UFEFF

UFEFF, referred to by the Unicode code point U+FEFF, historically functions as a Byte Order Mark (BOM) at the start of text streams to indicate the endianness of the encoding. It is most commonly associated with UTF-16 and UTF-32, where the BOM helps a decoder determine whether the stream is big-endian or little-endian. In UTF-8, the BOM appears as the three-byte sequence EF BB BF and is optional; many software components ignore it.

For UTF-16, the BOM bytes are FE FF for big-endian and FF FE for little-endian, placed at the very start of the file. If a BOM is missing, the decoder must infer endianness from context or transport; misinterpretation can lead to garbled text. The BOM is thus a critical signal only at the start of a stream; once the encoding is established, the BOM has no further role.

When U+FEFF is interpreted as text rather than metadata, it is the character ZERO WIDTH NO-BREAK SPACE, an invisible formatting character. It can be used to prevent line breaks in certain contexts, but it is not intended as a visible space. In practice, its presence inside a text stream can be confusing if software treats it as data rather than as an encoding marker.

Practical issues: BOM handling varies across tools and platforms. Some Unix tools or protocols do not expect a BOM and can misread files that include one. Conversely, omitting a BOM can prevent proper UTF-16/UTF-32 decoding. Many editors and languages provide options to preserve, remove, or normalize a BOM.

Standards and naming: The Unicode Standard defines U+FEFF as the BOM. When used as a character outside of a BOM context, it is named ZERO WIDTH NO-BREAK SPACE. The related code point U+FFFE is the mirrored sequence used historically for detecting endianness and should not appear in text as a character.