What are good regular expressions?

Question

I have worked for 5 years mainly in java desktop applications accessing Oracle databases and I have never used regular expressions. Now I enter Stack Overflow and I see a lot of

Answer 1

Coolest regular expression ever:

/^1?$|^(11+?)\1+$/

It tests if a number is prime. And it works!!

N.B.: to make it work, a bit of set-up is needed; the number that we want to test has to be converted into a string of “1”s first, then we can apply the expression to test if the string does not contain a prime number of “1”s:

def is_prime(n)
  str = "1" * n
  return str !~ /^1?$|^(11+?)\1+$/ 
end

There’s a detailled and very approachable explanation over at Avinash Meetoo’s blog.