Check out example codes for "c++ isalphanum". It will help you in understanding the concepts better.

Code Example 1

string test = "Ab4 :";
std::isalnum(test[0]);	//true
std::isalnum(test[1]);	//true
std::isalnum(test[2]);	//true
std::isalnum(test[3]);	//false
std::isalnum(test[4]);	//false

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