DTD of 006_02xml Constraint

XML technology, you can write a document to constrain an XML writing specification, a document called a constraint.

Common constraint Techniques:

　　XML DTD

Xdr

SOX

XML Schema

Quick start of DTD:

　　DTD (document type definition): Documentation type definitions.

　　

The syntax of a DTD document mainly involves the following definition:

Defining elements

Use the element keyword in a DTD document to declare an XML element.

Syntax: <! Element name Usage Rules >

Usage Rules:

(#PCDATA): Indicates that the body content of an element can only be normal text. (Parsed Character Data)

Empty: Used to indicate that the body of the element is null. such as <br/>

Any: Used to indicate that the body content of an element is of any type.

(child Element): Indicates the child element contained in the element

Define child elements and describe their relationships:

If the child elements are separated by commas, the XML document must be written in the order in which they are declared.

such as: <! ELEMENT FILE (Title,author,email)

If the child element uses the "|" Separate, explain any choice.

such as: <! ELEMENT FILE (title| author| EMAIL)

With +, *,? To indicate the number of occurrences of an element

If there is no +* behind the element?: Indicates that only one occurrence must occur

+: Indicates at least one occurrence, one or more times

*: Denotes dispensable, 0 times, one or more times

?: Indicates that there can or may not, and some words can only be once. 0 or one time

such as: <! ELEMENT MYFILE (title*, AUTHOR?, EMAIL) * | COMMENT) >

Defining properties

Use the Attlist keyword in a DTD document to declare attributes for an element.

Syntax: <! Attlist element Name Property Name 1 property value type set Description Property Name 2 property value type set description ... >

Property value type:

CDATA: Indicates that the value of the property is a normal text string

Enumerated (DTD does not have this keyword): Represents an enumeration and can only be selected from the enumeration list, such as (chicken | beef | pork | fish)

ID: Indicates that the value of the property cannot be duplicated

Setup instructions

#REQUIRED: Indicates that the property must appear

#IMPLIED: Indicates that the property is optional

#FIXED: Indicates that the value of the property is a fixed value. Syntax: #FIXED "fixed value"

Direct value: Indicates that the value of the property is the default value

Defining entities

Defining an entity is specifying a name for a piece of content so that it can refer to what it represents by using that name.

Use the Entity keyword in a DTD document to declare an entity.

Entities can be divided into: reference entities and parameter entities, with different syntax

Referencing entities

Concept: defined in a DTD, used in XML

Syntax: <! Entity entity name "Solid content" >

The reference method (note is used in XML):& entity name;

Parameter entity

Concept: defined in DTD, used in DTD

Syntax: <! Entities% entity name "Entity Content" >

Reference method (note is used in DTD):% entity name;

DTD of 006_02xml Constraint