Conveniently convert CamelCase to words_with_underscores using a small emacs hack

I currently cope with refactoring in an upstream project to which I maintain some changes which upstream does not merge. One nasty part is that the project converted from CamelCase for function names to words_with_underscores. And that created lots of merge errors.

Today I finally decided to speed up my work.

The first thing I needed was a function to convert a string in CamelCase to words_with_underscores. Since I’m lazy, I used google, and that turned up the CamelCase page of Emacswiki - and with it the following string functions:

(defun split-name (s)
  (split-string
   (let ((case-fold-search nil))
     (downcase
      (replace-regexp-in-string "\\([a-z]\\)\\([A-Z]\\)" "\\1 \\2" s)))
   "[^A-Za-z0-9]+"))
(defun underscore-string (s) (mapconcat 'downcase   (split-name s) "_"))

Quite handy - and elegantly executed. Now I just need to make this available for interactive use. For this, Emacs Lisp offers many useful ways to turn Editor information into program information, called interactive codes - in my case the region-code: "r". This gives the function the beginning and the end of the currently selected region as arguments.

With this, I created an interactive function which de-camelcases and underscores the selected region:

(defun underscore-region (begin end) (interactive "r")
  (let* ((word (buffer-substring begin end))
         (underscored (underscore-string word)))
    (save-excursion
      (widen) ; break out of the subregion so we can fix every usage of the function
      (replace-string word underscored nil (point-min) (point-max)))))

And now we’re almost there. Just create a macro which searches for a function, selects its name, de-camelcaeses and underscores it and then replaces every usage of the CamelCase name by the underscored name. This isn’t perfect refactoring (can lead to errors), but it’s fast and I see every change it does.

C-x C-(
C-s def 
M-x mark-word
M-x underscore-region
C-x C-)

That’s it, now just call the macro repeatedly.

C-x eeeeee…

Now check the diff to fix where this 13 lines hack got something wrong ( like changing __init__ into _init_ - I won’t debug this, you’ve been warned ☺).

Happy Hacking!