sub assert_valid_isbn {
    local($_) = @_;
    s/^ISBN\s*// or die "'$_' does not begin with 'ISBN'";
    y/- 0-9//cd and die "'$_[0]' contains invalid characters";
    /-/ && / / and die "'$_[0]' contains both hyphens and spaces";
    y/-/ /; # convert hyphens to spaces.
    my @g = split;
    @g == 4 or die "'$_[0]' is broken into the wrong number of groups";
    join('',@g) == 10 or die "'$_[0]' contains the wrong number of digits.";
  }