34:  How do I search for, delete, or replace unprintable (8-bit or control)  !
 characters?
  
  To search for a single character that appears in the buffer as, for
  example, `\237', you can type "C-s C-q 2 3 7".  (This assumes the value of
  search-quote-char is 17 (ie., C-q).)  Searching for ALL unprintable
  characters is best done with a "regexp" search.  The easiest regexp to use
  for the unprintable chars is the complement of the regexp for the
  printable chars.
  
  Regexp for the printable chars: [\t\n\r\f -~]
  
  Regexp for the unprintable chars: [^\t\n\r\f -~]
  
  To type some of these special characters in an interactive argument to
  isearch-forward-regexp or re-search-forward, you need to use C-q.  (`\t',
  `\n', `\r', and `\f' stand respectively for TAB, LFD, RET, and C-l.)  So,
  to search for unprintable characters using re-search-forward:
  
    M-x re-search-forward RET [^ TAB C-q LFD C-q RET C-q C-l SPC -~] RET
  
  Using isearch-forward-regexp:
  
    M-C-s [^ TAB RET C-q RET C-q C-l SPC -~]
  
  To delete all unprintable characters, simply use a replace-regexp:
  
    M-x replace-regexp RET [^ TAB C-q LFD C-q RET C-q C-l SPC -~] RET RET
  
  Replacing is similar to the above.  {I need to write the text for this     +
  part of the answer!}                                                       +
  
  Notes:
  
  * With isearch, you can type RET to get a quoted LFD (not a quoted RET).
  
  * You don't need to quote TAB with either isearch or typing something in
    the minibuffer.
  
  Here are the Emacs Lisp forms of the above regexps:
  
    ;; regexp matching all printable characters:
    "[\t\n\r\f -~]"
  
    ;; regexp matching all unprintable characters:
    "[^\t\n\r\f -~]"