DTD - Quick Guide

XML Document Type Declaration, commonly known as DTD, is a way to describe precisely the XML language. DTDs check the validity of structure and vocabulary of an XML document against the grammatical rules of the appropriate XML language.
An XML document can be defined as −
Well-formed − If the XML document adheres to all the general XML rules such as tags must be properly nested, opening and closing tags must be balanced, and empty tags must end with '/>', then it is called as well-formed.
OR
Well-formed − If the XML document adheres to all the general XML rules such as tags must be properly nested, opening and closing tags must be balanced, and empty tags must end with '/>', then it is called as well-formed.
OR
Valid − An XML document said to be valid when it is not only well-formed, but it also conforms to available DTD that specifies which tags it uses, what attributes those tags can contain, and which tags can occur inside other tags, among other properties.
Valid − An XML document said to be valid when it is not only well-formed, but it also conforms to available DTD that specifies which tags it uses, what attributes those tags can contain, and which tags can occur inside other tags, among other properties.
The following diagram represents that a DTD is used to structure the XML document −
DTD can be classified on its declaration basis in the XML document, such as −
Internal DTD
Internal DTD
External DTD
External DTD
When a DTD is declared within the file it is called Internal DTD and if it is declared in a separate file it is called External DTD.
We will learn more about these in the chapter DTD Syntax
Following are some important points that a DTD describes −
the elements that can appear in an XML document.
the elements that can appear in an XML document.
the order in which they can appear.
the order in which they can appear.
optional and mandatory elements.
optional and mandatory elements.
element attributes and whether they are optional or mandatory.
element attributes and whether they are optional or mandatory.
whether attributes can have default values.
whether attributes can have default values.
Documentation − You can define your own format for the XML files. Looking at this document a user/developer can understand the structure of the data.
Documentation − You can define your own format for the XML files. Looking at this document a user/developer can understand the structure of the data.
Validation − It gives a way to check the validity of XML files by checking whether the elements appear in the right order, mandatory elements and attributes are in place, the elements and attributes have not been inserted in an incorrect way, and so on.
Validation − It gives a way to check the validity of XML files by checking whether the elements appear in the right order, mandatory elements and attributes are in place, the elements and attributes have not been inserted in an incorrect way, and so on.
It does not support the namespaces. Namespace is a mechanism by which element and attribute names can be assigned to groups. However, in a DTD namespaces have to be defined within the DTD, which violates the purpose of using namespaces.
It does not support the namespaces. Namespace is a mechanism by which element and attribute names can be assigned to groups. However, in a DTD namespaces have to be defined within the DTD, which violates the purpose of using namespaces.
It supports only the text string data type.
It supports only the text string data type.
It is not object oriented. Hence, the concept of inheritance cannot be applied on the DTDs.
It is not object oriented. Hence, the concept of inheritance cannot be applied on the DTDs.
Limited possibilities to express the cardinality for elements.
Limited possibilities to express the cardinality for elements.
An XML DTD can be either specified inside the document, or it can be kept in a separate document and then the document can be linked to the DTD document to use it.
Basic syntax of a DTD is as follows −
<!DOCTYPE element DTD identifier
[
   declaration1
   declaration2
   ........
]>
In the above syntax −
DTD starts with <!DOCTYPE delimiter.
DTD starts with <!DOCTYPE delimiter.
An element tells the parser to parse the document from the specified root element.
An element tells the parser to parse the document from the specified root element.
DTD identifier is an identifier for the document type definition, which may be the path to a file on the system or URL to a file on the internet. If the DTD is pointing to external path, it is called external subset.
DTD identifier is an identifier for the document type definition, which may be the path to a file on the system or URL to a file on the internet. If the DTD is pointing to external path, it is called external subset.
The square brackets [ ] enclose an optional list of entity declarations called internal subset.
The square brackets [ ] enclose an optional list of entity declarations called internal subset.
A DTD is referred to as an internal DTD if elements are declared within the XML files. To reference it as internal DTD, standalone attribute in XML declaration must be set to yes. This means the declaration works independent of external source.
The syntax of internal DTD is as shown −
<!DOCTYPE root-element [element-declarations]>
where root-element is the name of root element and element-declarations is where you declare the elements.
Following is a simple example of internal DTD −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>

