Incorrect answer in RE2::FullMatch

Question

I am trying to use RE2::FullMatch but I get incorrect answer.

When I do this, I get true:

re2::RE2 regex("[sS]omename");
std::string input="somename";
re2::RE2::FullMatch(input, regex);

But if I want to extracts the result into variable, I get false

re2::RE2 regex("[sS]omename");
std::string input="somename";
std::string match_result;
re2::RE2::FullMatch(input, regex, &match_result);

What can be the problem in the second way ?