From 4cb6cbb6faa23cf8c6c687ab5738bea2259d9918 Mon Sep 17 00:00:00 2001
From: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>
Date: Fri, 5 Dec 2025 18:45:15 +0200
Subject: [PATCH] [3.14] Revert "gh-140797: Forbid capturing groups in
 re.Scanner lexicon patterns (GH-140944) (GH-140982)" (#142309)

---
 Lib/re/__init__.py                             |  5 +----
 Lib/test/test_re.py                            | 18 ------------------
 ...5-12-05-17-58-29.gh-issue-140797.YxB27u.rst |  3 +++
 3 files changed, 4 insertions(+), 22 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-12-05-17-58-29.gh-issue-140797.YxB27u.rst

diff --git a/Lib/re/__init__.py b/Lib/re/__init__.py
index e0d6c844b4fa..af2808a77da6 100644
--- a/Lib/re/__init__.py
+++ b/Lib/re/__init__.py
@@ -399,12 +399,9 @@ class Scanner:
         s = _parser.State()
         s.flags = flags
         for phrase, action in lexicon:
-            sub_pattern = _parser.parse(phrase, flags)
-            if sub_pattern.state.groups != 1:
-                raise ValueError("Cannot use capturing groups in re.Scanner")
             gid = s.opengroup()
             p.append(_parser.SubPattern(s, [
-                (SUBPATTERN, (gid, 0, 0, sub_pattern)),
+                (SUBPATTERN, (gid, 0, 0, _parser.parse(phrase, flags))),
                 ]))
             s.closegroup(gid, p[-1])
         p = _parser.SubPattern(s, [(BRANCH, (None, p))])
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index b7ed86849cb1..f6e797b3785d 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -1639,24 +1639,6 @@ class ReTests(unittest.TestCase):
                          (['sum', 'op=', 3, 'op*', 'foo', 'op+', 312.5,
                            'op+', 'bar'], ''))
 
-    def test_bug_gh140797(self):
-        # gh140797: Capturing groups are not allowed in re.Scanner
-
-        msg = r"Cannot use capturing groups in re\.Scanner"
-        # Capturing group throws an error
-        with self.assertRaisesRegex(ValueError, msg):
-            Scanner([("(a)b", None)])
-
-        # Named Group
-        with self.assertRaisesRegex(ValueError, msg):
-            Scanner([("(?P<name>a)", None)])
-
-        # Non-capturing groups should pass normally
-        s = Scanner([("(?:a)b", lambda scanner, token: token)])
-        result, rem = s.scan("ab")
-        self.assertEqual(result,['ab'])
-        self.assertEqual(rem,'')
-
     def test_bug_448951(self):
         # bug 448951 (similar to 429357, but with single char match)
         # (Also test greedy matches.)
diff --git a/Misc/NEWS.d/next/Library/2025-12-05-17-58-29.gh-issue-140797.YxB27u.rst b/Misc/NEWS.d/next/Library/2025-12-05-17-58-29.gh-issue-140797.YxB27u.rst
new file mode 100644
index 000000000000..ebbe06fddfb3
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-12-05-17-58-29.gh-issue-140797.YxB27u.rst
@@ -0,0 +1,3 @@
+Revert changes to the undocumented :class:`!re.Scanner` class. Capturing
+groups are still allowed for backward compatibility, although using them can
+lead to incorrect result. They will be forbidden in future Python versions.
-- 
2.47.3