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Regular Expression Reference: Lookaround and Conditionals

FeatureSyntaxDescriptionExampleJGsoft Python JavaScript VBScript XRegExp .NET Java ICU RE2 Perl PCRE PCRE2 PHP Delphi R Ruby std::regex Boost Tcl POSIX GNU Oracle XML XPath
Positive lookahead (?=regex) Matches at a position where the pattern inside the lookahead can be matched. Matches only the position. It does not consume any characters or expand the match. In a pattern like one(?=two)three, both two and three have to match beginning at the position where the match of one ends (which is only possible if two and three represent regexes that can match the same text rather than the literal words). t(?=s) matches the second t in streets. YESYESYESYESYESYESYESYESnoYESYESYESYESYESYESYESECMAECMAYESnonononono
Negative lookahead (?!regex) Similar to positive lookahead, except that negative lookahead only succeeds if the regex inside the lookahead fails to match. t(?!s) matches the first t in streets. YESYESYESYESYESYESYESYESnoYESYESYESYESYESYESYESECMAECMAYESnonononono
Positive lookbehind (?<=regex) Matches at a position if the pattern inside the lookbehind can be matched ending at that position. (?<=s)t matches the first t in streets. YESYESYESnoYESYESYESYESnoYESYESYESYESYESYES1.9noECMAnononononono
Negative lookbehind (?<!regex) Matches at a position if the pattern inside the lookbehind cannot be matched ending at that position. (?<!s)t matches the second t in streets. YESYESYESnoYESYESYESYESnoYESYESYESYESYESYES1.9noECMAnononononono
Lookbehind Any lookbehind with alternation Alternatives inside lookbehind can differ in length. (?<=is|e)t matches the second and fourth t in twisty streets. YESnoYESn/aYESYESYESYESn/a5.30YESYESYESYESYES1.9n/aECMA
1.38–1.43		n/an/an/an/an/an/a
Lookbehind Any lookbehind and bounded quantifier Quantifiers with a finite maximum number of repetitions can be used inside lookbehind. (?<=s\w{1,7})t matches only the fourth t in twisty streets. YESnoYESn/aYESYES6
4 fail		YESn/a5.36
5.30 fail		no10.43nono4.4.0non/anon/an/an/an/an/an/a
Lookbehind Any lookbehind and backreference Backreferences can be used inside lookbehind. Syntax prohibited in lookbehind is also prohibited in the referenced capturing group. (\w).+(?<=\1) matches twisty street in twisty streets. YES3.5YESn/aYESYESnonon/anono10.237.3.0no4.0.0non/anon/an/an/an/an/an/a
Lookbehind Any lookbehind The full regular expression syntax can be used inside lookbehind. The lookbehind’s regex is matched from right to left. (?<=s\w+)t matches only the fourth t in twisty streets. YESnoYESn/aYESYES13non/anononononononon/anon/an/an/an/an/an/a
Lookbehind Any lookbehind The lookbehind’s regex is matched from left to right, stepping through the subject string from left to right (from longest possible lookbehind to shortest possible lookbehind). (?<=bc|c|(abc))d captures abc as the first group when the regex matches d in abcd. nononon/anonononon/a5.30nonononononon/anon/an/an/an/an/an/a
Lookbehind Any lookbehind The lookbehind’s regex is matched from left to right, stepping through the subject string from right to left (from shortest possible lookbehind to longest possible lookbehind). (?<=bc|(c)|abc)d captures c as the first group when the regex matches d in abcd. nononon/anonoYESYESn/anononononononon/anon/an/an/an/an/an/a
Lookbehind (?<=regex) or (?<!regex) Lookbehind is atomic. Only relevant if the lookbehind contains a capturing group and the remainder of the regex has a backreference to that group. YESnoYESn/aYESYES6YESn/a5.36YESYESYESYESYESnon/anon/an/an/an/an/an/a
Positive lookahead (*positive_lookahead:regex) Matches at a position where the pattern inside the lookahead can be matched. Matches only the position. It does not consume any characters or expand the match. Atomic. t(*positive_lookahead:s) matches the second t in streets. nonononononononono5.28no10.337.4.0no4.0.0nonononononononono
Positive lookahead (*pla:regex) Short form of the above. t(*pla:s) matches the second t in streets. nonononononononono5.28no10.337.4.0no4.0.0nonononononononono
Negative lookahead (*negative_lookahead:regex) Similar to positive lookahead, except that negative lookahead only succeeds if the regex inside the lookahead fails to match. Atomic. t(*negative_lookahead:s) matches the first t in streets. nonononononononono5.28no10.337.4.0no4.0.0nonononononononono
Negative lookahead (*nla:regex) Short form of the above. t(*nla:s) matches the first t in streets. nonononononononono5.28no10.337.4.0no4.0.0nonononononononono
Positive lookbehind (*positive_lookbehind:regex) Matches at a position if the pattern inside the lookbehind can be matched ending at that position. Atomic. (*positive_lookbehind:s)t matches the first t in streets. nonononononononono5.28no10.337.4.0no4.0.0nonononononononono
Positive lookbehind (*plb:regex) Short form of the above. (*plb:s)t matches the first t in streets. nonononononononono5.28no10.337.4.0no4.0.0nonononononononono
Negative lookbehind (*negative_lookbehind:regex) Matches at a position if the pattern inside the lookbehind cannot be matched ending at that position. Atomic. (*negative_lookbehind:s)t matches the second t in streets. nonononononononono5.28no10.337.4.0no4.0.0nonononononononono
Negative lookbehind (*nlb:regex) Short form of the above. (*nlb:s)t matches the second t in streets. nonononononononono5.28no10.337.4.0no4.0.0nonononononononono
