UTF-8
UTF-8 (UCS Transformation Format—8-bit[1]) is a variable-width encoding that can represent every character in the Unicode character set. It was designed for backward compatibility with ASCII and to avoid the complications of endianness and byte order marks in UTF-16 and UTF-32.
UTF-8 has become the dominant character encoding for the World Wide Web, accounting for more than half of all Web pages.[2][3][4] The Internet Mail Consortium (IMC) recommends that all e-mail programs be able to display and create mail using UTF-8.[5] UTF-8 is also increasingly being used as the default character encoding in operating systems, programming languages, APIs, and software applications.[citation needed]
UTF-8 encodes each of the 1,112,064 code points in the Unicode character set using one to four 8-bit bytes (termed "octets" in the Unicode Standard). Code points with lower numerical values (i.e. earlier code positions in the Unicode character set, which tend to occur more frequently) are encoded using fewer bytes. The first 128 characters of Unicode, which correspond one-to-one with ASCII, are encoded using a single octet with the same binary value as ASCII, making valid ASCII text valid UTF-8-encoded Unicode as well.
The official IANA code for the UTF-8 character encoding is UTF-8.[6]
History
By early 1992 the search was on for a good byte-stream encoding of multi-byte character sets. 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, but did introduce the notion that bytes in the range of 0–127 continue representing the ASCII characters in UTF, thereby providing backward compatibility with ASCII.
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 multibyte sequences would include only bytes where the high bit was set. This original proposal, FSS-UTF (File System Safe UCS Transformation Format), was similar in concept to UTF-8, but lacked the crucial property of self-synchronization.[7][8]
In August 1992, this proposal was circulated by an IBM X/Open representative to interested parties. Ken Thompson of the Plan 9 operating system group at Bell Labs then made a small but crucial modification to the encoding, making it very slightly less bit-efficient than the previous proposal but allowing it to be self-synchronizing, meaning that it was no longer necessary to read from the beginning of the string to find code point boundaries. 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.[7]
UTF-8 was first officially presented at the USENIX conference in San Diego, from January 25 to 29, 1993.
In November 2003 UTF-8 was restricted by RFC 3629 to four bytes to match the constraints of the UTF-16 character encoding.
Google reported that in 2008 UTF-8 (misleadingly labelled "Unicode") became the most common encoding for HTML files.[9][10]
Description
The design of UTF-8 can be seen in this table of the scheme as originally proposed by Dave Prosser and subsequently modified by Ken Thompson (the xs are replaced by the bits of the code point):
| Bits of code point |
First code point |
Last code point |
Bytes in sequence |
Byte 1 | Byte 2 | Byte 3 | Byte 4 | Byte 5 | Byte 6 |
|---|---|---|---|---|---|---|---|---|---|
| 7 | U+0000 | U+007F | 1 | 0xxxxxxx
| |||||
| 11 | U+0080 | U+07FF | 2 | 110xxxxx |
10xxxxxx
| ||||
| 16 | U+0800 | U+FFFF | 3 | 1110xxxx |
10xxxxxx |
10xxxxxx
| |||
| 21 | U+10000 | U+1FFFFF | 4 | 11110xxx |
10xxxxxx |
10xxxxxx |
10xxxxxx
| ||
| 26 | U+200000 | U+3FFFFFF | 5 | 111110xx |
10xxxxxx |
10xxxxxx |
10xxxxxx |
10xxxxxx
| |
| 31 | U+4000000 | U+7FFFFFFF | 6 | 1111110x |
10xxxxxx |
10xxxxxx |
10xxxxxx |
10xxxxxx |
10xxxxxx
|
The original specification covered numbers up to 31 bits (the original limit of the Universal Character Set). In November 2003 UTF-8 was restricted by RFC 3629 to end at U+10FFFF, in order to match the constraints of the UTF-16 character encoding. This removed all 5- and 6-byte sequences, and about half of the 4-byte sequences.
The salient features of this scheme are as follows:
- One-byte codes are used only for the ASCII values 0 through 127. In this case the UTF-8 code has the same value as the ASCII code. The high-order bit of these codes is always 0.
