bug#31796: 27.1; dired-do-find-regexp-and-replace fails to find multiline regexps

[Dmitry Gutov <dgutov@yandex.ru>]

On 25.11.2020 22:30, Eli Zaretskii wrote:
> I'd like us to fix the current binding of Q so that it supports
> everything the previous command did.

Just how much of "everything" are we talking about?

For instance, a number of character classes in Emacs regexps are 
dependent on the syntax table. Like [:word:], for instance.

Even [:space:] is dependent on syntax, while it matches a fixed set of 
characters in Grep. So when searching across different file types we 
can't even "expand" such constructs into concrete characters to search for.

One approach I've considered is replacing such unsupported constructs 
with '.', or removing them entirely for constructs like \< and \_<. And 
then post-filter the resulting matches in Emacs.

For example, xref-references-in-directory uses a special case of this 
approach. In the general case though, I worry users would sometimes 
create regexps that result in an exponentially slow or just match-all 
regexp being passed to Grep, which would never finish, for no obvious 
reason.

Someone should try it, but it's a fair amount of work to handle all 
supported constructs, and to catch all (most?) the regexps which we 
can't support in this mode.