C# textbox license key auto formatting

Question

I'd like to provide users with a fancy license key textbox, that would insert the dashes. The key is supposed to be 20 characters long ( 4 groups of 5 characters )

I tried using regex, first:

Regex.Replace( text, ".{5}", "$0-" );

that is a problem as the dash is inserted even when there's no following characters, e.g AAAAA-, that results in not being able to remove the dash as it's automatically re-inserted.

then I thought about telling the regex there should be a following character

Regex.Replace( text, "(.{5})(.)", "$1-$2" );

but that splits the key into group of 5-6-6...

Any ideas?

Answer 1

Use a negative lookahead to avoid - to be added at the last. It matches each 5 digits of input string from the first but not the last 5 digits.

.{5}(?!$)

Replacement string

$0-

DEMO

string str = "12345678909876543212";
string result = Regex.Replace(str, @".{5}(?!$)", "$0-");
Console.WriteLine(result);
Console.ReadLine();

Output:

12345-67890-98765-43212

IDEONE