#ex_07-5
#Learning Perl, Appendix A, Exercise 7.5
while (<STDIN>) {
	  ($user, $pass, $uid, $gid, $gcos) = split(/:/);
  ($real) = split(/,/, $gcos);
  ($first) = split(/\s+/, $real);
  $names{$first} .= " $user";
}
foreach (keys %names) {
  $names = $names{$_};
  if ($names =~ /. /) {
    print "$_ is used by:$names\n";
  }
}