Yes, let's say that the dimension name is "URL."
You would create a calculated field and enter the following:
=LEFT([URL], FIND("?", [URL])-1)
This looks in the URL string for where the ? is located, then returns all the characters to the left of that position.
Note that this will always only return what is to the left of the first ? in your string.
Thanks. It looks like this is only returning a null value for all urls.