i guess you heard about Lucene library which written in java and used
world-wide by most of monster search engines, (wiki)
because of it's features and even efficiency, people started building
libraries of same kind in their native language, one of them is
CLucene(written in c++, i am using it).
So ferrect_index is one of such kind. It's a library basically used
for doing any text based search,
like for example, i have a blog application or search engine, where
the load is huge,
and i want to search the articles either by title, url, link, header,
content, type, etc.etc.,
In such cases you can go for this kind of library. I don't know
whether google search engine using this or not,
but in google search engine you can give some thing like intitle:
just shared whatever i know, sorry if it's not answering you question.