如何在Grep中使用Backreference

Question

我有一个带反向引用的正则表达式。如何在bash脚本中使用它？

例如，我想打印与(.*)匹配的内容

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

如果将其应用于

CONSTRAINT `fk_dm` FOREIGN KEY

我想输出

fk_dm

Answer 1

$ 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 

Answer 2

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