Emacs diff-region

diff_region.el

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;; diff-region* - Diff two regions
;;
;;  To compare two regions, select the first region
;; and run `diff-region`.  The region is now copied
;; to a seperate diff-ing buffer.  Next, navigate
;; to the next region in question (even in another file).
;; Mark the region and run `diff-region-now`, the diff
;; of the two regions will be displayed by ediff.
;;
;;  You can re-select the first region at any time
;; by re-calling `diff-region`.
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(defun diff-region ()
  "Select a region to compare"
  (interactive)
  (when (use-region-p)  ; there is a region
        (let (buf)
          (setq buf (get-buffer-create "*Diff-regionA*"))
          (save-current-buffer
            (set-buffer buf)
            (erase-buffer))
          (append-to-buffer buf (region-beginning) (region-end)))
        )
  (message "Now select other region to compare and run `diff-region-now`")
  )

(defun diff-region-now ()
  "Compare current region with region already selected by `diff-region`"
  (interactive)
  (when (use-region-p)
        (let (bufa bufb)
          (setq bufa (get-buffer-create "*Diff-regionA*"))
          (setq bufb (get-buffer-create "*Diff-regionB*"))
          (save-current-buffer
            (set-buffer bufb)
            (erase-buffer))
          (append-to-buffer bufb (region-beginning) (region-end))
          (ediff-buffers bufa bufb))
        )
  )

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I always seem to find myself wanting to diff two regions in Emacs, not two buffers, not to files, not a buffer and a file, just two regions. This pair of functions is intended to mimic WinMerge's Compare/Compare To... functionality. You first select a region with diff-region, which gets saved off in a hidden buffer. You are then free to navigate/edit/work until you have a corresponding region to diff. Running diff-region-now copies the new region to another hidden buffer, and the the diff of the two buffers is displayed by ediff.

This seemed to small to wrap into a full package, but at some point it might be nice. For now, just paste the code into your .emacs file - upon restart/evaluation you will have access to diff-region and diff-region-now.

Thanks! It works great! You should consider adding this to the Emacs wiki :)

Great idea but it's already implemented in GNU Emacs: ediff-regions-linewise and ediff-regions-wordwise. Nonetheless, I discovered this thanks to you when I misstyped "diff-region" as "ediff-region". :)

Very nice, exactly what I needed now!
@vifon I think the ediff-regions-* functions are different, you can't compare regions in the same buffer with them, can you?

@rashack: Sure you can. Just select the same buffer in both prompts.

works great! thank you!