Utf 8 with bom что это

The byte-order mark (BOM) in HTML

Quick check

Intended audience: XHTML/HTML coders (using editors or scripting), script developers (PHP, JSP, etc.), CSS coders, Web project managers, and anyone who needs to better understand what the BOM is, and how it affects HTML.

Question

What is the byte-order mark, and what do I need to know about it when creating HTML?

Answer

What is a byte-order mark?

At the beginning of a page that uses a Unicode character encoding you may find some bytes that represent the Unicode code point U+FEFF BYTE ORDER MARK (abbreviated as ).

The name BYTE ORDER MARK is an alias for the original character name ZERO WIDTH NO-BREAK SPACE (ZWNBSP). With the introduction of U+2060 WORD JOINER, there’s no longer a need to ever use U+FEFF for its ZWNSP effect, so from that point on, and with the availability of a formal alias, the name ZERO WIDTH NO-BREAK SPACE is no longer helpful, and we will use the alias here.

The BOM, when correctly used, is invisible.

Before UTF-8 was introduced in early 1993, the expected way for transferring Unicode text was using 16-bit code units using an encoding called UCS-2 which was later extended to UTF-16. 16-bit code units can be expressed as bytes in two ways: the most significant byte first ( big-endian ) or the least significant byte first ( little-endian ). To communicate which byte order was in use, U+FEFF (the byte-order mark) was used at the start of the stream as a magic number that is not logically part of the text the stream represents.

The picture below shows the bytes used in a sequence of two-byte characters. Each 2-digit hexadecimal number represents a byte in the stream of text. You can see that the order of the two bytes that represent a single character is reversed for big endian vs. little endian storage. The byte-order mark indicates which order is used, so that applications can immediately decode the content.

What do I need to know about the BOM?

Most of the time you will not have to worry about the byte-order mark in UTF-8. You will find that some editors (such as Notepad on Windows) will always add a BOM when you save a file with the UTF-8 encoding, others will offer you a choice.

In HTML5 browsers are required to recognize the UTF-8 BOM and use it to detect the encoding of the page, and recent versions of major browsers handle the BOM as expected when used for UTF-8 encoded pages.

The UTF-8 BOM offers reliable encoding detection, since it is extremely short and stable, works in XML and HTML, and works whether your page is read over the network or not (unlike HTTP declarations). However, bear in mind that it is always a good idea to declare the encoding of your page using the meta element, in addition to the BOM, so that the encoding is apparent to people looking at the source text.

Also there are a number of situations where the BOM, particularly because it is invisible, may cause a problem. See the section below for more information about those.

If you use a UTF-16 encoding for your page (and we strongly recommend that you don’t), there are some additional considerations.

Detecting the BOM

You can find out whether a page contains a BOM at the start or further down in the content by using the W3C Internationalization Checker. A BOM at the start of the page will be reported in the Information panel. A BOM that is included in the page lower down (typically due to content being added to the page from an external source) will be reported in the Detailed Report section.

You can try looking for a UTF-8 signature in your content in your editor, but if your editor handles the BOM correctly you probably won’t be able to see it. With a binary editor capable of displaying the hexadecimal byte values in the file, the UTF-8 signature displays as EF BB BF.

If your editor or browser applies the wrong character encoding to a UTF-8 encoded file with a BOM, you are likely to see a sequence of bytes at the start of the file. These are the bytes that compose BOM represented as the characters those bytes represent in that encoding. With the Latin 1 (ISO 8859-1) character encoding, the signature displays as characters .

Alternatively, your editor may tell you in a status bar or a menu what encoding your file is in, including information about the presence or not of the UTF-8 signature. For example, if you use Save As in Dreamweaver and your file has a BOM at the start you will see a check mark in the box labeled ‘Include Unicode Signature (BOM)’. You can also specify in your preferences (see illustration) whether new documents should use a BOM by default.

Potential issues with the UTF-8 BOM

What follows are some situations where the byte-order mark has been known to cause problems.

In general, these issues are fading away as people adopt newer versions of browsers and editing tools. It is worth knowing about them if your user base still uses older technology. However, this is not solely about legacy issues.

PHP includes

At the time this article was written, if you include some external file in a page using PHP and that file starts with a BOM, it may create blank lines.

This is because the BOM is not stripped before inclusion into the page, and acts like a character occupying a line of text. See an example. In the example, a blank line containing the BOM appears above the first item of included text.

You should ensure that the included files do not start with a BOM.

