title: Regex to match string containing letters and only underscore
body: <p>Well my question is simple, I want to match a string with following attributes</p>
  
  <ul>
  <li>No white space</li>
  <li>Must start with a letter</li>
  <li>Must not contain any other special characters other than underscore</li>
  <li>May contain numbers</li>
  </ul>
  
  <p>Please help in creating such a regex.</p>