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🧠 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:
🧾 1. Why Validate XML?
Reason |
Benefit |
Enforce data rules |
Ensure correct data
types/format |
Interoperability |
Make XML
portable across systems |
Catch errors early |
Prevent malformed data |
Improve documentation |
Define
expected structure clearly |
📑 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 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
Declaration Type |
Syntax Example |
Element |
<!ELEMENT title
(#PCDATA)> |
Attribute List |
<!ATTLIST
book genre CDATA #IMPLIED> |
Sequence |
<!ELEMENT book
(title, author)> |
Choice |
`<!ELEMENT
book (title |
Optional Element |
<!ELEMENT book
(title?, author)> |
Repeating Element |
<!ELEMENT
book (title+)> |
🧬 3. XSD – XML Schema
Definition
🔹 What is XSD?
XSD is more powerful and flexible than DTD. It
defines:
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
Feature |
DTD |
XSD |
Syntax |
Not XML |
Written in XML |
Data types |
Limited |
Rich (string,
date, decimal) |
Namespaces |
Not supported |
Supported |
Custom types |
No |
Yes (simple
and complex) |
Usage today |
Legacy and simple use
cases |
Modern and
enterprise-grade |
🔧 4. Data Types in XSD
XSD Type |
Description |
xs:string |
Any string of characters |
xs:integer |
Whole numbers |
xs:decimal |
Decimal numbers |
xs:date |
Date (YYYY-MM-DD) |
xs:boolean |
true or false |
✅ 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
XSD Notation |
Meaning |
minOccurs="0" |
Optional |
maxOccurs="1" |
Appears only
once |
maxOccurs="unbounded" |
Can repeat many times |
xml
<xs:element
name="tag" type="xs:string" minOccurs="0"
maxOccurs="unbounded"/>
📐 7. XSD Validation Tools
Tool Name |
Description |
XML Validator |
Online web tool to check
structure |
Visual Studio |
IDE with
schema-aware XML editing |
Oxygen XML Editor |
Professional tool for
validation |
XMLSpy |
Enterprise
XML modeling tool |
📁 8. Real-World Use Cases
Industry |
XML Schema
Application |
Healthcare |
HL7 Clinical Documents |
Banking |
ISO 20022
Payment Messages |
Publishing |
EPUB files (chapters,
metadata) |
Government |
UBL
(Invoices, Purchase Orders) |
Android |
Layout and Manifest
validation |
🧪 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
Feature |
DTD |
XSD |
Syntax |
Non-XML |
XML-based |
Validation Type |
Structural |
Structural +
Data types |
Readability |
Shorter, less strict |
Verbose but flexible |
Used In |
Legacy
projects, simple XML |
Modern apps,
industry standards |
A: XML stands for eXtensible Markup Language.
A: Yes, <Tag> and <tag> are treated as different elements.
A: Absolutely. That's why it's called "extensible."
A: XML stores and structures data, while HTML displays it.
A: Its structured format and readability make it ideal for settings/configs.
A: Yes. Many enterprise and legacy APIs use SOAP, which is XML-based.
A: Not at all. While JSON is preferred for web APIs, XML is widely used in enterprise, publishing, and government systems.
A: You can validate it using DTD or XSD files or an XML validator tool.
A: Not directly. It needs to be base64 encoded first.
A: Notepad++, VS Code, XMLSpy, Eclipse, and Oxygen XML Editor are popular.
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