Unicode’s Universal Character Set has a potential capacity to support over 1 million characters. Each UCS character is mapped to a code point which is an integer between 0 and 1,114,111 used to represent each character within the internal logic of text processing software (1,114,112 = 220 + 216 or 17 × 216, or hexadecimal 110000 code points).
As of Unicode 5.2.0, 246,943 (22.2%) of these code points are assigned, including 107,361 (9.6%) encoded characters, 137,468 (12.3%) reserved for private use, 2,048 for surrogates, and 66 designated noncharacters, leaving 867,169 (77.8%) unassigned. The number of encoded characters is made up as follows:
(See the summary table for a more detailed breakdown).
Unicode characters can be categorized in many ways. Every character is assigned a script or a symbol (though many are assigned the common or inherited scripts where they inherit the script from the adjacent character). In Unicode a script is a coherent writing system that includes letters but also may include script-specific punctuation, diacritic and other marks and numerals and symbols. A single script supports one or more languages. Symbols, including control characters, are relevant for their meaning, not their speech.
Characters are assigned in blocks of characters. A block is a single group of code points. Every character is also assigned a general category and subcategory. The general categories are: letter, mark, number, punctuation, symbol, or control (in other words a formatting or non-graphical character).
The blocks of characters are assigned according to various planes. Most characters are currently assigned to the first plane: the Basic Multilingual Plane. This is to help ease the transition for legacy software since the Basic Multilingual Plane is addressable with just two octet bytes. The characters outside the first plane usually have very specialized or rare use.
The first 256 code points correspond with those of ISO 8859-1, the most popular 8-bit character encoding in the Western world. As a result, the first 128 characters are also identical to ASCII. Though Unicode refers to these as a Latin script block, these two blocks contain many characters that are commonly useful outside of the Latin script. In general, not all characters in a given block need be of the same script, and a given script can occur in several different blocks.
All available codepoints are located on 17 Planes, each plane corresponding with the value of the hexadecimal digits (0–9, A–F) preceding the four final ones: hence U+24321 is in Plane 2, U+4321 is in Plane 0 (implicitly read U+04321), and U+10A200 would be in Plane 16 (for Hex 10=decimal 16). Within one plane, the maximum range of possible codepoints is Hex 0000–FFFF, about 65,000. Some planes only allow a limited number of this range.
| Unicode planes and code point (character) ranges | ||||||||
|---|---|---|---|---|---|---|---|---|
| Basic | Supplementary | |||||||
| Plane 0: Basic Multilingual Plane |
Plane 1: Supplementary Multilingual Plane |
Plane 2: Supplementary Ideographic Plane |
Planes 3–13: Unassigned |
Plane 14: Supplementary Special-purpose Plane |
Planes 15–16: Supplementary Private Use Area |
|||
| 0000–FFFF | 10000–1FFFF | 20000–2FFFF | 30000–DFFFF | E0000–EFFFF | F0000–10FFFF | |||
| BMP | SMP | SIP | — | SSP | S PUA A/B | |||
| 0000–0FFF 1000–1FFF 2000–2FFF 3000–3FFF 4000–4FFF 5000–5FFF 6000–6FFF 7000–7FFF |
8000–8FFF 9000–9FFF A000–AFFF B000–BFFF C000–CFFF D000–DFFF E000–EFFF F000–FFFF |
10000–10FFF 11000–11FFF 12000–12FFF 13000–13FFF 16000–16FFF |
1B000–1BFFF 1D000–1DFFF 1F000–1FFFF |
20000–20FFF 21000–21FFF 22000–22FFF 23000–23FFF 24000–24FFF 25000–25FFF 26000–26FFF 27000–27FFF |
28000–28FFF 29000–29FFF 2A000–2AFFF 2B000–2BFFF 2F000–2FFFF |
E0000–E0FFF | 15: PUA-A F0000–FFFFF 16: PUA-B 100000–10FFFF |
|
The latest Unicode repertoire codifies over a hundred thousand characters. Most of those represent graphemes for processing as linear text. Some, however, either do not represent graphemes, or, as graphemes, require exceptional treatment. Unlike the ASCII control characters and other characters included for legacy round-trip capabilities, these other special-purpose characters endow plain text with important semantics.
Some special characters can alter the layout of text, such as the zero-width joiner and zero-width non-joiner, while others do not affect text layout at all, but instead affect the way text strings are collated, matched or otherwise processed. Other special-purpose characters, such as the mathematical invisibles, generally have no effect on text rendering, though sophisticated text layout software may choose to subtly adjust spacing around them.
