title: What is the pattern for empty string?
body: <p>I need to validate input: valid variants are either number or empty string. What is the correspondent regular expression?</p>
  
  <pre><code>String pattern = "\d+|&lt;what shoudl be here?&gt;";
  </code></pre>
  
  <p>UPD: dont suggest "\d*" please, I'm just curious how to tell "empty string" in regexp.</p>