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PCRE2SYNTAX(3) Library Functions Manual PCRE2SYNTAX(3)
PCRE2 - Perl-compatible regular expressions (revised API)
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.
\x where x is non-alphanumeric is a literal x
\Q...\E treat enclosed characters as literal
Note that white space inside \Q...\E is always treated as
literal, even if PCRE2_EXTENDED is set, causing most other white
space to be ignored.
With one exception, wherever brace characters { and } are
required to enclose data for constructions such as \g{2} or
\k{name}, space and/or horizontal tab characters that follow { or
precede } are allowed and are ignored. In the case of
quantifiers, they may also appear before or after the comma. The
exception is \u{...} which is not Perl-compatible and is
recognized only when PCRE2_EXTRA_ALT_BSUX is set. This is an
ECMAScript compatibility feature, and follows ECMAScript's
behaviour.
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 a non-control 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..
\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).
. 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, but there are some option settings that can restrict
individual sequences to matching only ASCII 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.
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
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 Universally-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.
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
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
\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
[...] 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.
? 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
{,m} zero up to m, greedy
{,m}+ zero up to m, possessive
{,m}? zero up to m, lazy
\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
\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.
expr|expr|expr...
(...) 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 non-capture group
(*atomic:...) atomic non-capture group
(?#....) comment (not nestable)
Changes of these options within a group are automatically
cancelled at the end of the group.
(?a) all ASCII options
(?aD) restrict \d to ASCII in UCP mode
(?aS) restrict \s to ASCII in UCP mode
(?aW) restrict \w to ASCII in UCP mode
(?aP) restrict all POSIX classes to ASCII in UCP mode
(?aT) restrict POSIX digit classes to ASCII in UCP
mode
(?i) caseless
(?J) allow duplicate named groups
(?m) multiline
(?n) no auto capture
(?r) restrict caseless to either ASCII or non-ASCII
(?s) single line (dotall)
(?U) default ungreedy (lazy)
(?x) ignore white space except in classes or \Q...\E
(?xx) as (?x) but also ignore space and tab in
classes
(?-...) unset the given option(s)
(?^) unset imnrsx options
(?aP) implies (?aT) as well, though this has no additional
effect. However, it means that (?-aP) is really (?-PT) which
disables all ASCII restrictions for POSIX classes.
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.
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)
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
(?=...) )
(*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 have a limit for the
number of characters it matches. If any branch can match a
variable number of characters, the maximum for each branch is
limited to a value set by the caller of pcre2_compile() or
defaulted. The default is set when PCRE2 is built (ultimate
default 255). If every branch matches a fixed number of
characters, the limit for each branch is 65535 characters.
These assertions are specific to PCRE2 and are not Perl-
compatible.
(?*...) )
(*napla:...) ) synonyms
(*non_atomic_positive_lookahead:...) )
(?<*...) )
(*naplb:...) ) synonyms
(*non_atomic_positive_lookbehind:...) )
(*script_run:...) ) script run, can be backtracked
into
(*sr:...) )
(*atomic_script_run:...) ) atomic script run
(*asr:...) )
\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)
(?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)
(?(condition)yes-pattern)
(?(condition)yes-pattern|no-pattern)
(?(n) absolute reference condition
(?(+n) relative reference condition (PCRE2
extension)
(?(-n) relative reference condition (PCRE2
extension)
(?(<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.
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.
(?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.
pcre2pattern(3), pcre2api(3), pcre2callout(3), pcre2matching(3),
pcre2(3).
Philip Hazel
Retired from University Computing Service
Cambridge, England.
Last updated: 12 October 2023
Copyright (c) 1997-2023 University of Cambridge.
This page is part of the PCRE (Perl Compatible Regular
Expressions) project. Information about the project can be found
at ⟨http://www.pcre.org/⟩. If you have a bug report for this
manual page, see
⟨http://bugs.exim.org/enter_bug.cgi?product=PCRE⟩. This page was
obtained from the tarball fetched from
⟨https://github.com/PhilipHazel/pcre2.git⟩ on 2024-06-14. If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
PCRE2 10.43 12 October 2023 PCRE2SYNTAX(3)
Pages that refer to this page: grep(1), pcre2grep(1), pcre2pattern(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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