why can the character's order in regex expression affect sed? - sed

Question

The tv.txt file is as following:
mms://live21.gztv.com/gztv_gz 广州台[可于Totem/VLC/MPlayer播放，记得把高宽比设置成4:3]
mms://live21.gztv.com/gztv_news 广州新闻台·直播广州(可于Totem/VLC/MPlayer播放，记得把高宽比设置成4:3)
mms://live21.gztv.com/gztv_kids 广州少儿台(可于Totem/VLC/MPlayer播放，记得把高宽比设置成4:3)
mms://live21.gztv.com/gztv_econ 广州经济台
I want to group it into three groups.
sed -r 's/([^ ]*)\s([^][()]*)((\(.+\))*|(\[.+\])*)/\3/' tv.txt
got the result:
[可于Totem/VLC/MPlayer播放，记得把高宽比设置成4:3]
(可于Totem/VLC/MPlayer播放，记得把高宽比设置成4:3)
(可于Totem/VLC/MPlayer播放，记得把高宽比设置成4:3)
When I write it into
sed -r 's/([^ ]*)\s([^][()]*)((\(.+\))*|(\[.+\])*)/\3/' tv.txt
It can't work.
The only difference is [^][()] and [^[]()]; neither of the [^\[\]()] ,escape characters can not make it run properly.
I want to know the reason.

Answer 1

The POSIX rules for getting ] into a character class are a little arcane, but they make sense when you think about it hard.
For a positive (non-negated) character class, the ] must be the first character:
[]and]
This recognizes any character a, n, d or ] as part of the character class.
For a negated character class, the ] must be the first character after the ^:
[^]and]
This recognizes any character except a, n, d or ] as part of the character class.
Otherwise, the first ] after the [ marks the end of the character class. Inside a character class, most of the normal regex special characters lose their special meaning, and others (notably - minus) acquire special meanings. (If you want a - in a character class, it has to be 'first' or last, where 'first' means 'after the optional ^ and only if ] is not present'.)
In your examples:
[^][()] — this is a negated character class that recognizes any character except [, ], ( or ), but
[^[]()] — this is a negated character class that recognizes any character except [, followed by whatever () symbolizes in the regex family you're using, and ] which represents itself.