org-columns-compile-format: Fix regexp

* lisp/org-colview.el (org-columns-compile-format): Refactor
string-based regexp to `rx' form.  Fix regexp: When an empty
parentheses () is in the column format string then the regexp
can't capture the operator which is in curly brackets {}.
(org-columns-new): prevent adding empty parantheses '()' to 'column
format string' when 'column title' is empty.
* testing/lisp.test-org-colview.el (test-org-colview/compile-format):
New test case: check if operator will be captured when there is
empty parantheses '()' in 'column format string'.

Reported-by: Sławomir Grochowski <slawomir.grochowski@gmail.com>
Link: https://list.orgmode.org/orgmode/877ccczt83.fsf@gmail.com/

lisp/org-colview.el

@@ -935,8 +935,10 @@ details."
 			 (mapcar #'list (org-buffer-property-keys t nil t))
 			 nil nil (nth 0 spec))))
 		   (list prop
-			 (read-string (format "Column title [%s]: " prop)
-				      (nth 1 spec))
+                         ;; Discard useless whitespace-only titles.
+			 (org-string-nw-p
+                          (read-string (format "Column title [%s]: " prop)
+				       (nth 1 spec)))
 			 ;; Use `read-string' instead of `read-number'
 			 ;; to allow empty width.
 			 (let ((w (read-string
@@ -1183,14 +1185,18 @@ This function updates `org-columns-current-fmt-compiled'."
   (setq org-columns-current-fmt-compiled nil)
   (let ((start 0))
     (while (string-match
-	    "%\\([0-9]+\\)?\\([[:alnum:]_-]+\\)\\(?:(\\([^)]+\\))\\)?\
-\\(?:{\\([^}]+\\)}\\)?\\s-*"
-	    fmt start)
+            (rx "%"
+                (optional (group (+ digit)))
+                (group (one-or-more (in alnum "_-")))
+                (optional "(" (group (zero-or-more (not (any ")")))) ")")
+                (optional "{" (group (zero-or-more (not (any "}")))) "}")
+                (zero-or-more space))
+    	    fmt start)
       (setq start (match-end 0))
       (let* ((width (and (match-end 1) (string-to-number (match-string 1 fmt))))
 	     (prop (match-string-no-properties 2 fmt))
-	     (title (or (match-string-no-properties 3 fmt) prop))
-	     (operator (match-string-no-properties 4 fmt)))
+	     (title (or (org-string-nw-p (match-string-no-properties 3 fmt)) prop))
+	     (operator (org-string-nw-p (match-string-no-properties 4 fmt))))
 	(push (if (not operator) (list (upcase prop) title width nil nil)
 		(let (printf)
 		  (when (string-match ";" operator)

testing/lisp/test-org-colview.el

@@ -90,7 +90,12 @@
   (should
    (equal `(("ITEM" "ITEM" nil "+" "%.1f"))
           (org-columns-compile-format
-           "%ITEM{+;%.1f}"))))
+           "%ITEM{+;%.1f}")))
+  (should
+   ;; Bug https://list.orgmode.org/orgmode/877ccczt83.fsf@gmail.com/
+   (equal '(("ITEM" "ITEM" nil "X" nil))
+          (org-columns-compile-format
+           "%ITEM(){X}"))))
 
 (ert-deftest test-org-colview/substring-below-width ()
   "Test `org-columns--truncate-below-width'."