question about array indexing

Hey,
I'm wondering if this isn't a bug somewhere in the parser. I had to add
additional parenthesis around the regexp_split_to_array.

$ with x as (select 'a b c' a)
select
regexp_split_to_array(a, ' ')
from x;

regexp_split_to_array
-----------------------
{a,b,c}
(1 row)

$ with x as (select 'a b c' a)
select
regexp_split_to_array(a, ' ')[1]
from x;

ERROR: syntax error at or near "["
LINE 3: regexp_split_to_array(a, ' ')[1]
^
$ with x as (select 'a b c' a)
select
(regexp_split_to_array(a, ' '))[1]
from x;
regexp_split_to_array
-----------------------
a
(1 row)

regards,
Szymon