in reply to Re: Reliable way to detect base64 encoded strings
in thread Reliable way to detect base64 encoded strings

This works nicely. I don't have a header to work with so this good. There are a few things I don't quite understand in that regex though, would you mind commenting each line so I can get my head around it? Thanks a lot!
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Re^3: Reliable way to detect base64 encoded strings
by ikegami (Patriarch) on Jun 29, 2009 at 22:14 UTC
    It's actually really straightforward.
    • Start of input
    • Followed by any number of groups of 4 characters from [A-Za-z0-9+/],
    • Followed by one of the following:
      • [always matches]
      • Four characters where
        • The first and second match /[A-Za-z0-9+/]/
        • The third matches /[AEIMQUYcgkosw048]/
        • The fourth is a "="
      • Four characters where
        • The first matches /[A-Za-z0-9+/]/
        • The second matches /[AQgw]/
        • The third and fourth are both a "="
    • Followed by the end of input

    It's probably a bit simpler after the update I just did for you:

    • Start of input
    • Followed by any number of groups of 4 characters from [A-Za-z0-9+/],
    • Followed by zero or one of the following:
      • Four characters where
        • The first and second match /[A-Za-z0-9+/]/
        • The third matches /[AEIMQUYcgkosw048]/
        • The fourth is a "="
      • Four characters where
        • The first matches /[A-Za-z0-9+/]/
        • The second matches /[AQgw]/
        • The third and fourth are both a "="
    • Followed by the end of input
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      Your explanation makes it very clear... thanks again!
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      Regex are not my favorite part of Perl....even if they are powerful. I'm trying to use your way to detect if a string is base64...so I was taking what you had and putting an if around it. I'm sure I'm just not getting it....could you give me some pointers? 

      if($string_whole =~ m 
/
    ^ 
    (?: [A-Za-z0-9+/]{4} )*
    (?: [A-Za-z0-9+/]{2} [AEIMQUYcgkosw048] = 
    |   [A-Za-z0-9+/] [AQgw] ==
    )?
    \z
/x
) $&Log ("it found base64-$i");

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      Thanks in advance for your expertise.
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        You have the right idea, but 

        if (...) $&Log ("it found base64-$i");

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        is not valid Perl. You want 
        if($string_whole =~ m/
    ^ 
    (?: [A-Za-z0-9+/]{4} )*
    (?: [A-Za-z0-9+/]{2} [AEIMQUYcgkosw048] = 
    |   [A-Za-z0-9+/] [AQgw] ==
    )?
    \z
/x) {
   Log("$string_whole is valid base64");
}

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