$re = '/Insurance.*\[(\d+)\]/m';
$str = '<td class="th-clr-cel th-clr-td th-clr-pad th-clr-cel-dis" style="width:226px; text-align:left; ">
<span class="th-tx th-tx-value th-tx-nowrap" style="width:100%; " title="Social Insurance Number (SIN)" id="C29_W120_V121_builidnumber_table[5].type_text" f2="C;40">
Social Insurance Number (SIN)
</span>
';
preg_match_all($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