Change no brace sub/superscript syntax

* lisp/org.el (org-match-substring-regexp): Simplify syntax when not
using braces.  Also make the variable a defconst.
(org-match-substring-with-braces-regexp): Make the variable a defconst
and remove a useless group in regexp.
* lisp/org-element.el (org-element-subscript-parser,
  org-element-superscript-parser): Adapt to group changes.

lisp/org-element.el

@@ -3385,13 +3385,12 @@ Return a list whose CAR is `subscript' and CDR a plist with
 Assume point is at the underscore."
   (save-excursion
     (unless (bolp) (backward-char))
-    (let ((bracketsp (if (looking-at org-match-substring-with-braces-regexp)
-			 t
-		       (not (looking-at org-match-substring-regexp))))
+    (looking-at org-match-substring-regexp)
+    (let ((bracketsp (match-beginning 4))
 	  (begin (match-beginning 2))
-	  (contents-begin (or (match-beginning 5)
+	  (contents-begin (or (match-beginning 4)
 			      (match-beginning 3)))
-	  (contents-end (or (match-end 5) (match-end 3)))
+	  (contents-end (or (match-end 4) (match-end 3)))
 	  (post-blank (progn (goto-char (match-end 0))
 			     (skip-chars-forward " \t")))
 	  (end (point)))
@@ -3434,12 +3433,12 @@ Return a list whose CAR is `superscript' and CDR a plist with
 Assume point is at the caret."
   (save-excursion
     (unless (bolp) (backward-char))
-    (let ((bracketsp (if (looking-at org-match-substring-with-braces-regexp) t
-		       (not (looking-at org-match-substring-regexp))))
+    (looking-at org-match-substring-regexp)
+    (let ((bracketsp (match-beginning 4))
 	  (begin (match-beginning 2))
-	  (contents-begin (or (match-beginning 5)
+	  (contents-begin (or (match-beginning 4)
 			      (match-beginning 3)))
-	  (contents-end (or (match-end 5) (match-end 3)))
+	  (contents-end (or (match-end 4) (match-end 3)))
 	  (post-blank (progn (goto-char (match-end 0))
 			     (skip-chars-forward " \t")))
 	  (end (point)))

lisp/org.el

@@ -5638,21 +5638,20 @@ stacked delimiters is N.  Escaping delimiters is not possible."
 	    next (concat "\\(?:" nothing left next right "\\)+" nothing)))
     (concat left "\\(" re "\\)" right)))
 
-(defvar org-match-substring-regexp
+(defconst org-match-substring-regexp
   (concat
    "\\(\\S-\\)\\([_^]\\)\\("
-   "\\(" (org-create-multibrace-regexp "{" "}" org-match-sexp-depth) "\\)"
+   "\\(?:" (org-create-multibrace-regexp "{" "}" org-match-sexp-depth) "\\)"
    "\\|"
-   "\\(" (org-create-multibrace-regexp "(" ")" org-match-sexp-depth) "\\)"
+   "\\(?:" (org-create-multibrace-regexp "(" ")" org-match-sexp-depth) "\\)"
    "\\|"
-   "\\(\\(?:\\*\\|[-+]?[^-+*!@#$%^_ \t\r\n,:\"?<>~;./{}=()]+\\)\\)\\)")
+   "\\(?:\\*\\|[+-]?[[:alnum:].,\\]*[[:alnum:]]\\)\\)")
   "The regular expression matching a sub- or superscript.")
 
-(defvar org-match-substring-with-braces-regexp
+(defconst org-match-substring-with-braces-regexp
   (concat
-   "\\(\\S-\\)\\([_^]\\)\\("
-   "\\(" (org-create-multibrace-regexp "{" "}" org-match-sexp-depth) "\\)"
-   "\\)")
+   "\\(\\S-\\)\\([_^]\\)"
+   "\\(" (org-create-multibrace-regexp "{" "}" org-match-sexp-depth) "\\)")
   "The regular expression matching a sub- or superscript, forcing braces.")
 
 (defun org-make-link-regexps ()
@@ -6089,8 +6088,6 @@ by a #."
 
 (defvar org-latex-and-related-regexp nil
   "Regular expression for highlighting LaTeX, entities and sub/superscript.")
-(defvar org-match-substring-regexp)
-(defvar org-match-substring-with-braces-regexp)
 
 (defun org-compute-latex-and-related-regexp ()
   "Compute regular expression for LaTeX, entities and sub/superscript.