Original author(s)Mike Lesk, Eric Schmidt
Initial release1975; 48 years ago (1975)
Written inC
Operating systemUnix, Unix-like, Plan 9
LicensePlan 9: MIT License

Lex is a computer program that generates lexical analyzers ("scanners" or "lexers").[1][2]

Lex is commonly used with the yacc parser generator. Lex, originally written by Mike Lesk and Eric Schmidt[3] and described in 1975,[4][5] is the standard lexical analyzer generator on many Unix systems, and an equivalent tool is specified as part of the POSIX standard.[6]

Lex reads an input stream specifying the lexical analyzer and writes source code which implements the lexical analyzer in the C programming language.

In addition to C, some old versions of Lex could generate a lexer in Ratfor.[7]

Open source

Although originally distributed as proprietary software, some versions of Lex are now open-source. Open-source versions of Lex, based on the original proprietary code, are now distributed with open-source operating 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.

Structure of a Lex file

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:

Example of a Lex file

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.

/*** Definition section ***/

/* C code to be copied verbatim */
#include <stdio.h>

    /*** Rules section ***/

    /* [0-9]+ matches a string of one or more digits */
[0-9]+  {
            /* yytext is a string containing the matched text. */
            printf("Saw an integer: %s\n", yytext);

.|\n    {   /* Ignore all other characters. */   }

/*** C Code section ***/

int main(void)
    /* Call the lexer, then quit. */
    return 0;

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:


the program will print:

Saw an integer: 123
Saw an integer: 2
Saw an integer: 6

Using Lex with other programming tools

Using Lex with parser generators

Lex and parser generators, such as Yacc or Bison, are commonly used together. Parser generators use a formal grammar to parse an input stream, something which Lex cannot do using simple regular expressions, as Lex is limited to simple finite state automata.[clarification needed]

It is typically preferable to have a parser, one generated by Yacc for instance, accept a stream of tokens (a "token-stream") as input, rather than having to process a stream of characters (a "character-stream") directly. Lex is often used to produce such a token-stream.

Scannerless parsing refers to parsing the input character-stream directly, without a distinct lexer.

Lex and make

make is a utility that can be used to maintain programs involving Lex. Make assumes that a file that has an extension of .l is a Lex source file. The make internal macro LFLAGS can be used to specify Lex options to be invoked automatically by make.[8]

See also


  1. ^ Levine, John R.; Mason, Tony; Brown, Doug (1992). lex & yacc (2 ed.). O'Reilly. pp. 1–2. ISBN 1-56592-000-7.
  2. ^ Levine, John (August 2009). flex & bison. O'Reilly Media. p. 304. ISBN 978-0-596-15597-1.
  3. ^ Lesk, M.E.; Schmidt, E. "Lex – A Lexical Analyzer Generator". Archived from the original on 2012-07-28. Retrieved August 16, 2010.
  4. ^ Lesk, M.E.; Schmidt, E. (July 21, 1975). "Lex – A Lexical Analyzer Generator" (PDF). UNIX TIME-SHARING SYSTEM:UNIX PROGRAMMER’S MANUAL, Seventh Edition, Volume 2B. bell-labs.com. Retrieved Dec 20, 2011.
  5. ^ Lesk, M.E. (October 1975). "Lex – A Lexical Analyzer Generator". Comp. Sci. Tech. Rep. No. 39. Murray Hill, New Jersey: Bell Laboratories.
  6. ^ The Open Group Base Specifications Issue 7, 2018 edition § Shell & Utilities § Utilities § lex
  7. ^ John R. Levine; John Mason; Doug Brown (1992). Lex & Yacc. O'Reilly.
  8. ^ "make". The Open Group Base Specifications. The IEEE and The Open Group (6). 2004. IEEE Std 1003.1, 2004 Edition.