How in this textbox'a code to make it possible to press the backspace key (erase)?

Question

C++ / C#

you need to modify the code so that the backspace key is active (it works in the textbox)

private void TextBox1_KeyPress(object sender, KeyPressEventArgs e)
        {
            
            string Symbol = e.KeyChar.ToString();

            if (!Regex.Match(Symbol, @"[а-яА-Я]").Success )
            {
                e.Handled = true;
            }

        }

Answer 1

Use KeyDown instead of KeyPress. KeyDown receives not a printable character, but a key code, including control keys.
In your case, both events need to be called. KeyDown is called first, and if it hasn't done any processing work (e.Handled = true is not specified), then the KeyPress handler will be called.
Here is an example code for handling backspace:

private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
    if (e.KeyCode == Keys.Back)
    {
        var selectionStart = textBox1.SelectionStart;
        if (textBox1.SelectionLength > 0)
        {
            textBox1.Text = textBox1.Text.Substring(0, selectionStart) + textBox1.Text.Substring(selectionStart + textBox1.SelectionLength);
            textBox1.SelectionStart = selectionStart;
        }
        else if (selectionStart > 0)
        {
            textBox1.Text = textBox1.Text.Substring(0, selectionStart - 1) + textBox1.Text.Substring(selectionStart);
            textBox1.SelectionStart = selectionStart - 1;
        }
        
        e.Handled = true;
    }
}