Searching an HTML / XML Document

Basic Searching

Let’s suppose you have the following document:

[shows.xml]
<root>
  <sitcoms>
    <sitcom>
      <name>Married with Children</name>
      <characters>
        <character>Al Bundy</character>
        <character>Bud Bundy</character>
        <character>Marcy Darcy</character>
      </characters>
    </sitcom>
    <sitcom>
      <name>Perfect Strangers</name>
      <characters>
        <character>Larry Appleton</character>
        <character>Balki Bartokomous</character>
      </characters>
    </sitcom>
  </sitcoms>
  <dramas>
    <drama>
      <name>The A-Team</name>
      <characters>
        <character>John "Hannibal" Smith</character>
        <character>Templeton "Face" Peck</character>
        <character>"B.A." Baracus</character>
        <character>"Howling Mad" Murdock</character>
      </characters>
    </drama>
  </dramas>
</root>

Let’s further suppose that you want a list of all the characters in all the shows in this document.

@doc = Nokogiri::XML(File.open("shows.xml"))

@doc.xpath("//character")
# => ["<character>Al Bundy</character>",
#    "<character>Bud Bundy</character>",
#    "<character>Marcy Darcy</character>",
#    "<character>Larry Appleton</character>",
#    "<character>Balki Bartokomous</character>",
#    "<character>John \"Hannibal\" Smith</character>",
#    "<character>Templeton \"Face\" Peck</character>",
#    "<character>\"B.A.\" Baracus</character>",
#    "<character>\"Howling Mad\" Murdock</character>"]

The Node methods xpath and css actually return a NodeSet, which acts very much like an array, and contains matching nodes from the document.

characters[0].to_s # => "<character>Al Bundy</character>"

You can use any XPath or CSS query you like (see the chapter on XPath and CSS syntax for more information).

@doc.xpath("//dramas//character")
# => ["<character>John \"Hannibal\" Smith</character>",
#    "<character>Templeton \"Face\" Peck</character>",
#    "<character>\"B.A.\" Baracus</character>",
#    "<character>\"Howling Mad\" Murdock</character>"]

Notably, you can even use CSS queries in an XML document!

characters = @doc.css("sitcoms name") # => ["<name>Married with Children</name>", "<name>Perfect Strangers</name>"]

CSS queries are often the easiest and most succinct way to express what you’re looking for, so don’t be afraid to use them!

Single Results

If you know you’re going to get only a single result back, you can use the shortcuts at_css and at_xpath instead of having to access the first element of a NodeSet.

@doc.css("dramas name").first # => "<name>The A-Team</name>"
@doc.at_css("dramas name")    # => "<name>The A-Team</name>"

Namespaces

Just like our Ruby code, XML can suffer from name collisions. For example, an autoparts dealer can sell tires and so can a bike dealer. Both of them may use a “tire” tag to describe the tires they sell. However, we need to be able to tell the difference between a car tire and a bike tire. This is where namespaces come to the rescue.

Namespaces associate tags with a unique URL. Let’s take a look at the autoparts store’s XML versus the bike stores:

[parts.xml]
<parts>
  <!-- Alice's Auto Parts Store -->
  <inventory xmlns="http://alicesautoparts.com/">
    <tire>all weather</tire>
    <tire>studded</tire>
    <tire>extra wide</tire>
  </inventory>

  <!-- Bob's Bike Shop -->
  <inventory xmlns="http://bobsbikes.com/">
    <tire>street</tire>
    <tire>mountain</tire>
  </inventory>
</parts>

Since the URLs are unique, we can associate our query with a URL and get only the tires belonging to that url:

@doc = Nokogiri::XML(File.read("parts.xml"))

car_tires = @doc.xpath('//car:tire', 'car' => 'http://alicesautoparts.com/')
# => ["<tire>all weather</tire>",
#    "<tire>studded</tire>",
#    "<tire>extra wide</tire>"]

bike_tires = @doc.xpath('//bike:tire', 'bike' => 'http://bobsbikes.com/')
# => ["<tire>street</tire>", "<tire>mountain</tire>"]

To make this namespace registration a bit easier, nokogiri will automatically register any namespaces it finds on the root node for you. Nokogiri will associate the name in the declaration with the supplied URL. If we stick to this naming convention, we can shorten up our code.

Let’s take this atom feed for example:

[atom.xml]
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">

  <title>Example Feed</title>
  <link href="http://example.org/"/>
  <updated>2003-12-13T18:30:02Z</updated>
  <author>
    <name>John Doe</name>
  </author>
  <id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>

  <entry>
    <title>Atom-Powered Robots Run Amok</title>
    <link href="http://example.org/2003/12/13/atom03"/>
    <id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
    <updated>2003-12-13T18:30:02Z</updated>
    <summary>Some text.</summary>
  </entry>

</feed>

If we stick to the convention, we can grab all title tags like this

@doc.xpath('//xmlns:title') # => ["<title>Example Feed</title>", "<title>Atom-Powered Robots Run Amok</title>"]

Don’t be fooled though. You do not have to use XPath to get the benefits of namespaces. CSS selectors can be used as well. CSS just uses the pipe symbol to indicate a namespace search.

Let’s see the previous search rewritten to use CSS:

@doc.css('xmlns|title') # => ["<title>Example Feed</title>", "<title>Atom-Powered Robots Run Amok</title>"]

When using CSS, if the namespace is called “xmlns”, you can even omit the namespace name. That means your CSS will reduce to:

@doc.css('title') # => ["<title>Example Feed</title>", "<title>Atom-Powered Robots Run Amok</title>"]

Dealing with namespaces is a broad topic. If you need more examples, be sure to check out this blog post or send an email to the mailing list, and we can help out.