Javascript Chinese character byte judgment

function checkstr(str) {
num=
var arr=(/[^\\\\\\\\\\\\\\\\x00-\\\\\\\\\\\\\\\\x80]/ig)
if(arr!=null)num+=
return num
}

function CheckUserName(UserName) {
if(checkstr(UserName) > 30 || checkstr(UserName) < 4) {

} else {

    }
}