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revision 392 by ph10, Tue Mar 17 21:30:30 2009 UTC revision 406 by ph10, Mon Mar 23 12:05:43 2009 UTC
# Line 224  UTF-8 AND UNICODE PROPERTY SUPPORT Line 224  UTF-8 AND UNICODE PROPERTY SUPPORT
224         includes  Unicode  property support, because to do otherwise would slow         includes  Unicode  property support, because to do otherwise would slow
225         down PCRE in many common cases. If you really want to test for a  wider         down PCRE in many common cases. If you really want to test for a  wider
226         sense  of,  say,  "digit",  you must use Unicode property tests such as         sense  of,  say,  "digit",  you must use Unicode property tests such as
227         \p{Nd}.         \p{Nd}. Note that this also applies to \b, because  it  is  defined  in
228           terms of \w and \W.
229    
230         7. Similarly, characters that match the POSIX named  character  classes         7.  Similarly,  characters that match the POSIX named character classes
231         are all low-valued characters.         are all low-valued characters.
232    
233         8.  However,  the Perl 5.10 horizontal and vertical whitespace matching         8. However, the Perl 5.10 horizontal and vertical  whitespace  matching
234         escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-         escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
235         acters.         acters.
236    
237         9.  Case-insensitive  matching  applies only to characters whose values         9. Case-insensitive matching applies only to  characters  whose  values
238         are less than 128, unless PCRE is built with Unicode property  support.         are  less than 128, unless PCRE is built with Unicode property support.
239         Even  when  Unicode  property support is available, PCRE still uses its         Even when Unicode property support is available, PCRE  still  uses  its
240         own character tables when checking the case of  low-valued  characters,         own  character  tables when checking the case of low-valued characters,
241         so  as not to degrade performance.  The Unicode property information is         so as not to degrade performance.  The Unicode property information  is
242         used only for characters with higher values. Even when Unicode property         used only for characters with higher values. Even when Unicode property
243         support is available, PCRE supports case-insensitive matching only when         support is available, PCRE supports case-insensitive matching only when
244         there is a one-to-one mapping between a letter's  cases.  There  are  a         there  is  a  one-to-one  mapping between a letter's cases. There are a
245         small  number  of  many-to-one  mappings in Unicode; these are not sup-         small number of many-to-one mappings in Unicode;  these  are  not  sup-
246         ported by PCRE.         ported by PCRE.
247    
248    
# Line 251  AUTHOR Line 252  AUTHOR
252         University Computing Service         University Computing Service
253         Cambridge CB2 3QH, England.         Cambridge CB2 3QH, England.
254    
255         Putting an actual email address here seems to have been a spam  magnet,         Putting  an actual email address here seems to have been a spam magnet,
256         so  I've  taken  it away. If you want to email me, use my two initials,         so I've taken it away. If you want to email me, use  my  two  initials,
257         followed by the two digits 10, at the domain cam.ac.uk.         followed by the two digits 10, at the domain cam.ac.uk.
258    
259    
260  REVISION  REVISION
261    
262         Last updated: 12 April 2008         Last updated: 18 March 2009
263         Copyright (c) 1997-2008 University of Cambridge.         Copyright (c) 1997-2009 University of Cambridge.
264  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
265    
266    
# Line 3268  BACKSLASH Line 3269  BACKSLASH
3269         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-         \s, or \w, and always match \D, \S, and \W. This is true even when Uni-
3270         code  character  property  support is available. These sequences retain         code  character  property  support is available. These sequences retain
3271         their original meanings from before UTF-8 support was available, mainly         their original meanings from before UTF-8 support was available, mainly
3272         for efficiency reasons.         for  efficiency  reasons. Note that this also affects \b, because it is
3273           defined in terms of \w and \W.
3274    
3275         The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to         The sequences \h, \H, \v, and \V are Perl 5.10 features. In contrast to
3276         the other sequences, these do match certain high-valued  codepoints  in         the  other  sequences, these do match certain high-valued codepoints in
3277         UTF-8 mode.  The horizontal space characters are:         UTF-8 mode.  The horizontal space characters are:
3278    
3279           U+0009     Horizontal tab           U+0009     Horizontal tab
# Line 3305  BACKSLASH Line 3307  BACKSLASH
3307           U+2029     Paragraph separator           U+2029     Paragraph separator
3308    
3309         A "word" character is an underscore or any character less than 256 that         A "word" character is an underscore or any character less than 256 that
3310         is a letter or digit. The definition of  letters  and  digits  is  con-         is  a  letter  or  digit.  The definition of letters and digits is con-
3311         trolled  by PCRE's low-valued character tables, and may vary if locale-         trolled by PCRE's low-valued character tables, and may vary if  locale-
3312         specific matching is taking place (see "Locale support" in the  pcreapi         specific  matching is taking place (see "Locale support" in the pcreapi
3313         page).  For  example,  in  a French locale such as "fr_FR" in Unix-like         page). For example, in a French locale such  as  "fr_FR"  in  Unix-like
3314         systems, or "french" in Windows, some character codes greater than  128         systems,  or "french" in Windows, some character codes greater than 128
3315         are  used for accented letters, and these are matched by \w. The use of         are used for accented letters, and these are matched by \w. The use  of
3316         locales with Unicode is discouraged.         locales with Unicode is discouraged.
3317    
3318     Newline sequences     Newline sequences
3319    
3320         Outside a character class, by default, the escape sequence  \R  matches         Outside  a  character class, by default, the escape sequence \R matches
3321         any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8         any Unicode newline sequence. This is a Perl 5.10 feature. In non-UTF-8
3322         mode \R is equivalent to the following:         mode \R is equivalent to the following:
3323    
3324           (?>\r\n|\n|\x0b|\f|\r|\x85)           (?>\r\n|\n|\x0b|\f|\r|\x85)
3325    
3326         This is an example of an "atomic group", details  of  which  are  given         This  is  an  example  of an "atomic group", details of which are given
3327         below.  This particular group matches either the two-character sequence         below.  This particular group matches either the two-character sequence
3328         CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,         CR  followed  by  LF,  or  one  of  the single characters LF (linefeed,
3329         U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage         U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
3330         return, U+000D), or NEL (next line, U+0085). The two-character sequence         return, U+000D), or NEL (next line, U+0085). The two-character sequence
3331         is treated as a single unit that cannot be split.         is treated as a single unit that cannot be split.
3332    
3333         In  UTF-8  mode, two additional characters whose codepoints are greater         In UTF-8 mode, two additional characters whose codepoints  are  greater
3334         than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-         than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
3335         rator,  U+2029).   Unicode character property support is not needed for         rator, U+2029).  Unicode character property support is not  needed  for
3336         these characters to be recognized.         these characters to be recognized.
3337    
3338         It is possible to restrict \R to match only CR, LF, or CRLF (instead of         It is possible to restrict \R to match only CR, LF, or CRLF (instead of
3339         the  complete  set  of  Unicode  line  endings)  by  setting the option         the complete set  of  Unicode  line  endings)  by  setting  the  option
3340         PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.         PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
3341         (BSR is an abbrevation for "backslash R".) This can be made the default         (BSR is an abbrevation for "backslash R".) This can be made the default
3342         when PCRE is built; if this is the case, the  other  behaviour  can  be         when  PCRE  is  built;  if this is the case, the other behaviour can be
3343         requested  via  the  PCRE_BSR_UNICODE  option.   It is also possible to         requested via the PCRE_BSR_UNICODE option.   It  is  also  possible  to
3344         specify these settings by starting a pattern string  with  one  of  the         specify  these  settings  by  starting a pattern string with one of the
3345         following sequences:         following sequences:
3346    
3347           (*BSR_ANYCRLF)   CR, LF, or CRLF only           (*BSR_ANYCRLF)   CR, LF, or CRLF only
# Line 3348  BACKSLASH Line 3350  BACKSLASH
3350         These override the default and the options given to pcre_compile(), but         These override the default and the options given to pcre_compile(), but
3351         they can be overridden by options given to pcre_exec(). Note that these         they can be overridden by options given to pcre_exec(). Note that these
3352         special settings, which are not Perl-compatible, are recognized only at         special settings, which are not Perl-compatible, are recognized only at
3353         the very start of a pattern, and that they must be in  upper  case.  If         the  very  start  of a pattern, and that they must be in upper case. If
3354         more  than  one  of  them is present, the last one is used. They can be         more than one of them is present, the last one is  used.  They  can  be
3355         combined with a change of newline convention, for  example,  a  pattern         combined  with  a  change of newline convention, for example, a pattern
3356         can start with:         can start with:
3357    
3358           (*ANY)(*BSR_ANYCRLF)           (*ANY)(*BSR_ANYCRLF)
# Line 3360  BACKSLASH Line 3362  BACKSLASH
3362     Unicode character properties     Unicode character properties
3363    
3364         When PCRE is built with Unicode character property support, three addi-         When PCRE is built with Unicode character property support, three addi-
3365         tional escape sequences that match characters with specific  properties         tional  escape sequences that match characters with specific properties
3366         are  available.   When not in UTF-8 mode, these sequences are of course         are available.  When not in UTF-8 mode, these sequences are  of  course
3367         limited to testing characters whose codepoints are less than  256,  but         limited  to  testing characters whose codepoints are less than 256, but
3368         they do work in this mode.  The extra escape sequences are:         they do work in this mode.  The extra escape sequences are:
3369    
3370           \p{xx}   a character with the xx property           \p{xx}   a character with the xx property
3371           \P{xx}   a character without the xx property           \P{xx}   a character without the xx property
3372           \X       an extended Unicode sequence           \X       an extended Unicode sequence
3373    
3374         The  property  names represented by xx above are limited to the Unicode         The property names represented by xx above are limited to  the  Unicode
3375         script names, the general category properties, and "Any", which matches         script names, the general category properties, and "Any", which matches
3376         any character (including newline). Other properties such as "InMusical-         any character (including newline). Other properties such as "InMusical-
3377         Symbols" are not currently supported by PCRE. Note  that  \P{Any}  does         Symbols"  are  not  currently supported by PCRE. Note that \P{Any} does
3378         not match any characters, so always causes a match failure.         not match any characters, so always causes a match failure.
3379    
3380         Sets of Unicode characters are defined as belonging to certain scripts.         Sets of Unicode characters are defined as belonging to certain scripts.
3381         A character from one of these sets can be matched using a script  name.         A  character from one of these sets can be matched using a script name.
3382         For example:         For example:
3383    
3384           \p{Greek}           \p{Greek}
3385           \P{Han}           \P{Han}
3386    
3387         Those  that are not part of an identified script are lumped together as         Those that are not part of an identified script are lumped together  as
3388         "Common". The current list of scripts is:         "Common". The current list of scripts is:
3389    
3390         Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,         Arabic,  Armenian,  Balinese,  Bengali,  Bopomofo,  Braille,  Buginese,
3391         Buhid,   Canadian_Aboriginal,   Cherokee,  Common,  Coptic,  Cuneiform,         Buhid,  Canadian_Aboriginal,  Cherokee,  Common,   Coptic,   Cuneiform,
3392         Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,         Cypriot, Cyrillic, Deseret, Devanagari, Ethiopic, Georgian, Glagolitic,
3393         Gothic,  Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hira-         Gothic, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew,  Hira-
3394         gana, Inherited, Kannada,  Katakana,  Kharoshthi,  Khmer,  Lao,  Latin,         gana,  Inherited,  Kannada,  Katakana,  Kharoshthi,  Khmer, Lao, Latin,
3395         Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,         Limbu,  Linear_B,  Malayalam,  Mongolian,  Myanmar,  New_Tai_Lue,  Nko,
3396         Ogham, Old_Italic, Old_Persian, Oriya, Osmanya,  Phags_Pa,  Phoenician,         Ogham,  Old_Italic,  Old_Persian, Oriya, Osmanya, Phags_Pa, Phoenician,
3397         Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,         Runic,  Shavian,  Sinhala,  Syloti_Nagri,  Syriac,  Tagalog,  Tagbanwa,
3398         Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.         Tai_Le, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Ugaritic, Yi.
3399    
3400         Each character has exactly one general category property, specified  by         Each  character has exactly one general category property, specified by
3401         a two-letter abbreviation. For compatibility with Perl, negation can be         a two-letter abbreviation. For compatibility with Perl, negation can be
3402         specified by including a circumflex between the opening brace  and  the         specified  by  including a circumflex between the opening brace and the
3403         property name. For example, \p{^Lu} is the same as \P{Lu}.         property name. For example, \p{^Lu} is the same as \P{Lu}.
3404    
3405         If only one letter is specified with \p or \P, it includes all the gen-         If only one letter is specified with \p or \P, it includes all the gen-
3406         eral category properties that start with that letter. In this case,  in         eral  category properties that start with that letter. In this case, in
3407         the  absence of negation, the curly brackets in the escape sequence are         the absence of negation, the curly brackets in the escape sequence  are
3408         optional; these two examples have the same effect:         optional; these two examples have the same effect:
3409    
3410           \p{L}           \p{L}
# Line 3454  BACKSLASH Line 3456  BACKSLASH
3456           Zp    Paragraph separator           Zp    Paragraph separator
3457           Zs    Space separator           Zs    Space separator
3458    
3459         The special property L& is also supported: it matches a character  that         The  special property L& is also supported: it matches a character that
3460         has  the  Lu,  Ll, or Lt property, in other words, a letter that is not         has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
3461         classified as a modifier or "other".         classified as a modifier or "other".
3462    
3463         The Cs (Surrogate) property applies only to  characters  in  the  range         The  Cs  (Surrogate)  property  applies only to characters in the range
3464         U+D800  to  U+DFFF. Such characters are not valid in UTF-8 strings (see         U+D800 to U+DFFF. Such characters are not valid in UTF-8  strings  (see
3465         RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-         RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
3466         ing  has  been  turned off (see the discussion of PCRE_NO_UTF8_CHECK in         ing has been turned off (see the discussion  of  PCRE_NO_UTF8_CHECK  in
3467         the pcreapi page).         the pcreapi page).
3468    
3469         The long synonyms for these properties  that  Perl  supports  (such  as         The  long  synonyms  for  these  properties that Perl supports (such as
3470         \p{Letter})  are  not  supported by PCRE, nor is it permitted to prefix         \p{Letter}) are not supported by PCRE, nor is it  permitted  to  prefix
3471         any of these properties with "Is".         any of these properties with "Is".
3472    
3473         No character that is in the Unicode table has the Cn (unassigned) prop-         No character that is in the Unicode table has the Cn (unassigned) prop-
3474         erty.  Instead, this property is assumed for any code point that is not         erty.  Instead, this property is assumed for any code point that is not
3475         in the Unicode table.         in the Unicode table.
3476    
3477         Specifying caseless matching does not affect  these  escape  sequences.         Specifying  caseless  matching  does not affect these escape sequences.
3478         For example, \p{Lu} always matches only upper case letters.         For example, \p{Lu} always matches only upper case letters.
3479    
3480         The  \X  escape  matches  any number of Unicode characters that form an         The \X escape matches any number of Unicode  characters  that  form  an
3481         extended Unicode sequence. \X is equivalent to         extended Unicode sequence. \X is equivalent to
3482    
3483           (?>\PM\pM*)           (?>\PM\pM*)
3484    
3485         That is, it matches a character without the "mark"  property,  followed         That  is,  it matches a character without the "mark" property, followed
3486         by  zero  or  more  characters with the "mark" property, and treats the         by zero or more characters with the "mark"  property,  and  treats  the
3487         sequence as an atomic group (see below).  Characters  with  the  "mark"         sequence  as  an  atomic group (see below).  Characters with the "mark"
3488         property  are  typically  accents  that affect the preceding character.         property are typically accents that  affect  the  preceding  character.
3489         None of them have codepoints less than 256, so  in  non-UTF-8  mode  \X         None  of  them  have  codepoints less than 256, so in non-UTF-8 mode \X
3490         matches any one character.         matches any one character.
3491    
3492         Matching  characters  by Unicode property is not fast, because PCRE has         Matching characters by Unicode property is not fast, because  PCRE  has
3493         to search a structure that contains  data  for  over  fifteen  thousand         to  search  a  structure  that  contains data for over fifteen thousand
3494         characters. That is why the traditional escape sequences such as \d and         characters. That is why the traditional escape sequences such as \d and
3495         \w do not use Unicode properties in PCRE.         \w do not use Unicode properties in PCRE.
