Importance of \ [ ] + ? in grep

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# 1  
Old 09-16-2009
Importance of \ [ ] + ? in grep

What is the significance of special characters used in grep command.
Like \ [ ] + ?

What is the meaning of the below command:

grep -o "0078\([0-9]\)\+" $i | sort | uniq

Can i replace + with ?


Last edited by Franklin52; 09-16-2009 at 08:49 AM.. Reason: Correcting code tags
# 2  
Old 09-16-2009
\( \) is used to remember the pattern so that you can recall it later
"+" means one or more matched pattern
"?" means one or zero matched pattern
So you probably can't replace + with ?

Take a look at this : Regular Expressions
# 3  
Old 09-16-2009
Thanks thanhdat.

Just want to know whether my understandin of the below expression is correct.

grep -o "0034\([0-9]\)\+" => This will fetch all numbers starting with 0034
grep -o "0034[0-9]\+" => This will fetch anything that follows 0034. Say for example: 003483744 0034abcd
But the first one picks only 0034, followed by numbers. Am I Right?

# 4  
Old 09-16-2009
Did you test on your system, what did those commands return ?
In my test (with debian 5) it should be :
$cat test
1222 345345

$egrep -o "12[0-9]+" test

And this command should be translated to : find all numbers starting with 12
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