The translator package of RubyEx is a C++ Bison/Flex parser written from scratch. It constructs an AST and emits byte-code (there is relevant discussion on byte-code).
This package attempts to parse Ruby source-code in a manner as compatible with the official Ruby implementation as possible, targetting 1.8, with some of 1.9's features. It is barely complete, though the framework is completely set up, and relatively complex constructs are parsed correctly.