How to make TextBox continually update with user's input?

I have a TextBox in my form, the user should enter some string there.

After each 2 characters, it should automatically add a "|" in there. But I don't know how to do that. Here's my code:

private void textBox1_TextChanged(object sender, EventArgs e)
{
    string input = textBox1.Text; int count = 0;
    foreach (char c in input)
    {
        count++;
        if (count == 2)
        {
            input += '|';
            count = 0;
            textBox1.Text = input;
        }
    }
}

• The user should only enter the characters in hexadecimals (meaning 0-9, a-f).

I've tried the e.Keychar on the Keypress event, but it doesn't work, if the user use Copy-Paste and Ctrl-C, Ctrl-V.