requires an external lexical analyzer to perform the first tokenization stage (word analysis), which is then followed by the parsing stage proper. Lexical analyzer generators, such as Lex or Flex, are widely available. The IEEE POSIX P1003.2 standard defines the functionality and requirements for both Lex and Yacc. Some versions of AT&T Yacc have become open source. For example, source code is available with the standard distributions of Plan 9. Impact Yacc and similar programs (largely reimplementations) have been very popular. Yacc itself used to be available as the default parser generator on most Unix systems, though it has since been supplanted by more recent, largely compatible, programs such as Berkeley Yacc, GNU Bison, MKS Yacc, and Abraxas PCYACC. An updated version of the original AT&T Yacc is included as part of Sun's OpenSolaris project. Each offers slight improvements and additional features over the original Yacc, but the concept and basic syntax have remained the same. Among the languages that were first implemented with Yacc are AWK, C++, eqn and Pic. Yacc was also used on Unix to implement the Portable C Compiler, as well as parsers for such programming languages as FORTRAN 77, Ratfor, APL, bc, m4, etc. Yacc has also been rewritten for other languages, including OCaml, Ratfor, ML, Ada, Pascal, Java, Python, Ruby, Go, Common Lisp and Erlang. Berkeley Yacc: The Berkeley implementation of Yacc quickly became more popular than AT&T Yacc itself because of its performance and lack of reuse restrictions. LALR parser: The underlying parsing algorithm in Yacc-generated parsers. Bison: The GNU version of Yacc. Lex (and Flex lexical analyser), a token parser commonly used in conjunction with Yacc (and Bison). BNF is a metasyntax used to express context-free grammars: that is a formal way to describe context-free languages. PLY (Python Lex-Yacc) is an alternative implementation of Lex and Yacc in Python. See also Compiler-compiler hoc (programming language) References External links Compiling tools Parser generators Unix programming tools Unix SUS2008 utilities Plan 9 commands Inferno (operating system) commands