HTML::Pipeline Build Status

GitHub HTML processing filters and utilities. This module includes a small framework for defining DOM based content filters and applying them to user provided content. Read an introduction about this project in this blog post.


Add this line to your application's Gemfile:

gem 'html-pipeline'

And then execute:

$ bundle

Or install it yourself as:

$ gem install html-pipeline


This library provides a handful of chainable HTML filters to transform user content into markup. A filter takes an HTML string or Nokogiri::HTML::DocumentFragment, optionally manipulates it, and then outputs the result.

For example, to transform Markdown source into Markdown HTML:

require 'html/pipeline'

filter ="Hi **world**!")

Filters can be combined into a pipeline which causes each filter to hand its output to the next filter's input. So if you wanted to have content be filtered through Markdown and be syntax highlighted, you can create the following pipeline:

pipeline = [
result = <<-CODE
This is *great*:




<p>This is <em>great</em>:</p>

<div class="highlight">
<pre><span class="n">some_code</span><span class="p">(</span><span class="ss">:first</span><span class="p">)</span>

To generate CSS for HTML formatted code, use the pygments.rb #css method. pygments.rb is a dependency of the SyntaxHighlightFilter.

Some filters take an optional context and/or result hash. These are used to pass around arguments and metadata between filters in a pipeline. For example, if you don't want to use GitHub formatted Markdown, you can pass an option in the context hash:

filter ="Hi **world**!", :gfm => false)


We define different pipelines for different parts of our app. Here are a few paraphrased snippets to get you started:

# The context hash is how you pass options between different filters.
# See individual filter source for explanation of options.
context = {
  :asset_root => "",
  :base_url   => ""

# Pipeline providing sanitization and image hijacking but no mention
# related features.
SimplePipeline = [
  TableOfContentsFilter, # add 'name' anchors to all headers and generate toc list
], context

# Pipeline used for user provided content on the web
MarkdownPipeline = [
], context.merge(:gfm => true) # enable github formatted markdown

# Define a pipeline based on another pipeline's filters
NonGFMMarkdownPipeline =,
  context.merge(:gfm => false))

# Pipelines aren't limited to the web. You can use them for email
# processing also.
HtmlEmailPipeline = [
], {}

# Just emoji.
EmojiPipeline = [
], context


  • MentionFilter - replace @user mentions with links
  • AbsoluteSourceFilter - replace relative image urls with fully qualified versions
  • AutolinkFilter - auto_linking urls in HTML
  • CamoFilter - replace http image urls with camo-fied https versions
  • EmailReplyFilter - util filter for working with emails
  • EmojiFilter - everyone loves emoji!
  • HttpsFilter - HTML Filter for replacing http github urls with https versions.
  • ImageMaxWidthFilter - link to full size image for large images
  • MarkdownFilter - convert markdown to html
  • PlainTextInputFilter - html escape text and wrap the result in a div
  • SanitizationFilter - whitelist sanitize user markup
  • SyntaxHighlightFilter - code syntax highlighter
  • TextileFilter - convert textile to html
  • TableOfContentsFilter - anchor headings with name attributes and generate Table of Contents html unordered list linking headings


Filter gem dependencies are not bundled; you must bundle the filter's gem dependencies. The below list details filters with dependencies. For example, SyntaxHighlightFilter uses github-linguist to detect and highlight languages. For example, to use the SyntaxHighlightFilter, add the following to your Gemfile:

gem 'github-linguist'
  • AutolinkFilter - rinku
  • EmailReplyFilter - escape_utils, email_reply_parser
  • EmojiFilter - gemoji
  • MarkdownFilter - github-markdown
  • PlainTextInputFilter - escape_utils
  • SanitizationFilter - sanitize
  • SyntaxHighlightFilter - github-linguist
  • TextileFilter - RedCloth

Note: See Gemfile :test block for version requirements.


Full reference documentation can be found here.


To write a custom filter, you need a class with a call method that inherits from HTML::Pipeline::Filter.

For example this filter adds a base url to images that are root relative:

require 'uri'

class RootRelativeFilter < HTML::Pipeline::Filter

  def call"img").each do |img|
      next if img['src'].nil?
      src = img['src'].strip
      if src.start_with? '/'
        img["src"] = URI.join(context[:base_url], src).to_s


Now this filter can be used in a pipeline: [ RootRelativeFilter ], { :base_url => '' }

3rd Party Extensions

If you have an idea for a filter, propose it as an issue first. This allows us discuss whether the filter is a common enough use case to belong in this gem, or should be built as an external gem.


Filters and Pipelines can be set up to be instrumented when called. The pipeline must be setup with an ActiveSupport::Notifications compatible service object and a name. New pipeline objects will default to the HTML::Pipeline.default_instrumentation_service object.

# the AS::Notifications-compatible service object
service = ActiveSupport::Notifications

# instrument a specific pipeline
pipeline = [MarkdownFilter], context
pipeline.setup_instrumentation "MarkdownPipeline", service

# or set default instrumentation service for all new pipelines
HTML::Pipeline.default_instrumentation_service = service
pipeline = [MarkdownFilter], context
pipeline.setup_instrumentation "MarkdownPipeline"

Filters are instrumented when they are run through the pipeline. A call_filter.html_pipeline event is published once the filter finishes. The payload should include the filter name. Each filter will trigger its own instrumentation call.

service.subscribe "call_filter.html_pipeline" do |event, start, ending, transaction_id, payload|
  payload[:pipeline] #=> "MarkdownPipeline", set with `setup_instrumentation`
  payload[:filter] #=> "MarkdownFilter"
  payload[:context] #=> context Hash
  payload[:result] #=> instance of result class
  payload[:result][:output] #=> output HTML String or Nokogiri::DocumentFragment

The full pipeline is also instrumented:

service.subscribe "call_pipeline.html_pipeline" do |event, start, ending, transaction_id, payload|
  payload[:pipeline] #=> "MarkdownPipeline", set with `setup_instrumentation`
  payload[:filters] #=> ["MarkdownFilter"]
  payload[:doc] #=> HTML String or Nokogiri::DocumentFragment
  payload[:context] #=> context Hash
  payload[:result] #=> instance of result class
  payload[:result][:output] #=> output HTML String or Nokogiri::DocumentFragment


Please review the Contributing Guide.

  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Added some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request

To see what has changed in recent versions, see the CHANGELOG.


Thanks to all of these contributors.

Project is a member of the OSS Manifesto.

Releasing A New Version

This section is for gem maintainers to cut a new version of the gem.

  • update lib/html/pipeline/version.rb to next version number X.X.X following semver.
  • update Get latest changes with git log --oneline vLAST_RELEASE..HEAD | grep Merge
  • on the master branch, run script/release