Reading the section regular expressions in EmacsWiki appears this:

You can use a tool to construct regexps. For example, you can use ‘rx’ like this:

(rx (or (and "\*" (*? anything) "*/") (and "//" (*? anything) eol)))

To produce this regexp (which matches C-style multiline and single line comments):


I get the or will match either C-style multiline or C/C++ single line comments.
But I do not get the and parts.

  1. I am not sure how anything works and am assuming that will match any char.
    It might be something equivalent to .*. Is it?

  2. If I got it right, *? is the non-greedy variant of * operation.
    Which really confuses me... The smallest possible match for anything?

  3. Why the C-style multi-line opening is shown as "\*"? I suspect it is a typo on EmacsWiki and it should be "/*".

  • 1
    See also the documentation for rx under (info "(elisp) Rx Notation").
    – Basil
    Commented Oct 19, 2020 at 11:39
  • . matches any char except newline. So .* matches zero or more non-newline chars. .\\|\n matches any char (including newline).
    – Drew
    Commented Oct 19, 2020 at 16:34

1 Answer 1


anything matches any character, while regexp . matches any character except a newline, the rx representation is not-newline.

*? matches zero-or-more, non-greedy. Let's extract comment from C code:

/* comment */ char *s = "*/";
;; * is zero-or-more, greedy
(let ((string "/* comment */ char *s = \"*/\""))
  (when (string-match (rx "/*" (* anything) "*/") string)
    (match-string 0 string)))
;; => "/* comment */ char *s = \"*/"

;; *? is zero-or-more, non-greedy
(let ((string "/* comment */ char *s = \"*/\""))
  (when (string-match (rx "/*" (*? anything) "*/") string)
    (match-string 0 string)))
;; => "/* comment */"

Check the doc via C-h f rx or (info "(elisp) Rx Notation").

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.