Regular Expression Opposite

Question

Is it possible to write a regex that returns the converse of a desired result? Regexes are usually inclusive - finding matches. I want to be able to transform a regex into

Answer 1

Java Regexps have an interesting way of doing this (can test here) where you can create a greedy optional match for the string you want, and then match data after it. If the greedy match fails, it's optional so it doesn't matter, if it succeeds, it needs some extra data to match the second expression and so fails.

It looks counter-intuitive, but works.

Eg (foo)?+.+ matches bar, foox and xfoo but won't match foo (or an empty string).

It might be possible in other dialects, but couldn't get it to work myself (they seem more willing to backtrack if the second match fails?)