Before s///:     ,  ,  ,  ,  ,  ,  , 6
After 1st pass:  ,,|  ,  ,  ,  ,  , 6
After 2nd pass:  ,,  ,,|  ,  ,  , 6
After 3rd pass:  ,,  ,,  ,,|  , 6

##</code><code>##

1 while s{, +,}{,,};

##</code><code>##

s{, +(?=,)}{,}g;