21.3 XML Syntax
| Format | Meaning |
|---|---|
| DOCTYPE | Bold text indicates literal characters that must appear as written within the document (e.g., DOCTYPE ). |
| encoding- name | Italicized text indicates that the user must replace the text with real data. The item indicates what type of data should be inserted (e.g., encoding-name = en-us). |
|
| The vertical bar indicates that only one out of a list of possible values can be selected. |
| [ ] | Square brackets indicate that a particular portion of the syntax is optional. |
21.3.1 Global Syntax Structures
Every XML document is broken into two primary sections: the prolog and the document element. A few documents may also have comments or processing instructions that follow the root element in a sort of epilog (an unofficial term ). The prolog contains structural information about the particular type of XML document you are writing, including the XML declaration and document type declaration. The prolog is optional, and if a document does not need to be validated against a DTD, it can be omitted completely. The only required structure in a well- formed XML document is the top-level document element itself.
The following syntax structures are common to the entire XML document. Unless otherwise noted within a subsequent reference item, the following structures can appear anywhere within an XML document.
| Characters |
XML documents are inherently text documents, which are composed of characters. To ensure that documents are portable across disparate computer systems and can contain content in as many written human languages as possible, XML parsers are required to implement the Unicode standard. This does not mean that all XML documents must be saved and edited in Unicode, but it does mean that the XML parser must be able to convert your document from its native character encoding to Unicode. All XML parsers are required to support (as a minimum) either UTF-8 or UTF-16 as input encoding formats. For more information on encoding formats and Unicode, see Chapter 27.
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| Whitespace |
XML 1.0 defines whitespace as a space, tab, carriage return, or line feed. XML 1.1 also includes the newline character NEL ( #x85 ) and Unicode line separator ( #x2028 ) in whitespace. Whitespace serves the same purpose in XML as it does in most programming and natural languages: to separate tokens and language elements from one another. To an XML parser, all whitespace in element content is significant and will be passed to the client application. Whitespace within tagsfor instance, between attributesis not significant. Consider the following example:
<p> This sentence has extraneous line breaks.</p>
After parsing, the character data from this example element is passed to the underlying application as:
This sentence has extraneous line breaks.
Although XML specifies that all whitespace in element content be preserved for use by the client application, an additional facility is available to the XML author to further hint that an element's character data's space and formatting should be preserved. For more information, see the discussion of the xml:space attribute in >Special Attributes later in this chapter.
To simplify the lives of software developers, parsers are expected to normalize all occurrences of the carriage return ( #xD ) character to a single line feed ( #xA ) character. When the carriage return character appears directly before a line feed, it is simply removed. This results in a document that contains only single line feed characters to mark line ends. In XML 1.1, this normalization to a line feed character also occurs for the Unicode characters #x85 (NEXT LINE, NEL) and #x2028 (LINE SEPARATOR).
| Names |
XML 1.0 names must adhere to the following lexical conventions:
-
Begin with a letter, ideograph, _ , or : character.
-
After the first character, be composed only of letters , digits, ., - , _ , and : characters.
In this context, a letter is any Unicode character that matches the Letter production from the XML 1.0 EBNF grammar at the end of this chapter.
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According to the XML 1.0 specification, the : character may be used freely within names, although the character is now officially reserved as part of the "Namespaces in XML" recommendation. Even if a document does not use namespaces, the colon should still not be used within identifiers to maintain compatibility with namespace-aware parsers. See the Section 21.3.4 in this chapter for more information about how namespace-aware identifiers are formed.
Names should also avoid starting with the three-letter sequence X, M, L (in any case combination), unless specifically sanctioned by an XML specification.
| Character References |
&# decimal-number ; &#x hexadecimal-number ;
All XML parsers are based on the Unicode character set, no matter what the external encoding of the XML file is. It is theoretically possible to author documents directly in Unicode, but many text-editing, storage, and delivery systems do not fully support the Unicode character set. To allow XML authors to include Unicode characters in their documents' content without forcing them to abandon their existing editing tools, XML provides the character reference mechanism.
A character reference allows an author to insert a Unicode character by number (either decimal or hexadecimal) into the output stream produced by the parser to an XML application. Consider an XML document that includes the following character data:
© 2002 O'Reilly & Associates
In this example, the parser would replace the character reference with the actual Unicode character and pass it to the client application:
2002 O'Reilly & Associates
Character references may not be used in element or attribute names, although they may be used in attribute values. Note that hexadecimal character references are case-insensitive (i.e., &xa9; is equivalent to &xA9; ).
| Predefined Entities |
Besides user-defined entity references, XML includes the five named entity references shown in Table 21-1 that can be used without being declared. These references are a subset of those available in HTML documents.
