Like tadman++ noted the problem is that once you use it, all regexes anywhere in the script become slower - not likely to be tolerable in the context of the Everything engine.. And this one in particular is pretty simple: $title =~ s{((?:Re: ){2,})}{"Re(" . length($1)/4 . "): "}ge; Works a treat for your test cases.

Makeshifts last the longest.