Error exception startindex cannot be less than zero

Question

I have textbox1 and i need to allow this textbox1 to accept Arabic characters only

no digit accept also no English characters accept

so that i write in key_up event as following

private void textBox1_KeyUp(object sender, KeyEventArgs e)
        {
            if (textBox1.Text.Any(char.IsDigit))
            {

                textBox1.Text = textBox1.Text.Remove(textBox1.Text.Length - 1);
                textBox1.Focus();
            }

            QrClasses qr = new MatrixBarcode.QrClasses();
            bool b = qr.HasArabicCharacters(textBox1.Text);

            if (b == false)
            {

                    textBox1.Text = textBox1.Text.Remove(textBox1.Text.Length - 1);



                textBox1.Focus();

            }
            else
            {
                textBox1.Enabled = true;
            }
        }

public bool HasArabicCharacters(string text)

        {

            Regex regex = new Regex(

                "[\u0600-\u06ff]|[\u0750-\u077f]|[\ufb50-\ufc3f]|[\ufe70-\ufefc]");

            return regex.IsMatch(text);
        }

I get Error exception startindex cannot be less than zero (index out of range)

in line

textBox1.Text = textBox1.Text.Remove(textBox1.Text.Length - 1);

when try to enter Arabic letters

so that how to solve this problem