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Regular Expression Reference: Capturing Groups and Backreferences

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
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	ECMA extended egrep awk	ECMA extended egrep awk	YES	no	YES	no	YES	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	basic grep	basic grep	no	YES	no	YES	no	no	no	no
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	YES	YES	YES	YES	YES	YES	ECMA	ECMA	YES	no	no	no	no	no	no	YES
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	YES	YES	YES	YES	YES	YES	ECMA basic grep	ECMA basic grep	YES	YES	no	YES	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	YES	YES	YES	YES	YES	YES	ECMA	no	YES	no	no	no	no	no	no	YES
Backreference	`\k<1>` through `\k<99>`	Substituted with the text matched between the 1st through 99th numbered capturing group.	`(abc\|def)=\k<1>` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	YES	no	no	no	no	no	no	no	no	no	YES	no	1.9	no	ECMA 1.47–1.85	no	no	no	no	no	no	no	no
Backreference	`\k'1'` through `\k'99'`	Substituted with the text matched between the 1st through 99th numbered capturing group.	`(abc\|def)=\k'1'` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	YES	no	no	no	no	no	no	no	no	no	no	no	1.9	no	ECMA 1.47–1.85	no	no	no	no	no	no	no	no
Backreference	`\g1` through `\g99`	Substituted with the text matched between the 1st through 99th numbered capturing group.	`(abc\|def)=\g1` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	no	no	5.10	7.0	YES	5.2.2	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Backreference	`\g{1}` through `\g{99}`	Substituted with the text matched between the 1st through 99th numbered capturing group.	`(abc\|def)=\g{1}` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	no	no	5.10	7.0	YES	5.2.2	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Backreference	`\g<1>` through `\g<99>`	Substituted with the text matched between the 1st through 99th numbered capturing group.	`(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	ECMA 1.47–1.85	no	no	no	no	no	no	no	no
Backreference	`\g'1'` through `\g'99'`	Substituted with the text matched between the 1st through 99th numbered capturing group.	`(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	ECMA 1.47–1.85	no	no	no	no	no	no	no	no
Backreference	`(?P=1)` through `(?P=99)`	Substituted with the text matched between the 1st through 99th numbered capturing group.	`(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	1.9	no	ECMA 1.47–1.85	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	1.9	no	ECMA 1.47–1.85	no	no	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	5.10	7.0	YES	5.2.2	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	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	5.10	7.0	YES	5.2.2	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	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	ECMA 1.47–1.85	no	no	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	ECMA 1.47–1.85	no	no	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	non‑ECMA	YES	YES	YES	YES	YES	YES	YES	ignored	ignored	ignored	YES	YES	ECMA basic grep ignored	ECMA 1.47–1.85	YES	YES	n/a	YES	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	YES	error	error	error	error	error	error	error	error	error	error	1.8 only	ECMA basic grep error	ECMA basic grep error	error	error	n/a	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	YES	YES	YES	YES	YES	YES	YES	YES	ignored	YES	ignored	error	fail	ECMA basic grep error	ECMA 1.78–1.85 fail	error	error	n/a	error	error	error	n/a	error
Forward reference	Any numbered backreference	Backreferences can be used before the group they reference.	`(\2?(a)){3}` matches `aaaaaa`.	YES	non‑ECMA	YES	YES	YES	YES	YES	YES	YES	ignored	error	error	error	YES	ECMA basic grep error	ECMA 1.78–1.85	error	error	n/a	error	error	error	n/a	error
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