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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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"UTS18": {
title: "Unicode Technical Standard #18: Unicode Regular Expressions",
href: "http://unicode.org/reports/tr18/",
authors: [ "Mark Davis", "Andy Heninger" ]
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"Encoding": {
title: "Encoding",
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title: "Information Technology - Universal Multiple- Octet Coded CharacterSet (UCS) - Part 1: Architecture and Basic Multilingual Plane",
authors: [ "ISO/IEC10646-1:1993" ],
note: "The current specification also takes into consideration the first five amendments to ISO/IEC 10646-1:1993. Useful roadmaps (http://www.egt.ie/standards/iso10646/ucs-roadmap.html) show which scripts sit at which numeric ranges."
},
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title: "Unicode Technical Standard #10: Unicode Collation Algorithm",
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title: "Unicode Standard Annex #29: Unicode Text Segmentation",
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title: "Unicode Technical Standard #39: Unicode Security Mechanisms",
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title: "The Multilingual World Wide Web, Chapter 2: The WWW As A Multilingual Application",
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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, and 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
and 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>Unicode code points are denoted as <code class="kw" translate="no">U+hhhh</code>,
where <code class="kw" translate="no">hhhh</code> is a sequence of at
least four, and at most six hexadecimal digits. For example, the
character <span class="qchar">€</span> <span class="uname" translate="no">EURO
SIGN</span> has the code point <span class="uname" translate="no">U+20AC</span>.</p>
<p>Some characters that are used in the various examples might not
appear as intended unless you have the appropriate font. Care has been
taken 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 can include values that are not typically thought of as
"markup", such as the name of a field in an HTTP header, as well as
all of the characters that form the structure 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 value|user values">user 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 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 Unicode 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">
<p>The Hindi word for Unicode <q>यूनिकोड</q> is composed of a
sequence of seven Unicode characters from the Devanagari script (<span
class="uname" translate="no">U+092F U+0942 U+0928 U+093F U+0915
U+094B U+0921</span>). However, most users would identify this
word as containing four units of text—यू, नि, को, and ड. 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>
</aside>
<section>
<h5>Terminology Examples</h5>
<p>This section illustrates some of the terminology defined above.</p>
<div style="background-color:white;text-align: left; border-style: solid; border-width:3px; padding-left: 50px; padding-right: 50px; padding-top: 10px; width: 80%">
<p> <span class="markup"><<span class="vocabulary">html</span> <span class="vocabulary">lang</span>="en" <span class="vocabulary">dir</span>="<span class="vocabulary">ltr</span>"><br>
<<span class="vocabulary">head</span>></span></p>
<p><span class="markup"> <<span class="vocabulary">meta</span> <span class="vocabulary">charset</span>="UTF-8"><br>
<<span class="vocabulary">title</span>></span><span class="shakespeare">Shakespeare</span><span
class="markup"></<span class="vocabulary">title</span>><br>
</<span class="vocabulary">head</span>><br>
<<span class="vocabulary">body</span>><br>
<<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>"><br>
<br>
<<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>><br>
</<span class="vocabulary">body</span>><br>
</<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 resource.</li>
<li><a>Syntactic content</a> is shown in a <span class="markup">monospaced font</span>.</li>
<li><a>Natural language content</a> is shown in a <span class="shakespeare">bold
blue font with a gray background</span>.</li>
<li>User values are shown in <span class="userValue">italics</span>.</li>
<li><a>Vocabulary</a> is shown with <span class="vocabulary">red underlining</span>.</li>
<li>All of the text above (all text in a text file) makes up a
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.</li>
</ul>
</section>
</section>
<section id="conformance">
<p>This specification places conformance criteria on specifications, on
software (implementations) and on Web content. To aid the reader, all
conformance criteria are preceded by <span class="qterm">[X]</span>
where <span class="qchar">X</span> is one of <span class="qchar">S</span>
for specifications, <span class="qchar">I</span> for software
implementations, and <span class="qchar">C</span> for Web content.
