[=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.
alnumAlphanumeric characters
alphaAlphabetic characters
blankWhitespace characters
cntrlControl characters
digitDigit characters
graphGraphic characters
lowerLowercase alphabetic characters
printGraphic or space characters
punctPunctuation characters
spaceSpace, tab, newline, and carriage return
upperUppercase alphabetic characters
xdigitHexadecimal digit characters
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>