Decode a hex string into a UTF-8 string. Try this hex string as an example: e299a5205765204d616b652054756d6d792048617070792120e299a5

utf8.rb

#!/usr/bin/env ruby

def usage
  puts "usage: #{$0} <hex_string>"
  exit 1
end

def continuous_byte? (byte)
  (0x80..0xBF).include?(byte)
end

def leading_byte? (byte)
  !continuous_byte?(byte)
end

def decode_utf8_hex_string (hexstr)
  # Convert string into array of integers, each representing 2 hex chars (1 byte)
  bytes = hexstr.scan(/../).map { |unit| unit.hex }

  # Create an array of integers representing individual UTF-8 chars
  chars = bytes.inject([]) do |array, byte|
    if leading_byte?(byte)
      # This is a leading byte.  Push it onto the array
      array.push(byte)
    else
      # Continuous byte.  Append to last leading or continuous byte
      array[-1] << 8
      array[-1] += byte
    end
    array
  end

  # Return utf8 string
  utf8 = chars.pack("U*")
end

def main (args)
  usage unless args.length == 1
  puts decode_utf8_hex_string(args[0])
end

main(ARGV.dup) if __FILE__ == $0