gawk '/BEGIN\(EXTRACT\)/ { if (depth++) {
                            print "nested extracts not allowed on line: " NR
                            depth = 1
                          }
                          print ""
                          next
                        }
     /END\(EXTRACT\)/   { if (--depth) {
                            print "too many END(EXTRACT) s on line " NR
                            depth = 0
                          }
                          next
                        }
     depth > 0' "$*"