^.?$|^(..+?)\1+$
<answer>
Matches strings of any character repeated a non-prime number of times
3 points
*
By now, I have just one, so thanks for the assist. There’s always that one (sometimes puzzling) downvote on anything factual.
The pumping lemma, for anyone unfamiliar. It’s a consequence of the fact an FSM is finite, so you can construct a repeatable y just by exhausting the FSM’s ability to “remember” how much it’s seen.