Decodes bytes as encoding, or answers null when encoding is not one of the five.
The names are Shift_JIS, EUC-JP, GBK, Big5 and EUC-KR, as the standard spells them, in any letter case. The engine's detector names the encoding with one of them.
Shift_JIS
EUC-JP
GBK
Big5
EUC-KR