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

##</code><code>##

$ perl -MMIME::Base64 -le'print decode_base64("This sentence would be found valid up to the final period")' | od -t x1
0000000 4e 18 ac b1 e9 ed 7a 77 1e c2 8b a5 75 b7 9f a2
0000020 e9 dd bd a9 62 76 ea 6d a2 d8 5e 7e 29 da 96 97
0000040 ab 8a 87 0a
0000044