How to search and replace a multi-line string in a file on Linux

Last updated on July 4, 2020 by Dan Nanni

Suppose you have a text file that looks like the following.

Beginning of a text file.

This is a cat.
This is a dog.
This is a guinea pig.
This is a gecko.
This is a hamster.
This is a parrot.
This is a rabbit.
This is a ferret.
This is a turtle.

You want to replace the highlighted multiple lines with the following one-line, but without using a text editor.

Beginning of a text file.

This is a cat.
This is a dog.
These were removed.
This is a parrot.
This is a rabbit.
This is a ferret.
This is a turtle.

Search and Replace a Multi-line String Pattern

When it comes to finding and replacing a multi-line string pattern, Perl's regular expression based pattern matching comes in handy.

The following one-liner can get the job done.

$ perl -i -0pe 's/<multi-line-string-pattern>/<replacement-string>/' input.txt

A couple of notes in the above expression:

• The -i option tells Perl to perform in-place editing, meaning that Perl reads the input file, makes substitutions, and writes the results back to the same input file. If you want to dry-run the changes, you can omit this option, in which case the results will be shown to stdout without modifying the input file.
• The -0 option turns Perl into "file slurp" mode, where Perl reads the entire input file in one shot (intead of line by line). This enables multi-line search and replace.
• The -pe option allows you to run Perl code (pattern matching and replacement in this case) and display output from the command line.

In this particular example, the following command will replace a multi-line string as required, and display the result (without applying the change to the input file).

$ perl -0pe 's/This is a guinea pig.nThis is a gecko.nThis is a hamster.n/These were removed.n/' input.txt

Once confirming that the result is correct, you can apply the change to the input file by adding -i option:

$ perl -i -0pe 's/This is a guinea pig.nThis is a gecko.nThis is a hamster.n/These were removed.n/' input.txt

Here are some variations:

To find a multi-line string pattern in a case-insensitive fashion:

$ perl -i -0pe 's/<multi-line-string-pattern>/<replacement-string>/i' input.txt

To remove a multi-line string pattern:

$ perl -i -0pe 's/<multi-line-string-pattern>//' input.txt

Search and Replace a Multi-line String in Multiple Files

If you want to find and replace a multi-line string in more than one files, you can use a combination of find and xargs as follows.

$ find ~/doc -name "*.txt" | xargs perl -i -0pe 's/<multi-line-string-pattern>/<replacement-string>/'

This one-liner searches for all text files in ~/doc folder, and apply multi-line string replacement in each file.

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