# File lib/active_support/cache.rb, line 443
        def key_matcher(pattern, options)
          prefix = options[:namespace].is_a?(Proc) ? options[:namespace].call : options[:namespace]
          if prefix
            source = pattern.source
            if source.start_with?('^')
              source = source[1, source.length]
            else
              source = ".*#{source[0, source.length]}"
            end
            Regexp.new("^#{Regexp.escape(prefix)}:#{source}", pattern.options)
          else
            pattern
          end
        end