Fontifying variable references within string literals in Emacs font-lock-mode

"a #{foo} a"

(define-derived-mode temp-mode fundamental-mode "Temp"
  "Temporary major mode."
  (set (make-local-variable 'font-lock-defaults)
       '((temp-mode-font-lock-keywords) nil nil nil nil)))

(defconst temp-mode-font-lock-keywords
  (list (list "$[A-Za-z0-9]+" 0 font-lock-variable-name-face t)))

(setq ruby-font-lock-syntactic-keywords
  '(
    ;; #{ }, #$hoge, #@foo are not comments
    ("\(#\)[{$@]" 1 (1 . nil))