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Regular Expression Reference: Groups and Backreferences

Feature	Syntax	Description	Example	JGsoft	Python	JavaScript	VBScript	XRegExp	.NET	Java	ICU	RE2	Perl	PCRE	PCRE2	PHP	Delphi	R	Ruby	std::regex	Boost	Tcl	POSIX	GNU	Oracle	XML	XPath
Capturing group	`(regex)`	Parentheses group the regex between them. They capture the text matched by the regex inside them into a numbered group that can be reused with a numbered backreference. They allow you to apply regex operators to the entire grouped regex.	`(abc){3}` matches `abcabcabc`. First group matches `abc`.	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	ECMA extended egrep awk	ECMA extended egrep awk	YES	extended	extended	YES	YES	YES
Capturing group	`\(regex\)`	Escaped parentheses group the regex between them. They capture the text matched by the regex inside them into a numbered group that can be reused with a numbered backreference. They allow you to apply regex operators to the entire grouped regex.	`\(abc\){3}` matches `abcabcabc`. First group matches `abc`.	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	basic grep	basic grep	no	basic	basic	no	no	no
Capturing group		100 or more capturing groups supported in a single regular expression.		YES	3.5	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	YES	no	YES	YES	VC’12	YES	YES	YES	YES	YES	YES	YES
Non-capturing group	`(?:regex)`	Non-capturing parentheses group the regex so you can apply regex operators, but do not capture anything.	`(?:abc){3}` matches `abcabcabc`. No groups.	YES	YES	YES	YES	YES	YES	YES	YES	default	YES	YES	YES	YES	YES	YES	YES	ECMA	ECMA	YES	no	no	no	no	3
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	no	no	no	no	no	no	YES	7.2	YES	YES	YES	YES	no	no	ECMA 1.42	no	no	no	no	no	no
Atomic group	`(?>regex)`	Atomic groups prevent the regex engine from backtracking into the group after a match has been found for the group. If the remainder of the regex fails then the engine may backtrack over the group if a quantifier or alternation makes it optional or backtrack to something prior to the group. 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	3.11	no	no	no	YES	YES	YES	no	YES	YES	YES	YES	YES	YES	YES	no	ECMA	no	no	no	no	no	no
Atomic group	`(*atomic:regex)`	Alternate syntax for the above.	`a(*atomic:bc\|b)c` matches `abcc` but not `abc`	no	no	no	no	no	no	no	no	no	5.28	no	10.33	7.4.0	no	4.0.0	no	no	no	no	no	no	no	no	no
Comment	`(?#comment)`	Everything between `(?#` and `)` is ignored by the regex engine.	`a(?#foobar)b` matches `ab`	YES	YES	no	no	YES	YES	no	YES	no	YES	YES	YES	YES	YES	YES	YES	no	ECMA	YES	no	no	no	no	no
Backreference	`\1` through `\9`	Substituted with the text matched between the 1st through 9th numbered capturing group.	`(abc\|def)=\1` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	YES	YES	YES	YES	YES	YES	YES	no	YES	YES	YES	YES	YES	YES	YES	ECMA basic grep	ECMA basic grep	YES	basic	YES	YES	no	YES
Backreference	`\10` through `\99`	Substituted with the text matched between the 10th through 99th numbered capturing group.		YES	YES	YES	YES	YES	YES	YES	YES	no	YES	YES	YES	YES	YES	YES	YES	ECMA	no	YES	no	no	no	no	YES
Backreference	`\10` through `\99`	Substituted with the text matched between the 1st through 9th numbered capturing group followed by a literal digit if the regex has less than 10 capturing groups.	`(a)\17` matches `aa7`.	YES	no	no	YES	no	ECMA	YES	YES	no	no	no	no	no	no	no	no	no	ECMA basic grep	no	basic	YES	YES	no	YES
Backreference	`\100` through `\999`	Substituted with the text matched between the 100th through 999th numbered capturing group.		no	no	YES	no	YES	YES	YES	YES	no	YES	YES	YES	YES	no	YES	YES	ECMA VC’12	no	YES	no	no	no	no	YES
Backreference	`\100` through `\999`	Substituted with the text matched between the 10th through 99th numbered capturing group followed by a literal digit if the regex has less than 100 capturing groups.		YES	no	no	no	no	ECMA	YES	YES	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	YES
Backreference	`\k<1>`, `\k<2>`, etc.	Substituted with the text matched by the capturing group with the specified number.	`(abc\|def)=\k<1>` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	no	no	no	YES	YES	no	no	no	no	no	no	no	no	no	1.9	no	ECMA 1.47	no	no	no	no	no	no
Backreference	`\k'1'`, `\k'2'`, etc.	Substituted with the text matched by the capturing group with the specified number.	`(abc\|def)=\k'1'` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	no	no	no	no	YES	no	no	no	no	no	no	no	no	no	1.9	no	ECMA 1.47	no	no	no	no	no	no
