Compilable Regex Version

#include <iostream>
#include <string>
using namespace std;

template <const char Expression[], char C = Expression[0], size_t index = 0>
struct myRegex
{
    myRegex(string&& _str): str(_str) {}

    bool isMatch()
    {
        return str.front()==C ? myRegex<Expression, Expression[index+1], index+1>(str.substr(1)).isMatch() : false;
    }

private:
    string str;
};

template <const char Expression[], size_t index>
struct myRegex<Expression, '?', index>
{
    myRegex(string&& _str): str(_str) {}

    bool isMatch()
    {
        return myRegex<Expression, Expression[index+1], index+1>(str.substr(1)).isMatch();
    }

private:
    string str;
};

template <const char Expression[], size_t index>
struct myRegex<Expression, '\0', index>
{
    myRegex(string&& _str): str(_str) {}

    bool isMatch()
    {
        return true;
    }

private:
    string str;
};

extern constexpr char Expression[] = "a?b";
int main(void)
{
    cout << myRegex<Expression>("a+b").isMatch();
}