Simple Regular Expression Matching with NFA

'.' Matches any single character.
'*' Matches zero or more of the preceding element.

The matching should cover the entire input string (not partial).

The function prototype should be:
bool isMatch(const char *s, const char *p)

Some examples:
isMatch("aa","a") → false
isMatch("aa","aa") → true
isMatch("aaa","aa") → false
isMatch("aa", "a*") → true
isMatch("aa", ".*") → true
isMatch("ab", ".*") → true
isMatch("aab", "c*a*b") → true


Solution with state machine. 
Runtime O(nm) where n=len(s), m=len(p)

//
// Created by Sasinda Premarathna on 4/22/17.
//
#include <string>
#include <list>
#include <iostream>
#include <set>

using namespace std;

class Regex {
public:
    bool isMatch(string s, string p) {
        set<int> currStates;
        list<int> midxes;
        bool stop_state;

        updateCurrentStates(currStates, p, -1);
        for(int i=0; i<s.length(); i++ ){
            matches(currStates, p, s[i],midxes );
            if(midxes.size()==0){
                return false;
            }
            currStates.clear();
            for(int m :midxes){
                updateCurrentStates(currStates, p, m);
            }
            midxes.clear();
        }
        return currStates.find(p.length())!=currStates.end();
    }

private:
    /**
     * return true if final stop state is also reached
     * @param matchStates
     * @param p
     * @param index
     * @return
     */
    void updateCurrentStates(set<int> &matchStates, string &p, int index){
        bool stop_state= false;
        int i=index+1;
        for( ; i <= p.length(); i++) {
           if(i<p.length() && p[i]=='*'){
                i--;
               //if i<0 throw exception invalid pattern. and ** case as well.
           }
           matchStates.insert(i);
           if(i+1< p.length() && p[i+1]!='*'){
               break;
           } else{
               i++;
           }
        }

        //debug print
        cout<< "matched state:";
        for (int n : matchStates) {
            std::cout << ","<< n;
        }
        cout << "\n";


    }

    /**
     * return the next state in the state machine. -1 if this does not match to current state.
     * @param matchStates
     * @param p
     * @param c
     * @return
     */
     void matches(set<int> &matchStates, string &p, char c, list<int> &result){
        for(int i:matchStates) {
            if(i>=p.length())continue;
            if (c == p[i] || p[i] == '.') {
                result.push_back(i);
            }
        }
    }
};

int main(){

    Regex s;
//    printf("%i\n", s.isMatch("a", "ab*"));
//    printf("%i\n", s.isMatch("a", "ab*b*"));
//    printf("%i\n", s.isMatch("", "a*b*b*"));
//    printf("%i\n", s.isMatch("", ".*"));
//    printf("%i\n", s.isMatch("", "."));
//    printf("%i\n", s.isMatch("aab", "c*a*b"));
//    printf("%i\n", s.isMatch("aab", ".*"));
//    printf("%i\n", s.isMatch("aab", "..*"));
//    printf("%i\n", s.isMatch("aabb", ".*b"));
    printf("%i\n", s.isMatch("aabc", ".*bc"));
//    printf("%i\n", s.isMatch("aa", "a"));
}