<!DOCTYPE address [
   <!ELEMENT address (name,company,phone)>
   <!ELEMENT name (#PCDATA)>
   <!ELEMENT company (#PCDATA)>
   <!ELEMENT phone (#PCDATA)>
]>

<address>
   <name>Tanmay Patil</name>
   <company>TutorialsPoint</company>
   <phone>(011) 123-4567</phone>
</address>
Let us go through the above code −
Start Declaration − Begin the XML declaration with following statement.
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>
DTD − Immediately after the XML header, the document type declaration follows, commonly referred to as the DOCTYPE −
<!DOCTYPE address [
The DOCTYPE declaration has an exclamation mark (!) at the start of the element name. The DOCTYPE informs the parser that a DTD is associated with this XML document.
DTD Body − The DOCTYPE declaration is followed by body of the DTD, where you declare elements, attributes, entities, and notations −
<!ELEMENT address (name,company,phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone_no (#PCDATA)>
Several elements are declared here that make up the vocabulary of the <name> document. <!ELEMENT name (#PCDATA)> defines the element name to be of type "#PCDATA". Here #PCDATA means parse-able text data.
End Declaration − Finally, the declaration section of the DTD is closed using a closing bracket and a closing angle bracket (]>). This effectively ends the definition, and thereafter, the XML document follows immediately.
The document type declaration must appear at the start of the document (preceded only by the XML header) - it is not permitted anywhere else within the document.
The document type declaration must appear at the start of the document (preceded only by the XML header) - it is not permitted anywhere else within the document.
Similar to the DOCTYPE declaration, the element declarations must start with an exclamation mark.
Similar to the DOCTYPE declaration, the element declarations must start with an exclamation mark.
The Name in the document type declaration must match the element type of the root element.
The Name in the document type declaration must match the element type of the root element.
In external DTD elements are declared outside the XML file. They are accessed by specifying the system attributes which may be either the legal .dtd file or a valid URL. To reference it as external DTD, standalone attribute in the XML declaration must be set as  no. This means, declaration includes information from the external source.
Following is the syntax for external DTD −
<!DOCTYPE root-element SYSTEM "file-name">
where file-name is the file with .dtd extension.
The following example shows external DTD usage −
<?xml version = "1.0" encoding = "UTF-8" standalone = "no" ?>
<!DOCTYPE address SYSTEM "address.dtd">

<address>
  <name>Tanmay Patil</name>
  <company>TutorialsPoint</company>
  <phone>(011) 123-4567</phone>
</address>
The content of the DTD file address.dtd are as shown −
<!ELEMENT address (name,company,phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone (#PCDATA)>
You can refer to an external DTD by either using system identifiers or public identifiers.
System Identifiers
A system identifier enables you to specify the location of an external file containing DTD declarations. Syntax is as follows −
<!DOCTYPE name SYSTEM "address.dtd" [...]>
As you can see it contains keyword SYSTEM and a URI reference pointing to the location of the document.
Public Identifiers
Public identifiers provide a mechanism to locate DTD resources and are written as below −
<!DOCTYPE name PUBLIC "-//Beginning XML//DTD Address Example//EN">
As you can see, it begins with keyword PUBLIC, followed by a specialized identifier. Public identifiers are used to identify an entry in a catalog. Public identifiers can follow any format, however, a commonly used format is called Formal Public Identifiers, or FPIs.
This chapter will discuss about XML Components from DTD perspective. A DTD will basically contain declarations of the following XML components −
Element
Element
Attributes
Attributes
Entities
Entities
XML elements can be defined as building blocks of an XML document. Elements can behave as a container to hold text, elements, attributes, media objects or mix of all.
Each XML document contains one or more elements, the boundaries of which are either delimited by start-tags and end-tags, or empty elements.
Below is a simple example of XML elements
<name>
   Tutorials Point
</name>
As you can see we have defined a <name> tag. There's a text between start and end tag of <name>. Elements, when used in an XML-DTD, need to be declared which will be discussed in detail in the chapter DTD Elements.
Attributes are part of the XML elements. An element can have any number of unique attributes. Attributes give more information about the XML element or more precisely it defines a property of the element. An XML attribute is always a name-value pair.
Below is a simple example of XML attributes −
<img src = "flower.jpg"/>
Here img is the element name whereas src is an attribute name and flower.jpg is a value given for the attribute src.

