First note on XML, more is needed.
Some definitions:
- XML
- eXtensible Markup Language
- DTD
- Document Type Definition. This describes the tags that can be used in a XML documents
- XSD
- XML Schema Definition
Basic rules
- XML documents start with a XML-declaration. Specifying the xml version used.
<?xml version=”1.0″ standalone=”no” encoding=”UTF-8″?>
Standalone - The entire document is inside tags
- Tags are in lowercase
- All tags must be closed like <tagname>content</tagname> or <tagname />
- Tags must be correctly nested
- All attribute values must be quoted (” or ‘)
Links:
- http://www.w3.org/XML/
- http://www.xml.com/
- http://www.xml.org/
- http://xml.coverpages.org/xml.html
- http://www.ofx.net (attempt to put banking information in XML)
How an XML document looks
| <?xml version=”1.0″ standalone=”no” encoding=”UTF-8″?> | XML declaration, defines the version of XML used |
| <!DOCTYPE typename SYSTEM “typename.dtd”> | Decalration of documenttype, define which dtd is to be used for this document. |
DTD
How a DTD document looks
| <!- comments -> | Comments, for better understanding an readability . |
| <!ENTITY entityname “Entity content”> | Entities can be called as &entityname; For content you can use ‘SYSTEM “URI”‘ to point to a local file, ‘PUBLIC “URI” points to an external resource. |
| <!ELEMENT elementname (subelement1, subelement2)> | Define a tag and what can be inside it |
| <!ELEMENT elementname (#PCDATA)> | Define a tag holding ‘parser controlled’ data (ascii) |
| <!ELEMENT elementname (subelement1*)> | Define a tag that can hold subelement 0 or more times (use + for one or more, ? for one or zero, |
| <!ELEMENT elementname <(subelement1 | #PCDATA)*>> | Define a tag that can hold subelement1 or PCDATA, the * at the end is mandatory for mixed data. |
| <!ATTLIST elementname attributename type option “value”> | Define an attribute for elementname see below for types and options. |
Attribute types can be one of
- CDATA, character data (ascii)
- “value1″|”value2″|”value3”, Enumerated, a limited choice of values
- IDREF and IDREFS,
- ENTITY and ENTITIES,
- NMTOKEN and NMTOKENS,
- NOTATION, use data from a notation-declaration in the DTD
Attribute options can be one of
- #REQUIRED, mandatory attribute.
- #IMPLIED, optional attribute, no default value
- #FIXED “value”, the attribute is optional, if it is there it will have “value”
- text, the default value for the attribute
XSD
XSDs are another, more flexible way to define what a XML-document will look like. W3C used dtd’s for all there recommendations.