get_group

# File lib/rexml/parsers/xpathparser.rb, line 594
      def get_group string
        ind = 0
        depth = 0
        st = string[0,1]
        en = (st == "(" ? ")" : "]")
        begin
          case string[ind,1]
          when st
            depth += 1
          when en
            depth -= 1
          end
          ind += 1
        end while depth > 0 and ind < string.length
        return nil unless depth==0
        [string[ind..-1], string[0..ind-1]]
      end