[xapian](http://xapian.org/) engine, its
[omega](http://xapian.org/docs/omega/overview.html) frontend, and the
[[!cpan Search::Xapian]], [[!cpan Digest::SHA]], and [[!cpan HTML::Scrubber]]
-perl modules.
+perl modules (on debian, check that you have packages `xapian-omega`, `libsearch-xapian-perl`, `libdigest-sha-perl` and `libhtml-scrubber-perl` installed).
The [[ikiwiki/searching]] page describes how to write search queries.