YACC Program to check whether the arithmetic expression is valid or not

validexp.l

//LEX Program

/* declaration section in this sections we will declare the different value and include the header file which we are using in this program to run this program */

%{
#include"y.tab.h"
extern yylval;
%}

/* defined section */

%%
[0-9]+ {yylval=atoi(yytext); return NUMBER;}   //this is send to the yacc code as token INTEGER
[a-zA-Z]+ {return ID;}                                          //this is send to the yacc code as token  ID
[\t]+ ;
\n {return 0;}
. {return yytext[0];}
%%

  validexp.y 

//yacc program

//decelration section

%{
#include<stdio.h>
%}

//definition section


%token NUMBER ID                       // token from lex file
%left '+' '-'                                           // left associative 
%left '*' '/'
%%

expr: expr '+' expr                                        // grammer production rule 
     |expr '-' expr
     |expr '*' expr
     |expr '/' expr
     |'-'NUMBER
     |'-'ID
     |'('expr')'
     |NUMBER
     |ID
     ;
%%

//main function

main()
{
printf("Enter the expression\n");
yyparse();
printf("\nExpression is valid\n");
exit(0);
}

//if error occured 

int yyerror(char *s)
{
printf("\nExpression is invalid");
exit(0);
}

download the file 

how to run 

    

     save the file  as given above 

     compile the yacc code "yacc -d validexp.y"

     compile the lex code  "flex  validexp.l" 

     now compile c file code  "cc lex.yy.c  y.tab.c  -o validexp  -ll"

     execute the file  "./validexp"