Table 21-1. Predefined entities
| Entity | Character | XML declaration |
|---|---|---|
| < | < | <!ENTITY lt " &#60;"> |
| > | > | <!ENTITY gt ">"> |
| & | & | <!ENTITY amp " &#38;"> |
| ' | ' | <!ENTITY apos "'"> |
| " | " | <!ENTITY quot """> |
The < and & entities must be used wherever < or & appear in element content or attribute values. The > entity is frequently used wherever > appears in document content, but it is only mandatory to avoid putting the sequence ]]> into content. ' and " are generally used only within attribute values to avoid conflicts between the value and the quotes used to contain the value.
Although the parser must recognize these entities regardless of whether they have been declared, you can declare them in your DTD without generating errors.
The presence of these "special" predefined entities creates a conundrum within an XML document. Because it is possible to use these references without declaring them, it is possible to have a valid XML document that includes references to entities that were never declared. The XML specification actually encourages document authors to declare these entities to support older SGML parsers that don't predefine these entities. In practical terms, declaring these entities only adds unnecessary complexity to your document.
| CDATA (Character Data) Sections |
<![CDATA[ unescaped character & markup data ]]>
XML documents consist of markup and character data. The < or & characters cannot be included inside normal character data without using a character or entity reference, such as & or & . By using a reference, the resulting < and & characters are not recognized as markup by the parser, but they will become part of the data stream to the parser's client application.
For large blocks of character dataparticularly if the data contains markup, such as an HTML or XML fragmentthe CDATA section can be used. Within a CDATA block, every character between the opening and closing strings is treated as character data. Thus, special characters can be included in a CDATA section with impunity, except for the CDATA closing sequence, ]]> .
CDATA sections are very useful for tasks such as enclosing XML or HTML documents inside of tutorials explaining how to use markup, but it is difficult to process the contents of CDATA sections using XSLT, the DOM, or SAX as anything other than text.
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| Entities |
An XML entity can best be understood as a macro replacement facility, in which the replacement can be either parsed (the text becomes part of the XML document) or unparsed. If unparsed, the entity declaration points to external binary data that cannot be parsed. Additionally, the replacement text for parsed entities can come from a string or the contents of an external file. During parsing, a parsed entity reference is replaced by the substitution text that is specified in the entity declaration. The replacement text is then reparsed until no more entity or character references remain .
To simplify document parsing, two distinct types of entities are used in different situations: general and parameter. The basic syntax for referencing both entity types is almost identical, but specific rules apply to where each type can be used.
| Parameter Entity References |
% name ;
When an XML parser encounters a parameter entity reference within a document's DTD, it replaces the reference with the entity's text. Whether the replacement text is included as a literal or included from an external entity, the parser continues parsing the replacement text as if it had always been a part of the document. This parsing has interesting implications for nested entity references:
<!ENTITY % YEAR "2001"> <!ENTITY COPYRIGHT "© %YEAR;"> . . . <copyright_notice>©RIGHT;</copyright_notice>
After the necessary entity replacements are made, the previous example would yield the following canonical element:
<copyright_notice> 2001</copyright_notice>
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Parameter entity references are recognized only within the DTD; therefore, the % character has no significance within character data and does not need to be escaped.
| General Entity References |
& name ;
General entity references are recognized only within the parsed character data in the body of an XML document. They may appear within the parsed character data contained between element start- and end-tags, or within the value of an attribute. They are not recognized within a document's DTD (except inside default values for attributes) or within CDATA sections.
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| Comments |
<!-- comment text -->
Comments can appear anywhere in a document or DTD, outside of other markup tags. XML parsers are not required to preserve contents of comment blocks, so they should be used only to store information that is not a part of your application. In reality, most information you might consider storing in a comment block probably should be made an official part of your XML application. Rather than storing data that will be read and acted on by an application in a comment, as is frequently done in HTML documents, you should store it within the element structure of the actual XML document. Enhancing the readability of a complex DTD or temporarily disabling blocks of markup are effective uses of comments.
The character sequence -- cannot be included within a comment block, except as part of the tag closing text. Because comments cannot be nested, commenting out a comment block is impossible .
| Processing Instructions |
<? target processing-instruction data ?>
Processing instructions provide an escape mechanism that allows an XML application to include instructions to an XML processor that are not validated. The processing instruction target can be any legal XML name, except xml in any combination of upper- and lowercase (see Chapter 2). Linking to a stylesheet to provide formatting instructions for a document is a common use of this mechanism. According to the principles of XML, formatting instructions should remain separate from the actual content of a document, but some mechanism must associate the two. Processing instructions are significant only to applications that recognize them.
The notation facility can indicate exactly what type of processing instruction is included, and each individual XML application must decide what to do with the additional data. No action is required by an XML parser when it recognizes that a particular processing instruction matches a declared notation. When this facility is used, applications that do not recognize the public or system identifiers of a given processing instruction target should realize that they could not properly interpret its data portion.
Character Encoding AutodetectionThe XML declaration (possibly preceded by a Unicode byte-order mark) must be the very first item in a document so that the XML parser can determine which character encoding was used to store the document. A chicken-and-egg problem exists, involving the XML declaration's encoding="... " clause: the parser can't parse the clause if it doesn't know what character encoding the document uses. However, since the first five characters of the document must be the string <?xml (if it includes an XML declaration), the parser can read the first few bytes of a document and, in most cases, determine the character encoding before it has read the encoding declaration. |
| XML Declaration |
<?xml version=" version_number " [ encoding=" encoding-name " ][ standalone=" yes no " ] ?>
The XML declaration serves several purposes. It tells the parser what version of the specification was used, how the document is encoded, and whether the document is completely self-contained or not.
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The XML declaration, if included, must be the first thing that appears in an XML document. Nothing, except possibly a Unicode byte-order mark, may appear before this structure's initial < character. If no XML declaration is present, the document is assumed to conform to the XML 1.0 Recommendation.
| Version Information |
... version=" version_number " ...
The version information attribute denotes which version of the XML specification was used to create the current document. At this time, the only possible version numbers are 1.0 and 1.1 .
| Encoding Declaration |
... encoding=" encoding-name " ...
The encoding declaration, if present, indicates which character-encoding was used to store the document. Although all XML documents are ultimately handled as Unicode by the parser, the external storage scheme may be anything from an ASCII text file using the Latin-1 character set (ISO-8859-1) to a file with native Japanese characters.
XML parsers may also recognize other encodings, but the XML specification only requires that they recognize UTF-8 and UTF-16 encoded documents. Most parsers also support additional character encodings. For a thorough discussion of character-encoding schemes, see Chapter 27.
| Standalone Declaration |
... standalone=" yes no " ...
If a document is completely self contained (the DTD, if there is one, is contained completely within the original document), then the standalone="yes " declaration may be used. If this declaration is not given, the value no is assumed, and all external entities are read and parsed.
From the standpoint of an XML application developer, this flag has no effect on how a document is parsed. However, if it is given, it must be accurate. Setting standalone="yes " when a document does require DTD declarations that are not present in the main document file is a violation of XML validity rules.
21.3.2 DTD (Document Type Definition)
Chapter 2 explained the difference between well-formed and valid documents. Well-formed documents that include and conform to a given DTD are considered valid. Documents that include a DTD and violate the rules of that DTD are invalid. The DTD is comprised of both the internal subset (declarations contained directly within the document) and the external subset (declarations that are included from outside the main document).
| Parameter Entities |
The parameter entity mechanism is a simple macro replacement facility that is only valid within the context of the DTD. Parameter entities are declared and then referenced from within the DTD or possibly from within other entity declarations. The source of the entity replacement text can be either a literal string or the contents of an external file. Parameter entities simplify maintenance of large, complex documents by allowing authors to build libraries of commonly used entity declarations.
| Parameter Entity Declarations |
<!ENTITY % name " Replacement text ."> <!ENTITY % name SYSTEM " system-literal "> <!ENTITY % name PUBLIC " pubid-literal " " system-literal ">
Parameter entities are declared within the document's DTD and must be declared before they are used. The declaration provides two key pieces of information:
-
The name of the entity, which is used when it is referenced
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The replacement text, either directly or indirectly through a link to an external entity
Be aware that an XML parser performs some preprocessing on the replacement text before it is used in an entity reference. Most importantly, parameter entity references in the replacement text are recursively expanded before the final version of the replacement text is stored. Character references are also replaced immediately with the specified character. This replacement can lead to unexpected side effects, particularly when constructing parameter entities that declare other parameter entities. For full disclosure of how entity replacement is implemented by an XML parser and what kinds of unexpected side effects can occur, see Appendix D of the XML 1.0 specification. The specification is available on the World Wide Web Consortium web site (http://www.w3.org/TR/REC-xml#sec-entexpand).
| Parsed General Entities |
<!ENTITY name " Replacement text ."> <!ENTITY name SYSTEM " system-literal "> <!ENTITY name PUBLIC " pubid-literal " " system-literal ">
Parsed general entities are declared within the document type definition and then referenced within the document's text and attribute content. When the document is parsed, the entity's replacement text is substituted for the entity reference. The parser then resumes parsing, starting with the text that was just replaced.
Parsed general entities are declared within the DTD using a superset of the syntax used to declare parameter entities.
Internal entities store the replacement text inline as a literal string. The replacement text within an internal entity is included completely in the entity declaration itself, obviating the need for an external file to contain the replacement text. This situation closely resembles the string replacement macro facilities found in many popular programming languages and environments:
<!ENTITY name " Replacement text ">
When a parsed general entity is referenced, the contents of the external entity are included in the document, and the XML parser resumes parsing, starting with the newly included text.
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| Text Declarations |
<?xml [ version=" version_number " ] encoding=" encoding-name "?>
Files that contain external parsed entities must include a text declaration if the entity file uses a character encoding other than UTF-8 or UTF-16, or if its contents conform to the XML 1.1 Recommendation. This declaration would be followed by the replacement text of the external parsed entity. Entities with no text declaration are assumed to conform to the XML 1.0 Recommendation.
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| External Subset |
The document type declaration can include part or all of the document type definition from an external file. This external portion of the DTD is referred to as the external DTD subset and may contain markup declarations, conditional sections, and parameter entity references. It must include a text declaration if the DTD requires features of XML 1.1 or if the character encoding is not UTF-8 or UTF-16:
<?xml [ version=" version_number " ] encoding=" encoding-name "?>
This declaration (if present) would then be followed by a series of complete DTD markup statements, including ELEMENT , ATTLIST , ENTITY , and NOTATION declarations, as well as conditional sections, and processing instructions. For example:
<!ELEMENT furniture_item (desc, %extra_tags; user_tags?, parts_list, assembly+)> <!ATTLIST furniture_item xmlns CDATA #FIXED "http://namespaces.oreilly.com/furniture/" > ...
| Internal DTD Subset |
The internal DTD subset is the portion of the document type definition included directly within the document type declaration between the [ and ] characters. The internal DTD subset can contain markup declarations and parameter entity references, but not conditional sections. A single document may have both internal and external DTD subsets , which, when taken together, form the complete document type definition. The following example shows an internal subset, which appears between the [ and ] characters:
<!DOCTYPE furniture_item SYSTEM "furniture.dtd" [ <!ENTITY % bookcase_ex SYSTEM "Bookcase_ex.ent"> %bookcase_ex; <!ENTITY bookcase_pic SYSTEM "bookcase.gif" NDATA gif> <!ENTITY parts_list SYSTEM "parts_list.ent"> ]>
| Element Type Declaration |
Element type declarations provide a template for the actual element instances that appear within an XML document. The declaration determines what type of content, if any, can be contained within elements with the given name. The following sections describe the various element content options available.
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| Empty Element Type |
<!ELEMENT name EMPTY>
Elements that are declared empty cannot contain content or nested elements. Within the document, empty elements may use one of the following two syntax forms:
< name [attribute = " value " ...] /> < name [attribute = " value " ...] ></ name >
| Any Element Type |
<!ELEMENT name ANY>
This content specifier acts as a wildcard, allowing elements of this type to contain character data or instances of any valid element types that are declared in the DTD.
| Mixed Content Element Type |
<!ELEMENT name (#PCDATA [ name] +)*> <!ELEMENT name (#PCDATA)>
Element declarations that include the #PCDATA token can include text content mixed with other nested elements that are declared in the optional portion of the element declaration. If the #PCDATA token is used, it is not possible to limit the number of times or sequence in which other nested elements are mixed with the parsed character data. If only text content is desired, the asterisk is optional.
