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Regex Reference: Balancing Groups, Recursion, and Subroutines

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
Balancing group	`(?<capture-subtract>regex)` where “capture” and “subtract” are group names and “regex” is any regex	The name “subtract” must be used as the name of a capturing group elsewhere in the regex. If this group has captured matches that haven’t been subtracted yet, then the balancing group subtracts one capture from “subtract”, attempts to match “regex”, and stores its match into the group “capture”. If “capture” is omitted, the same happens without storing the match. If “regex” is omitted, the balancing group succeeds without advancing through the string. If the group “subtract” has no matches to subtract, then the balancing group fails to match, regardless of whether “regex” is specified or not.	`^(?<l>\w)+\w? (\k<l>(?<-l>))+ (?(l)(?!))$` matches any palindrome word	V2	YES	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no
Balancing group	`(?'capture-subtract'regex)` where “capture” and “subtract” are group names and “regex” is any regex	The name “subtract” must be used as the name of a capturing group elsewhere in the regex. If this group has captured matches that haven’t been subtracted yet, then the balancing group subtracts one capture from “subtract”, attempts to match “regex”, and stores its match into the group “capture”. If “capture” is omitted, the same happens without storing the match. If “regex” is omitted, the balancing group succeeds without advancing through the string. If the group “subtract” has no matches to subtract, then the balancing group fails to match, regardless of whether “regex” is specified or not.	`^(?'l'\w)+\w? (\k'l'(?'-l'))+ (?(l)(?!))$` matches any palindrome word	V2	YES	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no	no
Recursion	`(?R)`	Recursion of the entire regular expression.	`a(?R)?z` matches `az`, `aazz`, `aaazzz`, etc.	V2	no	no	5.10	YES	YES	YES	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Recursion	`(?0)`	Recursion of the entire regular expression.	`a(?0)?z` matches `az`, `aazz`, `aaazzz`, etc.	V2	no	no	5.10	YES	YES	YES	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Recursion	`\g<0>`	Recursion of the entire regular expression.	`a\g<0>?z` matches `az`, `aazz`, `aaazzz`, etc.	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	2.0	no	no	no	no	no	no	no	no	no	no
Recursion	`\g'0'`	Recursion of the entire regular expression.	`a\g'0'?z` matches `az`, `aazz`, `aaazzz`, etc.	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	2.0	no	no	no	no	no	no	no	no	no	no
Subroutine call	`(?1)` where 1 is the number of a capturing group	Recursion of a capturing group or subroutine call to a capturing group.	`a(b(?1)?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	no	4 only	5.10	YES	YES	YES	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Subroutine call	`\g<1>` where 1 is the number of a capturing group	Recursion of a capturing group or subroutine call to a capturing group.	`a(b\g<1>?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	1.9	no	no	no	no	no	no	no	no	no	no
Subroutine call	`\g'1'` where 1 is the number of a capturing group	Recursion of a capturing group or subroutine call to a capturing group.	`a(b\g'1'?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	1.9	no	no	no	no	no	no	no	no	no	no
Relative subroutine call	`(?-1)` where -1 is a negative integer	Recursion of or subroutine call to a 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 subroutine call.	`a(b(?-1)?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	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
Relative subroutine call	`\g<-1>` where -1 is a negative integer	Recursion of or subroutine call to a 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 subroutine call.	`a(b\g<-1>?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	1.9	no	no	no	no	no	no	no	no	no	no
Relative subroutine call	`\g'-1'` where -1 is a negative integer	Recursion of or subroutine call to a 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 subroutine call.	`a(b\g'-1'?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	1.9	no	no	no	no	no	no	no	no	no	no
Forward subroutine call	`(?+1)` where +1 is a positive integer	Recursion of or subroutine call to a 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 subroutine call.	`(?+1)x([ab])` matches `axa`, `axb`, `bxa`, and `bxb`	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
Forward subroutine call	`\g<+1>` where +1 is a positive integer	Recursion of or subroutine call to a 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 subroutine call.	`\g<+1>x([ab])` matches `axa`, `axb`, `bxa`, and `bxb`	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	2.0	no	no	no	no	no	no	no	no	no	no
Forward subroutine call	`\g'+1'` where +1 is a positive integer	Recursion of or subroutine call to a 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 subroutine call.	`\g'+1'x([ab])` matches `axa`, `axb`, `bxa`, and `bxb`	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	2.0	no	no	no	no	no	no	no	no	no	no
Named subroutine call	`(?&name)` where “name” is the name of a capturing group	Recursion of a capturing group or subroutine call to a capturing group.	`a(?<x>b(?&x)?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	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
Named subroutine call	`(?P>name)` where “name” is the name of a capturing group	Recursion of a capturing group or subroutine call to a capturing group.	`a(?P<x>b(?P>x)?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	no	no	5.10	YES	YES	YES	YES	YES	no	no	no	no	no	no	ECMA 1.42–1.85	no	no	no	no	no	no	no	no
