Regular Expression Reference: Named 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
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	no	YES	no	YES	YES	7	YES	default 2023/09	YES	7.0	YES	YES	YES	YES	1.9	no	ECMA 1.42	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	no	no	no	no	YES	no	no	no	YES	7.0	YES	YES	YES	YES	1.9	no	ECMA 1.42	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	YES	no	no	YES	no	no	no	default	YES	YES	YES	YES	YES	YES	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	error	error	n/a	error	YES	7 error	error	default	YES	6.7 option	option	option	option	option	1.9	n/a	ECMA 1.42	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	n/a	n/a	n/a	n/a	YES	n/a	n/a	no	no	no	no	no	no	no	no	n/a	no	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	n/a	n/a	n/a	n/a	no	n/a	n/a	default	no	6.7–8.33	no	5.5.0–5.5.9	XE–XE6	2.14.0–3.0.2	no	n/a	no	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	n/a	n/a	n/a	n/a	no	n/a	n/a	no	YES	8.36	YES	5.6.9	10.2	3.1.3	no	n/a	ECMA 1.47	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	n/a	n/a	n/a	n/a	no	n/a	n/a	no	no	no	no	no	no	no	no	n/a	ECMA 1.42–1.46	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	n/a	n/a	n/a	n/a	no	n/a	n/a	no	no	no	no	no	no	no	1.9	n/a	no	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, the regex engine returns the text matched by the last group with that name to participate in the match.	no	n/a	n/a	n/a	n/a	no	n/a	n/a	no	no	no	no	YES	YES	no	1.9	n/a	no	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	no	YES	no	YES	YES	7	YES	no	YES	7.0	YES	YES	YES	YES	1.9	no	ECMA 1.47	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	no	no	no	no	YES	no	no	no	YES	7.0	YES	YES	YES	YES	1.9	no	ECMA 1.47	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	no	no	no	no	no	no	YES	7.2	YES	YES	YES	YES	no	no	ECMA 1.47	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	no	no	no	no	no	no	YES	7.2	YES	YES	YES	YES	no	no	ECMA 1.42	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	YES	no	no	no	no	no	no	no	YES	YES	YES	YES	YES	YES	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	YES	ignored	n/a	ignored	non‐ECMA	7	YES	n/a	YES	YES	YES	YES	YES	YES	1.9	n/a	ECMA 1.47	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	2.4–3.4 fail	ignored	n/a	ignored	YES	7	YES	n/a	YES	6.5 4.5 error	YES	YES	YES	YES	1.9 fail	n/a	ECMA 1.78 fail	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	error	ignored	n/a	error	YES	7 error	error	n/a	YES	6.7 4.5 error	YES	YES	YES	YES	1.9 error	n/a	ECMA 1.42 error	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	error	error	n/a	error	YES	7 error	error	default	error	4.5–8.33	error	error	XE–XE6	2.14.0–3.0.2	1.9 error	n/a	ECMA 1.42	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	n/a	n/a	n/a	n/a	YES	n/a	n/a	default error	n/a	4.5–8.33	n/a	n/a	XE–XE6	2.14.0–3.0.2	n/a	n/a	ECMA 1.42 error	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円` matches `abc=abc` or `def=def`, but not `abc=def` or `def=abc`.	no	n/a	n/a	n/a	n/a	YES	n/a	n/a	no	n/a	no	n/a	n/a	no	no	n/a	n/a	no	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	error	n/a	error	error	7 error	error	default error	error	error	error	error	error	error	1.9 error	n/a	ECMA 1.42	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	n/a	n/a	ECMA 1.42 error	n/a	n/a	n/a	n/a	n/a	n/a
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