| Constrained Child Nodes |
<!ELEMENT name ( child_node_regexp ) [ ? * + ] >
XML provides a simple regular-expression syntax that can be used to limit the order and number of child elements within a parent element. This language includes the following operators:
| Operator | Meaning |
|---|---|
| Name | Matches an element of the given name |
| () | Groups expressions for processing as sets of sequences (using the comma as a separator) or choices (using as a separator) |
| ? | Indicates that the preceding name or expression can occur zero or one times at this point in the document |
| * | Indicates that the preceding name or expression can occur zero or more times at this point in the document |
| + | Indicates that the preceding name or expression must occur one or more times at this point in the document |
| Attribute List Declaration |
<!ATTLIST element_name [attribute_name attribute_type default_decl] *>
In a valid XML document, it is necessary to declare the attribute names, types, and default values that are used with each element type.
The attribute name must obey the rules for XML names, and no duplicate attribute names may exist within a single declaration.
Attributes are declared as having a specific type. Depending on the declared type, a validating XML parser will constrain the values that appear in instances of those attributes within a document. The following table lists the various attribute types and their meanings:
| Attribute type | Meaning |
|---|---|
| CDATA | Simple character data. |
| ID | A unique ID value within the current XML document. No two ID attribute values within a document can have the same value, and no element can have two attributes of type ID . |
| IDREF, IDREFS | A single reference to an element ID ( IDREF ) or a list of IDs ( IDREFS ), separated by spaces. Every ID token must refer to a valid ID located somewhere within the document that appears as the ID type attribute's value. |
| ENTITY, ENTITIES | A single reference to a declared unparsed external entity ( ENTITY ) or a list of references ( ENTITIES ), separated by whitespace. |
| NMTOKEN, NMTOKENS | A single name token value ( NMTOKEN ) or a list of name tokens ( NMTOKENS ), separated by spaces. |
| NOTATION Attribute Type |
... NOTATION ( notation [ notation] *) ...
The NOTATION attribute mechanism lets XML document authors indicate that the character content of some elements obey the rules of some formal language other than XML. The following short sample document shows how notations might be used to specify the type of programming language stored in the code_fragment element:
<?xml version="1.0"?> <!DOCTYPE code_fragment [ <!NOTATION java_code PUBLIC "Java source code"> <!NOTATION c_code PUBLIC "C source code"> <!NOTATION perl_code PUBLIC "Perl source code"> <!ELEMENT code_fragment (#PCDATA)> <!ATTLIST code_fragment code_lang NOTATION (java_code c_code perl_code) #REQUIRED> ]> <code_fragment code_lang="c_code"> main() { printf("Hello, world."); } </code_fragment> | Enumeration Attribute Type |
... ( name_token [ name_token] *) ...
This syntax limits the possible values of the given attribute to one of the name tokens from the provided list:
<!ELEMENT door EMPTY> <!ATTLIST door state (open closed missing) "open"> . . . <door state="closed"/>
| Default Values |
If an optional attribute is not present on a given element, a default value may be provided to be passed by the XML parser to the client application. The following table shows various forms of the attribute default value clause and their meanings:
| Default value clause | Explanation |
|---|---|
| #REQUIRED | A value must be provided for this attribute. |
| #IMPLIED | A value may or may not be provided for this attribute. |
| [#FIXED ] " default value " | If this attribute has no explicit value, the XML parser substitutes the given default value. If the #FIXED token is provided, this attribute's value must match the given default value. In either case, the parent element always has an attribute with this name. |
The #FIXED modifier indicates that the attribute may contain only the value given in the attribute declaration. Although redundant, it is possible to provide an explicit attribute value on an element when the attribute was declared as #FIXED . The only restriction is that the attribute value must exactly match the value given in the #FIXED declaration.
| Special Attributes |
Some attributes are significant to XML:
-
- xml:space
-
The xml:space attribute tells an XML application whether the whitespace within the specified element is significant:
<!ATTLIST element_name xml:space (defaultpreserve) default_decl > <!ATTLIST element_name xml:space (default) #FIXED 'default' > <!ATTLIST element_name xml:space (preserve) #FIXED 'preserve' >
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- xml:lang
-
The xml:lang attribute allows a document author to specify the human language for an element's character content. If used in a valid XML document, the document type definition must include an attribute type declaration with the xml:lang attribute name. See Chapter 5 for an explanation of language support in XML.
| Notation Declaration |
<!NOTATION notation_name SYSTEM " system-literal "> <!NOTATION notation_name PUBLIC " pubid-literal "> <!NOTATION notation_name PUBLIC " pubid-literal " " system-literal ">
Notation declarations are used to provide information to an XML application about the format of the document's unparsed content. Notations are used by unparsed external entities, processing instructions, and some attribute values.
