sed something negative

[maclenin]

Quick question.

I have declared the following variable within a bash script...


iP=[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}

...which I have used in conjunction with sed to whittle the contents of a file down to a single line, similar to:


,'laptop1','192.168.1.100','MA:CA:DD:RE:SS:00','21 :40:12','100'

I am now trying, using sed, to whittle the line, further, to a single string:


192.168.1.100

This is one method (http://www.cyberciti.biz/faq/sed-remove-all-except-digits-numbers/) with which I have been toying:


sed -r 's/[^\('$iP')//g' -i file

Essentially, I'd like to remove (using sed) all from the line, but for the '$iP' string....

Thanks for any suggestions!

[mörgæs]

Moved to Programming Talk.

[gmargo]

I could not get this to work with sed, since I think sed is always "greedy".

However, here it is in Perl. Note the quotes around the iP definition, to keep the backslashes:



$ iP='[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}'
$ echo ",'laptop1','192.168.1.100','MA:CA:DD:RE:SS:00','21 :40:12','100'" | perl -p -e "s/.*?($iP).*/\$1/"
192.168.1.100

[maclenin]

mörgæs: þakka þér -

gmargo: thank you - will give perl a go (being a perl virgin, this is all very exciting).... I'll keep wondering, however, whether there's a sed option - and if I encounter one, will post it back.

This also came to mind: http://xkcd.com/208/....

[mörgæs]

Það var lítið.

[Arndt]

I seem to need to escape the { and }, but this way almost works:


sed 's/.*[^0-9.]\([0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\).*/\1/'

Why almost? Because it fails if the address is at the start of the line.

grep has an option to output only the matching part:


grep -o '[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}\.[0-9]\{1,3\}'

[Vaphell]

you can go with -r for sed, which makes the regex related symbols work without escaping

$ echo ",'laptop1','192.168.1.100','MA:CA:DD:RE:SS:00','21 :40:12','100'" | sed -r 's/.*[^0-9.]([0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}).*/\1/'
192.168.1.100

perl -pe and sed are pretty much equivalent in this case (there are only minute syntax differences)

[Simian Man]

My mom always sed that if you can't sed something positive, you shouldn't sed anything at all.

[maclenin]

...also sed Arndt, Vaphell and Simian Man.... Thank you for the comments.

One final (and related) query.

Why does...

sed -r 's/.*('$iP').*/\1/'
...yield: 8.1.100

...and...


sed -r 's/.*[^0-9.]('$iP').*/\1/'
...yield: 192.168.1.100

Great help and wisdom, all!

[Vaphell]

because of a little screwup with . :-)
. means any char so it matched stuff other than dots and whole thing went bonkers (your \. wasn't preserved because you didn't put the ip in quotes to prevent the change from \. to .)

$ ip=1\.1\.1\.1; echo $ip
1.1.1.1
$ ip='1\.1\.1\.1'; echo $ip
1\.1\.1\.1

[.] is more foolproof, you explicitly enumerate dot and problem with proper escaping goes away

$ echo ",'laptop1','192.168.1.100','MA:CA:DD:RE:SS:00','21 :40:12','100'" |
sed -r 's/.*([0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}).*/\1/'
8.1.100
$ echo ",'laptop1','192.168.1.100','MA:CA:DD:RE:SS:00','21 :40:12','100'" |
sed -r 's/.*([0-9]{1,3}[.][0-9]{1,3}[.][0-9]{1,3}[.][0-9]{1,3}).*/\1/'
2.168.1.100
but there is another issue - * is greedy (it consumes as much as possible) so it ate '19' and the pattern was still matched


.*([0-9]{1,3}[.][0-9]{1,3}[.][0-9]{1,3}[.][0-9]{1,3}).*
some gibberish 19(2.168.1.100) ...
everything is ok - that 1 digit is matched by [0-9]{1,3}
version with bare dots worked like this

.*[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}
gibberish 198.1.100


You prevent that by specifying that the last char before $ip has to be "non-digit,non-dot" thus removing ambiguity completely and pushing '19' into the $ip territory. It also works with those shoddily specified dots almost by accident (though not rly, not enough 'moving parts' to produce very different result) :)
Another option is to play with non-greedy version of * (but sadly it appears it is not supported by sed). Question mark changes the behavior so non-greedy version of 'string made of anything' is .*? which was used in the perl example