$re = '/<(?!(?:/\s*)?[a-z]+(?:\s+[^>]*)?/?>)|(?<!<(?:/\s*)?[a-z]+(?:\s+[^>]*)?/?)>/i';
$str = '<
text
>
<ul>
<li>[Message processing time] < [time to send ack to Azure Service Bus] (about less than 100ms per message)</li>
<li>[Total process time of a group of messages] < [Message Lock Time] (default: 1 min)<br><b>Strictly REQUIRED</b> to avoid lock loss and messages processed more than one time
</li>
</ul>
<>
> <
5 > 3 < 2 > 1
<a href="www.google.com">placeholder!!</a>
<hello >
< ciao>
<hello>
<hello></hello>
</b>
< /b>
';
$subst = "&[gl]t;";
$result = preg_replace($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