bug#16546: C-s vs. whitespace highlighting

[Juri Linkov <juri@jurta.org>]

> IMO, that behavior would make sense only when the whitespace chars are
> entered interactively (i.e. typed on the keyboard), because if the
> search string comes from another place (copy&paste, grabbed from the
> buffer, ...), I don't think it would be a good idea to temporarily set
> `search-whitespace-regexp' to nil.  And BTW, that reasoning applies
> equally to case-fold-search.

I agree that when the user types several spaces in a row explicitly
then spaces should be treated as intended by the user.

Actually, whitespace search already does this in regexp mode.
Relevant lines from regex.c:

	    /* If the spaces are followed by a repetition op,
	       treat them normally.  */
	    if (p1 != pend
		&& (*p1 == '*' || *p1 == '+' || *p1 == '?'
		    || (*p1 == '\\' && p1 + 1 != pend && p1[1] == '{')))
	      goto normal_char;

This means that when the user types ` +' then `search-whitespace-regexp'
is ignored.  I think the same should be implemented for a non-regexp search
as well, so typing `   ' will transform the input to ` +' and ignored.

> IOW, if I copy some text "foo<any-whitespace>bar" from one place and
> then go to Emacs and do "C-s C-y", I definitely want a "lax" search
> wrt whitespace.

Indeed.

> At least, that behavior should be configurable, IMO.

For better configurability I'm using such implementation:

  (defun search-forward-lax-whitespace (string &optional bound noerror count)
    (re-search-forward (search-whitespace-regexp (regexp-quote string)) bound noerror count))
  (defun search-backward-lax-whitespace (string &optional bound noerror count)
    (re-search-backward (search-whitespace-regexp (regexp-quote string)) bound noerror count))
  (defun re-search-forward-lax-whitespace (regexp &optional bound noerror count)
    (re-search-forward (search-whitespace-regexp regexp) bound noerror count))
  (defun re-search-backward-lax-whitespace (regexp &optional bound noerror count)
    (re-search-backward (search-whitespace-regexp regexp) bound noerror count))

where in `search-whitespace-regexp' you can do any whitespace handling.