Listing 5. Partial Test for Valid Local Part Content

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;
   }
}