If attributes are used in an XML DTD then these need to be declared which will be discussed in detail in the chapter DTD Attributes
Entities are placeholders in XML. These can be declared in the document prolog or in a DTD. Entities can be primarily categorized as −
Built-in entities
Built-in entities
Character entities
Character entities
General entities
General entities
Parameter entities
Parameter entities
There are five built-in entities that play in well-formed XML, they are −
ampersand: &amp;
ampersand: &amp;
Single quote: &apos;
Single quote: &apos;
Greater than: &gt;
Greater than: &gt;
Less than: &lt;
Less than: &lt;
Double quote: "
Double quote: "
We will study more about entity declarations in XML DTD in detail in the chapter DTD Entities
XML elements can be defined as building blocks of an XML document. Elements can behave as a container to hold text, elements, attributes, media objects or mix of all.
A DTD element is declared with an ELEMENT declaration. When an XML file is validated by DTD, parser initially checks for the root element and then the child elements are validated.
All DTD element declarations have this general form −
<!ELEMENT elementname (content)>
ELEMENT declaration is used to indicate the parser that you are about to define an element.
ELEMENT declaration is used to indicate the parser that you are about to define an element.
elementname is the element name (also called the generic identifier) that you are defining.
elementname is the element name (also called the generic identifier) that you are defining.
content defines what content (if any) can go within the element.
content defines what content (if any) can go within the element.
Content of elements declaration in a DTD can be categorized as below −
Empty content
Empty content
Element content
Element content
Mixed content
Mixed content
Any content
Any content
This is a special case of element declaration. This element declaration does not contain any content. These are declared with the keyword EMPTY.
Syntax
Following is the syntax for empty element declaration −
<!ELEMENT elementname EMPTY >
In the above syntax −
ELEMENT is the element declaration of category EMPTY
ELEMENT is the element declaration of category EMPTY
elementname is the name of empty element.
elementname is the name of empty element.
Example
Following is a simple example demonstrating empty element declaration −
<?xml version = "1.0"?>

<!DOCTYPE hr[
   <!ELEMENT address EMPTY>    
]>
<address />
In this example address is declared as an empty element. The markup for address element would appear as <address />.
In element declaration with element content, the content would be allowable elements within parentheses. We can also include more than one element.
Syntax
Following is a syntax of element declaration with element content −
<!ELEMENT elementname (child1, child2...)>
ELEMENT is the element declaration tag
ELEMENT is the element declaration tag
elementname is the name of the element.
elementname is the name of the element.
child1, child2.. are the elements and each element must have its own definition within the DTD.
child1, child2.. are the elements and each element must have its own definition within the DTD.
Example
Below example demonstrates a simple example for element declaration with element content −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>

<!DOCTYPE address [
   <!ELEMENT address (name,company,phone)>
   <!ELEMENT name (#PCDATA)>
   <!ELEMENT company (#PCDATA)>
   <!ELEMENT phone (#PCDATA)>
]>

