- preg_replace: documentation ( source)
<?php
$str = 'This is <a href="http://foo.com">This is #foo</a>. simple #html-stuff text <a href="http://foobar.com#baz">#simple</a> <span class="simple">simple #simple text text</span>';
$str = preg_replace('/(?!<a[^<>]*>[^#]+)#(\b[-_\w]+\b)(?![^<]*<\/a>)/', '<a href="/tags/$1">#$1</a>', $str);
echo $str;