UTF-8: Переменная ширина кодировки Unicode, совместимая с ASCII

ASCII compatible variable width encoding of Unicode, using one to four bytes

UTF 8 is a variable length character encoding standard used for electronic communication. Defined by the Unicode Standard, the name is derived from Unicode Transformation Format 8 bit. UTF 8 is capable of encoding all 1,112,064 valid Unicode code points using one to four one byte (8 bit) code units. Code points with lower numerical values, which tend to occur more frequently, are encoded using fewer bytes. It was designed for backward compatibility with ASCII: the first 128 characters of Unicode, which correspond one to one with ASCII, are encoded using a single byte with the same binary value as ASCII, so that valid ASCII text is valid UTF 8 encoded Unicode as well. UTF 8 was designed as a superior alternative to UTF 1, a proposed variable length encoding with partial ASCII compatibility which lacked some features including self synchronization and fully ASCII compatible handling of characters such as slashes. Ken Thompson and Rob Pike produced the first implementation for the Plan 9 operating system in September 1992. This led to its adoption by X/Open as its specification for FSS UTF, which would first be officially presented at USENIX in January 1993 and subsequently adopted by the Internet Engineering Task Force (IETF) in RFC 2277 (BCP 18) for future internet standards work, replacing Single Byte Character Sets such as Latin 1 in older RFCs. UTF 8 results in fewer internationalization issues Spellings with a space e. g. "UTF 8" should not be used. The official Internet Assigned Numbers Authority also lists csUTF8 as the only alias, which is rarely used. In Windows, UTF 8 is codepage 65001 (i. e. CP UTF8 in source code). In MySQL, UTF 8 is called utf8mb4 (with utf8mb3, and its alias utf8, being a subset encoding for characters in the Basic Multilingual Plane). In HP PCL, the Symbol ID for UTF 8 is 18N. In Oracle Database (since version 9.0), AL32UTF8 means UTF 8. See also CESU 8 for an almost synonym with UTF 8 that rarely should be used. UTF 8 BOM and UTF 8 NOBOM are sometimes used for text files which contain or do not contain a byte order mark (BOM), respectively. In Japan especially, UTF 8 encoding without a BOM is sometimes called UTF 8N.

The following table summarizes usage of UTF 8 code units (individual bytes or octets) in a code page format. The upper half is for bytes used only in single byte codes, so it looks like a normal code page; the lower half is for continuation bytes and leading bytes and is explained further in the legend below.

In principle, it would be possible to inflate the number of bytes in an encoding by padding the code point with leading 0s. To encode the euro sign € from the above example in four bytes instead of three, it could be padded with leading 0s until it was 21 bits long
, and encoded as (or in hexadecimal). This is called an overlong encoding. The standard specifies that the correct encoding of a code point uses only the minimum number of bytes required to hold the significant bits of the code point. Longer encodings are called overlong and are not valid UTF 8 representations of the code point. This rule maintains a one to one correspondence between code points and their valid encodings, so that there is a unique valid encoding for each code point. This ensures that string comparisons and searches are well defined.

UTF 8 has been the most common encoding for the World Wide Web since 2008. as of 2024, UTF 8 is used by 98.2% of surveyed web sites. and even states "UTF 16 [ ] is a unique burden that Windows places on code that targets multiple platforms".

