Regex vs. Email Addresses: A Battle You’ll Always Lose

/^[a-zA-Z0-9_.-]+@[a-zA-Z0-9-]+.[a-zA-Z0-9-.]+$/

function checkEmail($email) {
  if (
    preg_match(
      "/^([a-zA-Z0-9])+([a-zA-Z0-9\._-])*@([a-zA-Z0-9_-])+([a-zA-Z0-9\._-]+)+$/",
      $email
    )
  ) {
    list($username, $domain) = split('@', $email);

    if (!checkdnsrr($domain, 'MX')) {
      return false;
    }

    return true;
  }

  return false;
}

<?php

class MailChecker {
    public static function isValid($email) {
        $mail_errors = 0;
        $atIndex = strrpos($email, "@");

        if (is_bool($atIndex) && !$atIndex) {
            // no "@"? Return false immediately.
            return false;
        }

	    $domain = substr($email, $atIndex + 1);
	    $local = substr($email, 0, $atIndex);

	    // 1.) Check local part for common errors
	    $mail_errors |= MailChecker::mail_check_local($local);
	    // 2.) Check domain for common errors
	    $mail_errors |= MailChecker::mail_check_domain($domain);
	    // 3.) Check local part for quotation errors
	    $mail_errors |= MailChecker::mail_check_quoted($local);

        // 4.) Check, if domain exists.
	    if ($mail_errors == 0) {
            $mail_errors |= MailChecker::mail_check_dns($domain);
        }

	    return $mail_errors;
    }

    private static function mail_check_local($local) {
        $local_errors = 0;
        $localLen = strlen($local);

        if ($localLen < 1 || $localLen > 64) {
            // local part length exceeded
            $local_errors |= 1;
        } else if ($local[0] == '.' || $local[$localLen - 1] == '.') {
            // local part starts or ends with '.'
            $local_errors |= 2;
        } else if (strpos($local, '..')) {
            // local part has two consecutive dots
            $local_errors |= 4;
        }

	    return $local_errors;
    }

    private static function mail_check_domain($domain) {
        $domain_errors = 0;
        $domainLen = strlen($domain);

        if ($domainLen < 1 || $domainLen > 255) {
            // domain part length exceeded
            $domain_errors |= 8;
        } else if (!preg_match('/^[A-Za-z0-9\\-\\.]+$/', $domain)) {
            // character not valid in domain part
            $domain_errors |= 16;
        } else if (strpos($domain, '..')) {
            // domain part has two consecutive dots
            $domain_errors |= 32;
        } else if (!strpos($domain, '.')) {
            // there is no dot at all?
            $domain_errors |= 64;
        }

	    return $domain_errors;
    }

    private static function mail_check_quoted($local) {
        $quoted_errors = 0;

        if (!preg_match('/^(\\\\.|[A-Za-z0-9!#%&`_=\\/$\'*+?^{}|~.-])+$/', str_replace("\\\\", "", $local))) {
            // character not valid in local part unless
            // local part is quoted
            if (!preg_match('/^"(\\\\"|[^"])+"$/', str_replace("\\\\", "", $local))) {
                $quoted_errors |= 128;
            }
        }

        return $quoted_errors;
    }

    /*
     * Validate an email address.
     * Provide an email address by raw input.
     * Returns 0 (true) if email address has an
     * RFC 2822 / 2821 / 822 conform format
     * and the domain does exist with
     * either A, AAAA or MX records.
     */

    private static function mail_check_dns($domain) {
        $domain_errors = 0;

        if (!(checkdnsrr($domain, "MX") || checkdnsrr($domain, "A") || checkdnsrr($domain, "AAAA"))) {
            // domain not found in DNS
            $domain_errors |= 256;
        }

        return $domain_errors;
    }
}