Did you ever saw a char and thought: "Damn, 1 byte for a single char is pretty darn inefficient"? No? Well I did. So what I decided to do instead is to pack 5 chars, convert each char to a 2 digit integer and then concat those 5 2 digit ints together into one big unsigned int and boom, I saved 5 chars using only 4 instead of 5 bytes. The reason this works is, because one unsigned int is a ten digit long number and so I can save one char using 2 digits. In theory you could save 32 different chars using this technique (the first two digits of an unsigned int are 42 and if you dont want to account for a possible 0 in the beginning you end up with 32 chars). If you would decide to use all 10 digits you could save exactly 3 chars. Why should anyone do that? Idk. Is it way to much work to be useful? Yes. Was it funny? Yes.
Anyone whos interested in the code: Heres how I did it in C: https://pastebin.com/hDeHijX6
Yes I know, the code is probably bad, but I do not care. It was just a funny useless idea I had.
The original switch statement is great as it is. Easy to understand at a quick glance.
Yes, it's easy to understand, but this ifelse is much safer because it "handles" uppercase and digits by just returning 0. If you'd call OP's function with something like 'A', you'll get nonsense data because it doesn't have a default. (I think it will return whatever value currently resides in eax)
Can easily be dealt with by adding a default case. You also figured out this undefined behavior because you easily understood the switch statement.
Fair.
A single line comment would make it as easy to understand, and much more flexible if you wanted to add handling upper case letters or digits. Or even remap the values to a more standard 6 bit encoding.