Here's the command I popped into the Terminal last year:

defaults write com.iconfactory.Twitterrific tweetTextFilter -string "McCain|Obama"

The first portion of that command tells you that you're setting Twitterrific's secret tweetTextFilter preference. The last, quoted part is the actual filter. And here's your big piece of grep knowledge: you use the pipe character (|) to mean "or." So in this case, McCain|Obama means, eliminate any tweet with the word McCain or the word Obama in it.

Similarly, if there's someone who sends you messages on Twitter that you'd rather not see, but whom you don't wish to block, you can filter those messages out via the tweetScreenNameFilter setting, all with the same format.

defaults write com.iconfactory.Twitterrific tweetScreenNameFilter -string "dude109|guy19"