Regex: Insert a value in a capture group based on a conditional match?

Question

First, this topic is a general regex question, but applies to a specific product - VirtualURL.NET (.NET HttpModule). I am using this product to do regex URL rewriting.

I have a situation which I am attempting to accomplish in a single regex rule. I am matching against the following URLs:

~/directory/page.aspx?id=idval

~/directory/page.aspx?id=idval&key1=value1&key2=value2

with the expression:

~/directory/page.aspx\?id=(?'ID'[a-zA-Z0-9_%\-+\.]*)(?'Delimiter'&*)(?'QueryString'(?(Delimiter).*|\s))

The result is for the first URL, the Delimiter capture group is empty and not needed; however, for the second URL, the Delimiter group contains an ampersand. When I supply the statement the product uses for replacement based on the capture groups, I need the Delimiter group to be a question mark, such that the resulting URLs are (respectively):

~/newdirectory/idval

~/newdirectory/idval?key1=value1&key2=value2

The output with the expression as is with the product is (as expected):

~/newdirectory/idval? <-- I want to remove the ? from this output

~/newdirectory/idval?key1=value1&key2=value2

In the case where additional querystring elements beyond 'id' are not in the original URL, I am asking if it is possible to check the condition of a capture group in the regex statement and insert a value in the capture group (i.e. if the & is captured, replace it with a ?; if it's not captured, leave blank). Is there a way in the regex statement itself to replace/append to the contents of a capture group?

NOTE: With the use of this product, I am not directly calling the Regex.Replace method - I am supplying the Regex match and replacement statements to the product. So for the above regex, my replacement statement is:

~/newdirectory/${ID}?${QueryString} <-- I want to remove the hard-coded ? in this statement.

I know attempting to solve this in regex may not be the best solution, but I am trying to work within the constraints of using a third-party product. Additionally, splitting this single rewriting rule into two rules is not desirable - I have 8 other rules that need to handle the ID field in this same manner, and I'd like to avoid doubling my ruleset.

Thanks, JJ