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