Perl grep

last modified August 24, 2023

Perl grep tutorial shows how to filter values with grep function in Perl.

The grep function evaluates a block or an expression for each element of a list and returns a list consisting of those elements for which the expression evaluated to true. It sets $_ locally to each element. In scalar context, it returns the number of times the expression was true.

Perl grep simple example

We start with a simple example.

simple.pl

#!/usr/bin/perl

use v5.34.0; use warnings;

my @vals = (-3, -1, 0, 3, -4, 2, 3, 4, -7);

my @res = grep { $_ < 0 } @vals; say “@res”;

my $n = grep { $_ < 0 } @vals; say “there are $n negative values”;

The example uses grep on a list of integers.

my @res = grep { $_ < 0 } @vals; say “@res”;

Here we filter out all negative values.

my $n = grep { $_ < 0 } @vals; say “there are $n negative values”;

In a scalar context, we get the number of elements that match the given condition.

$ ./simple.pl -3 -1 -4 -7 there are 4 negative values

Perl grep regex

Regular expressions can be used to filter data.

digits.pl

#!/usr/bin/perl

use v5.34.0; use warnings;

my @vals = (“sky”, 0, -1, “cup”, “breed”, “345”, 12);

my @res = grep /\d/, @vals; say “@res”;

In the example, we use the \d regular expression to filter out all digits from the list.

$ ./digits.pl 0 -1 345 12

In the next example, we grep a list of words.

words.pl

#!/usr/bin/perl

use v5.34.0; use warnings;

my @words = qw/sky war cup cloud abyss water car book note falcon/;

my @res = grep /^[w,c]/, @words; say “@res”;

my @res2 = grep /^…$/, @words; say “@res2”;

We find out all words that start with either ‘w’ or ‘c’ and all words that have exactly three characters.

$ ./words.pl war cup cloud water car sky war cup car

Perl grep file

We can use grep with a file handle.

words.txt

sky water rock falcon cloud war nice cup wrong

We have a small text file.

fhandle.pl

#!/usr/bin/perl

use v5.34.0; use warnings;

open my $fh, “<”, “words.txt” or die “cannot open file $!”;

my @res = grep /^w/, <$fh>; say join “”, @res;

close $fh;

We grep a file handle to find all words that start with ‘w’;

$ ./fhandle.pl water war wrong

In this article we have worked with the grep function in Perl.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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