$re = '/\/a\/b\/c\/([^\W_]+)(?:_([0-9]+))?\/p1=(.*)\/p2=(.*)\/p3=(.*)\/(.*)/m';
$str = '/a/b/c/d_9000/p1=<val1>/p2=<val2>/p3=<val3>/<val4>
/a/b/c/d/p1=<val1>/p2=<val2>/p3=<val3>/<val4>';
preg_match_all($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