UTF-8

UTF-8 (8-bit Unicode Transformation Format) is a variable-width character encoding 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. The encoding is defined by the Unicode Standard, and was originally designed by Ken Thompson and Rob Pike.[2][3] The name is derived from Unicode (or Universal Coded Character Set) Transformation Format – 8-bit.[4]

UTF-8

Standard	Unicode Standard
Classification	Unicode Transformation Format, extended ASCII, variable-width encoding
Extends	US-ASCII
Transforms / Encodes	ISO 10646 (Unicode)
Preceded by	UTF-1

Use of the main encodings on the web from 2001 to 2012 as recorded by Google,[1] with UTF-8 overtaking all others in 2008 and over 60% of the web in 2012. The ASCII-only figure includes all web pages that only contain ASCII characters, regardless of the declared header.

It was designed for backward compatibility with ASCII. Code points with lower numerical values, 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 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 is by far the most common encoding for the World Wide Web, accounting for over 95% of all web pages, and up to 100% for some languages, as of 2020.[5]

Use

Since 2009, UTF-8 has been the most common encoding for the World Wide Web.[5] The World Wide Web Consortium recommends UTF-8 as the default encoding in XML and HTML,[6] UTF-8 is the recommendation from the WHATWG for HTML and DOM specifications,[7] and the Internet Mail Consortium recommends that all e-mail programs be able to display and create mail using UTF-8.[8][9]

UTF-8 accounts for 83% to 100% of all web pages as of July 2020, depending on language or country.[5] On average, it accounts for 95.3% of all web pages and 97% of the top 1,000 highest ranked web pages,[5] some of which are simply ASCII, as ASCII is a subset of UTF-8.

Several languages, such as Punjabi, Tagalog, Lao, Marathi, and Kannada have 100.0% use of UTF-8 on the web, while use for Japanese at 90.4% is the lowest among popular languages.[10] Arabic has 96.4% UTF-8 use on the web.[11]

The next-most popular multi-byte encoding is the Chinese standard GB 2312 and with their extension to it GBK (which are both interpreted by web browsers as GB 18030, having support for the same letters as UTF-8) have a combined 12.8% share in China[12][13] and a 0.4% share world-wide. Big5 is another popular Chinese encoding with 0.1% share world-wide. Those (except GB 18030 and UTF-16) and all other encodings were designed for specific languages, and do not support all Unicode characters. These other encodings are more storage-efficient for the specific languages they were designed for, such as the single-byte Windows-1251 is more efficient for Cyrillic script and is used for 10.8% of Russian web sites.[14] 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 Shift JIS has 0.2% global share).[15][16][1]

For local text files UTF-8 usage is lower, and many legacy single-byte encodings remain in use. This is primarily due to editors that will not display or write UTF-8 unless the first character in a file is a byte order mark, making it impossible for other software to use UTF-8 without being rewritten to ignore the byte order mark on input and add it on output. UTF-16 files are also fairly common on Windows, but not elsewhere.

Internally in software usage is even lower, with UCS-2 and UTF-32 in use, particularly in Windows but also Python, JavaScript, Qt, and many other software libraries. This is due to a belief that direct indexing of code points is more important than 8-bit compatibility. UTF-16 is also used due to being compatible with UCS-2, even though it does not have direct indexing. Microsoft now recommends UTF-8 for Windows programs,[17] while previously they emphasized "Unicode" (meaning UTF-16) Win32 API, this may mean internal use of UTF-8 will increase in the future.

Description

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
1	U+0000	U+007F	0xxxxxxx
2	U+0080	U+07FF	110xxxxx	10xxxxxx
3	U+0800	U+FFFF	1110xxxx	10xxxxxx	10xxxxxx
4	U+10000	[18]U+10FFFF	11110xxx	10xxxxxx	10xxxxxx	10xxxxxx

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,[19] 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).

