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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 supported by PCRE are
8  described below. Regular expressions are also described in the Perl  described below. Regular expressions are also described in the Perl
9  documentation and in a number of other books, some of which have copious  documentation and in a number of books, some of which have copious examples.
10  examples. Jeffrey Friedl's "Mastering Regular Expressions", published by  Jeffrey Friedl's "Mastering Regular Expressions", published by O'Reilly, covers
11  O'Reilly, covers them in great detail. The description here is intended as  regular expressions in great detail. This description of PCRE's regular
12  reference documentation.  expressions is intended as reference material.
13    .P
14  The basic operation of PCRE is on strings of bytes. However, there is also  The original operation of PCRE was on strings of one-byte characters. However,
15  support for UTF-8 character strings. To use this support you must build PCRE to  there is now also support for UTF-8 character strings. To use this, you must
16  include UTF-8 support, and then call \fBpcre_compile()\fR with the PCRE_UTF8  build PCRE to include UTF-8 support, and then call \fBpcre_compile()\fP with
17  option. How this affects the pattern matching is mentioned in several places  the PCRE_UTF8 option. How this affects pattern matching is mentioned in several
18  below. There is also a summary of UTF-8 features in the  places below. There is also a summary of UTF-8 features in the
19  .\" HTML <a href="pcre.html#utf8support">  .\" HTML <a href="pcre.html#utf8support">
20  .\" </a>  .\" </a>
21  section on UTF-8 support  section on UTF-8 support
22  .\"  .\"
23  in the main  in the main
24  .\" HREF  .\" HREF
25  \fBpcre\fR  \fBpcre\fP
26  .\"  .\"
27  page.  page.
28    .P
29    The remainder of this document discusses the patterns that are supported by
30    PCRE when its main matching function, \fBpcre_exec()\fP, is used.
31    From release 6.0, PCRE offers a second matching function,
32    \fBpcre_dfa_exec()\fP, which matches using a different algorithm that is not
33    Perl-compatible. The advantages and disadvantages of the alternative function,
34    and how it differs from the normal function, are discussed in the
35    .\" HREF
36    \fBpcrematching\fP
37    .\"
38    page.
39    .
40    .
41    .SH "CHARACTERS AND METACHARACTERS"
42    .rs
43    .sp
44  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
45  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
46  corresponding characters in the subject. As a trivial example, the pattern  corresponding characters in the subject. As a trivial example, the pattern
47    .sp
48    The quick brown fox    The quick brown fox
49    .sp
50  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
51  regular expressions comes from the ability to include alternatives and  caseless matching is specified (the PCRE_CASELESS option), letters are matched
52  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
53  \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
54    always possible. For characters with higher values, the concept of case is
55    supported if PCRE is compiled with Unicode property support, but not otherwise.
56    If you want to use caseless matching for characters 128 and above, you must
57    ensure that PCRE is compiled with Unicode property support as well as with
58    UTF-8 support.
59    .P
60    The power of regular expressions comes from the ability to include alternatives
61    and repetitions in the pattern. These are encoded in the pattern by the use of
62    \fImetacharacters\fP, which do not stand for themselves but instead are
63  interpreted in some special way.  interpreted in some special way.
64    .P
65  There are two different sets of meta-characters: those that are recognized  There are two different sets of metacharacters: those that are recognized
66  anywhere in the pattern except within square brackets, and those that are  anywhere in the pattern except within square brackets, and those that are
67  recognized in square brackets. Outside square brackets, the meta-characters are  recognized within square brackets. Outside square brackets, the metacharacters
68  as follows:  are as follows:
69    .sp
70    \\      general escape character with several uses    \e      general escape character with several uses
71    ^      assert start of string (or line, in multiline mode)    ^      assert start of string (or line, in multiline mode)
72    $      assert end of string (or line, in multiline mode)    $      assert end of string (or line, in multiline mode)
73    .      match any character except newline (by default)    .      match any character except newline (by default)
# Line 58  as follows: Line 82  as follows:
82    +      1 or more quantifier    +      1 or more quantifier
83           also "possessive quantifier"           also "possessive quantifier"
84    {      start min/max quantifier    {      start min/max quantifier
85    .sp
86  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
87  a character class the only meta-characters are:  a character class the only metacharacters are:
88    .sp
89    \\      general escape character    \e      general escape character
90    ^      negate the class, but only if the first character    ^      negate the class, but only if the first character
91    -      indicates character range    -      indicates character range
92    .\" JOIN
93    [      POSIX character class (only if followed by POSIX    [      POSIX character class (only if followed by POSIX
94             syntax)             syntax)
95    ]      terminates the character class    ]      terminates the character class
96    .sp
97  The following sections describe the use of each of the meta-characters.  The following sections describe the use of each of the metacharacters.
98    .
99    .
100  .SH BACKSLASH  .SH BACKSLASH
101  .rs  .rs
102  .sp  .sp
103  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
104  non-alphameric character, it takes away any special meaning that character may  non-alphanumeric character, it takes away any special meaning that character
105  have. This use of backslash as an escape character applies both inside and  may have. This use of backslash as an escape character applies both inside and
106  outside character classes.  outside character classes.
107    .P
108  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.
109  This escaping action applies whether or not the following character would  This escaping action applies whether or not the following character would
110  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
111  non-alphameric with backslash to specify that it stands for itself. In  non-alphanumeric with backslash to specify that it stands for itself. In
112  particular, if you want to match a backslash, you write \\\\.  particular, if you want to match a backslash, you write \e\e.
113    .P
114  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
115  pattern (other than in a character class) and characters between a # outside  pattern (other than in a character class) and characters between a # outside
116  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
117  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.
118  pattern.  .P
   
119  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
120  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
121  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
122  Perl, $ and @ cause variable interpolation. Note the following examples:  Perl, $ and @ cause variable interpolation. Note the following examples:
123    .sp
124    Pattern            PCRE matches   Perl matches    Pattern            PCRE matches   Perl matches
125    .sp
126    \\Qabc$xyz\\E        abc$xyz        abc followed by the  .\" JOIN
127      \eQabc$xyz\eE        abc$xyz        abc followed by the
128                                        contents of $xyz                                        contents of $xyz
129    \\Qabc\\$xyz\\E       abc\\$xyz       abc\\$xyz    \eQabc\e$xyz\eE       abc\e$xyz       abc\e$xyz
130    \\Qabc\\E\\$\\Qxyz\\E   abc$xyz        abc$xyz    \eQabc\eE\e$\eQxyz\eE   abc$xyz        abc$xyz
131    .sp
132  The \\Q...\\E sequence is recognized both inside and outside character classes.  The \eQ...\eE sequence is recognized both inside and outside character classes.
133    .
134    .
135    .\" HTML <a name="digitsafterbackslash"></a>
136    .SS "Non-printing characters"
137    .rs
138    .sp
139  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
140  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
141  non-printing characters, apart from the binary zero that terminates a pattern,  non-printing characters, apart from the binary zero that terminates a pattern,
142  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 usually easier to
143  use one of the following escape sequences than the binary character it  use one of the following escape sequences than the binary character it
144  represents:  represents:
145    .sp
146    \\a        alarm, that is, the BEL character (hex 07)    \ea        alarm, that is, the BEL character (hex 07)
147    \\cx       "control-x", where x is any character    \ecx       "control-x", where x is any character
148    \\e        escape (hex 1B)    \ee        escape (hex 1B)
149    \\f        formfeed (hex 0C)    \ef        formfeed (hex 0C)
150    \\n        newline (hex 0A)    \en        newline (hex 0A)
151    \\r        carriage return (hex 0D)    \er        carriage return (hex 0D)
152    \\t        tab (hex 09)    \et        tab (hex 09)
153    \\ddd      character with octal code ddd, or backreference    \eddd      character with octal code ddd, or backreference
154    \\xhh      character with hex code hh    \exhh      character with hex code hh
155    \\x{hhh..} character with hex code hhh... (UTF-8 mode only)    \ex{hhh..} character with hex code hhh..
156    .sp
157  The precise effect of \\cx is as follows: if x is a lower case letter, it  The precise effect of \ecx is as follows: if x is a lower case letter, it
158  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.
159  Thus \\cz becomes hex 1A, but \\c{ becomes hex 3B, while \\c; becomes hex  Thus \ecz becomes hex 1A, but \ec{ becomes hex 3B, while \ec; becomes hex
160  7B.  7B.
161    .P
162  After \\x, from zero to two hexadecimal digits are read (letters can be in  After \ex, from zero to two hexadecimal digits are read (letters can be in
163  upper or lower case). In UTF-8 mode, any number of hexadecimal digits may  upper or lower case). Any number of hexadecimal digits may appear between \ex{
164  appear between \\x{ and }, but the value of the character code must be less  and }, but the value of the character code must be less than 256 in non-UTF-8
165  than 2**31 (that is, the maximum hexadecimal value is 7FFFFFFF). If characters  mode, and less than 2**31 in UTF-8 mode (that is, the maximum hexadecimal value
166  other than hexadecimal digits appear between \\x{ and }, or if there is no  is 7FFFFFFF). If characters other than hexadecimal digits appear between \ex{
167  terminating }, this form of escape is not recognized. Instead, the initial  and }, or if there is no terminating }, this form of escape is not recognized.
168  \\x will be interpreted as a basic hexadecimal escape, with no following  Instead, the initial \ex will be interpreted as a basic hexadecimal escape,
169  digits, giving a byte whose value is zero.  with no following digits, giving a character whose value is zero.
170    .P
171  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
172  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
173  way they are handled. For example, \\xdc is exactly the same as \\x{dc}.  example, \exdc is exactly the same as \ex{dc}.
174    .P
175  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
176  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
177  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
178  (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
179  character that follows is itself an octal digit.  follows is itself an octal digit.
180    .P
181  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.
182  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
183  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
184  previous capturing left parentheses in the expression, the entire sequence is  previous capturing left parentheses in the expression, the entire sequence is
185  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
186  later, following the discussion of parenthesized subpatterns.  .\" HTML <a href="#backreferences">
187    .\" </a>
188    later,
189    .\"
190    following the discussion of
191    .\" HTML <a href="#subpattern">
192    .\" </a>
193    parenthesized subpatterns.
194    .\"
195    .P
196  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
197  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
198  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
199  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
200  For example:  character specified in octal must be less than \e400. In UTF-8 mode, values up
201    to \e777 are permitted. For example:
202    \\040   is another way of writing a space  .sp
203    \\40    is the same, provided there are fewer than 40    \e040   is another way of writing a space
204    .\" JOIN
205      \e40    is the same, provided there are fewer than 40
206              previous capturing subpatterns              previous capturing subpatterns
207    \\7     is always a back reference    \e7     is always a back reference
208    \\11    might be a back reference, or another way of  .\" JOIN
209      \e11    might be a back reference, or another way of
210              writing a tab              writing a tab
211    \\011   is always a tab    \e011   is always a tab
212    \\0113  is a tab followed by the character "3"    \e0113  is a tab followed by the character "3"
213    \\113   might be a back reference, otherwise the  .\" JOIN
214      \e113   might be a back reference, otherwise the
215              character with octal code 113              character with octal code 113
216    \\377   might be a back reference, otherwise  .\" JOIN
217      \e377   might be a back reference, otherwise
218              the byte consisting entirely of 1 bits              the byte consisting entirely of 1 bits
219    \\81    is either a back reference, or a binary zero  .\" JOIN
220      \e81    is either a back reference, or a binary zero
221              followed by the two characters "8" and "1"              followed by the two characters "8" and "1"
222    .sp
223  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
224  zero, because no more than three octal digits are ever read.  zero, because no more than three octal digits are ever read.
