C++ regex error when declaring in h file. How to decide?

Question

C++ / C#

I have a program, I want to declare a library regex object, but I do it in a .h file. It doesn't work. And in the main function everything is ok.
Here is the code:

#include "Parsing.h"

Parsing::Parsing(){
  
}

void Parsing::Parse(){
  Html = req.buffer;
  
  if (regex_match(Html.c_str(), result, regular)){
    cout << "true!";
  }
}

#pragma once

#include <regex>
#include "Request.h"

class Parsing{
public:
  Parsing();

  Request req;
  void Parse();
  string Html = "";
  cmatch result;
  regex regular("");
};

The Parse() function is called in the main function.