custom-functions.el

(defun underscore-to-camelcase ()
  "Convert underscores to camelCase"
  (interactive)
  (destructuring-bind (beg . end) (bounds-of-thing-at-point 'word)
    (goto-char beg)
    (while (re-search-forward "_\\([a-z]\\)" end t)
      (replace-match (upcase (match-string 1))))))

(defun camelcase-to-underscore ()
  "Convert underscores to camelCase"
  (interactive)
  (let ((case-fold-search nil))
    (destructuring-bind (beg . end) (bounds-of-thing-at-point 'word)
      (goto-char beg)
      (while (re-search-forward "\\([a-z0-9]\\)\\([A-Z]\\)" end t)
        (replace-match (concat (match-string-no-properties 1)
                             "_"
                             (downcase (match-string-no-properties 2))))))))

(defun swap-underscore-camelcase ()
  "convert udnerscores to camelCase if word contains underscores
and the other way around otherwise"
  (interactive)
  (save-excursion
    (destructuring-bind (beg . end) (bounds-of-thing-at-point 'word)
      (goto-char beg)
      (cond
       ((save-excursion (re-search-forward "_" end t))
        (underscore-to-camelcase))
       (t (camelcase-to-underscore))))))