RegEx to parse or validate Base64 data

Question

From the RFC 4648:

Base encoding of data is used in many situations to store or transfer data in environments that, perhaps for legacy reasons, are restricted to US-ASCII data.

So it depends on the purpose of usage of the encoded data if the data should be considered as dangerous.

But if you’re just looking for a regular expression to match Base64 encoded words, you can use the following:

^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$

^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$

This one is good, but will match an empty String

This one does not match empty string :

^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=|[A-Za-z0-9+/]{4})$

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.

The answers presented so far fail to check that the Base64 string has all pad bits set to 0, as required for it to be the canonical representation of Base64 (which is important in some environments, see https://www.rfc-editor.org/rfc/rfc4648#section-3.5) and therefore, they allow aliases that are different encodings for the same binary string. This could be a security problem in some applications.

Here is the regexp that verifies that the given string is not just valid base64, but also the canonical base64 string for the binary data:

^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/][AQgw]==|[A-Za-z0-9+/]{2}[AEIMQUYcgkosw048]=)?$

The cited RFC considers the empty string as valid (see https://www.rfc-editor.org/rfc/rfc4648#section-10) therefore the above regex also does.

The equivalent regular expression for base64url (again, refer to the above RFC) is:

^(?:[A-Za-z0-9_-]{4})*(?:[A-Za-z0-9_-][AQgw]==|[A-Za-z0-9_-]{2}[AEIMQUYcgkosw048]=)?$