using System.Text.RegularExpressions;
public static void Main()
string input = "itsarun.sunny@yahoo.com";
string pattern = @"(?<=[\w]{1})[\w-\._\+%]*(?=[\w]{1}@)";
string maskedEmail = Regex.Replace(input, pattern, m => new string('*', m.Length));
System.Console.WriteLine(maskedEmail);