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+XML Expressions and Patterns
+============================
+
+{\bf By Burak Emir}\bigskip\bigskip
+
+
+This chapter describes the syntactic structure of XML expressions and patterns.
+It follows as closely as possible the XML 1.0 specification \cite{w3c:xml},
+changes being mandated by the possibility of embedding Scala code fragments.
+
+\section{XML expressions}
+XML expressions are expressions generated by the following production, where the 
+opening bracket `<' of the first element must be in a position to start the lexical
+[XML mode](#xml-mode).
+
+\syntax\begin{lstlisting}
+XmlExpr ::= XmlContent {Element}
+\end{lstlisting}
+Well-formedness constraints of the XML specification apply, which
+means for instance that start tags and end tags must match, and
+attributes may only be defined once, with the exception of constraints
+related to entity resolution.
+
+The following productions describe Scala's extensible markup language,
+designed as close as possible to the W3C extensible markup language
+standard. Only the productions for attribute values and character data
+are changed. Scala does not support declarations, CDATA
+sections or processing instructions. Entity references are not
+resolved at runtime.
+
+\syntax\begin{lstlisting}
+Element       ::=    EmptyElemTag
+                |    STag Content ETag                                       
+
+EmptyElemTag  ::=    `<' Name {S Attribute} [S] `/>'                         
+
+STag          ::=    `<' Name {S Attribute} [S] `>'                          
+ETag          ::=    `</' Name [S] '>'                                        
+Content       ::=    [CharData] {Content1 [CharData]}
+Content1      ::=    XmlContent
+                |    Reference
+                |    ScalaExpr
+XmlContent    ::=    Element
+                |    CDSect
+                |    PI
+                |    Comment
+\end{lstlisting}
+
+If an XML expression is a single element, its value is a runtime
+representation of an XML node (an instance of a subclass of 
+\lstinline@scala.xml.Node@). If the XML expression consists of more
+than one element, then its value is a runtime representation of a
+sequence of XML nodes (an instance of a subclass of 
+\lstinline@scala.Seq[scala.xml.Node]@).
+
+If an XML expression is an entity reference, CDATA section, processing 
+instructions or a comments, it is represented by an instance of the 
+corresponding Scala runtime class.
+
+By default, beginning and trailing whitespace in element content is removed, 
+and consecutive occurrences of whitespace are replaced by a single space
+character \U{0020}. This behavior can be changed to preserve all whitespace
+with a compiler option.
+
+\syntax\begin{lstlisting}
+Attribute  ::=    Name Eq AttValue                                    
+
+AttValue      ::=    `"' {CharQ | CharRef} `"'
+                |    `'' {CharA | CharRef} `''
+                |    ScalaExpr
+
+ScalaExpr     ::=    Block
+
+CharData      ::=   { CharNoRef } $\mbox{\rm\em without}$ {CharNoRef}`{'CharB {CharNoRef} 
+                                  $\mbox{\rm\em and without}$ {CharNoRef}`]]>'{CharNoRef}
+\end{lstlisting}
+XML expressions may contain Scala expressions as attribute values or
+within nodes. In the latter case, these are embedded using a single opening 
+brace `\{' and ended by a closing brace `\}'. To express a single opening braces 
+within XML text as generated by CharData, it must be doubled. Thus, `\{\{'
+represents the XML text `\{' and does not introduce an embedded Scala
+expression.
+
+\syntax\begin{lstlisting}
+BaseChar, Char, Comment, CombiningChar, Ideographic, NameChar, S, Reference
+              ::=  $\mbox{\rm\em ``as in W3C XML''}$
+
+Char1         ::=  Char $\mbox{\rm\em without}$ `<' | `&'
+CharQ         ::=  Char1 $\mbox{\rm\em without}$ `"'
+CharA         ::=  Char1 $\mbox{\rm\em without}$ `''
+CharB         ::=  Char1 $\mbox{\rm\em without}$ '{'
+
+Name          ::=  XNameStart {NameChar}
+
+XNameStart    ::= `_' | BaseChar | Ideographic 
+                 $\mbox{\rm\em (as in W3C XML, but without }$ `:'
+
+\end{lstlisting}
+\section{XML patterns}\label{sec:xml-pats}
+XML patterns are patterns generated by the following production, where
+the opening bracket `<' of the element patterns must be in a position
+to start the lexical [XML mode](#xml-mode).
+
+\syntax\begin{lstlisting}
+XmlPattern  ::= ElementPattern 
+\end{lstlisting}%{ElementPattern}
+Well-formedness constraints of the XML specification apply.
+
+An XML pattern has to be a single element pattern. It %expects the type of , and 
+matches exactly those runtime
+representations of an XML tree
+that have the same structure as described by the pattern. %If an XML pattern 
+%consists of more than one element, then it expects the type of sequences
+%of runtime representations of XML trees, and matches every sequence whose 
+%elements match the sequence described by the pattern.
+XML patterns may contain Scala patterns(\sref{sec:pattern-match}).
+
+Whitespace is treated the same way as in XML expressions. Patterns 
+that are entity references, CDATA sections, processing 
+instructions and comments match runtime representations which are the
+the same.
+
+By default, beginning and trailing whitespace in element content is removed, 
+and consecutive occurrences of whitespace are replaced by a single space
+character \U{0020}. This behavior can be changed to preserve all whitespace
+with a compiler option.
+
+\syntax\begin{lstlisting}
+ElemPattern   ::=    EmptyElemTagP
+                |    STagP ContentP ETagP                                    
+
+EmptyElemTagP ::=    `<' Name [S] `/>'
+STagP         ::=    `<' Name [S] `>'                          
+ETagP         ::=    `</' Name [S]`>'                                        
+ContentP      ::=    [CharData] {(ElemPattern|ScalaPatterns) [CharData]}
+ContentP1     ::=    ElemPattern
+                |    Reference
+                |    CDSect
+                |    PI
+                |    Comment
+                |    ScalaPatterns
+ScalaPatterns ::=    `{' Patterns `}'
+\end{lstlisting}
+