How To Use Backreference in Grep

Question

I have a regular expression with a backreference. How can use it in a bash script?

Such as I want to print what matches to (.*)

grep -E "CONSTRAINT \`(.*)\` FOREIGN KEY" temp.txt

If apply it to

CONSTRAINT `fk_dm` FOREIGN KEY

I want to output

fk_dm

score 19 · Accepted Answer · answered Jan 11 '12 at 11:58

19

$ echo 'CONSTRAINT `helloworld` FOREIGN KEY' | grep -oP '(?<=CONSTRAINT `).*(?=` FOREIGN KEY)'
helloworld

-o, --only-matching       show only the part of a line matching PATTERN
-P, --perl-regexp         PATTERN is a Perl regular expression

(?=pattern)
    is a positive look-ahead assertion
(?!pattern)
    is a negative look-ahead assertion
(?<=pattern)
    is a positive look-behind assertion
(?<!pattern)
    is a negative look-behind assertion

answered Jan 11 '12 at 11:58

kev

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Could you elaborate on ?<= and ?= – metdos Jan 11 '12 at 12:01
What if I want to print multiple matches, can I do this? – metdos Jan 11 '12 at 12:03
Your should use `substitution`. – kev Jan 11 '12 at 12:12
Actually I noticed that this returns all instances. – metdos Jan 11 '12 at 13:26
great example. have to resurrect this if you don't mind. – jredd Aug 07 '17 at 02:14

score -1 · Answer 2 · answered Jan 11 '12 at 11:53

-1

grep -E 'CONSTRAINT \`(.*)\` FOREIGN KEY' temp.txt

answered Jan 11 '12 at 11:53

Eugen Rieck

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You repeated part of the OP's question, changing only the quotes which have no effect in this case. – Dennis Williamson Jan 11 '12 at 17:29
They do: The OP asked "How can use it in a bash script?" and the single quotes are the answer. – Eugen Rieck Jan 11 '12 at 17:31
The backticks are escaped in the original so they aren't seen as command substitution. – Dennis Williamson Jan 11 '12 at 17:41

How To Use Backreference in Grep

2 Answers2