<address>
   <name>Tanmay Patil</name>
   <company>TutorialsPoint</company>
   <phone>(011) 123-4567</phone>
</address>
In the above example, address is the parent element and name, company and phone_no are its child elements.
Below table shows the list of operators and syntax rules which can be applied in defining child elements −
<!ELEMENT address (name+)>
Child element name can occur one or more times inside the element name address.
<!ELEMENT address (name*)>
Child element name can occur zero or more times inside the element name address.
<!ELEMENT address (name?)>
Child element name can occur zero or one time inside the element name address.
<!ELEMENT address (name, company)>
Sequence of child elements name, company, which must occur in the same order inside the element name address.
<!ELEMENT address (name | company)>
It allows you to choose either of child elements i.e. name or company, which must occur in inside the element name address.
We need to follow certain rules if there is more than one element content −
Sequences  −  Often the elements within DTD documents must appear in a distinct order. If this is the case, you define the content using a sequence.
The declaration indicates that the <address> element must have exactly three children - <name>, <company>, and <phone> - and that they must appear in this order. For example −

Sequences  −  Often the elements within DTD documents must appear in a distinct order. If this is the case, you define the content using a sequence.
The declaration indicates that the <address> element must have exactly three children - <name>, <company>, and <phone> - and that they must appear in this order. For example −
<!ELEMENT address (name,company,phone)>
Choices − Suppose you need to allow one element or another, but not both. In such cases you must use the pipe (|) character. The pipe functions as an exclusive OR. For example −

Choices − Suppose you need to allow one element or another, but not both. In such cases you must use the pipe (|) character. The pipe functions as an exclusive OR. For example −
<!ELEMENT address (mobile | landline)>
This is the combination of (#PCDATA) and children elements. PCDATA stands for parsed character data, that is, text that is not markup. Within mixed content models, text can appear by itself or it can be interspersed between elements. The rules for mixed content models are similar to the element content as discussed in the previous section.
Syntax
Following is a generic syntax for mixed element content −
<!ELEMENT elementname (#PCDATA|child1|child2)*>
ELEMENT is the element declaration tag.
ELEMENT is the element declaration tag.
elementname is the name of the element.
elementname is the name of the element.
PCDATA is the text that is not markup. #PCDATA must come first in the mixed content declaration.
PCDATA is the text that is not markup. #PCDATA must come first in the mixed content declaration.
child1, child2.. are the elements and each element must have its own definition within the DTD.
child1, child2.. are the elements and each element must have its own definition within the DTD.
The operator (*) must follow the mixed content declaration if children elements are included
The operator (*) must follow the mixed content declaration if children elements are included
The (#PCDATA) and children element declarations must be separated by the (|) operator.
The (#PCDATA) and children element declarations must be separated by the (|) operator.
Example
Following is a simple example demonstrating the mixed content element declaration in a DTD.
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>

<!DOCTYPE address [
   <!ELEMENT address (#PCDATA|name)*>
   <!ELEMENT name (#PCDATA)>
]>

<address>
   Here's a bit of text mixed up with the child element.
   <name>
      Tanmay Patil
   </name>
</address>
You can declare an element using the ANY keyword in the content. It is most often referred to as mixed category element. ANY is useful when you have yet to decide the allowable contents of the element.
Syntax
Following is the syntax for declaring elements with ANY content −
<!ELEMENT elementname ANY>
Here, the ANY keyword indicates that text (PCDATA) and/or any elements declared within the DTD can be used within the content of the <elementname> element. They can be used in any order any number of times. However, the ANY keyword does not allow you to include elements that are not declared within the DTD.
Example
Following is a simple example demonstrating the element declaration with ANY content −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes" ?>

