space in multiline interactive search / differences between Emacs 23 and 24

[David Madore]

Dear group,

Suppose I open a file containing exactly two lines as follows:

foo
bar

and I run either an interactive search with C-s foo bar (typing a space
between "foo" and "bar") or else an interactive regexp search with C-M-s
foo bar (still typing a space between "foo" and "bar).

Under Emacs 23 (at least as provided by my Ubuntu 14.04 LTS under the
name emacs23 23.4+1-4.1ubuntu1), the interactive regexp search succeeds
(i.e., the space character matches the ^J line separator) and the simple
(non-regexp) interactive search fails (i.e., the space character does
not match the ^J line separator).  Under Emacs 24 (24.3+1-2ubuntu1),
it's exactly the other way around: the simple search succeeds (=space
matches ^J) and the regexp search does not.

What is the reason for this difference, and why is it exactly opposite
in Emacs 23 and Emacs 24?  (I couldn't find anything in the NEWS file.)
 How can I configure whether space matches ^J in simple versus regexp
interactive search?

(Googling "Emacs multiline interactive search" returns nothing, which I
find strange.)

-- 
     David A. Madore
    (david+news@madore.org,
     http://www.madore.org/~david/ )