$ perl -E '
    my @x = (qw{A AA AAA _ __ ___ -A A- A.A A. .A A-A A_A}, "A\n", "A\tA");
    my $re = qr{\A[A-Za-z0-9](?:[A-Za-z0-9_.-]*?[A-Za-z0-9]|)\z};
    say "|$_| is ", /$re/ ? "OK" : "BAD" for @x
'
|A| is OK
|AA| is OK
|AAA| is OK
|_| is BAD
|__| is BAD
|___| is BAD
|-A| is BAD
|A-| is BAD
|A.A| is OK
|A.| is BAD
|.A| is BAD
|A-A| is OK
|A_A| is OK
|A
| is BAD
|A      A| is BAD