Your question is not very clear. All of the solutions offered above assume, more or less, that you want to remove any non-numeric characters from your string. However, if what you really want is to remove everything up to and including the underscore, regardless of whether it's numeric or whatever, you could do this:

use strict;
use warnings;
my $old_value = 'UPNOON33_A303.doc';
my $new_value = (split /_/, $old_value)[-1];
print $new_value;
[download]

hth

dave