Python 2.7手册中的官方解释为：
[] Used to indicate a set of characters. Characters can be listed individually, or a range of characters can be indicated by giving two characters and separating them by a '-'. Special characters are not active inside sets. For example, [akm$] will match any of the characters 'a', 'k', 'm', or '$'; [a-z] will match any lowercase letter, and [a-zA-Z0-9] matches any letter or digit. Character classes such as \w or \S (defined below) are also acceptable inside a range, although the characters they match depends on whether LOCALE or UNICODE mode is in force. If you want to include a ']' or a '-' inside a set, precede it with a backslash, or place it as the first character. The pattern []] will match ']', for example.
You can match the characters not within a range by complementing the set. This is indicated by including a '^' as the first character of the set; '^' elsewhere will simply match the '^' character. For example, [^5] will match any character except '5', and [^^] will match any character except '^'.
Note that inside [] the special forms and special characters lose their meanings and only the syntaxes described here are valid. For example, +, *, (, ), and so on are treated as literals inside [], and backreferences cannot be used inside [].
下面就来详细解释具体的含义：
1.中括号 [] 用来匹配一组的字符，一个集合内的字符，其中，此处所说的组或者集合，是你自己写在中括号内的，自己定义的组合。
最简单的举例为，比如你就只需要匹配字符abc中的任意一个字符，那么此处的abc，就是你自己定义的集合。
2. 另外最常见的如，匹配小写字母a到z中任何一个，那就可以从a到z，都列出来。当前这样写很蠢，更加正常的做法是写成，起始字符，中间是短横线’-‘分隔，加上结束字符，即写成a-z，就表示a到z中的所有的字符了。