maybe a bug

line 2770:

protected function isXMLish($string) {
// Long strings will exhaust the regex engine, so we
// grab a representative string.
$test = substr($string, 0, 255);
return (strpos($string, '<') !== FALSE && strpos($string, '>') !== FALSE);
//return preg_match(ML_EXP, $test) > 0;
}