我不懂MySQL中匹配字符类到底怎么理解？[:upper:]表示任意大写字母，同[A-Z]，通过下面这一句得到出现过大写字母的prod_name:

SELECT prod_name FROM Products WHERE prod_name REGEXP '[:upper:]';

使用[[:upper:]]则会全部输出。

同样的道理，在使用数字字符类[:digit:]时，需要双括号才能匹配到出现过数字的prod_name：

SELECT prod_name FROM Products WHERE prod_name REGEXP '[[:digit:]]';

而使用单括号则不行，请问这是为什么？该怎么理解？是否需要指定正则匹配出现的次数？如何指定？

mysql> SELECT prod_name FROM Products;

+----------------+

| prod_name |

+----------------+

| .5 ton anvil |

| 1 ton anvil |

| 2 ton anvil |

| Detonator |

| Bird seed |

| Carrots |

| Fuses |

| JetPack 1000 |

| JetPack 2000 |

| Oil can |

| Safe |

| Sling |

| TNT (1 stick) |

| TNT (5 sticks) |

+----------------+

14 rows in set (0.00 sec)

mysql> SELECT prod_name FROM Products WHERE prod_name REGEXP '[:upper:]';

+--------------+

| prod_name |

+--------------+

| Detonator |

| Bird seed |

| Carrots |

| Fuses |

| JetPack 1000 |

| JetPack 2000 |

| Safe |

+--------------+

7 rows in set (0.01 sec)

mysql> SELECT prod_name FROM Products WHERE prod_name REGEXP '[[:upper:]]';

+----------------+

| prod_name |

+----------------+

| .5 ton anvil |

| 1 ton anvil |

| 2 ton anvil |

| Detonator |

| Bird seed |

| Carrots |

| Fuses |

| JetPack 1000 |

| JetPack 2000 |

| Oil can |

| Safe |

| Sling |

| TNT (1 stick) |

| TNT (5 sticks) |

+----------------+

14 rows in set (0.01 sec)

mysql> SELECT prod_name FROM Products WHERE prod_name REGEXP '[[:digit:]]';

+----------------+

| prod_name |

+----------------+

| .5 ton anvil |

| 1 ton anvil |

| 2 ton anvil |

| JetPack 1000 |

| JetPack 2000 |

| TNT (1 stick) |

| TNT (5 sticks) |

+----------------+

7 rows in set (0.00 sec)

mysql> SELECT prod_name FROM Products WHERE prod_name REGEXP '[:digit:]';

+----------------+

| prod_name |

+----------------+

| .5 ton anvil |

| 1 ton anvil |

| 2 ton anvil |

| Detonator |

| Bird seed |

| Carrots |

| JetPack 1000 |

| JetPack 2000 |

| Oil can |

| Sling |

| TNT (1 stick) |

| TNT (5 sticks) |

+----------------+

12 rows in set (0.00 sec)