Returns a copy of +self+ with each invalid byte sequence replaced
by the given +replacement_string+.

With no block given, replaces each invalid sequence
with the given +default_replacement_string+
(by default, <tt>"�"</tt> for a Unicode encoding, <tt>'?'</tt> otherwise):

  "foo\x81\x81bar"scrub                             # => "foo��bar"
  "foo\x81\x81bar".force_encoding('US-ASCII').scrub # => "foo??bar"
  "foo\x81\x81bar".scrub('xyzzy')                   # => "fooxyzzyxyzzybar"

With a block given, calls the block with each invalid sequence,
and replaces that sequence with the return value of the block:

  "foo\x81\x81bar".scrub {|sequence| p sequence; 'XYZZY' } # => "fooXYZZYXYZZYbar"

Output :

  "\x81"
  "\x81"

Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].