using System;

using System.Text.RegularExpressions;

public class Program

{

    static Regex redactEmailRegex = new Regex("(?<=.).(?=.*@)", RegexOptions.Compiled); 

    public static void Main()

    {

        var input = "abc@abc.com";

        var result = redactEmailRegex.Replace(input, "*");

        result.Dump();

    }

}