Hey, it's been a while

Kinda have a small issue. I'm trying get the actual hostname of a domain the problem i face is subdomains.

For example, i'm trying to produce a regex that could at least get all the follow to result in domain.com:

sub.domain.com
domain.co.uk
domain.com
sub.domain.co.uk
sub.sub.domain.gov.uk

etc.. Any ideas?

Thanks!