カーソル手前の単語群をアンダースコアでつなげる

join-words-by-underscore.el

(defun join-words-by-underscore ()
  (interactive)
  (save-excursion
    (save-restriction
      (looking-back "[\s-\]*\\(\[a-zA-Z0-9\s-\]+\\)"  nil t)
      (narrow-to-region (match-beginning 1) (match-end 1))
      (beginning-of-line)
      (replace-string " " "_"))))