- Code points larger than 127 are represented by multi-byte sequences, composed of a leading byte and one or more continuation bytes. The leading byte has two or more high-order 1s followed by a 0, while continuation bytes all have '10' in the high-order position.
- The number of high-order 1s in the leading byte of a multi-byte sequence indicates the number of bytes in the sequence, so that the length of the sequence can be determined without examining the continuation bytes.
- The remaining bits of the encoding are used for the bits of the code point being encoded, padded with high-order 0s if necessary. The high-order bits go in the lead byte, lower-order bits in succeeding continuation bytes. The number of bytes in the encoding is the minimum required to hold all the significant bits of the code point.
- Single bytes, leading bytes, and continuation bytes do not share values. This makes the scheme self-synchronizing, allowing the start of a character to be found by backing up at most five bytes (three bytes in actual UTF‑8 as explained below).
The first 128 characters (US-ASCII) need one byte. The next 1,920 characters need two bytes to encode. This covers the remainder of almost all Latin-derived alphabets, and also Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic, Syriac and Tāna 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[11]). Four bytes are needed for characters in the other planes of Unicode, which include less common CJK characters and various historic scripts and mathematical symbols.
Examples
Let us consider how to encode the Euro sign, €.
- The Unicode code point for "€" is U+20AC.
- According to the scheme table above, this will take three bytes to encode, since it is between U+0800 and U+FFFF.
- Hexadecimal
20ACis binary0010000010101100. The two leading zeros are added because, as the scheme table shows, a three-byte encoding needs exactly sixteen bits from the code point. - Because it is a three-byte encoding, the leading byte starts with three 1s, then a 0 (
1110...) - The remaining bits of this byte are taken from the code point (
11100010), leaving ...000010101100. - Each of the continuation bytes starts with
10and takes six bits of the code point (so10000010, then10101100).
The three bytes 11100010 10000010 10101100 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.
| Character | Binary code point | Binary UTF-8 | Hexadecimal UTF-8 | |
|---|---|---|---|---|
| $ | U+0024
|
0100100
|
00100100
|
24
|
| ¢ | U+00A2
|
000 10100010
|
11000010 10100010
|
C2 A2
|
| € | U+20AC
|
00100000 10101100
|
11100010 10000010 10101100
|
E2 82 AC
|
| 𤭢 | U+24B62
|
00010 01001011 01100010
|
11110000 10100100 10101101 10100010
|
F0 A4 AD A2
|
Codepage layout
Legend: Yellow cells are control characters, blue cells are punctuation, purple cells are digits and green cells are ASCII letters.
Orange cells with a large dot are continuation bytes. The hexadecimal number shown after a "+" plus sign is the value of the 6 bits they add.
White cells are the start bytes for a sequence of multiple 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 start byte. When a start byte could form both overlong and valid encodings, the lowest non-overlong-encoded code point is shown, marked by an asterisk "*".
Red cells must never appear in a valid UTF-8 sequence. The first two (C0 and C1) could only be used for overlong encoding of basic ASCII characters. The remaining red cells indicate start bytes of sequences that could only encode numbers larger than the 0x10FFFF limit of Unicode. The byte 244 (hex 0xF4) could also encode some values greater than 0x10FFFF; such a sequence is also invalid.
Overlong encodings
The standard specifies that the correct encoding of a code point use 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. Allowing multiple encodings would make testing for string equality difficult to define.
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—000000010000010101100. The leading byte prefix for a four-byte encoding is 11110, and so the complete, overlong encoding is 11110000 10000010 10000010 10101100 (or F0 82 82 AC in hexadecimal).
Although overlong encodings are forbidden in UTF-8, at least one derivative makes use of the form. Modified UTF-8 requires the Unicode code point U+0000 (the NUL character) to be encoded in the overlong form 11000000 10000000 (hex C0 80), rather than 00000000 (hex 00). This allows the byte 00 to be used as a string terminator.
