Can I use Nokogiri to validate XML or HTML documents?

Yes, Nokogiri is a popular Ruby library that can be used for parsing and working with XML and HTML documents. In addition to its parsing capabilities, Nokogiri can be used to validate XML or XHTML documents against a Document Type Definition (DTD), XML Schema Definition (XSD), or RelaxNG schema. However, Nokogiri does not validate HTML documents against a schema.

Validating XML with Nokogiri:

To validate an XML document using Nokogiri, you will need to have a DTD, XSD, or RelaxNG schema file that defines the rules for the document structure. Here's how you can perform validation in Ruby with Nokogiri:

Using a DTD:

require 'nokogiri'

# Parse the XML document
xml = Nokogiri::XML(File.read('document.xml'))

# Validate the document against the DTD
dtd = Nokogiri::XML::DTD.new(File.read('mydtd.dtd'))
puts xml.validate(dtd)

Using an XSD:

require 'nokogiri'

# Parse the XML document
xml = Nokogiri::XML(File.read('document.xml'))

# Parse the XSD schema
xsd = Nokogiri::XML::Schema(File.read('myschema.xsd'))

# Validate the document against the XSD schema
errors = xsd.validate(xml)
errors.each do |error|
  puts error.message
end

Using RelaxNG:

require 'nokogiri'

# Parse the XML document
xml = Nokogiri::XML(File.read('document.xml'))

# Parse the RelaxNG schema
rng = Nokogiri::XML::RelaxNG(File.read('myschema.rng'))

# Validate the document against the RelaxNG schema
errors = rng.validate(xml)
errors.each do |error|
  puts error.message
end

Validating HTML with Nokogiri:

While Nokogiri does an excellent job of parsing HTML, including fixing malformed markup, it does not have built-in support for validating HTML against a schema like DTD or WHATWG HTML standards. However, you can use Nokogiri to ensure that the HTML is well-formed by parsing it and checking for any parsing errors.

require 'nokogiri'

# Parse the HTML document
html = Nokogiri::HTML(File.read('document.html'))

# Check for parsing errors
if html.errors.empty?
  puts 'HTML is well-formed.'
else
  html.errors.each do |error|
    puts error.message
  end
end

For full HTML validation, you would need to use other tools like the W3C Markup Validation Service, which you can use either through its web interface or through its API for automated validation.

Keep in mind that Nokogiri's validation features are for XML documents and not intended for HTML document validation against a schema. For HTML5, there is no official schema like DTD or XSD to validate against, which is why tools like the W3C validator use heuristics and custom implementations to check the validity of HTML5 documents.