Some of the important features of this encoding are as follows:

• Backward compatibility: Backwards compatibility with ASCII and the enormous amount of software designed to process ASCII-encoded text was the main driving force behind the design of UTF-8. In UTF-8, single bytes with values in the range of 0 to 127 map directly to Unicode code points in the ASCII range. Single bytes in this range represent characters, as they do in ASCII. Moreover, 7-bit bytes (bytes where the most significant bit is 0) never appear in a multi-byte sequence, and no valid multi-byte sequence decodes to an ASCII code-point. A sequence of 7-bit bytes is both valid ASCII and valid UTF-8, and under either interpretation represents the same sequence of characters. Therefore, the 7-bit bytes in a UTF-8 stream represent all and only the ASCII characters in the stream. Thus, many text processors, parsers, protocols, file formats, text display programs, etc., which use ASCII characters for formatting and control purposes, will continue to work as intended by treating the UTF-8 byte stream as a sequence of single-byte characters, without decoding the multi-byte sequences. ASCII characters on which the processing turns, such as punctuation, whitespace, and control characters will never be encoded as multi-byte sequences. It is therefore safe for such processors to simply ignore or pass-through the multi-byte sequences, without decoding them. For example, ASCII whitespace may be used to tokenize a UTF-8 stream into words; ASCII line-feeds may be used to split a UTF-8 stream into lines; and ASCII NUL characters can be used to split UTF-8-encoded data into null-terminated strings. Similarly, many format strings used by library functions like "printf" will correctly handle UTF-8-encoded input arguments.
• Fallback and auto-detection: UTF-8 provided backwards compatibility for 7-bit ASCII, but much software and data uses 8-bit extended ASCII encodings designed prior to the adoption of Unicode to represent the character sets of European languages. Part of the popularity of UTF-8 is due to the fact that it provides a form of backward compatibility for these as well. A UTF-8 processor which erroneously receives an extended ASCII file as input can "fall back" or replace 8-bit bytes using the appropriate code-point in the Unicode Latin-1 Supplement block, when the 8-bit byte appears outside a valid multi-byte sequence. The bytes in extended ASCII encodings of “real world” text are typically not legal UTF-8 multi-byte sequences. This is because the bytes which introduce multi-byte sequences in UTF-8 are primarily accented letters (mostly vowels) in the common extended ASCII encodings, and the UTF-8 continuation bytes are punctuation and symbol characters. To appear as a valid UTF-8 multi-byte sequence, a series of 2 to 4 extended ASCII 8-bit characters would have to be an unusual combination of symbols and accented letters (such as an accented vowel followed immediately by certain punctuation). In short, real-world extended ASCII character sequences which look like valid UTF-8 multi-byte sequences are unlikely. Fallback errors will be false negatives, and these will be rare. Moreover, in many applications, such as text display, the consequence of incorrect fallback is usually slight. Only legibility is affected, and only for a few characters. These two things make fallback feasible, if somewhat imperfect. Indeed, as discussed further below, the HTML5 standard requires that erroneous bytes in supposed UTF-8 data be replaced upon display on the assumption that they are Windows-1252 characters. The presence of invalid 8-bit characters outside valid multi-byte sequences can also be used to "auto-detect" that an encoding is actually an extended ASCII encoding rather than UTF-8, and decode it accordingly. A UTF-8 stream may simply contain errors, resulting in the auto-detection scheme producing false positives; but auto-detection is successful in the majority of cases, especially with longer texts, and is widely used.
• Prefix code: The first byte indicates the number of bytes in the sequence. Reading from a stream can instantaneously decode each individual fully received sequence, without first having to wait for either the first byte of a next sequence or an end-of-stream indication. The length of multi-byte sequences is easily determined by humans as it is simply the number of high-order 1s in the leading byte. An incorrect character will not be decoded if a stream ends mid-sequence.
• Self-synchronization: The leading bytes and the continuation bytes do not share values (continuation bytes start with the bits 10 while single bytes start with 0 and longer lead bytes start with 11). This means a search will not accidentally find the sequence for one character starting in the middle of another character. It also means the start of a character can be found from a random position by backing up at most 3 bytes to find the leading byte. An incorrect character will not be decoded if a stream starts mid-sequence, and a shorter sequence will never appear inside a longer one.
• Sorting order: The chosen values of the leading bytes means that a list of UTF-8 strings can be sorted in code point order by sorting the corresponding byte sequences.

