-prefix argument, the rectangle is actually killed, shifting remaining text."
- (interactive "r\ncRectangle to register: \nP")
+prefix argument, the rectangle is actually killed, shifting remaining text.
+
+Interactively, reads the register using `register-read-with-preview'."
+ (interactive (list (region-beginning) (region-end)
+ (register-read-with-preview "Rectangle to register: ")
+ current-prefix-arg))