The parser groups tokens into syntactical units. The output of the parser is a parse tree representation of the program. Context-free grammars are used to define the program structure recognized by a parser. The parser is implemented as a push-down automata.

Yacc and Bison are tools for generating bottom-up parsers in C. Bison is a faster version of Yacc. Jack is a tool for generating scanners and top-down parsers in Java.