^.?$|^(..+?)\1+$
<answer>
Matches strings of any character repeated a non-prime number of times
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Empty input Or input of exactly 1 character Or input of at least 2 characters, followed by at least 1 something (idk what \1 matches)
Did I get it (almost)?
\1 is group 1 which is inside
(), so second part is repeated 2 or more times of 2 or more char.Interesting.
So that means match any string that is made entirely of a single repeating sequence, where repititon is possible.
