XML Fundamentals Transparency No. 1 XML Fundamentals Cheng-Chia Chen November 2004

XML Fundamentals

Transparency No. 1

XML Fundamentals

Cheng-Chia Chen

November 2004

XML Fundamentals

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Well-formed XML Document

An XML document is a sequence of characters: Each character is an atomic unit of text as specified by ISO

/IEC 10646 [unicode]. can be opened/edited with any program that knows how to

read/write a text file usually given a .xml extension file name MIME media type: application/xml or text/xml

Ex:

<?xml version=“1.0” encoding=“UTF-8”>

<student> 張得功 </student>

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Characters used in XML

A character is an atomic unit of text as specified by ISO/IEC 10646 [ISO/IEC 10646].

Legal characters are tab, carriage return, line feed, and the legal graphic characters of Unicode and ISO/IEC 10646.

Character Range

[2] Char ::= #x9 | #xA | #xD | [#x20-#xD7FF]

| [#xE000-#xFFFD] | [#x10000-#x10FFFF]

/* any Unicode character, excluding the surrogate blocks, FFFE, and FFFF. */

character encoding may vary from entity to entity. All XML processors must accept the UTF-8 and UTF-16 e

ncodings.

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Whitespace

White Space:

[3] S ::= (#x20 | #x9 | #xD | #xA)+S (white space) consists of one or more space (#x20) cha

racters, tabs, carriage returns or line feeds.Whitespace can used to separate otherwise indistinguis

hable parts of an XML Document. <student age=“15”>…</student> <studentage=“15”>…</student>

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XML Declaration

<?xml version=“1.0” encoding=“Big5” standalone=“no” ?>

Besides using file extension name, an xml document may use an XML declaration to identify itself as an XML document.

If used, it should occur first (no proceding whitespace allowed) in the document.

Version of the

XML specification

1.0 or 1.1

character encoding of

the document, expressed

in Latin characters, e.g.,

UTF-8, UTF-16,

iso-8859-1,

no: parsing affected

by external

DTD subset

yes: not affected .

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Elements, tags and character data

The previous example is composed of a single element named student Start-tag: <student> End-tag: </student>

Everything between start-tag and end-tag is called content Content encompasses real information Whitespace is part of the content, though many application

s will choose to ignore it<student> and </student> are markup張得功 and its surrounding whitespace are character dat

a

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Structure of an element

Each XML document contains one or more elements, the boundaries of which are either delimited by start-tags and end-tags, or, for empty elements, by an empty-element tag.

Each element has a type, identified by name, and may have a set of attribute specifications. The name used in start-tag and end-tag must be identical. Note: xml is case sensitive, so <student> != <Student>

Each attribute specification has a name and a value. Element

[39] element ::= EmptyElemTag | STag content ETag

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Element (cont’d)

The text between the start-tag and end-tag is called the element's content:

Content of Elements

[43] content ::= CharData? ((element | Reference | CDSect | PI | Comment) CharData?)*

If an element is empty, it must be represented either by a start-tag immediately followed by an end-tag or by an empty-element tag.

Tags for Empty Elements [44] EmptyElemTag ::= '<' Name (S Attribute)* S? '/>'

Empty element tags may be used for any element which has no content, whether or not it is declared using the keyword EMPTY.

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Examples of empty elements

<IMG align="left” src="http://www.w3.org/Icons/WWW/w3c_home"

/>

 

 

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Start tag with attribute ( in document) and end tag

<tag attributeName = “ attrbute-value “ … >

</tag>

name of the

attribute

value or values

of the attribute

name(or type)

of the element

single or double

quotes,

‘ or “ must match

Each element

may contain zero

or more attributes

start tag and end

tag name must match

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Attributes

Attach additional information to elementsAn attribute is a name-value pair attached to an element’

s start-tag One element can have more than one attribute Name and value are separated by = and optional whitespac

e Attribute value is enclosed in double or single quotation m

arks <tel type=“office”>02-29381111</tel> Attribute order is not significant <student age=“20” gender=“male”> 趙得勝 </student>

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Start Tag

Start-tag

[40] STag ::= '<' Name (S Attribute)* S? '>'

[ WFC: Unique Att Spec ]

[41] Attribute ::= Name Eq AttValue

Example:

<termdef id=“dt-dog” term=“dog”>End-tag

[42] ETag ::= '</' Name S? '>’

Example:

</termdef> </termdef > vs </ termdef> < /termdef>

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Use attribute or element ?

Should one use child elements or attributes to hold information? Attributes are for metadata about the element, while eleme

nts are for the information itselfEach element may have no more than one attribute with

a given nameThe value of attribute is simply a text string – limited in s

tructureAn element-based structure is a lot more flexible and ext

ensibleIf you are designing your own XML vocabulary, it is up to

you to decide when to use which

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XML Names

Rules for naming elements, attributes…Names and Tokens [4] NameChar ::= Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender [5] Name ::= (Letter | '_' | ':') (NameChar)* [6] Names ::= Name ( #x20 Name)* [7] Nmtoken ::= (NameChar)+ [8] Nmtokens ::= Nmtoken (#x20 Nmtoken)*

Names beginning with (x|M)(m|M)(l|L) are reserved.Name is used for naming elements, attributes, entities et

c.Nmtoken (Nmtokens) is used for values of special attribu

tes(ID,IDREFS,NMTOKEN,NMTOKENS).

