"(?>pattern)" WARNING: This extended regular expression fea- ture is considered highly experimental, and may be changed or deleted without notice. An "independent" subexpression, one which matches the substring that a standalone "pat- tern" would match if anchored at the given posi- tion, and it matches nothing other than this substring. This construct is useful for opti- mizations of what would otherwise be "eternal" matches, because it will not backtrack (see the section on "Backtracking"). It may also be use- ful in places where the "grab all you can, and do not give anything back" semantic is desirable. For example: "^(?>a*)ab" will never match, since "(?>a*)" (anchored at the beginning of string, as above) will match all characters "a" at the beginning of string, leaving no "a" for "ab" to match. In contrast, "a*ab" will match the same as "a+b", since the match of the subgroup "a*" is influenced by the following group "ab" (see the section on "Backtracking"). In particular, "a*" inside "a*ab" will match fewer characters than a standalone "a*", since this makes the tail match. An effect similar to "(?>pattern)" may be achieved by writing "(?=(pattern))\1". This matches the same substring as a standalone "a+", and the following "\1" eats the matched string; it therefore makes a zero-length assertion into an analogue of "(?>...)". (The difference between these two constructs is that the second one uses a capturing group, thus shifting ordi- nals of backreferences in the rest of a regular expression.) ...
-- Randal L. Schwartz, Perl hacker
In reply to •Re: Avoiding regex backtracking
by merlyn
in thread Avoiding regex backtracking
by Aristotle
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