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

<answer>

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

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

This is brilliantly disgusting.

Literal interpretation of the regex

The regex matches either a line with a single character or a line with a sequence of two or more characters that’s repeated two or more times. For some examples: the regex matches “a”, “b”, “abab”, “ababab”, “aaaa”, and “bbbbbb”, but does not match “aa”, “bb”, “aaa”, “ab”, “aba”, or “ababa”.

Hint for the special thing it matches

For a line with a single character repeated n times, what does matching (or not matching) this regex say about the number n?

You forgot empty line. Since first part is ^.?$ it’s one or zero of any character.

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