in reply to mmv-like hack in perl?

This won't work: 
my $pattern = q/(...)(...)/;
my $replace = q/$2-$1/;
s/$pattern/$replace/g;

[download]
because the $replace is inserted into the "double-quoted" string on the right side, and not again rescanned to see if it has any variables to be replaced. So you get a literal $1 and so on.

However, if you trust your invoker, you could try something like: 

my $pattern = q/(...)(...)/;
my $replace = q/$2-$1/;
s/$pattern/qq{$replace}/eeg;

[download]
as long as the curly braces balance nicely.

-- Randal L. Schwartz, Perl hacker
Be sure to read my standard disclaimer if this is a reply.

Replies are listed 'Best First'.
Re: •Re: mmv-like hack in perl?
by mpd (Monk) on Apr 14, 2003 at 05:20 UTC
    Thank you for your help. I think I'm still having a problem with the line:

    s/$pattern/qq{$replace}/eeg;

    Here's the snippet that's failing:

    # get strings from GUI.  
my $old = my $new = $_->children->get; # get old filename
my $pattern = $form->{'entry2'}->get_text();
# e.g. $pattern == "^(\d{2})\. (.*)$" after this
my $replace = $form->{'entry1'}->get_text();
# e.g. $replace == "two-$2, one-$1" after this

# is the following match necessary? $1, etc
# need to be assigned somehow...
if($old =~ /$pattern/) {
    $new =~ s/$pattern/qq{$replace}/eeg;
    print STDERR $old." => ".$new."\n";
}

    [download]

    After the $new =~ line, $new is empty, every time. Am I missing something? Thanks again

[reply]
[d/l]
[select]
      $new = $old;
$new =~ s/$pattern/qq{$replace}/eeg;

      [download]
      But you're scaring me here. I said that the string had to be from a trusted source. As soon as you start talking "GUI", that makes me think that the user-ID of the person typing the string is not necessarily the user-ID of the person running the code.

      Let me reemphasize. Using this code as-is will allow any arbitrary Perl code to be executed, including shelling out to execute arbitrary system commands. Clear? It's not just about mangling a string. You must either trust the invoker, or not use this code.

      -- Randal L. Schwartz, Perl hacker
      Be sure to read my standard disclaimer if this is a reply.

[reply]
[d/l]
        Perfectly clear. That's what I want to do, because this is just a toy for me to play with. I don't plan on releasing this piece of my junky code. I will likely end up parsing it myself as Aristotle suggests below. If something clever comes out, I will post that bit of code, of course.

        Thanks again.

[reply]

In Section Seekers of Perl Wisdom