How do I call a method in application helper from a view?

I defined a custom method in application_helper.rb file like the following:

def rxtrnk(line)
    rxTRNK = /\w{9,12}/m
    trnks = Array.new
    i = 0
    while i <= line.size
        if line[i].match(rxTRNK)
            trnks[i] = line[i].scan(rxTRNK)
        end
        i += 1
    end

    return trnks
  end

Then I tried to call it from a view like so:

<% @yo = rxtrnk(@rts)%>

But I get an error page like this:

NoMethodError in TrunksController#routesperswitch

undefined method `rxtrnk' for #<TrunksController:0x7f2dcf88>

I know this is a very newbie question, but I couldn't find solution from googling :( Thanks for your help

edit/ here is the full application_helper.rb

module ApplicationHelper
     def rxtrnk(line)
    rxTRNK = /\w{9,12}/m
    trnks = Array.new
    i = 0
    while i <= line.size
        if line[i].match(rxTRNK)
            trnks[i] = line[i].scan(rxTRNK)
        end
        i += 1
    end

    return trnks
  end

end