I've thought about how to do a kind of regex-induction, where you give it matches and it gives you one or more expressions that will match it. But, after some thinking, even with a reasonable language bias, getting useful expression comes down to getting the user to type in a collection of positive an negative match examples. This kind of tool would be really nice to use, but the interaction has to be more than giving a single positive example.

A an iterative process where you start from a single positive example and then iteratively refine it with clarifying examples would work without being overwhelming. However, I'm really trying to get the user to better understand regexes to the point where they could do that process for themselves, mentally, and not depend on a super intelligent tool to do it for them.