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Regular Expression Reference: Special Groups

Feature	Syntax	Description	Example	JGsoft	.NET	Java	Perl	PCRE	PCRE2	PHP	Delphi	R	JavaScript	VBScript	XRegExp	Python	Ruby	std::regex	Boost	Tcl ARE	POSIX BRE	POSIX ERE	GNU BRE	GNU ERE	Oracle	XML	XPath
Comment	`(?#comment)`	Everything between `(?#` and `)` is ignored by the regex engine.	`a(?#foobar)b` matches `ab`	YES	YES	no	YES	YES	YES	YES	YES	YES	no	no	YES	YES	YES	no	ECMA	YES	no	no	no	no	no	no	no
Branch reset group	`(?\|regex)`	If the regex inside the branch reset group has multiple alternatives with capturing groups, then the capturing group numbers are the same in all the alternatives.	`(x)(?\|(a)\|(bc)\|(def))\2` matches `xaa`, `xbcbc`, or `xdefdef` with the first group capturing `x` and the second group capturing `a`, `bc`, or `def`	V2	no	no	5.10	7.2	YES	5.2.4	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Atomic group	`(?>regex)`	Atomic groups prevent the regex engine from backtracking back into the group after a match has been found for the group. If the remainder of the regex fails, the engine may backtrack over the group if a quantifier or alternation makes it optional. But it will not backtrack into the group to try other permutations of the group.	`a(?>bc\|b)c` matches `abcc` but not `abc`	YES	YES	YES	YES	YES	YES	YES	YES	YES	no	no	no	no	YES	no	ECMA	no	no	no	no	no	no	no	no
Positive lookahead	`(?=regex)`	Matches at a position where the pattern inside the lookahead can be matched. Matches only the position. It does not consume any characters or expand the match. In a pattern like `one(?=two)three`, both `two` and `three` have to match at the position where the match of `one` ends.	`t(?=s)` matches the second `t` in `streets`.	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	ECMA	ECMA	YES	no	no	no	no	no	no	no
Negative lookahead	`(?!regex)`	Similar to positive lookahead, except that negative lookahead only succeeds if the regex inside the lookahead fails to match.	`t(?!s)` matches the first `t` in `streets`.	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	ECMA	ECMA	YES	no	no	no	no	no	no	no
Positive lookbehind	`(?<=regex)`	Matches at a position if the pattern inside the lookbehind can be matched ending at that position.	`(?<=s)t` matches the first `t` in `streets`.	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	no	YES	YES	1.9	no	ECMA	no	no	no	no	no	no	no	no
Negative lookbehind	`(?<!regex)`	Matches at a position if the pattern inside the lookbehind cannot be matched ending at that position.	`(?<!s)t` matches the second `t` in `streets`.	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	no	YES	YES	1.9	no	ECMA	no	no	no	no	no	no	no	no
Lookbehind	`(?<=regex\|longer regex)`	Alternatives inside lookbehind can differ in length.	`(?<=is\|e)t` matches the second and fourth `t` in `twisty streets`.	YES	YES	YES	5.30	YES	YES	YES	YES	YES	YES	n/a	YES	no	1.9	n/a	ECMA 1.38–1.43	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Lookbehind	`(?<=x{n,m})`	Quantifiers with a finite maximum number of repetitions can be used inside lookbehind.	`(?<=s\w{1,7})t` matches only the fourth `t` in `twisty streets`.	YES	YES	6 4 fail	5.30 fail	no	no	no	no	no	YES	n/a	YES	no	no	n/a	no	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Lookbehind	`(?<=regex)`	The full regular expression syntax can be used inside lookbehind.	`(?<=s\w+)t` matches only the fourth `t` in `twisty streets`.	YES	YES	13	no	no	no	no	no	no	YES	n/a	YES	no	no	n/a	no	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Lookbehind	`(group)(?<=\1)`	Backreferences can be used inside lookbehind. Syntax prohibited in lookbehind is also prohibited in the referenced capturing group.	`(\w).+(?<=\1)` matches `twisty street` in `twisty streets`.	YES	YES	no	no	no	10.23	7.3.0	no	4.0.0	YES	n/a	YES	3.5	no	n/a	no	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Keep text out of the regex match	`\K`	The text matched by the part of the regex to the left of the `\K` is omitted from the overall regex match. Other than that the regex is matched normally from left to right. Capturing groups to the left of the `\K` capture as usual.	`s\Kt` matches only the first `t` in `streets`.	V2	no	no	5.10	7.2	YES	5.2.4	YES	YES	no	no	no	no	2.0	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Lookaround conditional	`(?(?=regex)then\|else)` where `(?=regex)` is any valid lookaround and `then` and `else` are any valid regexes	If the lookaround succeeds, the “then” part must match for the overall regex to match. If the lookaround fails, the “else” part must match for the overall regex to match. The lookaround is zero-length. The “then” and “else” parts consume their matches like normal regexes.	`(?(?<=a)b\|c)` matches the second `b` and the first `c` in `babxcac`	YES	YES	no	YES	YES	YES	YES	YES	YES	no	no	no	no	no	no	ECMA	no	no	no	no	no	no	no	no
