Regex Replace exclude first and nth character

Question

I am trying to mask a string name with * (asterisks) and exclude both the first and nth (5th) characters.

Example:

UserFirstName -> U****F*******

I managed to exclude the first character with (?!^).:

var regex = new Regex("(?!^).");
var result = regex.Replace(stringUserName, "*");

output:

UserFirstName -> U************

How can I also exclude the character in the 5th position?

Answer 1

You may use

(?!^)(?<!^.{4}).

See the regex demo

Pattern details

• (?!^) - (it is equal to (?<!^) lookbehind that you may use instead) a negative lookahead that fails the position at the start of string
• (?<!^.{4}) - a negative lookbehind that fails the match if, immediately to the left of the current position, there are any four characters other than a newline char from the start of the string
• . - any single char other than a newline char.

C# demo:

string text = "UserFirstName";
int SkipIndex = 5;
string pattern = $@"(?!^)(?<!^.{{{SkipIndex-1}}}).";
Console.WriteLine(Regex.Replace(text, pattern, "*"));

Output: U***F********

Answer 2

Without Regex, extra explanation not required ;)

var text = "UserFirstName";
var skip = new[] { 0, 4 }.ToHashSet();

var masked = text.Select((c, index) => skip.Contains(index) ? c : '*').ToArray();
var output = new String(masked);

Console.WriteLine (output); // U***F********

c# Demo