5.4 Universal Transformation Format, 8 bits

Even if UTF-8 does not originally come from IETF, there is now RFC 2279 to describe it. In letters sent on 1995-01-21 and 1995-04-20, Markus Kuhn writes:

UTF-8 is an ASCII compatible multi-byte encoding of the ISO 10646 universal character set (UCS). UCS is a 31-bit superset of all other character set standards. The first 256 characters of UCS are identical to those of ISO 8859-1 (Latin-1). The UCS-2 encoding of UCS is a sequence of bigendian 16-bit words, the UCS-4 encoding is a sequence of bigendian 32-bit words. The UCS-2 subset of ISO 10646 is also known as “Unicode”. As both UCS-2 and UCS-4 require heavy modifications to traditional ASCII oriented system designs (e.g. Unix), the UTF-8 encoding has been designed for these applications.

In UTF-8, only ASCII characters are encoded using bytes below 128. All other non-ASCII characters are encoded as multi-byte sequences consisting only of bytes in the range 128-253. This avoids critical bytes like NUL and / in UTF-8 strings, which makes the UTF-8 encoding suitable for being handled by the standard C string library and being used in Unix file names. Other properties include the preserved lexical sorting order and that UTF-8 allows easy self-synchronisation of software receiving UTF-8 strings.

UTF-8 is the most common external surface of UCS, each character uses from one to six bytes, and is able to encode all 2^31 characters of the UCS. It is implemented as a charset, with the following properties:

• Strict 7-bit ASCII is completely invariant under UTF-8, and those are the only one-byte characters. UCS values and ASCII values coincide. No multi-byte characters ever contain bytes less than 128. NUL is NUL. A multi-byte character always starts with a byte of 192 or more, and is always followed by a number of bytes between 128 to 191. That means that you may read at random on disk or memory, and easily discover the start of the current, next or previous character. You can count, skip or extract characters with this only knowledge.
• If you read the first byte of a multi-byte character in binary, it contains many ‘1’ bits in successions starting with the most significant one (from the left), at least two. The length of this ‘1’ sequence equals the byte size of the character. All succeeding bytes start by ‘10’. This is a lot of redundancy, making it fairly easy to guess that a file is valid UTF-8, or to safely state that it is not.
• In a multi-byte character, if you remove all leading ‘1’ bits of the first byte of a multi-byte character, and the initial ‘10’ bits of all remaining bytes (so keeping 6 bits per byte for those), the remaining bits concatenated are the UCS value.

These properties also have a few nice consequences:

• Conversion to/from values is algorithmically simple, and reasonably speedy.
• A sequence of N bytes can hold characters needing up to 2 + 5N bits in their UCS representation. Here, N is a number between 1 and 6. So, UTF-8 is most economical when mapping ASCII (1 byte), followed by UCS-2 (1 to 3 bytes) and UCS-4 (1 to 6 bytes).
• The lexicographic sorting order of UCS strings is preserved.
• Bytes with value 254 or 255 never appear, and because of that, these are sometimes used when escape mechanisms are needed.

In some case, when little processing is done on a lot of strings, one may choose for efficiency reasons to handle UTF-8 strings directly even if variable length, as it is easy to get start of characters. Character insertion or replacement might require moving the remainder of the string in either direction. In most cases, it is faster and easier to convert from UTF-8 to UCS-2 or UCS-4 prior to processing.

This charset is available in Recode under the name UTF-8. Accepted aliases are UTF-2, UTF-FSS, FSS_UTF, TF-8 and u8.