Question

I found a nice piece of regex code that checks for a prime number. I think I understand it but i’m still a little confused. Here’s the code: /^1?$|^(11+?)\1+$/

Can someone explain (step by step) exactly what is happening both with the regex code and how it actually relates to knowing if a number is prime or not?

Answer 1

The basic premise is that this regular expression examines a ones representation of the number (e.g. 5 = 11111). By checking for the presence of ones (1) in certain positions or groupings it can identify the number as prime.

Additional References:

• Credit where credit is due –
  http://montreal.pm.org/tech/neil_kandalgaonkar.shtml
• Great explanation – http://www.noulakaz.net/weblog/2007/03/18/a-regular-expression-to-check-for-prime-numbers/