$re = '/^https:\/\/[^\/]+\/([^.]*)\.jpg/i';
$str = 'https://mydomainname.net/productimages/1679/T716AP1_lg.jpg?w=125&h=125&tstamp=05/19/2016%2015:08:30
';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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