al">[</span> <span class="identifier">letter</span> <span class="special">|</span> <span class="identifier">digit</span> <span class="special">]*</span>
<span class="identifier">integer</span>    <span class="special">:=</span> <span class="identifier">digit</span><span class="special">+</span>
</pre>
<p>
        Higher level parts of practical grammars tend to be more complex and can't
        be implemented using plain regular expressions. We need to store information
        on the built-in hardware stac