#include <iostream>
#include <regex>
#include <string>
#include <vector>
 
std::vector<std::pair<std::string, std::size_t>> Extract(const std::string& s)
{
	std::vector<std::pair<std::string, std::size_t>> res;
	const std::regex reg{R"(([a-zA-Z]+)(\d*))"};
 
	for (auto it = std::sregex_iterator(s.begin(), s.end(), reg); it != std::sregex_iterator(); ++it)
	{
	    auto m = *it;
		res.push_back({m[1], m[2].length() == 0 ? 1 : std::stoi(m[2])});
	}
	return res;
}
 
int main() {
	for (auto p : Extract("C6H12O6")) {
		std::cout << p.first << ": " << p.second << std::endl;
	}
	std::cout << std::endl;
	for (auto p : Extract("H2O")) {
		std::cout << p.first << ": " << p.second << std::endl;
	}	
}

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