PHPMailer::isValidHost( string $host ): bool

4536    public static function isValidHost($host)4537    {4538        //Simple syntax limits4539        if (4540            empty($host)4541            || !is_string($host)4542            || strlen($host) > 2564543            || !preg_match('/^([a-z\d.-]*|\[[a-f\d:]+\])$/i', $host)4544        ) {4545            return false;4546        }4547        //Looks like a bracketed IPv6 address4548        if (strlen($host) > 2 && substr($host, 0, 1) === '[' && substr($host, -1, 1) === ']') {4549            return filter_var(substr($host, 1, -1), FILTER_VALIDATE_IP, FILTER_FLAG_IPV6) !== false;4550        }4551        //If removing all the dots results in a numeric string, it must be an IPv4 address.4552        //Need to check this first because otherwise things like `999.0.0.0` are considered valid host names4553        if (is_numeric(str_replace('.', '', $host))) {4554            //Is it a valid IPv4 address?4555            return filter_var($host, FILTER_VALIDATE_IP, FILTER_FLAG_IPV4) !== false;4556        }4557        //Is it a syntactically valid hostname (when embedded in a URL)?4558        return filter_var('https://' . $host, FILTER_VALIDATE_URL) !== false;4559    }