Which characters are allowed in MySQL/MariaDB user account names?
User names are stored on the mysql.user table. On my system (mariadb 10.3), that table is created as a CHAR(80) with the utf8 character set. You can check yours by running:

MariaDB [(none)]&gt; SHOW CREATE TABLE mysql.user\G
*************************** 1. row ***************************
       Table: user
Create Table: CREATE TABLE `user` (
  `Host` char(60) COLLATE utf8_bin NOT NULL DEFAULT '',
  `User` char(80) COLLATE utf8_bin NOT NULL DEFAULT '',


That means, for my version, usernames can be between 0 and 80 characters that are valid utf8 strings but not outside the basic multilingual plane (only 3-byte characters).

So control and wildcard characters are valid, but for example, not emojis:

MariaDB [(none)]&gt; CREATE USER 'a*b';
Query OK, 0 rows affected (0.001 sec)

MariaDB [(none)]&gt; CREATE USER 'a
    '&gt; b';
Query OK, 0 rows affected (0.001 sec)

MariaDB [(none)]&gt; CREATE USER '';
ERROR 1470 (HY000): String '????' is too long for user name (should be no longer than 80)

MariaDB [(none)]&gt; CREATE USER 'Robert\'); DROP TABLE Students;-- ';
Query OK, 0 rows affected (0.001 sec)

MariaDB [(none)]&gt; SELECT user FROM mysql.user;
+----------------------------------+
| user                             |
+----------------------------------+
| Robert'); DROP TABLE Students;-- |
| a
b                              |
| a*b                              |
| root                             |
| root                             |
| root                             |
| root                             |
+----------------------------------+
7 rows in set (0.000 sec)


Check the type in your system, I guess it could change depending on the version/vendor or be different in the future/past.