BUG or Feature? Conditional expressions cannot directly contain regex options in .NET 5 and later versions

[longxya]

Description

(?(wtf)(?i)wtf) and (?(?i:wtf)wtf) can't run in .NET v5、v6、v7、v8、v9 cause it directly contain regex options. But it works in lower versions.

Reproduction Steps

using System;
using System.Text.RegularExpressions;

var pattern = @"(?(wtf)(?i)wtf)";
//pattern = @"(?(wtf)(?i:wtf))";// this behavior same as the expression above
var input = @"11";
try
{
	Match matchCompiled = new Regex(pattern, RegexOptions.None).Match(input);
	Console.WriteLine($"Interpreted: {matchCompiled.Success}");
}catch(Exception ex)
{
	Console.WriteLine(ex.Message);
}
try
{
	Match matchCompiled = new Regex(pattern, RegexOptions.Compiled).Match(input);
	Console.WriteLine($"Compiled: {matchCompiled.Success}");
}catch(Exception ex)
{
	Console.WriteLine(ex.Message);
}

Output：

Invalid pattern '(?(wtf)(?i)wtf)' at offset 10. Unrecognized grouping construct.
Invalid pattern '(?(wtf)(?i)wtf)' at offset 10. Unrecognized grouping construct.

Expected behavior

Output:

Interpreted: True
Compiled: True

Actual behavior

OUtput:

Invalid pattern '(?(wtf)(?i)wtf)' at offset 10. Unrecognized grouping construct.
Invalid pattern '(?(wtf)(?i)wtf)' at offset 10. Unrecognized grouping construct.

Regression?

No response

Known Workarounds

No response

Configuration

No response

Other information

No response

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[stephentoub]

@danmoseley, this was disabled in dotnet/corefx#16609. It's not clear to me why, other than as an easy fix for addressing some parsing errors. Was it intentionally blocked off, including prohibiting quantifiers as part of the conditional's if/else expressions and not just the test expression?

[CyrusNajmabadi]

If things change here, please let me know so i can keep Roslyn's regex parser up to date. Thanks!