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

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

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

revision 73 by nigel, Sat Feb 24 21:40:30 2007 UTC revision 678 by ph10, Sun Aug 28 15:23:03 2011 UTC
# Line 1  Line 1 
1  .TH PCRE 3  .TH PCREPATTERN 3
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 other books, some of which have copious  .\" HREF
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  \fBpcresyntax\fP
 O'Reilly, covers them in great detail. The description here is intended as  
 reference documentation.  
   
 The basic operation of PCRE is on strings of bytes. However, there is also  
 support for UTF-8 character strings. To use this support you must build PCRE to  
 include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  
 option. How this affects the 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    The original operation of PCRE was on strings of one-byte characters. However,
24    there is now also support for UTF-8 character strings. To use this,
25    PCRE must be built to include UTF-8 support, and you must call
26    \fBpcre_compile()\fP or \fBpcre_compile2()\fP with the PCRE_UTF8 option. There
27    is also a special sequence that can be given at the start of a pattern:
28    .sp
29      (*UTF8)
30    .sp
31    Starting a pattern with this sequence is equivalent to setting the PCRE_UTF8
32    option. This feature is not Perl-compatible. How setting UTF-8 mode affects
33    pattern matching is mentioned in several places below. There is also a summary
34    of UTF-8 features in the
35    .\" HREF
36    \fBpcreunicode\fP
37    .\"
38    page.
39    .P
40    Another special sequence that may appear at the start of a pattern or in
41    combination with (*UTF8) is:
42    .sp
43      (*UCP)
44    .sp
45    This has the same effect as setting the PCRE_UCP option: it causes sequences
46    such as \ed and \ew to use Unicode properties to determine character types,
47    instead of recognizing only characters with codes less than 128 via a lookup
48    table.
49    .P
50    If a pattern starts with (*NO_START_OPT), it has the same effect as setting the
51    PCRE_NO_START_OPTIMIZE option either at compile or matching time. There are
52    also some more of these special sequences that are concerned with the handling
53    of newlines; they are described below.
54    .P
55    The remainder of this document discusses the patterns that are supported by
56    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
57    From release 6.0, PCRE offers a second matching function,
58    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
59    Perl-compatible. Some of the features discussed below are not available when
60    \fBpcre_dfa_exec()\fP is used. The advantages and disadvantages of the
61    alternative function, and how it differs from the normal function, are
62    discussed in the
63  .\" HREF  .\" HREF
64  \fBpcre\fR  \fBpcrematching\fP
65  .\"  .\"
66  page.  page.
67    .
68    .
69    .\" HTML <a name="newlines"></a>
70    .SH "NEWLINE CONVENTIONS"
71    .rs
72    .sp
73    PCRE supports five different conventions for indicating line breaks in
74    strings: a single CR (carriage return) character, a single LF (linefeed)
75    character, the two-character sequence CRLF, any of the three preceding, or any
76    Unicode newline sequence. The
77    .\" HREF
78    \fBpcreapi\fP
79    .\"
80    page has
81    .\" HTML <a href="pcreapi.html#newlines">
82    .\" </a>
83    further discussion
84    .\"
85    about newlines, and shows how to set the newline convention in the
86    \fIoptions\fP arguments for the compiling and matching functions.
87    .P
88    It is also possible to specify a newline convention by starting a pattern
89    string with one of the following five sequences:
90    .sp
91      (*CR)        carriage return
92      (*LF)        linefeed
93      (*CRLF)      carriage return, followed by linefeed
94      (*ANYCRLF)   any of the three above
95      (*ANY)       all Unicode newline sequences
96    .sp
97    These override the default and the options given to \fBpcre_compile()\fP or
98    \fBpcre_compile2()\fP. For example, on a Unix system where LF is the default
99    newline sequence, the pattern
100    .sp
101      (*CR)a.b
102    .sp
103    changes the convention to CR. That pattern matches "a\enb" because LF is no
104    longer a newline. Note that these special settings, which are not
105    Perl-compatible, are recognized only at the very start of a pattern, and that
106    they must be in upper case. If more than one of them is present, the last one
107    is used.
108    .P
109    The newline convention affects the interpretation of the dot metacharacter when
110    PCRE_DOTALL is not set, and also the behaviour of \eN. However, it does not
111    affect what the \eR escape sequence matches. By default, this is any Unicode
112    newline sequence, for Perl compatibility. However, this can be changed; see the
113    description of \eR in the section entitled
114    .\" HTML <a href="#newlineseq">
115    .\" </a>
116    "Newline sequences"
117    .\"
118    below. A change of \eR setting can be combined with a change of newline
119    convention.
120    .
121    .
122    .SH "CHARACTERS AND METACHARACTERS"
123    .rs
124    .sp
125  A regular expression is a pattern that is matched against a subject string from  A regular expression is a pattern that is matched against a subject string from
126  left to right. Most characters stand for themselves in a pattern, and match the  left to right. Most characters stand for themselves in a pattern, and match the
127  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
128    .sp
129    The quick brown fox    The quick brown fox
130    .sp
131  matches a portion of a subject string that is identical to itself. The power of  matches a portion of a subject string that is identical to itself. When
132  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
133  repetitions in the pattern. These are encoded in the pattern by the use of  independently of case. In UTF-8 mode, PCRE always understands the concept of
134  \fImeta-characters\fR, which do not stand for themselves but instead are  case for characters whose values are less than 128, so caseless matching is
135    always possible. For characters with higher values, the concept of case is
136    supported if PCRE is compiled with Unicode property support, but not otherwise.
137    If you want to use caseless matching for characters 128 and above, you must
138    ensure that PCRE is compiled with Unicode property support as well as with
139    UTF-8 support.
140    .P
141    The power of regular expressions comes from the ability to include alternatives
142    and repetitions in the pattern. These are encoded in the pattern by the use of
143    \fImetacharacters\fP, which do not stand for themselves but instead are
144  interpreted in some special way.  interpreted in some special way.
145    .P
146  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
147  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
148  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
149  as follows:  are as follows:
150    .sp
151    \\      general escape character with several uses    \e      general escape character with several uses
152    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
153    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
154    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 163  as follows:
163    +      1 or more quantifier    +      1 or more quantifier
164           also "possessive quantifier"           also "possessive quantifier"
165    {      start min/max quantifier    {      start min/max quantifier
166    .sp
167  Part of a pattern that is in square brackets is called a "character class". In  Part of a pattern that is in square brackets is called a "character class". In
168  a character class the only meta-characters are:  a character class the only metacharacters are:
169    .sp
170    \\      general escape character    \e      general escape character
171    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
172    -      indicates character range    -      indicates character range
173    .\" JOIN
174    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
175             syntax)             syntax)
176    ]      terminates the character class    ]      terminates the character class
177    .sp
178  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
179    .
180    .
181  .SH BACKSLASH  .SH BACKSLASH
182  .rs  .rs
183  .sp  .sp
184  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
185  non-alphameric character, it takes away any special meaning that character may  character that is not a number or a letter, it takes away any special meaning
186  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
187  outside character classes.  both inside and outside character classes.
188    .P
189  For example, if you want to match a * character, you write \\* in the pattern.  For example, if you want to match a * character, you write \e* in the pattern.
190  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
191  otherwise be interpreted as a meta-character, so it is always safe to precede a  otherwise be interpreted as a metacharacter, so it is always safe to precede a
192  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
193  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
194    .P
195    In UTF-8 mode, only ASCII numbers and letters have any special meaning after a
196    backslash. All other characters (in particular, those whose codepoints are
197    greater than 127) are treated as literals.
198    .P
199  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the  If a pattern is compiled with the PCRE_EXTENDED option, whitespace in the
200  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
201  a character class and the next newline character are ignored. An escaping  a character class and the next newline are ignored. An escaping backslash can
202  backslash can be used to include a whitespace or # character as part of the  be used to include a whitespace or # character as part of the pattern.
203  pattern.  .P
   
204  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
205  can do so by putting them between \\Q and \\E. This is different from Perl in  can do so by putting them between \eQ and \eE. This is different from Perl in
206  that $ and @ are handled as literals in \\Q...\\E sequences in PCRE, whereas in  that $ and @ are handled as literals in \eQ...\eE sequences in PCRE, whereas in
207  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
208    .sp
209    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
210    .sp
211    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
212      \eQabc$xyz\eE        abc$xyz        abc followed by the
213                                        contents of $xyz                                        contents of $xyz
214    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
215    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
216    .sp
217  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
218    An isolated \eE that is not preceded by \eQ is ignored. If \eQ is not followed
219    by \eE later in the pattern, the literal interpretation continues to the end of
220    the pattern (that is, \eE is assumed at the end). If the isolated \eQ is inside
221    a character class, this causes an error, because the character class is not
222    terminated.
223    .
224    .
225    .\" HTML <a name="digitsafterbackslash"></a>
226    .SS "Non-printing characters"
227    .rs
228    .sp
229  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
230  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
231  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
232  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
233  use one of the following escape sequences than the binary character it  one of the following escape sequences than the binary character it represents:
234  represents:  .sp
235      \ea        alarm, that is, the BEL character (hex 07)
236    \\a        alarm, that is, the BEL character (hex 07)    \ecx       "control-x", where x is any ASCII character
237    \\cx       "control-x", where x is any character    \ee        escape (hex 1B)
238    \\e        escape (hex 1B)    \ef        formfeed (hex 0C)
239    \\f        formfeed (hex 0C)    \en        linefeed (hex 0A)
240    \\n        newline (hex 0A)    \er        carriage return (hex 0D)
241    \\r        carriage return (hex 0D)    \et        tab (hex 09)
242    \\t        tab (hex 09)    \eddd      character with octal code ddd, or back reference
243    \\ddd      character with octal code ddd, or backreference    \exhh      character with hex code hh
244    \\xhh      character with hex code hh    \ex{hhh..} character with hex code hhh..
245    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)  .sp
246    The precise effect of \ecx is as follows: if x is a lower case letter, it
 The precise effect of \\cx is as follows: if x is a lower case letter, it  
247  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.  is converted to upper case. Then bit 6 of the character (hex 40) is inverted.
248  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  Thus \ecz becomes hex 1A (z is 7A), but \ec{ becomes hex 3B ({ is 7B), while
249  7B.  \ec; becomes hex 7B (; is 3B). If the byte following \ec has a value greater
250    than 127, a compile-time error occurs. This locks out non-ASCII characters in
251  After \\x, from zero to two hexadecimal digits are read (letters can be in  both byte mode and UTF-8 mode. (When PCRE is compiled in EBCDIC mode, all byte
252  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  values are valid. A lower case letter is converted to upper case, and then the
253  appear between \\x{ and }, but the value of the character code must be less  0xc0 bits are flipped.)
254  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  .P
255  other than hexadecimal digits appear between \\x{ and }, or if there is no  After \ex, from zero to two hexadecimal digits are read (letters can be in
256  terminating }, this form of escape is not recognized. Instead, the initial  upper or lower case). Any number of hexadecimal digits may appear between \ex{
257  \\x will be interpreted as a basic hexadecimal escape, with no following  and }, but the value of the character code must be less than 256 in non-UTF-8
258  digits, giving a byte whose value is zero.  mode, and less than 2**31 in UTF-8 mode. That is, the maximum value in
259    hexadecimal is 7FFFFFFF. Note that this is bigger than the largest Unicode code
260    point, which is 10FFFF.
261    .P
262    If characters other than hexadecimal digits appear between \ex{ and }, or if
263    there is no terminating }, this form of escape is not recognized. Instead, the
264    initial \ex will be interpreted as a basic hexadecimal escape, with no
265    following digits, giving a character whose value is zero.
266    .P
267  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
268  syntaxes for \\x when PCRE is in UTF-8 mode. There is no difference in the  syntaxes for \ex. There is no difference in the way they are handled. For
269  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
270    .P
271  After \\0 up to two further octal digits are read. In both cases, if there  After \e0 up to two further octal digits are read. If there are fewer than two
272  are fewer than two digits, just those that are present are used. Thus the  digits, just those that are present are used. Thus the sequence \e0\ex\e07
273  sequence \\0\\x\\07 specifies two binary zeros followed by a BEL character  specifies two binary zeros followed by a BEL character (code value 7). Make
274  (code value 7). Make sure you supply two digits after the initial zero if the  sure you supply two digits after the initial zero if the pattern character that
275  character that follows is itself an octal digit.  follows is itself an octal digit.
276    .P
277  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.
278  Outside a character class, PCRE reads it and any following digits as a decimal  Outside a character class, PCRE reads it and any following digits as a decimal
279  number. If the number is less than 10, or if there have been at least that many  number. If the number is less than 10, or if there have been at least that many
280  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
281  taken as a \fIback reference\fR. A description of how this works is given  taken as a \fIback reference\fP. A description of how this works is given
282  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
283    .\" </a>
284    later,
285    .\"
286    following the discussion of
287    .\" HTML <a href="#subpattern">
288    .\" </a>
289    parenthesized subpatterns.
290    .\"
291    .P
292  Inside a character class, or if the decimal number is greater than 9 and there  Inside a character class, or if the decimal number is greater than 9 and there
293  have not been that many capturing subpatterns, PCRE re-reads up to three octal  have not been that many capturing subpatterns, PCRE re-reads up to three octal
294  digits following the backslash, and generates a single byte from the least  digits following the backslash, and uses them to generate a data character. Any
295  significant 8 bits of the value. Any subsequent digits stand for themselves.  subsequent digits stand for themselves. In non-UTF-8 mode, the value of a
296  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
297    to \e777 are permitted. For example:
298    \\040   is another way of writing a space  .sp
299    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
300    .\" JOIN
301      \e40    is the same, provided there are fewer than 40
302              previous capturing subpatterns              previous capturing subpatterns
303    \\7     is always a back reference    \e7     is always a back reference
304    \\11    might be a back reference, or another way of  .\" JOIN
305      \e11    might be a back reference, or another way of
306              writing a tab              writing a tab
307    \\011   is always a tab    \e011   is always a tab
308    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
309    \\113   might be a back reference, otherwise the  .\" JOIN
310      \e113   might be a back reference, otherwise the
311              character with octal code 113              character with octal code 113
312    \\377   might be a back reference, otherwise  .\" JOIN
313      \e377   might be a back reference, otherwise
314              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
315    \\81    is either a back reference, or a binary zero  .\" JOIN
316      \e81    is either a back reference, or a binary zero
317              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
318    .sp
319  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
320  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
321    .P
322  All the sequences that define a single byte value or a single UTF-8 character  All the sequences that define a single character value can be used both inside
323  (in UTF-8 mode) can be used both inside and outside character classes. In  and outside character classes. In addition, inside a character class, the
324  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08). The sequences
325  backspace character (hex 08). Outside a character class it has a different  \eB, \eN, \eR, and \eX are not special inside a character class. Like any other
326  meaning (see below).  unrecognized escape sequences, they are treated as the literal characters "B",
327    "N", "R", and "X" by default, but cause an error if the PCRE_EXTRA option is
328  The third use of backslash is for specifying generic character types:  set. Outside a character class, these sequences have different meanings.
329    .
330    \\d     any decimal digit  .
331    \\D     any character that is not a decimal digit  .SS "Absolute and relative back references"
332    \\s     any whitespace character  .rs
333    \\S     any character that is not a whitespace character  .sp
334    \\w     any "word" character  The sequence \eg followed by an unsigned or a negative number, optionally
335    \\W     any "non-word" character  enclosed in braces, is an absolute or relative back reference. A named back
336    reference can be coded as \eg{name}. Back references are discussed
337  Each pair of escape sequences partitions the complete set of characters into  .\" HTML <a href="#backreferences">
338  two disjoint sets. Any given character matches one, and only one, of each pair.  .\" </a>
339    later,
340  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  .\"
341  \\w, and always match \\D, \\S, and \\W.  following the discussion of
342    .\" HTML <a href="#subpattern">
343  For compatibility with Perl, \\s does not match the VT character (code 11).  .\" </a>
344  This makes it different from the the POSIX "space" class. The \\s characters  parenthesized subpatterns.
345  are HT (9), LF (10), FF (12), CR (13), and space (32).  .\"
346    .
