Variable: TeX-comment-start-regexp
TeX-comment-start-regexp is a buffer-local variable defined in tex.el.
Documentation
Regular expression matching a comment starter.
Unlike the variable comment-start-skip it should not match any
whitespace after the comment starter or any character before it.
Source Code
;; Defined in ~/.emacs.d/elpa/auctex-14.1.2/tex.el
;;; Comments
(defvar-local TeX-comment-start-regexp "%"
"Regular expression matching a comment starter.
Unlike the variable `comment-start-skip' it should not match any
whitespace after the comment starter or any character before it.")