Examples

Consider the encoding of the Euro sign, €:

1. The Unicode code point for "€" is U+20AC.
2. According to the scheme table above, this will take three bytes to encode, since it is between U+0800 and U+FFFF.
3. Hexadecimal 20AC is binary 0010 0000 1010 1100. The two leading zeros are added because, as the scheme table shows, a three-byte encoding needs exactly sixteen bits from the code point.
4. Because the encoding will be three bytes long, its leading byte starts with three 1s, then a 0 (1110...)
5. The four most significant bits of the code point are stored in the remaining low order four bits of this byte (1110 0010), leaving 12 bits of the code point yet to be encoded (...0000 1010 1100).
6. All continuation bytes contain exactly six bits from the code point. So the next six bits of the code point are stored in the low order six bits of the next byte, and 10 is stored in the high order two bits to mark it as a continuation byte (so 1000 0010).
7. Finally the last six bits of the code point are stored in the low order six bits of the final byte, and again 10 is stored in the high order two bits (1010 1100).

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.

Character		Code point		UTF-8
		Octal	Binary	Binary	Octal	Hexadecimal
$	U+0024	044	010 0100	00100100	044	24
¢	U+00A2	0242	000 1010 0010	11000010 10100010	302 242	C2 A2
ह	U+0939	004471	0000 1001 0011 1001	11100000 10100100 10111001	340 244 271	E0 A4 B9
€	U+20AC	020254	0010 0000 1010 1100	11100010 10000010 10101100	342 202 254	E2 82 AC
한	U+D55C	152534	1101 0101 0101 1100	11101101 10010101 10011100	355 225 234	ED 95 9C
𐍈	U+10348	0201510	0 0001 0000 0011 0100 1000	11110000 10010000 10001101 10001000	360 220 215 210	F0 90 8D 88

Since UTF-8 uses groups of six bits, it is sometimes useful to use octal notation which uses 3-bit groups. With a calculator which can convert between hexadecimal and octal it can be easier to manually create or interpret UTF-8 compared with using binary.

• Octal 0–177 (hex 0–7F) is coded with an unchanged single byte.
• Octal 0200–3777 (hex 80–7FF) shall be coded with two bytes. xxyy will be 3xx 2yy.
• Octal 4000–77777 (hex 800–7FFF) shall be coded with three bytes. xyyzz will be 34x 2yy 2zz.
• Octal 100000–177777 (hex 8000–FFFF) shall be coded with three bytes. 1xyyzz will be 35x 2yy 2zz.
• Octal 200000–4177777 (hex 10000–10FFFF) shall be coded with four bytes. wxxyyzz will be 36w 2xx 2yy 2zz.

When converting UTF-8 into code points, the following rules apply:

• Octal 302–337 is the first of two bytes. 3xx 2yy will be xxyy in octal.
• Octal 340–347 is the first of three bytes. 34x 2yy 2zz will be xyyzz.
• Octal 350–357 is the first of three bytes. 35x 2yy 2zz will be 1xyyzz.
• Octal 360–364 is the first of four bytes. 36w 2xx 2yy 2zz will be wxxyyzz.
• Octal 200–277 are continuation bytes, and others beginning with 3 are invalid.

Codepage layout

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.

