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<!DOCTYPE html>
<html dir="ltr" lang="en">
<head>
<meta charset="UTF-8">
<title>Character Model for the World Wide Web: String Matching</title>
<link rel="canonical" href="http://www.w3.org/TR/2015/WD-charmod-norm-20151119/"/>
<!-- local styles. Includes the styles from http://www.w3.org/International/docs/styleguide -->
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<body>
<section id="abstract">
<p>This document builds upon on <cite>Character Model for the World Wide
Web 1.0: Fundamentals </cite>[[!CHARMOD]] to provide authors of
specifications, software developers, and content developers a common
reference on string identity matching on the World Wide Web and thereby
increase interoperability. </p>
</section>
<section id="sotd">
<div class="note">
<p>This version of the document represents a significant change from the
<a href="http://www.w3.org/TR/2012/WD-charmod-norm-20120501/">earlier
editions</a>. Much of the content is changed and the recommendations
are significantly altered. This fact is reflected in a change to the
name of the document from "Character Model: Normalization".</p>
</div>
<div class="note">
<p data-lang="en" style="font-weight: bold; font-size: 120%">Sending
comments on this document</p>
<p data-lang="en">If you wish to make comments regarding this document,
please raise them as <a href="https://github.com/w3c/charmod-norm/issues"
style="font-size: 120%;">github issues</a>. When reviewing the document,
please refer to the latest <a href="https://w3c.github.io/charmod-norm">editor's
copy</a>. Only send comments by email if you are unable to raise issues on github (see
links below). All comments are welcome.</p>
<p data-lang="en">To make it easier to track comments, please raise
separate issues or emails for each comment, and point to the section
you are commenting on using a URL.</p>
</div>
</section>
<section id="intro">
<h2>Introduction</h2>
<section id="goals">
<h3>Goals and Scope</h3>
<p>The goal of the Character Model for the World Wide Web is to facilitate use of the Web by all people, regardless of their language, script, writing system, or cultural conventions, in accordance with the <a href="http://www.w3.org/Consortium/mission"><cite>W3C goal of universal access</cite></a>. One basic prerequisite to achieve this goal is to be able to transmit and process the characters used around the world in a well-defined and well-understood way.</p>
<p class="note">This document builds on <cite>Character Model for the World Wide Web: Fundamentals</cite> [[!CHARMOD]]. Understanding the concepts in that document are important to being able to understand nd apply this document successfully.</p>
<p>This part of the Character Model for the World Wide Web covers string
matching—the process by which a specification or implementation defines
whether two string values are the same or different from one another. It
describes the ways in which texts that are semantically equivalent can
be encoded differently and the impact this has on matching operations
important to formal languages (such as those used in the formats and
protocols that make up the Web).</p>
<p>The main target audience of this specification is W3C specification
developers. This specification and parts of it can be referenced from
other W3C specifications and it defines conformance criteria for W3C
specifications, as well as other specifications.</p>
<p>Other audiences of this specification include software developers,
content developers, and authors of specifications outside the W3C.
Software developers and content developers implement and use W3C
specifications. This specification defines some conformance criteria
for implementations (software) and content that implement and use W3C
specifications. It also helps software developers and content
developers to understand the character-related provisions in W3C
specifications.</p>
<p>The character model described in this specification provides authors
of specifications, software developers, and content developers with a
common reference for consistent, interoperable text manipulation on
the World Wide Web. Working together, these three groups can build a
globally accessible Web.</p>
</section>
<section id="structure">
<h3>Structure of this Document</h3>
<p>This document defines one of the basic building blocks for the Web related
to this problem by defining rules and processes for String
Identity Matching in document formats. These rules are designed for
the identifiers and structural markup (<a href="#def_syntactic_content" class="termref">syntactic content</a>)
used in document formats to ensure consistent processing of each and are
targeted to Specification writers. This
section is targeted to implementers.</p>
<p>This document is divided into two main sections.</p>
<p>The <a href="#problemStatement">first section</a> lays out the
problems involved in string matching; the effects of Unicode and case
folding on these problems; and outlines the various issues and
normalization mechanisms that might be used to address these issues.</p>
<p>The <a href="#identityMatching">second section</a> provides
requirements and recommendations for string identity matching for use
in <span class="qterm">formal languages</span>, such as many of the
document formats defined in W3C Specifications. This primarily is
concerned with making the Web functional and providing document
authors with consistent results. </p>
</section>
<section id="background">
<h3>Background</h3>
<p>This section provides some historical background on the topics
addressed in this specification.</p>
<p>At the core of the character model is the Universal Character Set
(UCS), defined jointly by the <cite>Unicode Standard</cite>
[[!Unicode]] and ISO/IEC 10646 [[!ISO10646]]. In this document, <dfn>Unicode</dfn>
is used as a synonym for the Universal Character Set. A successful
character model allows Web documents authored in the world's writing
systems, scripts, and languages (and on different platforms) to be
exchanged, read, and searched by the Web's users around the world.</p>
<p>The first few chapters of the <cite>Unicode Standard</cite>
[[!Unicode]] provide useful background reading.</p>
<p>For information about the requirements that informed the development
of important parts of this specification, see <cite>Requirements for
String Identity Matching and String Indexing</cite> [[CHARREQ]].</p>
</section>
<section id="terminology">
<h3>Terminology and Notation</h3>
<p>This section contains terminology and notation specific to this document.</p>
<p>The Web is built on text-based formats and protocols. In order to
describe string matching or searching effectively, it is necessary to
establish terminology that allows us to talk about the different kinds
of text within a given format or protocol, as the requirements and
details vary significantly. </p>
<p>A <dfn data-lt="Unicode code point|Unicode code points|code point|code points">Unicode code point</dfn> (or "code point") refers to the numeric value assigned to each Unicode character. Unicode code points range from 0 to 0x10FFFF<sub>16</sub>. (See Section 4.1 in [[!CHARMOD]] for a deeper discussion of character encoding terminology.)</p>
<p>Unicode code points are denoted as <code class="kw" translate="no">U+<em>hhhh</em></code>, where <code class="kw" translate="no"><em>hhhh</em></code> is a sequence of at least four, and at most six hexadecimal digits. For example, the character <span class="codepoint"><span lang="en">€</span> [<span class="uname">U+20AC EURO SIGN</span>]</span> has the code point <span class="uname" translate="no">U+20AC</span>.</p>
<p>Some characters used in this document's examples might not appear as intended on your specific device or display. Usually this is due to lack of a script-specific font installed locally or due to other limitations of your specific rendering system. This document uses a Webfont to provide fallback glyphs for many of the non-Latin characters, but your device might not support displaying the font. To the degree possible, the editors have tried to ensure that the examples nevertheless remain understandable.</p>
<p>A <dfn data-lt="legacy character encoding|legacy character encodings">legacy character encoding</dfn> is a character encoding not based on the Unicode character set.</p>
<p>A <dfn data-lt="transcoder|transcoders">transcoder</dfn> is a process that converts
text between two character encodings. Most commonly in this document it
refers to a process that converts from a <a>legacy character encoding</a>
to a <a href="http://www.w3.org/TR/2005/REC-charmod-20050215/#Unicode_Encoding_Form">Unicode encoding form</a>,
such as UTF-8.</p>
<p><dfn id="def_syntactic_content">Syntactic content</dfn> is any text in a document format or protocol that belongs to the structure of the format or protocol. This definition includes values that are typically thought of as "markup" but can also include other values, such as the name of a field in an HTTP header. Syntactic content consists of all of the characters that form the <em>structure</em> of a format or protocol. For example, <span class="qchar"><</span> and <span class="qchar">></span> (as well as the element name and various attributes they surround) are part of the syntactic content in an HTML document.</p>
<p>Syntactic content usually is defined by a specification or specifications and
includes both the defined, reserved keywords for the given protocol or
format as well as string tokens and identifiers that are defined by
document authors to form the structure of the document (rather than
the "content" of the document).</p>
<aside class="example">
<p><cite>XML</cite> [[XML10]] defines specific elements, attributes,
and values that are reserved across all XML documents. Thus, the
word <code class="kw" translate="no">encoding</code> has a defined
meaning inside the XML document declaration: it is a reserved name.
