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PCRESYNTAX

Section: C Library Functions (3)
Updated: 08 January 2014
Index Return to Main Contents
 

NAME

PCRE - Perl-compatible regular expressions  

PCRE REGULAR EXPRESSION SYNTAX SUMMARY

The full syntax and semantics of the regular expressions that are supported by PCRE are described in the pcrepattern documentation. This document contains a quick-reference summary of the syntax.  

QUOTING


  \x         where x is non-alphanumeric is a literal x
  \Q...\E    treat enclosed characters as literal  

CHARACTERS


  \a         alarm, that is, the BEL character (hex 07)
  \cx        "control-x", where x is any ASCII character
  \e         escape (hex 1B)
  \f         form feed (hex 0C)
  \n         newline (hex 0A)
  \r         carriage return (hex 0D)
  \t         tab (hex 09)
  \0dd       character with octal code 0dd
  \ddd       character with octal code ddd, or backreference
  \o{ddd..}  character with octal code ddd..
  \xhh       character with hex code hh
  \x{hhh..}  character with hex code hhh..

Note that \0dd is always an octal code, and that \8 and \9 are the literal characters "8" and "9".  

CHARACTER TYPES


  .          any character except newline;
               in dotall mode, any character whatsoever
  \C         one data unit, even in UTF mode (best avoided)
  \d         a decimal digit
  \D         a character that is not a decimal digit
  \h         a horizontal white space character
  \H         a character that is not a horizontal white space character
  \N         a character that is not a newline
  \p{xx}     a character with the xx property
  \P{xx}     a character without the xx property
  \R         a newline sequence
  \s         a white space character
  \S         a character that is not a white space character
  \v         a vertical white space character
  \V         a character that is not a vertical white space character
  \w         a "word" character
  \W         a "non-word" character
  \X         a Unicode extended grapheme cluster

By default, \d, \s, and \w match only ASCII characters, even in UTF-8 mode or in the 16- bit and 32-bit libraries. However, if locale-specific matching is happening, \s and \w may also match characters with code points in the range 128-255. If the PCRE_UCP option is set, the behaviour of these escape sequences is changed to use Unicode properties and they match many more characters.  

GENERAL CATEGORY PROPERTIES FOR \p and \P


  C          Other
  Cc         Control
  Cf         Format
  Cn         Unassigned
  Co         Private use
  Cs         Surrogate


  L          Letter
  Ll         Lower case letter
  Lm         Modifier letter
  Lo         Other letter
  Lt         Title case letter
  Lu         Upper case letter
  L&         Ll, Lu, or Lt


  M          Mark
  Mc         Spacing mark
  Me         Enclosing mark
  Mn         Non-spacing mark


  N          Number
  Nd         Decimal number
  Nl         Letter number
  No         Other number


  P          Punctuation
  Pc         Connector punctuation
  Pd         Dash punctuation
  Pe         Close punctuation
  Pf         Final punctuation
  Pi         Initial punctuation
  Po         Other punctuation
  Ps         Open punctuation


  S          Symbol
  Sc         Currency symbol
  Sk         Modifier symbol
  Sm         Mathematical symbol
  So         Other symbol


  Z          Separator
  Zl         Line separator
  Zp         Paragraph separator
  Zs         Space separator  

PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P


  Xan        Alphanumeric: union of properties L and N
  Xps        POSIX space: property Z or tab, NL, VT, FF, CR
  Xsp        Perl space: property Z or tab, NL, VT, FF, CR
  Xuc        Univerally-named character: one that can be
               represented by a Universal Character Name
  Xwd        Perl word: property Xan or underscore

Perl and POSIX space are now the same. Perl added VT to its space character set at release 5.18 and PCRE changed at release 8.34.  

SCRIPT NAMES FOR \p AND \P

Arabic, Armenian, Avestan, Balinese, Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham, Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Duployan, Egyptian_Hieroglyphs, Elbasan, Ethiopic, Georgian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Imperial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian, Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Khojki, Khudawadi, Lao, Latin, Lepcha, Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian, Mahajani, Malayalam, Mandaic, Manichaean, Meetei_Mayek, Mende_Kikakui, Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian, Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene, Pau_Cin_Hau, Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic, Samaritan, Saurashtra, Sharada, Shavian, Siddham, Sinhala, Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.  