Invalid byte sequences
Not all sequences of bytes are valid UTF-8. A UTF-8 decoder should be prepared for:
- the red invalid bytes in the above table
- an unexpected continuation byte
- a start byte not followed by enough continuation bytes
- an Overlong Encoding as described above
- A 4-byte sequence (starting with 0xF4) that decodes to a value greater than U+10FFFF
Many earlier decoders would happily try to decode these. 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[12] and Apache's Tomcat servlet container.[13]
RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."[14] 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."
Many UTF-8 decoders throw exceptions on encountering errors.[15] This can turn what would otherwise be harmless errors (producing a message such as "no such file") into a denial of service bug. For instance, Python 3.0 would exit immediately if the command line or environment variables contained invalid UTF-8,[16] so it was impossible for any Python program to detect and recover from such an error.
An increasingly popular option is to detect errors with a separate API, and for converters to translate the first byte to a replacement and continue parsing with the next byte. These error bytes will always have the high bit set. Popular replacements include:
- The replacement character "�" (U+FFFD)
- The invalid Unicode code points U+DC80..U+DCFF where the low 8 bits are the byte's value.
- The Unicode code points U+0080..U+00FF with the same value as the byte, thus interpreting the bytes according to ISO-8859-1.
- The Unicode code point for the character represented by the byte in CP1252. This is similar to using ISO-8859-1, except that some characters in the range 0x80..0x9F are mapped into different Unicode code points. For example, 0x80 becomes the Euro sign, U+20AC.
These replacement algorithms are "lossy": an invalid string is converted to the same sequence of code points that a valid UTF-8 string could have.
Invalid code points
According to the UTF-8 definition (RFC 3629) the high and low surrogate halves used by UTF-16 (U+D800 through U+DFFF) are not legal Unicode values, and their UTF-8 encoding should be treated as an invalid byte sequence.
Whether an actual application should do this is debatable, as it makes it impossible to store invalid UTF-16 (that is, UTF-16 with unpaired surrogate halves) in a UTF-8 string. This is necessary to store unchecked UTF-16 such as Windows filenames as UTF-8. It is also incompatible with CESU encoding (described below).
Official name and variants
The official name 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.
Alternatively, the name "utf-8" may be used by all standards conforming to the Internet Assigned Numbers Authority (IANA) list (which include CSS, HTML, XML, and HTTP headers),[17] as the declaration is case insensitive.[18]
Other descriptions that omit the hyphen or replace it with a space, such as "utf8" or "UTF 8", are not accepted as correct by the governing standards.[14] 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 or UTF-8-NOBOM are sometimes used to refer to text files which contain or lack a BOM. In Japan especially, "UTF-8 encoding without BOM" is sometimes called "UTF-8N".[19][20]
Derivatives
The following implementations show slight differences from the UTF-8 specification. They are incompatible with the UTF-8 specification.
CESU-8
Many programs added UTF-8 conversions for UCS-2 data and did not alter this UTF-8 conversion when UCS-2 was replaced with the surrogate-pair using UTF-16. In such programs each half of a UTF-16 surrogate pair is encoded as its own 3-byte UTF-8 encoding, resulting in 6-byte sequences rather than 4 bytes for characters outside the Basic Multilingual Plane. Oracle and MySQL databases use this, as well as Java and Tcl as described below, and probably many Windows programs where the programmers were unaware of the complexities of UTF-16. Although this non-optimal encoding is generally not deliberate, a supposed benefit is that it preserves UTF-16 binary sorting order when CESU-8 is binary sorted.
Modified UTF-8
In Modified UTF-8,[21] the null character (U+0000) is encoded as 0xC0,0x80; this is not valid UTF-8[22] because it is not the shortest possible representation. Modified UTF-8 strings never contain any actual null bytes but can contain all Unicode code points including U+0000,[23] 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 Java programming language supports standard UTF-8 when reading and writing strings through InputStreamReader and OutputStreamWriter. However it uses Modified UTF-8 for object serialization,[24] for the Java Native Interface,[25] and for embedding constant strings in class files.[26]
The dex format defined by Dalvik also uses the same modified UTF-8 to represent string values.[27]
Tcl also uses the same modified UTF-8[28] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.
Extending from 31 bit to 36 bit range
Extending the accepted input pattern from 6 bytes to 7 bytes would allow over 70 billion code points to be encoded;[29] however, this would require an initial byte value of 0xFE to be accepted as a 7-byte sequence indicator (see under Advantages in section "Compared to single-byte encodings").
Byte order mark
Many Windows programs (including Windows Notepad) add the bytes 0xEF, 0xBB, 0xBF at the start of any document saved as UTF-8. This is the UTF-8 encoding of the Unicode byte order mark (BOM), and is commonly referred to as a UTF-8 BOM, even though it is not relevant to byte order. Translation of an encoding to UTF-8 can optionally retain the BOM. Software that is not aware of multibyte encodings may display the BOM as "" at the start of the document.
The Unicode Standard neither requires nor recommends the use of the BOM for UTF-8.[30] The presence of the UTF-8 BOM may cause interoperability problems with existing software that could otherwise handle UTF-8; for example:
- Programming language parsers not explicitly designed for UTF-8 can often handle UTF-8 in string constants and comments, but cannot parse the BOM at the start of the file.
- Programs that identify file types by leading characters may fail to identify the file if a BOM is present even if the user of the file could skip the BOM. An example is the Unix shebang syntax. Another example is Internet Explorer which will render pages in standards mode only when it starts with a document type declaration.
If compatibility with existing programs is not important, the BOM could be used to identify UTF-8 encoding, but such use should not be necessary as UTF-8 can be identified with very high reliability since other encodings are extremely unlikely to contain valid UTF-8 byte sequences. Programs that insert information at the start of a file will break this identification (one example is offline browsers that add the originating URL to the start of the file).
Advantages and disadvantages
 | This article contains a pro and con list. (November 2012) |
 | This section needs additional citations for verification. (October 2009) |
General
Advantages
- The ASCII characters are represented by themselves as single bytes that do not appear anywhere else, which makes UTF-8 work with the majority of existing APIs that take bytes strings but only treat a small number of ASCII codes specially. This removes the need to write a new Unicode version of every API, and makes it much easier to convert existing systems to UTF-8 than any other Unicode encoding.
- UTF-8 is the only encoding for XML entities that does not require a BOM or an indication of the encoding.[31]
- UTF-8 and UTF-16 are the standard encodings for Unicode text in HTML documents, with UTF-8 as the preferred and most used encoding.
- UTF-8 strings can be fairly reliably recognized as such by a simple heuristic algorithm.[32] The probability of a random string of bytes which is not pure ASCII being valid UTF-8 is 3.9% for a two-byte sequence,[33] and decreases exponentially for longer sequences. ISO/IEC 8859-1 is even less likely to be mis-recognized as UTF-8: the only non-ASCII characters in it would have to be in sequences starting with either an accented letter or the multiplication symbol and ending with a symbol. This is an advantage that most other encodings do not have, causing errors (mojibake) if the receiving application isn't told and can't guess the correct encoding. Even word-based UTF-16 can be mistaken for byte encodings (like in the "bush hid the facts" bug).
- Sorting a set of UTF-8 encoded strings as strings of unsigned bytes yields the same order as sorting the corresponding Unicode strings lexicographically by codepoint.
- Other byte-based encodings can pass through the same API. This means, however, that the encoding must be identified. Because the other encodings are unlikely to be valid UTF-8, a reliable way to implement this is to assume UTF-8 and switch to a legacy encoding only if several invalid UTF-8 byte sequences are encountered.
Disadvantages
- A UTF-8 parser that is not compliant with current versions of the standard might 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 eight-bit representation.[14]
Compared to single-byte encodings
Advantages
- 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 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).
