1 while s/(\d)(\d\d\d)(?!\d)/$1,$2)/;

##</code><code>##

1 while s/(\d)(\d\d\d)(?:\d)/$1,$2)/;

##</code><code>##

1 while s/(\d)(\d\d\d)(?:,|$)/$1,$2/;

##</code><code>##

1 while s/(\d)(\d\d\d)(?:\D)/$1,$2/;