XML also allows a user to define elements and attributes for a given
document, for example, by using a DTD. In a document that uses a DTD that defines an
element called <code class="kw"><muffin></code>, <span class="qterm">muffin</span>
is a part of the syntactic content.</p>
</aside>
<p><dfn>Natural language content</dfn> refers to the language-bearing
content in a document and <b>not</b> to any of the surrounding or embedded
syntactic content that form part of the document structure. You can think
of it as the actual "content" of the document or the "message" in a
given protocol. Note that syntactic content can contain natural
language content, such as when an [[HTML]] <code class="kw">img</code> element
has an <code class="kw">alt</code> attribute containing a description of
the image.</p>
<p>A <dfn data-lt="resource|resources">resource</dfn>, in the context of this document, is a given
document, file, or protocol "message" which includes both the <a>natural
language content</a> as well as the <a href="#def_syntactic_content" class="termref">syntactic content</a>
such as identifiers surrounding or containing it. For example, in an
HTML document that also has some CSS and a few <code class="kw" translate="no">script</code>
tags with embedded JavaScript, the entire HTML document, considered as
a file, is a resource. This term is intentionally similar to the term
'resource' as used in [[RFC3986]], although here the term is applied
loosely. </p>
<p>A <dfn data-lt="user-supplied value|user-supplied values">user-supplied value</dfn> is unreserved syntactic
content in a <a>vocabulary</a> that is assigned by users, as distinct
from reserved keywords in a given format
or protocol. For example, CSS class names are part of the syntax of a CSS
style sheet. They are not reserved keywords, predefined by any CSS specification.
They are subject to the syntactic rules of CSS. And they may (or may not)
consist of natural language tokens.</p>
<p>A <dfn id="def_vocabulary">vocabulary</dfn> provides the list of
reserved names as well as the set of rules and specifications
controlling how <a>user-supplied values</a> (such as identifiers) can be assigned in a
format or protocol. This can include restrictions on range, order, or
type of characters that can appear in different places. For example,
HTML defines the names of its elements and attributes, as well as
enumerated attribute values, which defines the "vocabulary" of HTML
<a href="#def_syntactic_content" class="termref">syntactic content</a>.
Another example would be ECMAScript, which restricts the range of
characters that can appear at the start or in the body of an identifier
or variable name. It applies different rules for other cases, such as to
the values of string literals.</p>
<p>A <dfn data-lt="grapheme|graphemes">grapheme</dfn> is a sequence of
one or more characters in a visual representation of some text
that a typical user would perceive as being a single unit (<q>character</q>).
Graphemes are important for a number of text operations such as
sorting or text selection, so it is necessary to be able to compute
the boundaries between each user-perceived character. Unicode defines
the default mechanism for computing graphemes in <cite>Unicode
Standard Annex #29: Text Segmentation</cite> [[!UAX29]] and calls
this approximation a <dfn>grapheme cluster</dfn>. There are two types
of default grapheme cluster defined. Unless otherwise noted, grapheme
cluster in this document refers to an extended default grapheme
cluster. (A discussion of grapheme clusters is also given in Section 2
of the <cite>Unicode Standard</cite>, [[!Unicode]]. Cf. near the end of
<a href="http://www.unicode.org/versions/Unicode8.0.0/ch02.pdf">Section 2.11</a>
in version 8.0 of The Unicode Standard)</p>
<p>Because different natural languages have different needs, grapheme clusters
can also sometimes require tailoring. For example, a Slovak user might
wish to treat the default pair of grapheme clusters "ch" as a single
grapheme cluster. Note that the interaction between the language of
string content and the end-user's preferences might be complex.</p>
<aside class="example" id=graphemeExample>
<p>The Hindi word for Unicode <q>यूनिकोड</q> is composed of seven Unicode characters from the Devanagari script.
