PCRE2SYNTAX(3)

NAME
PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY

The full syntax and semantics of the regular expressions that are supported by PCRE2 are described in the pcre2pattern 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

ESCAPED CHARACTERS

This table applies to ASCII and Unicode environments.

  \a alarm, that is, the BEL character (hex 07)
  \cx "control-x", where x is any ASCII printing 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..
  \U "U" if PCRE2_ALT_BSUX is set (otherwise is an error)
  \uhhhh character with hex code hhhh (if PCRE2_ALT_BSUX is set)
  \xhh character with hex code hh
  \x{hhh..} character with hex code hhh..

Note that \0dd is always an octal code. The treatment of backslash followed by a non-zero digit is complicated; for details see the section "Non-printing characters" in the pcre2pattern documentation, where details of escape processing in EBCDIC environments are also given.

When \x is not followed by {, from zero to two hexadecimal digits are read, but if PCRE2_ALT_BSUX is set, \x must be followed by two hexadecimal digits to be recognized as a hexadecimal escape; otherwise it matches a literal "x". Likewise, if \u (in ALT_BSUX mode) is not followed by four hexadecimal digits, it matches a literal "u".

CHARACTER TYPES

  . any character except newline;
               in dotall mode, any character whatsoever
  \C one code 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

\C is dangerous because it may leave the current matching point in the middle of a UTF-8 or UTF-16 character. The application can lock out the use of \C by setting the PCRE2_NEVER_BACKSLASH_C option. It is also possible to build PCRE2 with the use of \C permanently disabled.

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 PCRE2_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

PCRE2 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.

SCRIPT NAMES FOR \p AND \P

Ahom, Anatolian_Hieroglyphs, 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, Hatran, 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, Multani, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki, Old_Hungarian, 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, SignWriting, 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 PCRE2, POSIX character set names recognize only ASCII characters by default, but some of them use Unicode properties if PCRE2_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 an internal newline in multiline mode
                (after any newline if PCRE2_ALT_CIRCUMFLEX is set)
  \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
  (?n) no auto capture
  (?s) single line (dotall)
  (?U) default ungreedy (lazy)
  (?x) extended: ignore white space except in classes
  (?xx) as (?x) but also ignore space and tab in classes
  (?-...) 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. For the first three, d is a decimal number.

  (*LIMIT_DEPTH=d) set the backtracking limit to d
  (*LIMIT_HEAP=d) set the heap size limit to d kilobytes
  (*LIMIT_MATCH=d) set the match limit to d
  (*NOTEMPTY) set PCRE2_NOTEMPTY when matching
  (*NOTEMPTY_ATSTART) set PCRE2_NOTEMPTY_ATSTART when matching
  (*NO_AUTO_POSSESS) no auto-possessification (PCRE2_NO_AUTO_POSSESS)
  (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR)
  (*NO_JIT) disable JIT optimization
  (*NO_START_OPT) no start-match optimization (PCRE2_NO_START_OPTIMIZE)
  (*UTF) set appropriate UTF mode for the library in use
  (*UCP) set PCRE2_UCP (use Unicode properties for \d etc)

Note that LIMIT_DEPTH, LIMIT_HEAP, and LIMIT_MATCH can only reduce the value of the limits set by the caller of pcre2_match() or pcre2_dfa_match(), not increase them. LIMIT_RECURSION is an obsolete synonym for LIMIT_DEPTH. The application can lock out the use of (*UTF) and (*UCP) by setting the PCRE2_NEVER_UTF or PCRE2_NEVER_UCP options, respectively, at compile time.

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
  (*NUL) the NUL character (binary zero)

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 (PCRE2 extension)
  \g-n relative reference by number
  \g{+n} relative reference by number (PCRE2 extension)
  \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 (PCRE2 extension)
  \g'+n' call subpattern by relative number (PCRE2 extension)
  \g<-n> call subpattern by relative number (PCRE2 extension)
  \g'-n' call subpattern by relative number (PCRE2 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 (PCRE2, deprecated)
  (?(R) overall recursion condition
  (?(Rn) specific numbered group recursion condition
  (?(R&name) specific named group recursion condition
  (?(DEFINE) define subpattern for reference
  (?(VERSION[>]=n.m) test PCRE2 version
  (?(assert) assertion condition

Note the ambiguity of (?(R) and (?(Rn) which might be named reference conditions or recursion tests. Such a condition is interpreted as a reference condition if the relevant named group exists.

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 (assumed number 0)
  (?Cn) callout with numerical data n
  (?C"text") callout with string data

The allowed string delimiters are ` ' " ^ % # $ (which are the same for the start and the end), and the starting delimiter { matched with the ending delimiter }. To encode the ending delimiter within the string, double it.

SEE ALSO

pcre2pattern(3), pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2(3).

AUTHOR

Philip Hazel
University Computing Service
Cambridge, England.

REVISION

Last updated: 17 June 2017
Copyright (c) 1997-2017 University of Cambridge.