Is there a programatic way to find the domain name from a given hostname?
given -> www.yahoo.co.jp return -> yahoo.co.jp
The approach that works but is very slow is:
split on "." and remove 1 group from the left, join and query an SOA record using dnspython when a valid SOA record is returned, consider that a domain
Is there a cleaner/faster way to do this without using regexps?
There's no trivial definition of which "domain name" is the parent of any particular "host name".
Your current method of traversing up the tree until you see an
SOA record is actually the most correct.
Technically, what you're doing there is finding a "zone cut", and in the vast majority of cases that will correspond to the point at which the domain was delegated from its TLD.
Any method that relies on mere text parsing of the host name without reference to the DNS is doomed to failure.
Alternatively, make use of the centrally maintained lists of delegation-centric domains from http://publicsuffix.org/, but beware that these lists can be incomplete and/or out of date.
See also this question where all of this has been gone over before...