How can I use a variable in the replacement side of the Perl substitution operator?

I would like to do the following:

$find = "start (.*) end";
$replace = "foo \1 bar";

$var = "start middle end";
$var =~ s/$find/$replace/;

I would expect $var to contain "foo middle bar", but it does not work. Neither does:

$replace = 'foo \1 bar';

Somehow I am missing something regarding the escaping.

Solution 1:

On the replacement side, you must use $1, not \1.

And you can only do what you want by making replace an evalable expression that gives the result you want and telling s/// to eval it with the /ee modifier like so:

$find="start (.*) end";
$replace='"foo $1 bar"';

$var = "start middle end";
$var =~ s/$find/$replace/ee;

print "var: $var\n";

To see why the "" and double /e are needed, see the effect of the double eval here:

$ perl
$foo = "middle";
$replace='"foo $foo bar"';
print eval('$replace'), "\n";
print eval(eval('$replace')), "\n";
__END__
"foo $foo bar"
foo middle bar

(Though as ikegami notes, a single /e or the first /e of a double e isn't really an eval(); rather, it tells the compiler that the substitution is code to compile, not a string. Nonetheless, eval(eval(...)) still demonstrates why you need to do what you need to do to get /ee to work as desired.)

Solution 2:

Deparse tells us this is what is being executed:

$find = 'start (.*) end';
$replace = "foo \cA bar";
$var = 'start middle end';
$var =~ s/$find/$replace/;

However,

 /$find/foo \1 bar/

Is interpreted as :

$var =~ s/$find/foo $1 bar/;

Unfortunately it appears there is no easy way to do this.

You can do it with a string eval, but thats dangerous.

The most sane solution that works for me was this:

$find = "start (.*) end"; 
$replace = 'foo \1 bar';

$var = "start middle end"; 

sub repl { 
    my $find = shift; 
    my $replace = shift; 
    my $var = shift;

    # Capture first 
    my @items = ( $var =~ $find ); 
    $var =~ s/$find/$replace/; 
    for( reverse 0 .. $#items ){ 
        my $n = $_ + 1; 
        #  Many More Rules can go here, ie: \g matchers  and \{ } 
        $var =~ s/\\$n/${items[$_]}/g ;
        $var =~ s/\$$n/${items[$_]}/g ;
    }
    return $var; 
}

print repl $find, $replace, $var; 

A rebuttal against the ee technique:

As I said in my answer, I avoid evals for a reason.

$find="start (.*) end";
$replace='do{ print "I am a dirty little hacker" while 1; "foo $1 bar" }';

$var = "start middle end";
$var =~ s/$find/$replace/ee;

print "var: $var\n";

this code does exactly what you think it does.

If your substitution string is in a web application, you just opened the door to arbitrary code execution.

Good Job.

Also, it WON'T work with taints turned on for this very reason.

$find="start (.*) end";
$replace='"' . $ARGV[0] . '"';

$var = "start middle end";
$var =~ s/$find/$replace/ee;

print "var: $var\n"


$ perl /tmp/re.pl  'foo $1 bar'
var: foo middle bar
$ perl -T /tmp/re.pl 'foo $1 bar' 
Insecure dependency in eval while running with -T switch at /tmp/re.pl line 10.

However, the more careful technique is sane, safe, secure, and doesn't fail taint. ( Be assured tho, the string it emits is still tainted, so you don't lose any security. )

Solution 3:

As others have suggested, you could use the following:

my $find = 'start (.*) end';
my $replace = 'foo $1 bar';   # 'foo \1 bar' is an error.
my $var = "start middle end";
$var =~ s/$find/$replace/ee;

The above is short for the following:

my $find = 'start (.*) end';
my $replace = 'foo $1 bar';
my $var = "start middle end";
$var =~ s/$find/ eval($replace) /e;

I prefer the second to the first since it doesn't hide the fact that eval(EXPR) is used. However, both of the above silence errors, so the following would be better:

my $find = 'start (.*) end';
my $replace = 'foo $1 bar';
my $var = "start middle end";
$var =~ s/$find/ my $r = eval($replace); die $@ if $@; $r /e;

But as you can see, all of the above allow for the execution of arbitrary Perl code. The following would be far safer:

use String::Substitution qw( sub_modify );

my $find = 'start (.*) end';
my $replace = 'foo $1 bar';
my $var = "start middle end";
sub_modify($var, $find, $replace);