$re = '~NEM12([^<]*(?:<(?!/CSVIntervalData>)[^<]*)*)</CSVIntervalData>~';
$str = '<.CSVIntervalData>100,NEM12,201807290900,WBAYM,EEQ 200,3030910307,B1E1K1Q1,03,B1,N1,91111580,kWh,30, 300,20180728,.278,.278,.278,.278,.278,.278,.278,.278,.278,.278,.278,.278,.278,.056,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,.074,.278,.278,.278,.278,.278,.278,.278,.278,.278,.278,.278,.278,.278,E75,,,20180729000320, 900 </CSVIntervalData>';
preg_match_all($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