Masking of Email Address in Java

If you're bad at regular expressions, don't use them :) I don't know if you've ever heard the quote:

Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.

(source)

You might get a working regular expression here, but will you understand it today? tomorrow? in six months' time? And will your colleagues?

An easy alternative is using a StringBuilder, and I'd argue that it's a lot more straightforward to understand what is going on here:

StringBuilder sb = new StringBuilder(email);
for (int i = 3; i < sb.length() && sb.charAt(i) != '@'; ++i) {
  sb.setCharAt(i, '*');
}
email = sb.toString();

"Starting at the third character, replace the characters with a * until you reach the end of the string or @."

(You don't even need to use StringBuilder: you could simply manipulate the elements of email.toCharArray(), then construct a new string at the end).

Of course, this doesn't work correctly for email addresses where the local part is shorter than 3 characters - it would actually then mask the domain.

Your Look-ahead is kind of complicated. Try this code :

public static void main(String... args) throws Exception {
    String s = "";
    s= s.replaceAll("(?<=.{3}).(?=.*@)", "*");
    System.out.println(s);
}

O/P :

nil********@gmail.com

I like this one because I just want to hide 4 characters, it also dynamically decrease the hidden chars to 2 if the email address is too short:

public static String maskEmailAddress(final String email) {
    final String mask = "*****";
    final int at = email.indexOf("@");
    if (at > 2) {
        final int maskLen = Math.min(Math.max(at / 2, 2), 4);
        final int start = (at - maskLen) / 2;
        return email.substring(0, start) + mask.substring(0, maskLen) + email.substring(start + maskLen);
    }
    return email;
}

Sample outputs:

    >    my****
         >    i**

//In Kotlin

val email = ""
val maskedEmail = email.replace(Regex("(?<=.{3}).(?=.*@)"), "*")

    public static string GetMaskedEmail(string emailAddress)
    {
        string _emailToMask = emailAddress;
        try
        {
            if (!string.IsNullOrEmpty(emailAddress))
            {
                var _splitEmail = emailAddress.Split(Char.Parse("@"));
                var _user = _splitEmail[0];
                var _domain = _splitEmail[1];

                if (_user.Length > 3)
                {
                    var _maskedUser = _user.Substring(0, 3) + new String(Char.Parse("*"), _user.Length - 3);
                    _emailToMask = _maskedUser + "@" + _domain;
                }
                else
                {
                    _emailToMask = new String(Char.Parse("*"), _user.Length) + "@" + _domain;
                }
            }
        }
        catch (Exception) { }
        return _emailToMask;
    }