function countInstance(mainStr,srchStr){
var count = 0;
var offset = 0;// 搜索的起始位置
do{
offset = mainStr.indexOf(srchStr,offset);
count += (offset != -1)? 1:0;
}while(offset++ != -1);
return count;
}
alert(countInstance("wwweeewwwweee",'we')); //2
