Question

I’m writing a regular expression that can interactively validate SMTP responses codes, once the SMTP dialog is completed it should pass the following regex (some parentheses added for better readability):

^(220)(250){3,}(354)(250)(221)$

Or with(out) authentication:

^(220)(250)((334){2}(235))?(250){2,}(354)(250)(221)$

I’m trying to do rewrite the above regexes so that I can interactively check if the dialog is going as expected, otherwise politely send a QUIT command and close the connection saving bandwidth and time, but I’m having a hard time writing an optimal regex. So far I’ve managed to come up with:

^(220(250(334(235(250(354(250(221)?)?)?){0,})?){0,2})?)?$

Which, besides only matching authenticated connections, has some bugs… For instance, it matches:

220250334235250354250221
220250334334235250354250221

I’ve also tried the following modification:

^(220(250)?)?((334(235)?){2})?(250(354(250(221)?)?)?){0,}$

This one accepts non-authenticated responses but it fails to match 220250334 and wrongly matches 220250334334235250354250221 (at least 2 250 are needed before the 354 response code).

Can someone help me out with this? Thanks in advance.

---

An example of what I’m trying to do:

$smtp = fsockopen('mail.example.com', 25);
$result = null;
$commands = array('HELO', 'AUTH LOGIN', 'user', 'pass', 'MAIL FROM', 'RCPT TO', 'RCPT TO', 'DATA', "\r\n.", 'QUIT');

foreach ($commands as $command)
{
    $result .= substr(fgets($smtp), 0, 3);

    if (preg_match('~^(220(250)?)?((334){1,2}(235)?)?(250(354(250(221)?)?)?){0,}$~S', $result) > 0)
    {
        fwrite($smtp, $command . "\r\n");
    }

    else
    {
        fwrite($smtp, "QUIT\r\n");
        fclose($smtp);
        break;
    }
}

Which should act as a replacement for the following procedural code:

$smtp = fsockopen('mail.example.com', 25);
$result = substr(fgets($smtp), 0, 3); // 220

if ($result == '220')
{
    fwrite($smtp, 'HELO' . "\r\n");
    $result = substr(fgets($smtp), 0, 3); // 220

    if ($result == '250')
    {
        fwrite($smtp, 'AUTH LOGIN' . "\r\n");
        $result = substr(fgets($smtp), 0, 3); // 334

        if ($result == '334')
        {
            fwrite($smtp, 'user' . "\r\n");
            $result = substr(fgets($smtp), 0, 3); // 334

            if ($result == '334')
            {
                fwrite($smtp, 'pass' . "\r\n");
                $result = substr(fgets($smtp), 0, 3); // 235

                if ($result == '235')
                {
                    fwrite($smtp, 'MAIL FROM' . "\r\n");
                    $result = substr(fgets($smtp), 0, 3); // 250

                    if ($result == '250')
                    {
                        foreach ($to as $mail)
                        {
                            fwrite($smtp, 'RCPT TO' . "\r\n");
                            $result = substr(fgets($smtp), 0, 3); // 250

                            if ($result != '250')
                            {
                                fwrite($smtp, 'QUIT' . "\r\n");
                                $result = substr(fgets($smtp), 0, 3); // 221
                                fclose($smtp);

                                break;
                            }
                        }

                        if ($result == '250')
                        {
                            fwrite($smtp, 'DATA' . "\r\n");
                            $result = substr(fgets($smtp), 0, 3); // 354

                            if ($result == '354')
                            {
                                fwrite($smtp, "\r\n.\r\n");
                                $result = substr(fgets($smtp), 0, 3); // 250

                                if ($result == '250')
                                {
                                    fwrite($smtp, 'QUIT' . "\r\n");
                                    $result = substr(fgets($smtp), 0, 3); // 221
                                    fclose($smtp);

                                    if ($result == '221')
                                    {
                                        echo 'SUCESS!';
                                    }
                                }

                                else
                                {
                                    fwrite($smtp, 'QUIT' . "\r\n");
                                    $result = substr(fgets($smtp), 0, 3); // 221
                                    fclose($smtp);
                                }
                            }

                            else
                            {
                                fwrite($smtp, 'QUIT' . "\r\n");
                                $result = substr(fgets($smtp), 0, 3); // 221
                                fclose($smtp);
                            }
                        }
                    }

                    else
                    {
                        fwrite($smtp, 'QUIT' . "\r\n");
                        $result = substr(fgets($smtp), 0, 3); // 221
                        fclose($smtp);
                    }
                }

                else
                {
                    fwrite($smtp, 'QUIT' . "\r\n");
                    $result = substr(fgets($smtp), 0, 3); // 221
                    fclose($smtp);
                }
            }

            else
            {
                fwrite($smtp, 'QUIT' . "\r\n");
                $result = substr(fgets($smtp), 0, 3); // 221
                fclose($smtp);
            }
        }

        else
        {
            fwrite($smtp, 'QUIT' . "\r\n");
            $result = substr(fgets($smtp), 0, 3); // 221
            fclose($smtp);
        }
    }

    else
    {
        fwrite($smtp, 'QUIT' . "\r\n");
        $result = substr(fgets($smtp), 0, 3); // 221
        fclose($smtp);
    }
}

else
{
    fwrite($smtp, 'QUIT' . "\r\n");
    $result = substr(fgets($smtp), 0, 3); // 221
    fclose($smtp);
}

Answer 1

It’s amazing how regular expressions become so much easier after a good night of sleep, here it is:

(?>220(?>250(?>(?>334){1,2}(?>235)?)?(?>(?>250){1,}(?>354(?>250(?>221)?)?)?)?)?)?

Which can be simplified to this:

^220(?>250(?>(?>334){1,2}(?>235)?)?(?>(?>250){1,}(?>354(?>250)?)?)?)?$

Since the first response code (220) is not optional and we will always send the last QUIT command.