Character Model for the World Wide Web: Normalization Version 2.0

W3C Working Draft 2 May 2013

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Addison Phillips, Invited Expert


Based on Character Model for the World Wide Web 1.0: Fundamentals [CharMod], this Architectural Specification provides authors of specifications, software developers, and content developers with a common reference on string identity matching on the Web. The goal of this specification is to improve interoperable text manipulation on the World Wide Web.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This version of this document represents a significant edit and replacement from the previous edition of Character Model. Much of the content is changed and the recommendations are significantly altered.

This is an updated W3C Working Draft of this document. The main difference from previous versions of this document is that it no longer proposes to rely exclusively on Early Uniform Normalization. Work to address the comments received during the second Last Call relevant to this document is still ongoing. We ask reviewers with outstanding comments to wait for us to finalise the disposition of their comments. A list of last call comments with their status can be found in the disposition of comments (public version, Members only version). Comments may be submitted by email to www-international@w3.org (public archive).

This document is published as part of the W3C Internationalization Activity by the Internationalization Core Working Group. The Working Group expects to advance this Working Draft to Recommendation Status (see W3C document maturity levels).

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced under the 5 February 2004 W3C Patent Policy. The Working Group maintains a public list of patent disclosures relevant to this document; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) with respect to this specification should disclose the information in accordance with section 6 of the W3C Patent Policy

Table of Contents

EDNOTE: This TOC is out of date and does not reflect document structure.

1 Introduction
    1.1 Goals and Scope
    1.2 Background
    1.3 Terminology and Notation
2 Conformance
3 Normalization
    3.1 Motivation
        3.1.1 Why do we need character normalization?
        3.1.2 The choice of Normalization Form
    3.2 Definitions for W3C Text Normalization
        3.2.1 Normalizing Transcoder
        3.2.2 Unicode-normalized text
        3.2.3 Include-normalized text
        3.2.4 Fully-normalized text
        3.2.5 Normalization-sensitive operations
        3.2.6 Text-processing component
        3.2.7 Certified and suspect text
    3.3 Examples
        3.3.1 General examples
        3.3.2 Examples of XML in a Unicode encoding form
        3.3.3 Examples of restrictions on the use of combining characters
    3.4 Responsibility for Normalization
4 String Identity Matching


A References
    A.1 Normative References
    A.2 Other References
B Composing Characters (Non-Normative)
C Acknowledgements (Non-Normative)

1 Introduction

1.1 Goals and Scope

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 W3C goal of universal access. 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.

The main target audience of this specification is W3C specification developers. This specification and parts of it can be referenced from other W3C specifications. It defines conformance criteria for W3C specifications as well as other specifications.

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.

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.

This part of the Character Model for the World Wide Web covers string identity matching in content, which includes considerations for case folding and the use of Unicode normalization.

Other parts of the Character Model address the fundamental aspects of the model ([CharMod]) and Internationalized Resource Identifiers (IRI) conventions ([CharIRI]).

At the core of the character model is the Universal Character Set (UCS), defined jointly by the Unicode Standard [Unicode] and ISO/IEC 10646 [ISO/IEC 10646]. In this document, Unicode is used as a synonym for the Universal Character Set. The model will allow Web documents authored in the world's scripts (and on different platforms) to be exchanged, read, and searched by Web users around the world.

1.2 Background

This section provides some historical background on the topics addressed in this specification.

Starting with Internationalization of the Hypertext Markup Language [RFC 2070], the Web community has recognized the need for a character model for the World Wide Web. The first step towards building this model was the adoption of Unicode as the document character set for HTML.

The choice of Unicode was motivated by the fact that Unicode:

  • is the only universal character repertoire available,

  • provides a way of referencing characters independent of the encoding of the text,

  • is being updated/completed carefully,

  • is widely accepted and implemented by industry.

W3C adopted Unicode as the document character set for HTML in [HTML 4.0]. The same approach was later used for specifications such as XML 1.0 [XML 1.0] and CSS2 [CSS2]. W3C specifications and applications now use Unicode as the common reference character set.

When data transfer on the Web remained mostly unidirectional (from server to browser), and where the main purpose was to render documents, the use of Unicode without specifying additional details was sufficient. However, the Web has grown:

  • Data transfers among servers, proxies, and clients, in all directions, have increased.

  • Non-ASCII characters [ISO/IEC 646] are being used in more and more places.

  • Data transfers between different protocol/format elements (such as element/attribute names, URI components, and textual content) have increased.

  • More and more APIs are defined, not just protocols and formats.

In short, the Web may be seen as a single, very large application (see [Nicol]), rather than as a collection of small independent applications.

While these developments strengthen the requirement that Unicode be the basis of a character model for the Web, they also create the need for additional specifications on the application of Unicode to the Web. Some aspects of Unicode that require additional specification for the Web include:

  • Choice of Unicode encoding forms (UTF-8, UTF-16, UTF-32).

  • Counting characters, measuring string length in the presence of variable-length character encodings and combining characters.

  • Duplicate encodings of characters (e.g. precomposed vs decomposed).

  • Use of control codes for various purposes (e.g. bidirectionality control, symmetric swapping, etc.).

It should be noted that such aspects also exist for legacy encodings (where legacy encoding is taken to mean any character encoding not based on Unicode), and in many cases have been inherited by Unicode in one way or another from such legacy encodings.