Positive lookahead (*non_atomic_positive_lookahead:regex) Non-atomic lookahead can be backtracked into to try different alternatives if the remainder of the regex fails to match. Only relevant if the lookahead contains a capturing group and the remainder of the regex has a backreference to that group. nonononononononononono10.347.4.6no4.2.0nonononononononono
Positive lookahead (*napla:regex) Short form of the above. nonononononononononono10.347.4.6no4.2.0nonononononononono
Positive lookbehind (*non_atomic_positive_lookbehind:regex) Non-atomic lookbehind can be backtracked into to try different alternatives if the remainder of the regex fails to match. Only relevant if the lookbehind contains a capturing group and the remainder of the regex has a backreference to that group. nonononononononononono10.347.4.6no4.2.0nonononononononono
Positive lookbehind (*naplb:regex) Short form of the above. nonononononononononono10.347.4.6no4.2.0nonononononononono
Keep text out of the regex match \K The text matched by the part of the regex to the left of the \K is omitted from the overall regex match. Other than that the regex is matched normally from left to right. Capturing groups to the left of the \K capture as usual. s\Kt matches only the first t in streets. V2nonononononononoYES7.2YESYESYESYES2.0noECMA
1.42		nononononono
Lookaround conditional (?(?=regex)then|else) where (?=regex) is any valid lookaround and then and else are any valid regexes If the lookaround succeeds, the “then” part must match for the overall regex to match. If the lookaround fails, the “else” part must match for the overall regex to match. The lookaround is zero-length. The “then” and “else” parts consume their matches like normal regexes. (?(?<=a)b|c) matches the second b and the first c in babxcac YESnonononoYESnononoYESYESYESYESYESYESnonoECMAnononononono
Implicit lookahead conditional (?(regex)then|else) where regex, then, and else are any valid regexes and regex is not the name of a capturing group If “regex” is not the name of a capturing group, then it is interpreted as a lookahead as if you had written (?(?=regex)then|else). If the lookahead succeeds, the “then” part must match for the overall regex to match. If the lookahead fails, the “else” part must match for the overall regex to match. The lookaround is zero-length. The “then” and “else” parts consume their matches like normal regexes. (?(\d{2})7|c) matches the first 7 and the c in 747c nononononoYESnononononononononononononononononono
Named conditional (?(name)then|else) where name is the name of a capturing group and then and else are any valid regexes If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match. (?<one>a)?(?(one)b|c) matches ab, the first c, and the second c in babxcac YESYESnononoYESnononono6.7YESYESYESYESnonononononononono
Named conditional (?(<name>)then|else) where name is the name of a capturing group and then and else are any valid regexes If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match. (?<one>a)?(?(<one>)b|c) matches ab, the first c, and the second c in babxcac V2nonononononononoYES7.0YESYESYESYES2.0noECMA
1.42		nononononono
Named conditional (?('name')then|else) where name is the name of a capturing group and then and else are any valid regexes If the capturing group with the given name took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match. (?'one'a)?(?('one')b|c) matches ab, the first c, and the second c in babxcac V2nonononononononoYES7.0YESYESYESYES2.0noECMA
1.42		nononononono
Conditional (?(1)then|else) where 1 is the number of a capturing group and then and else are any valid regexes If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match. (a)?(?(1)b|c) matches ab, the first c, and the second c in babxcac YESYESnononoYESnononoYESYESYESYESYESYES2.0noECMAnononononono
Relative conditional (?(-1)then|else) where -1 is a negative integer and then and else are any valid regexes Conditional that tests 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 immediately before the conditional. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match. (a)?(?(-1)b|c) matches ab, the first c, and the second c in babxcac V2nonononononononono7.2YESYESYESYESnonononononononono
Forward conditional (?(+1)then|else) where +1 is a positive integer and then and else are any valid regexes Conditional that tests 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 left to right starting at the “then” part of conditional. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match. ((?(+1)b|c)(d)?){2} matches cc and cdb in bdbdccxcdcxcdb V2nonononononononono7.2YESYESYESYESnonononononononono
Conditional (?(+1)then|else) where 1 is the number of a capturing group and then and else are any valid regexes The + is ignored and the number is taken as an absolute reference to a capturing group. If the referenced capturing group took part in the match attempt thus far, the “then” part must match for the overall regex to match. If the capturing group did not take part in the match thus far, the “else” part must match for the overall regex to match. (a)?(?(+1)b|c) matches ab, the first c, and the second c in babxcac no2.4–3.10nononononononononononononononononononononono
FeatureSyntaxDescriptionExampleJGsoft Python JavaScript VBScript XRegExp .NET Java ICU RE2 Perl PCRE PCRE2 PHP Delphi R Ruby std::regex Boost Tcl POSIX GNU Oracle XML XPath