Byte order mark

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A byte order mark (BOM) is a sequence of bytes used to indicate Unicode encoding of a text file. The underlying character code, U+FEFF, takes one of the following forms depending on the character encoding.[1]

Bytes Encoding Form
EF BB BF UTF-8
FE FF UTF-16, big-endian
FF FE UTF-16, little-endian
00 00 FE FF UTF-32, big-endian
FF FE 00 00 UTF-32, little-endian

BOM use is optional. If used, it must be at the very beginning of the text. The BOM gives the producer of the text a way to describe the encoding such as UTF-8 or UTF-16, and in the case of UTF-16 and UTF-32, its endianness. The BOM is important for text interchange, when files move between systems that use different byte orders or different encodings, rather than in normal text handling in a closed environment.

As UTF-8 has become the most common text encoding, EFBBBF (shown here as three hexadecimal values) is the most commonly occurring BOM form, also known as the UTF-8 signature. HTML5 browsers are required to recognize the UTF-8 BOM and use it to detect the encoding of the page.[2] Software may alternatively recognize UTF-8 encoding by looking for bytes with the high order bit set (values 0x80 through 0xFF) followed by bytes that define valid UTF-8 sequences.

The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8, but warns that it may be encountered at the start of a file.[3]

Most modern software applications recognize a BOM and may insert it when saving a text file with UTF encoding. The presence of the UTF-8 BOM may cause problems with some software, especially legacy software not designed to handle UTF-8, in which case it may appear as the characters "".

  1. "FAQ - UTF-8, UTF-16, UTF-32 & BOM". unicode.org. Retrieved 2019-10-24.
  2. "The byte-order mark (BOM) in HTML". www.w3.org. Retrieved 2019-10-24.
  3. "The Unicode Standard – Chapter 2" (PDF). p. 30.