Handling popup whose value in the URL changes

Question

I'm using WatiN testing tool. How to handle the popup whose value in the URL changes each time the popup appears. In the below URL http://localhost/admin/ILT/SessionsPick.aspx?Pick_Sessions=1f37d165-c64b-4371-9060-9ebed89605eb&opener=ILT_NewTraining.aspx&TableId=undefined&TrainingId=undefined

in this URL the value of Pick_sessions= "1f37d165-c64b-4371-9060-9ebed89605eb" changes each time the popup appears.SO please suggest some solution for it. I tried replacing the value with regular expression like this one

pick session= [a-f0-9]{8}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{4}-[a-f0-9]{12}/i} and i used many other regex so please suggest some solution.