RE: search-whitespace-regexp

["Drew Adams" <drew.adams@oracle.com>]

    So how about M-SPC toggling this flag in isearch...?

I vote yes.

    It is relatively easy for a user to get rid of the magic space
    for regexp search: ... search-whitespace-regexp ...
    I suggest implementing an analogous mechanism for string search
    (I'm not certain what the variable should be called, though):

If we use two different variables, which makes sense for flexibility and
code, it still might be good to find a way to toggle either using the same
function. That way, the same key binding (e.g. `C-s M-SPC', `C-M-s M-SPC')
could work for (toggle) both.

WRT the current 21.3 behavior and the issue of compatibility:

I think the default behavior wrt this should be the same for both regexp and
regular isearch: no "magic" whitespace search. This corresponds to what I
think most people expect, out-of-the-box, for both string and regexp
searches: respect the input exactly.

This is incompatible with 21.3, but it is compatible with 20.7 (which had no
magic whitespace isearch). I don't think we should worry about compatibility
here, anyway (it is easy for users to change the behavior) - we should just
DTRT.