parslet is a Parsing Expression Grammar based[1] parser generator 
library. Uff. Now that is out of our system, here goes what it really 
does: It makes writing parsers pleasant for the rest of us. No code 
generation, clear access to data, unit testable.

* http://kschiess.github.com/parslet/index.html
* https://github.com/kschiess/parslet

Installation:

gem install parslet

Changes:

* For deep grammars, this release is several orders of magnitude
   faster. Really! Benchmark at [2].
* Export to citrus and treetop dialects.
* You can apply visitors to the grammar now.
* Bug fixes and code cleanup.

Synopsis:

   require 'parslet'
   class Mini < Parslet::Parser
     rule(:integer) { match('[0-9]').repeat(1) }
     root(:integer)
   end

   Mini.new.parse("132432") # => 132432

Please, ladies and gents, amuse yourselves!

[1] http://en.wikipedia.org/wiki/Parsing_expression_grammar
[2] http://blog.absurd.li/2011/02/02/parslet_and_its_friends.html