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From: Francesco Potorti` <pot@gnu.org>
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Subject: Re: please implement query-exchange
Date: 19 Nov 2002 23:10:51 +0100
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Untested:

(defun query-exchange ( &optional fstr sstr )
  "Exchanges all occurences of fstr with sstr and vice-versa in the region."
  (interactive)
  (if (interactive-p)
      (progn
	(setq fstr (read-from-minibuffer "First string: "))
	(setq sstr (read-from-minibuffer "Second string: "))))
  (save-excursion
    (goto-char (region-beginning)
    (while (re-search-forward 
             (concat "\\(" fstr "\\)\\|\\(" sstr "\\)) (region-end) t)
      (if (nilp (match-beginning 1))
        (replace-match fstr)
       (replace-match sstr)))))))