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Problem with occurence of square brackets

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Top Forums Shell Programming and Scripting Problem with occurence of square brackets
# 8  
Old 06-12-2014
I decided to use neutronscott's solution, which I understand except the effect of these two expressions:

Code:
sed 's/[^[]*[[]\([^]]*\)]\([[][^]]*]\)*\([^]]*\)$/\1 -- \3/g' infile
            ^^^            ^^^

A group which consists of a single square bracket? I would have written the single square bracket without the enclosure but this does not work obviously.
Last edited by zaxxon; 06-12-2014 at 05:53 AM.. Reason: typo
# 9  
Old 06-12-2014
Quote:
Originally Posted by zaxxon
I decided to use neutronscott's solution, which I understand except the effect of these two expressions:

Code:
sed 's/[^[]*[[]\([^]]*\)]\([[][^]]*]\)*\([^]]*\)$/\1 -- \3/g' infile
            ^^^            ^^^

A group which consists of a single square bracket? I would have written the single square bracket without the enclosure but this does not work obviously.
If you understood the meaning of a repetition of a non-matching bracket expression (such as [^[]* which matches zero or more occurrences of any character except [), I'm surprised that you didn't understand the meaning of the matching bracket expression [[] which matches one occurrence of the [ character. Similarly, [^]] matches any character other than ] and []] matches a ].

You have to use the bracket expression [[] or escape the opening bracket \[ to distinguish it as a character to be matched (rather than the start of a bracket expression). In some contexts, you do not need to use a bracket expression or an escape to specify a closing bracket, but the meaning is is the same if you use []] and it is symmetric with the [[] if you have an editor that pairs up opening and closing parentheses, braces, and brackets.
These 2 Users Gave Thanks to Don Cragun For This Post:
CarloM zaxxon
# 10  
Old 06-12-2014
Quote:
You have to use the bracket expression [[] or escape the opening bracket \[
Ok, I would have written it escaped. So far I never used the grouping to avoid escaping - I was not aware this is an "allowed" usage of the square brackets.
It's now clear to me, thanks.
Last edited by zaxxon; 06-12-2014 at 08:48 AM.. Reason: typo
# 11  
Old 06-12-2014
Both suggestions could be reduced a bit, when taking into account sed's greedy matchin property, notably \[.*\] matches everything between the first square bracket until the last, from the point of where sed's is looking at that moment. Thus:

CarloM's approach, with the two dashes inserted:
Code:
sed 's/[^[]*\[\([^]]*\)\]/\1 --/; s/\[.*\]//' file

And NeutronScott's approach..
Code:
sed 's/[^[]*[[]\([^]]*\)]\(\[.*\]\)*/\1 --/' file

(the original will fail with more than two square bracket episodes)
These 3 Users Gave Thanks to Scrutinizer For This Post:
CarloM neutronscott zaxxon
# 12  
Old 06-12-2014
Yes, sorry. It's just clever escaping. Like one would ps | grep [s]omething. I began to use this more because in awk, depending on quoting/context, often times you need to escape your escapes since they're really processed twice, and it gets ugly so I tend to avoid \ when possible now. Smilie

Quote:
Originally Posted by Scrutinizer
And NeutronScott's approach..
Code:
sed 's/[^[]*[[]\([^]]*\)]\(\[.*\]\)*/\1 --/' file

(the original will fail with more than two square bracket episodes)
The * after the grouping allows it to repeat. But I see mine doesn't perform correctly in the last two cases.

Code:
mute@thedoctor:~$ cat input
this is spam [i need this][this is spam][another one][last] and i need this too
this is spam [i need this] probably everything [here] too
this is spam [i need this] and probably i need ] everything here too?
mute@thedoctor:~$ sed 's/[^[]*[[]\([^]]*\)]\([[][^]]*]\)*\([^]]*\)$/\1 -- \3/g' input
i need this --  and i need this too
this is spam [here --  too
this is spam [i need this] and probably i need ] everything here too?
mute@thedoctor:~$ sed 's/[^[]*[[]\([^]]*\)]\(\[.*\]\)*/\1 --/' input
i need this -- and i need this too
i need this -- probably everything [here] too
i need this -- and probably i need ] everything here too?

I also didn't think of not needing to match & sub the last part. That's definitely shorter.
# 13  
Old 06-12-2014
Quote:
Originally Posted by Scrutinizer
Both suggestions could be reduced a bit, when taking into account sed's greedy matchin property, notably \[.*\] matches everything between the first square bracket until the last, from the point of where sed's is looking at that moment. Thus:

CarloM's approach, with the two dashes inserted:
Code:
sed 's/[^[]*\[\([^]]*\)\]/\1 --/; s/\[.*\]//' file

Note that that isn't just slightly shorter, it also corrects the output to match Zaxxon's requirement - my original produced different output since it left any non-leading text not inside brackets.
# 14  
Old 06-12-2014
Yet another approach:
Code:
sed 's/]\(.*]\)*/ --/; s/.*\[//' file

Last edited by Scrutinizer; 06-12-2014 at 02:55 PM..
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