Turn hashtags in text into links (excludes hashtags already in links)

hashtags.php

<?php

$str = <<<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;


echo preg_replace('/(?!<a[^<>]*>[^#]+)#(\b[-_\w]+\b)(?![^<]*<\/a>)/', '<a href="/tags/$1">#$1</a>', $str);