Finding keywords in a sentence

I'm trying to implement few algorithms for measuring similarity between two sentences. I think it would be helpful if I had some kind of expanded representation of a sentence.

We could get this expanded representation with google/bing search, but putting whole sentence as a keyword doesn't give 开发者_如何学编程good results, especially if sentence is long. So I was thinking that finding keywords in a sentence and using them as search terms would improve our results.

Anybody aware of an algorithm for finding keywords in a sentence?

Thank you!

Lucene can handle that extraction as well as the comparison.