Perl: Use backreferences in a replacement string variable

Question

I am performing a string substitution in Perl, but I have both the pattern and the replacement strings stored as scalar variables outside the regular expression operators. The problem is that I want the replacement string to be able to use backreferences.

I hope the code below will illustrate the matter more clearly.

my $pattern = 'I have a pet (\w+).';
my $replacement = 'My pet $1 is a good boy.';
my $original_string = 'I have a pet dog.';

# Not Working
my $new_string = $original_string =~ s/$pattern/$replacement/r;

# Working
#my $new_string = $original_string =~ s/$pattern/My pet $1 is a good boy./r;

# Expected: "My pet dog is a good boy."
# Actual: "My pet $1 is a good boy."
print "$new_string\n";

Answer 1

s/$pattern/My pet $1 is a good boy./

is short for

s/$pattern/ "My pet $1 is a good boy." /e

The replacement expression ( "My pet $1 is a good boy." ) is a string literal that interpolates $1 .

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This means that

s/$pattern/$replacement/

is short for

s/$pattern/ "$replacement" /e

The replacement expression ( "$replacement" ) is a string literal that interpolates $replacement (not $1 ).

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While it may be hindering you, it's a good thing that perl isn't in the habit of executing the contents of variables as Perl code. :)

You can use gsub_copy from String::Substitution to solve your problem.

use String::Subtitution qw( gsub_copy );

my $pattern         = 'I have a pet (\w+)\.';
my $replacement     = 'My pet $1 is a good boy.';
my $original_string = 'I have a pet dog.';

my $new_string = gsub_copy($original_string, $pattern, $replacement);

Answer 2

That $1 in the replacement string is just successive chars $ and 1 , and to make it into a variable for the first capture you'd have to go through bad hoops. ^†

How about an alternative

my string = q(a pet dog);

my $pattern = qr/a pet (\w+)/;

my $new = $string =~ s/$pattern/ repl($1) /er;


sub repl {
    my ($capture) = @_;
    return "$capture is a good boy";
}

where the sub is really just

sub repl { "$_[0] is a good boy" }

It's a little more but then it's more capable and flexible.

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^† Or, as it turns out per ikegami's answer, use String::Substitution which wraps up all the involved 'niceties' into a single call