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SESK: Stuff Everyone Should Know · sesk.info/tech-utf8-ascii-compatible

A plain ASCII text file is read as UTF-8. What happens?

It reads perfectly, because ASCII bytes are already valid UTF-8.

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  1. It reads perfectly ✔
  2. Every character turns to gibberish
  3. It must be converted first

Why: UTF-8 keeps the exact one-byte values of all 128 ASCII characters. Therefore, every valid ASCII byte sequence is also valid UTF-8 and needs no conversion.

Huh, didn't know that: In UTF-8, bytes of multibyte characters all have the top bit set, so they can never be mistaken for ASCII characters like a slash.

The dad joke
Why did ASCII feel at home in UTF-8? Because nothing about it had to change.
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References

Answer: It reads perfectly. UTF-8 keeps the exact one-byte values of all 128 ASCII characters. Therefore, every valid ASCII byte sequence is also valid UTF-8 and needs no conversion.

  1. RFC 3629: UTF-8, a transformation format of ISO 10646 www.rfc-editor.org/rfc/rfc3629.txt
    US-ASCII characters are encoded in one octet having the normal US-ASCII value
    Checked 2026-10-10.

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