DTD Simple to use

Dtd:document Type Definition

DTD is a simple XML constrained schema language

DTD documents must be encoded in Utf-8 or Unicode

Annotations in the same way as HTML, XML documents

References to DTD documents: There are two ways to follow an XML declaration statement (externally introduced and embedded internally)

<?xml standalone= "No"?>
<! DOCTYPE ... >

Externally introduced DTD mode:

There are two ways of referencing: the first is to reference the local DTD file, and the second is the public DTD file (Doctype-name is typically named with the root element)

1 <! DOCTYPE doctype-name SYSTEM "Dtd-url" > 2 <! DOCTYPE Doctype-name  Public "Dtd-name" "Dtd-url">

Internally embedded DTD mode:

1 <! DOCTYPE doctype-name [2   3]>

A simple DTD document is as follows:

<! ELEMENT Bookshelf (book+)>2     <!  ELEMENT Book (name,author,price)><!  ELEMENT name (#PCDATA)><!  ELEMENT author (#PCDATA)><!  ELEMENT Price (#PCDATA)>              

DTD element definition:<! Element name usage rules >

The following rules apply:

Empty: Define NULL elements

Any: In any form

(#PCDATA): normal text

(<ELE1> <ELE2> <ELE3> ...) : unordered element Group

(<ELE1>,<ELE2>,<ELE3> ...) : Ordered element group (requires child elements to be <ELE1>, <ELE2>, <ele3> The order in which they appear)

(<ELE1>|<ELE2>|<ELE3> ...) : Only one element

The following three kinds of elements can be added after the wildcard character (+: appear one or more times;?: appears 0 or more times; *:0 or multiple times; there is nothing to indicate and only once)

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

The DTD attribute definition:

<! attlist element name

Att1 Property Type setting description

...

>

The property types are as follows:

CDATA: String

Enumerated

Id

...

The setup instructions are as follows:

#REQUIRED: Must be set

#IMPLIED: Optional

#FIXED ' fixed ': fixed value

"Default": Setting defaults

such as:<! Attlist meat Kind (chicken|beaf|fish| ...) "Fish">

DTD entity definition:<! Entity name "Content">

DTD simple use