$re = '/\b(?<pat>[whlb])(?:\s*x\s*(?<pat>[whlb]))+\b/m';
$str = 'The measurements are (w x h x l): 5x7x3cm
The measurements, in form wxhxl: 5x7x3cm
Measurement options are (wxhxl), (hxlxb): Some random stuff
w x h x l x l x h';
preg_match_all($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