Backreference	`\g1`, `\g2`, etc.	Substituted with the text matched by the capturing group with the specified number.	`(abc\|def)=\g1` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	no	no	no	no	no	no	no	no	YES	7.0	YES	YES	YES	YES	no	no	ECMA 1.42	no	no	no	no	no	no
Backreference	`\g{1}`, `\g{2}`, etc.	Substituted with the text matched by the capturing group with the specified number.	`(abc\|def)=\g{1}` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	no	no	no	no	no	no	no	no	YES	7.0	YES	YES	YES	YES	no	no	ECMA 1.42	no	no	no	no	no	no
Backreference	`\g<1>`, `\g<2>`, etc.	Substituted with the text matched by the capturing group with the specified number.	`(abc\|def)=\g<1>` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	ECMA 1.47	no	no	no	no	no	no
Backreference	`\g'1'`, `\g'2'`, etc.	Substituted with the text matched by the capturing group with the specified number.	`(abc\|def)=\g'1'` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	ECMA 1.47	no	no	no	no	no	no
Backreference	`(?P=1)`, `(?P=2)`, etc.	Substituted with the text matched by the capturing group with the specified number.	`(abc\|def)=(?P=1)` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no
Relative Backreference	`\k<-1>`, `\k<-2>`, etc.	Substituted with the text matched by 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 at the backreference.	`(a)(b)(c)(d)\k<-3>` matches `abcdb`.	V2	no	no	no	no	no	no	no	no	no	no	no	no	no	no	1.9	no	ECMA 1.47	no	no	no	no	no	no
Relative Backreference	`\k'-1'`, `\k'-2'`, etc.	Substituted with the text matched by 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 at the backreference.	`(a)(b)(c)(d)\k'-3'` matches `abcdb`.	V2	no	no	no	no	no	no	no	no	no	no	no	no	no	no	1.9	no	ECMA 1.47	no	no	no	no	no	no
Relative Backreference	`\g-1`, `\g-2`, etc.	Substituted with the text matched by 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 at the backreference.	`(a)(b)(c)(d)\g-3` matches `abcdb`.	no	no	no	no	no	no	no	no	no	YES	7.0	YES	YES	YES	YES	no	no	ECMA 1.42	no	no	no	no	no	no
Relative Backreference	`\g{-1}`, `\g{-2}`, etc.	Substituted with the text matched by 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 at the backreference.	`(a)(b)(c)(d)\g{-3}` matches `abcdb`.	no	no	no	no	no	no	no	no	no	YES	7.0	YES	YES	YES	YES	no	no	ECMA 1.42	no	no	no	no	no	no
Relative Backreference	`\g<-1>`, `\g<-2>`, etc.	Substituted with the text matched by 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 at the backreference.	`(a)(b)(c)(d)\g<-3>` matches `abcdb`.	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	ECMA 1.47	no	no	no	no	no	no
Relative Backreference	`\g'-1'`, `\g'-2'`, etc.	Substituted with the text matched by 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 at the backreference.	`(a)(b)(c)(d)\g'-3'` matches `abcdb`.	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	ECMA 1.47	no	no	no	no	no	no
Failed backreference	Any numbered backreference	Backreferences to groups that did not participate in the match attempt fail to match.	`(a)?\1` matches `aa` but fails to match `b`.	YES	YES	ignored	ignored	ignored	non‑ECMA	YES	YES	n/a	YES	YES	YES	YES	YES	YES	YES	ECMA basic grep ignored	ECMA 1.47	YES	basic	YES	YES	n/a	ignored
Invalid backreference	Any numbered backreference	Backreferences to groups that do not exist at all are valid but fail to match anything.	`(a)?\2\|b` matches `b` in `aab`.	error	error	error	error	error	error	YES	error	n/a	error	error	error	error	error	error	1.8 only	ECMA basic grep error	ECMA basic grep error	error	basic error	error	error	n/a	error
Nested backreference	Any numbered backreference	Backreferences can be used inside the group they reference.	`(a\1?){3}` matches `aaaaaa`.	YES	error	ignored	YES	ignored	YES	YES	YES	n/a	YES	YES	YES	YES	YES	YES	fail	ECMA basic grep error	ECMA 1.78 fail	error	basic error	error	YES	n/a	ignored
Forward reference	Any numbered backreference	Backreferences can be used before the group they reference.	`(\2?(a)){3}` matches `aaaaaa`.	YES	error	ignored	error	error	non‑ECMA	YES	YES	n/a	YES	YES	YES	YES	YES	YES	YES	ECMA basic grep error	ECMA 1.78	error	basic error	error	error	n/a	error
Feature	Syntax	Description	Example	JGsoft	Python	JavaScript	VBScript	XRegExp	.NET	Java	ICU	RE2	Perl	PCRE	PCRE2	PHP	Delphi	R	Ruby	std::regex	Boost	Tcl	POSIX	GNU	Oracle	XML	XPath