ios正则匹配用户密码6-20位数字和字母组合

• (BOOL)checkPassword:(NSString *) password
  {
  NSString *pattern = @"^(?![0-9]+$)(?![a-zA-Z]+$)[a-zA-Z0-9]{6,20}";
  NSPredicate *pred = [NSPredicate predicateWithFormat:@"SELF MATCHES%@",pattern];
  BOOL isMatch = [pred evaluateWithObject:password];
  return isMatch;

}