UTF-8

	_0	_1	_2	_3	_4	_5	_6	_7	_8	_9	_A	_B	_C	_D	_E	_F
0_	NUL 0000	SOH 0001	STX 0002	ETX 0003	EOT 0004	ENQ 0005	ACK 0006	BEL 0007	BS 0008	HT 0009	LF 000A	VT 000B	FF 000C	CR 000D	SO 000E	SI 000F
1_	DLE 0010	DC1 0011	DC2 0012	DC3 0013	DC4 0014	NAK 0015	SYN 0016	ETB 0017	CAN 0018	EM 0019	SUB 001A	ESC 001B	FS 001C	GS 001D	RS 001E	US 001F
2_	SP 0020	! 0021	" 0022	# 0023	$ 0024	% 0025	& 0026	' 0027	( 0028	) 0029	* 002A	+ 002B	, 002C	- 002D	. 002E	/ 002F
3_	0 0030	1 0031	2 0032	3 0033	4 0034	5 0035	6 0036	7 0037	8 0038	9 0039	: 003A	; 003B	< 003C	= 003D	> 003E	? 003F
4_	@ 0040	A 0041	B 0042	C 0043	D 0044	E 0045	F 0046	G 0047	H 0048	I 0049	J 004A	K 004B	L 004C	M 004D	N 004E	O 004F
5_	P 0050	Q 0051	R 0052	S 0053	T 0054	U 0055	V 0056	W 0057	X 0058	Y 0059	Z 005A	[ 005B	\ 005C	] 005D	^ 005E	_ 005F
6_	` 0060	a 0061	b 0062	c 0063	d 0064	e 0065	f 0066	g 0067	h 0068	i 0069	j 006A	k 006B	l 006C	m 006D	n 006E	o 006F
7_	p 0070	q 0071	r 0072	s 0073	t 0074	u 0075	v 0076	w 0077	x 0078	y 0079	z 007A	{ 007B	| 007C	} 007D	~ 007E	DEL 007F
8_	• +00	• +01	• +02	• +03	• +04	• +05	• +06	• +07	• +08	• +09	• +0A	• +0B	• +0C	• +0D	• +0E	• +0F
9_	• +10	• +11	• +12	• +13	• +14	• +15	• +16	• +17	• +18	• +19	• +1A	• +1B	• +1C	• +1D	• +1E	• +1F
A_	• +20	• +21	• +22	• +23	• +24	• +25	• +26	• +27	• +28	• +29	• +2A	• +2B	• +2C	• +2D	• +2E	• +2F
B_	• +30	• +31	• +32	• +33	• +34	• +35	• +36	• +37	• +38	• +39	• +3A	• +3B	• +3C	• +3D	• +3E	• +3F
2 C_	2 0000	2 0040	Latin 0080	Latin 00C0	Latin 0100	Latin 0140	Latin 0180	Latin 01C0	Latin 0200	IPA 0240	IPA 0280	IPA 02C0	accents 0300	accents 0340	Greek 0380	Greek 03C0
2 D_	Cyril 0400	Cyril 0440	Cyril 0480	Cyril 04C0	Cyril 0500	Armeni 0540	Hebrew 0580	Hebrew 05C0	Arabic 0600	Arabic 0640	Arabic 0680	Arabic 06C0	Syriac 0700	Arabic 0740	Thaana 0780	N'Ko 07C0
3 E_	Indic 0800	Misc. 1000	Symbol 2000	Kana… 3000	CJK 4000	CJK 5000	CJK 6000	CJK 7000	CJK 8000	CJK 9000	Asian A000	Hangul B000	Hangul C000	Hangul D000	PUA E000	Forms F000
4 F_	SMP… 10000	񀀀 40000	򀀀 80000	SSP… C0000	SPU… 100000	4 140000	4 180000	4 1C0000	5 200000	5 1000000	5 2000000	5 3000000	6 4000000	6 40000000

     Blue cells are 7-bit (single-byte) sequences. They must not be followed by a continuation byte.

     Orange cells with a large dot are continuation bytes. The hexadecimal number shown after the + symbol is the value of the 6 bits they add.

     White cells are the leading 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 leading byte.

     Red cells must never appear in a valid UTF-8 sequence. The first two red cells (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), and FE and FF were never defined for any purpose in UTF-8.

     Pink cells are the leading bytes for a sequence of multiple bytes, of which some, but not all, possible continuation sequences are valid. 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.

Overlong encodings

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 – 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 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. This ensures that string comparisons and searches are well-defined.

Modified UTF-8 uses the two-byte overlong encoding of U+0000 (the NUL character) as 11000000 10000000 (hexadecimal C0 80) instead of 00000000 (hexadecimal 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 non-continuation byte before the end of the character
• the string ending before the end of the character (which can happen in simple string truncation)
• an overlong encoding as described above
• a sequence that decodes to an invalid code point as described below

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[20] and Apache's Tomcat servlet container.[21] RFC 3629 states "Implementations of the decoding algorithm MUST protect against decoding invalid sequences."[22] 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."

