|Classification||Unicode Transformation Format, extended ASCII, variable-width encoding|
|Transforms / Encodes||ISO 10646 (Unicode)|
UTF-8 is a variable-width character encoding used for electronic communication. Defined by the Unicode Standard, the name is derived from Unicode (or Universal Coded Character Set) Transformation Format – 8-bit.
UTF-8 is capable of encoding all 1,112,064[nb 1] valid character code points in Unicode 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. Since ASCII bytes do not occur when encoding non-ASCII code points into UTF-8, UTF-8 is safe to use within most programming and document languages that interpret certain ASCII characters in a special way, such as "/" (slash) in filenames, "\" (backslash) in escape sequences, and "%" in printf.
UTF-8 was designed as a superior alternative to UTF-1, a proposed variable-width 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 is the recommendation from the WHATWG for HTML and DOM specifications, and the Internet Mail Consortium recommends that all e-mail programs be able to display and create mail using UTF-8.
Google reported that in 2008, UTF-8 (labelled "Unicode") became the most common encoding for HTML files.
Since 2009, UTF-8 has been the most common encoding for the World Wide Web. The World Wide Web Consortium recommends UTF-8 as the default encoding in XML and HTML (and not just using UTF-8, also stating it in metadata), "even when all characters are in the ASCII range .. Using non-UTF-8 encodings can have unexpected results". Many other standards only support UTF-8, e.g. open JSON exchange requires it.
As of September 2020 (This takes into account that ASCII is valid UTF-8.) Several languages have 100.0% use of UTF-8 on the web, such as Punjabi, Tagalog, Lao, Marathi, Kannada, Kurdish, Pashto, Javanese, Greenlandic (Kalaallisut) and Iranian languages and sign languages., UTF-8 accounts on average for 95.4% of all web pages and 97% of the top 1,000 highest ranked web pages.
In locales where UTF-8 is used alongside another encoding, the latter is typically more efficient for the associated language. The Chinese standard and with its extension GBK (which are both interpreted by web browsers as GB 18030, having support for the same letters as UTF-8) have a combined 14.5% share in China and a 0.5% share world-wide. Big5 is another popular Chinese encoding with 0.1% share world-wide. The single-byte Windows-1251 is twice as efficient for the Cyrillic script and is used for 10.6% of Russian web sites. E.g. Greek and Hebrew encodings are also twice as efficient, but still those languages have over 95% use of UTF-8.EUC-KR is more efficient for Korean text and is used for 17.3% of South Korean websites. Shift JIS and EUC-JP have a 10.5% share on Japanese websites (the more popular has 0.2% global share). With the exception of GB 18030 and UTF-16, these encodings were designed for specific languages, and do not support all Unicode characters. As of September 2020 , the Breton language has the lowest UTF-8 use on the Web of any tracked language, with 79.4% use.
International Components for Unicode (ICU) has historically used UTF-16, and still does only for Java; while for C/C++ UTF-8 is now supported as the "Default Charset", including the correct handling of "illegal UTF-8".
Since the restriction of the Unicode code-space to 21-bit values in 2003, UTF-8 is defined to encode code points in one to four bytes, depending on the number of significant bits in the numerical value of the code point. The following table shows the structure of the encoding. The x characters are replaced by the bits of the code point.
|Number of bytes||First code point||Last code point||Byte 1||Byte 2||Byte 3||Byte 4|
The first 128 characters (US-ASCII) need one byte. The next 1,920 characters need two bytes to encode, which covers the remainder of almost all Latin-script alphabets, and also Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic, Syriac, Thaana and N'Ko alphabets, as well as Combining Diacritical Marks. Three bytes are needed for characters in the rest of the Basic Multilingual Plane, which contains virtually all characters in common use, including most Chinese, Japanese and Korean characters. Four bytes are needed for characters in the other planes of Unicode, which include less common CJK characters, various historic scripts, mathematical symbols, and emoji (pictographic symbols).
Consider the encoding of the Euro sign, EUR:
The three bytes 1110 0010 1000 0010 1010 1100 can be more concisely written in hexadecimal, as E2 82 AC.
The following table summarises this conversion, as well as others with different lengths in UTF-8. The colors indicate how bits from the code point are distributed among the UTF-8 bytes. Additional bits added by the UTF-8 encoding process are shown in black.
|¢||U+00A2||0242||000 1010 0010||11000010 10100010||302 242||C2 A2|
|?||U+0939||004471||0000 1001 0011 1001||11100000 10100100 10111001||340 244 271||E0 A4 B9|
|EUR||U+20AC||020254||0010 0000 1010 1100||11100010 10000010 10101100||342 202 254||E2 82 AC|
|?||U+D55C||152534||1101 0101 0101 1100||11101101 10010101 10011100||355 225 234||ED 95 9C|
|?||U+10348||0201510||0 0001 0000 0011 0100 1000||11110000 10010000 10001101 10001000||360 220 215 210||F0 90 8D 88|
UTF-8's use of six bits per byte to represent the actual characters being encoded means that octal notation (which uses 3-bit groups) can aid in the comparison of UTF-8 sequences with one another.
