searching for specific strings

Hello,

I need to be able to do some searches from my access.log. The partial string
in the log that shows us when a customer is searching for an item looks like
this: substring=searchitem.

When a search is not performed the string looks like this: substring=" ".

How can I do a search and have the search not match substring=" ". I need it
to only match when there is a value there instead of the " ".