How can I make a custom font-lock keyword regex?

Question

I'm new to emacs (spacemacs) and have been trying to get org-mode to highlight quoted strings "like this" and quoted strings 'like this'. I followed the example here, and came up with this:

  ;; Fancy custom syntax highlighting of quoted strings!
  (add-hook 'org-mode-hook
            (lambda ()
              (font-lock-add-keywords nil '(("\".*?\"" 1 font-lock-constant-face t)
                                            ("'.*?'" 1 font-lock-doc-face t)))))

This seems to break syntax highlighting all together, and doesn't succeed in higlighting my quoted strings at all. What am I doing wrong?

Answer 1

For strings and comments it's better to use syntactic fontification. The syntax table for a given major mode has to be defined in variable <major-mode-name>-syntax-table.

This entry from the Emacs manual helps clarifying how Font Lock works: Font Lock mode finds text to highlight in two ways: through syntactic parsing based on the syntax table, and through searching (usually for regular expressions). Syntactic fontification happens first; it finds comments and string constants and highlights them. Search-based fontification happens second.

Now, there are a couple of problems with your code:

• Your regexp is badly written. To escape characters within a regexp string you have to use \\, so "\".*?\"" should be "\\".*?\\"".

• Don't put the code on a hook, just add (font-lock-add-keywords 'org-mode ...) to your init file. Otherwise, whenever you enter Org mode you'll be adding that entry to font-lock-keywords.

• The fontification definition is badly writen. The number is used for groups, which you have none and so you should be using ("\\".*?\\"" 0 'font-lock-constant-face t). Read the documentation of font-lock-keywords for further details. It's pretty helpful.

• If you need to use more complex regexps take a look at package rx.