$re = '/\[[^][]*(?(?=\[)(?R))[^][]*\]/';
$str = '[2016-12-26 dfgdfg16:48:57 +0100] Copying SSL certificates, CSRs, and keys...[2016-12-26 16:48:57 +0100] Done
[2016-12-26 16:48:57 +0100] Copying SSL certificates, CSRs, and keys...[2016-12-26 16:48:57 +0100] Done';
$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