var str="abcdafgahiAbcAfgAhi";
var newstr = str.replace("a","o"); //obcdafgahiAbcAfgAhi (只替换一次)
newstr = str.replace(/a/g,"o"); //obcdofgohiAbcAfgAhi (g代表全局)
newstr = str.replace(/a/gi,"o"); //obcdofgohiobcofgohi (i代表忽略大小写)
原文：http://www.cnblogs.com/ndgail/p/6904807.html