That's what I coded.

Puzzled over the subtractor vs the repeats, until
I figured out one need to do both.

Here's my solution - now I'll be able to look
at everyone elses :)

Vance
--- roman.rb ---
#! /usr/bin/env ruby

def is_arabic?(n)
   return n =~ /^[0-9]*$/
end

def to_roman(n)
  r = ''
  rd = %w{M D C L X V I}
  val = [1000, 500, 100, 50, 10, 5, 1]
  val.each_with_index { |v, i|
    c = (n - (n % v))/v
    if c < 4 && c > 0
      c.times { r << rd[i] }
      n -= (c * v)
    end
    s = (i % 2 == 0) ? i+2 : i+1
    if (s < 7) && (n >= (val[i] - val[s]))
        r << rd[s] << rd[i]
        n -= (val[i] - val[s])
    end
  }
  return r
end

def to_arabic(n)
val = { 'I' => 1,   'V' => 5,   'X' => 10, 'L' => 50,
        'C' => 100, 'D' => 500, 'M' => 1000 }
  sum = 0
  lastval = 1000
  n.each_byte {|c|
    sum += val[c.chr]
    sum -= (lastval * 2) if val[c.chr] > lastval
    lastval = val[c.chr]
  }
  return sum

end