Extend backward link searching function

Now support CUSTOM_ID as well, and check arguments for plausibility.

site-lisp/db-org.el

@@ -595,16 +595,30 @@ This is done only if the value of this variable is not null."
 
 ;;; Find items by link to current headline
 
-(defun db/org-find-items-linking-to-id (&optional id)
+;; TODO: expand: let the user choose the item that is to be linked to
+;; interactively
+(defun db/org-find-items-linking-by-id (&optional id custom-id)
   "List all Org Mode items that link to ID.
 Uses `org-search-view' to conduct the actual search.  ID must be
-a UUID as generated by, e.g., `org-id-get-create'.  If ID is not
-given, use the ID of the current item, if in Org Mode."
-  (interactive (list (when (derived-mode-p 'org-mode)
-                       (org-id-get))))
-  (if id
-      (org-search-view nil (format "[id:%s]" id))
-    (user-error "No ID given and not in Org Mode.")))
+a UUID as generated by, e.g., `org-id-get-create', and CUSTOM-ID
+must consist of ASCII letters, numbers, and hyphens only.  If ID
+and CUSTOM-ID are not given, use the values of the current item
+if in Org Mode."
+  (interactive (when (derived-mode-p 'org-mode)
+                 (list (org-id-get) (org-entry-get nil "CUSTOM_ID"))))
+
+  (unless (string-match-p  "^[a-f0-9]\\{8\\}-[a-f0-9]\\{4\\}-[a-f0-9]\\{4\\}-[a-f0-9]\\{4\\}-[a-f0-9]\\{12\\}$" id)
+    (user-error "Given ID is not a valid UUID: %s" id))
+  (unless (string-match-p "[-a-zA-Z0-9]" custom-id)
+    ;; sorry, only ASCII right now …
+    (user-error "CUSTOM_ID must consist of alphanumeric charaters only"))
+
+  (let ((query (cond
+                ((and id custom-id) (format "{\\[\\[id:%s\\]\\|\\[\\[#%s\\]}" id custom-id))
+                (id (format "[[id:%s]" id))
+                (custom-id (format "[[#%s]" custom-id))
+                (t (user-error "No ID given and not in Org Mode.")))))
+    (org-search-view nil query)))
 
 
 ;;; End