Add a regex test for a loop followed by a positive lookahead

[stephentoub]

Locally I made a change (allowing loops to be made atomic based on a positive lookahead after them) that should have failed tests but didn't. This adds a test that would have caught the issue.

[Copilot]

Pull Request Overview

This PR adds a regression test for regex patterns that combine loops with positive lookahead assertions. The test ensures that a specific pattern involving a*(?=b)\bb correctly matches against the input "ab", which would help catch issues where loop optimizations incorrectly interact with lookahead assertions.

Comments suppressed due to low confidence (1)

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Match.Tests.cs:101

• The test case pattern a*(?=b)\bb appears to test a very specific edge case. Consider adding additional test cases with different variations of loops and positive lookaheads (e.g., a+(?=b)\bb, a{2,}(?=b)\bb) to ensure comprehensive coverage of the optimization behavior being tested.

                yield return (@"a*(?=b)\bb", "ab", RegexOptions.None, 0, 2, true, "ab");

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