Improve org-link-plain-re

* lisp/ol.el (org-link-make-regexps): Allow URLs with up to two
levels of nested brackets.

* lisp/org.el (org-link-plain-re): Update docstring.

* testing/lisp/test-ol.el: Add tests for the plain link regular
expression.

(org-link-plain-re): Update docstring.  Now,

Now, URLs like [1] can be matched.  The new regexp is based on [2].
The docstring of `org-link-plain-re'  explicitly mentions that the
regexp must contain groups for the link type and the path.

[1] https://doi.org/10.1016/0160-791x(79)90023-x
[2] https://daringfireball.net/2010/07/improved_regex_for_matching_urls

lisp/ol.el

@@ -512,7 +512,10 @@ links more efficient."
   "Matches link with angular brackets, spaces are allowed.")
 
 (defvar org-link-plain-re nil
-  "Matches plain link, without spaces.")
+  "Matches plain link, without spaces.
+Group 1 must contain the link type (i.e. https).
+Group 2 must contain the link path (i.e. //example.com).
+Used by `org-element-link-parser'.")
 
 (defvar org-link-bracket-re nil
   "Matches a link in double brackets.")
@@ -800,15 +803,33 @@ This should be called after the variable `org-link-parameters' has changed."
 	  (format "<%s:\\([^>\n]*\\(?:\n[ \t]*[^> \t\n][^>\n]*\\)*\\)>"
 		  types-re)
 	  org-link-plain-re
-	  (concat
-	   "\\<" types-re ":"
-	   "\\([^][ \t\n()<>]+\\(?:([[:word:]0-9_]+)\\|\\([^[:punct:] \t\n]\\|/\\)\\)\\)")
-	  ;;	 "\\([^]\t\n\r<>() ]+[^]\t\n\r<>,.;() ]\\)")
-	  org-link-bracket-re
-	  (rx (seq "[["
-		   ;; URI part: match group 1.
-		   (group
-		    (one-or-more
+          (let* ((non-space-bracket "[^][ \t\n()<>]")
+	         (parenthesis
+		  `(seq "("
+		        (0+ (or (regex ,non-space-bracket)
+			        (seq "("
+				     (0+ (regex ,non-space-bracket))
+				     ")")))
+		        ")")))
+	    ;; Heuristics for an URL link inspired by
+	    ;; https://daringfireball.net/2010/07/improved_regex_for_matching_urls
+	    (rx-to-string
+	     `(seq word-start
+                   ;; Link type: match group 1.
+		   (regexp ,types-re)
+		   ":"
+                   ;; Link path: match group 2.
+                   (group
+		    (1+ (or (regex ,non-space-bracket)
+			    ,parenthesis))
+		    (or (regexp "[^[:punct:] \t\n]")
+		        ?/
+		        ,parenthesis)))))
+          org-link-bracket-re
+          (rx (seq "[["
+	           ;; URI part: match group 1.
+	           (group
+	            (one-or-more
                      (or (not (any "[]\\"))
 			 (and "\\" (zero-or-more "\\\\") (any "[]"))
 			 (and (one-or-more "\\") (not (any "[]"))))))

testing/lisp/test-ol.el

@@ -491,5 +491,115 @@
 	      (org-previous-link))
 	    (buffer-substring (point) (line-end-position))))))
 
+
+;;; Link regexps
+
+
+(defmacro test-ol-parse-link-in-text (text)
+  "Return list of :type and :path of link parsed in TEXT.
+\"<point>\" string must be at the beginning of the link to be parsed."
+  (declare (indent 1))
+  `(org-test-with-temp-text ,text
+     (list (org-element-property :type (org-element-link-parser))
+           (org-element-property :path (org-element-link-parser)))))
+
+(ert-deftest test-ol/plain-link-re ()
+  "Test `org-link-plain-re'."
+  (should
+   (equal
+    '("https" "//example.com")
+    (test-ol-parse-link-in-text
+        "(<point>https://example.com)")))
+  (should
+   (equal
+    '("https" "//example.com/qwe()")
+    (test-ol-parse-link-in-text
+        "(Some text <point>https://example.com/qwe())")))
+  (should
+   (equal
+    '("https" "//doi.org/10.1016/0160-791x(79)90023-x")
+    (test-ol-parse-link-in-text
+        "<point>https://doi.org/10.1016/0160-791x(79)90023-x")))
+  (should
+   (equal
+    '("file" "aa")
+    (test-ol-parse-link-in-text
+        "The <point>file:aa link")))
+  (should
+   (equal
+    '("file" "a(b)c")
+    (test-ol-parse-link-in-text
+        "The <point>file:a(b)c link")))
+  (should
+   (equal
+    '("file" "a()")
+    (test-ol-parse-link-in-text
+        "The <point>file:a() link")))
+  (should
+   (equal
+    '("file" "aa((a))")
+    (test-ol-parse-link-in-text
+        "The <point>file:aa((a)) link")))
+  (should
+   (equal
+    '("file" "aa(())")
+    (test-ol-parse-link-in-text
+        "The <point>file:aa(()) link")))
+  (should
+   (equal
+    '("file" "/a")
+    (test-ol-parse-link-in-text
+        "The <point>file:/a link")))
+  (should
+   (equal
+    '("file" "/a/")
+    (test-ol-parse-link-in-text
+        "The <point>file:/a/ link")))
+  (should
+   (equal
+    '("http" "//")
+    (test-ol-parse-link-in-text
+        "The <point>http:// link")))
+  (should
+   (equal
+    '("file" "ab")
+    (test-ol-parse-link-in-text
+        "The (some <point>file:ab) link")))
+  (should
+   (equal
+    '("file" "aa")
+    (test-ol-parse-link-in-text
+        "The <point>file:aa) link")))
+  (should
+   (equal
+    '("file" "aa")
+    (test-ol-parse-link-in-text
+        "The <point>file:aa( link")))
+  (should
+   (equal
+    '("http" "//foo.com/more_(than)_one_(parens)")
+    (test-ol-parse-link-in-text
+        "The <point>http://foo.com/more_(than)_one_(parens) link")))
+  (should
+   (equal
+    '("http" "//foo.com/blah_(wikipedia)#cite-1")
+    (test-ol-parse-link-in-text
+        "The <point>http://foo.com/blah_(wikipedia)#cite-1 link")))
+  (should
+   (equal
+    '("http" "//foo.com/blah_(wikipedia)_blah#cite-1")
+    (test-ol-parse-link-in-text
+        "The <point>http://foo.com/blah_(wikipedia)_blah#cite-1 link")))
+  (should
+   (equal
+    '("http" "//foo.com/unicode_(✪)_in_parens")
+    (test-ol-parse-link-in-text
+        "The <point>http://foo.com/unicode_(✪)_in_parens link")))
+  (should
+   (equal
+    '("http" "//foo.com/(something)?after=parens")
+    (test-ol-parse-link-in-text
+        "The <point>http://foo.com/(something)?after=parens link"))))
+
 (provide 'test-ol)
 ;;; test-ol.el ends here