#!/usr/bin/perl

if ('abc' =~ qr/a(.)c/) {
    print "a$1$1c", "\n";
}

sub expand {
    my ($rx, $from, $to) = @_;
    if ($from =~ $rx) {
        print eval {$to}, "\n";
    }
}

expand(qr/a(.)c/, 'abc', 'a$1$1c');

##</code><code>##

abbc
abbc

##</code><code>##

abbc
a$1$1c