Add additional regex gtests for contains, count, findall, and replace

[davidwendt]

Description

Adds some additional gtests for regex patterns to help validate optimizations in follow on PRs.

This is part of splitting out some of the work for #21936

Checklist

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

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ℹ️ Recent review info

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Run ID: c8ab7ad0-9610-4116-a022-f815a5b19210

📥 Commits

Reviewing files that changed from the base of the PR and between 0b8ef6a and 8954762.

📒 Files selected for processing (3)

• cpp/tests/strings/contains_tests.cpp
• cpp/tests/strings/findall_tests.cpp
• cpp/tests/strings/replace_regex_tests.cpp

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📝 Walkthrough

Summary by CodeRabbit

• Tests
  • Extended test coverage for regex operations to validate alternation priority, bounded repetition limits, newline character handling, and lazy quantifier behavior across multiple string processing functions.

Walkthrough

This PR extends regex behavior coverage in CUDF string functions by adding unit tests across three test files. Tests validate nullable alternation branches, bounded repetition constraints, newline handling in dot patterns, alternation priority semantics, and lazy quantifier behavior in contains_re, count_re, findall, and replace_re operations.

Changes

String Regex Edge Case Coverage

Layer / File(s)	Summary
Fundamental regex semantics in contains_re and count_re cpp/tests/strings/contains_tests.cpp	Five new test cases validate core regex rules: alternation with nullable branches (a(bc|de|fg|)h), bounded repetition (ab{0,4}cv), dot behavior differences under default vs. EXT_NEWLINE modes, alternation leftmost-first priority (a|aa, foo|foobar), and lazy quantifier shortest-match semantics (x.*?x, x.+?x).
Alternation priority across string operations cpp/tests/strings/findall_tests.cpp, cpp/tests/strings/replace_regex_tests.cpp	New tests in findall and replace_re verify that alternation leftmost-first matching is consistently applied when alternatives overlap at the same starting position, using patterns like foo|foobar and cat|catch.

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Estimated Code Review Effort

🎯 2 (Simple) | ⏱️ ~12 minutes

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Possibly Related PRs

• rapidsai/cudf#22429: The new contains_re/count_re test cases with explicitly empty alternation branches (|) exercise the same empty-match handling logic changed in this PR.
• rapidsai/cudf#22800: The new tests with counted quantifiers at zero minimum (e.g., ab{0,4}cv) exercise the zero-repeat expansion logic fixed in regex_parser::expand_counted_items in regcomp.cpp.

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Suggested Reviewers

• igorpeshansky
• vyasr
• wence-
• qbacpey

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% 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
Title check	✅ Passed	The title accurately summarizes the main change: adding new regex tests for four string operations (contains, count, findall, and replace).
Description check	✅ Passed	The description clearly explains the purpose of the PR—adding regex gtests to validate optimizations—and relates directly to the changeset of new test cases.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[lamarrr]

LGTM!

[davidwendt]

/merge