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fix(pseudos): avoid overlapping whitespace in nth parsing - #1909
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Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
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Included review availability: This review used your included allowance. Your plan provides up to 4 included reviews per hour; 3 remain after this review. 📝 WalkthroughWalkthroughThe ChangesNth-child whitespace parsing
Priority: ⬇️ Low Estimated code review effort: 2 (Simple) | ~8 minutes Change: Bug fix · Severity of issue fixed: Low Merge Risk: ⚪ Minimal · up to The change updates nth-of whitespace parsing and adds a targeted long-whitespace regression check. No material merge risk is identified in the supplied context. Pre-merge checks |
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| Title check | |
The title clearly and concisely describes the main change: preventing overlapping whitespace during nth pseudo-selector parsing. |
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Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files. |
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src/pseudo-selectors/filters.ts
ESLint skipped: missing config or dependency (missing-dependency). The ESLint configuration references a package that is not available in the sandbox.
test/pseudo-classes.ts
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A rabbit checks the parser’s pace,
As spaces stretch across the place.
The regex finds the boundary clear,
The expected error soon appears.
The rabbit hops, its test complete,
And leaves neat tracks with nimble feet.
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🟢 Approval recommended
The focused regex change preserves parsing behavior while resolving the demonstrated performance issue with regression coverage.
0 open findings
What changed in this PR
Prevents excessive regex backtracking when parsing invalid :nth-child(... of ...) arguments.
Changes:
- Reworks
nthOfRegexto eliminate overlapping whitespace matching. - Adds a regression test using a long whitespace argument.
| File | Description |
|---|---|
src/pseudo-selectors/filters.ts |
Makes nth-of parsing fail efficiently. |
test/pseudo-classes.ts |
Verifies prompt rejection of pathological input. |
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| expect(() => | ||
| CSSselect.compile(`:nth-child(${" ".repeat(350_000)})`), | ||
| ).toThrow("n-th rule couldn't be parsed"); | ||
| }, 2000); |
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If this regex regresses to synchronous backtracking again, the parser blocks the event loop, so Vitest cannot fire this 2-second timeout; the test eventually completes and passes once compile() throws, even if that takes minutes. Vitest's own reproduction confirms that synchronous work completing after a test timeout is still reported as passing. As a result, this test does not enforce the performance property described by its name and comment; measure elapsed time explicitly or run the parse in a worker/subprocess that can be terminated.
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The
:nth-child(… of …)argument was split with^(.+?)\s+of\s+(.+)$. Both.+?and\s+can claim the same whitespace, so an argument made of a long run of spaces made the engine try every split before giving up, taking seconds for a few thousand spaces.^(.+?\s)of\s+(.+)$lets the lazy group own the whitespace, so there is one way to match and failure is immediate.A test parses a 350,000-space argument and expects the parse error within a short timeout; on the previous pattern that input took minutes.
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Summary by CodeRabbit
:nth-childexpressions with whitespace beforeof, preventing excessive delays on very long inputs.