It is showing! However you don't see it as such.

Let's analyze it:

Code:

sed "s:\(\ [0-9]*\ \):\1:"

It matches space200space and encapsulates it, right?

Code:

sed "s:\(\ [0-9]*\ \):\1:"

It replaces it with what it was encapsulated, as TEST proved it. Nevertheless, since 200 is replaced by 200, you don't think it did.

You have to match the whole and disregard what you don't want.

Code:

echo "192.22.22.22 [23ddddd] \"33dffwef\" 200 300 dsdsd"  | sed "s:.*\(\ [0-9]*\ \)[0-9].*:\1:"

That matches the space200space.
If you want space300space

Code:

echo "192.22.22.22 [23ddddd] \"33dffwef\" 200 300 dsdsd"  | sed "s:.*\(\ [0-9]*\ \).*:\1:"

Removing the [0-9] will do it, because of the greedy nature of the first .*