Regular Expression That Checks If A Number Is Prime
tl;dr if you know a little regex   - convert your number to a string of that length (1 = 1, 2 = 11, 3 = 111)
   - handle 0 / 1 separately
   - ^(..+?)\1+$
   - trick; the WHOLE thing has to match to return (^$)
   - first \1 match is "11", ergo string must be "11", "1111", "111111" to match 
   - second \1 match "111", ergo string must be "111", "111111", "111111111" to match
   - and so on.  if you find before length of string, it was not prime.

Clever trick.  Look forward to being asked it in your next google interview :)