How to use the /e modifier for code evaluation in substitution?

The /e modifier in Perl’s substitution operator s/// is a powerful but sometimes confusing feature that allows you to execute Perl code during a substitution. Instead of replacing the matched text with a static string, /e tells Perl to treat the replacement part as a Perl expression, evaluate it, and replace the match with the result of that evaluation.

How the /e Modifier Works

Normally, a substitution looks like this:

s/pattern/replacement/;

This replaces pattern with the string replacement. But if you add the /e modifier, Perl evaluates replacement as if it were Perl code each time the pattern matches.

For example:

$text =~ s/(\d+)/$1 * 2/e;

When $text contains numbers, those numbers are doubled because Perl evaluates $1 * 2.

Why Use /e?

• Dynamic replacements: Compute replacements based on matched content.
• Complex transformations: You can run any code, including calling functions or evaluating expressions.
• Flexible conditional replacements: Execute different code depending on the match.

Multiple /e Modifiers

Since Perl 5.10, you can use /ee (double e), to evaluate the substitution result twice, which is helpful if the evaluated code returns another piece of code to be evaluated again.

Common Pitfalls

• Security risk: Running arbitrary code from untrusted input with /e is dangerous and may allow code injection.
• Context awareness: The code runs in scalar context, so be careful when returning lists or arrays.
• Quoting issues: Using quotes inside the replacement string can be tricky, so consider using single quotes around the pattern/replacement to avoid interpolation surprises.

Runnable Example

The following example doubles every number in a string using the /e modifier:

use strict;
use warnings;

my $text = "I have 3 apples and 7 oranges.";
print "Original text: $text\n";

# Double each number found
$text =~ s/(\d+)/$1 * 2/e;

print "Modified text: $text\n";

Output:

Original text: I have 3 apples and 7 oranges.
Modified text: I have 6 apples and 14 oranges.

Explanation of the Code

• s/(\d+)/$1 * 2/e: Finds one or more digits (\d+), captures them in $1.
• The replacement part $1 * 2 is evaluated as Perl code because of /e.
• The resulting number from the multiplication replaces the matched digits.

The use of the /e modifier lets you leverage Perl’s "There's more than one way to do it" (TMTOWTDI) philosophy by enabling code execution right within regex substitutions, offering flexibility beyond static replacements.