is that what you want?

<ul class="list_box">\s(<li><a\s*href=\"(.*)\">(.*)</a><\/li>\s)*</ul>

=

to get all the url senses, you have to take two steps

Code

<?php
$strPattern = '/<ul class="list_box">\s(?<lists>.*?)<\/ul>/s';
$strSubject = '<ul class="list_box">
<li><a href="xxxx1.html">xxxxx</a></li>
<li><a href="xxxx2.html">xxxxx</a></li>
<li><a href="xxxx3.html">xxxxx</a></li>
<li><a href="xxxx4.html">xxxxx</a></li>
<li><a href="xxxx5.html">xxxxx</a></li>
<li><a href="xxxx6.html">xxxxx</a></li>
</ul>';
preg_match_all($strPattern, $strSubject, $arrMatches);

$strPattern2 = '/<li><a\s*href=\"(?<url>.*?)\">(.*?)<\/a><\/li>\s/s';
preg_match_all($strPattern2, $arrMatches['lists'][0], $arrMatches2);
var_dump($arrMatches2['url']);

result

array(6) {
  [0]=>
  string(10) "xxxx1.html"
  [1]=>
  string(10) "xxxx2.html"
  [2]=>
  string(10) "xxxx3.html"
  [3]=>
  string(10) "xxxx4.html"
  [4]=>
  string(10) "xxxx5.html"
  [5]=>
  string(10) "xxxx6.html"
}

$html = <<< HTML
<ul class="list_box">
    <li><a href="xxxx.html">xxxxx</a></li>
    <li><a href="xxxx.html">xxxxx</a></li>
    <li><a href="xxxx.html">xxxxx</a></li>
    <li><a href="xxxx.html">xxxxx</a></li>
    <li><a href="xxxx.html">xxxxx</a></li>
    <li><a href="xxxx.html">xxxxx</a></li>
</ul>
HTML;

$pattern_href = '/href=[\"\'](.*)[\"\']/i';
preg_match_all($pattern_href, $html, $href);
var_dump($href[1]);

$pattern_title = "/[\"\"]\>(.*)\<\/a\>/i';
preg_match_all($pattern_title, $html, $title);
var_dump($title[1]);