How To Use Backreference in Grep

$ 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

Tags:

Unix

Regex

Grep

Text Processing

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