UTF-8

UTF-8 (UCS Transformation Format — 8-bit) 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 (compare UTF-16 and UTF-32).

For these and other reasons, UTF-8 has become the dominant character encoding for the World-Wide Web, accounting for more than half of all Web pages. 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. The Internet Mail Consortium (IMC) recommends that all e-mail programs be able to display and create mail using UTF-8. 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 in practice) are encoded using fewer bytes, making the encoding scheme moderately efficient. In particular, the first 128 characters of the Unicode character set, which correspond one-to-one with ASCII, are encoded using a single octet with the same binary value as the corresponding ASCII character, making valid ASCII text valid UTF-8-encoded Unicode text as well.

The official IANA code for the UTF-8 character encoding is UTF-8.

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 ASCII range of 0–127 represent themselves in UTF, thereby providing backward compatibility.

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.

UTF-8 was first officially presented at the USENIX conference in San Diego, from January 25–29, 1993.

The original specification allowed for sequences of up to six bytes, covering numbers up to 31 bits (the original limit of the Universal Character Set). In November 2003 UTF-8 was restricted by RFC 3629 to four bytes covering only the range U+0000 to U+10FFFF, in order to match the constraints of the UTF-16 character encoding.

Design

The design of UTF‑8 as originally proposed by Dave Prosser and subsequently modified by Ken Thompson was intended to satisfy two objectives:

1. To be backward-compatible with ASCII; and
2. To enable encoding of up to at least 2³¹ characters (the theoretical limit of the first draft proposal for the Universal Character Set).

Being backward-compatible with ASCII implied that every valid ASCII character (a 7-bit character set) also be a valid UTF‑8 character sequence, specifically, a one-byte UTF‑8 character sequence whose binary value equals that of the corresponding ASCII character:

Bits	Last code point	Byte 1
7	U+007F	0xxxxxxx

Prosser's and Thompson's challenge was to extend this scheme to handle code points with up to 31 bits. The solution proposed by Prosser as subsequently modified by Thompson was as follows:

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:

1. Every valid ASCII character is also a valid UTF‑8 encoded Unicode character with the same binary value. (Thus, valid ASCII text is also valid UTF‑8-encoded Unicode text.)
2. For every UTF‑8 byte sequence corresponding to a single Unicode character, the first byte unambiguously indicates the length of the sequence in bytes.
3. All continuation bytes (byte nos. 2–6 in the table above) have 10 as their two most-significant bits (bits 7–6); in contrast, the first byte never has 10 as its two most-significant bits. As a result, it is immediately obvious whether any given byte anywhere in a (valid) UTF‑8 stream represents the first byte of a byte sequence corresponding to a single character, or a continuation byte of such a byte sequence.
4. As a consequence of no. 3 above, starting with any arbitrary byte anywhere in a (valid) UTF‑8 stream, it is necessary to back up by only at most five bytes in order to get to the beginning of the byte sequence corresponding to a single character (three bytes in actual UTF‑8 as explained in the next section). If it is not possible to back up, or a byte is missing because of e.g. a communication failure, one single character can be discarded, and the next character be correctly read.
5. Starting with the second row in the table above (two bytes), every additional byte extends the maximum number of bits by five (six additional bits from the additional continuation byte, minus one bit lost in the first byte).
6. Prosser's and Thompson's scheme was sufficiently general to be extended beyond 6-byte sequences (however, this would have allowed FE or FF bytes to occur in valid UTF-8 text—see under Advantages in section "Compared to single byte encodings" below—and indefinite extension would lose the desirable feature that the length of a sequence can be determined from the start byte only).

Description

UTF-8 is a variable-width encoding, with each character represented by one to four bytes. If the character is encoded by just one byte, the high-order bit is 0 and the other bits give the code value (in the range 0..127). If the character is encoded by a sequence of more than one byte, the first byte has as many leading "1" bits as the total number of bytes in the sequence, followed by a "0" bit, and the succeeding bytes are all marked by a leading "10" bit pattern. The remaining bits in the byte sequence are concatenated to form the Unicode code point value (in the range 80_hex to 10FFFF_hex). Thus a byte with lead bit "0" is a single-byte code, a byte with multiple leading "1" bits is the first of a multi-byte sequence, and a byte with a leading "10" bit pattern is a continuation byte of a multi-byte sequence. The format of the bytes thus allows the beginning of each sequence to be detected without decoding from the beginning of the string. UTF-16 limits Unicode to 10FFFF_hex; therefore UTF-8 is not defined beyond that value, even if it could easily be defined to reach 7FFFFFFF_hex.