These markers indicate the relevance of the conformance criteria and
allow the reader to quickly locate relevant conformance criteria by
searching through this document.</p>
<p>Specifications conform to this document if they:</p>
<ol type="1">
<li>
<p> do not violate any conformance criteria preceded by [S] where
the imperative is MUST or MUST NOT,</p>
</li>
<li>
<p>document the reason for any deviation from criteria where the
imperative is <span class="rfc2119">SHOULD</span>, <span class="rfc2119">SHOULD
NOT</span>, or <span class="rfc2119">RECOMMENDED</span>,</p>
</li>
<li>
<p> make it a conformance requirement for implementations to conform
to this document,</p>
</li>
<li>
<p> make it a conformance requirement for content to conform to this
document.</p>
</li>
</ol>
<p>Software conforms to this document if it does not violate any
conformance criteria preceded by [I].</p>
<p>Content conforms to this document if it does not violate any
conformance criteria preceded by [C].</p>
<div class="note">
<p><span class="note-head">NOTE: </span>Requirements placed on
specifications might indirectly cause requirements to be placed on
implementations or content that claim to conform to those
specifications.</p>
</div>
<p>Where this specification contains a procedural description, it is to
be understood as a way to specify the desired external behavior.
Implementations can 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 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>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 user-defined values permitted by the
format or protocol. For example, this occurs when matching element
names
between an HTML document and its associated style sheet. Consider this
HTML fragment: </p>
<aside class="example">
<pre><style type="text/css">
SPAN.hello {
text-decoration: underline;
}
</style>
<span class="hello">Hello World!</span>
</pre>
</aside>
<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>
<p><dfn>Case folding</dfn> is the process of making two texts identical
which differ in case but are otherwise "the same".</p>
<p>Case folding might, at first, appear simple. However there are
variations that need to be considered when treating the full range of
Unicode in diverse languages. For more information,
<cite>[[!Unicode]]</cite> Chapter 5 (in v8.0, <a href="">Section 5.18</a>)
discusses case mappings in detail.</p>
<p>Unicode defines the default case fold mapping for each Unicode code point.
Since most scripts do not provide a case distinction, 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>
<p>In addition to the <code class="kw">common</code> case folding mappings, a few characters
have a case fold mapping that would normally map one
Unicode character to more than one during case folding. These are called the <code class="kw">full</code> case fold mappings.
Together with the <code class="kw">common</code> case fold mappings, these provide the
default case fold mapping for all of Unicode. This case fold mapping is referred to in this
document as <dfn id="dfn-UnicodeC+F">Unicode C+F</dfn>.
</p>
<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. This <code class="kw">simple</code> case fold mapping, referred to in this document
as <dfn id="UnicodeC+S">Unicode C+S</dfn>, is not appropriate for the Web. </p>
<aside class="example">
<p>One well-known example of a 'full' 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 'full' mapping of this character is to two ASCII letters 's'.
There is no 'simple' mapping for this letter. </p>
<p>Other examples 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 <code>U+1F9B</code> (<span class="uname" translate="no">GREEK CAPITAL LETTER ETA WITH DASIA AND VARIA AND PROSGEGRAMMENI</span>) and it's <code class="kw">full</code> and <code class="kw">simple</code> case fold mappings:</p>
<table style="width: 100%">
<tr>
<td class="bigtext">ᾛ</td>
<td><code>U+1F9B</code> <span class="uname">GREEK CAPITAL LETTER ETA WITH DASIA AND VARIA AND PROSGEGRAMMINI</span></td>
</tr>
<tr>
<td class="bigtext">ἣι</td>
<td><em>Full:</em> <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="bigtext">ᾓ</td>
<td><em>Simple:</em> <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>
<p>Another aspect of case folding is that it can be language sensitive.
Unicode defines default case 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">
<p>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>. When rendered into upper
case, this word appears like this: <span class="qterm"><code>DİYARBAKIR</code></span>.
