Scalar and List Context with the Binding Operator

Perl Regular Expressions – Part 6

Perl Course

Foreword: In this part of the series, I illustrate the scalar and list context behavior, of the binding operator.

By: Chrysanthus Date Published: 6 Oct 2015

Introduction

This is part 6 of my series, Perl Regular Expressions. In this part of the series, I illustrate the scalar and list context behavior, of the binding operator. You should have read the previous parts of the series before coming here, as this is a continuation.

Recall: The number 0, the string '0', the string "", the empty list (), and undef are all false in a Boolean context. If a value is not false, it is true. So, if a value is not equivalent to 0, or '0', or "", or (), or undef, then it is true. For example, –1 is true, 1 is true, and 2 is true. In fact, all numbers are true except 0. The string, "something" is true. In fact any non-empty string is true, including " ", which has just a space character.

Scalar Context
In scalar context, the binding expression returns 1 for true and 0 for false. Scalar context is when the expression should be either successful or failed; that is, whether the search item was found or not. List context is when the expression should return a list of at least one string (value). The following is a simple scalar context expression:

    "Hello World" =~ /World/

This expression returns 1, for matching. It would return undef if there was no matching. The return value can be assign to a variable as follows:

    my $var = "Hello World" =~ /World/;

If you print $var, you will have 1 (or undef if matching did not occur – undef is not printable). You can place the expression in a condition as follows:

if ("Hello World" =~ /World/)
  {
    print "Matched";
  }
else
  {
    print "Not Matched";
  }

In this case the if-condition boils down to 1 (or undef). In Perl, 1 is true and undef is false.

List Context
List context is when the binding expression should return at list one string (value). When you use capturing groups in your regex, a list would be returned. When you use the global (g) modifier, you would also return a list.

When you know the number of return values, you can receive the list with a list of scalar variables. When you do not know the number, you should receive the list with an array.

The following code receives the list of captured strings (groups) with a list of scalar variables:

use strict;

my ($var1, $var2) = ("I am a boy and you are a girl." =~ /(boy).*(girl)/);

print $var1, "\n";

print $var2, "\n";

The output is:

boy
girl

The following code is the same as the above but an array is used to receive the captured strings:

use strict;

my @arr = "I am a boy and you are a girl." =~ /(boy).*(girl)/;

print $arr[0], "\n";

print $arr[1], "\n";

The following code receives the list of globally matched strings with a list of scalar variables:

use strict;

my $subject = "A cat is an animal. A rat is an animal. A bat is a creature.";

my ($var1, $var2, $var3) = $subject =~ /[cbr]at/g;

print "$var1, $var2, $var3";

The output is:

    cat, rat, bat

The following code receives the list of globally matched strings with an array:

use strict;

my $subject = "A cat is an animal. A rat is an animal. A bat is a creature.";

my @arr = $subject =~ /[cbr]at/g;

print "$_ " foreach @arr;

The output is:

    cat rat bat

That is it for this part of the series. We stop here and continue in the next part.

Chrys

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