$re = '/Size: (?:\d*X[SL]|L|M|S|\d+W)(?:-(?:\d*X[LS]|L|M|S|\d+W))*(?:,|$)/m';
$str = 'Athletics, Fitted, Mesh, Feature_Moisture Wicking, Material_Polyester 100%, , Material_Polyester 100%, Material_Polyester Over 50%, School, Style_Short Sleeves, Size_2XL, Size_L, Size_M, Size_S, Size_XL, Size_XS, Size: XS-S-M-L-XL-2XL, Uniforms, Unisex, V-Neck, VisibleLogos, Youth
Size: 28W-30W-32W-34W-36W-38W-40W,
Size: 28W-30W-32W-34W
Size: 28W-30W-32W-34W-36W-38W-40W, Size: 28W-30W-32W-34W';
preg_match_all($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