using System;
using System.Linq;
using System.Text.RegularExpressions;
public class Test
{
public static void Main()
{
var s = "XYYXYX";
var res = Regex.Replace(s, @"X(Y+)X", m => string.Format("X{0}X",m.Groups[1].Value.Replace("Y", "Z")));
Console.WriteLine(res);
}
}
dXNpbmcgU3lzdGVtOwp1c2luZyBTeXN0ZW0uTGlucTsKdXNpbmcgU3lzdGVtLlRleHQuUmVndWxhckV4cHJlc3Npb25zOwoKcHVibGljIGNsYXNzIFRlc3QKewoJcHVibGljIHN0YXRpYyB2b2lkIE1haW4oKQoJewoJCXZhciBzID0gIlhZWVhZWCI7CgkJdmFyIHJlcyA9IFJlZ2V4LlJlcGxhY2UocywgQCJYKFkrKVgiLCBtID0+IHN0cmluZy5Gb3JtYXQoIlh7MH1YIixtLkdyb3Vwc1sxXS5WYWx1ZS5SZXBsYWNlKCJZIiwgIloiKSkpOwoJCUNvbnNvbGUuV3JpdGVMaW5lKHJlcyk7Cgl9Cn0=