check if e-mail passed as argument is valid

main.cpp

#include <iostream>
#include <string>
#include <algorithm> 

bool is_digits(const std::string &s)
{   
    return std::any_of(s.begin(), s.end(), ::isdigit);
}

bool is_all_lower(const std::string &s)
{
        return std::all_of(s.begin(), s.end(), ::islower);
}


std::string isValid(std::string email)
{
    std::size_t first_delimeter = email.find("@");
    std::size_t second_delimeter = email.find(".");
    
    if(first_delimeter != std::string::npos && second_delimeter != std::string::npos)
    {
        std::string name = email.substr(0, first_delimeter);
        std::string provider = email.substr(first_delimeter + 1, (second_delimeter - first_delimeter) - 1);
        std::string adress = email.substr(second_delimeter + 1);
    
        if(name.size() == 5 && !is_digits(name) && is_all_lower(name)){
            if(provider == "hogwarts" && adress == "com"){
                return "Yes";
            }
        }      
    }
    
    return "NO";
}

int main(const int argc, const char **argv)
{
    if(argc < 2){
        std::cout << "usage: " << argv[0] << " [email protected]" << std::endl;
        return EXIT_FAILURE;
    }
    
    std::cout << isValid(argv[1]) << std::endl;
    
    return EXIT_SUCCESS;
}