using System;
using System.Text.RegularExpressions;
public class Test
{
    public static void Main()
    {   
        string msg= "ABC";
        var charSetOccurences = new Regex(@"(?=([A-Z]{2}))", RegexOptions.IgnoreCase);
        var charSetMatches = charSetOccurences.Matches(msg);
        foreach (Match match in charSetMatches)
        {
            string charSet = match.Groups[1].Value;
            Console.WriteLine(charSet);
        }
    }
}

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