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Nokogumbo - a Nokogiri interface to the Gumbo HTML5 parser.

NOTICE: End of life

Nokogumbo has been merged into Nokogiri v1.12.0. Please update to Nokogiri v1.12.0 or later, and remove Nokogumbo as an explicit dependency.

Please note that the final release of Nokogumbo, v2.0.5 (2021-03-19), will not support Ruby 3.2.0 and later. For Ruby 3.2 support, please upgrade to Nokogiri v1.12.0 or later and remove Nokogumbo as an explicit dependency.


Nokogumbo provides the ability for a Ruby program to invoke our version of the Gumbo HTML5 parser and to access the result as a Nokogiri::HTML::Document.

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require 'nokogumbo'
doc = Nokogiri.HTML5(string)

To parse an HTML fragment, a fragment method is provided.

require 'nokogumbo'
doc = Nokogiri::HTML5.fragment(string)

Because HTML is often fetched via the web, a convenience interface to HTTP get is also provided:

require 'nokogumbo'
doc = Nokogiri::HTML5.get(uri)

Parsing options

The document and fragment parsing methods,

  • Nokogiri.HTML5(html, url = nil, encoding = nil, options = {})
  • Nokogiri::HTML5.parse(html, url = nil, encoding = nil, options = {})
  • Nokogiri::HTML5::Document.parse(html, url = nil, encoding = nil, options = {})
  • Nokogiri::HTML5.fragment(html, encoding = nil, options = {})
  • Nokogiri::HTML5::DocumentFragment.parse(html, encoding = nil, options = {}) support options that are different from Nokogiri's.

The three currently supported options are :max_errors, :max_tree_depth and :max_attributes, described below.

Error reporting

Nokogumbo contains an experimental parse error reporting facility. By default, no parse errors are reported but this can be configured by passing the :max_errors option to ::parse or ::fragment.

require 'nokogumbo'
doc = Nokogiri::HTML5.parse('<span/>Hi there!</span foo=bar />', max_errors: 10)
doc.errors.each do |err|

This prints the following.

1:1: ERROR: Expected a doctype token
<span/>Hi there!</span foo=bar />
1:1: ERROR: Start tag of nonvoid HTML element ends with '/>', use '>'.
<span/>Hi there!</span foo=bar />
1:17: ERROR: End tag ends with '/>', use '>'.
<span/>Hi there!</span foo=bar />
1:17: ERROR: End tag contains attributes.
<span/>Hi there!</span foo=bar />

Using max_errors: -1 results in an unlimited number of errors being returned.

The errors returned by #errors are instances of Nokogiri::XML::SyntaxError.

The HTML standard defines a number of standard parse error codes. These error codes only cover the "tokenization" stage of parsing HTML. The parse errors in the "tree construction" stage do not have standardized error codes (yet).

As a convenience to Nokogumbo users, the defined error codes are available via the Nokogiri::XML::SyntaxError#str1 method.

require 'nokogumbo'
doc = Nokogiri::HTML5.parse('<span/>Hi there!</span foo=bar />', max_errors: 10)
doc.errors.each do |err|
  puts("#{err.line}:#{err.column}: #{err.str1}")

This prints the following.

1:1: generic-parser
1:1: non-void-html-element-start-tag-with-trailing-solidus
1:17: end-tag-with-trailing-solidus
1:17: end-tag-with-attributes

Note that the first error is generic-parser because it's an error from the tree construction stage and doesn't have a standardized error code.

For the purposes of semantic versioning, the error messages, error locations, and error codes are not part of Nokogumbo's public API. That is, these are subject to change without Nokogumbo's major version number changing. These may be stabilized in the future.

Maximum tree depth

The maximum depth of the DOM tree parsed by the various parsing methods is configurable by the :max_tree_depth option. If the depth of the tree would exceed this limit, then an ArgumentError is thrown.

This limit (which defaults to Nokogumbo::DEFAULT_MAX_TREE_DEPTH = 400) can be removed by giving the option max_tree_depth: -1.

html = '<!DOCTYPE html>' + '<div>' * 1000
doc = Nokogiri.HTML5(html)
# raises ArgumentError: Document tree depth limit exceeded
doc = Nokogiri.HTML5(html, max_tree_depth: -1)

Attribute limit per element

The maximum number of attributes per DOM element is configurable by the :max_attributes option. If a given element would exceed this limit, then an ArgumentError is thrown.

This limit (which defaults to Nokogumbo::DEFAULT_MAX_ATTRIBUTES = 400) can be removed by giving the option max_attributes: -1.

html = '<!DOCTYPE html><div ' + (1..1000).map { |x| "attr-#{x}" }.join(' ') + '>'
# "<!DOCTYPE html><div attr-1 attr-2 attr-3 ... attr-1000>"
doc = Nokogiri.HTML5(html)
# raises ArgumentError: Attributes per element limit exceeded
doc = Nokogiri.HTML5(html, max_attributes: -1)

HTML Serialization

After parsing HTML, it may be serialized using any of the Nokogiri serialization methods. In particular, #serialize, #to_html, and #to_s will serialize a given node and its children. (This is the equivalent of JavaScript's Element.outerHTML.) Similarly, #inner_html will serialize the children of a given node. (This is the equivalent of JavaScript's Element.innerHTML.)

doc = Nokogiri::HTML5("<!DOCTYPE html><span>Hello world!</span>")
puts doc.serialize
# Prints: <!DOCTYPE html><html><head></head><body><span>Hello world!</span></body></html>

Due to quirks in how HTML is parsed and serialized, it's possible for a DOM tree to be serialized and then re-parsed, resulting in a different DOM. Mostly, this happens with DOMs produced from invalid HTML. Unfortunately, even valid HTML may not survive serialization and re-parsing.