Notice that the ASCII letter <span class="qchar">i</span> maps to <span class="uname" translate="no">U+0130 LATIN CAPITAL LETTER I WITH DOT ABOVE</span>, while the letter <span class="qchar">ı</span> (<span class="uname" translate="no">U+0131 LATIN SMALL LETTER DOTLESS I</span>) maps to the ASCII uppercase <span class="qchar">I</span>. Similarly a lower case casefolding of I to i would change the meaning of the text in Turkish, even thought this is the expected mapping in other languages, such as English or German.</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 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><dfn data-lt="case-sensitive">Case sensitive matching</dfn>: code
points are compared directly with no case folding.</p>
<p><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 Unicode C+F Unicode-defined language-independent default case
folding form mentioned 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. <span class="requirement">In these
cases, tailoring of the Unicode case-fold mappings above to match
the expectations of that language SHOULD be specified and applied.</span>
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 id="unicodeNormalization">
<h3>Unicode Normalization</h3>
<p>A different kind of variation can occur in Unicode text: sometimes
several different Unicode code point sequences can represent the same
logical character.
When searching or matching text by comparing code points, variations
in encoding could cause text values otherwise expected to match not to
match. </p>
<p>Consider the character Ǻ. 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 an HTML document could
use to represent this character:</p>
<ul class="dropExampleList">
<li class="dropExampleItem"><span class="dropExample">Ǻ</span> <span class="uname" translate="no">U+01FA</span>—A "precomposed" character.</li>
<li class="dropExampleItem"><span class="dropExample">Ǻ</span><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>)</li>
<li class="dropExampleItem"><span class="dropExample">Ǻ</span><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>)</li>
<li class="dropExampleItem"><span class="dropExample">Ǻ</span><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>)</li>
<li class="dropExampleItem"><span class="dropExample">Ａ̊́</span><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>)</li>
</ul>
<p>Each of the above strings contains the same apparent
<span class="quote">meaning</span> as <span class="qchar">Ǻ</span> (<span class="uname" translate="no">U+01FA
LATIN CAPITAL LETTER A WITH RING ABOVE AND ACUTE</span>), but each
one is encoded slightly differently. More variations are possible,
but are omitted for brevity.</p>
<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 data-lt="resource|resources">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 data-lt="resource">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. When correctly displayed,
these should always have the same visual appearance and behavior.
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 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="qchar">Ç</span>
<span class="uname" translate="no">U+00C7</span> is canonically
equivalent to the base character <span class="qchar">C</span> <span
class="uname" translate="no">U+0043</span> followed by the
combining cedilla character <span class="qchar">̧</span> <span class="uname"
translate="no">U+0327</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="qterm">q̣̇</span>(<span
class="uname" translate="no">U+0071 U+0323 U+0307</span>) and <span
class="qterm">q̣̇</span>(<span class="uname" translate="no">U+0071
U+0307 U+0323</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 have a positional meaning.</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="qchar">Ω</span> <span class="uname" translate="no">U+2126</span>
is canonically equivalent (and identical in appearance) to the
Greek letter Omega <span class="qchar">Ω</span> <span class="uname"
translate="no">U+03A9</span>.</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="qchar">가</span> <span class="uname"
translate="no">U+AC00</span> is canonically equivalent to its
constituent <em>jamo</em> characters <span class="qchar">ᄀ</span> <span
class="uname" translate="no">U+1100</span> and <span class="qchar">ᅡ</span> <span
class="uname" translate="no">U+1161</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>
<p>The following table illustrates various kinds of compatibility
equivalence in Unicode:</p>
<table class="data">
<thead>
<tr>
<th colspan="5">Compatibility Equivalance</th>
</tr>
</thead>
<tbody>
<tr>
<td class="long"><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" colspan="2"> <span class="sampleCharacter">ℌ</span></td>
<td style="text-align: center" colspan="2"> <span class="sampleCharacter">ℍ</span></td>
</tr>
<tr>
