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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 Engineering Task Force (IETF) requires all Internet protocols to identify the encoding used for character data, and the supported character encodings must include UTF-8.[5] The Internet Mail Consortium (IMC) recommends that all e-mail programs be able to display and create mail using UTF-8.[6] 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[7] 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 in practice) are encoded using fewer bytes.[8] 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
.[9]
Contents |
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.
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 crucial modification to the encoding to allow it to be self-synchronizing, meaning that it was not 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. 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.[10]
UTF-8 was first officially presented at the USENIX conference in San Diego, from January 25–29, 1993.
In November 2003 UTF-8 was restricted by RFC 3629 to four bytes in order to match the constraints of the UTF-16 character encoding.
The design of UTF‑8 is most easily seen in the table of the scheme as originally proposed by Dave Prosser and subsequently modified by Ken Thompson (the x
's are replaced by the bits of the code point):
Bits | Last code point | Byte 1 | Byte 2 | Byte 3 | Byte 4 | Byte 5 | Byte 6 |
---|---|---|---|---|---|---|---|
7 | U+007F | 0xxxxxxx |
|||||
11 | U+07FF | 110xxxxx |
10xxxxxx |
||||
16 | U+FFFF | 1110xxxx |
10xxxxxx |
10xxxxxx |
|||
21 | U+1FFFFF | 11110xxx |
10xxxxxx |
10xxxxxx |
10xxxxxx |
||
26 | U+3FFFFFF | 111110xx |
10xxxxxx |
10xxxxxx |
10xxxxxx |
10xxxxxx |
|
31 | U+7FFFFFFF | 1111110x |
10xxxxxx |
10xxxxxx |
10xxxxxx |
10xxxxxx |
10xxxxxx |
The salient features of the above scheme are as follows:
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). 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.
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.
Examples:
Character | Binary code point | Binary UTF-8 | Hexadecimal UTF-8 | |
---|---|---|---|---|
$ | U+0024 |
00100100 |
00100100 |
24 |
¢ | U+00A2 |
00000000 10100010 |
11000010 10100010 |
C2 A2 |
€ | U+20AC |
00100000 10101100 |
11100010 10000010 10101100 |
E2 82 AC |
𤭢 | U+24B62 |
00000010 01001011 01100010 |
11110000 10100100 10101101 10100010 |
F0 A4 AD A2 |
UTF-8 | ||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
_0 | _1 | _2 | _3 | _4 | _5 | _6 | _7 | _8 | _9 | _A | _B | _C | _D | _E | _F | |
0_ |
NUL 0000 0 |
SOH 0001 1 |
STX 0002 2 |
ETX 0003 3 |
EOT 0004 4 |
ENQ 0005 5 |
ACK 0006 6 |
BEL 0007 7 |
BS 0008 8 |
HT 0009 9 |
LF 000A 10 |
VT 000B 11 |
FF 000C 12 |
CR 000D 13 |
SO 000E 14 |
SI 000F 15 |
1_ |
DLE 0010 16 |
DC1 0011 17 |
DC2 0012 18 |
DC3 0013 19 |
DC4 0014 20 |
NAK 0015 21 |
SYN 0016 22 |
ETB 0017 23 |
CAN 0018 24 |
EM 0019 25 |
SUB 001A 26 |
ESC 001B 27 |
FS 001C 28 |
GS 001D 29 |
RS 001E 30 |
US 001F 31 |
2_ |
SP 0020 32 |
! 0021 33 |
" 0022 34 |
# 0023 35 |
$ 0024 36 |
% 0025 37 |
& 0026 38 |
' 0027 39 |
( 0028 40 |
) 0029 41 |
* 002A 42 |
+ 002B 43 |
, 002C 44 |
- 002D 45 |
. 002E 46 |
/ 002F 47 |
3_ |
0 0030 48 |
1 0031 49 |
2 0032 50 |
3 0033 51 |
4 0034 52 |
5 0035 53 |
6 0036 54 |
7 0037 55 |
8 0038 56 |
9 0039 57 |
: 003A 58 |
; 003B 59 |
< 003C 60 |
= 003D 61 |
> 003E 62 |
? 003F 63 |
4_ |
@ 0040 64 |
A 0041 65 |
B 0042 66 |
C 0043 67 |
D 0044 68 |
E 0045 69 |
F 0046 70 |
G 0047 71 |
H 0048 72 |
I 0049 73 |
J 004A 74 |
K 004B 75 |
L 004C 76 |
M 004D 77 |
N 004E 78 |
O 004F 79 |
5_ |
P 0050 80 |
Q 0051 81 |
R 0052 82 |
S 0053 83 |
T 0054 84 |
U 0055 85 |
V 0056 86 |
W 0057 87 |
X 0058 88 |
Y 0059 89 |
Z 005A 90 |
[ 005B 91 |
\ 005C 92 |
] 005D 93 |
^ 005E 94 |
_ 005F 95 |
6_ |
` 0060 96 |
a 0061 97 |
b 0062 98 |
c 0063 99 |
d 0064 100 |
e 0065 101 |
f 0066 102 |
g 0067 103 |
h 0068 104 |
i 0069 105 |
j 006A 106 |
k 006B 107 |
l 006C 108 |
m 006D 109 |
n 006E 110 |
o 006F 111 |
7_ |
p 0070 112 |
q 0071 113 |
r 0072 114 |
s 0073 115 |
t 0074 116 |
u 0075 117 |
v 0076 118 |
w 0077 119 |
x 0078 120 |
y 0079 121 |
z 007A 122 |
{ 007B 123 |
| 007C 124 |
} 007D 125 |
~ 007E 126 |
DEL 007F 127 |
8_ |
• +00 128 |
• +01 129 |
• +02 130 |
• +03 131 |
• +04 132 |
• +05 133 |
• +06 134 |
• +07 135 |
• +08 136 |
• +09 137 |
• +0A 138 |
• +0B 139 |
• +0C 140 |
• +0D 141 |
• +0E 142 |
• +0F 143 |
9_ |
• +10 144 |
• +11 145 |
• +12 146 |
• +13 147 |
• +14 148 |
• +15 149 |
• +16 150 |
• +17 151 |
• +18 152 |
• +19 153 |
• +1A 154 |
• +1B 155 |
• +1C 156 |
• +1D 157 |
• +1E 158 |
• +1F 159 |
A_ |
• +20 160 |
• +21 161 |
• +22 162 |
• +23 163 |
• +24 164 |
• +25 165 |
• +26 166 |
• +27 167 |
• +28 168 |
• +29 169 |
• +2A 170 |
• +2B 171 |
• +2C 172 |
• +2D 173 |
• +2E 174 |
• +2F 175 |
B_ |
• +30 176 |
• +31 177 |
• +32 178 |
• +33 179 |
• +34 180 |
• +35 181 |
• +36 182 |
• +37 183 |
• +38 184 |
• +39 185 |
• +3A 186 |
• +3B 187 |
• +3C 188 |
• +3D 189 |
• +3E 190 |
• +3F 191 |
2-byte C_ |
2-byte inval (0000) 192 |
2-byte inval (0040) 193 |
Latin-1 0080 194 |
Latin-1 00C0 195 |
Latin Ext-A 0100 196 |
Latin Ext-A 0140 197 |
Latin Ext-B 0180 198 |
Latin Ext-B 01C0 199 |
Latin Ext-B 0200 200 |
IPA 0240 201 |
IPA 0280 202 |
Spaci Modif 02C0 203 |
Combi Diacr 0300 204 |
Combi Diacr 0340 205 |
Greek 0380 206 |
Greek 03C0 207 |
2-byte D_ |
Cyril 0400 208 |
Cyril 0440 209 |
Cyril 0480 210 |
Cyril 04C0 211 |
Cyril 0500 212 |
Armen 0540 213 |
Hebrew 0580 214 |
Hebrew 05C0 215 |
Arabic 0600 216 |
Arabic 0640 217 |
Arabic 0680 218 |
Arabic 06C0 219 |
Syriac 0700 220 |
Arabic 0740 221 |
Thaana 0780 222 |
N'Ko 07C0 223 |
3-byte E_ |
Indic 0800* 224 |
Misc. 1000 225 |
Symbol 2000 226 |
Kana CJK 3000 227 |
CJK 4000 228 |
CJK 5000 229 |
CJK 6000 230 |
CJK 7000 231 |
CJK 8000 232 |
CJK 9000 233 |
Asian A000 234 |
Hangul B000 235 |
Hangul C000 236 |
Hangul Surr D000 237 |
Priv Use E000 238 |
Forms F000 239 |
4-byte F_ |
Ancient Sym,CJK 10000* 240 |
unall 40000 241 |
unall 80000 242 |
Tags Priv C0000 243 |
Priv Use 100000 244 |
4-byte inval 140000 245 |
4-byte inval 180000 246 |
4-byte inval 1C0000 247 |
5-byte inval 200000* 248 |
5-byte inval 1000000 249 |
5-byte inval 2000000 250 |
5-byte inval 3000000 251 |
6-byte inval 4000000* 252 |
6-byte inval 40000000 253 |
254 |
255 |
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 codepoint 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.
