Regex to extract subdomain from URL?

/(http:\/\/)?(([^.]+)\.)?domain\.com/

Then $3 (or \3) will contain "subdomain" if one was supplied.

If you want to have the subdomain in the first group, and your regex engine supports non-capturing groups (shy groups), use this as suggested by palindrom:

/(?:http:\/\/)?(?:([^.]+)\.)?domain\.com/

Purely the subdomain string (result is $1):

^http://([^.]+)\.domain\.com

Making http:// optional (result is $2):

^(http://)?([^.]+)\.domain\.com

Making the http:// and the subdomain optional (result is $3):

(http://)?(([^.]+)\.)?domain\.com

It should just be

\Qhttp://\E(\w+)\.domain\.com

The sub domain will be the first group.

The problem with the above regex is: if you do not know what the protocol is, or what the domain suffix is, you will get some unexpected results. Here is a little regex accounts for those situations. :D

/(?:http[s]*\:\/\/)*(.*?)\.(?=[^\/]*\..{2,5})/i  //javascript

This should always return your subdomain (if present) in group 1. Here it is in a Javascript example, but it should also work for any other engine that supports positive look-ahead assertions:

// EXAMPLE of use
var regex = /(?:http[s]*\:\/\/)*(.*?)\.(?=[^\/]*\..{2,5})/i
  , whoKnowsWhatItCouldBe = [
                        "www.mydomain.com/whatever/my-site" //matches: www
                      , "mydomain.com"// does not match
                      , "http://mydomain.com" // does not match
                      , "https://mydomain.com"// does not match
                      , "banana.com/somethingelse" // does not match
                      , "https://banana.com/somethingelse.org" // does not match
                      , "http://what-ever.mydomain.mu" //matches: what-ever
                      , "dev-www.thisdomain.com/whatever" // matches: dev-www
                      , "hot-MamaSitas.SomE_doma-in.au.xxx"//matches: hot-MamaSitas
                  , "http://hot-MamaSitas.SomE_doma-in.au.xxx" // matches: hot-MamaSitas
                  , "пуст.пустыня.ru" //even non english chars! Woohoo! matches: пуст
                  , "пустыня.ru" //does not match
                  ];

// Run a loop and test it out.
for ( var i = 0, length = whoKnowsWhatItCouldBe.length; i < length; i++ ){
    var result = whoKnowsWhatItCouldBe[i].match(regex);
    if(result != null){
      // YAY! We have a match!
    } else {
      // Boo... No subdomain was found
    }
}