Implement more Polars string Expressions

[mroeschke]

Description

Implements the following string expressions

Expr.str.to_integer
Expr.str.count_matches(literal=True)
Expr.str.extract(group_index=0)
Expr.str.strptime/to_date/to_datetime(cache=True) (just ignores the cache=True keyword which I think is OK)
Expr.str.zfill w/ null widths

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]

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📥 Commits

Reviewing files that changed from the base of the PR and between 199c51c and 6049894.

📒 Files selected for processing (1)

• python/cudf_polars/cudf_polars/testing/inject_gpu_engine.py

🚧 Files skipped from review as they are similar to previous changes (1)

• python/cudf_polars/cudf_polars/testing/inject_gpu_engine.py

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📝 Walkthrough

Summary by CodeRabbit

• New Features
  • Added GPU to_integer support with strict/non-strict parsing, consistent null handling, and supported base validation.
  • Extended string operations: count_matches(literal=...), extract(group_index=0), and zfill with per-row null widths.
  • Improved datetime parsing/inference, including stricter format-specifier validation and fractional-seconds precision detection.
• Bug Fixes
  • Updated strptime validation to allow the cache option.
• Tests
  • Expanded string-function test coverage (including strict failure cases) and adjusted expected failure/skip outcomes for known engine differences.

Walkthrough

This PR adds GPU support for str.to_integer, relaxes regex and string validation for literal matches and group_index=0, updates datetime parsing and inference handling, fixes null-width str.zfill, and revises GPU engine expected-failure mappings.

Changes

String Function Changes

Layer / File(s)	Summary
ToInteger validation and evaluation python/cudf_polars/cudf_polars/dsl/expressions/string.py, python/cudf_polars/tests/expressions/test_stringfunction.py	Adds Name.ToInteger support, validates base==10 as a literal, implements parsing via is_integer/to_integers with strict-mode error handling and null masking, and adds tests for strict, non-strict, and unsupported-base cases.
CountMatches literal pattern support python/cudf_polars/cudf_polars/dsl/expressions/string.py, python/cudf_polars/tests/expressions/test_stringfunction.py	Escapes literal patterns during validation instead of rejecting them, and updates tests for multiple literal patterns and the empty-pattern unsupported case.
Extract group_index==0 support python/cudf_polars/cudf_polars/dsl/expressions/string.py, python/cudf_polars/tests/expressions/test_stringfunction.py	Wraps group_index==0 patterns in an outer capture group during validation, remaps the runtime group index during evaluation, and updates tests to treat group_index=0 as a success case.
Strptime cache and datetime inference python/cudf_polars/cudf_polars/dsl/expressions/string.py, python/cudf_polars/tests/expressions/test_stringfunction.py	Ignores the cache option in Strptime, validates datetime format specifiers, updates fractional-second inference regexes, and extends datetime format inference for fractional precision; tests cover default cache behavior and the translation-error condition.
ZFill null width handling python/cudf_polars/cudf_polars/dsl/expressions/string.py, python/cudf_polars/tests/expressions/test_stringfunction.py	Replaces null per-row widths with a placeholder during zfill computation, then reapplies a mask so rows with null widths remain null; adds a test for column null widths.
Streaming engine xfail mapping python/cudf_polars/cudf_polars/testing/inject_gpu_engine.py	Updates the GPU-engine expected-failure map for one Iceberg test and adds one streaming-engine expected-failure entry for a predicate pushdown test.

Estimated code review effort: 4 (Complex) | ~45 minutes

Possibly related PRs

• rapidsai/cudf#22605: Also updates python/cudf_polars/cudf_polars/testing/inject_gpu_engine.py expected-failure and skip mappings.

Suggested labels: feature request

Suggested reviewers: Matt711, TomAugspurger

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title matches the main change set: adding more Polars string expression support.
Description check	✅ Passed	The description clearly lists the string expressions implemented and aligns with the changeset.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[mroeschke]

Closing as these expressions will be split into individual PRs