$re = '/<li>(?:.(?!</li>))+Code:(?:.*?)</li>/is';
$str = '<ul><li><span>some words here.</span></li><li><span>other words here.</span></li><li><span>Code: 55555.</span></li></ul>';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php