You may also find that the BOM causes problems for an ordinary PHP page. When sending custom HTTP headers the code to set the header must be called before output begins. A BOM at the start of the file causes the page to begin output before the header command is interpreted, and may lead to error messages and other problems in the displayed page.

Processing with program code

You need to be careful to take the BOM into account in scripts or program code that automatically process files that start with a BOM. For example, when pattern matching at the start of a file that begins with a BOM you need additional code to test for the presence of the BOM and ignore it if found.

The UTF-8 encoding without a BOM has the property that a document which contains only characters from the US-ASCII range is encoded byte-for-byte the same way as the same document encoded using the US-ASCII encoding. Such a document can be processed and understood when encoded either as UTF-8 or as US-ASCII. Adding a BOM inserts additional non-ASCII bytes, so this is no longer true. If you have processes or scripts that assume that the content is comprised of US-ASCII characters only, you will need to avoid the BOM.

HTTP precedence

Changes introduced with HTML5 mean that the byte-order mark overrides any encoding declaration in the HTTP header when detecting the encoding of an HTML page. This can be very useful when the author of the page cannot control the character encoding setting of the server, or is unaware of its effect, and the server is declaring pages to be in an encoding other than UTF-8. If the BOM has a higher precedence than the HTTP headers, the page should be correctly identified as UTF-8.

At the time of writing, not all browsers do this, so you should not rely on all readers of your page benefitting from this just yet.

Previous versions of Internet Explorer gave the BOM precedence over HTTP, but IE10 and IE11 give a higher precedence to HTTP. It is hoped that the next version of Internet Explorer will revert to the previous behaviour, which will then be in line with the other major browsers.

In browsers where the HTTP header still overrides the byte-order mark and the server is declaring pages to have a non-Unicode character encoding, you are likely to find unexpected characters at the start of the page (such as  in a page labelled in HTTP as ISO 8859-1) as well as problems displaying non-ASCII characters on the page.

Other issues

If you use applications or scripts in the back end of your site you should check that they are also able to recognize and handle the BOM.

We strongly recommend that you don’t change the encoding of a UTF-8 file from a Unicode encoding to a non-Unicode encoding, but if, for some exceptional reason, you do you must ensure that the BOM is removed. If you don’t, either the browser will continue to treat your content as UTF-8, or you will see strange characters at the beginning of the page.

Removing the BOM

If you need to remove the BOM, check whether your editor allows you to specify whether a UTF-8 signature is added or kept while you save the file. Such an editor provides a way of removing the signature by simply reading the file in then saving it out again. For example, in editors such as Notepad++ on Windows and TextWrangler on the Mac, it is possible to select the encoding from a list while using the Save As function. The list has options to save as UTF-8 with or without the BOM. Just choose the option without the BOM and save.

One of the benefits of using a script is that you can remove the signature quickly, and from multiple files. In fact the script could be run automatically as part of your process. If you use Perl, you could use a simple script created by Martin Dürst.

Note: You should check the process impact of removing the signature. It may be that some part of your content development process relies on the use of the signature to indicate that a file is in UTF-8. Bear in mind also that pages with a high proportion of Latin characters may look correct superficially but that occasional characters outside the ASCII range (U+0000 to U+007F) may be incorrectly encoded.

Additional information

Here are some additional notes for those who are encoding their HTML pages using UTF-16. Note that, for HTML it’s recommended that you use UTF-8 and that you avoid UTF-16. So for most people this section will be academic.

According to RFC 2718 and the Unicode Standard, if you declare the character encoding of your page using HTTP as either «UTF-16LE» or «UTF-16BE» then you should not use a byte-order mark at the beginning of the page. Only if the page is labelled in HTTP using IANA charset name «UTF-16» is a byte-order mark appropriate.

Note that this is solely about the labeling of the content. Of course, the actual sequence of bytes is the same, whether you label content as UTF-16 and add a BOM, or whether you label it as UTF-16LE or UTF-16BE.

The HTML5 specification currently disallows the use of any other, text-based in-document encoding declaration for pages using the UTF-16 encoding. In effect, this means that the BOM is, itself, the declaration that you have to add.

The byte-order mark is also used for text labeled as UTF-32, and should not be used for text labeled as UTF-32BE or UTF-32LE. The use of UTF-32 for HTML content, however, is strongly discouraged and some implementations have removed support for it, so we haven’t even mentioned it until now.

