var oldSid = new RegExp('sid=[0-9]{1,5}x[0-9]{1,5}');
var newSid = 'sid=500x500';
var oldUrl = 'media.php?sid=200x100&mid=100x100';
var newUrl = oldUrl.replace(oldSid, newSid);
print(newUrl);
dmFyIG9sZFNpZCA9IG5ldyBSZWdFeHAoJ3NpZD1bMC05XXsxLDV9eFswLTldezEsNX0nKTsgCnZhciBuZXdTaWQgPSAnc2lkPTUwMHg1MDAnOwp2YXIgb2xkVXJsID0gJ21lZGlhLnBocD9zaWQ9MjAweDEwMCZtaWQ9MTAweDEwMCc7CnZhciBuZXdVcmwgPSBvbGRVcmwucmVwbGFjZShvbGRTaWQsIG5ld1NpZCk7CnByaW50KG5ld1VybCk7