就参考这吧: https://dev.mysql.com/doc/refman/5.7/en/regexp.html

[.characters.]

Within a bracket expression (written using [ and ]), matches the sequence of characters of that collating element.

mysql> SELECT '~' REGEXP '[[.~.]]';                     -> 1

为啥不是单[？结果也是 1 呀
mysql> SELECT '~' REGEXP '[.~.]';

[=character_class=]

Within a bracket expression (written using [ and ]), [=character_class=] represents an equivalence class.

里面没举例，可否写个例子？

[:character_class:]

Within a bracket expression (written using [ and ]), [:character_class:] represents a character class that matches all characters belonging to that class.

mysql> SELECT 'justalnums' REGEXP '[[:alnum:]]+';       -> 1

为啥不是单[？结果也是 1 呀
mysql> SELECT 'justalnums' REGEXP '[:alnum:]+';

[[:<:]], [[:>:]]

These markers stand for word boundaries. They match the beginning and end of words, respectively.

怎么需要 2 个来分别表示？