XML - Extensible Markup Language

Learn to structure data with XML and its associated technologies

🎯 What is XML?

XML (eXtensible Markup Language) is a markup language designed to store and transport data in a structured way that is readable by both humans and machines.

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Self-descriptive

Tag names describe the content

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Independent

Doesn't depend on any platform or programming language

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Extensible

You can create your own tags according to your needs

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W3C Standard

Maintained by the World Wide Web Consortium

📝 Basic Syntax of XML

ejemplo.xml 
<?xml version="1.0" encoding="UTF-8"?>
<biblioteca>
    <libro id="001">
        <titulo>El Quijote</titulo>
        <autor>Miguel de Cervantes</autor>
        <anio>1605</anio>
        <genero>Novela</genero>
    </libro>
    <libro id="002">
        <titulo>Cien años de soledad</titulo>
        <autor>Gabriel García Márquez</autor>
        <anio>1967</anio>
        <genero>Realismo mágico</genero>
    </libro>
</biblioteca>

Key Elements:

  • Declaration XML: First line with version and encoding
  • Root element: A single element that contains everything (<biblioteca>)
  • Tags: Must always be closed (<titulo>...</titulo>)
  • Attributes: Additional information in elements (id="001")
  • Nesting: Elements inside other elements

📜 Essential Rules of XML

1

XML Declaration

Every XML document must start with a declaration specifying the version and encoding.

<?xml version="1.0" encoding="UTF-8"?>
2

Root Element Single

There can only be one root element containing all the other elements.

3

Tags Closed

All tags must be closed, either with a closing tag or as self-contained tags.

<nombre>valor</nombre>
4

Case Sensitive

XML distinguishes between uppercase and lowercase in tag names.

5

Attributes in Quotes

Attribute values must always be enclosed in single or double quotes.

id="123"
6

Nesting Correct

Elements must be nested correctly without overlapping.

🔧 Technologies Associated with XML

DTD - Document Type Definition

Schema that defines the structure of a document XML.

<!ELEMENT biblioteca (libro+)>
<!ELEMENT libro (titulo, autor, anio, genero)>
<!ATTLIST libro id CDATA #REQUIRED>
<!ELEMENT titulo (#PCDATA)>

XSD - XML Schema Definition

A more powerful and expressive schema than DTD, also in XML.

<xs:element name="libro">
    <xs:complexType>
        <xs:sequence>
            <xs:element name="titulo" type="xs:string"/>
            <xs:element name="autor" type="xs:string"/>
        </xs:sequence>
        <xs:attribute name="id" type="xs:integer"/>
    </xs:complexType>
</xs:element>

XPath - XML Path Language

Language to navigate and select nodes in XML.

/biblioteca/libro[titulo='El Quijote']
//autor[contains(text(), 'Cervantes')]
/biblioteca/libro[@id='001']/titulo

XSLT - XML Transformations

Language to transform XML documents into other formats (HTML, XML, text).

<xsl:template match="/">
    <html>
        <xsl:for-each select="biblioteca/libro">
            <div class="libro">
                <h2><xsl:value-of select="titulo"/></h2>
            </div>
        </xsl:for-each>
    </html>
</xsl:template>

XQuery - XML Query Language

Language to query XML (similar to SQL for databases).

for $libro in /biblioteca/libro
where $libro/anio > 1900
return $libro/titulo

🎮 Interactive Examples

Select an example to see it in action and learn by practicing:

✍️ XML Interactive Editor

Write your own XML here and validate it in real time:

Write your XML:

Result:

XML Write "Validate"

🏢 Common Use Cases

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Web Services

SOAP and RESTful APIs frequently use XML to exchange data

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Settings

Configuration files in applications and frameworks

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Storage

XML Native databases and data persistence

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Documentation

Formats like DocBook for technical documentation