Hi,
as far as I understood this, the OP searched for this (although it is not a regex)

#!/usr/bin/perl
use strict;
use warnings;

my $key;
my %data;

while ($key = <DATA>) {
    chomp $key;
    my $val = <DATA>; chomp $val;
    push (@{$data{$key}}, $val);
}

print "First occurrence of U: " . $data{'U'}[0] . "\n";
print "Second occurrence of Pu: " . $data{'Pu'}[1] . "\n";

__DATA__
NP
111
U
222
Pu
333
NP
2-111
U
2-222
Pu
2-333
[download]

Regards,
svenXY