Parse TOML files using only C++ standard regex module

tomlparse.cpp

// Parse TOML file with standard regex module, no validation
// Author: Ugo Varetto

#include <fstream>
#include <iostream>
#include <regex>
#include <string>
#include <unordered_map>

using namespace std;

using Dict = unordered_map<std::string, std::string>;
using Toml = unordered_map<std::string, Dict>;

namespace {
void Trim(string& s) {
    auto i = s.find("#");
    if (i != string::npos) s.erase(i);
    i = s.find_last_not_of(" \r\n\t");
    if(i != string::npos) s.erase(++i);
}
}  // namespace

Toml ParseTomlFile(const string& filename) {
    ifstream is(filename);
    if (!is) {
        throw std::invalid_argument("Cannot open file " + filename);
        return {};
    }
    string line;
    const string section = "^\\s*\\[\\s*([^\\]]+)\\]\\s*$";
    int lineCount = 0;
    string curSection;
    Toml toml;
    Dict curSectionData;
    string lastKey;
    while (getline(is, line)) {
        ++lineCount;
        if (line.size() == 0 || regex_search(line, regex{"^\\s*#"})) {
            continue;
        }
        smatch sm;
        if (regex_search(line, sm, regex{section})) {
            if (curSection.size() != 0) {
                toml[curSection] = curSectionData;
                curSectionData.clear();
                lastKey.clear();
            }
            curSection = sm[1];
        } else {
            smatch sm;
            if (regex_search(line, sm, regex{"\\s*(\\w+)\\s*=\\s*(.*)"})) {
                string value = sm[2];
                Trim(value);
                curSectionData[sm[1]] = value;
                lastKey = sm[1];
            } else {
                if (lastKey.empty()) {
                    throw std::runtime_error("Toml parsing error, line " +
                                             to_string(lineCount));
                }
                if (line.size() == 0) continue;
                Trim(line);
                curSectionData[lastKey] += line;
            }
        }
    }
    if (toml.find(curSection) == toml.end()) {
        toml[curSection] = curSectionData;
    }

    return toml;
}

int main(int argc, char const* argv[]) {
    if (argc != 2) {
        cerr << "usage: " << argv[0] << " toml filename" << endl;
        return 1;
    }
    auto toml = ParseTomlFile(argv[1]);
    for (auto kv : toml) {
        cout << "[" << kv.first << "]" << endl;
        for (auto i : kv.second) {
            cout << "\t" << i.first << ": " << i.second << endl;
        }
    }
    return 0;
}