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Utf 8 with bom что это

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Что такое ВОМ и кодировка файлов UTF-8 без BOM

Многие владельцы сайтов, в частности работающих на блоговом движке WordPress, редактируют PHP-файлы в блокноте, который прилагается к ОС Windows.

Поскольку сам движок WordPress работает с кодировкой UTF-8, а файлы шаблона, как правило, содержат русские буквы, т.е. кириллицу, то для правильного ее отображения такие файлы также необходимо сохранять в кодировке UTF-8.

Однако блокнот Windows при сохранении в кодировке UTF-8 добавляет в PHP-файлы невидимые символы, называемые BOM (кстати говоря, невидимые они именно в блокноте Винды), которые в результате приводят к тому, что либо на сайте отображается какая-то PHP-ошибка, либо “съезжает” дизайн.

После этого сохраните файл и загрузите его на свой сайт через FTP.

Если вы не знаете, какой из файлов сохранен в неправильной UTF-8 кодировке, то воспользуйтесь специальным скриптом, который найдет все файлы с BOM, которые необходимо пересохранить вышеописанным способом.

Все, на этом задачу с кодировкой можно считать решенной.

Понравилась статья. Простенько и со вкусом. Буду пользоваться и другим рекомендую.
Только не забывайте, что если вы уже видите крикозябли в виде черных квадратиков, нужно в начале выбрать пункт «Преобразовать в UTF-8 без ВОМ». Вот так:

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UTF-8, UTF-16, UTF-32 & BOM

General questions, relating to UTF or Encoding Forms

UTF-8 FAQ

UTF-16 FAQ

UTF-32 FAQ

Byte Order Mark (BOM) FAQ

General questions, relating to UTF or Encoding Form

Q: Is Unicode a 16-bit encoding?

A: No. The first version of Unicode was a 16-bit encoding, from 1991 to 1995, but starting with Unicode 2.0 (July, 1996), it has not been a 16-bit encoding. The Unicode Standard encodes characters in the range U+0000..U+10FFFF, which amounts to a 21-bit code space. Depending on the encoding form you choose (UTF-8, UTF-16, or UTF-32), each character will then be represented either as a sequence of one to four 8-bit bytes, one or two 16-bit code units, or a single 32-bit code unit.

Q: Can Unicode text be represented in more than one way?

A: Yes, there are several possible representations of Unicode data, including UTF-8, UTF-16 and UTF-32. In addition, there are compression transformations such as the one described in the UTS #6: A Standard Compression Scheme for Unicode (SCSU).

A: A Unicode transformation format (UTF) is an algorithmic mapping from every Unicode code point (except surrogate code points) to a unique byte sequence. The ISO/IEC 10646 standard uses the term “UCS transformation format” for UTF; the two terms are merely synonyms for the same concept.

Each UTF is reversible, thus every UTF supports lossless round tripping: mapping from any Unicode coded character sequence S to a sequence of bytes and back will produce S again. To ensure round tripping, a UTF mapping must map all code points (except surrogate code points) to unique byte sequences. This includes reserved (unassigned) code points and the 66 noncharacters (including U+FFFE and U+FFFF).

The SCSU compression method, even though it is reversible, is not a UTF because the same string can map to very many different byte sequences, depending on the particular SCSU compressor. [AF]

Q: Where can I get more information on encoding forms?

A: For the formal definition of UTFs see Section 3.9, Unicode Encoding Forms in The Unicode Standard. For more information on encoding forms see UTR #17: Unicode Character Encoding Model. [AF]

Q: How do I write a UTF converter?

A: The freely available open source project International Components for Unicode (ICU) has UTF conversion built into it. The latest version may be downloaded from the ICU Project web site. [AF]

Q: Are there any byte sequences that are not generated by a UTF? How should I interpret them?

A: None of the UTFs can generate every arbitrary byte sequence. For example, in UTF-8 every byte of the form 110xxxxx₂ must be followed with a byte of the form 10xxxxxx₂. A sequence such as is illegal, and must never be generated. When faced with this illegal byte sequence while transforming or interpreting, a UTF-8 conformant process must treat the first byte 110xxxxx₂ as an illegal termination error: for example, either signaling an error, filtering the byte out, or representing the byte with a marker such as FFFD (REPLACEMENT CHARACTER). In the latter two cases, it will continue processing at the second byte 0xxxxxxx₂.

A conformant process must not interpret illegal or ill-formed byte sequences as characters, however, it may take error recovery actions. No conformant process may use irregular byte sequences to encode out-of-band information.

