gh-140797: Forbid capturing groups in re.Scanner lexicon patterns #140944

Abhi210 · 2025-11-03T11:16:30Z

This PR adds validation to re.Scanner.__init__ that rejects lexicon patterns containing capturing groups. If a user-supplied pattern contains any capturing groups, Scanner now raises ValueError with a clear message advising the use of non-capturing groups (?:...) instead. Further, tests were added to assert ValueError for lexicons containing capturing groups and a passing test for non-capturing group.

Issue: Crash when using multiple capturing groups in re.Scanner #140797

Lib/re/__init__.py

Lib/test/test_re.py

Lib/re/__init__.py

It adds validation to re.Scanner.init that rejects lexicon patterns containing capturing groups. If a user-supplied pattern contains any capturing groups, Scanner now raises ValueError with a clear message advising the use of non-capturing groups (?:...) instead.

wjssz

After you update the patch, you may ask core developer Serhiy to review.

Lib/re/__init__.py

Lib/test/test_re.py

wjssz

It seems that you are a beginner, please be patient.
After my review is completed, then ask core developer Serhiy to review.

wjssz · 2025-11-04T05:43:32Z

Lib/re/__init__.py

        for phrase, action in lexicon:
+            sub_pattern = _parser.parse(phrase, flags)
+            if sub_pattern.state.groups != 1:
+                raise ValueError(


You can write in one line. A line should <= 80 characters.

raise ValueError("Can not use capturing groups in re.Scanner.")

Thank you for the suggestion. I have resolved it now

wjssz · 2025-11-04T05:48:37Z

Lib/test/test_re.py

+
+        #Capturing group throws an error
+        lex = [("(a)b", None)]
+        with self.assertRaises(ValueError):


You may check exception message.

msg = "Can not use capturing groups in re.Scanner" with self.assertRaisesRegex(ValueError, msg): ...

Thank you for the suggestion! I have resolved it now. Need to learn a lot!

wjssz · 2025-11-04T06:44:45Z

Lib/test/test_re.py

                           'op+', 'bar'], ''))

+    def test_bug_140797(self):
+        #bug 140797: remove capturing groups compilation form re.Scanner


Please add a space after # in comments.

- #Capturing group throws an error + # Capturing group throws an error

And add a space after , in functions arguments.

- with self.assertRaisesRegex(ValueError,msg): + with self.assertRaisesRegex(ValueError, msg):

Then looks good to me.

Oops! Thank you again! Resolved

wjssz · 2025-11-04T06:58:48Z

Lib/test/test_re.py

                         (['sum', 'op=', 3, 'op*', 'foo', 'op+', 312.5,
                           'op+', 'bar'], ''))

+    def test_bug_140797(self):


My negligence, please use test_bug_gh140797 as the name.
If no "gh", It may refer to the previous bug tracker.
Sorry for this.

- # bug 140797: remove capturing groups compilation form re.Scanner + # gh140797: capturing groups is not allowed in re.Scanner

Done! Thank you again for your time and suggestions!

wjssz · 2025-11-04T07:12:54Z

Lib/test/test_re.py

+        # Capturing group throws an error
+        lex = [("(a)b", None)]
+        with self.assertRaisesRegex(ValueError, msg):
+            Scanner(lex)


This saves a line...
My fault again.

- Scanner(lex) + Scanner([("(a)b", None)])

Done! Testing sure takes the time 😂

wjssz · 2025-11-04T07:29:55Z

I have checked basic problems, @serhiy-storchaka please review.

serhiy-storchaka

Thank you for your PR @Abhi210 and thank you for your review @wjssz.

There are some minor nitpicks, overall LGTM.

serhiy-storchaka · 2025-11-04T08:01:04Z

Lib/re/__init__.py

        for phrase, action in lexicon:
+            sub_pattern = _parser.parse(phrase, flags)
+            if sub_pattern.state.groups != 1:
+                raise ValueError("Can not use capturing groups in re.Scanner")


"Cannot". This is the most commonly used variant.

Done! Thank you

Lib/test/test_re.py

serhiy-storchaka · 2025-11-04T08:07:31Z

Misc/NEWS.d/next/Library/2025-11-03-16-23-54.gh-issue-140797.DuFEeR.rst

@@ -0,0 +1,4 @@
+The re.Scanner class now forbids regular expressions containing capturing


Mention that that class is undocumented. You can also use some formatting, even if the link does not work: :class:`!re.Scanner`.

Done! Thank you 😊

…s- Rev-7

serhiy-storchaka

Two more nitpicks and LGTM. 👍

Lib/test/test_re.py

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

miss-islington-app · 2025-11-04T10:54:56Z

Thanks @Abhi210 for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

miss-islington-app · 2025-11-04T10:54:56Z

Thanks @Abhi210 for the PR, and @serhiy-storchaka for merging it 🌮🎉.. I'm working now to backport this PR to: 3.13.
🐍🍒⛏🤖

…ns (pythonGH-140944) (cherry picked from commit fa9c3ee) Co-authored-by: Abhishek Tiwari <Abhi210@users.noreply.github.com>

bedevere-app · 2025-11-04T10:55:05Z

GH-140982 is a backport of this pull request to the 3.14 branch.

…ns (pythonGH-140944) (cherry picked from commit fa9c3ee) Co-authored-by: Abhishek Tiwari <Abhi210@users.noreply.github.com>

bedevere-app · 2025-11-04T10:55:11Z

GH-140983 is a backport of this pull request to the 3.13 branch.

…rns (GH-140944) (GH-140983) (cherry picked from commit fa9c3ee) Co-authored-by: Abhishek Tiwari <Abhi210@users.noreply.github.com>

…rns (GH-140944) (GH-140982) (cherry picked from commit fa9c3ee) Co-authored-by: Abhishek Tiwari <Abhi210@users.noreply.github.com>

bedevere-app bot added the awaiting review label Nov 3, 2025

bedevere-app bot mentioned this pull request Nov 3, 2025

Crash when using multiple capturing groups in re.Scanner #140797

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pythongh-140797 forbid capturing groups in re.Scanner lexicon pattern…

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…s- Rev-7

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Abhi210 and others added 2 commits November 4, 2025 15:29

Update Lib/test/test_re.py

d1d582e

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

Update Lib/test/test_re.py

f4f7d4d

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>

serhiy-storchaka merged commit fa9c3ee into python:main Nov 4, 2025
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		@@ -0,0 +1,4 @@
		The re.Scanner class now forbids regular expressions containing capturing

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gh-140797: Forbid capturing groups in re.Scanner lexicon patterns #140944

gh-140797: Forbid capturing groups in re.Scanner lexicon patterns #140944

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