Editing Lex (software) (section)

==Example of a Lex file==
The following is an example Lex file for the [[Flex lexical analyser|flex]] version of Lex. It recognizes strings of numbers (positive integers) in the input, and simply prints them out.

<syntaxhighlight lang="c">/*** 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. */
    yylex();
    return 0;
}</syntaxhighlight>

If this input is given to <code>flex</code>, it will be converted into a C file, {{nowrap | <code>lex.yy.c</code>}}. This can be compiled into an executable which matches and outputs strings of integers. For example, given the input:
 abc123z.!&*2gj6
the program will print:
 Saw an integer: 123
 Saw an integer: 2
 Saw an integer: 6