$re = '`<span[^>]+class="[^"]*\btxt_arial_8pt_black\b[^"]*"[^>]*>(.*)</span>`s';
$str = '<span class="txt_arial_8pt_black">Antes <span>Durante</span> Depois</span>
<span class="txt_arial_8pt_black">outro</span><br>';
preg_match_all($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