3496    
3497     Resetting the match start     Resetting the match start
3498    
3499         The escape sequence \K, which is a Perl 5.10 feature, causes any previ-         The escape sequence \K, which is a Perl 5.10 feature, causes any previ-
3500         ously  matched  characters  not  to  be  included  in the final matched         ously matched characters not  to  be  included  in  the  final  matched
3501         sequence. For example, the pattern:         sequence. For example, the pattern:
3502    
3503           foo\Kbar           foo\Kbar
3504    
3505         matches "foobar", but reports that it has matched "bar".  This  feature         matches  "foobar",  but reports that it has matched "bar". This feature
3506         is  similar  to  a lookbehind assertion (described below).  However, in         is similar to a lookbehind assertion (described  below).   However,  in
3507         this case, the part of the subject before the real match does not  have         this  case, the part of the subject before the real match does not have
3508         to  be of fixed length, as lookbehind assertions do. The use of \K does         to be of fixed length, as lookbehind assertions do. The use of \K  does
3509         not interfere with the setting of captured  substrings.   For  example,         not  interfere  with  the setting of captured substrings.  For example,
3510         when the pattern         when the pattern
3511    
3512           (foo)\Kbar           (foo)\Kbar
# Line 3513  BACKSLASH Line 3515  BACKSLASH
3515    
3516     Simple assertions     Simple assertions
3517    
3518         The  final use of backslash is for certain simple assertions. An asser-         The final use of backslash is for certain simple assertions. An  asser-
3519         tion specifies a condition that has to be met at a particular point  in         tion  specifies a condition that has to be met at a particular point in
3520         a  match, without consuming any characters from the subject string. The         a match, without consuming any characters from the subject string.  The
3521         use of subpatterns for more complicated assertions is described  below.         use  of subpatterns for more complicated assertions is described below.
3522         The backslashed assertions are:         The backslashed assertions are:
3523    
3524           \b     matches at a word boundary           \b     matches at a word boundary
# Line 3527  BACKSLASH Line 3529  BACKSLASH
3529           \z     matches only at the end of the subject           \z     matches only at the end of the subject
3530           \G     matches at the first matching position in the subject           \G     matches at the first matching position in the subject
3531    
3532         These  assertions may not appear in character classes (but note that \b         These assertions may not appear in character classes (but note that  \b
3533         has a different meaning, namely the backspace character, inside a char-         has a different meaning, namely the backspace character, inside a char-
3534         acter class).         acter class).
3535    
3536         A  word  boundary is a position in the subject string where the current         A word boundary is a position in the subject string where  the  current
3537         character and the previous character do not both match \w or  \W  (i.e.         character  and  the previous character do not both match \w or \W (i.e.
3538         one  matches  \w  and the other matches \W), or the start or end of the         one matches \w and the other matches \W), or the start or  end  of  the
3539         string if the first or last character matches \w, respectively.         string if the first or last character matches \w, respectively.
3540    
3541         The \A, \Z, and \z assertions differ from  the  traditional  circumflex         The  \A,  \Z,  and \z assertions differ from the traditional circumflex
3542         and dollar (described in the next section) in that they only ever match         and dollar (described in the next section) in that they only ever match
3543         at the very start and end of the subject string, whatever  options  are         at  the  very start and end of the subject string, whatever options are
3544         set.  Thus,  they are independent of multiline mode. These three asser-         set. Thus, they are independent of multiline mode. These  three  asser-
3545         tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which         tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
3546         affect  only the behaviour of the circumflex and dollar metacharacters.         affect only the behaviour of the circumflex and dollar  metacharacters.
3547         However, if the startoffset argument of pcre_exec() is non-zero,  indi-         However,  if the startoffset argument of pcre_exec() is non-zero, indi-
3548         cating that matching is to start at a point other than the beginning of         cating that matching is to start at a point other than the beginning of
3549         the subject, \A can never match. The difference between \Z  and  \z  is         the  subject,  \A  can never match. The difference between \Z and \z is
3550         that \Z matches before a newline at the end of the string as well as at         that \Z matches before a newline at the end of the string as well as at
3551         the very end, whereas \z matches only at the end.         the very end, whereas \z matches only at the end.
3552    
3553         The \G assertion is true only when the current matching position is  at         The  \G assertion is true only when the current matching position is at
3554         the  start point of the match, as specified by the startoffset argument         the start point of the match, as specified by the startoffset  argument
3555         of pcre_exec(). It differs from \A when the  value  of  startoffset  is         of  pcre_exec().  It  differs  from \A when the value of startoffset is
3556         non-zero.  By calling pcre_exec() multiple times with appropriate argu-         non-zero. By calling pcre_exec() multiple times with appropriate  argu-
3557         ments, you can mimic Perl's /g option, and it is in this kind of imple-         ments, you can mimic Perl's /g option, and it is in this kind of imple-
3558         mentation where \G can be useful.         mentation where \G can be useful.
3559    
3560         Note,  however,  that  PCRE's interpretation of \G, as the start of the         Note, however, that PCRE's interpretation of \G, as the  start  of  the
3561         current match, is subtly different from Perl's, which defines it as the         current match, is subtly different from Perl's, which defines it as the
3562         end  of  the  previous  match. In Perl, these can be different when the         end of the previous match. In Perl, these can  be  different  when  the
3563         previously matched string was empty. Because PCRE does just  one  match         previously  matched  string was empty. Because PCRE does just one match
3564         at a time, it cannot reproduce this behaviour.         at a time, it cannot reproduce this behaviour.
3565    
3566         If  all  the alternatives of a pattern begin with \G, the expression is         If all the alternatives of a pattern begin with \G, the  expression  is
3567         anchored to the starting match position, and the "anchored" flag is set         anchored to the starting match position, and the "anchored" flag is set
3568         in the compiled regular expression.         in the compiled regular expression.
3569    
# Line 3569  BACKSLASH Line 3571  BACKSLASH
3571  CIRCUMFLEX AND DOLLAR  CIRCUMFLEX AND DOLLAR
3572    
3573         Outside a character class, in the default matching mode, the circumflex         Outside a character class, in the default matching mode, the circumflex
3574         character is an assertion that is true only  if  the  current  matching         character  is  an  assertion  that is true only if the current matching
3575         point  is  at the start of the subject string. If the startoffset argu-         point is at the start of the subject string. If the  startoffset  argu-
3576         ment of pcre_exec() is non-zero, circumflex  can  never  match  if  the         ment  of  pcre_exec()  is  non-zero,  circumflex can never match if the
3577         PCRE_MULTILINE  option  is  unset. Inside a character class, circumflex         PCRE_MULTILINE option is unset. Inside a  character  class,  circumflex
3578         has an entirely different meaning (see below).         has an entirely different meaning (see below).
3579    
3580         Circumflex need not be the first character of the pattern if  a  number         Circumflex  need  not be the first character of the pattern if a number
3581         of  alternatives are involved, but it should be the first thing in each         of alternatives are involved, but it should be the first thing in  each
3582         alternative in which it appears if the pattern is ever  to  match  that         alternative  in  which  it appears if the pattern is ever to match that
3583         branch.  If all possible alternatives start with a circumflex, that is,         branch. If all possible alternatives start with a circumflex, that  is,
3584         if the pattern is constrained to match only at the start  of  the  sub-         if  the  pattern  is constrained to match only at the start of the sub-
3585         ject,  it  is  said  to be an "anchored" pattern. (There are also other         ject, it is said to be an "anchored" pattern.  (There  are  also  other
3586         constructs that can cause a pattern to be anchored.)         constructs that can cause a pattern to be anchored.)
3587    
3588         A dollar character is an assertion that is true  only  if  the  current         A  dollar  character  is  an assertion that is true only if the current
3589         matching  point  is  at  the  end of the subject string, or immediately         matching point is at the end of  the  subject  string,  or  immediately
3590         before a newline at the end of the string (by default). Dollar need not         before a newline at the end of the string (by default). Dollar need not
3591         be  the  last  character of the pattern if a number of alternatives are         be the last character of the pattern if a number  of  alternatives  are
3592         involved, but it should be the last item in  any  branch  in  which  it         involved,  but  it  should  be  the last item in any branch in which it
3593         appears. Dollar has no special meaning in a character class.         appears. Dollar has no special meaning in a character class.
3594    
3595         The  meaning  of  dollar  can be changed so that it matches only at the         The meaning of dollar can be changed so that it  matches  only  at  the
3596         very end of the string, by setting the  PCRE_DOLLAR_ENDONLY  option  at         very  end  of  the string, by setting the PCRE_DOLLAR_ENDONLY option at
3597         compile time. This does not affect the \Z assertion.         compile time. This does not affect the \Z assertion.
3598    
3599         The meanings of the circumflex and dollar characters are changed if the         The meanings of the circumflex and dollar characters are changed if the
3600         PCRE_MULTILINE option is set. When  this  is  the  case,  a  circumflex         PCRE_MULTILINE  option  is  set.  When  this  is the case, a circumflex
3601         matches  immediately after internal newlines as well as at the start of         matches immediately after internal newlines as well as at the start  of
3602         the subject string. It does not match after a  newline  that  ends  the         the  subject  string.  It  does not match after a newline that ends the
3603         string.  A dollar matches before any newlines in the string, as well as         string. A dollar matches before any newlines in the string, as well  as
3604         at the very end, when PCRE_MULTILINE is set. When newline is  specified         at  the very end, when PCRE_MULTILINE is set. When newline is specified
3605         as  the  two-character  sequence CRLF, isolated CR and LF characters do         as the two-character sequence CRLF, isolated CR and  LF  characters  do
3606         not indicate newlines.         not indicate newlines.
3607    
3608         For example, the pattern /^abc$/ matches the subject string  "def\nabc"         For  example, the pattern /^abc$/ matches the subject string "def\nabc"
3609         (where  \n  represents a newline) in multiline mode, but not otherwise.         (where \n represents a newline) in multiline mode, but  not  otherwise.
3610         Consequently, patterns that are anchored in single  line  mode  because         Consequently,  patterns  that  are anchored in single line mode because
3611         all  branches  start  with  ^ are not anchored in multiline mode, and a         all branches start with ^ are not anchored in  multiline  mode,  and  a
3612         match for circumflex is  possible  when  the  startoffset  argument  of         match  for  circumflex  is  possible  when  the startoffset argument of
3613         pcre_exec()  is  non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if         pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is  ignored  if
3614         PCRE_MULTILINE is set.         PCRE_MULTILINE is set.
3615    
3616         Note that the sequences \A, \Z, and \z can be used to match  the  start         Note  that  the sequences \A, \Z, and \z can be used to match the start
3617         and  end of the subject in both modes, and if all branches of a pattern         and end of the subject in both modes, and if all branches of a  pattern
3618         start with \A it is always anchored, whether or not  PCRE_MULTILINE  is         start  with  \A it is always anchored, whether or not PCRE_MULTILINE is
3619         set.         set.
3620    
3621    
3622  FULL STOP (PERIOD, DOT)  FULL STOP (PERIOD, DOT)
3623    
3624         Outside a character class, a dot in the pattern matches any one charac-         Outside a character class, a dot in the pattern matches any one charac-
3625         ter in the subject string except (by default) a character  that  signi-         ter  in  the subject string except (by default) a character that signi-
3626         fies  the  end  of  a line. In UTF-8 mode, the matched character may be         fies the end of a line. In UTF-8 mode, the  matched  character  may  be
3627         more than one byte long.         more than one byte long.
3628    
3629         When a line ending is defined as a single character, dot never  matches         When  a line ending is defined as a single character, dot never matches
3630         that  character; when the two-character sequence CRLF is used, dot does         that character; when the two-character sequence CRLF is used, dot  does
3631         not match CR if it is immediately followed  by  LF,  but  otherwise  it         not  match  CR  if  it  is immediately followed by LF, but otherwise it
3632         matches  all characters (including isolated CRs and LFs). When any Uni-         matches all characters (including isolated CRs and LFs). When any  Uni-
3633         code line endings are being recognized, dot does not match CR or LF  or         code  line endings are being recognized, dot does not match CR or LF or
3634         any of the other line ending characters.         any of the other line ending characters.
3635    
3636         The  behaviour  of  dot  with regard to newlines can be changed. If the         The behaviour of dot with regard to newlines can  be  changed.  If  the
3637         PCRE_DOTALL option is set, a dot matches  any  one  character,  without         PCRE_DOTALL  option  is  set,  a dot matches any one character, without
3638         exception. If the two-character sequence CRLF is present in the subject         exception. If the two-character sequence CRLF is present in the subject
3639         string, it takes two dots to match it.         string, it takes two dots to match it.
3640    
3641         The handling of dot is entirely independent of the handling of  circum-         The  handling of dot is entirely independent of the handling of circum-
3642         flex  and  dollar,  the  only relationship being that they both involve         flex and dollar, the only relationship being  that  they  both  involve
3643         newlines. Dot has no special meaning in a character class.         newlines. Dot has no special meaning in a character class.
3644    
3645    
3646  MATCHING A SINGLE BYTE  MATCHING A SINGLE BYTE
3647    
3648         Outside a character class, the escape sequence \C matches any one byte,         Outside a character class, the escape sequence \C matches any one byte,
3649         both  in  and  out  of  UTF-8 mode. Unlike a dot, it always matches any         both in and out of UTF-8 mode. Unlike a  dot,  it  always  matches  any
3650         line-ending characters. The feature is provided in  Perl  in  order  to         line-ending  characters.  The  feature  is provided in Perl in order to
3651         match  individual bytes in UTF-8 mode. Because it breaks up UTF-8 char-         match individual bytes in UTF-8 mode. Because it breaks up UTF-8  char-
3652         acters into individual bytes, what remains in the string may be a  mal-         acters  into individual bytes, what remains in the string may be a mal-
3653         formed  UTF-8  string.  For this reason, the \C escape sequence is best         formed UTF-8 string. For this reason, the \C escape  sequence  is  best
3654         avoided.         avoided.
3655    
3656         PCRE does not allow \C to appear in  lookbehind  assertions  (described         PCRE  does  not  allow \C to appear in lookbehind assertions (described
3657         below),  because  in UTF-8 mode this would make it impossible to calcu-         below), because in UTF-8 mode this would make it impossible  to  calcu-
3658         late the length of the lookbehind.         late the length of the lookbehind.
3659    
3660    
# Line 3661  SQUARE BRACKETS AND CHARACTER CLASSES Line 3663  SQUARE BRACKETS AND CHARACTER CLASSES
3663         An opening square bracket introduces a character class, terminated by a         An opening square bracket introduces a character class, terminated by a
3664         closing square bracket. A closing square bracket on its own is not spe-         closing square bracket. A closing square bracket on its own is not spe-
3665         cial. If a closing square bracket is required as a member of the class,         cial. If a closing square bracket is required as a member of the class,
3666         it  should  be  the first data character in the class (after an initial         it should be the first data character in the class  (after  an  initial
3667         circumflex, if present) or escaped with a backslash.         circumflex, if present) or escaped with a backslash.
3668    
3669         A character class matches a single character in the subject.  In  UTF-8         A  character  class matches a single character in the subject. In UTF-8
3670         mode,  the character may occupy more than one byte. A matched character         mode, the character may occupy more than one byte. A matched  character
3671         must be in the set of characters defined by the class, unless the first         must be in the set of characters defined by the class, unless the first
3672         character  in  the  class definition is a circumflex, in which case the         character in the class definition is a circumflex, in  which  case  the
3673         subject character must not be in the set defined by  the  class.  If  a         subject  character  must  not  be in the set defined by the class. If a
3674         circumflex  is actually required as a member of the class, ensure it is         circumflex is actually required as a member of the class, ensure it  is
3675         not the first character, or escape it with a backslash.         not the first character, or escape it with a backslash.
3676    
3677         For example, the character class [aeiou] matches any lower case  vowel,         For  example, the character class [aeiou] matches any lower case vowel,
3678         while  [^aeiou]  matches  any character that is not a lower case vowel.         while [^aeiou] matches any character that is not a  lower  case  vowel.
3679         Note that a circumflex is just a convenient notation for specifying the         Note that a circumflex is just a convenient notation for specifying the
3680         characters  that  are in the class by enumerating those that are not. A         characters that are in the class by enumerating those that are  not.  A
3681         class that starts with a circumflex is not an assertion: it still  con-         class  that starts with a circumflex is not an assertion: it still con-
3682         sumes  a  character  from the subject string, and therefore it fails if         sumes a character from the subject string, and therefore  it  fails  if
3683         the current pointer is at the end of the string.         the current pointer is at the end of the string.
3684    
3685         In UTF-8 mode, characters with values greater than 255 can be  included         In  UTF-8 mode, characters with values greater than 255 can be included
3686         in  a  class as a literal string of bytes, or by using the \x{ escaping         in a class as a literal string of bytes, or by using the  \x{  escaping
3687         mechanism.         mechanism.
3688    
3689         When caseless matching is set, any letters in a  class  represent  both         When  caseless  matching  is set, any letters in a class represent both
3690         their  upper  case  and lower case versions, so for example, a caseless         their upper case and lower case versions, so for  example,  a  caseless
3691         [aeiou] matches "A" as well as "a", and a caseless  [^aeiou]  does  not         [aeiou]  matches  "A"  as well as "a", and a caseless [^aeiou] does not
3692         match  "A", whereas a caseful version would. In UTF-8 mode, PCRE always         match "A", whereas a caseful version would. In UTF-8 mode, PCRE  always
3693         understands the concept of case for characters whose  values  are  less         understands  the  concept  of case for characters whose values are less
3694         than  128, so caseless matching is always possible. For characters with         than 128, so caseless matching is always possible. For characters  with
3695         higher values, the concept of case is supported  if  PCRE  is  compiled         higher  values,  the  concept  of case is supported if PCRE is compiled
3696         with  Unicode  property support, but not otherwise.  If you want to use         with Unicode property support, but not otherwise.  If you want  to  use
3697         caseless matching for characters 128 and above, you  must  ensure  that         caseless  matching  for  characters 128 and above, you must ensure that
3698         PCRE  is  compiled  with Unicode property support as well as with UTF-8         PCRE is compiled with Unicode property support as well  as  with  UTF-8
3699         support.         support.
