title: Excluding a string in a regex expression
body: <p>I currently have the following regular expression:</p>
  
  <pre><code>/(_|[a-z]|[A-Z])(_|[a-z]|[A-Z]|[0-9])*/
  </code></pre>
  
  <p>I would like the expression not to match with "PI", however I failed to do so.
  To clarify, I would like the following to be valid:
  _PI, abcPI, PIpipipi
  I just dont want to accept PI when its on its own.</p>