Q: Which of the UTFs do I need to support?

A: UTF-8 is most common on the web. UTF-16 is used by Java and Windows. UTF-8 and UTF-32 are used by Linux and various Unix systems. The conversions between all of them are algorithmically based, fast and lossless. This makes it easy to support data input or output in multiple formats, while using a particular UTF for internal storage or processing. [AF]

Q: What are some of the differences between the UTFs?

A: The following table summarizes some of the properties of each of the UTFs.

In the table indicates that the byte order is determined by a byte order mark, if present at the beginning of the data stream, otherwise it is big-endian. [AF]

Q: Why do some of the UTFs have a BE or LE in their label, such as UTF-16LE?

A: UTF-16 and UTF-32 use code units that are two and four bytes long respectively. For these UTFs, there are three sub-flavors: BE, LE and unmarked. The BE form uses big-endian byte serialization (most significant byte first), the LE form uses little-endian byte serialization (least significant byte first) and the unmarked form uses big-endian byte serialization by default, but may include a byte order mark at the beginning to indicate the actual byte serialization used. [AF]

Q: Is there a standard method to package a Unicode character so it fits an 8-Bit ASCII stream?

A: There are three or four options for making Unicode fit into an 8-bit format.

a) Use UTF-8. This preserves ASCII, but not Latin-1, because the characters >127 are different from Latin-1. UTF-8 uses the bytes in the ASCII only for ASCII characters. Therefore, it works well in any environment where ASCII characters have a significance as syntax characters, e.g. file name syntaxes, markup languages, etc., but where the all other characters may use arbitrary bytes.
Example: “Latin Small Letter s with Acute” (015B) would be encoded as two bytes: C5 9B.

b) Use Java or C style escapes, of the form \uXXXXX or \xXXXXX. This format is not standard for text files, but well defined in the framework of the languages in question, primarily for source files.
Example: The Polish word “wyjście” with character “Latin Small Letter s with Acute” (015B) in the middle (ś is one character) would look like: “wyj\u015Bcie».

c) Use the &#xXXXX; or &#DDDDD; numeric character escapes as in HTML or XML. Again, these are not standard for plain text files, but well defined within the framework of these markup languages.
Example: “wyjście” would look like “ wyjście «

d) Use SCSU. This format compresses Unicode into 8-bit format, preserving most of ASCII, but using some of the control codes as commands for the decoder. However, while ASCII text will look like ASCII text after being encoded in SCSU, other characters may occasionally be encoded with the same byte values, making SCSU unsuitable for 8-bit channels that blindly interpret any of the bytes as ASCII characters.
Example: “ wyjÛcie” where indicates the byte 0x12 and “Û” corresponds to byte 0xDB. [AF]

Q: Which of these approaches is the best?

A: That depends on the circumstances: Of these four approaches, d) uses the least space, but cannot be used transparently in most 8-bit environments. a) is the most widely supported in plain text files and b) and c) use the most space, but are widely supported for program source files in Java and C, or within HTML and XML files respectively. [AF]

Q: Which of these formats is the most standard?

A: All four require that the receiver can understand that format, but a) is considered one of the three equivalent Unicode Encoding Forms and therefore standard. The use of b), or c) out of their given context would definitely be considered non-standard, but could be a good solution for internal data transmission. The use of SCSU is itself a standard (for compressed data streams) but few general purpose receivers support SCSU, so it is again most useful in internal data transmission. [AF]

UTF-8 FAQ

Q: What is the definition of UTF-8?

A: UTF-8 is the byte-oriented encoding form of Unicode. For details of its definition, see Section 2.5, Encoding Forms and Section 3.9, Unicode Encoding Forms ” in The Unicode Standard. See, in particular, Table 3-6 UTF-8 Bit Distribution and Table 3-7 Well-formed UTF-8 Byte Sequences, which give succinct summaries of the encoding form. Make sure you refer to the latest version of the Unicode Standard, as the Unicode Technical Committee has tightened the definition of UTF-8 over time to more strictly enforce unique sequences and to prohibit encoding of certain invalid characters. There is an Internet RFC 3629 about UTF-8. UTF-8 is also defined in Annex D of ISO/IEC 10646. See also the question above, How do I write a UTF converter?

Q: Is the UTF-8 encoding scheme the same irrespective of whether the underlying processor is little endian or big endian?

