ode>match('[0-9]').repeat(1)</code>.
This says: &#8220;match a character that is in the range <code>0-9</code>, then
match any number of those, but at least match one.&#8221;</p>
<pre class="sh_ruby"><code> 
  root(:integer)
</code></pre>
<p>That second line just says: Start parsing at the rule called
<code>:integer</code>.</p>
<h2>Addition</h2>
<p>Let&#8217;s go for simple addition. We&#8217;ll have to allow for spaces in our input,
since those help make code readable.</p>
<pre class="sh_ruby"><code> 
  ru