Simple eclipse search problem

I use the eclipse File Search option very much to search all files in my workspace for a certain content. But how do I specify that it should only return hits from a fixed search criteria? As an example I would like to find all occurrences of the string:

com.mystuff.data

but I also get all the hits for:

com.mystuff.data.ui

How do I make a "this-string-only-search" when searching files in my workspace??