A: Yes. Since UTF-8 is interpreted as a sequence of bytes, there is no endian problem as there is for encoding forms that use 16-bit or 32-bit code units. Where a BOM is used with UTF-8, it is only used as an encoding signature to distinguish UTF-8 from other encodings — it has nothing to do with byte order. [AF]

Q: Is the UTF-8 encoding scheme the same irrespective of whether the underlying system uses ASCII or EBCDIC encoding?

A: There is only one definition of UTF-8. It is precisely the same, whether the data were converted from ASCII or EBCDIC based character sets. However, byte sequences from standard UTF-8 won’t interoperate well in an EBCDIC system, because of the different arrangements of control codes between ASCII and EBCDIC. UTR #16: UTF-EBCDIC defines is a specialized UTF that will interoperate in EBCDIC systems. [AF]

Q: How do I convert a UTF-16 surrogate pair such as to UTF-8? As one 4-byte sequence or as two separate 3-byte sequences?

A: The definition of UTF-8 requires that supplementary characters (those using surrogate pairs in UTF-16) be encoded with a single 4-byte sequence. However, there is a widespread practice of generating pairs of 3-byte sequences in older software, especially software which pre-dates the introduction of UTF-16 or that is interoperating with UTF-16 environments under particular constraints. Such an encoding is not conformant to UTF-8 as defined. See UTR #26: Compatability Encoding Scheme for UTF-16: 8-bit (CESU) for a formal description of such a non-UTF-8 data format. When using CESU-8, great care must be taken that data is not accidentally treated as if it was UTF-8, due to the similarity of the formats. [AF]

Q: How do I convert an unpaired UTF-16 surrogate to UTF-8?

A different issue arises if an unpaired surrogate is encountered when converting ill-formed UTF-16 data. By representing such an unpaired surrogate on its own as a 3-byte sequence, the resulting UTF-8 data stream would become ill-formed. While it faithfully reflects the nature of the input, Unicode conformance requires that encoding form conversion always results in a valid data stream. Therefore a converter must treat this as an error. [AF]

UTF-16 FAQ

A: UTF-16 uses a single 16-bit code unit to encode the most common 63K characters, and a pair of 16-bit code units, called surrogates, to encode the 1M less commonly used characters in Unicode.

Originally, Unicode was designed as a pure 16-bit encoding, aimed at representing all modern scripts. (Ancient scripts were to be represented with private-use characters.) Over time, and especially after the addition of over 14,500 composite characters for compatibility with legacy sets, it became clear that 16-bits were not sufficient for the user community. Out of this arose UTF-16. [AF]

Q: What are surrogates?

A: Surrogates are code points from two special ranges of Unicode values, reserved for use as the leading, and trailing values of paired code units in UTF-16. Leading, also called high, surrogates are from D800₁₆ to DBFF₁₆, and trailing, or low, surrogates are from DC00₁₆ to DFFF₁₆. They are called surrogates, since they do not represent characters directly, but only as a pair.

Q: What’s the algorithm to convert from UTF-16 to character codes?

A: The Unicode Standard used to contain a short algorithm, now there is just a bit distribution table. Here are three short code snippets that translate the information from the bit distribution table into C code that will convert to and from UTF-16.

Using the following type definitions

the first snippet calculates the high (or leading) surrogate from a character code C.

where X, U and W correspond to the labels used in Table 3-5 UTF-16 Bit Distribution. The next snippet does the same for the low surrogate.

Finally, the reverse, where hi and lo are the high and low surrogate, and C the resulting character

A caller would need to ensure that C, hi, and lo are in the appropriate ranges. [AF]

Q: Isn’t there a simpler way to do this?

A: There is a much simpler computation that does not try to follow the bit distribution table.

Q: Why are some people opposed to UTF-16?

A: People familiar with variable width East Asian character sets such as Shift-JIS ( SJIS) are understandably nervous about UTF-16, which sometimes requires two code units to represent a single character. They are well acquainted with the problems that variable-width codes have caused. However, there are some important differences between the mechanisms used in SJIS and UTF-16:

In SJIS, there is overlap between the leading and trailing code unit values, and between the trailing and single code unit values. This causes a number of problems:

It causes false matches. For example, searching for an “a” may match against the trailing code unit of a Japanese character.

It prevents efficient random access. To know whether you are on a character boundary, you have to search backwards to find a known boundary.

It makes the text extremely fragile. If a unit is dropped from a leading-trailing code unit pair, many following characters can be corrupted.

In UTF-16, the code point ranges for high and low surrogates, as well as for single units are all completely disjoint. None of these problems occur:

There are no false matches.

