A "Well Formed" XML document has correct XML syntax.
A "Well Formed" XML document is a document that conforms to the
XML syntax rules that were described in the previous chapters:
XML documents must have a root element
XML elements must have a closing tag
XML tags are case sensitive
XML elements must be properly nested
XML attribute values must always be quoted
<?xml version="1.0" encoding="ISO-8859-1"?>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
Valid XML Documents
A "Valid" XML document also conforms to a DTD.
A "Valid" XML document is a "Well Formed" XML document, which
also conforms to the rules of a Document Type Definition (DTD):
<?xml version="1.0" encoding="ISO-8859-1"?>
<!DOCTYPE note SYSTEM "InternalNote.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
XML DTD
A DTD defines the legal elements of an XML document.
The function of a DTD is to define the legal building blocks of an XML document. It defines the document structure with a list of legal elements.
A DTD can be declared inline in your XML document, or as an external reference.
Internal DTD
This is an XML document with a Document Type Definition:
<?xml version="1.0"?>
<!DOCTYPE note [
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
]>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
The DTD is interpreted like this:
!ELEMENT note (in line 2) defines the element "note" as having four elements: "to,from,heading,body".
!ELEMENT to (in line 3) defines the "to" element to be of the type "CDATA".
!ELEMENT from (in line 4) defines the "from" element to be of the type "CDATA"
and so on.....
External DTD
This is a copy of the file "note.dtd" containing the Document Type Definition:
<?xml version="1.0"?>
<!DOCTYPE note SYSTEM "note.dtd">
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
This is a copy of the file "note.dtd" containing the Document Type Definition:
<?xml version="1.0"?>
<!ELEMENT note (to,from,heading,body)>
<!ELEMENT to (#PCDATA)>
<!ELEMENT from (#PCDATA)>
<!ELEMENT heading (#PCDATA)>
<!ELEMENT body (#PCDATA)>
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Keywords: valid xml, xml schema, xml document, w3c xml schema, xml parser, xml DTD, Internal DTD ,External DTD