CHARACTER CLASSES


  [...]       positive character class
  [^...]      negative character class
  [x-y]       range (can be used for hex characters)
  [[:xxx:]]   positive POSIX named set
  [[:^xxx:]]  negative POSIX named set


  alnum       alphanumeric
  alpha       alphabetic
  ascii       0-127
  blank       space or tab
  cntrl       control character
  digit       decimal digit
  graph       printing, excluding space
  lower       lower case letter
  print       printing, including space
  punct       printing, excluding alphanumeric
  space       white space
  upper       upper case letter
  word        same as \w
  xdigit      hexadecimal digit

In PCRE, POSIX character set names recognize only ASCII characters by default, but some of them use Unicode properties if PCRE_UCP is set. You can use \Q...\E inside a character class.  

QUANTIFIERS


  ?           0 or 1, greedy
  ?+          0 or 1, possessive
  ??          0 or 1, lazy
  *           0 or more, greedy
  *+          0 or more, possessive
  *?          0 or more, lazy
  +           1 or more, greedy
  ++          1 or more, possessive
  +?          1 or more, lazy
  {n}         exactly n
  {n,m}       at least n, no more than m, greedy
  {n,m}+      at least n, no more than m, possessive
  {n,m}?      at least n, no more than m, lazy
  {n,}        n or more, greedy
  {n,}+       n or more, possessive
  {n,}?       n or more, lazy  

ANCHORS AND SIMPLE ASSERTIONS


  \b          word boundary
  \B          not a word boundary
  ^           start of subject
               also after internal newline in multiline mode
  \A          start of subject
  $           end of subject
               also before newline at end of subject
               also before internal newline in multiline mode
  \Z          end of subject
               also before newline at end of subject
  \z          end of subject
  \G          first matching position in subject  

MATCH POINT RESET


  \K          reset start of match

\K is honoured in positive assertions, but ignored in negative ones.  

ALTERNATION


  expr|expr|expr...  

CAPTURING


  (...)           capturing group
  (?<name>...)    named capturing group (Perl)
  (?'name'...)    named capturing group (Perl)
  (?P<name>...)   named capturing group (Python)
  (?:...)         non-capturing group
  (?|...)         non-capturing group; reset group numbers for
                   capturing groups in each alternative  

ATOMIC GROUPS


  (?>...)         atomic, non-capturing group  

COMMENT


  (?#....)        comment (not nestable)  

OPTION SETTING


  (?i)            caseless
  (?J)            allow duplicate names
  (?m)            multiline
  (?s)            single line (dotall)
  (?U)            default ungreedy (lazy)
  (?x)            extended (ignore white space)
  (?-...)         unset option(s)

The following are recognized only at the very start of a pattern or after one of the newline or \R options with similar syntax. More than one of them may appear.


  (*LIMIT_MATCH=d) set the match limit to d (decimal number)
  (*LIMIT_RECURSION=d) set the recursion limit to d (decimal number)
  (*NO_AUTO_POSSESS) no auto-possessification (PCRE_NO_AUTO_POSSESS)
  (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE)
  (*UTF8)         set UTF-8 mode: 8-bit library (PCRE_UTF8)
  (*UTF16)        set UTF-16 mode: 16-bit library (PCRE_UTF16)
  (*UTF32)        set UTF-32 mode: 32-bit library (PCRE_UTF32)
  (*UTF)          set appropriate UTF mode for the library in use
  (*UCP)          set PCRE_UCP (use Unicode properties for \d etc)

Note that LIMIT_MATCH and LIMIT_RECURSION can only reduce the value of the limits set by the caller of pcre_exec(), not increase them.  

NEWLINE CONVENTION

These are recognized only at the very start of the pattern or after option settings with a similar syntax.


  (*CR)           carriage return only
  (*LF)           linefeed only
  (*CRLF)         carriage return followed by linefeed
  (*ANYCRLF)      all three of the above
  (*ANY)          any Unicode newline sequence  

WHAT \R MATCHES

These are recognized only at the very start of the pattern or after option setting with a similar syntax.


