downcase-word-with-camelcase.el

(defun downcase-word-with-camelcase ()
  "This works similar to downcase-word, but additionally inserts
   an underscore before upper case chars within camel cased words, so that:
   FooBar => foo_bar"
  (interactive)
  (progn
    (if (looking-at-p "\\>") (search-forward-regexp "\\<"))
    (let ((start (point)))
      (search-forward-regexp "\\>")
      (let ((end (point)))
        (let ((word (buffer-substring start end)))
          (delete-region start end)
          (insert
          (downcase (let ((case-fold-search nil))
            (replace-regexp-in-string "\\([a-z]\\)\\([A-Z]\\)" "\\1_\\2" word))))
)))))

(global-set-key (kbd "M-l") 'downcase-word-with-camelcase)