Schlechte Idee: E-Mailadressen mit regex prüfen

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

    if ($localLen &lt; 1 || $localLen &gt; 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 &lt; 1 || $domainLen &gt; 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!#%&amp;`_=\\/$\'*+?^{}|~.-])+$/',
        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;
    }

    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;
    }

    /*
     * Validate an email adress.
     * Provide an email adress by raw input.
     * Returns 0 (true) if email adress has an
     * RFC 2822 / 2821 / 822 conform format
     * and the domain does exist with
     * either A, AAAA or MX records.
     */
    public static function isValid($email)
    {
    $mail_errors = 0;
    $atIndex = strrpos($email, "@");

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

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

    // 1.) Check local part for common errors
    $mail_errors |= self::mail_check_local($local);
    // 2.) Check domain for common errors
    $mail_errors |= self::mail_check_domain($domain);
    // 3.) Check local part for quotation errors
    $mail_errors |= self::mail_check_quoted($local);
    // 4.) Check, if domain exists.
    if ($mail_errors == 0)
    {
        $mail_errors |= self::mail_check_dns($domain);
    }

    return $mail_errors;
    }

}
?>

<?php
require("mailcheck.php"); // your favorite here

function testEmail($email)
{
  echo $email;
  $pass = MailChecker::isValid($email);
  if ($pass == 0)
  {
    echo " is valid.\n";
  }
  else
  {
    echo " is not valid because of $pass\n";
  }

  return $pass;
}

$pass = 0;

echo "All of these should succeed:\n";
$pass |= testEmail("dclo@us.ibm.com");
$pass |= testEmail("abc\\@def@example.com");
$pass |= testEmail("abc\\\\@example.com");
$pass |= testEmail("Fred\\ Bloggs@example.com");
$pass |= testEmail("Joe.\\\\Blow@example.com");
$pass |= testEmail("\"Abc@def\"@example.com");
$pass |= testEmail("\"Fred Bloggs\"@example.com");
$pass |= testEmail("customer/department=shipping@example.com");
$pass |= testEmail("\$A12345@example.com");
$pass |= testEmail("!def!xyz%abc@example.com");
$pass |= testEmail("_somename@example.com");
$pass |= testEmail("user+mailbox@example.com");
$pass |= testEmail("peter.piper@example.com");
$pass |= testEmail("Doug\\ \\\"Ace\\\"\\ Lovell@example.com");
$pass |= testEmail("\"Doug \\\"Ace\\\" L.\"@example.com");

echo "\nAll of these should fail:\n";
$pass |= !testEmail("abc@def@example.com");
$pass |= !testEmail("abc\\\\@def@example.com");
$pass |= !testEmail("abc\\@example.com");
$pass |= !testEmail("@example.com");
$pass |= !testEmail("doug@");
$pass |= !testEmail("\"qu@example.com");
$pass |= !testEmail("ote\"@example.com");
$pass |= !testEmail(".dot@example.com");
$pass |= !testEmail("dot.@example.com");
$pass |= !testEmail("two..dot@example.com");
$pass |= !testEmail("\"Doug \"Ace\" L.\"@example.com");
$pass |= !testEmail("Doug\\ \\\"Ace\\\"\\ L\\.@example.com");
$pass |= !testEmail("hello world@example.com");
$pass |= !testEmail("gatsby@f.sc.ot.t.f.i.tzg.era.l.d.");
$pass |= !testEmail("hello@world");
$pass |= !testEmail("hello@gmail");

echo "\nThe email validation ";

if ($pass == 0)
{
   echo "passes all tests.\n";
}
else
{
   echo "is deficient.\n";
}
?>