declare @n table( n int primary key);
insert @n
select 1
 union all 
select 2
 union all 
select 3
 union all 
select 4
 union all 
select 5
 union all 
select 6
 union all 
select 7
 union all 
select 8
 union all 
select 9
 union all 
select 10
;

declare @s nvarchar(10);
set @s = N'--1f,c345';


with s as ( select s = @s )
  select * from s;

with s as ( select s = @s )
   , c as ( select c = substring(s, n, 1), n from s inner join @n on n between 1 and len(s) )
  select * from c;

with s as ( select s = @s )
   , c as ( select c = substring(s, n, 1), n from s inner join @n on n between 1 and len(s) )
   , d as ( select c, n from c where c in (N'1', N'2', N'3', N'4', N'5') )
  select * from d;

with s as ( select s = @s )
   , c as ( select c = substring(s, n, 1), n from s inner join @n on n between 1 and len(s) )
   , d as ( select c, n from c where c in (N'1', N'2', N'3', N'4', N'5') )
  select nums = (select c + '' from d order by n for xml path(''));