The remainder of this specification presents additional requirements to ensure an interoperable character model for the Web, taking into account earlier work (from W3C, ISO and IETF).

The first few chapters of the Unicode Standard [Unicode] provide very useful background reading. The policies adopted by the IETF for on the use of character sets on the Internet are documented in [RFC 2277].

For information about the requirements that informed the development of important parts of this specification, see Requirements for String Identity Matching and String Indexing [CharReq].

2 Conformance

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY" and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC 2119].

NOTE: RFC 2119 makes it clear that requirements that use SHOULD are not optional and must be complied with unless there are specific reasons not to: "This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course."

This specification places conformance criteria on specifications, on software and on Web content. To aid the reader, all conformance criteria are preceded by '[X]' where 'X' is one of 'S' for specifications, 'I' for software implementations, and 'C' 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.

Specifications conform to this document if they:

  1. do not violate any conformance criteria preceded by [S],

  2. document the reason for any deviation from criteria where the imperative is SHOULD, SHOULD NOT, or RECOMMENDED,

  3. make it a conformance requirement for implementations to conform to this document,

  4. make it a conformance requirement for content to conform to this document.

Software conforms to this document if it does not violate any conformance criteria preceded by [I].

Content conforms to this document if it does not violate any conformance criteria preceded by [C].

NOTE: Requirements placed on specifications might indirectly cause requirements to be placed on implementations or content that claim to conform to those specifications.

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.

3 String Identity Matching

This chapter discusses how string matching and string search should be implemented in formal languages and content, and, consequently, how Unicode normalization and Unicode case folding affect such processing.

One important operation that depends on the use of consistent character sequences is string identity matching [CharReq], which is a subset of the more general problem of string matching. There are various degrees of specificity for string matching, from approximate matching such as regular expressions or phonetic matching, to more specific matches such as case-insensitive or accent-insensitive matching and finally to identity matching.

Examples of string identity matching abound: parsing element and attribute names in Web documents, matching CSS selectors to the nodes in a document, matching font names in a style sheet to the names known to the operating system, matching URI pieces to the resources in a server, matching strings embedded in an ECMAScript program to strings typed in by a Web form user, matching the parts of an XPath expression (element names, attribute names and values, content, etc.) to what is found in an XML instance, etc.

String identity matching can take several forms. The most trivial of these is performed by comparing two strings code unit-by-code unit (in multbyte encodings, this is byte-by-byte). The existence on the Web of multiple means of representing characters means that this process is actually less trivial than it appears. Binary comparison does not work if the strings are not in the same character encoding (e.g. an EBCDIC style sheet being directly applied to an ASCII document, or a font specification in a Shift_JIS style sheet directly used on a system that maintains font names in UTF-16) or if they are in the same character encoding but if the 'same' string contains variations allowed for by the use of combining characters or by the constructs of Web languages. Even when a consistent character encoding based on Unicode is used, defining string equality is hampered by the availability of multiple Unicode representations of the same canonical character or grapheme.

Incorrect string identity matching can have far reaching consequences, including the creation of security holes. Consider a contract, encoded in XML, for buying goods: each item sold is described in a Stück element; unfortunately, "Stück" is subject to different representations in the character encoding of the contract. Suppose that the contract is viewed and signed by means of a user agent that looks for Stück elements, extracts them (matching on the element name), presents them to the user and adds up their prices. If different instances of the Stück element happen to be represented differently in a particular contract, then the buyer and seller may see (and sign) different contracts if their respective user agents perform string identity matching differently, which is fairly likely in the absence of a well-defined specification for string matching. The absence of a well-defined specification would also mean that there would be no way to resolve the ensuing contractual dispute.

Ideally identity would only occur if the compared strings contained no user-identifiable distinctions. That is, strings do not match when they differ in case or accentuation, but do match when they differ only in non-semantically significant ways such as character encoding, use of character escapes (of potentially different kinds), or use of precomposed vs. decomposed character sequences.

3.1 Considerations for Identity Matching

3.1.1 Case Sensitivity

One main consideration in string identity matching is whether the comparison is case sensitive or case insensitive. Case sensitive matching is RECOMMENDED as the default for new specifications and formats. However, cases exist in which case-insensitivity is desirable.

Case sensitive matching compares Unicode code points and consists of the following steps:

Step 1 ensures 1) that the identity matching process can produce correct results using the next three steps and 2) that a minimum of effort is spent on solving the problem.

NOTE: The expansion of character escapes and includes (step 3 above) is dependent on context, i.e. on which markup or programming language is considered to apply when the string matching operation is performed. Consider a search for the string 'suçon' in an XML document containing suçon but not suçon. If the search is performed in a plain text editor, the context is plain text (no markup or programming language applies), the ç character escape is not recognized, hence not expanded and the search fails. If the search is performed in an XML browser, the context is XML, the character escape (defined by XML) is expanded and the search succeeds.