3700    
3701         Characters that might indicate line breaks are  never  treated  in  any         Characters  that  might  indicate  line breaks are never treated in any
3702         special  way  when  matching  character  classes,  whatever line-ending         special way  when  matching  character  classes,  whatever  line-ending
3703         sequence is in  use,  and  whatever  setting  of  the  PCRE_DOTALL  and         sequence  is  in  use,  and  whatever  setting  of  the PCRE_DOTALL and
3704         PCRE_MULTILINE options is used. A class such as [^a] always matches one         PCRE_MULTILINE options is used. A class such as [^a] always matches one
3705         of these characters.         of these characters.
3706    
3707         The minus (hyphen) character can be used to specify a range of  charac-         The  minus (hyphen) character can be used to specify a range of charac-
3708         ters  in  a  character  class.  For  example,  [d-m] matches any letter         ters in a character  class.  For  example,  [d-m]  matches  any  letter
3709         between d and m, inclusive. If a  minus  character  is  required  in  a         between  d  and  m,  inclusive.  If  a minus character is required in a
3710         class,  it  must  be  escaped  with a backslash or appear in a position         class, it must be escaped with a backslash  or  appear  in  a  position
3711         where it cannot be interpreted as indicating a range, typically as  the         where  it cannot be interpreted as indicating a range, typically as the
3712         first or last character in the class.         first or last character in the class.
3713    
3714         It is not possible to have the literal character "]" as the end charac-         It is not possible to have the literal character "]" as the end charac-
3715         ter of a range. A pattern such as [W-]46] is interpreted as a class  of         ter  of a range. A pattern such as [W-]46] is interpreted as a class of
3716         two  characters ("W" and "-") followed by a literal string "46]", so it         two characters ("W" and "-") followed by a literal string "46]", so  it
3717         would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a         would  match  "W46]"  or  "-46]". However, if the "]" is escaped with a
3718         backslash  it is interpreted as the end of range, so [W-\]46] is inter-         backslash it is interpreted as the end of range, so [W-\]46] is  inter-
3719         preted as a class containing a range followed by two other  characters.         preted  as a class containing a range followed by two other characters.
3720         The  octal or hexadecimal representation of "]" can also be used to end         The octal or hexadecimal representation of "]" can also be used to  end
3721         a range.         a range.
3722    
3723         Ranges operate in the collating sequence of character values. They  can         Ranges  operate in the collating sequence of character values. They can
3724         also   be  used  for  characters  specified  numerically,  for  example         also  be  used  for  characters  specified  numerically,  for   example
3725         [\000-\037]. In UTF-8 mode, ranges can include characters whose  values         [\000-\037].  In UTF-8 mode, ranges can include characters whose values
3726         are greater than 255, for example [\x{100}-\x{2ff}].         are greater than 255, for example [\x{100}-\x{2ff}].
3727    
3728         If a range that includes letters is used when caseless matching is set,         If a range that includes letters is used when caseless matching is set,
3729         it matches the letters in either case. For example, [W-c] is equivalent         it matches the letters in either case. For example, [W-c] is equivalent
3730         to  [][\\^_`wxyzabc],  matched  caselessly,  and  in non-UTF-8 mode, if         to [][\\^_`wxyzabc], matched caselessly,  and  in  non-UTF-8  mode,  if
3731         character tables for a French locale are in  use,  [\xc8-\xcb]  matches         character  tables  for  a French locale are in use, [\xc8-\xcb] matches
3732         accented  E  characters in both cases. In UTF-8 mode, PCRE supports the         accented E characters in both cases. In UTF-8 mode, PCRE  supports  the
3733         concept of case for characters with values greater than 128  only  when         concept  of  case for characters with values greater than 128 only when
3734         it is compiled with Unicode property support.         it is compiled with Unicode property support.
3735    
3736         The  character types \d, \D, \p, \P, \s, \S, \w, and \W may also appear         The character types \d, \D, \p, \P, \s, \S, \w, and \W may also  appear
3737         in a character class, and add the characters that  they  match  to  the         in  a  character  class,  and add the characters that they match to the
3738         class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-         class. For example, [\dABCDEF] matches any hexadecimal digit. A circum-
3739         flex can conveniently be used with the upper case  character  types  to         flex  can  conveniently  be used with the upper case character types to
3740         specify  a  more  restricted  set of characters than the matching lower         specify a more restricted set of characters  than  the  matching  lower
3741         case type. For example, the class [^\W_] matches any letter  or  digit,         case  type.  For example, the class [^\W_] matches any letter or digit,
3742         but not underscore.         but not underscore.
3743    
3744         The  only  metacharacters  that are recognized in character classes are         The only metacharacters that are recognized in  character  classes  are
3745         backslash, hyphen (only where it can be  interpreted  as  specifying  a         backslash,  hyphen  (only  where  it can be interpreted as specifying a
3746         range),  circumflex  (only  at the start), opening square bracket (only         range), circumflex (only at the start), opening  square  bracket  (only
3747         when it can be interpreted as introducing a POSIX class name - see  the         when  it can be interpreted as introducing a POSIX class name - see the
3748         next  section),  and  the  terminating closing square bracket. However,         next section), and the terminating  closing  square  bracket.  However,
3749         escaping other non-alphanumeric characters does no harm.         escaping other non-alphanumeric characters does no harm.
3750    
3751    
3752  POSIX CHARACTER CLASSES  POSIX CHARACTER CLASSES
3753    
3754         Perl supports the POSIX notation for character classes. This uses names         Perl supports the POSIX notation for character classes. This uses names
3755         enclosed  by  [: and :] within the enclosing square brackets. PCRE also         enclosed by [: and :] within the enclosing square brackets.  PCRE  also
3756         supports this notation. For example,         supports this notation. For example,
3757    
3758           [01[:alpha:]%]           [01[:alpha:]%]
# Line 3773  POSIX CHARACTER CLASSES Line 3775  POSIX CHARACTER CLASSES
3775           word     "word" characters (same as \w)           word     "word" characters (same as \w)
3776           xdigit   hexadecimal digits           xdigit   hexadecimal digits
3777    
3778         The  "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),         The "space" characters are HT (9), LF (10), VT (11), FF (12), CR  (13),
3779         and space (32). Notice that this list includes the VT  character  (code         and  space  (32). Notice that this list includes the VT character (code
3780         11). This makes "space" different to \s, which does not include VT (for         11). This makes "space" different to \s, which does not include VT (for
3781         Perl compatibility).         Perl compatibility).
3782    
3783         The name "word" is a Perl extension, and "blank"  is  a  GNU  extension         The  name  "word"  is  a Perl extension, and "blank" is a GNU extension
3784         from  Perl  5.8. Another Perl extension is negation, which is indicated         from Perl 5.8. Another Perl extension is negation, which  is  indicated
3785         by a ^ character after the colon. For example,         by a ^ character after the colon. For example,
3786    
3787           [12[:^digit:]]           [12[:^digit:]]
3788    
3789         matches "1", "2", or any non-digit. PCRE (and Perl) also recognize  the         matches  "1", "2", or any non-digit. PCRE (and Perl) also recognize the
3790         POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but         POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
3791         these are not supported, and an error is given if they are encountered.         these are not supported, and an error is given if they are encountered.
3792    
# Line 3794  POSIX CHARACTER CLASSES Line 3796  POSIX CHARACTER CLASSES
3796    
3797  VERTICAL BAR  VERTICAL BAR
3798    
3799         Vertical  bar characters are used to separate alternative patterns. For         Vertical bar characters are used to separate alternative patterns.  For
3800         example, the pattern         example, the pattern
3801    
3802           gilbert|sullivan           gilbert|sullivan
3803    
3804         matches either "gilbert" or "sullivan". Any number of alternatives  may         matches  either "gilbert" or "sullivan". Any number of alternatives may
3805         appear,  and  an  empty  alternative  is  permitted (matching the empty         appear, and an empty  alternative  is  permitted  (matching  the  empty
3806         string). The matching process tries each alternative in turn, from left         string). The matching process tries each alternative in turn, from left
3807         to  right, and the first one that succeeds is used. If the alternatives         to right, and the first one that succeeds is used. If the  alternatives
3808         are within a subpattern (defined below), "succeeds" means matching  the         are  within a subpattern (defined below), "succeeds" means matching the
3809         rest of the main pattern as well as the alternative in the subpattern.         rest of the main pattern as well as the alternative in the subpattern.
3810    
3811    
3812  INTERNAL OPTION SETTING  INTERNAL OPTION SETTING
3813    
3814         The  settings  of  the  PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and         The settings of the  PCRE_CASELESS,  PCRE_MULTILINE,  PCRE_DOTALL,  and
3815         PCRE_EXTENDED options (which are Perl-compatible) can be  changed  from         PCRE_EXTENDED  options  (which are Perl-compatible) can be changed from
3816         within  the  pattern  by  a  sequence  of  Perl option letters enclosed         within the pattern by  a  sequence  of  Perl  option  letters  enclosed
3817         between "(?" and ")".  The option letters are         between "(?" and ")".  The option letters are
3818    
3819           i  for PCRE_CASELESS           i  for PCRE_CASELESS
# Line 3821  INTERNAL OPTION SETTING Line 3823  INTERNAL OPTION SETTING
3823    
3824         For example, (?im) sets caseless, multiline matching. It is also possi-         For example, (?im) sets caseless, multiline matching. It is also possi-
3825         ble to unset these options by preceding the letter with a hyphen, and a         ble to unset these options by preceding the letter with a hyphen, and a
3826         combined setting and unsetting such as (?im-sx), which sets  PCRE_CASE-         combined  setting and unsetting such as (?im-sx), which sets PCRE_CASE-
3827         LESS  and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,         LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and  PCRE_EXTENDED,
3828         is also permitted. If a  letter  appears  both  before  and  after  the         is  also  permitted.  If  a  letter  appears  both before and after the
3829         hyphen, the option is unset.         hyphen, the option is unset.
3830    
3831         The  PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA         The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and  PCRE_EXTRA
3832         can be changed in the same way as the Perl-compatible options by  using         can  be changed in the same way as the Perl-compatible options by using
3833         the characters J, U and X respectively.         the characters J, U and X respectively.
3834    
3835         When  an option change occurs at top level (that is, not inside subpat-         When an option change occurs at top level (that is, not inside  subpat-
3836         tern parentheses), the change applies to the remainder of  the  pattern         tern  parentheses),  the change applies to the remainder of the pattern
3837         that follows.  If the change is placed right at the start of a pattern,         that follows.  If the change is placed right at the start of a pattern,
3838         PCRE extracts it into the global options (and it will therefore show up         PCRE extracts it into the global options (and it will therefore show up
3839         in data extracted by the pcre_fullinfo() function).         in data extracted by the pcre_fullinfo() function).
3840    
3841         An  option  change  within a subpattern (see below for a description of         An option change within a subpattern (see below for  a  description  of
3842         subpatterns) affects only that part of the current pattern that follows         subpatterns) affects only that part of the current pattern that follows
3843         it, so         it, so
3844    
3845           (a(?i)b)c           (a(?i)b)c
3846    
3847         matches abc and aBc and no other strings (assuming PCRE_CASELESS is not         matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
3848         used).  By this means, options can be made to have  different  settings         used).   By  this means, options can be made to have different settings
3849         in  different parts of the pattern. Any changes made in one alternative         in different parts of the pattern. Any changes made in one  alternative
3850         do carry on into subsequent branches within the  same  subpattern.  For         do  carry  on  into subsequent branches within the same subpattern. For
3851         example,         example,
3852    
3853           (a(?i)b|c)           (a(?i)b|c)
3854    
3855         matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the         matches "ab", "aB", "c", and "C", even though  when  matching  "C"  the
3856         first branch is abandoned before the option setting.  This  is  because         first  branch  is  abandoned before the option setting. This is because
3857         the  effects  of option settings happen at compile time. There would be         the effects of option settings happen at compile time. There  would  be
3858         some very weird behaviour otherwise.         some very weird behaviour otherwise.
3859    
3860         Note: There are other PCRE-specific options that  can  be  set  by  the         Note:  There  are  other  PCRE-specific  options that can be set by the
3861         application  when  the  compile  or match functions are called. In some         application when the compile or match functions  are  called.  In  some
3862         cases the pattern can contain special  leading  sequences  to  override         cases  the  pattern  can  contain special leading sequences to override
3863         what  the  application  has set or what has been defaulted. Details are         what the application has set or what has been  defaulted.  Details  are
3864         given in the section entitled "Newline sequences" above.         given in the section entitled "Newline sequences" above.
3865    
3866    
# Line 3871  SUBPATTERNS Line 3873  SUBPATTERNS
3873    
3874           cat(aract|erpillar|)           cat(aract|erpillar|)
3875    
3876         matches  one  of the words "cat", "cataract", or "caterpillar". Without         matches one of the words "cat", "cataract", or  "caterpillar".  Without
3877         the parentheses, it would match  "cataract",  "erpillar"  or  an  empty         the  parentheses,  it  would  match  "cataract", "erpillar" or an empty
3878         string.         string.
3879    
3880         2.  It  sets  up  the  subpattern as a capturing subpattern. This means         2. It sets up the subpattern as  a  capturing  subpattern.  This  means
3881         that, when the whole pattern  matches,  that  portion  of  the  subject         that,  when  the  whole  pattern  matches,  that portion of the subject
3882         string that matched the subpattern is passed back to the caller via the         string that matched the subpattern is passed back to the caller via the
3883         ovector argument of pcre_exec(). Opening parentheses are  counted  from         ovector  argument  of pcre_exec(). Opening parentheses are counted from
3884         left  to  right  (starting  from 1) to obtain numbers for the capturing         left to right (starting from 1) to obtain  numbers  for  the  capturing
3885         subpatterns.         subpatterns.
3886    
3887         For example, if the string "the red king" is matched against  the  pat-         For  example,  if the string "the red king" is matched against the pat-
3888         tern         tern
3889    
3890           the ((red|white) (king|queen))           the ((red|white) (king|queen))
# Line 3890  SUBPATTERNS Line 3892  SUBPATTERNS
3892         the captured substrings are "red king", "red", and "king", and are num-         the captured substrings are "red king", "red", and "king", and are num-
3893         bered 1, 2, and 3, respectively.         bered 1, 2, and 3, respectively.
3894    
3895         The fact that plain parentheses fulfil  two  functions  is  not  always         The  fact  that  plain  parentheses  fulfil two functions is not always
3896         helpful.   There are often times when a grouping subpattern is required         helpful.  There are often times when a grouping subpattern is  required
3897         without a capturing requirement. If an opening parenthesis is  followed         without  a capturing requirement. If an opening parenthesis is followed
3898         by  a question mark and a colon, the subpattern does not do any captur-         by a question mark and a colon, the subpattern does not do any  captur-
3899         ing, and is not counted when computing the  number  of  any  subsequent         ing,  and  is  not  counted when computing the number of any subsequent
3900         capturing  subpatterns. For example, if the string "the white queen" is         capturing subpatterns. For example, if the string "the white queen"  is
3901         matched against the pattern         matched against the pattern
3902    
3903           the ((?:red|white) (king|queen))           the ((?:red|white) (king|queen))
# Line 3903  SUBPATTERNS Line 3905  SUBPATTERNS
3905         the captured substrings are "white queen" and "queen", and are numbered         the captured substrings are "white queen" and "queen", and are numbered
3906         1 and 2. The maximum number of capturing subpatterns is 65535.         1 and 2. The maximum number of capturing subpatterns is 65535.
3907    
3908         As  a  convenient shorthand, if any option settings are required at the         As a convenient shorthand, if any option settings are required  at  the
3909         start of a non-capturing subpattern,  the  option  letters  may  appear         start  of  a  non-capturing  subpattern,  the option letters may appear
3910         between the "?" and the ":". Thus the two patterns         between the "?" and the ":". Thus the two patterns
3911    
3912           (?i:saturday|sunday)           (?i:saturday|sunday)
3913           (?:(?i)saturday|sunday)           (?:(?i)saturday|sunday)
3914    
3915         match exactly the same set of strings. Because alternative branches are         match exactly the same set of strings. Because alternative branches are
3916         tried from left to right, and options are not reset until  the  end  of         tried  from  left  to right, and options are not reset until the end of
3917         the  subpattern is reached, an option setting in one branch does affect         the subpattern is reached, an option setting in one branch does  affect
3918         subsequent branches, so the above patterns match "SUNDAY"  as  well  as         subsequent  branches,  so  the above patterns match "SUNDAY" as well as
3919         "Saturday".         "Saturday".
