Make query-replace skip certain matches

Question

Lets say I have these strings: "foo", "fooo" and "fo". And this line of code:

(query-replace-regexp "\\<fo+\\>" "foo" nil (point-min) (point-max))

I'd like to skip the query-replacement for the matched string "foo" because it would be replaced by an identical string.

In other words I'd like to skip query-replacements if the matched string matches a string (or a regexp).

I figured out it could be done with a lambda function but I have no idea how to implement this idea. May be something like:

(query-replace-regexp "\\<fo+\\>" 
                      '((lambda (data count)
                          (if (string= (match-string 0) "foo")
                              (skipping-function)
                            "foo")))
                      nil (point-min) (point-max))

a sort of skipping-function should do the trick.

Answer 1

For your particular example, you can write a regexp to matching "fo" and "fooo" but not "foo", e.g.,

(rx bow (or "fo" (and "foo" (1+ "o"))) eow)

C-M-% (query-replace-regexp) is mainly for interactive use, it might be easier to start from scratch, e.g.,

(defun my-query-replace-regexp (regexp replacement)
  (interactive "sRegexp: \nsReplacement: ")
  (while (re-search-forward regexp nil t)
    (if (string= (match-string 0) replacement)
        (message "[DEBUG] Skipping")
      (when (y-or-n-p "Replace this match? ")
        (replace-match replacement)))))