C++ source code to judge if a string is an real number.

is_real.cpp

/***************************************************************
  Check real number

  $ g++ -std=c++17 -Wall -O2 --pedantic-errors -o is_real is_real.cpp

    DATE          AUTHOR          VERSION
    2020.08.24    mk-mode.com     1.00 新規作成

  Copyright(C) 2020 mk-mode.com All Rights Reserved.
***************************************************************/
#include <iostream>   // for cout
#include <regex>      // for regex etc.

namespace funcs {
  bool is_real(std::string s) {
    std::smatch m;
    std::regex re("^[+-]?(\\d+\\.\\d*|\\.\\d+)$");

    try {
      if (!std::regex_search(s, m, re))
        return false;
    } catch (std::regex_error& e) {
      //std::cout << "[EXCEPTION] Regex error! ("
      //          << e.what() << ")" << std::endl;
      return false;
    }

    return true;
  }
}

int main(int argc, char* argv[]) {
  std::string buf;

  try {
    while (true) {
      std::cout << "n? ";
      getline(std::cin, buf);
      if (buf.empty())
        break;
      std::cout << buf << ": ";
      if (!funcs::is_real(buf))
        std::cout << "NOT ";
      std::cout << "REAL NUMBER\n---" << std::endl;
    }
  } catch (...) {
    std::cerr << "EXCEPTION!" << std::endl;
    return EXIT_FAILURE;
  }

  return EXIT_SUCCESS;
}