using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        string input = "testa.user@yahoo.co.in";

        string output = PartialEmailAndMobileMasking(input);

        Console.WriteLine(output);

    }

    private static readonly Regex Pattern = new Regex("{(.+?)}", RegexOptions.Compiled);

    private static readonly string str = @"([^.])";

public static string PartialEmailAndMobileMasking(string input)

{

    string[] arr = input.Split('@');

    return AlternateMasking(arr[0])+"@"+AlternateMasking(arr[1]);

}

    private static string AlternateMasking(string input)

    {   

        int i = 0;          

        string newtext = new Regex(str).Replace(input, m => i++ % 2 == 0 ? Convert.ToString(m) : "*" );

        return newtext;         

    }

}