Naturally, fixing a bug in my comment system and mentioning it here yesterday merely encouraged the quiet malcontents to bring up their petty annoyances! I’m kidding: keep the bug reports coming. Platypus pointed out that the email validation didn’t deal properly with his domain name.

So I’ve improved the email validation code. I know what a lot of you are thinking: why validate the email at all? I do it because it’s a good way to prevent spam. Maybe someday I’ll get rid of it, but for now it stays.

The problem in Platypus’s case is that I look up MX records for the domain, and pl.atyp.us doesn’t have one, but atup.us does. This is the code I’m now using:

function IsValidEmail($email)
{
    // I got this originally from
    // http://www.developer.com/lang/php/article.php/10941_3290141_2
    // Create the syntactical validation regular expression
    $atom_re = "[a-z0-9!#$%&'*+\\/=?^_`{|}~-]+";
    $regexp = "/^(" . $atom_re . ")(\\." . $atom_re .
                ")*@([a-z0-9-]+)(\\.[a-z0-9-]+)*(\\.[a-z]{2,4})$/i";

    // Presume that the email is invalid
    $valid = 0;

    // Validate the syntax
    if (preg_match($regexp, $email)) {
        if (function_exists("getmxrr")) {
            list($username, $domaintld) = split("@", $email);
            while (substr_count($domaintld, ".") > 0) {
                // Validate the domain
                if (getmxrr($domaintld, $mxrecords)) {
                    $valid = 1;
                    break;
                }

                // Didn't find an MX record.
                // If we have a subdomain, move up the hierarchy.
                list($dummy, $domaintld) = split(".", $domaintld, 2);
            }
        }
        else {
            // Couldn't check the domain with getmxrr, assume the best.
            $valid = 1;
        }
    }

    return $valid;
}

Anyone else have a complaint?

Updated: I’ve since improved the code to deal with some of the issues in the comment thread here: Email validation again.