Unicode does not specify the division of labor between font and text layout software (or "engine") when rendering Unicode text. Because the more complex font formats, such as OpenType or Apple Advanced Typography, provide for contextual substitution and positioning of glyphs, a simple text layout engine might rely entirely on the font for all decisions of glyph choice and placement. In the same situation a more complex engine may combine information from the font with its own rules to achieve its own idea of best rendering. To implement all recommendations of the Unicode specification, a text engine must be prepared to work with fonts of any level of sophistication, since contextual substitution and positioning rules do not exist in some font formats and are optional in the rest. The fraction slash is an example: complex fonts may or may not supply positioning rules in the presence of the fraction slash character to create a fraction, while fonts in simple formats cannot.
When appearing at the head of a text file or stream, the byte order mark (BOM) U+FEFF hints at the encoding form and its byte order.
If the stream’s first byte is 0xFE and the second 0xFF, then the stream’s text is not likely to be encoded in UTF-8, since those bytes are meaningless in UTF-8. It is also not likely to be UTF-16 in little-endian byte order because 0xFE, 0xFF read as a 16-bit little endian word would be U+FFFE, which is meaningless. The sequence also has no meaning in any arrangement of UTF-32 encoding, so, in summary, it serves as a fairly reliable indication that the text stream is encoded as UTF-16 in big-endian byte order. Conversely, if the first two bytes are 0xFF, 0xFE, then the text stream may be assumed to be encoded as UTF-16LE because, read as a 16-bit little-endian value, the bytes yield the expected 0xFEFF byte order mark.
The UTF-8 sequence corresponding to U+FEFF is 0xEF, 0xBB, 0xBF. This sequence has no meaning in other Unicode encoding forms, so it may serve to indicate that that stream is encoded as UTF-8.
The Unicode specification does not require the use of byte order marks in text streams. It further states that they should not be used in situations where some other method of signaling the encoding form is already in use.
The zero-width joiner (U+200D) and zero-width non-joiner (U+200C) control the joining and ligation of glyphs. The joiner does not cause characters that would not otherwise join or ligate to do so, but when paired with the non-joiner these characters can be used to control the joining and ligating properties of the surrounding two joining or ligating characters. The Combining Grapheme Joiner (U+034F) is used to distinguish two base characters as one common base or digraph, mostly for underlying text processing, collation of strings, case folding and so on.
The most common word separator is a space (U+0020). However, there are other word joiners and separators that also indicate a break between words and participate in line-breaking algorithms. The No-Break Space (U+00A0) also produces a baseline advance without a glyph but inhibits rather than enabling a line-break. The Zero Width Space (U+200B) allows a line-break but provides no space: in a sense joining, rather than separating, two words. Finally, the Word Joiner (U+2060) inhibits line breaks and also involves none of the white space produced by a baseline advance.
| Baseline Advance | No Baseline Advance | |
| Allow Line-break (Separators) |
Space U+0020 | Zero Width Space U+200B |
| Inhibit Line-break (Joiners) |
No-Break Space U+00A0 | Word Joiner U+2060 |
These provide Unicode with native paragraph and line separators independent of the legacy encoded ASCII control characters such as carriage return (U+000A), linefeed (U+000D), and Next Line (U+0085). Unicode does not provide for other ASCII formatting control characters which presumably then are not part of the Unicode plain text processing model. These legacy formatting control characters include Tab (U+0009), Line Tabulation or Vertical Tab (U+000B), and Form Feed (U+000C) which is also thought of as a page break.
The space character (U+0020) typically input by the space bar on a keyboard serves semantically as a word separator in many languages. For legacy reasons, the UCS also includes spaces of varying sizes that are compatibility equivalents for the space character. While these spaces of varying width are important in typography, the Unicode processing model calls for such visual effects to be handled by rich text, markup and other such protocols. They are included in the Unicode repertoire primarily to handle lossless roundtrip transcoding from other character set encodings. These spaces include:
Aside from the original ASCII space, the other spaces are all compatibility characters. In this context this means that they effectively add no semantic content to the text, but instead provide styling control. Within Unicode, this non-semantic styling control is often referred to as rich text and is outside the thrust of Unicode’s goals. Rather than using different spaces in different contexts, this styling should instead be handled through intelligent text layout software.
Three other writing-system-specific word separators are:
Several characters are designed to help control line-breaks either by discouraging them (no-break characters) or suggesting line breaks such as the soft hyphen (U+00AD) (sometimes called the "shy hyphen"). Such characters, though designed for styling, are probably indispensable for the intricate types of line-breaking they make possible.
Break Inhibiting
The break inhibiting characters are meant to be equivalent to a character sequence wrapped in the Word Joiner U+2060. However, the Word Joiner may be appended before or after any character that would allow a line-break to inhibit such line-breaking.
Break Enabling
Both the break inhibiting and break enabling characters participate with other punctuation and whitespace characters to enable text imaging systems to determine line breaks within the Unicode Line Breaking Algorithm.
Primarily for mathematics, the Invisible Separator (U+2063) provides a separator between characters where punctuation or space may be omitted such as in a two-dimensional index like ij. Invisible Times (U+2062) and Function Application (U+2061) are useful in mathematics text where the multiplication of terms or the application of a function is implied without any glyph indicating the operation. Unicode 5.1 introduces the Mathematical Invisible Plus character as well (U+2064).
The fraction slash character (U+2044) has special behavior in the Unicode Standard (section 6.2, Other Punctuation):
The standard form of a fraction built using the fraction slash is defined as follows: any sequence of one or more decimal digits (General Category = Nd), followed by the fraction slash, followed by any sequence of one or more decimal digits. Such a fraction should be displayed as a unit, such as ¾. If the displaying software is incapable of mapping the fraction to a unit, then it can also be displayed as a simple linear sequence as a fallback (for example, 3/4).
By following this Unicode recommendation, text processing systems yield sophisticated symbols from plain text alone. Here the presence of the fraction slash character instructs the layout engine to synthesize a fraction from all consecutive digits preceding and following the slash. In practice, results vary because of the complicated interplay between fonts and layout engines. Simple text layout engines tend not to synthesize fractions all, and instead draw the glyphs as a linear sequence as described in the Unicode fallback scheme.
More sophisticated layout engines face two practical choices: they can follow Unicode’s recommendation, or they can rely on the font’s own instructions for synthesizing fractions. By ignoring the font’s instructions, the layout engine can guarantee Unicode’s recommended behavior. By following the font’s instructions, the layout engine can achieve better typography because placement and shaping of the digits will be tuned to that particular font at that particular size.
The problem with following the font’s instructions is that the simpler font formats have no way to specify fraction synthesis behavior. Meanwhile the more complex formats do not require the font to specify fraction synthesis behavior and therefore many do not. Most fonts of complex formats can instruct the layout engine to replace a plain text sequence such as "1⁄2" with the precomposed "½" glyph. But because many of them will not issue instructions to synthesize fractions, a plain text string such as "221⁄225" may well render as 22½25 (with the ½ being the substituted precomposed fraction, rather than synthesized). In the face of problems like this, those who wish to rely on the recommended Unicode behavior should choose fonts known to synthesize fractions or text layout software known to produce Unicode’s recommended behavior regardless of font.
Writing direction is the direction glyphs are placed on the page in relation to forward progression of characters in the Unicode string. English and other languages of Latin script have left-to-right writing direction. Several major writing scripts, such as Arabic and Hebrew, have right-to-left writing direction. The Unicode specification assigns a directional type to each character to inform text processors how sequences of characters should be ordered on the page.
While lexical characters (that is, letters) are normally specific to a single writing script, some symbols and punctuation marks are used across many writing scripts. Unicode could have created duplicate symbols in the repertoire that differ only by directional type, but chose instead to unify them and assign them a neutral directional type. They acquire direction at render time from adjacent characters. Some of these characters also have a bidi-mirrored property indicating the glyph should be rendered in mirror-image when used in right-to-left text.
The render-time directional type of a neutral character can remain ambiguous when the mark is placed on the boundary between directional changes. To address this, Unicode includes two characters that have strong directionality, have no glyph associated with them, and are ignorable by systems that do not process bidirectional text:
Surrounding a bidirectionally neutral character by the left-to-right mark will force the character to behave as a left-to-right character while surrounding it by the right-to-left mark will force it to behave as a right-to-left character. The behavior of these characters is detailed in Unicode’s Bidirectional Algorithm.
While Unicode is designed to handle multiple languages, multiple writing systems and even text that flows either left-to-right or right-to-left with minimal author intervention, there are special circumstances where the mix of bidirectional text can become intricate—requiring more author control. For these circumstances, Unicode includes five other characters to control the complex embedding of left-to-right text within right-to-left text and vice versa:
Unicode provides a list of characters it deems whitespace characters for interoperability support. Software Implementations and other standards may use the term to denote a slightly different set of characters. For example, Java does not consider U+00A0 NO-BREAK SPACE or U+0085 NEXT LINE to be whitespace, even though Unicode does. Whitespace characters are characters typically designated for programming environments. Often they have no syntactic meaning in such programming environments and are ignored by the machine interpreters. Unicode designates the legacy control characters U+0009 through U+000D and U+0085 as whitespace characters, as well as all characters whose General Category property value is Separator. There are 26 total whitespace characters as of Unicode 6.0.0.
The UCS includes 137,468 code points for private use in three different ranges, each called a Private Use Area (PUA). The Unicode standard recognizes code points within PUAs as legitimate Unicode character codes, but does not assign them any (abstract) character. Instead, individuals, organizations and software vendors are free to use them as they see fit. Within closed systems, characters in the PUA can operate unambiguously, allowing such systems to represent characters or glyphs not defined in Unicode. In public systems their use is more problematic, since there is no registry and no way to prevent several organizations from adopting the same code points for different purposes. One example of such a conflict is Apple’s use of U+F8FF for the Apple logo, versus the ConScript Unicode Registry’s use of U+F8FF as klingon mummification glyph in the Klingon script[1].
The Basic Multilingual Plane includes a PUA in the range from U+E000 to U+F8FF (6,400 code locations). Plane Fifteen and Plane Sixteen have a PUAs that consist of all but their final two code locations, which are designated non-characters. The PUA in Plane Fifteen is the range from U+F0000 to U+FFFFD (65,534 code locations). The PUA in Plane Sixteen is the range from U+100000 to U+10FFFD (65,534 code locations).
PUAs are a concept inherited from certain Asian encoding systems. These systems had private use areas to encode what the Japanese call gaiji (rare characters not normally found in fonts) in application-specific ways.
Schemes and initiatives that use the PUA include:
At the simplest level, each character in the UCS represents a code point and a particular semantic function: For graphical characters, the semantic function is often implied by its name, and the script or block it is included within. A graphical character may also have a recommended glyph that helps define the meaning of the character. Han characters, used in China, Japan, Korea, Vietnam and their respective diaspora, include many other rich properties that participate in defining the semantic role for a character.
However, the UCS and Unicode designate other code points for other purposes. Those code points may have no or few character properties associated with them.
The 2,048 surrogates are not characters, but are reserved for use in UTF-16 to specify code points outside the Basic Multilingual Plane. They are divided into leading or "high surrogates" (D800–DBFF) and trailing or "low surrogates" (DC00–DFFF). In UTF-16, they must always appear in pairs, as a high surrogate followed by a low surrogate, thus using 32 bits to denote one code point.
A surrogate pair denotes the code point
where H and L are the numeric values of the high and low surrogates respectively.
Since high surrogate values in the range DB80–DBFF always produce values in the Private Use planes, the high surrogate range can be further divided into (normal) high surrogates (D800–DB7F) and "high private use surrogates" (DB80–DBFF).
Unicode defines sixty-six code points as non-characters (labeled <not a character>), never to change. In these 66, the last two code points of each plane are included. So, noncharacters are: U+FFFE and U+FFFF on the BMP, U+1FFFE and U+1FFFF on Plane 1, and so on, up to U+10FFFE and U+10FFFF on Plane 16, for a total of 34 code points. In addition, there is a contiguous range of another 32 noncharacter code points in the BMP: U+FDD0..U+FDEF. Software implementations are therefore free to use these code points for internal use. However, these noncharacters should never be included in text interchange between implementations. One particularly useful example of a noncharacter is the code point U+FFFE. This code point has the reverse binary sequence of the byte order mark (U+FEFF). If a stream of text contains this noncharacter, this is a good indication the text has been interpreted with the incorrect endianness.
Every character in Unicode is defined by a large and growing set of properties. The properties facilitate text processing including collation or sorting of text, identifying words, sentences and graphemes, rendering or imaging text and so on. Below is a list of some of the core properties. There are many others documented in the Unicode Character Database.
