The problem is a little underspecified ... I will assume you are looking for the text between the last [ and the first ]. You need to:

ignore until you find [
read in buffer until you find [ or ]
if you found ] the buffer is your result
if you found [ reset buffer
{optional - if you reach the end and there's no ], scream "betrayal!"}
[download]

In Perl you do

$input = ...your text here...;
$result = /.*\[(.*^[)\].*/;
[download]

Match anything between [ and ], unless you find a [, otherwise you'd pick up anything between the first [ and the first ].

Hope this helps!