title: convert ABNF rules to regular expressions
body: <p>Please help me understand this <a href="https://www.rfc-editor.org/rfc/rfc5234" rel="nofollow noreferrer">ABNF</a> rule <code>([a-z]* [A-Z]* [0-9]*)*</code>.<br />
  I think it could be converted to the regex like this <code>[a-zA-Z0-9]*</code>. So the ABNF rule should match lowercase and/or uppercase letters and/or numbers in any order and their combinations. For example, below strings should be matched with the rule.</p>
  <pre><code>&quot;ABC&quot;, &quot;abc&quot;, &quot;abc12&quot;, &quot;aAbC876&quot;, &quot;123go&quot;, etc.
  </code></pre>
  <p>And if the ABNF rule is <code>([a-z]* [A-Z]* | [0-9]*)*</code>, it can also be converted to the same regex.</p>
  <p>Verifying regex is easy, but is there a tool or something that can verify my understanding about these ABNF rules or can anyone confirm or correct me please?</p>