[amp108]

Interesting, I wrote a quick script to use this:

#!/usr/bin/env ruby

def is_prime(n)

  ("1" \* n.to_i)!~ /^1?$|^(11+?)\1+$/

end

num = ARGV.shift

$stdout.puts is_prime(num)

... then decided to test it on a button-mashed number:

$ruby ~/prime.rb 767423

... and it hung.

Thought it was the length of the number, so I tried something smaller (101). Quick "true". Then I tried progressively more digits. I got to `ruby ~/prime.rb 7673121` (false) and it took less than a second.

But it doesn't like something about the number 767423.

[TrianguloY]

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.