An intermediate case would be an XML editor that purposefully provides a view of an XML document with entity references left unexpanded. In that case, a search over that pseudo-XML view will deliberately not expand entities: in that particular context, entity references are not considered includes and need not be expanded

Case-insensitive matching is sometimes useful in contexts where case may vary in a way that is not semantically meaningful or in which case distinctions cannot be controlled by the user. Case insensitive matching takes several forms:

3.1.2 Unicode Normalization

One non-semantically significant distinction that can occur in Unicode text is variation in the character sequence due to the use of combining marks or compatibility characters. Version 1.0 of this document focused on defining Early Uniform Normalization as a means of ensuring that string matching processes on the Web would be straightforward, performant, and consistent. Ideally, Unicode normalization would be used to ensure that character sequences that Unicode defines as canonically equivalent are also treated as canonically equivalent ("match") when used on the Web.

However, since W3C specifications and implementations generally do not enforce Unicode normalization, it is up to content authors to supply text that uses a consistent representation in order to obtain predictable results. Using or applying a Unicode normalization form for text is one way to accomplish this.

Why normalize your content?

When text is encoded in a computer system, it often can use one of many different character encodings. In addition, some character encodings allow multiple representations for the 'same' string. Web languages and format also have escape mechanisms that introduce even more equivalent representations. For instance, in ISO 8859-1 the letter 'ç' can only be represented as the single character E7 'ç', but in a Unicode encoding it can be represented as the single character U+00E7 'ç' or the sequence U+0063 'c' U+0327 '¸'. In HTML it could be additionally represented as ç or ç or ç or even using the combining mark ç or ç (and so forth).

The basis for most W3C document formats is text that includes some form of structural markup. This includes formats in diverse forms, such as HTML, CSS, JSON, RDF, and so forth. When processing document formats based on text, operations such as string matching, indexing, searching, sorting, regular expression matching, selection (and so forth) become sensitive to the different ways in which text might be represented in the document: the proper functioning of the Web (and of much other software) depends to a large extent on string matching.

Because the document formats, specifications, and implementations on the Web do not supply Unicode normalization of the content being exchanged or in their string matching algorithms, users and content must provide a consistent representation in order to avoid problems later. Since users can, on their own, have difficulty assuring that content uses a consistent representation, implementations and tools should consider the difficulties experienced by users who expect that visually and logically equivalent strings that are not considered to match and to provide a means for users to see differences and/or canonicalize them as appropriate.

3.1.3 The choice of Normalization Form

Given that there are many character sequences that content authors or applications could choose when inputting or exchanging text, what form is most appropriate for content on the Web? When providing text in a normalized form, there are different options for the normalization form to be used.

The Unicode Consortium provides four standard normalization forms (see Unicode Normalization Forms [UTR #15]). These forms differ in 1) whether they normalize towards decomposed characters (NFD, NFKD) or precomposed characters (NFC, NFKC) and 2) whether the normalization process erases compatibility distinctions (NFKD, NFKC) or not (NFD, NFC).

For use on the Web, it is important not to lose compatibility distinctions, which are often important to the content (see [UXML] Chapter 5 for a discussion). The NFKD and NFKC normalization forms are therefore excluded. Among the remaining two forms, NFC has the advantage that almost all legacy data (if transcoded trivially, one-to-one, to a Unicode encoding), as well as data created by current software, is already in this form; NFC also has a slight compactness advantage and is a better match to user expectations with respect to the character vs. grapheme issue. This document therefore recommends, when possible, that all content be stored and exchanged in Unicode Normalization Form C (NFC).

NOTE: Roughly speaking, NFC 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 encoding form is said to be in NFC if it doesn't contain any combining sequence that could be replaced and if any remaining combining sequence is in canonical order.

4 Requirements for String Identity Matching

In the Web environment, where multiple character encodings are used to represent strings, including some character encodings which allow multiple representations for the same thing, it's important to establish a consistent process for evaluating string identity.

C3xx [S][I] Case sensitive string identity matching MUST be performed as if the following steps were followed:

  1. Conversion to a common Unicode encoding form of the strings to be compared. A normalizing transcoder MUST be used for any such conversions.
  2. Expansion of all recognized character escapes and includes.
  3. Testing for bit-by-bit identity.

To avoid unnecessary conversions and, more importantly, to ensure predictability and correctness, it is necessary for all components of the Web to use the same identity testing mechanism. Conformance to the rule that follows meets this requirement and supports the above definition of identity.

Where caseless matching is desired, there are several implementation choices that a formal language needs to consider. If the namespace of strings to be compared is limited to the Basic Latin (ASCII) subset of Unicode, ASCII-case-insensitivity can be used.

If the namespace of strings to be compared is not limited, then Unicode case folding should be applied. Unicode case folding can take several forms. Unicode defines the "common" (C) case foldings for "simple" 1:1 mappings of a character to a case folded form. However, some characters have a 1:many mapping. For compatibility with certain types of implementation, Unicode defines both a "simple" (S) case fold (that is always 1:1) and a "full" case fold.

Because the "simple" case fold mapping removes information that can be important to forming an identity match, the "Common plus Full" (or CF) case fold is preferred for Unicode case insensitive matching.

A namespace is considered to be "ASCII-only" if and only if all tokens and identifiers are defined by the specification directly and these identifiers or tokens use only the Basic Latin subset of Unicode. If user-defined identifiers are permitted, the full range of Unicode characters (limited, as appropriate, for security or interchange concerns, see UAX#xx) SHOULD be allowed and Unicode case insensitivity used for identity matching.

Content Requirements

These requirements pertain to content and are intended as guidelines for content authors.

[C] Text content SHOULD be stored and exchanged in Unicode Normalization Form C (NFC).

NOTE: In order to be processed correctly content must use a consistent sequence of code points to represent text. While content can be in any normalization form or may use a de-normalized (but valid) Unicode character sequence, inconsistency of representation will cause implementations to treat the different sequence as "different". The best way to ensure consistent selection, access, extraction, processing, or display is to always use NFC.

[I] Implementations which transcode text from a legacy encoding to a Unicode encoding form SHOULD use a normalizing transcoder that produces Unicode Normalization Form C (NFC).

[C] Content developers SHOULD avoid composing characters at the beginning of constructs that might be significant, such as at the beginning of an entity that will be included, immediately after a construct that causes inclusion or immediately after markup.

[C] Identifiers in content SHOULD use consistent case (upper, lower, mixed case) to facilitate matching, even if case-insensitive matching is supported by the format or implementation.

Formal Language (markup or programming languages) Requirements

The requirements pertain to specifications for document formats or programming/scripting languaguages and their implementations.

[S][I] Specifications and implementations that define string matching as part of the definition of a format, protocol, or formal language (which might include operations such as parsing, matching, tokenizing, etc.) MUST define the criteria and matching forms used. These MUST be one of:

  • ASCII-only case-sensitive (ACS)
  • ASCII-only case-insensitive (ACI)
  • Unicode case-sensitive (UniCS)
  • Unicode case-insensitive using Unicode case-folding CF (UniCF)
  • Unicode case-insensitive locale-specific case-folding (UniLoc)

[S] Specifications of text-based formats and protocols MAY specify that the format or protocol requires content to be normalized using Unicode Normalization Form C (NFC).

Specificiations are generally discouraged from including a normalization requirement unless there are specific, clear reasons why the additional requirement is necessary. As other content on the Web is not generally normalized, content authors may rely on denormalized character sequences and a normalization step could negatively affect such content.

NOTE: specifying NFC requires additional care on the part of the spec developer, as content on the Web generally is not in a known normalization state. Boundary and error conditions for denormalized content ought to be carefully considered and well specified in these cases.

[S] Specifications SHOULD NOT specify case-insensitive comparison of strings.

[S] Specifications that specify case-insensitive comparison for non-ASCII namespaces SHOULD specify Unicode C+F case folding.

In some limited cases, locale- or language-specific tailoring might also be appropriate. However, such cases are generally linked to natural language processing operations. Because they produce potentially different results from the generic case folding rules, these should be avoided in formal languages, where predictability is at a premium.

[S] Specifications that specify case-insensitive comparison for ASCII-only namespaces MAY specify ASCII-only case-insensitive comparison.

This requirement applies to formal languages whose keywords are all ASCII and which do not allow user-defined names or identifiers. An example of this is HTML, which defines the use of ACI comparison for element and attribute names defined by the HTML specification.

[S][I] Specifications and implementations MUST NOT specify ASCII-only case-sensistive or case-insensitive (ASC or ACI) forms for values or constructs that permit non-ASCII characters.

Non-Normalizing Specification Requirements

These requirements pertain to any specification that specifies normalization explicitly (which should include all new specifications):

[S] Specifications that do not normalize MUST document or provide a health-warning if canonically equivalent but disjoint Unicode character sequences represent a security issue.

[S][I] Specifications and implementations MUST NOT assume that content is in any particular normalization form.

The normalization form or lack of normalization for any given content has to be considered intentional in these cases.

[S][I] For namespaces and values that are not restricted to Basic Latin (ASCII), case-insensitive matching MUST specify either UniCF or locale-sensitive string comparison.

[I] Implementations MUST NOT alter the normalization form of content being exchanged, read, parsed, or processed except when required to do so as a side-effect of transcoding the content to a Unicode character encoding, as content might depend on the de-normalized representation.

[S] Specifications MUST specify that string matching takes the form of "code point-by-code point" comparison of the Unicode character sequence, or, if a specific Unicode character encoding is specified, code unit-by-code unit comparison of the sequences.

Unicode Normalizing Specification Requirements

For specifications of text-based formats and protocols that define Unicode Normalization as a requirement, the following requirements apply:

[S] Specifications of text-based formats and protocols that, as part of their syntax definition, require that the text be in normalized form MUST define string matching in terms of normalized string comparison and MUST define the normalized form to be NFC.

[S] [I] A normalizing text-processing component which receives suspect text MUST NOT perform any normalization-sensitive operations unless it has first either confirmed through inspection that the text is in normalized form or it has re-normalized the text itself. Private agreements MAY, however, be created within private systems which are not subject to these rules, but any externally observable results MUST be the same as if the rules had been obeyed.

[I] A normalizing text-processing component which modifies text and performs normalization-sensitive operations MUST behave as if normalization took place after each modification, so that any subsequent normalization-sensitive operations always behave as if they were dealing with normalized text.

[S] Specifications of text-based languages and protocols SHOULD define precisely the construct boundaries necessary to obtain a complete definition of full-normalization. These definitions SHOULD include at least the boundaries between markup and character data as well as entity boundaries (if the language has any include mechanism) , SHOULD include any other boundary that may create denormalization when instances of the language are processed, but SHOULD NOT include character escapes designed to express arbitrary characters.