225    .P
226  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
227  (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
228  addition, inside a character class, the sequence \\b is interpreted as the  sequence \eb is interpreted as the backspace character (hex 08), and the
229  backspace character (hex 08). Outside a character class it has a different  sequences \eR and \eX are interpreted as the characters "R" and "X",
230  meaning (see below).  respectively. Outside a character class, these sequences have different
231    meanings
232  The third use of backslash is for specifying generic character types:  .\" HTML <a href="#uniextseq">
233    .\" </a>
234    \\d     any decimal digit  (see below).
235    \\D     any character that is not a decimal digit  .\"
236    \\s     any whitespace character  .
237    \\S     any character that is not a whitespace character  .
238    \\w     any "word" character  .SS "Absolute and relative back references"
239    \\W     any "non-word" character  .rs
240    .sp
241    The sequence \eg followed by a positive or negative number, optionally enclosed
242    in braces, is an absolute or relative back reference. Back references are
243    discussed
244    .\" HTML <a href="#backreferences">
245    .\" </a>
246    later,
247    .\"
248    following the discussion of
249    .\" HTML <a href="#subpattern">
250    .\" </a>
251    parenthesized subpatterns.
252    .\"
253    .
254    .
255    .SS "Generic character types"
256    .rs
257    .sp
258    Another use of backslash is for specifying generic character types. The
259    following are always recognized:
260    .sp
261      \ed     any decimal digit
262      \eD     any character that is not a decimal digit
263      \es     any whitespace character
264      \eS     any character that is not a whitespace character
265      \ew     any "word" character
266      \eW     any "non-word" character
267    .sp
268  Each pair of escape sequences partitions the complete set of characters into  Each pair of escape sequences partitions the complete set of characters into
269  two disjoint sets. Any given character matches one, and only one, of each pair.  two disjoint sets. Any given character matches one, and only one, of each pair.
270    .P
271  In UTF-8 mode, characters with values greater than 255 never match \\d, \\s, or  These character type sequences can appear both inside and outside character
272  \\w, and always match \\D, \\S, and \\W.  classes. They each match one character of the appropriate type. If the current
273    matching point is at the end of the subject string, all of them fail, since
274  For compatibility with Perl, \\s does not match the VT character (code 11).  there is no character to match.
275  This makes it different from the the POSIX "space" class. The \\s characters  .P
276  are HT (9), LF (10), FF (12), CR (13), and space (32).  For compatibility with Perl, \es does not match the VT character (code 11).
277    This makes it different from the the POSIX "space" class. The \es characters
278  A "word" character is any letter or digit or the underscore character, that is,  are HT (9), LF (10), FF (12), CR (13), and space (32). (If "use locale;" is
279  any character which can be part of a Perl "word". The definition of letters and  included in a Perl script, \es may match the VT character. In PCRE, it never
280  digits is controlled by PCRE's character tables, and may vary if locale-  does.)
281  specific matching is taking place (see  .P
282    A "word" character is an underscore or any character less than 256 that is a
283    letter or digit. The definition of letters and digits is controlled by PCRE's
284    low-valued character tables, and may vary if locale-specific matching is taking
285    place (see
286  .\" HTML <a href="pcreapi.html#localesupport">  .\" HTML <a href="pcreapi.html#localesupport">
287  .\" </a>  .\" </a>
288  "Locale support"  "Locale support"
289  .\"  .\"
290  in the  in the
291  .\" HREF  .\" HREF
292  \fBpcreapi\fR  \fBpcreapi\fP
293  .\"  .\"
294  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,
295  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
296    accented letters, and these are matched by \ew.
297  These character type sequences can appear both inside and outside character  .P
298  classes. They each match one character of the appropriate type. If the current  In UTF-8 mode, characters with values greater than 128 never match \ed, \es, or
299  matching point is at the end of the subject string, all of them fail, since  \ew, and always match \eD, \eS, and \eW. This is true even when Unicode
300  there is no character to match.  character property support is available. The use of locales with Unicode is
301    discouraged.
302  The fourth use of backslash is for certain simple assertions. An assertion  .
303    .
304    .SS "Newline sequences"
305    .rs
306    .sp
307    Outside a character class, the escape sequence \eR matches any Unicode newline
308    sequence. This is an extension to Perl. In non-UTF-8 mode \eR is equivalent to
309    the following:
310    .sp
311      (?>\er\en|\en|\ex0b|\ef|\er|\ex85)
312    .sp
313    This is an example of an "atomic group", details of which are given
314    .\" HTML <a href="#atomicgroup">
315    .\" </a>
316    below.
317    .\"
318    This particular group matches either the two-character sequence CR followed by
319    LF, or one of the single characters LF (linefeed, U+000A), VT (vertical tab,
320    U+000B), FF (formfeed, U+000C), CR (carriage return, U+000D), or NEL (next
321    line, U+0085). The two-character sequence is treated as a single unit that
322    cannot be split.
323    .P
324    In UTF-8 mode, two additional characters whose codepoints are greater than 255
325    are added: LS (line separator, U+2028) and PS (paragraph separator, U+2029).
326    Unicode character property support is not needed for these characters to be
327    recognized.
328    .P
329    Inside a character class, \eR matches the letter "R".
330    .
331    .
332    .\" HTML <a name="uniextseq"></a>
333    .SS Unicode character properties
334    .rs
335    .sp
336    When PCRE is built with Unicode character property support, three additional
337    escape sequences to match character properties are available when UTF-8 mode
338    is selected. They are:
339    .sp
340      \ep{\fIxx\fP}   a character with the \fIxx\fP property
341      \eP{\fIxx\fP}   a character without the \fIxx\fP property
342      \eX       an extended Unicode sequence
343    .sp
344    The property names represented by \fIxx\fP above are limited to the Unicode
345    script names, the general category properties, and "Any", which matches any
346    character (including newline). Other properties such as "InMusicalSymbols" are
347    not currently supported by PCRE. Note that \eP{Any} does not match any
348    characters, so always causes a match failure.
349    .P
350    Sets of Unicode characters are defined as belonging to certain scripts. A
351    character from one of these sets can be matched using a script name. For
352    example:
353    .sp
354      \ep{Greek}
355      \eP{Han}
356    .sp
357    Those that are not part of an identified script are lumped together as
358    "Common". The current list of scripts is:
359    .P
360    Arabic,
361    Armenian,
362    Balinese,
363    Bengali,
364    Bopomofo,
365    Braille,
366    Buginese,
367    Buhid,
368    Canadian_Aboriginal,
369    Cherokee,
370    Common,
371    Coptic,
372    Cuneiform,
373    Cypriot,
374    Cyrillic,
375    Deseret,
376    Devanagari,
377    Ethiopic,
378    Georgian,
379    Glagolitic,
380    Gothic,
381    Greek,
382    Gujarati,
383    Gurmukhi,
384    Han,
385    Hangul,
386    Hanunoo,
387    Hebrew,
388    Hiragana,
389    Inherited,
390    Kannada,
391    Katakana,
392    Kharoshthi,
393    Khmer,
394    Lao,
395    Latin,
396    Limbu,
397    Linear_B,
398    Malayalam,
399    Mongolian,
400    Myanmar,
401    New_Tai_Lue,
402    Nko,
403    Ogham,
404    Old_Italic,
405    Old_Persian,
406    Oriya,
407    Osmanya,
408    Phags_Pa,
409    Phoenician,
410    Runic,
411    Shavian,
412    Sinhala,
413    Syloti_Nagri,
414    Syriac,
415    Tagalog,
416    Tagbanwa,
417    Tai_Le,
418    Tamil,
419    Telugu,
420    Thaana,
421    Thai,
422    Tibetan,
423    Tifinagh,
424    Ugaritic,
425    Yi.
426    .P
427    Each character has exactly one general category property, specified by a
428    two-letter abbreviation. For compatibility with Perl, negation can be specified
429    by including a circumflex between the opening brace and the property name. For
430    example, \ep{^Lu} is the same as \eP{Lu}.
431    .P
432    If only one letter is specified with \ep or \eP, it includes all the general
433    category properties that start with that letter. In this case, in the absence
434    of negation, the curly brackets in the escape sequence are optional; these two
435    examples have the same effect:
436    .sp
437      \ep{L}
438      \epL
439    .sp
440    The following general category property codes are supported:
441    .sp
442      C     Other
443      Cc    Control
444      Cf    Format
445      Cn    Unassigned
446      Co    Private use
447      Cs    Surrogate
448    .sp
449      L     Letter
450      Ll    Lower case letter
451      Lm    Modifier letter
452      Lo    Other letter
453      Lt    Title case letter
454      Lu    Upper case letter
455    .sp
456      M     Mark
457      Mc    Spacing mark
458      Me    Enclosing mark
459      Mn    Non-spacing mark
460    .sp
461      N     Number
462      Nd    Decimal number
463      Nl    Letter number
464      No    Other number
465    .sp
466      P     Punctuation
467      Pc    Connector punctuation
468      Pd    Dash punctuation
469      Pe    Close punctuation
470      Pf    Final punctuation
471      Pi    Initial punctuation
472      Po    Other punctuation
473      Ps    Open punctuation
474    .sp
475      S     Symbol
476      Sc    Currency symbol
477      Sk    Modifier symbol
478      Sm    Mathematical symbol
479      So    Other symbol
480    .sp
481      Z     Separator
482      Zl    Line separator
483      Zp    Paragraph separator
484      Zs    Space separator
485    .sp
486    The special property L& is also supported: it matches a character that has
487    the Lu, Ll, or Lt property, in other words, a letter that is not classified as
488    a modifier or "other".
489    .P
490    The long synonyms for these properties that Perl supports (such as \ep{Letter})
491    are not supported by PCRE, nor is it permitted to prefix any of these
492    properties with "Is".
493    .P
494    No character that is in the Unicode table has the Cn (unassigned) property.
495    Instead, this property is assumed for any code point that is not in the
496    Unicode table.
497    .P
498    Specifying caseless matching does not affect these escape sequences. For
499    example, \ep{Lu} always matches only upper case letters.
500    .P
501    The \eX escape matches any number of Unicode characters that form an extended
502    Unicode sequence. \eX is equivalent to
503    .sp
504      (?>\ePM\epM*)
505    .sp
506    That is, it matches a character without the "mark" property, followed by zero
507    or more characters with the "mark" property, and treats the sequence as an
508    atomic group
509    .\" HTML <a href="#atomicgroup">
510    .\" </a>
511    (see below).
512    .\"
513    Characters with the "mark" property are typically accents that affect the
514    preceding character.
515    .P
516    Matching characters by Unicode property is not fast, because PCRE has to search
517    a structure that contains data for over fifteen thousand characters. That is
518    why the traditional escape sequences such as \ed and \ew do not use Unicode
519    properties in PCRE.
