$re = '/<!--((?:(?!-->).)*)-->((?:(?!<!--)[\s\S])+)<!--((?:(?!-->).)*)-->/mi';
$str = '<!-- Text --><b>Text</b>
<a href="google.com">Link</a>
<div class="col"><h1>Nested Content</h1><p>More content</p>
</div>
<!-- END of Text -->';
$subst = "";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php