Implicit lookahead conditional	`(?(regex)then\|else)` where `regex`, `then`, and `else` are any valid regexes and `regex` is not the name of a capturing group	If “regex” is not the name of a capturing group, then it is interpreted as a lookahead as if you had written `(?(?=regex)then\|else)`. If the lookahead succeeds, the “then” part must match for the overall regex to match. If the lookahead fails, the “else” part must match for the overall regex to match. The lookaround is zero-length. The “then” and “else” parts consume their matches like normal regexes.	`(?(\d{2})7\|c)` matches the first `7` and the `c` in `747c`	no	YES	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no
Named conditional	`(?(name)then\|else)` where `name` is the name of a capturing group and `then` and `else` are any valid regexes	If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.	`(?<one>a)?(?(one)b\|c)` matches `ab`, the first `c`, and the second `c` in `babxcac`	YES	YES	no	no	6.7	YES	5.2.0	YES	YES	no	no	no	YES	no	no	no	no	no	no	no	no	no	no	no
Named conditional	`(?(<name>)then\|else)` where `name` is the name of a capturing group and `then` and `else` are any valid regexes	If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.	`(?<one>a)?(?(<one>)b\|c)` matches `ab`, the first `c`, and the second `c` in `babxcac`	V2	no	no	5.10	7.0	YES	5.2.2	YES	YES	no	no	no	no	2.0	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Named conditional	`(?('name')then\|else)` where `name` is the name of a capturing group and `then` and `else` are any valid regexes	If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.	`(?'one'a)?(?('one')b\|c)` matches `ab`, the first `c`, and the second `c` in `babxcac`	V2	no	no	5.10	7.0	YES	5.2.2	YES	YES	no	no	no	no	2.0	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Conditional	`(?(1)then\|else)` where `1` is the number of a capturing group and `then` and `else` are any valid regexes	If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.	`(a)?(?(1)b\|c)` matches `ab`, the first `c`, and the second `c` in `babxcac`	YES	YES	no	YES	YES	YES	YES	YES	YES	no	no	no	YES	2.0	no	ECMA	no	no	no	no	no	no	no	no
Relative conditional	`(?(-1)then\|else)` where `-1` is a negative integer and `then` and `else` are any valid regexes	Conditional that tests the capturing group that can be found by counting as many opening parentheses of named or numbered capturing groups as specified by the number from right to left starting immediately before the conditional. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.	`(a)?(?(-1)b\|c)` matches `ab`, the first `c`, and the second `c` in `babxcac`	V2	no	no	no	7.2	YES	5.2.4	YES	YES	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no
Forward conditional	`(?(+1)then\|else)` where `+1` is a positive integer and `then` and `else` are any valid regexes	Conditional that tests the capturing group that can be found by counting as many opening parentheses of named or numbered capturing groups as specified by the number from left to right starting at the “then” part of conditional. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.	`((?(+1)b\|c)(d)?){2}` matches `cc` and `cdb` in `bdbdccxcdcxcdb`	V2	no	no	no	7.2	YES	5.2.4	YES	YES	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no
Conditional	`(?(+1)then\|else)` where `1` is the number of a capturing group and `then` and `else` are any valid regexes	The `+` is ignored and the number is taken as an absolute reference to a capturing group. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match.	`(a)?(?(+1)b\|c)` matches `ab`, the first `c`, and the second `c` in `babxcac`	no	no	no	no	no	no	no	no	no	no	no	no	YES	no	no	no	no	no	no	no	no	no	no	no
Feature	Syntax	Description	Example	JGsoft	.NET	Java	Perl	PCRE	PCRE2	PHP	Delphi	R	JavaScript	VBScript	XRegExp	Python	Ruby	std::regex	Boost	Tcl ARE	POSIX BRE	POSIX ERE	GNU BRE	GNU ERE	Oracle	XML	XPath