[fixfon]

Closes #15775

tac -r looked for the separator with an unanchored Regex::find_at and then threw the match away unless it started at the current position. On a long record with no separator, each position scanned to the end of the record, so the run time grew quadratically.

buffer_tac_regex now uses regex_automata::meta::Regex and runs an anchored search at each position. The regex is built with the same syntax settings as before. TacError::InvalidRegex holds the new build error, and a small helper in error.rs keeps the error messages unchanged. regex-automata was already in the dependency tree through regex.

Tests: test_regex_long_record_is_not_quadratic times out on the old code and passes now; test_invalid_regex_error_messages pins the existing error messages.

Run locally: cargo fmt --check -p uu_tac and cargo test --locked --offline --features tac --no-default-features test_tac (35 passed). Tested on macOS (arm64).

[codspeed]

Merging this PR will degrade performance by 9.25%

2 warnings, 1 note

⚠️ 122 benchmarks are dominated by system calls

System calls cannot be simulated consistently and are left out of the measure. The reported value understates the real cost. Switch to the Walltime instrument to measure them.

Learn more about system calls

See affected benchmarks

⚠️ Base and head ran on different runner types

Some benchmarks with significant performance changes were compared across different runner types, so the changes might not come from the code. Run base and head on the same runner type, or use CodSpeed Macro Runners for more stable results.

Learn more about Macro Runners

Open the report in CodSpeed to investigate

ℹ️ 1 benchmark has no flamegraph

The flamegraph wasn't generated: most of the time was spent in runtime internals, which would leave nothing useful to analyze.

See affected benchmarks

⚡ 1 improved benchmark
❌ 3 regressed benchmarks
✅ 397 untouched benchmarks
⏩ 70 skipped benchmarks¹

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Memory	cp_recursive_deep_tree[(120, 4)]	532.6 KB	699.2 KB	-23.82%
❌	CPU Simulation	ls_recursive_wide_tree[(10000, 1000)]	66.3 ms	72.9 ms	-9.05%
❌	Memory	du_deep_tree[(100, 3)]	76.5 KB	82.4 KB	-7.14%
⚡	CPU Simulation	du_deep_tree[(100, 3)]	1.8 ms	1.7 ms	+5.43%

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing fixfon:fix/15775-tac-regex-quadratic (03dec80) with main (1542121)²}

Footnotes

1. 70 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (32b2d60) during the generation of this report, so 1542121 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[fixfon]

Squashed into one commit and removed the issue reference from the message.