The regexp basically tests all dividers of the number (from 2 to the number-1), if it founds one, it says false and stops, if nothing is found, it says true. 1

The regexp basically tests all dividers of the number (from 2 to the number-1), if it founds one, it says false and stops, if nothing is found, it says true. 101 is prime, but it does "only" a hundred checks approx. 7673121 is divisible by 3, so as soon as it tries to match groups of 3 it succeeds and stops. 767423 is prime, so it needs to do hundreds of millions of checks, that's probably why it hungs. Basically: the runtime of the algorithm should be equivalent to the minimum divisor excluding 1 of the number.