UTF 8 can encode any Unicode character, avoiding the need to figure out and set a "code page" or otherwise indicate what character set is in use, and allowing output in multiple scripts at the same time. For many scripts there have been more than one single byte encoding in usage, so even knowing the script was insufficient information to display it correctly. The bytes 0xFE and 0xFF do not appear, so a valid UTF 8 stream never matches the UTF 16 byte order mark (BOM) and thus cannot be confused with it. The absence of 0xFF (0377) also eliminates the need to escape this byte in Telnet (and FTP control connection). UTF 8 encoded text is larger than specialized single byte encodings except for plain ASCII characters. In the case of scripts which used 8 bit character sets with non Latin characters encoded in the upper half (such as most Cyrillic and Greek alphabet code pages), characters in UTF 8 will be double the size. For some scripts, such as Thai and Devanagari (which is used by various South Asian languages), characters will triple in size. There are even examples where a single byte turns into a composite character in Unicode and is thus six times larger in UTF 8. This has caused objections in India and other countries. It is possible in UTF 8 (or any other multi byte encoding) to split or truncate a string in the middle of a character. If the two pieces are not re appended later before interpretation as characters, this can introduce an invalid sequence at both the end of the previous section and the start of the next, and some decoders will not preserve these bytes and result in data loss. Because UTF 8 is self synchronizing this will however never introduce a different valid character, and it is also fairly easy to move the truncation point backward to the start of a character. If the code points are all the same size, measurements of a fixed number of them is easy. Due to ASCII era documentation where "character" is used as a synonym for "byte" this is often considered important. However, by measuring string positions using bytes instead of "characters" most algorithms can be easily and efficiently adapted for UTF 8. Searching for a string within a long string can for example be done byte by byte; the self synchronization property prevents false positives.

UTF 8 can encode any Unicode character. Files in different scripts can be displayed correctly without having to choose the correct code page or font. For instance, Chinese and Arabic can be written in the same file without specialized markup or manual settings that specify an encoding. UTF 8 is self synchronizing: character boundaries are easily identified by scanning for well defined bit patterns in either direction. If bytes are lost due to error or corruption, one can always locate the next valid character and resume processing. If there is a need to shorten a string to fit a specified field, the previous valid character can easily be found. Many multi byte encodings such as Shift JIS are much harder to resynchronize. This also means that byte oriented string searching algorithms can be used with UTF 8 (as a character is the same as a "word" made up of that many bytes), optimized versions of byte searches can be much faster due to hardware support and lookup tables that have only 256 entries. Self synchronization does however require that bits be reserved for these markers in every byte, increasing the size. Efficient to encode using simple bitwise operations. UTF 8 does not require slower mathematical operations such as multiplication or division (unlike Shift JIS, GB 2312 and other encodings). UTF 8 will take more space than a multi byte encoding designed for a specific script. East Asian legacy encodings generally used two bytes per character yet take three bytes per character in UTF 8.

Byte encodings and UTF 8 are represented by byte arrays in programs, and often nothing needs to be done to a function when converting source code from a byte encoding to UTF 8. UTF 16 is represented by 16 bit word arrays, and converting to UTF 16 while maintaining compatibility with existing ASCII based programs (such as was done with Windows) requires every API and data structure that takes a string to be duplicated, one version accepting byte strings and another version accepting UTF 16. If backward compatibility is not needed, all string handling still must be modified. Text encoded in UTF 8 will be smaller than the same text encoded in UTF 16 if there are more code points below U+0080 than in the range U+0800 U+FFFF. This is true for all modern European languages. It is often true even for languages like Chinese, due to the large number of spaces, newlines, digits, and HTML markup in typical files. Most communication (e. g. HTML and IP) and storage (e. g. for Unix) was designed for a stream of bytes. A UTF 16 string must use a pair of bytes for each code unit:
The order of those two bytes becomes an issue and must be specified in the UTF 16 protocol, such as with a byte order mark (BOM). If an odd number of bytes is missing from UTF 16, the whole rest of the string will be meaningless text. Any bytes missing from UTF 8 will still allow the text to be recovered accurately starting with the next character after the missing bytes.

The following implementations show slight differences from the UTF 8 specification. They are incompatible with the UTF 8 specification and may be rejected by conforming UTF 8 applications.

