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Scheme Regex literals in Emacs

· 3 min read
Jakub T. Jankiewicz
LIPS maintainer

LIPS Scheme support regular expression literals, but it's not the only one implementation that support those. Other implementation includes Gauche and Kawa.

Unfortunetlly, you can't easily use those regular expressions in GNU Emacs, my main editor of choice.

The problem

The main problem is when using vertical bar character inside Scheme code in Emacs. GNU Emacs thinks that the vertical bar is part of the symbol:

(let ((str "foo bar")
      (re #/foo|bar/)) ;; | ))
  (str.match re))
;; ==> #("foo")

This blog (the same as whole website) use modified PrismJS Scheme mode that supports regular expressions. But in GNU Emacs there was a need to add | after a comment and close the lists that were ignored by Emacs scheme mode (because they were inside symbol).

The solution

I asked a question on emacs-devel mailing list, on how to solve this problem. I didn't get any reply for days, then suddenly someone reply with this emacs lisp code snippet.

(defun scheme-regex-patch ()
   (lambda (start end)
     (goto-char start)
       ;; For #/regexp/ syntax
        (1 "|")
        (3 "|"))
       ;; For #; comment syntax
        (1 (prog1 "< cn"
              (point) end)))))
      (point) end))))

(add-hook 'scheme-mode-hook 'scheme-regex-patch)

This solution worked great, until I've found that it don't properly handle Scheme expression comments #;, that are part of the solution. In the meantime on the mailing list there was discussion about this feature (probably because it's part of GNU Kawa) to integrate with builtin scheme.el. So soon you may not need a hack like this when working with regular expressions.

This is a proposed solution after I said that the code doesn't work for Scheme expression comments.

(defun scheme-regex-patch ()
    (lambda (beg end)
      (goto-char beg)
      (scheme-syntax-propertize-sexp-comment2 end)
      (scheme-syntax-propertize-regexp end)
        ("\\(#\\);" (1 (prog1 "< cn"
                         (scheme-syntax-propertize-sexp-comment2 end))))
        ("\\(#\\)/" (1 (when (null (nth 8 (save-excursion
                                             (match-beginning 0)))))
                          (match-beginning 1)
                          (match-end 1)
                          'syntax-table (string-to-syntax "|"))
                         (scheme-syntax-propertize-regexp end)
       (point) end))))

(defun scheme-syntax-propertize-sexp-comment2 (end)
  (let ((state (syntax-ppss)))
    (when (eq 2 (nth 7 state))
      ;; It's a sexp-comment.  Tell parse-partial-sexp where it ends.
      (condition-case nil
            (goto-char (+ 2 (nth 8 state)))
            ;; FIXME: this doesn't handle the case where the sexp
            ;; itself contains a #; comment.
            (forward-sexp 1)
            (put-text-property (1- (point)) (point)
                               'syntax-table (string-to-syntax "> cn")))
        (scan-error (goto-char end))))))

(defun scheme-syntax-propertize-regexp (end)
  (let* ((state (syntax-ppss))
         (within-str (nth 3 state))
         (start-delim-pos (nth 8 state)))
    (when (and within-str
               (char-equal ?# (char-after start-delim-pos)))
           (re-search-forward "/" end 'move)
           (eq -1
               (% (save-excursion
                    (skip-chars-backward "\\\\")) 2))))
      (when (< (point) end)
           (match-beginning 0)
           (match-end 0)
           'syntax-table (string-to-syntax "|")))))))

(add-hook 'scheme-mode-hook 'scheme-regex-patch)

You can read the whole discussion on emacs-devel mailing list archive.