email address hiding some characters with c#, regex

Because your rules are quite simple it might be easier to just use substring to get the characters before and after the @ and then replace them.

Something along the lines of

            int index = email.IndexOf('@');                
            string returnValue = email.Replace(email.Substring(index - 3, 3), "***").Replace(email.Substring(index+1,3), "***");

Although you'll need to first validate that the email address contains enough characters before the @ and change accordingly.

Similar to other responses, but also different. Accepts the .co.uk addresses too.

using System;
using System.Collections.Generic;
using System.Text.RegularExpressions;

public class Test
{
        public static void Main()
        {
                String regex = @"(.{2}).+@.+(.{2}(?:\..{2,3}){1,2})";
                String replace = "$1*@*$2";
                List<String> tests = new List<String>(new String[]{
                        "[email protected]",
                        "[email protected]",
                        "[email protected]",
                        "[email protected]",
                        "[email protected]"
                });
                tests.ForEach(email =>
                {
                        Console.WriteLine(Regex.Replace(email, regex, replace));
                });
        }
}

Results in:

jo*@*le.com
ji*@*ob.com
su*@*co.uk
[email protected]
[email protected]

Though I'm not 100% sure what you want to do with names that only have 2 letters on either side (thus the last two results). But that's my bid. Example