a) in fact is does return 1 and 0, that isn't exactly true
and false in php 4 anymore, either documentation or regex
code needs a small update... moving to change request

b) your problem is another, your regex is wrong, try echoing
your pattern before passing it to ereg() and you will see
that php ate up your brackets (eg. [0-9]2)

it should read like this:

if (!ereg('([0-9]{'.$id_size.'})',"$id_usr")){
  echo "Error ID";
}

ereg() does return false when no matches are made, or the length of the match.