正则表达式判断是否截断文本

var cutStrForNum = function (str,num) { 
    var len = 0; 
    for (var i = 0; i < str.length; i++) { 
        if (str[i].match(/[^x00-xff]/ig) != null) //全角 
            len += 2; 
        else
            len += 1; 
    } 
    if (len >=num) { 
        newStr = str.substring(0,num) + "..."; 
    } 
    return newStr; 
}