bug#64128: regexp parser zero-width assertion bugs

[Paul Eggert <eggert@cs.ucla.edu>]

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On 2023-06-19 12:52, Mattias Engdegård wrote:

> Sure, we can turn \b and \B into group B assertions, but the patch was more conservative in nature.

OK, but we still need to fix this, as \b and \B should not be a special 
case for following "*".

> I think we have to preserve \`* meaning \`\* for compatibility, historical or not, because it's something we keep sighting in the wild.

That makes some sense, in that \` is like ^, and ^ is already a special 
case (this is true even in POSIX BREs).

In other words, how about if we change the groups from your list:

Group A: ^ $ \` \' \b \B
Group B: \< \> \_< \_> \=

to this:

Group A: ^ \`
Group B: $ \' \b \B \< \> \_< \_> \=

where "*" is ordinary after Group A, and special after Group B and there 
is no other squirrelly behavior. And similarly for the other repetition 
operators.

Attached is a proposed doc change for this, which I have not installed. 
Of course the code and etc/NEWS would need changing too.

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From 18f6e0c85a7313d221da868e6bf55af32828112b Mon Sep 17 00:00:00 2001
From: Paul Eggert <eggert@cs.ucla.edu>
Date: Mon, 19 Jun 2023 13:35:48 -0700
Subject: [PATCH] Document proposed regex fix (bug#64128)

* doc/lispref/searching.texi (Regexp Special):
Say that repetition operators are not special after \`,
and that they work as expected after other backslash escapes.
---
 doc/lispref/searching.texi | 6 +-----
 1 file changed, 1 insertion(+), 5 deletions(-)

diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi
index 28230cea64..7c9893054d 100644
--- a/doc/lispref/searching.texi
+++ b/doc/lispref/searching.texi
@@ -546,15 +546,11 @@ Regexp Special
 
 For historical compatibility, a repetition operator is treated as ordinary
 if it appears at the start of a regular expression
-or after @samp{^}, @samp{\(}, @samp{\(?:} or @samp{\|}.
+or after @samp{^}, @samp{\`}, @samp{\(}, @samp{\(?:} or @samp{\|}.
 For example, @samp{*foo} is treated as @samp{\*foo}, and
 @samp{two\|^\@{2\@}} is treated as @samp{two\|^@{2@}}.
 It is poor practice to depend on this behavior; use proper backslash
 escaping anyway, regardless of where the repetition operator appears.
-Also, a repetition operator should not immediately follow a backslash escape
-that matches only empty strings, as Emacs has bugs in this area.
-For example, it is unwise to use @samp{\b*}, which can be omitted
-without changing the documented meaning of the regular expression.
 
 As a @samp{\} is not special inside a bracket expression, it can
 never remove the special meaning of @samp{-}, @samp{^} or @samp{]}.
-- 
2.39.2