</p>
<p>Most users would identify this word as containing four units of text. Each of the first three graphemes consists of two characters: a syllable and a modifying vowel character. So the word contains seven Unicode characters, but only four graphemes:</p>
<table>
<tr>
<td>Word</td>
<td colspan=7 class="bigtext">यूनिकोड</td>
</tr>
<tr>
<td>Graphemes</td>
<td class="bigtext" colspan=2>यू</td>
<td class="bigtext" colspan=2>नि</td>
<td class="bigtext" colspan=2>को</td>
<td class="bigtext">ड</td>
</tr>
<tr>
<td>Code Points</td>
<td class="bigtext">य</td>
<td class="bigtext">ू</td>
<td class="bigtext">न</td>
<td class="bigtext">ि</td>
<td class="bigtext">क</td>
<td class="bigtext">ो</td>
<td class="bigtext">ड</td>
</tr>
<tr>
<td></td>
<td style="text-align:center">U+092F</td>
<td style="text-align:center">U+0942</td>
<td style="text-align:center">U+0928</td>
<td style="text-align:center">U+093F</td>
<td style="text-align:center">U+0915</td>
<td style="text-align:center">U+094B</td>
<td style="text-align:center">U+0921</td>
</tr>
</table>
</aside>
<section>
<h5>Terminology Examples</h5>
<p>This section illustrates some of the terminology defined above. For illustration purposes we'll use the following small HTML file as an example (line numbers added for reference):</p>
<div class=terminologyExample>
<p class=syntaxExample><span class=lnum>1 </span> <span class="markup"><<span class="vocabulary">html</span> <span class="vocabulary">lang</span>="<span class=userValue>en</span>" <span class="vocabulary">dir</span>="<span class="vocabulary">ltr</span>"></span></p>
<p class=syntaxExample><span class=lnum>2 </span><span class="markup"><<span class="vocabulary">head</span>></span></p>
<p class=syntaxExample><span class=lnum>3 </span><span class="markup"> <<span class="vocabulary">meta</span> <span class="vocabulary">charset</span>="<span class=userValue>UTF-8</span>"></span></p>
<p class=syntaxExample><span class=lnum>4 </span><span class=markup> <<span class="vocabulary">title</span>></span><span class="shakespeare">Shakespeare</span><span class="markup"></<span class="vocabulary">title</span>></span></p>
<p class=syntaxExample><span class=lnum>5 </span></<span class="vocabulary">head</span>></p>
<p class=syntaxExample><span class=lnum>6 </span><<span class="vocabulary">body</span>></p>
<p class=syntaxExample><span class=lnum>7 </span><span class=markup> <<span class="vocabulary">img</span> <span class="vocabulary">src</span>="<span class="userValue">shakespeare.jpg</span>" <span class="vocabulary">alt</span>="<span class="userValue"><span class="shakespeare">William Shakespeare</span></span>" <span class="vocabulary">id</span>="<span class="userValue">shakespeare_image</span>"></span></p>
<p class=syntaxExample><span class=lnum>8 </span><span class=markup> <<span class="vocabulary">p</span>></span><span class="shakespeare">What<span class="markup">&#x2019;</span>s in a name? That which we call a rose by any other name would smell as sweet.</span><span class="markup"></<span class="vocabulary">p</span>></span></p>
<p class=syntaxExample><span class=lnum>9 </span></<span class="vocabulary">body</span>></p>
<p class=syntaxExample><span class=lnum>10 </span><span class=markup></<span class="vocabulary">html</span>></span> </p>
</div>
<ul style="text-align:left">
<li>Everything inside the black rectangle (that is, in this HTML file) is part of the <a>resource</a>.</li>
<li><a>Syntactic content</a> in this case includes all of the HTML markup. There are only two strings that are <strong>not</strong> part of the syntactic content: the word <em>"Shakespeare"</em> on line 4 and the sentence <em>"What’s in a name? That which we call a rose by any other name would smell as sweet."</em> on line 8. (The HTML entity <q><kbd>&#x2019;</kbd></q> embedded in the sentence on line 8 <em>is</em> part of the syntactic content.)</li>
<li><a>Natural language content</a> is shown in a <span class="shakespeare">bold blue font with a gray background</span>. In addition to the non-syntactic content, the <kbd>alt</kbd> value on line 7 (<em><q>William Shakespeare</q></em>) contains natural language text.</li>
<li>User-supplied values are shown in <span class="userValue">italics</span>. In this case there are three user-supplied values on line 7: the values of the <kbd>src</kbd>, <kbd>alt</kbd>, and <kbd>id</kbd> attributes of the <kbd>img</kbd> tag. In addition, the value of the <kbd>lang</kbd> attribute on line 1 and the <kbd>charset</kbd> attribute on line 3 are user-supplied values.</li>
<li><a>Vocabulary</a> is shown with <span class="vocabulary">red underlining</span>. The vocabulary of an HTML document are the elements and attributes (as well as some of the attribute values, such as the value <kbd>ltr</kbd> for the attribute <kbd>dir</kbd> in the example above) defined in [[HTML5]].</li>
</ul>
<p class=note>All of the text above (all text in a text file) makes up the resource. It's possible that a given resource will contain no natural language content at all (consider an HTML document consisting of four empty <code>div</code> elements styled to be orange rectangles). It's also possible that a resource will contain <em>no</em> syntactic content and consist solely of natural language content: for example, a plain text file with a soliloquy from <cite>Hamlet</cite> in it. Notice too that the HTML entity <code>&#x2019;</code> appears in the natural language content and belongs to both the natural language content and the syntactic content in this resource.</p>
</section>
</section>
<section id="conformance">
<p>This document describes best practices for the authors of other specifications, as well as recommendations for implementations and content authors. These best practices can also be found in the Internationalization Working Group's document <cite>[[!INTERNATIONAL-SPECS]]</cite>, which is intended to serve as a general reference for all Internationalization best practices in W3C specifications.</p>
<p class=requirement>When a best practice or recommendation appears in this document, it has been styled to like this paragraph. Recommendations for specifications and spec authors are preceded by <span class=qrec>[S]</span>. Recommendations for implementations and software developers are preceeded by <span class=qrec>[I]</span>. Recommendations for content and content authors are preceeded by <span class=qrec>[C]</span>.</p>
<p>Best practices in this document use [[!RFC2119]] keywords in order to clarify the Internationalization Working Group's intentions regarding a specific recommendation. Following the recommendations in this document can help avoid issues during the W3C's "wide review" process, during implementation, or in the content that authors produce. This document is not, itself, normative and can be revised from time to time.</p>
<p>Specifications can claim conformance to this document if they:</p>
<ol type="1">
<li>do not violate any conformance criteria preceded by <span class=qrec>[S]</span> where the imperative is MUST or MUST NOT</li>
<li>document the reason for any deviation from criteria where the imperative is SHOULD, SHOULD NOT, or RECOMMENDED</li>
<li>make it a conformance requirement for implementations to conform to this document</li>
<li>make it a conformance requirement for content to conform to this document</li>
</ol>
<p class=note>Requirements placed on specifications might indirectly cause requirements to be placed on implementations or content that claim to conform to those specifications.</p>
<p>Where this specification contains a procedural description, it is to be understood as a way to specify the desired external behavior. Implementations MAY use other means of achieving the same results, as long as observable behavior is not affected.</p>
</section>
</section>
<section id="problemStatement">
<h2>The String Matching Problem</h2>
