% 
 { 
    int i, j, flag; 
    % 
 } 
  
% % 
    [a - z A - z 0 - 9]* 
{ 
    for (i = 0, j = yyleng - 1; i <= j; i++, j--) { 
        if (yytext[i] == yytext[j]) { 
            flag = 1; 
        } 
        else { 
            flag = 0; 
            break; 
        } 
    } 
    if (flag == 1) 
        printf("Given string is Palindrome"); 
    else
        printf("Given string is not Palindrome"); 
} 
% % 
  
    // driver code 
    int main() 
{ 
    printf("Enter a string :"); 
    yylex(); 
    return 0; 
} 
  
int yywrap() 
{ 
    return 1; 
} 

Standard output is empty

ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Can't ignore goal at this port
ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: Can't ignore goal at this port
ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ?