[:character_class:] represents a character class that matches all characters belonging to that class : character_class « Regular Expressions « MySQL Tutorial

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3. character_class

[=character_class=] represents an equivalence class.

It matches all characters with the same collation value, including itself.

For example, if o and (+) are the members of an equivalence class, then [[=o=]], [[=(+)=]], and [o(+)] are all synonymous.

An equivalence class may not be used as an endpoint of a range.

The following table lists the standard class names.

mysql> SELECT 'ABC' REGEXP '[[:alnum:]]+';
+-----------------------------+
| 'ABC' REGEXP '[[:alnum:]]+' |
+-----------------------------+
|                           1 |
+-----------------------------+
1 row in set (0.00 sec)

mysql>
mysql> SELECT 'ABC123' REGEXP '[[:alnum:]]+';
+--------------------------------+
| 'ABC123' REGEXP '[[:alnum:]]+' |
+--------------------------------+
|                              1 |
+--------------------------------+
1 row in set (0.00 sec)

mysql>