<p>The Web is primarily made up of document formats and protocols based on
character data. These formats or protocols can be viewed as a set of <a>resources</a> consisting mainly of text files that include some form of structural markup or <a>syntactic content</a>. Processing such syntactic content or document data requires
string-based operations such as matching (including regular expressions), indexing, searching, sorting,
and so forth.</p>
<p>Users, particularly implementers, sometimes have naïve expectations regarding the matching or non-matching
of similar strings or of the efficacy of different transformations they might apply to text, particularly to
syntactic content, but including many types of text processing on the Web.</p>
<p>Because fundamentally the Web is sensitive to the different ways in which text might be represented in a
document, failing to consider the different ways in which the same text can be represented can confuse
users or cause unexpected or frustrating results. In the sections below, this document examines the different
types of text variation that affect both user perception of text on the Web and the string processing on which
the Web relies.</p>
<section id="definitionCaseFolding">
<h3>Case Mapping and Case Folding</h3>
<p>Some scripts and writing systems make a distinction between UPPER,
lower, and Title case characters. Most scripts, such as the Brahmic
scripts of India, the Arabic script, and the scripts used to
write Chinese, Japanese, or Korean do not have a case distinction, but
some important ones do. Examples of such scripts include the Latin
script used in the majority of this document, as well as scripts such
as Greek, Armenian, and Cyrillic. </p>
<p><dfn>Case mapping</dfn> is the process of transforming characters to a specific case, such as upper, lower, or titlecase. For those scripts that have a case distinction, Unicode defines a <em>default</em> UPPER, lower, and Title case character mapping for each Unicode code point. Case mapping, at first, appears simple. However there are variations that need to be considered when treating the full range of Unicode in diverse languages.</p>
<aside class="note">
<p>For more information, <cite>[[!Unicode]]</cite> Chapter 5 (in v8.0, <a href="">Section 5.18</a>) discusses case mappings and case folding in detail. </p>
</aside>
<aside class="example">
<p>For example here is a character with mappings to all three case variations. These mappings are defined in the Unicode Character Database (UCD).</p>
<table>
<tr>
<th>Uppercase</th>
<th>Lowercase</th>
<th>Titlecase</th>
</tr>
<tr>
<td class="exampleChar">DŽ</td>
<td class="exampleChar">dž</td>
<td class="exampleChar">Dž</td>
</tr>
<tr>
<td>U+01C4</td>
<td>U+01C6</td>
<td>U+01C5</td>
</tr>
</table>
</aside>
<p><dfn>Case folding</dfn> is the process of making two texts which differ only in case identical for comparison purposes, that is, it is meant for the purpose of string matching. This is distinct from <a>case mapping</a>, which is primarily meant for display purposes. As with the default case mappings, Unicode defines default case fold mappings for each Unicode code point. Unicode defines two forms of case fold mapping, which we'll examine below.</p>
<p>Since most scripts do not have a case distinction, as with case mappings, most Unicode code points do not require a case fold mapping. For those characters that
have a case fold mapping, the majority have a simple, straight-forward mapping to a single matching (generally lowercase) code point. Unicode
calls these the <code class="kw">common</code> case fold mappings, as they are shared by Unicode's case fold mappings.
</p>
<aside class="example">
<p>Here are some examples of <code class="kw">common</code> case fold mappings:</p>
<table>
<tr>
<td class="exampleChar">A</td>
<td>⇒</td>
<td class="exampleChar">a</td>
<td><span class="uname">LATIN CAPITAL LETTER A</span> to <span class="uname">LATIN SMALL LETTER A</span></td>
</tr>
<tr>
<td class="exampleChar">Θ</td>
<td>⇒</td>
<td class="exampleChar">θ</td>
<td><span class="uname">GREEK CAPITAL LETTER THETA</span> to <span class="uname">GREEK SMALL LETTER THETA</span></td>
</tr>
<tr>
<td class="exampleChar">Д</td>
<td>⇒</td>
<td class="exampleChar">д</td>
<td><span class="uname">CYRILLIC CAPITAL LETTER DE</span> to <span class="uname">CYRILLIC SMALL LETTER DE</span></td>
</tr>
</table>
</aside>
<p>A few characters have a case fold mapping that map one Unicode code point to two or more code points during case folding. These are called the <code class="kw">full</code> case fold mappings. The <code class=kw>full</code> and <code class=kw>common</code> case fold mappings are used together to provide the default case fold mapping for all of Unicode. We refer to this form of case folding as <dfn>full casefolding</dfn> or <dfn>Unicode full</dfn> in this document.
</p>
<aside class="example">
<p>One well-known example of a <code class="kw">full</code> case fold mapping is the character <span class="qchar">ß</span>
<span class="uname" translate="no">U+00DF LATIN SMALL LETTER SHARP S</span>, a letter that is commonly
used in the German language. The <code class="kw">full</code> case fold mapping and the lower case mapping of this character is to two ASCII letters 's'. The upper case mapping is to "SS".
</p>
<table>
<tr>
<td class="exampleChar">ß</td>
<td>⇒</td>
<td class="exampleChar">ss</td>
<td><span class="uname">LATIN SMALL LETTER SHARP S</span> to <span class="uname">LATIN SMALL LETTER S</span> + <span class="uname">LATIN SMALL LETTER S</span></td>
</tr>
</table>
</aside>
<p>Because some applications cannot allocate additional storage when performing a case fold operation, Unicode provides a <code class="kw">simple</code> case fold mapping that maps characters that would normally map to more or
fewer code points to use a single code point for comparison purposes instead. Unlike the full mapping, this mapping invariably alters the content (and potentially the meaning) of the text. As with <a>full casefolding</a>, the <dfn>simple casefolding</dfn> or <dfn>Unicode simple</dfn> casefold, is a combination of <code class=kw>simple</code> and <code class=kw>common</code> mappings so as to cover the full range of Unicode. <a>Unicode simple</a> is not appropriate for use on the Web. </p>
<aside class="example">
<p>Examples of <code class=kw>full</code> versus <code class=kw>simple</code> case fold variations can be found in the Greek script, where several precomposed characters have multi-character case fold mappings. The table below shows one such example, the character <span class="codepoint"><span lang="en">ᾛ</span> [<span class="uname">U+1F9B GREEK CAPITAL LETTER ETA WITH DASIA AND VARIA AND PROSGEGRAMMENI</span>]</span> and its <code class="kw">full</code> and <code class="kw">simple</code> case fold mappings:</p>
<table style="width: 100%">
<tr>
<td class="exampleChar">ᾛ</td>
<td>⇒</td>
<td class="exampleChar">ἣι</td>
<td><code class=kw>full</code> case fold: <code>U+1F23 U+03B9</code> <span class="uname">GREEK SMALL LETTER ETA WITH DASIA AND VARIA</span> + <span class="uname">GREEK SMALL LETTER IOTA</span></td>
</tr>
<tr>
<td class="exampleChar">ᾛ</td>
<td>⇒</td>
<td class="exampleChar">ᾓ</td>
<td><code class=kw>simple</code> case fold: <code>U+1F93</code> <span class="uname" translate="no">GREEK SMALL LETTER ETA WITH DASIA AND VARIA AND PROSGEGRAMMENI</span></td>
</tr>
</table>
</aside>
<p>Note that case folding removes information from a string which cannot
be recovered later. For example, two <span class="qchar">s</span> letters in German do not necessarily represent <span class="qchar">ß</span> in unfolded text.</p>
<section id="caseMappingLanguageSensitivity">
<h3>Language Sensitivity</h3>
<p>Another aspect of case mapping and case folding is that it can be language sensitive.
Unicode defines <em>default</em> case mappings and case fold mappings for each encoded character, but
these are only defaults and are not appropriate in all cases. Some
languages need case-folding to be tailored to meet specific linguistic
needs. One common example of this are Turkic languages written in the
Latin script:</p>
<aside class="example">
<table>
<tr>
<th colspan=4>Default Folding</th>
</tr>
<tr>
<td class="exampleChar">I</td>
<td>⇒</td>
<td class="exampleChar">i</td>
<td>Default folding of letter I</td>
</tr>
<tr>
<th colspan=4>Turkic Language Folding</th>
</tr>
<tr>
<td class="exampleChar">I</td>
<td>⇒</td>
<td class="exampleChar">ı</td>
<td>Turkic language folding of dotless (ASCII) letter I</td>
</tr>
<tr>
<td class="exampleChar">İ</td>
<td>⇒</td>
<td class="exampleChar">i</td>
<td>Turkic language folding of dotted letter I</td>
</tr>
</table>
</aside>
<p>While the example above (and this document in general) focuses on case folding for the purpose of matching, note that case mapping is also language-specific. The name of the second largest city in Turkey is "<code>Diyarbakır</code>", which
contains both the dotted and dotless letters <span class="qchar">i</span>.
</p>
<aside class="example">
<p><span class="exampleChar">Diyarbakır</span> ⇒ <code>text-transform: uppercase</code> ⇒ <span class="exampleChar">DİYARBAKIR</span></p>
<p>Notice that the ASCII letter <span class="codepoint"><span lang="en">i</span> [<span class="uname">U+0069 LATIN SMALL LETTER I</span>]</span> maps to <span class="codepoint"><span lang="en">İ</span> [<span class="uname">U+0130 LATIN CAPITAL LETTER I WITH DOT ABOVE</span>]</span>, while the letter <span class="codepoint"><span lang="en">ı</span> [<span class="uname">U+0131 LATIN SMALL LETTER DOTLESS I</span>]</span> maps to the ASCII uppercase <span class="codepoint"><span lang="en">I</span> [<span class="uname">U+0049 LATIN CAPITAL LETTER I</span>]</span>. Failure to apply this localized case mapping would change the meaning of the text in Turkish, even though this is the expected mapping in other languages, such as English or German.</p>
<p>This language-specific tailoring can also be applied to case folding. For example, if the uppercase text needed to be matched against some set of strings in a case-insensitive way:</p>
<p><span class="exampleChar">DİYARBAKIR</span> ⇒ <code>case fold</code> ⇒ <span class="exampleChar">diyarbakır</span></p>
</aside>
</section>
<section id="caseFoldApplication">
<h3>Uses for Case Folding</h3>
<p>Some document formats or protocols seek to aid interoperability or provide an aid to content authors by ignoring case variations in the <a data-lt="vocabulary">vocabulary</a> they define or in <a>user-supplied values</a> permitted by the format or protocol.</p>
<aside class="example">
<p>One example where this occurs is when matching element names between an HTML document and its associated style sheet. Consider this HTML fragment: </p>
<pre><style type="text/css">
SPAN.hello {
text-decoration: underline;
}
</style>
<span class="hello">Hello World!</span>
</pre>
<p>The <code class="kw" translate="no">SPAN</code> in the stylesheet
matches the <code class="kw" translate="no">span</code> element in the
document, even though the stylesheet uses uppercase and the HTML markup
does not.</p>
</aside>
<p>Sometimes case can vary in a way that is not semantically meaningful
or is not fully under the user's control. This is particularly true
when searching a document, but may sometimes also apply
when defining rules for matching user- or content-generated values,
such as identifiers. In these situations, case-<em>in</em>sensitive
matching might be desirable instead.</p>
<p>When defining a <a>vocabulary</a>, one important consideration is whether the values are restricted to the ASCII [[ASCII]] subset of Unicode or if the vocabulary permits the use of characters (such as accents on Latin letters or a broad range of Unicode including non-Latin scripts) that potentially have more complex case folding requirements. To address these different requirements, there are four types of casefold matching defined by this document for the purposes of string identity matching in document formats or protocols:</p>
<p id="case-sensitive"><dfn data-lt="case-sensitive">Case sensitive matching</dfn>: code points are compared directly with no case folding.</p>
<p id="aci"><dfn data-lt="ASCII case-insensitive">ASCII case-insensitive matching</dfn>
compares a sequence of code points as if all ASCII code points in the
range 0x41 to 0x5A (A to Z) were mapped to the corresponding code
points in the range 0x61 to 0x7A (a to z). When a vocabulary is itself
constrained to ASCII, ASCII case-insensitive matching can be required.
</p>
<p id="uci"><dfn data-lt="Unicode case-insensitive">Unicode case-insensitive matching</dfn> compares a sequence of code points as if the <a>Unicode full</a> casefolding (see above) had been applied to both input sequences.</p>
<p><dfn>Language-sensitive case-sensitive matching</dfn> is useful in the rare case where a document format or protocol contains information about the language of the syntactic content and where language-sensitive case folding might sensibly be applied. These case-fold mappings are defined in the <cite>Common Locale Data Repository</cite> [[UAX35]] project of the Unicode Consortium.</p>
<p>For advice on how to handle case folding see <a href="#handlingCaseFolding"></a>.</p>
</section>
</section>
<section id="unicodeNormalization">
<h3>Unicode Normalization</h3>
<p>A different kind of variation can occur in Unicode text: sometimes several different <a>Unicode code point</a> sequences can be used to represent the same abstract character. When searching or matching text by comparing code points, these variations in encoding cause text values not to match that users expect to be the same. </p>
<aside class=example id=aringExample title="Encoding Variations">
<p>Consider the character <span class="codepoint"><span lang="en">Ǻ</span> [<span class="uname">U+01FA LATIN CAPITAL LETTER A WITH RING ABOVE AND ACUTE</span>]</span>. One way to encode this character is as <span class="uname" translate="no"> U+01FA LATIN LETTER CAPITAL A WITH RING ABOVE AND ACUTE</span>. Here are some of the different character sequences that a document could use to represent this character:</p>
<table>
<tr>
<td class=exampleChar style="width:10%">Ǻ</td>
<td><span class="uname" translate="no">U+01FA</span>—A "precomposed" character.</td>
</tr>
<tr>
<td class=exampleChar>Ǻ</td>
<td><span class="uname" translate="no">A + U+030A + U+0301</span>— A <span class="qterm">base</span> letter <span class="qchar">A</span> followed by two combining marks (<span class="uname" translate="no">U+030A COMBINING RING ABOVE</span> and <span class="uname" translate="no">U+0301 COMBINING ACUTE ACCENT</span>)</td>
</tr><tr>
<td class=exampleChar>Ǻ</td>
<td><span class="uname" translate="no">U+00C5 + U+0301</span>—An accented letter (<span class="uname" translate="no">U+00C5 LATIN CAPITAL LETTER A WITH RING ABOVE</span>) followed by a combining accent (<span class="uname" translate="no">U+0301 COMBINING ACUTE ACCENT</span>)</td>
</tr><tr>
<td class=exampleChar>Ǻ</td>
<td><span class="uname" translate="no">U+212B + U+0301</span>—A compatibility character (<span class="uname" translate="no">U+212B ANGSTROM SIGN</span>) followed by a combining accent (<span class="uname" translate="no">U+0301 COMBINING ACUTE ACCENT</span>)</td>
</tr><tr>
<td class=exampleChar>Ａ̊́</td>
<td><span class="uname" translate="no">U+FF21 + U+030A + U+0301</span>— A compatibility character <span class="uname" translate="no">U+FF21 FULLWIDTH LATIN LETTER CAPITAL A</span>) followed by two combining marks (<span class="uname" translate="no">U+030A COMBINING RING ABOVE</span> and <span class="uname" translate="no">U+0301 COMBINING ACUTE ACCENT</span>)</td>
</tr>
</table>
<p>Each of the above strings contains the same apparent <span class="quote">meaning</span> as <span class="codepoint"><span lang="en">Ǻ</span> [<span class="uname">U+01FA LATIN CAPITAL LETTER A WITH RING ABOVE AND ACUTE</span>]</span>, but each one is encoded slightly differently. More variations are possible, but are omitted for brevity.</p>
</aside>
<p>Because applications need to find the semantic equivalence in texts
that use different code point sequences, Unicode defines a means of
making two semantically equivalent texts identical: the Unicode
Normalization Forms [[!UAX15]].</p>
<aside class="advisement">
<div class="note-title marker"><span>Note Well</span></div>
<p>Unicode Normalization does not guarantee that two
identical-appearing strings that are in a given Unicode Normalization Form use the same sequence of code points.
