RPN_rec.rb

class RPNCalculator
  def evaluate(string)
    return string.to_i if string.split(' ').count == 1
    sarray = string.split(' ')
        
    #Check for Valid setup
    syms = 0
    nums = 0
    valchars = true
    sarray.each do |c|
      syms += 1 if c =~ /[\D]/ && c.length == 1
      nums += 1 if c =~ /[\d]/
      if c =~ /[a-zA-Z]/ then
        valchars = false
        break
      end
    end
    raise ArgumentError.new("The String contains Invalid Characters") if valchars == false  
    raise ArgumentError.new("The String Does not contain the proper number of symbols/numbres") if nums - syms != 1
    
    i = 1
    until sarray[i] =~ /[\D]/ && sarray[i].length == 1 do
     i +=1
    end
    
    if sarray[i] == '+'
     sarray[i-2] = sarray[i-2].to_i + sarray[i - 1].to_i
    elsif sarray[i] == '*'
      sarray[i-2] = sarray[i-2].to_i * sarray[i - 1].to_i
    else
      sarray[i-2] = sarray[i-2].to_i - sarray[i - 1].to_i
    end
    
    sarray.delete_at(i)
    sarray.delete_at(i-1)
    evaluate(sarray.join(' '))
  end
end