Fix RegexParser to ignore vertical tabs with RegexOptions.IgnorePatternWhitespace

[Copilot]

Fix: Regex parser to ignore vertical tabs in pattern with RegexOptions.IgnorePatternWhitespace

Plan:

• Investigate the issue and reproduce the bug
• Build baseline (clr+libs) successfully
• Confirm the bug: vertical tab (\v, 0xB) is not ignored with IgnorePatternWhitespace
• Fix the Category array in RegexParser.cs by changing position 0xB from 0 to W
• Update s_metachars to include vertical tab character
• Add test case to existing test file (Regex.Match.Tests.cs)
• Build and test the fix
• Verify all tests pass
• Guard test for .NET Framework platform

Changes Made:

1. RegexParser.cs (line 1932): Changed Category[0xB] from 0 to W to mark vertical tab as whitespace
2. RegexParser.cs (line 1942): Added \v to s_metachars SearchValues
3. Regex.Match.Tests.cs (line 584-588): Added test case for vertical tab with IgnorePatternWhitespace option, guarded with if (!PlatformDetection.IsNetFramework) to prevent failures on .NET Framework

Test Results:

✅ FunctionalTests: 29,291 tests passed, 0 failures
✅ UnitTests: 1,005 tests passed, 0 failures
✅ New test case: Verified passing across all regex engines (Interpreter, Compiled, NonBacktracking, SourceGenerated) on .NET Core

Technical Details:

• Position 0xB in the Category array corresponds to the vertical tab character ('\v')
• The fix aligns with .NET's char.IsWhiteSpace('\v') which returns true
• PCRE2 and other regex engines also treat vertical tab as whitespace in ignore pattern whitespace mode
• The vertical tab character is now properly ignored when using RegexOptions.IgnorePatternWhitespace on .NET Core
• Test is guarded to not run on .NET Framework where the fix is not applicable

Original prompt

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This section details on the original issue you should resolve

<issue_title>Regex parser doesn't ignore vertical tabs in patter when using RegexOptions.IgnorePatternWhiteSpace</issue_title>
<issue_description>While porting PCRE2 tests suite, one of the failing tests is:

bool isMatch = Regex.IsMatch("ab", "a\vb", RegexOptions.IgnorePatternWhitespace);
Assert.True(isMatch);

This should probably be true and match other engines like PCRE as \v character should be ignored as whitespace and just have the pattern be ab

cc: @stephentoub </issue_description>

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

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Issue Details

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While porting PCRE2 tests suite, one of the failing tests is:

bool isMatch = Regex.IsMatch("ab", "a\vb", RegexOptions.IgnorePatternWhitespace);
Assert.True(isMatch);

This should probably be true and match other engines like PCRE as \v character should be ignored as whitespace and just have the pattern be ab

cc: @stephentoub

Author:	joperezr
Assignees:	-
Labels:	area-System.Text.RegularExpressions
Milestone:	-

@stephentoub This does seem like a bug; `char.IsWhitespace('\v')` is true, as is `Regex.IsMatch("\v", @"\s")`. Presumably it'd be as simple as changing the `Category[0xB]` value in the following table to be 'X': https://github.com/dotnet/runtime/blob/aac729ff906a31f327823587748687c0308a4043/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexParser.cs#L2060-L2062 We might want to dig a tad deeper, as it feels a little deliberate that cell was left as 0 rather than X.

Fixes #73206

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Pull Request Overview

This PR fixes a bug in the regex parser where vertical tab (\v, character 0xB) was not being treated as whitespace when using RegexOptions.IgnorePatternWhitespace, making it inconsistent with other .NET APIs and regex engines like PCRE2.

• Updated the ASCII character category table to mark vertical tab as whitespace
• Added vertical tab to the metacharacters SearchValues for proper escaping
• Added test case to verify vertical tab is properly ignored with IgnorePatternWhitespace option

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
RegexParser.cs	Updated character category table to treat vertical tab as whitespace and added \v to metacharacters
Regex.Match.Tests.cs	Added test case verifying vertical tab is ignored with IgnorePatternWhitespace option