Two popular parser generators have or will have ruby support:

ANTLR 3.0 (still in alpha and likely to remain so for another year or
two) will have ruby support too apparently:
http://split-s.blogspot.com/2005/12/antlr-for-ruby.html
They are even working on an antlr 3.0 grammar for ruby itself:
http://rubyforge.org/projects/rubygrammar

Also (not in alpha), there is a ruby version of Coco:
http://www.ssw.uni-linz.ac.at/Research/Projects/Coco/
http://www.zenspider.com/ZSS/Products/CocoR/
http://rubyforge.org/projects/coco-ruby/