my $re = qr{
    \A                    # Assert start of string
    [A-Za-z0-9]           # Must start with one of these
    (?:                   # Followed by either
        [A-Za-z0-9_.-]*?      # Zero or more of these
        [A-Za-z0-9]           # But ending with one of these
    |                     # OR
                              # Nothing
    )
    \z                    # Assert end of string
}x;