$re = '/(?<=^[A-RT-Z]*S[A-Z]*,[^,\n]*,)[^,\n]+(?=,)/m';
$str = 'BB,21,1.750000,0.000000,
AS,21.5,3.250000,-0.187500
BS,21,3.250000,-0.187500
CS, ,1.750000,-0.375000
DS,1,30.375000,-0.375000
QA,aa,30.375000,-0.375000';
preg_match_all($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