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AttValues (attribute value literal)

are those that can occur as an attribute value.

[10] AttValue ::= '"' ([^<&"] | Reference)* '"'

| "'" ([^<&'] | Reference)* "'"

Enclosed by double or single quotes.Can contain entity/char references or any char data but <

and &.

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Comments

Comments may appear 1. anywhere in a document outside other markup; 2. within the document type declaration at places

allowed by the grammar. They are not part of the document's character data. The string "--" (double-hyphen) must not occur within

comments. Comments

[15] Comment ::= ''Example:



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Processing Instructions (PIs)

Processing instructions (PIs) allow documents to contain instructions for applications.

Processing Instructions:

[16] PI ::= '<?' PITarget (S (Char* - (Char* '?>' Char*)))? '?>'

[17] PITarget ::= Name - (('X' | 'x') ('M' | 'm') ('L' | 'l'))

The PI begins with a target (PITarget) used to identify the application.

The target names "XML", "xml", and so on are reserved for standardization.

Ex: <?xml-stylesheet type=“text/css” href=“style.css” ?>

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Processing Instruction and comment

<?PItarget ***other staff*** ?>



may contain any characters

except the string “--”

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XML Document

[1]document ::= prolog element Misc*

elemet is called the root or document element of the document

[22] prolog ::= XMLDecl? Misc* (doctypedecl Misc*)?

[23] XMLDecl ::= '<?xml' VersionInfo EncodingDecl?

SDDecl? S? '?>'

[27] Misc ::= Comment | PI | S

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Character references

What if the character data inside an element contains < ? <expr> x+1 < z </expr>

Instead of using ‘<‘, we can use its character code (60) reference: < --- decimal #60 < --- hexadecimal #x3c

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Entity reference

Numeric code is hard to remember. Can use a name to denote a char or a sequence of chars Such name is called entity.

Entity reference – If xxx is an entity => &xxx; is it entity reference when parsing an XML document, xml processor replaces t

he entity reference with the actual characters to which the entity reference refers

XML predefines 5 entity references – you can define more < – the less-than sign (<) & – the ampersand (&) > – the greater-than sign(>) " – the straight, double quotation marks (") ' – the straight single quote (')

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CDATA Section

What if my element content has a lot of special characters ? Ex: <expr> x < y && z < 1 </expr>

Solution 1: <expr> x < y &amps;&amps; z < 1 </expr> Hard to read

Solution 2: <expr><![CDATA[ x < y && z < 1 ]]></expr>

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CDATA Sections

CDATA sections may occur anywhere character data may occur; used to escape blocks of text containing characters which would othe

rwise be recognized as markup. begin with the string "<![CDATA[" and end with the string "]]>":

CDATA Sections

[18] CDSect ::= CDStart CData CDEnd

[19] CDStart ::= '<![CDATA['

[20] CData ::= (Char* - (Char* ']]>' Char*))

[21] CDEnd ::= ']]>' Within a CDATA section, only the CDEnd ']]>' string is recognized a

s markup, so that left angle brackets ‘<‘ and ampersands ‘&’ may and must occur in their literal form.

Example: <![CDATA[<greeting>Hello, world!</greeting>]]>

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Character Data and Markup

XML Document consists of intermingled character data and markup. Markup takes the form of start-tags, end-tags, empty-element tags, entity references, character references, comments, CDATA section delimiters, document type declarations, processing instructions, XML declarations, text declarations and white space outside root element

All text that is not markup constitutes the character data of the document. I.e., it may occur in the content of an element or In the content of an CDATA Section.

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Character Data and Markup (cont’d)

In the content of elements, character data is any string of characters which does not contain the start-delimiter of any markup.

In a CDATA section, character data is any string of characters not including the CDATA-section-close delimiter, "]]>".

To allow attribute values to contain both single and double quotes, the apostrophe or single-quote character (') may be represented as "'", and the double-quote character (") as """.

Character Data :

[14] CharData ::= [^<&]* - ([^<&]* ']]>' [^<&]*) i.e., Any string containing none of <, & and ]]>.

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Possible contents of an element content

Element

[39] element ::= EmptyElemTag | STag content ETag

Content of Elements

[43] content ::= CharData? ((element | Reference | CDSect | PI | Comment) CharData?)*

In addition to char data and child elements, an element may contain as children also references, PIs, comments or CDATA sections.

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Rules for well-formed XML Documents

1: balance start and end tags The set of tags is unlimited but all start tags must have mat

ching end tags

Example of legal XML <student>

<name> DeTsi Wang</name><email> [email protected]</email><age> 20 </age></student>

2: There must be exactly one root element

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Rule 3: Proper element nesting All tags must be nested correctly. Like HTML, XML can inte

rmix tags and text, but tags may not overlap each other. Legal XML

<student><name> DeTsi Wang</name><email> [email protected]</email><age> 20 </age>

</student> Illegal XML

This text is bold and italic and italic

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Rule 4: Attribute values must be single or double quoted Legal

<tag attribute=“value”><tag attribute=‘value’>

Illegal 

Rule 5: An element may not have two attributes with the same name 

Rule 6: Comments and processing instructions may not appear inside tags size = “6” />

Rule 7: No unescaped < or & signs may occur in the character data of an element or attributes 20&3