Python: Fix false positive for unmatchable dollar/caret #20495
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Our previous modelling did not account for the fact that a lookahead can potentially extend all the way to the end of the input (and similarly, that a lookbehind can extend all the way to the beginning). To fix this, I extended `firstPart` and `lastPart` to handle lookbehinds and lookaheads correctly, and added some test cases (all of which yield no new results). Fixes #20429.
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Pull Request Overview
This PR fixes false positives in CodeQL queries that check for unmatchable $ and ^ anchors in regular expressions by improving the handling of lookahead and lookbehind assertions. The fix ensures that anchors inside these assertions are correctly analyzed for their potential to match the beginning or end of input.
Key changes:
- Extended lookahead/lookbehind predicate signatures to include content boundaries
- Updated
firstPartandlastPartlogic to handle assertions that can reach string boundaries - Added test cases covering various lookahead and lookbehind scenarios
Reviewed Changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| python/ql/lib/semmle/python/regexp/internal/ParseRegExp.qll | Core fix: extended assertion predicates and added logic for handling assertions in first/last part detection |
| python/ql/lib/semmle/python/regexp/RegexTreeView.qll | Updated calls to assertion predicates to match new signatures |
| python/ql/test/query-tests/Expressions/Regex/test.py | Added test cases for lookahead and lookbehind assertions |
| python/ql/test/library-tests/regex/FirstLast.expected | Updated expected test results reflecting the improved analysis |
| python/ql/src/change-notes/2025-09-19-fix-unmatchable-dollar-and-caret-in-assertions.md | Added release notes documenting the fix |
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Our previous modelling did not account for the fact that a lookahead can potentially extend all the way to the end of the input (and similarly, that a lookbehind can extend all the way to the beginning).
To fix this, I extended
firstPartandlastPartto handle lookbehinds and lookaheads correctly, and added some test cases (all of which yield no new results).Fixes #20429.