The best I can come up with is

#! /usr/bin/perl

use strict;

while( <DATA> ) {
    s/(?<!\d), *|, *(?=\D)/, /g;
    print;
}
__DATA__
1,2,a,b
1,a,2,b
1,a,b,2
a,1,2,b
__produces__
1,2, a, b
1, a, 2, b
1, a, b, 2
a, 1,2, b
[download]

But the use of alternation strikes me a bit as cheating. There is no doubt ABWTDI, but I can't see it for the moment.