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DTD - XML building blocks
The building blocks of XML documentsXML documents (and HTML documents) are made up by the following building blocks: Elements, Tags, Attributes, Entities, PCDATA, and CDATA This is a brief explanation of each of the building blocks: ElementsElements are the main building blocks of both XML and HTML documents. Examples of HTML elements are "body" and "table". Examples of XML elements could be "note" and "message". Elements can contain text, other elements, or be empty. Examples of empty HTML elements are "hr", "br" and "img". TagsTags are used to markup elements. A starting tag like <element_name> mark up the beginning of an element, and an ending tag like </element_name> mark up the end of an element. Examples: AttributesAttributes provide extra information about elements. Attributes are placed inside the start tag of an element. Attributes come in name/value pairs. The following "img" element has an additional information about a source file:
The name of the element is "img". The name of the attribute is "src". The value of the attribute is "computer.gif". Since the element itself is empty it is closed by a " /". PCDATAPCDATA means parsed character data. Think of character data as the text found between the start tag and the end tag of an XML element. PCDATA is text that will be parsed by a parser. Tags inside the text will be treated as markup and entities will be expanded. CDATACDATA also means character data. CDATA is text that will NOT be parsed by a parser. Tags inside the text will NOT be treated as markup and entities will not be expanded. EntitiesEntities as variables used to define common text. Entity references are references to entities. Most of you will known the HTML entity reference: " " that is used to insert an extra space in an HTML document. Entities are expanded when a document is parsed by an XML parser. The following entities are predefined in XML:
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