gistfile1

(defun replace-in-matching-lines (sought-re old-re new)
  (interactive "sMatch lines containing:\nsOld regexp:\nsReplacement:")
  (while (re-search-forward sought-re (point-max) t)
    (save-restriction
      (narrow-to-region (line-beginning-position)
                        (line-end-position))
      (goto-char (point-min))           ;i.e., beginning of line
      (while (re-search-forward old-re (point-max)
                                t)
        (replace-match new)))
    (goto-char (line-end-position))))