Lex (software)

Lex (software)

Lex is a computer program that generates lexical analyzers ("scanners" or "lexers"). Lex is commonly used with the yacc parser generator. Lex, originally written by Mike Lesk and Eric Schmidt and described in 1975, is the standard lexical analyzer generator on many Unix systems, and a tool exhibiting its behavior is specified as part of the POSIX standard.

Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language.

Though originally distributed as proprietary software, some versions of Lex are now open source. Open source versions of Lex, based on the original AT&T code are now distributed as open source systems such as OpenSolaris and Plan 9 from Bell Labs. One popular open source version of Lex, called flex, or the "fast lexical analyzer", is not derived from proprietary code.