正则表达式：/^([A-Za-z0-9]{8,20})$

function containSpecial(s){
var isSpecialCharacter;
var reg = /^[a-zA-Z0-9_-]/;
if(reg.test(s)){
isSpecialCharacter = false;
}else{
isSpecialCharacter = true;
}
return isSpecialCharacter;
}