You have an extra % at the start of the line which should probably be ^
You do not need to escape K or > with backslash.
That said, I think you should capture the K along with the number, because your code is going to break when it sees B or M or G there. If you capture 9728K using something like (?<someSize>[0-9]+[BKMGT]).
You then need to convert that to a number. I once did that will a mutate filter (replacing K with 000 etc -- good enough for what I needed right then). I thought I recently saw a filter that could do that, but I cannot find it now. It may have been an elasticsearch mapper, but I cannot find that either. This thread has some suggestions around that, including ruby code.