Setting the field seperator in awk(1)

[John A. Palkovic]

I would like to set the field separator in awk(1) so that in addition
to the default separator I could include a comma with white space,
i.e. 

	a, b
	a,b
	a 	,	b

as well as the default case, which is just white space. I'm afraid
that I don't understand regular expressions well enough to do this
since all my attempts so far have failed. 

What is the regexp for the default field separator in awk(1)?