$re = '/\",\"thumb\":\"(http.:[^"]*)\"/m';
$str = '","thumb":"https://images-na.ssl-images-amazon.com/images/I/51azLW7w-TL._US40_.jpg","large';
preg_match_all($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