Split the sentences by ',' and remove surrounding spaces

I have this code:

var r = /(?:^\s*([^\s]*)\s*)(?:,\s*([^\s]*)\s*){0,}$/
var s = "   a   ,  b  , c "
var m = s.match(r)
m => ["   a   ,  b  , c ", "a", "c"]

Looks like the whole string has been matched, but where has "b" gone? I would rather expect to get:

["   a   ,  b  , c ", "a", "b", "c"]

so that I can do m.shift() with a result like s.split(',') but also with whitespaces removed.

Do I have a mistake in the regexp or do I misunderstand String.prototype.match?