diff --git a/lisp/oc-csl.el b/lisp/oc-csl.el
index 3d1388075..07916cd0b 100644
--- a/lisp/oc-csl.el
+++ b/lisp/oc-csl.el
@@ -270,11 +270,12 @@ If nil then the Chicago author-date style is used as a fallback.")
 (defconst org-cite-csl--label-regexp
   ;; Prior to Emacs-27.1 argument of `regexp' form must be a string literal.
   ;; It is the reason why `rx' is avoided here.
-  (rx-to-string `(seq word-start
-                      (regexp ,(regexp-opt (mapcar #'car org-cite-csl--label-alist) t))
-                      (0+ digit)
-                      (or word-start line-end (any ?\s ?\t)))
-                t)
+  (rx-to-string
+   `(seq word-start
+         (regexp ,(regexp-opt (mapcar #'car org-cite-csl--label-alist) t))
+         (0+ digit)
+         (or word-start line-end space ? ))
+   t)
   "Regexp matching a label in a citation reference suffix.
 Label is in match group 1.")