string-match code fragment #3 see http://codefork.com/blog/index.php/2013/04/11/string-match-and-dealing-with-state/

gistfile1.el

(defun mask-ssn (num)
  (save-match-data
    (concat "XXX-XX-" (nth 2 (split-string num "-")))))

(replace-regexp-in-string "[0-9]\\{3\\}-[0-9]\\{2\\}-[0-9]\\{4\\}"
                          'mask-ssn
                          "Here's a number: 999-22-1111. Here's another: 222-11-9999.")

;; evaluates to "Here's a number: XXX-XX-1111. Here's another: XXX-XX-9999."
;; Hooray!