3920    
3921    
3922  DUPLICATE SUBPATTERN NUMBERS  DUPLICATE SUBPATTERN NUMBERS
3923    
3924         Perl 5.10 introduced a feature whereby each alternative in a subpattern         Perl 5.10 introduced a feature whereby each alternative in a subpattern
3925         uses the same numbers for its capturing parentheses. Such a  subpattern         uses  the same numbers for its capturing parentheses. Such a subpattern
3926         starts  with (?| and is itself a non-capturing subpattern. For example,         starts with (?| and is itself a non-capturing subpattern. For  example,
3927         consider this pattern:         consider this pattern:
3928    
3929           (?|(Sat)ur|(Sun))day           (?|(Sat)ur|(Sun))day
3930    
3931         Because the two alternatives are inside a (?| group, both sets of  cap-         Because  the two alternatives are inside a (?| group, both sets of cap-
3932         turing  parentheses  are  numbered one. Thus, when the pattern matches,         turing parentheses are numbered one. Thus, when  the  pattern  matches,
3933         you can look at captured substring number  one,  whichever  alternative         you  can  look  at captured substring number one, whichever alternative
3934         matched.  This  construct  is useful when you want to capture part, but         matched. This construct is useful when you want to  capture  part,  but
3935         not all, of one of a number of alternatives. Inside a (?| group, paren-         not all, of one of a number of alternatives. Inside a (?| group, paren-
3936         theses  are  numbered as usual, but the number is reset at the start of         theses are numbered as usual, but the number is reset at the  start  of
3937         each branch. The numbers of any capturing buffers that follow the  sub-         each  branch. The numbers of any capturing buffers that follow the sub-
3938         pattern  start after the highest number used in any branch. The follow-         pattern start after the highest number used in any branch. The  follow-
3939         ing example is taken from the Perl documentation.  The  numbers  under-         ing  example  is taken from the Perl documentation.  The numbers under-
3940         neath show in which buffer the captured content will be stored.         neath show in which buffer the captured content will be stored.
3941    
3942           # before  ---------------branch-reset----------- after           # before  ---------------branch-reset----------- after
3943           / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x           / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
3944           # 1            2         2  3        2     3     4           # 1            2         2  3        2     3     4
3945    
3946         A  backreference  or  a  recursive call to a numbered subpattern always         A backreference or a recursive call to  a  numbered  subpattern  always
3947         refers to the first one in the pattern with the given number.         refers to the first one in the pattern with the given number.
3948    
3949         An alternative approach to using this "branch reset" feature is to  use         An  alternative approach to using this "branch reset" feature is to use
3950         duplicate named subpatterns, as described in the next section.         duplicate named subpatterns, as described in the next section.
3951    
3952    
3953  NAMED SUBPATTERNS  NAMED SUBPATTERNS
3954    
3955         Identifying  capturing  parentheses  by number is simple, but it can be         Identifying capturing parentheses by number is simple, but  it  can  be
3956         very hard to keep track of the numbers in complicated  regular  expres-         very  hard  to keep track of the numbers in complicated regular expres-
3957         sions.  Furthermore,  if  an  expression  is  modified, the numbers may         sions. Furthermore, if an  expression  is  modified,  the  numbers  may
3958         change. To help with this difficulty, PCRE supports the naming of  sub-         change.  To help with this difficulty, PCRE supports the naming of sub-
3959         patterns. This feature was not added to Perl until release 5.10. Python         patterns. This feature was not added to Perl until release 5.10. Python
3960         had the feature earlier, and PCRE introduced it at release  4.0,  using         had  the  feature earlier, and PCRE introduced it at release 4.0, using
3961         the  Python syntax. PCRE now supports both the Perl and the Python syn-         the Python syntax. PCRE now supports both the Perl and the Python  syn-
3962         tax.         tax.
3963    
3964         In PCRE, a subpattern can be named in one of three  ways:  (?<name>...)         In  PCRE,  a subpattern can be named in one of three ways: (?<name>...)
3965         or  (?'name'...)  as in Perl, or (?P<name>...) as in Python. References         or (?'name'...) as in Perl, or (?P<name>...) as in  Python.  References
3966         to capturing parentheses from other parts of the pattern, such as back-         to capturing parentheses from other parts of the pattern, such as back-
3967         references,  recursion,  and conditions, can be made by name as well as         references, recursion, and conditions, can be made by name as  well  as
3968         by number.         by number.
3969    
3970         Names consist of up to  32  alphanumeric  characters  and  underscores.         Names  consist  of  up  to  32 alphanumeric characters and underscores.
3971         Named  capturing  parentheses  are  still  allocated numbers as well as         Named capturing parentheses are still  allocated  numbers  as  well  as
3972         names, exactly as if the names were not present. The PCRE API  provides         names,  exactly as if the names were not present. The PCRE API provides
3973         function calls for extracting the name-to-number translation table from         function calls for extracting the name-to-number translation table from
3974         a compiled pattern. There is also a convenience function for extracting         a compiled pattern. There is also a convenience function for extracting
3975         a captured substring by name.         a captured substring by name.
3976    
3977         By  default, a name must be unique within a pattern, but it is possible         By default, a name must be unique within a pattern, but it is  possible
3978         to relax this constraint by setting the PCRE_DUPNAMES option at compile         to relax this constraint by setting the PCRE_DUPNAMES option at compile
3979         time.  This  can  be useful for patterns where only one instance of the         time. This can be useful for patterns where only one  instance  of  the
3980         named parentheses can match. Suppose you want to match the  name  of  a         named  parentheses  can  match. Suppose you want to match the name of a
3981         weekday,  either as a 3-letter abbreviation or as the full name, and in         weekday, either as a 3-letter abbreviation or as the full name, and  in
3982         both cases you want to extract the abbreviation. This pattern (ignoring         both cases you want to extract the abbreviation. This pattern (ignoring
3983         the line breaks) does the job:         the line breaks) does the job:
3984    
# Line 3986  NAMED SUBPATTERNS Line 3988  NAMED SUBPATTERNS
3988           (?<DN>Thu)(?:rsday)?|           (?<DN>Thu)(?:rsday)?|
3989           (?<DN>Sat)(?:urday)?           (?<DN>Sat)(?:urday)?
3990    
3991         There  are  five capturing substrings, but only one is ever set after a         There are five capturing substrings, but only one is ever set  after  a
3992         match.  (An alternative way of solving this problem is to use a "branch         match.  (An alternative way of solving this problem is to use a "branch
3993         reset" subpattern, as described in the previous section.)         reset" subpattern, as described in the previous section.)
3994    
3995         The  convenience  function  for extracting the data by name returns the         The convenience function for extracting the data by  name  returns  the
3996         substring for the first (and in this example, the only)  subpattern  of         substring  for  the first (and in this example, the only) subpattern of
3997         that  name  that  matched.  This saves searching to find which numbered         that name that matched. This saves searching  to  find  which  numbered
3998         subpattern it was. If you make a reference to a non-unique  named  sub-         subpattern  it  was. If you make a reference to a non-unique named sub-
3999         pattern  from elsewhere in the pattern, the one that corresponds to the         pattern from elsewhere in the pattern, the one that corresponds to  the
4000         lowest number is used. For further details of the interfaces  for  han-         lowest  number  is used. For further details of the interfaces for han-
4001         dling named subpatterns, see the pcreapi documentation.         dling named subpatterns, see the pcreapi documentation.
4002    
4003         Warning: You cannot use different names to distinguish between two sub-         Warning: You cannot use different names to distinguish between two sub-
4004         patterns with the same number (see the previous section)  because  PCRE         patterns  with  the same number (see the previous section) because PCRE
4005         uses only the numbers when matching.         uses only the numbers when matching.
4006    
4007    
4008  REPETITION  REPETITION
4009    
4010         Repetition  is  specified  by  quantifiers, which can follow any of the         Repetition is specified by quantifiers, which can  follow  any  of  the
4011         following items:         following items:
4012    
4013           a literal data character           a literal data character
# Line 4018  REPETITION Line 4020  REPETITION
4020           a back reference (see next section)           a back reference (see next section)
4021           a parenthesized subpattern (unless it is an assertion)           a parenthesized subpattern (unless it is an assertion)
4022    
4023         The general repetition quantifier specifies a minimum and maximum  num-         The  general repetition quantifier specifies a minimum and maximum num-
4024         ber  of  permitted matches, by giving the two numbers in curly brackets         ber of permitted matches, by giving the two numbers in  curly  brackets
4025         (braces), separated by a comma. The numbers must be  less  than  65536,         (braces),  separated  by  a comma. The numbers must be less than 65536,
4026         and the first must be less than or equal to the second. For example:         and the first must be less than or equal to the second. For example:
4027    
4028           z{2,4}           z{2,4}
4029    
4030         matches  "zz",  "zzz",  or  "zzzz". A closing brace on its own is not a         matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a
4031         special character. If the second number is omitted, but  the  comma  is         special  character.  If  the second number is omitted, but the comma is
4032         present,  there  is  no upper limit; if the second number and the comma         present, there is no upper limit; if the second number  and  the  comma
4033         are both omitted, the quantifier specifies an exact number of  required         are  both omitted, the quantifier specifies an exact number of required
4034         matches. Thus         matches. Thus
4035    
4036           [aeiou]{3,}           [aeiou]{3,}
# Line 4037  REPETITION Line 4039  REPETITION
4039    
4040           \d{8}           \d{8}
4041    
4042         matches  exactly  8  digits. An opening curly bracket that appears in a         matches exactly 8 digits. An opening curly bracket that  appears  in  a
4043         position where a quantifier is not allowed, or one that does not  match         position  where a quantifier is not allowed, or one that does not match
4044         the  syntax of a quantifier, is taken as a literal character. For exam-         the syntax of a quantifier, is taken as a literal character. For  exam-
4045         ple, {,6} is not a quantifier, but a literal string of four characters.         ple, {,6} is not a quantifier, but a literal string of four characters.
4046    
4047         In UTF-8 mode, quantifiers apply to UTF-8  characters  rather  than  to         In  UTF-8  mode,  quantifiers  apply to UTF-8 characters rather than to
4048         individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-         individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
4049         acters, each of which is represented by a two-byte sequence. Similarly,         acters, each of which is represented by a two-byte sequence. Similarly,
4050         when Unicode property support is available, \X{3} matches three Unicode         when Unicode property support is available, \X{3} matches three Unicode
4051         extended sequences, each of which may be several bytes long  (and  they         extended  sequences,  each of which may be several bytes long (and they
4052         may be of different lengths).         may be of different lengths).
4053    
4054         The quantifier {0} is permitted, causing the expression to behave as if         The quantifier {0} is permitted, causing the expression to behave as if
4055         the previous item and the quantifier were not present. This may be use-         the previous item and the quantifier were not present. This may be use-
4056         ful  for  subpatterns that are referenced as subroutines from elsewhere         ful for subpatterns that are referenced as subroutines  from  elsewhere
4057         in the pattern. Items other than subpatterns that have a {0} quantifier         in the pattern. Items other than subpatterns that have a {0} quantifier
4058         are omitted from the compiled pattern.         are omitted from the compiled pattern.
4059    
4060         For  convenience, the three most common quantifiers have single-charac-         For convenience, the three most common quantifiers have  single-charac-
4061         ter abbreviations:         ter abbreviations:
4062    
4063           *    is equivalent to {0,}           *    is equivalent to {0,}
4064           +    is equivalent to {1,}           +    is equivalent to {1,}
4065           ?    is equivalent to {0,1}           ?    is equivalent to {0,1}
4066    
4067         It is possible to construct infinite loops by  following  a  subpattern         It  is  possible  to construct infinite loops by following a subpattern
4068         that can match no characters with a quantifier that has no upper limit,         that can match no characters with a quantifier that has no upper limit,
4069         for example:         for example:
4070    
4071           (a?)*           (a?)*
4072    
4073         Earlier versions of Perl and PCRE used to give an error at compile time         Earlier versions of Perl and PCRE used to give an error at compile time
4074         for  such  patterns. However, because there are cases where this can be         for such patterns. However, because there are cases where this  can  be
4075         useful, such patterns are now accepted, but if any  repetition  of  the         useful,  such  patterns  are now accepted, but if any repetition of the
4076         subpattern  does in fact match no characters, the loop is forcibly bro-         subpattern does in fact match no characters, the loop is forcibly  bro-
4077         ken.         ken.
4078    
4079         By default, the quantifiers are "greedy", that is, they match  as  much         By  default,  the quantifiers are "greedy", that is, they match as much
4080         as  possible  (up  to  the  maximum number of permitted times), without         as possible (up to the maximum  number  of  permitted  times),  without
4081         causing the rest of the pattern to fail. The classic example  of  where         causing  the  rest of the pattern to fail. The classic example of where
4082         this gives problems is in trying to match comments in C programs. These         this gives problems is in trying to match comments in C programs. These
4083         appear between /* and */ and within the comment,  individual  *  and  /         appear  between  /*  and  */ and within the comment, individual * and /
4084         characters  may  appear. An attempt to match C comments by applying the         characters may appear. An attempt to match C comments by  applying  the
4085         pattern         pattern
4086    
4087           /\*.*\*/           /\*.*\*/
# Line 4088  REPETITION Line 4090  REPETITION
4090    
4091           /* first comment */  not comment  /* second comment */           /* first comment */  not comment  /* second comment */
4092    
4093         fails, because it matches the entire string owing to the greediness  of         fails,  because it matches the entire string owing to the greediness of
4094         the .*  item.         the .*  item.
4095    
4096         However,  if  a quantifier is followed by a question mark, it ceases to         However, if a quantifier is followed by a question mark, it  ceases  to
4097         be greedy, and instead matches the minimum number of times possible, so         be greedy, and instead matches the minimum number of times possible, so
4098         the pattern         the pattern
4099    
4100           /\*.*?\*/           /\*.*?\*/
4101    
4102         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
4103         quantifiers is not otherwise changed,  just  the  preferred  number  of         quantifiers  is  not  otherwise  changed,  just the preferred number of
4104         matches.   Do  not  confuse this use of question mark with its use as a         matches.  Do not confuse this use of question mark with its  use  as  a
4105         quantifier in its own right. Because it has two uses, it can  sometimes         quantifier  in its own right. Because it has two uses, it can sometimes
4106         appear doubled, as in         appear doubled, as in
4107    
4108           \d??\d           \d??\d
# Line 4108  REPETITION Line 4110  REPETITION
4110         which matches one digit by preference, but can match two if that is the         which matches one digit by preference, but can match two if that is the
4111         only way the rest of the pattern matches.         only way the rest of the pattern matches.
4112    
4113         If the PCRE_UNGREEDY option is set (an option that is not available  in         If  the PCRE_UNGREEDY option is set (an option that is not available in
4114         Perl),  the  quantifiers are not greedy by default, but individual ones         Perl), the quantifiers are not greedy by default, but  individual  ones
4115         can be made greedy by following them with a  question  mark.  In  other         can  be  made  greedy  by following them with a question mark. In other
4116         words, it inverts the default behaviour.         words, it inverts the default behaviour.
4117    
4118         When  a  parenthesized  subpattern  is quantified with a minimum repeat         When a parenthesized subpattern is quantified  with  a  minimum  repeat
4119         count that is greater than 1 or with a limited maximum, more memory  is         count  that is greater than 1 or with a limited maximum, more memory is
4120         required  for  the  compiled  pattern, in proportion to the size of the         required for the compiled pattern, in proportion to  the  size  of  the
4121         minimum or maximum.         minimum or maximum.
4122    
4123         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-         If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
4124         alent  to  Perl's  /s) is set, thus allowing the dot to match newlines,         alent to Perl's /s) is set, thus allowing the dot  to  match  newlines,
4125         the pattern is implicitly anchored, because whatever  follows  will  be         the  pattern  is  implicitly anchored, because whatever follows will be
4126         tried  against every character position in the subject string, so there         tried against every character position in the subject string, so  there
4127         is no point in retrying the overall match at  any  position  after  the         is  no  point  in  retrying the overall match at any position after the
4128         first.  PCRE  normally treats such a pattern as though it were preceded         first. PCRE normally treats such a pattern as though it  were  preceded
4129         by \A.         by \A.
4130    
4131         In cases where it is known that the subject  string  contains  no  new-         In  cases  where  it  is known that the subject string contains no new-
4132         lines,  it  is  worth setting PCRE_DOTALL in order to obtain this opti-         lines, it is worth setting PCRE_DOTALL in order to  obtain  this  opti-
4133         mization, or alternatively using ^ to indicate anchoring explicitly.         mization, or alternatively using ^ to indicate anchoring explicitly.
4134    
4135         However, there is one situation where the optimization cannot be  used.         However,  there is one situation where the optimization cannot be used.
4136         When  .*   is  inside  capturing  parentheses that are the subject of a         When .*  is inside capturing parentheses that  are  the  subject  of  a
4137         backreference elsewhere in the pattern, a match at the start  may  fail         backreference  elsewhere  in the pattern, a match at the start may fail
4138         where a later one succeeds. Consider, for example:         where a later one succeeds. Consider, for example:
4139    
4140           (.*)abc\1           (.*)abc\1
4141    
4142         If  the subject is "xyz123abc123" the match point is the fourth charac-         If the subject is "xyz123abc123" the match point is the fourth  charac-
4143         ter. For this reason, such a pattern is not implicitly anchored.         ter. For this reason, such a pattern is not implicitly anchored.