347  A "word" character is any letter or digit or the underscore character, that is,  .
348  any character which can be part of a Perl "word". The definition of letters and  .SS "Absolute and relative subroutine calls"
349  digits is controlled by PCRE's character tables, and may vary if locale-  .rs
350  specific matching is taking place (see  .sp
351    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
352    a number enclosed either in angle brackets or single quotes, is an alternative
353    syntax for referencing a subpattern as a "subroutine". Details are discussed
354    .\" HTML <a href="#onigurumasubroutines">
355    .\" </a>
356    later.
357    .\"
358    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
359    synonymous. The former is a back reference; the latter is a
360    .\" HTML <a href="#subpatternsassubroutines">
361    .\" </a>
362    subroutine
363    .\"
364    call.
365    .
366    .
367    .\" HTML <a name="genericchartypes"></a>
368    .SS "Generic character types"
369    .rs
370    .sp
371    Another use of backslash is for specifying generic character types:
372    .sp
373      \ed     any decimal digit
374      \eD     any character that is not a decimal digit
375      \eh     any horizontal whitespace character
376      \eH     any character that is not a horizontal whitespace character
377      \es     any whitespace character
378      \eS     any character that is not a whitespace character
379      \ev     any vertical whitespace character
380      \eV     any character that is not a vertical whitespace character
381      \ew     any "word" character
382      \eW     any "non-word" character
383    .sp
384    There is also the single sequence \eN, which matches a non-newline character.
385    This is the same as
386    .\" HTML <a href="#fullstopdot">
387    .\" </a>
388    the "." metacharacter
389    .\"
390    when PCRE_DOTALL is not set.
391    .P
392    Each pair of lower and upper case escape sequences partitions the complete set
393    of characters into two disjoint sets. Any given character matches one, and only
394    one, of each pair. The sequences can appear both inside and outside character
395    classes. They each match one character of the appropriate type. If the current
396    matching point is at the end of the subject string, all of them fail, because
397    there is no character to match.
398    .P
399    For compatibility with Perl, \es does not match the VT character (code 11).
400    This makes it different from the the POSIX "space" class. The \es characters
401    are HT (9), LF (10), FF (12), CR (13), and space (32). If "use locale;" is
402    included in a Perl script, \es may match the VT character. In PCRE, it never
403    does.
404    .P
405    A "word" character is an underscore or any character that is a letter or digit.
406    By default, the definition of letters and digits is controlled by PCRE's
407    low-valued character tables, and may vary if locale-specific matching is taking
408    place (see
409  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
410  .\" </a>  .\" </a>
411  "Locale support"  "Locale support"
412  .\"  .\"
413  in the  in the
414  .\" HREF  .\" HREF
415  \fBpcreapi\fR  \fBpcreapi\fP
416  .\"  .\"
417  page). For example, in the "fr" (French) locale, some character codes greater  page). For example, in a French locale such as "fr_FR" in Unix-like systems,
418  than 128 are used for accented letters, and these are matched by \\w.  or "french" in Windows, some character codes greater than 128 are used for
419    accented letters, and these are then matched by \ew. The use of locales with
420  These character type sequences can appear both inside and outside character  Unicode is discouraged.
421  classes. They each match one character of the appropriate type. If the current  .P
422  matching point is at the end of the subject string, all of them fail, since  By default, in UTF-8 mode, characters with values greater than 128 never match
423  there is no character to match.  \ed, \es, or \ew, and always match \eD, \eS, and \eW. These sequences retain
424    their original meanings from before UTF-8 support was available, mainly for
425  The fourth use of backslash is for certain simple assertions. An assertion  efficiency reasons. However, if PCRE is compiled with Unicode property support,
426    and the PCRE_UCP option is set, the behaviour is changed so that Unicode
427    properties are used to determine character types, as follows:
428    .sp
429      \ed  any character that \ep{Nd} matches (decimal digit)
430      \es  any character that \ep{Z} matches, plus HT, LF, FF, CR
431      \ew  any character that \ep{L} or \ep{N} matches, plus underscore
432    .sp
433    The upper case escapes match the inverse sets of characters. Note that \ed
434    matches only decimal digits, whereas \ew matches any Unicode digit, as well as
435    any Unicode letter, and underscore. Note also that PCRE_UCP affects \eb, and
436    \eB because they are defined in terms of \ew and \eW. Matching these sequences
437    is noticeably slower when PCRE_UCP is set.
438    .P
439    The sequences \eh, \eH, \ev, and \eV are features that were added to Perl at
440    release 5.10. In contrast to the other sequences, which match only ASCII
441    characters by default, these always match certain high-valued codepoints in
442    UTF-8 mode, whether or not PCRE_UCP is set. The horizontal space characters
443    are:
444    .sp
445      U+0009     Horizontal tab
446      U+0020     Space
447      U+00A0     Non-break space
448      U+1680     Ogham space mark
449      U+180E     Mongolian vowel separator
450      U+2000     En quad
451      U+2001     Em quad
452      U+2002     En space
453      U+2003     Em space
454      U+2004     Three-per-em space
455      U+2005     Four-per-em space
456      U+2006     Six-per-em space
457      U+2007     Figure space
458      U+2008     Punctuation space
459      U+2009     Thin space
460      U+200A     Hair space
461      U+202F     Narrow no-break space
462      U+205F     Medium mathematical space
463      U+3000     Ideographic space
464    .sp
465    The vertical space characters are:
466    .sp
467      U+000A     Linefeed
468      U+000B     Vertical tab
469      U+000C     Formfeed
470      U+000D     Carriage return
471      U+0085     Next line
472      U+2028     Line separator
473      U+2029     Paragraph separator
474    .
475    .
476    .\" HTML <a name="newlineseq"></a>
477    .SS "Newline sequences"
478    .rs
479    .sp
480    Outside a character class, by default, the escape sequence \eR matches any
481    Unicode newline sequence. In non-UTF-8 mode \eR is equivalent to the following:
482    .sp
483      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
484    .sp
485    This is an example of an "atomic group", details of which are given
486    .\" HTML <a href="#atomicgroup">
487    .\" </a>
488    below.
489    .\"
490    This particular group matches either the two-character sequence CR followed by
491    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
492    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
493    line, U+0085). The two-character sequence is treated as a single unit that
494    cannot be split.
495    .P
496    In UTF-8 mode, two additional characters whose codepoints are greater than 255
497    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
498    Unicode character property support is not needed for these characters to be
499    recognized.
500    .P
501    It is possible to restrict \eR to match only CR, LF, or CRLF (instead of the
502    complete set of Unicode line endings) by setting the option PCRE_BSR_ANYCRLF
503    either at compile time or when the pattern is matched. (BSR is an abbrevation
504    for "backslash R".) This can be made the default when PCRE is built; if this is
505    the case, the other behaviour can be requested via the PCRE_BSR_UNICODE option.
506    It is also possible to specify these settings by starting a pattern string with
507    one of the following sequences:
508    .sp
509      (*BSR_ANYCRLF)   CR, LF, or CRLF only
510      (*BSR_UNICODE)   any Unicode newline sequence
511    .sp
512    These override the default and the options given to \fBpcre_compile()\fP or
513    \fBpcre_compile2()\fP, but they can be overridden by options given to
514    \fBpcre_exec()\fP or \fBpcre_dfa_exec()\fP. Note that these special settings,
515    which are not Perl-compatible, are recognized only at the very start of a
516    pattern, and that they must be in upper case. If more than one of them is
517    present, the last one is used. They can be combined with a change of newline
518    convention; for example, a pattern can start with:
519    .sp
520      (*ANY)(*BSR_ANYCRLF)
521    .sp
522    They can also be combined with the (*UTF8) or (*UCP) special sequences. Inside
523    a character class, \eR is treated as an unrecognized escape sequence, and so
524    matches the letter "R" by default, but causes an error if PCRE_EXTRA is set.
525    .
526    .
527    .\" HTML <a name="uniextseq"></a>
528    .SS Unicode character properties
529    .rs
530    .sp
531    When PCRE is built with Unicode character property support, three additional
532    escape sequences that match characters with specific properties are available.
533    When not in UTF-8 mode, these sequences are of course limited to testing
534    characters whose codepoints are less than 256, but they do work in this mode.
535    The extra escape sequences are:
536    .sp
537      \ep{\fIxx\fP}   a character with the \fIxx\fP property
538      \eP{\fIxx\fP}   a character without the \fIxx\fP property
539      \eX       an extended Unicode sequence
540    .sp
541    The property names represented by \fIxx\fP above are limited to the Unicode
542    script names, the general category properties, "Any", which matches any
543    character (including newline), and some special PCRE properties (described
544    in the
545    .\" HTML <a href="#extraprops">
546    .\" </a>
547    next section).
548    .\"
549    Other Perl properties such as "InMusicalSymbols" are not currently supported by
550    PCRE. Note that \eP{Any} does not match any characters, so always causes a
551    match failure.
552    .P
553    Sets of Unicode characters are defined as belonging to certain scripts. A
554    character from one of these sets can be matched using a script name. For
555    example:
556    .sp
557      \ep{Greek}
558      \eP{Han}
559    .sp
560    Those that are not part of an identified script are lumped together as
561    "Common". The current list of scripts is:
562    .P
563    Arabic,
564    Armenian,
565    Avestan,
566    Balinese,
567    Bamum,
568    Bengali,
569    Bopomofo,
570    Braille,
571    Buginese,
572    Buhid,
573    Canadian_Aboriginal,
574    Carian,
575    Cham,
576    Cherokee,
577    Common,
578    Coptic,
579    Cuneiform,
580    Cypriot,
581    Cyrillic,
582    Deseret,
583    Devanagari,
584    Egyptian_Hieroglyphs,
585    Ethiopic,
586    Georgian,
587    Glagolitic,
588    Gothic,
589    Greek,
590    Gujarati,
591    Gurmukhi,
592    Han,
593    Hangul,
594    Hanunoo,
595    Hebrew,
596    Hiragana,
597    Imperial_Aramaic,
598    Inherited,
599    Inscriptional_Pahlavi,
600    Inscriptional_Parthian,
601    Javanese,
602    Kaithi,
603    Kannada,
604    Katakana,
605    Kayah_Li,
606    Kharoshthi,
607    Khmer,
608    Lao,
609    Latin,
610    Lepcha,
611    Limbu,
612    Linear_B,
613    Lisu,
614    Lycian,
615    Lydian,
616    Malayalam,
617    Meetei_Mayek,
618    Mongolian,
619    Myanmar,
620    New_Tai_Lue,
621    Nko,
622    Ogham,
623    Old_Italic,
624    Old_Persian,
625    Old_South_Arabian,
626    Old_Turkic,
627    Ol_Chiki,
628    Oriya,
629    Osmanya,
630    Phags_Pa,
631    Phoenician,
632    Rejang,
633    Runic,
634    Samaritan,
635    Saurashtra,
636    Shavian,
637    Sinhala,
638    Sundanese,
639    Syloti_Nagri,
640    Syriac,
641    Tagalog,
642    Tagbanwa,
643    Tai_Le,
644    Tai_Tham,
645    Tai_Viet,
646    Tamil,
647    Telugu,
648    Thaana,
649    Thai,
650    Tibetan,
651    Tifinagh,
652    Ugaritic,
653    Vai,
654    Yi.
655    .P
656    Each character has exactly one Unicode general category property, specified by
657    a two-letter abbreviation. For compatibility with Perl, negation can be
658    specified by including a circumflex between the opening brace and the property
659    name. For example, \ep{^Lu} is the same as \eP{Lu}.
660    .P
661    If only one letter is specified with \ep or \eP, it includes all the general
662    category properties that start with that letter. In this case, in the absence
663    of negation, the curly brackets in the escape sequence are optional; these two
664    examples have the same effect:
665    .sp
666      \ep{L}
667      \epL
668    .sp
669    The following general category property codes are supported:
670    .sp
671      C     Other
672      Cc    Control
673      Cf    Format
674      Cn    Unassigned
675      Co    Private use
676      Cs    Surrogate
677    .sp
678      L     Letter
679      Ll    Lower case letter
680      Lm    Modifier letter
681      Lo    Other letter
682      Lt    Title case letter
683      Lu    Upper case letter
684    .sp
685      M     Mark
686      Mc    Spacing mark
687      Me    Enclosing mark
688      Mn    Non-spacing mark
689    .sp
690      N     Number
691      Nd    Decimal number
692      Nl    Letter number
693      No    Other number
694    .sp
695      P     Punctuation
696      Pc    Connector punctuation
697      Pd    Dash punctuation
698      Pe    Close punctuation
699      Pf    Final punctuation
700      Pi    Initial punctuation
701      Po    Other punctuation
702      Ps    Open punctuation
703    .sp
704      S     Symbol
705      Sc    Currency symbol
706      Sk    Modifier symbol
707      Sm    Mathematical symbol
708      So    Other symbol
709    .sp
710      Z     Separator
711      Zl    Line separator
712      Zp    Paragraph separator
713      Zs    Space separator
714    .sp
715    The special property L& is also supported: it matches a character that has
716    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
717    a modifier or "other".
718    .P
719    The Cs (Surrogate) property applies only to characters in the range U+D800 to
720    U+DFFF. Such characters are not valid in UTF-8 strings (see RFC 3629) and so
721    cannot be tested by PCRE, unless UTF-8 validity checking has been turned off
722    (see the discussion of PCRE_NO_UTF8_CHECK in the
723    .\" HREF
724    \fBpcreapi\fP
725    .\"
726    page). Perl does not support the Cs property.
727    .P
728    The long synonyms for property names that Perl supports (such as \ep{Letter})
729    are not supported by PCRE, nor is it permitted to prefix any of these
730    properties with "Is".
731    .P
732    No character that is in the Unicode table has the Cn (unassigned) property.
733    Instead, this property is assumed for any code point that is not in the
734    Unicode table.
735    .P
736    Specifying caseless matching does not affect these escape sequences. For
737    example, \ep{Lu} always matches only upper case letters.
738    .P
739    The \eX escape matches any number of Unicode characters that form an extended
740    Unicode sequence. \eX is equivalent to
741    .sp
742      (?>\ePM\epM*)
743    .sp
744    That is, it matches a character without the "mark" property, followed by zero
745    or more characters with the "mark" property, and treats the sequence as an
746    atomic group
747    .\" HTML <a href="#atomicgroup">
748    .\" </a>
749    (see below).
750    .\"
751    Characters with the "mark" property are typically accents that affect the
752    preceding character. None of them have codepoints less than 256, so in
753    non-UTF-8 mode \eX matches any one character.
754    .P
755    Note that recent versions of Perl have changed \eX to match what Unicode calls
756    an "extended grapheme cluster", which has a more complicated definition.
757    .P
758    Matching characters by Unicode property is not fast, because PCRE has to search
759    a structure that contains data for over fifteen thousand characters. That is
760    why the traditional escape sequences such as \ed and \ew do not use Unicode
761    properties in PCRE by default, though you can make them do so by setting the
762    PCRE_UCP option for \fBpcre_compile()\fP or by starting the pattern with
763    (*UCP).
764    .
765    .
766    .\" HTML <a name="extraprops"></a>
767    .SS PCRE's additional properties
768    .rs
769    .sp
770    As well as the standard Unicode properties described in the previous
771    section, PCRE supports four more that make it possible to convert traditional
772    escape sequences such as \ew and \es and POSIX character classes to use Unicode
773    properties. PCRE uses these non-standard, non-Perl properties internally when
774    PCRE_UCP is set. They are:
775    .sp
776      Xan   Any alphanumeric character
777      Xps   Any POSIX space character
778      Xsp   Any Perl space character
779      Xwd   Any Perl "word" character
780    .sp
781    Xan matches characters that have either the L (letter) or the N (number)
782    property. Xps matches the characters tab, linefeed, vertical tab, formfeed, or
783    carriage return, and any other character that has the Z (separator) property.
