declare @s varchar(max) = 'aaaaabbbbbcccccdddddddd'

select
	t.s
	,STRING_AGG(a.xz,'\') WITHIN GROUP (ORDER BY a.number) as x
from (select @s as s) t
cross apply (select SUBSTRING(t.s, v.number*5+1, 5) as xz, v.number from master..spt_values v where [type] = 'P' and v.number <= CEILING(len(t.s)/5.0)-1) a
group by t.s