using System;

using System.Collections.Generic;

using System.Text.RegularExpressions;

public class Program

{

    static void Main(string[] args)

    {

        var str = "Something @Model.xxx something @Model.yyy";

        var result = Regex.Replace(str, @"@Model\.([xy]{3})", match => $"$${match.Groups[1]}$$");

        Console.Write(result);

    }

}