Parsing and processing XML in Ruby

So I have a little XML document, foobar.xml

<?xml version="1.0" standalone="no"?>
<!DOCTYPE foo SYSTEM "bar.dtd">
<title _FORMAT="XXX.XXX" _QUANTITY="1" _DEVICENAME="XXX" _JOBNAME="FOOBAR">
    <subtitle>
    <variable name="x">A-1234567</variable>
    </subtitle>
</title>

and a little Python,

    with open('foobar.xml', 'rt') as f:
        tree = ElementTree.parse(f)
    # Loop over all elements in 'tree' in section order
    for node in tree.getiterator():
        if node.tag == "variable":
            for z in range(number):
                if len(str(start)) == 7:
                    accession = "A-" + str(start)
                # This simply adds leading zeros if there are < 7 digits
                elif len(str(start)) < 7:
                    accession = "A-" + ("0" * (7 - len(str(start))) + str(start))
                start += 1
                # Assign 'accession' to the node text
                node.text = acces开发者_高级运维sion
                tree.write("foobar.xml")
        else:
            continue

that beautifully finds the node tag I'm interested in, and processes it by iterating over a range, each time replacing the node text and writing the XML to a file. There's only one problem: I need this to happen in Ruby.

So far I have

doc = Document.new(File.new("foobar.xml"))
doc.elements.each() do |element|
  element.elements.each() do |child|
    child.elements.each() do |sub| # probably wrong
      for z in 0...$number
        if $start.to_s.length == 7 
          accession = "A-" + $start.to_s
        else
          accession = "A-" + ("0" * (7 - $start.to_s.length)) + $start.to_s
        end
      $start += 1
      # need to assign here and write to file or assign to variable
      end
    end
  end
end

This is my first time processing XML in Ruby and I'm failing to really understand the syntax. My goal is essentially replicate the Python, by changing the node text for each iteration of the loop, and then writing that out to an XML file. Any suggestions are much appreciated.

I prefer Nokogiri for my XML parsing in Ruby; it's fast and efficient to use (and lets you use CSS-style selectors instead of XPath if you like):

require 'nokogiri'
$number = 3
$start  = 134341

my_xml = IO.read('foobar.xml')
doc = Nokogiri::XML(my_xml)
doc.css('variable').each do |el|
  $number.times do
    # Pads to a 7-digit number: see `ri Kernel#sprintf`
    el.content = "A-%07d" % $start
    File.open( "foobar-#{$start}.xml", 'w' ) do |f|
      f << doc
    end
    $start += 1
  end
end

I modified the above to write out a unique file; surely you don't write the same file again and again, right?

For completeness, here's an REXML-based solution:

require 'rexml/document'
$number = 3
$start  = 1312

doc = REXML::Document.new(my_xml)
REXML::XPath.each(doc,'//variable') do |el|
  $number.times do
    el.text = "A-%07d" % $start
    File.open( "f-#{$start}.xml", 'w' ){ |f| f << doc }
    $start += 1
  end
end