Mastering XML: A Complete Guide to Structuring, Storing, and Exchanging Data

📘 Chapter 3: Validation Using DTD and XSD

🧠 What is XML Validation?

XML validation ensures that your XML document conforms to a specific structure and set of rules. This helps catch errors early and maintain data consistency, especially in applications that exchange information between systems.

Two main types of validation are:

• DTD (Document Type Definition)
• XSD (XML Schema Definition)

🧾 1. Why Validate XML?

📑 2. DTD – Document Type Definition

🔹 What is DTD?

DTD defines the structure and legal elements/attributes of an XML document. It can be either:

• Internal DTD (embedded in the XML file)
• External DTD (referenced via URL or path)

✅ Internal DTD Example

xml

<?xml version="1.0"?>

<!DOCTYPE book [

  <!ELEMENT book (title, author, price)>

  <!ELEMENT title (#PCDATA)>

  <!ELEMENT author (#PCDATA)>

  <!ELEMENT price (#PCDATA)>

]>

<book>

  <title>XML Basics</title>

  <author>Jane Doe</author>

  <price>29.99</price>

</book>

✅ External DTD Example

book.dtd:

dtd

<!ELEMENT book (title, author, price)>

<!ELEMENT title (#PCDATA)>

<!ELEMENT author (#PCDATA)>

<!ELEMENT price (#PCDATA)>

book.xml:

xml

<?xml version="1.0"?>

<!DOCTYPE book SYSTEM "book.dtd">

<book>

  <title>XML Basics</title>

  <author>Jane Doe</author>

  <price>29.99</price>

</book>

📋 DTD Declarations

🧬 3. XSD – XML Schema Definition

🔹 What is XSD?

XSD is more powerful and flexible than DTD. It defines:

• Element types
• Data types (string, integer, date, etc.)
• Attribute types
• Sequence, choice, and constraints
• Custom complex types

XSD is written in XML syntax, making it extensible and tool-friendly.

✅ Basic XSD Example

books.xsd:

xml

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">

  <xs:element name="book">

    <xs:complexType>

      <xs:sequence>

        <xs:element name="title" type="xs:string"/>

        <xs:element name="author" type="xs:string"/>

        <xs:element name="price" type="xs:decimal"/>

      </xs:sequence>

    </xs:complexType>

  </xs:element>

</xs:schema>

book.xml:

xml

<?xml version="1.0"?>

<book xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

      xsi:noNamespaceSchemaLocation="books.xsd">

  <title>XML Mastery</title>

  <author>Alan Turing</author>

  <price>49.99</price>

</book>

📋 XSD vs DTD

🔧 4. Data Types in XSD

✅ Restricting Values

xml

<xs:element name="status">

  <xs:simpleType>

    <xs:restriction base="xs:string">

      <xs:enumeration value="active"/>

      <xs:enumeration value="inactive"/>

    </xs:restriction>

  </xs:simpleType>

</xs:element>

✅ Pattern Matching (RegEx)

xml

<xs:element name="zipCode">

  <xs:simpleType>

    <xs:restriction base="xs:string">

      <xs:pattern value="\d{5}"/>

    </xs:restriction>

  </xs:simpleType>

</xs:element>

🧱 5. Complex Types

✅ Complex Type with Attributes

xml

<xs:element name="product">

  <xs:complexType>

    <xs:sequence>

      <xs:element name="name" type="xs:string"/>

      <xs:element name="price" type="xs:decimal"/>

    </xs:sequence>

    <xs:attribute name="id" type="xs:integer" use="required"/>

  </xs:complexType>

</xs:element>

📎 6. Optional and Repeating Elements

xml

<xs:element name="tag" type="xs:string" minOccurs="0" maxOccurs="unbounded"/>

📐 7. XSD Validation Tools

📁 8. Real-World Use Cases

🧪 9. Practice Example

✅ XML

xml

<employee id="1001">

  <name>John Doe</name>

  <position>Manager</position>

  <salary>5000</salary>

</employee>

✅ XSD

xml

<xs:element name="employee">

  <xs:complexType>

    <xs:sequence>

      <xs:element name="name" type="xs:string"/>

      <xs:element name="position" type="xs:string"/>

      <xs:element name="salary" type="xs:decimal"/>

    </xs:sequence>

    <xs:attribute name="id" type="xs:integer" use="required"/>

  </xs:complexType>

</xs:element>

📚 Summary Table