How can I parse an HTML file using Nokogiri?

Nokogiri is a Ruby gem that allows you to parse HTML as well as XML in Ruby. It's very powerful and provides a simple interface for navigating and manipulating the DOM of the document.

Here's a step-by-step guide on how to parse an HTML file using Nokogiri:

Install the Nokogiri Gem

Before you can use Nokogiri, you need to install the gem. You can do this using the following command:

   gem install nokogiri

Require Nokogiri in Your Ruby Script

At the top of your Ruby script, you'll need to require the Nokogiri library so that you can use it:

   require 'nokogiri'

Read the HTML File

You'll need to read the HTML content that you want to parse. This can be done by reading from a file, or by fetching content directly from the web.

   # If you have an HTML file, you can read it into a string
   html_content = File.open('path/to/your/htmlfile.html') { |f| f.read }
   # Or you can use a string with HTML content directly
   # html_content = "<html>...</html>"

Parse the HTML Content with Nokogiri

Once you have your HTML content in a string, you can parse it using Nokogiri's HTML parser:

   doc = Nokogiri::HTML(html_content)

Navigate and Manipulate the Document

Now that you have a Nokogiri::HTML::Document object, you can use Nokogiri's methods to navigate and manipulate the DOM.

Finding Elements by CSS Selectors

Use the css method to find elements using CSS selectors:

 titles = doc.css('h1, h2, h3') # Select all <h1>, <h2>, and <h3> tags
 titles.each do |title|
   puts title.content
 end

Finding Elements by XPath Selectors

Use the xpath method to find elements using XPath selectors:

 paragraphs = doc.xpath('//p') # Select all <p> tags
 paragraphs.each do |paragraph|
   puts paragraph.content
 end

Extracting Attributes

To get the value of an attribute from an element, you can use the [] method:

 links = doc.css('a')
 links.each do |link|
   puts link['href'] # Print the 'href' attribute
 end

Modifying Elements

You can also change the content of elements:

 doc.css('h1').each do |h1|
   h1.content = "Modified Title"
 end

Serialize the Document

After manipulating the document, you may want to convert it back to HTML:

   html_output = doc.to_html

Here's a complete example that combines all the steps:

require 'nokogiri'

# Read the HTML file into a string
html_content = File.open('path/to/your/htmlfile.html') { |f| f.read }

# Parse the HTML content with Nokogiri
doc = Nokogiri::HTML(html_content)

# Find and print titles
titles = doc.css('h1, h2, h3')
titles.each do |title|
  puts title.content
end

# Modify all <h1> tags
doc.css('h1').each do |h1|
  h1.content = "Modified Title"
end

# Serialize the document to HTML
html_output = doc.to_html

Remember to replace 'path/to/your/htmlfile.html' with the actual path to your HTML file. Nokogiri is a robust and flexible tool that can handle a wide variety of HTML parsing and manipulation tasks, making it a go-to library for many Ruby developers dealing with HTML data.

How can I parse an HTML file using Nokogiri?

Related Questions

What is XPath and how does Nokogiri support it?

How does Nokogiri handle XML namespaces when querying documents?

Can Nokogiri parse XML documents as well as HTML?

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