What I want to do

I have a set of substitutions where both the pattern and the replacements are read from a config file at run time. Handling the patterns with qr// is no sweat, but the replacements pose more of a challenge, specifically in dealing with capture variables ($1 and friends).

What I've tried

use strict;
use warnings;


my $repl = '$1';
{ 
    print "attempt 1 ";
    my $str = "a,A,c,d";
    $str =~ s/([a-z]),/$repl/ig;
    print "yields: $str\n";
}

{ 
    no strict 'refs';
    print "attempt 2 ";
    my $str = "a,A,c,d";
    $str =~ s/([a-z]),/$$repl/ig;
    print "yields: $str\n";
}

{ 
    print "attempt 3 ";
    my $str = "a,A,c,d";
    $str =~ s/([a-z]),/eval "$repl"/ieg;
    print "yields: $str\n";
}


my $code = 'sub {$_[0] =~ s/([a-z]),/'. $repl . '/ig}';
my $re = eval ($code);
if ($@) { print "eval failed: $@" }
{ 
    print "attempt 4 ";
    my $str = "a,A,c,d";
    $re->($str);
    print "yields: $str\n";
}

__END__

attempt 1 yields: $1$1$1d
Use of uninitialized value in substitution (s///) at zwomp.pl line 17.
attempt 2 yields: d
attempt 3 yields: aAcd
attempt 4 yields: aAcd

[download]
Attempts 3 and 4 work correctly. 3 is useless to me because this will be in a long running loop, and my goal is to expunge string evals from the loop. 4 is passable, since the eval can be done outside of the loop. That being said, 2 looks like it should work, or could be made to work, but I can't get it to. The uninitialized value warning leads me to think it's grabbing $1 from outside the s//, but I'm not really sure how to correct for this. Does anyone have any advice, or any other thoughts on ways to approach this problem?

For the curious, why I'm doing this

I'm building a text transformation engine, not entirely unlike a very simple parser (much too simple for Parse::RecDescent to be helpful). In essence, it has a collection of transformations which it applies to named input "tokens" to get named output "tokens". For example, it might have a rule to the effect of
URL: 
    domain: 
        match: http://(\w+)\.(\w+)\.
        replace: $2

[download]
which should be read (in some kind of pseudo-code) as:
apply s/http://(\w+)\.(\w+)\./$2/ to 'URL' to produce 'domain'

[download]
The transformations themselves come out of a config file because several different languages need to implement a consistent set of transformations. This way, as long as each language has a reliable way of reading the input file and generating translation rules, and changes to the rules are made only in the config file, new rules or modifications will be applied in all places automatically. The goal is to prevent inconsistency between the different (language) translation processes.

Any thoughts on this would be greatly appreciated.

In reply to Dynamically generating the replacement part of a substitution by chaos_cat

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