as a subroutine, such options cannot be changed for
2649    different calls. For example, consider this pattern:
2650  .sp  .sp
2651    (abc)(?i:(?-1))    (abc)(?i:(?-1))
2652  .sp  .sp
# Line 1842  It matches "abcabc". It does not match " Line 2654  It matches "abcabc". It does not match "
2654  processing option does not affect the called subpattern.  processing option does not affect the called subpattern.
2655  .  .
2656  .  .
2657    .\" HTML <a name="onigurumasubroutines"></a>
2658    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2659    .rs
2660    .sp
2661    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2662    a number enclosed either in angle brackets or single quotes, is an alternative
2663    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2664    are two of the examples used above, rewritten using this syntax:
2665    .sp
2666      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2667      (sens|respons)e and \eg'1'ibility
2668    .sp
2669    PCRE supports an extension to Oniguruma: if a number is preceded by a
2670    plus or a minus sign it is taken as a relative reference. For example:
2671    .sp
2672      (abc)(?i:\eg<-1>)
2673    .sp
2674    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2675    synonymous. The former is a back reference; the latter is a subroutine call.
2676    .
2677    .
2678  .SH CALLOUTS  .SH CALLOUTS
2679  .rs  .rs
2680  .sp  .sp
# Line 1852  same pair of parentheses when there is a Line 2685  same pair of parentheses when there is a
2685  .P  .P
2686  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
2687  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
2688  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
2689    (8-bit library) or \fIpcre[16|32]_callout\fP (16-bit or 32-bit library).
2690  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2691  .P  .P
2692  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 1862  For example, this pattern has two callou Line 2696  For example, this pattern has two callou
2696  .sp  .sp
2697    (?C1)abc(?C2)def    (?C1)abc(?C2)def
2698  .sp  .sp
2699  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
2700  automatically installed before each item in the pattern. They are all numbered  automatically installed before each item in the pattern. They are all numbered
2701  255.  255. If there is a conditional group in the pattern whose condition is an
2702  .P  assertion, an additional callout is inserted just before the condition. An
2703  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:
2704  set), the external function is called. It is provided with the number of the  .sp
2705  callout, the position in the pattern, and, optionally, one item of data    (?(?C9)(?=a)abc|def)
2706  originally supplied by the caller of \fBpcre_exec()\fP. The callout function  .sp
2707  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
2708  description of the interface to the callout function is given in the  condition.
2709    .P
2710    During matching, when PCRE reaches a callout point, the external function is
2711    called. It is provided with the number of the callout, the position in the
2712    pattern, and, optionally, one item of data originally supplied by the caller of
2713    the matching function. The callout function may cause matching to proceed, to
2714    backtrack, or to fail altogether. A complete description of the interface to
2715    the callout function is given in the
2716  .\" HREF  .\" HREF
2717  \fBpcrecallout\fP  \fBpcrecallout\fP
2718  .\"  .\"
2719  documentation.  documentation.
2720  .  .
2721  .  .
2722    .\" HTML <a name="backtrackcontrol"></a>
2723    .SH "BACKTRACKING CONTROL"
2724    .rs
2725    .sp
2726    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2727    are still described in the Perl documentation as "experimental and subject to
2728    change or removal in a future version of Perl". It goes on to say: "Their usage
2729    in production code should be noted to avoid problems during upgrades." The same
2730    remarks apply to the PCRE features described in this section.
2731    .P
2732    The new verbs make use of what was previously invalid syntax: an opening
2733    parenthesis followed by an asterisk. They are generally of the form
2734    (*VERB) or (*VERB:NAME). Some may take either form, possibly behaving
2735    differently depending on whether or not a name is present. A name is any
2736    sequence of characters that does not include a closing parenthesis. The maximum
2737    length of name is 255 in the 8-bit library and 65535 in the 16-bit and 32-bit
2738    libraries. If the name is empty, that is, if the closing parenthesis
2739    immediately follows the colon, the effect is as if the colon were not there.
2740    Any number of these verbs may occur in a pattern.
2741    .P
2742    Since these verbs are specifically related to backtracking, most of them can be
2743    used only when the pattern is to be matched using one of the traditional
2744    matching functions, because these use a backtracking algorithm. With the
2745    exception of (*FAIL), which behaves like a failing negative assertion, the
2746    backtracking control verbs cause an error if encountered by a DFA matching
2747    function.
