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Understanding sed

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# 8  
Old 03-02-2013
Discrepancies like these are why every now and then a space probe crashes into the surface of a planet instead of safely entering orbit.

Thank you for living up to your nick, Scrutinizer. Your observations make us all better.

From POSIX: 9.3.6 BREs Matching Multiple Characters:
Quote:
A subexpression repeated by an <asterisk> ( '*' ) or an interval expression shall not match a null expression unless this is the only match for the repetition or it is necessary to satisfy the exact or minimum number of occurrences for the interval expression.
GNU sed 4.1.5:
Code:
$ echo :BEFORE: | sed 's/[^:]*/AFTER/'
AFTER:BEFORE

I expect :AFTER:. It's behaving as if the match is anchored.

Do other implementations (including newer GNU sed) agree in this as well? I don't have access at the moment to my *BSD stuff, only a windows machine and an old linux laptop.

Regards,
Alister
# 9  
Old 03-03-2013
Code:
FreeBSD 9.0-RELEASE
$ echo :BEFORE: | sed 's/[^:]*/AFTER/'
AFTER:BEFORE:

Code:
Linux 3.5.0-26-generic
GNU sed version 4.2.1
$ echo :BEFORE: | sed 's/[^:]*/AFTER/'
AFTER:BEFORE:

This User Gave Thanks to RudiC For This Post:
alister
# 10  
Old 03-03-2013
@alister:
There seems to be no difference with a single (first) substitution or a global substitution, but only with the flag with a specific number. The difference is about a zero match after a previous match (was it already covered by a previous match or not?). Compare:

Code:
$ echo :BEFORE: | gsed 's/[^:]*/AFTER/'
AFTER:BEFORE:
$ echo :BEFORE: | sed 's/[^:]*/AFTER/'
AFTER:BEFORE:

$ echo :BEFORE: | gsed 's/[^:]*/AFTER/2'
:AFTER:
$ echo :BEFORE: | sed 's/[^:]*/AFTER/2'
:AFTER:

$ echo :BEFORE: | gsed 's/[^:]*/AFTER/3'
:BEFORE:AFTER
$ echo :BEFORE: | sed 's/[^:]*/AFTER/3'
:BEFOREAFTER:

$ echo :BEFORE: | gsed 's/[^:]*/AFTER/4'
:BEFORE:
$ echo :BEFORE: | sed 's/[^:]*/AFTER/4'
:BEFORE:AFTER

$ echo :BEFORE: | gsed 's/[^:]*/AFTER/5'
:BEFORE:
$ echo :BEFORE: | sed 's/[^:]*/AFTER/5'
:BEFORE:

$ echo :BEFORE: | gsed 's/[^:]*/AFTER/g'
AFTER:AFTER:AFTER
$ echo :BEFORE: | sed 's/[^:]*/AFTER/g'
AFTER:AFTER:AFTER

$ echo x:BEFORE: | gsed 's/[^:]*/AFTER/2'
x:AFTER:
$ echo x:BEFORE: | sed 's/[^:]*/AFTER/2'
xAFTER:BEFORE:

Last edited by Scrutinizer; 03-03-2013 at 02:02 PM..
This User Gave Thanks to Scrutinizer For This Post:
alister
# 11  
Old 03-03-2013
That sed implementation isn't self-consistent. Using numeric s-command flags, there are 4 matches:
Quote:
Originally Posted by Scrutinizer
Code:
$ echo :BEFORE: | sed 's/[^:]*/AFTER/'
AFTER:BEFORE:

$ echo :BEFORE: | sed 's/[^:]*/AFTER/2'
:AFTER:

$ echo :BEFORE: | sed 's/[^:]*/AFTER/3'
:BEFOREAFTER:

$ echo :BEFORE: | sed 's/[^:]*/AFTER/4'
:BEFORE:AFTER

Using the s-command's g flag, there are only 3:
Quote:
Originally Posted by Scrutinizer
Code:
$ echo :BEFORE: | sed 's/[^:]*/AFTER/g'
AFTER:AFTER:AFTER

Regards,
Alister
# 12  
Old 03-03-2013
I agree this seems to be inconsistent, but it appears to be the case in several sed implementations...
# 13  
Old 03-03-2013
So then we are all in agreement: every sed implementation sucks. This thread can now be closed. Smilie

Regards,
Alister
# 14  
Old 03-06-2013
The correct output (GNU sed) is:
Code:
echo :BEFORE: | sed 's/[^:]*/AFTER/3'
:BEFORE:AFTER

How could the output possibly be:
Code:
:BEFOREAFTER:

What's the possible logic? What sed version are you using?

All the other outputs make sense. It's only the one with n=3 that seems off.
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