[I] Authoring tool implementations for a formal language that does not mandate full-normalization SHOULD either prevent users from creating content with composing characters at the beginning of constructs that may be significant, such as at the beginning of an entity that will be included, immediately after a construct that causes inclusion or immediately after markup, or SHOULD warn users when they do so.

[S] Where operations can produce denormalized output from normalized text input, specifications of API components (functions/methods) that implement these operations MUST define whether normalization is the responsibility of the caller or the callee. Specifications MAY state that performing normalization is optional for some API components; in this case the default SHOULD be that normalization is performed, and an explicit option SHOULD be used to switch normalization off. Specifications SHOULD NOT make the implementation of normalization optional.

[S] Specifications that define a mechanism (for example an API or a defining language) for producing textual data object SHOULD require that the final output of this mechanism be normalized.


3.2 Definitions for W3C Text Normalization

For use on the Web, this document defines Web-related text normalization forms by starting with Unicode Normalization Form C (NFC), and additionally addressing the issues of legacy encodings, character escapes, includes, and character and markup boundaries. Examples illustrating some of these definitions can be found in 3.3 Examples. This section defines various useful terms and concepts that can be applied to normalization of Web content, particularly when implementing more complex string processing that is normalization sensitive.

3.2.1 Normalizing Transcoder

A normalizing transcoder is a transcoder that converts from a legacy encoding to a Unicode encoding form and ensures that the result is in Unicode Normalization Form C (see 3.2.2 Unicode-normalized text). For most legacy encodings, it is possible to construct a normalizing transcoder (by using any transcoder followed by a normalizer); it is not possible to do so if the encoding's repertoire contains characters not represented in Unicode.

3.2.2 Unicode-normalized text

Text is, for the purposes of this specification, Unicode-normalized if it is in a Unicode encoding form and is in Unicode Normalization Form C, according to a version of Unicode Standard Annex #15: Unicode Normalization Forms [UTR #15] at least as recent as the oldest version of the Unicode Standard that contains all the characters actually present in the text, but no earlier than version 3.2 [Unicode 3.2].

3.2.3 Include-normalized text

Markup languages, style languages and programming languages often offer facilities for including a piece of text inside another. An include is an instance of a syntactic device specified in a language to include text at the position of the include, replacing the include itself. Examples of includes are entity references in XML, @import rules in CSS and the #include preprocessor statement in C/C++. Character escapes are a special case of includes where the included entity is predetermined by the language.

Text is include-normalized if:

  1. the text is Unicode-normalized and does not contain any character escapes or includes whose expansion would cause the text to become no longer Unicode-normalized; or

  2. the text is in a legacy encoding and, if it were transcoded to a Unicode encoding form by a normalizing transcoder, the resulting text would satisfy clause 1 above.

NOTE: A consequence of this definition is that legacy text (i.e. text in a legacy encoding) is always include-normalized unless i) a normalizing transcoder cannot exist for that encoding (e.g. because the repertoire contains characters not in Unicode) or ii) the text contains character escapes or includes which, once expanded, result in un-normalized text.

NOTE: The specification of include-normalization relies on the syntax for character escapes and includes defined by the (computer) language in use. For plain text (no character escapes or includes) in a Unicode encoding form, include-normalization and Unicode-normalization are equivalent.

3.2.4 Fully-normalized text

Formal languages define constructs, which are identifiable pieces, occurring in instances of the language, such as comments, identifiers, element tags, processing instructions, runs of character data, etc. During the normal processing of include-normalized text, these various constructs may be moved, removed (e.g. removing comments) or merged (e.g. merging all the character data within an element as done by the string() function of XPath), creating opportunities for text to become denormalized. The software performing those operations, or other software down the line that needs to perform normalization-sensitive operations, then has to re-normalize the result, which is a burden. One way to avoid such denormalization is to make sure that the various important constructs never begin with a character such that appending that character to a normalized string can cause the string to become denormalized. A composing character is a character that is one or both of the following:

  1. the second character in the canonical decomposition mapping of some character that is not listed in the Composition Exclusion Table defined in [UTR #15], or

  2. of non-zero canonical combining class as defined in [Unicode] .

Please consult Appendix B Composing Characters for a discussion of composing characters, which are not exactly the same as Unicode combining characters.

Text is fully-normalized if:

  1. the text is in a Unicode encoding form, is include-normalized and none of the constructs comprising the text begin with a composing character or a character escape representing a composing character; or

  2. the text is in a legacy encoding and, if it were transcoded to a Unicode encoding form by a normalizing transcoder, the resulting text would satisfy clause 1 above.

NOTE: Full-normalization is specified against the context of a (computer) language (or the absence thereof), which specifies the form of character escapes and includes and the separation into constructs. For plain text (which has no includes, no constructs, and no character escapes) in a Unicode encoding form, full-normalization and Unicode-normalization are equivalent.

Identification of the constructs that should be prohibited from beginning with a composing character (the relevant constructs) is language-dependent.