2748    .P
2749    The behaviour of these verbs in
2750    .\" HTML <a href="#btrepeat">
2751    .\" </a>
2752    repeated groups,
2753    .\"
2754    .\" HTML <a href="#btassert">
2755    .\" </a>
2756    assertions,
2757    .\"
2758    and in
2759    .\" HTML <a href="#btsub">
2760    .\" </a>
2761    subpatterns called as subroutines
2762    .\"
2763    (whether or not recursively) is documented below.
2764    .
2765    .
2766    .\" HTML <a name="nooptimize"></a>
2767    .SS "Optimizations that affect backtracking verbs"
2768    .rs
2769    .sp
2770    PCRE contains some optimizations that are used to speed up matching by running
2771    some checks at the start of each match attempt. For example, it may know the
2772    minimum length of matching subject, or that a particular character must be
2773    present. When one of these optimizations bypasses the running of a match, any
2774    included backtracking verbs will not, of course, be processed. You can suppress
2775    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2776    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2777    pattern with (*NO_START_OPT). There is more discussion of this option in the
2778    section entitled
2779    .\" HTML <a href="pcreapi.html#execoptions">
2780    .\" </a>
2781    "Option bits for \fBpcre_exec()\fP"
2782    .\"
2783    in the
2784    .\" HREF
2785    \fBpcreapi\fP
2786    .\"
2787    documentation.
2788    .P
2789    Experiments with Perl suggest that it too has similar optimizations, sometimes
2790    leading to anomalous results.
2791    .
2792    .
2793    .SS "Verbs that act immediately"
2794    .rs
2795    .sp
2796    The following verbs act as soon as they are encountered. They may not be
2797    followed by a name.
2798    .sp
2799       (*ACCEPT)
2800    .sp
2801    This verb causes the match to end successfully, skipping the remainder of the
2802    pattern. However, when it is inside a subpattern that is called as a
2803    subroutine, only that subpattern is ended successfully. Matching then continues
2804    at the outer level. If (*ACCEPT) in triggered in a positive assertion, the
2805    assertion succeeds; in a negative assertion, the assertion fails.
2806    .P
2807    If (*ACCEPT) is inside capturing parentheses, the data so far is captured. For
2808    example:
2809    .sp
2810      A((?:A|B(*ACCEPT)|C)D)
2811    .sp
2812    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2813    the outer parentheses.
2814    .sp
2815      (*FAIL) or (*F)
2816    .sp
2817    This verb causes a matching failure, forcing backtracking to occur. It is
2818    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2819    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2820    Perl features that are not present in PCRE. The nearest equivalent is the
2821    callout feature, as for example in this pattern:
2822    .sp
2823      a+(?C)(*FAIL)
2824    .sp
2825    A match with the string "aaaa" always fails, but the callout is taken before
2826    each backtrack happens (in this example, 10 times).
2827    .
2828    .
2829    .SS "Recording which path was taken"
2830    .rs
2831    .sp
2832    There is one verb whose main purpose is to track how a match was arrived at,
2833    though it also has a secondary use in conjunction with advancing the match
2834    starting point (see (*SKIP) below).
2835    .sp
2836      (*MARK:NAME) or (*:NAME)
2837    .sp
2838    A name is always required with this verb. There may be as many instances of
2839    (*MARK) as you like in a pattern, and their names do not have to be unique.
2840    .P
2841    When a match succeeds, the name of the last-encountered (*MARK:NAME),
2842    (*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to the
2843    caller as described in the section entitled
2844    .\" HTML <a href="pcreapi.html#extradata">
2845    .\" </a>
2846    "Extra data for \fBpcre_exec()\fP"
2847    .\"
2848    in the
2849    .\" HREF
2850    \fBpcreapi\fP
2851    .\"
2852    documentation. Here is an example of \fBpcretest\fP output, where the /K
2853    modifier requests the retrieval and outputting of (*MARK) data:
2854    .sp
2855        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2856      data> XY
2857       0: XY
2858      MK: A
2859      XZ
2860       0: XZ
2861      MK: B
2862    .sp
2863    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2864    indicates which of the two alternatives matched. This is a more efficient way
2865    of obtaining this information than putting each alternative in its own
2866    capturing parentheses.
2867    .P
2868    If a verb with a name is encountered in a positive assertion that is true, the
2869    name is recorded and passed back if it is the last-encountered. This does not
2870    happen for negative assertions or failing positive assertions.