4144    
4145         When a capturing subpattern is repeated, the value captured is the sub-         When a capturing subpattern is repeated, the value captured is the sub-
# Line 4146  REPETITION Line 4148  REPETITION
4148           (tweedle[dume]{3}\s*)+           (tweedle[dume]{3}\s*)+
4149    
4150         has matched "tweedledum tweedledee" the value of the captured substring         has matched "tweedledum tweedledee" the value of the captured substring
4151         is "tweedledee". However, if there are  nested  capturing  subpatterns,         is  "tweedledee".  However,  if there are nested capturing subpatterns,
4152         the  corresponding captured values may have been set in previous itera-         the corresponding captured values may have been set in previous  itera-
4153         tions. For example, after         tions. For example, after
4154    
4155           /(a|(b))+/           /(a|(b))+/
# Line 4157  REPETITION Line 4159  REPETITION
4159    
4160  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS  ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
4161    
4162         With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")         With  both  maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
4163         repetition,  failure  of what follows normally causes the repeated item         repetition, failure of what follows normally causes the  repeated  item
4164         to be re-evaluated to see if a different number of repeats  allows  the         to  be  re-evaluated to see if a different number of repeats allows the
4165         rest  of  the pattern to match. Sometimes it is useful to prevent this,         rest of the pattern to match. Sometimes it is useful to  prevent  this,
4166         either to change the nature of the match, or to cause it  fail  earlier         either  to  change the nature of the match, or to cause it fail earlier
4167         than  it otherwise might, when the author of the pattern knows there is         than it otherwise might, when the author of the pattern knows there  is
4168         no point in carrying on.         no point in carrying on.
4169    
4170         Consider, for example, the pattern \d+foo when applied to  the  subject         Consider,  for  example, the pattern \d+foo when applied to the subject
4171         line         line
4172    
4173           123456bar           123456bar
4174    
4175         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
4176         action of the matcher is to try again with only 5 digits  matching  the         action  of  the matcher is to try again with only 5 digits matching the
4177         \d+  item,  and  then  with  4,  and  so on, before ultimately failing.         \d+ item, and then with  4,  and  so  on,  before  ultimately  failing.
4178         "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides         "Atomic  grouping"  (a  term taken from Jeffrey Friedl's book) provides
4179         the  means for specifying that once a subpattern has matched, it is not         the means for specifying that once a subpattern has matched, it is  not
4180         to be re-evaluated in this way.         to be re-evaluated in this way.
4181    
4182         If we use atomic grouping for the previous example, the  matcher  gives         If  we  use atomic grouping for the previous example, the matcher gives
4183         up  immediately  on failing to match "foo" the first time. The notation         up immediately on failing to match "foo" the first time.  The  notation
4184         is a kind of special parenthesis, starting with (?> as in this example:         is a kind of special parenthesis, starting with (?> as in this example:
4185    
4186           (?>\d+)foo           (?>\d+)foo
4187    
4188         This kind of parenthesis "locks up" the  part of the  pattern  it  con-         This  kind  of  parenthesis "locks up" the  part of the pattern it con-
4189         tains  once  it  has matched, and a failure further into the pattern is         tains once it has matched, and a failure further into  the  pattern  is
4190         prevented from backtracking into it. Backtracking past it  to  previous         prevented  from  backtracking into it. Backtracking past it to previous
4191         items, however, works as normal.         items, however, works as normal.
4192    
4193         An  alternative  description  is that a subpattern of this type matches         An alternative description is that a subpattern of  this  type  matches
4194         the string of characters that an  identical  standalone  pattern  would         the  string  of  characters  that an identical standalone pattern would
4195         match, if anchored at the current point in the subject string.         match, if anchored at the current point in the subject string.
4196    
4197         Atomic grouping subpatterns are not capturing subpatterns. Simple cases         Atomic grouping subpatterns are not capturing subpatterns. Simple cases
4198         such as the above example can be thought of as a maximizing repeat that         such as the above example can be thought of as a maximizing repeat that
4199         must  swallow  everything  it can. So, while both \d+ and \d+? are pre-         must swallow everything it can. So, while both \d+ and  \d+?  are  pre-
4200         pared to adjust the number of digits they match in order  to  make  the         pared  to  adjust  the number of digits they match in order to make the
4201         rest of the pattern match, (?>\d+) can only match an entire sequence of         rest of the pattern match, (?>\d+) can only match an entire sequence of
4202         digits.         digits.
4203    
4204         Atomic groups in general can of course contain arbitrarily  complicated         Atomic  groups in general can of course contain arbitrarily complicated
4205         subpatterns,  and  can  be  nested. However, when the subpattern for an         subpatterns, and can be nested. However, when  the  subpattern  for  an
4206         atomic group is just a single repeated item, as in the example above, a         atomic group is just a single repeated item, as in the example above, a
4207         simpler  notation,  called  a "possessive quantifier" can be used. This         simpler notation, called a "possessive quantifier" can  be  used.  This
4208         consists of an additional + character  following  a  quantifier.  Using         consists  of  an  additional  + character following a quantifier. Using
4209         this notation, the previous example can be rewritten as         this notation, the previous example can be rewritten as
4210    
4211           \d++foo           \d++foo
# Line 4213  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE Line 4215  ATOMIC GROUPING AND POSSESSIVE QUANTIFIE
4215    
4216           (abc|xyz){2,3}+           (abc|xyz){2,3}+
4217    
4218         Possessive  quantifiers  are  always  greedy;  the   setting   of   the         Possessive   quantifiers   are   always  greedy;  the  setting  of  the
4219         PCRE_UNGREEDY option is ignored. They are a convenient notation for the         PCRE_UNGREEDY option is ignored. They are a convenient notation for the
4220         simpler forms of atomic group. However, there is no difference  in  the         simpler  forms  of atomic group. However, there is no difference in the
4221         meaning  of  a  possessive  quantifier and the equivalent atomic group,         meaning of a possessive quantifier and  the  equivalent  atomic  group,
4222         though there may be a performance  difference;  possessive  quantifiers         though  there  may  be a performance difference; possessive quantifiers
4223         should be slightly faster.         should be slightly faster.
4224    
4225         The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-         The possessive quantifier syntax is an extension to the Perl  5.8  syn-
4226         tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first         tax.   Jeffrey  Friedl  originated the idea (and the name) in the first
4227         edition of his book. Mike McCloskey liked it, so implemented it when he         edition of his book. Mike McCloskey liked it, so implemented it when he
4228         built Sun's Java package, and PCRE copied it from there. It  ultimately         built  Sun's Java package, and PCRE copied it from there. It ultimately
4229         found its way into Perl at release 5.10.         found its way into Perl at release 5.10.
4230    
4231         PCRE has an optimization that automatically "possessifies" certain sim-         PCRE has an optimization that automatically "possessifies" certain sim-
4232         ple pattern constructs. For example, the sequence  A+B  is  treated  as         ple  pattern  constructs.  For  example, the sequence A+B is treated as
4233         A++B  because  there is no point in backtracking into a sequence of A's         A++B because there is no point in backtracking into a sequence  of  A's
4234         when B must follow.         when B must follow.
4235    
4236         When a pattern contains an unlimited repeat inside  a  subpattern  that         When  a  pattern  contains an unlimited repeat inside a subpattern that
4237         can  itself  be  repeated  an  unlimited number of times, the use of an         can itself be repeated an unlimited number of  times,  the  use  of  an
4238         atomic group is the only way to avoid some  failing  matches  taking  a         atomic  group  is  the  only way to avoid some failing matches taking a
4239         very long time indeed. The pattern         very long time indeed. The pattern
4240    
4241           (\D+|<\d+>)*[!?]           (\D+|<\d+>)*[!?]
4242    
4243         matches  an  unlimited number of substrings that either consist of non-         matches an unlimited number of substrings that either consist  of  non-
4244         digits, or digits enclosed in <>, followed by either ! or  ?.  When  it         digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
4245         matches, it runs quickly. However, if it is applied to         matches, it runs quickly. However, if it is applied to
4246    
4247           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa           aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
4248    
4249         it  takes  a  long  time  before reporting failure. This is because the         it takes a long time before reporting  failure.  This  is  because  the
4250         string can be divided between the internal \D+ repeat and the  external         string  can be divided between the internal \D+ repeat and the external
4251         *  repeat  in  a  large  number of ways, and all have to be tried. (The         * repeat in a large number of ways, and all  have  to  be  tried.  (The
4252         example uses [!?] rather than a single character at  the  end,  because         example  uses  [!?]  rather than a single character at the end, because
4253         both  PCRE  and  Perl have an optimization that allows for fast failure         both PCRE and Perl have an optimization that allows  for  fast  failure
4254         when a single character is used. They remember the last single  charac-         when  a single character is used. They remember the last single charac-
4255         ter  that  is required for a match, and fail early if it is not present         ter that is required for a match, and fail early if it is  not  present
4256         in the string.) If the pattern is changed so that  it  uses  an  atomic         in  the  string.)  If  the pattern is changed so that it uses an atomic
4257         group, like this:         group, like this:
4258    
4259           ((?>\D+)|<\d+>)*[!?]           ((?>\D+)|<\d+>)*[!?]
# Line 4263  BACK REFERENCES Line 4265  BACK REFERENCES
4265    
4266         Outside a character class, a backslash followed by a digit greater than         Outside a character class, a backslash followed by a digit greater than
4267         0 (and possibly further digits) is a back reference to a capturing sub-         0 (and possibly further digits) is a back reference to a capturing sub-
4268         pattern  earlier  (that is, to its left) in the pattern, provided there         pattern earlier (that is, to its left) in the pattern,  provided  there
4269         have been that many previous capturing left parentheses.         have been that many previous capturing left parentheses.
4270    
4271         However, if the decimal number following the backslash is less than 10,         However, if the decimal number following the backslash is less than 10,
4272         it  is  always  taken  as a back reference, and causes an error only if         it is always taken as a back reference, and causes  an  error  only  if
4273         there are not that many capturing left parentheses in the  entire  pat-         there  are  not that many capturing left parentheses in the entire pat-
4274         tern.  In  other words, the parentheses that are referenced need not be         tern. In other words, the parentheses that are referenced need  not  be
4275         to the left of the reference for numbers less than 10. A "forward  back         to  the left of the reference for numbers less than 10. A "forward back
4276         reference"  of  this  type can make sense when a repetition is involved         reference" of this type can make sense when a  repetition  is  involved
4277         and the subpattern to the right has participated in an  earlier  itera-         and  the  subpattern to the right has participated in an earlier itera-
4278         tion.         tion.
4279    
4280         It  is  not  possible to have a numerical "forward back reference" to a         It is not possible to have a numerical "forward back  reference"  to  a
4281         subpattern whose number is 10 or  more  using  this  syntax  because  a         subpattern  whose  number  is  10  or  more using this syntax because a
4282         sequence  such  as  \50 is interpreted as a character defined in octal.         sequence such as \50 is interpreted as a character  defined  in  octal.
4283         See the subsection entitled "Non-printing characters" above for further         See the subsection entitled "Non-printing characters" above for further
4284         details  of  the  handling of digits following a backslash. There is no         details of the handling of digits following a backslash.  There  is  no
4285         such problem when named parentheses are used. A back reference  to  any         such  problem  when named parentheses are used. A back reference to any
4286         subpattern is possible using named parentheses (see below).         subpattern is possible using named parentheses (see below).
4287    
4288         Another  way  of  avoiding  the ambiguity inherent in the use of digits         Another way of avoiding the ambiguity inherent in  the  use  of  digits
4289         following a backslash is to use the \g escape sequence, which is a fea-         following a backslash is to use the \g escape sequence, which is a fea-
4290         ture  introduced  in  Perl  5.10.  This  escape  must be followed by an         ture introduced in Perl 5.10.  This  escape  must  be  followed  by  an
4291         unsigned number or a negative number, optionally  enclosed  in  braces.         unsigned  number  or  a negative number, optionally enclosed in braces.
4292         These examples are all identical:         These examples are all identical:
4293    
4294           (ring), \1           (ring), \1
4295           (ring), \g1           (ring), \g1
4296           (ring), \g{1}           (ring), \g{1}
4297    
4298         An  unsigned number specifies an absolute reference without the ambigu-         An unsigned number specifies an absolute reference without the  ambigu-
4299         ity that is present in the older syntax. It is also useful when literal         ity that is present in the older syntax. It is also useful when literal
4300         digits follow the reference. A negative number is a relative reference.         digits follow the reference. A negative number is a relative reference.
4301         Consider this example:         Consider this example:
# Line 4301  BACK REFERENCES Line 4303  BACK REFERENCES
4303           (abc(def)ghi)\g{-1}           (abc(def)ghi)\g{-1}
4304    
4305         The sequence \g{-1} is a reference to the most recently started captur-         The sequence \g{-1} is a reference to the most recently started captur-
4306         ing  subpattern  before \g, that is, is it equivalent to \2. Similarly,         ing subpattern before \g, that is, is it equivalent to  \2.  Similarly,
4307         \g{-2} would be equivalent to \1. The use of relative references can be         \g{-2} would be equivalent to \1. The use of relative references can be
4308         helpful  in  long  patterns,  and  also in patterns that are created by         helpful in long patterns, and also in  patterns  that  are  created  by
4309         joining together fragments that contain references within themselves.         joining together fragments that contain references within themselves.
4310    
4311         A back reference matches whatever actually matched the  capturing  sub-         A  back  reference matches whatever actually matched the capturing sub-
4312         pattern  in  the  current subject string, rather than anything matching         pattern in the current subject string, rather  than  anything  matching
4313         the subpattern itself (see "Subpatterns as subroutines" below for a way         the subpattern itself (see "Subpatterns as subroutines" below for a way
4314         of doing that). So the pattern         of doing that). So the pattern
4315    
4316           (sens|respons)e and \1ibility           (sens|respons)e and \1ibility
4317    
4318         matches  "sense and sensibility" and "response and responsibility", but         matches "sense and sensibility" and "response and responsibility",  but
4319         not "sense and responsibility". If caseful matching is in force at  the         not  "sense and responsibility". If caseful matching is in force at the
4320         time  of the back reference, the case of letters is relevant. For exam-         time of the back reference, the case of letters is relevant. For  exam-
4321         ple,         ple,
4322    
4323           ((?i)rah)\s+\1           ((?i)rah)\s+\1
4324    
4325         matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the         matches  "rah  rah"  and  "RAH RAH", but not "RAH rah", even though the
4326         original capturing subpattern is matched caselessly.         original capturing subpattern is matched caselessly.
4327    
4328         There  are  several  different ways of writing back references to named         There are several different ways of writing back  references  to  named
4329         subpatterns. The .NET syntax \k{name} and the Perl syntax  \k<name>  or         subpatterns.  The  .NET syntax \k{name} and the Perl syntax \k<name> or
4330         \k'name'  are supported, as is the Python syntax (?P=name). Perl 5.10's         \k'name' are supported, as is the Python syntax (?P=name). Perl  5.10's
4331         unified back reference syntax, in which \g can be used for both numeric         unified back reference syntax, in which \g can be used for both numeric
4332         and  named  references,  is  also supported. We could rewrite the above         and named references, is also supported. We  could  rewrite  the  above
4333         example in any of the following ways:         example in any of the following ways:
4334    
4335           (?<p1>(?i)rah)\s+\k<p1>           (?<p1>(?i)rah)\s+\k<p1>
# Line 4335  BACK REFERENCES Line 4337  BACK REFERENCES
4337           (?P<p1>(?i)rah)\s+(?P=p1)           (?P<p1>(?i)rah)\s+(?P=p1)
4338           (?<p1>(?i)rah)\s+\g{p1}           (?<p1>(?i)rah)\s+\g{p1}
4339    
4340         A subpattern that is referenced by  name  may  appear  in  the  pattern         A  subpattern  that  is  referenced  by  name may appear in the pattern
4341         before or after the reference.         before or after the reference.
4342    
4343         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
4344         subpattern has not actually been used in a particular match,  any  back         subpattern  has  not actually been used in a particular match, any back
4345         references to it always fail. For example, the pattern         references to it always fail. For example, the pattern
4346    
4347           (a|(bc))\2           (a|(bc))\2
4348    
4349         always  fails if it starts to match "a" rather than "bc". Because there         always fails if it starts to match "a" rather than "bc". Because  there
4350         may be many capturing parentheses in a pattern,  all  digits  following         may  be  many  capturing parentheses in a pattern, all digits following
4351         the  backslash  are taken as part of a potential back reference number.         the backslash are taken as part of a potential back  reference  number.
4352         If the pattern continues with a digit character, some delimiter must be         If the pattern continues with a digit character, some delimiter must be
4353         used  to  terminate  the back reference. If the PCRE_EXTENDED option is         used to terminate the back reference. If the  PCRE_EXTENDED  option  is
4354         set, this can be whitespace.  Otherwise an  empty  comment  (see  "Com-         set,  this  can  be  whitespace.  Otherwise an empty comment (see "Com-
4355         ments" below) can be used.         ments" below) can be used.
