Cucumber Expressions

Cucumber Expressions is an alternative to Regular Expressions with a more intuitive syntax.

Try Cucumber Expressions in your browser.

Cucumber supports both Cucumber Expressions and Regular Expressions for defining Step Definitions, but you cannot mix Cucumber Expression syntax with Regular Expression syntax in the same expression.

On platforms that don't have a literal syntax for regular expressions (such as Java), Cucumber will create a Cucumber Expression by default. To use Regular Expressions, add anchors (starting with ^ and ending with $) or forward slashes (/). For more information, see Cucumber Expression - Java Heuristics.

Introduction

Let's write a Cucumber Expression that matches the following Gherkin step (the Given keyword has been removed here, as it's not part of the match).

I have 42 cucumbers in my belly

The simplest Cucumber Expression that matches that text would be the text itself, but we can also write a more generic expression, with an int output parameter:

I have {int} cucumbers in my belly

When the text is matched against that expression, the number 42 is extracted from the {int} output parameter and passed as an argument to the step definition.

The following text would not match the expression:

I have 42.5 cucumbers in my belly

This is because 42.5 has a decimal part, and doesn't fit into an int. Let's change the output parameter to float instead:

I have {float} cucumbers in my belly

Now the expression will match the text, and the float 42.5 is extracted.

Parameter types

Text between curly braces reference a parameter type. Cucumber comes with the following built-in parameter types:

Cucumber-JVM

The anonymous parameter type will be converted to the parameter type of the step definition using an object mapper. Cucumber comes with a built-in object mapper that can handle all numeric types as well as. Enum.

To automatically convert to other types it is recommended to install an object mapper. See configuration to learn how.

Custom Parameter types

Cucumber Expressions can be extended so they automatically convert output parameters to your own types. Consider this Cucumber Expression:

I have a {color} ball

If we want the {color} output parameter to be converted to a Color object, we can define a custom parameter type in Cucumber's configuration.

The table below explains the various arguments you can pass when defining a parameter type.

Java

@ParameterType("red|blue|yellow")  // regexp
public Color color(String color){  // type, name (from method)
    return new Color(color);       // transformer function
}

Kotlin

@ParameterType("red|blue|yellow")   // regexp
fun color(color: String): Color {   // name (from method), type
    return Color(color)             // transformer function
}                                    

Scala

ParameterType("color", "red|blue|yellow") { color: String => // name, regexp
    Color(color)                                             // transformer function, type
}                                    

JavaScript / TypeScript

import { defineParameterType } from '@cucumber/cucumber'

defineParameterType({
    name: 'color',
    regexp: /red|blue|yellow/,
    transformer: s => new Color(s)
})

The transformer function may return a Promise.

Ruby

ParameterType(
  name:        'color',
  regexp:      /red|blue|yellow/,
  type:        Color,
  transformer: ->(s) { Color.new(s) }
)

.NET / SpecFlow

[StepArgumentTransformation("(red|blue|yellow)")]
public Color ConvertColor(string colorValue)
{
    return new Color(colorValue);
}

Python

ParameterType(
  name=        'color',
  regexp=      "red|blue|yellow",
  type=  Color,
  transformer= lambda s: Color(),
)

Note: Currently the parameter name cannot be customized, so the custom parameters can only be used with the type name, e.g. {Color}.

Optional text

It's grammatically incorrect to say 1 cucumbers, so we should make the plural s optional. That can be done by surrounding the optional text with parentheses:

I have {int} cucumber(s) in my belly

That expression would match this text:

I have 1 cucumber in my belly

It would also match this text:

I have 42 cucumbers in my belly

In Regular Expressions, parentheses indicate a capture group, but in Cucumber Expressions they mean optional text.

Alternative text

Sometimes you want to relax your language, to make it flow better. For example:

I have {int} cucumber(s) in my belly/stomach

This would match either of those texts:

I have 1 cucumber in my belly
I have 1 cucumber in my stomach
I have 42 cucumbers in my stomach
I have 42 cucumbers in my belly

Alternative text only works when there is no whitespace between the alternative parts.

Escaping

If you ever need to match () or {} literally, you can escape the opening ( or { with a backslash:

I have {int} \{what} cucumber(s) in my belly \(amazing!)

This expression would match the following examples:

I have 1 {what} cucumber in my belly (amazing!)
I have 42 {what} cucumbers in my belly (amazing!)

You may have to escape the \ character itself with another \, depending on your programming language. For example, in Java, you have to use escape character \ with another backslash.

I have {int} \\{what} cucumber(s) in my belly \\(amazing!)

Then this expression would match the following example:

I have 1 \{what} cucumber in my belly \(amazing!)
I have 42 \{what} cucumbers in my belly \(amazing!)

There is currently no way to escape a / character - it will always be interpreted as alternative text.

Architecture

See ARCHITECTURE.md

Acknowledgements

The Cucumber Expression syntax is inspired by similar expression syntaxes in other BDD tools, such as Turnip, Behat and Behave.

Big thanks to Jonas Nicklas, Konstantin Kudryashov and Jens Engel for implementing those libraries.

The Tiny-Compiler-Parser tutorial by Yehonathan Sharvit inspired the design of the Cucumber expression parser.