The location of the character boundary can be directly determined from each code unit value.

A dropped surrogate will corrupt only a single character.

The vast majority of SJIS characters require 2 units, but characters using single units occur commonly and often have special importance, for example in file names.

With UTF-16, relatively few characters require 2 units. The vast majority of characters in common use are single code units. Even in East Asian text, the incidence of surrogate pairs should be well less than 1% of all text storage on average. (Certain documents, of course, may have a higher incidence of surrogate pairs, just as phthisique is an fairly infrequent word in English, but may occur quite often in a particular scholarly text.) [AF]

Q: Will UTF-16 ever be extended to more than a million characters?

A: No. Both Unicode and ISO 10646 have policies in place that formally limit future code assignment to the integer range that can be expressed with current UTF-16 (0 to 1,114,111). Even if other encoding forms (i.e. other UTFs) can represent larger integers, these policies mean that all encoding forms will always represent the same set of characters. Over a million possible codes is far more than enough for the goal of Unicode of encoding characters, not glyphs. Unicode is not designed to encode arbitrary data. If you wanted, for example, to give each “instance of a character on paper throughout history” its own code, you might need trillions or quadrillions of such codes; noble as this effort might be, you would not use Unicode for such an encoding. [AF]

Q: Are there any 16-bit values that are invalid?

A: Unpaired surrogates are invalid in UTFs. These include any value in the range D800₁₆ to DBFF₁₆ not followed by a value in the range DC00₁₆ to DFFF₁₆, or any value in the range DC00₁₆ to DFFF₁₆ not preceded by a value in the range D800₁₆ to DBFF₁₆. [AF]

Q: What about noncharacters? Are they invalid?

A: Not at all. Noncharacters are valid in UTFs and must be properly converted. For more details on the definition and use of noncharacters, as well as their correct representation in each UTF, see the Noncharacters FAQ.

Q: Because most supplementary characters are uncommon, does that mean I can ignore them?

A: Most supplementary characters (expressed with surrogate pairs in UTF-16) are not too common. However, that does not mean that supplementary characters should be neglected. Among them are a number of individual characters that are very popular, as well as many sets important to East Asian procurement specifications. Among the notable supplementary characters are:

many popular emoji and emoticons

symbols used for interoperating with Wingdings and Webdings

numerous small sets of CJK characters important for procurement, including personal and place names

variation selectors used for all ideographic variation sequences

numerous minority scripts important for some user communities

some highly salient historic scripts, such as Egyptian hieroglyphics

Ken Lunde has an interesting presentation file on this topic, with a Top Ten list: Why Support Beyond-BMP Code Points?

Q: How should I handle supplementary characters in my code?

A: Compared with BMP characters as a whole, the supplementary characters occur less commonly in text. This remains true now, even though many thousands of supplementary characters have been added to the standard, and a few individual characters, such as popular emoji, have become quite common. The relative frequency of BMP characters, and of the ASCII subset within the BMP, can be taken into account when optimizing implementations for best performance: execution speed, memory usage, and data storage.

Such strategies are particularly useful for UTF-16 implementations, where BMP characters require one 16-bit code unit to process or store, whereas supplementary characters require two.

Strategies that optimize for the BMP are less useful for UTF-8 implementations, but if the distribution of data warrants it, an optimization for the ASCII subset may make sense, as that subset only requires a single byte for processing and storage in UTF-8.

Q: What is the difference between UCS-2 and UTF-16?

A: UCS-2 is obsolete terminology which refers to a Unicode implementation up to Unicode 1.1, before surrogate code points and UTF-16 were added to Version 2.0 of the standard. This term should now be avoided.

UCS-2 does not describe a data format distinct from UTF-16, because both use exactly the same 16-bit code unit representations. However, UCS-2 does not interpret surrogate code points, and thus cannot be used to conformantly represent supplementary characters.

Sometimes in the past an implementation has been labeled «UCS-2» to indicate that it does not support supplementary characters and doesn’t interpret pairs of surrogate code points as characters. Such an implementation would not handle processing of character properties, code point boundaries, collation, etc. for supplementary characters. [AF]

UTF-32 FAQ

A: Any Unicode character can be represented as a single 32-bit unit in UTF-32. This single 4 code unit corresponds to the Unicode scalar value, which is the abstract number associated with a Unicode character. UTF-32 is a subset of the encoding mechanism called UCS-4 in ISO 10646. For more information, see Section 3.9, Unicode Encoding Forms in The Unicode Standard. [AF]

Q: Should I use UTF-32 (or UCS-4) for storing Unicode strings in memory?

