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most common XPath expressions, simply to introduce the concept.
The expression @type="unordered" specifies an attribute named type whose value is "unordered".
So an expression like LIST/@type specifies the type attribute of a LIST element.
But now for something a little different! In XPath, the square-bracket notation ([]) normally associated
with indexing is extended to specify selection-criteria. For example, the expression
LIST[@type="unordered"] selects all LIST elements whose type value is "unordered".
Similar expressions exist for elements, where each element has an associated string-value. (You'll see
how the string-value is determined for a complicated element in a little while. For now, we'll stick with
super-simple elements that have a single text string.)
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Suppose you model what's going on in your organization with an XML structure that consists of
PROJECT elements and ACTIVITY elements that have a text string with the project name, multiple
PERSON elements to list the people involved and, optionally, a STATUS element that records the
projects status. Here are some more examples that use the extended square-bracket notation:
/PROJECT[.="MyProject"] selects a PROJECT named "MyProject".
/PROJECT[STATUS] -- selects all projects that have a STATUS child element.
/PROJECT[STATUS="Critical"] -- selects all projects that have a STATUS child element
with the string-value "Critical".
Combining Index Addresses
The XPath specification defines quite a few addressing mechanisms, and they can be combined in many
different ways. As a result, XPath delivers a lot of expressive power for a relatively simple specification.
This section illustrates two more interesting combinations:
LIST[@type="ordered"][3] -- selects all LIST elements of type "ordered", and returns the
third.
LIST[3][@type="ordered"] -- selects the third LIST element, but only if it is of "ordered"
type.
Note:
Many more combinations of address operators are listed in section 2.5 of the XPath
Specification. This is arguably the most useful section of the spec for defining an XSLT
transform.
Wildcards
By definition, an unqualified XPath expression selects a set of XML nodes that matches that specified
pattern. For example, /HEAD matches all top-level HEAD entries, while /HEAD[1] matches only the
first. But XPath expressions can also contain one of several wildcards to broaden the scope of the pattern
matching:
matches any element node (not attributes or
*
text)
matches all nodes of any kind: element nodes,
text nodes, attribute nodes,
node()processing instruction nodes, namespace
nodes, and comment nodes.
@* matches all attribute nodes
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1. Intro to XSLT
In the project database example, for instance, /*/PERSON[.="Fred"] matches any PROJECT or
ACTIVITY element that includes Fred.
Extended-Path Addressing
So far, all of the patterns we've seen have specified an exact number of levels in the hierarchy. For
example, /HEAD specifies any HEAD element at the first level in the hierarchy, while /*/* specifies
any element at the second level in the hierarchy. To specify an indeterminate level in the hierarchy, use a
double forward slash (//). For example, the XPath expression //PARA selects all paragraph
elements in a document, wherever they may be found.
The // pattern can also be used within a path. So the expression /HEAD/LIST//PARA indicates all
paragraph elements in a subtree that begins from /HEAD/LIST.
XPath Data Types and Operators
XPath expressions yield either a set of nodes,: a string, a boolean (true/false value), or a number.
Expressions can also be created using one of several operations on these values:
| Alternative. So PARA|LIST selects all PARA and LIST elements.
or, and Returns the or/and of two boolean values.
=, != Equal or not equal, for booleans, strings, and numbers.
Less than, greater than, less than or equal to, greater than or equal to -- for
, =
numbers.
Add, subtract, multiply, floating-point divide, and modulus (remainder)
+, -, *, div,
operations
mod
(e.g. 6 mod 4 = 2)
Finally, expressions can be grouped in parentheses, so you don't have to worry about operator
precedence. (Which, for those of you who who are good at such things, is roughly the same as that shown
in the table.)
String-Value of an Element
Before going on, it's worthwhile to understand how the string-value of more complex element is
determined. We'll do that now.
The string-value of an element is the concatenation of all descendent text nodes, no matter how deep. So,
for a "mixed-model" XML data element like this:
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This paragraph contains a bold word
the string-value of is "This paragraph contains a bold word". In particular, note that is a
child of and that the text contained in all children is concatenated to form the string-value.
Also, it is worth understanding that the text in the abstract data model defined by XPath is fully
normalized. So whether the XML structure contains the entity reference "<" or "
section, the element's string-value will contain the "
XML with an XSLT stylesheet, occurrences of "
CDATA section. Similarly, occurrence of "&" will need to be converted to &.
XPath Functions
This section ends with an overview of the XPath functions. You can use XPath functions to select a
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