UTF8 validity check

#!/usr/bin/env ruby -wKU
# http://blog.grayproductions.net/articles/the_unicode_character_set_and_encodings

module UTF8Checksum
  def is_utf8?
    where_we_were = pos
    begin
      loop do
        break if eof?
        first_byte = "%08b" % read(1)[0]
        unless first_byte[0] == ?0
          bytes_left  = first_byte[/\A1+/].size - 1
          extra_bytes = read(bytes_left)
          unless extra_bytes and extra_bytes.size == bytes_left and
                 extra_bytes.split("").all? { |b| ("%08b" % b[0]) =~ /\A10/ }
            return false
          end
        end
      end
      return true
    ensure
      seek(where_we_were)
    end
  end
end

class IO
  include UTF8Checksum
end
ARGF.extend(UTF8Checksum)

class String
  def is_utf8?
    require "stringio"
    StringIO.new(self).extend(UTF8Checksum).is_utf8?
  end
end

if __FILE__ == $PROGRAM_NAME
  answer = ARGF.is_utf8?
  p answer
  exit(answer ? 0 : 1)
end