<!DOCTYPE address [
   <!ELEMENT address ANY>
]>

<address>
   Here's a bit of sample text
</address>
In this chapter we will discuss about DTD Attributes. Attribute gives more information about an element or more precisely it defines a property of an element. An XML attribute is always in the form of a name-value pair. An element can have any number of unique attributes.
Attribute declaration is very much similar to element declarations in many ways except one; instead of declaring allowable content for elements, you declare a list of allowable attributes for each element. These lists are called ATTLIST declaration.
Basic syntax of DTD attributes declaration is as follows −
<!ATTLIST element-name attribute-name attribute-type attribute-value>
In the above syntax −
The DTD attributes start with <!ATTLIST keyword if the element contains the attribute.
The DTD attributes start with <!ATTLIST keyword if the element contains the attribute.
element-name specifies the name of the element to which the attribute applies.
element-name specifies the name of the element to which the attribute applies.
attribute-name specifies the name of the attribute which is included with the element-name.
attribute-name specifies the name of the attribute which is included with the element-name.
attribute-type defines the type of attributes. We will discuss more on this in the following sections.
attribute-type defines the type of attributes. We will discuss more on this in the following sections.
attribute-value takes a fixed value that the attributes must define. We will discuss more on this in the following sections.
attribute-value takes a fixed value that the attributes must define. We will discuss more on this in the following sections.
Below is a simple example for attribute declaration in DTD −
<?xml version = "1.0"?>

<!DOCTYPE address [
   <!ELEMENT address ( name )>
   <!ELEMENT name ( #PCDATA )>
   <!ATTLIST name id CDATA #REQUIRED>
]>

<address>
   <name id = "123">Tanmay Patil</name>
</address>
Let us go through the above code −
Begin with the XML declaration with the following statement −
Begin with the XML declaration with the following statement −
<?xml version = "1.0"?>
Immediately following the XML header is the document type declaration, commonly referred to as the DOCTYPE as shown below −
The DOCTYPE informs the parser that a DTD is associated with this XML document. The DOCTYPE declaration has an exclamation mark (!) at the start of the element name.
Immediately following the XML header is the document type declaration, commonly referred to as the DOCTYPE as shown below −
The DOCTYPE informs the parser that a DTD is associated with this XML document. The DOCTYPE declaration has an exclamation mark (!) at the start of the element name.
<!DOCTYPE address [
Following is the body of DTD. Here we have declared element and attribute −
Following is the body of DTD. Here we have declared element and attribute −
<!ELEMENT address ( name )>
<!ELEMENT name ( #PCDATA )>
Attribute id for the element name is defined as given below −
Here attribute type is CDATA and its value is #REQUIRED.
Attribute id for the element name is defined as given below −
Here attribute type is CDATA and its value is #REQUIRED.
<!ATTLIST name id CDATA #REQUIRED>
All attributes used in an XML document must be declared in the Document Type Definition (DTD) using an Attribute-List Declaration
All attributes used in an XML document must be declared in the Document Type Definition (DTD) using an Attribute-List Declaration
Attributes may only appear in start or empty tags.
Attributes may only appear in start or empty tags.
The keyword ATTLIST must be in upper case
The keyword ATTLIST must be in upper case
No duplicate attribute names will be allowed within the attribute list for a given element.
No duplicate attribute names will be allowed within the attribute list for a given element.
When declaring attributes, you can specify how the processor should handle the data that appears in the value. We can categorize attribute types in three main categories −
String type
String type
Tokenized types
Tokenized types
Enumerated types
Enumerated types
Following table provides a summary of the different attribute types −
CDATA
CDATA is character data (text and not markup). It is a String Attribute Type. 
ID
It is a unique identifier of the attribute. It should not appear more than once. It is a Tokenized Attribute Type.
IDREF
It is used to reference an ID of another element. It is used to establish connections between elements. It is a Tokenized Attribute Type.
IDREFS
It is used to reference multiple ID's. It is a Tokenized Attribute Type.
ENTITY
It represents an external entity in the document. It is a Tokenized Attribute Type.
ENTITIES
It represents a list of external entities in the document. It is a Tokenized Attribute Type.
NMTOKEN
It is similar to CDATA and the attribute value consists of a valid XML name. It is a Tokenized Attribute Type.
NMTOKENS
It is similar to CDATA and the attribute value consists a list of valid XML name. It is a Tokenized Attribute Type.
NOTATION
An element will be referenced to a notation declared in the DTD document. It is an Enumerated Attribute Type.
Enumeration
It allows defining a specific list of values where one of the values must match. It is an Enumerated Attribute Type.
Within each attribute declaration, you must specify how the value will appear in the document. You can specify if an attribute −
can have a default value
can have a default value
can have a fixed value
can have a fixed value
is required
is required
is implied
is implied
It contains the default value. The values can be enclosed in single quotes(') or double quotes(").
Syntax
Following is the syntax of value −
<!ATTLIST element-name attribute-name attribute-type "default-value">
where default-value is the attribute value defined.
Example
Following is a simple example of attribute declaration with default value −
<?xml version = "1.0"?>

