DTD Building Blocks

🧱 DTD Building Blocks

DTD consists of fundamental building blocks: elements, attributes, entities, PCDATA, and CDATA. These components work together to define complete XML document structure and validation rules.


<!-- Main building blocks -->
<!ELEMENT person (name,age)>
<!ATTLIST person id CDATA #REQUIRED>
<!ENTITY company "Tech Corp">

Main Building Blocks

📦

Elements

Define the structure and content containers in XML documents

<!ELEMENT book (title,author)>

🏷️

Attributes

Add properties and metadata to elements

<!ATTLIST book isbn CDATA #REQUIRED>

🔤

Entities

Store reusable text and special characters

<!ENTITY author "John Doe">

📝

PCDATA

Parsed character data for text content

<!ELEMENT title (#PCDATA)>

🔹 Elements

Elements are the primary building blocks that define the structure of your XML document. They specify what content is allowed and in what order.

<!-- Empty element -->
<!ELEMENT br EMPTY>

<!-- Element with text content -->
<!ELEMENT name (#PCDATA)>

<!-- Element with child elements -->
<!ELEMENT person (firstname,lastname,age)>

<!-- Element with mixed content -->
<!ELEMENT description (#PCDATA|bold|italic)*>

Example Usage:

<person>
  <firstname>John</firstname>
  <lastname>Doe</lastname>
  <age>30</age>
</person>

🔹 Attributes

Attributes provide additional information about elements. They are declared using ATTLIST and can have different types and default values for flexibility.

<!-- Basic attribute -->
<!ATTLIST person id CDATA #REQUIRED>

<!-- Multiple attributes -->
<!ATTLIST book
  isbn CDATA #REQUIRED
  edition CDATA #IMPLIED
  language CDATA "English">

<!-- Enumerated attribute -->
<!ATTLIST payment method (cash|card|online) "card">

Example Usage:

<person id="P001">John Doe</person>
<book isbn="123-456" edition="2nd" language="English">
<payment method="online">

🔹 Entities

Entities are shortcuts for frequently used text or special characters. They help maintain consistency and reduce repetition throughout your XML documents.

<!-- Text entity -->
<!ENTITY company "TechCorp Industries">
<!ENTITY copyright "Copyright © 2025">

<!-- Special character entities -->
<!ENTITY nbsp "&#160;">
<!ENTITY copy "&#169;">

Example Usage:

<footer>
  &copyright; &company;
</footer>

<!-- Output: Copyright © 2025 TechCorp Industries -->

🔹 PCDATA vs CDATA

Understanding the difference between PCDATA and CDATA is crucial for proper XML document structure and content handling in your applications.

PCDATA (Parsed Character Data):

Text that will be parsed by the XML parser
Special characters must be escaped (< > &)
Can contain entity references

<!ELEMENT message (#PCDATA)>
<message>Price: &lt; $100</message>

CDATA (Character Data):

Text that will NOT be parsed
Special characters don't need escaping
Used for code snippets or special content

<script><![CDATA[
  if (x < 10 && y > 5) {
    alert("Valid");
  }
]]></script>

🔹 Complete Example

Here's a comprehensive example showing all building blocks working together in a real-world scenario for a product catalog system.

<?xml version="1.0"?>
<!DOCTYPE catalog [
  <!ELEMENT catalog (product+)>
  <!ELEMENT product (name,description,price)>
  <!ELEMENT name (#PCDATA)>
  <!ELEMENT description (#PCDATA)>
  <!ELEMENT price (#PCDATA)>
  
  <!ATTLIST product
    id CDATA #REQUIRED
    category (electronics|books|clothing) #REQUIRED
    available (yes|no) "yes">
  
  <!ENTITY currency "USD">
  <!ENTITY store "TechMart">
]>

<catalog>
  <product id="P001" category="electronics" available="yes">
    <name>Laptop</name>
    <description>High-performance laptop from &store;</description>
    <price>999 &currency;</price>
  </product>
  <product id="P002" category="books">
    <name>XML Guide</name>
    <description>Complete XML tutorial</description>
    <price>29 &currency;</price>
  </product>
</catalog>