kpeg

https://github.com/evanphx/kpeg

KPeg is a simple PEG library for Ruby. It provides an API as well as native grammar to build the grammar. KPeg strives to provide a simple, powerful API without being too exotic. KPeg supports direct left recursion of rules via the {OMeta memoization}[http://www.vpri.org/pdf/tr2008003_experimenting.pdf] trick.