$re = '/^([^,]+,){10}(?<YourNewField>([^_]+_)+)/';
$str = 'RSN,interstitial/live_rsn_desktop_live ,Autozone/RSN_RSN_372462,Autozone/RSN_900014269,DIGITAL- 4Q17-2Q18 NBA Lakers Streaming_101917-042218_Live Stream,Autozone/RSN_ZONA1801_RSN,RSN APP,73369465,RSNAPP_LIVE,XXXXXXXXXXXX Network,Autozone/RSN_RSN_Live Stream,2/15/2018,620';
preg_match_all($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