XML DTD

A well-formed XML document follows basic syntax rules. But what if a project requires more control — ensuring that specific elements always appear, or that certain attributes are mandatory? This is where a Document Type Definition (DTD) comes in. A DTD defines the structure and legal building blocks of an XML document, acting as a blueprint that the document must follow.

When an XML document conforms to its DTD, it is called a valid XML document (as opposed to merely well-formed).

What Does a DTD Define?

A DTD specifies:

Which elements are allowed and what they can contain.
What attributes each element can have and whether they are required or optional.
The order and number of child elements allowed within a parent.
Any special entities (shortcuts for commonly used text or characters).

Types of DTD

A DTD can be placed in two locations:

1. Internal DTD

The DTD rules are written directly inside the XML document, between the XML declaration and the root element.

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note [
  <!ELEMENT note (to, from, subject, body)>
  <!ELEMENT to (#PCDATA)>
  <!ELEMENT from (#PCDATA)>
  <!ELEMENT subject (#PCDATA)>
  <!ELEMENT body (#PCDATA)>
]>
<note>
  <to>Sarah</to>
  <from>James</from>
  <subject>Meeting Tomorrow</subject>
  <body>Please confirm your attendance.</body>
</note>

2. External DTD

The DTD rules are stored in a separate file with a .dtd extension and referenced from the XML document. This is the preferred approach when multiple XML files share the same structure.

File: note.dtd

<!ELEMENT note (to, from, subject, body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT subject (#PCDATA)>
<!ELEMENT body (#PCDATA)>

XML File referencing the external DTD:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
  <to>Sarah</to>
  <from>James</from>
  <subject>Meeting Tomorrow</subject>
  <body>Please confirm your attendance.</body>
</note>

Declaring Elements in a DTD

Elements are declared using the <!ELEMENT> declaration. The general syntax is:

<!ELEMENT elementname (content)>

Content Types

Content Type	DTD Syntax	Meaning
Text only	`(#PCDATA)`	Element contains plain text
Specific child elements	`(child1, child2)`	Must appear in that exact order
Empty element	`EMPTY`	Element has no content
Any content	`ANY`	Element can have any content
Choice	`(a \| b)`	Either a or b

Occurrence Indicators

Symbols placed after a child element name control how many times it can appear:

Symbol	Meaning
(none)	Exactly once
`?`	Zero or one time (optional)
`*`	Zero or more times
`+`	One or more times

Element Declaration Examples

<!-- Library must contain one or more book elements -->
<!ELEMENT library (book+)>

<!-- A book must have title and author, optionally a summary -->
<!ELEMENT book (title, author, summary?)>

<!-- title, author, and summary are text-only -->
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT summary (#PCDATA)>

Declaring Attributes in a DTD

Attributes for an element are declared using <!ATTLIST>. The syntax is:

<!ATTLIST elementname attributename type default>

Common Attribute Types

Type	Description
`CDATA`	Any text string
`ID`	A unique identifier within the document
`IDREF`	A reference to an ID of another element
`(a\|b\|c)`	Enumerated list — value must be one from the list

Default Value Keywords

Keyword	Meaning
`#REQUIRED`	Attribute must always be provided
`#IMPLIED`	Attribute is optional with no default
`#FIXED "value"`	Attribute always has this fixed value
`"defaultvalue"`	Attribute defaults to this value if not specified

Attribute Declaration Examples

<!-- id is required, status has a default of "active", type is optional -->
<!ATTLIST employee
  id       ID       #REQUIRED
  status   CDATA    "active"
  type     (full-time | part-time | contract) #IMPLIED
>

Declaring Entities in a DTD

Entities in a DTD are like shortcuts — they define a name that stands for a piece of text. When the entity name is used in the XML document, the parser replaces it with the defined text.

<!ENTITY companyName "Bright Solutions Ltd.">

Using the entity in the XML document:

<footer>Copyright 2024 &companyName;</footer>

The parser replaces &companyName; with Bright Solutions Ltd. when processing the document.

Complete DTD and XML Example

File: library.dtd

<!ELEMENT library (book+)>
<!ELEMENT book (title, author, year, genre?)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT year (#PCDATA)>
<!ELEMENT genre (#PCDATA)>
<!ATTLIST book id ID #REQUIRED>

XML File: library.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE library SYSTEM "library.dtd">
<library>
  <book id="B001">
    <title>The Midnight Garden</title>
    <author>Laura Chen</author>
    <year>2019</year>
    <genre>Fiction</genre>
  </book>
  <book id="B002">
    <title>Data Structures Simplified</title>
    <author>Mark Osei</author>
    <year>2021</year>
  </book>
</library>

Limitations of DTD

DTD does not support data types — it cannot specify that a value must be a number or a date.
DTD uses its own syntax, not XML syntax.
DTD has limited support for namespaces.
DTD cannot define complex constraints on element values.

These limitations led to the development of XML Schema (XSD), which is a more powerful and flexible validation system covered in the next topic.

Key Points

A DTD defines the allowed structure of an XML document.
A document that follows its DTD is called a valid XML document.
DTDs can be internal (written inside the XML file) or external (stored in a separate .dtd file).
Elements are declared with <!ELEMENT> and attributes with <!ATTLIST>.
Occurrence indicators (?, *, +) control how many times a child element appears.
Entities (<!ENTITY>) define reusable text shortcuts.
DTD has limitations in data typing and namespace support, which XML Schema addresses.

Previous lessons

Back to courses

Next lessons