784    Xsp is the same as Xps, except that vertical tab is excluded. Xwd matches the
785    same characters as Xan, plus underscore.
786    .
787    .
788    .\" HTML <a name="resetmatchstart"></a>
789    .SS "Resetting the match start"
790    .rs
791    .sp
792    The escape sequence \eK causes any previously matched characters not to be
793    included in the final matched sequence. For example, the pattern:
794    .sp
795      foo\eKbar
796    .sp
797    matches "foobar", but reports that it has matched "bar". This feature is
798    similar to a lookbehind assertion
799    .\" HTML <a href="#lookbehind">
800    .\" </a>
801    (described below).
802    .\"
803    However, in this case, the part of the subject before the real match does not
804    have to be of fixed length, as lookbehind assertions do. The use of \eK does
805    not interfere with the setting of
806    .\" HTML <a href="#subpattern">
807    .\" </a>
808    captured substrings.
809    .\"
810    For example, when the pattern
811    .sp
812      (foo)\eKbar
813    .sp
814    matches "foobar", the first substring is still set to "foo".
815    .P
816    Perl documents that the use of \eK within assertions is "not well defined". In
817    PCRE, \eK is acted upon when it occurs inside positive assertions, but is
818    ignored in negative assertions.
819    .
820    .
821    .\" HTML <a name="smallassertions"></a>
822    .SS "Simple assertions"
823    .rs
824    .sp
825    The final use of backslash is for certain simple assertions. An assertion
826  specifies a condition that has to be met at a particular point in a match,  specifies a condition that has to be met at a particular point in a match,
827  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
828  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
829  assertions are  .\" HTML <a href="#bigassertions">
830    .\" </a>
831    \\b     matches at a word boundary  below.
832    \\B     matches when not at a word boundary  .\"
833    \\A     matches at start of subject  The backslashed assertions are:
834    \\Z     matches at end of subject or before newline at end  .sp
835    \\z     matches at end of subject    \eb     matches at a word boundary
836    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
837      \eA     matches at the start of the subject
838  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
839  different meaning, namely the backspace character, inside a character class).            also matches before a newline at the end of the subject
840      \ez     matches only at the end of the subject
841      \eG     matches at the first matching position in the subject
842    .sp
843    Inside a character class, \eb has a different meaning; it matches the backspace
844    character. If any other of these assertions appears in a character class, by
845    default it matches the corresponding literal character (for example, \eB
846    matches the letter B). However, if the PCRE_EXTRA option is set, an "invalid
847    escape sequence" error is generated instead.
848    .P
849  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
850  and the previous character do not both match \\w or \\W (i.e. one matches  and the previous character do not both match \ew or \eW (i.e. one matches
851  \\w and the other matches \\W), 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
852  first or last character matches \\w, respectively.  first or last character matches \ew, respectively. In UTF-8 mode, the meanings
853    of \ew and \eW can be changed by setting the PCRE_UCP option. When this is
854  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  done, it also affects \eb and \eB. Neither PCRE nor Perl has a separate "start
855  dollar (described below) in that they only ever match at the very start and end  of word" or "end of word" metasequence. However, whatever follows \eb normally
856  of the subject string, whatever options are set. Thus, they are independent of  determines which it is. For example, the fragment \eba matches "a" at the start
857  multiline mode.  of a word.
858    .P
859  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
860  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  dollar (described in the next section) in that they only ever match at the very
861  matching is to start at a point other than the beginning of the subject, \\A  start and end of the subject string, whatever options are set. Thus, they are
862  can never match. The difference between \\Z and \\z is that \\Z matches before  independent of multiline mode. These three assertions are not affected by the
863  a newline that is the last character of the string as well as at the end of the  PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
864  string, whereas \\z matches only at the end.  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
865    argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
866  The \\G assertion is true only when the current matching position is at the  at a point other than the beginning of the subject, \eA can never match. The
867  start point of the match, as specified by the \fIstartoffset\fR argument of  difference between \eZ and \ez is that \eZ matches before a newline at the end
868  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  of the string as well as at the very end, whereas \ez matches only at the end.
869  non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  .P
870    The \eG assertion is true only when the current matching position is at the
871    start point of the match, as specified by the \fIstartoffset\fP argument of
872    \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
873    non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
874  arguments, you can mimic Perl's /g option, and it is in this kind of  arguments, you can mimic Perl's /g option, and it is in this kind of
875  implementation where \\G can be useful.  implementation where \eG can be useful.
876    .P
877  Note, however, that PCRE's interpretation of \\G, as the start of the current  Note, however, that PCRE's interpretation of \eG, as the start of the current
878  match, is subtly different from Perl's, which defines it as the end of the  match, is subtly different from Perl's, which defines it as the end of the
879  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
880  string was empty. Because PCRE does just one match at a time, it cannot  string was empty. Because PCRE does just one match at a time, it cannot
881  reproduce this behaviour.  reproduce this behaviour.
882    .P
883  If all the alternatives of a pattern begin with \\G, the expression is anchored  If all the alternatives of a pattern begin with \eG, the expression is anchored
884  to the starting match position, and the "anchored" flag is set in the compiled  to the starting match position, and the "anchored" flag is set in the compiled
885  regular expression.  regular expression.
886    .
887  .SH CIRCUMFLEX AND DOLLAR  .
888    .SH "CIRCUMFLEX AND DOLLAR"
889  .rs  .rs
890  .sp  .sp
891  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
892  character is an assertion which is true only if the current matching point is  character is an assertion that is true only if the current matching point is
893  at the start of the subject string. If the \fIstartoffset\fR argument of  at the start of the subject string. If the \fIstartoffset\fP argument of
894  \fBpcre_exec()\fR 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
895  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
896  meaning (see below).  meaning
897    .\" HTML <a href="#characterclass">
898    .\" </a>
899    (see below).
900    .\"
901    .P
902  Circumflex need not be the first character of the pattern if a number of  Circumflex need not be the first character of the pattern if a number of
903  alternatives are involved, but it should be the first thing in each alternative  alternatives are involved, but it should be the first thing in each alternative
904  in which it appears if the pattern is ever to match that branch. If all  in which it appears if the pattern is ever to match that branch. If all
# Line 290  possible alternatives start with a circu Line 906  possible alternatives start with a circu
906  constrained to match only at the start of the subject, it is said to be an  constrained to match only at the start of the subject, it is said to be an
907  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
908  to be anchored.)  to be anchored.)
909    .P
910  A dollar character is an assertion which is true only if the current matching  A dollar character is an assertion that is true only if the current matching
911  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
912  character that is the last character in the string (by default). Dollar need  at the end of the string (by default). Dollar need not be the last character of
913  not be the last character of the pattern if a number of alternatives are  the pattern if a number of alternatives are involved, but it should be the last
914  involved, but it should be the last item in any branch in which it appears.  item in any branch in which it appears. Dollar has no special meaning in a
915  Dollar has no special meaning in a character class.  character class.
916    .P
917  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
918  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
919  does not affect the \\Z assertion.  does not affect the \eZ assertion.
920    .P
921  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
922  PCRE_MULTILINE option is set. When this is the case, they match immediately  PCRE_MULTILINE option is set. When this is the case, a circumflex matches
923  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
924  addition to matching at the start and end of the subject string. For example,  string. It does not match after a newline that ends the string. A dollar
925  the pattern /^abc$/ matches the subject string "def\\nabc" in multiline mode,  matches before any newlines in the string, as well as at the very end, when
926  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
927  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
928  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
929  \fBpcre_exec()\fR is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if  For example, the pattern /^abc$/ matches the subject string "def\enabc" (where
930  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
931    patterns that are anchored in single line mode because all branches start with
932  Note that the sequences \\A, \\Z, and \\z can be used to match the start and  ^ are not anchored in multiline mode, and a match for circumflex is possible
933    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
934    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
935    .P
936    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
937  end of the subject in both modes, and if all branches of a pattern start with  end of the subject in both modes, and if all branches of a pattern start with
938  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
939    .
940  .SH FULL STOP (PERIOD, DOT)  .
941    .\" HTML <a name="fullstopdot"></a>
942    .SH "FULL STOP (PERIOD, DOT) AND \eN"
943  .rs  .rs
944  .sp  .sp
945  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
946  the subject, including a non-printing character, but not (by default) newline.  the subject string except (by default) a character that signifies the end of a
947  In UTF-8 mode, a dot matches any UTF-8 character, which might be more than one  line. In UTF-8 mode, the matched character may be more than one byte long.
948  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
949  dots match newlines as well. The handling of dot is entirely independent of the  When a line ending is defined as a single character, dot never matches that
950  handling of circumflex and dollar, the only relationship being that they both  character; when the two-character sequence CRLF is used, dot does not match CR
951  involve newline characters. Dot has no special meaning in a character class.  if it is immediately followed by LF, but otherwise it matches all characters
952    (including isolated CRs and LFs). When any Unicode line endings are being
953  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
954  .rs  characters.
955  .sp  .P
956  Outside a character class, the escape sequence \\C matches any one byte, both  The behaviour of dot with regard to newlines can be changed. If the PCRE_DOTALL
957  in and out of UTF-8 mode. Unlike a dot, it always matches a newline. The  option is set, a dot matches any one character, without exception. If the
958  feature is provided in Perl in order to match individual bytes in UTF-8 mode.  two-character sequence CRLF is present in the subject string, it takes two dots
959  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
960  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
961    The handling of dot is entirely independent of the handling of circumflex and
962  PCRE does not allow \\C to appear in lookbehind assertions (see below), because  dollar, the only relationship being that they both involve newlines. Dot has no
963  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
964    .P
965  .SH SQUARE BRACKETS  The escape sequence \eN behaves like a dot, except that it is not affected by
966    the PCRE_DOTALL option. In other words, it matches any character except one
967    that signifies the end of a line.
968    .
969    .
970    .SH "MATCHING A SINGLE BYTE"
971    .rs
972    .sp
973    Outside a character class, the escape sequence \eC matches any one byte, both
974    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
975    characters. The feature is provided in Perl in order to match individual bytes
976    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes, the
977    rest of the string may start with a malformed UTF-8 character. For this reason,
978    the \eC escape sequence is best avoided.
979    .P
980    PCRE does not allow \eC to appear in lookbehind assertions
981    .\" HTML <a href="#lookbehind">
982    .\" </a>
983    (described below),
984    .\"
985    because in UTF-8 mode this would make it impossible to calculate the length of
986    the lookbehind.
987    .
988    .
989    .\" HTML <a name="characterclass"></a>
990    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
991  .rs  .rs
992  .sp  .sp
993  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
994  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.
995  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
996  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
997  escaped with a backslash.  a member of the class, it should be the first data character in the class
998    (after an initial circumflex, if present) or escaped with a backslash.
999    .P
1000  A character class matches a single character in the subject. In UTF-8 mode, the  A character class matches a single character in the subject. In UTF-8 mode, the
1001  character may occupy more than one byte. A matched character must be in the set  character may be more than one byte long. A matched character must be in the
1002  of characters defined by the class, unless the first character in the class  set of characters defined by the class, unless the first character in the class
1003  definition is a circumflex, in which case the subject character must not be in  definition is a circumflex, in which case the subject character must not be in
1004  the set defined by the class. If a circumflex is actually required as a member  the set defined by the class. If a circumflex is actually required as a member
1005  of the class, ensure it is not the first character, or escape it with a  of the class, ensure it is not the first character, or escape it with a
1006  backslash.  backslash.
1007    .P
1008  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
1009  [^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
1010  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
1011  are in the class by enumerating those that are not. It is not an assertion: it  are in the class by enumerating those that are not. A class that starts with a
1012  still consumes a character from the subject string, and fails if the current  circumflex is not an assertion; it still consumes a character from the subject
1013  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
1014    string.
1015    .P
1016  In UTF-8 mode, characters with values greater than 255 can be included in a  In UTF-8 mode, characters with values greater than 255 can be included in a
1017  class as a literal string of bytes, or by using the \\x{ escaping mechanism.  class as a literal string of bytes, or by using the \ex{ escaping mechanism.
1018    .P
1019  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
1020  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
1021  "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
1022  caseful version would. PCRE does not support the concept of case for characters  caseful version would. In UTF-8 mode, PCRE always understands the concept of
1023  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
1024    always possible. For characters with higher values, the concept of case is
1025  The newline character is never treated in any special way in character classes,  supported if PCRE is compiled with Unicode property support, but not otherwise.
1026  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  If you want to use caseless matching in UTF8-mode for characters 128 and above,
1027  such as [^a] will always match a newline.  you must ensure that PCRE is compiled with Unicode property support as well as
1028    with UTF-8 support.
1029    .P
1030    Characters that might indicate line breaks are never treated in any special way
1031    when matching character classes, whatever line-ending sequence is in use, and
1032    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
1033    such as [^a] always matches one of these characters.
1034    .P
1035  The minus (hyphen) character can be used to specify a range of characters in a  The minus (hyphen) character can be used to specify a range of characters in a
1036  character class. For example, [d-m] matches any letter between d and m,  character class. For example, [d-m] matches any letter between d and m,
1037  inclusive. If a minus character is required in a class, it must be escaped with  inclusive. If a minus character is required in a class, it must be escaped with
1038  a backslash or appear in a position where it cannot be interpreted as  a backslash or appear in a position where it cannot be interpreted as
1039  indicating a range, typically as the first or last character in the class.  indicating a range, typically as the first or last character in the class.
1040    .P
1041  It is not possible to have the literal character "]" as the end character of a  It is not possible to have the literal character "]" as the end character of a
1042  range. A pattern such as [W-]46] is interpreted as a class of two characters  range. A pattern such as [W-]46] is interpreted as a class of two characters
1043  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or  ("W" and "-") followed by a literal string "46]", so it would match "W46]" or
1044  "-46]". However, if the "]" is escaped with a backslash it is interpreted as  "-46]". However, if the "]" is escaped with a backslash it is interpreted as
1045  the end of range, so [W-\\]46] is interpreted as a single class containing a  the end of range, so [W-\e]46] is interpreted as a class containing a range
1046  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
1047  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
1048    .P
1049  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
1050  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
1051  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
1052  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
1053    .P
1054  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
1055  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
1056  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
1057  locale are in use, [\\xc8-\\xcb] matches accented E characters in both cases.  tables for a French locale are in use, [\exc8-\excb] matches accented E
1058    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
1059  The character types \\d, \\D, \\s, \\S, \\w, and \\W may also appear in a  characters with values greater than 128 only when it is compiled with Unicode
1060  character class, and add the characters that they match to the class. For  property support.
1061  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
1062  conveniently be used with the upper case character types to specify a more  The character escape sequences \ed, \eD, \eh, \eH, \ep, \eP, \es, \eS, \ev,
1063  restricted set of characters than the matching lower case type. For example,  \eV, \ew, and \eW may appear in a character class, and add the characters that
1064  the class [^\\W_] matches any letter or digit, but not underscore.  they match to the class. For example, [\edABCDEF] matches any hexadecimal
1065    digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of \ed, \es, \ew
1066  All non-alphameric characters other than \\, -, ^ (at the start) and the  and their upper case partners, just as it does when they appear outside a
1067  terminating ] are non-special in character classes, but it does no harm if they  character class, as described in the section entitled
1068  are escaped.  .\" HTML <a href="#genericchartypes">
1069    .\" </a>
1070  .SH POSIX CHARACTER CLASSES  "Generic character types"
1071    .\"
1072    above. The escape sequence \eb has a different meaning inside a character
1073    class; it matches the backspace character. The sequences \eB, \eN, \eR, and \eX
1074    are not special inside a character class. Like any other unrecognized escape
1075    sequences, they are treated as the literal characters "B", "N", "R", and "X" by
1076    default, but cause an error if the PCRE_EXTRA option is set.
