title: Regex for MongoDB ObjectID
body: <p>With reference to <a href="https://stackoverflow.com/a/11989159/645458">this</a> SO question, I have a scenario where I only need to match a hex string with a-f included. All else should not match. Example:</p>
  
  <pre><code>checkForHexRegExp.test("112345679065574883030833"); // =&gt; false
  checkForHexRegExp.test("FFFFFFFFFFFFFFFFFFFFFFFF"); // =&gt; false
  checkForHexRegExp.test("45cbc4a0e4123f6920000002"); // =&gt; true
  </code></pre>
  
  <p>My use case is that I am working with a set of hex strings and would like to only validate as true those that are mongodb objectIDs.</p>