Mask a string, works for various lengths

MaskString.cs

public static string MaskString(string inputString)
{
    string pattern = "^(.{2}?)(.+)(.{2}?)$";
    char maskChar = '*';
    if (string.IsNullOrEmpty(inputString))
    {
        return inputString;
    }

    if (inputString.Length <= 4)
    {
        return new string(maskChar, inputString.Length);
    }

    pattern = "^(.{2}?)(.+)(.{2}?)$";
    return Regex.Replace(inputString, pattern, (match) => Regex.Replace(String.Format("{0}{1}{2}",
        match.Groups[1].Value,
        new string(maskChar, match.Groups[2].Value.Length), // X times the 'X' char
        match.Groups[3].Value), ".{2}", "$0"));
}