title: Regular expression for user id
body: <p>I am new to regular expressions.</p>
  
  <p>I have a requirement to write a regular expression with the following criteria</p>
  
  <ul>
  <li>User ID cannot be all numeric</li>
  <li>User ID cannot have special characters</li>
  <li>Minimum length of 8 characters</li>
  <li>Maximum length of 20 characters</li>
  <li>Can be all alphabetic</li>
  <li>Can be alphanumeric</li>
  </ul>
  
  <p>I have written the following expression but it does not work</p>
  
  <pre><code>^[a-zA-Z\\d*]{8,20}$
  </code></pre>