on get double from key down

Question

How do I only get a double from a key down ?
so if the user type 4.56
I want to display 4.56
but if they type 4.35 then backspace
I want 4.3
and
not 4.36Back

here is my code:

if (e.Key == Key.Enter || e.Key == Key.Return) return;  
                string letter;  
                if (e.Key == Key.OemPeriod)  
                    letter = ".";  
                else  
                    letter  = e.Key.ToString();  
      
                if (letter.Contains("D"))  
                    letter = Regex.Replace(letter, "D", String.Empty);  
                SliderTxt.Text = SliderTxt.Text + letter;  
  

Answer 1

Or do you want something like the following?
Xaml:

 <TextBox x:Name="tb2"   Width="200" Height="105"  PreviewTextInput="txt_PreviewTextInput"/>  

Codebehind:

 private void txt_PreviewTextInput(object sender, TextCompositionEventArgs e)  
    {   

      foreach (char ch in e.Text)  
        if (!(Char.IsDigit(ch) || ch.Equals('.')))  
        {  
          e.Handled = true;  
        }  
    }  

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Answer 2

All
thanks for the help but I got it to work now.