0xFEFF

0xFEFF is the hexadecimal encoding of the Unicode code point U+FEFF. In Unicode, this code point is named ZERO WIDTH NO-BREAK SPACE (ZWNBSP), and it has a special role as the byte order mark (BOM) in text streams. The BOM is used to indicate the encoding form and endianness of the data, particularly for UTF-16 and UTF-32.

As a BOM, 0xFEFF signals endianness at the start of a file or stream. In UTF-16, the BOM is represented by the two-byte sequences FE FF (big-endian) or FF FE (little-endian). In UTF-8, a BOM can be encoded as the three-byte sequence EF BB BF; its presence is optional and many programs treat it as noncontent, often removing it before processing text.

When not used as a BOM, U+FEFF is the zero-width no-break space character. It is a formatting character that is invisible in rendering and is intended not to cause line breaks at its position. In practice, software treats U+FEFF at the start of a file as a BOM, but if it appears elsewhere in the text, it is handled as ZWNBSP.

Distinctions and cautions: 0xFFFE is the byte-reversed pattern commonly associated with an invalid or misinterpreted BOM and should not be confused with 0xFEFF. The BOM’s handling varies by platform and encoding; some systems insert or remove it, which can affect text processing and data interchange.

In summary, 0xFEFF denotes a code point central to encoding detection (as a BOM) and, in other contexts, a nonprinting space character, with practical implications for file interoperability and text processing.