bug#58992: 28.2; "lax space matching" no longer works

[Gregory Heytings <gregory@heytings.org>]

>> commit 74d091a0a665da5dc01989d1b06a61ee21b975b2
>> Author: Lars Ingebrigtsen <larsi@gnus.org>
>> Date:   Fri Sep 10 12:27:28 2021 +0200
>>
>>     Change the default value of search-whitespace-regexp
>>
>>     * lisp/isearch.el (search-whitespace-regexp): Change the default
>>     to always exclude newlines from the set (bug#21278).  It used to
>>     be mode-dependent whether newlines were included or not, and this
>>     was confusing as a user interface.
>>
>> which changed the default value of search-whitespace-regexp from 
>> "\\s-+" to "[ \t]".
>
> This is still buggy in Emacs 28.2 if I change the value:
>
> This works in Fundamental mode, but not in Lisp mode.
>

As the commit message explains, "It used to be mode-dependent whether 
newlines were included or not, and this was confusing as a user 
interface."  Hence your confusion.  You will see the same in Emacs 27 and 
28: RET is space in fundamental mode and not space in text mode.

>
> BTW, I don't understand what "\\s-+" means.
>

It means "match any character whose syntax is "space".