string-match code fragment #5 see http://codefork.com/blog/index.php/2013/04/11/string-match-and-dealing-with-state/

gistfile1.el

(defun my-replace (regex rep str &optional start)
  (save-match-data
    (let ((found-pos (string-match regex str (or start 0))))
      (if found-pos
          (let* ((match (substring str found-pos (match-end 0)))
                 (replacement (if (stringp rep)
                                  rep
                                (or (funcall rep match) ""))))
            (my-replace regex
                        rep
                        (concat (substring str 0 found-pos)
                                replacement
                                (substring str (+ found-pos (length match))))
                        (+ found-pos (length replacement))))
        ;; else, we're done
        str))))