A simple approach to easily incrementing (or decrementing) the next number in an Emacs buffer. This seems to be helpful when editing odd XML data files.

increment-number.el

(defun increment-decrement (amount)
  (let* ((for-number "[0-9][0-9]*")
         (end (re-search-forward for-number))
         (start (match-beginning 0))
         (str (buffer-substring start end))
         (num (+ amount (string-to-number str))))

    (delete-region start end)
    (insert (number-to-string num))
    (goto-char start)))

(defun increment (start end)
  "Searches forward in the buffer and increments the first number
   it finds."
  (interactive "r")
  (increment-decrement 1))

(defun decrement (start end)
  "Searches forward in the buffer and decrement the first number
   it finds."
  (interactive "r")
  (increment-decrement -1))

(global-set-key (kbd "C-c <up>") 'increment)
(global-set-key (kbd "C-c <down>") 'decrement)