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GNU bison is a parser generator that is part of the GNU project. Bison converts a grammar description for a context-free grammar into a C, C++, or Java program which can parse a sequence of tokens that conforms to that grammar (a LALR parser). It can also produce “Generalized Left-to-right Rightmost” (GLR) parsers for ambiguous grammars.
In POSIX mode, Bison is compatible with Yacc, but it offers several improvements over this earlier program. It is sometimes used in conjunction with flex, an automatic lexical analyser.
Bison is distributed as free software and is available in source code form. The output from bison used to also be protected under the GPL, due to the inclusion of the yyparse() function from the original source code in the output. However, an exception was made, to allow proprietary use of the output.