Introduction to XML DTD. XML Schema Languages

ElementName specifies the name of the element to include in the tag. The attributes associated with the element are then listed. Each attribute declaration consists of three main components: a name, a data type, and a flag that defines the characteristics of that attribute. Declarations of other attributes may be placed instead of the ellipsis(...).

We already saw a simple attribute declaration in Listing 14.2:

However, as can be seen from the above general definition, simultaneous declaration of several attributes is allowed. Let's say that in addition to the category attribute, you want to associate an additional difficulty attribute with the recipe element. Both attributes are declared in the same list:

It is not necessary to format your ads this way; however, multi-line declarations are clearer than single-line declarations. Additionally, since both attributes are required, the reci re tag cannot be limited to just one attribute; it must include both attributes at once. For example, the following tag would be considered invalid:

Why? Because it is missing the category attribute. A valid tag must contain both attributes:

Special conditions for attribute processing are described by three flags listed in Table. 14.2.

Table 14.2. Attribute flags

Attribute Types

An element attribute can be declared with a specific type. The attribute types are described below.

CDATA attributes

Very often attributes contain general character data. These attributes are called CDATA attributes. The following example was already encountered at the beginning of this section:

ID, IDREF, and IDREFS Attributes

The idea of ​​unambiguously representing data (for example, information about a user or product stored in a database) through identifiers has been encountered several times in previous chapters of the book. Identifiers are also often used in XML because cross-referencing between documents is not limited to common tasks data processing, but also in World Wide Web(hyperlinks).

Element identifiers are assigned to the ID attribute. Let's say you want to associate a unique ID with each recipe. The corresponding DTD fragment might look like this:

The declaration of the recipe element in the document might then look like this:

The recipe is uniquely identified by the identifier ital003. Note that the redpe-id attribute is of type ID, so ital003 cannot be used as the value of the recipe-id attribute of another element, otherwise the document will be considered syntactically invalid. Now let's say that you later want to reference this recipe from another document - say, from a user's list of favorite recipes. This is where cross-references and the IDREF attribute come into play. The IDREF attribute is assigned an identifier that is used to reference an element, similar to how a URL is used to identify a page in a hyperlink. Consider the following XML code snippet:

When the XML document is processed, the element is replaced with a more descriptive link to the recipe with the specified identifier (for example, the name of the recipe). It will likely be formatted as a hyperlink to make it easier to navigate to said recipe.

Enumerable attributes

When declaring an attribute, you can list all the valid values ​​accepted by the attribute. In our example, this would be convenient because you can immediately define a list of valid categories. The above declaration is written as follows:

Note that when using valid value lists, you do not need to include the CDATA type in the declaration because all values ​​listed are in CDATA format.

Enumerated attributes with default value

Sometimes it is convenient to declare a default value for an attribute. Chances are you've done this before when building forms with drop-down lists. For example, if most of the recipes in your cookbook are Italian, the recipe attribute will often be categorized as Italian. In this case, the Italian category can be assigned as the default:

If the category attribute is not explicitly specified, it defaults to Italian.

ENTITY and ENTITIES attributes

Data in XML documents is not always text - the document can also contain binary information (for example, graphics). Such data can be referenced using the entity attribute. For example, in the description of the description element you can specify the recipePicture attribute with a graphic image:

You can also declare several entities at once by replacing ENTITY with ENTITIES. Values ​​are separated by spaces.

NMTOKEN and NMTOKENS attributes

NMTOKEN attributes are strings of characters included in a limited set. Declaring an attribute of type NMTOKEN assumes that the attribute value matches established restrictions. Typically, the value of the NMTOKEN attribute consists of one word:

You can declare multiple attributes at once by replacing NMTOKEN with NMTOKENS. Values ​​are separated by spaces.

Entity Declarations

An entity declaration is similar to the define command in some programming languages, including PHP. Entity references were briefly mentioned in the previous section, "Introducing XML Syntax." Just in case, let me remind you that an entity link is used as a replacement for another piece of content. When an XML document is processed, all occurrences of an entity are replaced by the content that it represents. There are two types of entities: internal and external.

