oc: rx compatibility with Emacs < 27.1

* lisp/oc.el (org-cite-adjust-note):
* lisp/oc-basic.el (org-cite-basic-goto):
* lisp/oc-biblatex.el (org-cite-biblatex-prepare-preamble):
* lisp/oc-csl.el (org-cite-csl--label-regexp): Avoid `rx' forms
introduced in Emacs-27.1 to keep compatibility with older versions.

Use `rx-to-string' instead of `rx' due to lack of support of  `regexp'
and `literal' forms with a variable as an argument.

lisp/oc-basic.el

@@ -667,7 +667,9 @@ present in the citation."
     (org-open-file file '(4))
     (pcase (file-name-extension file)
       ("json"
-       (let ((regexp (rx "\"id\":" (0+ (any "[ \t]")) "\"" (literal key) "\"")))
+       ;; `rx' can not be used with Emacs <27.1 since `literal' form
+       ;; is not supported.
+       (let ((regexp (rx-to-string `(seq "\"id\":" (0+ (any "[ \t]")) "\"" ,key "\"") t)))
          (goto-char (point-min))
          (re-search-forward regexp)
          (search-backward "{")))

lisp/oc-biblatex.el

@@ -260,7 +260,7 @@ to the document, and set styles."
       ;; Then set options.
       (goto-char (match-beginning 0))
       (let ((re (rx "\\usepackage"
-                    (opt (group "[" (*? anychar) "]"))
+                    (opt (group "[" (*? anything) "]"))
                     "{biblatex}")))
         (cond
          ;; No "biblatex" package loaded.  Insert "usepackage" command

lisp/oc-csl.el

@@ -253,10 +253,13 @@ If nil then the Chicago author-date style is used as a fallback.")
   "Alist mapping locator names to locators.")
 
 (defconst org-cite-csl--label-regexp
-  (rx word-start
-      (regexp (regexp-opt (mapcar #'car org-cite-csl--label-alist) t))
-      (0+ digit)
-      (or word-start line-end (any ?\s ?\t)))
+  ;; 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)
   "Regexp matching a label in a citation reference suffix.
 Label is in match group 1.")
 

lisp/oc.el

@@ -878,14 +878,19 @@ the same object, call `org-cite-adjust-punctuation' first."
   (when org-cite-adjust-note-numbers
     (pcase-let* ((rule (or rule (org-cite--get-note-rule info)))
                  (punct-re (regexp-opt (or punct org-cite-punctuation-marks)))
+                 ;; with Emacs <27.1. Argument of `regexp' form (PUNCT-RE this case)
+                 ;; must be a string literal.
                  (previous-punct-re
-                  (rx (opt (group (0+ (any blank ?\n)) (regexp punct-re)))
-                      (opt (0+ (any blank ?\n)) (group ?\"))
-                      (opt (group (1+ (any blank ?\n))))
-                      string-end))
+                  (rx-to-string `(seq (opt (group (regexp ,(rx (0+ (any blank ?\n))))
+                                                  (regexp ,punct-re)))
+                                      (regexp ,(rx (opt (0+ (any blank ?\n)) (group ?\"))
+                                                   (opt (group (1+ (any blank ?\n))))
+                                                   string-end)))
+                                t))
                  (next-punct-re
-                  (rx string-start
-                      (group (0+ (any blank ?\n)) (regexp punct-re))))
+                  (rx-to-string `(seq string-start
+                                      (group (0+ (any blank ?\n)) (regexp ,punct-re)))
+                                t))
                  (next (org-export-get-next-element citation info))
                  (final-punct
                   (and (stringp next)
@@ -928,7 +933,9 @@ the same object, call `org-cite-adjust-punctuation' first."
                      (concat final-punct "\"") previous nil nil 2))
                    (new-next
                     (replace-regexp-in-string
-                     (rx string-start (literal final-punct))
+                     ;; Before Emacs-27.1 `literal' `rx' form with a variable
+                     ;; as an argument is not available.
+                     (rx-to-string `(seq string-start ,final-punct) t)
                      "" next)))
                (org-element-set-element previous new-prev)
                (org-element-set-element next new-next)