%{
#include <stdio.h>
int count = 0;
%}

%%
[A-Z]      { count++; }        // Match capital letters
.|\n       { /* Ignore others */ }
%%

int main() {
    printf("Enter a string:\n");
    yylex();  // Start lexical analysis
    printf("The number of capitalized letters: %d\n", count);
    return 0;
}

int yywrap() {
    return 0;
}

Standard output is empty

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