#!/bin/sh

if [ $# -eq 0 -o "${1:-}" = "-h" ] ; then
  echo "usage: $0 [-v] pattern [files]"
  return 1
fi

# Support FreeBSD sed and GNU sed
case $uname in
  FreeBSD|Darwin) FLAGS="-El" ;;
  *) EREG_FLAG="-ru" ;;
esac

func="p"
if [ "$1" = "-v" ] ; then
  shift
  func="!p"
fi
pattern="$1"
shift

sed $FLAGS -ne "/^ *$/!H; /^ *$/ {x; /$pattern/$func; d;}; $ { x; /$pattern/$func; d; }" "$@"