520    .
521    .
522    .\" HTML <a name="smallassertions"></a>
523    .SS "Simple assertions"
524    .rs
525    .sp
526    The final use of backslash is for certain simple assertions. An assertion
527  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,
528  without consuming any characters from the subject string. The use of  without consuming any characters from the subject string. The use of
529  subpatterns for more complicated assertions is described below. The backslashed  subpatterns for more complicated assertions is described
530  assertions are  .\" HTML <a href="#bigassertions">
531    .\" </a>
532    \\b     matches at a word boundary  below.
533    \\B     matches when not at a word boundary  .\"
534    \\A     matches at start of subject  The backslashed assertions are:
535    \\Z     matches at end of subject or before newline at end  .sp
536    \\z     matches at end of subject    \eb     matches at a word boundary
537    \\G     matches at first matching position in subject    \eB     matches when not at a word boundary
538      \eA     matches at the start of the subject
539  These assertions may not appear in character classes (but note that \\b has a    \eZ     matches at the end of the subject
540              also matches before a newline at the end of the subject
541      \ez     matches only at the end of the subject
542      \eG     matches at the first matching position in the subject
543    .sp
544    These assertions may not appear in character classes (but note that \eb has a
545  different meaning, namely the backspace character, inside a character class).  different meaning, namely the backspace character, inside a character class).
546    .P
547  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
548  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
549  \\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
550  first or last character matches \\w, respectively.  first or last character matches \ew, respectively.
551    .P
552  The \\A, \\Z, and \\z assertions differ from the traditional circumflex and  The \eA, \eZ, and \ez assertions differ from the traditional circumflex and
553  dollar (described below) in that they only ever match at the very start and end  dollar (described in the next section) in that they only ever match at the very
554  of the subject string, whatever options are set. Thus, they are independent of  start and end of the subject string, whatever options are set. Thus, they are
555  multiline mode.  independent of multiline mode. These three assertions are not affected by the
556    PCRE_NOTBOL or PCRE_NOTEOL options, which affect only the behaviour of the
557  They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. If the  circumflex and dollar metacharacters. However, if the \fIstartoffset\fP
558  \fIstartoffset\fR argument of \fBpcre_exec()\fR is non-zero, indicating that  argument of \fBpcre_exec()\fP is non-zero, indicating that matching is to start
559  matching is to start at a point other than the beginning of the subject, \\A  at a point other than the beginning of the subject, \eA can never match. The
560  can never match. The difference between \\Z and \\z is that \\Z matches before  difference between \eZ and \ez is that \eZ matches before a newline at the end
561  a newline that is the last character of the string as well as at the end of the  of the string as well as at the very end, whereas \ez matches only at the end.
562  string, whereas \\z matches only at the end.  .P
563    The \eG assertion is true only when the current matching position is at the
564  The \\G assertion is true only when the current matching position is at the  start point of the match, as specified by the \fIstartoffset\fP argument of
565  start point of the match, as specified by the \fIstartoffset\fR argument of  \fBpcre_exec()\fP. It differs from \eA when the value of \fIstartoffset\fP is
566  \fBpcre_exec()\fR. It differs from \\A when the value of \fIstartoffset\fR is  non-zero. By calling \fBpcre_exec()\fP multiple times with appropriate
 non-zero. By calling \fBpcre_exec()\fR multiple times with appropriate  
567  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
568  implementation where \\G can be useful.  implementation where \eG can be useful.
569    .P
570  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
571  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
572  previous match. In Perl, these can be different when the previously matched  previous match. In Perl, these can be different when the previously matched
573  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
574  reproduce this behaviour.  reproduce this behaviour.
575    .P
576  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
577  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
578  regular expression.  regular expression.
579    .
580  .SH CIRCUMFLEX AND DOLLAR  .
581    .SH "CIRCUMFLEX AND DOLLAR"
582  .rs  .rs
583  .sp  .sp
584  Outside a character class, in the default matching mode, the circumflex  Outside a character class, in the default matching mode, the circumflex
585  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
586  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
587  \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
588  option is unset. Inside a character class, circumflex has an entirely different  option is unset. Inside a character class, circumflex has an entirely different
589  meaning (see below).  meaning
590    .\" HTML <a href="#characterclass">
591    .\" </a>
592    (see below).
593    .\"
594    .P
595  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
596  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
597  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 599  possible alternatives start with a circu
599  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
600  "anchored" pattern. (There are also other constructs that can cause a pattern  "anchored" pattern. (There are also other constructs that can cause a pattern
601  to be anchored.)  to be anchored.)
602    .P
603  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
604  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
605  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
606  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
607  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
608  Dollar has no special meaning in a character class.  character class.
609    .P
610  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
611  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
612  does not affect the \\Z assertion.  does not affect the \eZ assertion.
613    .P
614  The meanings of the circumflex and dollar characters are changed if the  The meanings of the circumflex and dollar characters are changed if the
615  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
616  after and immediately before an internal newline character, respectively, in  immediately after internal newlines as well as at the start of the subject
617  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
618  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
619  but not otherwise. Consequently, patterns that are anchored in single line mode  PCRE_MULTILINE is set. When newline is specified as the two-character
620  because all branches start with ^ are not anchored in multiline mode, and a  sequence CRLF, isolated CR and LF characters do not indicate newlines.
621  match for circumflex is possible when the \fIstartoffset\fR argument of  .P
622  \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
623  PCRE_MULTILINE is set.  \en represents a newline) in multiline mode, but not otherwise. Consequently,
624    patterns that are anchored in single line mode because all branches start with
625  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
626    when the \fIstartoffset\fP argument of \fBpcre_exec()\fP is non-zero. The
627    PCRE_DOLLAR_ENDONLY option is ignored if PCRE_MULTILINE is set.
628    .P
629    Note that the sequences \eA, \eZ, and \ez can be used to match the start and
630  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
631  \\A it is always anchored, whether PCRE_MULTILINE is set or not.  \eA it is always anchored, whether or not PCRE_MULTILINE is set.
632    .
633  .SH FULL STOP (PERIOD, DOT)  .
634    .SH "FULL STOP (PERIOD, DOT)"
635  .rs  .rs
636  .sp  .sp
637  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
638  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
639  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.
640  byte long, except (by default) for newline. If the PCRE_DOTALL option is set,  .P
641  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
642  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
643  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
644    (including isolated CRs and LFs). When any Unicode line endings are being
645  .SH MATCHING A SINGLE BYTE  recognized, dot does not match CR or LF or any of the other line ending
646  .rs  characters.
647  .sp  .P
648  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
649  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
650  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
651  Because it breaks up UTF-8 characters into individual bytes, what remains in  to match it.
652  the string may be a malformed UTF-8 string. For this reason it is best avoided.  .P
653    The handling of dot is entirely independent of the handling of circumflex and
654  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
655  in UTF-8 mode it makes it impossible to calculate the length of the lookbehind.  special meaning in a character class.
656    .
657  .SH SQUARE BRACKETS  .
658    .SH "MATCHING A SINGLE BYTE"
659    .rs
660    .sp
661    Outside a character class, the escape sequence \eC matches any one byte, both
662    in and out of UTF-8 mode. Unlike a dot, it always matches any line-ending
663    characters. The feature is provided in Perl in order to match individual bytes
664    in UTF-8 mode. Because it breaks up UTF-8 characters into individual bytes,
665    what remains in the string may be a malformed UTF-8 string. For this reason,
666    the \eC escape sequence is best avoided.
667    .P
668    PCRE does not allow \eC to appear in lookbehind assertions
669    .\" HTML <a href="#lookbehind">
670    .\" </a>
671    (described below),
672    .\"
673    because in UTF-8 mode this would make it impossible to calculate the length of
674    the lookbehind.
675    .
676    .
677    .\" HTML <a name="characterclass"></a>
678    .SH "SQUARE BRACKETS AND CHARACTER CLASSES"
679  .rs  .rs
680  .sp  .sp
681  An opening square bracket introduces a character class, terminated by a closing  An opening square bracket introduces a character class, terminated by a closing
# Line 348  square bracket. A closing square bracket Line 683  square bracket. A closing square bracket
683  closing square bracket is required as a member of the class, it should be the  closing square bracket is required as a member of the class, it should be the
684  first data character in the class (after an initial circumflex, if present) or  first data character in the class (after an initial circumflex, if present) or
685  escaped with a backslash.  escaped with a backslash.
686    .P
687  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
688  character may occupy more than one byte. A matched character must be in the set  character may occupy more than one byte. A matched character must be in the set
689  of characters defined by the class, unless the first character in the class  of characters defined by the class, unless the first character in the class
# Line 356  definition is a circumflex, in which cas Line 691  definition is a circumflex, in which cas
691  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
692  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
693  backslash.  backslash.
694    .P
695  For example, the character class [aeiou] matches any lower case vowel, while  For example, the character class [aeiou] matches any lower case vowel, while
696  [^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
697  circumflex is just a convenient notation for specifying the characters which  circumflex is just a convenient notation for specifying the characters that
698  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
699  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
700  pointer is at the end of the string.  string, and therefore it fails if the current pointer is at the end of the
701    string.
702    .P
703  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
704  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.
705    .P
706  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
707  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
708  "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
709  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
710  with values greater than 255.  case for characters whose values are less than 128, so caseless matching is
711    always possible. For characters with higher values, the concept of case is
712  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.
713  whatever the setting of the PCRE_DOTALL or PCRE_MULTILINE options is. A class  If you want to use caseless matching for characters 128 and above, you must
714  such as [^a] will always match a newline.  ensure that PCRE is compiled with Unicode property support as well as with
715    UTF-8 support.
716    .P
717    Characters that might indicate line breaks are never treated in any special way
718    when matching character classes, whatever line-ending sequence is in use, and
719    whatever setting of the PCRE_DOTALL and PCRE_MULTILINE options is used. A class
720    such as [^a] always matches one of these characters.
721    .P
722  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
723  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,
724  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
725  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
726  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.
727    .P
728  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
729  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
730  ("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
731  "-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
732  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
733  range followed by two separate characters. The octal or hexadecimal  followed by two other characters. The octal or hexadecimal representation of
734  representation of "]" can also be used to end a range.  "]" can also be used to end a range.
735    .P
736  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
737  used for characters specified numerically, for example [\\000-\\037]. In UTF-8  used for characters specified numerically, for example [\e000-\e037]. In UTF-8
738  mode, ranges can include characters whose values are greater than 255, for  mode, ranges can include characters whose values are greater than 255, for
739  example [\\x{100}-\\x{2ff}].  example [\ex{100}-\ex{2ff}].
740    .P
741  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
742  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
743  [][\\^_`wxyzabc], matched caselessly, and if character tables for the "fr"  [][\e\e^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if character
744  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
745    characters in both cases. In UTF-8 mode, PCRE supports the concept of case for
746  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
747  character class, and add the characters that they match to the class. For  property support.
