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UTF-8 (8-bit Unicode Transformation Format) is a lossless, variable-length character encoding for Unicode created by Rob Pike and Ken Thompson. It uses groups of bytes to represent the Unicode standard for the alphabets of many of the world's languages. UTF-8 is especially useful for transmission over 8-bit mail systems.

It uses 1 to 4 bytes per character, 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. Additionally, data compression can still be performed independently of the use of UTF-8.

The IETF requires all Internet protocols to identify the encoding used for character data with UTF-8 as at least one supported encoding.

Description

UTF-8 is currently standardized as RFC 3629 (UTF-8, a transformation format of ISO 10646).

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 110 or 1110, 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 Latin alphabet characters with diacritics, 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.)

In fact, UTF-8 is able to use a sequence of up to six bytes and cover the whole area 0x00-0x7FFFFFFF (31 bits), but UTF-8 was restricted by RFC 3629 to only use the area covered by the formal Unicode definition, 0x00-0x10FFFF, in November 2003. Before this, only the bytes 0xFE and 0xFF did not occur in a UTF-8 encoded text. After this limit was introduced, the number of unused bytes in a UTF-8 stream increased to 13 bytes: 0xC0, 0xC1, 0xF5-0xFF. Even though this new definitition limits the available encoding area severely, the problem with overlong sequences (different ways of encoding the same character, which can be a security risk) is eliminated, because an overlong sequence will contain some of these bytes that are not used and therefore will not be a valid sequence.

Rationale behind UTF-8's mechanics

As a consequence of the exact mechanics of UTF-8, the following properties of multi-byte sequences hold:

• The most significant bit of a single-byte character is always 0.
• The most significant bits of the first byte of a multi-byte sequence determine the length of the sequence. These most significant bits are 110 for two-byte sequences; 1110 for three-byte sequences, etc.
• The remaining bytes in a multi-byte sequence have 10 as their two most significant bits.

UTF-8 was designed to satisfy these properties in order to guarantee that no byte sequence of one character is contained within a longer byte sequence of another character. This ensures that byte-wise sub-string matching can be applied to search for words or phrases within a text; some older variable-length 8-bit encodings (such as Shift-JIS) did not have this property and thus made string-matching algorithms rather complicated. Although it is argued that this property adds redundancy to UTF-8-encoded text, the advantages outweigh this concern; besides, data compression is not one of Unicode's aims and must be considered independently.

Advantages

• Of course, the most notable advantage of any Unicode Transformation Format over legacy encodings is that it can encode any character.
• 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 or UTF-32 in text where 7-bit ASCII characters are common.
• A byte sequence for one character never occurs as part of a longer sequence for another character as it did in older variable-length encodings like Shift-JIS (see the previous section on this).
• The first byte of a multi-byte sequence is enough to determine the length of the multi-byte sequence (just count the number of leading set bits). This makes it extremely simple to extract a substring from a given string without elaborate parsing.
• 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 the single-byte character 0x00 (see null-terminated string). 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 ASCII's 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).
• 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.

History

UTF-8 was invented by Ken Thompson on September 2, 1992 on a placemat in a New Jersey diner with Rob Pike. The day after, Pike and Thompson implemented it and updated their Plan 9 operating system to use it throughout.

UTF-8 was first officially presented on the USENIX conference in San Diego January 25-29 1993.