正则实现String的trim方法

ES5里面的trim()方法是去掉字符串的前后空格，自己写个正则兼容一下不支持ES5的浏览器

var re_f = /^s+/g;
var re_e = /s+$/g;
var s = " asd ";
console.log(s);
console.log(" asd");
var s_f = s.replace(re_f,"");
console.log(s_f);
console.log(s_f.length);
var s_e = s_f.replace(re_e,"");
console.log(s_e.length);