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

If you want to learn about regular expressions, I recommend Mastering Regular Expressions. It goes all the way from the very basic concepts, all the way up to talking about how different engines work underneath. The last 4 chapters also gives a dedicated chapter to each of PHP, .Net, Perl, and Java. I learned a lot from it, and still use it as a reference.

Answer 2

Validating strong passwords:

This one will validate a password with a length of 5 to 10 alphanumerical characters, with at least one upper case, one lower case and one digit:

^(?=.*[A-Z])(?=.*[a-z])(?=.*[0-9])[a-zA-Z0-9]{5,10}$

Answer 3

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.