mysql> create table test(num varchar(16));
Query OK, 0 rows affected (0.32 sec)

mysql> insert into test (num)
    -> select '15' union all
    -> select '15k1' union all
    -> select '15k2' union all
    -> select '15A' union all
    -> select '15B' union all
    -> select '15/1' union all
    -> select '15/2' ;
Query OK, 7 rows affected (0.07 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> select * from test
    -> order by
    -> case
    -> when num = cast(num+0 as char) then 1
    -> when locate('k', num) then 2
    -> when locate('/', num) then 4
    -> else 3 end ,
    -> num
    -> ;
+------+
| num  |
+------+
| 15   |
| 15k1 |
| 15k2 |
| 15A  |
| 15B  |
| 15/1 |
| 15/2 |
+------+
7 rows in set, 2 warnings (0.01 sec)

mysql> insert into test select replace(num,'15','31') from test order by num;
Query OK, 7 rows affected (0.09 sec)
Records: 7  Duplicates: 0  Warnings: 0

mysql> select * from test;
+------+
| num  |
+------+
| 15   |
| 15k1 |
| 15k2 |
| 15A  |
| 15B  |
| 15/1 |
| 15/2 |
| 31   |
| 31/1 |
| 31/2 |
| 31A  |
| 31B  |
| 31k1 |
| 31k2 |
+------+
14 rows in set (0.00 sec)

mysql> select * from test
    -> order by
    -> num+0,
    -> case
    -> when num = cast(num+0 as char) then 1
    -> when locate('k', num) then 2
    -> when locate('/', num) then 4
    -> else 3 end ,
    -> num
    -> ;
+------+
| num  |
+------+
| 15   |
| 15k1 |
| 15k2 |
| 15A  |
| 15B  |
| 15/1 |
| 15/2 |
| 31   |
| 31k1 |
| 31k2 |
| 31A  |
| 31B  |
| 31/1 |
| 31/2 |
+------+
14 rows in set, 8 warnings (0.00 sec)