// 使用slice寫出的 自定義方法，綁定在String的原型鏈上
String.prototype.splice = function(start, newStr) {
      return this.slice(0, start) + newStr + this.slice(start);
};

let imgArr = ['http://23.45.56.89:8080/11.jpg','http://23.45.56.89:8080/12.jpg','http://23.45.56.89:8080/13.jpg']
let noSrc = `<img src="" /></div><div><br><div><img src="" /></div><div><img src="" /></div><b>whatsApp</b>`;

//進行循環匹配，替換
   for(let i=0;i<imgArr.length;i++){
      //indexOf檢測位置
      let index = noSrc.indexOf('src=""');
      let a = noSrc.splice(index+5,imgArr[i]);
      //賦值回去 ，因為必須對原來的字符串進行修改
      noSrc = a;
   } 

 var s = "The rain in Spain falls rain mainly in the rain plain"; 
 var re = new RegExp("rain","g");
 var arr = s.match(re);
 alert(arr.length)