Coda-like swapping in Emacs

gistfile1.cl

(defun coda-swap (expr)
  (interactive "sExpr: ")
  (query-replace-regexp
   (replace-regexp-in-string "$[1-2]" "\\\\([0-9a-zA-Z]*\\\\)" expr)
   (replace-regexp-in-string "$[1-2]" (lambda (m) (if (equal m "$1") "\\\\2" "\\\\1")) expr)))

;; Example:
; enter
; width="$1" height="$2"
; and it will replace
; width="240" height="320"
; with
; width="320" height="240"