//程序目的，去掉圖片路徑中的域名
var str = '<img src="http://news.163.com/sports/yao.jpg" src="http://news.163.com/sports/yao.jpg">';
var reg1 = /(/<img)(.*(?=(http|https)/:////))((http|https)/:////[^//]*)/gim;
str.match(reg1);
alert(str.replace(RegExp.$4,''));

import java.util.regex.Matcher;
import java.util.regex.Pattern;
public class RegExpTest {
    public static void main(String[] args){
        Pattern p = Pattern.compile("((?<=//<img.*(?=(http|https)))(http|https))");
        Matcher matcher = p.matcher("<img src="http://news.163.com/sports/yao.jpg" src="http://news.163.com/sports/yao.jpg"/>");
        System.out.println(matcher.matches());
    }
}

var str2 = '客戶端名稱';
var reg2 = /(客戶端名(?:稱)?)/;
str2.match(reg2);
alert(RegExp.$1); //客戶端名稱
alert(RegExp.$2); //空字符串
//不使用?:
reg2 = /(客戶端名(稱)?)/;
str2.match(reg2);
alert(RegExp.$1); //客戶端名稱
alert(RegExp.$2); //稱