您可以使用以下正则表达式

[.*?][^]]*]

DEMO

您的代码将是

<?php
$str = "This is a test string. I want to get value between [this] and [/this]";
$regex = '~[.*?][^]]*]~';
if (preg_match($regex, $str, $m)) {
    $yourmatch = $m[0]; 
    echo $yourmatch;
    }
?> //=>  [this] and [/this]