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Cucumber Core is the inner hexagon for the Ruby flavour of Cucumber.

It contains the core domain logic to execute Cucumber features. It has no user interface, just a Ruby API. If you're interested in how Cucumber works, or in building other tools that work with Gherkin documents, you've come to the right place.

An overview

The entry-point is a single method on the module Cucumber::Core called #execute. Here's what it does:

  1. Parses the plain-text Gherkin documents into an AST
  2. Compiles the AST down to test cases
  3. Passes the test cases through any filters
  4. Executes the test cases, emitting events as it goes

We've introduced a number of concepts here, so let's go through them in detail.


The Abstract Syntax Tree or AST is an object graph that represents the Gherkin documents you've passed into the core. Things like Feature, Scenario and ExamplesTable.

These are immutable value objects.

Test cases

Your gherkin might contain scenarios, as well as examples from tables beneath a scenario outline.

Test cases represent the general case of both of these. We compile the AST down to instances of Cucumber::Core::Test::Case, each containing a number of instances of Cucumber::Core::Test::Step. It's these that are then filtered and executed.

Test cases and their test steps are also immutable value objects.


Once we have the test cases, and they've been activated by the mappings, you may want to pass them through a filter or two. Filters can be used to do things like activate, sort, replace or remove some of the test cases or their steps before they're executed.


Events are how you find out what is happening during your test run. As the test cases and steps are executed, the runner emits events to signal what's going on.

The following events are emitted during a run:

That's probably best illustrated with an example.


Here's an example of how you might use Cucumber::Core#execute

require 'cucumber/core'
require 'cucumber/core/filter'

# This is the most complex part of the example. The filter takes test cases as input,
# activates each step with an action block, then passes a new test case with those activated
# steps in it on to the next filter in the chain.
class ActivateSteps <
  def test_case(test_case)
    test_steps = do |step|



  def activate(step)
    case step.text
    when /fail/
      step.with_action { raise Failure }
    when /pass/
      step.with_action {}

# Create a Gherkin document to run
feature =, <<-GHERKIN)
    Given passing
    And failing
    And undefined

# Create a runner class that uses the Core's DSL
class MyRunner
  include Cucumber::Core

# Now execute the feature, using the filter we built, and subscribing to
# an event so we can print the output.[feature], []) do |events|
  events.on(:test_step_finished) do |event|
    test_step, result = event.test_step, event.result
    puts "#{test_step.text} #{result}"

If you run this little Ruby script, you should see the following output:

passing ✓
failing ✗
undefined ?

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