use strict;
use warnings;

print filter($_), "\n" for <DATA>;

sub filter {
	my $str = shift;
	defined $str or return '';
	my $acceptable = 'a-z0-9_.';
	
	$str =~ s/[^$acceptable]//gi;

	return $str;
}

__DATA__
Hello, world!
te-st%$/*.
All_of_this_is_OK.
!@#$%^&*()12345

##</code><code>##

Helloworld
test.
All_of_this_is_OK.
12345