toggle camelcase and snakecase.

toggle-camelcase-snakecase.el

(defun toggle-camelcase-and-snakecase ()
  (interactive)

  (let (start end bounds)
    (if (and transient-mark-mode mark-active)
        (progn
          (setq start (mark))
          (setq end   (point)))
      (let ((bounds (bounds-of-thing-at-point 'symbol)))
        (when bounds
          (setq start (car bounds))
          (setq end   (cdr bounds))))
      )
    (when (and start end) (toggle-camelcase-and-snakecase-innter (min start end) (max start end)))
    ))

(defun toggle-camelcase-and-snakecase-innter (start end)
  (save-excursion
    (save-restriction
      (setq case-fold-search nil)
      (narrow-to-region start end)
      (if (string-match "_" (buffer-substring start end))
          (progn ;; snake case => camel case
            (goto-char (point-min))
            (while (re-search-forward "_\\(.\\)" nil t)
              (replace-match (upcase (match-string 1)))))
        (progn ;; camel case => snake case
          (goto-char (point-min))
          (while (re-search-forward "\\([a-z]\\)\\([A-Z]\\)" nil t)
            (replace-match (concat (match-string 1) "_" (downcase (match-string 2))))))))))