样例字符串：
.....
<div>等你等等你</div>
<h2>xxx</h2>
<ul class="mlist">
<li>alsjdflasjlfdjlasdfjjsadfjalsfj<span>张三</span><a href="xxxxxx"></a></li>
<li>alsjdflasjlfdjlasdfjjsadfjalsfj<span>张三</span><a href="xxxxxx"></a></li>
<li>alsjdflasjlfdjlasdfjjsadfjalsfj<span>张三</span><a href="xxxxxx"></a></li>
<li>alsjdflasjlfdjlasdfjjsadfjalsfj<span>张三</span><a href="xxxxxx"></a></li>
</ul>
<div class="answercount">
<div class="diggit">
<div class="diggnum unanswered">0</div>
<div class="diggword">回答数</div>
</div>
<div class="clear">
</div>
</div>
我要的数据是ul class="mlist" 下面的li的集合，如：
<li>alsjdflasjlfdjlasdfjjsadfjalsfj<span>张三</span><a href="xxxxxx"></a></li>
<li>alsjdflasjlfdjlasdfjjsadfjalsfj<span>张三</span><a href="xxxxxx"></a></li>
<li>alsjdflasjlfdjlasdfjjsadfjalsfj<span>张三</span><a href="xxxxxx"></a></li>
<li>alsjdflasjlfdjlasdfjjsadfjalsfj<span>张三</span><a href="xxxxxx"></a></li>`
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