A non prime number of times... It looks like the string of characters could repeat number of times because the whole capture group repeats. I don't see a prime constraint.
The capture group must be the same each time it repeats, so the number of characters stays the same. So X groups of Y characters = string of length X*Y. X and Y can be anything so any string length that can be made by multiplying two numbers-- which is every non-prime string length-- is matched. 0 and 1 are handled specially at the start.