Regex grouping matches with C++ 11 regex library

Your regular expression is incorrect because neither capture group does what you want. The first is looking to match a single character from the set [a-zA-Z0-9] followed by <space>:, which works for single character usernames, but nothing else. The second capture group will always be empty because you're looking for zero or more characters, but also specifying the match should not be greedy, which means a zero character match is a valid result.

Fixing both of these your regex becomes

std::regex rgx("WEBMSG #([a-zA-Z0-9]+) :(.*)");

But simply instantiating a regex and a match_results object does not produce matches, you need to apply a regex algorithm. Since you only want to match part of the input string the appropriate algorithm to use in this case is regex_search.

std::regex_search(s, matches, rgx);

Putting it all together

    std::string s{R"(
tХB:[email protected] Connected
tХB:[email protected] WEBMSG #Username :this is a message
tХB:[email protected] Status: visible
)"};

    std::regex rgx("WEBMSG #([a-zA-Z0-9]+) :(.*)");
    std::smatch matches;

    if(std::regex_search(s, matches, rgx)) {
        std::cout << "Match found\n";

        for (size_t i = 0; i < matches.size(); ++i) {
            std::cout << i << ": '" << matches[i].str() << "'\n";
        }
    } else {
        std::cout << "Match not found\n";
    }

Live demo

"WEBMSG #([a-zA-Z0-9]) :(.*?)"

This regex will match only strings, which contain username of 1 character length and any message after semicolon, but second group will be always empty, because tries to find the less non-greedy match of any characters from 0 to unlimited.

This should work:

"WEBMSG #([a-zA-Z0-9]+) :(.*)"