I saw this and was going to reply with something to the effect of Set::IntSpan but I was too late and mine wouldn't have had as clever a trick with that closure coderef . . . bah.

Also as far as the original question, IMHO most times if you're thinking about embedding code inside your regexen you're pretty well already deep into (quasi-apocryphical) JWZ-quote / xkcd 1171 territory.