how to check for alphanumeric in c++ code example

Example 1: c++ check if string contains non alphanumeric

#include <algorithm>

any_of(str.begin(), str.end(), [](const char& c) -> bool { return !isalnum(c); });

Example 2: c++ isalphanum

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