Internal Entities

Internal entities are like string variables that associate a name with a piece of text. For example, if you wanted to define a name for a link to copyright information, you could declare an entity like this:

During document processing, all instances of &Copyright are replaced with the text “Copyright 2000 YourCompanyName. All Rights Reserved." All XML code in the replacement text is treated as if it were present in the original document.

Internal entities are useful in situations where you plan to use the entity in a relatively small way. large quantities XML documents. If you have a large number of documents, it is better to use external entities.

External Entities

External entities are used to reference content located in another file. Entities of this type may contain text information, but can also refer to binary data (for example, graphics). Returning to the previous example, let's say you decide to save your copyright information in a separate file to make it easier to edit in the future. The link to the created file looks like this:

When the XML document is subsequently processed, all &Copyright references are replaced with the contents of the copyright.xml document. All XML code in the replacement text is treated as if it were present in the original document.

External entities are also useful for referencing graphic images. For example, if you want to include a graphical logo in an XML document, create an external entity:

XML Resources

Although the above material is quite enough for understanding basic structure XML documents, this description is not complete. Below are links to Internet resources containing more detailed information:

The rest of the chapter shows you how to use PHP to process XML documents. At first glance, the task seems very difficult (lexical analysis of any documents of any type causes a lot of difficulties).

But once you get acquainted with the basic strategy for working with XML in PHP, everything turns out to be surprisingly simple.

Document schema description

A DTD describes the document layout for a particular markup language through a set of declarations (parameter objects, elements, and attributes) that describe its class (or type) in terms of the syntactic constraints of that document. A DTD can also declare constructs that are always necessary to define the structure of a document, but can, however, affect the interpretation of certain documents.

Declaring parameter objects

A parameter object declaration defines a macro of a particular type that can be referenced and expanded somewhere in the DTD. These macros may not appear in the document itself, but only in the DTD. If a parameter object is referenced by its DTD name, it is expanded into a string that specifies the contents of that object.

The fontstyle parameter object contains a group of tags TT | I | B | BIG | SMALL.

"#PCDATA | %fontstyle; | %phrase; | %special; | %formctrl;">

The inline parameter object contains text data and four more parameter objects fontstyle , phrase , special and formctrl .

Declaration of elements

Element declarations provide a list of allowed element names in a document, and also specify information regarding tags (whether they are required) and the content model for each element.

Various keywords and the characters define the contents of the element:

• EMPTY - empty content
• ANY - any content
• , - indicates the order
• | - separation of alternatives
• () - grouping
• * - any number of elements (zero or more)
• + - at least one element (one or more)
• ? - optional presence of an element (zero or one)
• If not *, + or ? - there must be only one element

(DT|DD) +>

A DL element must contain one or more DT or DD elements in any order.

(%block;|SCRIPT) + -(FORM) >

A FORM element must contain one or more elements with a block parameter object or SCRIPT elements in any order, but it is not possible to contain another FORM element.

Defining Attributes

Each element in a DTD document can have a list of attributes associated with it. To do this, use the!ATTLIST directive, which specifies the name of the element to which the list of attributes can be mapped and the parameters of each attribute: its name, type and default properties.

For example:

name CDATA #REQUIRED>

This example defines name attribute for the MAP element. It is mandatory.

There are these types of attributes:

• CDATA (Character set of data) - the attribute value can be any character data
• ID - the attribute value must be a unique identifier of the element
• IDREF - the value of the element is a reference to the element by its ID
• IDREFS - the same as IDREF, but with the ability to link not by one identifier, but by several
• NMTOKEN - the attribute value can be a sequence of characters, somewhat similar to a name (hence the name - name token). This is a string that contains any combination of those characters that are allowed for XML names.
• NMTOKENS - the attribute value is a list of values
• ENTITY - the value is used to reference an external entity.
• ENTITIES - allows you to specify a list of external entities, separated by spaces.
• NOTATION - the attribute value can be one of the previously defined notations
• NOTATIONS - allows you to specify a list of notations.
• Listings and NOTATION-listings
• ENUMERATION - specifies a list of possible value alternatives.

There are the following default properties:

1. IMPLIED - the attribute value is not required to be specified;
2. REQUIRED - the attribute value must be specified;
3. FIXED - the value of this attribute is specified as a constant in the DTD and cannot be changed in the document;
4. some specific value that is used by default.

