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What is a well-formed XML document?
XML, or Extensible Markup Language, is a widely used format for storing and transporting structured data. An XML document consists of a series of elements enclosed in tags, which define its structure and hierarchy. A well-formed XML document refers to one that adheres to the syntax rules of XML and is correctly structured.
In order for an XML document to be considered well-formed, the following requirements must be met:
1. XML Declaration: The document must start with an XML declaration, which specifies the version of XML being used and the character encoding.
2. Proper Nesting: All elements must be properly nested within one another, without overlapping or crossing over. Each opening tag must have a corresponding closing tag, and tags should not be left unclosed.
3. Empty Elements: Elements with no content must be explicitly indicated as empty by using a self-closing tag or an empty element tag.
4. Attributes: Element attributes must be enclosed in quotes (either single or double) and values should be properly escaped if necessary.
5. Reserved Characters: Reserved characters in XML (such as <, >, &, ‘, “) must be properly escaped using predefined entities.
6. Entities: Entities, such as predefined character entities (<, >, &, etc.) or character references (€, €, etc.), should be correctly used if special characters need to be represented.
7. Unique Root Element: XML documents must have a single, unique root element that encapsulates all other elements.
Explaining the Basic Concepts of Valid XML Documents
Valid XML documents go a step beyond being well-formed and adhere to a specific XML schema or document type definition (DTD). The purpose of an XML schema or DTD is to define the structure, data types, and constraints of the XML document. It acts as a blueprint for creating and validating XML files.
Here are a few key concepts related to valid XML documents:
1. XML Schema or DTD: An XML schema serves as a grammar or blueprint that defines the structure, data types, and constraints of the XML document. It specifies the elements, attributes, and other rules that the XML document must adhere to. DTD (Document Type Definition) is an older and simpler way to define XML structures.
2. Element Validation: XML schema or DTD allows for the validation of individual elements. It specifies rules such as data types, minimum and maximum occurrences, or patterns that an element’s content must adhere to.
3. Document Validation: An XML document can be validated against an XML schema or DTD to ensure it conforms to the defined rules and structure. This validation helps identify any inconsistencies or errors in the XML document.
4. Namespace: XML namespaces are used to avoid conflicts between element names in different XML vocabularies. By defining namespaces, different XML files can use elements with the same name but from different vocabularies without any ambiguity.
5. Document Type Declaration (DOCTYPE): DOCTYPE declaration, when used, defines the DTD or XML schema to which the XML document conforms. It is placed at the beginning of the XML document, before the root element.
Valid XML documents ensure data integrity, interoperability, and compatibility between different systems that exchange XML-based information. By defining the structure and rules of XML, valid documents can be accurately processed and interpreted by software applications.
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