Remember this is a homework problem. We are supposed to be helping the supplicant to reach a solution by giving hints, and insightful analysis of the problem.

If you want it to be a home-work problem, and to give insightful analysis, you shouldn't ask for a one-liner, and you shouldn't consider Abigail's (infamous) regexp as an answer. It's obscure, inefficient, uses an unreasonable amount of memory, and quite quickly hits limits.