From perlre: 
"(?<=pattern)"
                 A zero-width positive look-behind assertion.
                 For example, "/(?<=\t)\w+/" matches a word that
                 follows a tab, without including the tab in $&.
                 Works only for fixed-width look-behind.

"(?<!pattern)"
                 A zero-width negative look-behind assertion.
                 For example "/(?<!bar)foo/" matches any occur-
                 rence of "foo" that does not follow "bar".
                 Works only for fixed-width look-behind.
Liz

In reply to Re: regex match question by liz
in thread regex match question by Anonymous Monk

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