ruby - How does this custom route matcher in Sinatra example work? -

in sinatra readme, there section called custom route matchers following example:

class allbutpattern   match = struct.new(:captures)    def initialize(except)     @except   = except     @captures = match.new([])   end    def match(str)     @captures unless @except === str   end end  def all_but(pattern)   allbutpattern.new(pattern) end  all_but("/index")   # ... end 

would helpful enough talk me through how works? bit i'm not sure why example has match struct , captures are. user can't set @captures instance variable, @except one; how captures used?

when route processed, takes argument get (or post or whatever), , sends object's match method path argument. expects either nil mean didn't match, or array of captures. object string or regex , both have match method.

sinatra calls on captures method of object when processing route. example uses struct easy way set , respond object respond captures, , puts in array, that's captures return. it's empty because string hasn't been checked captures, it's negative filter. such, i'd prefer the use of before filter this, finding clear , useful examples difficult.