Perl defines an extended syntax for regular expressions. The syntax is a pair of parentheses with a question mark as the first thing within the parentheses. The character after the question mark gives the function of the extension. The extensions are:
(?#text)A comment. The text is ignored.
(?:...)(?imsx-imsx:...)This groups things like
(...)but doesn’t make backreferences.(?=...)A zero-width positive lookahead assertion. For example,
/w+(?= )/matches a word followed by a tab, without including the tab in$&.(?!...)A zero-width negative lookahead assertion. For example,
/foo(?!bar)/matches any occurrence offoothat isn’t followed bybar.(?<...)A zero-width positive lookbehind assertion. For example,
/(?<bad)boy/matches the wordboythat followsbad, without includingbadin$&. This works only for fixed-width lookbehind.(?{code})An experimental regular expression feature to evaluate any embedded Perl code. This evaluation always succeeds, and
codeis not interpolated.(?<!=...)A zero-width negative lookbehind assertion. For example,
/(?<!=bad)boy/matches any occurrence ofboythat doesn’t followbad. This works only for fixed-width lookbehind.(?>...)Matches the substring that the standalone pattern would match if anchored at the given position.
(?(condition)yes-pattern|no-pattern)(?(condition)yes-pattern)Matches a pattern determined by a condition.
conditionshould be either an integer, which is true if the pair of parentheses corresponding to the integer has matched, or a lookahead, lookbehind, or evaluate, zero-width assertion.no-patternwill be used to match if the condition was not meant, but it is also optional.(?imsx-imsx)One or more embedded pattern-match modifiers. Modifiers are switched off if they follow a
-(dash). The modifiers are defined as follows :