A: This depends. If you frequently need to access APIs that require string parameters to be in UTF-32, it may be more convenient to work with UTF-32 strings all the time. However, the downside of UTF-32 is that it forces you to use 32-bits for each character, when only 21 bits are ever needed. The number of significant bits needed for the average character in common texts is much lower, making the ratio effectively that much worse. In many situations that does not matter, and the convenience of having a fixed number of code units per character can be the deciding factor.

Increasing the storage for the same number of characters does have its cost in applications dealing with large volume of text data: it can mean exhausting cache limits sooner; it can result in noticeably increased read/write times or in reaching bandwidth limits; and it requires more space for storage. What a number of implementations do is to represent strings with UTF-16, but individual character values with UTF-32.

The chief selling point for Unicode is providing a representation for all the world’s characters, eliminating the need for juggling multiple character sets and avoiding the associated data corruption problems. These features were enough to swing industry to the side of using Unicode (UTF-16). While a UTF-32 representation does make the programming model somewhat simpler, the increased average storage size has real drawbacks, making a complete transition to UTF-32 less compelling. [AF]

Q: How about using UTF-32 interfaces in my APIs?

A: Except in some environments that store text as UTF-32 in memory, most Unicode APIs are using UTF-16. With UTF-16 APIs the low level indexing is at the storage or code unit level, with higher-level mechanisms for graphemes or words specifying their boundaries in terms of the code units. This provides efficiency at the low levels, and the required functionality at the high levels.

If its ever necessary to locate the n th character, indexing by character can be implemented as a high level operation. However, while converting from such a UTF-16 code unit index to a character index or vice versa is fairly straightforward, it does involve a scan through the 16-bit units up to the index point. In a test run, for example, accessing UTF-16 storage as characters, instead of code units resulted in a 10× degradation. While there are some interesting optimizations that can be performed, it will always be slower on average. Therefore locating other boundaries, such as grapheme, word, line or sentence boundaries proceeds directly from the code unit index, not indirectly via an intermediate character code index.

Q: Doesn’t it cause a problem to have only UTF-16 string APIs, instead of UTF-32 char APIs?

A: Almost all international functions (upper-, lower-, titlecasing, case folding, drawing, measuring, collation, transliteration, grapheme-, word-, linebreaks, etc.) should take string parameters in the API, not single code-points (UTF-32). Single code-point APIs almost always produce the wrong results except for very simple languages, either because you need more context to get the right answer, or because you need to generate a sequence of characters to return the right answer, or both.

Storing a single code point in a struct or class instead of a string, would exclude support for graphemes, such as “ch” for Slovak, where a single code point may not be sufficient, but a character sequence is needed to express what is required. In other words, most API parameters and fields of composite data types should not be defined as a character, but as a string. And if they are strings, it does not matter what the internal representation of the string is.

Given that any industrial-strength text and internationalization support API has to be able to handle sequences of characters, it makes little difference whether the string is internally represented by a sequence of UTF-16 code units, or by a sequence of code-points ( = UTF-32 code units). Both UTF-16 and UTF-8 are designed to make working with substrings easy, by the fact that the sequence of code units for a given code point is unique. [AF]

Q: Are there exceptions to the rule of exclusively using string parameters in APIs?

A: The main exception are very low-level operations such as getting character properties (e.g. General Category or Canonical Class in the UCD). For those it is handy to have interfaces that convert quickly to and from UTF-16 and UTF-32, and that allow you to iterate through strings returning UTF-32 values (even though the internal format is UTF-16).

Q: How do I convert a UTF-16 surrogate pair such as to UTF-32? As one 4-byte sequence or as two 4-byte sequences?

A: The definition of UTF-32 requires that supplementary characters (those using surrogate pairs in UTF-16) be encoded with a single 4-byte sequence.

Q: How do I convert an unpaired UTF-16 surrogate to UTF-32?

A: If an unpaired surrogate is encountered when converting ill-formed UTF-16 data, any conformant converter must treat this as an error. By representing such an unpaired surrogate on its own, the resulting UTF-32 data stream would become ill-formed. While it faithfully reflects the nature of the input, Unicode conformance requires that encoding form conversion always results in valid data stream. [AF]

Byte Order Mark (BOM) FAQ

A: A byte order mark (BOM) consists of the character code U+FEFF at the beginning of a data stream, where it can be used as a signature defining the byte order and encoding form, primarily of unmarked plaintext files. Under some higher level protocols, use of a BOM may be mandatory (or prohibited) in the Unicode data stream defined in that protocol. [AF]

Q: Where is a BOM useful?

A: A BOM is useful at the beginning of files that are typed as text, but for which it is not known whether they are in big or little endian format—it can also serve as a hint indicating that the file is in Unicode, as opposed to in a legacy encoding and furthermore, it act as a signature for the specific encoding form used. [AF]

Q: What does ‘endian’ mean?

A: Data types longer than a byte can be stored in computer memory with the most significant byte (MSB) first or last. The former is called big-endian, the latter little-endian. When data is exchanged, bytes that appear in the «correct» order on the sending system may appear to be out of order on the receiving system. In that situation, a BOM would look like 0xFFFE which is a noncharacter, allowing the receiving system to apply byte reversal before processing the data. UTF-8 is byte oriented and therefore does not have that issue. Nevertheless, an initial BOM might be useful to identify the datastream as UTF-8. [AF]

Q: When a BOM is used, is it only in 16-bit Unicode text?

A: No, a BOM can be used as a signature no matter how the Unicode text is transformed: UTF-16, UTF-8, or UTF-32. The exact bytes comprising the BOM will be whatever the Unicode character U+FEFF is converted into by that transformation format. In that form, the BOM serves to indicate both that it is a Unicode file, and which of the formats it is in. Examples:

Q: Can a UTF-8 data stream contain the BOM character (in UTF-8 form)? If yes, then can I still assume the remaining UTF-8 bytes are in big-endian order?

A: Yes, UTF-8 can contain a BOM. However, it makes no difference as to the endianness of the byte stream. UTF-8 always has the same byte order. An initial BOM is only used as a signature — an indication that an otherwise unmarked text file is in UTF-8. Note that some recipients of UTF-8 encoded data do not expect a BOM. Where UTF-8 is used transparently in 8-bit environments, the use of a BOM will interfere with any protocol or file format that expects specific ASCII characters at the beginning, such as the use of «#!» of at the beginning of Unix shell scripts. [AF]

Q: What should I do with U+FEFF in the middle of a file?

A: In the absence of a protocol supporting its use as a BOM and when not at the beginning of a text stream, U+FEFF should normally not occur. For backwards compatibility it should be treated as ZERO WIDTH NON-BREAKING SPACE (ZWNBSP), and is then part of the content of the file or string. The use of U+2060 WORD JOINER is strongly preferred over ZWNBSP for expressing word joining semantics since it cannot be confused with a BOM. When designing a markup language or data protocol, the use of U+FEFF can be restricted to that of Byte Order Mark. In that case, any U+FEFF occurring in the middle of a file can be treated as an unsupported character. [AF]

Q: I am using a protocol that has BOM at the start of text. How do I represent an initial ZWNBSP?

A: Use U+2060 WORD JOINER instead.

Q: How do I tag data that does not interpret U+FEFF as a BOM?

A: Use the tag UTF-16BE to indicate big-endian UTF-16 text, and UTF-16LE to indicate little-endian UTF-16 text. If you do use a BOM, tag the text as simply UTF-16. [MD]

Q: Why wouldn’t I always use a protocol that requires a BOM?

A: Where the data has an associated type, such as a field in a database, a BOM is unnecessary. In particular, if a text data stream is marked as UTF-16BE, UTF-16LE, UTF-32BE or UTF-32LE, a BOM is neither necessary nor permitted. Any U+FEFF would be interpreted as a ZWNBSP.

Do not tag every string in a database or set of fields with a BOM, since it wastes space and complicates string concatenation. Moreover, it also means two data fields may have precisely the same content, but not be binary-equal (where one is prefaced by a BOM).

Q: How I should deal with BOMs?

A: Here are some guidelines to follow:

Some protocols allow optional BOMs in the case of untagged text. In those cases,

Where a text data stream is known to be plain text, but of unknown encoding, BOM can be used as a signature. If there is no BOM, the encoding could be anything.

Where a text data stream is known to be plain Unicode text (but not which endian), then BOM can be used as a signature. If there is no BOM, the text should be interpreted as big-endian.

Some byte oriented protocols expect ASCII characters at the beginning of a file. If UTF-8 is used with these protocols, use of the BOM as encoding form signature should be avoided.

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