[ruby-talk:6438] variable in regular expression

hello,

I have recently bought the new book about ruby and are new trying to learn.
I have one question about variable in regular expressions

I have a little script which I want to use to rename files in a directory:
the code is a follows:

old = ARGV[1]
new = ARGV[2]
dir = Dir.new(".");
while line = dir.read
  next if line == "." or line == ".."
  puts line
  line.sub!(/old/,  new)    #!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! Error
  puts line
end


in the line with "line.sub" there is an error. I want to use the values
in the variable "old" and "new" in this method, but I can not figure out
how to to so.

In Perl is goes like this:

$old = $ARGV[0];
$new = $ARGV[1];
opendir(DIR, ".");
while(defined ($line = readdir(DIR))) {
  next if $line eq "." or $line eq "..";
  print "$line\n";
  $line =~ s/$old/$new/;
  print "$line\n";
}

here I just put a "$"-sign in front of the variables and the regex-parser
interpolates the variable.

can anyone tell me, how to do this in ruby????
thanks a lot

markus