Extract domain from a hostname

Hi,

I'd like to extract the domain from a given hostname. For example:

hostname: this.is.a.hostname.domain.com
result: domain.com

hostname: domain.com
result: domain.com

My current regexp (php) is:

/([a-zA-Z0-9\-_]+\.[a-zA-Z0-9\-_]{2,3})$/

I'm very bad at regexp so i need some help. This current example works for domains that end in com, ru or something that contains 2 or 3 characters. But how do i specify that the ending of the domain can be:
either: 2 or 3 characters long
or: 2 or 3 characters long, followed by a dot, followed by to or 3 more characters (like co.uk)

How should i modify this expression?

Also there's a problem of knowing if the co.uk part is actually a domain ending or is it adomain itself (like ab.com).

I'm a little rusty with my regex and there might be something simpler you could use, but try:

/(([a-z0-9]+)(-[a-z0-9]+)?)(\.[a-z]{2}\.[a-z]{2}|\.[a-z]{2,4})$/i

Also, AFAIK, domain names don't typically include underscores. You're out of luck if you're trying to match ip addresses with this expression, or if the user ends the domain with / or whatever. I don't really know where you're getting your data from, though, so it's difficult to say whether or not this will be a problem for you.

I tried this against the following (some of which are random domains, some real), and the expression worked out against all of them:

domain.name
domain.com
domain.ch
domain.co.uk
domain-name.com
my-domain-name.com
123-reg.co.uk
www.123-reg.co.uk
ab.com
my.really.obscure.domain.com
http://www.google.com

You could just use parse_url. Remember, in PHP, there's almost always a built-in function for what you're trying to do.

-Dan