Gammo - A pure-Ruby HTML5 parser
Gammo provides a pure Ruby HTML5-compliant parser and CSS selector / XPath support for traversing the DOM tree built by Gammo. The implementation of the HTML5 parsing algorithm in Gammo conforms the WHATWG specification. Given an HTML string, Gammo parses it and builds DOM tree based on the tokenization and tree-construction algorithm defined in WHATWG parsing algorithm, these implementations are provided without any external dependencies.
Gammo, its naming is inspired by Gumbo. But Gammo is a fried tofu fritter made with vegetables.
require 'gammo'
require 'open-uri'
parser = URI.open('https://google.com') { |f| Gammo.new(f.read) }
document = parser.parse #=> #<Gammo::Node::Document>
puts document.css('title').first.inner_text #=> 'Google'- Overview
- Tokenizaton
- Parsing
- Node
- DOM Tree Traversal
- Performance
- References
- License
- Release History
Overview
Features
- Tokenization: Gammo has a tokenizer for implementing the tokenization algorithm.
- Parsing: Gammo provides a parser which implements the parsing algorithm by the above tokenization and the tree-construction algorithm.
- Node: Gammo provides the nodes which implement WHATWG DOM specification partially.
- DOM Tree Traversal: Gammo provides a way of DOM tree traversal (CSS selector / XPath).
- Performance: Gammo does not prioritize performance, and there are a few potential performance notes.
Tokenizaton
Gammo::Tokenizer implements the tokenization algorithm in WHATWG. You can get tokens in order by calling Gammo::Tokenizer#next_token.
Here is a simple example for performing only the tokenizer.
def dump_for(token)
puts "data: #{token.data}, class: #{token.class}"
end
tokenizer = Gammo::Tokenizer.new('<!doctype html><input type="button"><frameset>')
dump_for tokenizer.next_token #=> data: html, class: Gammo::Tokenizer::DoctypeToken
dump_for tokenizer.next_token #=> data: input, class: Gammo::Tokenizer::StartTagToken
dump_for tokenizer.next_token #=> data: frameset, class: Gammo::Tokenizer::StartTagToken
dump_for tokenizer.next_token #=> data: end of string, class: Gammo::Tokenizer::ErrorTokenThe parser described below depends on this tokenizer, it applies the WHATWG parsing algorithm to the tokens extracted by this tokenization in order.
Token types
The tokens generated by the tokenizer will be categorized into one of the following types:
| Token type | Description |
|---|---|
Gammo::Tokenizer::ErrorToken |
Represents an error token, it usually means end-of-string. |
Gammo::Tokenizer::TextToken |
Represents a text token like "foo" which is inner text of elements. |
Gammo::Tokenizer::StartTagToken |
Represents a start tag token like <a>. |
Gammo::Tokenizer::EndTagToken |
Represents an end tag token like </a>. |
Gammo::Tokenizer::SelfClosingTagToken |
Represents a self closing tag token like <img /> |
Gammo::Tokenizer::CommentToken |
Represents a comment token like <!-- comment -->. |
Gammo::Tokenizer::DoctypeToken |
Represents a doctype token like <!doctype html>. |
Parsing
Gammo::Parser implements processing in the tree-construction stage based on the tokenization described above.
A successfully parsed parser has the document accessor as the root document (this is the same as the return value of the Gammo::Parser#parse). From the document accessor, you can traverse the DOM tree constructed by the parser.
require 'gammo'
require 'pp'
document = Gammo.new('<!doctype html><input type="button">').parse
def dump_for(node, strm)
strm << node.to_h
return unless node && (child = node.first_child)
while child
dump_for(child, (strm.last[:children] ||= []))
child = child.next_sibling
end
strm
end
pp dump_for(document, [])Notes
Currently, it's not possible to traverse the DOM tree with css selector or xpath like Nokogiri. However, Gammo plans to implement these features in the future.
Node
The nodes generated by the parser will be categorized into one of the following types:
| Node type | Description |
|---|---|
Gammo::Node::Error |
Represents error node, it usually means end-of-string. |
Gammo::Node::Text |
Represents the text node like "foo" which is inner text of elements. |
Gammo::Node::Document |
Represents the root document type. It's always returned by Gammo::Parser#document. |
Gammo::Node::Element |
Represents any elements of HTML like <p>. |
Gammo::Node::Comment |
Represents comments like <!-- foo --> |
Gammo::Node::Doctype |
Represents doctype like <!doctype html> |
For some nodes such as Gammo::Node::Element and Gammo::Node::Document, they contain pointers to nodes that can be referenced by itself, such as Gammo::Node#next_sibling or Gammo::Node#first_child. In addition, APIs such as Gammo::Node#append_child and Gammo::Node#remove_child that perform operations defined in DOM living standard are also provided.
