Question

Both puts Nokogiri::XML(xml) and puts Nokogiri.parse(xml) return the same XML.
And yes, they both return XML:

 > Nokogiri::XML(xml).class
 => Nokogiri::XML::Document  
 > Nokogiri.parse(xml).class
 => Nokogiri::XML::Document 

Yet when comparing the returned values, I get:

 > Nokogiri.parse(xml) === Nokogiri::XML(xml)
 => false 
 > Nokogiri.parse(xml) == Nokogiri::XML(xml)
 => false 

What is difference between Nokogiri::XML and Nokogiri.parse?

Answer 1

Nokogiri.parse just attempts to detect if the string passed to it is HTML, then creates/returns either a Nokogiri::XML or Nokogiri::HTML instance with a preset ParseOption:

def parse string, url = nil, encoding = nil, options = nil
  doc =
    if string.respond_to?(:read) ||
      string =~ /^\s*<[^Hh>]*html/i # Probably html
      Nokogiri.HTML(
        string,
        url,
        encoding, options || XML::ParseOptions::DEFAULT_HTML
      )
    else
      Nokogiri.XML(string, url, encoding,
                    options || XML::ParseOptions::DEFAULT_XML)
    end
  yield doc if block_given?
  doc
end

https://github.com/sparklemotion/nokogiri/blob/master/lib/nokogiri.rb#L66

The comparisons return false because == isn’t comparing the contents of the object, just that they’re different instances:

>> Nokogiri.parse(xml) == Nokogiri.parse(xml)
=> false