"(?>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.) 
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-- Randal L. Schwartz, Perl hacker