string-replace

;; straight-forward solution doing just string manipulation
;; it's rather heavy handed
(defun string-replace (from to string &optional re)
  "Replace all occurrences of FROM with TO in STRING.
All arguments are strings.
When optional fourth argument is non-nil, treat the from as a regular expression."
  (let ((pos 0)
        (res "")
        (from (if re from (regexp-quote from))))
    (while (< pos (length string))
      (if (setq beg (string-match from string pos))
          (progn
            (setq res (concat res
                              (substring string pos (match-beginning 0))
                              to))
            (setq pos (match-end 0)))
        (progn
          (setq res (concat res (substring string pos (length string))))
          (setq pos (length string)))))
    res))
 
;; the more emacs way to do it is to use 
;; string-replace - which has the drawback of not knowing *where*
;; the replacement was done, so if you apply this technique to strings
;; you can end up in an infinite loop
;; try:   (string-replace "a" "aa" "a string contains a")
;; the "trick" is to put the string into a temporary buffer, like so:
 
(defun string-replace-2 (this withthat in)
  "replace THIS with WITHTHAT' in the string IN"
  (with-temp-buffer
    (insert in)
    (goto-char (point-min))
    (while (search-forward this nil t)
      (replace-match withthat nil t))
    (buffer-substring (point-min) (point-max))))