        failure to match should result in an exception. For example, when parsing
        an e-mail address, a name, an "@" and a domain name must be matched
        or the address is invalid.
      </p>
<p>
        Two X3 mechanisms facilitate parser expectations:
      </p>
<div class="orderedlist"><ol class="orderedlist" type="1">
<li class="listitem">
            The expectation operator (<code class="computeroutput"><span class="identifier">a</span>
            <span class="special">&gt;</span> <span c