Adding new syntax highlighting for some major modes does not work

Question

My question is similar to this one and that one: I would like to add highlighting of some characters that are not highlighted in some programming modes, like python-mode and fortran-mode.

So, following Lindydancer's answer to the first question cited above, I try the following in my init file:

(defun python-fortran-common-hook ()
    (font-lock-add-keywords nil
                '(;
                  ("+" (0 font-lock-negation-char-face))
                  ("-" (0 font-lock-negation-char-face))
                  ("*" (0 font-lock-negation-char-face))
                  ("/" (0 font-lock-negation-char-face))
                  ("=" (0 font-lock-negation-char-face))
                  ("<" (0 font-lock-negation-char-face))
                  (">" (0 font-lock-negation-char-face))
                  ("," (0 font-lock-negation-char-face))
                  (";" (0 font-lock-negation-char-face))
                  (":" (0 font-lock-negation-char-face))
                  )))
(add-hook 'python-mode-hook #'python-fortran-common-hook)
(add-hook 'fortran-mode-hook #'python-fortran-common-hook)

The problem is that for some reason this does not work (the characters I want to highlight still have the default highlighting), and my question is why.

Note 1: I tried the same with an add-hook form, and it works. But I would greatly appreciate to have the above form, not to have to write and add-hook for each mode that I want to have this highlighting. I am aware of prog-mode, but there are modes in it for which I don't want this highlighting.

Note 2: Of course, if there is a better way to achieve what I want, I would be glad to hear about it.

Answer 1

I just tried this, for me it works straight out of the box. Try to restart your Emacs, as it's easy to end up with intermediate versions of font-lock rules in hooks and other places when trying out new rules.

(Just one minor note. * has a special meaning in regexp:s so you should write "\\*" instead of "*".)

If you don't want to write this for each mode, you can use something like (even though I don't think you gain much from it):

(dolist (hook '(python-mode-hook fortran-mode-hook))
  (add-hook hook #'python-fortran-common-hook))

When it comes to if you can do this better, you could join all rules into one using the regexp [...] construct:

("[-+*/=<>,;:]" (0 font-lock-negation-char-face))

Note: If you want - to be interpreted literally when put inside square brackets [ and ], you have to put it just after the openning bracket [ or just before the closing one ], otherwise it is interpreted specially. Have a look also here if you need ], \ or ^ literally inside square brackets.