I want to replace a url querystring parameter with a new value if it already exists or add it on if not.

e.g.

The current url could be:
a. www.mysite.com/whatever.asp?page=5&version=1 OR
b. www.mysite.com/whatever.asp?version=1

I need the resulting url to be www.mysite.com/whatever.asp?page=1&version=1

I suspect I can use string.replace with a regex to do this the most intelligent way but am hoping for a little help with it from someone more experienced with regexs :) Thanks!