Don't use quotes ""
if [[ "$output" =~ ^CMD\[.*?\]$ ]]; then
Update : (in response to @frhd)
Well, the regex operator =~ expects an unquoted regular expression on its RHS and does only a sub-string match unless the anchors ^ (start of input) and $ (end of input) are also used to make it match the whole of the LHS.
Quotations "" override this behaviour and force a simple string match instead i.e. the matcher starts looking for all these characters \[.*?\] literally.
