%{
#include <stdio.h>
#include <string.h>

// Function to check if a given string is a palindrome
int is_palindrome(char *str) {
    int len = strlen(str);
    for (int i = 0; i < len / 2; i++) {
        if (str[i] != str[len - i - 1]) {
            return 0; // Not a palindrome
        }
    }
    return 1; // Is a palindrome
}
%}

%%
\n               { /* Ignore empty lines */ }
.*\n             { 
                    yytext[strlen(yytext) - 1] = '\0'; // Remove newline character
                    if (is_palindrome(yytext)) {
                        printf("Palindrome: %s\n", yytext);
                    } else {
                        printf("Not a palindrome: %s\n", yytext);
                    }
                }
.|\n             { /* Ignore any other input */ }
%%

int main() {
    printf("Enter a string: ");
    yylex();
    return 0;
}

Standard output is empty

ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? [Illegal port specification]
   Exception: (3) program ? ERROR: Can't ignore goal at this port
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: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ? ERROR: Can't ignore goal at this port
ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ?    Call: (3) program ? ERROR: Unknown option (h for help)
   Call: (3) program ? ERROR: Unknown option (h for help)
   Call: (3) program ? ERROR: Can't retry 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: No previous search
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Can't ignore goal at this port
ERROR: '$runtoplevel'/0: Undefined procedure: program/0
   Exception: (3) program ?    Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ?    Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: No previous search
   Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ? ERROR: '$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 ?    Exception: (3) program ? ERROR: Unknown option (h for help)
   Exception: (3) program ?    Call: (3) program ? ERROR: Unknown option (h for help)
   Call: (3) program ? EOF: exit