mask all digits except first 6 and last 4 digits of a string( length varies )

Question

I have a card number as a string, for example:

string  ClsCommon.str_CardNumbe r = \"3456123434561234\";

The length of this card number can

Answer 1

How about replacing a specific matched group using Regex :

        string cardNumber = "3456123434561234";
        var pattern = "^(.{6})(.+)(.{4})$";
        var maskedNumber = Regex.Replace(cardNumber, pattern, (match) =>
        {
           return Regex.Replace(String.Format("{0}{1}{2}",
           match.Groups[1].Value, // the first 6 digits
           new String('X', match.Groups[2].Value.Length), // X times the 'X' char
           match.Groups[3].Value) /*the last 4 digits*/,".{4}", "$0 "); //finally add a separator every 4 char
        });