ed
</code></pre>
<p>Here&#8217;s the error message provided by that exception: &#8220;Expected at least 1 of
[0-9] at line 1 char 1.&#8221; parslet tries to find a number there, but can&#8217;t find
one.</p>
<p>There are just two lines to the definition of this parser, let&#8217;s go through
them:</p>
<pre class="sh_ruby"><code> 
  rule(:integer) { match('[0-9]').repeat(1) }
</code></pre>
<p><code>rule</code> lets you create a new parser rule. Inside the block of that
<code>:integer</code> rule, you find <c