Find primes in regexp
Posted by Jonas Elfström Fri, 30 Mar 2007 05:33:00 GMT
In an earlier post the example code did find prime numbers. Recently I stumbled over a really cool regexp hack that also deals with primes. This is how you execute that regexp in Ruby:
puts 'Prime' unless ('1' * 43) =~ /^1$|^(11+?)\1+$/Change 43 to whatever you like and you will get Prime as output if it's a prime number. EDIT: As you can see in the comments Neil Kandalonkar explained how the regexp by Abigail works.
As the page makes clear, credit should go to Abigail. I merely explained it.
Sorry, now it has been corrected.
Seems Neils site has gone missing. Here’s another nice explanation: http://paddy3118.blogspot.com/2009/08/story-of-regexp-and-primes.html