[C] isalnum / isalnum.c / isalnum in c

man isalnum

int	isalnum(int c)
{
	if ((c >= 0x30 && c <= 0x39) || (c >= 0x41 && c <= 0x5a)
		|| (c >= 0x61 && c <= 0x7a))
		return (c);
	return (0);
}