XML Expressions and Patterns
By Burak Emir
This chapter describes the syntactic structure of XML expressions and patterns. It follows as closely as possible the XML 1.0 specification, changes being mandated by the possibility of embedding Scala code fragments.
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.
XmlExpr ::= XmlContent {Element}
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.
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
If an XML expression is a single element, its value is a runtime
representation of an XML node (an instance of a subclass of
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
scala.Seq[scala.xml.Node]
).
If an XML expression is an entity reference, CDATA section, processing instruction, or a comment, 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 \u0020
. This behavior can be changed to preserve all whitespace
with a compiler option.
Attribute ::= Name Eq AttValue
AttValue ::= ‘"’ {CharQ | CharRef} ‘"’
| ‘'’ {CharA | CharRef} ‘'’
| ScalaExpr
ScalaExpr ::= Block
CharData ::= { CharNoRef } $\textit{ without}$ {CharNoRef}`{'CharB {CharNoRef}
$\textit{ and without}$ {CharNoRef}`]]>'{CharNoRef}
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.
BaseChar, Char, Comment, CombiningChar, Ideographic, NameChar, S, Reference
::= $\textit{“as in W3C XML”}$
Char1 ::= Char $\textit{ without}$ ‘<’ | ‘&’
CharQ ::= Char1 $\textit{ without}$ ‘"’
CharA ::= Char1 $\textit{ without}$ ‘'’
CharB ::= Char1 $\textit{ without}$ ‘{’
Name ::= XNameStart {NameChar}
XNameStart ::= ‘_’ | BaseChar | Ideographic
$\textit{ (as in W3C XML, but without }$ ‘:’$)$
XML patterns
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.
XmlPattern ::= ElementPattern
Well-formedness constraints of the XML specification apply.
An XML pattern has to be a single element pattern. It matches exactly those runtime representations of an XML tree that have the same structure as described by the pattern. XML patterns may contain Scala patterns.
Whitespace is treated the same way as in XML expressions.
By default, beginning and trailing whitespace in element content is removed,
and consecutive occurrences of whitespace are replaced by a single space
character \u0020
. This behavior can be changed to preserve all whitespace
with a compiler option.
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 ‘}’