← Back to Curriculum

XML Syntax and Structure

📚 Lesson 2 of 15 ⏱️ 30 min

XML Syntax and Structure

30 min

XML elements must have opening and closing tags, except for self-closing tags (empty elements). Every opening tag like `<element>` must have a corresponding closing tag `</element>`. Empty elements (elements with no content) can use self-closing syntax: `<element />`. This strict tag pairing ensures XML parsers can correctly identify element boundaries. Understanding proper tag usage is fundamental to creating well-formed XML. Malformed XML (unclosed tags) will be rejected by parsers.

XML attributes provide additional information about elements, appearing within the opening tag. Attributes are name-value pairs like `id="123"` or `type="text"`. Attributes are useful for metadata (IDs, types, flags) while element content holds actual data. Each attribute name must be unique within an element. Attribute values must be quoted (single or double quotes). Understanding attributes enables you to add metadata to elements effectively.

XML is case-sensitive and requires proper nesting of elements, meaning `<Book>` and `<book>` are different elements, and inner elements must be completely contained within outer elements. Case sensitivity means element names must match exactly between opening and closing tags. Proper nesting means you cannot close an outer element before closing all inner elements. Understanding case sensitivity and nesting prevents common XML errors. These rules ensure XML can be parsed unambiguously.

XML comments use `<!-- comment -->` syntax and are ignored by parsers. Comments are useful for documentation and temporarily disabling code. Comments cannot contain `--` and cannot be nested. Understanding comments helps you document XML effectively. Comments are preserved in some contexts but ignored during parsing.

XML processing instructions use `<?target data?>` syntax to provide instructions to applications. The XML declaration `<?xml version="1.0"?>` is a special processing instruction. Processing instructions are application-specific and are passed through by parsers. Understanding processing instructions enables you to provide metadata to applications. Processing instructions are less common in modern XML usage.

XML character data (CDATA) sections use `<![CDATA[...]]>` to include text that might otherwise be interpreted as markup. CDATA sections are useful for including code, XML examples, or text with many special characters. Everything inside CDATA is treated as literal text. Understanding CDATA enables you to include special content without escaping. Best practices include using meaningful element and attribute names, keeping structure consistent, validating against schemas, and understanding that XML syntax is strict but enables reliable parsing.

Key Concepts

  • XML elements must have opening and closing tags (or be self-closing).
  • XML attributes provide metadata about elements.
  • XML is case-sensitive and requires proper nesting.
  • XML comments use <!-- --> syntax.
  • CDATA sections include literal text without escaping.

Learning Objectives

Master

  • Creating properly structured XML elements and attributes
  • Understanding XML case sensitivity and nesting rules
  • Using comments and CDATA sections
  • Following XML syntax rules correctly

Develop

  • Understanding markup language syntax
  • Designing consistent XML structures
  • Appreciating XML's strict syntax rules

Tips

  • Always match opening and closing tags exactly (case-sensitive).
  • Use attributes for metadata, elements for content.
  • Ensure proper nesting—close inner elements before outer elements.
  • Use CDATA sections for content that might be interpreted as markup.

Common Pitfalls

  • Mismatched case in opening and closing tags, causing parse errors.
  • Improper nesting, closing outer elements before inner elements.
  • Forgetting to quote attribute values, causing syntax errors.
  • Using XML for simple data that JSON would handle better.

Summary

  • XML elements must have matching opening and closing tags.
  • XML attributes provide metadata about elements.
  • XML is case-sensitive and requires proper nesting.
  • Understanding XML syntax enables creating well-formed documents.
  • XML's strict syntax ensures reliable parsing.

Exercise

Create an XML document with various elements and attributes.

<?xml version="1.0" encoding="UTF-8"?>
<employees>
  <employee id="E001" department="IT" salary="75000">
    <name>Alice Johnson</name>
    <position>Software Developer</position>
    <email>alice@company.com</email>
    <skills>
      <skill>Java</skill>
      <skill>Python</skill>
      <skill>JavaScript</skill>
    </skills>
  </employee>
  <employee id="E002" department="HR" salary="65000">
    <name>Bob Wilson</name>
    <position>HR Manager</position>
    <email>bob@company.com</email>
    <skills>
      <skill>Recruitment</skill>
      <skill>Employee Relations</skill>
    </skills>
  </employee>
</employees>

Code Editor

Output