Rats! is a parser generator for C-like languages (though currently it only generates parsers in Java). It has been designed so that grammars are concise and easily extensible. To this end, grammars are organized into modules that can be easily changed by adding, removing, or modifying alternatives in individual productions and composed with each other through module parameters. Furthermore, it relies on ordered choices to avoid ambiguities, supports syntactic predicates for unlimited lookahead, and integrates lexing with parsing. Finally, it has extensive support for automatically generating abstract syntax trees, thus greatly reducing the need for explicit semantic actions.
Release highlights are support for formatting-preserving source refactorings through parse trees, support for parsing and pretty printing Java 5, (an improved abstract syntax tree for Java, and improved support for type checking C and Java ASTs.