Edit to original answer: Even though this is answer (as of the time of this comment) is the selected answer, the original version of this answer is outdated.
I’m adding an update here to help others avoid getting sidetracked by this answer like I did.
As the other answer mentions, Ruby >= 2.5 added the Hash#slice method which was previously only available in Rails.
Example:
> { one: 1, two: 2, three: 3 }.slice(:one, :two)
=> {:one=>1, :two=>2}
End of edit. What follows is the original answer which I guess will be useful if you’re on Ruby < 2.5 without Rails, although I imagine that case is pretty uncommon at this point.
If you’re using Ruby, you can use the select method. You’ll need to convert the key from a Symbol to a String to do the regexp match. This will give you a new Hash with just the choices in it.
choices = params.select { |key, value| key.to_s.match(/^choice\d+/) }
or you can use delete_if and modify the existing Hash e.g.
params.delete_if { |key, value| !key.to_s.match(/choice\d+/) }
or if it is just the keys and not the values you want then you can do:
params.keys.select { |key| key.to_s.match(/^choice\d+/) }
and this will give the just an Array of the keys e.g. [:choice1, :choice2, :choice3]