Some implementations of decoders throw exceptions on errors.[23] 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.[24]

An alternative practice is to replace errors with a replacement character. Replacement requires defining how many bytes are in the error:

• Using the length defined by the lead byte (so that E1,A0,C0 is one error) is a bad idea, as a missing byte will cause the next character to become an error as well.
• Since Unicode 6[25] (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.[26] The standard also recommends replacing each error with the replacement character "�" (U+FFFD).
• Another popular practice is to turn each byte into an error. In this case E1,A0,C0 is three errors. This requires the UTF-8 parser to back up 2 bytes when an error is detected. The primary advantage is that there are now only 128 different errors. This allows the decoder to define 128 different error replacements such as:
  • The invalid Unicode code points U+DC80–U+DCFF where the low eight bits are the byte's value.[27] Sometimes it is called UTF-8B.[28] This has the nice property that it makes invalid UTF-8 into invalid UTF-16.
  • The Unicode code points U+0080–U+00FF with the same value as the byte, thus interpreting the bytes according to ISO-8859-1. Care must be taken so that the C1 control codes such as NEL 0x0085 do not cause further code to misbehave.
  • The Unicode code point for the character represented by the byte in CP1252, which is similar to using ISO-8859-1, except most bytes in the range 0x80–0x9F turn into printing characters. For example, 0x80 becomes the Euro sign, U+20AC. This makes text where legacy encodings are mixed with UTF-8 readable, and thus it is commonly done in browsers.

The large number of invalid byte sequences provides the advantage of making it easy to have a program accept both UTF-8 and legacy encodings such as ISO-8859-1. Software can check for UTF-8 correctness, and if that fails assume the input to be in the legacy encoding. It is technically true that this may detect an ISO-8859-1 string as UTF-8, but this is very unlikely if it contains any 8-bit bytes as they all have to be in unusual patterns of two or more in a row, such as "£".

Invalid code points

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. To preserve these invalid UTF-16 sequences, their corresponding UTF-8 encodings are sometimes allowed by implementations despite the above rule. There are attempts to define this behavior formally (see WTF-8 and CESU below).

Byte order mark

Main article: Byte order mark

Many Windows programs (including older versions of 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 byte order is irrelevant to UTF-8. While ASCII text encoded using UTF-8 normally is backwards compatible with ASCII, this is not true when Unicode Standard recommendations are ignored and a BOM is added. Non-UTF-8 software may show the BOM as three garbage characters, e.g. "" in software interpreting the document as ISO 8859-1 or Windows-1252, and "" if interpreted as code page 437. This is an example of mojibake, the output of garbled text when text is decoded using an unintended character encoding.

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 transcoded from another encoding.[29] The presence of the UTF-8 BOM may cause problems with existing software that can 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 UTF-8 BOM at the start of the file.
• Programs that identify file types by leading characters may fail to identify the file if a UTF-8 BOM is present even if the user of the file can handle the BOM. An example is the Unix shebang syntax, another is Internet Explorer which will render pages in standards mode only when it starts with a document type declaration.
• Programs that insert information at the start of a file will break use of the BOM to identify UTF-8 (one example is offline browsers that add the originating URL to the start of the file.)

Nonetheless, some software such as text editors will refuse to correctly display or interpret UTF-8 unless the text starts with a byte order mark or it contains only ASCII.

Official name and variants

The official Internet Assigned Numbers Authority (IANA) code for the encoding is "UTF-8".[30] 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 IANA list (which include CSS, HTML, XML, and HTTP headers),[31] as the declaration is case insensitive.[30]

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.[22] 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.[32]

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".[33][34]

Supported Windows versions, i.e. Windows 7 and later, have codepage 65001, as a synonym for UTF-8 (with better support than in older Windows),[35] and Microsoft has a script for Windows 10, to enable it by default for its program Microsoft Notepad.[36]

In PCL, UTF-8 is called Symbol-ID "18N" (PCL supports 183 character encodings, called Symbol Sets, which potentially could be reduced to one, 18N, that is UTF-8).[37]

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, 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.

UTF-1

Number of bytes	First code point	Last code point	Byte 1	Byte 2	Byte 3	Byte 4	Byte 5
1	U+0000	U+009F	00–9F
2	U+00A0	U+00FF	A0	A0–FF
2	U+0100	U+4015	A1–F5	21–7E, A0–FF
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.[38][39][40][41]

FSS-UTF proposal (1992)

Number of bytes	First code point	Last code point	Byte 1	Byte 2	Byte 3	Byte 4	Byte 5
1	U+0000	U+007F	0xxxxxxx
2	U+0080	U+207F	10xxxxxx	1xxxxxxx
3	U+2080	U+8207F	110xxxxx	1xxxxxxx	1xxxxxxx
4	U+82080	U+208207F	1110xxxx	1xxxxxxx	1xxxxxxx	1xxxxxxx
5	U+2082080	U+7FFFFFFF	11110xxx	1xxxxxxx	1xxxxxxx	1xxxxxxx	1xxxxxxx

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.[40]

FSS-UTF (1992) / UTF-8 (1993)[2]

Number of bytes	First code point	Last code point	Byte 1	Byte 2	Byte 3	Byte 4	Byte 5	Byte 6
1	U+0000	U+007F	0xxxxxxx
2	U+0080	U+07FF	110xxxxx	10xxxxxx
3	U+0800	U+FFFF	1110xxxx	10xxxxxx	10xxxxxx
4	U+10000	U+1FFFFF	11110xxx	10xxxxxx	10xxxxxx	10xxxxxx
5	U+200000	U+3FFFFFF	111110xx	10xxxxxx	10xxxxxx	10xxxxxx	10xxxxxx
6	U+4000000	U+7FFFFFFF	1111110x	10xxxxxx	10xxxxxx	10xxxxxx	10xxxxxx	10xxxxxx

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.[42]

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.

Google reported that in 2008, UTF-8 (labelled "Unicode") became the most common encoding for HTML files.[43] By 2018, most languages have use of UTF-8 up in the low to high 90%, including Greek at 97.1%.[44] A few have even 100.0% use such as Kurdish, Pashto, Javanese, Kalaallisut (Greenlandic) and Iranian languages[45] and Sign Languages.[46] Exceptions include mainly Asian languages with Chinese at 88.0%,[47] Japanese at 86.7% (while Mongolian is at 99.7%[48]) and Breton at 70%.[49]

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",[50] including the correct handling of "illegal UTF-8".[51]

Standards

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
• RFC 5198 defines UTF-8 NFC for Network Interchange (2008)
• ISO/IEC 10646:2014 §9.1 (2014)[52]
• The Unicode Standard, Version 11.0 (2018)[53]

They supersede the definitions given in the following obsolete works:

• The Unicode Standard, Version 2.0, Appendix A (1996)
• ISO/IEC 10646-1:1993 Amendment 2 / Annex R (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)[54]
• The Unicode Standard, Version 5.0 (2006)[55]
• The Unicode Standard, Version 6.0 (2010)[56]

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.

Comparison with single-byte encodings

• 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).
• UTF-8 encoded text is larger than specialized single-byte encodings except for plain ASCII characters. In the case of scripts which used 8-bit character sets with non-Latin characters encoded in the upper half (such as most Cyrillic and Greek alphabet code pages), characters in UTF-8 will be double the size. For some scripts, such as Thai and Devanagari (which is used by various South Asian languages), characters will triple in size. There are even examples where a single byte turns into a composite character in Unicode and is thus six times larger in UTF-8. This has caused objections in India and other countries.
• It is possible in UTF-8 (or any other variable-length encoding) to split or truncate a string in the middle of a character. If the two pieces are not re-appended later before interpretation as characters, this can introduce an invalid sequence at both the end of the previous section and the start of the next, and some decoders will not preserve these bytes and result in data loss. Because UTF-8 is self-synchronizing this will however never introduce a different valid character, and it is also fairly easy to move the truncation point backwards to the start of a character.
• If the code points are all the same size, measurements of a fixed number of them is easy. Due to ASCII-era documentation where "character" is used as a synonym for "byte" this is often considered important. However, by measuring string positions using bytes instead of "characters" most algorithms can be easily and efficiently adapted for UTF-8. Searching for a string within a long string can for example be done byte by byte; the self-synchronization property prevents false positives.

