Using Of Substring With Dynamic Values For Length Parameter

Hi everybody,

I want to use substring with dynamic
values for length parameter
but result is different compare to
static parameter.

declare
@x smallint,
@y smallint,
@string varchar(250)


set @x = 0
set @string = '17898880219800alex3.5'

select @y = charindex('',@string,@x)
-- result varchar(5)
select substring(@string,@x,5)

select @y
-- result varchar(250) !!!!!!

select substring(@string,@x,@y)

Any Idea why ?