title: Custom .is_a? in Ruby
body: <p>Is it possible to create a custom <code>is_a?</code> in Ruby?</p>
  
  <p>I have a string that can be either a UID or a nickname that I need to check and decide.</p>
  
  <p>UID: <code>001ABC123</code> (starts with 3 numeric and ends in 6 alphanumeric characters)</p>
  
  <p>Nickname: <code>john</code> (can be anything but never starts with a number)</p>
  
  <p>I want to be able to do:</p>
  
  <pre><code>t = '001ABC123'
  
  if t.is_a? UID
    ...
  end
  </code></pre>
  
  <p>Is it possible?</p>