Comparison with other multi-byte encodings

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

Comparison with UTF-16

See also: Comparison of Unicode encodings

• Byte encodings and UTF-8 are represented by byte arrays in programs, and often nothing needs to be done to a function when converting source code from a byte encoding to UTF-8. UTF-16 is represented by 16-bit word arrays, and converting to UTF-16 while maintaining compatibility with existing ASCII-based programs (such as was done with Windows) requires every API and data structure that takes a string to be duplicated, one version accepting byte strings and another version accepting UTF-16. If backward compatibility is not needed, all string handling still must be modified.
• Text encoded in UTF-8 will be smaller than the same text encoded in UTF-16 if there are more code points below U+0080 than in the range U+0800..U+FFFF. This is true for all modern European languages.
  • Text in (for example) Chinese, Japanese or Devanagari will take more space in UTF-8 if there are more of these characters than there are ASCII characters. This is likely when data mainly consist of pure prose, but is lessened by the degree to which the context uses ASCII whitespace, digits, and punctuation.[nb 2]
  • Most of the rich text formats (including HTML) contain a large proportion of ASCII characters for the sake of formatting, thus the size usually will be reduced significantly compared with UTF-16, even when the language mostly uses 3-byte long characters in UTF-8.[nb 3]
• Most communication (e.g. HTML and IP) and storage (e.g. for Unix) was designed for a stream of bytes. A UTF-16 string must use a pair of bytes for each code unit:
  • The order of those two bytes becomes an issue and must be specified in the UTF-16 protocol, such as with a byte order mark.
  • 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.

Derivatives

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.

CESU-8

Main article: 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 three-byte UTF-8 encoding, resulting in six-byte sequences rather than four bytes for characters outside the Basic Multilingual Plane. This is primarily an issue on operating systems which extensively use UTF-16 internally, such as Microsoft Windows.

Oracle Database uses this non-standard behaviour for the UTF8 name, and refers to standards-compliant UTF-8 as AL32UTF8.[57] Java and Tcl include a closely related behaviour as described below.

MySQL calls this utf8mb3, because it transforms UCS-2 codes to three bytes or fewer. Although version 5.5 adds support for UTF-16 and for full UTF-8 (labelled utf8mb4), the label utf8 is still implemented as an alias for utf8bm3, although this is intended to change in the future.[58][59][60]

Although this non-optimal encoding is generally not deliberate, a supposed benefit is that it preserves UTF-16 binary collation order. Unicode Technical Report #26 codifies it and gives it the name CESU-8, with the intention of firmly distinguishing it from UTF-8, but discourages its use in open interchange.[61] Its use is prohibited in HTML5 documents.[62][63][64]

Modified UTF-8

In Modified UTF-8 (MUTF-8),[65] 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,[66] 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 (if it is the platform's default character set or as requested by the program). However it uses Modified UTF-8 for object serialization[67] among other applications of DataInput and DataOutput, for the Java Native Interface,[68] and for embedding constant strings in class files.[69] The dex format defined by Dalvik also uses the same modified UTF-8 to represent string values.[70] Tcl also uses the same modified UTF-8[71] as Java for internal representation of Unicode data, but uses strict CESU-8 for external data.

WTF-8

WTF-8 (Wobbly Transformation Format – 8-bit) is an extension of UTF-8 where the encodings of unpaired surrogate halves (U+D800 through U+DFFF) are allowed.[72] This is necessary to store possibly-invalid UTF-16, such as Windows filenames. Many systems that deal with UTF-8 work this way without considering it a different encoding, as it is simpler.

The term "WTF-8" has also been used humorously to refer to erroneously doubly-encoded UTF-8[73][74] sometimes with the implication that CP1252 bytes are the only ones encoded.[75]

PEP 383

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 (see above) 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.[76] Another encoding called MirBSD OPTU-8/16 converts them to U+EF80...U+EFFF in a Private Use Area.[77] 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;[78] as the Windows filesystem API uses UTF-16, the need to support invalid UTF-8 is less there.[76]

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.[78]

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 email
• Unicode and HTML
• Universal Character Set
• UTF-8 in URIs
• UTF-EBCDIC
• UTF-9 and UTF-18
• UTF-16/UCS-2

Notes

1. 17 planes times 2¹⁶ code points per plane, minus 2¹¹ technically-invalid surrogates.
2. The 2010-11-22 version of 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.
3. The 2010-10-27 version of UTF-8 (in Japanese) 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 यूनिकोड (Unicode in Hindi) required 119 KB in UTF-16 and 76 KB in UTF-8.

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External links

• Original UTF-8 paper (or pdf) for Plan 9 from Bell Labs
• 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
• Characters, Symbols and the Unicode Miracle – Computerphile on YouTube