1077    .P
1078    A circumflex can conveniently be used with the upper case character types to
1079    specify a more restricted set of characters than the matching lower case type.
1080    For example, the class [^\eW_] matches any letter or digit, but not underscore,
1081    whereas [\ew] includes underscore. A positive character class should be read as
1082    "something OR something OR ..." and a negative class as "NOT something AND NOT
1083    something AND NOT ...".
1084    .P
1085    The only metacharacters that are recognized in character classes are backslash,
1086    hyphen (only where it can be interpreted as specifying a range), circumflex
1087    (only at the start), opening square bracket (only when it can be interpreted as
1088    introducing a POSIX class name - see the next section), and the terminating
1089    closing square bracket. However, escaping other non-alphanumeric characters
1090    does no harm.
1091    .
1092    .
1093    .SH "POSIX CHARACTER CLASSES"
1094  .rs  .rs
1095  .sp  .sp
1096  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
1097  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
1098  this notation. For example,  this notation. For example,
1099    .sp
1100    [01[:alpha:]%]    [01[:alpha:]%]
1101    .sp
1102  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
1103  are  are:
1104    .sp
1105    alnum    letters and digits    alnum    letters and digits
1106    alpha    letters    alpha    letters
1107    ascii    character codes 0 - 127    ascii    character codes 0 - 127
1108    blank    space or tab only    blank    space or tab only
1109    cntrl    control characters    cntrl    control characters
1110    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
1111    graph    printing characters, excluding space    graph    printing characters, excluding space
1112    lower    lower case letters    lower    lower case letters
1113    print    printing characters, including space    print    printing characters, including space
1114    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits and space
1115    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
1116    upper    upper case letters    upper    upper case letters
1117    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
1118    xdigit   hexadecimal digits    xdigit   hexadecimal digits
1119    .sp
1120  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
1121  space (32). Notice that this list includes the VT character (code 11). This  space (32). Notice that this list includes the VT character (code 11). This
1122  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
1123  compatibility).  compatibility).
1124    .P
1125  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
1126  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
1127  after the colon. For example,  after the colon. For example,
1128    .sp
1129    [12[:^digit:]]    [12[:^digit:]]
1130    .sp
1131  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX  matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the POSIX
1132  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
1133  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
1134    .P
1135  In UTF-8 mode, characters with values greater than 255 do not match any of  By default, in UTF-8 mode, characters with values greater than 128 do not match
1136  the POSIX character classes.  any of the POSIX character classes. However, if the PCRE_UCP option is passed
1137    to \fBpcre_compile()\fP, some of the classes are changed so that Unicode
1138  .SH VERTICAL BAR  character properties are used. This is achieved by replacing the POSIX classes
1139    by other sequences, as follows:
1140    .sp
1141      [:alnum:]  becomes  \ep{Xan}
1142      [:alpha:]  becomes  \ep{L}
1143      [:blank:]  becomes  \eh
1144      [:digit:]  becomes  \ep{Nd}
1145      [:lower:]  becomes  \ep{Ll}
1146      [:space:]  becomes  \ep{Xps}
1147      [:upper:]  becomes  \ep{Lu}
1148      [:word:]   becomes  \ep{Xwd}
1149    .sp
1150    Negated versions, such as [:^alpha:] use \eP instead of \ep. The other POSIX
1151    classes are unchanged, and match only characters with code points less than
1152    128.
1153    .
1154    .
1155    .SH "VERTICAL BAR"
1156  .rs  .rs
1157  .sp  .sp
1158  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
1159  the pattern  the pattern
1160    .sp
1161    gilbert|sullivan    gilbert|sullivan
1162    .sp
1163  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
1164  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
1165  The matching process tries each alternative in turn, from left to right,  process tries each alternative in turn, from left to right, and the first one
1166  and the first one that succeeds is used. If the alternatives are within a  that succeeds is used. If the alternatives are within a subpattern
1167  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
1168  pattern as well as the alternative in the subpattern.  .\" </a>
1169    (defined below),
1170  .SH INTERNAL OPTION SETTING  .\"
1171    "succeeds" means matching the rest of the main pattern as well as the
1172    alternative in the subpattern.
1173    .
1174    .
1175    .SH "INTERNAL OPTION SETTING"
1176  .rs  .rs
1177  .sp  .sp
1178  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
1179  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
1180  Perl option letters enclosed between "(?" and ")". The option letters are  the pattern by a sequence of Perl option letters enclosed between "(?" and ")".
1181    The option letters are
1182    .sp
1183    i  for PCRE_CASELESS    i  for PCRE_CASELESS
1184    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
1185    s  for PCRE_DOTALL    s  for PCRE_DOTALL
1186    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
1187    .sp
1188  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
1189  unset these options by preceding the letter with a hyphen, and a combined  unset these options by preceding the letter with a hyphen, and a combined
1190  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
1191  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
1192  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
1193  unset.  unset.
1194    .P
1195  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
1196  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
1197  If the change is placed right at the start of a pattern, PCRE extracts it into  J, U and X respectively.
1198  the global options (and it will therefore show up in data extracted by the  .P
1199  \fBpcre_fullinfo()\fR function).  When one of these option changes occurs at top level (that is, not inside
1200    subpattern parentheses), the change applies to the remainder of the pattern
1201  An option change within a subpattern affects only that part of the current  that follows. If the change is placed right at the start of a pattern, PCRE
1202  pattern that follows it, so  extracts it into the global options (and it will therefore show up in data
1203    extracted by the \fBpcre_fullinfo()\fP function).
1204    .P
1205    An option change within a subpattern (see below for a description of
1206    subpatterns) affects only that part of the subpattern that follows it, so
1207    .sp
1208    (a(?i)b)c    (a(?i)b)c
1209    .sp
1210  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).  matches abc and aBc and no other strings (assuming PCRE_CASELESS is not used).
1211  By this means, options can be made to have different settings in different  By this means, options can be made to have different settings in different
1212  parts of the pattern. Any changes made in one alternative do carry on  parts of the pattern. Any changes made in one alternative do carry on
1213  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
1214    .sp
1215    (a(?i)b|c)    (a(?i)b|c)
1216    .sp
1217  matches "ab", "aB", "c", and "C", even though when matching "C" the first  matches "ab", "aB", "c", and "C", even though when matching "C" the first
1218  branch is abandoned before the option setting. This is because the effects of  branch is abandoned before the option setting. This is because the effects of
1219  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
1220  behaviour otherwise.  behaviour otherwise.
1221    .P
1222  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  \fBNote:\fP There are other PCRE-specific options that can be set by the
1223  same way as the Perl-compatible options by using the characters U and X  application when the compile or match functions are called. In some cases the
1224  respectively. The (?X) flag setting is special in that it must always occur  pattern can contain special leading sequences such as (*CRLF) to override what
1225  earlier in the pattern than any of the additional features it turns on, even  the application has set or what has been defaulted. Details are given in the
1226  when it is at top level. It is best put at the start.  section entitled
1227    .\" HTML <a href="#newlineseq">
1228    .\" </a>
1229    "Newline sequences"
1230    .\"
1231    above. There are also the (*UTF8) and (*UCP) leading sequences that can be used
1232    to set UTF-8 and Unicode property modes; they are equivalent to setting the
1233    PCRE_UTF8 and the PCRE_UCP options, respectively.
1234    .
1235    .
1236    .\" HTML <a name="subpattern"></a>
1237  .SH SUBPATTERNS  .SH SUBPATTERNS
1238  .rs  .rs
1239  .sp  .sp
1240  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
1241  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
1242    .sp
1243  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
1244    .sp
1245    cat(aract|erpillar|)    cat(aract|erpillar|)
1246    .sp
1247  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches "cataract", "caterpillar", or "cat". Without the parentheses, it would
1248  parentheses, it would match "cataract", "erpillar" or the empty string.  match "cataract", "erpillar" or an empty string.
1249    .sp
1250  2. It sets up the subpattern as a capturing subpattern (as defined above).  2. It sets up the subpattern as a capturing subpattern. This means that, when
1251  When the whole pattern matches, that portion of the subject string that matched  the whole pattern matches, that portion of the subject string that matched the
1252  the subpattern is passed back to the caller via the \fIovector\fR argument of  subpattern is passed back to the caller via the \fIovector\fP argument of
1253  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
1254  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns. For example, if the
1255    string "the red king" is matched against the pattern
1256  For example, if the string "the red king" is matched against the pattern  .sp
   
1257    the ((red|white) (king|queen))    the ((red|white) (king|queen))
1258    .sp
1259  the captured substrings are "red king", "red", and "king", and are numbered 1,  the captured substrings are "red king", "red", and "king", and are numbered 1,
1260  2, and 3, respectively.  2, and 3, respectively.
1261    .P
1262  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
1263  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
1264  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
1265  and a colon, the subpattern does not do any capturing, and is not counted when  and a colon, the subpattern does not do any capturing, and is not counted when
1266  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
1267  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
1268    .sp
1269    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
1270    .sp
1271  the captured substrings are "white queen" and "queen", and are numbered 1 and  the captured substrings are "white queen" and "queen", and are numbered 1 and
1272  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
1273  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
1274  As a convenient shorthand, if any option settings are required at the start of  As a convenient shorthand, if any option settings are required at the start of
1275  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
1276  the ":". Thus the two patterns  the ":". Thus the two patterns
1277    .sp
1278    (?i:saturday|sunday)    (?i:saturday|sunday)
1279    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
1280    .sp
1281  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
1282  from left to right, and options are not reset until the end of the subpattern  from left to right, and options are not reset until the end of the subpattern
1283  is reached, an option setting in one branch does affect subsequent branches, so  is reached, an option setting in one branch does affect subsequent branches, so
1284  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
1285    .
1286  .SH NAMED SUBPATTERNS  .
1287    .\" HTML <a name="dupsubpatternnumber"></a>
1288    .SH "DUPLICATE SUBPATTERN NUMBERS"
1289    .rs
1290    .sp
1291    Perl 5.10 introduced a feature whereby each alternative in a subpattern uses
1292    the same numbers for its capturing parentheses. Such a subpattern starts with
1293    (?| and is itself a non-capturing subpattern. For example, consider this
1294    pattern:
1295    .sp
1296      (?|(Sat)ur|(Sun))day
1297    .sp
1298    Because the two alternatives are inside a (?| group, both sets of capturing
1299    parentheses are numbered one. Thus, when the pattern matches, you can look
1300    at captured substring number one, whichever alternative matched. This construct
1301    is useful when you want to capture part, but not all, of one of a number of
1302    alternatives. Inside a (?| group, parentheses are numbered as usual, but the
1303    number is reset at the start of each branch. The numbers of any capturing
1304    parentheses that follow the subpattern start after the highest number used in
1305    any branch. The following example is taken from the Perl documentation. The
1306    numbers underneath show in which buffer the captured content will be stored.
1307    .sp
1308      # before  ---------------branch-reset----------- after
1309      / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
1310      # 1            2         2  3        2     3     4
1311    .sp
1312    A back reference to a numbered subpattern uses the most recent value that is
1313    set for that number by any subpattern. The following pattern matches "abcabc"
1314    or "defdef":
1315    .sp
1316      /(?|(abc)|(def))\e1/
1317    .sp
1318    In contrast, a recursive or "subroutine" call to a numbered subpattern always
1319    refers to the first one in the pattern with the given number. The following
1320    pattern matches "abcabc" or "defabc":
1321    .sp
1322      /(?|(abc)|(def))(?1)/
1323    .sp
1324    If a
1325    .\" HTML <a href="#conditions">
1326    .\" </a>
1327    condition test
1328    .\"
1329    for a subpattern's having matched refers to a non-unique number, the test is
1330    true if any of the subpatterns of that number have matched.
1331    .P
1332    An alternative approach to using this "branch reset" feature is to use
1333    duplicate named subpatterns, as described in the next section.
1334    .
1335    .
1336    .SH "NAMED SUBPATTERNS"
1337  .rs  .rs
1338  .sp  .sp
1339  Identifying capturing parentheses by number is simple, but it can be very hard  Identifying capturing parentheses by number is simple, but it can be very hard
1340  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
1341  if an expression is modified, the numbers may change. To help with the  if an expression is modified, the numbers may change. To help with this
1342  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
1343  not provide. The Python syntax (?P<name>...) is used. Names consist of  added to Perl until release 5.10. Python had the feature earlier, and PCRE
1344  alphanumeric characters and underscores, and must be unique within a pattern.  introduced it at release 4.0, using the Python syntax. PCRE now supports both
1345    the Perl and the Python syntax. Perl allows identically numbered subpatterns to
1346  Named capturing parentheses are still allocated numbers as well as names. The  have different names, but PCRE does not.
1347  PCRE API provides function calls for extracting the name-to-number translation  .P
1348  table from a compiled pattern. For further details see the  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
1349    (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
1350    parentheses from other parts of the pattern, such as
1351    .\" HTML <a href="#backreferences">
1352    .\" </a>
1353    back references,
1354    .\"
1355    .\" HTML <a href="#recursion">
1356    .\" </a>
1357    recursion,
1358    .\"
1359    and
1360    .\" HTML <a href="#conditions">
1361    .\" </a>
1362    conditions,
1363    .\"
1364    can be made by name as well as by number.
1365    .P
1366    Names consist of up to 32 alphanumeric characters and underscores. Named
1367    capturing parentheses are still allocated numbers as well as names, exactly as
1368    if the names were not present. The PCRE API provides function calls for
1369    extracting the name-to-number translation table from a compiled pattern. There
1370    is also a convenience function for extracting a captured substring by name.
1371    .P
1372    By default, a name must be unique within a pattern, but it is possible to relax
1373    this constraint by setting the PCRE_DUPNAMES option at compile time. (Duplicate
1374    names are also always permitted for subpatterns with the same number, set up as
1375    described in the previous section.) Duplicate names can be useful for patterns
1376    where only one instance of the named parentheses can match. Suppose you want to
1377    match the name of a weekday, either as a 3-letter abbreviation or as the full
1378    name, and in both cases you want to extract the abbreviation. This pattern
1379    (ignoring the line breaks) does the job:
1380    .sp
1381      (?<DN>Mon|Fri|Sun)(?:day)?|
1382      (?<DN>Tue)(?:sday)?|
1383      (?<DN>Wed)(?:nesday)?|
1384      (?<DN>Thu)(?:rsday)?|
1385      (?<DN>Sat)(?:urday)?
1386    .sp
1387    There are five capturing substrings, but only one is ever set after a match.
1388    (An alternative way of solving this problem is to use a "branch reset"
1389    subpattern, as described in the previous section.)
1390    .P
1391    The convenience function for extracting the data by name returns the substring
1392    for the first (and in this example, the only) subpattern of that name that
1393    matched. This saves searching to find which numbered subpattern it was.
1394    .P
1395    If you make a back reference to a non-unique named subpattern from elsewhere in
1396    the pattern, the one that corresponds to the first occurrence of the name is
1397    used. In the absence of duplicate numbers (see the previous section) this is
1398    the one with the lowest number. If you use a named reference in a condition
1399    test (see the
1400    .\"
1401    .\" HTML <a href="#conditions">
1402    .\" </a>
1403    section about conditions
1404    .\"
1405    below), either to check whether a subpattern has matched, or to check for
1406    recursion, all subpatterns with the same name are tested. If the condition is
1407    true for any one of them, the overall condition is true. This is the same
1408    behaviour as testing by number. For further details of the interfaces for
1409    handling named subpatterns, see the
1410  .\" HREF  .\" HREF
1411  \fBpcreapi\fR  \fBpcreapi\fP
1412  .\"  .\"
1413  documentation.  documentation.