Not all sequences of bytes are valid UTF-8. A UTF-8 decoder should be prepared for:
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[11] and Apache's Tomcat servlet container.[12]
RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."[13] 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,[14] since such errors suggest the input is not a UTF-8 string at all. 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 contained invalid UTF-8,[15] so it was impossible to write a Python program that could handle such input.
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. Popular replacements are:
Replacing errors is "lossy": more than one UTF-8 string converts to the same Unicode result. Therefore the original UTF-8 should be stored, and translation should only be used when displaying the text to the user.
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 the UTF-8 encoding of them is an invalid byte sequence and thus should be treated as described above.
Whether an actual application should do this with surrogate halves is debatable.[who?] Allowing them allows lossless storage of invalid UTF-16, and allows CESU encoding (described below) to be decoded. There are other code points that are far more important to detect and reject, such as the reversed-BOM U+FFFE, or the C1 controls, caused by improper conversion of CP1252 text or double-encoding of UTF-8. These are invalid in HTML.
The official name is "UTF-8". All letters are upper-case, and the name is hyphenated. This spelling is used in all the 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),[16] as the declaration is case insensitive.[17]
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.[18] 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.
MySQL omits the hyphen in the following query:
SET NAMES 'utf8'
The following implementations show slight differences from the UTF-8 specification. They are incompatible with the UTF-8 specification.
Many pieces of software added UTF-8 conversions for UCS-2 data and did not alter their UTF-8 conversion when UCS-2 was replaced with the surrogate-pair supporting UTF-16. The result is that 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 for characters outside the Basic Multilingual Plane. Oracle databases use this, as well as Java and Tcl as described below, and probably a great deal of other Windows software where the programmers were unaware of the complexities of UTF-16. Although most usage is by accident, a supposed benefit is that this preserves UTF-16 binary sorting order when CESU-8 is binary sorted.
In Modified UTF-8,[19] the null character (U+0000) is encoded as 0xC0,0x80; this is not valid UTF-8[20] 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,[21] 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,[22] for the Java Native Interface,[23] and for embedding constant strings in class files.[24] Tcl also uses the same modified UTF-8[25] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.
Extending the accepted input pattern from 6 bytes to 7 bytes would allow over 70 billion code points to be encoded;[26] 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").
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. The BOM can also appear if another encoding with a BOM is translated to UTF-8 without stripping it. Older text editors may display the BOM as "" at the start of the document.
The Unicode standard recommends against the BOM for UTF-8.[27] The presence of the UTF-8 BOM may cause interoperability problems with existing software that could otherwise handle UTF-8; for example:
If compatibility with existing programs is not important, the BOM could be used to identify UTF-8 encoding. Because checking if text is valid UTF-8 is very reliable (the majority of random byte sequences are not valid UTF-8) such use should not be necessary. 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).
In Japan especially, "UTF-8 encoding without BOM" is sometimes called "UTF-8N".[citation needed]
This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (October 2009) |
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.
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