scrub 'pure', 'p!u-r-*e'; # returns pure

##</code><code>##

sub string_clean {
  my ($pure, $dirty) = @_;

  my @pure  = (split //, $pure) ;
  my @dirty = (split //, $dirty) ;
  my %pure  = map { ($_ => 1) } @pure;

  my @cleaned = grep { $pure{$_} } @dirty;
  return join '', @cleaned;
}

##</code><code>##

scrub 'impure' => 'pure' ; # yields 'pure'