I have this loop:

(catch 'QUIT
      (search-forward-regexp "\\([^\n\\]\\(?:\\\\\\\\\\)*\\|^\\(?:\\\\\\\\\\)+\\)%.*\n"
                 nil t)
          (let ((b (make-marker))
        (e (make-marker))
        (set-marker b (match-beginning 0))
        (set-marker e (point))

        (setq MatchedStringOverlay (make-overlay b e))
          (overlay-put MatchedStringOverlay 'face '(:background "OliveDrab1"))
          (setq ACTION (read-char "Options:
- [y] erase string and the corresponding \"newline\"
- [l] erase string keeping the corresponding \"newline\"
- [n] skip
- [q] quit:
           ((char-equal ACTION ?y)
            (replace-match "\\1"))
           ((char-equal ACTION ?l)
            (replace-match "\\1\n"))
           ((char-equal ACTION ?n)
           ((char-equal ACTION ?q)
            (throw 'QUIT nil))
          (remove-overlays b e))

that performs some interactive replacements on buffer.

Now it happens that when I do undo after the loop, the command "undoes" all the replacements I made within the loop. How can I make each replacement a "single item" in buffer-undo-list?

I've searched in the docs, in the internet and I read the code of undo command but I've not been able to find how to get what I need.

I think I should add a nil in buffer-undo-list to mark the boundary between two change groups (see here). So I tried

(add-to-list 'buffer-undo-list nil)

just before the first cond group but it does not work.

  • 1
    +1 for trying and succeeding to find the answer yourself. You might consider adding here how you found it - that info might help others, along with your question.
    – Drew
    Jun 23, 2017 at 13:33
  • @Drew I just continued searching for related posts. Thank you.
    – Gabriele
    Jun 23, 2017 at 15:28
  • 1
    OK; thx. Next time, you might also consider asking Emacs: As @phils suggested. C-h i, choose the Elisp manual, then i undo TAB. (On the web, the doc you want is here.)
    – Drew
    Jun 23, 2017 at 17:11

1 Answer 1


You're looking for undo-boundary:

undo-boundary is a built-in function in ‘src/undo.c’.


Mark a boundary between units of undo.
An undo command will stop at this point,
but another undo command will undo to the previous boundary.

See also C-hig (elisp)Undo RET

  • 1
    I've just found this solution by myself. But thank you! This works for me!
    – Gabriele
    Jun 23, 2017 at 12:21

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