gem install builder

require 'builder'  
 
x = Builder::XmlMarkup.new(:target =>
 $stdout, :indent => 1) 
#":target =＞$stdout"参数：指示输出内
容将被写向标准输出控制台 
#":indent =＞1"参数：XML输出形式将被缩
进一个空格字符x.instruct! :xml,
:version =>'1.1',:encoding => 'gb2312' 
x.comment! "书本信息" 
 
x.library("shelf" => "Recent Acquisitions") { 
x.section("name" => "ruby"){ 
x.book("isbn" => "0672310001"){ 
x.title "Programming Ruby"  
x.author "Yukihiro " 
x.description "Programming Ruby - 
The Pragmatic Programmer's Guide" 
} 
} 
} 

< "1.1" encoding="gb2312"?> 
< !-- 书本信息 --> 
< library shelf="Recent Acquisitions"> 
< section name="ruby"> 
< book isbn="0672310001"> 
< title>Programming Ruby< /title> 
< author>Yukihiro < /author> 
< description>Programming Ruby - The 
Pragmatic Programmer's Guide
< /description> 
< /book> 
< /section> 
< /library> 
< inspect/> 
#< IO:0x2a06ae8> 

<library shelf="Recent Acquisitions"> 
  <section name="Ruby"> 
    <book isbn="0672328844"> 
    <title>The Ruby Way</title> 
    <author>Hal Fulton</author> 
    <description> 
      Second edition. The book you are now reading. 
      Ain't recursion grand"Space"> 
    <book isbn="0684835509"> 
      <title>The Case for Mars</title> 
      <author>Robert Zubrin</author> 
      <description>Pushing toward a second home for the human 
        race. 
      </description> 
    </book> 
    <book isbn="074325631X"> 
      <title>First Man: The Life of Neil A. Armstrong</title> 
      <author>James R. Hansen</author> 
      <description>Definitive biography of the first man on 
        the moon. 
      </description> 
    </book> 
  </section> 
</library>

require 'rexml/document' 
include REXML 
 
input = File.new("books.xml") 
doc = Document.new(input) 
 
root = doc.root 
puts root.attributes["shelf"]   # Recent Acquisitions 
 
doc.elements.each("library/section") { |e| puts e.attributes["name"] } 
# Output: 
#  Ruby 
#  Space 
 
doc.elements.each("*/section/book") { |e| puts e.attributes["isbn"] } 
# Output: 
#  0672328844 
#  0321445619 
#  0684835509 
#  074325631X 
 
sec2 = root.elements[2] 
author = sec2.elements[1].elements["author"].text    # Robert Zubrin 

require 'rexml/document' 
require 'rexml/streamlistener' 
include REXML 
 
class MyListener 
 include REXML::StreamListener 
 def tag_start(*args) 
  puts "tag_start: #{args.map {|x| x.inspect}.join(', ')}" 
 end 
 
 def text(data) 
  return if data =~ /^\w*$/   # whitespace only 
  abbrev = data[0..40] + (data.length > 40 "..." : "") 
  puts " text  :  #{abbrev.inspect}" 
 end 
end 
 
list = MyListener.new 
source = File.new "books.xml" 
Document.parse_stream(source, list) 

tag_start: "library", {"shelf"=>"Recent Acquisitions"} 
tag_start: "section", {"name"=>"Ruby"} 
tag_start: "book", {"isbn"=>"0672328844"} 
tag_start: "title", {} 
 text  :  "The Ruby Way" 
......................................... 

book1 = XPath.first(doc, "//book")  # Info for first book found 
p book1 
 
# Print out all titles 
XPath.each(doc, "//title") { |e| puts e.text } 
 
# Get an array of all of the "author" elements in the document. 
names = XPath.match(doc, "//author").map {|x| x.text } 
p names 

<book isbn='0672328844'> ... </> 
The Ruby Way 
The Case for Mars 
First Man: The Life of Neil A. Armstrong 
["Hal Fulton", "Robert Zubrin", "James R. Hansen"]