2871    .P
2872    After a partial match or a failed match, the last encountered name in the
2873    entire match process is returned. For example:
2874    .sp
2875        re> /X(*MARK:A)Y|X(*MARK:B)Z/K
2876      data> XP
2877      No match, mark = B
2878    .sp
2879    Note that in this unanchored example the mark is retained from the match
2880    attempt that started at the letter "X" in the subject. Subsequent match
2881    attempts starting at "P" and then with an empty string do not get as far as the
2882    (*MARK) item, but nevertheless do not reset it.
2883    .P
2884    If you are interested in (*MARK) values after failed matches, you should
2885    probably set the PCRE_NO_START_OPTIMIZE option
2886    .\" HTML <a href="#nooptimize">
2887    .\" </a>
2888    (see above)
2889    .\"
2890    to ensure that the match is always attempted.
2891    .
2892    .
2893    .SS "Verbs that act after backtracking"
2894    .rs
2895    .sp
2896    The following verbs do nothing when they are encountered. Matching continues
2897    with what follows, but if there is no subsequent match, causing a backtrack to
2898    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2899    the verb. However, when one of these verbs appears inside an atomic group or an
2900    assertion that is true, its effect is confined to that group, because once the
2901    group has been matched, there is never any backtracking into it. In this
2902    situation, backtracking can "jump back" to the left of the entire atomic group
2903    or assertion. (Remember also, as stated above, that this localization also
2904    applies in subroutine calls.)
2905    .P
2906    These verbs differ in exactly what kind of failure occurs when backtracking
2907    reaches them. The behaviour described below is what happens when the verb is
2908    not in a subroutine or an assertion. Subsequent sections cover these special
2909    cases.
2910    .sp
2911      (*COMMIT)
2912    .sp
2913    This verb, which may not be followed by a name, causes the whole match to fail
2914    outright if there is a later matching failure that causes backtracking to reach
2915    it. Even if the pattern is unanchored, no further attempts to find a match by
2916    advancing the starting point take place. If (*COMMIT) is the only backtracking
2917    verb that is encountered, once it has been passed \fBpcre_exec()\fP is
2918    committed to finding a match at the current starting point, or not at all. For
2919    example:
2920    .sp
2921      a+(*COMMIT)b
2922    .sp
2923    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2924    dynamic anchor, or "I've started, so I must finish." The name of the most
2925    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2926    match failure.
2927    .P
2928    If there is more than one backtracking verb in a pattern, a different one that
2929    follows (*COMMIT) may be triggered first, so merely passing (*COMMIT) during a
2930    match does not always guarantee that a match must be at this starting point.
2931    .P
2932    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2933    unless PCRE's start-of-match optimizations are turned off, as shown in this
2934    \fBpcretest\fP example:
2935    .sp
2936        re> /(*COMMIT)abc/
2937      data> xyzabc
2938       0: abc
2939      xyzabc\eY
2940      No match
2941    .sp
2942    PCRE knows that any match must start with "a", so the optimization skips along
2943    the subject to "a" before running the first match attempt, which succeeds. When
2944    the optimization is disabled by the \eY escape in the second subject, the match
2945    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2946    starting points.
2947    .sp
2948      (*PRUNE) or (*PRUNE:NAME)
2949    .sp
2950    This verb causes the match to fail at the current starting position in the
2951    subject if there is a later matching failure that causes backtracking to reach
2952    it. If the pattern is unanchored, the normal "bumpalong" advance to the next
2953    starting character then happens. Backtracking can occur as usual to the left of
2954    (*PRUNE), before it is reached, or when matching to the right of (*PRUNE), but
2955    if there is no match to the right, backtracking cannot cross (*PRUNE). In
2956    simple cases, the use of (*PRUNE) is just an alternative to an atomic group or
2957    possessive quantifier, but there are some uses of (*PRUNE) that cannot be
2958    expressed in any other way. In an anchored pattern (*PRUNE) has the same effect
2959    as (*COMMIT).
2960    .P
2961    The behaviour of (*PRUNE:NAME) is the not the same as (*MARK:NAME)(*PRUNE).
2962    It is like (*MARK:NAME) in that the name is remembered for passing back to the
2963    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
2964    .sp
2965      (*SKIP)
2966    .sp
2967    This verb, when given without a name, is like (*PRUNE), except that if the
2968    pattern is unanchored, the "bumpalong" advance is not to the next character,
2969    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2970    signifies that whatever text was matched leading up to it cannot be part of a
2971    successful match. Consider:
2972    .sp
2973      a+(*SKIP)b
2974    .sp
2975    If the subject is "aaaac...", after the first match attempt fails (starting at
2976    the first character in the string), the starting point skips on to start the
2977    next attempt at "c". Note that a possessive quantifer does not have the same
2978    effect as this example; although it would suppress backtracking during the
2979    first match attempt, the second attempt would start at the second character
2980    instead of skipping on to "c".