Disadvantages
- UTF-8 encoded text is larger than specialized single-byte encodings except for plain ASCII characters. For scripts which used 8-bit character sets, non-Latin characters encoded in the upper half (as in most Cyrillic and Greek alphabet code pages) will double in size in UTF-8. For some scripts such as Thai and Hindi's Devanagari, characters will triple in size. This has caused objections in India and other countries.[citation needed]
- It is possible in UTF-8 (or any other multi-byte encoding) to split or truncate a string in the middle of a character, which may result in an invalid string. This will not happen in correct handling of UTF-8.
- 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.[citation needed]
- Some software such as text editors will refuse to correctly display or interpret UTF-8 unless the text starts with a Byte Order Mark, and will insert such a mark. This has the effect of making it impossible to use UTF-8 with any older software than can handle ASCII-like encodings but cannot handle the byte order mark. This is considered an incorrect implementation of the text editor, not the older software.
Compared to other multi-byte encodings
Advantages
- UTF-8 uses the codes 0–127 only for the ASCII characters. This means that UTF-8 is an ASCII extension and can with limited change be supported by software that supports an ASCII extension and handles non-ASCII characters as free text.
- 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 supported (in the same text) without special codes inserted or manual settings to switch the 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 beginning of the next valid character and resume processing. Many multi-byte encodings are much harder to resynchronize.
- Any byte oriented string searching algorithm can be used with UTF-8 data, since the sequence of bytes for a character cannot occur anywhere else. Some older variable-length encodings (such as Shift JIS) did not have this property and thus made string-matching algorithms rather complicated. In Shift JIS the end byte of a character and the first byte of the next character could look like another legal character, something that can't happen in UTF-8.
- Efficient to encode using simple bit operations. UTF-8 does not require slower mathematical operations such as multiplication or division (unlike the obsolete UTF-1 encoding).
Disadvantages
- For certain scripts UTF-8 will take more space than an older multi-byte encoding. East Asian scripts generally have two bytes per character in their multi-byte encodings yet take three bytes per character in UTF-8.
Compared to UTF-16
Advantages
- A text byte stream cannot be losslessly converted to UTF-16, due to the possible presence of errors in the byte stream encoding. This causes unexpected and often severe problems attempting to use existing data in a system that uses UTF-16 as an internal encoding. Results are security bugs, DoS if bad encoding throws an exception, and data loss when different byte streams convert to the same UTF-16. Due to the ASCII compatibility and high degree of pattern recognition in UTF-8, random byte streams can be passed losslessly through a system using it, as interpretation can be deferred until display.
- Converting to UTF-16 while maintaining compatibility with existing programs (such as was done with Windows) requires every API and data structure that takes a string to be duplicated. Invalid encodings make the duplicated APIs not exactly map to each other, often making it impossible to do some action with one of them.
- Characters outside the basic multilingual plane are not a special case. UTF-16 is often mistaken to be the obsolete constant-length UCS-2 encoding, leading to code that works for most text but suddenly fails for non-BMP characters.
- 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 of all modern European languages. As HTML markup, Arabic numbers, spaces and line terminators are code points below U+0080, this is often true even for Asian scripts.
- Most communication and storage 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.
- 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. If any partial character is removed the corruption is always recognizable.
Disadvantages
- Characters U+0800 through U+FFFF use three bytes in UTF-8, but only two in UTF-16. As a result, text in (for example) Chinese, Japanese or Hindi could take more space in UTF-8 if there are more of these characters than there are ASCII characters. This happens for pure text[34] but rarely for HTML documents or documents in XML based formats such as .docx or .odt. For example, both the Japanese UTF-8 and the Hindi Unicode articles on Wikipedia take more space in UTF-16 than in UTF-8.[35]
See also
- Alt code
- Character encodings in HTML
- Comparison of e-mail clients#Features
- Comparison of Unicode encodings
- GB 18030
- Iconv—a standardized API used to convert between different character encodings
- ISO/IEC 8859
- Specials (Unicode block)
- Unicode and e-mail
- Unicode and HTML
- Universal Character Set
- UTF-8 in URIs
- UTF-9 and UTF-18
- UTF-16/UCS-2
References
- ^ "Chapter 2. General Structure". The Unicode Standard (6.0 ed.). Mountain View, California, USA: The Unicode Consortium. ISBN 978-1-936213-01-6.