748  example, [\\dABCDEF] matches any hexadecimal digit. A circumflex can  .P
749    The character types \ed, \eD, \ep, \eP, \es, \eS, \ew, and \eW may also appear
750    in a character class, and add the characters that they match to the class. For
751    example, [\edABCDEF] matches any hexadecimal digit. A circumflex can
752  conveniently be used with the upper case character types to specify a more  conveniently be used with the upper case character types to specify a more
753  restricted set of characters than the matching lower case type. For example,  restricted set of characters than the matching lower case type. For example,
754  the class [^\\W_] matches any letter or digit, but not underscore.  the class [^\eW_] matches any letter or digit, but not underscore.
755    .P
756  All non-alphameric characters other than \\, -, ^ (at the start) and the  The only metacharacters that are recognized in character classes are backslash,
757  terminating ] are non-special in character classes, but it does no harm if they  hyphen (only where it can be interpreted as specifying a range), circumflex
758  are escaped.  (only at the start), opening square bracket (only when it can be interpreted as
759    introducing a POSIX class name - see the next section), and the terminating
760  .SH POSIX CHARACTER CLASSES  closing square bracket. However, escaping other non-alphanumeric characters
761    does no harm.
762    .
763    .
764    .SH "POSIX CHARACTER CLASSES"
765  .rs  .rs
766  .sp  .sp
767  Perl supports the POSIX notation for character classes, which uses names  Perl supports the POSIX notation for character classes. This uses names
768  enclosed by [: and :] within the enclosing square brackets. PCRE also supports  enclosed by [: and :] within the enclosing square brackets. PCRE also supports
769  this notation. For example,  this notation. For example,
770    .sp
771    [01[:alpha:]%]    [01[:alpha:]%]
772    .sp
773  matches "0", "1", any alphabetic character, or "%". The supported class names  matches "0", "1", any alphabetic character, or "%". The supported class names
774  are  are
775    .sp
776    alnum    letters and digits    alnum    letters and digits
777    alpha    letters    alpha    letters
778    ascii    character codes 0 - 127    ascii    character codes 0 - 127
779    blank    space or tab only    blank    space or tab only
780    cntrl    control characters    cntrl    control characters
781    digit    decimal digits (same as \\d)    digit    decimal digits (same as \ed)
782    graph    printing characters, excluding space    graph    printing characters, excluding space
783    lower    lower case letters    lower    lower case letters
784    print    printing characters, including space    print    printing characters, including space
785    punct    printing characters, excluding letters and digits    punct    printing characters, excluding letters and digits
786    space    white space (not quite the same as \\s)    space    white space (not quite the same as \es)
787    upper    upper case letters    upper    upper case letters
788    word     "word" characters (same as \\w)    word     "word" characters (same as \ew)
789    xdigit   hexadecimal digits    xdigit   hexadecimal digits
790    .sp
791  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
792  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
793  makes "space" different to \\s, which does not include VT (for Perl  makes "space" different to \es, which does not include VT (for Perl
794  compatibility).  compatibility).
795    .P
796  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
797  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
798  after the colon. For example,  after the colon. For example,
799    .sp
800    [12[:^digit:]]    [12[:^digit:]]
801    .sp
802  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
803  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
804  supported, and an error is given if they are encountered.  supported, and an error is given if they are encountered.
805    .P
806  In UTF-8 mode, characters with values greater than 255 do not match any of  In UTF-8 mode, characters with values greater than 128 do not match any of
807  the POSIX character classes.  the POSIX character classes.
808    .
809  .SH VERTICAL BAR  .
810    .SH "VERTICAL BAR"
811  .rs  .rs
812  .sp  .sp
813  Vertical bar characters are used to separate alternative patterns. For example,  Vertical bar characters are used to separate alternative patterns. For example,
814  the pattern  the pattern
815    .sp
816    gilbert|sullivan    gilbert|sullivan
817    .sp
818  matches either "gilbert" or "sullivan". Any number of alternatives may appear,  matches either "gilbert" or "sullivan". Any number of alternatives may appear,
819  and an empty alternative is permitted (matching the empty string).  and an empty alternative is permitted (matching the empty string). The matching
820  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
821  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
822  subpattern (defined below), "succeeds" means matching the rest of the main  .\" HTML <a href="#subpattern">
823  pattern as well as the alternative in the subpattern.  .\" </a>
824    (defined below),
825  .SH INTERNAL OPTION SETTING  .\"
826    "succeeds" means matching the rest of the main pattern as well as the
827    alternative in the subpattern.
828    .
829    .
830    .SH "INTERNAL OPTION SETTING"
831  .rs  .rs
832  .sp  .sp
833  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and  The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
834  PCRE_EXTENDED options can be changed from within the pattern by a sequence of  PCRE_EXTENDED options can be changed from within the pattern by a sequence of
835  Perl option letters enclosed between "(?" and ")". The option letters are  Perl option letters enclosed between "(?" and ")". The option letters are
836    .sp
837    i  for PCRE_CASELESS    i  for PCRE_CASELESS
838    m  for PCRE_MULTILINE    m  for PCRE_MULTILINE
839    s  for PCRE_DOTALL    s  for PCRE_DOTALL
840    x  for PCRE_EXTENDED    x  for PCRE_EXTENDED
841    .sp
842  For example, (?im) sets caseless, multiline matching. It is also possible to  For example, (?im) sets caseless, multiline matching. It is also possible to
843  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
844  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and  setting and unsetting such as (?im-sx), which sets PCRE_CASELESS and
845  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also  PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED, is also
846  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
847  unset.  unset.
848    .P
849  When an option change occurs at top level (that is, not inside subpattern  When an option change occurs at top level (that is, not inside subpattern
850  parentheses), the change applies to the remainder of the pattern that follows.  parentheses), the change applies to the remainder of the pattern that follows.
851  If the change is placed right at the start of a pattern, PCRE extracts it into  If the change is placed right at the start of a pattern, PCRE extracts it into
852  the global options (and it will therefore show up in data extracted by the  the global options (and it will therefore show up in data extracted by the
853  \fBpcre_fullinfo()\fR function).  \fBpcre_fullinfo()\fP function).
854    .P
855  An option change within a subpattern affects only that part of the current  An option change within a subpattern (see below for a description of
856  pattern that follows it, so  subpatterns) affects only that part of the current pattern that follows it, so
857    .sp
858    (a(?i)b)c    (a(?i)b)c
859    .sp
860  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).
861  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
862  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
863  into subsequent branches within the same subpattern. For example,  into subsequent branches within the same subpattern. For example,
864    .sp
865    (a(?i)b|c)    (a(?i)b|c)
866    .sp
867  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
868  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
869  option settings happen at compile time. There would be some very weird  option settings happen at compile time. There would be some very weird
870  behaviour otherwise.  behaviour otherwise.
871    .P
872  The PCRE-specific options PCRE_UNGREEDY and PCRE_EXTRA can be changed in the  The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA can be
873  same way as the Perl-compatible options by using the characters U and X  changed in the same way as the Perl-compatible options by using the characters
874  respectively. The (?X) flag setting is special in that it must always occur  J, U and X respectively.
875  earlier in the pattern than any of the additional features it turns on, even  .
876  when it is at top level. It is best put at the start.  .
877    .\" HTML <a name="subpattern"></a>
878  .SH SUBPATTERNS  .SH SUBPATTERNS
879  .rs  .rs
880  .sp  .sp
881  Subpatterns are delimited by parentheses (round brackets), which can be nested.  Subpatterns are delimited by parentheses (round brackets), which can be nested.
882  Marking part of a pattern as a subpattern does two things:  Turning part of a pattern into a subpattern does two things:
883    .sp
884  1. It localizes a set of alternatives. For example, the pattern  1. It localizes a set of alternatives. For example, the pattern
885    .sp
886    cat(aract|erpillar|)    cat(aract|erpillar|)
887    .sp
888  matches one of the words "cat", "cataract", or "caterpillar". Without the  matches one of the words "cat", "cataract", or "caterpillar". Without the
889  parentheses, it would match "cataract", "erpillar" or the empty string.  parentheses, it would match "cataract", "erpillar" or an empty string.
890    .sp
891  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
892  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
893  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
894  \fBpcre_exec()\fR. Opening parentheses are counted from left to right (starting  \fBpcre_exec()\fP. Opening parentheses are counted from left to right (starting
895  from 1) to obtain the numbers of the capturing subpatterns.  from 1) to obtain numbers for the capturing subpatterns.
896    .P
897  For example, if the string "the red king" is matched against the pattern  For example, if the string "the red king" is matched against the pattern
898    .sp
899    the ((red|white) (king|queen))    the ((red|white) (king|queen))
900    .sp
901  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,
902  2, and 3, respectively.  2, and 3, respectively.
903    .P
904  The fact that plain parentheses fulfil two functions is not always helpful.  The fact that plain parentheses fulfil two functions is not always helpful.
905  There are often times when a grouping subpattern is required without a  There are often times when a grouping subpattern is required without a
906  capturing requirement. If an opening parenthesis is followed by a question mark  capturing requirement. If an opening parenthesis is followed by a question mark
907  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
908  computing the number of any subsequent capturing subpatterns. For example, if  computing the number of any subsequent capturing subpatterns. For example, if
909  the string "the white queen" is matched against the pattern  the string "the white queen" is matched against the pattern
910    .sp
911    the ((?:red|white) (king|queen))    the ((?:red|white) (king|queen))
912    .sp
913  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
914  2. The maximum number of capturing subpatterns is 65535, and the maximum depth  2. The maximum number of capturing subpatterns is 65535.
915  of nesting of all subpatterns, both capturing and non-capturing, is 200.  .P
   
916  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
917  a non-capturing subpattern, the option letters may appear between the "?" and  a non-capturing subpattern, the option letters may appear between the "?" and
918  the ":". Thus the two patterns  the ":". Thus the two patterns
919    .sp
920    (?i:saturday|sunday)    (?i:saturday|sunday)
921    (?:(?i)saturday|sunday)    (?:(?i)saturday|sunday)
922    .sp
923  match exactly the same set of strings. Because alternative branches are tried  match exactly the same set of strings. Because alternative branches are tried
924  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
925  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
926  the above patterns match "SUNDAY" as well as "Saturday".  the above patterns match "SUNDAY" as well as "Saturday".
927    .
928  .SH NAMED SUBPATTERNS  .
929    .SH "NAMED SUBPATTERNS"
930  .rs  .rs
931  .sp  .sp
932  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
933  to keep track of the numbers in complicated regular expressions. Furthermore,  to keep track of the numbers in complicated regular expressions. Furthermore,
934  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
935  difficulty, PCRE supports the naming of subpatterns, something that Perl does  difficulty, PCRE supports the naming of subpatterns. This feature was not
936  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
937  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
938    the Perl and the Python syntax.
939  Named capturing parentheses are still allocated numbers as well as names. The  .P
940  PCRE API provides function calls for extracting the name-to-number translation  In PCRE, a subpattern can be named in one of three ways: (?<name>...) or
941  table from a compiled pattern. For further details see the  (?'name'...) as in Perl, or (?P<name>...) as in Python. References to capturing
942    parentheses from other parts of the pattern, such as
943    .\" HTML <a href="#backreferences">
944    .\" </a>
945    backreferences,
946    .\"
947    .\" HTML <a href="#recursion">
948    .\" </a>
949    recursion,
950    .\"
951    and
952    .\" HTML <a href="#conditions">
953    .\" </a>
954    conditions,
955    .\"
956    can be made by name as well as by number.
