Can anyone recommend any open-source faceted search tools?
Bonus for:
- being:
- easy to install (via yum, apt-get, brew, etc)
- accessible from the command line / shell
- accessible as a library
- actively developed
- not requiring (though allowing) an external database
- allowing search through regular expressions
- emacs integration
Both ElasticSearch and Solr are services, and both are built from Apache Lucene, a set of Java libraries providing indexed search, with facets and many other features. Xapian is a C++ library with similar functionality, although it doesn't seem to have the same level of popularity as Lucene.
Solr is something you can implement faceted search with, but I'm not sure what you'd want from emacs integration...
Faceted search typically helps you drill down to find a set of objects or other discrete entities; not really for searching within such an entity.
In other words you need to have a set of objects/entities that are already fairly well-structured or have reasonably coherent attributes on them from which to build the facets.
In the worst-case, a faceted search view of your computer's filesystem might only be able to show you facets based on datetimes (created/modified), filesize, filename extension, permissions etc. but nothing about the content of the files.
If you've drilled down to a collection of .mp3 files, your faceted search engine might then be able to offer facets based on MP3 tag data.
A good example is RS Components and Digi-Key. I can't pull one up now, but sometimes I find that these suppliers won't show you many facets until you've drilled down into a subset of their product range that has a common set of attributes - at which point you can suddenly filter by current rating, voltage, MTBF, etc. whereas at some broader category view you might only have brand-name, stock status and lead-time facets.
Apache Solr is also extremely good. I use it at work for enterprise search and know it meets all but your last requirement (emacs integration) but a search for "solr emacs integration" suggests even that is possible.
Bonus for: