Unicode what is bom

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. A different issue arises if an unpaired surrogate is encountered when converting ill-formed UTF 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. A: UTF uses a single bit code unit to encode the most common 63K characters, and a pair of bit code units, called surrogates, to encode the 1M less commonly used characters in Unicode.

Originally, Unicode was designed as a pure 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, composite characters for compatibility with legacy sets, it became clear that bits were not sufficient for the user community. Out of this arose UTF 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 They are called surrogates, since they do not represent characters directly, but only as a pair.

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

They are well acquainted with the problems that variable-width codes have caused. 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. 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, 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. 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, relatively few characters require 2 units.

The vast majority of characters in common use are single code units. 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. Both Unicode and ISO have policies in place that formally limit future code assignment to the integer range that can be expressed with current UTF 0 to 1,, Even if other encoding forms i.

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. A: Unpaired surrogates are invalid in UTFs. 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 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:. 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 implementations, where BMP characters require one 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 This term should now be avoided.

UCS-2 does not describe a data format distinct from UTF, because both use exactly the same 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.

This single 4 code unit corresponds to the Unicode scalar value, which is the abstract number associated with a Unicode character. For more information, see Section 3. A: This depends. However, the downside of UTF is that it forces you to use bits for each character, when only 21 bits are ever needed. 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.

You can also specify in your preferences see illustration whether new documents should use a BOM by default. 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. 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. 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. 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. 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 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.

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.

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. Unicode is a group of standards developed in the s and '90s in order to integrate all of the major computer languages into one coding lexicon.

Before UTF-8 was introduced in , Unicode text was transferred using bit code units. These units had a quality called endianness, which essentially identified the byte order either by least significant first or most significant first. The byte order mark is generally an optional feature in typical, closed-environment text-processing, however it is needed in situations involving text interchange. By: Justin Stoltzfus Contributor, Reviewer.

By: Satish Balakrishnan. Dictionary Dictionary Term of the Day. Gorilla Glass.