How do you do so that when you search for "alien vs predator" you also get results with the string "alienS vs predator" with the "S"
example http://www.torrentz.com/search?q=alien+vs+predator
how have they implemented this?
is this advanced search engine stuff?
This is known as Word Stemming. When the text is indexed, words are "stemmed" to their "roots". So fighting becomes fight, skiing becomes ski, runs becomes run, etc. The same thing is done to the text that a user enters at search time, so when the search terms are compared to the values in the index, they match.
The Lucene project supports this. I wouldn't consider it an advanced feature. Especially with the expectations that Google has set.