find pattern in string regular expression c++

find pattern in string regular expression c++

I have the following string : E:\501_Document_60_1_R.xml
I am trying to find the pattern "_R"
I am using the following : boost::regex rgx("[R]");
But it's not working : "Empty Match"
thank you.
Code:
vector<string> findMono(string s)
{
vector<string> vec;
boost::regex rgx("[R]");
boost::smatch match;
boost::sregex_iterator begin {s.begin(), s.end(), rgx},
end {};
for (boost::sregex_iterator& i = begin; i != end; ++i)
{
boost::smatch m = *i;
vec.push_back(m.str());
}
return vec;
}