- ^ Davis, Mark (28 January 2010). "Unicode nearing 50% of the web". Official Google Blog. Google. Retrieved 5 December 2010.
- ^ "UTF-8 Usage Statistics". BuiltWith. Retrieved March 28, 2011.
- ^ "Usage of character encodings for websites". W3Techs. Retrieved March 30, 2010.
- ^ "Using International Characters in Internet Mail". Internet Mail Consortium. August 1, 1998. Retrieved November 8, 2007.
- ^ "CHARACTER SETS". Internet Assigned Numbers Authority. November 4, 2010. Retrieved 5 December 2010.
- ^ a b Pike, Rob (30 Apr 2003). "UTF-8 history". Retrieved September 7, 2012.
- ^ Pike, Rob (September 6, 2012). "UTF-8 turned 20 years old yesterday". Retrieved September 7, 2012.
- ^ Davis, Mark (5 May 2008). "Moving to Unicode 5.1". Retrieved 2013-03-01.
- ^ Goodger, David (6 May 2008). "Unicode misinformation". Retrieved 2013-03-01.
- ^ Allen, Julie D.; Anderson, Deborah; Becker, Joe; Cook, Richard, eds. (2012). "The Unicode Standard, Version 6.1". Mountain View, California: Unicode Consortium.
The Basic Multilingual Plane (BMP, or Plane 0) contains the common-use characters for all the modern scripts of the world as well as many historical and rare characters. By far the majority of all Unicode characters for almost all textual data can be found in the BMP.
{{cite journal}}: Cite journal requires|journal=(help); Unknown parameter|displayeditors=ignored (|display-editors=suggested) (help) - ^ Marin, Marvin (October 17, 2000). "Web Server Folder Traversal MS00-078".
- ^ "National Vulnerability Database - Summary for CVE-2008-2938".
- ^ a b c Yergeau, F. (2003). "RFC 3629". Internet Engineering Task Force.
{{cite journal}}:|contribution=ignored (help); Cite journal requires|journal=(help) - ^ decode() method of Java UTF8 object
- ^ "Non-decodable Bytes in System Character Interfaces".
- ^ Dürst, Martin. "Setting the HTTP charset parameter". W3C. Retrieved February 8, 2013.
- ^ "Character Sets". Internet Assigned Numbers Authority. January 23, 2013. Retrieved February 8, 2013.
- ^ "BOM - suikawiki" (in Japanese). Retrieved 2013-04-26.
- ^ Mark Davis. "Forms of Unicode". IBM. Archived from the original on 6 May 2005. Retrieved 18 September 2013.
- ^ "Java SE 6 documentation for Interface java.io.DataInput, subsection on Modified UTF-8". Sun Microsystems. 2008. Retrieved May 22, 2009.
- ^ "Request for Comments 3629: "UTF-8, a transformation format of ISO 10646"". 2003. Retrieved May 22, 2009.
[...] the overlong UTF-8 sequence C0 80 [...]", "[...] the illegal two-octet sequence C0 80 [...]
- ^ "The Java Virtual Machine Specification, 2nd Edition, section 4.4.7: "The CONSTANT_Utf8_info Structure"". Sun Microsystems. 1999. Retrieved May 24, 2009.
[...] Java virtual machine UTF-8 strings never have embedded nulls.
- ^ "Java Object Serialization Specification, chapter 6: Object Serialization Stream Protocol, section 2: Stream Elements". Sun Microsystems. 2005. Retrieved May 22, 2009.
[...] encoded in modified UTF-8.
- ^ "Java Native Interface Specification, chapter 3: JNI Types and Data Structures, section: Modified UTF-8 Strings". Sun Microsystems. 2003. Retrieved May 22, 2009.
The JNI uses modified UTF-8 strings to represent various string types.
- ^ "The Java Virtual Machine Specification, 2nd Edition, section 4.4.7: "The CONSTANT_Utf8_info Structure"". Sun Microsystems. 1999. Retrieved May 23, 2009.
