alias filter_numbers {
  var %string $1-, %e 256
  while (%e) {
    if ($chr(%e) !isnum) { %string = $remove(%string, $chr(%e)) }
    dec %e
  }
  return %string
}

alias filter_letters {
  var %string $1-, %e 256
  while (%e) {
    if ($chr(%e) !isalpha) { %string = $remove(%string, $chr(%e)) }
    dec %e
  }
  return %string
}

alias filter_chars {
  var %string $1-, %e 256
  while (%e) {
    if (($chr(%e) isalpha) || ($chr(%e) isnum)) { %string = $remove(%string, $chr(%e)) }
    dec %e
  }
  return %string
}

winner? :P