Basic XML Concepts Web Programming Maymester 2004 XML

Basic XML Concepts Web Programming Maymester 2004

XML fundamental concepts • XML is not a language, but a structure for defining application-specific minilanguages • Tags, elements and attributes • Tag names can be ANYTHING (depends on application

Example XML document

XML/HTML entities • Some symbols cannot appear in document content because they confuse parsing • Entities represent these symbols < > < > & & " “ (double quote)

Well-Formed XML document • Every tag has a matching closing tag • Tags are properly nested • You can have an empty element, which is its own matching close tag • <danger voltage=“ 480” /> • All attribute values appear in quotes

Valid XML document • A valid document must be well-formed • In addition, the document conforms to a language definition • The language definition specifies possible tags, their contents, and their attributes • Two types of language definitions: – Document Type Definition (DTD) – XML Schema

Document Type Definition • Example: a letter contains a source and destination address, a salutation, one or more paragraphs, and a closing

DTD Attribute Definitions • REQUIRED: must always appear • IMPLIED: optional • FIXED: required, and always has same value

DTD Data Types & Quantifiers • #PCDATA =“parsed character data”, can contain no XML markup chars • Use entities to replace markup chars • “X+” means one or more instances of X • “X*” means zero or more instances of X • “X? ” means zero or one instances of X

Where to put DTDs • Can be embedded in the DOCTYPE declaration, as in previous example • SYSTEM DTD refers to a file on the same machine: • PUBLIC DTD gives a URL to the DTD:

XML Schema • Document definition file is also an XML document, unlike DTD • Gives much tighter control over contents of XML document • See http: //www. w 3. org/TR/2000/CRxmlschema-0 -20001024/ for details