WIP Highlight `printf` format string specifier token. Used some code from: http://www.emacswiki.org/emacs/AddKeywords#toc7

highlight-format-string-spec.el

(defvar font-lock-format-specifier-face 
  'font-lock-format-specifier-face)

(defface font-lock-format-specifier-face
  '((t (:inherit font-lock-preprocessor-face)))
  "Font Lock mode face used to highlight format specifiers."
  :group 'font-lock-faces)

(defun highlight-format-specifier ()
  (font-lock-add-keywords
   nil '(("\".*\\(%\\(?:[-+0-9\\$.]+\\)?[bdiuoxXDOUfeEgGcCsSpn]\\)[^\"]*\""
          1 font-lock-preprocessor-face t))))

(add-hook 'prog-mode-hook 'highlight-format-specifier)

Differencies from the Emacs Wiki version:

• Simpler regex, doesn't do validation of any sort. This is more flexible since the specifier might differ between languages.
• Only highlight token inside string.