^[A-Za-z0-9]{6,20}$
^$:分别匹配字符串的开始和结束
^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])[a-zA-Z\d]{6,20}$a-z])(?=.*[A-Z])[a-zA-Z\d]{6,20}$
^(?=.*\d)(?=.*[a-z])(?=.*[A-Z])[a-zA-Z\d]{6,20}$
(?=.*\d)表示字符串中有数字,(?=.*[a-z])(?=.*[A-Z])则分别表示字符串中含有小写字母和大写字母 例如:[a-zA-Z\d]{6,20}:表示含有字母和数字,6到20位