正则表达式困扰

               String input ="abac";

		String regex = "a(?=b)";//这里换成a(?<=b)为什么不一样，"b"也是在a的后面啊

		Pattern p = Pattern.compile(regex);

		Matcher m = p.matcher(input);

		while(m.find()) {

			println(m.group()+" start "+m.start()+" end " +m.end());

			

		}

                String regex = "<((?!(br)|(?!p)).)*>";//这么写不行，应该怎么写

		String input = "aaaa<font color=\"red\"><p><br>bbbbb<br></p>";

		Pattern p = Pattern.compile(regex);

                Matcher m = p.matcher(input);

	         while(m.find()){

		 System.out.println(m.replaceAll(""));

                 }