4356    
4357         A  back reference that occurs inside the parentheses to which it refers         A back reference that occurs inside the parentheses to which it  refers
4358         fails when the subpattern is first used, so, for example,  (a\1)  never         fails  when  the subpattern is first used, so, for example, (a\1) never
4359         matches.   However,  such references can be useful inside repeated sub-         matches.  However, such references can be useful inside  repeated  sub-
4360         patterns. For example, the pattern         patterns. For example, the pattern
4361    
4362           (a|b\1)+           (a|b\1)+
4363    
4364         matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-         matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
4365         ation  of  the  subpattern,  the  back  reference matches the character         ation of the subpattern,  the  back  reference  matches  the  character
4366         string corresponding to the previous iteration. In order  for  this  to         string  corresponding  to  the previous iteration. In order for this to
4367         work,  the  pattern must be such that the first iteration does not need         work, the pattern must be such that the first iteration does  not  need
4368         to match the back reference. This can be done using alternation, as  in         to  match the back reference. This can be done using alternation, as in
4369         the example above, or by a quantifier with a minimum of zero.         the example above, or by a quantifier with a minimum of zero.
4370    
4371    
4372  ASSERTIONS  ASSERTIONS
4373    
4374         An  assertion  is  a  test on the characters following or preceding the         An assertion is a test on the characters  following  or  preceding  the
4375         current matching point that does not actually consume  any  characters.         current  matching  point that does not actually consume any characters.
4376         The  simple  assertions  coded  as  \b, \B, \A, \G, \Z, \z, ^ and $ are         The simple assertions coded as \b, \B, \A, \G, \Z,  \z,  ^  and  $  are
4377         described above.         described above.
4378    
4379         More complicated assertions are coded as  subpatterns.  There  are  two         More  complicated  assertions  are  coded as subpatterns. There are two
4380         kinds:  those  that  look  ahead of the current position in the subject         kinds: those that look ahead of the current  position  in  the  subject
4381         string, and those that look  behind  it.  An  assertion  subpattern  is         string,  and  those  that  look  behind  it. An assertion subpattern is
4382         matched  in  the  normal way, except that it does not cause the current         matched in the normal way, except that it does not  cause  the  current
4383         matching position to be changed.         matching position to be changed.
4384    
4385         Assertion subpatterns are not capturing subpatterns,  and  may  not  be         Assertion  subpatterns  are  not  capturing subpatterns, and may not be
4386         repeated,  because  it  makes no sense to assert the same thing several         repeated, because it makes no sense to assert the  same  thing  several
4387         times. If any kind of assertion contains capturing  subpatterns  within         times.  If  any kind of assertion contains capturing subpatterns within
4388         it,  these are counted for the purposes of numbering the capturing sub-         it, these are counted for the purposes of numbering the capturing  sub-
4389         patterns in the whole pattern.  However, substring capturing is carried         patterns in the whole pattern.  However, substring capturing is carried
4390         out  only  for  positive assertions, because it does not make sense for         out only for positive assertions, because it does not  make  sense  for
4391         negative assertions.         negative assertions.
4392    
4393     Lookahead assertions     Lookahead assertions
# Line 4395  ASSERTIONS Line 4397  ASSERTIONS
4397    
4398           \w+(?=;)           \w+(?=;)
4399    
4400         matches  a word followed by a semicolon, but does not include the semi-         matches a word followed by a semicolon, but does not include the  semi-
4401         colon in the match, and         colon in the match, and
4402    
4403           foo(?!bar)           foo(?!bar)
4404    
4405         matches any occurrence of "foo" that is not  followed  by  "bar".  Note         matches  any  occurrence  of  "foo" that is not followed by "bar". Note
4406         that the apparently similar pattern         that the apparently similar pattern
4407    
4408           (?!foo)bar           (?!foo)bar
4409    
4410         does  not  find  an  occurrence  of "bar" that is preceded by something         does not find an occurrence of "bar"  that  is  preceded  by  something
4411         other than "foo"; it finds any occurrence of "bar" whatsoever,  because         other  than "foo"; it finds any occurrence of "bar" whatsoever, because
4412         the assertion (?!foo) is always true when the next three characters are         the assertion (?!foo) is always true when the next three characters are
4413         "bar". A lookbehind assertion is needed to achieve the other effect.         "bar". A lookbehind assertion is needed to achieve the other effect.
4414    
4415         If you want to force a matching failure at some point in a pattern, the         If you want to force a matching failure at some point in a pattern, the
4416         most  convenient  way  to  do  it  is with (?!) because an empty string         most convenient way to do it is  with  (?!)  because  an  empty  string
4417         always matches, so an assertion that requires there not to be an  empty         always  matches, so an assertion that requires there not to be an empty
4418         string must always fail.         string must always fail.
4419    
4420     Lookbehind assertions     Lookbehind assertions
4421    
4422         Lookbehind  assertions start with (?<= for positive assertions and (?<!         Lookbehind assertions start with (?<= for positive assertions and  (?<!
4423         for negative assertions. For example,         for negative assertions. For example,
4424    
4425           (?<!foo)bar           (?<!foo)bar
4426    
4427         does find an occurrence of "bar" that is not  preceded  by  "foo".  The         does  find  an  occurrence  of "bar" that is not preceded by "foo". The
4428         contents  of  a  lookbehind  assertion are restricted such that all the         contents of a lookbehind assertion are restricted  such  that  all  the
4429         strings it matches must have a fixed length. However, if there are sev-         strings it matches must have a fixed length. However, if there are sev-
4430         eral  top-level  alternatives,  they  do  not all have to have the same         eral top-level alternatives, they do not all  have  to  have  the  same
4431         fixed length. Thus         fixed length. Thus
4432    
4433           (?<=bullock|donkey)           (?<=bullock|donkey)
# Line 4434  ASSERTIONS Line 4436  ASSERTIONS
4436    
4437           (?<!dogs?|cats?)           (?<!dogs?|cats?)
4438    
4439         causes an error at compile time. Branches that match  different  length         causes  an  error at compile time. Branches that match different length
4440         strings  are permitted only at the top level of a lookbehind assertion.         strings are permitted only at the top level of a lookbehind  assertion.
4441         This is an extension compared with  Perl  (at  least  for  5.8),  which         This  is  an  extension  compared  with  Perl (at least for 5.8), which
4442         requires  all branches to match the same length of string. An assertion         requires all branches to match the same length of string. An  assertion
4443         such as         such as
4444    
4445           (?<=ab(c|de))           (?<=ab(c|de))
4446    
4447         is not permitted, because its single top-level  branch  can  match  two         is  not  permitted,  because  its single top-level branch can match two
4448         different  lengths,  but  it is acceptable if rewritten to use two top-         different lengths, but it is acceptable if rewritten to  use  two  top-
4449         level branches:         level branches:
4450    
4451           (?<=abc|abde)           (?<=abc|abde)
4452    
4453         In some cases, the Perl 5.10 escape sequence \K (see above) can be used         In some cases, the Perl 5.10 escape sequence \K (see above) can be used
4454         instead  of  a lookbehind assertion; this is not restricted to a fixed-         instead of a lookbehind assertion; this is not restricted to  a  fixed-
4455         length.         length.
4456    
4457         The implementation of lookbehind assertions is, for  each  alternative,         The  implementation  of lookbehind assertions is, for each alternative,
4458         to  temporarily  move the current position back by the fixed length and         to temporarily move the current position back by the fixed  length  and
4459         then try to match. If there are insufficient characters before the cur-         then try to match. If there are insufficient characters before the cur-
4460         rent position, the assertion fails.         rent position, the assertion fails.
4461    
4462         PCRE does not allow the \C escape (which matches a single byte in UTF-8         PCRE does not allow the \C escape (which matches a single byte in UTF-8
4463         mode) to appear in lookbehind assertions, because it makes it  impossi-         mode)  to appear in lookbehind assertions, because it makes it impossi-
4464         ble  to  calculate the length of the lookbehind. The \X and \R escapes,         ble to calculate the length of the lookbehind. The \X and  \R  escapes,
4465         which can match different numbers of bytes, are also not permitted.         which can match different numbers of bytes, are also not permitted.
4466    
4467         Possessive quantifiers can  be  used  in  conjunction  with  lookbehind         Possessive  quantifiers  can  be  used  in  conjunction with lookbehind
4468         assertions  to  specify  efficient  matching  at the end of the subject         assertions to specify efficient matching at  the  end  of  the  subject
4469         string. Consider a simple pattern such as         string. Consider a simple pattern such as
4470    
4471           abcd$           abcd$
4472    
4473         when applied to a long string that does  not  match.  Because  matching         when  applied  to  a  long string that does not match. Because matching
4474         proceeds from left to right, PCRE will look for each "a" in the subject         proceeds from left to right, PCRE will look for each "a" in the subject
4475         and then see if what follows matches the rest of the  pattern.  If  the         and  then  see  if what follows matches the rest of the pattern. If the
4476         pattern is specified as         pattern is specified as
4477    
4478           ^.*abcd$           ^.*abcd$
4479    
4480         the  initial .* matches the entire string at first, but when this fails         the initial .* matches the entire string at first, but when this  fails
4481         (because there is no following "a"), it backtracks to match all but the         (because there is no following "a"), it backtracks to match all but the
4482         last  character,  then all but the last two characters, and so on. Once         last character, then all but the last two characters, and so  on.  Once
4483         again the search for "a" covers the entire string, from right to  left,         again  the search for "a" covers the entire string, from right to left,
4484         so we are no better off. However, if the pattern is written as         so we are no better off. However, if the pattern is written as
4485    
4486           ^.*+(?<=abcd)           ^.*+(?<=abcd)
4487    
4488         there  can  be  no backtracking for the .*+ item; it can match only the         there can be no backtracking for the .*+ item; it can  match  only  the
4489         entire string. The subsequent lookbehind assertion does a  single  test         entire  string.  The subsequent lookbehind assertion does a single test
4490         on  the last four characters. If it fails, the match fails immediately.         on the last four characters. If it fails, the match fails  immediately.
4491         For long strings, this approach makes a significant difference  to  the         For  long  strings, this approach makes a significant difference to the
4492         processing time.         processing time.
4493    
4494     Using multiple assertions     Using multiple assertions
# Line 4495  ASSERTIONS Line 4497  ASSERTIONS
4497    
4498           (?<=\d{3})(?<!999)foo           (?<=\d{3})(?<!999)foo
4499    
4500         matches  "foo" preceded by three digits that are not "999". Notice that         matches "foo" preceded by three digits that are not "999". Notice  that
4501         each of the assertions is applied independently at the  same  point  in         each  of  the  assertions is applied independently at the same point in
4502         the  subject  string.  First  there  is a check that the previous three         the subject string. First there is a  check  that  the  previous  three
4503         characters are all digits, and then there is  a  check  that  the  same         characters  are  all  digits,  and  then there is a check that the same
4504         three characters are not "999".  This pattern does not match "foo" pre-         three characters are not "999".  This pattern does not match "foo" pre-
4505         ceded by six characters, the first of which are  digits  and  the  last         ceded  by  six  characters,  the first of which are digits and the last
4506         three  of  which  are not "999". For example, it doesn't match "123abc-         three of which are not "999". For example, it  doesn't  match  "123abc-
4507         foo". A pattern to do that is         foo". A pattern to do that is
4508    
4509           (?<=\d{3}...)(?<!999)foo           (?<=\d{3}...)(?<!999)foo
4510    
4511         This time the first assertion looks at the  preceding  six  characters,         This  time  the  first assertion looks at the preceding six characters,
4512         checking that the first three are digits, and then the second assertion         checking that the first three are digits, and then the second assertion
4513         checks that the preceding three characters are not "999".         checks that the preceding three characters are not "999".
4514    
# Line 4514  ASSERTIONS Line 4516  ASSERTIONS
4516    
4517           (?<=(?<!foo)bar)baz           (?<=(?<!foo)bar)baz
4518    
4519         matches an occurrence of "baz" that is preceded by "bar" which in  turn         matches  an occurrence of "baz" that is preceded by "bar" which in turn
4520         is not preceded by "foo", while         is not preceded by "foo", while
4521    
4522           (?<=\d{3}(?!999)...)foo           (?<=\d{3}(?!999)...)foo
4523    
4524         is  another pattern that matches "foo" preceded by three digits and any         is another pattern that matches "foo" preceded by three digits and  any
4525         three characters that are not "999".         three characters that are not "999".
4526    
4527    
4528  CONDITIONAL SUBPATTERNS  CONDITIONAL SUBPATTERNS
4529    
4530         It is possible to cause the matching process to obey a subpattern  con-         It  is possible to cause the matching process to obey a subpattern con-
4531         ditionally  or to choose between two alternative subpatterns, depending         ditionally or to choose between two alternative subpatterns,  depending
4532         on the result of an assertion, or whether a previous capturing  subpat-         on  the result of an assertion, or whether a previous capturing subpat-
4533         tern  matched  or not. The two possible forms of conditional subpattern         tern matched or not. The two possible forms of  conditional  subpattern
4534         are         are
4535    
4536           (?(condition)yes-pattern)           (?(condition)yes-pattern)
4537           (?(condition)yes-pattern|no-pattern)           (?(condition)yes-pattern|no-pattern)
4538    
4539         If the condition is satisfied, the yes-pattern is used;  otherwise  the         If  the  condition is satisfied, the yes-pattern is used; otherwise the
4540         no-pattern  (if  present)  is used. If there are more than two alterna-         no-pattern (if present) is used. If there are more  than  two  alterna-
4541         tives in the subpattern, a compile-time error occurs.         tives in the subpattern, a compile-time error occurs.
4542    
4543         There are four kinds of condition: references  to  subpatterns,  refer-         There  are  four  kinds of condition: references to subpatterns, refer-
4544         ences to recursion, a pseudo-condition called DEFINE, and assertions.         ences to recursion, a pseudo-condition called DEFINE, and assertions.
4545    
4546     Checking for a used subpattern by number     Checking for a used subpattern by number
4547    
4548         If  the  text between the parentheses consists of a sequence of digits,         If the text between the parentheses consists of a sequence  of  digits,
4549         the condition is true if the capturing subpattern of  that  number  has         the  condition  is  true if the capturing subpattern of that number has
4550         previously  matched.  An  alternative notation is to precede the digits         previously matched. An alternative notation is to  precede  the  digits
4551         with a plus or minus sign. In this case, the subpattern number is rela-         with a plus or minus sign. In this case, the subpattern number is rela-
4552         tive rather than absolute.  The most recently opened parentheses can be         tive rather than absolute.  The most recently opened parentheses can be
4553         referenced by (?(-1), the next most recent by (?(-2),  and  so  on.  In         referenced  by  (?(-1),  the  next most recent by (?(-2), and so on. In
4554         looping constructs it can also make sense to refer to subsequent groups         looping constructs it can also make sense to refer to subsequent groups
4555         with constructs such as (?(+2).         with constructs such as (?(+2).
4556    
4557         Consider the following pattern, which  contains  non-significant  white         Consider  the  following  pattern, which contains non-significant white
4558         space to make it more readable (assume the PCRE_EXTENDED option) and to         space to make it more readable (assume the PCRE_EXTENDED option) and to
4559         divide it into three parts for ease of discussion:         divide it into three parts for ease of discussion:
4560    
4561           ( \( )?    [^()]+    (?(1) \) )           ( \( )?    [^()]+    (?(1) \) )
4562    
4563         The first part matches an optional opening  parenthesis,  and  if  that         The  first  part  matches  an optional opening parenthesis, and if that
4564         character is present, sets it as the first captured substring. The sec-         character is present, sets it as the first captured substring. The sec-
4565         ond part matches one or more characters that are not  parentheses.  The         ond  part  matches one or more characters that are not parentheses. The
4566         third part is a conditional subpattern that tests whether the first set         third part is a conditional subpattern that tests whether the first set
4567         of parentheses matched or not. If they did, that is, if subject started         of parentheses matched or not. If they did, that is, if subject started
4568         with an opening parenthesis, the condition is true, and so the yes-pat-         with an opening parenthesis, the condition is true, and so the yes-pat-
4569         tern is executed and a  closing  parenthesis  is  required.  Otherwise,         tern  is  executed  and  a  closing parenthesis is required. Otherwise,
4570         since  no-pattern  is  not  present, the subpattern matches nothing. In         since no-pattern is not present, the  subpattern  matches  nothing.  In
4571         other words,  this  pattern  matches  a  sequence  of  non-parentheses,         other  words,  this  pattern  matches  a  sequence  of non-parentheses,
4572         optionally enclosed in parentheses.         optionally enclosed in parentheses.
4573    
4574         If  you  were  embedding  this pattern in a larger one, you could use a         If you were embedding this pattern in a larger one,  you  could  use  a
4575         relative reference:         relative reference:
4576    
4577           ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...           ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...
4578    
4579         This makes the fragment independent of the parentheses  in  the  larger         This  makes  the  fragment independent of the parentheses in the larger
4580         pattern.         pattern.
