If one really wants to do it with a regex, a much better way would be

$fullname = "C:\\apple\\index.htm";
my ($file) = $fullname =~ m!([^/\\]+)$!;
print $file;
[download]

Zero backtracking here. However, Macs are still going to be a problem (paths are separated with doublecolons there, right?), so File::Basename is what one should use.

use File::Basename; # I ain't afraid of no ghosts..
$fullname = "C:\\apple\\index.htm";
my $file = basename $fullname;
print $file;
[download]

____________
Makeshifts last the longest.