bison - GNU parser generator.

Bison is a general-purpose parser generator that converts an annotated
context-free grammar into an LALR(1) or GLR parser for that grammar.
Once you are proficient with Bison, you can use it to develop a wide
range of language parsers, from those used in simple desk calculators
to complex programming languages.

For more information, visit:

http://www.gnu.org/software/bison/bison.html