%{ 
#include <stdio.h> 
#include <ctype.h> 
void print_token(const char *token_type, const char *value); 
%} 
%% 
"int"|"float"|"return" {print_token("KEYWORD", yytext);} 
[a-zA-Z_][a-zA-Z0-9_]* {print_token("IDENTIFIER", yytext);} 
[0-9]+ {print_token("NUMBER", yytext);} 
"+"|"-"|"*"|"/" {print_token("OPERATOR", yytext);} 
"(" {print_token("LEFT_PAREN", yytext);} 
")" {print_token("RIGHT_PAREN", yytext);} 
";" {print_token("SEMICOLON", yytext);} 
[ \t\n]+; 
.{print_token("UNKNOWN", yytext);} 
%% 
void print_token(const char *token_type, const char *value) { 
printf("Token: %-12s Value: %s\n", token_type, value); 
} 
int main() { 
yylex();

Standard output is empty

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 ? 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: 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: 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: 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: 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: Unknown option (h for help)
   Exception: (3) program ?    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 ? EOF: exit