  def convert_string(str)
    ruby_19 { str = convert_to_ascii_if_ascii(str) }

    encoding = 0

    str.sub!(/^"/,'')   # Remove leading  "
    str.sub!(/"$/,'')   # Remove trailing "
    str.gsub!(/""/,'"') # Substitute Excel's escaped double quote "" for "

    # number of characters in str
    length = ruby_18 { str.gsub(/[^\WA-Za-z_\d]/, ' ').length } ||
             ruby_19 { str.length }

    # Handle utf8 strings
    if is_utf8?(str)
      str = utf8_to_16le(str)
      ruby_19 { str.force_encoding('BINARY') }
      encoding = 1
    end

    exit "String in formula has more than 255 chars\n" if length > 255

    [@ptg['ptgStr'], length, encoding].pack("CCC") + str
  end