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uencoder

Byte Order Mark

The byte order mark (BOM) is the U+FEFF character placed at the start of Unicode text to indicate the encoding form and byte order.

Purpose

It lets a decoder auto-detect the text encoding and byte order without external metadata.

Mechanism

Different encodings produce different BOM byte sequences: UTF-8 = EF BB BF, UTF-16LE = FF FE, UTF-16BE = FE FF, UTF-32LE = FF FE 00 00, UTF-32BE = 00 00 FE FF. A decoder reads the first 2-4 bytes and matches them.

Use cases

  • Encoding detection in text editors
  • Cross-system text file exchange
  • Encoding identification for database imports
  • CSV file handling in Excel

Examples

  • Input 
    UTF-16LE file
    Output 
    FF FE ...

    Leading bytes FF FE indicate little-endian UTF-16

Common errors

Mistaking a UTF-8 BOM for data content
Detect and strip the BOM when reading

Related terms

BOM
The BOM (byte order mark) is the Unicode character U+FEFF, placed at the start of a text stream to indicate the encoding form and byte order.
GLOSSARY
Endianness
Endianness is the byte ordering of multi-byte values in memory: big-endian stores the most significant byte first, little-endian the least significant byte first.
GLOSSARY
UTF-16
UTF-16 is Unicode's variable-length encoding form, using 2 or 4 bytes per code point: BMP characters take one 16-bit code unit and supplementary-plane characters take a surrogate pair (2 code units).
GLOSSARY
UTF-8
UTF-8 is Unicode's variable-length encoding form, using 1 to 4 bytes per code point; it is fully backward compatible with ASCII and is the de facto standard for the web and modern systems.
GLOSSARY

Related tools

Encoding Detector
Check text encoding: ASCII subset, UTF-8 BOM, and non-ASCII characters. Text input is always re-encoded as UTF-8, so legacy encodings (GBK/Big5/Shift-JIS) cannot be distinguished — the tool honestly reports the UTF-8 byte form. Free, private, 100% client-side.
TOOL
Charset Reference
Reference table of character encodings. Free, private, 100% client-side.
TOOL

References