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A help in regexp and grep

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# 1  
Old 02-27-2013
A help in regexp and grep

I have test string value , something like the one below

If I type
echo $str | grep -o -e '\|+'

it doesnt give me anything .
But on the contrary
echo $str | grep -o -e '|'

display the only one pipe character(|) thats there in the string above .
The way I understood Unix regexp and grep command is
+ : matches one / more occurence of the preceeding character
-o : displays all the occurences of patter in the file / standard input

So why the first pattern yeilds nothing but the second does ?
Does grep -o option match literal values only ?
Please help .


Last edited by Scrutinizer; 02-27-2013 at 07:29 AM.. Reason: code tags
# 2  
Old 02-27-2013
You need to escape the '+', not the pipe character:
echo $str | grep -o -e '|\+'

# 3  
Old 02-27-2013
Single quoting supersedes escaping, so it is looking for \|+ or |\+verbatim. Put the expression in double quotes. Interpreting the + needs extended regular expressions ("ERE"), so use grep -E. As | is the branch separator in ERE, you need to escape it. The -e option is dispensabe. So the following will work:
$ echo $str | grep -Eo  "\|+"

# 4  
Old 02-27-2013
@Rudi, the double quotes or single quotes do not make a difference in this case:
$ echo "$str" |  grep -Eo '\|+'

This User Gave Thanks to Scrutinizer For This Post:
# 5  
Old 02-27-2013
Rats! You're right. Writing a thesis and then the basics are wrong... still it needs the -E
# 6  
Old 02-27-2013
@Subbeh : Tried echo $str | grep -o -e '|\+' , works good , but fails while using the following command
echo $str | grep -o -e '|\*'
# 7  
Old 02-27-2013
@kumarjt, Subbeh, that would only work with GNU grep (and then -e would be unnecessary and \+ would be a GNU extension on Basic Regular Expression and * would not need a \ )
IMO it is better to use RudiC's suggestion and use the -E option (Extended Regular Expression)

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