Unicode Technical Report #26 assigns the name CESU 8 to a nonstandard variant of UTF 8, in which Unicode characters in supplementary planes are encoded using six bytes, rather than the four bytes required by UTF 8. CESU 8 encoding treats each half of a four byte UTF 16 surrogate pair as a two byte UCS 2 character, yielding two three byte UTF 8 characters, which together represent the original supplementary character. Unicode characters within the Basic Multilingual Plane appear as they would normally in UTF 8. The Report was written to acknowledge and formalize the existence of data encoded as CESU 8, despite the Unicode Consortium discouraging its use, and notes that a possible intentional reason for CESU 8 encoding is preservation of UTF 16 binary collation. CESU 8 encoding can result from converting UTF 16 data with supplementary characters to UTF 8, using conversion methods that assume UCS 2 data, meaning they are unaware of four byte UTF 16 supplementary characters. It is primarily an issue on operating systems which extensively use UTF 16 internally, such as Microsoft Windows. In Oracle Database, the character set uses CESU 8 encoding, and is deprecated. The character set uses standards compliant UTF 8 encoding, and is preferred. CESU 8 is prohibited for use in HTML5 documents.

In MySQL, the character set is defined to be UTF 8 encoded data with a maximum of three bytes per character, meaning only Unicode characters in the Basic Multilingual Plane (i. e. from UCS 2) are supported. Unicode characters in supplementary planes are explicitly not supported. is deprecated in favor of the character set, which uses standards compliant UTF 8 encoding. is an alias for , but is intended to become an alias to in a future release of MySQL. Modified UTF 8 strings never contain any actual null bytes but can contain all Unicode code points including U+0000, which allows such strings (with a null byte appended) to be processed by traditional null terminated string functions. All known Modified UTF 8 implementations also treat the surrogate pairs as in CESU 8. In normal usage, the language supports standard UTF 8 when reading and writing strings through and (if it is the platform's default character set or as requested by the program). However it uses Modified UTF 8 for object serialization among other applications of and , for the Java Native Interface, and for embedding constant strings in class files. The dex format defined by Dalvik also uses the same modified UTF 8 to represent string values. Tcl also uses the same modified UTF 8 as Java for internal representation of Unicode data, but uses strict CESU 8 for external data.

In WTF 8 (Wobbly Transformation Format, 8 bit) unpaired surrogate halves (U+D800 through U+DFFF) are allowed. This is necessary to store possibly invalid UTF 16, such as Windows filenames. Many systems that deal with UTF 8 work this way without considering it a different encoding, as it is simpler. The term "WTF 8" has also been used humorously to refer to erroneously doubly encoded UTF 8 sometimes with the implication that CP1252 bytes are the only ones encoded.

Version 3 of the Python programming language treats each byte of an invalid UTF 8 bytestream as an error (see also changes with new UTF 8 mode in Python 3.7); this gives 128 different possible errors. Extensions have been created to allow any byte sequence that is assumed to be UTF 8 to be losslessly transformed to UTF 16 or UTF 32, by translating the 128 possible error bytes to reserved code points, and transforming those code points back to error bytes to output UTF 8. The most common approach is to translate the codes to U+DC80 U+DCFF which are low (trailing) surrogate values and thus "invalid" UTF 16, as used by Python's PEP 383 (or "surrogateescape") approach. Another encoding called MirBSD OPTU 8/16 converts them to U+EF80 U+EFFF in a Private Use Area. In either approach, the byte value is encoded in the low eight bits of the output code point. These encodings are very useful because they avoid the need to deal with "invalid" byte strings until much later, if at all, and allow "text" and "data" byte arrays to be the same object. If a program wants to use UTF 16 internally these are required to preserve and use filenames that can use invalid UTF 8; as the Windows filesystem API uses UTF 16, the need to support invalid UTF 8 is less there. For the encoding to be reversible, the standard UTF 8 encodings of the code points used for erroneous bytes must be considered invalid. This makes the encoding incompatible with WTF 8 or CESU 8 (though only for 128 code points). When re encoding it is necessary to be careful of sequences of error code points which convert back to valid UTF 8, which may be used by malicious software to get unexpected characters in the output, though this cannot produce ASCII characters so it is considered comparatively safe, since malicious sequences (such as cross site scripting) usually rely on ASCII characters.