Checking for and inserting strings into a POST

Yeah I am not looking to replace any data I am looking to insert a field into the post that may be missing:

 

 

This iRule will need to scan POST data from /thispage.php and POST data from /overhere/thispage.php. If "&thisParam=" is not present in the POST data the iRule will need to insert "&thisParam=" to the post data.

 

 

I can see how to replace data fairly easily, but I am unsure as to how to look for data and then insert. Keep in mind that the post fields may be different every time, some may already have &thisParam= and others will not.

 

 

Thanks!