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<a name="Empty-Rules"></a>
<div class="header">
<p>
Next: <a href="Recursion.html#Recursion" accesskey="n" rel="next">Recursion</a>, Previous: <a href="Rules-Syntax.html#Rules-Syntax" accesskey="p" rel="prev">Rules Syntax</a>, Up: <a href="Rules.html#Rules" accesskey="u" rel="up">Rules</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Terms.html#Index-of-Terms" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Empty-Rules-1"></a>
<h4 class="subsection">3.3.2 Empty Rules</h4>
<a name="index-empty-rule"></a>
<a name="index-rule_002c-empty"></a>
<a name="index-_0025empty"></a>

<p>A rule is said to be <em>empty</em> if its right-hand side (<var>components</var>)
is empty.  It means that <var>result</var> can match the empty string.  For
example, here is how to define an optional semicolon:
</p>
<div class="example">
<pre class="example">semicolon.opt: | &quot;;&quot;;
</pre></div>

<p>It is easy not to see an empty rule, especially when <code>|</code> is used.  The
<code>%empty</code> directive allows to make explicit that a rule is empty on
purpose:
</p>
<div class="example">
<pre class="example">semicolon.opt:
  %empty
| &quot;;&quot;
;
</pre></div>

<p>Flagging a non-empty rule with <code>%empty</code> is an error.  If run with
<samp>-Wempty-rule</samp>, <code>bison</code> will report empty rules without
<code>%empty</code>.  Using <code>%empty</code> enables this warning, unless
<samp>-Wno-empty-rule</samp> was specified.
</p>
<p>The <code>%empty</code> directive is a Bison extension, it does not work with
Yacc.  To remain compatible with POSIX Yacc, it is customary to write a
comment &lsquo;<samp>/* empty */</samp>&rsquo; in each rule with no components:
</p>
<div class="example">
<pre class="example">semicolon.opt:
  /* empty */
| &quot;;&quot;
;
</pre></div>





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