Fix lazy quantifier priority handling in Glushkov regex engine

[davidwendt]

Description

Closes #23287.

The Glushkov eligibility checker previously rejected an ACCEPT item only when it appeared before the first character-consuming frontier item. This missed Thompson-priority frontiers such as [LF, ACCEPT, CR-repeat], where a successful accept has higher priority than a later continuation. Flattening that ordering into Glushkov bit positions caused a reluctant \r+? delimiter to consume a second \r, producing greedy behavior in split_record_re.

This PR:

• rejects a Glushkov frontier whenever an ACCEPT item is followed by a later CHAR_POS, conservatively falling back to the Thompson engine;
• preserves safe frontiers that end in ACCEPT;
• adds StringsSplitTest.SplitRecordRegexLazyQuantifier to verify the delimiter length and resulting split records.

The change affects only patterns whose Thompson-priority ordering cannot be represented faithfully by the Glushkov fast path. Supported patterns continue to use Glushkov.

Validation

• Focused StringsSplitTest.SplitRecordRegexLazyQuantifier: 1/1 passed.
• Focused regression with LIBCUDF_DISABLE_GLUSHKOV=1: 1/1 passed.
• Full STRINGS_TEST: 540/540 passed.
• Clean local spark-rapids-jni package using this cuDF checkout: BUILD SUCCESS; a second same-toolchain rebuild also completed successfully.
• NVIDIA/cudf-spark, Scala 2.13 / Spark 4.0.1, RegularExpressionTranspilerSuite: 97 succeeded, 0 failed, 6 pre-existing canceled tests; Maven BUILD SUCCESS. The original string split fuzz - anchor focused failure passed.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between 56f5503 and 79422b6.

📒 Files selected for processing (3)

• cpp/src/strings/regex/glushkov_regcomp.cpp
• cpp/src/strings/regex/glushkov_regcomp.hpp
• cpp/tests/strings/split_tests.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Bug Fixes

  • Improved regular expression priority handling for patterns where acceptance occurs before a later character match.
  • Added clearer documentation for patterns that cannot be represented faithfully by the regex engine.

• Tests

  • Added coverage for lazy versus greedy quantifiers when splitting records with regular expressions.

Walkthrough

The Glushkov frontier eligibility check now rejects ACCEPT entries followed by later character positions. Its failure condition is documented, and split-record regex tests cover differing lazy and greedy quantifier results.

Changes

Glushkov lazy quantifier handling

Layer / File(s)	Summary
Frontier conflict detection cpp/src/strings/regex/glushkov_regcomp.cpp, cpp/src/strings/regex/glushkov_regcomp.hpp	Rule 1 detects an ACCEPT before a later CHAR position, and the compiler documentation records this frontier incompatibility as a null-return condition.
Split-record regression coverage cpp/tests/strings/split_tests.cpp	Adds lazy-versus-greedy quantifier assertions for carriage-return input and updates the SPDX copyright year.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related issues

• rapidsai/cudf issue 23290 — Covers the same lazy-quantifier regression and Glushkov frontier conflict detection changes.

Possibly related PRs

• rapidsai/cudf#21936 — Introduced the Glushkov fast path and its frontier conflict-detection machinery.

Suggested labels: improvement

Suggested reviewers: bdice, lamarrr

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	⚠️ Warning	The PR includes an unrelated 2019-2026 copyright year update in the test file.	Remove the standalone copyright year change or split it into a separate maintenance PR.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main fix: lazy quantifier priority handling in the Glushkov regex engine.
Description check	✅ Passed	The description matches the PR and explains the bug, fix, and validation.
Linked Issues check	✅ Passed	The code and test changes implement the linked bug fix, fallback behavior, and regression coverage described in #23287.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[igorpeshansky]

LGTM

[davidwendt]

/merge