js string 截断中英文字串

String.prototype.len=function(){                 
	return this.replace(/[^\x00-\xff]/g,"rr").length;          
}
String.prototype.sub = function(n){    
	var r = /[^\x00-\xff]/g;    
	if(this.replace(r, "mm").length <= n) 
		return this;   

	var m = n;//Math.floor(n/2);    
	for(var i=m; i<this.length; i++) {    
		if(this.substr(0, i).replace(r, "mm").length>=n) { 
			return this.substr(0, i); 
		}
	} 
	return this;   
};