Cucumber arity mismatch: trying to bind several matching groups to a single stepdef parameter

Question

I'm trying to create a regular expression that will match the following:

Given I'm a user
Given I am a user
Given Dylan is a user

And this is my step definition:

@Given("^(?:(I)'m|(I) am|(.+) is) a user$")
public void aUserExists(String username) throws Throwable {
}

It's matching what I need, but seems like Cucumber is trying to assign each capturing group in the non-capturing group to a parameter, while I want it to assign just one:

cucumber.runtime.CucumberException: Arity mismatch: Step Definition 'hello.MyStepdefs.aUserExists(String) in [...] with pattern [^(?:(I)'m|(I) am|(.+) is) a user$] is declared with 1 parameters. However, the gherkin step has 3 arguments [I, null, null].

I'm coming from PHP background and this regular expression worked great with Behat. Maybe I'm not getting something about Java regular expressions or Cucumber behavior.

---

Update

Since all the current answers are having the same problem, I'm updating the question.

I don't want to match a string like:

Given I is a user

I want to match I only when it's followed by 'm or am. I hope it's not an impossible thing to do with Java regular expressions and Cucumber.

Answer 1

I am not sure if that is what you are looking for but if you want to capture user name in group one without creating more groups then maybe try

"^(\\w+)(?:(?<=I\\b)(?:'m| am)|(?<!^I\\b) is) a user$"

Answer 2

You could rewrite the regex to use only one capturing group, though it gets ugly:

@Given("^(I(?='m| am)|.+(?= is))(?:'m| am| is) a user$")

Answer 3

Arity mismatch: does nothing to do with your regex, rather your Given I'm a user line is mismatching with your method signature public void aUserExists(String username) throws Throwable { } take out the string argument and try again.