What it is really doing is checking if the length of a string is a prime number or not. And as coded, it is checking specifically if the length of a string consisting of some number of '1' characters is prime or not.
I've done some Codewars problems where you have to write a regex to check whether an input number is divisible by a specific number, so I was really interested to see how that would work for primes.
- it is turning the number into a string of 1s 3 => 111 4 => 1111 and so on
- then trying to simulate divisibility test by trying to find perfect groups of '11' to simulate division by 2 failing which (that is the backtracking part) and trying find perfect groups of '111' to simulate division by 3 and so on...