Code point range	Binary code point	UTF-8 bytes	Example
U+0000 to U+007F	0xxxxxxx	0xxxxxxx	character "$" = code point U+0024 = 00100100 → 00100100 → hexadecimal 24
U+0080 to U+07FF	00000yyy yyxxxxxx	110yyyyy 10xxxxxx	character "¢" = code point U+00A2 = 00000000 10100010 → 11000010 10100010 → hexadecimal C2 A2
U+0800 to U+FFFF	zzzzyyyy yyxxxxxx	1110zzzz 10yyyyyy 10xxxxxx	character "€" = code point U+20AC = 00100000 10101100 → 11100010 10000010 10101100 → hexadecimal E2 82 AC
U+010000 to U+10FFFF	000wwwzz zzzzyyyy yyxxxxxx	11110www 10zzzzzz 10yyyyyy 10xxxxxx	character "𤭢" = code point U+024B62 = 00000010 01001011 01100010 → 11110000 10100100 10101101 10100010 → hexadecimal F0 A4 AD A2

So the first 128 characters (US-ASCII) need one byte. The next 1,920 characters need two bytes to encode. This includes Latin letters with diacritics and characters from the Greek, Cyrillic, Coptic, Armenian, Hebrew, Arabic, Syriac and Tāna alphabets. Three bytes are needed for 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.

Codepage layout

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
C_	2 (0000) 192	2 (0040) 193	2 0080 194	2 00C0 195	2 0100 196	2 0140 197	2 0180 198	2 01C0 199	2 0200 200	2 0240 201	2 0280 202	2 02C0 203	2 0300 204	2 0340 205	2 0380 206	2 03C0 207
D_	2 0400 208	2 0440 209	2 0480 210	2 04C0 211	2 0500 212	2 0540 213	2 0580 214	2 05C0 215	2 0600 216	2 0640 217	2 0680 218	2 06C0 219	2 0700 220	2 0740 221	2 0780 222	2 07C0 223
E_	3 0800* 224	3 1000 225	3 2000 226	3 3000 227	3 4000 228	3 5000 229	3 6000 230	3 7000 231	3 8000 232	3 9000 233	3 A000 234	3 B000 235	3 C000 236	3 D000 237	3 E000 238	3 F000 239
F_	4 10000* 240	4 40000 241	4 80000 242	4 C0000 243	4 100000 244	4 140000 245	4 180000 246	4 1C0000 247	5 200000* 248	5 1000000 249	5 2000000 250	5 3000000 251	6 4000000* 252	6 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 containing a large single-digit number are the start bytes for a sequence of that many bytes. The unbolded hexadecimal code point number 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.

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
• a sequence that decodes to a value that should use a shorter sequence (an "overlong form").

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 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."

Many UTF-8 decoders throw exceptions on encountering errors, 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, 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:

• The replacement character "�" (U+FFFD)
• The symbol for substitute "␦" (U+2426) (ISO 2047)
• The "?" or "¿" character (U+003F or U+00BF)
• The invalid Unicode code points U+DC80..U+DCFF where the low 8 bits are the byte's value.
• Interpret the bytes according to another encoding (often ISO-8859-1 or CP1252).

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.

Invalid code points

UTF-8 may only legally be used to encode valid Unicode scalar values. According to the Unicode standard the high and low surrogate halves used by UTF-16 (U+D800 through U+DFFF) and values above U+10FFFF are not legal Unicode values, and the UTF-8 encoding of them is an invalid byte sequence and should be treated as described above.^{[citation needed]}

Whether an actual application should do this with surrogate halves is debatable. 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.

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 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), as the declaration is case insensitive.

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. 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'

Derivatives

The following implementations show slight differences from the UTF-8 specification. They are incompatible with the UTF-8 specification.

CESU-8

Main article: CESU-8

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.

