Based on your question and the comments to Jakub's answer, it looks like you are trying to write a single Step that can cover multiple user journeys through your site. SpecFlow isn't really designed for this, and it's probably an indication that you should try and improve the structure of your scenarios/features.
To answer your question directly, I don't believe there is a way to deduce boolean values based on the existence of certain strings in your step definition.
If you want to persist down this route, then your original example is probably your best bet.
I would recommend that you don't take this approach, however, and instead look at restructuring your step definitions so that you can chain them together, and re-use them across scenarios. I'm actually struggling to think of an example step definition that would fit your solution.
An example of a multi-step approach might look like this:
Given I have logged in as an existing user //1
And I have started my 6-step registration process //2
And I have filled in valid address values on step 1 //3
And I have left the fields blank on step 2 //4
... etc
When I save my registration
And your steps would be:
- navigate to login page, log in as valid user
- navigate to step 1
- fill in fields with valid input, click 'next'
- click 'next'
You just need to ensure that each step is as independent of the others as possible, so you could replace one step with a subtly different one (for a new scenario) without affecting the others.
With this approach you can still end up with complex (and potentially quite verbose) scenarios, but I think that's a better solution than trying to be clever and pack as much into a single step definition. You'll probably end up with scenarios that are unreadable, and the code will probably be a pain to read/maintain as well.
twice == false?