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Too easy ......very very easy
this kata is not worth 5kyu at least to me, too easy
a really great kata for C, but might be too easy in other languages.
you're accepting all characters above ch 48 except 95... Which means you are accepting 69 of ASCIIs 128 characters... There are not 69 alphanumeric characters.
You can use a built-in method, or you can challenge yourself by using Regex, which is the obvious intent of the kata. Still not difficult.
Good test of basic methods knowledge, not a 5kyu challenge though. Might be better if there was some way to ban isalnum usage.
This kata needs to be changed from 5kyu to (6kyu | 7kyu)
Needs random testcases for ruby.
C translation kumited
please scrutinize for approval