Thank you for pointing this out. I am shocked to find this in the documentation:

“The “_” and “%” wildcards are permitted when specifying database names in GRANT statements that grant privileges at the global or database levels. This means, for example, that if you want to use a “_” character as part of a database name, you should specify it as “\_” in the GRANT statement, to prevent the user from being able to access additional databases matching the wildcard pattern; for example, GRANT … ON `foo\_bar`.* TO ”

News to me.

The current code The code proposed in this post does not use “\_”, and in this respect @Mrten’s warning is irrelevant. But since I will have to fix the code, the warning WILL become relevant in future code. Also, this means a search would have to take place using LIKE instead of “=”.
Will update in common_schema.

root@mysql-5.1.51> create database a_b;

root@mysql-5.1.51> create database acb;

root@mysql-5.1.51> grant all privileges on a_b.* to ‘shu’@localhost;

Which schema would ‘shu’@localhost have access to? Both.

If you want the user to only have access to a_b you would have to do the following.

root@mysql-5.1.51> grant all privileges on a\_b.* to ‘shu’@localhost;

root@mysql-5.1.51> create database a_b;

root@mysql-5.1.51> grant all privileges on a_b.* to 'shu'@localhost;

root@mysql-5.1.51> select * from sql_grants where user='shu' \G
*************************** 1. row ***************************
           GRANTEE: 'shu'@'localhost'
              user: shu
              host: localhost
        priv_level: *.*
   priv_level_name: user
     object_schema: NULL
       object_name: NULL
current_privileges: USAGE
      IS_GRANTABLE: NO
         sql_grant: GRANT USAGE ON *.* TO 'shu'@'localhost' IDENTIFIED BY PASSWORD ''
        sql_revoke:
     sql_drop_user: DROP USER 'shu'@'localhost'
*************************** 2. row ***************************
           GRANTEE: 'shu'@'localhost'
              user: shu
              host: localhost
        priv_level: `a_b`.*
   priv_level_name: schema
     object_schema: NULL
       object_name: a_b
current_privileges: ALTER, ALTER ROUTINE, CREATE, CREATE ROUTINE, CREATE TEMPORARY TABLES, CREATE VIEW, DELETE, DROP, EVENT, EXECUTE, INDEX, INSERT, LOCK TABLES, REFERENCES, SELECT, SHOW VIEW, TRIGGER, UPDATE
      IS_GRANTABLE: NO
         sql_grant: GRANT ALTER, ALTER ROUTINE, CREATE, CREATE ROUTINE, CREATE TEMPORARY TABLES, CREATE VIEW, DELETE, DROP, EVENT, EXECUTE, INDEX, INSERT, LOCK TABLES, REFERENCES, SELECT, SHOW VIEW, TRIGGER, UPDATE ON `a_b`.* TO 'shu'@'localhost'
        sql_revoke: REVOKE ALTER, ALTER ROUTINE, CREATE, CREATE ROUTINE, CREATE TEMPORARY TABLES, CREATE VIEW, DELETE, DROP, EVENT, EXECUTE, INDEX, INSERT, LOCK TABLES, REFERENCES, SELECT, SHOW VIEW, TRIGGER, UPDATE ON `a_b`.* FROM 'shu'@'localhost'
     sql_drop_user: DROP USER 'shu'@'localhost'
2 rows in set (0.01 sec)

root@mysql-5.1.51> select * from information_schema.schemata;
+--------------+--------------------+----------------------------+------------------------+----------+
| CATALOG_NAME | SCHEMA_NAME        | DEFAULT_CHARACTER_SET_NAME | DEFAULT_COLLATION_NAME | SQL_PATH |
+--------------+--------------------+----------------------------+------------------------+----------+
| NULL         | a_b                | utf8                       | utf8_general_ci        | NULL     |
...
+--------------+--------------------+----------------------------+------------------------+----------+
16 rows in set (0.00 sec)

REVOKE [….] ON `it\_test`.* FROM […]

Do a dry run!

REVOKE [….] ON `it\_test`.* FROM […]