The following table summarizes usage of UTF-8 code units (individual bytes or octets) in a code page format. The upper half (0_ to 7_) is for bytes used only in single-byte codes, so it looks like a normal code page; the lower half is for continuation bytes (8_ to B_) and leading bytes (C_ to F_), and is explained further in the legend below.
 The hexadecimal number shown after the + symbol is the value of the 6 bits they add.Orange cells with a large dot are continuation bytes.
 the length shown at the left edge of the row. The text shows the Unicode blocks encoded by sequences starting with this byte, and the hexadecimal code point shown in the cell is the lowest character value encoded using that leading byte.White cells are the leading bytes for a sequence of multiple bytes,
C0 and C1) could be used only for a 2-byte encoding of a 7-bit ASCII character which should be encoded in 1 byte; as described below, such "overlong" sequences are disallowed. The red cells in the F_ row (F5 to FD) indicate leading bytes of 4-byte or longer sequences that cannot be valid because they would encode code points larger than the U+10FFFF limit of Unicode (a limit derived from the maximum code point encodable in UTF-16 ). They, FE, and FF do not match any allowed character pattern and are therefore not valid start bytes.Red cells must never appear in a valid UTF-8 sequence. The first two red cells (
E0 and F0 could start overlong encodings, in this case the lowest non-overlong-encoded code point is shown. F4 can start code points greater than U+10FFFF which are invalid. ED can start the encoding of a code point in the range U+D800-U+DFFF; these are invalid since they are reserved for UTF-16 surrogate halves.Pink cells are the leading bytes for a sequence of multiple bytes, of which some, but not all, possible continuation sequences are valid.
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 EUR from the above example in four bytes instead of three, it could be padded with leading 0s until it was 21 bits long – 000 000010 000010 101100, and encoded as 11110000 10000010 10000010 10101100 (or F0 82 82 AC 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.
Not all sequences of bytes are valid UTF-8. A UTF-8 decoder should be prepared for:
Many of the first UTF-8 decoders would decode these, ignoring incorrect bits and accepting overlong results. Carefully crafted invalid UTF-8 could make them either skip or create ASCII characters such as NUL, slash, or quotes. Invalid UTF-8 has been used to bypass security validations in high-profile products including Microsoft's IIS web server and Apache's Tomcat servlet container. RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."The Unicode Standard requires decoders to "...treat any ill-formed code unit sequence as an error condition. This guarantees that it will neither interpret nor emit an ill-formed code unit sequence."
Since RFC 3629 (November 2003), the high and low surrogate halves used by UTF-16 (U+D800 through U+DFFF) and code points not encodable by UTF-16 (those after U+10FFFF) are not legal Unicode values, and their UTF-8 encoding must be treated as an invalid byte sequence. Not decoding unpaired surrogate halves makes it impossible to store invalid UTF-16 (such as Windows filenames or UTF-16 that has been split between the surrogates) as UTF-8.
Some implementations of decoders throw exceptions on errors. This has the disadvantage that it can turn what would otherwise be harmless errors (such as a "no such file" error) into a denial of service. For instance early versions of Python 3.0 would exit immediately if the command line or environment variables contained invalid UTF-8. An alternative practice is to replace errors with a replacement character. Since Unicode 6 (October 2010), the standard (chapter 3) has recommended a "best practice" where the error ends as soon as a disallowed byte is encountered. In these decoders E1,A0,C0 is two errors (2 bytes in the first one). This means an error is no more than three bytes long and never contains the start of a valid character, and there are 21,952 different possible errors. The standard also recommends replacing each error with the replacement character "?" (U+FFFD).
If the UTF-16 Unicode byte order mark (BOM) character is at the start of a UTF-8 file, the first three bytes will be 0xEF, 0xBB, 0xBF.
The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8, but warns that it may be encountered at the start of a file trans-coded from another encoding. While ASCII text encoded using UTF-8 is backward compatible with ASCII, this is not true when Unicode Standard recommendations are ignored and a BOM is added. Nevertheless, there was and still is software that always inserts a BOM when writing UTF-8, and refuses to correctly interpret UTF-8 unless the first character is a BOM (or the file only contains ASCII).
The official Internet Assigned Numbers Authority (IANA) code for the encoding is "UTF-8". All letters are upper-case, and the name is hyphenated. This spelling is used in all the Unicode Consortium documents relating to the encoding.
Other descriptions, such as those that omit the hyphen or replace it with a space, i.e. "utf8" or "UTF 8", are not accepted as correct by the governing standards. Despite this, most agents such as browsers can understand them, and so standards intended to describe existing practice (such as HTML5) may effectively require their recognition.
Unofficially, UTF-8-BOM and UTF-8-NOBOM are sometimes used to refer to text files which respectively contain and lack a byte order mark (BOM). In Japan especially, UTF-8 encoding without BOM is sometimes called "UTF-8N".