DOM Tree Traversal
CSS selector and XPath-1.0 are the way for traversing DOM tree built by Gammo.
XPath 1.0 (experimental)
Gammo has an original lexer/parser for XPath 1.0, it's provided as a helper in the DOM tree built by Gammo. Here is a simple example:
document = Gammo.new('<!doctype html><input type="button">').parse
node_set = document.xpath('//input[@type="button"]') #=> "<Gammo::XPath::NodeSet>"
node_set.length #=> 1
node_set.first #=> "<Gammo::Node::Element>"Since this is implemented by full scratch, Gammo is providing this support as a very experimental feature. Please file an issue if you find bugs.
Example
Before proceeding at the details of XPath support, let's have a look at a few simple examples. Given a sample HTML text and its DOM tree:
document = Gammo.new(<<-EOS).parse
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<h1>namusyaka.com</h1>
<p class="description">Here is a sample web site.</p>
<ul>
<li>hello</li>
<li>world</li>
</ul>
<ul id="links">
<li>Google <a href="https://google.com/">google.com</a></li>
<li>GitHub <a href="https://github.com/namusyaka">github.com/namusyaka</a></li>
</ul>
</body>
</html>
EOSThe following XPath expression gets all li elements and prints those text contents:
document.xpath('//li').each do |elm|
puts elm.inner_text
endThe following XPath expression gets all li elements under the ul element having the id=links attribute:
document.xpath('//ul[@id="links"]/li').each do |elm|
puts elm.inner_text
endThe following XPath expression gets each text node for each li element under the ul element having the id=links attribute:
document.xpath('//ul[@id="links"]/li/text()').each do |elm|
puts elm.data
endAxis Specifiers
In the combination with Gammo, the axis specifier indicates navigation direction within the DOM tree built by Gammo. Here is list of axes. As you can see, Gammo fully supports the all of axes.
| Full Syntax | Abbreviated Syntax | Supported | Notes |
|---|---|---|---|
ancestor |
yes | ||
ancestor-or-self |
yes | ||
attribute |
@ |
yes | @abc is the alias for attribute::abc |
child |
|
yes | abc is the short for child::abc |
descendant |
yes | ||
descendant-or-self |
// |
yes | // is the alias for /descendant-or-self::node()/ |
following |
yes | ||
following-sibling |
yes | ||
namespace |
yes | ||
parent |
.. |
yes | .. is the alias for parent::node() |
preceding |
yes | ||
preceding-sibling |
yes | ||
self |
. |
yes | . is the alias for self::node() |
Node Test
Node tests consist of specific node names or more general expressions. Although particular syntax like : should work for specifying namespace prefix in XPath, Gammo does not support it yet as it's not a core feature in HTML5.
| Full Syntax | Supported | Notes |
|---|---|---|
text() |
yes | Finds a node of type text, e.g. hello in <p>hello <a href="https://hello">world</a></p> |
comment() |
yes | Finds a node of type comment, e.g. <!-- comment --> |
node() |
yes | Finds any node at all. |
Also note that the processing-instruction is not supported. There is no plan to support it.
Operators
- The
/,//and[]are used in the path expression. - The union operator
|forms the union of two node sets. - The boolean operators:
and,or - The arithmetic operators:
+,-,*,divandmod - Comparison operators:
=,!=,<,>,<=,>=
Functions
XPath 1.0 defines four data types (nodeset, string, number, boolean) and there are various functions based on the types. Gammo supports those functions partially, please check it to be supported before using functions.
