Oracle number to string missing 0

I saw a few examples on the Internet:

select to_char(a,'99D90'),
       to_char(a,'90D90'),
       to_char(a,'FM90D99'),
       rtrim(to_char(a,'FM90D99'),to_char(0,'D'))
from(
       select 50 a from dual
       union all select 50.57 from dual
       union all select 5.57 from dual
       union all select 0.35 from dual
       union all select 0.4 from dual
)

SELECT DECODE(SUBSTR(a,1,1),'.','0'||a,a) from dual