Range of valid character for a base 64 encoding

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Here is what I could turn up: RFC 4648

It includes this convenient table:

                  Table 1: The Base 64 Alphabet

 Value Encoding  Value Encoding  Value Encoding  Value Encoding
     0 A            17 R            34 i            51 z
     1 B            18 S            35 j            52 0
     2 C            19 T            36 k            53 1
     3 D            20 U            37 l            54 2
     4 E            21 V            38 m            55 3
     5 F            22 W            39 n            56 4
     6 G            23 X            40 o            57 5
     7 H            24 Y            41 p            58 6
     8 I            25 Z            42 q            59 7
     9 J            26 a            43 r            60 8
    10 K            27 b            44 s            61 9
    11 L            28 c            45 t            62 +
    12 M            29 d            46 u            63 /
    13 N            30 e            47 v
    14 O            31 f            48 w         (pad) =
    15 P            32 g            49 x
    16 Q            33 h            50 y

So a regular expression that matches any character that should never appear in Base 64 encodings would be:

[^A-Za-z0-9+/=]

However, as kapeps answer points out, this is only the recommendation. Specific implementations might choose a different set of 64 characters. (In fact, even the linked RFC contains an alternative table for URL and filename safe encoding, which replaces character 62 and 63 with - and _ respectively). So I guess it really depends on the implementation that created the encoding.

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