Common
XML stands for eXtensible Markup Language. |
XML documents are structured, tagged documents composed of elements, attributes, processing instructions, namespace declarations, comments, and text. |
Elements
<root></root>
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<root />
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<root>child data</root>
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There can be exactly one root element of an XML document. (The document element) |
Elements have a name and may also have children; Child data may be text, elements, attributes, or other types of XML structures. |
Element names are case-sensitive |
Elements must be correctly nested: |
<outer><inner>Content</inner></outer>
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<outer><inner>Content</outer></inner>
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Element names must start with a letter or underscore. The following characters may contain letters, digits, periods, hyphens, underscores, or colons. |
Attributes
key='value'
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key="value"
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Elements can be annotated with attributes. Attributes cannot exist in a serialized document without a parent element. |
Attributes are name/value pairs that are separated by an equals sign, with a value surrounded by matching sing or double quotes. |
The attributes for an element are serialized within the start tag of an element. |
<User login="J.Smith" enrolled="2014-12-14" />
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<User enrolled="2014-12-14">J.Smith</User>
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Attribute names follow the same rules as element names.
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Namespaces
<prefix:localname xmlns:prefix="URI" />
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<tagname xmlns="URI" />
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Namespaces resolve ambiguity when tag names collide. |
Namespace declarations appear in an element start tag and are typically mapped to shorter prefix. |
Namespace declarations are scoped to the element they are defined in, and all descendants thereof. |
All elements in an XML document are QNames (qualified names); A QName is a local name with an optional prefix. |
Both the prefix and the local name are NCNames (No-colon names). |
The namespace of an element with a given prefix is the namespace specified by the in-scope namespace declaration for that prefix. (May be overridden) |
The namespace of un-prefixed elements is the default namespace. |
Attribute names are QNames; unprefixed attributes are in no namespace, even if a default namespace is in scope. |
Comments
<!-- Commented text -->
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Comments can contain spaces, text, elements, and line breaks, and can therefore span multiple lines of XML |
You may not use a double hyphen (--) within a comment itself; therefore, you cannot nest comments |
Character References
&#DecimalUnicodeValue; |
&#xHexadecimalUnicodeValue; |
Character references can only be used for element and attribute content |
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Processing-Instructions
<?xml version="1.0"?>
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<?target data?>
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Only processing instructions and comments may appear before the document lement |
The processing instruction target xml is reserved |
No processing instruction may start with the string xml or any recapitalization thereof |
Predefined Entities
There are five predefined entities used in the XML markup which must be escaped in order to appear in XML content |
& |
& |
< |
< |
> |
> |
" |
" |
' |
' |
You may not use (<) or (&) anywhere in your XML document, except to begin a tag or an entity, respectively |
CDATA
<![CDATA[Almost any content here]]>
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Use a CDATA section to include content that you do not want the XML parser to interpret |
CDATA stands for (unparsed) character data, as opposed to PCDATA (parsed character data) |
You may not nest CDATA sections |
CDATA sections may be used anywhere withing the root element of an XML document |
If you need to include the string ]]> in the CDATA text, and you are not closing the CDATA section, the closing > must be written as > |
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