Although Unicode normalization is not required of W3C specifications, .... (health warning)

As specified in 3.4 Responsibility for Normalization, it is the responsibility of the specification for a language to specify exactly what constitutes a relevant construct. This may be done by specifying important boundaries, taking into account which operations would benefit the most from being protected against denormalization. The relevant constructs are then defined as the spans of text between the boundaries. At a minimum, for those languages which have these notions, the important boundaries are entity (include) boundaries as well as the boundaries between most markup and character data. Many languages will benefit from defining more boundaries and therefore finer-grained full-normalization constructs.

NOTE: In general, it will be advisable not to include character escapes designed to express arbitrary characters among the relevant constructs; the reason is that including them would prevent the expression of combining sequences using character escapes (e.g. 'q̌' for q-caron), which is especially important in legacy encodings that lack the desired combining marks.

NOTE: Full-normalization is closed under concatenation: the concatenation of two fully-normalized strings is also fully-normalized. As a result, a side benefit of including entity boundaries in the set of boundaries important for full-normalization is that the state of normalization of a document that includes entities can be assessed without expanding the includes, if the included entities are known to be fully-normalized. If all the entities are known to be include-normalized and not to start with a composing character, then it can be concluded that including the entities would not denormalize the document.

3.2.5 Normalization-sensitive operations

An operation is normalization-sensitive if its output(s) are different depending on the state of normalization of the input(s); if the output(s) are textual, they are deemed different only if they would remain different were they to be normalized. These operations are any that involve comparison of characters or character counting, as well as some other operations such as ‘delete first character’ or ‘delete last character’.

3.2.6 Text-processing component

A text-processing component is a component that recognizes data as text. This specification does not specify the boundaries of a text-processing component, which may be as small as one line of code or as large as a complete application. A text-processing component may receive text, produce text, or both.

3.2.7 Certified and suspect text

In the following definitions, the word 'normalized' may stand for either 'include-normalized' or 'fully-normalized', depending on which is most appropriate for the specification or implementation under consideration.

Certified text is text which satisfies at least one of the following conditions:

  1. it has been confirmed through inspection that the text is in normalized form

  2. the source of the text (a text-processing component ) is known to produce only normalized text.

Suspect text is text which is not certified.

NOTE: To normalize text, it is in general sufficient to store the last seen character, but in certain cases (a sequence of combining marks) a buffer of theoretically unlimited length is necessary. However, for normalization checking no such buffer is necessary, only a few variables. C Resources for Normalization points to some compact code that shows how to check normalization without an expanding buffer.

3.3 Examples

In some of the following examples, '¸' is used to depict the character U+0327 COMBINING CEDILLA, for the purposes of illustration. Had a real U+0327 been used instead of this spacing (non-combining) variant, some browsers might combine it with a preceding 'c', resulting in a display indistinguishable from a U+00E7 'ç' and a loss of understandability of the examples. In addition, if the sequence c + combining cedilla were present, this document would not be include-normalized.

It is also assumed that the example strings are relevant constructs for the purposes of full-normalization.

3.3.1 General examples

The string suçon (U+0073 U+0075 U+00E7 U+006F U+006E) encoded in a Unicode encoding form, is Unicode-normalized, include-normalized and fully-normalized. The same string encoded in a legacy encoding for which there exists a normalizing transcoder would be both include-normalized and fully-normalized but not Unicode-normalized (since not in a Unicode encoding form).

In an XML or HTML context, the string suçon is also include-normalized, fully-normalized and, if encoded in a Unicode encoding form, Unicode-normalized. Expanding ç yields suçon as above, which contains no replaceable combining sequence.

The string suc¸on (U+0073 U+0075 U+0063 U+0327 U+006F U+006E), where U+0327 is the COMBINING CEDILLA, encoded in a Unicode encoding form, is not Unicode-normalized (since the combining sequence '' (U+0063 U+0327) should appear instead as the precomposed 'ç' (U+00E7)). As a consequence this string is neither include-normalized (since in a Unicode encoding form but not Unicode-normalized) nor fully-normalized (since not include-normalized). Note however that the string sub¸on (U+0073 U+0075 U+0062 U+0327 U+006F U+006E) in a Unicode encoding form is Unicode-normalized since there is no precomposed form of 'b' plus cedilla. It is also include-normalized and fully-normalized.

In plain text the string suçon is Unicode-normalized, since plain text doesn't recognize that ̧ represents a character in XML or HTML and considers it just a sequence of non-replaceable characters.

In an XML or HTML context, however, expanding ̧ yields the string suc¸on (U+0073 U+0075 U+0063 U+0327 U+006F U+006E) which is not Unicode-normalized ('' is replaceable by 'ç'). As a consequence the string is neither include-normalized nor fully-normalized. As another example, if the entity reference &word-end; refers to an entity containing ¸on (U+0327 U+006F U+006E), then the string suc&word-end; is not include-normalized for the same reasons.

In an XML or HTML context, expanding ̧ in the string sub̧on yields the string sub¸on which is Unicode-normalized since there is no precomposed character for 'b cedilla' in NFC. This string is therefore also include-normalized. Similarly, the string sub&word-end; (with &word-end; as above) is include-normalized, for the same reasons.

In an XML or HTML context, the strings ¸on (U+0327 U+006F U+006E) and ̧on are not fully-normalized, as they begin with a composing character (after expansion of the character escape for the second). However, both are Unicode-normalized (if expressed in a Unicode encoding form) and include-normalized.

