Question

I need a little help with some Emacs syntax highlighting troubles. I’d like to set the colour of all C/C++ operators. I am having trouble because “/” is used for division (which I want to colour) but also used to start comments (which I don’t want to colour myself, the default code colours them fine.) I can’t seem to figure out how to tell Emacs to only colour a single forward-slash when it is not surrounded by any other forward slashes.

This is what I have now:

(add-hook 'c-mode-common-hook (lambda ()
  (font-lock-add-keywords nil         
    '(("[<>:&*=+^%!~,.?;/-]" 0 font-lock-warning-face t)))
))

But it won’t correctly highlight something like this:

i = 1 / 2
// Test
i=1/2//test

Of course if there’s already a way of assigning a colour/face to punctuation/operators and I’ve just missed it please let me know!

Answer 1

Each font long entry has the following format:

(SUBEXP FACENAME [OVERRIDE [LAXMATCH]])

The t in your rule corresponds to the OVERRIDE entry. If you drop it, your rule will not re-highlight something that already has been highlighted, for example comments and strings.

There is no general package for highlighting all operators, but cwarn-mode will highlight = and += etc. that are embedded inside larger expressions, as well as the ; in situations where it shouldn’t be, like if (condition);. It’s implemented using functions rather than regexp:s, and I consider it a good reference if you plan to implement relatively advanced font-lock rules.