Splitting Transactions Conditionally

I am working on configuring business transactions for an application that uses one constant URI for all of its pages.  I have had some success splitting transactions using a custom expression on the HttpRequest object.  However, there are some occasions where the site passes my target parameter with a different name.  Is there any way to add logic to the expression so I can perform a check to see which parameter exists and select the one that does?

For example, one of the following expressions will return my desired value from the HttpRequest depending on the page displayed.

${Form.Get(parmName1).Split(char[]/_).[1]}

${Form.Get(parmName2).Split(char[]/_).[1]}

I would like to be able to do:

if ${Form.Get(parmName1).Split(char[]/_).[1]} exists

then return ${Form.Get(parmName1).Split(char[]/_).[1]}

else return ${Form.Get(parmName2).Split(char[]/_).[1]}

Is logic possible inside of custom expressions?  Thanks for the help.