A simple English language number parser. How's the coding style? How would I extend this for a broader range of natural language number expressions?

number_parser.rb

#
# Parse textual numbers into actual numbers
#
class NumberParser
  ATOMS = {
    'zero' => 0,
    'one' => 1,
    'two' => 2,
    'three' => 3,
    'four' => 4,
    'five' => 5,
    'six' => 6,
    'seven' => 7,
    'eight' => 8,
    'nine' => 9,
    'ten' => 10,
    'eleven' => 11,
    'twelve' => 12,
    'thirteen' => 13,
    'fourteen' => 14,
    'fifteen' => 15,
    'sixteen' => 16,
    'seventeen' => 17,
    'eighteen' => 18,
    'nineteen' => 19,
    'twenty' => 20,
    'thirty' => 30,
    'forty' => 40,
    'fifty' => 50,
    'sixty' => 60,
    'seventy' => 70,
    'eighty' => 80,
    'ninety' => 90
  }

 
  #
  # Return true if the text is a number.
  #
  def number?(possible_number)
    begin
      parse(possible_number)
      return true
    rescue
      return false
    end
  end
  
  
  #
  # Convert text into a number.  E.g., 
  # "forty-one" -> 41
  #
  def parse(text)
    # Clean up the input
    phrase = text.downcase.strip

    # Handle the easy case: it's an atom.
    result = ATOMS[phrase]
    unless result.nil?
      return result
    end
    
    # Compounds are e.g. "forty-one"
    result = handle_compound(phrase)
    unless result.nil?
      return result
    end
    
    # Handle a hundred-based number.
    # E.g., "five hundred eleven"
    result = handle_hundred_based(phrase)
    unless result.nil?
      return result
    end
    
    raise "Couldn't parse the number: \"#{text}\""
  end  
  
  
  private
  
  
  def handle_hundred_based(tok)
    if tok =~ /^([a-z\-]+) hundred ([a-z\-]+)$/
      return parse($1) * 100 + parse($2)
    end

    if tok =~ /^([a-z\-]+) hundred$/
      return parse($1) * 100
    end

    return nil
  end
  
  
  #
  # Convert a compound phrase into
  # a number or return nil if I 
  # can't.
  #
  # E.g. "forty-five" -> 45
  #
  def handle_compound(tok)
    if tok =~ /^([a-z]+)-([a-z]+)$/
      return ATOMS[$1] + ATOMS[$2]
    else
      return nil
    end
  end
  
end