PHP-Beispielcode zur E-Mail-Verifizierung mithilfe regulärer Ausdrücke

<?php
function validateEmail($email)
{
 $isValid = true;
 $atIndex = strrpos($email, "@");
 if (is_bool($atIndex) && !$atIndex)
 {
  $isValid = false;
 }
 else
 {
  $domain = substr($email, $atIndex+1);
  $local = substr($email, 0, $atIndex);
  $localLen = strlen($local);
  $domainLen = strlen($domain);
  if ($localLen < 1 || $localLen > 64)
  {
   // local part length exceeded
   $isValid = false;
  }
  else if ($domainLen < 1 || $domainLen > 255)
  {
   // domain part length exceeded
   $isValid = false;
  }
  else if ($local[0] == &#39;.&#39; || $local[$localLen-1] == &#39;.&#39;)
  {
   // local part starts or ends with &#39;.&#39;
   $isValid = false;
  }
  else if (preg_match(&#39;/\\.\\./&#39;, $local))
  {
   // local part has two consecutive dots
   $isValid = false;
  }
  else if (!preg_match(&#39;/^[A-Za-z0-9\\-\\.]+$/&#39;, $domain))
  {
   // character not valid in domain part
   $isValid = false;
  }
  else if (preg_match(&#39;/\\.\\./&#39;, $domain))
  {
   // domain part has two consecutive dots
   $isValid = false;
  }
  else if(!preg_match(&#39;/^(\\\\.|[A-Za-z0-9!#%&`_=\\/$\&#39;*+?^{}|~.-])+$/&#39;, str_replace("\\\\","",$local)))
  {
   // character not valid in local part unless 
   // local part is quoted
   if (!preg_match(&#39;/^"(\\\\"|[^"])+"$/&#39;, str_replace("\\\\","",$local)))
   {
   $isValid = false;
   }
  }
  if ($isValid && !(checkdnsrr($domain,"MX") || checkdnsrr($domain,"A")))
  {
   // domain not found in DNS
   $isValid = false;
  }
 }
 return $isValid;
}
?>