title: Why are uuids restricted to [89AB] in the first character of the 4th group?
body: <p>The regex for validating uuid v4 is the following :</p>
  <pre><code>/^[0-9A-F]{8}-[0-9A-F]{4}-[4][0-9A-F]{3}-[89AB][0-9A-F]{3}-[0-9A-F]{12}$/i
  </code></pre>
  <p>As you can see, the 4th group contains <code>[89AB][0-9A-F]{3}</code> instead of <code>[0-9A-F]{4}</code></p>
  <p>Do you know why that is ? Is there any historical reason for this choice ?</p>
  <p>Yesterday I found out that the library that I use doesn't generate correct uuids because of the 4th group 1st character which was randomly choosen, and I wonder why there is such a restriction since this makes implementations a bit more complex.</p>