regex: Plug a regex literal crash

[cosmo0920]

Closes #11937.

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Testing
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Backporting

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Fluent Bit is licensed under Apache 2.0, by submitting this pull request I understand that this code will be released under the terms of that license.

Summary by CodeRabbit

Bug Fixes

• Fixed an out-of-bounds access issue when processing short or empty regex patterns, improving stability and preventing potential crashes.

Tests

• Added comprehensive test coverage for regex patterns with literal slashes to validate edge case handling.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 07df8bfa-f778-49ff-9629-df86813568ef

📥 Commits

Reviewing files that changed from the base of the PR and between b7f4f89 and ae73a25.

📒 Files selected for processing (2)

• src/flb_regex.c
• tests/internal/regex.c

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📝 Walkthrough

Walkthrough

The PR fixes a crash in regex pattern parsing by preventing out-of-bounds access when validating pattern delimiters on short inputs. A length guard is added before delimiter detection, and comprehensive test cases document expected behavior for edge cases including single-slash and literal-slash patterns.

Changes

Regex Pattern Delimiter Parsing Fix

Layer / File(s)	Summary
Bounds-check fix in str_to_regex() src/flb_regex.c	str_to_regex() adds len > 1 guard before checking first and last characters for / delimiters, preventing out-of-bounds access on empty or single-character patterns while preserving delimiter-parsing for valid /pat/ forms.
Regex pattern delimiter edge-case tests tests/internal/regex.c	Three new test functions validate literal / patterns, slash-delimited patterns like /tmp/, and escaped slash delimiters like //tmp//, with assertions on match behavior with and without surrounding delimiters. Tests are registered in TEST_LIST.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

A regex that stumbles on single slashes bare,
Now guards against bounds with protective care,
Tests confirm that / stands alone with pride,
And //tmp// keeps its delimiters wide,
One-line fix, three tests—a rabbit's delight! 🐰

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 20.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the primary change: fixing a crash in regex literal handling when processing patterns like a single '/'.
Linked Issues check	✅ Passed	The PR successfully addresses the crash requirement from issue #11937 by preventing out-of-bounds access when parsing single '/' patterns.
Out of Scope Changes check	✅ Passed	All changes are directly related to fixing the regex literal crash issue and adding test coverage for the fix.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch cosmo0920-plug-regex-literal-crash

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[cosmo0920]

This Windows ARM64 fail https://github.com/fluent/fluent-bit/actions/runs/27324468341/job/80722539551?pr=11938 will be resolved with:
#11934
For 4.2:
#11935