#include <boost/regex.hpp>
#include <cstdlib>
#include <string>
#include <iostream>
#include <random>
#include <fstream>

using namespace std;

int to_int(int c) {
  if (not isxdigit(c)) return -1; // error: non-hexadecimal digit found
  if (isdigit(c)) return c - '0';
  if (isupper(c)) c = tolower(c);
  return c - 'a' + 10;
}

template<class InputIterator, class OutputIterator> int
unhexlify(InputIterator first, InputIterator last, OutputIterator ascii) {
  while (first != last) {
    int top = to_int(*first++);
    int bot = to_int(*first++);
    if (top == -1 or bot == -1)
      return -1; // error
    *ascii++ = (top << 4) + bot;
  }
  return 0;
}

int main(int argc, char* argv[]){
    if (argc < 2){
        //usage: regex_validator 1C.........
        return 1;
    }
    char* arg = argv[1];

    char data[4096*400];
    size_t size = sizeof(data); //Declare size of data
    ifstream urandom("/home/nik/data", ios::in|ios::binary); //Open stream
    if(urandom) //Check if stream is open
    {
        urandom.read(reinterpret_cast<char*>(&data), size);
    }else {
        return 1;
    }

    size_t expr_len = (strlen(arg)-2)/2;
    char expr[expr_len];
    unhexlify(arg+2, arg+strlen(arg)-1, expr);
    try{
        boost::regex regex(reinterpret_cast<char*>(expr),
        reinterpret_cast<char*>(expr) + expr_len);
        std::cout << "Going to test: " << expr << std::endl;
        boost::cmatch what;
        if (boost::regex_match(reinterpret_cast<const char*>(data), reinterpret_cast<const char*>(data) + size , what, regex)) {
            std::cout << "WTF IT MATCHED" << std::endl;
        } else {
            std::cout << "DID NOT MATCH" << std::endl;
        }
    } catch(...){
        std::cerr << "Crashed :(" << std::endl;
        return 1;
    }
    return 0;
}

Payload data:

GET /post?id=-1%20UNION%20SELECT%201%2C%22Benvenuto%20Hacker!%22%2C1%2C%22Hai%20scoperto%20una%20sql-injection%22%2C%22Qui%20sotto%20trovi%20la%20tua%20password%22%2C%20password%20FROM%20users%20WHERE%20username%20%3D%20%27admin%27 HTTP/1.1
Host: localhost:4600
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:101.0) Gecko/20100101 Firefox/101.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Connection: keep-alive
Referer: http://localhost:4500/post?id=1
Upgrade-Insecure-Requests: 1
Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: same-origin
Sec-Fetch-User: ?1

Output:

./regex_validator 1C2e2a474554205c2f706f73742e2a5c3f2e2a5b265d3f69643d2e2a285b5e302d395d2b292e2a5b26205d2e2a5b205d3f485454502e2a5b5c6e5d3f2e2a
Going to test: .*GET \/post.*\?.*[&]?id=.*([^0-9]+).*[& ].*[ ]?HTTP.*[\n]?.*V
Crashed :(