Windows 7 and later, i.e. all supported Windows versions, have codepage 65001, as a synonym for UTF-8 (with better support than in older Windows), and Microsoft has a script for Windows 10, to enable it by default for its program Microsoft Notepad.
The International Organization for Standardization (ISO) set out to compose a universal multi-byte character set in 1989. The draft ISO 10646 standard contained a non-required annex called UTF-1 that provided a byte stream encoding of its 32-bit code points. This encoding was not satisfactory on performance grounds, among other problems, and the biggest problem was probably that it did not have a clear separation between ASCII and non-ASCII: new UTF-1 tools would be backward compatible with ASCII-encoded text, but UTF-1-encoded text could confuse existing code expecting ASCII (or extended ASCII), because it could contain continuation bytes in the range 0x21-0x7E that meant something else in ASCII, e.g., 0x2F for '/', the Unix path directory separator, and this example is reflected in the name and introductory text of its replacement. The table below was derived from a textual description in the annex.
|Byte 1||Byte 2||Byte 3||Byte 4||Byte 5|
|3||U+4016||U+38E2D||F6-FB||21-7E, A0-FF||21-7E, A0-FF|
|5||U+38E2E||U+7FFFFFFF||FC-FF||21-7E, A0-FF||21-7E, A0-FF||21-7E, A0-FF||21-7E, A0-FF|
In July 1992, the X/Open committee XoJIG was looking for a better encoding. Dave Prosser of Unix System Laboratories submitted a proposal for one that had faster implementation characteristics and introduced the improvement that 7-bit ASCII characters would only represent themselves; all multi-byte sequences would include only bytes where the high bit was set. The name File System Safe UCS Transformation Format (FSS-UTF) and most of the text of this proposal were later preserved in the final specification.
|Byte 1||Byte 2||Byte 3||Byte 4||Byte 5|
In August 1992, this proposal was circulated by an IBM X/Open representative to interested parties. A modification by Ken Thompson of the Plan 9 operating system group at Bell Labs made it somewhat less bit-efficient than the previous proposal but crucially allowed it to be self-synchronizing, letting a reader start anywhere and immediately detect byte sequence boundaries. It also abandoned the use of biases and instead added the rule that only the shortest possible encoding is allowed; the additional loss in compactness is relatively insignificant, but readers now have to look out for invalid encodings to avoid reliability and especially security issues. Thompson's design was outlined on September 2, 1992, on a placemat in a New Jersey diner with Rob Pike. In the following days, Pike and Thompson implemented it and updated Plan 9 to use it throughout, and then communicated their success back to X/Open, which accepted it as the specification for FSS-UTF.
|Byte 1||Byte 2||Byte 3||Byte 4||Byte 5||Byte 6|
UTF-8 was first officially presented at the USENIX conference in San Diego, from January 25 to 29, 1993. The Internet Engineering Task Force adopted UTF-8 in its Policy on Character Sets and Languages in RFC 2277 (BCP 18) for future Internet standards work, replacing Single Byte Character Sets such as Latin-1 in older RFCs.
In November 2003, UTF-8 was restricted by RFC 3629 to match the constraints of the UTF-16 character encoding: explicitly prohibiting code points corresponding to the high and low surrogate characters removed more than 3% of the three-byte sequences, and ending at U+10FFFF removed more than 48% of the four-byte sequences and all five- and six-byte sequences.
There are several current definitions of UTF-8 in various standards documents:
They supersede the definitions given in the following obsolete works:
They are all the same in their general mechanics, with the main differences being on issues such as allowed range of code point values and safe handling of invalid input.
Some of the important features of this encoding are as follows:
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 MySQL, the
utf8mb3 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 are supported. Unicode characters in supplementary planes are explicitly not supported.
utf8mb3 is deprecated in favor of the
utf8mb4 character set, which uses standards-compliant UTF-8 encoding.
utf8 is an alias for
utf8mb3, but is intended to become an alias to
utf8mb4 in a future release of MySQL.
It is possible, though unsupported, to store CESU-8 encoded data in
utf8mb3, by handling UTF-16 data with supplementary characters as though it is UCS-2. In this way,
utf8mb3 and Oracle Database
AL32UTF8 behave similarly, with the key difference being that
AL32UTF8 specifies support for supplementary characters via CESU-8 encoding, while
utf8mb3 explicitly excludes supplementary characters.
Modified UTF-8 (MUTF-8) originated in the Java programming language. In Modified UTF-8, the null character (U+0000) uses the two-byte overlong encoding 11000000 10000000 (hexadecimal C0 80), instead of 00000000 (hexadecimal 00). 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
OutputStreamWriter (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
DataOutput, 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.
This section contains a list of miscellaneous information. (August 2020)
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.
Version 3 of the Python programming language treats each byte of an invalid UTF-8 bytestream as an error; this gives 128 different possible errors. Extensions have been created to allow any byte sequence that is assumed to be UTF-8 to be lossless 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.