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

Answer 1

There is now another general way in 2018: magnars/s.el: The long lost Emacs string manipulation library. - github.com, some examples regarding OP's question:

1. whatever case to snake case(underscore seperated):

   (s-snake-case "some words") ;; => "some_words"
   (s-snake-case "dashed-words") ;; => "dashed_words"
   (s-snake-case "camelCasedWords") ;; => "camel_cased_words"
   

2. whatever case to lower camel case:

   (s-lower-camel-case "some words") ;; => "someWords"
   (s-lower-camel-case "dashed-words") ;; => "dashedWords"
   (s-lower-camel-case "under_scored_words") ;; => "underScoredWords"
   

see more examples at its repo.