4581    
4582     Checking for a used subpattern by name     Checking for a used subpattern by name
4583    
4584         Perl  uses  the  syntax  (?(<name>)...) or (?('name')...) to test for a         Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
4585         used subpattern by name. For compatibility  with  earlier  versions  of         used  subpattern  by  name.  For compatibility with earlier versions of
4586         PCRE,  which  had this facility before Perl, the syntax (?(name)...) is         PCRE, which had this facility before Perl, the syntax  (?(name)...)  is
4587         also recognized. However, there is a possible ambiguity with this  syn-         also  recognized. However, there is a possible ambiguity with this syn-
4588         tax,  because  subpattern  names  may  consist entirely of digits. PCRE         tax, because subpattern names may  consist  entirely  of  digits.  PCRE
4589         looks first for a named subpattern; if it cannot find one and the  name         looks  first for a named subpattern; if it cannot find one and the name
4590         consists  entirely  of digits, PCRE looks for a subpattern of that num-         consists entirely of digits, PCRE looks for a subpattern of  that  num-
4591         ber, which must be greater than zero. Using subpattern names that  con-         ber,  which must be greater than zero. Using subpattern names that con-
4592         sist entirely of digits is not recommended.         sist entirely of digits is not recommended.
4593    
4594         Rewriting the above example to use a named subpattern gives this:         Rewriting the above example to use a named subpattern gives this:
# Line 4597  CONDITIONAL SUBPATTERNS Line 4599  CONDITIONAL SUBPATTERNS
4599     Checking for pattern recursion     Checking for pattern recursion
4600    
4601         If the condition is the string (R), and there is no subpattern with the         If the condition is the string (R), and there is no subpattern with the
4602         name R, the condition is true if a recursive call to the whole  pattern         name  R, the condition is true if a recursive call to the whole pattern
4603         or any subpattern has been made. If digits or a name preceded by amper-         or any subpattern has been made. If digits or a name preceded by amper-
4604         sand follow the letter R, for example:         sand follow the letter R, for example:
4605    
4606           (?(R3)...) or (?(R&name)...)           (?(R3)...) or (?(R&name)...)
4607    
4608         the condition is true if the most recent recursion is into the  subpat-         the  condition is true if the most recent recursion is into the subpat-
4609         tern  whose  number or name is given. This condition does not check the         tern whose number or name is given. This condition does not  check  the
4610         entire recursion stack.         entire recursion stack.
4611    
4612         At "top level", all these recursion test conditions are  false.  Recur-         At  "top  level", all these recursion test conditions are false. Recur-
4613         sive patterns are described below.         sive patterns are described below.
4614    
4615     Defining subpatterns for use by reference only     Defining subpatterns for use by reference only
4616    
4617         If  the  condition  is  the string (DEFINE), and there is no subpattern         If the condition is the string (DEFINE), and  there  is  no  subpattern
4618         with the name DEFINE, the condition is  always  false.  In  this  case,         with  the  name  DEFINE,  the  condition is always false. In this case,
4619         there  may  be  only  one  alternative  in the subpattern. It is always         there may be only one alternative  in  the  subpattern.  It  is  always
4620         skipped if control reaches this point  in  the  pattern;  the  idea  of         skipped  if  control  reaches  this  point  in the pattern; the idea of
4621         DEFINE  is that it can be used to define "subroutines" that can be ref-         DEFINE is that it can be used to define "subroutines" that can be  ref-
4622         erenced from elsewhere. (The use of "subroutines" is described  below.)         erenced  from elsewhere. (The use of "subroutines" is described below.)
4623         For  example,  a pattern to match an IPv4 address could be written like         For example, a pattern to match an IPv4 address could be  written  like
4624         this (ignore whitespace and line breaks):         this (ignore whitespace and line breaks):
4625    
4626           (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )           (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
4627           \b (?&byte) (\.(?&byte)){3} \b           \b (?&byte) (\.(?&byte)){3} \b
4628    
4629         The first part of the pattern is a DEFINE group inside which a  another         The  first part of the pattern is a DEFINE group inside which a another
4630         group  named "byte" is defined. This matches an individual component of         group named "byte" is defined. This matches an individual component  of
4631         an IPv4 address (a number less than 256). When  matching  takes  place,         an  IPv4  address  (a number less than 256). When matching takes place,
4632         this  part  of  the pattern is skipped because DEFINE acts like a false         this part of the pattern is skipped because DEFINE acts  like  a  false
4633         condition.         condition.
4634    
4635         The rest of the pattern uses references to the named group to match the         The rest of the pattern uses references to the named group to match the
4636         four  dot-separated  components of an IPv4 address, insisting on a word         four dot-separated components of an IPv4 address, insisting on  a  word
4637         boundary at each end.         boundary at each end.
4638    
4639     Assertion conditions     Assertion conditions
4640    
4641         If the condition is not in any of the above  formats,  it  must  be  an         If  the  condition  is  not  in any of the above formats, it must be an
4642         assertion.   This may be a positive or negative lookahead or lookbehind         assertion.  This may be a positive or negative lookahead or  lookbehind
4643         assertion. Consider  this  pattern,  again  containing  non-significant         assertion.  Consider  this  pattern,  again  containing non-significant
4644         white space, and with the two alternatives on the second line:         white space, and with the two alternatives on the second line:
4645    
4646           (?(?=[^a-z]*[a-z])           (?(?=[^a-z]*[a-z])
4647           \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )           \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )
4648    
4649         The  condition  is  a  positive  lookahead  assertion  that  matches an         The condition  is  a  positive  lookahead  assertion  that  matches  an
4650         optional sequence of non-letters followed by a letter. In other  words,         optional  sequence of non-letters followed by a letter. In other words,
4651         it  tests  for the presence of at least one letter in the subject. If a         it tests for the presence of at least one letter in the subject.  If  a
4652         letter is found, the subject is matched against the first  alternative;         letter  is found, the subject is matched against the first alternative;
4653         otherwise  it  is  matched  against  the  second.  This pattern matches         otherwise it is  matched  against  the  second.  This  pattern  matches
4654         strings in one of the two forms dd-aaa-dd or dd-dd-dd,  where  aaa  are         strings  in  one  of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
4655         letters and dd are digits.         letters and dd are digits.
4656    
4657    
4658  COMMENTS  COMMENTS
4659    
4660         The  sequence (?# marks the start of a comment that continues up to the         The sequence (?# marks the start of a comment that continues up to  the
4661         next closing parenthesis. Nested parentheses  are  not  permitted.  The         next  closing  parenthesis.  Nested  parentheses are not permitted. The
4662         characters  that make up a comment play no part in the pattern matching         characters that make up a comment play no part in the pattern  matching
4663         at all.         at all.
4664    
4665         If the PCRE_EXTENDED option is set, an unescaped # character outside  a         If  the PCRE_EXTENDED option is set, an unescaped # character outside a
4666         character  class  introduces  a  comment  that continues to immediately         character class introduces a  comment  that  continues  to  immediately
4667         after the next newline in the pattern.         after the next newline in the pattern.
4668    
4669    
4670  RECURSIVE PATTERNS  RECURSIVE PATTERNS
4671    
4672         Consider the problem of matching a string in parentheses, allowing  for         Consider  the problem of matching a string in parentheses, allowing for
4673         unlimited  nested  parentheses.  Without the use of recursion, the best         unlimited nested parentheses. Without the use of  recursion,  the  best
4674         that can be done is to use a pattern that  matches  up  to  some  fixed         that  can  be  done  is  to use a pattern that matches up to some fixed
4675         depth  of  nesting.  It  is not possible to handle an arbitrary nesting         depth of nesting. It is not possible to  handle  an  arbitrary  nesting
4676         depth.         depth.
4677    
4678         For some time, Perl has provided a facility that allows regular expres-         For some time, Perl has provided a facility that allows regular expres-
4679         sions  to recurse (amongst other things). It does this by interpolating         sions to recurse (amongst other things). It does this by  interpolating
4680         Perl code in the expression at run time, and the code can refer to  the         Perl  code in the expression at run time, and the code can refer to the
4681         expression itself. A Perl pattern using code interpolation to solve the         expression itself. A Perl pattern using code interpolation to solve the
4682         parentheses problem can be created like this:         parentheses problem can be created like this:
4683    
# Line 4685  RECURSIVE PATTERNS Line 4687  RECURSIVE PATTERNS
4687         refers recursively to the pattern in which it appears.         refers recursively to the pattern in which it appears.
4688    
4689         Obviously, PCRE cannot support the interpolation of Perl code. Instead,         Obviously, PCRE cannot support the interpolation of Perl code. Instead,
4690         it supports special syntax for recursion of  the  entire  pattern,  and         it  supports  special  syntax  for recursion of the entire pattern, and
4691         also  for  individual  subpattern  recursion. After its introduction in         also for individual subpattern recursion.  After  its  introduction  in
4692         PCRE and Python, this kind of recursion was  introduced  into  Perl  at         PCRE  and  Python,  this  kind of recursion was introduced into Perl at
4693         release 5.10.         release 5.10.
4694    
4695         A  special  item  that consists of (? followed by a number greater than         A special item that consists of (? followed by a  number  greater  than
4696         zero and a closing parenthesis is a recursive call of the subpattern of         zero and a closing parenthesis is a recursive call of the subpattern of
4697         the  given  number, provided that it occurs inside that subpattern. (If         the given number, provided that it occurs inside that  subpattern.  (If
4698         not, it is a "subroutine" call, which is described  in  the  next  sec-         not,  it  is  a  "subroutine" call, which is described in the next sec-
4699         tion.)  The special item (?R) or (?0) is a recursive call of the entire         tion.) The special item (?R) or (?0) is a recursive call of the  entire
4700         regular expression.         regular expression.
4701    
4702         In PCRE (like Python, but unlike Perl), a recursive subpattern call  is         In  PCRE (like Python, but unlike Perl), a recursive subpattern call is
4703         always treated as an atomic group. That is, once it has matched some of         always treated as an atomic group. That is, once it has matched some of
4704         the subject string, it is never re-entered, even if it contains untried         the subject string, it is never re-entered, even if it contains untried
4705         alternatives and there is a subsequent matching failure.         alternatives and there is a subsequent matching failure.
4706    
4707         This  PCRE  pattern  solves  the nested parentheses problem (assume the         This PCRE pattern solves the nested  parentheses  problem  (assume  the
4708         PCRE_EXTENDED option is set so that white space is ignored):         PCRE_EXTENDED option is set so that white space is ignored):
4709    
4710           \( ( (?>[^()]+) | (?R) )* \)           \( ( (?>[^()]+) | (?R) )* \)
4711    
4712         First it matches an opening parenthesis. Then it matches any number  of         First  it matches an opening parenthesis. Then it matches any number of
4713         substrings  which  can  either  be  a sequence of non-parentheses, or a         substrings which can either be a  sequence  of  non-parentheses,  or  a
4714         recursive match of the pattern itself (that is, a  correctly  parenthe-         recursive  match  of the pattern itself (that is, a correctly parenthe-
4715         sized substring).  Finally there is a closing parenthesis.         sized substring).  Finally there is a closing parenthesis.
4716    
4717         If  this  were  part of a larger pattern, you would not want to recurse         If this were part of a larger pattern, you would not  want  to  recurse
4718         the entire pattern, so instead you could use this:         the entire pattern, so instead you could use this:
4719    
4720           ( \( ( (?>[^()]+) | (?1) )* \) )           ( \( ( (?>[^()]+) | (?1) )* \) )
4721    
4722         We have put the pattern into parentheses, and caused the  recursion  to         We  have  put the pattern into parentheses, and caused the recursion to
4723         refer to them instead of the whole pattern.         refer to them instead of the whole pattern.
4724    
4725         In  a  larger  pattern,  keeping  track  of  parenthesis numbers can be         In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be
4726         tricky. This is made easier by the use of relative references. (A  Perl         tricky.  This is made easier by the use of relative references. (A Perl
4727         5.10  feature.)   Instead  of  (?1)  in the pattern above you can write         5.10 feature.)  Instead of (?1) in the  pattern  above  you  can  write
4728         (?-2) to refer to the second most recently opened parentheses preceding         (?-2) to refer to the second most recently opened parentheses preceding
4729         the  recursion.  In  other  words,  a  negative number counts capturing         the recursion. In other  words,  a  negative  number  counts  capturing
4730         parentheses leftwards from the point at which it is encountered.         parentheses leftwards from the point at which it is encountered.
4731    
4732         It is also possible to refer to  subsequently  opened  parentheses,  by         It  is  also  possible  to refer to subsequently opened parentheses, by
4733         writing  references  such  as (?+2). However, these cannot be recursive         writing references such as (?+2). However, these  cannot  be  recursive
4734         because the reference is not inside the  parentheses  that  are  refer-         because  the  reference  is  not inside the parentheses that are refer-
4735         enced.  They  are  always  "subroutine" calls, as described in the next         enced. They are always "subroutine" calls, as  described  in  the  next
4736         section.         section.
4737    
4738         An alternative approach is to use named parentheses instead.  The  Perl         An  alternative  approach is to use named parentheses instead. The Perl
4739         syntax  for  this  is (?&name); PCRE's earlier syntax (?P>name) is also         syntax for this is (?&name); PCRE's earlier syntax  (?P>name)  is  also
4740         supported. We could rewrite the above example as follows:         supported. We could rewrite the above example as follows:
4741    
4742           (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )           (?<pn> \( ( (?>[^()]+) | (?&pn) )* \) )
4743    
4744         If there is more than one subpattern with the same name,  the  earliest         If  there  is more than one subpattern with the same name, the earliest
4745         one is used.         one is used.
4746    
4747         This  particular  example pattern that we have been looking at contains         This particular example pattern that we have been looking  at  contains
4748         nested unlimited repeats, and so the use of atomic grouping for  match-         nested  unlimited repeats, and so the use of atomic grouping for match-
4749         ing  strings  of non-parentheses is important when applying the pattern         ing strings of non-parentheses is important when applying  the  pattern
4750         to strings that do not match. For example, when this pattern is applied         to strings that do not match. For example, when this pattern is applied
4751         to         to
4752    
4753           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()           (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
4754    
4755         it  yields "no match" quickly. However, if atomic grouping is not used,         it yields "no match" quickly. However, if atomic grouping is not  used,
4756         the match runs for a very long time indeed because there  are  so  many         the  match  runs  for a very long time indeed because there are so many
4757         different  ways  the  + and * repeats can carve up the subject, and all         different ways the + and * repeats can carve up the  subject,  and  all
4758         have to be tested before failure can be reported.         have to be tested before failure can be reported.
4759    
4760         At the end of a match, the values set for any capturing subpatterns are         At the end of a match, the values set for any capturing subpatterns are
4761         those from the outermost level of the recursion at which the subpattern         those from the outermost level of the recursion at which the subpattern
4762         value is set.  If you want to obtain  intermediate  values,  a  callout         value  is  set.   If  you want to obtain intermediate values, a callout
4763         function  can be used (see below and the pcrecallout documentation). If         function can be used (see below and the pcrecallout documentation).  If
4764         the pattern above is matched against         the pattern above is matched against
4765    
4766           (ab(cd)ef)           (ab(cd)ef)
4767    
4768         the value for the capturing parentheses is  "ef",  which  is  the  last         the  value  for  the  capturing  parentheses is "ef", which is the last
4769         value  taken  on at the top level. If additional parentheses are added,         value taken on at the top level. If additional parentheses  are  added,
4770         giving         giving
4771    
4772           \( ( ( (?>[^()]+) | (?R) )* ) \)           \( ( ( (?>[^()]+) | (?R) )* ) \)
4773              ^                        ^              ^                        ^
4774              ^                        ^              ^                        ^
4775    
4776         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
4777         parentheses.  If there are more than 15 capturing parentheses in a pat-         parentheses. If there are more than 15 capturing parentheses in a  pat-
4778         tern, PCRE has to obtain extra memory to store data during a recursion,         tern, PCRE has to obtain extra memory to store data during a recursion,
4779         which  it  does  by  using pcre_malloc, freeing it via pcre_free after-         which it does by using pcre_malloc, freeing  it  via  pcre_free  after-
4780         wards. If  no  memory  can  be  obtained,  the  match  fails  with  the         wards.  If  no  memory  can  be  obtained,  the  match  fails  with the
4781         PCRE_ERROR_NOMEMORY error.         PCRE_ERROR_NOMEMORY error.
4782    
4783         Do  not  confuse  the (?R) item with the condition (R), which tests for         Do not confuse the (?R) item with the condition (R),  which  tests  for
4784         recursion.  Consider this pattern, which matches text in  angle  brack-         recursion.   Consider  this pattern, which matches text in angle brack-
4785         ets,  allowing for arbitrary nesting. Only digits are allowed in nested         ets, allowing for arbitrary nesting. Only digits are allowed in  nested
4786         brackets (that is, when recursing), whereas any characters are  permit-         brackets  (that is, when recursing), whereas any characters are permit-
4787         ted at the outer level.         ted at the outer level.
4788    
4789           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >           < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >
4790    
4791         In  this  pattern, (?(R) is the start of a conditional subpattern, with         In this pattern, (?(R) is the start of a conditional  subpattern,  with
4792         two different alternatives for the recursive and  non-recursive  cases.         two  different  alternatives for the recursive and non-recursive cases.
