$re = '/(?<=\bGASTEC).*(?:NO\s|,\s)(?<id>\b[1-9][ABHLMT][ABLMT]{0,1}|[1-9][0-9]{1,2}\s?[^-Pm ][BDLPT]{0,2})(?:\s|,).*$/mi';
$str = 'GASTEC TUBE, BENZENE 1-10 PPM, 121L, //MUST MATCH
121L TUBE, BENZENE 1-10 PPM, GASTEC, //MUST MATCH <<<<-----
GASTEC TUBE FOR H2S, 0.5-15 PPM, //NO MATCH
GASTEC TUBE FOR CO, 1-25 PPM,//NO MATCH
KITAGAWA 128SD//NO MATCH
GASTEC TUBE FOR ETHYL ALCOHOL NO 112L , 100-2000 PPM,//MUST MATCH
GASTEC TUBES NO 111 METHYL ALCOHOL QC NO: 80777 MEASURING RANGE: 0.02-4.5% SCALE RANGE: 0.02-1.5%//MUST MATCH
GASTEC TUBES NO 4H BENZENE QC NO: 90985 MEASURING RANGE: 0.2-66 PPM SCALE RANGE: 0.2-20PPM//MUST MATCH
MSA TUBES NO 111 METHYL ALCOHOL QC NO: 80777 MEASURING RANGE: 0.02-4.5 SCALE RANGE: 0.02-1.5 (2)//NO MATCH
GASTEC TUBES NO 121SP BENZENE QC NO: 90985 MEASURING RANGE: 0.2-66 PPM SCALE RANGE: 0.2-20PPM (1)//MUST MATCH
GASTEC TUBE FOR BENZENE (RANGE 0.1-10 PPM) , 5 TEST S EACH PACKET, //NO MATCH
DRAGER TUBES NO 121SP BENZENE QC NO: 90985 MEASURING RANGE: 0.2-66 PPM SCALE RANGE: 0.2-20PPM 1 //NO MATCH
TOXIC GAS DETECTION TUBES, GASTEC General Deck Stores, Man: IMPA //NO MATCH
';
preg_match_all($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