title: Regular Expression pattern for alphanumeric username
body: <p>I want to enter the user username as <strong>alphanumeric string</strong>.</p>
  <ul>
  <li>Both letters and numbers should be in string.</li>
  <li>String should start with with alphabets and end with numbers.</li>
  <li>It can contain only underscores as special charachter(optional).</li>
  </ul>
  <p>My code</p>
  <pre class="lang-html prettyprint-override"><code> &lt;input type=&quot;text&quot; name=&quot;username&quot; id=&quot;username&quot; placeholder=&quot;Choose Username&quot; 
    pattern=&quot;^[a-zA-Z0-9_]+${7,25}&quot; autocomplete=&quot;off&quot; required&gt;
  </code></pre>