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);
}
}
dXNpbmcgU3lzdGVtOwp1c2luZyBTeXN0ZW0uTGlucTsKdXNpbmcgU3lzdGVtLlRleHQuUmVndWxhckV4cHJlc3Npb25zOwoKCnB1YmxpYyBjbGFzcyBUZXN0CnsKCXB1YmxpYyBzdGF0aWMgdm9pZCBNYWluKCkKCXsKCQl2YXIgaW5wdXRTdHJpbmcgPSAiNEFCQzNEIjsKCQl2YXIgcmVzID0gUmVnZXguUmVwbGFjZShpbnB1dFN0cmluZywgIihbMC05XSspKFtBLVpdKSIsCgkJICAgIHggPT4gU3RyaW5nLkNvbmNhdChFbnVtZXJhYmxlLlJlcGVhdCh4Lkdyb3Vwc1syXS5WYWx1ZSwgSW50MzIuUGFyc2UoeC5Hcm91cHNbMV0uVmFsdWUpKSkKCQkpOyAKCQlDb25zb2xlLldyaXRlTGluZShyZXMpOwoJfQp9