2981    .sp
2982      (*SKIP:NAME)
2983    .sp
2984    When (*SKIP) has an associated name, its behaviour is modified. When it is
2985    triggered, the previous path through the pattern is searched for the most
2986    recent (*MARK) that has the same name. If one is found, the "bumpalong" advance
2987    is to the subject position that corresponds to that (*MARK) instead of to where
2988    (*SKIP) was encountered. If no (*MARK) with a matching name is found, the
2989    (*SKIP) is ignored.
2990    .P
2991    Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It ignores
2992    names that are set by (*PRUNE:NAME) or (*THEN:NAME).
2993    .sp
2994      (*THEN) or (*THEN:NAME)
2995    .sp
2996    This verb causes a skip to the next innermost alternative when backtracking
2997    reaches it. That is, it cancels any further backtracking within the current
2998    alternative. Its name comes from the observation that it can be used for a
2999    pattern-based if-then-else block:
3000    .sp
3001      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
3002    .sp
3003    If the COND1 pattern matches, FOO is tried (and possibly further items after
3004    the end of the group if FOO succeeds); on failure, the matcher skips to the
3005    second alternative and tries COND2, without backtracking into COND1. If that
3006    succeeds and BAR fails, COND3 is tried. If subsequently BAZ fails, there are no
3007    more alternatives, so there is a backtrack to whatever came before the entire
3008    group. If (*THEN) is not inside an alternation, it acts like (*PRUNE).
3009    .P
3010    The behaviour of (*THEN:NAME) is the not the same as (*MARK:NAME)(*THEN).
3011    It is like (*MARK:NAME) in that the name is remembered for passing back to the
3012    caller. However, (*SKIP:NAME) searches only for names set with (*MARK).
3013    .P
3014    A subpattern that does not contain a | character is just a part of the
3015    enclosing alternative; it is not a nested alternation with only one
3016    alternative. The effect of (*THEN) extends beyond such a subpattern to the
3017    enclosing alternative. Consider this pattern, where A, B, etc. are complex
3018    pattern fragments that do not contain any | characters at this level:
3019    .sp
3020      A (B(*THEN)C) | D
3021    .sp
3022    If A and B are matched, but there is a failure in C, matching does not
3023    backtrack into A; instead it moves to the next alternative, that is, D.
3024    However, if the subpattern containing (*THEN) is given an alternative, it
3025    behaves differently:
3026    .sp
3027      A (B(*THEN)C | (*FAIL)) | D
3028    .sp
3029    The effect of (*THEN) is now confined to the inner subpattern. After a failure
3030    in C, matching moves to (*FAIL), which causes the whole subpattern to fail
3031    because there are no more alternatives to try. In this case, matching does now
3032    backtrack into A.
3033    .P
3034    Note that a conditional subpattern is not considered as having two
3035    alternatives, because only one is ever used. In other words, the | character in
3036    a conditional subpattern has a different meaning. Ignoring white space,
3037    consider:
3038    .sp
3039      ^.*? (?(?=a) a | b(*THEN)c )
3040    .sp
3041    If the subject is "ba", this pattern does not match. Because .*? is ungreedy,
3042    it initially matches zero characters. The condition (?=a) then fails, the
3043    character "b" is matched, but "c" is not. At this point, matching does not
3044    backtrack to .*? as might perhaps be expected from the presence of the |
3045    character. The conditional subpattern is part of the single alternative that
3046    comprises the whole pattern, and so the match fails. (If there was a backtrack
3047    into .*?, allowing it to match "b", the match would succeed.)
3048    .P
3049    The verbs just described provide four different "strengths" of control when
3050    subsequent matching fails. (*THEN) is the weakest, carrying on the match at the
3051    next alternative. (*PRUNE) comes next, failing the match at the current
3052    starting position, but allowing an advance to the next character (for an
3053    unanchored pattern). (*SKIP) is similar, except that the advance may be more
3054    than one character. (*COMMIT) is the strongest, causing the entire match to
3055    fail.
3056    .
3057    .