Notation information is not significant to the XML parser, but it is preserved for use by the client application. The public and system identifiers are made available to the client application so that it may correctly interpret non-XML data and processing instructions.
| Conditional Sections |
The conditional section markup provides support for conditionally including and excluding content at parse time within an XML document's external subset. Conditional sections are not allowed within a document's internal subset. The following example illustrates a likely application of conditional sections:
<!ENTITY % debug 'IGNORE' > <!ENTITY % release 'INCLUDE' > <!ELEMENT addend (#PCDATA)> <!ELEMENT result (#PCDATA)> <![%debug;[ <!ELEMENT sum (addend+, result)> ]]> <![%release;[ <!ELEMENT sum (result)> ]]>
21.3.3 Document Body
Elements are an XML document's lifeblood. They provide the structure for character data and attribute values that make up a particular instance of an XML document type definition. The !ELEMENT and !ATTLIST declarations from the DTD restrict the possible contents of an element within a valid XML document. Combining elements and/or attributes that violate these restrictions generates an error in a validating parser.
| Start-Tags and End-Tags |
< element_name [attribute_name =" attribute value " ] *> ... </ element_name >
Elements that have content (either character data, other elements, or both) must start with a start-tag and end with an element end-tag.
| Empty-Element Tags |
< element_name [attribute_name =" attribute value " ] *></ element_name > < element_name [attribute_name =" attribute value " ] * />
Empty elements have no content and are written using either the start- and end-tag syntax mentioned previously or the empty-element syntax. The two forms are functionally identical, but the empty-element syntax is more succinct and more frequently used.
| Attributes |
attribute_name =" attribute value " attribute_name =' attribute value '
Elements may include attributes. The order of attributes within an element tag is not significant and is not guaranteed to be preserved by an XML parser. Attribute values must appear within either single or double quotations. Attribute values within a document must conform to the rules explained in the Section 21.4 of this chapter.
Note that whitespace may appear around the = character.
The value that appears in the quoted string is tested for validity, depending on the attribute type provided in the ATTLIST declaration for the element type. Attribute values can contain general entity references, but cannot contain references to external parsed entities. See the Section 21.4 of this chapter for more information about attribute-value restrictions.
21.3.4 Namespaces
Although namespace support was not part of the original XML 1.0 Recommendation, Namespaces in XML was approved less than a year later (January 14, 1999). Namespaces are used to uniquely identify the element and attribute names of a given XML application from those of other applications. See Chapter 4 for more detailed information.
The following sections describe how namespaces impact the formation and interpretation of element and attribute names within an XML document.
| Unqualified Names |
name
An unqualified name is an XML element or attribute name that is not associated with a namespace. This could be because it has no namespace prefix and no default namespace has been declared. All unprefixed attribute names are unqualified because they are never automatically associated with a default namespace. XML parsers that do not implement namespace support (of which there are very few) or parsers that have been configured to ignore namespaces will always return unqualified names to their client applications. Two unqualified names are considered to be the same if they are lexically identical.
| Qualified Names |
[ prefix : ] local_part
A qualified name is an element or attribute name that is associated with an XML namespace. There are three possible types of qualified names:
-
Unprefixed element names that are contained within the scope of a default namespace declaration
-
Prefixed element names
-
Prefixed attribute names
Unlike unqualified names, qualified names are considered the same only if their namespace URIs (from their namespace declarations) and their local parts match.
| Default Namespace Declaration |
xmlns=" namespace_URI "
When this attribute is included in an element start-tag, it and any unprefixed elements contained within it are automatically associated with the namespace URI given. If the xmlns attribute is set to the empty string, any effective default namespace is ignored, and unprefixed elements are not associated with any namespace.
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| Namespace Prefix Declaration |
xmlns: prefix = " namespace_URI "
This declaration associates the namespace URI given with the prefix name given. Once it has been declared, the prefix may qualify the current element name, attribute names, or any other element or attribute name within the scope of the element that declares it. Nested elements may redefine a given prefix, using a different namespace URI if desired, and XML 1.1 documents may undefine a namespace prefix by setting it to an empty string:
xmlns: prefix= ""
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