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

(progn (replace-regexp "\\([A-Z]\\)" "_\\1" nil (region-beginning) (region-end))
       (downcase-region (region-beginning) (region-end)))