Move cursor after decimal seprator when press a specific button in WPF

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C#

.NET

WPF

Hello I would like to implement a function in WPF for my textbox. I would like you to choose the number of digits before the comma (e.g. 3), arrived with the cursor near the decimal separator, at the press of the button . (point) the cursor jumps after the decimal separator automatically.

What I have tried:

public static void MaskNumericInput(object sender, TextCompositionEventArgs e)
    {
        e.Handled = !TextIsNumeric(e.Text);
    }
    public static void MaskDoubleInput(object sender, TextCompositionEventArgs e)
    {
        Regex regex = new Regex("^[.][0-9]+$|^[0-9]*[.]{0,1}[0-9]*$");
        e.Handled = !regex.IsMatch((sender as TextBox).Text.Insert((sender as TextBox).SelectionStart, e.Text));
        //var dou = 0d;

        //e.Handled = !IsTextAllowed(e.Text);
        //e.Handled = true;
    }
    private static bool TextIsNumeric(string input)
    {
        return input.All(c => Char.IsDigit(c) || Char.IsControl(c));
    }
    private static readonly Regex _regex = new Regex("[^0-9.-]+"); //regex that matches disallowed text
    private static bool IsTextAllowed(string text)
    {
        return !_regex.IsMatch(text);
    }