在屏幕键盘上使用正则表达式

Question

Ok i'm really getting stuck here guys :/. I've got a (custom!) onscreen keyboard for which I wanna use regular expressions.

I Successfully created a regex to only allow 0-9 and a comma for a regular keyboard.

Of course this isn't working for my on-screen keyboard.

I have a code snippet that works...

BUT I also have a space button, a backspace button, and 2 buttons to move the caret around.

When I use the below regex code and click the space, backspace or the caret buttons I get this error:

Object reference not set to an instance of an object.

What am I doing wrong?

    // the regex code   
    private Control keyb = null;
    private void EditProdPriceEx_GotFocus(object sender, RoutedEventArgs e)
    {
        string AllowedChars = "[^0-9,]";
        if (Regex.IsMatch(EditProdPriceEx.Text, AllowedChars))
        {
            keyb = (Control)sender;
        }
    }

Space button:

    private void KnopSpatie_Click(object sender, RoutedEventArgs e)
    {
        TextBox tb = keyb as TextBox;
        int selStart = tb.SelectionStart; // here is where i get the error
        string before = tb.Text.Substring(0, selStart);
        string after = tb.Text.Substring(before.Length);
        tb.Text = string.Concat(before, " ", after);
        tb.SelectionStart = before.Length + 1;
    }

Backspace Button:

    private void KnopWissen_Click(object sender, RoutedEventArgs e)
    {
        TextBox tb = keyb as TextBox;
        int cursorPosition = tb.SelectionStart; // here is where i get the error
        if (cursorPosition > 0)
        {
            tb.Text = tb.Text.Substring(0, cursorPosition -1) + tb.Text.Substring(cursorPosition);
            tb.SelectionStart = cursorPosition - 1;
        }
    }

Move caret left button:

    private void KnopLinks_Click(object sender, RoutedEventArgs e)
    {
        TextBox tb = keyb as TextBox;
        if (tb.SelectionStart > 0) // here is where i get the error
        {
            int selStart = tb.SelectionStart;
            string before = tb.Text.Substring(0, selStart);
            string after = tb.Text.Substring(before.Length);
            tb.SelectionStart = before.Length - 1;
        }
    }

Move caret right button:

    private void KnopRechts_Click(object sender, RoutedEventArgs e)
    {
        TextBox tb = keyb as TextBox;

        if (tb.SelectionStart >= 0) // here is where i get the error
        {
            int selStart = tb.SelectionStart;
            string before = tb.Text.Substring(0, selStart);
            string after = tb.Text.Substring(before.Length);
            tb.SelectionStart = before.Length + 1;
        }
    }

Answer 1

I think you must try this. In the first Code.

// the regex code   
private Control keyb = null;
private void EditProdPriceEx_GotFocus(object sender, RoutedEventArgs e)
{
    string AllowedChars = "[^0-9,]";
    if (Regex.IsMatch(EditProdPriceEx.Text, AllowedChars))
    {
        keyb = (Control)sender;
    }
}

You made keyb null, and is not set by any key you mention (space and others), I think this produce the reference not set error. Can you check that?.