title: ASP.NET Regular Expression Validator
body: <p>I need a Regular Expression that can validate an exact 3 character(alpha only) code but also a blank field to set as the validation expression of a ASP.NET RegEx validator control.
  I am currently using <code>^[a-zA-Z]{3}$</code>
  and this works out well to match the code but of course doesn't match a blank.
  I have been looking at using something like this:
  <code>^(?:|)[a-zA-Z]{3}$</code></p>