[...] differences between this format and the "standard" UTF-8 format.
- ^ "dex — Dalvik Executable Format". Retrieved April 9, 2013.
[T]he dex format encodes its string data in a de facto standard modified UTF-8 form, hereafter referred to as MUTF-8.
- ^ "Tcler's Wiki: UTF-8 bit by bit (Revision 6)". April 25, 2009. Retrieved May 22, 2009.
In orthodox UTF-8, a NUL byte(\x00) is represented by a NUL byte. [...] But [...] we [...] want NUL bytes inside [...] strings [...]
- ^ "Computational Methods in Linguistic Research". University of Pennsylvania Department of Linguistics. 2004. Retrieved February 8, 2013.
- ^ "The Unicode Standard - Chapter 2" (PDF). p. 30.
- ^ "Extensible Markup Language (XML) 1.0 (Fifth Edition)". W3C. 26 November 26, 2008. Retrieved February 8, 2013.
{{cite web}}: Check date values in:|date=(help) - ^ Dürst, Martin. "Multilingual Forms". W3C. Retrieved February 8, 2013.
- ^ There are 256 × 256 − 128 × 128 = 49152 2-byte sequences with at least one high bit set, only 1920 encode valid UTF-8 characters (the range U+0080 to U+07FF), 1920 ÷ 49152 = 3.9%
- ^ Although the difference may not be great: the 2010-11-22 version of hi:यूनिकोड (Unicode in Hindi), when the pure text was pasted to Notepad, generated 19 KB when saved as UTF-16 and 22 KB when saved as UTF-8.
- ^ The 2010-10-27 version of ja:UTF-8 generated 169 KB when converted with Notepad to UTF-16, and only 101 KB when converted back to UTF-8. The 2010-11-22 version of hi:यूनिकोड (Unicode in Hindi) required 119 KB in UTF-16 and 76 KB in UTF-8.
External links
There are several current definitions of UTF-8 in various standards documents:
- RFC 3629 / STD 63 (2003), which establishes UTF-8 as a standard Internet protocol element
- The Unicode Standard, Version 6.0, §3.9 D92, §3.10 D95 (2011)
- ISO/IEC 10646:2003 Annex D (2003)
They supersede the definitions given in the following obsolete works:
- ISO/IEC 10646-1:1993 Amendment 2 / Annex R (1996)
- The Unicode Standard, Version 5.0, §3.9 D92, §3.10 D95 (2007)
- The Unicode Standard, Version 4.0, §3.9–§3.10 (2003)
- The Unicode Standard, Version 2.0, Appendix A (1996)
- RFC 2044 (1996)
- RFC 2279 (1998)
- The Unicode Standard, Version 3.0, §2.3 (2000) plus Corrigendum #1 : UTF-8 Shortest Form (2000)
- Unicode Standard Annex #27: Unicode 3.1 (2001)
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.
- Original UTF-8 paper (or pdf) for Plan 9 from Bell Labs
- RFC 5198 defines UTF-8 NFC for Network Interchange
- UTF-8 test pages by Andreas Prilop, Jost Gippert and the World Wide Web Consortium
- Unix/Linux: UTF-8/Unicode FAQ, Linux Unicode HOWTO, UTF-8 and Gentoo
- The Unicode/UTF-8-character table displays UTF-8 in a variety of formats (with Unicode and HTML encoding information)
- Clear introduction to UTF-8 by Tom Scott
| Early telecommunications | |
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| ISO/IEC 8859 |
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| Bibliographic use | |
| National standards | |
| ISO/IEC 2022 | |
| Mac OS Code pages ("scripts") | |
| DOS code pages | |
| IBM AIX code pages | |
| Windows code pages | |
| EBCDIC code pages | |
| DEC terminals (VTx) | |
| Platform specific |
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| Unicode / ISO/IEC 10646 | |
| TeX typesetting system | |
| Miscellaneous code pages | |
| Control character | |
| Related topics | |