postgres sql help

hi, i am fairly new in postgresql, so if anyone can help me would be great

if i simply do:

select ver_no

from version

order by ver_no

the result will be something like this:

.1.3.1

.1.3.2.5.

.1.4.1.7.12

.1.4.11.14.7.

.1.4.3.109.1.

.1.4.8.66.

so as you can see first 3 lines are ok, but how to make 1.4.3.109 come
before 1.4.11 because the third level "3" is smaller than "11". i
understand they are stored as char so i used split_part function to
separate each numbers between "." in a separate column. but when i try
to convert those column into integer, i am getting an error msg saying:
ERROR: invalid input syntax for type numeric: " "

here is my code:

select ver_no, duedate, status,

to_number(split_part(ver_no, '.', 2), '9999') a,

to_number(split_part(ver_no, '.', 3), '9999') b,

to_number(split_part(ver_no, '.', 4), '9999') c,

to_number(split_part(ver_no, '.', 5), '9999') d

from version

order by a,b,c,d

I am not sure if i am heading towards wrong direction but can someone please suggest or give me some other ideas to sort this.

PS: i found some good solution in SQL Server but those commands are not used in postgreSQL.

Thanks in advance

- James