new Regex(@"[0-9a-zA-Z\+/=]{20,}");

'(^Content-Transfer-Encoding: base64$)(^$)(^(([A-Za-z0-9+/=]){4}){1,19}$)*(^$)'

^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$


my $sanitized_str = join q{}, grep {!/[^A-Za-z0-9+\/=]/} split /\n/, $str;