Association of a document with a specific DTD

To associate a document with a specific DTD, you must specify the Document Type Declaration element at the beginning of the document text.

Depending on the location of the DTD, the Document Type Declaration can be of two types:

• Internal DTD subset

A set of DTD declarations is contained within the body of the document itself. For example:

]> ]>

• External DTD subset

A set of DTD declarations is located in a separate text file with the extension .dtd In this case, a link to the file can be made through a public identifier and (or) through a system identifier. For example:

"http://www.w3.org/TR/html4/strict.dtd">

Example

An example of a very simple XML DTD describing a list of people:

(person*) > (name, birthdate?, gender?, socialsecuritynumber?) > (#PCDATA) > (#PCDATA) > (#PCDATA) >

Starting from the first line:

1. Element Contains any number of elements Sign<*>means that 0, 1 or more elements are possible Inside an element .
2. Element Contains elements , , And . Signmeans the element is optional. Element does not contain, which means that the element must contain element .
3. Element contains data.
4. Element contains data.
5. Element contains data.
6. Element contains data.

An example XML document using this DTD:

> > > Fred Bloggs > > 27/11/2008> > Male > > 1234567890> > >

See also

Wikimedia Foundation. 2010.

See what "DTD" is in other dictionaries:

DTD- , die in einer ASCII Datei (ASCII) abgelegte Beschreibung der Struktur von Dokumenten, welche alle vom selben Typ sind. Eine DTD wird nach den Regeln der international anerkannten… … Universal-Lexikon

DTD- may stand for: Contents 1 Media 2 Music 3 Sports 4 Technologies 4.1 Computing ... Wikipedia

DTD- steht für: Inhaltsverzeichnis 1 Medien 2 Music 3 Technologien 3.1 Computer 3.1.1 Spiele … Deutsch Wikipedia

Dtd- steht für: Darwin Digital Television, eine australische Fernsehstation Delta Tau Delta, eine US amerikanische Studentenorganization Document Type Definition, siehe Dokumenttypdefinition … Deutsch Wikipedia

DTD- (dē tē dēʹ) n. A set of rules for marking up a document in SGML. * * * …Universalium

DTD- (document type definition) specification written in the Standard Generalized Markup Language and containing information about the format of a particular document (Computers) ... English contemporary dictionary

• describe what exactly the markup is;
• describe exactly what the markup means.

The most well-known schema description languages ​​are the following:

• DTD (Document Type Definition) is a document type definition language that was originally used as a language for describing the structure of an SGML document.
• XDR (XML Data Reduced is an XML schema dialect developed by Microsoft that was supported in Internet Explorer versions 4 and 5.
• XML Schema or simply XSD ( schema definition language XML) is a W3C recommendation since 2001.

Let's take a closer look at the first two of them. A third circuit description language is discussed in laboratory work 11.

DTD scheme

The DTD schema provides sample markup of the document, which indicates availability, order following And location elements and them attributes in the XML document.

In terms of a DTD, the content model of an XML document can be described as follows:

Every element the document can be one of the following types:

Attributes found inside document tags are described separately using the syntax:

In this case, an attribute in a DTD can have one of three types:

• Line
• Labeled attribute
• Attribute with enumeration

In addition to the attribute type, you can also set its modality:

Consider as an example the description of attributes string type for an element describing some message:

If this element contains attributes with transfer, then their description may look, for example, as follows:

Labeled element attributes can be of four types:

Finally, the following sequence occurrence indicators can be used in the DTD:

As an example, here is a DTD diagram describing the structure of an electronic mailbox:

This is the next article in the series “XML Basics” and in it we will look at the basics of describing the structure XML data using DTD. It's quite old way descriptions XML structures-documents, but it is still in use, so we will still consider it.

I also want to point out that this great way show how XML checks document content, grammar, etc. We will look at a newer and more advanced way of describing the structure of XML documents using XML Schema technology in the next article, but for now let’s move on directly to studying the XML DTD.

In this article we will look at several important points. This is what an XML DTD is and what it is needed for, let's talk about the disadvantages of DTD, and also learn how to independently compose your own DTD for validating XML documents. All this, as usual, will be presented step by step, as briefly and clearly as possible in order to save your time.

