Text box validation in C# with wpf

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C#

WPF

I have a text box in which user enters hours like ex: 10.5 or 2.5 , so when the user just enters 10 or 2, then the text box will display ".0" as appended to 10 or 2.
and finally the text box will displayed as value as 10.0 or 2.0.

So the maximum lenghtof textBox is 4. so when the user enters values like 1111, the textbox will be appended with ".0". How to restrict the lenght of textbox so that it will accept only values like ex: 1.2 or 10.5 or 12.0 or 1.0. and not to accept values like 1111.0

am using the follwoing code to code to append ".0"

private void txtPlannedHours_LostKeyboardFocus(object sender, KeyboardFocusChangedEventArgs e)
        {
            txtPlannedHours.Text = Convert.ToDouble(txtPlannedHours.Text).ToString("0.0");
        }

And am using the following regular expression:
Regex re = new Regex("^\\d{1,2}([.]\\d{1})$");

can anyone please suggest me?