See <a href="#normalizationLimitations"></a> for more information.</p>
</aside>
<p><a>Resources</a> are often susceptible to the
effects of these variations because their specifications and
implementations on the Web do not require Unicode Normalization of the
text, nor do they take into consideration the string matching
algorithms used when processing the syntactic content and natural language content later. For this
reason, content developers need to ensure that they have provided a
consistent representation in order to avoid problems later.</p>
<p>However, it can be difficult for users to assure that a given <a>resource</a>
or set of resources uses a consistent textual representation because
the differences are usually not visible when viewed as text. Tools and
implementations thus need to consider the difficulties experienced by
users when visually or logically equivalent strings that "ought to"
match (in the user's mind) are considered to be distinct values.
Providing a means for users to see these differences and/or normalize
them as appropriate makes it possible for end users to avoid failures
that spring from invisible differences in their source documents. For
example, the W3C Validator warns when an HTML document is not fully in
Unicode Normalization Form C.</p>
<section id="canonical_compatibility">
<h4>Canonical vs. Compatibility Equivalence</h4>
<p>Unicode defines two types of equivalence between characters: <em>canonical equivalence</em> and <em>compatibility equivalence</em>.</p>
<p><dfn>Canonical equivalence</dfn> is a fundamental equivalency between Unicode characters or sequences of Unicode characters that represent the same abstract character. Equivalent sequences ideally should have the same visual appearance (although there are many factors that can cause them to appear somewhat differently) and they should be treated and processed equivalently. Generally speaking, two canonically equivalent Unicode texts should be considered to be identical as text. Unicode defines a process called <em>canonical decomposition</em> that removes these primary distinctions between two differently-encoded but canonically equivalent texts.</p>
<p>Examples of canonical equivalence defined by Unicode include:</p>
<ul class="dropExampleList">
<li class="dropExampleItem"><span class="dropExample">Ç<span style="font-size:75%">
vs.</span>Ç</span> <em>Precomposed versus combining
sequences.</em> Some characters can be composed from a base
character followed by one or more combining characters. The same
characters are sometimes also encoded as a distinct "precomposed"
character. In this example, the character <span class="codepoint"><span lang="en">Ç</span> [<span class="uname">U+00C7 LATIN CAPITAL LETTER C WITH CEDILLA</span>]</span> is canonically equivalent to the character sequence starting with the base character <span class="codepoint"><span lang="en">C</span> [<span class="uname">U+0043 LATIN CAPITAL LETTER C</span>]</span> followed by <span class="codepoint"><span lang="en">◌̧</span> [<span class="uname">U+0327 COMBINING CEDILLA</span>]</span>. Such equivalence can extend to characters with multiple combining marks.</li>
<li class="dropExampleItem"><span class="dropExample">q̣̇<span style="font-size:75%">
vs.</span>q̣̇</span> <em>Order of combining marks.</em> When a base character is modified by multiple combining marks, the order of the combining marks might not represent a distinct character. Here the sequence <span class="codepoint"><span lang="en">q</span> [<span class="uname">U+0071 LATIN SMALL LETTER Q</span>]</span> <span class="codepoint"><span lang="en"> ̇</span> [<span class="uname">U+0307 COMBINING DOT ABOVE</span>]</span> <span class="codepoint"><span lang="en"> ̣</span> [<span class="uname">U+0323 COMBINING DOT BELOW</span>]</span> and <span class="codepoint"><span lang="en">q</span> [<span class="uname">U+0071 LATIN SMALL LETTER Q</span>]</span> <span class="codepoint"><span lang="en"> ̣</span> [<span class="uname">U+0323 COMBINING DOT BELOW</span>]</span> <span class="codepoint"><span lang="en"> ̇</span> [<span class="uname">U+0307 COMBINING DOT ABOVE</span>]</span> are equivalent, even though the combining marks are in a different order. Note that this example is chosen carefully: the dot-above character and dot-below character are on opposite "sides" of the base character. The order of combining diacritics on the same side often has a positional meaning (although there are cases where the order doesn't matter to the presentation).</li>
<li class="dropExampleItem"><span class="dropExample">Ω<span style="font-size:75%">
vs.</span>Ω</span> <em>Singleton mappings.</em> These result from the need to separately encode otherwise equivalent characters to support legacy character encodings. In this example, the Ohm symbol <span class="codepoint"><span lang="en">Ω</span> [<span class="uname">U+2126 OHM SYMBOL</span>]</span> is canonically equivalent (and identical in appearance) to the Greek letter Omega <span class="codepoint"><span lang="en">Ω</span> [<span class="uname">U+03A9 GREEK CAPITAL LETTER OMEGA</span>]</span>. (Another example of a singleton is <span class="codepoint"><span lang="en">Å</span> [<span class="uname">U+212B ANGSTROM SIGN</span>]</span> in the <a href="#aringExample">encoding variations example</a> above)</li>
<li class="dropExampleItem"><span class="dropExample">가<span style="font-size:75%">
vs.</span>가</span> <em>Hangul.</em> The Hangul script is
used to write the Korean language. This script is constructed
logically, with each syllable being a roughly-square <a>grapheme</a>
formed from specific sub-parts that represent consonants and
vowels. These specific sub-parts, called <em>jamo</em>, are
encoded in Unicode. So too are the precomposed syllables. Thus the
syllable <span class="codepoint"><span lang="en">가</span> [<span class="uname">U+AC00 [Hangul Syllable, First]</span>]</span> is canonically equivalent to its constituent <em>jamo</em> characters <span class="codepoint"><span lang="en">ᄀ</span> [<span class="uname">U+1100 HANGUL CHOSEONG KIYEOK</span>]</span> <span class="codepoint"><span lang="en">ᅡ</span> [<span class="uname">U+1161 HANGUL JUNGSEONG A</span>]</span>.</li>
</ul>
<p><dfn>Compatibility equivalence</dfn> is a weaker equivalence
between Unicode characters or sequences of Unicode characters that represent the
same abstract character, but may have a different visual appearance
or behavior. Generally the process called <em>compatibility decomposition</em> removes
formatting variations, such as superscript, subscript, rotated,
circled, and so forth, but other variations also occur. In many
cases, characters with compatibility decompositions represent a
distinction of a semantic nature; replacing the use of distinct
characters with their compatibility decomposition can therefore
change the meaning of the text. Texts that are equivalent after
compatibility decomposition often were not perceived as being