<!DOCTYPE address [
   <!ELEMENT address ( name )>
   <!ELEMENT name ( #PCDATA )>
   <!ATTLIST name id CDATA "0">
]>

<address>
   <name id = "123">
      Tanmay Patil
   </name>
</address>
In this example we have name element with attribute id whose default value is 0. The default value is been enclosed within the double quotes.
#FIXED keyword followed by the fixed value is used when you want to specify that the attribute value is constant and cannot be changed. A common use of fixed attributes is specifying version numbers.
Syntax
Following is the syntax of fixed values −
<!ATTLIST element-name attribute-name attribute-type #FIXED "value" >
where #FIXED is an attribute value defined.
Example
Following is a simple example of attribute declaration with FIXED value −
<?xml version = "1.0"?>

<!DOCTYPE address [
  <!ELEMENT address (company)*>
  <!ELEMENT company (#PCDATA)>
  <!ATTLIST company name NMTOKEN #FIXED "tutorialspoint">
]>

<address>
  <company name = "tutorialspoint">we are a free online teaching faculty</company>
</address>
In this example we have used the keyword #FIXED where it indicates that the value "tutorialspoint" is the only value for the attribute name of element <company>. If we try to change the attribute value then it gives an error.
Following is an invalid DTD −
<?xml version = "1.0"?>

<!DOCTYPE address [
  <!ELEMENT address (company)*>
  <!ELEMENT company (#PCDATA)>
  <!ATTLIST company name NMTOKEN #FIXED "tutorialspoint">
]>

<address>
  <company name = "abc">we are a free online teaching faculty</company>
</address>
Whenever you want specify that an attribute is required, use #REQUIRED keyword.
Syntax
Following is the syntax of #REQUIRED −
<!ATTLIST element-name attribute-name attribute-type #REQUIRED>
where #REQUIRED is an attribute type defined.
Example
Following is a simple example of DTD attribute declaration with #REQUIRED keyword −
<?xml version = "1.0"?>

<!DOCTYPE address [
   <!ELEMENT address ( name )>
   <!ELEMENT name ( #PCDATA )>
   <!ATTLIST name id CDATA #REQUIRED>
]>

<address>
   <name id = "123">
      Tanmay Patil
   </name>
</address>
In this example we have used #REQUIRED keyword to specify that the attribute id must be provided for the element-name name
When declaring attributes you must always specify a value declaration. If the attribute you are declaring has no default value, has no fixed value, and is not required, then you must declare that the attribute as implied. Keyword #IMPLIED is used to specify an attribute as implied.
Syntax
Following is the syntax of #IMPLIED −
<!ATTLIST element-name attribute-name attribute-type #IMPLIED>
where #IMPLIED is an attribute type defined.
Example
Following is a simple example of #IMPLIED
<?xml version = "1.0"?>

<!DOCTYPE address [
   <!ELEMENT address ( name )>
   <!ELEMENT name ( #PCDATA )>
   <!ATTLIST name id CDATA #IMPLIED>
]>

