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Oh boy what a beautiful regex. I'm sure it does something logical and easy to understand.
(lemmy.world)
^.?$|^(..+?)\1+$
Matches strings of any character repeated a non-prime number of times
I've never had to use it for important stuff tbh. But alongside a regex tester and a sample of the stuff I intend to use it on, I've had good results with an incremental approach where I tell the LLM what I want to change with the expression until I'm satisfied