From fa9c3eefd475f0647a69bf3f49db8100848fb6a9 Mon Sep 17 00:00:00 2001
From: Abhishek Tiwari <Abhi210@users.noreply.github.com>
Date: Tue, 4 Nov 2025 16:24:28 +0530
Subject: [PATCH] gh-140797: Forbid capturing groups in re.Scanner lexicon
 patterns (GH-140944)

---
 Lib/re/__init__.py                             |  5 ++++-
 Lib/test/test_re.py                            | 18 ++++++++++++++++++
 ...5-11-03-16-23-54.gh-issue-140797.DuFEeR.rst |  2 ++
 3 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2025-11-03-16-23-54.gh-issue-140797.DuFEeR.rst

diff --git a/Lib/re/__init__.py b/Lib/re/__init__.py
index a5316391297f..ecec16e9005f 100644
--- a/Lib/re/__init__.py
+++ b/Lib/re/__init__.py
@@ -397,9 +397,12 @@ 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, _parser.parse(phrase, flags))),
+                (SUBPATTERN, (gid, 0, 0, sub_pattern)),
                 ]))
             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 5fc95087f2b6..9f6f04bf6b83 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -1639,6 +1639,24 @@ 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/2025-11-03-16-23-54.gh-issue-140797.DuFEeR.rst b/Misc/NEWS.d/next/Library/2025-11-03-16-23-54.gh-issue-140797.DuFEeR.rst
new file mode 100644
index 000000000000..493b740261e6
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-11-03-16-23-54.gh-issue-140797.DuFEeR.rst
@@ -0,0 +1,2 @@
+The undocumented :class:`!re.Scanner` class now forbids regular expressions containing capturing groups in its lexicon patterns. Patterns using capturing groups could
+previously lead to crashes with segmentation fault. Use non-capturing groups (?:...) instead.
-- 
2.47.3