4793         The (?R) item is the actual recursive call.         The (?R) item is the actual recursive call.
4794    
4795    
4796  SUBPATTERNS AS SUBROUTINES  SUBPATTERNS AS SUBROUTINES
4797    
4798         If the syntax for a recursive subpattern reference (either by number or         If the syntax for a recursive subpattern reference (either by number or
4799         by name) is used outside the parentheses to which it refers,  it  oper-         by  name)  is used outside the parentheses to which it refers, it oper-
4800         ates  like a subroutine in a programming language. The "called" subpat-         ates like a subroutine in a programming language. The "called"  subpat-
4801         tern may be defined before or after the reference. A numbered reference         tern may be defined before or after the reference. A numbered reference
4802         can be absolute or relative, as in these examples:         can be absolute or relative, as in these examples:
4803    
# Line 4807  SUBPATTERNS AS SUBROUTINES Line 4809  SUBPATTERNS AS SUBROUTINES
4809    
4810           (sens|respons)e and \1ibility           (sens|respons)e and \1ibility
4811    
4812         matches  "sense and sensibility" and "response and responsibility", but         matches "sense and sensibility" and "response and responsibility",  but
4813         not "sense and responsibility". If instead the pattern         not "sense and responsibility". If instead the pattern
4814    
4815           (sens|respons)e and (?1)ibility           (sens|respons)e and (?1)ibility
4816    
4817         is used, it does match "sense and responsibility" as well as the  other         is  used, it does match "sense and responsibility" as well as the other
4818         two  strings.  Another  example  is  given  in the discussion of DEFINE         two strings. Another example is  given  in  the  discussion  of  DEFINE
4819         above.         above.
4820    
4821         Like recursive subpatterns, a "subroutine" call is always treated as an         Like recursive subpatterns, a "subroutine" call is always treated as an
4822         atomic  group. That is, once it has matched some of the subject string,         atomic group. That is, once it has matched some of the subject  string,
4823         it is never re-entered, even if it contains  untried  alternatives  and         it  is  never  re-entered, even if it contains untried alternatives and
4824         there is a subsequent matching failure.         there is a subsequent matching failure.
4825    
4826         When  a  subpattern is used as a subroutine, processing options such as         When a subpattern is used as a subroutine, processing options  such  as
4827         case-independence are fixed when the subpattern is defined. They cannot         case-independence are fixed when the subpattern is defined. They cannot
4828         be changed for different calls. For example, consider this pattern:         be changed for different calls. For example, consider this pattern:
4829    
4830           (abc)(?i:(?-1))           (abc)(?i:(?-1))
4831    
4832         It  matches  "abcabc". It does not match "abcABC" because the change of         It matches "abcabc". It does not match "abcABC" because the  change  of
4833         processing option does not affect the called subpattern.         processing option does not affect the called subpattern.
4834    
4835    
4836  ONIGURUMA SUBROUTINE SYNTAX  ONIGURUMA SUBROUTINE SYNTAX
4837    
4838         For compatibility with Oniguruma, the non-Perl syntax \g followed by  a         For  compatibility with Oniguruma, the non-Perl syntax \g followed by a
4839         name or a number enclosed either in angle brackets or single quotes, is         name or a number enclosed either in angle brackets or single quotes, is
4840         an alternative syntax for referencing a  subpattern  as  a  subroutine,         an  alternative  syntax  for  referencing a subpattern as a subroutine,
4841         possibly  recursively. Here are two of the examples used above, rewrit-         possibly recursively. Here are two of the examples used above,  rewrit-
4842         ten using this syntax:         ten using this syntax:
4843    
4844           (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )           (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
4845           (sens|respons)e and \g'1'ibility           (sens|respons)e and \g'1'ibility
4846    
4847         PCRE supports an extension to Oniguruma: if a number is preceded  by  a         PCRE  supports  an extension to Oniguruma: if a number is preceded by a
4848         plus or a minus sign it is taken as a relative reference. For example:         plus or a minus sign it is taken as a relative reference. For example:
4849    
4850           (abc)(?i:\g<-1>)           (abc)(?i:\g<-1>)
4851    
4852         Note  that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not         Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are  not
4853         synonymous. The former is a back reference; the latter is a  subroutine         synonymous.  The former is a back reference; the latter is a subroutine
4854         call.         call.
4855    
4856    
4857  CALLOUTS  CALLOUTS
4858    
4859         Perl has a feature whereby using the sequence (?{...}) causes arbitrary         Perl has a feature whereby using the sequence (?{...}) causes arbitrary
4860         Perl code to be obeyed in the middle of matching a regular  expression.         Perl  code to be obeyed in the middle of matching a regular expression.
4861         This makes it possible, amongst other things, to extract different sub-         This makes it possible, amongst other things, to extract different sub-
4862         strings that match the same pair of parentheses when there is a repeti-         strings that match the same pair of parentheses when there is a repeti-
4863         tion.         tion.
4864    
4865         PCRE provides a similar feature, but of course it cannot obey arbitrary         PCRE provides a similar feature, but of course it cannot obey arbitrary
4866         Perl code. The feature is called "callout". The caller of PCRE provides         Perl code. The feature is called "callout". The caller of PCRE provides
4867         an  external function by putting its entry point in the global variable         an external function by putting its entry point in the global  variable
4868         pcre_callout.  By default, this variable contains NULL, which  disables         pcre_callout.   By default, this variable contains NULL, which disables
4869         all calling out.         all calling out.
4870    
4871         Within  a  regular  expression,  (?C) indicates the points at which the         Within a regular expression, (?C) indicates the  points  at  which  the
4872         external function is to be called. If you want  to  identify  different         external  function  is  to be called. If you want to identify different
4873         callout  points, you can put a number less than 256 after the letter C.         callout points, you can put a number less than 256 after the letter  C.
4874         The default value is zero.  For example, this pattern has  two  callout         The  default  value is zero.  For example, this pattern has two callout
4875         points:         points:
4876    
4877           (?C1)abc(?C2)def           (?C1)abc(?C2)def
4878    
4879         If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are         If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
4880         automatically installed before each item in the pattern. They  are  all         automatically  installed  before each item in the pattern. They are all
4881         numbered 255.         numbered 255.
4882    
4883         During matching, when PCRE reaches a callout point (and pcre_callout is         During matching, when PCRE reaches a callout point (and pcre_callout is
4884         set), the external function is called. It is provided with  the  number         set),  the  external function is called. It is provided with the number
4885         of  the callout, the position in the pattern, and, optionally, one item         of the callout, the position in the pattern, and, optionally, one  item
4886         of data originally supplied by the caller of pcre_exec().  The  callout         of  data  originally supplied by the caller of pcre_exec(). The callout
4887         function  may cause matching to proceed, to backtrack, or to fail alto-         function may cause matching to proceed, to backtrack, or to fail  alto-
4888         gether. A complete description of the interface to the callout function         gether. A complete description of the interface to the callout function
4889         is given in the pcrecallout documentation.         is given in the pcrecallout documentation.
4890    
4891    
4892  BACKTRACKING CONTROL  BACKTRACKING CONTROL
4893    
4894         Perl  5.10 introduced a number of "Special Backtracking Control Verbs",         Perl 5.10 introduced a number of "Special Backtracking Control  Verbs",
4895         which are described in the Perl documentation as "experimental and sub-         which are described in the Perl documentation as "experimental and sub-
4896         ject  to  change or removal in a future version of Perl". It goes on to         ject to change or removal in a future version of Perl". It goes  on  to
4897         say: "Their usage in production code should be noted to avoid  problems         say:  "Their usage in production code should be noted to avoid problems
4898         during upgrades." The same remarks apply to the PCRE features described         during upgrades." The same remarks apply to the PCRE features described
4899         in this section.         in this section.
4900    
4901         Since these verbs are specifically related  to  backtracking,  most  of         Since  these  verbs  are  specifically related to backtracking, most of
4902         them  can  be  used  only  when  the  pattern  is  to  be matched using         them can be  used  only  when  the  pattern  is  to  be  matched  using
4903         pcre_exec(), which uses a backtracking algorithm. With the exception of         pcre_exec(), which uses a backtracking algorithm. With the exception of
4904         (*FAIL), which behaves like a failing negative assertion, they cause an         (*FAIL), which behaves like a failing negative assertion, they cause an
4905         error if encountered by pcre_dfa_exec().         error if encountered by pcre_dfa_exec().
4906    
4907         The new verbs make use of what was previously invalid syntax: an  open-         The  new verbs make use of what was previously invalid syntax: an open-
4908         ing parenthesis followed by an asterisk. In Perl, they are generally of         ing parenthesis followed by an asterisk. In Perl, they are generally of
4909         the form (*VERB:ARG) but PCRE does not support the use of arguments, so         the form (*VERB:ARG) but PCRE does not support the use of arguments, so
4910         its  general  form is just (*VERB). Any number of these verbs may occur         its general form is just (*VERB). Any number of these verbs  may  occur
4911         in a pattern. There are two kinds:         in a pattern. There are two kinds:
4912    
4913     Verbs that act immediately     Verbs that act immediately
# Line 4914  BACKTRACKING CONTROL Line 4916  BACKTRACKING CONTROL
4916    
4917            (*ACCEPT)            (*ACCEPT)
4918    
4919         This verb causes the match to end successfully, skipping the  remainder         This  verb causes the match to end successfully, skipping the remainder
4920         of  the pattern. When inside a recursion, only the innermost pattern is         of the pattern. When inside a recursion, only the innermost pattern  is
4921         ended immediately. PCRE differs  from  Perl  in  what  happens  if  the         ended  immediately.  PCRE  differs  from  Perl  in  what happens if the
4922         (*ACCEPT)  is inside capturing parentheses. In Perl, the data so far is         (*ACCEPT) is inside capturing parentheses. In Perl, the data so far  is
4923         captured: in PCRE no data is captured. For example:         captured: in PCRE no data is captured. For example:
4924    
4925           A(A|B(*ACCEPT)|C)D           A(A|B(*ACCEPT)|C)D
4926    
4927         This matches "AB", "AAD", or "ACD", but when it matches "AB",  no  data         This  matches  "AB", "AAD", or "ACD", but when it matches "AB", no data
4928         is captured.         is captured.
4929    
4930           (*FAIL) or (*F)           (*FAIL) or (*F)
4931    
4932         This  verb  causes the match to fail, forcing backtracking to occur. It         This verb causes the match to fail, forcing backtracking to  occur.  It
4933         is equivalent to (?!) but easier to read. The Perl documentation  notes         is  equivalent to (?!) but easier to read. The Perl documentation notes
4934         that  it  is  probably  useful only when combined with (?{}) or (??{}).         that it is probably useful only when combined  with  (?{})  or  (??{}).
4935         Those are, of course, Perl features that are not present in  PCRE.  The         Those  are,  of course, Perl features that are not present in PCRE. The
4936         nearest  equivalent is the callout feature, as for example in this pat-         nearest equivalent is the callout feature, as for example in this  pat-
4937         tern:         tern:
4938    
4939           a+(?C)(*FAIL)           a+(?C)(*FAIL)
4940    
4941         A match with the string "aaaa" always fails, but the callout  is  taken         A  match  with the string "aaaa" always fails, but the callout is taken
4942         before each backtrack happens (in this example, 10 times).         before each backtrack happens (in this example, 10 times).
4943    
4944     Verbs that act after backtracking     Verbs that act after backtracking
4945    
4946         The following verbs do nothing when they are encountered. Matching con-         The following verbs do nothing when they are encountered. Matching con-
4947         tinues with what follows, but if there is no subsequent match, a  fail-         tinues  with what follows, but if there is no subsequent match, a fail-
4948         ure  is  forced.   The  verbs  differ  in  exactly what kind of failure         ure is forced.  The verbs  differ  in  exactly  what  kind  of  failure
4949         occurs.         occurs.
4950    
4951           (*COMMIT)           (*COMMIT)
4952    
4953         This verb causes the whole match to fail outright if the  rest  of  the         This  verb  causes  the whole match to fail outright if the rest of the
4954         pattern  does  not match. Even if the pattern is unanchored, no further         pattern does not match. Even if the pattern is unanchored,  no  further
4955         attempts to find a match by advancing the start point take place.  Once         attempts  to find a match by advancing the start point take place. Once
4956         (*COMMIT)  has been passed, pcre_exec() is committed to finding a match         (*COMMIT) has been passed, pcre_exec() is committed to finding a  match
4957         at the current starting point, or not at all. For example:         at the current starting point, or not at all. For example:
4958    
4959           a+(*COMMIT)b           a+(*COMMIT)b
4960    
4961         This matches "xxaab" but not "aacaab". It can be thought of as  a  kind         This  matches  "xxaab" but not "aacaab". It can be thought of as a kind
4962         of dynamic anchor, or "I've started, so I must finish."         of dynamic anchor, or "I've started, so I must finish."
4963    
4964           (*PRUNE)           (*PRUNE)
4965    
4966         This  verb causes the match to fail at the current position if the rest         This verb causes the match to fail at the current position if the  rest
4967         of the pattern does not match. If the pattern is unanchored, the normal         of the pattern does not match. If the pattern is unanchored, the normal
4968         "bumpalong"  advance to the next starting character then happens. Back-         "bumpalong" advance to the next starting character then happens.  Back-
4969         tracking can occur as usual to the left of (*PRUNE), or  when  matching         tracking  can  occur as usual to the left of (*PRUNE), or when matching
4970         to  the right of (*PRUNE), but if there is no match to the right, back-         to the right of (*PRUNE), but if there is no match to the right,  back-
4971         tracking cannot cross (*PRUNE).  In simple cases, the use  of  (*PRUNE)         tracking  cannot  cross (*PRUNE).  In simple cases, the use of (*PRUNE)
4972         is just an alternative to an atomic group or possessive quantifier, but         is just an alternative to an atomic group or possessive quantifier, but
4973         there are some uses of (*PRUNE) that cannot be expressed in  any  other         there  are  some uses of (*PRUNE) that cannot be expressed in any other
4974         way.         way.
4975    
4976           (*SKIP)           (*SKIP)
4977    
4978         This  verb  is like (*PRUNE), except that if the pattern is unanchored,         This verb is like (*PRUNE), except that if the pattern  is  unanchored,
4979         the "bumpalong" advance is not to the next character, but to the  posi-         the  "bumpalong" advance is not to the next character, but to the posi-
4980         tion  in  the  subject where (*SKIP) was encountered. (*SKIP) signifies         tion in the subject where (*SKIP) was  encountered.  (*SKIP)  signifies
4981         that whatever text was matched leading up to it cannot  be  part  of  a         that  whatever  text  was  matched leading up to it cannot be part of a
4982         successful match. Consider:         successful match. Consider:
4983    
4984           a+(*SKIP)b           a+(*SKIP)b
4985    
4986         If  the  subject  is  "aaaac...",  after  the first match attempt fails         If the subject is "aaaac...",  after  the  first  match  attempt  fails
4987         (starting at the first character in the  string),  the  starting  point         (starting  at  the  first  character in the string), the starting point
4988         skips on to start the next attempt at "c". Note that a possessive quan-         skips on to start the next attempt at "c". Note that a possessive quan-
4989         tifer does not have the same effect in this example; although it  would         tifer  does not have the same effect in this example; although it would
4990         suppress  backtracking  during  the  first  match  attempt,  the second         suppress backtracking  during  the  first  match  attempt,  the  second
4991         attempt would start at the second character instead of skipping  on  to         attempt  would  start at the second character instead of skipping on to
4992         "c".         "c".
4993    
4994           (*THEN)           (*THEN)
4995    
4996         This verb causes a skip to the next alternation if the rest of the pat-         This verb causes a skip to the next alternation if the rest of the pat-
4997         tern does not match. That is, it cancels pending backtracking, but only         tern does not match. That is, it cancels pending backtracking, but only
4998         within  the  current  alternation.  Its name comes from the observation         within the current alternation. Its name  comes  from  the  observation
4999         that it can be used for a pattern-based if-then-else block:         that it can be used for a pattern-based if-then-else block:
5000    
5001           ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...           ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
5002    
5003         If the COND1 pattern matches, FOO is tried (and possibly further  items         If  the COND1 pattern matches, FOO is tried (and possibly further items
5004         after  the  end  of  the group if FOO succeeds); on failure the matcher         after the end of the group if FOO succeeds);  on  failure  the  matcher
5005         skips to the second alternative and tries COND2,  without  backtracking         skips  to  the second alternative and tries COND2, without backtracking
5006         into  COND1.  If  (*THEN)  is  used outside of any alternation, it acts         into COND1. If (*THEN) is used outside  of  any  alternation,  it  acts
5007         exactly like (*PRUNE).         exactly like (*PRUNE).
5008    
5009    
# Line 5019  AUTHOR Line 5021  AUTHOR
5021    
5022  REVISION  REVISION
5023    
5024         Last updated: 08 March 2009         Last updated: 18 March 2009
5025         Copyright (c) 1997-2009 University of Cambridge.         Copyright (c) 1997-2009 University of Cambridge.
5026  ------------------------------------------------------------------------------  ------------------------------------------------------------------------------
5027    

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