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.