$ perl -E '
$str = q{1,2,3,,5,6,,,9,10,,,,14,15,,,,,,,,,,,,,};
$str =~ s{(?<=,)(?=,)}{0}g;
say $str;'
1,2,3,0,5,6,0,0,9,10,0,0,0,14,15,0,0,0,0,0,0,0,0,0,0,0,0,

##</code><code>##

$ perl -E '
$str = q{1,2,3,,5,6,,,9,10,,,,14,15,,,,,,,,,,,,,};
$str =~ s{(?<=,)(?=,|\z)}{0}g;
say $str;'
1,2,3,0,5,6,0,0,9,10,0,0,0,14,15,0,0,0,0,0,0,0,0,0,0,0,0,0