How can I highlight different keywords according to a suffix?

Question

I want to highlight foo in foo:: without highlighting foo::<bar>, using font-lock-keywords. This is the result I want:

Initially I tried this:

(defvar example-font-lock-keywords
  `((,(rx (group (1+ word)) "::" (not (any "<"))) 1 font-lock-type-face)))

(define-derived-mode example-mode prog-mode "Example"
  (setq-local font-lock-defaults '(example-font-lock-keywords)))

But that gave the following:

Instead I tried two separate keywords:

(defvar example-font-lock-keywords
  `((,(rx (group (1+ word)) "::") 1 font-lock-type-face)
    ;; This is ignored if I use `default', but other faces work!
    (,(rx (group (1+ word)) "::<") 1 font-lock-variable-name-face t)))

(define-derived-mode example-mode prog-mode "Example"
  (setq-local font-lock-defaults '(example-font-lock-keywords)))

The second pattern is used if I specify font-lock-variable-name-face, but if I use default, the whole pattern is ignored!

Answer 1

The problem is that your regular expression eats the character behind the second colon and the next search starts behind that character. That is why the first letter of module2 is not highlighted.

You may only look at the character behind the second colon but you may not include it into the regular expression.

Basis for a working version:

(defvar example-font-lock-keywords
  `((example-font-lock-matcher 1 font-lock-type-face)))

(defun example-font-lock-matcher (limit)
  "Matcher matching \"keyword::\" but only if we are not looking at < afterwards."
  (let (match)
    (while (and (setq match (re-search-forward "\\_<\\(\\(?:\\sw\\|\\s_\\)+\\)::" limit t))
        (eq (char-after) ?<)))
    match))

(define-derived-mode example-mode prog-mode "Example"
  (setq-local font-lock-defaults '(example-font-lock-keywords)))

I think that solves your question, but you have still work to do. For an instance you should consider whether there may be some whitespace in between the colon and the <.

If you actually always want to highlight the first keyword in the case that the two colons are followed by a further keyword the following version would be even more appropriate:

(defvar example-font-lock-keywords
  `(("\\_<\\(\\(?:\\sw\\|\\s_\\)+\\)::[[:space:]]*\\_<" 1 font-lock-type-face)))

(define-derived-mode example-mode prog-mode "Example"
  (setq-local font-lock-defaults '(example-font-lock-keywords)))

---

Allow me one further remark. It would be nice if you would not only post a picture of the text to be highlighted but also the text itself. This would ease the reconstruction of the problem for the people who want to help you. I do so here for other potential helpers:

use module1::function;
use module1::module2::function;

foo::<bar>