Wait for release a key to go on in a loop

Question

I have this loop:

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

        (setq MatchedStringOverlay (make-overlay b e))
        (unwind-protect 
        (progn
          (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:
"))
          (cond 
           ((char-equal ACTION ?y)
            (replace-match "\\1"))
           ((char-equal ACTION ?l)
            (replace-match "\\1\n"))
           ((char-equal ACTION ?n)
            nil)
           ((char-equal ACTION ?q)
            (throw 'QUIT nil))
           )
          )
          (remove-overlays b e))
        )))
  )

It work fine for my purposes but it would be better if I could get this behaviour: Let's say I chose the [y] option, I'd like to perform the replacement but "pause" the loop until the [y] key is released. The best would be if, while pressing [y] I could undo the action pressing another key (e.g. [u]).

I'm, obviously, not asking you to write the code for me, but I'd like to know if it is possible to get what I figured out and, if so, what to look at.

EDIT. I figured out a simpler way to get way to get what I need, buy using lowcase or Uppercase version of char but I'm also interested to learn about events.

Answer 1

Release of a keyboard key is not an event that is detected by Emacs. (But release of a mouse button is an event seen by Emacs.)

Maybe change from reading and acting on a single character to acting on two keys, separately: Act on the first key by doing what you describe first: "perform the replacement but "pause" the loop until" the second key is pressed. Act on the second key to confirm the real action and continue the loop.