Node set functions
| Function Name | Supported | Specification |
|---|---|---|
last() |
yes | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-last |
position() |
yes | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-position |
count(node-set) |
yes | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-count |
String Functions
Boolean Functions
| Function Name | Supported | Specification |
|---|---|---|
boolean(object) |
yes | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-boolean |
not(object) |
yes | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-not |
true() |
yes | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-true |
false() |
yes | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-false |
lang() |
no | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-lang |
Number Functions
| Function Name | Supported | Specification |
|---|---|---|
number(object?) |
no | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-number |
sum(node-set) |
no | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-sum |
floor(number) |
no | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-floor |
ceiling(number) |
yes | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-ceiling |
round(number) |
no | https://www.w3.org/TR/1999/REC-xpath-19991116/#function-round |
CSS Selector (experimental)
Gammo has an original lexer/parser for CSS Selector, it's provided as a helper in the DOM tree built by Gammo. Here is a simple example:
document = Gammo.new('<!doctype html><input type="button">').parse
node_set = document.css('input[type="button"]') #=> "<Gammo::CSSSelector::NodeSet>"
node_set.length #=> 1
node_set.first #=> "<Gammo::Node::Element>"Since this is implemented by full scratch, Gammo is providing this support as a very experimental feature. Please file an issue if you find bugs.
Example
Before proceeding at the details of CSS Selector support, let's have a look at a few simple examples. Given a sample HTML text and its DOM tree:
document = Gammo.new(<<-EOS).parse
<!DOCTYPE html>
<html>
<head>
</head>
<body>
<h1>namusyaka.com</h1>
<p class="description">Here is a sample web site.</p>
<ul>
<li>hello</li>
<li>world</li>
</ul>
<ul id="links">
<li>Google <a href="https://google.com/">google.com</a></li>
<li>GitHub <a href="https://github.com/namusyaka">github.com/namusyaka</a></li>
</ul>
</body>
</html>
EOSThe following CSS selector gets all li elements and prints thoese text contents:
document.css('li').each do |elm|
puts elm.inner_text
endThe following CSS selector gets all li elements under the ul element having the id=links attribute:
document.xpath('ul#links li').each do |elm|
puts elm.inner_text
endGroups of selectors
Gammo supports groups of selectors, this means you can use , to traverse DOM tree by multiple selectors.
require 'gammo'
@doc = Gammo.new(<<-EOS).parse
<!DOCTYPE html>
<html>
<head>
<title>hello</title>
<meta charset="utf8">
</head>
<body>
<p id="hello">hello</p>
<p id="world">world</p>
EOS
@doc.css('#hello, #world').map(&:inner_text).join(' ') #=> 'hello world'Simple selectors
Type selector & Universal selector
Gammo supports the basic grammar of type selector and universal selector, but not namespaces.
Attribute selectors
See more details: 6.3. Attribute selectors
| Syntax | Supported |
|---|---|
[att] |
yes |
[att=val] |
yes |
[att~=val] |
yes |
[att|=val] |
yes |
Class selectors
Supported. See more details: 6.4. Class selectors
ID selectors
Supported. See more details: 6.5. ID selectors
Pseudo-classes
Partially supported. See the table below.
| Class name | Supported | Can support? |
|---|---|---|
:link |
no | no |
:visited |
no | no |
:hover |
no | no |
:active |
no | no |
:focus |
no | no |
:target |
no | no |
:lang |
no | yes |
:enabled |
yes | yes |
:disabled |
yes | yes |
:checked |
yes | yes |
:root |
yes | yes |
:nth-child |
yes | yes |
:nth-last-child |
no | yes |
:nth-of-type |
no | yes |
:nth-last-of-type |
no | yes |
:first-child |
no | yes |
:last-child |
no | yes |
:first-of-type |
no | yes |
:last-of-type |
no | yes |
:only-child |
no | yes |
:only-of-type |
no | yes |
:empty |
no | yes |
:not |
yes | yes |
Combinators
See more details: 8. Combinators
| Syntax | Supported | Desc |
|---|---|---|
h1 em |
yes | Descendant combinator |
h1 > em |
yes | Child combinator |
math + p |
yes | Next-sibling combinator |
h1 ~ pre |
yes | Subsequent-sibling combinator |
Performance
As mentioned in the features at the beginning, Gammo doesn't prioritize its performance. Thus, for example, Gammo is not suitable for very performance-sensitive applications (e.g. performing Gammo parsing synchronously from an incoming request from an end user). Instead, the goal is to work well with batch processing such as crawlers. Gammo places the highest priority on making it easy to parse HTML by peforming it without depending on native-extensions and external gems.
References
This was developed with reference to the following softwares.
- x/net/html: I've been working on this package, it gave me strong reason to make this happen.
- Blink: Blink gave me great impression about tree construction.
- html5lib-tests: Gammo relies on this test.
License
The gem is available as open source under the terms of the MIT License.