Converting from camelcase to _ in emacs

Question

Is there an emacs function to convert a camel-cased word to underscore? Something, like:

longVariableName

M-x to-underscore

long_variable_name

Answer 1

Use the string-inflection package, available on MELPA, or at https://github.com/akicho8/string-inflection.

Useful keyboard shortcuts, copied from https://www.emacswiki.org/emacs/CamelCase :

;; Cycle between snake case, camel case, etc.
(require 'string-inflection)
(global-set-key (kbd "C-c i") 'string-inflection-cycle)
(global-set-key (kbd "C-c C") 'string-inflection-camelcase)        ;; Force to CamelCase
(global-set-key (kbd "C-c L") 'string-inflection-lower-camelcase)  ;; Force to lowerCamelCase
(global-set-key (kbd "C-c J") 'string-inflection-java-style-cycle) ;; Cycle through Java styles

Answer 2

I use the following for toggling between camelcase and underscores:

(defun toggle-camelcase-underscores ()
  "Toggle between camelcase and underscore notation for the symbol at point."
  (interactive)
  (save-excursion
    (let* ((bounds (bounds-of-thing-at-point 'symbol))
           (start (car bounds))
           (end (cdr bounds))
           (currently-using-underscores-p (progn (goto-char start)
                                                 (re-search-forward "_" end t))))
      (if currently-using-underscores-p
          (progn
            (upcase-initials-region start end)
            (replace-string "_" "" nil start end)
            (downcase-region start (1+ start)))
        (replace-regexp "\\([A-Z]\\)" "_\\1" nil (1+ start) end)
        (downcase-region start (cdr (bounds-of-thing-at-point 'symbol)))))))