<address>
   <name />
</address>
In this example we have used the keyword #IMPLIED as we do not want to specify any attributes to be included in element name. It is optional.
Entities are used to define shortcuts to special characters within the XML documents. Entities can be primarily of four types −
Built-in entities
Built-in entities
Character entities
Character entities
General entities
General entities
Parameter entities
Parameter entities
In general, entities can be declared internally or externally. Let us understand each of these and their syntax as follows −
If an entity is declared within a DTD it is called as internal entity.
Syntax
Following is the syntax for internal entity declaration −
<!ENTITY entity_name "entity_value">
In the above syntax −
entity_name is the name of entity followed by its value within the double quotes or single quote.
entity_name is the name of entity followed by its value within the double quotes or single quote.
entity_value holds the value for the entity name.
entity_value holds the value for the entity name.
The entity value of the Internal Entity is de-referenced by adding prefix  &  to the entity name i.e. &entity_name.
The entity value of the Internal Entity is de-referenced by adding prefix  &  to the entity name i.e. &entity_name.
Example
Following is a simple example for internal entity declaration −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes"?>

<!DOCTYPE address [
   <!ELEMENT address (#PCDATA)>
   <!ENTITY name "Tanmay patil">
   <!ENTITY company "TutorialsPoint">
   <!ENTITY phone_no "(011) 123-4567">
]>

<address>
   &name;
   &company;
   &phone_no;
</address>
In the above example, the respective entity names name, company and phone_no  are replaced by their values in the XML document. The entity values are de-referenced by adding prefix  &  to the entity name.
Save this file as sample.xml and open it in any browser, you will notice that the entity values for name, company, phone_no are replaced respectively.
If an entity is declared outside a DTD it is called as external entity. You can refer to an external Entity by either using system identifiers or public identifiers.
Syntax
Following is the syntax for External Entity declaration −
<!ENTITY name SYSTEM "URI/URL">
In the above syntax −
name is the name of entity.
name is the name of entity.
SYSTEM is the keyword.
SYSTEM is the keyword.
URI/URL is the address of the external source enclosed within the double or single quotes.
URI/URL is the address of the external source enclosed within the double or single quotes.
Types
You can refer to an external DTD by either using −
System Identifiers  − A system identifier enables you to specify the location of an external file containing DTD declarations.
As you can see it contains keyword SYSTEM and a URI reference pointing to the document's location. Syntax is as follows −

System Identifiers  − A system identifier enables you to specify the location of an external file containing DTD declarations.
As you can see it contains keyword SYSTEM and a URI reference pointing to the document's location. Syntax is as follows −
<!DOCTYPE name SYSTEM "address.dtd" [...]>
Public Identifiers  − Public identifiers provide a mechanism to locate DTD resources and are written as below −
As you can see, it begins with keyword PUBLIC, followed by a specialized identifier. Public identifiers are used to identify an entry in a catalog. Public identifiers can follow any format; however, a commonly used format is called Formal Public Identifiers, or FPIs.

Public Identifiers  − Public identifiers provide a mechanism to locate DTD resources and are written as below −
As you can see, it begins with keyword PUBLIC, followed by a specialized identifier. Public identifiers are used to identify an entry in a catalog. Public identifiers can follow any format; however, a commonly used format is called Formal Public Identifiers, or FPIs.
<!DOCTYPE name PUBLIC "-//Beginning XML//DTD Address Example//EN">
Example
Let us understand the external entity with the following example −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes"?>
<!DOCTYPE address SYSTEM "address.dtd">

<address>
   <name>
      Tanmay Patil
   </name>
   
   <company>
      TutorialsPoint
   </company>
   
   <phone>
      (011) 123-4567
   </phone>
</address>
Below is the content of the DTD file address.dtd −
<!ELEMENT address (name, company, phone)>
<!ELEMENT name (#PCDATA)>
<!ELEMENT company (#PCDATA)>
<!ELEMENT phone (#PCDATA)>
All XML parsers must support built-in entities. In general, you can use these entity references anywhere. You can also use normal text within the XML document, such as in element contents and attribute values.
There are five built-in entities that play their role in well-formed XML, they are −
ampersand: &amp;
ampersand: &amp;
Single quote: &apos;
Single quote: &apos;
Greater than: &gt;
Greater than: &gt;
Less than: &lt;
Less than: &lt;
Double quote: "
Double quote: "
Following example demonstrates the built-in entity declaration −
<?xml version = "1.0"?>