identical beforehand and SHOULD NOT be treated as equivalent by a formal
language.</p>
<aside class=example>
<p>The following table illustrates various kinds of compatibility equivalence in Unicode:</p>
<table class=data>
<thead>
<tr>
<th>Compatibility Equivalance</th>
<th style="text-align:center">Original Character</th>
<th></th>
<th style="text-align:center; width:30%">Compatibility Mapping</th>
</tr>
</thead>
<tbody>
<tr>
<td rowspan=2><strong>Font variants</strong>—characters that have a specific visual appearance (generally associated with a specialized use, such as in mathematics).</td>
<td style="text-align: center"><span class="exampleChar">ℌ</span><br><span class=uname>U+201C</span></td>
<td style="text-align: center" rowspan=2>⇒</td>
<td style="text-align: center" rowspan=2><span class="exampleChar">H</span><br><span class=uname>U+0048</span></td>
</tr>
<tr>
<td style="text-align: center"><span class="exampleChar">ℍ</span><br><span class=uname>U+210D</span></td>
</tr>
<tr>
<td><strong>Breaking versus non-breaking</strong>—variations in breaking or joining rules, such as the difference between a <span class="qterm">normal</span> and a non-breaking space.</td>
<td style="text-align: center"><span class="uname" translate="no">NON-BREAKING SPACE</span><br><span class=uname>U+00A0</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="uname" translate="no">SPACE<br>U+0020</span></td>
</tr>
<tr>
<td rowspan=4><strong>Presentation forms of Arabic</strong>— characters that encode the specific shapes (initial, medial, final, isolated) needed by visual legacy encodings of the Arabic script.</td>
<td style="text-align: center"><span class="exampleChar">ﻥ</span><br><span class=uname>U+FEE5</span></td>
<td style="text-align: center" rowspan=4>⇒</td>
<td style="text-align: center" rowspan=4><span class="exampleChar">ن</span><br><span class=uname>U+0646</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">ﻦ</span><br><span class=uname>U+FEE6</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">ﻧ</span><br><span class=uname>U+FEE7</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">ﻨ</span><br><span class=uname>U+FEE8</span></td>
</tr>
<tr>
<td rowspan=4><strong>Circled</strong>—numbers, letters, and other characters in a circled, bullet, or other presentational form; often used for lists, footnotes, and specialized presentation</td>
<td style="text-align: center"><span class="exampleChar">①</span><br><span class=uname>U+2460</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">1</span><br><span class=uname>U+0031</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">⑩</span><br><span class=uname>U+2469</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">10</span><br><span class=uname>U+0031 U+0030</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">Ⓐ</span><br><span class=uname>U+24B6</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">A</span><br><span class=uname>U+0041</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">㊞</span><br><span class=uname>U+329E</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">印</span><br><span class=uname>U+5370</span></td>
</tr>
<tr>
<td rowspan=4><strong>Width variation, size, rotated presentation forms</strong>—narrow vs. wide presentational forms of characters (such as those associated with legacy multibyte encodings), as well as "rotated" presentation forms necessary for vertical text.</td>
<td style="text-align: center"><span class="exampleChar">ｶ</span><br><span class=uname>U+FF76</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">カ</span><br><span class=uname>U+30AB</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">︷</span><br><span class=uname>U+FE37</span></td>
<td style="text-align: center" rowspan=2>⇒</td>
<td style="text-align: center" rowspan=2><span class="exampleChar">{</span><br><span class=uname>U+007B</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">｛</span><br><span class=uname>U+FF5B</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">Ａ</span><br><span class=uname>U+FF21</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">A</span><br><span class=uname>U+0041</span></td>
</tr>
<tr>
<td rowspan=4><strong>Superscripts/subscripts</strong>—superscript or subscript letters, numbers, and symbols.</td>
<td style="text-align: center"><span class="exampleChar">⁹</span><br><span class=uname>U+2079</span></td>
<td style="text-align: center" rowspan=2>⇒</td>
<td style="text-align: center" rowspan=2><span class="exampleChar">9</span><br><span class=uname>U+0039</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">₉</span><br><span class=uname>U+2089</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">ª</span><br><span class=uname>U+00AA</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">a</span><br><span class=uname>U+0061</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">₊</span><br><span class=uname>U+208A</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">+</span><br><span class=uname>U+002B</span></td>
</tr>
<tr>
<td rowspan=4><strong><span class="quote">Squared</span> characters</strong>—East Asian (particularly kana) sequences encoded as a presentation form to fit in a single ideographic "cell" in text.</td>
<td style="text-align: center"><span class="exampleChar">㌀</span><br><span class=uname>U+3300</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">アパート</span><br><span class=uname>U+30A2 U+30D1 U+30FC U+30C8</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">㍐</span><br><span class=uname>U+3350</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">ユアン</span><br><span class=uname>U+30E6 U+30A2 U+30F3</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">㉐</span><br><span class=uname>U+3250</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">PTE</span><br><span class=uname>U+0050 U+0054 U+0045</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">㎉</span><br><span class=uname>U+3389</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">kcal</span><br><span class=uname>U+006B U+0063 U+0061 U+006C</span></td>
</tr>
<tr>
<td rowspan=4><strong>Fractions</strong>—precomposed vulgar fractions, often encoded for compatibility with font glyph sets.</td>
<td style="text-align: center"><span class="exampleChar">¼</span><br><span class=uname>U+00BC</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">1⁄4</span><br><span class=uname>U+0031 U+2044 U+0034</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">½</span><br><span class=uname>U+00BD</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">1⁄2</span><br><span class=uname>U+0031 U+2044 U+0032</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">⅟</span><br><span class=uname>U+215F</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">1⁄</span><br><span class=uname>U+0031 U+2044</span></td>
</tr><tr>
<td style="text-align: center"><span class="exampleChar">↉</span><br><span class=uname>U+2189</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">0⁄3</span><br><span class=uname>U+0030 U+2044 U+0033</span></td>
