PHP正则验证Email的方法

<?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] == '.' || $local[$locallen-1] == '.')
  {
   // local part starts or ends with '.'
   $isvalid = false;
  }
  else if (preg_match('/\.\./', $local))
  {
   // local part has two consecutive dots
   $isvalid = false;
  }
  else if (!preg_match('/^[a-za-z0-9\-\.]+$/', $domain))
  {
   // character not valid in domain part
   $isvalid = false;
  }
  else if (preg_match('/\.\./', $domain))
  {
   // domain part has two consecutive dots
   $isvalid = false;
  }
  else if(!preg_match('/^(\\.|[a-za-z0-9!#%&`_=\/$'*+?^{}|~.-])+$/', str_replace("\\","",$local)))
  {
   // character not valid in local part unless 
   // local part is quoted
   if (!preg_match('/^"(\\"|[^"])+"$/', str_replace("\\","",$local)))
   {
   $isvalid = false;
   }
  }
  if ($isvalid && !(checkdnsrr($domain,"mx") || checkdnsrr($domain,"a")))
  {
   // domain not found in dns
   $isvalid = false;
  }
 }
 return $isvalid;
}
?>