Domain validation methodshttp://search.cpan.org/dist/Data-Validate-Domain
This module collects domain validation routines to make input validation,
and untainting easier and more readable.
All functions return an untainted value if the test passes, and undef if
it fails. This means that you should always check for a defined status
explicitly. Don't assume the return will be true. (e.g. is_username('0'))
The value to test is always the first (and often only) argument.