3058    .SS "More than one backtracking verb"
3059    .rs
3060    .sp
3061    If more than one backtracking verb is present in a pattern, the one that is
3062    backtracked onto first acts. For example, consider this pattern, where A, B,
3063    etc. are complex pattern fragments:
3064    .sp
3065      (A(*COMMIT)B(*THEN)C|ABD)
3066    .sp
3067    If A matches but B fails, the backtrack to (*COMMIT) causes the entire match to
3068    fail. However, if A and B match, but C fails, the backtrack to (*THEN) causes
3069    the next alternative (ABD) to be tried. This behaviour is consistent, but is
3070    not always the same as Perl's. It means that if two or more backtracking verbs
3071    appear in succession, all the the last of them has no effect. Consider this
3072    example:
3073    .sp
3074      ...(*COMMIT)(*PRUNE)...
3075    .sp
3076    If there is a matching failure to the right, backtracking onto (*PRUNE) causes
3077    it to be triggered, and its action is taken. There can never be a backtrack
3078    onto (*COMMIT).
3079    .
3080    .
3081    .\" HTML <a name="btrepeat"></a>
3082    .SS "Backtracking verbs in repeated groups"
3083    .rs
3084    .sp
3085    PCRE differs from Perl in its handling of backtracking verbs in repeated
3086    groups. For example, consider:
3087    .sp
3088      /(a(*COMMIT)b)+ac/
3089    .sp
3090    If the subject is "abac", Perl matches, but PCRE fails because the (*COMMIT) in
3091    the second repeat of the group acts.
3092    .
3093    .
3094    .\" HTML <a name="btassert"></a>
3095    .SS "Backtracking verbs in assertions"
3096    .rs
3097    .sp
3098    (*FAIL) in an assertion has its normal effect: it forces an immediate backtrack.
3099    .P
3100    (*ACCEPT) in a positive assertion causes the assertion to succeed without any
3101    further processing. In a negative assertion, (*ACCEPT) causes the assertion to
3102    fail without any further processing.
3103    .P
3104    The other backtracking verbs are not treated specially if they appear in a
3105    positive assertion. In particular, (*THEN) skips to the next alternative in the
3106    innermost enclosing group that has alternations, whether or not this is within
3107    the assertion.
3108    .P
3109    Negative assertions are, however, different, in order to ensure that changing a
3110    positive assertion into a negative assertion changes its result. Backtracking
3111    into (*COMMIT), (*SKIP), or (*PRUNE) causes a negative assertion to be true,
3112    without considering any further alternative branches in the assertion.
3113    Backtracking into (*THEN) causes it to skip to the next enclosing alternative
3114    within the assertion (the normal behaviour), but if the assertion does not have
3115    such an alternative, (*THEN) behaves like (*PRUNE).
3116    .
3117    .
3118    .\" HTML <a name="btsub"></a>
3119    .SS "Backtracking verbs in subroutines"
3120    .rs
3121    .sp
3122    These behaviours occur whether or not the subpattern is called recursively.
3123    Perl's treatment of subroutines is different in some cases.
3124    .P
3125    (*FAIL) in a subpattern called as a subroutine has its normal effect: it forces
3126    an immediate backtrack.
3127    .P
3128    (*ACCEPT) in a subpattern called as a subroutine causes the subroutine match to
3129    succeed without any further processing. Matching then continues after the
3130    subroutine call.
3131    .P
3132    (*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine cause
3133    the subroutine match to fail.
3134    .P
3135    (*THEN) skips to the next alternative in the innermost enclosing group within
3136    the subpattern that has alternatives. If there is no such group within the
3137    subpattern, (*THEN) causes the subroutine match to fail.
3138    .
3139    .
3140  .SH "SEE ALSO"  .SH "SEE ALSO"
3141  .rs  .rs
3142  .sp  .sp
3143  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).  \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
3144    \fBpcresyntax\fP(3), \fBpcre\fP(3), \fBpcre16(3)\fP, \fBpcre32(3)\fP.
3145  .  .
3146  .  .
3147  .SH AUTHOR  .SH AUTHOR
# Line 1898  Cambridge CB2 3QH, England. Line 3158  Cambridge CB2 3QH, England.
3158  .rs  .rs
3159  .sp  .sp
3160  .nf  .nf
3161  Last updated: 29 May 2007  Last updated: 08 October 2013
3162  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2013 University of Cambridge.
3163  .fi  .fi

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