using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        var re = new Regex("((?<a>a)|(?<b>b))");

        var ma = re.Match("a");

        Console.WriteLine("a in a: " + ma.Groups["a"].Success);

        Console.WriteLine("b in a: " + ma.Groups["b"].Success);

​

        ma = re.Match("b");

        Console.WriteLine("a in b: " + ma.Groups["a"].Success);

        Console.WriteLine("b in b: " + ma.Groups["b"].Success);

    }

}