Matches a valid domain name.
Contributed by @itsbrunodev
^[a-z0-9]+([\-\.]{1}[a-z0-9]+)*\.[a-z]{2,6}$# Valid domain names
example.com
sub.example.co.uk
example-domain.com
subdomain.example.com
# Invalid domain names
-example.com
example..com
example.c