PHP remove all attribuite from html tag

gistfile1.php

<?php

$str = '
<p class="class_style" style="font-size: medium; line-height: normal; letter-spacing: normal;">content</p>
<span class="another_class_style" style="font-size: medium; line-height: normal; letter-spacing: normal;">content</span>
<ul class="another_class_style" style="background:#006;"></ul>
<li class="another_class_style" style=" list-style:circle; color:#930;">content</li>';

$clean = preg_replace('/ .*".*"/', '', $str);

echo $clean;