What set of chars is php's uniqid composed of?
<p>I would like to prepare simple regular expression for php's <code>uniqid</code>. I checked  <a href="http://php.net/manual/en/function.uniqid.php" rel="nofollow noreferrer">uniqid</a> manual looking for set of chars used as return value. But the documentation only mention that:</p>

<blockquote>
  <p>@return string the unique identifier, as a string.</p>
</blockquote>

<p>And</p>

<blockquote>
  <p>With an empty prefix, the returned string will be 13 characters long. If more_entropy is true, it will be 23 characters.</p>
</blockquote>

<p>I would like to know what characters can I expect in the return value. Is it a hex string? How to know for sure? Where to find something more about the uniqid function?</p>