In particular, a newline at the start of pre, listing, and textarea elements is ignored by the parser.

doc = Nokogiri::HTML5(<<-EOF)
<!DOCTYPE html>
# Prints: <pre>Content</pre>

In this case, the original HTML is semantically equivalent to the serialized version. If the pre, listing, or textarea content starts with two newlines, the first newline will be stripped on the first parse and the second newline will be stripped on the second, leading to semantically different DOMs. Passing the parameter preserve_newline: true will cause two or more newlines to be preserved. (A single leading newline will still be removed.)

doc = Nokogiri::HTML5(<<-EOF)
<!DOCTYPE html>

puts'/html/body/listing').serialize(preserve_newline: true)
# Prints: <listing>
# Content</listing>


Nokogumbo always parses HTML using UTF-8; however, the encoding of the input can be explicitly selected via the optional encoding parameter. This is most useful when the input comes not from a string but from an IO object.

When serializing a document or node, the encoding of the output string can be specified via the :encoding options. Characters that cannot be encoded in the selected encoding will be encoded as HTML numeric entities.

frag = Nokogiri::HTML5.fragment('<span>아는 길도 물어가라</span>')
html = frag.serialize(encoding: 'US-ASCII')
puts html
# Prints: <span>&#xc544;&#xb294; &#xae38;&#xb3c4; &#xbb3c;&#xc5b4;&#xac00;&#xb77c;</span>
frag = Nokogiri::HTML5.fragment(html)
puts frag.serialize
# Prints: <span>아는 길도 물어가라</span>

(There's a bug in all current versions of Ruby that can cause the entity encoding to fail. Of the mandated supported encodings for HTML, the only encoding I'm aware of that has this bug is 'ISO-2022-JP'. I recommend avoiding this encoding.)


require 'nokogumbo'
puts Nokogiri::HTML5.get('').search('ol li')[2].text


  • The Nokogiri::HTML5.fragment function takes a string and parses it as a HTML5 document. The <html>, <head>, and <body> elements are removed from this document, and any children of these elements that remain are returned as a Nokogiri::HTML::DocumentFragment.

  • The Nokogiri::HTML5.parse function takes a string and passes it to the gumbo_parse_with_options method, using the default options. The resulting Gumbo parse tree is then walked.

    • If the necessary Nokogiri and libxml2 headers can be found at installation time then an xmlDoc tree is produced and a single Nokogiri Ruby object is constructed to wrap the xmlDoc structure. Nokogiri only produces Ruby objects as necessary, so all searching is done using the underlying libxml2 libraries.
    • If the necessary headers are not present at installation time, then Nokogiri Ruby objects are created for each Gumbo node. Other than memory usage and CPU time, the results should be equivalent.
  • The Nokogiri::HTML5.get function takes care of following redirects, https, and determining the character encoding of the result, based on the rules defined in the HTML5 specification for doing so.

  • Instead of uppercase element names, lowercase element names are produced.

  • Instead of returning unknown as the element name for unknown tags, the original tag name is returned verbatim.

Flavors of Nokogumbo

Nokogumbo uses libxml2, the XML library underlying Nokogiri, to speed up parsing. If the libxml2 headers are not available, then Nokogumbo resorts to using Nokogiri's Ruby API to construct the DOM tree.

Nokogiri can be configured to either use the system library version of libxml2 or use a bundled version. By default (as of Nokogiri version 1.8.4), Nokogiri will use a bundled version.

To prevent differences between versions of libxml2, Nokogumbo will only use libxml2 if the build process can find the exact same version used by Nokogiri. This leads to three possibilities

  1. Nokogiri is compiled with the bundled libxml2. In this case, Nokogumbo will (by default) use the same version of libxml2.
  2. Nokogiri is compiled with the system libxml2. In this case, if the libxml2 headers are available, then Nokogumbo will (by default) use the system version and headers.
  3. Nokogiri is compiled with the system libxml2 but its headers aren't available at build time for Nokogumbo. In this case, Nokogumbo will use the slower Ruby API.

Using libxml2 can be required by passing -- --with-libxml2 to bundle exec rake or to gem install. Using libxml2 can be prohibited by instead passing -- --without-libxml2.

Functionally, the only difference between using libxml2 or not is in the behavior of Nokogiri::XML::Node#line. If it is used, then #line will return the line number of the corresponding node. Otherwise, it will return 0.


git clone
cd nokogumbo
bundle install
rake gem
gem install pkg/nokogumbo*.gem

Related efforts

  • ruby-gumbo -- a ruby binding for the Gumbo HTML5 parser.
  • lua-gumbo -- a lua binding for the Gumbo HTML5 parser.