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Editing Fortran in Emacs with f90-mode, I would like the “word commands” M-b, M-f, M-d, M-DEL and so on to act on “words” as separated by underscores. On the other hand, for the syntax highlighting, commands like C-M-f, and so on, the underscores obviously must not break apart identifiers.

subword-mode partially does this: M-f and M-d act like I want, but M-b and M-DEL still treat the whole identifier as one word. To illustrate: if point is after some_thing, M-b (subword-backward) will move it to s; after that, M-f (subword-forward) will move point to _.

In f90-mode, _ has syntax class "w" (word). If I change it to "_" (symbol) or "." (punctuation), I get the desired behavior, but then some_thing will also be two words to font lock and completion. E.g., program my_prog; end will be completed only to end my, and write_me would be incorrectly fontified (write as a keyword).

How do I get both things at once?

PS: This is GNU Emacs 24.


EDIT: Here is the complete code I ended up using, building on cadrians answer (thanks!)

(let ((adv (cons 'advice
                 (lambda ()
                   (let ((os (char-syntax ?_)))
                     (modify-syntax-entry ?_ "_")
                     ad-do-it
                     (modify-syntax-entry ?_ (string os))))))
      (fun '(subword-forward subword-kill subword-backward
             subword-backward-kill subword-downcase subword-upcase
             subword-transpose)))
  (dolist (f fun)
    (ad-add-advice f (list 'underscore-wrap nil t adv)
                   'around 'last)
    (ad-activate f)))

I added a let so as not to change the syntax permanently, and used ad-add-advice to be able to put it in a loop.

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One solution is to use defadvice:

(defadvice forward-word (around eif-forward-word activate)
  "forward-word, with the underscore not being a letter"
  (interactive "p")
  (modify-syntax-entry ?_  "_")
  ad-do-it
  (modify-syntax-entry ?_  "w"))

Modify forward-word, backword-word, left-word, and right-word.

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