The following table consolidates the above examples. Normalized forms are indicated using 'Y', a hyphen means 'not normalized'.

suçonUnicodePlain textYYY
LegacyPlain text-YY
suçonUnicodePlain textYYY
LegacyPlain text-YY
suc¸onUnicodePlain text---
suçonUnicodePlain textYYY
LegacyPlain text-YY
¸onUnicodePlain textYY-
̧onUnicodePlain textYYY
LegacyPlain text-YY

3.3.2 Examples of XML in a Unicode encoding form

Here is another summary table, with more examples but limited to XML in a Unicode encoding form. The following list describes what the entities contain and special character usage. Normalized forms are indicated using 'Y'. There is no precomposed 'b with cedilla' in NFC.


NOTE: From the last example in the table above, it follows that it is impossible to produce a normalized XML or HTML document containing the character U+0338 COMBINING LONG SOLIDUS OVERLAY immediately following an element tag, comment, CDATA section or processing instruction, since the U+0338 '/' combines with the '>' (yielding U+226F NOT GREATER-THAN). It is noteworthy that U+0338 COMBINING LONG SOLIDUS OVERLAY also combines with '<', yielding U+226E NOT LESS-THAN. Consequently, U+0338 COMBINING LONG SOLIDUS OVERLAY should remain excluded from the initial character of XML identifiers.

3.3.3 Examples of restrictions on the use of combining characters

Include-normalization and full-normalization create restrictions on the use of combining characters. The following examples discuss various such potential restrictions and how they can be addressed.

Full-normalization prevents the markup of an isolated combining mark, for example for styling it differently from its base character (Benoi<span style='color: blue'>^</span>t, where '^' represents a combining circumflex). However, the equivalent effect can be achieved by assigning a class to the accents in an SVG font or using equivalent technology. View an example using SVG (SVG-enabled browsers only).

Full-normalization prevents the use of entities for expressing composing characters. This limitation can be circumvented by using character escapes or by using entities representing complete combining character sequences. With appropriate entity definitions, instead of A&acute;, write &Aacute; (or better, use 'Á' directly).

A References

A.1 Normative References

Martin J. Dürst, François Yergeau, Richard Ishida, Misha Wolf, Tex Texin. Character Model for the World Wide Web 1.0: Fundamentals. W3C Recommendation 15 February 2005. Available at http://www.w3.org/TR/2005/REC-charmod-20050215/. The latest version of CharMod is available at http://www.w3.org/TR/charmod/.
ISO/IEC 10646
ISO/IEC 10646-1:2000, Information technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part 1: Architecture and Basic Multilingual Plane and ISO/IEC 10646-2:2001, Information technology -- Universal Multiple-Octet Coded Character Set (UCS) -- Part 2: Supplementary Planes, as, from time to time, amended, replaced by a new edition or expanded by the addition of new parts. The latest version of UCS Part 1 and Part 2 is available at http://www.iso.ch .
ISO/IEC 646:1991, Information technology -- ISO 7-bit coded character set for information interchange. This standard defines an International Reference Version (IRV) which corresponds exactly to what is widely known as ASCII or US-ASCII. ISO/IEC 646 was based on the earlier standard ECMA-6. ECMA has maintained its standard up to date with respect to ISO/IEC 646. An electronic copy of ECMA is available at http://www.ecma-international.org/publications/standards/Ecma-006.htm .
RFC 2119
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. IETF RFC 2119 March 1997. Available at http://www.ietf.org/rfc/rfc2119.txt.
RFC 2396
T. Berners-Lee, R. Fielding, L. Masinter. Uniform Resource Identifiers (URI): Generic Syntax. IETF RFC 2396 August 1998. Available at http://www.ietf.org/rfc/rfc2396.txt.
The Unicode Consortium. The Unicode Standard, Version 4. ISBN 0-321-18578-1, as updated from time to time by the publication of new versions. The latest version of Unicode and additional information on versions of the standard and of the Unicode Character Database is available at http://www.unicode.org/unicode/standard/versions/.
Unicode 6.3
The Unicode Consortium. The Unicode Standard, Version 6.3.0 is defined by The Unicode Standard, Version 6.3, ISBN 0-201-61633-5, as amended by the Unicode Standard Annex #27: Unicode 3.1 (see http://www.unicode.org/reports/tr27/) and by the Unicode Standard Annex #28: Unicode 3.2 (see http://www.unicode.org/reports/tr28).
UTR #15
Mark Davis, Martin Dürst. Unicode Normalization Forms Unicode Standard Annex #15 March 2005. Available at http://www.unicode.org/reports/tr15/tr15-25.html. The latest version of Unicode Normalization Forms is available at http://www.unicode.org/reports/tr15/.

