$re = '/(?:<span data-prefix="([^"]+)">([^<]+)<\/span>)+/';
$str = '<div class="set">
<span data-prefix="[1]">Duck</span>
<span data-prefix="[2]">Dog</span>
<span data-prefix="[3]">Cat</span>
</div>';
preg_match_all($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