RFC 2279 (rfc2279) - Page 3 of 10

UTF-8, a transformation format of ISO 10646

RFC 2279                         UTF-8                      January 1998


   -  The first octet of a multi-octet sequence indicates the number of
      octets in the sequence.

   -  The octet values FE and FF never appear.

   -  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 Boyer-Moore fast search algorithm can be used with UTF-8 data.

   -  UTF-8 strings can be fairly reliably recognized as such by a
      simple algorithm, i.e. the probability that a string of characters
      in any other encoding appears as valid UTF-8 is low, diminishing
      with increasing string length.

   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 and in
   the Unicode Standard, version 2.0 [UNICODE].  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.




Yergeau                     Standards Track