Doing XPath using Perl - perl

Question

I am coding with Perl on a Window 7 machine. I am able to extract data from the XML using the XPath code below
use strict;
use warning;
use XML::LibXML;
my $parser = XML::LibXML->new();
my $doc = $parser->parse_file($newfile);
my $query = "/tradenet/message/header/unique_ref_no/date/text( )";
my($node) = $doc->findnodes($query);
$node->setData("$file_seq_number");
However, when i use the same code on a different XML, the xpath from the second document looks as below:
/TradenetResponse/OutboundMessage/out:OutwardPermit/out:Declaration/out:Header/cac:UniqueReferenceNumber/cbc:SequenceNumeric
Together with the Perl code, this is what the extraction code looks like:
my $parser = XML::LibXML->new();
my $doc = $parser->parse_file($newfile);
my $query = "/TradenetResponse/OutboundMessage/out:OutwardPermit/out:Declaration/out:Header/cac:UniqueReferenceNumber/cbc:SequenceNumeric/text( )";
my($node) = $doc->findnodes($query);
$node->setData("$file_seq_number");
Using the second code, I am unable to retrieve the data from the second XML. I receive this error "Can't call method "setData"on an undefined value at Perl.pl line 5".
Does the ":" character in the second XPATH address affecting the code?

Answer 1

You have to define what out, cac, and cbc mean in order for the XPath query to find the appropriate nodes:
my $doc = $parser->parse_file($newfile);
my $xpath_context = XML::LibXML::XPathContext->new($doc->documentElement());
# These URIs need to be the same as the ones in the source document
$xpath_context->registerNs('out', 'http://example.com/out.xsd');
$xpath_context->registerNs('cac', 'http://example.com/cac.xsd');
$xpath_context->registerNs('cbc', 'http://example.com/cbc.xsd');
my $query = "/TradenetResponse/OutboundMessage/out:OutwardPermit/out:Declaration/out:Header/cac:UniqueReferenceNumber/cbc:SequenceNumeric/text( )";
my ($node) = $xpath_context->findnodes($query);
As promised, here is a working example. First, the test input file:
<?xml version="1.0"?>
<!-- input.xml -->
<TradenetResponse xmlns:a="http://example.com/out.xsd"
xmlns:b="http://example.com/cac.xsd"
xmlns:c="http://example.com/cbc.xsd">
<OutboundMessage>
<a:OutwardPermit>
<a:Declaration>
<a:Header>
<b:UniqueReferenceNumber>
<c:SequenceNumeric>1234</c:SequenceNumeric>
</b:UniqueReferenceNumber>
</a:Header>
</a:Declaration>
</a:OutwardPermit>
</OutboundMessage>
</TradenetResponse>
And here is the working Perl script:
#!/usr/bin/perl
# parse.pl
use strict;
use warnings;
use XML::LibXML;
my $parser = XML::LibXML->new();
my $newfile = "input.xml";
my $doc = $parser->parse_file($newfile);
my $xpath_context = XML::LibXML::XPathContext->new($doc->documentElement());
# These URIs need to be the same as the ones in the source document
$xpath_context->registerNs('out', 'http://example.com/out.xsd');
$xpath_context->registerNs('cac', 'http://example.com/cac.xsd');
$xpath_context->registerNs('cbc', 'http://example.com/cbc.xsd');
# Query wrapped for clarity
my $query = "/TradenetResponse/OutboundMessage/out:OutwardPermit" .
"/out:Declaration/out:Header/cac:UniqueReferenceNumber" .
"/cbc:SequenceNumeric/text()";
my ($node) = $xpath_context->findnodes($query);
print "Value: " . $node->getData() . "\n";
The output for me is:
sean#localhost:~xmltest$ ./parse.pl
Value: 1234