title: How to get all unique HTML tags on a webpage using regular expression?
body: <p>I have html source code from an HTML page:</p>
  <pre><code>import requests
  
  text = requests.get(&quot;https://en.wikipedia.org/wiki/Collatz_conjecture&quot;).text
  </code></pre>
  <p>What I would like to do is to get a count of the number of unique HTML tags on this page.</p>
  <p>For example: <code>&lt;head&gt;</code>, <code>&lt;title&gt;</code>. Closing tags do not count (<code>&lt;head&gt;</code> and <code>&lt;/head&gt;</code> would be counted only once).</p>
  <p>Yes, I know this is much easier using HTML parsers such as Beautiful Soup but I would like to accomplish this using only Regular Expression.</p>
  <p>I've brute force counted this and the answer is in the ballpark of around 60 unique tags.
  How would I go about doing this?</p>
  <p>I've already tried using <code>re.findall()</code>, to no avail.</p>
  <p>Since the answer is around 60, I would like the output to be:</p>
  <pre><code>&quot;Number of unique HTML tags: 60&quot;
  </code></pre>