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

Question

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.

Answer 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.)

Answer 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. )