regexp-engine is a regexp engine written entirely in Ruby that makes it possible to search in text with advanced search patterns. It supports Perl 5 and some Perl 6 syntax and is fairly compatible with Ruby's native regexp engine, passing almost 98% of the Rubicon tests. The implementation is simple and contains no optimizations.
deplate converts wiki-like markup to LaTeX (standard classes, koma, dramatist, sweave), HTML/PHP (single page, chunked/website, HTML, or s5-based slideshow), DocBook (article, book, man/ref page), and really plain text. Currently supported input formats are viki and Ruby's rdoc. The viki markup supports footnotes, citations, index, table of contents, embedded LaTeX for mathematics, integration with R for dynamically generated figures and tables, and more. Output can be customized via page templates.