If you refer to the documentation you'll find an alternate syntax for dbscan which might be useful in your case.
[idx,corepts] = dbscan(data,eps,minpts);
Here, corepts is a logical vector containing core points identified by dbscan. A value 1 in any row of corepts indicates that the corresponding observation is a core point. Otherwise it has a value 0.
If what you meant by corepoints is the data of core points, and if you want cluster numbers of these core points you may do
[idx,corepts] = dbscan(data,eps,minpts);
corepoints = data(corepts,:);
clust_nums = idx(corepts,:);