Modified UTF-8

In Modified UTF-8, the null character (U+0000) is encoded as 0xC0,0x80; this is not valid UTF-8 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, 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, for the Java Native Interface, and for embedding constant strings in class files. Tcl also uses the same modified UTF-8 as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.

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. The BOM can also appear if another encoding with a BOM is translated to UTF-8 without stripping it.

The presence of the UTF-8 BOM may cause interoperability problems with existing software that could otherwise handle UTF-8; for example:

• Older text editors may display the BOM as "" at the start of the document, even if the UTF-8 file contains only ASCII and would otherwise display correctly.
• 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. Or conversely they will identify the file when the user cannot handle the BOM. An example is the Unix shebang syntax.
• Programs that insert information at the start of a file will result in a file with the BOM somewhere in the middle of it (this is also a problem with the UTF-16 BOM). One example is offline browsers that add the originating URL to the start of the file.

If compatibility with existing programs is not important, the BOM could be used to identify if a file is in UTF-8 versus a legacy encoding, but this is still problematic, due to many instances where the BOM is added or removed without actually changing the encoding, or various encodings are concatenated together. Checking if the text is valid UTF-8 is more reliable than using BOM. The Unicode standard recommends against the BOM for UTF-8

Advantages and disadvantages

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.
• 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. The chance of a random string of bytes being valid UTF-8 and not pure ASCII is 3.9% for a two-byte sequence, 0.41% for a three-byte sequence and 0.026% for a four-byte sequence. 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 of UTF-8 strings as arrays of unsigned bytes will produce the same results as sorting them based on Unicode code points.
• 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.

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 languages at the same time. For many languages there has been more than one single-byte encoding in usage, so even knowing the language 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 (\377) also eliminates the need to escape this byte in Telnet (and FTP control connection).

Disadvantages

• UTF-8 encoded text is larger than the appropriate single-byte encoding except for plain ASCII characters. In the case of languages which used 8-bit character sets with non-Latin alphabets encoded in the upper half (such as most Cyrillic and Greek alphabet code pages), letters in UTF-8 will be double the size. For some languages such as Thai and Hindi's Devanagari, letters will be triple the size (this has caused objections in India and other countries).
• It is possible in UTF-8 (or any other multi-byte encoding) to split a string in the middle of a character, which may result in an invalid string if the pieces are not concatenated later.
• 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]}.

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 languages 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 found when searching either forwards or backwards. If bytes are lost due to error or corruption, one can always locate the beginning of the next character and thus limit the damage. 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 languages 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 is often smaller than (or the same size as) the same text encoded in UTF-16.
  • This is always true for text using only code points below U+0800 (which includes all modern European languages), as each code point's UTF-8 encoding is one or two bytes then.
  • Even if text contains code points not below U+0800, it might contain so many code points below U+0080 (which UTF-8 encodes in one byte) that the UTF-8 encoding is still smaller. As HTML markup and line terminators are code points below U+0080, most HTML source is smaller if encoded in UTF-8 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

• A simplistic parser for UTF-16 is unlikely to convert invalid sequences to ASCII.^{[citation needed]} Since the dangerous characters in most situations are ASCII, a simplistic UTF-16 parser is much less dangerous than a simplistic UTF-8 parser.
• 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, but rarely for HTML documents. For example, both the Japanese UTF-8 and the Hindi Unicode articles on Wikipedia take more space in UTF-16 than in UTF-8 .
• In UCS-2 (but not UTF-16) Unicode code points are all the same size, making measurements of a fixed number of them easy. Due to ASCII-era documentation where "character" is used as a synonym for "byte", this is often considered important. Most UTF-16 implementations, including Windows, measure non-BMP characters as 2 units in UTF-16, as this is the only practical way to handle the strings. A similar variability in character size applies to UTF-8.

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

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
• How to configure e-mail clients to send UTF-8 text
• 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)
• Unicode and Multilingual Web Browsers from Alan Wood's Unicode Resources describes support and additional configuration of Unicode/UTF-8 in modern browsers
• JSP Wiki Browser Compatibility page details specific problems with UTF-8 in older browsers
• Mathematical Symbols in Unicode
• Graphical View of UTF-8 in ICU's Converter Explorer