Accordion

An accordion is a string (yes, in the real world accordions are musical instruments, but let's forget about it for a while) which can be represented as a concatenation of: an opening bracket (ASCII code $$$091$$$), a colon (ASCII code $$$058$$$), some (possibly zero) vertical line characters (ASCII code $$$124$$$), another colon, and a closing bracket (ASCII code $$$093$$$). The length of the accordion is the number of characters in it.

For example, [::], [:||:] and [:|||:] are accordions having length $$$4$$$, $$$6$$$ and $$$7$$$. (:|:), {:||:}, [:], ]:||:[ are not accordions.

You are given a string $$$s$$$. You want to transform it into an accordion by removing some (possibly zero) characters from it. Note that you may not insert new characters or reorder existing ones. Is it possible to obtain an accordion by removing characters from $$$s$$$, and if so, what is the maximum possible length of the result?