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Possible to grep string based on surrounding strings?

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# 1  
Old 05-15-2012
Possible to grep string based on surrounding strings?
I was wondering if it was possible to grep a pattern based on the surround text. For example, if i have an input file like this:

Code:
titleA
titleB
titlex
titleC
titleD
titlex
titleE

And I want to grep "title" and save the results only if it is not followed with a "titlex". My output would look like this:
Code:
titleA
titleC
titleE

Is this possible? I feel like awk should come into play, but I'm not great at awk at the moment.
# 2  
Old 05-15-2012
your expected output will have following two also rt?
Code:
titleB
titleD

try this
Code:
grep -v titlex infile

# 3  
Old 05-15-2012
Quote:
Originally Posted by 47shailesh
your expected output will have following two also rt?
Code:
titleB
titleD

try this
Code:
grep -v titlex infile

I don't want to have titleB and titleD in my output. With "grep -v" I would get titleA, titleB, titleC, titleD, and titleE.

I found the following, which would produced an output of just titleB and titleD, I just need to make it do the opposite!

Code:
awk '/titlex/ { print prv_line; next } { prv_line = $0 }' input.txt

# 4  
Old 05-15-2012
grep is not a programming language, it can't understand 'if x then do y'. It can't even remember lines for later. awk is a programming language, though, and can do both.

Perhaps something like
Code:
$ cat regafter.awk

# Recall N lines ago up to 9 lines
function last(N)
{
        if(N>L) return("");
        return(LINE[(L-N)%10]);
}

{ LINE[(++L)%10]=$0 } # Remember line for later

# If this line and the last line don't match titlex, print last line.
(last(1) ~ /title[^xX]/) && /title[^xX]/        { print last(1) }
# Do the same test for the last line by itself.
END {   if(last(0) ~ /title[^xX]/) print last(0); }

$ awk -f regafter.awk data

titleA
titleC
titleE

$

If awk doesn't work, try nawk or gawk.
# 5  
Old 05-15-2012
NEVERMIND. THIS DOES NOT WORK CORRECTLY when titlex is the first line in the file or when there are consecutive instances of titlex. I leave it here only for your amusment.


Corona's awk solution is more efficient, since it only reads the data once, but here's a simple ed solution.

Code:
ed -s data <<EOED
g/titlex/-,.d
g/title/
Q
EOED

Or, if you prefer a less readable oneliner
Code:
printf %s\\n g/titlex/-,.d g/title/ Q | ed -s data

Regards,
Alister
Last edited by alister; 05-15-2012 at 12:48 PM.. Reason: Solution is incorrect.
# 6  
Old 05-15-2012
Quote:
Originally Posted by Corona688
grep is not a programming language, it can't understand 'if x then do y'. It can't even remember lines for later. awk is a programming language, though, and can do both.

Perhaps something like
Code:
$ cat regafter.awk

# Recall N lines ago up to 9 lines
function last(N)
{
        if(N>L) return("");
        return(LINE[(L-N)%10]);
}

{ LINE[(++L)%10]=$0 } # Remember line for later

# If this line and the last line don't match titlex, print last line.
(last(1) ~ /title[^xX]/) && /title[^xX]/        { print last(1) }
# Do the same test for the last line by itself.
END {   if(last(0) ~ /title[^xX]/) print last(0); }

$ awk -f regafter.awk data

titleA
titleC
titleE

$

If awk doesn't work, try nawk or gawk.
Thanks corona. If I want to substitute "titlex" with another string, how can I do that? In the code, I replaced "title[^xX]" with another string to test, but it outputs the new searched string.
# 7  
Old 05-15-2012
The regex I put in accepts titleG but not titleX or titlex. If you give it a regex that doesn't reject titleX, it of course won't reject titleX...

Please show what you did.

Better yet, show the actual input you have and actual output you want, since your sample data doesn't seem to be it.
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