solution for octal problem

octal.rb

class Octal
  attr_reader :digits

  def initialize(input)
    @invalid = (input =~ /\D|8|9/)
    @digits = input.chars.map(&:to_i)
  end

  def to_decimal
    return 0 if @invalid
    digits.reverse.each_with_index.inject(0){|sum, (n, index)| sum + n * (8 ** index) } 
  end
end