Change "\..." entities into "\_ "

* lisp/org-element.el (org-element--set-regexps):
(org-element-entity-parser): Update regexps.

* lisp/org-entities.el (org-entities): Modify syntax for space
  entities.

* lisp/org.el (org-fontify-entities): Update comment.

* lisp/org-clock.el (org-clocktable-indent-string): Apply syntax
  change.

* etc/ORG-NEWS (title): Update announce.

Use a less verbose syntax, which cannot interfere with file paths.

etc/ORG-NEWS

@@ -316,18 +316,10 @@ It is now possible to supply and optional formatting argument to
 *** New option ~org-export-with-title~
 It is possible to suppress the title insertion with ~#+OPTIONS:
 title:nil~ or globally using the variable ~org-export-with-title~.
-*** New entities family: "\...."
-"\..." are used to insert up to 20 contiguous spaces in various
+*** New entities family: "\_ "
+"\_ " are used to insert up to 20 contiguous spaces in various
 back-ends.  In particular, this family can be used to introduce
 leading spaces within table cells.
-
-Also, unlike to other entities, these can be used mid-word, e.g.,
-
-  wo\..rd
-
-Curly brackets can still be added before a dot, e.g.,
-
-  This is the end\...{}.
 *** New MathJax configuration options.
 Org uses the MathJax CDN by default.  See the manual and the docstring
 of ~org-html-mathjax-options~ for details.

lisp/org-clock.el

@@ -2644,7 +2644,7 @@ from the dynamic block definition."
   "Return indentation string according to LEVEL.
 LEVEL is an integer.  Indent by two spaces per level above 1."
   (if (= level 1) ""
-    (concat "\\" (make-string (- (* 2 level) 3) ?.) " ")))
+    (concat "\\_" (make-string (* 2 (1- level)) ?\s))))
 
 (defun org-clocktable-steps (params)
   "Step through the range to make a number of clock tables."

lisp/org-element.el

@@ -207,7 +207,7 @@ specially in `org-element--object-lex'.")
 		      "\\$"
 		      ;; Objects starting with "\": line break,
 		      ;; entity, latex fragment.
-		      "\\\\\\(?:[a-zA-Z[(.]\\|\\\\[ \t]*$\\)"
+		      "\\\\\\(?:[a-zA-Z[(]\\|\\\\[ \t]*$\\|_ +\\)"
 		      ;; Objects starting with raw text: inline Babel
 		      ;; source block, inline Babel call.
 		      "\\(?:call\\|src\\)_"))
@@ -2722,7 +2722,7 @@ a plist with `:begin', `:end', `:latex', `:latex-math-p',
 
 Assume point is at the beginning of the entity."
   (catch 'no-object
-    (when (looking-at "\\\\\\(?:\\(?1:\\.+\\)\\(?2:{}\\)?\\|\\(?1:there4\\|sup[123]\\|frac[13][24]\\|[a-zA-Z]+\\)\\(?2:$\\|{}\\|[^[:alpha:]]\\)\\)")
+    (when (looking-at "\\\\\\(?:\\(?1:_ +\\)\\|\\(?1:there4\\|sup[123]\\|frac[13][24]\\|[a-zA-Z]+\\)\\(?2:$\\|{}\\|[^[:alpha:]]\\)\\)")
       (save-excursion
 	(let* ((value (or (org-entity-get (match-string 1))
 			  (throw 'no-object nil)))

lisp/org-entities.el

@@ -515,12 +515,12 @@ packages to be loaded, add these packages to `org-latex-packages-alist'."
      ("diamond" "\\diamondsuit" t "⋄" "[diamond]" "[diamond]" "◆")
      ("Diamond" "\\diamondsuit" t "⋄" "[diamond]" "[diamond]" "◆")
      ("loz" "\\lozenge" t "◊" "[lozenge]" "[lozenge]" "⧫"))
-   ;; Add "\...."-entity family for spaces.
-   (let (space-entities html-spaces)
+   ;; Add "\_ "-entity family for spaces.
+   (let (space-entities html-spaces (entity "_"))
      (dotimes (n 20 (nreverse space-entities))
        (let ((n (+ 1 n))
 	     (spaces (make-string n ?\s)))
-	 (push (list (make-string n ?.)
+	 (push (list (setq entity (concat entity " "))
 		     (format "\\hspace*{%sem}" (* n .5))
 		     nil
 		     (setq html-spaces (concat " " html-spaces))

lisp/org.el

@@ -6456,9 +6456,9 @@ needs to be inserted at a specific position in the font-lock sequence.")
   (let (ee)
     (when org-pretty-entities
       (catch 'match
-	;; \...-family is left out on purpose.  Only the first one,
-	;; i.e., "\.", could be fontified anyway, and it would be
-	;; confusing when adding more dots.
+	;; "\_ "-family is left out on purpose.  Only the first one,
+	;; i.e., "\_ ", could be fontified anyway, and it would be
+	;; confusing when adding more underscores.
 	(while (re-search-forward
 		"\\\\\\(there4\\|sup[123]\\|frac[13][24]\\|[a-zA-Z]+\\)\\($\\|{}\\|[^[:alpha:]\n]\\)"
 		limit t)