Need help to use preg_replace

<a href="http://theconversation.com">The Conversation</a> as <a href="http://theconversation.com" class="external_link">The Conversation</a>

{exp:low_replace find="you" replace="we"}
text you want processed
{/exp:low_replace}

$result = preg_replace('/(<a\b[^><]*)>/i', '$1 class="external_link">', $str);