First, I don't think the drop filter works the way you imagine it to work. From the documentation
Drops everything that gets to this filter.
So drop will drop the entire "event" or "document" (what ever the correct term is. I don't think you can use it to "drop" specific parts of a document or field.
Second, your are trying to match the start of a line. The message field only has one line start. But even if you would get a match, the whole document would be dropped anyway.
This is as far as I know. I'm definitely not an expert on the drop filter...