Core files may from time to time end up in the source tree of the JDK, adding noise in git etc. Because we have valid files and directories called "core*" in the tree it's non-trivial to exclude any and all core files, but as a special case we can at least ignore core.[0-9]* which is the typical format used for linux core files. We don't have any such files/directories at the moment and it's unlikely we ever will.