my $str = join(" ", @a) . "&" . join(" ", @b);

if ($str =~ /(?:\b\w+\b\s*)*?
             ((?:\b\w+\b\s*)+)
             (?:\s*\b\w+\b\s*)*?
             &
             (?:\b\w+\b\s*)*?
             \1
             (?:\s*\b\w+\b)*?/x) {
	my $result = $1;
	$result =~ s/\s*$//;
	print "found '$result'\n";
}