RFC 2044 - UTF-8, a transformation format of Unicode and ISO 10646

UTF-8, a transformation format of Unicode and ISO 10646
Status of this Memo T
his memo provides information for the Internet community. This memo does not specify an Internet standard of any kind. Distribution of this memo is unlimited.
 Abstract
The Unicode Standard, version 1.1, and ISO/IEC 10646-1:1993 jointly define a 16 bit character set which encompasses most of the world's writing systems. 16-bit characters, however, are not compatible with many current applications and protocols, and this has led to the development of a few so-called UCS transformation formats (UTF), each with different characteristics. UTF-8, the object of this memo, has the characteristic of preserving the full US-ASCII range: US-ASCII characters are encoded in one octet having the usual US-ASCII value, and any octet with such a value can only be an US-ASCII character. This provides compatibility with file systems, parsers and other software that rely on US-ASCII values but are transparent to other values.
1. Introduction
The Unicode Standard, version 1.1 [UNICODE], and ISO/IEC 10646-1:1993 [ISO-10646] jointly define a 16 bit character set, UCS-2, which encompasses most of the world's writing systems. ISO 10646 further defines a 31-bit character set, UCS-4, with currently no assignments outside of the region corresponding to UCS-2 (the Basic Multilingual Plane, BMP). The UCS-2 and UCS-4 encodings, however, are hard to use in many current applications and protocols that assume 8 or even 7 bit characters. Even newer systems able to deal with 16 bit characters cannot process UCS-4 data. This situation has led to the development of so-called UCS transformation formats (UTF), each with different characteristics.
UTF-1 has only historical interest, having been removed from ISO 10646. UTF-7 has the quality of encoding the full Unicode repertoire using only octets with the high-order bit clear (7 bit US-ASCII values, [US-ASCII]), and is thus deemed a mail-safe encoding ([ RFC1642]). UTF-8, the object of this memo, uses all bits of an octet, but has the quality of preserving the full US-ASCII range: US-ASCII characters are encoded in one octet having the normal US- ASCII value, and any octet with such a value can only stand for an US-ASCII character, and nothing else. UTF-16 is a scheme for transforming a subset of the UCS-4 repertoire into a pair of UCS-2 values from a reserved range. UTF-16 impacts UTF-8 in that UCS-2 values from the reserved range must be treated specially in the UTF-8 transformation. UTF-8 encodes UCS-2 or UCS-4 characters as a varying number of octets, where the number of octets, and the value of each, depend on the integer value assigned to the character in ISO 10646.
This transformation format has the following characteristics (all values are in hexadecimal):
- Character values from 0000 0000 to 0000 007F (US-ASCII repertoire) correspond to octets 00 to 7F (7 bit US-ASCII values).
- US-ASCII values do not appear otherwise in a UTF-8 encoded charac- ter stream. This provides compatibility with file systems or other software (e.g. the printf() function in C libraries) that parse based on US-ASCII values but are transparent to other val- ues.
- Round-trip conversion is easy between UTF-8 and either of UCS-4, UCS-2 or Unicode.
- The first octet of a multi-octet sequence indicates the number of octets in the sequence.
- Character boundaries are easily found from anywhere in an octet stream.
- The lexicographic sorting order of UCS-4 strings is preserved. Of course this is of limited interest since the sort order is not culturally valid in either case.
- The octet values FE and FF never appear.
UTF-8 was originally a project of the X/Open Joint Internationalization Group XOJIG with the objective to specify a File System Safe UCS Transformation Format [FSS-UTF] that is compatible with UNIX systems, supporting multilingual text in a single encoding. The original authors were Gary Miller, Greger Leijonhufvud and John Entenmann. Later, Ken Thompson and Rob Pike did significant work for the formal UTF-8. A description can also be found in Unicode Technical Report #4 [UNI- CODE]. The definitive reference, including provisions for UTF-16 data within UTF-8, is Annex R of ISO/IEC 10646-1 [ISO-10646].
2. UTF-8 definition
In UTF-8, characters are encoded using sequences of 1 to 6 octets. The only octet of a "sequence" of one has the higher-order bit set to 0, the remaining 7 bits being used to encode the character value. In a sequence of n octets, n>1, the initial octet has the n higher-order bits set to 1, followed by a bit set to 0. The remaining bit(s) of that octet contain bits from the value of the character to be encoded. The following octet(s) all have the higher-order bit set to 1 and the following bit set to 0, leaving 6 bits in each to contain bits from the character to be encoded. The table below summarizes the format of these different octet types. The letter x indicates bits available for encoding bits of the UCS-4 character value. UCS-4 range (hex.) UTF-8 octet sequence (binary)
0000 0000-0000 007F 0xxxxxxx
0000 0080-0000 07FF 110xxxxx 10xxxxxx
0000 0800-0000 FFFF 1110xxxx 10xxxxxx 10xxxxxx
0001 0000-001F FFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
0020 0000-03FF FFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
0400 0000-7FFF FFFF 1111110x 10xxxxxx ... 10xxxxxx

Encoding from UCS-4 to UTF-8 proceeds as follows:
1) Determine the number of octets required from the character value and the first column of the table above. 2) Prepare the high-order bits of the octets as per the second column of the table.
3) Fill in the bits marked x from the bits of the character value, starting from the lower-order bits of the character value and putting them first in the last octet of the sequence, then the next to last, etc. until all x bits are filled in. The algorithm for encoding UCS-2 (or Unicode) to UTF-8 can be obtained from the above, in principle, by simply extending each UCS-2 character with two zero-valued octets. However, UCS-2 val- ues between D800 and DFFF, being actually UCS-4 characters trans- formed through UTF-16, need special treatment: the UTF-16 trans- formation must be undone, yielding a UCS-4 character that is then transformed as above.
Decoding from UTF-8 to UCS-4 proceeds as follows:
1) Initialize the 4 octets of the UCS-4 character with all bits set to 0.
2) Determine which bits encode the character value from the number of octets in the sequence and the second column of the table above (the bits marked x).
3) Distribute the bits from the sequence to the UCS-4 character, first the lower-order bits from the last octet of the sequence and proceeding to the left until no x bits are left. If the UTF-8 sequence is no more than three octets long, decoding can proceed directly to UCS-2 (or equivalently Unicode). A more detailed algorithm and formulae can be found in [FSS_UTF], [UNICODE] or Annex R to [ISO-10646].

3. Examples The Unicode sequence "A<NOT IDENTICAL TO><ALPHA>." (0041, 2262, 0391, 002E) may be encoded as follows: 41 E2 89 A2 CE 91 2E The Unicode sequence "Hi Mom <WHITE SMILING FACE>!" (0048, 0069, 0020, 004D, 006F, 006D, 0020, 263A, 0021) may be encoded as follows: 48 69 20 4D 6F 6D 20 E2 98 BA 21 The Unicode sequence representing the Han characters for the Japanese word "nihongo" (65E5, 672C, 8A9E) may be encoded as follows: E6 97 A5 E6 9C AC E8 AA 9E