using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text.RegularExpressions;

public class Test
{
	public static void Main()
	{
		var text = "0:1:a:b";
    	var results = Regex.Match(text, @"^(?:([01ab])(?::\b|$))+$")?
    		.Groups[1].Captures.Cast<Capture>().Select(c => c.Value);
        Console.WriteLine(string.Join(", ", results));
	}
}