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---
language: xml
filename: learnxml.xml
contributors:
- ["João Farias", "https://github.com/JoaoGFarias"]
---
XML is a markup language designed to store and transport data.
Unlike HTML, XML does not specify how to display or to format data, just carry it.
* XML Syntax
```xml
<!-- Comments in XML are like this -->
<?xml version="1.0" encoding="UTF-8"?>
<bookstore>
<book category="COOKING">
<title lang="en">Everyday Italian</title>
<author>Giada De Laurentiis</author>
<year>2005</year>
<price>30.00</price>
</book>
<book category="CHILDREN">
<title lang="en">Harry Potter</title>
<author>J K. Rowling</author>
<year>2005</year>
<price>29.99</price>
</book>
<book category="WEB">
<title lang="en">Learning XML</title>
<author>Erik T. Ray</author>
<year>2003</year>
<price>39.95</price>
</book>
</bookstore>
<!-- Above is a typical XML file.
It starts with a declaration, informing some metadata (optional).
XML uses a tree structure. Above, the root node is 'bookstore', which has
three child nodes, all 'books'. Those nodes has more child nodes, and so on...
Nodes are created using open/close tags, and childs are just nodes between
the open and close tags.-->
<!-- XML carries two kind of data:
1 - Attributes -> That's metadata about a node.
Usually, the XML parser uses this information to store the data properly.
It is characterized by appearing with the format name="value" within the opening
tag.
2 - Elements -> That's pure data.
That's what the parser will retrieve from the XML file.
Elements appear between the open and close tags. -->
<!-- Below, an element with two attributes -->
<file type="gif" id="4293">computer.gif</file>
```
* Well-Formated Document x Validation
A XML document is well-formated if it is syntactically correct.
However, it is possible to inject more constraints in the document,
using document definitions, such as DTD and XML Schema.
A XML document which follows a document definition is called valid,
regarding that document.
With this tool, you can check the XML data outside the application logic.
```xml
<!-- Below, you can see an simplified version of bookstore document,
with the addition of DTD definition.-->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note SYSTEM "Bookstore.dtd">
<bookstore>
<book category="COOKING">
<title >Everyday Italian</title>
<price>30.00</price>
</book>
</bookstore>
<!-- This DTD could be something like:-->
<!DOCTYPE note
[
<!ELEMENT bookstore (book+)>
<!ELEMENT book (title,price)>
<!ATTLIST book category CDATA "Literature">
<!ELEMENT title (#PCDATA)>
<!ELEMENT price (#PCDATA)>
]>
<!-- The DTD starts with a declaration.
Following, the root node is declared, requiring 1 or more child nodes 'book'.
Each 'book' should contain exactly one 'title' and 'price' and an attribute
called 'category', with "Literature" as its default value.
The 'title' and 'price' nodes contain a parsed character data.-->
<!-- The DTD could be declared inside the XML file itself.-->
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE note
[
<!ELEMENT bookstore (book+)>
<!ELEMENT book (title,price)>
<!ATTLIST book category CDATA "Literature">
<!ELEMENT title (#PCDATA)>
<!ELEMENT price (#PCDATA)>
]>
<bookstore>
<book category="COOKING">
<title >Everyday Italian</title>
<price>30.00</price>
</book>
</bookstore>
```
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