The Foundation: XML, XSL, X-Link (2/4) - exploring XML

The Pattern Matcher

Let's dissect the example style sheet of my last column to explain the three elements:

<xsl:template match="/">

This tells the XSL processor to apply the subsequent template (not shown here) when it finds a match to the match argument, in this case the document root. The match argument needs to be specified in XPath syntax, a related W3C specification for referencing parts of an XML document. It bears strong resemblance to directory path specifications, with some odd additions:

<xsl:template match="shoppingcart/item">

This example matches a item tag enclosed by a shoppingcart tag, anywhere in a document tree.

<xsl:template match="/shoppingcart/item">

(Note the leading slash before shoppingcart). This example matches the same construct as above, but only at the top level of the document, directly underneath the document root.

<xsl:template match="price[@unit='USD']">

This checks for an attribute named unit on the element price with a value of 'USD.' It would not match a price of any other currency.

Here are some more examples, with increasing obscurity:

* matches any element
@* matches any attribute
@class matches any class attribute (not any element that has a class attribute)
chapter|appendix matches any chapter element and any appendix element
appendix//para matches any para element with an appendix ancestor element
text() matches any text node
id("me") matches the element with unique ID me
para[1] matches any para element that is the first para child element of its parent
*[position()=1 and self::para] matches any para element that is the first child element of its parent
para[last()=1] matches any para element that is the only para child element of its parent
items/item[position()>1] matches any item element that has a items parent and that is not the first item child of its parent
item[position() mod 2 = 1] would be true for any item element that is an odd-numbered item child of its parent.
div[@class="appendix"]//p matches any p element with a div ancestor element that has a class attribute with value appendix

The rules engine

The rules engine needs to resolve conflicts when more than one pattern matches to decide which one takes precedence. A complete discussion of precedence rules is beyond the scope of this arcticle, but two rules of thumb are handy to remember:

A more specific match takes precedence over a less specific one, so in case of an item in a shoppingcart "shoppingcart/item" takes precedence over "item."
In the case of multiple included and embedded style sheets, the latest, innermost definition takes precedence

In practice it is a good idea to avoid conflicts altogether and stay away from overriding rules. It is hard to debug, believe me.... Needless to say our little shopping cart example in column 1 does not have rule conflicts of any kind.

The template processor

The template processor executes a set of instructions in consequence to a pattern match. These instructions create new elements in the output tree, either implicitly by using non-XSL content like

<HTML>
  <HEAD>
    <TITLE>Your shopping cart</TITLE>
...

or explicitly using <xsl:element>, <xsl:attribute>, or <xsl:text>. XSL instructions like

<xsl:for-each select="shoppingcart/item">

allow you to iterate over a list of nodes, and enable selective processing. Finally

<xsl:apply-templates>

hands over control to the pattern matcher again, to search for the next rule to be fired. More examples and detail will follow in later installments of this column.

URL: https://www.webreference.com/xml/column2/2.html
Created: Dec. 20, 1999
Revised: Dec. 21, 1999

The Foundation: XML, XSL, X-Link (2/4) - exploring XML

The Pattern Matcher

The rules engine

The template processor

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