Neither a ":" nor a "." will show up in valid Base64, so I think you can unambiguously throw away the http://www.stackoverflow.com
line. In Perl, say, something like
my $sanitized_str = join q{}, grep {!/[^A-Za-z0-9+\/=]/} split /\n/, $str;
say decode_base64($sanitized_str);
might be what you want. It produces
This is simple ASCII Base64 for StackOverflow exmaple.