using System;
using System.Linq;
using System.Text.RegularExpressions;


public class Test
{
	public static void Main()
	{
		var inputString = "4ABC3D";
		var res = Regex.Replace(inputString, "([0-9]+)([A-Z])",
		    x => String.Concat(Enumerable.Repeat(x.Groups[2].Value, Int32.Parse(x.Groups[1].Value)))
		); 
		Console.WriteLine(res);
	}
}