arithmetic.l

// lex file
%{ 
	/* Definition section*/
	#include "y.tab.h" 
	extern yylval;
%}

%% 
[0-9]+ { 
			yylval = atoi(yytext); 
			return NUMBER; 
			} 

[a-zA-Z]+ { return ID; } 
[ \t]+		 ; /*For skipping whitespaces*/

\n		 { return 0; } 
.		 { return yytext[0]; } 

%% 


// yacc file


%{
	/* Definition section */
#include <stdio.h>
%}

%token NUMBER ID
// setting the precedence 
// and associativity of operators 
%left '+' '-'
%left '*' '/'

/* Rule Section */
%% 
E : T	 {
				printf("Result = %d\n", $$);
				return 0;
			}

T : 
	T '+' T { $$ = $1 + $3; }
	| T '-' T { $$ = $1 - $3; }
	| T '*' T { $$ = $1 * $3; }
	| T '/' T { $$ = $1 / $3; }
	| '-' NUMBER { $$ = -$2; }
	| '-' ID { $$ = -$2; }
	| '(' T ')' { $$ = $2; }
	| NUMBER { $$ = $1; }
	| ID { $$ = $1; };
% %

int main() {
	printf("Enter the expression\n");
	yyparse();
}

/* For printing error messages */
int yyerror(char* s) {
	printf("\nExpression is invalid\n");
}