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/*
 * Return an error message if the given pattern argument or its underlying regular expression
 * are not syntactically valid. Otherwise (if they are valid), false is returned.
 */
function preg_pattern_error($pattern)
{

   /*
    * To tell if the pattern has errors, we simply try to use it.
    * To detect and capture the error is not so simple, especially if we want to be sociable and not
    * tramp on global state (e.g., the value of $php_errormsg). So, if 'track_errors' is on, we preserve
    * the $php_errormsg value and restore it later. If 'track_errors' is not on, we turn it on (because
    * we need it) but turn it off when we're done.
    */
   if ($old_track = ini_get("track_errors"))
       $old_message = isset($php_errormsg) ? $php_errormsg : false; 
   else 
       ini_set('track_errors', 1);
   /* We're now sure that track_errors is on. */

   unset($php_errormsg);
   @ preg_match($pattern, ""); /* actually give the pattern a try! */
   $return_value = isset($php_errormsg) ? $php_errormsg : false;

   /* We've now captured what we need; restore global state to what it was. */
   if ($old_track)
       $php_errormsg = isset($old_message) ? $old_message : false;
   else
       ini_set('track_errors', 0);

   return $return_value;

}