Add vertical tab support to Regex.Escape

[prozolic]

Add support for escaping vertical tab character (\v) in Regex.Escape.
Regex.Unescape already supported \v, but Regex.Escape was missing this functionality.

• Handle \v character (0x0B) in RegexParser.EscapeImpl
• Use \v instead of \u000B in RegexParser.ScanCharEscape
• Add test for vertical tab character in Regex.Escape/Unescape

Related to PR #120625

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[Copilot]

Pull request overview

This pull request adds support for escaping the vertical tab character (\v) in Regex.Escape, completing the functionality that was already present in Regex.Unescape. This change builds on PR #120625, which added vertical tab support to the regex parser for IgnorePatternWhitespace.

Changes:

• Added vertical tab (\v) handling in RegexParser.EscapeImpl to escape the character as \v
• Changed ScanCharEscape to use \v literal instead of \u000B for consistency
• Updated comment to document \v as part of whitespace characters
• Added comprehensive tests for both Escape and Unescape with vertical tab

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
Regex.EscapeUnescape.Tests.cs	Added new test methods Escape_VerticalTab and Unescape_VerticalTab to verify vertical tab escaping/unescaping behavior
RegexParser.cs	Added case for \v in EscapeImpl, changed \u000B to \v in ScanCharEscape, and updated whitespace comment

[stephentoub]

Regex.Unescape already supported \v, but Regex.Escape was missing this functionality.

There are many more characters Unescape recognizes that Escape doesn't. Why is this one in particular important to add to Escape?

[prozolic]

In #120625, \v was made to be treated as whitespace like other escape sequences (\t\n\f\r).
During that process, \v was added to s_metachars, which changed the behavior of Regex.Escape when \v is included, so I thought the same handling as \t\n\f\r was necessary.

https://github.com/dotnet/runtime/blob/main/src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/RegexParser.cs#L1941-L1942

The mention of Regex.Unescape may have been unnecessary, sorry about that.

[stephentoub]

It was unintended for Regex.Escape to be impacted by that PR. I'll fix it. Thanks.