<?php
$string='[li id="1"]text1[div]text2[/div][/li]';
$pattern=':\[([^\]]+)\]:';
// your code goes here
PD9waHAKJHN0cmluZz0nW2xpIGlkPSIxIl10ZXh0MVtkaXZddGV4dDJbL2Rpdl1bL2xpXSc7CiRwYXR0ZXJuPSc6XFsoW15cXV0rKVxdOic7CmVjaG8gcHJlZ19yZXBsYWNlKCRwYXR0ZXJuLCI8JDE+Iiwkc3RyaW5nKTsKCi8vIHlvdXIgY29kZSBnb2VzIGhlcmU=