^.?$|^(..+?)\1+$
<answer>
Matches strings of any character repeated a non-prime number of times
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Whatever you do, don’t get in a time machine back to 1998 and become a Unix sysadmin.
(Though we didn’t have CL-PPCRE then. It’s really the best thing that ever happened to regex.)
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.
I was a sysadmin with some Linux usage in 1998, does that count?
