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Regular Expression Reference: Named 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
Named capturing group	`(?<name>regex)`	Captures the text matched by “regex” into the group “name”. The name can contain letters and numbers but must start with a letter.	`(?<x>abc){3}` matches `abcabcabc`. The group x matches `abc`.	YES	YES	7	5.10	7.0	YES	5.2.2	YES	YES	YES	no	YES	no	1.9	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Named capturing group	`(?'name'regex)`	Captures the text matched by “regex” into the group “name”. The name can contain letters and numbers but must start with a letter.	`(?'x'abc){3}` matches `abcabcabc`. The group x matches `abc`.	YES	YES	no	5.10	7.0	YES	5.2.2	YES	YES	no	no	no	no	1.9	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Named capturing group	`(?P<name>regex)`	Captures the text matched by “regex” into the group “name”. The name can contain letters and numbers but must start with a letter.	`(?P<x>abc){3}` matches `abcabcabc`. The group x matches `abc`.	YES	no	no	5.10	YES	YES	YES	YES	YES	no	no	YES	YES	no	no	no	no	no	no	no	no	no	no	no
Duplicate named group	Any named group	Two named groups can share the same name.	`(?<x>a)\|(?<x>b)` matches `a` or `b`.	YES	YES	7 error	5.10	6.7 option	option	5.2.0 option	option	option	error	n/a	error	error	1.9	n/a	ECMA 1.42–1.85	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Duplicate named group	Any named group	Named groups that share the same name are treated as one an the same group, so there are no pitfalls when using backreferences to that name.		YES	YES	n/a	no	no	no	no	no	no	n/a	n/a	n/a	n/a	no	n/a	no	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Duplicate named group	Any named group	If a regex has multiple groups with the same name, backreferences using that name point to the leftmost group in the regex with that name.		no	no	n/a	no	6.7–8.33	no	5.2.0–5.5.9	XE–XE6	2.14.0–3.0.2	n/a	n/a	n/a	n/a	no	n/a	no	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Duplicate named group	Any named group	If a regex has multiple groups with the same name, backreferences using that name point to the leftmost group with that name that has actually participated in the match attempt when the backreference is evaluated.		no	no	n/a	5.10	8.36	YES	5.6.9	10.2	3.1.3	n/a	n/a	n/a	n/a	no	n/a	ECMA 1.47–1.85	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Duplicate named group	Any named group	If a regex has multiple groups with the same name, backreferences using that name point to the rightmost group with that name that appears to the left of the backreference in the regex.		no	no	n/a	no	no	no	no	no	no	n/a	n/a	n/a	n/a	no	n/a	ECMA 1.42–1.46	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Duplicate named group	Any named group	If a regex has multiple groups with the same name, backreferences using that name can match the text captured by any group with that name that appears to the left of the backreference in the regex.		no	no	n/a	no	no	no	no	no	no	n/a	n/a	n/a	n/a	1.9	n/a	no	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Named backreference	`\k<name>`	Substituted with the text matched by the named group “name”.	`(?<x>abc\|def)=\k<x>` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	YES	7	5.10	7.0	YES	5.2.2	YES	YES	YES	no	YES	no	1.9	no	ECMA 1.47–1.85	no	no	no	no	no	no	no	no
Named backreference	`\k'name'`	Substituted with the text matched by the named group “name”.	`(?'x'abc\|def)=\k'x'` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	YES	no	5.10	7.0	YES	5.2.2	YES	YES	no	no	no	no	1.9	no	ECMA 1.47–1.85	no	no	no	no	no	no	no	no
Named backreference	`\k{name}`	Substituted with the text matched by the named group “name”.	`(?'x'abc\|def)=\k{x}` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	no	no	5.10	7.2	YES	5.2.4	YES	YES	no	no	no	no	no	no	ECMA 1.47–1.85	no	no	no	no	no	no	no	no
Named backreference	`\g{name}`	Substituted with the text matched by the named group “name”.	`(?'x'abc\|def)=\g{x}` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	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
Named backreference	`(?P=name)`	Substituted with the text matched by the named group “name”.	`(?P<x>abc\|def)=(?P=x)` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	no	no	5.10	YES	YES	YES	YES	YES	no	no	no	YES	no	no	no	no	no	no	no	no	no	no	no
Failed backreference	Any named backreference	Backreferences to groups that did not participate in the match attempt fail to match.	`(?<x>a)?\k<x>` matches `aa` but fails to match `b`.	YES	non‑ECMA	7	5.10	YES	YES	YES	YES	YES	ignored	n/a	ignored	YES	1.9	n/a	ECMA 1.47–1.85	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Nested backreference	Any named backreference	Backreferences can be used inside the group they reference.	`(?<x>a\k<x>?){3}` matches `aaaaaa`.	YES	YES	7	5.10	6.5	YES	5.1.3	YES	YES	ignored	n/a	ignored	2.4–3.4 fail	1.9 fail	n/a	ECMA 1.78–1.85 fail	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Forward reference	Any named backreference	Backreferences can be used before the group they reference.	`(\k<x>?(?<x>a)){3}` matches `aaaaaa`.	YES	YES	7 error	5.10	6.7	YES	5.2.0	YES	YES	ignored	n/a	error	error	1.9 error	n/a	ECMA 1.42–1.85 error	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Named capturing group	Any named capturing group	A number is a valid name for a capturing group.	`(?<17>abc){3}` matches `abcabcabc`. The group named “17” matches `abc`.	YES	YES	7 error	5.10 error	4.0–8.33	error	5.0.0–5.1.2	XE–XE6	2.14.0–3.0.2	error	n/a	error	error	1.9 error	n/a	ECMA 1.42–1.85	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Named capturing group	Any capturing group with a number as its name	If the name of the group is a number, that becomes the group’s name and the group’s number.	`(?<17>abc\|def)=\17` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	YES	n/a	n/a	no	n/a	no	no	no	n/a	n/a	n/a	n/a	n/a	n/a	no	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Named backreference	Any named backreference	A number is a valid name for a backreference which then points to a group with that number as its name.	`(?<17>abc\|def)=\k<17>` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	YES	YES	n/a	n/a	4.0–8.33	n/a	5.0.0–5.1.2	XE–XE6	2.14.0–3.0.2	n/a	n/a	n/a	n/a	n/a	n/a	ECMA 1.42–1.85 error	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Named capturing group	Any named capturing group	A negative number is a valid name for a capturing group.	`(?<-17>abc){3}` matches `abcabcabc`. The group named “-17” matches `abc`.	error	error	7 error	5.10 error	error	error	error	error	error	error	n/a	error	error	1.9 error	n/a	ECMA 1.42–1.85	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Named backreference	Any named backreference	A negative number is a valid name for a backreference which then points to a group with that negative number as its name.		n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	ECMA 1.42–1.85 error	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
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