C# - Using regex with if statements

Question

I have some code that checks a fields input against a regex, although for some reason (no matter what I put in the field, it returns flase. Is there something I have missed?

private void textBox5_Validating(object sender, CancelEventArgs e)
{
    String AllowedChars = @"^a-zA-Z0-9.$";
    if (Regex.IsMatch(textBox5.Text, AllowedChars))
    {
        MessageBox.Show("Valid");
    }
    else
    {
        MessageBox.Show("Invalid");
    }
}

Answer 1

The regex makes no sense to me. This one would (notice the square brackets used for defining an alphabet):

String AllowedChars = @"^[a-zA-Z0-9]*$";

Answer 2

What you want is to group those characters and allow 0 or more:

@"^[a-zA-Z0-9.]*$"

Otherwise, what you posted allows "a-zA-Z0-9" and one more character only.