lex/flex Tutorial

By Xah Lee. Date: . Last updated: .

warning: work in progress.

Structure of a Lex file

files are divided into three sections, separated by lines that contain only two percent signs, as follows:

Definition section
Rules section
C code section

Example of a Lex file

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

/*** Definition section ***/

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

/* This tells flex to read only one input file */
%option noyywrap

    /*** 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.

It is typically preferable to have a (Yacc-generated, say) parser be fed a token-stream as input, rather than having it consume the input 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.[6]

above is from or based on [ Lex (software) ] [ 2016-01-21 https://en.wikipedia.org/wiki/Lex_%28software%29 ] .

Lex Manual and Tutorial

in emacs, Ctrl+h i to see the full detailed manual that is also tutorial.