957    .P
958    Names consist of up to 32 alphanumeric characters and underscores. Named
959    capturing parentheses are still allocated numbers as well as names, exactly as
960    if the names were not present. The PCRE API provides function calls for
961    extracting the name-to-number translation table from a compiled pattern. There
962    is also a convenience function for extracting a captured substring by name.
963    .P
964    By default, a name must be unique within a pattern, but it is possible to relax
965    this constraint by setting the PCRE_DUPNAMES option at compile time. This can
966    be useful for patterns where only one instance of the named parentheses can
967    match. Suppose you want to match the name of a weekday, either as a 3-letter
968    abbreviation or as the full name, and in both cases you want to extract the
969    abbreviation. This pattern (ignoring the line breaks) does the job:
970    .sp
971      (?<DN>Mon|Fri|Sun)(?:day)?|
972      (?<DN>Tue)(?:sday)?|
973      (?<DN>Wed)(?:nesday)?|
974      (?<DN>Thu)(?:rsday)?|
975      (?<DN>Sat)(?:urday)?
976    .sp
977    There are five capturing substrings, but only one is ever set after a match.
978    The convenience function for extracting the data by name returns the substring
979    for the first (and in this example, the only) subpattern of that name that
980    matched. This saves searching to find which numbered subpattern it was. If you
981    make a reference to a non-unique named subpattern from elsewhere in the
982    pattern, the one that corresponds to the lowest number is used. For further
983    details of the interfaces for handling named subpatterns, see the
984  .\" HREF  .\" HREF
985  \fBpcreapi\fR  \fBpcreapi\fP
986  .\"  .\"
987  documentation.  documentation.
988    .
989    .
990  .SH REPETITION  .SH REPETITION
991  .rs  .rs
992  .sp  .sp
993  Repetition is specified by quantifiers, which can follow any of the following  Repetition is specified by quantifiers, which can follow any of the following
994  items:  items:
995    .sp
996    a literal data character    a literal data character
997    the . metacharacter    the dot metacharacter
998    the \\C escape sequence    the \eC escape sequence
999    escapes such as \\d that match single characters    the \eX escape sequence (in UTF-8 mode with Unicode properties)
1000      the \eR escape sequence
1001      an escape such as \ed that matches a single character
1002    a character class    a character class
1003    a back reference (see next section)    a back reference (see next section)
1004    a parenthesized subpattern (unless it is an assertion)    a parenthesized subpattern (unless it is an assertion)
1005    .sp
1006  The general repetition quantifier specifies a minimum and maximum number of  The general repetition quantifier specifies a minimum and maximum number of
1007  permitted matches, by giving the two numbers in curly brackets (braces),  permitted matches, by giving the two numbers in curly brackets (braces),
1008  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
1009  be less than or equal to the second. For example:  be less than or equal to the second. For example:
1010    .sp
1011    z{2,4}    z{2,4}
1012    .sp
1013  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
1014  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
1015  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
1016  quantifier specifies an exact number of required matches. Thus  quantifier specifies an exact number of required matches. Thus
1017    .sp
1018    [aeiou]{3,}    [aeiou]{3,}
1019    .sp
1020  matches at least 3 successive vowels, but may match many more, while  matches at least 3 successive vowels, but may match many more, while
1021    .sp
1022    \\d{8}    \ed{8}
1023    .sp
1024  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
1025  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
1026  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
1027  quantifier, but a literal string of four characters.  quantifier, but a literal string of four characters.
1028    .P
1029  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
1030  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
1031  which is represented by a two-byte sequence.  which is represented by a two-byte sequence. Similarly, when Unicode property
1032    support is available, \eX{3} matches three Unicode extended sequences, each of
1033    which may be several bytes long (and they may be of different lengths).
1034    .P
1035  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
1036  previous item and the quantifier were not present.  previous item and the quantifier were not present.
1037    .P
1038  For convenience (and historical compatibility) the three most common  For convenience, the three most common quantifiers have single-character
1039  quantifiers have single-character abbreviations:  abbreviations:
1040    .sp
1041    *    is equivalent to {0,}    *    is equivalent to {0,}
1042    +    is equivalent to {1,}    +    is equivalent to {1,}
1043    ?    is equivalent to {0,1}    ?    is equivalent to {0,1}
1044    .sp
1045  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
1046  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:
1047    .sp
1048    (a?)*    (a?)*
1049    .sp
1050  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
1051  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
1052  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
1053  match no characters, the loop is forcibly broken.  match no characters, the loop is forcibly broken.
1054    .P
1055  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
1056  possible (up to the maximum number of permitted times), without causing the  possible (up to the maximum number of permitted times), without causing the
1057  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
1058  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 */
1059  sequences /* and */ and within the sequence, individual * and / characters may  and within the comment, individual * and / characters may appear. An attempt to
1060  appear. An attempt to match C comments by applying the pattern  match C comments by applying the pattern
1061    .sp
1062    /\\*.*\\*/    /\e*.*\e*/
1063    .sp
1064  to the string  to the string
1065    .sp
1066    /* first command */  not comment  /* second comment */    /* first comment */  not comment  /* second comment */
1067    .sp
1068  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 .*
1069  item.  item.
1070    .P
1071  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
1072  greedy, and instead matches the minimum number of times possible, so the  greedy, and instead matches the minimum number of times possible, so the
1073  pattern  pattern
1074    .sp
1075    /\\*.*?\\*/    /\e*.*?\e*/
1076    .sp
1077  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
1078  quantifiers is not otherwise changed, just the preferred number of matches.  quantifiers is not otherwise changed, just the preferred number of matches.
1079  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
1080  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
1081    .sp
1082    \\d??\\d    \ed??\ed
1083    .sp
1084  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
1085  way the rest of the pattern matches.  way the rest of the pattern matches.
1086    .P
1087  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),
1088  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
1089  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
1090  default behaviour.  default behaviour.
1091    .P
1092  When a parenthesized subpattern is quantified with a minimum repeat count that  When a parenthesized subpattern is quantified with a minimum repeat count that
1093  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
1094  compiled pattern, in proportion to the size of the minimum or maximum.  compiled pattern, in proportion to the size of the minimum or maximum.
1095    .P
1096  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
1097  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
1098  implicitly anchored, because whatever follows will be tried against every  implicitly anchored, because whatever follows will be tried against every
1099  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
1100  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
1101  pattern as though it were preceded by \\A.  pattern as though it were preceded by \eA.
1102    .P
1103  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
1104  worth setting PCRE_DOTALL in order to obtain this optimization, or  worth setting PCRE_DOTALL in order to obtain this optimization, or
1105  alternatively using ^ to indicate anchoring explicitly.  alternatively using ^ to indicate anchoring explicitly.
1106    .P
1107  However, there is one situation where the optimization cannot be used. When .*  However, there is one situation where the optimization cannot be used. When .*
1108  is inside capturing parentheses that are the subject of a backreference  is inside capturing parentheses that are the subject of a backreference
1109  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
1110  succeed. Consider, for example:  succeeds. Consider, for example:
1111    .sp
1112    (.*)abc\\1    (.*)abc\e1
1113    .sp
1114  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
1115  this reason, such a pattern is not implicitly anchored.  this reason, such a pattern is not implicitly anchored.
1116    .P
1117  When a capturing subpattern is repeated, the value captured is the substring  When a capturing subpattern is repeated, the value captured is the substring
1118  that matched the final iteration. For example, after  that matched the final iteration. For example, after
1119    .sp
1120    (tweedle[dume]{3}\\s*)+    (tweedle[dume]{3}\es*)+
1121    .sp
1122  has matched "tweedledum tweedledee" the value of the captured substring is  has matched "tweedledum tweedledee" the value of the captured substring is
1123  "tweedledee". However, if there are nested capturing subpatterns, the  "tweedledee". However, if there are nested capturing subpatterns, the
1124  corresponding captured values may have been set in previous iterations. For  corresponding captured values may have been set in previous iterations. For
1125  example, after  example, after
1126    .sp
1127    /(a|(b))+/    /(a|(b))+/
1128    .sp
1129  matches "aba" the value of the second captured substring is "b".  matches "aba" the value of the second captured substring is "b".
1130    .
1131  .SH ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  .
1132    .\" HTML <a name="atomicgroup"></a>
1133    .SH "ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS"
1134  .rs  .rs
1135  .sp  .sp
1136  With both maximizing and minimizing repetition, failure of what follows  With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
1137  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
1138  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
1139  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
1140  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
1141  there is no point in carrying on.  the author of the pattern knows there is no point in carrying on.
1142    .P
1143  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
1144    .sp
1145    123456bar    123456bar
1146    .sp
1147  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
1148  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+
1149  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"
1150  (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
1151  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.
1152    .P
1153  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
1154  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
1155  special parenthesis, starting with (?> as in this example:  special parenthesis, starting with (?> as in this example:
1156    .sp
1157    (?>\\d+)bar    (?>\ed+)foo
1158    .sp
1159  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
1160  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
1161  backtracking into it. Backtracking past it to previous items, however, works as  backtracking into it. Backtracking past it to previous items, however, works as
1162  normal.  normal.
1163    .P
1164  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
1165  of characters that an identical standalone pattern would match, if anchored at  of characters that an identical standalone pattern would match, if anchored at
1166  the current point in the subject string.  the current point in the subject string.
1167    .P
1168  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as  Atomic grouping subpatterns are not capturing subpatterns. Simple cases such as
1169  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
1170  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
1171  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,
1172  (?>\\d+) can only match an entire sequence of digits.  (?>\ed+) can only match an entire sequence of digits.
1173    .P
1174  Atomic groups in general can of course contain arbitrarily complicated  Atomic groups in general can of course contain arbitrarily complicated
1175  subpatterns, and can be nested. However, when the subpattern for an atomic  subpatterns, and can be nested. However, when the subpattern for an atomic
1176  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
1177  notation, called a "possessive quantifier" can be used. This consists of an  notation, called a "possessive quantifier" can be used. This consists of an
1178  additional + character following a quantifier. Using this notation, the  additional + character following a quantifier. Using this notation, the
1179  previous example can be rewritten as  previous example can be rewritten as
1180    .sp
1181    \\d++bar    \ed++foo
1182    .sp
1183  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY  Possessive quantifiers are always greedy; the setting of the PCRE_UNGREEDY
1184  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
1185  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
1186  possessive quantifier and the equivalent atomic group.  quantifier and the equivalent atomic group, though there may be a performance
1187    difference; possessive quantifiers should be slightly faster.
1188  The possessive quantifier syntax is an extension to the Perl syntax. It  .P
1189  originates in Sun's Java package.  The possessive quantifier syntax is an extension to the Perl 5.8 syntax.
1190    Jeffrey Friedl originated the idea (and the name) in the first edition of his
1191    book. Mike McCloskey liked it, so implemented it when he built Sun's Java
1192    package, and PCRE copied it from there. It ultimately found its way into Perl
1193    at release 5.10.
1194    .P
1195    PCRE has an optimization that automatically "possessifies" certain simple
1196    pattern constructs. For example, the sequence A+B is treated as A++B because
1197    there is no point in backtracking into a sequence of A's when B must follow.
