<?php
$html = $original = <<<HTML
<br class="clearfix" /><ul class="tml-action-links">
<li><a href="http://localhost/robinhood/register/" rel="nofollow">Register</a></li>
<li><a href="http://localhost/robinhood/lostpassword/" rel="nofollow">Lost Password</a></li>
</ul>
HTML;
preg_match('(<ul class="tml-action-links">(.*?)<\/ul>)s', $html, $matches);
$html = $matches[0];
$html = str_replace('<li>', '<li class="peehaa">', $html);
echo $html;
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