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4.36: How can I expand variables in text strings?

(contributed by brian d foy)

If you can avoid it, don't, or if you can use a templating system, such
as "Text::Template" or "Template" Toolkit, do that instead. You might
even be able to get the job done with "sprintf" or "printf":

my $string = sprintf 'Say hello to %s and %s', $foo, $bar;

However, for the one-off simple case where I don't want to pull out a
full templating system, I'll use a string that has two Perl scalar
variables in it. In this example, I want to expand $foo and $bar to
their variable's values:

my $foo = 'Fred';
my $bar = 'Barney';
$string = 'Say hello to $foo and $bar';

One way I can do this involves the substitution operator and a double
"/e" flag. The first "/e" evaluates $1 on the replacement side and turns
it into $foo. The second /e starts with $foo and replaces it with its
value. $foo, then, turns into 'Fred', and that's finally what's left in
the string:

$string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'

The "/e" will also silently ignore violations of strict, replacing
undefined variable names with the empty string. Since I'm using the "/e"
flag (twice even!), I have all of the same security problems I have with
"eval" in its string form. If there's something odd in $foo, perhaps
something like "@{[ system "rm -rf /" ]}", then I could get myself in

To get around the security problem, I could also pull the values from a
hash instead of evaluating variable names. Using a single "/e", I can
check the hash to ensure the value exists, and if it doesn't, I can
replace the missing value with a marker, in this case "???" to signal
that I missed something:

my $string = 'This has $foo and $bar';

my %Replacements = (
foo => 'Fred',

# $string =~ s/\$(\w+)/$Replacements{$1}/g;
$string =~ s/\$(\w+)/
exists $Replacements{$1} ? $Replacements{$1} : '???'

print $string;


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