A search landed me at this page, but it wasn't quite what I was looking for. Posting here in case someone else looking for what I was lands at this page too.
There is two ways to do it if you only have a string value.
.NET way:
Same as @Canavar, but you can instantiate a new Uri Object
String URL = "http://localhost:1302/TESTERS/Default6.aspx";
System.Uri uri = new System.Uri(URL);
which means you can use the same methods, e.g.
string url = uri.AbsoluteUri;
// http://localhost:1302/TESTERS/Default6.aspx
string host = uri.host
// localhost
Regex way: