use strict;
use warnings;

my @strs = qw(id_1_1 id_2 banana id_1_2_);

for my $id (@strs) {
    if ($id =~ /(\d)(?!_)/) {
        print "Extracted $1 from $id\n";
    } else {
        print "no match for $id\n";
    }
}

##</code><code>##

Extracted 1 from id_1_1
Extracted 2 from id_2
no match for banana
no match for id_1_2_