. One way to think about XPath is that it does for XML instances a bit of what SQL does for RDBMSs--it's a kind of query language. It complements XSLT in particular; XSLT describes what changes to make, and XPath tells where in a document to make them (very roughly). [Also recommend http://www.w3schools.com/ ? ]Examples of XPath
<html>
<a href="http://wiki.tcl.tk/">Tcler's Wiki</a>
<a href="http://www.tcl.tk/">Tcl Developer Xchange</a>
<a id="getme" href="http://openacs.org/">OpenACS</a>
</html>Given the example XML above we could extract all <a> tags using the following XPath:
//aWe could also grab the "OpenACS" link with the following XPath:
//a[@id="getme"]More examples can be found at: http://www.zvon.org/xxl/XPathTutorial/General/examples.htmlJAC [Nice comments, JAC.][Explain XPath implementations in existing Tcl-XSLT bindings.]
tDOM since 1999 makes XPath queries on script level very easy. With the example from above:
package require tdom
set data {
<html>
<a href="http://wiki.tcl.tk/">Tcler's Wiki</a>
<a href="http://www.tcl.tk/">Tcl Developer Xchange</a>
<a id="getme" href="http://openacs.org/">OpenACS</a>
</html>}
set doc [dom parse $data]
set root [$doc documentElement]
set aNodes [$root selectNodes {//a[@id="getme"]}]
foreach node $aNodes {
puts "Visit [$node text] at [$node @href]"
}
$doc deleteTwo notes:The brackets '[ ]' have syntactical meaning both in tcl and in XPath expressions. Don't forget to protect the brackets in your XPath expressions.The XPath expression //a is not the best example one could choose. The '//' (which is the abbreviation for /descendant-or-self::node()/) is one of the most expensive XPath location steps for almost all known XPath engines. It means, that the XPath engine has to scan the whole tree beneath the node. Avoiding // - of course, if possible - could amazingly speed up your XPath queries or your XSLT stylesheets. Rolf Ade.TclXML implemented XPath in around 2001. It's possible the TclXML side is not as actively maintained. "DOM Level 3 has an official method for using XPath with a DOM tree, but I haven't had time to implement that yet." In any case, here's an example of TclDOM XPath programming that arose from a comp.lang.tcl request to make a list of all attributes across all tags:
package require dom
set xml_image {<?xml version = "1.0" encoding = "UTF-8" ?>
<via id="1" trivia_id="255" question="How much wood would a
woodchuck chuck?" answer_id="1" answer="A lot"
expDate="116494926"></via>}
set doc [dom::parse $xml_image]
set result {}
# "//@*" says, "all tags, all attributes." "//via/@*"
# gives all attributes of all "via" tags. And so on.
foreach node [dom::selectNode $doc //@*] {
lappend result [dom::node configure $node -localName] \
[dom::node stringValue $node]
}
puts $result
# The output is "id 1 trivia_id 255 question {How much wood would a woodchuck chuck?} answer_id 1 answer {A lot} expDate 116494926"Steve once did "some work on a higher-level package that allows XPath over a streaming interface ..." See the [xmlswitch] package for more on the subject.XPath users and students will want to have XGrep [1] at their sides. "XGrep is a grep like utility for XML documents" which uses XPath syntax for its searches.
RS 2005-06-13: Here's experimental code to fill in a default namespace into XPath expressions:
proc XPathNS {ns path} {
set res {}
foreach e [split $path /] {
if {$e eq "."} continue
if {![in {"" ..} $e] && ![has : $e]} {set e $ns:$e}
lappend res $e
}
join $res /
}
proc in {list el} {expr {[lsearch $list $el]>=0}}
proc has {substr str} {expr {[string first $substr $str]>=0}}#-- Testing:% XPathNS NN /a/b/c /NN:a/NN:b/NN:c % XPathNS NN /a/b/../c /NN:a/NN:b/../NN:c % XPathNS NN /a/./b/../c/foo:bar ;# preserve explicit namespaces, "foo" here /NN:a/NN:b/../NN:c/foo:bar
RS 2006-03-24: A little command line tool to do an XPath query on an XML file:
#!/usr/bin/env tclsh
set usage {
usage: xpath xmlfile query
}
if {[llength $argv] != 2} {puts stderr $usage; exit}
package require tdom
proc main argv {
foreach {xmlfile query} $argv break
set f [open $xmlfile]
set docel [[dom parse -channel $f doc] documentElement]
close $f
foreach node [$docel selectNodes $query] {
puts [$node asXML]
}
}
main $argv2006-05-22 From linux-magazine article issue 20Table 3: XPATH examples
| Query | Description |
|---|---|
| /option | The option element directly below the root node |
| //option | All elements in the document called option |
| //option[3] | The third option element |
| /table/* | All elements below table, where table must be located directly below the root node |
| //table[1] | The first table element in a document |
| //table[last()] | The last table element |
| //@colspan | All colspan attributes in a document |
| //td[@colspan] | All td elements with the attribute colspan |
| //table[@width] | All table elements that have a width attribute |
| //table[@width=690] | All table elements with a width attribute that has avalue of 690 |
| //*[count(tr)=2] | All elements with two tr child nodes |
| //tr/td|th | All td and th elements contained within a tr element |
| //table//img | All img elements contained within a table-element |
| //table[1]//img[2] | Second img element in the first table |
| //status[.='hit']/.. | All elements that contain a <status>hit</status> element |
| //p/br/following-sibling::text() | The bar in <p>foo<br/>bar</p> |
//table[tr/td/@width=690]
Tables that contains a tr with a td inside that is of width 690There are other ways of writing the above://table/tr[td/@width=690]/.. //table/tr/td[@width=690]/../.. //table/tr[td[@width=690]]As you can see, XPath is powerful, tricky, baroque, ... :-)
[anjalijaingmailcom] - 2010-03-26 07:15:12
<div class="tabUnselectedText" align="center">
<a href="javascript:renderPage('mainForm:consoleBeanId.3','Analysis' , 'analysisdetails.faces');">Analysis</a>
</div>What would be Xpath for element 'Analysis'?Lars H: That depends on what you consider to characterise it. From the fragment you quote, one possibility would be //div/a, but in a typical XHTML document that'll probably match lots of things.xpath.tcl
: A wrapper script that models an XML file as a filesystem to make using XPATH via Tdom feel like using file commands.