protected function _validateDomain($domain)
    {
        // Note the different use of $subdomains and $sub_domains
        $subdomains = explode('.', $domain);

        while (count($subdomains) > 0) {
            $sub_domains[] = $this->_splitCheck($subdomains, '.');
            for ($i = 0; $i < $this->index + 1; $i++)
                array_shift($subdomains);
        }

        foreach ($sub_domains as $sub_domain) {
            if (!$this->_validateSubdomain(trim($sub_domain)))
                return false;
        }

        // Managed to get here, so return input.
        return $domain;
    }