Perl Cookbook

Expanding Variables in User Input

Problem

You’ve read in a string with an embedded variable reference, such as:

You owe $debt to me.

Now you want to replace $debt in the string with its value.

Solution

Use a substitution with symbolic references if the variables are all globals:

$text =~ s/\$(\w+)/${$1}/g;

But use a double /ee if they might be lexical (my) variables:

$text =~ s/(\$\w+)/$1/gee;

Discussion

The first technique is basically “find what looks like a variable name, and then use symbolic dereferencing to interpolate its contents.” If $1 contains the string somevar, then ${$1} will be whatever $somevar contains. This won’t work if the use strict 'refs' pragma is in effect because that bans symbolic dereferencing.

Here’s an example:

use vars qw($rows $cols);
no strict 'refs';                   # for ${$1}/g below
my $text;

($rows, $cols) = (24, 80);
$text = q(I am $rows high and $cols long);  # like single quotes!
$text =~ s/\$(\w+)/${$1}/g;
print $text;

                  I am 24 high and 80 long

You may have seen the /e substitution modifier used to evaluate the replacement as code rather than as a string. It’s designed for situations such as doubling every whole number in a string:

$text = "I am 17 years old";
$text =~ s/(\d+)/2 * $1/eg;

When Perl is compiling your program and sees a /e on a substitute, it compiles the code in the replacement block along with the rest of your program, long before the substitution actually happens. When a substitution is made, $1 is replaced with the string that matched. The code to ...