Como verificar o tipo de um caractere em c++, biblioteca cctype - Curso de C++ #81 - YouTube
isalnum() Function in C : How to check if a character is alphanumeric or Not - YouTube
Looking at The Source Code for Function isalpha, isdigit, isalnum, isspace, islower, isupper, isxdigit, iscntrl, isprint, ispunct, isgraph, tolower, and, toupper in C Programming | Tachyon
C Language - C isalnum function checks its argument is either a letter of the alphabet or a digit