There is more than one correct answer. Here is one approach.

%{
#include <stdio.h>
%}
%%
frog	{printf("toad");
	}

"\n"	{printf("\n");
	}

. 	{printf("%c", yytext[0]);
        }
%%

int main()
{
  yyin = stdin;
  while(yylex() != 0) {}
  return 0;
}

int yywrap()
{
  return 1;
}