So let's begin.

What is a DTD in XML and why is it needed?

In short, a DTD in XML is used to check the grammar of a document and its conformance to a standard (one that the developer or yourself came up with). This allows the parser (processor) to determine at the processing stage whether the document meets our requirements. That is, the XML document is validated.

The need to check the grammar of XML documents is as follows:

• The XML document may not be intended for your system.
• The XML document may contain incorrect data.
• The XML document may contain errors in the structure ().

So, we figured out what an XML DTD is and why it is needed. Now let's briefly look at the disadvantages of DTDs, and then move on to the process of creating DTD files for validating XML documents.

Disadvantages of XML DTD

• Different from XML syntax language. This causes many problems, such as encoding problems or the inability to track errors.
• No data type checking. There is only one type in DTD - string.
• There is no DTD. You cannot match two or more DTD descriptions to a document.

It was short list DTD shortcomings, which were successfully corrected in XML schemas, which we will talk about in the following articles.

Declaring elements, attributes and entities in a DTD. Modifiers “*”, “?”, “+”

Special declarations and modifiers are used to declare elements, attributes, and entities in a DTD. To understand everything in detail, let's first look at the theoretical information, and then in the second part of the article we will move on to practical examples.

Defining an XML Element and a Sequence of XML Elements

The book element contains one title, author, price, and description elements.

Element Alternatives

The pricelist element contains the elements title, price and one of three elements to choose from – author, company or sample.

Empty elements

The none element must be empty.

Attribute Declaration

The pricelist element can contain two attributes - an id attribute and a name attribute. In this case, the id attribute is required, since #REQUIRED is specified, and the name attribute is optional (#IMPLIED is specified). In turn, CDATA indicates to the handler that there is no need to parse the contents of the attributes.

Defining Entities

If the entity “” is encountered, then “Dmitry Denisov” will be automatically substituted instead.

Modifiers (explain repetitions of elements)

* - zero or many.
? – zero or one.
+ - one or many.

The books element can contain one or more book elements.

Now let's look at what this all looks like with more practical examples.

Creating a DTD file for validating an XML document using the example of a book price list

Let us have the same price list of books that we use for examples in almost every article about XML. The XML document itself will look something like this.

&myname; Price 1 Description

Of course, the above example is not the ultimate dream, but it will do as an example. As you can see from the example, we have a root element pricelist, which contains nested book elements. Inside the book elements there are title, author, price and possibly description elements, which may contain some text data.

To validate this price list, we can use the following DTD document.

Now let's look at everything in more detail.

• — we declare the root element books and indicate in parentheses what it can contain. In this case, it can contain one or more book elements (the plus sign means one or more, see above).
• — define the book element. The book element can contain one title element, one or more author elements (plus sign), one price element, and one or no description elements (question mark).
• — define the title element. We specify #PCDATA as the content of the element. This means that the parser is required to parse what is inside this element.
• Similarly, we define the elements author, price, description.
• — we define the essence. First we write the entity itself, and then in quotes what will be displayed in its place. By default, only 3 entities are defined in XML. This is more (">" -<), меньше («<» — >) and ampersand (“&” - &). If you wish, you can create an unlimited number of entities using this method. The meanings can be not only words, but also entire sentences of significant length.

Connecting DTD for validating XML documents

Declarative method

This method is very rarely used, since its essence is to create self-sufficient documents. That is, the document will immediately contain both DTD and XML. The following construct is used to add a DTD to XML.

where instead of DOCUMENT we indicate the root element of the XML document.

For clarity, let's look at an example of a ready-made self-sufficient document with a declarative way to include a DTD.

]>

External DTD definition - connecting a DTD document

The essence this method is to connect a DTD file to an XML document using the following construct.

where DOCUMENT – we indicate the root element of the XML document.
file.dtd – link to the DTD file.

For clarity, consider the following example.

XML document

This concludes this article. We looked at all the main points when working with XML DTD and, I hope, I was able to explain everything clearly. If you don't want to miss other XML and XSLT tutorials, I recommend subscribing to the newsletter using the form below.

That's all. Good luck and success in learning XML!