Pegarus is, broadly, an implementation of LPEG on Rubinius. LPEG implements a Parsing Expression Grammar using a parsing machine rather than the Packrat algorithm. (See "A Text Pattern-Matching Tool based on Parsing Expression Grammars" by Roberto Ierusalimschy.) Pegarus actually implements an abstract syntax tree (AST) for the PEG. There are various options to execute the AST against a subject string. One option is a simple AST-walking evaluator. A second option is an implementation of the LPEG parsing machine. A third option is a compiler that targets Rubinius bytecode. 1. Running the specs Pegarus uses the mspec gem because of the facilities for tags and easily defining custom matchers and guards, since Pegarus targets more than one Ruby implementation. To run the specs, use the following steps: 1. Install the mspec gem 2. If you plan to use Rubinius, ensure 'rbx' is on your path 3. Run 'rake' to run the specs with $RUBY or 'rbx' 4. Set the environment variable RUBY to the name of the Ruby to use