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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>
+```