http://groups.google.com/group/gnu.emacs.help/browse_thread/thread/c7...%2C%22+swap#a2afe24ce98ed739

which presents the following code.

,----
| (defun query-swap-regexp (regexp-a regexp-b)
|   "Swap A and B regexp matches in current buffer or region."
|   (interactive "sRegexp A: \nsRegexp B: ")
|   (let ((match-a (save-excursion
|                  (re-search-forward regexp-a nil t)
|                  (match-string 0)))
|         (match-b (save-excursion
|                  (re-search-forward regexp-b nil t)
|                  (match-string 0))))
|     (query-replace-regexp
|      (concat "\\(\\(" regexp-a "\\)\\|" regexp-b "\\)")
|      `(replace-eval-replacement
|        replace-quote
|        (if (match-string 2) ,match-b ,match-a))
|      nil
|      (if (and transient-mark-mode mark-active) (region-beginning))
|      (if (and transient-mark-mode mark-active) (region-end)))))
`----

I am find it hard to understand due to lack of any example or
documentation explaining the operations of

replace-eval-replacement
replace-quote

For the latter, emacs says

No apropos matches for 'replace-quote'

and for the former,

replace-eval-replacement is a compiled Lisp function in `replace'.
(replace-eval-replacement EXPRESSION REPLACE-COUNT)
not documented

I cant run the above and play with it.

Thank for any help.
Bolega