A Document Type Definition (DTD) is a set of rules that defines the structure, elements, attributes, and data types of an SGML or XML document. It ensures that a document follows a consistent structure by specifying what elements are allowed, their relationships, and the type of content they can contain.
Syntax and Structure
A DTD consists of declarations that define elements, attributes, and entities.
Defining Elements
Elements are the building blocks of a document.
<!ELEMENT book (title, author, chapter+)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
<!ELEMENT chapter (#PCDATA)> <!ELEMENT>defines an element.#PCDATA(Parsed Character Data) means the element contains text.chapter+means one or more<chapter>elements are required.
Defining Attributes
Attributes provide additional information about elements.
<!ELEMENT book (title, author)>
<!ATTLIST book id ID #REQUIRED>- The
idattribute must be unique (IDtype) and is required (#REQUIRED).
Internal vs. External DTDs
Internal DTD (Defined inside the XML document):
<?xml version="1.0"?>
<!DOCTYPE book [
<!ELEMENT book (title, author)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>
]>
<book>
<title>XML Basics</title>
<author>John Doe</author>
</book>External DTD (Referenced from an external file)
<!DOCTYPE book SYSTEM "book.dtd">
<book>
<title>XML Basics</title>
<author>John Doe</author>
</book>The DTD file (book.dtd) contains:
<!ELEMENT book (title, author)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT author (#PCDATA)>This keeps the document clean and allows multiple files to share the same DTD.
Use Cases
- Early Web Development: Used in early HTML (SGML-based) document validation.
- XML Document Validation: Ensures well-structured XML documents.
- Data Exchange: Standardizes data structure for databases and APIs.