That's actually a much better problem description because you've a) nailed down the why behind what you're attempting to do which can dispel any lingering hints of X-Y problems, and b) given a more concrete target (convert Perl regexen into POSIX regexen as understood by Postgres).

You might look into building a convertor on top of YAPE::Regex. It would parse an existing regex and then walk it emitting a Postgres regex equivalent (or throwing an error if it encounters something like (?{}) which doesn't have an analogue).

Update: D'oh! Forgot that Pg can embed Perl; that'd be a much better route if you can swing it than translating. Go with the suggestion below if you can; parse and convert as a last resort.