When is a issue too complex for a regular expression?

Question

Please don\'t answer the obvious, but what are the limit signs that tell us a problem should not be solved using regular expressions?

For example: Why is a complete emai

Answer 1

Solve the problem with a regex, then give it to somebody else conversant in regexes. If they can't tell you what it does (or at least say with confidence that they understand) in about 10 minutes, it's too complex.