Filtering RSS Feeds/News

So, I need to build an application which will grab some news from some sources, but only news containing an specific query such as "Iraq war", and display in my main page.

I tried using Scrapi, Nokogiri, Scrubyt, Mechanize, but with no success (maybe cause I'm a RoR newbie?). So I went to Feedzirra and the likes, but I couldn't find a way to filter the feeds results for a specific query.

So, what is the best/easiest/cleaner way to do this?