How to filter RSS feed items?

How to filter RSS items and leave only certain items that I want?

Rules may be like:

satisfies all or any rules { contains keyword; does not contain keyword } or etc.

Two sServices that I find working well for me:

Yahoo Piples: https://pipes.yahoo.com/pipes/</s>

Pipes is a powerful composition tool to aggregate, manipulate, and mashup content from around the web. It can filter feeds and do much more that that such as combining content, sorting content, publishing in RSS or JSON. (Yahoo Pipes was shut down on Sep. 30, 2015)

Feed Rinse: http://www.feedrinse.com/

Feed Rinse you automatically filter out syndicated content that you aren’t interested in