How can I read every text filename in folder by regular expressions?

include<iostream>
#include<io.h>
#include<string>
#include<fstream>
using namespace std;
void dfsFolder(string folderPath, ostream &fout)
{
    _finddata_t FileInfo;
    //only can read current folder text file .I can't read the text in subfolder;
    //how can I read every text file included in sub-folder by regular expression?
    string strfind = folderPath + "\\*.txt";
    long Handle = _findfirst(strfind.c_str(), &FileInfo);

    if (Handle == -1L)
    {
        cerr << "can not match the folder path" << endl;
        exit(-1);
    }
    do{
        if (FileInfo.attrib & _A_SUBDIR)
        {
            string newpath = strfind + "\\" + FileInfo.name;
            dfsFolder(newpath,fout);
        }
        else
            fout << folderPath << "\\" << FileInfo.name  << endl;
    }while (_findnext(Handle, &FileInfo) == 0);
    _findclose(Handle);
}

int main()
{
    string path;
    cin>>path;
    dfsFolder(path,cout);
}