Extracting submatches using boost regex in c++

Question

I'm trying to extract submatches from a text file using boost regex. Currently I'm only returning the first valid line and the full line instead of the valid email address. I tried using the iterator and using submatches but I wasn't having success with it. Here is the current code:

if(Myfile.is_open()) {
    boost::regex pattern("^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,4})$");
    while(getline(Myfile, line)) {
            string::const_iterator start = line.begin();
            string::const_iterator end = line.end();
            boost::sregex_token_iterator i(start, end, pattern);
            boost::sregex_token_iterator j;
            while ( i != j) {
            cout << *i++ << endl;  

    } 
    Myfile.close(); 
}

Answer 1

Use boost::smatch.

boost::regex pattern("what(ever) ...");
boost::smatch result;
if (boost::regex_search(s, result, pattern)) {
    string submatch(result[1].first, result[1].second);
    // Do whatever ...
}

Answer 2

const string pattern = "(abc)(def)";  
const string target = "abcdef"; 

boost::regex regexPattern(pattern, boost::regex::extended); 
boost::smatch what; 

bool isMatchFound = boost::regex_match(target, what, regexPattern); 
if (isMatchFound) 
{ 
    for (unsigned int i=0; i < what.size(); i++) 
    { 
        cout << "WHAT " << i << " " << what[i] << endl; 
    } 
} 

The output is the following

WHAT 0 abcdef 
WHAT 1 abc 
WHAT 2 def 

Boost uses parenthesized submatches, and the first submatch is always the full matched string. regex_match has to match the entire line of input against the pattern, if you are trying to match a substring, use regex_search instead.

The example I used above uses the posix extended regex syntax, which is specified using the boost::regex::extended parameter. Omitting that parameter changes the syntax to use perl style regex syntax. Other regex syntax is available.