sed command to swap words not working

Question

I'm trying to swap words in each line using sed like so:

sed 's/\([^ ]*\), \([^ ]*\)/\2, \1/' sed.txt

And this is sed.txt:

FIRST LAST
FIRST LAST
FIRST LAST
FIRST LAST

But it's not working and I'm not sure why, my desired outcome is:

LAST FIRST
LAST FIRST
LAST FIRST
LAST FIRST

I know the example is insignificant but I'd like to understand sed more because I've only been using it for swapping words in their current position without changing positions.

Answer 1

sed 's/\([^ ]*\), \([^ ]*\)/\2, \1/' sed.txt

The problem is first comma because in source text you do not have it. Change it:

sed 's/\([^ ]*\) \([^ ]*\)/\2, \1/' sed.txt

and will work