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(efaq)Working with unprintable characters

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How do I search for, delete, or replace unprintable (eight-bit or control) characters?
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   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 (i.e., `C-q').)  Searching for *all*
unprintable characters is best done with a regular expression
("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 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> <LFD> C-q <RET> C-q C-l <SPC> -~]'

   To delete all unprintable characters, simply use 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.  To replace all unprintable
characters with a colon, use:

   M-x replace-regexp <RET> [^ <TAB> C-q <LFD> C-q <RET> C-q C-l <SPC>
-~] <RET> : <RET>

   * You don't need to quote <TAB> with either isearch or typing
     something in the minibuffer.