1198    .P
1199  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
1200  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
1201  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
1202  pattern  pattern
1203    .sp
1204    (\\D+|<\\d+>)*[!?]    (\eD+|<\ed+>)*[!?]
1205    .sp
1206  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
1207  digits enclosed in <>, followed by either ! or ?. When it matches, it runs  digits enclosed in <>, followed by either ! or ?. When it matches, it runs
1208  quickly. However, if it is applied to  quickly. However, if it is applied to
1209    .sp
1210    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
1211    .sp
1212  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
1213  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
1214  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
1215  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
1216  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
1217  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
1218  If the pattern is changed to  if it is not present in the string.) If the pattern is changed so that it uses
1219    an atomic group, like this:
1220    ((?>\\D+)|<\\d+>)*[!?]  .sp
1221      ((?>\eD+)|<\ed+>)*[!?]
1222    .sp
1223  sequences of non-digits cannot be broken, and failure happens quickly.  sequences of non-digits cannot be broken, and failure happens quickly.
1224    .
1225  .SH BACK REFERENCES  .
1226    .\" HTML <a name="backreferences"></a>
1227    .SH "BACK REFERENCES"
1228  .rs  .rs
1229  .sp  .sp
1230  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
1231  possibly further digits) is a back reference to a capturing subpattern earlier  possibly further digits) is a back reference to a capturing subpattern earlier
1232  (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
1233  previous capturing left parentheses.  previous capturing left parentheses.
1234    .P
1235  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
1236  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
1237  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
1238  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
1239  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
1240  details of the handling of digits following a backslash.  when a repetition is involved and the subpattern to the right has participated
1241    in an earlier iteration.
1242    .P
1243    It is not possible to have a numerical "forward back reference" to a subpattern
1244    whose number is 10 or more using this syntax because a sequence such as \e50 is
1245    interpreted as a character defined in octal. See the subsection entitled
1246    "Non-printing characters"
1247    .\" HTML <a href="#digitsafterbackslash">
1248    .\" </a>
1249    above
1250    .\"
1251    for further details of the handling of digits following a backslash. There is
1252    no such problem when named parentheses are used. A back reference to any
1253    subpattern is possible using named parentheses (see below).
1254    .P
1255    Another way of avoiding the ambiguity inherent in the use of digits following a
1256    backslash is to use the \eg escape sequence, which is a feature introduced in
1257    Perl 5.10. This escape must be followed by a positive or a negative number,
1258    optionally enclosed in braces. These examples are all identical:
1259    .sp
1260      (ring), \e1
1261      (ring), \eg1
1262      (ring), \eg{1}
1263    .sp
1264    A positive number specifies an absolute reference without the ambiguity that is
1265    present in the older syntax. It is also useful when literal digits follow the
1266    reference. A negative number is a relative reference. Consider this example:
1267    .sp
1268      (abc(def)ghi)\eg{-1}
1269    .sp
1270    The sequence \eg{-1} is a reference to the most recently started capturing
1271    subpattern before \eg, that is, is it equivalent to \e2. Similarly, \eg{-2}
1272    would be equivalent to \e1. The use of relative references can be helpful in
1273    long patterns, and also in patterns that are created by joining together
1274    fragments that contain references within themselves.
1275    .P
1276  A back reference matches whatever actually matched the capturing subpattern in  A back reference matches whatever actually matched the capturing subpattern in
1277  the current subject string, rather than anything matching the subpattern  the current subject string, rather than anything matching the subpattern
1278  itself (see  itself (see
# Line 832  itself (see Line 1281  itself (see
1281  "Subpatterns as subroutines"  "Subpatterns as subroutines"
1282  .\"  .\"
1283  below for a way of doing that). So the pattern  below for a way of doing that). So the pattern
1284    .sp
1285    (sens|respons)e and \\1ibility    (sens|respons)e and \e1ibility
1286    .sp
1287  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1288  "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
1289  back reference, the case of letters is relevant. For example,  back reference, the case of letters is relevant. For example,
1290    .sp
1291    ((?i)rah)\\s+\\1    ((?i)rah)\es+\e1
1292    .sp
1293  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
1294  capturing subpattern is matched caselessly.  capturing subpattern is matched caselessly.
1295    .P
1296  Back references to named subpatterns use the Python syntax (?P=name). We could  Back references to named subpatterns use the Perl syntax \ek<name> or \ek'name'
1297  rewrite the above example as follows:  or the Python syntax (?P=name). We could rewrite the above example in either of
1298    the following ways:
1299    (?<p1>(?i)rah)\\s+(?P=p1)  .sp
1300      (?<p1>(?i)rah)\es+\ek<p1>
1301      (?P<p1>(?i)rah)\es+(?P=p1)
1302    .sp
1303    A subpattern that is referenced by name may appear in the pattern before or
1304    after the reference.
1305    .P
1306  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
1307  subpattern has not actually been used in a particular match, any back  subpattern has not actually been used in a particular match, any back
1308  references to it always fail. For example, the pattern  references to it always fail. For example, the pattern
1309    .sp
1310    (a|(bc))\\2    (a|(bc))\e2
1311    .sp
1312  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". Because there may be
1313  many capturing parentheses in a pattern, all digits following the backslash are  many capturing parentheses in a pattern, all digits following the backslash are
1314  taken as part of a potential back reference number. If the pattern continues  taken as part of a potential back reference number. If the pattern continues
1315  with a digit character, some delimiter must be used to terminate the back  with a digit character, some delimiter must be used to terminate the back
1316  reference. If the PCRE_EXTENDED option is set, this can be whitespace.  reference. If the PCRE_EXTENDED option is set, this can be whitespace.
1317  Otherwise an empty comment can be used.  Otherwise an empty comment (see
1318    .\" HTML <a href="#comments">
1319    .\" </a>
1320    "Comments"
1321    .\"
1322    below) can be used.
1323    .P
1324  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
1325  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.
1326  However, such references can be useful inside repeated subpatterns. For  However, such references can be useful inside repeated subpatterns. For
1327  example, the pattern  example, the pattern
1328    .sp
1329    (a|b\\1)+    (a|b\e1)+
1330    .sp
1331  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
1332  the subpattern, the back reference matches the character string corresponding  the subpattern, the back reference matches the character string corresponding
1333  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
1334  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
1335  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
1336  minimum of zero.  minimum of zero.
1337    .
1338    .
1339    .\" HTML <a name="bigassertions"></a>
1340  .SH ASSERTIONS  .SH ASSERTIONS
1341  .rs  .rs
1342  .sp  .sp
1343  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
1344  matching point that does not actually consume any characters. The simple  matching point that does not actually consume any characters. The simple
1345  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
1346    .\" HTML <a href="#smallassertions">
1347    .\" </a>
1348    above.
1349    .\"
1350    .P
1351  More complicated assertions are coded as subpatterns. There are two kinds:  More complicated assertions are coded as subpatterns. There are two kinds:
1352  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
1353  that look behind it.  that look behind it. An assertion subpattern is matched in the normal way,
1354    except that it does not cause the current matching position to be changed.
1355  An assertion subpattern is matched in the normal way, except that it does not  .P
1356  cause the current matching position to be changed. Lookahead assertions start  Assertion subpatterns are not capturing subpatterns, and may not be repeated,
1357  with (?= for positive assertions and (?! for negative assertions. For example,  because it makes no sense to assert the same thing several times. If any kind
1358    of assertion contains capturing subpatterns within it, these are counted for
1359    \\w+(?=;)  the purposes of numbering the capturing subpatterns in the whole pattern.
1360    However, substring capturing is carried out only for positive assertions,
1361    because it does not make sense for negative assertions.
1362    .
1363    .
1364    .SS "Lookahead assertions"
1365    .rs
1366    .sp
1367    Lookahead assertions start with (?= for positive assertions and (?! for
1368    negative assertions. For example,
1369    .sp
1370      \ew+(?=;)
1371    .sp
1372  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
1373  the match, and  the match, and
1374    .sp
1375    foo(?!bar)    foo(?!bar)
1376    .sp
1377  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
1378  apparently similar pattern  apparently similar pattern
1379    .sp
1380    (?!foo)bar    (?!foo)bar
1381    .sp
1382  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
1383  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion  "foo"; it finds any occurrence of "bar" whatsoever, because the assertion
1384  (?!foo) is always true when the next three characters are "bar". A  (?!foo) is always true when the next three characters are "bar". A
1385  lookbehind assertion is needed to achieve this effect.  lookbehind assertion is needed to achieve the other effect.
1386    .P
1387  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
1388  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
1389  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.
1390    .
1391    .
1392    .\" HTML <a name="lookbehind"></a>
1393    .SS "Lookbehind assertions"
1394    .rs
1395    .sp
1396  Lookbehind assertions start with (?<= for positive assertions and (?<! for  Lookbehind assertions start with (?<= for positive assertions and (?<! for
1397  negative assertions. For example,  negative assertions. For example,
1398    .sp
1399    (?<!foo)bar    (?<!foo)bar
1400    .sp
1401  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
1402  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
1403  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
1404  all have to have the same fixed length. Thus  do not all have to have the same fixed length. Thus
1405    .sp
1406    (?<=bullock|donkey)    (?<=bullock|donkey)
1407    .sp
1408  is permitted, but  is permitted, but
1409    .sp
1410    (?<!dogs?|cats?)    (?<!dogs?|cats?)
1411    .sp
1412  causes an error at compile time. Branches that match different length strings  causes an error at compile time. Branches that match different length strings
1413  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
1414  extension compared with Perl (at least for 5.8), which requires all branches to  extension compared with Perl (at least for 5.8), which requires all branches to
1415  match the same length of string. An assertion such as  match the same length of string. An assertion such as
1416    .sp
1417    (?<=ab(c|de))    (?<=ab(c|de))
1418    .sp
1419  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
1420  lengths, but it is acceptable if rewritten to use two top-level branches:  lengths, but it is acceptable if rewritten to use two top-level branches:
1421    .sp
1422    (?<=abc|abde)    (?<=abc|abde)
1423    .sp
1424  The implementation of lookbehind assertions is, for each alternative, to  The implementation of lookbehind assertions is, for each alternative, to
1425  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
1426  match. If there are insufficient characters before the current position, the  match. If there are insufficient characters before the current position, the
1427  match is deemed to fail.  assertion fails.
1428    .P
1429  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)
1430  to appear in lookbehind assertions, because it makes it impossible to calculate  to appear in lookbehind assertions, because it makes it impossible to calculate
1431  the length of the lookbehind.  the length of the lookbehind. The \eX and \eR escapes, which can match
1432    different numbers of bytes, are also not permitted.
1433  Atomic groups can be used in conjunction with lookbehind assertions to specify  .P
1434  efficient matching at the end of the subject string. Consider a simple pattern  Possessive quantifiers can be used in conjunction with lookbehind assertions to
1435  such as  specify efficient matching at the end of the subject string. Consider a simple
1436    pattern such as
1437    .sp
1438    abcd$    abcd$
1439    .sp
1440  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
1441  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
1442  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
1443    .sp
1444    ^.*abcd$    ^.*abcd$
1445    .sp
1446  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
1447  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,
1448  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"
1449  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,
1450  if the pattern is written as  if the pattern is written as
1451    .sp
   ^(?>.*)(?<=abcd)  
   
 or, equivalently,  
   
1452    ^.*+(?<=abcd)    ^.*+(?<=abcd)
1453    .sp
1454  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
1455  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
1456  characters. If it fails, the match fails immediately. For long strings, this  characters. If it fails, the match fails immediately. For long strings, this
1457  approach makes a significant difference to the processing time.  approach makes a significant difference to the processing time.