1414    .P
1415    \fBWarning:\fP You cannot use different names to distinguish between two
1416    subpatterns with the same number because PCRE uses only the numbers when
1417    matching. For this reason, an error is given at compile time if different names
1418    are given to subpatterns with the same number. However, you can give the same
1419    name to subpatterns with the same number, even when PCRE_DUPNAMES is not set.
1420    .
1421    .
1422  .SH REPETITION  .SH REPETITION
1423  .rs  .rs
1424  .sp  .sp
1425  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
1426  items:  items:
1427    .sp
1428    a literal data character    a literal data character
1429    the . metacharacter    the dot metacharacter
1430    the \\C escape sequence    the \eC escape sequence
1431    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1432      the \eR escape sequence
1433      an escape such as \ed or \epL that matches a single character
1434    a character class    a character class
1435    a back reference (see next section)    a back reference (see next section)
1436    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (including assertions)
1437      a recursive or "subroutine" call to a subpattern
1438    .sp
1439  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1440  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1441  separated by a comma. The numbers must be less than 65536, and the first must  separated by a comma. The numbers must be less than 65536, and the first must
1442  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1443    .sp
1444    z{2,4}    z{2,4}
1445    .sp
1446  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special  matches "zz", "zzz", or "zzzz". A closing brace on its own is not a special
1447  character. If the second number is omitted, but the comma is present, there is  character. If the second number is omitted, but the comma is present, there is
1448  no upper limit; if the second number and the comma are both omitted, the  no upper limit; if the second number and the comma are both omitted, the
1449  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1450    .sp
1451    [aeiou]{3,}    [aeiou]{3,}
1452    .sp
1453  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1454    .sp
1455    \\d{8}    \ed{8}
1456    .sp
1457  matches exactly 8 digits. An opening curly bracket that appears in a position  matches exactly 8 digits. An opening curly bracket that appears in a position
1458  where a quantifier is not allowed, or one that does not match the syntax of a  where a quantifier is not allowed, or one that does not match the syntax of a
1459  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
1460  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1461    .P
1462  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual  In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to individual
1463  bytes. Thus, for example, \\x{100}{2} matches two UTF-8 characters, each of  bytes. Thus, for example, \ex{100}{2} matches two UTF-8 characters, each of
1464  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1465    support is available, \eX{3} matches three Unicode extended sequences, each of
1466    which may be several bytes long (and they may be of different lengths).
1467    .P
1468  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
1469  previous item and the quantifier were not present.  previous item and the quantifier were not present. This may be useful for
1470    subpatterns that are referenced as
1471  For convenience (and historical compatibility) the three most common  .\" HTML <a href="#subpatternsassubroutines">
1472  quantifiers have single-character abbreviations:  .\" </a>
1473    subroutines
1474    .\"
1475    from elsewhere in the pattern (but see also the section entitled
1476    .\" HTML <a href="#subdefine">
1477    .\" </a>
1478    "Defining subpatterns for use by reference only"
1479    .\"
1480    below). Items other than subpatterns that have a {0} quantifier are omitted
1481    from the compiled pattern.
1482    .P
1483    For convenience, the three most common quantifiers have single-character
1484    abbreviations:
1485    .sp
1486    *    is equivalent to {0,}    *    is equivalent to {0,}
1487    +    is equivalent to {1,}    +    is equivalent to {1,}
1488    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1489    .sp
1490  It is possible to construct infinite loops by following a subpattern that can  It is possible to construct infinite loops by following a subpattern that can
1491  match no characters with a quantifier that has no upper limit, for example:  match no characters with a quantifier that has no upper limit, for example:
1492    .sp
1493    (a?)*    (a?)*
1494    .sp
1495  Earlier versions of Perl and PCRE used to give an error at compile time for  Earlier versions of Perl and PCRE used to give an error at compile time for
1496  such patterns. However, because there are cases where this can be useful, such  such patterns. However, because there are cases where this can be useful, such
1497  patterns are now accepted, but if any repetition of the subpattern does in fact  patterns are now accepted, but if any repetition of the subpattern does in fact
1498  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1499    .P
1500  By default, the quantifiers are "greedy", that is, they match as much as  By default, the quantifiers are "greedy", that is, they match as much as
1501  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1502  rest of the pattern to fail. The classic example of where this gives problems  rest of the pattern to fail. The classic example of where this gives problems
1503  is in trying to match comments in C programs. These appear between the  is in trying to match comments in C programs. These appear between /* and */
1504  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1505  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1506    .sp
1507    /\\*.*\\*/    /\e*.*\e*/
1508    .sp
1509  to the string  to the string
1510    .sp
1511    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1512    .sp
1513  fails, because it matches the entire string owing to the greediness of the .*  fails, because it matches the entire string owing to the greediness of the .*
1514  item.  item.
1515    .P
1516  However, if a quantifier is followed by a question mark, it ceases to be  However, if a quantifier is followed by a question mark, it ceases to be
1517  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1518  pattern  pattern
1519    .sp
1520    /\\*.*?\\*/    /\e*.*?\e*/
1521    .sp
1522  does the right thing with the C comments. The meaning of the various  does the right thing with the C comments. The meaning of the various
1523  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1524  Do not confuse this use of question mark with its use as a quantifier in its  Do not confuse this use of question mark with its use as a quantifier in its
1525  own right. Because it has two uses, it can sometimes appear doubled, as in  own right. Because it has two uses, it can sometimes appear doubled, as in
1526    .sp
1527    \\d??\\d    \ed??\ed
1528    .sp
1529  which matches one digit by preference, but can match two if that is the only  which matches one digit by preference, but can match two if that is the only
1530  way the rest of the pattern matches.  way the rest of the pattern matches.
1531    .P
1532  If the PCRE_UNGREEDY option is set (an option which is not available in Perl),  If the PCRE_UNGREEDY option is set (an option that is not available in Perl),
1533  the quantifiers are not greedy by default, but individual ones can be made  the quantifiers are not greedy by default, but individual ones can be made
1534  greedy by following them with a question mark. In other words, it inverts the  greedy by following them with a question mark. In other words, it inverts the
1535  default behaviour.  default behaviour.
1536    .P
1537  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1538  is greater than 1 or with a limited maximum, more store is required for the  is greater than 1 or with a limited maximum, more memory is required for the
1539  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1540    .P
1541  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent  If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equivalent
1542  to Perl's /s) is set, thus allowing the . to match newlines, the pattern is  to Perl's /s) is set, thus allowing the dot to match newlines, the pattern is
1543  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1544  character position in the subject string, so there is no point in retrying the  character position in the subject string, so there is no point in retrying the
1545  overall match at any position after the first. PCRE normally treats such a  overall match at any position after the first. PCRE normally treats such a
1546  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1547    .P
1548  In cases where it is known that the subject string contains no newlines, it is  In cases where it is known that the subject string contains no newlines, it is
1549  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1550  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1551    .P
1552  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1553  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a back reference
1554  elsewhere in the pattern, a match at the start may fail, and a later one  elsewhere in the pattern, a match at the start may fail where a later one
1555  succeed. Consider, for example:  succeeds. Consider, for example:
1556    .sp
1557    (.*)abc\\1    (.*)abc\e1
1558    .sp
1559  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
1560  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1561    .P
1562  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1563  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1564    .sp
1565    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1566    .sp
1567  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1568  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1569  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1570  example, after  example, after
1571    .sp
1572    /(a|(b))+/    /(a|(b))+/
1573    .sp
1574  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1575    .
1576  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1577    .\" HTML <a name="atomicgroup"></a>
1578    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1579  .rs  .rs
1580  .sp  .sp
1581  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1582  normally causes the repeated item to be re-evaluated to see if a different  repetition, failure of what follows normally causes the repeated item to be
1583  number of repeats allows the rest of the pattern to match. Sometimes it is  re-evaluated to see if a different number of repeats allows the rest of the
1584  useful to prevent this, either to change the nature of the match, or to cause  pattern to match. Sometimes it is useful to prevent this, either to change the
1585  it fail earlier than it otherwise might, when the author of the pattern knows  nature of the match, or to cause it fail earlier than it otherwise might, when
1586  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1587    .P
1588  Consider, for example, the pattern \\d+foo when applied to the subject line  Consider, for example, the pattern \ed+foo when applied to the subject line
1589    .sp
1590    123456bar    123456bar
1591    .sp
1592  After matching all 6 digits and then failing to match "foo", the normal  After matching all 6 digits and then failing to match "foo", the normal
1593  action of the matcher is to try again with only 5 digits matching the \\d+  action of the matcher is to try again with only 5 digits matching the \ed+
1594  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"  item, and then with 4, and so on, before ultimately failing. "Atomic grouping"
1595  (a term taken from Jeffrey Friedl's book) provides the means for specifying  (a term taken from Jeffrey Friedl's book) provides the means for specifying
1596  that once a subpattern has matched, it is not to be re-evaluated in this way.  that once a subpattern has matched, it is not to be re-evaluated in this way.
1597    .P
1598  If we use atomic grouping for the previous example, the matcher would give up  If we use atomic grouping for the previous example, the matcher gives up
1599  immediately on failing to match "foo" the first time. The notation is a kind of  immediately on failing to match "foo" the first time. The notation is a kind of
1600  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1601    .sp
1602    (?>\\d+)foo    (?>\ed+)foo
1603    .sp
1604  This kind of parenthesis "locks up" the  part of the pattern it contains once  This kind of parenthesis "locks up" the  part of the pattern it contains once
1605  it has matched, and a failure further into the pattern is prevented from  it has matched, and a failure further into the pattern is prevented from
1606  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1607  normal.  normal.
1608    .P
1609  An alternative description is that a subpattern of this type matches the string  An alternative description is that a subpattern of this type matches the string
1610  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1611  the current point in the subject string.  the current point in the subject string.
1612    .P
1613  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1614  the above example can be thought of as a maximizing repeat that must swallow  the above example can be thought of as a maximizing repeat that must swallow
1615  everything it can. So, while both \\d+ and \\d+? are prepared to adjust the  everything it can. So, while both \ed+ and \ed+? are prepared to adjust the
1616  number of digits they match in order to make the rest of the pattern match,  number of digits they match in order to make the rest of the pattern match,
1617  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1618    .P
1619  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1620  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1621  group is just a single repeated item, as in the example above, a simpler  group is just a single repeated item, as in the example above, a simpler
1622  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1623  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1624  previous example can be rewritten as  previous example can be rewritten as
1625    .sp
1626    \\d++bar    \ed++foo
1627    .sp
1628    Note that a possessive quantifier can be used with an entire group, for
1629    example:
1630    .sp
1631      (abc|xyz){2,3}+
1632    .sp
1633  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1634  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
1635  atomic group. However, there is no difference in the meaning or processing of a  atomic group. However, there is no difference in the meaning of a possessive
1636  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1637    difference; possessive quantifiers should be slightly faster.
1638  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1639  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1640    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1641    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1642    package, and PCRE copied it from there. It ultimately found its way into Perl
1643    at release 5.10.
1644    .P
1645    PCRE has an optimization that automatically "possessifies" certain simple
1646    pattern constructs. For example, the sequence A+B is treated as A++B because
1647    there is no point in backtracking into a sequence of A's when B must follow.
1648    .P
1649  When a pattern contains an unlimited repeat inside a subpattern that can itself  When a pattern contains an unlimited repeat inside a subpattern that can itself
1650  be repeated an unlimited number of times, the use of an atomic group is the  be repeated an unlimited number of times, the use of an atomic group is the
1651  only way to avoid some failing matches taking a very long time indeed. The  only way to avoid some failing matches taking a very long time indeed. The
1652  pattern  pattern
1653    .sp
1654    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1655    .sp
1656  matches an unlimited number of substrings that either consist of non-digits, or  matches an unlimited number of substrings that either consist of non-digits, or
1657  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1658  quickly. However, if it is applied to  quickly. However, if it is applied to
1659    .sp
1660    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1661    .sp
1662  it takes a long time before reporting failure. This is because the string can  it takes a long time before reporting failure. This is because the string can
1663  be divided between the two repeats in a large number of ways, and all have to  be divided between the internal \eD+ repeat and the external * repeat in a
1664  be tried. (The example used [!?] rather than a single character at the end,  large number of ways, and all have to be tried. (The example uses [!?] rather
1665  because both PCRE and Perl have an optimization that allows for fast failure  than a single character at the end, because both PCRE and Perl have an
1666  when a single character is used. They remember the last single character that  optimization that allows for fast failure when a single character is used. They
1667  is required for a match, and fail early if it is not present in the string.)  remember the last single character that is required for a match, and fail early
1668  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1669    an atomic group, like this:
1670    ((?>\\D+)|<\\d+>)*[!?]  .sp
1671      ((?>\eD+)|<\ed+>)*[!?]
1672    .sp
1673  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1674    .
1675  .SH BACK REFERENCES  .
1676    .\" HTML <a name="backreferences"></a>
1677    .SH "BACK REFERENCES"
1678  .rs  .rs
1679  .sp  .sp
1680  Outside a character class, a backslash followed by a digit greater than 0 (and  Outside a character class, a backslash followed by a digit greater than 0 (and
1681  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1682  (that is, to its left) in the pattern, provided there have been that many  (that is, to its left) in the pattern, provided there have been that many
1683  previous capturing left parentheses.  previous capturing left parentheses.
1684    .P
1685  However, if the decimal number following the backslash is less than 10, it is  However, if the decimal number following the backslash is less than 10, it is
1686  always taken as a back reference, and causes an error only if there are not  always taken as a back reference, and causes an error only if there are not
1687  that many capturing left parentheses in the entire pattern. In other words, the  that many capturing left parentheses in the entire pattern. In other words, the
1688  parentheses that are referenced need not be to the left of the reference for  parentheses that are referenced need not be to the left of the reference for
1689  numbers less than 10. See the section entitled "Backslash" above for further  numbers less than 10. A "forward back reference" of this type can make sense
1690  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1691    in an earlier iteration.
1692    .P
1693    It is not possible to have a numerical "forward back reference" to a subpattern
1694    whose number is 10 or more using this syntax because a sequence such as \e50 is
1695    interpreted as a character defined in octal. See the subsection entitled
1696    "Non-printing characters"
1697    .\" HTML <a href="#digitsafterbackslash">
1698    .\" </a>
1699    above
1700    .\"
1701    for further details of the handling of digits following a backslash. There is
1702    no such problem when named parentheses are used. A back reference to any
1703    subpattern is possible using named parentheses (see below).
1704    .P
1705    Another way of avoiding the ambiguity inherent in the use of digits following a
1706    backslash is to use the \eg escape sequence. This escape must be followed by an
1707    unsigned number or a negative number, optionally enclosed in braces. These
1708    examples are all identical:
1709    .sp
1710      (ring), \e1
1711      (ring), \eg1
1712      (ring), \eg{1}
1713    .sp
1714    An unsigned number specifies an absolute reference without the ambiguity that
1715    is present in the older syntax. It is also useful when literal digits follow
1716    the reference. A negative number is a relative reference. Consider this
1717    example:
1718    .sp
1719      (abc(def)ghi)\eg{-1}
1720    .sp
1721    The sequence \eg{-1} is a reference to the most recently started capturing
1722    subpattern before \eg, that is, is it equivalent to \e2 in this example.
1723    Similarly, \eg{-2} would be equivalent to \e1. The use of relative references
1724    can be helpful in long patterns, and also in patterns that are created by
1725    joining together fragments that contain references within themselves.
