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+;;; syntax-tests.el --- tests for syntax.el  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2020-2021 Free Software Foundation, Inc.
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(require 'ert)
+(require 'syntax)
+
+(ert-deftest syntax-propertize--shift-groups-and-backrefs ()
+  "Test shifting of numbered groups and back-references in regexps."
+  ;; A numbered group must be shifted.
+  (should
+   (string=
+    (syntax-propertize--shift-groups-and-backrefs
+     "\\(?2:[abc]+\\)foobar" 2)
+    "\\(?4:[abc]+\\)foobar"))
+  ;; A back-reference \1 on a normal sub-regexp context must be
+  ;; shifted.
+  (should
+   (string=
+    (syntax-propertize--shift-groups-and-backrefs "\\(a\\)\\1" 2)
+    "\\(a\\)\\3"))
+  ;; Shifting must not happen if the \1 appears in a character class,
+  ;; or in a \{\} repetition construct (although \1 isn't valid there
+  ;; anyway).
+  (let ((rx-with-class "\\(a\\)[\\1-2]")
+        (rx-with-rep "\\(a\\)\\{1,\\1\\}"))
+    (should
+     (string=
+      (syntax-propertize--shift-groups-and-backrefs rx-with-class 2)
+      rx-with-class))
+    (should
+     (string=
+      (syntax-propertize--shift-groups-and-backrefs rx-with-rep 2)
+      rx-with-rep)))
+  ;; Now numbered groups and back-references in combination.
+  (should
+   (string=
+    (syntax-propertize--shift-groups-and-backrefs
+     "\\(?2:[abc]+\\)foo\\(\\2\\)" 2)
+    "\\(?4:[abc]+\\)foo\\(\\4\\)"))
+  ;; Emacs supports only the back-references \1,...,\9, so when a
+  ;; shift would result in \10 or more, an error must be signalled.
+  (should-error
+   (syntax-propertize--shift-groups-and-backrefs "\\(a\\)\\3" 7)))
+
+;; Local Variables:
+;; no-byte-compile: t
+;; End:
+
+;;; syntax-tests.el ends here.