Lex programming tool

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 an equivalent tool 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 coding.

The structure of a Lex file is intentionally similar to that of a yacc file; files are divided into three sections, separated by lines that contain only two percent signs, as follows

The following is an example Lex file for the flex version of Lex. It recognizes strings of numbers (positive integers) in the input, and simply prints them out.

If this input is given to flex, it will be converted into a C file, lex.yy.c. This can be compiled into an executable which matches and outputs strings of integers. For example, given the input:

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