title: WPF StringFormat to allow only positive numbers
body: <p>I only want to allow positive numbers as user input in my TextBox.</p>
  
  <p>If I input <code>-10</code> and use</p>
  
  <pre><code>&lt;TextBox Text="{Binding Width, StringFormat={}{0:0.00;0.00}}" /&gt;
  </code></pre>
  
  <p>then it always displays positive numbers, but the Data behind is still negative. </p>
  
  <p>How to do it right?</p>