public class Test {
    public static void main(String[] args) {
        //case1:除去“.”和重复出现的字符
        String line= "我我我..我要..要要要.....学学学学学...编编编....程程程程程程";
         line = line.replaceAll("\\.","");
        System.out.println(line.replaceAll("(.)\\1+","$1"));
        //case2:对IP地址进行排序
        String ip = "192.168.1.254 102.49.23.13 10.10.10.10 2.2.2.2 8.109.90.30";
        ip = ip.replaceAll("(\\d+)","00$1");
        ip = ip.replaceAll("0*(\\d{3})","$1");
        System.out.println("排序前："+ip);
        /*....排序过程已被省略...*/
        ip = ip.replaceAll("0*(\\d+)","$1");
        System.out.println("排序后:"+ip);
    }
}

2，多行匹配html

Matcher matcher = Pattern.compile("<div\\s*class=\\s*\"area\"\\s*id=\\s*\"area2\"\\s*>\\s*<h2>.*</h2>" +
                                          "\\s*<ul>\\s*<li>(?<re>.*?)?</li>\\s*</ul>\\s*</div>",Pattern.DOTALL).matcher(text);,

3，常用场景

非中文：
[^\u4E00-\u9FA5]+ 
提取网页content：
<Meta ([^>]*?)(keywords|description|KEYWORDS|DESCRIPTION).*?content="?(?<content>[^">]+)
去除标签：
<title[^>]*>.*?</title>|<script[^>]*>.*?</script>|<style[^>]*>.*?</style>|<!--.*?-->|&(#*)\w+;|\s+|<[^>]*>
提取网页编码：
<Meta([^>]*?)charset\s*=\s*(?<charset>[^">]+)