#! perl -sw
use strict;
my $re_commify = qr/^([+-]?\d+?)(\d{3})(?>((?:,|\.|$)\d?))(.*)$/;

for (-12..7) {
 	my $num= eval("-123456789E$_");

 	printf '%31.13f commified becomes ', $num;
 	1 while $num =~ s/$re_commify/$1,$2$3$4/;
 	printf "%s\n", $num;
}

__END__
c:\test>commify
               -0.0001234567890 commified becomes -0.000123456789
               -0.0012345678900 commified becomes -0.00123456789
               -0.0123456789000 commified becomes -0.0123456789
               -0.1234567890000 commified becomes -0.123456789
               -1.2345678900000 commified becomes -1.23456789
              -12.3456789000000 commified becomes -12.3456789
             -123.4567890000000 commified becomes -123.456789
            -1234.5678900000000 commified becomes -1,234.56789
           -12345.6789000000010 commified becomes -12,345.6789
          -123456.7890000000000 commified becomes -123,456.789
         -1234567.8899999999000 commified becomes -1,234,567.89
        -12345678.9000000000000 commified becomes -12,345,678.9
       -123456789.0000000000000 commified becomes -123,456,789
      -1234567890.0000000000000 commified becomes -1,234,567,890
     -12345678900.0000000000000 commified becomes -12,345,678,900
    -123456789000.0000000000000 commified becomes -123,456,789,000
   -1234567890000.0000000000000 commified becomes -1,234,567,890,000
  -12345678900000.0000000000000 commified becomes -12,345,678,900,000
 -123456789000000.0000000000000 commified becomes -123,456,789,000,000
-1234567890000000.0000000000000 commified becomes -1.23456789e+015

c:\test>