string fnames[] = {"foo.txt", "bar.txt", "test", "a0.txt", "AAA.txt"};

// In C++, `\` will be used as an escape character in the string.

// In order for `\.` to be passed as a regular expression,

// it is necessary to perform second escaping of `\`, thus we have `\\.`

regex txt_regex("[a-z]+\\.txt");

for (const auto &fname: fnames)

cout << fname << ": " << regex_match(fname, txt_regex) << endl;

regex base_regex("([a-z]+)\\.txt");

smatch base_match;

for(const auto &fname: fnames) {

if (regex_match(fname, base_match, base_regex)) {

// the first element of std::smatch matches the entire string

// the second element of std::smatch matches the first expression with brackets

if (base_match.size() == 2) {

string base = base_match[1].str();

cout << "sub-match[0]: " << base_match[0].str() << endl;

cout << fname << " sub-match[1]: " << base <<endl;

}

}

}

return 0;