//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