Поиск букв и цифр по строке(предложению)

#include <iostream>
#include <string>
#include <vector>
using namespace std;
 
std::vector<std::string> split(const std::string &string, char sep, bool keepEmptyParts = false) {
    std::vector<std::string> list;
    std::string::size_type start = 0;
    std::string::size_type end;
    while ((end = string.find(sep, start)) != std::string::npos) {
        if (start != end || keepEmptyParts)
            list.push_back(string.substr(start, end - start));
        start = end + 1;
    }
    if (start != string.size() || keepEmptyParts)
        list.push_back(string.substr(start));
    return list;
}
 
int main()
{
    string source;
    cout << "Input string : "; 
        getline(cin, source);
 
    size_t countRusKeys_small = 0;
    size_t countRusKeys_big = 0;
    size_t countEnKeys_small = 0;
    size_t countEnKeys_big = 0;
    size_t countNum = 0;
    size_t spec_symbol = 0;
 
    for (char c : source)
    if (c >= 'a' && c <= 'z')
        ++countEnKeys_small;
    else if (c >= 'A' && c <= 'Z')
        ++countEnKeys_big;
    else if (c >= 'а' && c <= 'я') 
        ++countRusKeys_small;
    else if (c >= 'А' && c <= 'Я')
        ++countRusKeys_big;
    else if (c >= '0' && c <= '9')
        ++countNum;
    else
        ++spec_symbol;
 
    cout << "count rus keys small = " << countRusKeys_small << endl;
    cout << "count rus keys big = " << countRusKeys_big << endl;
    cout << "count en keys small = " << countEnKeys_small << endl;
    cout << "count en keys big = " << countEnKeys_big << endl;
    cout << "count numeric = " << countNum << endl;
    cout << "count spec_symbol = " << spec_symbol << endl;
 
    return 0;
}