XML and DTDs: Understanding Document Type Definitions for Structuring XML Documents, Slides of Computer Science

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DTDs

XML and DTDs

• A DTD (Document Type Definition) describes the structure of

one or more XML documents. Specifically, a DTD describes:

• Elements
• Attributes, and
• Entities
• (We will discuss each of these in turn)
• An XML document is well-structured if it follows certain

simple syntactic rules

• An XML document is valid if it also specifies and conforms to a

DTD

Parsers

• An XML parser is an API that reads the content

of an XML document

– Currently popular APIs are DOM (Document

Object Model) and SAX (Simple API for XML)

• A validating parser is an XML parser that

compares the XML document to a DTD and

reports any errors

– Most browsers don’t use validating parsers

An XML example

This is the great American novel.</ paragraph> It was a dark and stormy night. Suddenly, a shot rang out!

• An XML document contains (and the DTD describes):
  • Elements, such as novel and paragraph, consisting of tags and content
  • Attributes, such as number="1", consisting of a name and a value
  • Entities (not used in this example)

ELEMENT descriptions

• Suffixes:

? optional foreword?

+ one or more chapter+

* zero or more appendix*

• Separators

, both, in order foreword?,

chapter+

| or section|chapter

• Grouping

( ) grouping (section|chapter)+Docsity.com

Elements without children

• The syntax is

– The name is the element name used in start and

end tags

– The category may be EMPTY:

• In the DTD:
• In the XML:
  
  or just
  

– In the XML, an empty element may not have any

content between the start tag and the end tag

– An empty element may (and usually does) have

attributes

Elements with children

• A category may describe one or more children:

• Parentheses are required, even if there is only one child
• A space must precede the opening parenthesis
• Commas (,) between elements mean that all children must appear, and must be in the order specified
• “|” separators means any one child may be used
• All child elements must themselves be declared
• Children may have children
• Parentheses can be used for grouping:

Elements with mixed content

• #PCDATA describes elements with only

character data

• #PCDATA can be used in an “or” grouping:

–

– This is called mixed content

– Certain (rather severe) restrictions apply:

• #PCDATA must be first
• The separators must be “|”
• The group must be starred (meaning zero or more)

An expanded DTD example

• -

<!ELEMENT novel

(foreword, chapter+, biography?, criticalEssay*)>

]>

Attributes and entities

• In addition to elements, a DTD may declare attributes and

entities

• This slide shows examples; we will discuss each in detail
• An attribute describes information that can be put within the

start tag of an element

• In XML:
• In DTD:
• An entity describes text to be substituted
• In XML: &copyright; In the DTD:

Important attribute types

• There are ten attribute types

• These are the most important ones:

– CDATA The value is character data

– (man|woman|child) The value is one

from this list

– ID The value is a unique

identifier

• ID values must be legal XML names and must be

unique within the document

– NMTOKEN The value is a legal XML

name Docsity.com

Less important attribute types

• IDREF The ID of another element

• IDREFS A list of other IDs

• NMTOKENS A list of valid XML names

• ENTITY An entity

• ENTITIES A list of entities

• NOTATION A notation

• xml: A predefined XML value

Entities

• There are exactly five predefined entities: <, >, &, ", and '
• Additional entities can be defined in the DTD:

• Entities can be defined in another document:

• Example of use in the XML:

This document is &copyright; 2002.

• Entities are a way to include fixed text (sometimes called “boilerplate”)
• Entities should not be confused with character references, which are

numerical values between & and

• Example: &233#; or &xE9#; to indicate the character é

Another example: XML

05/29/2002 Philadelphia, PA USA 84 51