Getfilename-GetExtension

 Here is simple way to get file name or extension of the file at specified string.

you need to use System.IO namespace

e.g.

 string path =  "http://revenmerchantservices.com/default.aspx";

string fileName  = System.IO.Path.GetFileName(path));

To get extension you can use GetExtension function

Now, you learned this technique hope you will use it instead of writting your own logic.