A simple program of LEX

/** Definition section **/

%{
#include <stdio.h>
%}

%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 other characters. */   }

%%
/** C Code section **/

int main(void)
{
    /* Call the lexer */
    yylex();
    return 0;
}

steps to execute

1. lex filename.l

2. cc ley.yy.c

3. ./a.out

<<<THEORY ABOUT LEX PROGRAM>>> 

1)check whether flex or flex-old is installed or not

2)if it is not installed use following command

sudo apt-get install flex

structure of lex file

it contains 3 diff. sections as follows:

1)Definition section which written in %{.....%}

2)Rule Section which is enclosed between %%....%%

it is assosiated with regular expression patterns with C statements.

3)Code section

which contains C statements and functions required to execute Lex file.

Execution of lex program

By using structure of lex program create lex file with .l as extension.

Step-1: lex filename.l

it will check whether lex program is lexically correct or not.

If it contains some errors we need to recompile the file using the same command.

Step-2: cc lex.yy.c

it converts the lex program into a C program name as lex.yy.c

This lex.yy.c file is executable

Step-3: ./a.out

This command will execute lex.yy.c file.

Metachar	Matches
.	Any character except new line.
/n	New line.
*	Zero or more copies of preceding expression
+	One or more copies of preceding expression
?	Zero or one copies of preceding expression
$	End of line
(ab)+	One or more copies of ab together
^	Beggining of line.
“a+b”	Literal a+b
[]	Character class.

                                   Table: pattern matching premitives

expression	Matches
abc	a,b,c
abc*	ab,abc,abcc,abccc ......
abc+	abc,abcc......
a(bc)+	abc,abcbc,abcbcbc.....
abc	one of (a b c)
a/z	Any character from a to z.
a(bc)^?	abc,a

Table:pattern matching Examples