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Extracting a regex with awk

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# 8  
Old 05-17-2009
From the grep man page:

-o, --only-matching
Show only the part of a matching line that matches PATTERN.


In the words of the Kool-Aid guy....Ohhh-Yeeeaah!
# 9  
Old 05-17-2009
Quote:
Originally Posted by Enobarbus37
I have a regexp that I wish to match against every line of a file using awk.
But I do not want to substitute it or select the line.
I want to pull the matched text out and put it in a different file, line by line.
What is the correct awk usage to *extract* a regexp and put it in another file?
Let assume that input.txt contains the text quoted above and " w[^ ]*" is the regexp:
Code:
colemar@deb:~$ awk '{match($0," w[^ ]*",a);print a[0]}' input.txt 
 wish
 want
 want

colemar@deb:~$

If you like to avoid empty lines:
Code:
colemar@deb:~$ awk 'match($0," w[^ ]*",a){print a[0]}' input.txt 
 wish
 want
 want
colemar@deb:~$

Now suppose you want to output only a subexpression, for example you want only the piece following the initial space:
Code:
colemar@deb:~$ awk 'match($0," (w[^ ]*)",a){print a[1]}' input.txt 
wish
want
want
colemar@deb:~$

The above works with GNU awk.
Traditional awk does not allow a third argument for match() and does not support parenthesized subexpressions. Instead you have to rely on the fact that match() sets RSTART and RLENGTH:

Code:
colemar@deb:~$ awk 'match($0," w[^ ]*"){print substr($0,RSTART,RLENGTH)}' input.txt 
 wish
 want
 want
colemar@deb:~$

Last edited by colemar; 05-18-2009 at 03:43 AM..
# 10  
Old 05-18-2009
An easy way will be to count the number of times pattern is coming through grep. and use this count in a loop and paste the pattern that many times

Code:
>output
count=`grep -n "and" file | wc -l`
while [ $count -gt 0 ]
do
echo "and" >> output
count=$(($count-1))
done

# 11  
Old 05-18-2009
Thank you one and all! I have enough to keep me very busy. A great pleasure to receive your replies. Thanks again.
# 12  
Old 05-18-2009
I should add that I had overlooked the -o option to grep which is certainly the first place to start. I apologize for that total rookie error.

My excuse? I was using the "grep" facility in BBEdit which does not include extensions. Sounds bad, doesn't it?

Never underestimate the strength of the Unix Kung Fu.

Encore une fois, mes excuses.
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