Subject: bug#74307: 30.0.92; emacs-lisp font-locking word regexp

From: Juri Linkov <[email-masked]>
Date: Thu, 05 Dec 2024 09:47:05 +0200

>> The problem is that this removes highlighting from the last character
>> because it doesn't get into the group:
>> 
>>   (rx (seq "[" (group-n 1 lisp-mode-symbol) (not "\\") "]"))
>>   => "\\[\\(?1:\\(?:\\w\\|\\s_\\|\\\\.\\)+\\)[^\\]]"
>> 
>> A possible solution is to move (not "\\") inside the group:
>> 
>>   (rx (seq "[" (group-n 1 (seq lisp-mode-symbol (not "\\"))) "]"))
>>   => "\\[\\(?1:\\(?:\\w\\|\\s_\\|\\\\.\\)+[^\\]\\)]"
>> 
>> But this removes highlighting completely from the reported case of
>> (setq foo "\\<foo\\>").  However, I guess it should not have highlighting
>> anyway because this is an incorrect syntax of `substitute-command-keys'
>> that should match only \\[], \\<>, \\{} or \\`' without the second \\
>
> Sorry, I don't understand: the change which was supposed to fix this
> was already installed.

Ah, so (setq foo "\\<foo\\>") should not be highlighted.  Ok, then
indeed (not "\\") should be inside the group.

> If you are saying it caused regressions, could you please show
> a recipe for reproducing those regressions?

A recipe is to put the following two lines into a buffer with
emacs-lisp-mode:

  (setq foo "\\<foo\\>")
  (setq foo "\\<foo>")

The first foo should not be highlighted, the second currently is
highlighted partially without the last character.  Here is the fix:

diff --git a/lisp/emacs-lisp/lisp-mode.el b/lisp/emacs-lisp/lisp-mode.el
index 99980a44ddf..95fbae48bb6 100644
--- a/lisp/emacs-lisp/lisp-mode.el
+++ b/lisp/emacs-lisp/lisp-mode.el
@@ -491,16 +491,16 @@ lisp-mode--search-key
          ;; Words inside \\[], \\<>, \\{} or \\`' tend to be for
          ;; `substitute-command-keys'.
          (,(rx "\\\\" (or (seq "["
-                               (group-n 1 lisp-mode-symbol) (not "\\") "]")
+                               (group-n 1 (seq lisp-mode-symbol (not "\\"))) "]")
                           (seq "`" (group-n 1
                                      ;; allow multiple words, e.g. "C-x a"
                                      lisp-mode-symbol (* " " lisp-mode-symbol))
                                "'")))
           (1 font-lock-constant-face prepend))
          (,(rx "\\\\" (or (seq "<"
-                               (group-n 1 lisp-mode-symbol) (not "\\") ">")
+                               (group-n 1 (seq lisp-mode-symbol (not "\\"))) ">")
                           (seq "{"
-                               (group-n 1 lisp-mode-symbol) (not "\\") "}")))
+                               (group-n 1 (seq lisp-mode-symbol) (not "\\")) "}")))
           (1 font-lock-variable-name-face prepend))
          ;; Ineffective backslashes (typically in need of doubling).
          ("\\(\\\\\\)\\([^\"\\]\\)"