Fix a greedy exponential regex

[elisescu]

Should close #4176

Summary by CodeRabbit

• Bug Fixes
  • Improved validation logic for question group formatting to handle edge cases more accurately, ensuring stricter and more reliable processing of group identifiers.

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📥 Commits

Reviewing files that changed from the base of the PR and between 1b883ba and d81836c.

📒 Files selected for processing (4)

• notifications/services.py
• notifications/templates/emails/post_cp_change.html
• notifications/templates/emails/post_cp_change.mjml
• questions/utils.py

📝 Walkthrough

Walkthrough

A regex pattern in the get_question_group_title function was refined to be more restrictive. The updated pattern no longer matches empty parentheses, requiring at least one non-parenthesis character within matched groups to prevent unintended captures.

Changes

Cohort / File(s)	Summary
Regex Pattern Refinement questions/utils.py	Modified regex in get_question_group_title from \((?:[^()]*|\([^()]*\))*\) to `((?:[^()]

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

🐰 A parenthesis dance, so tight and refined,
No empty pairs left behind,
One line changed, one line true,
Security regex, we thank you!
✨

🚥 Pre-merge checks | ✅ 3 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Linked Issues check	❓ Inconclusive	The linked issue #4176 references a code-scanning alert but provides no specific coding requirements or objectives to validate against the changes.	Provide detailed requirements from the code-scanning issue #4176 to verify whether the regex fix adequately addresses the identified security or performance concern.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix a greedy exponential regex' clearly describes the main change: fixing a regex pattern that had exponential complexity behavior.
Out of Scope Changes check	✅ Passed	The single regex change in questions/utils.py is directly related to fixing the greedy exponential regex mentioned in the PR title and linked issue.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix4176

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[github-actions]

🧹 Preview Environment Cleaned Up

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Cleanup triggered by PR close at 2026-02-03T11:49:36Z

[hlbmtc]

I think we can fully deprecate get_question_group_title, since the migration was applied a long time ago:

1. Deprecate CPChangeData.format_question_title
2. Deprecate get_question_group_title
3. Replace {{ item.format_question_title }} with {{ item.question.label }} in notifications/templates/emails/post_cp_change.html
4. Regenerate MJML

[elisescu]

@hlbmtc have another look now