  (*BSR_ANYCRLF)  CR, LF, or CRLF
  (*BSR_UNICODE)  any Unicode newline sequence  

LOOKAHEAD AND LOOKBEHIND ASSERTIONS


  (?=...)         positive look ahead
  (?!...)         negative look ahead
  (?<=...)        positive look behind
  (?<!...)        negative look behind

Each top-level branch of a look behind must be of a fixed length.  

BACKREFERENCES


  \n              reference by number (can be ambiguous)
  \gn             reference by number
  \g{n}           reference by number
  \g{-n}          relative reference by number
  \k<name>        reference by name (Perl)
  \k'name'        reference by name (Perl)
  \g{name}        reference by name (Perl)
  \k{name}        reference by name (.NET)
  (?P=name)       reference by name (Python)  

SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)


  (?R)            recurse whole pattern
  (?n)            call subpattern by absolute number
  (?+n)           call subpattern by relative number
  (?-n)           call subpattern by relative number
  (?&name)        call subpattern by name (Perl)
  (?P>name)       call subpattern by name (Python)
  \g<name>        call subpattern by name (Oniguruma)
  \g'name'        call subpattern by name (Oniguruma)
  \g<n>           call subpattern by absolute number (Oniguruma)
  \g'n'           call subpattern by absolute number (Oniguruma)
  \g<+n>          call subpattern by relative number (PCRE extension)
  \g'+n'          call subpattern by relative number (PCRE extension)
  \g<-n>          call subpattern by relative number (PCRE extension)
  \g'-n'          call subpattern by relative number (PCRE extension)  

CONDITIONAL PATTERNS


  (?(condition)yes-pattern)
  (?(condition)yes-pattern|no-pattern)


  (?(n)...        absolute reference condition
  (?(+n)...       relative reference condition
  (?(-n)...       relative reference condition
  (?(<name>)...   named reference condition (Perl)
  (?('name')...   named reference condition (Perl)
  (?(name)...     named reference condition (PCRE)
  (?(R)...        overall recursion condition
  (?(Rn)...       specific group recursion condition
  (?(R&name)...   specific recursion condition
  (?(DEFINE)...   define subpattern for reference
  (?(assert)...   assertion condition  

BACKTRACKING CONTROL

The following act immediately they are reached:


  (*ACCEPT)       force successful match
  (*FAIL)         force backtrack; synonym (*F)
  (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

The following act only when a subsequent match failure causes a backtrack to reach them. They all force a match failure, but they differ in what happens afterwards. Those that advance the start-of-match point do so only if the pattern is not anchored.


  (*COMMIT)       overall failure, no advance of starting point
  (*PRUNE)        advance to next starting character
  (*PRUNE:NAME)   equivalent to (*MARK:NAME)(*PRUNE)
  (*SKIP)         advance to current matching position
  (*SKIP:NAME)    advance to position corresponding to an earlier
                  (*MARK:NAME); if not found, the (*SKIP) is ignored
  (*THEN)         local failure, backtrack to next alternation
  (*THEN:NAME)    equivalent to (*MARK:NAME)(*THEN)  

CALLOUTS


  (?C)      callout
  (?Cn)     callout with data n  

SEE ALSO

pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).  

AUTHOR

Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
 

REVISION

Last updated: 08 January 2014
Copyright (c) 1997-2014 University of Cambridge.


 

Index

NAME
PCRE REGULAR EXPRESSION SYNTAX SUMMARY
QUOTING
CHARACTERS
CHARACTER TYPES
GENERAL CATEGORY PROPERTIES FOR \p and \P
PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P
SCRIPT NAMES FOR \p AND \P
CHARACTER CLASSES
QUANTIFIERS
ANCHORS AND SIMPLE ASSERTIONS
MATCH POINT RESET
ALTERNATION
CAPTURING
ATOMIC GROUPS
COMMENT
OPTION SETTING
NEWLINE CONVENTION
WHAT \R MATCHES
LOOKAHEAD AND LOOKBEHIND ASSERTIONS
BACKREFERENCES
SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
CONDITIONAL PATTERNS
BACKTRACKING CONTROL
CALLOUTS
SEE ALSO
AUTHOR
REVISION


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