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Extracting a regex matched with 'sed' without printing the surrounding characters

How do I make this only print test:

echo "atestb" | sed -n 's/\(test\)/\1/p'
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marked as duplicate by Gilles, Mat, Renan, jasonwryan, Stéphane Gimenez Aug 20 '12 at 17:06

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Must use sed? grep's -o switch looks like a shorter and cleaner way: echo "atestb" | grep -o 'test'. –  manatwork Jul 16 '12 at 7:54
    
I can't use sed because the real thing is like echo 'afoo="test"b' | sed -n 's/.*foo="\([^"]*\)".*/\1/p', where I only want to match test. –  Tyilo Jul 16 '12 at 15:53

1 Answer 1

up vote 9 down vote accepted

You need to match the whole line:

echo "atestb" | sed -n 's/.*\(test\).*/\1/p'

or

echo "atestb" | sed 's/.*\(test\).*/\1/'
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I had to add the -r or ` --regexp-extended` option otherwise I was getting invalid reference \1 on s' command's RHS ` error. –  Daniel Sokolowski Aug 11 at 16:12
    
@danielsokolowski: this example uses basic regular expressions (BRE) and thus capture groups are noted with escaped parentheses. If you add -r to the above you should get an error saying "invalid reference". What version of sed are you using? –  Thor Aug 11 at 20:46
    
sed (GNU sed) 4.2.2 on cygwin –  Daniel Sokolowski Aug 12 at 4:42

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