For an ordinary string, this method, +String#dump+,
returns a printable ASCII-only version of +self+, enclosed in double-quotes.

For a dumped string, method String#undump is the inverse of +String#dump+;
it returns a "restored" version of +self+,
where all the dumping changes have been undone.

In the simplest case, the dumped string contains the original string,
enclosed in double-quotes;
this example is done in +irb+ (interactive Ruby), which uses method `inspect` to render the results:

  s = 'hello'   # => "hello"
  s.dump        # => "\"hello\""
  s.dump.undump # => "hello"

Keep in mind that in the second line above:

- The outer double-quotes are put on by +inspect+,
  and _are_ _not_ part of the output of #dump.
- The inner double-quotes _are_ part of the output of +dump+,
  and are escaped by +inspect+ because they are within the outer double-quotes.

To avoid confusion, we'll use this helper method to omit the outer double-quotes:

  def dump(s)
    print "String:   ", s, "\n"
    print "Dumped:   ", s.dump, "\n"
    print "Undumped: ", s.dump.undump, "\n"
  end

So that for string <tt>'hello'</tt>, we'll see:

  String:    hello
  Dumped:    "hello"
  Undumped:  hello

In a dump, certain special characters are escaped:

  String:    "
  Dumped:    "\""
  Undumped:  "

  String:    \
  Dumped:    "\\"
  Undumped:  \

In a dump, unprintable characters are replaced by printable ones;
the unprintable characters are the whitespace characters (other than space itself);
here we see the ordinals for those characers, together with explanatory text:

  h = {
     7 => 'Alert (BEL)',
     8 => 'Backspace (BS)',
     9 => 'Horizontal tab (HT)',
    10 => 'Linefeed (LF)',
    11 => 'Vertical tab (VT)',
    12 => 'Formfeed (FF)',
    13 => 'Carriage return (CR)'
  }

In this example, the dumped output is printed by method #inspect,
and so contains both outer double-quotes and escaped inner double-quotes:

  s = ''
  h.keys.each {|i| s << i } # => [7, 8, 9, 10, 11, 12, 13]
  s                         # => "\a\b\t\n\v\f\r"
  s.dump                    # => "\"\\a\\b\\t\\n\\v\\f\\r\""

If +self+ is encoded in UTF-8 and contains Unicode characters,
each Unicode character is dumped as a Unicode escape sequence:

  String:    тест
  Dumped:    "\u0442\u0435\u0441\u0442"
  Undumped:  тест

  String:    こんにちは
  Dumped:    "\u3053\u3093\u306B\u3061\u306F"
  Undumped:  こんにちは

If the encoding of +self+ is not ASCII-compatible
(i.e., if <tt>self.encoding.ascii_compatible?</tt> returns +false+),
each ASCII-compatible byte is dumped as an ASCII character,
and all other bytes are dumped as hexadecimal;
also appends <tt>.dup.force_encoding(\"encoding\")</tt>,
where <tt><encoding></tt> is <tt>self.encoding.name</tt>:

  String:    hello
  Dumped:    "\xFE\xFF\x00h\x00e\x00l\x00l\x00o".dup.force_encoding("UTF-16")
  Undumped:  hello

  String:    тест
  Dumped:    "\xFE\xFF\x04B\x045\x04A\x04B".dup.force_encoding("UTF-16")
  Undumped:  тест

  String:    こんにちは
  Dumped:    "\xFE\xFF0S0\x930k0a0o".dup.force_encoding("UTF-16")
  Undumped:  こんにちは