How to mask a mobile number in .net core razor page?

Do you want to mask the value when user is providing or you want to mask a value which you are already holding in a variable?

If you want to perform on this on a value already hold by a variable then this would work:

var temp = Regex.Replace(str, "[0-9]", "*" );
str = temp + str.Substring(str.Length - 4, str.Length - (str.Length - 4));