Unfortunately (or perhaps fortunately), “regexes” as commonly implemented in programming languages are only loosely related to regular expressions from automata

Unfortunately (or perhaps fortunately), “regexes” as commonly implemented in programming languages are only loosely related to regular expressions from automata theory. With all their extensions, they can recognize much, much more than just regular languages, and I don’t think they’re closed under complement (though I’m not sure). However, most regex engines have a feature called negative lookahead assertions, (?!do not match), which would almost work in the way you suggest. You have to be careful about inputs like this though: “Inside a string”Tarzan”Again inside a string”