| Property | Example | Details |
| Name | LATIN CAPITAL LETTER A | This is a permanent name assigned by the joint cooperation of Unicode and the ISO UCS |
| Code Point | U+0041 | The Unicode code point is a number also permanently assigned along with the "Name" property and included in the companion UCS. The usual custom is to represent the code point as hexadecimal number with the prefix "U+" in front. |
| Representative Glyph | [7] | The representative glyphs are provided in code charts. |
| General Category | Uppercase_Letter | The general category is expressed as a two-letter sequence such as "Lu" for uppercase letter or "Nd", for decimal digit number. |
| Combining Class | Not_Reordered (0) | Since diacritics and other combining marks can be expressed with multiple characters in Unicode the "Combining Class" property allows characters to be differentiated by the type of combining character it represents. The combining class can be expressed as an integer between 0 and 255 or as a named value. The integer values allow the combining marks to be reordered into a canonical order to make string comparison of identical strings possible. |
| Bidirectional Category | Left_To_Right | Indicates the type of character for applying the Unicode bidirectional algorithm. |
| Bidirectional Mirrored | no | Indicates the character’s glyph must be reversed or mirrored within the bidirectional algorithm. Mirrored glyphs can be provided by font makers, extracted from other characters related through the “Bidirectional Mirroring Glyph” property or synthesized by the text rendering system. |
| Bidirectional Mirroring Glyph | N/A | This property indicates the code point of another character whose glyph can serve as the mirrored glyph for the present character when mirroring within the bidirectional algorithm. |
| Decimal Digit Value | NaN | For numerals, this property indicates the numeric value of the character. Decimal digits have all three values set to the same value, presentational rich text compatibility characters and other Arabic-Indic non-decimal digits typically have only the latter two properties set to the numeric value of the character while numerals unrelated to Arabic Indic digits such as Roman Numerals or Hanzhou/Suzhou numerals typically have only the "Numeric Value" indicated. |
| Digit Value | NaN | |
| Numeric Value | NaN | |
| Ideographic | False | Indicates the character is an ideograph. |
| Default Ignorable | False | Indicates the character is ignorable for implementations and that no glyph, last resort glyph, or replacement character need be displayed. |
| Deprecated | False | Unicode never removes characters from the repertoire, but on occasion Unicode has deprecated a small number of characters. |
| Bidirectional | Numeric Value | |||||||||
| Name | Code Point |
Repre- sentative Glyph[7] |
General Category |
Combining Class |
Category | Mirrored | Mirroring Glyph | Decimal | Digit | Numeric |
| DIGIT FOUR | U+0034 | 4 | Decimal_Number_Digit (Nd) | Not_Reordered (0) | European_Number | no | n/a | 4 | 4 | 4 |
| DEVANAGARI DIGIT FOUR | U+096A | ४ | Decimal_Number_Digit (Nd) | Not_Reordered (0) | Left_To_Right | no | n/a | 4 | 4 | 4 |
| CIRCLED DIGIT FOUR | U+2463 | ④ | Other_Number (Nd) | Not_Reordered (0) | Other_Neutral | no | n/a | n/a | 4 | 4 |
| ROMAN NUMERAL FOUR | U+2163 | Ⅳ | Letter_Number (Nd) | Not_Reordered (0) | Left_To_Right | no | n/a | n/a | n/a | 4 |
| LEFT CURLY BRACKET | U+007B | { | Open_Punctuation (Ps) | Not_Reordered (0) | Other_Neutral (On) | yes | “}” U+007D | NaN | NaN | NaN |
| COMBINING CIRCUMFLEX ACCENT | U+0302 | ̂ | Nonspacing_Mark (Mn) | Above (230) | Nonspacing_Mark (NSM) | no | n/a | NaN | NaN | NaN |
| COMBINING GRAVE ACCENT BELOW | U+0316 | ̖ | Nonspacing_Mark (Mn) | Below (220) | Nonspacing_Mark (NSM) | no | n/a | NaN | NaN | NaN |
| ARABIC LETTER BEH | U+0628 | ب | Other_Letter (Lo) | Not_Reordered (0) | Arabic_Letter (AL) | no | n/a | n/a | n/a | n/a |
| HEBREW LETTER BET | U+05D1 | ב | Other_Letter (Lo) | Not_Reordered (0) | Right_To_Left (R) | no | n/a | n/a | n/a | n/a |
| CJK UNIFIED IDEOGRAPH-4E0F (kDefinition = parapet; invisible) | U+4E0F | 丏 | Other_Letter (Lo) | Not_Reordered (0) | Left_To_Right (L) | no | n/a | n/a | n/a | n/a |
Characters include many other properties. Some properties are strings, some are booleans, some are relations to other characters. For example cased letters include properties that map those characters to their upper case, lower case and title case equivalents (title case is only used for ligatures). Some characters (canonical and compatibility decomposable characters) include mappings to canonical and compatibility equivalents. Characters have many boolean properties to indicate whether they are included as white space, or used as pattern syntax within programming languages and more. Many of these properties are exposed through regular expressions to perform complex queries on text. These properties are also used in the many Unicode text processing algorithms and also might be used by text imaging and font technologies to display text (like the bidirectional algorithm).
Unicode provides an online database to interactively query the entire Unicode character repertoire by the various properties.