<note>
   <description>I'm a technical writer & programmer</description>
<note>
As you can see here the &amp; character is replaced by & whenever the processor encounters this.
Character Entities are used to name some of the entities which are symbolic representation of information i.e characters that are difficult or impossible to type can be substituted by Character Entities.
Following example demonstrates the character entity declaration −
<?xml version = "1.0" encoding = "UTF-8" standalone = "yes"?>
<!DOCTYPE author[
   <!ELEMENT author (#PCDATA)>
   <!ENTITY writer "Tanmay patil">
   <!ENTITY copyright "&#169;">
]>
<author>&writer;&copyright;</author>
You will notice here we have used &#169; as value for copyright character. Save this file as sample.xml and open it in your browser and you will see that copyright is replaced by the character ©.
General entities must be declared within the DTD before they can be used within an XML document. Instead of representing only a single character, general entities can represent characters, paragraphs, and even entire documents.
To declare a general entity, use a declaration of this general form in your DTD −
<!ENTITY ename "text">
Following example demonstrates the general entity declaration −
<?xml version = "1.0"?>

<!DOCTYPE note [
   <!ENTITY source-text "tutorialspoint">
]>

<note>
   &source-text;
</note>
Whenever an XML parser encounters a reference to source-text entity, it will supply the replacement text to the application at the point of the reference.
The purpose of a parameter entity is to enable you to create reusable sections of replacement text.
Following is the syntax for parameter entity declaration −
<!ENTITY % ename "entity_value">
entity_value is any character that is not an '&', '%' or ' " '.
entity_value is any character that is not an '&', '%' or ' " '.
Following example demonstrates the parameter entity declaration. Suppose you have element declarations as below −
<!ELEMENT residence (name, street, pincode, city, phone)>
<!ELEMENT apartment (name, street, pincode, city, phone)>
<!ELEMENT office (name, street, pincode, city, phone)>
<!ELEMENT shop (name, street, pincode, city, phone)>
Now suppose you want to add additional eleement country, then  then you need to add it to all four declarations. Hence we can go for a parameter entity reference. Now using parameter entity reference the above example will be  −
<!ENTITY % area "name, street, pincode, city">
<!ENTITY % contact "phone">
Parameter entities are dereferenced in the same way as a general entity reference, only with a percent sign instead of an ampersand −
<!ELEMENT residence (%area;, %contact;)>
<!ELEMENT apartment (%area;, %contact;)>
<!ELEMENT office (%area;, %contact;)>
<!ELEMENT shop (%area;, %contact;)>
When the parser reads these declarations, it substitutes the entity's replacement text for the entity reference.
We use DTD to describe precisely the XML document. DTDs check the validity of structure and vocabulary of an XML document against the grammatical rules of the appropriate XML language. Now to check the validity of DTD, following procedures can be used −
Using XML DTD validation tools −  You can use some IDEs such as XML Spy (not free) and XMLStarlet(opensource) can be used to validate XML files against DTD document.
Using XML DTD validation tools −  You can use some IDEs such as XML Spy (not free) and XMLStarlet(opensource) can be used to validate XML files against DTD document.
Using XML DTD on-line validators −  W3C Markup Validation Service is designed to validate Web documents. Use the online validator to check the validaty of your XML DTD here.
Using XML DTD on-line validators −  W3C Markup Validation Service is designed to validate Web documents. Use the online validator to check the validaty of your XML DTD here.
Write your own XML validators with XML DTD validation API  −  Newer versions of JDK (above 1.4) support  XML DTD validation API. You can write your own validator code to check the validity of XML DTD validation.
Write your own XML validators with XML DTD validation API  −  Newer versions of JDK (above 1.4) support  XML DTD validation API. You can write your own validator code to check the validity of XML DTD validation.
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