Does every UTF-8 file have a BOM?

No. A BOM is optional in UTF-8 and isn't needed for it, since UTF-8 has no byte-order ambiguity to resolve. Some tools write one anyway for legacy compatibility, which is why a reader still needs to detect and strip it.

What happens if a BOM isn't stripped?

It gets treated as ordinary content. It typically shows up as an invisible or oddly-rendered character glued to the very first word of the extracted text. That can break an exact-match comparison or a first-line parser downstream.

Byte order mark (BOM)

A few invisible bytes at the start of a text file declaring its encoding and byte order. Helpful for a reader, a stray character if a parser doesn't strip it.

definition

A byte order mark (BOM) is an optional sequence of bytes placed at the start of a text file. It declares the file's encoding and, for multi-byte encodings, the byte order used.

Also called: BOM, UTF-8 BOM

in-plain-terms

UTF-16 can store each character's bytes in two different orders. A reader has no way to tell which one a file uses without being told. The BOM's job is to say so before any real content begins. UTF-8 doesn't need a BOM for this reason, since it has no byte-order ambiguity. But some tools write one anyway, mostly for legacy compatibility.

A reader that doesn't recognize and strip the BOM treats it as ordinary content. It shows up as an invisible or oddly-rendered character glued to the very first word of the file. This is a common, easy-to-miss extraction artifact.

why-it-matters

A stray BOM at the start of extracted text can silently break an exact-match comparison or a first-line parser. It can also break a chunk boundary that assumes the file starts cleanly.
Detecting and stripping the BOM correctly is also how a reader knows which encoding and byte order to apply to the rest of the file. Get it wrong and every character after it can decode incorrectly too.

how-to-check

Check the first few bytes of a text file for a UTF-8 or UTF-16 BOM signature before assuming the file starts with real content.

related-terms

faq

Does every UTF-8 file have a BOM?: No. A BOM is optional in UTF-8 and isn't needed for it, since UTF-8 has no byte-order ambiguity to resolve. Some tools write one anyway for legacy compatibility, which is why a reader still needs to detect and strip it.
What happens if a BOM isn't stripped?: It gets treated as ordinary content. It typically shows up as an invisible or oddly-rendered character glued to the very first word of the extracted text. That can break an exact-match comparison or a first-line parser downstream.

related-reading

Characters & encoding

See the term in real output.

Drop a file into the free reader and watch it happen.

Open the file reader →

Get an API key →