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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
"at least 2 characters repeated [at least] twice" implies the string's length is divisible by a number greater than 1.
Yes but the match goes for the first repetition the rest of the string isn't matched no matter the length, again don't find anything about prime numbers unless I checked something wrong. There is another guy who got it right it seems.