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.