Check if string is a specific character, and limit it to one character

Question

I'm making a calculator for my C# class. I'm not allowed to use buttons for operands. There's an operand text box in which the user is supposed to enter "+" for addition, "-" for subtraction, "/" for division, or "*" for multiplication.

I'm trying to find out how to make it where you can only enter one of those four characters, but it also only lets you input one at a time.

I'm pretty new to C#.

Answer 1

Thank you to "TheGeneral" for helping me figure this out, by giving me this link .

I'm not sure why I couldn't find this question on Google.

This is how I did it:

private void TxtOperator_TextChanged(object sender, EventArgs e)
    {
        Regex regex = new Regex(@"[^+^\-^\/^\*^\(^\)]");
        MatchCollection matches = regex.Matches(txtOperator.Text);
        if (matches.Count > 0)
        {
            MessageBox.Show("Please input an operator.");
            txtOperator.Text = "";
        }
    }