Question

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

比如,我要印刷与(*)对应的内容。

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

If apply it to

CONSTRAINT `fk_dm` FOREIGN KEY

我想产出

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

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