Matches any character that is (or is not, if ^ is used) either a, b, c, d or X: [a-dX], [^a-dX] : Not « Regular Expressions « MySQL Tutorial

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A character between two other characters forms a range.

For example, [0-9] matches any decimal digit.

To include a literal ] character, it must immediately follow the opening bracket [.

To include a literal - character, it must be written first or last.

Any character that does not have a defined special meaning inside a [] pair matches only itself.

mysql>
mysql> SELECT 'aXbc' REGEXP '[a-dXYZ]';
+--------------------------+
| 'aXbc' REGEXP '[a-dXYZ]' |
+--------------------------+
|                        1 |
+--------------------------+
1 row in set (0.00 sec)