1726    .P
1727  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1728  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1729  itself (see  itself (see
# Line 832  itself (see Line 1732  itself (see
1732  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1733  .\"  .\"
1734  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1735    .sp
1736    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1737    .sp
1738  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1739  "sense and responsibility". If caseful matching is in force at the time of the  "sense and responsibility". If caseful matching is in force at the time of the
1740  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1741    .sp
1742    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1743    .sp
1744  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original  matches "rah rah" and "RAH RAH", but not "RAH rah", even though the original
1745  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1746    .P
1747  Back references to named subpatterns use the Python syntax (?P=name). We could  There are several different ways of writing back references to named
1748  rewrite the above example as follows:  subpatterns. The .NET syntax \ek{name} and the Perl syntax \ek<name> or
1749    \ek'name' are supported, as is the Python syntax (?P=name). Perl 5.10's unified
1750    (?<p1>(?i)rah)\\s+(?P=p1)  back reference syntax, in which \eg can be used for both numeric and named
1751    references, is also supported. We could rewrite the above example in any of
1752    the following ways:
1753    .sp
1754      (?<p1>(?i)rah)\es+\ek<p1>
1755      (?'p1'(?i)rah)\es+\ek{p1}
1756      (?P<p1>(?i)rah)\es+(?P=p1)
1757      (?<p1>(?i)rah)\es+\eg{p1}
1758    .sp
1759    A subpattern that is referenced by name may appear in the pattern before or
1760    after the reference.
1761    .P
1762  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
1763  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1764  references to it always fail. For example, the pattern  references to it always fail by default. For example, the pattern
1765    .sp
1766    (a|(bc))\\2    (a|(bc))\e2
1767    .sp
1768  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
1769  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
1770  taken as part of a potential back reference number. If the pattern continues  unset value matches an empty string.
1771  with a digit character, some delimiter must be used to terminate the back  .P
1772  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  Because there may be many capturing parentheses in a pattern, all digits
1773  Otherwise an empty comment can be used.  following a backslash are taken as part of a potential back reference number.
1774    If the pattern continues with a digit character, some delimiter must be used to
1775    terminate the back reference. If the PCRE_EXTENDED option is set, this can be
1776    whitespace. Otherwise, the \eg{ syntax or an empty comment (see
1777    .\" HTML <a href="#comments">
1778    .\" </a>
1779    "Comments"
1780    .\"
1781    below) can be used.
1782    .
1783    .SS "Recursive back references"
1784    .rs
1785    .sp
1786  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
1787  when the subpattern is first used, so, for example, (a\\1) never matches.  when the subpattern is first used, so, for example, (a\e1) never matches.
1788  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1789  example, the pattern  example, the pattern
1790    .sp
1791    (a|b\\1)+    (a|b\e1)+
1792    .sp
1793  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of  matches any number of "a"s and also "aba", "ababbaa" etc. At each iteration of
1794  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1795  to the previous iteration. In order for this to work, the pattern must be such  to the previous iteration. In order for this to work, the pattern must be such
1796  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
1797  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
1798  minimum of zero.  minimum of zero.
1799    .P
1800    Back references of this type cause the group that they reference to be treated
1801    as an
1802    .\" HTML <a href="#atomicgroup">
1803    .\" </a>
1804    atomic group.
1805    .\"
1806    Once the whole group has been matched, a subsequent matching failure cannot
1807    cause backtracking into the middle of the group.
1808    .
1809    .
1810    .\" HTML <a name="bigassertions"></a>
1811  .SH ASSERTIONS  .SH ASSERTIONS
1812  .rs  .rs
1813  .sp  .sp
1814  An assertion is a test on the characters following or preceding the current  An assertion is a test on the characters following or preceding the current
1815  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1816  assertions coded as \\b, \\B, \\A, \\G, \\Z, \\z, ^ and $ are described above.  assertions coded as \eb, \eB, \eA, \eG, \eZ, \ez, ^ and $ are described
1817    .\" HTML <a href="#smallassertions">
1818    .\" </a>
1819    above.
1820    .\"
1821    .P
1822  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1823  those that look ahead of the current position in the subject string, and those  those that look ahead of the current position in the subject string, and those
1824  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1825    except that it does not cause the current matching position to be changed.
1826  An assertion subpattern is matched in the normal way, except that it does not  .P
1827  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns. If such an assertion
1828  with (?= for positive assertions and (?! for negative assertions. For example,  contains capturing subpatterns within it, these are counted for the purposes of
1829    numbering the capturing subpatterns in the whole pattern. However, substring
1830    \\w+(?=;)  capturing is carried out only for positive assertions, because it does not make
1831    sense for negative assertions.
1832    .P
1833    For compatibility with Perl, assertion subpatterns may be repeated; though
1834    it makes no sense to assert the same thing several times, the side effect of
1835    capturing parentheses may occasionally be useful. In practice, there only three
1836    cases:
1837    .sp
1838    (1) If the quantifier is {0}, the assertion is never obeyed during matching.
1839    However, it may contain internal capturing parenthesized groups that are called
1840    from elsewhere via the
1841    .\" HTML <a href="#subpatternsassubroutines">
1842    .\" </a>
1843    subroutine mechanism.
1844    .\"
1845    .sp
1846    (2) If quantifier is {0,n} where n is greater than zero, it is treated as if it
1847    were {0,1}. At run time, the rest of the pattern match is tried with and
1848    without the assertion, the order depending on the greediness of the quantifier.
1849    .sp
1850    (3) If the minimum repetition is greater than zero, the quantifier is ignored.
1851    The assertion is obeyed just once when encountered during matching.
1852    .
1853    .
1854    .SS "Lookahead assertions"
1855    .rs
1856    .sp
1857    Lookahead assertions start with (?= for positive assertions and (?! for
1858    negative assertions. For example,
1859    .sp
1860      \ew+(?=;)
1861    .sp
1862  matches a word followed by a semicolon, but does not include the semicolon in  matches a word followed by a semicolon, but does not include the semicolon in
1863  the match, and  the match, and
1864    .sp
1865    foo(?!bar)    foo(?!bar)
1866    .sp
1867  matches any occurrence of "foo" that is not followed by "bar". Note that the  matches any occurrence of "foo" that is not followed by "bar". Note that the
1868  apparently similar pattern  apparently similar pattern
1869    .sp
1870    (?!foo)bar    (?!foo)bar
1871    .sp
1872  does not find an occurrence of "bar" that is preceded by something other than  does not find an occurrence of "bar" that is preceded by something other than
1873  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1874  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1875  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1876    .P
1877  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
1878  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
1879  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.
1880    The backtracking control verb (*FAIL) or (*F) is a synonym for (?!).
1881    .
1882    .
1883    .\" HTML <a name="lookbehind"></a>
1884    .SS "Lookbehind assertions"
1885    .rs
1886    .sp
1887  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1888  negative assertions. For example,  negative assertions. For example,
1889    .sp
1890    (?<!foo)bar    (?<!foo)bar
1891    .sp
1892  does find an occurrence of "bar" that is not preceded by "foo". The contents of  does find an occurrence of "bar" that is not preceded by "foo". The contents of
1893  a lookbehind assertion are restricted such that all the strings it matches must  a lookbehind assertion are restricted such that all the strings it matches must
1894  have a fixed length. However, if there are several alternatives, they do not  have a fixed length. However, if there are several top-level alternatives, they
1895  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1896    .sp
1897    (?<=bullock|donkey)    (?<=bullock|donkey)
1898    .sp
1899  is permitted, but  is permitted, but
1900    .sp
1901    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1902    .sp
1903  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1904  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
1905  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
1906  match the same length of string. An assertion such as  length of string. An assertion such as
1907    .sp
1908    (?<=ab(c|de))    (?<=ab(c|de))
1909    .sp
1910  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
1911  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
1912    branches:
1913    .sp
1914    (?<=abc|abde)    (?<=abc|abde)
1915    .sp
1916    In some cases, the escape sequence \eK
1917    .\" HTML <a href="#resetmatchstart">
1918    .\" </a>
1919    (see above)
1920    .\"
1921    can be used instead of a lookbehind assertion to get round the fixed-length
1922    restriction.
1923    .P
1924  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1925  temporarily move the current position back by the fixed width and then try to  temporarily move the current position back by the fixed length and then try to
1926  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1927  match is deemed to fail.  assertion fails.
1928    .P
1929  PCRE does not allow the \\C escape (which matches a single byte in UTF-8 mode)  PCRE does not allow the \eC escape (which matches a single byte in UTF-8 mode)
1930  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1931  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1932    different numbers of bytes, are also not permitted.
1933  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1934  efficient matching at the end of the subject string. Consider a simple pattern  .\" HTML <a href="#subpatternsassubroutines">
1935  such as  .\" </a>
1936    "Subroutine"
1937    .\"
1938    calls (see below) such as (?2) or (?&X) are permitted in lookbehinds, as long
1939    as the subpattern matches a fixed-length string.
1940    .\" HTML <a href="#recursion">
1941    .\" </a>
1942    Recursion,
1943    .\"
1944    however, is not supported.
1945    .P
1946    Possessive quantifiers can be used in conjunction with lookbehind assertions to
1947    specify efficient matching of fixed-length strings at the end of subject
1948    strings. Consider a simple pattern such as
1949    .sp
1950    abcd$    abcd$
1951    .sp
1952  when applied to a long string that does not match. Because matching proceeds  when applied to a long string that does not match. Because matching proceeds
1953  from left to right, PCRE will look for each "a" in the subject and then see if  from left to right, PCRE will look for each "a" in the subject and then see if
1954  what follows matches the rest of the pattern. If the pattern is specified as  what follows matches the rest of the pattern. If the pattern is specified as
1955    .sp
1956    ^.*abcd$    ^.*abcd$
1957    .sp
1958  the initial .* matches the entire string at first, but when this fails (because  the initial .* matches the entire string at first, but when this fails (because
1959  there is no following "a"), it backtracks to match all but the last character,  there is no following "a"), it backtracks to match all but the last character,
1960  then all but the last two characters, and so on. Once again the search for "a"  then all but the last two characters, and so on. Once again the search for "a"
1961  covers the entire string, from right to left, so we are no better off. However,  covers the entire string, from right to left, so we are no better off. However,
1962  if the pattern is written as  if the pattern is written as
1963    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1964    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1965    .sp
1966  there can be no backtracking for the .* item; it can match only the entire  there can be no backtracking for the .*+ item; it can match only the entire
1967  string. The subsequent lookbehind assertion does a single test on the last four  string. The subsequent lookbehind assertion does a single test on the last four
1968  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1969  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1970    .
1971    .
1972    .SS "Using multiple assertions"
1973    .rs
1974    .sp
1975  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1976    .sp
1977    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1978    .sp
1979  matches "foo" preceded by three digits that are not "999". Notice that each of  matches "foo" preceded by three digits that are not "999". Notice that each of
1980  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1981  string. First there is a check that the previous three characters are all  string. First there is a check that the previous three characters are all
1982  digits, and then there is a check that the same three characters are not "999".  digits, and then there is a check that the same three characters are not "999".
1983  This pattern does \fInot\fR match "foo" preceded by six characters, the first  This pattern does \fInot\fP match "foo" preceded by six characters, the first
1984  of which are digits and the last three of which are not "999". For example, it  of which are digits and the last three of which are not "999". For example, it
1985  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1986    .sp
1987    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1988    .sp
1989  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1990  that the first three are digits, and then the second assertion checks that the  that the first three are digits, and then the second assertion checks that the
1991  preceding three characters are not "999".  preceding three characters are not "999".
1992    .P
1993  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1994    .sp
1995    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1996    .sp
1997  matches an occurrence of "baz" that is preceded by "bar" which in turn is not  matches an occurrence of "baz" that is preceded by "bar" which in turn is not
1998  preceded by "foo", while  preceded by "foo", while
1999    .sp
2000    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
2001    .sp
2002  is another pattern which matches "foo" preceded by three digits and any three  is another pattern that matches "foo" preceded by three digits and any three
2003  characters that are not "999".  characters that are not "999".
2004    .
2005  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
2006  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
2007  of assertion contains capturing subpatterns within it, these are counted for  .SH "CONDITIONAL SUBPATTERNS"
 the purposes of numbering the capturing subpatterns in the whole pattern.  
 However, substring capturing is carried out only for positive assertions,  
 because it does not make sense for negative assertions.  
   
 .SH CONDITIONAL SUBPATTERNS  
2008  .rs  .rs
2009  .sp  .sp
2010  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
2011  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
2012  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a specific capturing subpattern has
2013  or not. The two possible forms of conditional subpattern are  already been matched. The two possible forms of conditional subpattern are:
2014    .sp
2015    (?(condition)yes-pattern)    (?(condition)yes-pattern)
2016    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
2017    .sp
2018  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
2019  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
2020  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs. Each of the two alternatives may
2021    itself contain nested subpatterns of any form, including conditional
2022  There are three kinds of condition. If the text between the parentheses  subpatterns; the restriction to two alternatives applies only at the level of
2023  consists of a sequence of digits, the condition is satisfied if the capturing  the condition. This pattern fragment is an example where the alternatives are
2024  subpattern of that number has previously matched. The number must be greater  complex:
2025  than zero. Consider the following pattern, which contains non-significant white  .sp
2026  space to make it more readable (assume the PCRE_EXTENDED option) and to divide    (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
2027  it into three parts for ease of discussion:  .sp
2028    .P
2029    ( \\( )?    [^()]+    (?(1) \\) )  There are four kinds of condition: references to subpatterns, references to
2030    recursion, a pseudo-condition called DEFINE, and assertions.
2031    .
2032    .SS "Checking for a used subpattern by number"
2033    .rs
2034    .sp
2035    If the text between the parentheses consists of a sequence of digits, the
2036    condition is true if a capturing subpattern of that number has previously
2037    matched. If there is more than one capturing subpattern with the same number
2038    (see the earlier
2039    .\"
2040    .\" HTML <a href="#recursion">
2041    .\" </a>
2042    section about duplicate subpattern numbers),
2043    .\"
2044    the condition is true if any of them have matched. An alternative notation is
2045    to precede the digits with a plus or minus sign. In this case, the subpattern
2046    number is relative rather than absolute. The most recently opened parentheses
2047    can be referenced by (?(-1), the next most recent by (?(-2), and so on. Inside
2048    loops it can also make sense to refer to subsequent groups. The next
2049    parentheses to be opened can be referenced as (?(+1), and so on. (The value
2050    zero in any of these forms is not used; it provokes a compile-time error.)
2051    .P
2052    Consider the following pattern, which contains non-significant white space to
2053    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
2054    three parts for ease of discussion:
2055    .sp
2056      ( \e( )?    [^()]+    (?(1) \e) )
2057    .sp
2058  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
2059  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
2060  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
2061  conditional subpattern that tests whether the first set of parentheses matched  conditional subpattern that tests whether or not the first set of parentheses
2062  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,
2063  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
2064  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
2065  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
2066  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
2067    .P
2068  If the condition is the string (R), it is satisfied if a recursive call to the  If you were embedding this pattern in a larger one, you could use a relative
2069  pattern or subpattern has been made. At "top level", the condition is false.  reference:
2070  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
2071      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
2072  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
2073    This makes the fragment independent of the parentheses in the larger pattern.
2074    .
2075    .SS "Checking for a used subpattern by name"
2076    .rs
2077    .sp
2078    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
2079    subpattern by name. For compatibility with earlier versions of PCRE, which had
2080    this facility before Perl, the syntax (?(name)...) is also recognized. However,
2081    there is a possible ambiguity with this syntax, because subpattern names may
2082    consist entirely of digits. PCRE looks first for a named subpattern; if it
2083    cannot find one and the name consists entirely of digits, PCRE looks for a
2084    subpattern of that number, which must be greater than zero. Using subpattern
2085    names that consist entirely of digits is not recommended.
2086    .P
2087    Rewriting the above example to use a named subpattern gives this:
2088    .sp
2089      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
2090    .sp
2091    If the name used in a condition of this kind is a duplicate, the test is
2092    applied to all subpatterns of the same name, and is true if any one of them has
2093    matched.
2094    .
