===== yacc_exp.y =====
%{
#include <stdio.h>
#include <stdlib.h>
int valid = 1;
int yylex();
int yyerror(char *s);
%}
%token num id
%left '+' '-'
%left '*' '/'
%%
E : E '+' E
  | E '-' E
  | E '*' E
  | E '/' E
  | '-' E
  | '(' E ')'
  | num
  | id
  ;
%%
int yyerror(char *s){
    valid = 0;
    printf("\nInvalid expression!\n");
    return 0;
}
int main(){
    printf("Enter the expression:\n");
    yyparse();
    if (valid)
        printf("\nValid expression!\n");
    return 0;
}

===== yacc_exp.l =====
%{
#include "y.tab.h"
%}
%%
[0-9]+                { return num; }
[a-zA-Z_][a-zA-Z_0-9]* { return id; }
[+\-*/()]             { return yytext[0]; }
[ \t\n]               ;
.                     { return yytext[0]; }
%%
int yywrap() { return 1; }

===== output.txt =====
Enter the expression:
a+b*c
Valid expression!

===== run =====
nano yacc_exp.l
nano yacc_exp.y
yacc -d yacc_exp.y
lex yacc_exp.l
gcc y.tab.c lex.yy.c -o yacc_exp -ll -ly
./yacc_exp