I'm writing a major mode to handle syntax highlighting for a language that allows namespaces just like in C++ e.g. somewhere::out_there.

I'd like to highlight the text somewhere, but leave the namespace operator :: in the default face (as is done in cc-mode). So I'd like to match text that is followed by a pattern, without matching that pattern too.

I spent some time working with lookahead stuff, trying things like [a-zA-Z]+\(?::\) but it seems I've bumped into the optional group syntax \(?: ... \) that's just optionally including a single : in the match.

Searching around for methods to fix this, it seems that emacs doesn't actually support lookahead at all!

Since the solution may not be regex-based, let me give a bit more context: I'm setting font-lock-defaults and define-derived-mode, and my code follows this form:

;; define regexes for different text categories
(setq aql-numeric-regexp "[-+]?\\.?\\_<[0-9]*\\.?[0-9]+\\.?\\(?:[eE][-+]?[0-9]+\\)?\\_>\\.?")
(setq aql-bindvar-regexp "\\(^\\|[ \t]\\)@@?[a-zA-z_]+")

;; Associate categories with faces                                                                                                                                                                                                             
(setq aql-font-lock-keywords
    (,aql-numeric-regexp . font-lock-warning-face)
    (,aql-bindvar-regexp . font-lock-variable-name-face)

and then inside define-derived-mode I include:

(setq font-lock-defaults '((aql-font-lock-keywords)))
(set (make-local-variable 'font-lock-defaults) '(aql-font-lock-keywords nil t))

Any strategies for dealing with this sort of thing? Any workarounds that will enable me to approximate lookaheads? Or other methods to tell emacs what to highlight?

1 Answer 1


You can match for something like \(\_<[a-zA-Z_][a-zA-Z0-9_]*\):: and specify that group number 1 should be highlighted.

  • After digging around a bit, I figured out how to do what you suggest. I added (setq aql-namespace-regexp "\\([a-zA-Z]+\\)::") and then updated aql-font-lock-keywords with (,aql-namespace-regexp 1 font-lock-constant-face). Replacing the . with the group number is the key.
    – mattsilver
    Sep 5, 2016 at 16:15
  • You can also write it as (,aql-namespace-regexp . (1 font-lock-constant-face)) if you prefer.
    – troelskn
    Nov 26, 2016 at 22:33

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