While there are packages that handle this, I would like to highlight a single word in source code comments, using for example the warning face, when a function call returns t, using the word as input.

How would this be achieved?

  • 1
    How is static analysis (the regex kind) supposed to know what a function call returns?
    – wasamasa
    Commented Apr 2, 2020 at 6:48
  • 1
    There are many packages that do similar things, e.g. my doxygen package: github.com/Lindydancer/highlight-doxygen . Can you give an example of how the comment and code looks like? Commented Apr 2, 2020 at 8:47
  • In which programming language is the source code written? What function should return t? An Elisp function that gets some context information and gets called from font-lock or a function from the source code that is called in the program the source code belongs to? Is the program run in a debugger?
    – Tobias
    Commented Apr 2, 2020 at 18:51
  • Misunderstanding, clarifying the question.
    – ideasman42
    Commented Apr 2, 2020 at 23:03

1 Answer 1


First study the doc-string of font-lock-keywords. There you have MATCHER which can be a regexp but also can be a function that is called with one arg and should act like re-search-forward.

In the MATCHER function you have the chance to consider other conditions, as that one that your function that should return t.

You can use the form (MATCHER . (SUBEXP FACENAME OVERRIDE)) for the keyword entry and append it to font-lock-keywords with the help of font-lock-add-keywords.


(defun my-highlight-fixme-p (s)
  "Example predicate function for `my-highlight-mode'."
  (string-equal s "FIXME"))

(defvar my-highlight-regexp (regexp-opt '("FIXME" "TODO") 'words)
  "Regexp matching in comments.")

(defvar my-highlight-predicate #'my-highlight-fixme-p
  "Register your predicate function here.")

(defvar my-highlight-keywords '((my-highlight-highlight
                  0 ;; subexpression
                  'warning ;; facename
                  t ;; override syntactic fontification for comments
  "One of the possible constructs of keywords in `font-lock-keywords'.
The HIGHLIGHT element of the keyword should always be `my-highlight-highlight'.")

(defun my-highlight-highlight (bound)
  "MATCHER for the font lock keyword in `my-highlight-mode'."
  (let (found)
    (while (and
        (setq found (re-search-forward my-highlight-regexp bound t))
         (and (nth 4 (syntax-ppss)) ;;  inside comment?
          (funcall my-highlight-predicate (match-string 0))))))

(define-minor-mode my-highlight-mode
  "Highlight matches for `my-highlight-regexp' in comments
if they also fulfill `my-highlight-predicate'."
  :lighter " κ"
  (if my-highlight-mode
      (font-lock-add-keywords nil my-highlight-keywords 'append) ;; append and override
    (font-lock-remove-keywords nil my-highlight-keywords))
  • my-highlight-highlight should loop until it finds a suitable match, or reach bound. If not, it will stop after the first occurrence of warning. Commented Apr 3, 2020 at 9:12
  • @Lindydancer Better so?
    – Tobias
    Commented Apr 3, 2020 at 10:03
  • Yes, this looks right! Commented Apr 3, 2020 at 11:18

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