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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
And misses others you didn’t think about.
Yeah, I've found myself wasting quite a lot of time thinking of the 'perfect regex' for task X only to realise that I could have avoided doing so by simply taking a different approach.