[AUCTeX] adapt auctex to the latex3 syntax

[TeXnicien de Surface]

hello all

I would like to see a latex3 macro name such as \c_something:Nn
"correctly" recognised as a macro name (to obtain the same face as with 
\something)
I've tried to modify the syntax table with

(defun latex-syntaxe-latex3()
  "modifie la table de syntaxe pour faire de _ et : des lettres"
  (interactive)
  (modify-syntax-entry ?_ "w" LaTeX-mode-syntax-table)
  ;; to be completed for :
)

and that works *partially* i.e. I obtain the good face for "\c_" but it 
stops there and the color is not the same for the remaining part of the 
macro name.

What do I miss?

many thanks for any hint

GNU Emacs 23.3.1 on Ubuntu 12.04 / AucTeX 11.86

--
Le TeXnicien de Surface