FWIW we will likely open source our search engine/IR technology sometime next month (hold up is the documentation and cleaning up some parts of the codebase).

It's written in C++, and the core features are performance, simplicity(operation and extension of functionality) and a sane, simple API (the codebase is quite small as well, at about 14k lines of code).

Its similar in some regards to Lucene, but does away with most of the exotic features and instead is optimised for rich queries support, fast execution and extensibility.

It should be on GH sometime in April.