337
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 have to admit that using CL-PPCRE does not really help me understanding the regexp any better. But this may be because I deal with complex regexps for decades now, and I just read them.