Return optionals as part of a Cucumber Expression match about cucumber-expressions

[mpkorstanje]

Currently you can not assign special meaning to groups because every possible regex generated by a Cucumber expression is also a regular valid regular regex that may be provided by a user.

For example the regex:

A(?<Special-String> non-optional) pattern

Would be capturing when used with Cucumber, but non capturing in the Language Server. And while it is unlikely a user may have guessed Special-String, it is an indication that this abstraction is leaking.

Now I'm not opposed to adding named group support to the tree regex. These names could be used when generating step definitions based of regular regexes for example.

However the proposed Special-String should not be used in production code. So consider generating a different regex patterns when transforming the AST to a Cucumber expression while using the langue server.

This may require some refactoring.

[aslakhellesoy]

I wrote in cucumber/language-service#57 that we could change TreeRegexp. After thinking about it a bit more, I think the correct place to do this is in rewriteOptional:

cucumber-expressions/javascript/src/CucumberExpression.ts

Line 72 in 2841a19

return `(?:${regex})?`

We could pass a makeOptionalGroup: (regex: string, count: number) => string constructor argument to CucumberExpression (defaulting to this:

(regex, count) => `(?:${regex})?`

That way, the language server could pass its own:

(regex, count) => `(?<optional-${count}>${regex})`

[mpkorstanje]

That interface is very language server specific.

For example:

• count is not a concept that occurs in cucumber expressions. It exists only for the language server.
• This only and specifically replaces optionals.

Ideally we define the Cucumber expression API in its own domain terms.

So I think some refactoring is in order.

[mpkorstanje]

It does come to mind that currently we also assume that all capture groups are parameters. If you write optionals as capturing then match won't work anymore.

Maybe you can extract the code that rewrites the AST, add support for named groups to tree-regex and then use those in a class that is not CucumberExpression.