codegolf.stackexchange.com => True
codegolf.stackexchange.net => False
codegolf.stackexchange => True
codegolf.foo => False
codegolf => True
foo => False
codegolf.com => False
codegolf.foo.bar => Error
codegolf.stackexchange.com.foo => Error
codegolf.stackexchange.com.foo.bar => Error
foo.stackexchange.com => Error
foo.bar => Error
foo.bar.baz => Error