add-to-mode function

gistfile1.el

(defun add-to-mode (mode &rest files)
  "Turn on MODE's syntax highlighting for FILES
where FILES is either a file or a regular expression"
  (dolist (file files)
    (add-to-list 'auto-mode-alist (cons file mode))))

(add-to-mode 'ruby-mode
             "\\.gem\\(rc\\|spec\\)$"
             "\\.\\(irbrc\\|rake\\|ru\\)$"
             "\\(Cap\\|Gem\\|Rake\\|Puppet\\|Vagrant\\)file$")

(add-to-mode 'markdown-mode "\\.\\(md\\|markdown\\)$")