Webmaster Forum / Perl / General Perl Topics / March 2010

The obvious way is to match the optional leading '-' with a regular
expression, and pass the rest to hex().

sub signedhex
{
 return $_[0] =~ /(-?)(.*)/ ? ($1 ? -hex($2) : hex($2)) : undef;
}

If you want the result printed out the same way, an extra problem appears:
there is no printf format for signed hex.

sub formatsignedhex
{
 return ($_[0] < 0 ? '-' : '').sprintf("%x", abs($_[0]));
}

Usage would be like this:

my $total = 0;
$total += signedhex($_) for @ARGV;

print "total: ", formatsignedhex($total), "\n";

   if ( $num =~ s/^-// ) {
       $sum -= hex $num;
   }
   else {
       $sum += hex $num;
   }

Since you will need to parse the @ARGV values for '-', you might
as well validate the parameters as well. Using eval in a proper way
will make it safe and use Perls ability to interpolate "--" to + in
in a numeric computation.

-sln

c:\temp>perl aa.pl -02 -1 +44 -+-a b x -0 ffffffff -01

3 + (-02)h = 1
1 + (-1)h = 0
0 + (+44)h = 68
68 + (-+-a)h = 78
78 + (b)h = 89
89 + (x)h = <invalid parameter: 'x'>
89 + (-0)h = 89
89 + (ffffffff)h = 4294967384
4294967384 + (-01)h = 4294967383

c:\temp>

--------------------
use strict;
use warnings;

my $sum = 3;

for (@ARGV)
{
    print $sum," + ($_)","h = ";
    if (/^ ([-+]*?) ([0-9a-f]+$) /xi) {
        print $sum += eval "$1  hex '$2'", "\n";
    }
    else {
        print "<invalid parameter: '$_'>\n";
    }
}