Inconsistent Regex matching behavior in InvariantCulture

[veanes]

Description

RegexOptions.None and RegexOptions.Compiled give different answers in InvariantCulture when ignoring case and involving \u0130 (Turkish I with dot).
In this case Compiled gives the wrong match that starts with \u0130.
The expected match below is "II".

Configuration

.NET 5.0

Regression?

Not sure if this is a regression. In earlier versions System.Globalization.CultureInfo.CurrentCulture
cannot be set, I believe.

Other information

This is a repo:

using System;
using System.Text.RegularExpressions;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            string pattern = "(?i:iI+)";
            string input = "abc\u0130IIxyz";
            var culture = System.Globalization.CultureInfo.CurrentCulture;
            System.Globalization.CultureInfo.CurrentCulture = System.Globalization.CultureInfo.InvariantCulture;
            Regex re = new Regex(pattern);
            Regex reC = new Regex(pattern, RegexOptions.Compiled);
            System.Globalization.CultureInfo.CurrentCulture = culture;
            Console.WriteLine("correct:" + re.Match(input).Value);
            Console.WriteLine("incorrect:" + reC.Match(input).Value);
        }
    }
}

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Issue Details

---

Description

RegexOptions.None and RegexOptions.Compiled give different answers in InvariantCulture when ignoring case and involving \u0130 (Turkish I with dot).
In this case Compiled gives the wrong match that starts with \u0130.
The expected match below is "II".

Configuration

.NET 5.0

Regression?

Not sure if this is a regression. In earlier versions System.Globalization.CultureInfo.CurrentCulture
cannot be set, I believe.

Other information

This is a repo:

using System;
using System.Text.RegularExpressions;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            string pattern = "(?i:iI+)";
            string input = "abc\u0130IIxyz";
            var culture = System.Globalization.CultureInfo.CurrentCulture;
            System.Globalization.CultureInfo.CurrentCulture = System.Globalization.CultureInfo.InvariantCulture;
            Regex re = new Regex(pattern);
            Regex reC = new Regex(pattern, RegexOptions.Compiled);
            System.Globalization.CultureInfo.CurrentCulture = culture;
            Console.WriteLine("correct:" + re.Match(input).Value);
            Console.WriteLine("incorrect:" + reC.Match(input).Value);
        }
    }
}

Author:	veanes
Assignees:	-
Labels:	area-System.Text.RegularExpressions, untriaged
Milestone:	-

[GrabYourPitchforks]

Possible dupe of #58958.

[stephentoub]

This is not a regression. Same behavior repros on .NET Framework 4.8.

[jeffhandley]

@pgovind FYI on Stephen's findings.

This is not a regression. Same behavior repros on .NET Framework 4.8.

[pgovind]

Yea we have issues around culture when dealing with u\0130. The fundamental bug here is #36147. IMO, we should close this as a dupe of #36147.