A.2 Other References

Martin J. Dürst, François Yergeau, Richard Ishida, Misha Wolf, Tex Texin. Character Model for the World Wide Web 1.0: Resource Identifiers. W3C Canidate Recommendation 22 November 2004. Available at http://www.w3.org/TR/2004/CR-charmod-resid-20041122/. The latest version of CharIRI is available at http://www.w3.org/TR/charmod-resid/.
Martin J. Dürst. Requirements for String Identity Matching and String Indexing. W3C Working Draft 10 July 1998. Available at http://www.w3.org/TR/1998/WD-charreq-19980710. The latest version of CharReq is available at http://www.w3.org/TR/WD-charreq.
Bert Bos, Håkon Wium Lie, Chris Lilley, Ian Jacobs, Eds. Cascading Style Sheets, level 2. W3C Recommendation 12 May 1998. Available at http://www.w3.org/TR/1998/REC-CSS2-19980512/. The latest version of CSS2 is available at http://www.w3.org/TR/REC-CSS2/.
Johnny Stenback, Andy Heninger, Eds. Document Object Model (DOM) Level 3 Load and Save Specification. W3C Recommendation 7 April 2007. Available at http://www.w3.org/TR/2004/REC-DOM-Level-3-LS-20040407/. The latest version of DOM3 LS is available at http://www.w3.org/TR/DOM-Level-3-LS/.
HTML 4.0
Dave Raggett, Arnaud Le Hors, Ian Jacobs, Eds. HTML 4.0 Specification. W3C Recommendation 18 December 1997. Available at http://www.w3.org/TR/REC-html40-971218/. The latest version of HTML 4.0 is available at http://www.w3.org/TR/REC-html40/.
ISO/IEC 14651
ISO/IEC 14651:2000. Information technology -- International string ordering and comparison -- Method for comparing character strings and description of the common template tailorable ordering as, from time to time, amended, replaced by a new edition or expanded by the addition of new parts. The latest version of ISO/IEC 14651 is available at http://www.iso.ch/.
Gavin Nicol. The Multilingual World Wide Web, Chapter 2: The WWW As A Multilingual Application. Available at http://www.mind-to-mind.com/library/papers/multilingual/multilingual-www.html.
RFC 2070
François Yergeau, Gavin. Nicol, G. Adams, Martin Dürst. Internationalization of the Hypertext Markup Language. IETF RFC 2070 January 1997. Available at http://www.ietf.org/rfc/rfc2070.txt.
RFC 2277
H. Alvestrand. IETF Policy on Character Sets and Languages. IETF RFC 2277, BCP 18 January 1998. Available at http://www.ietf.org/rfc/rfc2277.txt.
Martin Dürst and Asmus Freytag. Unicode in XML and other Markup Languages. Unicode Technical Report #20 and W3C Note 13 June 2003. Available at http://www.unicode.org/reports/tr20/tr20-7.html. The latest version of UXML is available at http://www.unicode.org/reports/tr20/.
XML 1.0
Tim Bray, Jean Paoli, C. Michael Sperberg-McQueen, Eve Maler, Eds. Extensible Markup Language (XML) 1.0 (Third Edition). W3C Recommendation 4 February 2004. Available at http://www.w3.org/TR/2004/REC-xml-20040204. The latest version of XML 1.0 is available at http://www.w3.org/TR/REC-xml/.
XQuery Operators
Ashok Malhotra, Jim Melton, Jonathan Robie, Norman Walsh, Eds. XQuery 1.0 and XPath 2.0 Functions and Operators. W3C Working Draft 15 September 2005. Available at http://www.w3.org/TR/2005/WD-xpath-functions-20050915/. The latest version of XQuery Operators is available at http://www.w3.org/TR/xquery-operators/.
James Clark Ed., XSL Transformations (XSLT). W3C Recommendation 16 November 1999. Available at http://www.w3.org/TR/1999/REC-xslt-19991116. The latest version of XSLT is available at http://www.w3.org/TR/xslt.

B Composing Characters (Non-Normative)

As specified in 3.2.4 Fully-normalized text, a composing character is any character that is

  1. the second character in the canonical decomposition mapping of some character that is not listed in the Composition Exclusion Table defined in [UTR #15], or

  2. of non-zero canonical combining class (as defined in [Unicode]).

These two categories are highly but not exactly overlapping. The first category includes a few class-zero characters that do compose with a previous character in NFC; this is the case for some vowel and length marks in Brahmi-derived scripts, as well as for the modern non-initial conjoining jamos of the Korean Hangul script. The second category includes some combining characters that do not compose in NFC, for the simple reason that there is no precomposed character involving them. They must nevertheless be taken into account as composing characters because their presence may make reordering of combining marks necessary, to maintain normalization under concatenation or deletion. Therefore, composing characters as defined in 3.2.4 Fully-normalized text include all characters of non-zero canonical combining class plus the following (as of Unicode 3.2):

Unicode numberCharacterName
Brahmi-derived scripts
Hangul vowels
Hangul trailing consonants

NOTE: The characters in the second column of the above table may or may not appear, or may appear as blank rectangles, depending on the capabilities of your browser and on the fonts installed in your system.

C Acknowledgements (Non-Normative)

The W3C Internationalization Working Group and Interest Group, as well as others, provided many comments and suggestions.

Asmus Freytag and, in early stages, Ian Jacobs provided significant help in the authoring and editing process of the first version of the Character Model and their work thus contributes to this document.

The previous version of this document was edited by:

François Yergeau, Invited Expert (and before at Alis Technologies)
Martin J. Dürst, (until Dec 2004 while at W3C)
Richard Ishida, W3C (and before at Xerox)
Misha Wolf, (until Dec 2002 while at Reuters Ltd.)
Tex Texin, (until Dec 2004 while an Invited Expert, and before at Progress Software)