1458    .
1459    .
1460    .SS "Using multiple assertions"
1461    .rs
1462    .sp
1463  Several assertions (of any sort) may occur in succession. For example,  Several assertions (of any sort) may occur in succession. For example,
1464    .sp
1465    (?<=\\d{3})(?<!999)foo    (?<=\ed{3})(?<!999)foo
1466    .sp
1467  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
1468  the assertions is applied independently at the same point in the subject  the assertions is applied independently at the same point in the subject
1469  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
1470  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".
1471  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
1472  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
1473  doesn't match "123abcfoo". A pattern to do that is  doesn't match "123abcfoo". A pattern to do that is
1474    .sp
1475    (?<=\\d{3}...)(?<!999)foo    (?<=\ed{3}...)(?<!999)foo
1476    .sp
1477  This time the first assertion looks at the preceding six characters, checking  This time the first assertion looks at the preceding six characters, checking
1478  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
1479  preceding three characters are not "999".  preceding three characters are not "999".
1480    .P
1481  Assertions can be nested in any combination. For example,  Assertions can be nested in any combination. For example,
1482    .sp
1483    (?<=(?<!foo)bar)baz    (?<=(?<!foo)bar)baz
1484    .sp
1485  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
1486  preceded by "foo", while  preceded by "foo", while
1487    .sp
1488    (?<=\\d{3}(?!999)...)foo    (?<=\ed{3}(?!999)...)foo
1489    .sp
1490  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
1491  characters that are not "999".  characters that are not "999".
1492    .
1493  Assertion subpatterns are not capturing subpatterns, and may not be repeated,  .
1494  because it makes no sense to assert the same thing several times. If any kind  .\" HTML <a name="conditions"></a>
1495  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  
1496  .rs  .rs
1497  .sp  .sp
1498  It is possible to cause the matching process to obey a subpattern  It is possible to cause the matching process to obey a subpattern
1499  conditionally or to choose between two alternative subpatterns, depending on  conditionally or to choose between two alternative subpatterns, depending on
1500  the result of an assertion, or whether a previous capturing subpattern matched  the result of an assertion, or whether a previous capturing subpattern matched
1501  or not. The two possible forms of conditional subpattern are  or not. The two possible forms of conditional subpattern are
1502    .sp
1503    (?(condition)yes-pattern)    (?(condition)yes-pattern)
1504    (?(condition)yes-pattern|no-pattern)    (?(condition)yes-pattern|no-pattern)
1505    .sp
1506  If the condition is satisfied, the yes-pattern is used; otherwise the  If the condition is satisfied, the yes-pattern is used; otherwise the
1507  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
1508  subpattern, a compile-time error occurs.  subpattern, a compile-time error occurs.
1509    .P
1510  There are three kinds of condition. If the text between the parentheses  There are four kinds of condition: references to subpatterns, references to
1511  consists of a sequence of digits, the condition is satisfied if the capturing  recursion, a pseudo-condition called DEFINE, and assertions.
1512  subpattern of that number has previously matched. The number must be greater  .
1513  than zero. Consider the following pattern, which contains non-significant white  .SS "Checking for a used subpattern by number"
1514  space to make it more readable (assume the PCRE_EXTENDED option) and to divide  .rs
1515  it into three parts for ease of discussion:  .sp
1516    If the text between the parentheses consists of a sequence of digits, the
1517    ( \\( )?    [^()]+    (?(1) \\) )  condition is true if the capturing subpattern of that number has previously
1518    matched. An alternative notation is to precede the digits with a plus or minus
1519    sign. In this case, the subpattern number is relative rather than absolute.
1520    The most recently opened parentheses can be referenced by (?(-1), the next most
1521    recent by (?(-2), and so on. In looping constructs it can also make sense to
1522    refer to subsequent groups with constructs such as (?(+2).
1523    .P
1524    Consider the following pattern, which contains non-significant white space to
1525    make it more readable (assume the PCRE_EXTENDED option) and to divide it into
1526    three parts for ease of discussion:
1527    .sp
1528      ( \e( )?    [^()]+    (?(1) \e) )
1529    .sp
1530  The first part matches an optional opening parenthesis, and if that  The first part matches an optional opening parenthesis, and if that
1531  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
1532  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
# Line 1047  the condition is true, and so the yes-pa Line 1536  the condition is true, and so the yes-pa
1536  parenthesis is required. Otherwise, since no-pattern is not present, the  parenthesis is required. Otherwise, since no-pattern is not present, the
1537  subpattern matches nothing. In other words, this pattern matches a sequence of  subpattern matches nothing. In other words, this pattern matches a sequence of
1538  non-parentheses, optionally enclosed in parentheses.  non-parentheses, optionally enclosed in parentheses.
1539    .P
1540  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
1541  pattern or subpattern has been made. At "top level", the condition is false.  reference:
1542  This is a PCRE extension. Recursive patterns are described in the next section.  .sp
1543      ...other stuff... ( \e( )?    [^()]+    (?(-1) \e) ) ...
1544  If the condition is not a sequence of digits or (R), it must be an assertion.  .sp
1545    This makes the fragment independent of the parentheses in the larger pattern.
1546    .
1547    .SS "Checking for a used subpattern by name"
1548    .rs
1549    .sp
1550    Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a used
1551    subpattern by name. For compatibility with earlier versions of PCRE, which had
1552    this facility before Perl, the syntax (?(name)...) is also recognized. However,
1553    there is a possible ambiguity with this syntax, because subpattern names may
1554    consist entirely of digits. PCRE looks first for a named subpattern; if it
1555    cannot find one and the name consists entirely of digits, PCRE looks for a
1556    subpattern of that number, which must be greater than zero. Using subpattern
1557    names that consist entirely of digits is not recommended.
1558    .P
1559    Rewriting the above example to use a named subpattern gives this:
1560    .sp
1561      (?<OPEN> \e( )?    [^()]+    (?(<OPEN>) \e) )
1562    .sp
1563    .
1564    .SS "Checking for pattern recursion"
1565    .rs
1566    .sp
1567    If the condition is the string (R), and there is no subpattern with the name R,
1568    the condition is true if a recursive call to the whole pattern or any
1569    subpattern has been made. If digits or a name preceded by ampersand follow the
1570    letter R, for example:
1571    .sp
1572      (?(R3)...) or (?(R&name)...)
1573    .sp
1574    the condition is true if the most recent recursion is into the subpattern whose
1575    number or name is given. This condition does not check the entire recursion
1576    stack.
1577    .P
1578    At "top level", all these recursion test conditions are false. Recursive
1579    patterns are described below.
1580    .
1581    .SS "Defining subpatterns for use by reference only"
1582    .rs
1583    .sp
1584    If the condition is the string (DEFINE), and there is no subpattern with the
1585    name DEFINE, the condition is always false. In this case, there may be only one
1586    alternative in the subpattern. It is always skipped if control reaches this
1587    point in the pattern; the idea of DEFINE is that it can be used to define
1588    "subroutines" that can be referenced from elsewhere. (The use of "subroutines"
1589    is described below.) For example, a pattern to match an IPv4 address could be
1590    written like this (ignore whitespace and line breaks):
1591    .sp
1592      (?(DEFINE) (?<byte> 2[0-4]\ed | 25[0-5] | 1\ed\ed | [1-9]?\ed) )
1593      \eb (?&byte) (\e.(?&byte)){3} \eb
1594    .sp
1595    The first part of the pattern is a DEFINE group inside which a another group
1596    named "byte" is defined. This matches an individual component of an IPv4
1597    address (a number less than 256). When matching takes place, this part of the
1598    pattern is skipped because DEFINE acts like a false condition.
1599    .P
1600    The rest of the pattern uses references to the named group to match the four
1601    dot-separated components of an IPv4 address, insisting on a word boundary at
1602    each end.
1603    .
1604    .SS "Assertion conditions"
1605    .rs
1606    .sp
1607    If the condition is not in any of the above formats, it must be an assertion.
1608  This may be a positive or negative lookahead or lookbehind assertion. Consider  This may be a positive or negative lookahead or lookbehind assertion. Consider
1609  this pattern, again containing non-significant white space, and with the two  this pattern, again containing non-significant white space, and with the two
1610  alternatives on the second line:  alternatives on the second line:
1611    .sp
1612    (?(?=[^a-z]*[a-z])    (?(?=[^a-z]*[a-z])
1613    \\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} )
1614    .sp
1615  The condition is a positive lookahead assertion that matches an optional  The condition is a positive lookahead assertion that matches an optional
1616  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
1617  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
1618  subject is matched against the first alternative; otherwise it is matched  subject is matched against the first alternative; otherwise it is matched
1619  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
1620  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.
1621    .
1622    .
1623    .\" HTML <a name="comments"></a>
1624  .SH COMMENTS  .SH COMMENTS
1625  .rs  .rs
1626  .sp  .sp
1627  The sequence (?# marks the start of a comment which continues up to the next  The sequence (?# marks the start of a comment that continues up to the next
1628  closing parenthesis. Nested parentheses are not permitted. The characters  closing parenthesis. Nested parentheses are not permitted. The characters
1629  that make up a comment play no part in the pattern matching at all.  that make up a comment play no part in the pattern matching at all.
1630    .P
1631  If the PCRE_EXTENDED option is set, an unescaped # character outside a  If the PCRE_EXTENDED option is set, an unescaped # character outside a
1632  character class introduces a comment that continues up to the next newline  character class introduces a comment that continues to immediately after the
1633  character in the pattern.  next newline in the pattern.
1634    .
1635  .SH RECURSIVE PATTERNS  .
1636    .\" HTML <a name="recursion"></a>
1637    .SH "RECURSIVE PATTERNS"
1638  .rs  .rs
1639  .sp  .sp
1640  Consider the problem of matching a string in parentheses, allowing for  Consider the problem of matching a string in parentheses, allowing for
1641  unlimited nested parentheses. Without the use of recursion, the best that can  unlimited nested parentheses. Without the use of recursion, the best that can
1642  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
1643  is not possible to handle an arbitrary nesting depth. Perl has provided an  is not possible to handle an arbitrary nesting depth.
1644  experimental facility that allows regular expressions to recurse (amongst other  .P
1645  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
1646  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
1647  parentheses problem can be created like this:  expression at run time, and the code can refer to the expression itself. A Perl
1648    pattern using code interpolation to solve the parentheses problem can be
1649    $re = qr{\\( (?: (?>[^()]+) | (?p{$re}) )* \\)}x;  created like this:
1650    .sp
1651      $re = qr{\e( (?: (?>[^()]+) | (?p{$re}) )* \e)}x;
1652    .sp
1653  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
1654  recursively to the pattern in which it appears. Obviously, PCRE cannot support  recursively to the pattern in which it appears.
