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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_parser.tutorial.alternative_parsers"></a><a class="link" href="alternative_parsers.html" title="Alternative Parsers">Alternative
Parsers</a>
</h3></div></div></div>
<p>
Frequently, you need to parse something that might have one of several forms.
<code class="computeroutput"><span class="keyword">operator</span><span class="special">|</span></code>
is overloaded to form alternative parsers. For example:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">bp</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">;</span>
<span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">parser_1</span> <span class="special">=</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">int_</span> <span class="special">|</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">eps</span><span class="special">;</span>
</pre>
<p>
<code class="computeroutput"><span class="identifier">parser_1</span></code> matches an integer,
or if that fails, it matches <span class="emphasis"><em>epsilon</em></span>, the empty string.
This is equivalent to writing:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">bp</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">;</span>
<span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">parser_2</span> <span class="special">=</span> <span class="special">-</span><span class="identifier">bp</span><span class="special">::</span><span class="identifier">int_</span><span class="special">;</span>
</pre>
<p>
However, neither <code class="computeroutput"><span class="identifier">parser_1</span></code>
nor <code class="computeroutput"><span class="identifier">parser_2</span></code> is equivalent
to writing this:
</p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">bp</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">parser</span><span class="special">;</span>
<span class="keyword">auto</span> <span class="keyword">const</span> <span class="identifier">parser_3</span> <span class="special">=</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">eps</span> <span class="special">|</span> <span class="identifier">bp</span><span class="special">::</span><span class="identifier">int_</span><span class="special">;</span> <span class="comment">// Does not do what you think.</span>
</pre>
<p>
The reason is that alternative parsers try each of their subparsers, one
at a time, and stop on the first one that matches. <span class="emphasis"><em>Epsilon</em></span>
matches anything, since it is zero length and consumes no input. It even
matches the end of input. This means that <code class="computeroutput"><span class="identifier">parser_3</span></code>
is equivalent to <code class="computeroutput"><a class="link" href="../../boost/parser/eps.html" title="Global eps">eps</a></code>
by itself.
</p>
<div class="note"><table border="0" summary="Note">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../images/note.png"></td>
<th align="left">Note</th>
</tr>
<tr><td align="left" valign="top"><p>
For this reason, writing <code class="computeroutput"><a class="link" href="../../boost/parser/eps.html" title="Global eps">eps</a> <span class="special">|</span>
<span class="identifier">p</span></code> for any parser p is considered
a bug. Debug builds will assert when <code class="computeroutput"><a class="link" href="../../boost/parser/eps.html" title="Global eps">eps</a> <span class="special">|</span>
<span class="identifier">p</span></code> is encountered.
</p></td></tr>
</table></div>
<div class="warning"><table border="0" summary="Warning">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Warning]" src="../../images/warning.png"></td>
<th align="left">Warning</th>
</tr>
<tr><td align="left" valign="top"><p>
This kind of error is very common when <code class="computeroutput"><a class="link" href="../../boost/parser/eps.html" title="Global eps">eps</a></code> is involved, and also
very easy to detect. However, it is possible to write <code class="computeroutput"><span class="identifier">P1</span>
<span class="special">&gt;&gt;</span> <span class="identifier">P2</span></code>,
where <code class="computeroutput"><span class="identifier">P1</span></code> is a prefix of
<code class="computeroutput"><span class="identifier">P2</span></code>, such as <code class="computeroutput"><span class="identifier">int_</span> <span class="special">|</span> <span class="keyword">int</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span></code>, or <code class="computeroutput"><span class="identifier">repeat</span><span class="special">(</span><span class="number">4</span><span class="special">)[</span><span class="identifier">hex_digit</span><span class="special">]</span>
<span class="special">|</span> <span class="identifier">repeat</span><span class="special">(</span><span class="number">8</span><span class="special">)[</span><span class="identifier">hex_digit</span><span class="special">]</span></code>.
This is almost certainly an error, but is impossible to detect in the general
case — remember that <code class="computeroutput"><a class="link" href="../../boost/parser/rule.html" title="Struct template rule">rules</a></code> can be separately compiled,
and consider a pair of rules whose associated <code class="computeroutput"><span class="identifier">_def</span></code>
parsers are <code class="computeroutput"><span class="identifier">int_</span></code> and <code class="computeroutput"><span class="identifier">int_</span> <span class="special">&gt;&gt;</span>
<span class="identifier">int_</span></code>, respectively.
</p></td></tr>
</table></div>
</div>
<div class="copyright-footer">Copyright © 2020 T. Zachary Laine<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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