def preprocess(string)
      # combine CR+NULL into CR
      string = string.gsub(/#{CR}#{NULL}/no, CR) if @options["Telnetmode"]

      # combine EOL into "\n"
      string = string.gsub(/#{EOL}/no, "\n") unless @options["Binmode"]

      # remove NULL
      string = string.gsub(/#{NULL}/no, '') unless @options["Binmode"]

      string.gsub(/#{IAC}(
                   [#{IAC}#{AO}#{AYT}#{DM}#{IP}#{NOP}]|
                   [#{DO}#{DONT}#{WILL}#{WONT}]
                     [#{OPT_BINARY}-#{OPT_NEW_ENVIRON}#{OPT_EXOPL}]|
                   #{SB}[^#{IAC}]*#{IAC}#{SE}
                 )/xno) do
        if    IAC == $1  # handle escaped IAC characters
          IAC
        elsif AYT == $1  # respond to "IAC AYT" (are you there)
          self.write("nobody here but us pigeons" + EOL)
          ''
        elsif DO[0] == $1[0]  # respond to "IAC DO x"
          if OPT_BINARY[0] == $1[1]
            @telnet_option["BINARY"] = true
            self.write(IAC + WILL + OPT_BINARY)
          else
            self.write(IAC + WONT + $1[1..1])
          end
          ''
        elsif DONT[0] == $1[0]  # respond to "IAC DON'T x" with "IAC WON'T x"
          self.write(IAC + WONT + $1[1..1])
          ''
        elsif WILL[0] == $1[0]  # respond to "IAC WILL x"
          if    OPT_BINARY[0] == $1[1]
            self.write(IAC + DO + OPT_BINARY)
          elsif OPT_ECHO[0] == $1[1]
            self.write(IAC + DO + OPT_ECHO)
          elsif OPT_SGA[0]  == $1[1]
            @telnet_option["SGA"] = true
            self.write(IAC + DO + OPT_SGA)
          else
            self.write(IAC + DONT + $1[1..1])
          end
          ''
        elsif WONT[0] == $1[0]  # respond to "IAC WON'T x"
          if    OPT_ECHO[0] == $1[1]
            self.write(IAC + DONT + OPT_ECHO)
          elsif OPT_SGA[0]  == $1[1]
            @telnet_option["SGA"] = false
            self.write(IAC + DONT + OPT_SGA)
          else
            self.write(IAC + DONT + $1[1..1])
          end
          ''
        else
          ''
        end
      end
    end
Preprocess received data from the host.

Performs newline conversion and detects telnet command sequences.
Called automatically by #waitfor().  You should only use this
method yourself if you have read input directly using sysread()
or similar, and even then only if in telnet mode.