1655  the interpolation of Perl code. Instead, it supports some special syntax for  .P
1656  recursion of the entire pattern, and also for individual subpattern recursion.  Obviously, PCRE cannot support the interpolation of Perl code. Instead, it
1657    supports special syntax for recursion of the entire pattern, and also for
1658  The special item that consists of (? followed by a number greater than zero and  individual subpattern recursion. After its introduction in PCRE and Python,
1659  a closing parenthesis is a recursive call of the subpattern of the given  this kind of recursion was introduced into Perl at release 5.10.
1660  number, provided that it occurs inside that subpattern. (If not, it is a  .P
1661  "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
1662  (?R) is a recursive call of the entire regular expression.  closing parenthesis is a recursive call of the subpattern of the given number,
1663    provided that it occurs inside that subpattern. (If not, it is a "subroutine"
1664  For example, this PCRE pattern solves the nested parentheses problem (assume  call, which is described in the next section.) The special item (?R) or (?0) is
1665  the PCRE_EXTENDED option is set so that white space is ignored):  a recursive call of the entire regular expression.
1666    .P
1667    \\( ( (?>[^()]+) | (?R) )* \\)  In PCRE (like Python, but unlike Perl), a recursive subpattern call is always
1668    treated as an atomic group. That is, once it has matched some of the subject
1669    string, it is never re-entered, even if it contains untried alternatives and
1670    there is a subsequent matching failure.
1671    .P
1672    This PCRE pattern solves the nested parentheses problem (assume the
1673    PCRE_EXTENDED option is set so that white space is ignored):
1674    .sp
1675      \e( ( (?>[^()]+) | (?R) )* \e)
1676    .sp
1677  First it matches an opening parenthesis. Then it matches any number of  First it matches an opening parenthesis. Then it matches any number of
1678  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
1679  match of the pattern itself (that is a correctly parenthesized substring).  match of the pattern itself (that is, a correctly parenthesized substring).
1680  Finally there is a closing parenthesis.  Finally there is a closing parenthesis.
1681    .P
1682  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
1683  pattern, so instead you could use this:  pattern, so instead you could use this:
1684    .sp
1685    ( \\( ( (?>[^()]+) | (?1) )* \\) )    ( \e( ( (?>[^()]+) | (?1) )* \e) )
1686    .sp
1687  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
1688  them instead of the whole pattern. In a larger pattern, keeping track of  them instead of the whole pattern.
1689  parenthesis numbers can be tricky. It may be more convenient to use named  .P
1690  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
1691  the Python syntax that PCRE uses for named parentheses (Perl does not provide  is made easier by the use of relative references. (A Perl 5.10 feature.)
1692  named parentheses). We could rewrite the above example as follows:  Instead of (?1) in the pattern above you can write (?-2) to refer to the second
1693    most recently opened parentheses preceding the recursion. In other words, a
1694    (?<pn> \\( ( (?>[^()]+) | (?P>pn) )* \\) )  negative number counts capturing parentheses leftwards from the point at which
1695    it is encountered.
1696  This particular example pattern contains nested unlimited repeats, and so the  .P
1697  use of atomic grouping for matching strings of non-parentheses is important  It is also possible to refer to subsequently opened parentheses, by writing
1698  when applying the pattern to strings that do not match. For example, when this  references such as (?+2). However, these cannot be recursive because the
1699  pattern is applied to  reference is not inside the parentheses that are referenced. They are always
1700    "subroutine" calls, as described in the next section.
1701    .P
1702    An alternative approach is to use named parentheses instead. The Perl syntax
1703    for this is (?&name); PCRE's earlier syntax (?P>name) is also supported. We
1704    could rewrite the above example as follows:
1705    .sp
1706      (?<pn> \e( ( (?>[^()]+) | (?&pn) )* \e) )
1707    .sp
1708    If there is more than one subpattern with the same name, the earliest one is
1709    used.
1710    .P
1711    This particular example pattern that we have been looking at contains nested
1712    unlimited repeats, and so the use of atomic grouping for matching strings of
1713    non-parentheses is important when applying the pattern to strings that do not
1714    match. For example, when this pattern is applied to
1715    .sp
1716    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()    (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
1717    .sp
1718  it yields "no match" quickly. However, if atomic grouping is not used,  it yields "no match" quickly. However, if atomic grouping is not used,
1719  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
1720  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
1721  before failure can be reported.  before failure can be reported.
1722    .P
1723  At the end of a match, the values set for any capturing subpatterns are those  At the end of a match, the values set for any capturing subpatterns are those
1724  from the outermost level of the recursion at which the subpattern value is set.  from the outermost level of the recursion at which the subpattern value is set.
1725  If you want to obtain intermediate values, a callout function can be used (see  If you want to obtain intermediate values, a callout function can be used (see
1726  below and the  below and the
1727  .\" HREF  .\" HREF
1728  \fBpcrecallout\fR  \fBpcrecallout\fP
1729  .\"  .\"
1730  documentation). If the pattern above is matched against  documentation). If the pattern above is matched against
1731    .sp
1732    (ab(cd)ef)    (ab(cd)ef)
1733    .sp
1734  the value for the capturing parentheses is "ef", which is the last value taken  the value for the capturing parentheses is "ef", which is the last value taken
1735  on at the top level. If additional parentheses are added, giving  on at the top level. If additional parentheses are added, giving
1736    .sp
1737    \\( ( ( (?>[^()]+) | (?R) )* ) \\)    \e( ( ( (?>[^()]+) | (?R) )* ) \e)
1738       ^                        ^       ^                        ^
1739       ^                        ^       ^                        ^
1740    .sp
1741  the string they capture is "ab(cd)ef", the contents of the top level  the string they capture is "ab(cd)ef", the contents of the top level
1742  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE  parentheses. If there are more than 15 capturing parentheses in a pattern, PCRE
1743  has to obtain extra memory to store data during a recursion, which it does by  has to obtain extra memory to store data during a recursion, which it does by
1744  using \fBpcre_malloc\fR, freeing it via \fBpcre_free\fR afterwards. If no  using \fBpcre_malloc\fP, freeing it via \fBpcre_free\fP afterwards. If no
1745  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.  memory can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
1746    .P
1747  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.
1748  Consider this pattern, which matches text in angle brackets, allowing for  Consider this pattern, which matches text in angle brackets, allowing for
1749  arbitrary nesting. Only digits are allowed in nested brackets (that is, when  arbitrary nesting. Only digits are allowed in nested brackets (that is, when
1750  recursing), whereas any characters are permitted at the outer level.  recursing), whereas any characters are permitted at the outer level.
1751    .sp
1752    < (?: (?(R) \\d++  | [^<>]*+) | (?R)) * >    < (?: (?(R) \ed++  | [^<>]*+) | (?R)) * >
1753    .sp
1754  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
1755  different alternatives for the recursive and non-recursive cases. The (?R) item  different alternatives for the recursive and non-recursive cases. The (?R) item
1756  is the actual recursive call.  is the actual recursive call.
1757    .
1758    .
1759  .\" HTML <a name="subpatternsassubroutines"></a>  .\" HTML <a name="subpatternsassubroutines"></a>
1760  .SH SUBPATTERNS AS SUBROUTINES  .SH "SUBPATTERNS AS SUBROUTINES"
1761  .rs  .rs
1762  .sp  .sp
1763  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
1764  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
1765  subroutine in a programming language. An earlier example pointed out that the  subroutine in a programming language. The "called" subpattern may be defined
1766  pattern  before or after the reference. A numbered reference can be absolute or
1767    relative, as in these examples:
1768    (sens|respons)e and \\1ibility  .sp
1769      (...(absolute)...)...(?2)...
1770      (...(relative)...)...(?-1)...
1771      (...(?+1)...(relative)...
1772    .sp
1773    An earlier example pointed out that the pattern
1774    .sp
1775      (sens|respons)e and \e1ibility
1776    .sp
1777  matches "sense and sensibility" and "response and responsibility", but not  matches "sense and sensibility" and "response and responsibility", but not
1778  "sense and responsibility". If instead the pattern  "sense and responsibility". If instead the pattern
1779    .sp
1780    (sens|respons)e and (?1)ibility    (sens|respons)e and (?1)ibility
1781    .sp
1782  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
1783  strings. Such references must, however, follow the subpattern to which they  strings. Another example is given in the discussion of DEFINE above.
1784  refer.  .P
1785    Like recursive subpatterns, a "subroutine" call is always treated as an atomic
1786    group. That is, once it has matched some of the subject string, it is never
1787    re-entered, even if it contains untried alternatives and there is a subsequent
1788    matching failure.
1789    .P
1790    When a subpattern is used as a subroutine, processing options such as
1791    case-independence are fixed when the subpattern is defined. They cannot be
1792    changed for different calls. For example, consider this pattern:
1793    .sp
1794      (abc)(?i:(?-1))
1795    .sp
1796    It matches "abcabc". It does not match "abcABC" because the change of
1797    processing option does not affect the called subpattern.
1798    .
1799    .
1800  .SH CALLOUTS  .SH CALLOUTS
1801  .rs  .rs
1802  .sp  .sp
# Line 1201  Perl has a feature whereby using the seq Line 1804  Perl has a feature whereby using the seq
1804  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
1805  possible, amongst other things, to extract different substrings that match the  possible, amongst other things, to extract different substrings that match the
1806  same pair of parentheses when there is a repetition.  same pair of parentheses when there is a repetition.
1807    .P
1808  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
1809  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
1810  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.
1811  By default, this variable contains NULL, which disables all calling out.  By default, this variable contains NULL, which disables all calling out.
1812    .P
1813  Within a regular expression, (?C) indicates the points at which the external  Within a regular expression, (?C) indicates the points at which the external
1814  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
1815  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.
1816  For example, this pattern has two callout points:  For example, this pattern has two callout points:
1817    .sp
1818    (?C1)\dabc(?C2)def    (?C1)abc(?C2)def
1819    .sp
1820  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
1821    automatically installed before each item in the pattern. They are all numbered
1822    255.
1823    .P
1824    During matching, when PCRE reaches a callout point (and \fIpcre_callout\fP is
1825  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
1826  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
1827  \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
1828  fail altogether. A complete description of the interface to the callout  may cause matching to proceed, to backtrack, or to fail altogether. A complete
1829  function is given in the  description of the interface to the callout function is given in the
1830  .\" HREF  .\" HREF
1831  \fBpcrecallout\fR  \fBpcrecallout\fP
1832  .\"  .\"
1833  documentation.  documentation.
1834    .
1835  .in 0  .
1836  Last updated: 03 February 2003  .SH "SEE ALSO"
1837  .br  .rs
1838  Copyright (c) 1997-2003 University of Cambridge.  .sp
1839    \fBpcreapi\fP(3), \fBpcrecallout\fP(3), \fBpcrematching\fP(3), \fBpcre\fP(3).
1840    .
1841    .
1842    .SH AUTHOR
1843    .rs
1844    .sp
1845    .nf
1846    Philip Hazel
1847    University Computing Service
1848    Cambridge CB2 3QH, England.
1849    .fi
1850    .
1851    .
1852    .SH REVISION
1853    .rs
1854    .sp
1855    .nf
1856    Last updated: 09 May 2007
1857    Copyright (c) 1997-2007 University of Cambridge.
1858    .fi

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