Named subroutine call	`\g<name>` where “name” is the name of a capturing group	Recursion of a capturing group or subroutine call to a capturing group.	`a(?<x>b\g<x>?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	1.9	no	no	no	no	no	no	no	no	no	no
Named subroutine call	`\g'name'` where “name” is the name of a capturing group	Recursion of a capturing group or subroutine call to a capturing group.	`a(?'x'b\g'x'?y)z` matches `abyz`, `abbyyz`, `abbbyyyz`, etc.	V2	no	no	no	7.7	YES	5.2.7	YES	YES	no	no	no	no	1.9	no	no	no	no	no	no	no	no	no	no
Subroutine definitions	`(?(DEFINE)regex)` where “regex” is any regex	The DEFINE group does not take part in the matching process. Subroutine calls can be made to capturing groups inside the DEFINE group.	`(?(DEFINE)([ab])) x(?1)y(?1)z` matches `xayaz`, `xaybz`, `xbyaz`, and `xbybz`	V2	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
Subroutine calls capture	Subroutine call using Ruby-style `\g` syntax	A subroutine call to a capturing group makes that capturing group store the text matched during the subroutine call.	When `([ab])\g'1'` matches `ab` the first capturing group holds `b` after the match.	V2	n/a	n/a	n/a	no	no	no	no	no	n/a	n/a	n/a	n/a	1.9	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Subroutine calls capture	Subroutine call using syntax other than `\g`	A subroutine call to a capturing group makes that capturing group store the text matched during the subroutine call.	When `([ab])(?1)` matches `ab` the first capturing group holds `b` after the match.	no	n/a	no	no	no	no	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
Recursion isolates capturing groups	Any recursion or subroutine call	Each subroutine call has its own separate storage space for capturing groups. Backreferences inside the call cannot see text matched before the call, and backreferences after the call cannot see text matched inside the call. Backreferences inside recursion cannot see text matched at other recursion levels.	`(a)(\1)(?2)` never matches anything because `\1` always fails during the call made by `(?2)`.	no	n/a	no	5.10–5.18	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
Recursion reverts capturing groups	Recursion or subroutine call using Ruby-style `\g` syntax	When the regex engine exits from recursion or a subroutine call, it reverts all capturing groups to the text they had matched prior to entering the recursion or subroutine call.	When `(a)(([bc])\1)\g'2'` matches `abaca` the third group stores `b` after the match	no	n/a	n/a	n/a	7.7	YES	5.2.7	YES	YES	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	n/a	n/a
Recursion reverts capturing groups	Recursion or subroutine call using syntax other than `\g`	When the regex engine exits from recursion or a subroutine call, it reverts all capturing groups to the text they had matched prior to entering the recursion or subroutine call.	When `(a)(([bc])\1)(?2)` matches `abaca` the third group stores `b` after the match	V2	n/a	no	5.10	YES	YES	YES	YES	YES	n/a	n/a	n/a	n/a	n/a	n/a	ECMA 1.42–1.85	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Recursion does not isolate or revert capturing groups	Recursion or subroutine call using Ruby-style `\g` syntax	Capturing groups are not given any special treatment by recursion and subroutine calls, except perhaps that subroutine calls capture. Backreferences always see the text most recently matched by each capturing group, regardless of whether they are inside the same level of recursion or not.	When `(a)(([bc])\1)\g'2'` matches `abaca` the third group stores `c` after the match	V2	n/a	n/a	n/a	no	no	no	no	no	n/a	n/a	n/a	n/a	1.9	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Recursion does not isolate or revert capturing groups	Recursion or subroutine call using syntax other than `\g`	Capturing groups are not given any special treatment by recursion and subroutine calls, except perhaps that subroutine calls capture. Backreferences always see the text most recently matched by each capturing group, regardless of whether they are inside the same level of recursion or not.	When `(a)(([bc])\1)(?2)` matches `abaca` the third group stores `c` after the match	no	n/a	4 only	no	no	no	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
Recursion is atomic	Recursion or subroutine call using `(?P>…)`	Recursion and subroutine calls are atomic. Once the regex engine exits from them, it will not backtrack into it to try different permutations of the recursion or subroutine call.	`(a+)(?P>1)(?P>1)` can never match anything because the first `(?P>1)` matches all remaining a’s and the regex engine won’t backtrack into the first `(?P>1)` when the second one fails	V2	n/a	n/a	no	6.5	10.00–10.23	5.1.3–7.2.34	YES	2.14.0–3.6.3	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
Recursion is atomic	Recursion of the whole regex using syntax other than `(?P>0)`	Recursion of the whole regex is atomic. Once the regex engine exits from recursion, it will not backtrack into it to try different permutations of the recursion.	`aa$\|a(?R)a\|a` matches `a` in `aaa` when recursion is atomic; otherwise it would match the whole string.	no	n/a	n/a	no	6.5	YES	5.1.3–7.2.34	YES	YES	n/a	n/a	n/a	n/a	no	n/a	ECMA 1.42–1.85	n/a	n/a	n/a	n/a	n/a	n/a	n/a	n/a
Recursion is atomic	Subroutine call using syntax other than `(?P>…)`	Subroutine calls are atomic. Once the regex engine exits from them, it will not backtrack into it to try different permutations of the subroutine call.	`(a+)(?1)(?1)` can never match anything because the first `(?1)` matches all remaining a’s and the regex engine won’t backtrack into the first `(?1)` when the second one fails	no	n/a	4 only	no	6.5	10.00–10.23	5.1.3–7.2.34	YES	2.14.0–3.6.3	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
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