The UTF-8 BOM is a sequence of Bytes at the start of a text-stream (EF BB BF) that allows the reader to more reliably guess a file as being encoded in UTF-8.
Normally, the BOM is used to signal the endianness of an encoding, but since endianness is irrelevant to UTF-8, the BOM is unnecessary.
According to the Unicode standard, the BOM for UTF-8 files is not recommended:
2.6 Encoding Schemes
... Use of a BOM is neither required nor recommended for UTF-8, but may be encountered in contexts where UTF-8 data is converted from other encoding forms that use a BOM or where the BOM is used as a UTF-8 signature. See the “Byte Order Mark” subsection in Section 16.8, Specials, for more information.
The other excellent answers already answered that:
- There is no official difference between UTF-8 and BOM-ed UTF-8
- A BOM-ed UTF-8 string will start with the three following bytes.
EF BB BF
- Those bytes, if present, must be ignored when extracting the string from the file/stream.
But, as additional information to this, the BOM for UTF-8 could be a good way to "smell" if a string was encoded in UTF-8... Or it could be a legitimate string in any other encoding...
For example, the data [EF BB BF 41 42 43] could either be:
- The legitimate ISO-8859-1 string "ABC"
- The legitimate UTF-8 string "ABC"
So while it can be cool to recognize the encoding of a file content by looking at the first bytes, you should not rely on this, as show by the example above
Encodings should be known, not divined.
