Subject: 25.0.50; undo in query-replace w/o exit

From: Tino Calancha <[email-masked]>
Date: Wed, 14 Oct 2015 22:26:22 +0900 (JST)

In GNU Emacs 25.0.50.1 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.23)
  of 2015-10-14
Repository revision: 38f99a02b83e9e408970cb7abdb685725b2004f9

Quite often, performing query-replace, i just hit the [space] quicky
with no much attention. If i replace one string by mistake, i can
comeback to it with "^", but if i edit the string with "E" to reset it 
to its original value, the interactive query-replace session ends.

I found useful if we can undo such replacements without exiting the 
query-replace.

The attached patch implement such behaviour for `query-replace'.
Having this also working for `query-replace-regexp' would be very nice,
but harder to implement.

diff --git a/lisp/replace.el b/lisp/replace.el
index 3a908ac..e3c906b 100644
--- a/lisp/replace.el
+++ b/lisp/replace.el
@@ -1802,6 +1802,7 @@ query-replace-help
 C-l to clear the screen, redisplay, and offer same replacement again,
 ! to replace all remaining matches in this buffer with no more questions,
 ^ to move point back to previous match,
+U to undo previous replacement (queries w/ regexp not supported),
 E to edit the replacement string.
 In multi-buffer replacements type `Y' to replace all remaining
 matches in all remaining buffers with no more questions,
@@ -1831,6 +1832,8 @@ query-replace-map
     (define-key map "\C-l" 'recenter)
     (define-key map "!" 'automatic)
     (define-key map "^" 'backup)
+    (define-key map "u" 'undo)
+    (define-key map "U" 'undo)
     (define-key map "\C-h" 'help)
     (define-key map [f1] 'help)
     (define-key map [help] 'help)
@@ -1856,7 +1859,7 @@ query-replace-map
 `act-and-exit', `exit', `exit-prefix', `recenter', `scroll-up',
 `scroll-down', `scroll-other-window', `scroll-other-window-down',
 `edit', `edit-replacement', `delete-and-edit', `automatic',
-`backup', `quit', and `help'.
+`backup', `undo', `quit', and `help'.
 
 This keymap is used by `y-or-n-p' as well as `query-replace'.")
 
@@ -2332,6 +2335,32 @@ perform-replace
 			   (message "No previous match")
 			   (ding 'no-terminate)
 			   (sit-for 1)))
+			((eq def 'undo)
+			 (cond (regexp-flag
+					(message "Undo not supported in queries with regexp")
+					(ding 'no-terminate)
+					(sit-for 1))
+				   (t
+					 ;; bind locally swapped values (search-string/replacement)
+					(let* ((search-string-1   next-replacement)
+					       (next-replacement  search-string)
+                                               (search-string     search-string-1)
+                                               (elt               (if stack (pop stack) (match-data))))
+					  (goto-char (nth 0 elt))
+					  (setq replaced (nth 1 elt))
+					  (and replaced (goto-char (- (point) (length next-replacement))))
+
+					  (replace-match-maybe-edit
+					   next-replacement nocasify literal
+					   noedit real-match-data backward)
+
+					  (setq noedit nil
+                                                       replace-count (1- replace-count)
+                                                       replaced nil
+                                                       real-match-data
+                                                       (replace-match-data
+                                                        t real-match-data
+                                                        (nth 2 elt)))))))
 			((eq def 'act)
 			 (or replaced
 			     (setq noedit