code.cr

    pp string
    string.each_byte do |byte|
      p byte <= 0x1F
      case byte
      when '\t'.ord, ' '.ord, '"'.ord, '\\'.ord
        io << '\\'
      when byte <= 0x1F, 0x7F
        raise ArgumentError.new("String contained invalid character #{byte.chr.inspect}")
      end
      io.write_byte byte
    end

result.txt

string # => "foo\u{0}bar"
false
false
false
true
false
false
false