Why not just specify the name format to only return names that start with 'sub-', e.g.:
S = dir(fullfile(parent_path,'sub-*'));
D = {S([S.isdir]).name} % only folders starting with 'sub-'
If there are other folder names which also start with sub- but don't have the trailing number, then add this:
X = cellfun(@isempty,regexpi(D,'^sub-\d+'));
D(X) = []
