大的分组中存在小的分组时，小的分组是排在该大分组后面的分组，以此类推

 1     <script type="text/javascript">
 2         var str = "ABCDE";
 3         var reg = /(A+)((B|C|D)+)(E+)/gi;
 4         console.log(str.match(reg));//输出：["ABCDE"]
 5         console.log(reg.exec(str,‘i‘));//输出：["ABCDE", "A", "BCD", "D", "E"]
 6         console.log(RegExp.$1);//输出："A"
 7         console.log(RegExp.$2);//输出："BCD"
 8         console.log(RegExp.$3);//输出："D"
 9         console.log(RegExp.$4);//输出："E
10     </script>