Port RE2 regex tests to improve test coverage

[Copilot]

This PR ports functional regex tests from the RE2 test suite to improve .NET's regex test coverage, as requested in #120756.

Changes

Test Suite Additions

• Added RegexRe2Tests.cs: New test file containing 85 unique test cases ported from RE2's re2_test.cc and search_test.cc
• Test coverage: The ported tests execute ~340 times across all available regex engines (Interpreter, Compiled, NonBacktracking, SourceGenerated)
• Removed duplicates: Analyzed overlap with existing PCRE, Rust, and core regex tests; removed 57 duplicative test cases to avoid redundancy

Test Categories Covered

The ported tests validate:

• Complex matching patterns and alternations
• Anchors (^, $) in single-line and multiline modes with non-trivial cases
• Word boundaries (\b, \B) with ASCII and special characters
• UTF-8/Unicode character handling
• Escape sequences (octal \141, hexadecimal \x61, unicode \u0061)
• Case-insensitive matching ((?i))
• Non-trivial quantifier combinations ({n}, {n,}, {n,m})
• Backreferences (excluded for NonBacktracking engine)
• Edge cases and historical bug patterns from RE2

Compatibility Adjustments

Several RE2-specific patterns were excluded or adapted for .NET compatibility:

• Removed \C patterns: RE2's byte-matching construct is not supported in .NET
• Removed \Q...\E patterns: Quote meta syntax is not supported in .NET
• Converted \x{...} escapes: Changed to .NET-compatible \x (2 digits) or \u (4 digits) format
• Excluded backreferences for NonBacktracking: Uses RegexHelpers.IsNonBacktracking() check
• Removed Unicode word boundary test: .NET treats Unicode letters as word characters (unlike RE2/PCRE which use ASCII-only)
• Removed duplicative tests: Excluded basic patterns like "a", "a*", "a+", "a?", "^$" that are already well-covered in existing test files

License Attribution

• Updated THIRD-PARTY-NOTICES.TXT: Added RE2 BSD 3-Clause license to properly attribute the ported tests
• Updated .csproj: Added RegexRe2Tests.cs to the compilation

Testing

All 29,621 tests pass successfully, including the newly ported RE2 tests across all regex engines.

Fixes #120756

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Port re2 regex tests</issue_title>
<issue_description>re2 has lots of regex functional tests we should port over to improve our coverage:
https://github.com/google/re2/blob/d826d9fcb68c62996c1b7c0a45d604e22d814952/re2/testing/</issue_description>

<agent_instructions>You can see in https://github.com/dotnet/runtime/tree/main/src/libraries/System.Text.RegularExpressions/tests/FunctionalTests we have a file of tests ported from PCRE and a file of tests ported from Rust. Please similarly port the cited RE2 tests (or a newer version if there is one); any that don't pass but should need to be marked as [ActiveIssue(...)]... any that don't pass but that's expected should be removed.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

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Issue Details

---

re2 has lots of regex functional tests we should port over to improve our coverage:
https://github.com/google/re2/blob/d826d9fcb68c62996c1b7c0a45d604e22d814952/re2/testing/

Author:	stephentoub
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	.NET 7.0

@joperezr This will likely miss ZBB but will still make it before 7.0. Adjusting the milestone just because this is not technically a blocker for 7.0.

Fixes #61896

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Pull Request Overview

Ports a subset of RE2's functional regex tests into the .NET test suite to expand coverage across multiple regex engines.

• Adds 142 test cases derived from RE2's re2_test.cc and search_test.cc.
• Updates test project file to include the new test file and adds RE2's BSD 3-Clause license to third-party notices.
• Exercises diverse regex features (anchors, word boundaries, quantifiers, escapes, Unicode) across all available engines.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
THIRD-PARTY-NOTICES.TXT	Adds RE2 BSD 3-Clause license attribution for the ported tests.
System.Text.RegularExpressions.Tests.csproj	Includes the new RegexRe2Tests.cs file in the test compilation.
RegexRe2Tests.cs	Introduces the ported RE2-derived test cases and supporting data generation logic.

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