Provided by: libpcre2-dev_10.42-4ubuntu2_amd64 
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. An unrecognized escape sequence
causes an error.
\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..
\N{U+hh..} character with Unicode code point hh.. (Unicode mode only)
\xhh character with hex code hh
\x{hh..} character with hex code hh..
If PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX is set ("ALT_BSUX mode"), the following are also
recognized:
\U the character "U"
\uhhhh character with hex code hhhh
\u{hh..} character with hex code hh.. but only for EXTRA_ALT_BSUX
When \x is not followed by {, from zero to two hexadecimal digits are read, but in
ALT_BSUX mode \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 or (in EXTRA_ALT_BSUX mode) a sequence of
hex digits in curly brackets, it matches a literal "u".
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. \N{U+hh..} is synonymous with \x{hh..} in PCRE2 but is not supported in EBCDIC
environments. Note that \N not followed by an opening curly bracket has a different
meaning (see below).
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.
Property descriptions in \p and \P are matched caselessly; hyphens, underscores, and white
space are ignored, in accordance with Unicode's "loose matching" rules.
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
Lc Ll, Lu, or Lt
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.
BINARY PROPERTIES FOR \p AND \P
Unicode defines a number of binary properties, that is, properties whose only values are
true or false. You can obtain a list of those that are recognized by \p and \P, along with
their abbreviations, by running this command:
pcre2test -LP
SCRIPT MATCHING WITH \p AND \P
Many script names and their 4-letter abbreviations are recognized in \p{sc:...} or
\p{scx:...} items, or on their own with \p (and also \P of course). You can obtain a list
of these scripts by running this command:
pcre2test -LS
THE BIDI_CLASS PROPERTY FOR \p AND \P
\p{Bidi_Class:<class>} matches a character with the given class
\p{BC:<class>} matches a character with the given class
The recognized classes are:
AL Arabic letter
AN Arabic number
B paragraph separator
BN boundary neutral
CS common separator
EN European number
ES European separator
ET European terminator
FSI first strong isolate
L left-to-right
LRE left-to-right embedding
LRI left-to-right isolate
LRO left-to-right override
NSM non-spacing mark
ON other neutral
PDF pop directional format
PDI pop directional isolate
R right-to-left
RLE right-to-left embedding
RLI right-to-left isolate
RLO right-to-left override
S segment separator
WS which space
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
REPORTED MATCH POINT SETTING
\K set reported start of match
From release 10.38 \K is not permitted by default in lookaround assertions, for
compatibility with Perl. However, if the PCRE2_EXTRA_ALLOW_LOOKAROUND_BSK option is set,
the previous behaviour is re-enabled. When this option is set, \K is honoured in positive
assertions, but ignored in negative ones.
ALTERNATION
expr|expr|expr...
CAPTURING
(...) capture group
(?<name>...) named capture group (Perl)
(?'name'...) named capture group (Perl)
(?P<name>...) named capture group (Python)
(?:...) non-capture group
(?|...) non-capture group; reset group numbers for
capture groups in each alternative
In non-UTF modes, names may contain underscores and ASCII letters and digits; in UTF
modes, any Unicode letters and Unicode decimal digits are permitted. In both cases, a name
must not start with a digit.
ATOMIC GROUPS
(?>...) atomic non-capture group
(*atomic:...) atomic non-capture group
COMMENT
(?#....) comment (not nestable)
OPTION SETTING
Changes of these options within a group are automatically cancelled at the end of the
group.
(?i) caseless
(?J) allow duplicate named groups
(?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)
(?^) unset imnsx options
Unsetting x or xx unsets both. Several options may be set at once, and a mixture of
setting and unsetting such as (?i-x) is allowed, but there may be only one hyphen. Setting
(but no unsetting) is allowed after (?^ for example (?^in). An option setting may appear
at the start of a non-capture group, for example (?i:...).
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 * 1024 bytes
(*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
(?=...) )
(*pla:...) ) positive lookahead
(*positive_lookahead:...) )
(?!...) )
(*nla:...) ) negative lookahead
(*negative_lookahead:...) )
(?<=...) )
(*plb:...) ) positive lookbehind
(*positive_lookbehind:...) )
(?<!...) )
(*nlb:...) ) negative lookbehind
(*negative_lookbehind:...) )
Each top-level branch of a lookbehind must be of a fixed length.
NON-ATOMIC LOOKAROUND ASSERTIONS
These assertions are specific to PCRE2 and are not Perl-compatible.
(?*...) )
(*napla:...) ) synonyms
(*non_atomic_positive_lookahead:...) )
(?<*...) )
(*naplb:...) ) synonyms
(*non_atomic_positive_lookbehind:...) )
SCRIPT RUNS
(*script_run:...) ) script run, can be backtracked into
(*sr:...) )
(*atomic_script_run:...) ) atomic script run
(*asr:...) )
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 subroutine by absolute number
(?+n) call subroutine by relative number
(?-n) call subroutine by relative number
(?&name) call subroutine by name (Perl)
(?P>name) call subroutine by name (Python)
\g<name> call subroutine by name (Oniguruma)
\g'name' call subroutine by name (Oniguruma)
\g<n> call subroutine by absolute number (Oniguruma)
\g'n' call subroutine by absolute number (Oniguruma)
\g<+n> call subroutine by relative number (PCRE2 extension)
\g'+n' call subroutine by relative number (PCRE2 extension)
\g<-n> call subroutine by relative number (PCRE2 extension)
\g'-n' call subroutine 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 groups 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
All backtracking control verbs may be in the form (*VERB:NAME). For (*MARK) the name is
mandatory, for the others it is optional. (*SKIP) changes its behaviour if :NAME is
present. The others just set a name for passing back to the caller, but this is not a name
that (*SKIP) can see. 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
(*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
The effect of one of these verbs in a group called as a subroutine is confined to the
subroutine call.
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
Retired from University Computing Service
Cambridge, England.
REVISION
Last updated: 12 January 2022
Copyright (c) 1997-2022 University of Cambridge.