UTF-8

It uses a set of 1 to 4 bytes depending on the Unicode symbol. For example, only one UTF-8 byte is needed to encode the 128 US-ASCII characters in the Unicode range U+0000 to U+007F.

While it may seem inefficient to represent Unicode characters with as many as 4 bytes, UTF-8 allows legacy systems to transmit this ASCII superset.

Description

In summary, a Unicode character's bits are divided into several groups, which are then divided among the lower bit positions inside the UTF-8 bytes.

Characters smaller than 128_dec are encoded with a single byte that contains their value: these correspond exactly to the 128 7-bit ASCII characters.

In other cases, up to 4 bytes are required. The uppermost bit of these bytes is 1, to prevent confusion with 7-bit ASCII characters. Particularly characters lower than 32_dec traditionally called control characters, e.g. carriage return).

Code range hexadecimal	UTF-16	UTF-8 binary	Notes
000000 - 00007F	00000000 0xxxxxxx	0xxxxxxx	ASCII equivalence range; byte begins with zero
000080 - 0007FF	00000xxx xxxxxxxx	110xxxxx 10xxxxxx	first byte begins with 11, the following byte(s) begin with 10
000800 - 00FFFF	xxxxxxxx xxxxxxxx	1110xxxx 10xxxxxx 10xxxxxx
010000 - 10FFFF	110110xx xxxxxxxx 110111xx xxxxxxxx	11110xxx 10xxxxxx 10xxxxxx 10xxxxxx	UTF-16 requires surrogates; an offset of 0x10000 is subtracted, so the bit pattern is not identical with UTF-8

For example, the character alef (א), which is Unicode 0x05D0, is encoded into UTF-8 in this way:

• It falls into the range of 0x0080 to 0x07FF. The table shows it will be encoded using 2 bytes, 110xxxxx 10xxxxxx.
• Hexadecimal 0x05D0 is equivalent to binary 101-1101-0000.
• The 11 bits are put in their order into the position marked by "x"-s: 11010111 10010000.
• The final result is the two bytes, more conveniently expressed as the two hexadecimal bytes 0xD7 0x90. That's the letter aleph in UTF-8.

So the first 128 characters need one byte. The next 1920 characters need two bytes to encode. This includes Greek, Cyrillic, Coptic, Armenian, Hebrew, and Arabic characters. The rest of the UCS-2 characters use three bytes, and additional characters are encoded in 4 bytes. (An earlier UTF-8 specification allowed even higher code points to be represented, using 5 or 6 bytes, but this is no longer supported.)

Advantages

• Some Unicode symbols (including the Latin alphabet) in UTF-8 will take as little as 1 byte, although others may take up to 4. So UTF-8 will generally save space compared to UTF-16 (the simplest encoding of Unicode) in text where 7-bit ASCII characters are common.
• Most existing computer software (including operating systems) was not written with Unicode in mind, and using Unicode with them might create some compatibility issues. For example, the C standard library marks the end of a string with a character that has an 1-byte code 0x00 (hexadecimal). In UTF-16-encoded Unicode the English letter "A" will be coded as 0x0041. The library will consider the first byte 0x00 as the end of the string and will ignore anything after it. UTF-8, however, is designed so that encoded bytes never take on any of the special ASCII 'special character' values, preventing this and similar problems.
• UTF-8 strings can be sorted using standard byte-oriented sorting routines (however there will be no differentiation between stroke and capital letters with values exceeding 128).
• Since the top bit is always set in all the bytes of multiple-byte UTF-8 characters, most software designed to process ASCII or other 8-bit codes will never see any of them as a space, so white-space based tokenizing routines will continue to work correctly with UTF-8 encoded strings.
• Although encoded characters are variable length, their encoding is such that their boundaries can be delineated without elaborate parsing.
• UTF-8 is the default value for the XML format.

Disadvantages

• UTF-8 is variable-length; that means that different characters take sequences of different lengths to encode. The acuteness of this could be decreased, however, by creating an abstract interface to work with UTF-8 strings, and making it all transparent to the user.
• A badly-written (and non-standard-compliant) UTF-8 parser could accept a number of different pseudo-UTF-8 representations and convert them to the same Unicode output. This provides a way for information to leak past validation routines designed to process data in its 8-bit representation.
• Ideographs use 3 bytes in UTF-8, but only 2 in UTF-16. So Chinese/Japanese/Korean text will take up more space when represented in UTF-8.

External links

• RFC 3629, the UTF-8 standard
• UTF-8
• a UTF-8 test page
• another UTF-8 test page

Referenced By