</tr>
<tr>
<td rowspan=4><strong>Others</strong>—compatibility characters encoded for other reasons, generally for compatibility with legacy character encodings. Many of these characters are simply a sequence of characters encoded as a single presentational unit.</td>
<td style="text-align: center"> <span class="exampleChar">dž</span><br><span class=uname>U+01C6</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">dž</span><br><span class=uname>U+017E</span></td>
</tr><tr>
<td style="text-align: center"> <span class="exampleChar">⑴</span><br><span class=uname>U+2474</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">(1)</span><br><span class=uname>U+0028 U+0031 U+0029</span></td>
</tr><tr>
<td style="text-align: center"> <span class="exampleChar">⒈</span><br><span class=uname>U+2488</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">1.</span><br><span class=uname>U+0031 U+002E</span></td>
</tr><tr>
<td style="text-align: center"> <span class="exampleChar">⻳</span><br><span class=uname>U+2EF3</span></td>
<td style="text-align: center">⇒</td>
<td style="text-align: center"><span class="exampleChar">龟</span><br><span class=uname>U+9F9F</span></td>
</tr>
</tbody>
</table>
</aside>
<p>In the above table, it is important to note that the characters
illustrated are <em>actual Unicode codepoints</em>, not just presentational
variations due to context or style. Each character was
encoded into Unicode for compatibility with various legacy character
encodings. They should not be confused with the normal kinds of
presentational processing used on their non-compatibility
counterparts.</p>
<p>For example, most Arabic-script text uses the characters in the Arabic script block of Unicode (starting at <span class="uname" translate="no">U+0600</span>). The actual glyphs used to display the text are selected using fonts and text processing logic based on the position inside a word (initial, medial, final, or isolated), in a process called "shaping". In the table above, the four presentation forms of the Arabic letter <span class="codepoint"><span lang="ar" dir="rtl">ن</span> [<span class="uname">U+0646 ARABIC LETTER NOON</span>]</span> are shown. The characters shown are compatibility characters in the <span class="uname" translate="no">U+FE00</span> block, each of which represents a specific "positional" shape and each of the four code points shown have a compatibility decomposition to the <span class="quote">regular</span> Arabic letter <span class="codepoint"><span lang="ar" dir=rtl>ن</span> [<span class="uname">U+0646 ARABIC LETTER NOON</span>]</span>.</p>
<p>Similarly, the variations in half-width and full-width forms and rotated
characters (for use in vertical text) are encoded as separate code
points, mainly for compatibility with legacy character encodings. In
many cases these variations are associated with the Unicode properties
described in <cite>East Asian Width</cite> [[UAX11]]. See also <cite>Unicode
Vertical Text Layout</cite> [[UTR50]] for a discussion of vertical text
presentation forms.</p>
<p>In the case of characters with compatibility decompositions, such
as those shown above, the <span class="qchar">K</span> Unicode
Normalization forms convert the text to the "normal" or "expected"
Unicode code point. But the existence of these compatibility
characters cannot be taken to imply that similar appearance
variations produced in the normal course of text layout and
presentation are affected by Unicode Normalization. They are not.</p>
</section>
<section id="composition_decomposition">
<h4>Composition vs. Decomposition</h4>
<p>These two types of Unicode-defined equivalence are then grouped by
another pair of variations: "decomposition" and "composition". In
"decomposition", separable logical parts of a visual character are
broken out into a sequence of base characters and combining marks
and the resulting code points are put into a fixed, canonical order.
In "composition", the decomposition is performed and then any
combining marks are recombined, if possible, with their base
characters. Note that this does <strong>not</strong> mean that all
of the combining marks have been removed from the resulting
normalized text. </p>
<div class="note">
<p>Roughly speaking, <abbr title="Normalization Form C">NFC</abbr>
is defined such that each combining character sequence (a base
character followed by one or more combining characters) is
replaced, as far as possible, by a canonically equivalent
precomposed character. Text in a Unicode character encoding form
(such as UTF-8 or UTF-16) is said to be in NFC if it doesn't
contain any combining sequence that could be replaced with a
precomposed character and if any remaining combining sequence is
in canonical order.</p>
<p>Users are cautioned that the resulting character sequence can still contain combining marks: not all character sequences have a precomposed equivalent and some scripts depend on combining marks for encoding. There are even cases where a given base character and combining mark is not replaced with a precomposed character because the combination is "blocked" by another combining mark in the sequence. </p>
</div>
</section>
<section id="normalization_forms">
<h4>Unicode Normalization Forms</h4>
<p>There are four Unicode Normalization Forms. Each form is named using a letter code: </p>
<ul>
<li><strong>D</strong> (or NFD) stands for <em>canonical Decomposition</em>.</li>
<li><strong>C</strong> (or NFC) stands for <em>Composition</em>, which is canonical decomposition followed by composition.</li>
<li><strong>KD</strong> (or NFKD) stands for <em>Kompatibility decomposition</em> (K because the letter C is already used).</li>
<li><strong>KC</strong> (or NFKC) stands for compatibility decomposition followed by composition.</li>
</ul>
<aside class=example>
<p>Having converted a resource to a sequence of Unicode characters and unescaped any escape sequences, we can finally "normalize" the Unicode texts given in the <a href="#aringExample">example</a> above. Here are the resulting sequences in each Unicode Normalization form for the <span class=uname>U+01FA</span> example given earlier. Note that there are only three distinct code point sequences between the various normalized forms (<span class=uname>U+01FA</span>, <span class=uname>U+0041 U+030A U+0301</span>, and <span class=uname>U+FF21 U+030A U+0301</span>):</p>
<table style="border:1px solid black; vertical-align:center; table-layout:fixed; text-align:center; column-width:200px;">
<thead>
<tr>
<th class=tableHead>Original Codepoints</th>
<th class=tableHead>NFC</th>
<th class=tableHead>NFD</th>
<th class=tableHead>NFKC</th>
<th class=tableHead>NFKD</th>
</tr>
</thead>
<tr>
<td class=b-clear>Ǻ</td>
<td class=b-clear rowspan=7>Ǻ</td>
<td class=b-clear rowspan=7>Ǻ</td>
<td class=b-clear rowspan=9>Ǻ</td>
<td class=b-clear rowspan=9>Ǻ</td>
</tr>
<tr>
<td class=tableSub>U+01FA</td>
</tr>
<tr>
<td class=b-clear>Ǻ</td>
</tr>
<tr>