2095    .SS "Checking for pattern recursion"
2096    .rs
2097    .sp
2098    If the condition is the string (R), and there is no subpattern with the name R,
2099    the condition is true if a recursive call to the whole pattern or any
2100    subpattern has been made. If digits or a name preceded by ampersand follow the
2101    letter R, for example:
2102    .sp
2103      (?(R3)...) or (?(R&name)...)
2104    .sp
2105    the condition is true if the most recent recursion is into a subpattern whose
2106    number or name is given. This condition does not check the entire recursion
2107    stack. If the name used in a condition of this kind is a duplicate, the test is
2108    applied to all subpatterns of the same name, and is true if any one of them is
2109    the most recent recursion.
2110    .P
2111    At "top level", all these recursion test conditions are false.
2112    .\" HTML <a href="#recursion">
2113    .\" </a>
2114    The syntax for recursive patterns
2115    .\"
2116    is described below.
2117    .
2118    .\" HTML <a name="subdefine"></a>
2119    .SS "Defining subpatterns for use by reference only"
2120    .rs
2121    .sp
2122    If the condition is the string (DEFINE), and there is no subpattern with the
2123    name DEFINE, the condition is always false. In this case, there may be only one
2124    alternative in the subpattern. It is always skipped if control reaches this
2125    point in the pattern; the idea of DEFINE is that it can be used to define
2126    "subroutines" that can be referenced from elsewhere. (The use of
2127    .\" HTML <a href="#subpatternsassubroutines">
2128    .\" </a>
2129    "subroutines"
2130    .\"
2131    is described below.) For example, a pattern to match an IPv4 address such as
2132    "192.168.23.245" could be written like this (ignore whitespace and line
2133    breaks):
2134    .sp
2135      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
2136      \eb (?&byte) (\e.(?&byte)){3} \eb
2137    .sp
2138    The first part of the pattern is a DEFINE group inside which a another group
2139    named "byte" is defined. This matches an individual component of an IPv4
2140    address (a number less than 256). When matching takes place, this part of the
2141    pattern is skipped because DEFINE acts like a false condition. The rest of the
2142    pattern uses references to the named group to match the four dot-separated
2143    components of an IPv4 address, insisting on a word boundary at each end.
2144    .
2145    .SS "Assertion conditions"
2146    .rs
2147    .sp
2148    If the condition is not in any of the above formats, it must be an assertion.
2149  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
2150  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
2151  alternatives on the second line:  alternatives on the second line:
2152    .sp
2153    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
2154    \\d{2}-[a-z]{3}-\\d{2}  |  \\d{2}-\\d{2}-\\d{2} )    \ed{2}-[a-z]{3}-\ed{2}  |  \ed{2}-\ed{2}-\ed{2} )
2155    .sp
2156  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
2157  sequence of non-letters followed by a letter. In other words, it tests for the  sequence of non-letters followed by a letter. In other words, it tests for the
2158  presence of at least one letter in the subject. If a letter is found, the  presence of at least one letter in the subject. If a letter is found, the
2159  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
2160  against the second. This pattern matches strings in one of the two forms  against the second. This pattern matches strings in one of the two forms
2161  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.  dd-aaa-dd or dd-dd-dd, where aaa are letters and dd are digits.
2162    .
2163    .
2164    .\" HTML <a name="comments"></a>
2165  .SH COMMENTS  .SH COMMENTS
2166  .rs  .rs
2167  .sp  .sp
2168  The sequence (?# marks the start of a comment which continues up to the next  There are two ways of including comments in patterns that are processed by
2169  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,
2170  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
2171    subpattern name or number. The characters that make up a comment play no part
2172  If the PCRE_EXTENDED option is set, an unescaped # character outside a  in the pattern matching.
2173  character class introduces a comment that continues up to the next newline  .P
2174  character in the pattern.  The sequence (?# marks the start of a comment that continues up to the next
2175    closing parenthesis. Nested parentheses are not permitted. If the PCRE_EXTENDED
2176  .SH RECURSIVE PATTERNS  option is set, an unescaped # character also introduces a comment, which in
2177    this case continues to immediately after the next newline character or
2178    character sequence in the pattern. Which characters are interpreted as newlines
2179    is controlled by the options passed to \fBpcre_compile()\fP or by a special
2180    sequence at the start of the pattern, as described in the section entitled
2181    .\" HTML <a href="#newlines">
2182    .\" </a>
2183    "Newline conventions"
2184    .\"
2185    above. Note that the end of this type of comment is a literal newline sequence
2186    in the pattern; escape sequences that happen to represent a newline do not
2187    count. For example, consider this pattern when PCRE_EXTENDED is set, and the
2188    default newline convention is in force:
2189    .sp
2190      abc #comment \en still comment
2191    .sp
2192    On encountering the # character, \fBpcre_compile()\fP skips along, looking for
2193    a newline in the pattern. The sequence \en is still literal at this stage, so
2194    it does not terminate the comment. Only an actual character with the code value
2195    0x0a (the default newline) does so.
2196    .
2197    .
2198    .\" HTML <a name="recursion"></a>
2199    .SH "RECURSIVE PATTERNS"
2200  .rs  .rs
2201  .sp  .sp
2202  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
2203  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
2204  be done is to use a pattern that matches up to some fixed depth of nesting. It  be done is to use a pattern that matches up to some fixed depth of nesting. It
2205  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
2206  experimental facility that allows regular expressions to recurse (amongst other  .P
2207  things). It does this by interpolating Perl code in the expression at run time,  For some time, Perl has provided a facility that allows regular expressions to
2208  and the code can refer to the expression itself. A Perl pattern to solve the  recurse (amongst other things). It does this by interpolating Perl code in the
2209  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
2210    pattern using code interpolation to solve the parentheses problem can be
2211    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
2212    .sp
2213      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
2214    .sp
2215  The (?p{...}) item interpolates Perl code at run time, and in this case refers  The (?p{...}) item interpolates Perl code at run time, and in this case refers
2216  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
2217  the interpolation of Perl code. Instead, it supports some special syntax for  .P
2218  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
2219    supports special syntax for recursion of the entire pattern, and also for
2220  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
2221  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was subsequently introduced into Perl at release 5.10.
2222  number, provided that it occurs inside that subpattern. (If not, it is a  .P
2223  "subroutine" call, which is described in the next section.) The special item  A special item that consists of (? followed by a number greater than zero and a
2224  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
2225    provided that it occurs inside that subpattern. (If not, it is a
2226  For example, this PCRE pattern solves the nested parentheses problem (assume  .\" HTML <a href="#subpatternsassubroutines">
2227  the PCRE_EXTENDED option is set so that white space is ignored):  .\" </a>
2228    "subroutine"
2229    \\( ( (?>[^()]+) | (?R) )* \\)  .\"
2230    call, which is described in the next section.) The special item (?R) or (?0) is
2231    a recursive call of the entire regular expression.
2232    .P
2233    This PCRE pattern solves the nested parentheses problem (assume the
2234    PCRE_EXTENDED option is set so that white space is ignored):
2235    .sp
2236      \e( ( [^()]++ | (?R) )* \e)
2237    .sp
2238  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
2239  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
2240  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
2241  Finally there is a closing parenthesis.  Finally there is a closing parenthesis. Note the use of a possessive quantifier
2242    to avoid backtracking into sequences of non-parentheses.
2243    .P
2244  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
2245  pattern, so instead you could use this:  pattern, so instead you could use this:
2246    .sp
2247    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( [^()]++ | (?1) )* \e) )
2248    .sp
2249  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
2250  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
2251  parenthesis numbers can be tricky. It may be more convenient to use named  .P
2252  parentheses instead. For this, PCRE uses (?P>name), which is an extension to  In a larger pattern, keeping track of parenthesis numbers can be tricky. This
2253  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. Instead of (?1) in the
2254  named parentheses). We could rewrite the above example as follows:  pattern above you can write (?-2) to refer to the second most recently opened
2255    parentheses preceding the recursion. In other words, a negative number counts
2256    (?P<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  capturing parentheses leftwards from the point at which it is encountered.
2257    .P
2258  This particular example pattern contains nested unlimited repeats, and so the  It is also possible to refer to subsequently opened parentheses, by writing
2259  use of atomic grouping for matching strings of non-parentheses is important  references such as (?+2). However, these cannot be recursive because the
2260  when applying the pattern to strings that do not match. For example, when this  reference is not inside the parentheses that are referenced. They are always
2261  pattern is applied to  .\" HTML <a href="#subpatternsassubroutines">
2262    .\" </a>
2263    "subroutine"
2264    .\"
2265    calls, as described in the next section.
2266    .P
2267    An alternative approach is to use named parentheses instead. The Perl syntax
2268    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
2269    could rewrite the above example as follows:
2270    .sp
2271      (?<pn> \e( ( [^()]++ | (?&pn) )* \e) )
2272    .sp
2273    If there is more than one subpattern with the same name, the earliest one is
2274    used.
2275    .P
2276    This particular example pattern that we have been looking at contains nested
2277    unlimited repeats, and so the use of a possessive quantifier for matching
2278    strings of non-parentheses is important when applying the pattern to strings
2279    that do not match. For example, when this pattern is applied to
2280    .sp
2281    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
2282    .sp
2283  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,
2284  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
2285  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
2286  before failure can be reported.  before failure can be reported.
2287    .P
2288  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
2289  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
2290  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  
2291  .\" HREF  .\" HREF
2292  \fBpcrecallout\fR  \fBpcrecallout\fP
2293  .\"  .\"
2294  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
2295    .sp
2296    (ab(cd)ef)    (ab(cd)ef)
2297    .sp
2298  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
2299  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
2300    matched at the top level, its final value is unset, even if it is (temporarily)
2301    \\( ( ( (?>[^()]+) | (?R) )* ) \\)  set at a deeper level.
2302       ^                        ^  .P
2303       ^                        ^  If there are more than 15 capturing parentheses in a pattern, PCRE has to
2304    obtain extra memory to store data during a recursion, which it does by using
2305  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
2306  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
2307  has to obtain extra memory to store data during a recursion, which it does by  .P
 using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  
 memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  
   
2308  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.
2309  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
2310  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
2311  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
2312    .sp
2313    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
2314    .sp
2315  In this pattern, (?(R) is the start of a conditional subpattern, with two  In this pattern, (?(R) is the start of a conditional subpattern, with two
2316  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
2317  is the actual recursive call.  is the actual recursive call.
2318    .
2319    .
2320    .\" HTML <a name="recursiondifference"></a>
2321    .SS "Recursion difference from Perl"
2322    .rs
2323    .sp
2324    In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
2325    treated as an atomic group. That is, once it has matched some of the subject
2326    string, it is never re-entered, even if it contains untried alternatives and
2327    there is a subsequent matching failure. This can be illustrated by the
2328    following pattern, which purports to match a palindromic string that contains
2329    an odd number of characters (for example, "a", "aba", "abcba", "abcdcba"):
2330    .sp
2331      ^(.|(.)(?1)\e2)$
2332    .sp
2333    The idea is that it either matches a single character, or two identical
2334    characters surrounding a sub-palindrome. In Perl, this pattern works; in PCRE
2335    it does not if the pattern is longer than three characters. Consider the
2336    subject string "abcba":
2337    .P
2338    At the top level, the first character is matched, but as it is not at the end
2339    of the string, the first alternative fails; the second alternative is taken
2340    and the recursion kicks in. The recursive call to subpattern 1 successfully
2341    matches the next character ("b"). (Note that the beginning and end of line
2342    tests are not part of the recursion).
2343    .P
2344    Back at the top level, the next character ("c") is compared with what
2345    subpattern 2 matched, which was "a". This fails. Because the recursion is
2346    treated as an atomic group, there are now no backtracking points, and so the
2347    entire match fails. (Perl is able, at this point, to re-enter the recursion and
2348    try the second alternative.) However, if the pattern is written with the
2349    alternatives in the other order, things are different:
2350    .sp
2351      ^((.)(?1)\e2|.)$
2352    .sp
2353    This time, the recursing alternative is tried first, and continues to recurse
2354    until it runs out of characters, at which point the recursion fails. But this
2355    time we do have another alternative to try at the higher level. That is the big
2356    difference: in the previous case the remaining alternative is at a deeper
2357    recursion level, which PCRE cannot use.
2358    .P
2359    To change the pattern so that it matches all palindromic strings, not just
2360    those with an odd number of characters, it is tempting to change the pattern to
2361    this:
2362    .sp
2363      ^((.)(?1)\e2|.?)$
2364    .sp
2365    Again, this works in Perl, but not in PCRE, and for the same reason. When a
2366    deeper recursion has matched a single character, it cannot be entered again in
2367    order to match an empty string. The solution is to separate the two cases, and
2368    write out the odd and even cases as alternatives at the higher level:
2369    .sp
2370      ^(?:((.)(?1)\e2|)|((.)(?3)\e4|.))
2371    .sp
2372    If you want to match typical palindromic phrases, the pattern has to ignore all
2373    non-word characters, which can be done like this:
2374    .sp
2375      ^\eW*+(?:((.)\eW*+(?1)\eW*+\e2|)|((.)\eW*+(?3)\eW*+\e4|\eW*+.\eW*+))\eW*+$
2376    .sp
2377    If run with the PCRE_CASELESS option, this pattern matches phrases such as "A
2378    man, a plan, a canal: Panama!" and it works well in both PCRE and Perl. Note
2379    the use of the possessive quantifier *+ to avoid backtracking into sequences of
2380    non-word characters. Without this, PCRE takes a great deal longer (ten times or
2381    more) to match typical phrases, and Perl takes so long that you think it has
2382    gone into a loop.
2383    .P
2384    \fBWARNING\fP: The palindrome-matching patterns above work only if the subject
2385    string does not start with a palindrome that is shorter than the entire string.
2386    For example, although "abcba" is correctly matched, if the subject is "ababa",
2387    PCRE finds the palindrome "aba" at the start, then fails at top level because
2388    the end of the string does not follow. Once again, it cannot jump back into the
2389    recursion to try other alternatives, so the entire match fails.
2390    .
2391    .
2392  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
2393  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
2394  .rs  .rs
2395  .sp  .sp
2396  If the syntax for a recursive subpattern reference (either by number or by  If the syntax for a recursive subpattern reference (either by number or by
2397  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
2398  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
2399  pattern  before or after the reference. A numbered reference can be absolute or
2400    relative, as in these examples:
2401    (sens|respons)e and \\1ibility  .sp
2402      (...(absolute)...)...(?2)...
2403      (...(relative)...)...(?-1)...
2404      (...(?+1)...(relative)...
2405    .sp
2406    An earlier example pointed out that the pattern
2407    .sp
2408      (sens|respons)e and \e1ibility
2409    .sp
2410  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
2411  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
2412    .sp
2413    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
2414    .sp
2415  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
2416  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
2417  refer.  .P
2418    Like recursive subpatterns, a subroutine call is always treated as an atomic
2419    group. That is, once it has matched some of the subject string, it is never
2420    re-entered, even if it contains untried alternatives and there is a subsequent
2421    matching failure. Any capturing parentheses that are set during the subroutine
2422    call revert to their previous values afterwards.
2423    .P
2424    When a subpattern is used as a subroutine, processing options such as
2425    case-independence are fixed when the subpattern is defined. They cannot be
2426    changed for different calls. For example, consider this pattern:
2427    .sp
2428      (abc)(?i:(?-1))
2429    .sp
2430    It matches "abcabc". It does not match "abcABC" because the change of
2431    processing option does not affect the called subpattern.
2432    .
2433    .
