Parsing an HTML/XML Document

  1. From a String
  2. From a File
  3. From the Internets
  4. Parse Options
  5. Encoding

Parsing an HTML / XML Document

From a String

We’ve tried to make this easy on you. Really! We’re here to make your life easier.

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html_doc = Nokogiri::HTML("<html><body><h1>Mr. Belvedere Fan Club</h1></body></html>")
xml_doc  = Nokogiri::XML("<root><aliens><alien><name>Alf</name></alien></aliens></root>")

The variables html_doc and xml_doc are Nokogiri documents, which have all kinds of interesting properties and methods that you can read about here. We’ll cover the interesting bits in other chapters.

From a File

Note that you don’t need to read the file into a string variable. Nokogiri will do this for you.

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doc = File.open("blossom.xml") { |f| Nokogiri::XML(f) }

Clever Nokogiri! With the time you just saved, approach enlightenment by meditating on this koan.

From the Internets

I understand that there may be some HTML documents available on the World Wide Web.

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require 'open-uri'
doc = Nokogiri::HTML(open("http://www.threescompany.com/"))

Parse Options

Nokogiri offers quite a few options that affect how a document is parsed; you can read about them in the XML::ParseOptions docs.

Notably, Nokogiri will treat input as untrusted documents by default, thereby avoiding a class of vulnerabilities known as XXE or “XML eXternal Entity” processing. What this means is that Nokogiri won’t attempt to load external DTDs or access the network for any external resources.

Some commonly-used parse options are:

  • NONET - Prevent any network connections during parsing. Recommended for parsing untrusted documents. This is set by default!
  • RECOVER - Attempt to recover from errors. Recommended for parsing malformed or invalid documents. This is set by default!
  • NOBLANKS - Remove blank nodes
  • NOENT - Substitute entities
  • NOERROR - Suppress error reports
  • STRICT - Strict parsing; raise an error when parsing malformed documents
  • DTDLOAD and DTDVALID - If you want DTD validation
  • HUGE - use to skip hardcoded limits around document size or DOM depth; comes with a performance penalty

You could use them by handcrafting an artisanal bitmap (not recommended):

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doc = Nokogiri::XML(File.open("blossom.xml")) do |config|
  config.options = Nokogiri::XML::ParseOptions::STRICT | Nokogiri::XML::ParseOptions::NOBLANKS
end

But it’s more idiomatic to use the chainable shortcuts on the config object instead:

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doc = Nokogiri::XML(File.open("blossom.xml")) do |config|
  config.strict.noblanks
end

Notably, if you want to turn off an option that’s set by default, you can prefix a “no” to the config shortcut:

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doc = Nokogiri::XML(File.open("blossom.xml")) do |config|
  config.norecover
end

Leading to the perhaps-surprising (but logical!) nononet to turn networking back on:

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doc = Nokogiri::XML(File.open("blossom.xml")) do |config|
  config.nononet
end

Encoding

Strings are always stored as UTF-8 internally. Methods that return text values will always return UTF-8 encoded strings. Methods that return XML (like to_xml, to_html and inner_html) will return a string encoded like the source document.

WARNING

Some documents declare one particular encoding, but use a different one. So, which encoding should the parser choose?

Remember that data is just a stream of bytes. Only us humans add meaning to that stream. Any particular set of bytes could be valid characters in multiple encodings, so detecting encoding with 100% accuracy is not possible. libxml2 does its best, but it can’t be right 100% of the time.

If you want Nokogiri to handle the document encoding properly, your best bet is to explicitly set the encoding. Here is an example of explicitly setting the encoding to EUC-JP on the parser:

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doc = Nokogiri.XML('<foo><bar /><foo>', nil, 'EUC-JP')