$re = '/<span.*?>(.*)<\/span>/';
$str = '<span class=comment>First block with <span class=nest>nested</span> text.</span>
<stuff>
<more-badly-formatted-tags>
<td - out of nowhere>
<span class=comment>Other block with <span class=nest>nested</span> text.</span>';
preg_match_all($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