This section provides an overview of what Nokogiri is, and why a developer might want to use it.
It should also mention any large subjects within Nokogiri, and link out to the related topics. Since the Documentation for Nokogiri is new, you may need to create initial versions of those related topics.
How to check for parsing errors
Nokogiri is somewhat like a browser, in that it will attempt to provide something useful even if the incoming HTML or XML is malformed. Unfortunately it usually does it silently, but we can ask for a list of the errors using
require 'nokogiri' doc = Nokogiri::XML('<node><foo/>') doc.errors # => [#<Nokogiri::XML::SyntaxError: 1:13: FATAL: Premature end of data in tag node line 1>]
whereas, the correct XML would result in no errors:
doc = Nokogiri::XML('<node><foo/></node>') doc.errors # => 
This also applies to parsing HTML, but, because HTML is a relaxed form of XML, Nokogiri will pass over missing end-nodes often and will only report malformed nodes and more pathological errors:
doc = Nokogiri::HTML('<html><body>') doc.errors # =>  doc = Nokogiri::HTML('<html><body><p') doc.errors # => [#<Nokogiri::XML::SyntaxError: 1:15: ERROR: Couldn't find end of Start Tag p>]
If, after parsing, you can't find an node that you can see in your editor, this could be the cause of the problem. Sometimes it helps to pass the HTML through a formatter, and see if the nesting helps reveal the problem.
And, because Nokogiri tries to fix the problem but sometimes can't do it correctly, because it can be a very difficult thing for software to do, we'll have to pre-process the file and touch up lines prior to handing it off to Nokogiri. How to do that depends on the file and the problem. It can vary from simply finding a node and adding a trailing
>, to removing embedded malformed markup that was injected by a bad scraping routine, so it's up to the programmer how best to intercede.
How to extract text from a node or nodes
How to correctly extract text from nodes is one of the most popular questions we see, and almost invariably is made more difficult by misusing Nokogiri's "searching" methods.
Nokogiri supports using CSS and XPath selectors. These are equivalent:
doc.at('p').text # => "foo" doc.at('//p').text # => "foo" doc.search('p').size # => 2 doc.search('//p').size # => 2
The CSS selectors are extended with many of jQuery's CSS extensions for convenience.
search are generic versions of
at_xpath along with
xpath. Nokogiri makes an attempt to determine whether a CSS or XPath selector is being passed in. It's possible to create a selector that fools
search so occasionally it will misunderstand, which is why we have the more specific versions of the methods. In general I use the generic versions almost always, and only use the specific version if I think Nokogiri will misunderstand. This practice falls under the first entry in "Three Virtues".
If you are searching for one specific node and want its text, then use
at or one of its
require 'nokogiri' doc = Nokogiri::HTML(<<EOT) <html> <body> <p>foo</p> <p>bar</p> </body> </html> EOT doc.at('p').text # => "foo"
at is equivalent to
search(...).first, so you could use the longer-to-type version, but why?
If the text being extracted is concatenated after using
xpath then add
map(&:text) instead of simply using
require 'nokogiri' doc = Nokogiri::HTML(<<EOT) <html> <body> <p>foo</p> <p>bar</p> </body> </html> EOT doc.search('p').text # => "foobar" doc.search('p').map(&:text) # => ["foo", "bar"]
How to parse HTML or XML into a Nokogiri XML or HTML document
There isn't much to add to Nokogiri's "Parsing an HTML/XML Document" tutorial, which is an easy introduction to the subject, so start there, then return to this page to help fill in some gaps.
Nokogiri's basic parsing attempts to clean up a malformed document, sometimes adding missing closing tags, and will add some additional tags to make it correct.
This is an example of telling Nokogiri that the document being parsed is a complete HTML file, and Nokogiri discovering it isn't:
require 'nokogiri' doc = Nokogiri::HTML('<body></body>') puts doc.to_html
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd"> <html><body></body></html>
Notice that the DTD statement was added, along with a wrapping
If we want to avoid this we can parse the document as a DocumentFragment:
require 'nokogiri' doc = Nokogiri::HTML.fragment('<body></body>') puts doc.to_html
which now outputs only what was actually passed in:
There is an XML variant also:
require 'nokogiri' doc = Nokogiri::XML('<node />') puts doc.to_xml
<?xml version="1.0"?> <node/>
doc = Nokogiri::XML.fragment('<node />') puts doc.to_xml
A more verbose variation of
fragment is to use
DocumentFragment.parse, so sometimes you'll see it written that way.
Occasionally, Nokogiri will have to do some fix-ups to try to make sense of the document:
doc = Nokogiri::XML::DocumentFragment.parse('<node ><foo/>') puts doc.to_xml
With the modified code now being:
<node> <foo/> </node>
The same can happen with HTML.
Sometimes the document is mangled beyond Nokogiri's ability to fix it, but it will try anyway, resulting in a document that has a changed hierarchy. Nokogiri won't raise an exception, but it does provide a way to check for errors and the actions it took. See "How to check for parsing errors" for more information.
See the Nokogiri::XML::ParseOptions documentation for various options used when parsing.
Installation or Setup
Detailed instructions on getting Nokogiri set up or installed.