my_hex.rb

def my_hex(hex_str)
  result = 0
  hex_str.split('').reverse.each_with_index do |ch, i|
    low, high = ['1'..'9'], ['a'..'f']
    result += 16 ** i *
      case ch
      when *low  then ch.to_i
      when *high then ch.ord - 'a'.ord + 10
      else 0
      end
  end
  return result
end

s = '1d11e'
p my_hex(s)
# >> 119070

これ見てたら何となく16進数変換したくなった