Occasionally, you can't use just a `/g' to get all the changes to
   occur that you might want.  Here are two common cases:

      # put commas in the right places in an integer
      1 while s/(\d)(\d\d\d)(?!\d)/$1,$2/g;

      # expand tabs to 8-column spacing
      1 while s/\t+/' ' x (length($&)*8 - length($`)%8)/e;