2434    .\" HTML <a name="onigurumasubroutines"></a>
2435    .SH "ONIGURUMA SUBROUTINE SYNTAX"
2436    .rs
2437    .sp
2438    For compatibility with Oniguruma, the non-Perl syntax \eg followed by a name or
2439    a number enclosed either in angle brackets or single quotes, is an alternative
2440    syntax for referencing a subpattern as a subroutine, possibly recursively. Here
2441    are two of the examples used above, rewritten using this syntax:
2442    .sp
2443      (?<pn> \e( ( (?>[^()]+) | \eg<pn> )* \e) )
2444      (sens|respons)e and \eg'1'ibility
2445    .sp
2446    PCRE supports an extension to Oniguruma: if a number is preceded by a
2447    plus or a minus sign it is taken as a relative reference. For example:
2448    .sp
2449      (abc)(?i:\eg<-1>)
2450    .sp
2451    Note that \eg{...} (Perl syntax) and \eg<...> (Oniguruma syntax) are \fInot\fP
2452    synonymous. The former is a back reference; the latter is a subroutine call.
2453    .
2454    .
2455  .SH CALLOUTS  .SH CALLOUTS
2456  .rs  .rs
2457  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 2459  Perl has a feature whereby using the seq
2459  code to be obeyed in the middle of matching a regular expression. This makes it  code to be obeyed in the middle of matching a regular expression. This makes it
2460  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
2461  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
2462    .P
2463  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
2464  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
2465  function by putting its entry point in the global variable \fIpcre_callout\fR.  function by putting its entry point in the global variable \fIpcre_callout\fP.
2466  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
2467    .P
2468  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
2469  function is to be called. If you want to identify different callout points, you  function is to be called. If you want to identify different callout points, you
2470  can put a number less than 256 after the letter C. The default value is zero.  can put a number less than 256 after the letter C. The default value is zero.
2471  For example, this pattern has two callout points:  For example, this pattern has two callout points:
2472    .sp
2473    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
2474    .sp
2475  During matching, when PCRE reaches a callout point (and \fIpcre_callout\fR is  If the PCRE_AUTO_CALLOUT flag is passed to \fBpcre_compile()\fP, callouts are
2476    automatically installed before each item in the pattern. They are all numbered
2477    255.
2478    .P
2479    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
2480  set), the external function is called. It is provided with the number of the  set), the external function is called. It is provided with the number of the
2481  callout, and, optionally, one item of data originally supplied by the caller of  callout, the position in the pattern, and, optionally, one item of data
2482  \fBpcre_exec()\fR. The callout function may cause matching to backtrack, or to  originally supplied by the caller of \fBpcre_exec()\fP. The callout function
2483  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
2484  function is given in the  description of the interface to the callout function is given in the
2485  .\" HREF  .\" HREF
2486  \fBpcrecallout\fR  \fBpcrecallout\fP
2487  .\"  .\"
2488  documentation.  documentation.
2489    .
2490  .in 0  .
2491  Last updated: 03 February 2003  .\" HTML <a name="backtrackcontrol"></a>
2492  .br  .SH "BACKTRACKING CONTROL"
2493  Copyright (c) 1997-2003 University of Cambridge.  .rs
2494    .sp
2495    Perl 5.10 introduced a number of "Special Backtracking Control Verbs", which
2496    are described in the Perl documentation as "experimental and subject to change
2497    or removal in a future version of Perl". It goes on to say: "Their usage in
2498    production code should be noted to avoid problems during upgrades." The same
2499    remarks apply to the PCRE features described in this section.
2500    .P
2501    Since these verbs are specifically related to backtracking, most of them can be
2502    used only when the pattern is to be matched using \fBpcre_exec()\fP, which uses
2503    a backtracking algorithm. With the exception of (*FAIL), which behaves like a
2504    failing negative assertion, they cause an error if encountered by
2505    \fBpcre_dfa_exec()\fP.
2506    .P
2507    If any of these verbs are used in an assertion or subroutine subpattern
2508    (including recursive subpatterns), their effect is confined to that subpattern;
2509    it does not extend to the surrounding pattern, with one exception: a *MARK that
2510    is encountered in a positive assertion \fIis\fP passed back (compare capturing
2511    parentheses in assertions). Note that such subpatterns are processed as
2512    anchored at the point where they are tested.
2513    .P
2514    The new verbs make use of what was previously invalid syntax: an opening
2515    parenthesis followed by an asterisk. They are generally of the form
2516    (*VERB) or (*VERB:NAME). Some may take either form, with differing behaviour,
2517    depending on whether or not an argument is present. An name is a sequence of
2518    letters, digits, and underscores. If the name is empty, that is, if the closing
2519    parenthesis immediately follows the colon, the effect is as if the colon were
2520    not there. Any number of these verbs may occur in a pattern.
2521    .P
2522    PCRE contains some optimizations that are used to speed up matching by running
2523    some checks at the start of each match attempt. For example, it may know the
2524    minimum length of matching subject, or that a particular character must be
2525    present. When one of these optimizations suppresses the running of a match, any
2526    included backtracking verbs will not, of course, be processed. You can suppress
2527    the start-of-match optimizations by setting the PCRE_NO_START_OPTIMIZE option
2528    when calling \fBpcre_compile()\fP or \fBpcre_exec()\fP, or by starting the
2529    pattern with (*NO_START_OPT).
2530    .
2531    .
2532    .SS "Verbs that act immediately"
2533    .rs
2534    .sp
2535    The following verbs act as soon as they are encountered. They may not be
2536    followed by a name.
2537    .sp
2538       (*ACCEPT)
2539    .sp
2540    This verb causes the match to end successfully, skipping the remainder of the
2541    pattern. When inside a recursion, only the innermost pattern is ended
2542    immediately. If (*ACCEPT) is inside capturing parentheses, the data so far is
2543    captured. (This feature was added to PCRE at release 8.00.) For example:
2544    .sp
2545      A((?:A|B(*ACCEPT)|C)D)
2546    .sp
2547    This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is captured by
2548    the outer parentheses.
2549    .sp
2550      (*FAIL) or (*F)
2551    .sp
2552    This verb causes the match to fail, forcing backtracking to occur. It is
2553    equivalent to (?!) but easier to read. The Perl documentation notes that it is
2554    probably useful only when combined with (?{}) or (??{}). Those are, of course,
2555    Perl features that are not present in PCRE. The nearest equivalent is the
2556    callout feature, as for example in this pattern:
2557    .sp
2558      a+(?C)(*FAIL)
2559    .sp
2560    A match with the string "aaaa" always fails, but the callout is taken before
2561    each backtrack happens (in this example, 10 times).
2562    .
2563    .
2564    .SS "Recording which path was taken"
2565    .rs
2566    .sp
2567    There is one verb whose main purpose is to track how a match was arrived at,
2568    though it also has a secondary use in conjunction with advancing the match
2569    starting point (see (*SKIP) below).
2570    .sp
2571      (*MARK:NAME) or (*:NAME)
2572    .sp
2573    A name is always required with this verb. There may be as many instances of
2574    (*MARK) as you like in a pattern, and their names do not have to be unique.
2575    .P
2576    When a match succeeds, the name of the last-encountered (*MARK) is passed back
2577    to the caller via the \fIpcre_extra\fP data structure, as described in the
2578    .\" HTML <a href="pcreapi.html#extradata">
2579    .\" </a>
2580    section on \fIpcre_extra\fP
2581    .\"
2582    in the
2583    .\" HREF
2584    \fBpcreapi\fP
2585    .\"
2586    documentation. No data is returned for a partial match. Here is an example of
2587    \fBpcretest\fP output, where the /K modifier requests the retrieval and
2588    outputting of (*MARK) data:
2589    .sp
2590      /X(*MARK:A)Y|X(*MARK:B)Z/K
2591      XY
2592       0: XY
2593      MK: A
2594      XZ
2595       0: XZ
2596      MK: B
2597    .sp
2598    The (*MARK) name is tagged with "MK:" in this output, and in this example it
2599    indicates which of the two alternatives matched. This is a more efficient way
2600    of obtaining this information than putting each alternative in its own
2601    capturing parentheses.
2602    .P
2603    If (*MARK) is encountered in a positive assertion, its name is recorded and
2604    passed back if it is the last-encountered. This does not happen for negative
2605    assetions.
2606    .P
2607    A name may also be returned after a failed match if the final path through the
2608    pattern involves (*MARK). However, unless (*MARK) used in conjunction with
2609    (*COMMIT), this is unlikely to happen for an unanchored pattern because, as the
2610    starting point for matching is advanced, the final check is often with an empty
2611    string, causing a failure before (*MARK) is reached. For example:
2612    .sp
2613      /X(*MARK:A)Y|X(*MARK:B)Z/K
2614      XP
2615      No match
2616    .sp
2617    There are three potential starting points for this match (starting with X,
2618    starting with P, and with an empty string). If the pattern is anchored, the
2619    result is different:
2620    .sp
2621      /^X(*MARK:A)Y|^X(*MARK:B)Z/K
2622      XP
2623      No match, mark = B
2624    .sp
2625    PCRE's start-of-match optimizations can also interfere with this. For example,
2626    if, as a result of a call to \fBpcre_study()\fP, it knows the minimum
2627    subject length for a match, a shorter subject will not be scanned at all.
2628    .P
2629    Note that similar anomalies (though different in detail) exist in Perl, no
2630    doubt for the same reasons. The use of (*MARK) data after a failed match of an
2631    unanchored pattern is not recommended, unless (*COMMIT) is involved.
2632    .
2633    .
2634    .SS "Verbs that act after backtracking"
2635    .rs
2636    .sp
2637    The following verbs do nothing when they are encountered. Matching continues
2638    with what follows, but if there is no subsequent match, causing a backtrack to
2639    the verb, a failure is forced. That is, backtracking cannot pass to the left of
2640    the verb. However, when one of these verbs appears inside an atomic group, its
2641    effect is confined to that group, because once the group has been matched,
2642    there is never any backtracking into it. In this situation, backtracking can
2643    "jump back" to the left of the entire atomic group. (Remember also, as stated
2644    above, that this localization also applies in subroutine calls and assertions.)
2645    .P
2646    These verbs differ in exactly what kind of failure occurs when backtracking
2647    reaches them.
2648    .sp
2649      (*COMMIT)
2650    .sp
2651    This verb, which may not be followed by a name, causes the whole match to fail
2652    outright if the rest of the pattern does not match. Even if the pattern is
2653    unanchored, no further attempts to find a match by advancing the starting point
2654    take place. Once (*COMMIT) has been passed, \fBpcre_exec()\fP is committed to
2655    finding a match at the current starting point, or not at all. For example:
2656    .sp
2657      a+(*COMMIT)b
2658    .sp
2659    This matches "xxaab" but not "aacaab". It can be thought of as a kind of
2660    dynamic anchor, or "I've started, so I must finish." The name of the most
2661    recently passed (*MARK) in the path is passed back when (*COMMIT) forces a
2662    match failure.
2663    .P
2664    Note that (*COMMIT) at the start of a pattern is not the same as an anchor,
2665    unless PCRE's start-of-match optimizations are turned off, as shown in this
2666    \fBpcretest\fP example:
2667    .sp
2668      /(*COMMIT)abc/
2669      xyzabc
2670       0: abc
2671      xyzabc\eY
2672      No match
2673    .sp
2674    PCRE knows that any match must start with "a", so the optimization skips along
2675    the subject to "a" before running the first match attempt, which succeeds. When
2676    the optimization is disabled by the \eY escape in the second subject, the match
2677    starts at "x" and so the (*COMMIT) causes it to fail without trying any other
2678    starting points.
2679    .sp
2680      (*PRUNE) or (*PRUNE:NAME)
2681    .sp
2682    This verb causes the match to fail at the current starting position in the
2683    subject if the rest of the pattern does not match. If the pattern is
2684    unanchored, the normal "bumpalong" advance to the next starting character then
2685    happens. Backtracking can occur as usual to the left of (*PRUNE), before it is
2686    reached, or when matching to the right of (*PRUNE), but if there is no match to
2687    the right, backtracking cannot cross (*PRUNE). In simple cases, the use of
2688    (*PRUNE) is just an alternative to an atomic group or possessive quantifier,
2689    but there are some uses of (*PRUNE) that cannot be expressed in any other way.
2690    The behaviour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE) when the
2691    match fails completely; the name is passed back if this is the final attempt.
2692    (*PRUNE:NAME) does not pass back a name if the match succeeds. In an anchored
2693    pattern (*PRUNE) has the same effect as (*COMMIT).
2694    .sp
2695      (*SKIP)
2696    .sp
2697    This verb, when given without a name, is like (*PRUNE), except that if the
2698    pattern is unanchored, the "bumpalong" advance is not to the next character,
2699    but to the position in the subject where (*SKIP) was encountered. (*SKIP)
2700    signifies that whatever text was matched leading up to it cannot be part of a
2701    successful match. Consider:
2702    .sp
2703      a+(*SKIP)b
2704    .sp
2705    If the subject is "aaaac...", after the first match attempt fails (starting at
2706    the first character in the string), the starting point skips on to start the
2707    next attempt at "c". Note that a possessive quantifer does not have the same
2708    effect as this example; although it would suppress backtracking during the
2709    first match attempt, the second attempt would start at the second character
2710    instead of skipping on to "c".
2711    .sp
2712      (*SKIP:NAME)
2713    .sp
2714    When (*SKIP) has an associated name, its behaviour is modified. If the
2715    following pattern fails to match, the previous path through the pattern is
2716    searched for the most recent (*MARK) that has the same name. If one is found,
2717    the "bumpalong" advance is to the subject position that corresponds to that
2718    (*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with a
2719    matching name is found, normal "bumpalong" of one character happens (the
2720    (*SKIP) is ignored).
2721    .sp
2722      (*THEN) or (*THEN:NAME)
2723    .sp
2724    This verb causes a skip to the next alternation in the innermost enclosing
2725    group if the rest of the pattern does not match. That is, it cancels pending
2726    backtracking, but only within the current alternation. Its name comes from the
2727    observation that it can be used for a pattern-based if-then-else block:
2728    .sp
2729      ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
2730    .sp
2731    If the COND1 pattern matches, FOO is tried (and possibly further items after
2732    the end of the group if FOO succeeds); on failure the matcher skips to the
2733    second alternative and tries COND2, without backtracking into COND1. The
2734    behaviour of (*THEN:NAME) is exactly the same as (*MARK:NAME)(*THEN) if the
2735    overall match fails. If (*THEN) is not directly inside an alternation, it acts
2736    like (*PRUNE).
2737    .
2738    .P
2739    The above verbs provide four different "strengths" of control when subsequent
2740    matching fails. (*THEN) is the weakest, carrying on the match at the next
2741    alternation. (*PRUNE) comes next, failing the match at the current starting
2742    position, but allowing an advance to the next character (for an unanchored
2743    pattern). (*SKIP) is similar, except that the advance may be more than one
2744    character. (*COMMIT) is the strongest, causing the entire match to fail.
2745    .P
2746    If more than one is present in a pattern, the "stongest" one wins. For example,
2747    consider this pattern, where A, B, etc. are complex pattern fragments:
2748    .sp
2749      (A(*COMMIT)B(*THEN)C|D)
2750    .sp
2751    Once A has matched, PCRE is committed to this match, at the current starting
2752    position. If subsequently B matches, but C does not, the normal (*THEN) action
2753    of trying the next alternation (that is, D) does not happen because (*COMMIT)
2754    overrides.
2755    .
2756    .
2757    .SH "SEE ALSO"
2758    .rs
2759    .sp
2760    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3),
2761    \fBpcresyntax\fP(3), \fBpcre\fP(3).
2762    .
2763    .
2764    .SH AUTHOR
2765    .rs
2766    .sp
2767    .nf
2768    Philip Hazel
2769    University Computing Service
2770    Cambridge CB2 3QH, England.
2771    .fi
2772    .
2773    .
2774    .SH REVISION
2775    .rs
2776    .sp
2777    .nf
2778    Last updated: 24 August 2011
2779    Copyright (c) 1997-2011 University of Cambridge.
2780    .fi

Legend:
Removed from v.73  
changed lines
  Added in v.678

  ViewVC Help
Powered by ViewVC 1.1.5