^.?$|^(..+?)\1+$

<answer>

Matches strings of any character repeated a non-prime number of times

https://www.youtube.com/watch?v=5vbk0TwkokM

A non prime number of times… It looks like the string of characters could repeat number of times because the whole capture group repeats. I don’t see a prime constraint.

The capture group must be the same each time it repeats, so the number of characters stays the same. So X groups of Y characters = string of length X*Y. X and Y can be anything so any string length that can be made by multiplying two numbers-- which is every non-prime string length-- is matched. 0 and 1 are handled specially at the start.

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