exec 方法
   使用正则表达式模式对字符串执行查找，并返回该查找结果的第一个值（数组），如果匹配失败，返回null。  
   rgExp.exec(str)
参数：
   str               必选项。对其进行查找的 String 对象或字符串文字。
   rgExp          必选项。为包含正则表达式模式和可用标志的正则表达式对象。

返回数组包含：
   input：整个被查找的字符串的值；
   index：匹配结果所在的位置（位）；
   lastInput：下一次匹配结果的位置；
   arr：结果值，arr[0]全匹配结果，arr[1,2...]为表达式内()的子匹配，由左至右为1,2...。

例1:

<!DOCTYPE html>
<html>
<head>
    <title>ExecDemo</title>
    <script type="text/javascript" language="JavaScript">
        function ExecDemo(){
            var r, re;
            var s="The rain in Spain falls mainly in the plain";
            //创建正则表达式模式。 g 代表全部匹配 并且不含子匹配项
            re = /\w+/g;
            while((r = re.exec(s)) !=null){ //exec使r返回匹配的第一个，while循环一次将使re在g作用寻找下一个匹配。
                document.write(r.index + "-" + r.lastIndex + ":" + r + "<br/>");
                for(key in r){
                    document.write(key + "=>" + r[key] + "<br/>");
                }
                document.write("<br/>");
            }
            //测试exec的输出将包含子匹配项
            re = /[\w]*(ai)n/ig;
            r = re.exec(s);
            document.write("<br/>");
            document.write(r + "<br/>");
            for(key in r){
                document.write(key + "-" + r[key] + "<br/>");
            }
        }
    </script>
</head>
<body onload="ExecDemo()">
</body>
</html>

输出结果:

0-3:The
input=>The rain in Spain falls mainly in the plain
index=>0
lastIndex=>3
0=>The
4-8:rain
input=>The rain in Spain falls mainly in the plain
index=>4
lastIndex=>8
0=>rain
9-11:in
input=>The rain in Spain falls mainly in the plain
index=>9
lastIndex=>11
0=>in
12-17:Spain
input=>The rain in Spain falls mainly in the plain
index=>12
lastIndex=>17
0=>Spain
18-23:falls
input=>The rain in Spain falls mainly in the plain
index=>18
lastIndex=>23
0=>falls
24-30:mainly
input=>The rain in Spain falls mainly in the plain
index=>24
lastIndex=>30
0=>mainly
31-33:in
input=>The rain in Spain falls mainly in the plain
index=>31
lastIndex=>33
0=>in
34-37:the
input=>The rain in Spain falls mainly in the plain
index=>34
lastIndex=>37
0=>the
38-43:plain
input=>The rain in Spain falls mainly in the plain
index=>38
lastIndex=>43
0=>plain
rain,ai
input-The rain in Spain falls mainly in the plain
index-4
lastIndex-8
0-rain
1-ai