Fix flaky RegularExpressionAttribute timeout test

[danmoseley]

Fix flaky RegularExpressionAttribute timeout test

Rare CI failure in RegularExpressionAttributeTests.Validate_MatchingTimesOut_ThrowsRegexMatchTimeoutException:

Assert.Throws() Failure: Exception type was not an exact match
Expected: typeof(System.Text.RegularExpressions.RegexMatchTimeoutException)
Actual:   typeof(System.ComponentModel.DataAnnotations.ValidationException)
---- System.ComponentModel.DataAnnotations.ValidationException : The field Object must match the regular expression '(a[ab]+)+$'.

The test uses pattern (a[ab]+)+$ with a 1ms timeout, expecting catastrophic backtracking to cause a RegexMatchTimeoutException. The input was a hardcoded 28-character string ("aaaaaaaaaaaaaaaaaaaaaaaaaaaa>"). With regex engine improvements, the backtracking sometimes completes within the 1ms timeout window before the timeout check fires. When that happens, IsValid() returns false (no match) and Validate() wraps it as a ValidationException instead of the expected RegexMatchTimeoutException.

The fix increases the input length from 28 to 100 characters (new string('a', 100) + ">"), making the exponential backtracking work (O(2^n)) large enough that the 1ms timeout is reliably exceeded.

Note

This change was AI-generated with GitHub Copilot.

[Copilot]

Pull request overview

This PR fixes a rare CI flake in RegularExpressionAttributeTests.Validate_MatchingTimesOut_ThrowsRegexMatchTimeoutException by making the test input reliably trigger a RegexMatchTimeoutException even with recent regex engine performance improvements.

Changes:

• Increase the input length used in the timeout test from a fixed short literal to new string('a', 100) + ">" to more consistently exceed a 1ms regex timeout.

Show a summary per file

File	Description
src/libraries/System.ComponentModel.Annotations/tests/System/ComponentModel/DataAnnotations/RegularExpressionAttributeTests.cs	Strengthens the timeout test input to reduce flakiness and keep the expected exception stable.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 0

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-componentmodel-dataannotations
See info in area-owners.md if you want to be subscribed.

[danmoseley]

ha, it already had to get slowed-down once
danmoseley@520719c#diff-915a401b67bd29c3e593a4030b1958af52a2cd5133f0934eb95a200145d1e6f9L81

[tarekgh]

LGTM, Thanks!

[adamsitnik]

LGTM, thank you @danmoseley !