<td class="long"><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" colspan="4"><span class="uname"
translate="no">U+00A0 NON-BREAKING SPACE</span></td>
</tr>
<tr>
<td class="long"><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="sampleCharacter">ﻨ</span></td>
<td style="text-align: center"> <span class="sampleCharacter">ﻧ</span></td>
<td style="text-align: center"> <span class="sampleCharacter">ﻦ</span></td>
<td style="text-align: center"> <span class="sampleCharacter">ﻥ</span></td>
</tr>
<tr>
<td class="long"><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" colspan="1"> <span class="sampleCharacter">①</span></td>
<td style="text-align: center" colspan="1"> <span class="sampleCharacter">⑩</span></td>
<td style="text-align: center" colspan="1"> <span class="sampleCharacter">Ⓐ</span></td>
<td style="text-align: center" colspan="1"> <span class="sampleCharacter">㊞</span></td>
</tr>
<tr>
<td class="long"><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="sampleCharacter">ｶ</span></td>
<td style="text-align: center"><span class="sampleCharacter">カ</span></td>
<td style="text-align: center"><span class="sampleCharacter">︷</span></td>
<td style="text-align: center"><span class="sampleCharacter">{</span></td>
</tr>
<tr>
<td class="long"><strong>Superscripts/subscripts</strong>—superscript or
subscript letters, numbers, and symbols.</td>
<td style="text-align: center"> <span class="sampleCharacter">⁹</span></td>
<td style="text-align: center"> <span class="sampleCharacter">₉</span></td>
<td style="text-align: center"> <span class="sampleCharacter">ª</span></td>
<td style="text-align: center"> <span class="sampleCharacter">₊</span></td>
</tr>
<tr>
<td class="long"><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="sampleCharacter">㌀</span></td>
<td style="text-align: center"> <span class="sampleCharacter">㍐</span></td>
<td style="text-align: center"> <span class="sampleCharacter">㉐</span></td>
<td style="text-align: center"> <span class="sampleCharacter">㎉</span></td>
</tr>
<tr>
<td class="long"><strong>Fractions</strong>—precomposed vulgar fractions,
often encoded for compatibility with font glyph sets.</td>
<td style="text-align: center"> <span class="sampleCharacter">¼</span></td>
<td style="text-align: center"> <span class="sampleCharacter">½</span></td>
<td style="text-align: center"> <span class="sampleCharacter">⅟</span></td>
<td style="text-align: center"> <span class="sampleCharacter">↉</span></td>
</tr>
<tr>
<td class="long"><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="sampleCharacter">dž</span></td>
<td style="text-align: center"> <span class="sampleCharacter">⑴</span></td>
<td style="text-align: center"> <span class="sampleCharacter">⒈</span></td>
<td style="text-align: center"> <span class="sampleCharacter">⻳</span></td>
</tr>
</tbody>
</table>
<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="uname" translate="no">NOON</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="uname" translate="no">U+0646 NOON</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 are 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:
the letter 'C' stands for Composition; the letter 'D' for Decomposition;
and the
letter 'K' stands for Compatibility decomposition. 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 example
above. Here are the resulting sequences in each Unicode
Normalization form for the U+01FA example given earlier. Each different colored background is a different resulting code point sequence. </p>
<figure>
<div>
<table class="data">
<thead>
<tr>
<th>Original Codepoints</th>
<th>NFC</th>
<th>NFD</th>
<th>NFKC</th>
<th>NFKD</th>
</tr>
</thead>
<tbody>
<tr>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b2">Ǻ<br>
<span class="tableSub">U+0041 U+030A U+0301</span></td>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b2">Ǻ<br>
<span class="tableSub">U+0041 U+030A U+0301</span></td>
</tr>
<tr>
<td class="b-clear">Ǻ<br>
<span class="tableSub">U+00C5 U+0301</span></td>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b2">Ǻ<br>
<span class="tableSub">U+0041 U+030A U+0301</span></td>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b2">Ǻ<br>
<span class="tableSub">U+0041 U+030A U+0301</span></td>
</tr>
<tr>
<td class="b-clear">Ǻ<br>
<span class="tableSub">U+212B U+0301</span></td>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b2">Ǻ<br>
<span class="tableSub">U+0041 U+030A U+0301</span></td>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b2">Ǻ<br>
<span class="tableSub">U+0041 U+030A U+0301</span></td>
</tr>
<tr>
<td class="b2">Ǻ<br>
<span class="tableSub">U+0041 U+030A U+0301</span></td>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b2">Ǻ<br>
<span class="tableSub"> U+0041 U+030A U+0301</span></td>
<td class="b1">Ǻ<br>
<span class="tableSub">U+01FA</span></td>
<td class="b2">Ǻ<br>
<span class="tableSub">U+0041 U+030A U+0301</span></td>