title: Forcing text entry on textbox to be am/pm
body: <p>I'd like to create a textbox that forces the user to enter either "am" or "pm" or if they don't finish and move to another control having only entered an "a" or "p" automatically finish it off. Or maybe as soon as the user enters "a" or "p" it adds in the "m".</p>
  
  <p>I was thinking of using a regular expression to match the entry somehow. I already have a custom textbox that can check a regular expression, however I am unsure how to construct it.</p>
  
  <pre><code>RegularExpression="([AaPp][Mm])"
  </code></pre>
  
  <p>Could some help me with the expression please?</p>