title: Trigger jQuery UI autocomplete at minLength = 3 but 1 when only digits
body: <p>I have an autocomplete input field, and I want it to search on objects with a label (text) and an id (integer). </p>
  
  <p>The autocomplete must allow users to search either on the textual label or the id. The server part is ok, but on the client part, I would like to trigger the search at minLength=3 when the user enters at least one letter, but at the first character when there is only digits (if not, it misses all ids &lt;=99). </p>
  
  <p>Code: </p>
  
  <pre><code>$("#my_input").autocomplete({
      minLength : 1,
      delay:200,  source: function(request,response){ 
         ... 
  } });
  </code></pre>
  
  <p>How to prevent the autocomplete to trigger when less then 3 characters if there is at least 1 letter ?   </p>