This Ruby code raises an error if the XPath query fails because the attribute being queried did not exist for the given element.

  def map_pattr(node, fieldx, valuex)
    begin
    parameter = node.root.elements["parameter[@field='#{fieldx}']"]
    parameter.add_attribute('value', valuex)
    parameter
    
    rescue
      puts 'feedpopulated.rb: map_attr() the field ' + fieldx + ' was not found in params.'
      raise
    end
  end

Notice that a raise statement is used to ensure that the system error message is raised and any further code execution is halted.

Without adding a customized helpful message I would be left scratching my head trying to work out what the following system error message meant.

./feedpopulated.rb:27:in `map_pattr': undefined method `add_attribute' for nil:NilClass (NoMethodError)
     from ./feedpopulated.rb:51:in `create_record'
     from ./feedpopulated.rb:49:in `each'
     from ./feedpopulated.rb:49:in `create_record'
     from ./recordx.rb:91:in `call_create'
     from ./s3fileuploader_handler.rb:14:in `call'
     from ./s3fileuploader_handler.rb:40:in `invoke'
     from ./uploadtwitteraudio.rb:22:in `initialize'
     from /usr/lib/ruby/1.8/rexml/element.rb:890:in `each'
     from /usr/lib/ruby/1.8/rexml/xpath.rb:53:in `each'
     from /usr/lib/ruby/1.8/rexml/element.rb:890:in `each'
     from ./uploadtwitteraudio.rb:18:in `initialize'
     from ./uploadtwitteraudio.rb:72:in `new'
     from ./uploadtwitteraudio.rb:72

Reference: Programming Ruby: The Pragmatic Programmer's Guide - Exceptions, Catch, and Throw [ruby-doc.org]

A few XPath examples copied from XPath Syntax [w3schools.com]

Predicates

/bookstore/book[1] 	# Selects the first book element that is the child of the bookstore element. 
                        # Note: IE5 and later has implemented that [0] should be the first node, 
                        # but according to the W3C standard it should have been [1]!!
/bookstore/book[last()] 	# Selects the last book element that is the child of the bookstore element
/bookstore/book[last()-1] 	# Selects the last but one book element that is the child of the bookstore element
/bookstore/book[position()<3] 	# Selects the first two book elements that are children of the bookstore element
//title[@lang] 	# Selects all the title elements that have an attribute named lang

Selecting Unknown Nodes

/bookstore/* 	# Selects all the child nodes of the bookstore element
//* 	# Selects all elements in the document
//title[@*] 	# Selects all title elements which have any attribute

Selecting Several Paths

//book/title | //book/price  	# Selects all the title AND price elements of all book elements

This code is used to remove a message from the whiteboard queue containing a shape which has recently moved it's position (x and y coordinates) on the board. Code extract from whiteboardqueue.rb

  def call_delete_shape(params)
    #get the shape's message id.
    doc = Document.new(params)
    shape_id = doc.root.elements["param[@var='id']"].text.to_s
    initialize_doc

    doc_xml = Document.new(decode2(@doc_file.root.to_s))
    id = doc_xml.root.elements["records/message/body/*[@id='#{shape_id}']"].parent.parent.attributes.get_attribute('id')
    delete_record(id)
    save_file
  end

*update 10:04pm*
Added the link to the project code http://github.com/jrobertson/whiteboardqueue/tree

Following on from the post A simple XSLT example [dzone.com], this code lists all files which have the type 'rb' (equivalent to ls *.rb).

file: dir.xsl

<?xml version="1.0"?>

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
      
    <xsl:template match="dir">
    <div id="articles">
      <ul>
      <xsl:apply-templates select="records/file[@type='rb']"/>
      </ul>
    </div>
    </xsl:template>
    
    <xsl:template match="records/file[@type='rb']">
      <li><xsl:value-of select="."/></li>
    </xsl:template>
    
</xsl:stylesheet>

output:

<div id='articles'>
  <ul>
    <li>projxmlhelper.rb</li>
    <li>feedpopulated.rb</li>
    <li>squrl_handler.rb</li>
    <li>password_handler.rb</li>
    <li>category.rb</li>
    <li>gwd.rb</li>
  </ul>
</div>

*update 11:58am 26-Feb*
Here's a filter I will be using in my projects

<?xml version="1.0"?>

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
      
    <xsl:template match="dir">
    <div id="articles">
      <ul>
      <xsl:apply-templates select="records/file"/>
      </ul>
    </div>
    </xsl:template>
    
    <xsl:template match="records/file">
      <xsl:if test="@type=$type">
        <li><xsl:value-of select="."/></li>
      </xsl:if>
    </xsl:template>
    
</xsl:stylesheet>

The following JavaScript code uses XPath to select a specific element. Note: This code works for Firefox, but Internet Explorer has it's own implementation of XPath. see Introduction to using XPath in JavaScript [mozilla.org]

doc (xml document)

<codes>
  <codex value='bQ' index='Q'/>
  <codex value='S' index='R'/>
  <codex value='PU' index='S'/>
  <codex value='ji' index='T'/>
  <codex value='0' index='U'/>
  <codex value='33' index='V'/>
  <codex value='A' index='W'/>
  <codex value='tO' index='X'/>
  <codex value='fW' index='Y'/>
  <codex value='P' index='Z'/>
  <codex value='4h' index='a'/>
  <codex value='B' index='b'/>
  <codex value='m' index='c'/>
  <codex value='qf' index='d'/>
  <codex value='uJ' index='e'/>
</codes>

Assuming the doc object below contains the XML data from above.

var nodesSnapshot = doc.evaluate("codes/codex[@index='a']", doc, null, XPathResult.
  UNORDERED_NODE_SNAPSHOT_TYPE, null );
node = nodesSnapshot.snapshotItem(0);
msg = "The secret code for '" + node.getAttribute('index') + "' is " + node.getAttribute('value');
alert(msg);

output (value from the alert box)

"The secret code for 'a' is 4h"

see also: Reading an XML file using JavaScript