Byte order mark
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.
|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. Software may alternatively recognize UTF-8 encoding by looking for bytes with the high order bit set (values
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.
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 "ï»¿".