C++ Boost usage over a string - c++

Question

I have no idea about boost, could anybody please tell me what exactly this function is doing?
int
Function(const string& tempStr)
{
boost::regex expression ("result = ");
std::string::const_iterator start, end;
start = tempStr.begin();
end = tempStr.end();
boost::match_results<std::string::const_iterator> what;
boost::regex_constants::_match_flags flags = boost::match_default;
int count = 0;
while(regex_search(start, end, what, expression, flags)){
start = what[0].second;
count++;
}
cout << "Count :"<< count << endl;
return count;
}

Answer 1

match_results is a collection of sub_match objects. The first sub_match object (index 0) represents the full match in the target sequence (subsequent matches would correspond to the subexpressions matches). Your code is searching for result = matches and restarting the search each time from the end of the previous match (what[0].second)
int
Function(const string& tempStr)
{
boost::regex expression ("result = ");
std::string::const_iterator start, end;
start = tempStr.begin();
end = tempStr.end();
boost::match_results<std::string::const_iterator> what;
boost::regex_constants::_match_flags flags = boost::match_default;
int count = 0;
while(regex_search(start, end, what, expression, flags)){
start = what[0].second;
count++;
}
cout << "Count :"<< count << endl;
return count;
}
int main()
{
Function("result = 22, result = 33"); // Outputs 'Count: 2'
}
Live Example

Answer 2

The base of the functionality is searching for a regular expression match on tempStr.
Look at the regex_search documentation and notice what the match_result contains after it finishes (that's the 3rd parameter, or what in your code sample). From there understanding the while loop should be straightforward.

Answer 3

This function is a complicated way to count the number of occurrences of "result = " string. A simpler way would be:
boost::regex search_string("result = ");
auto begin = boost::make_regex_iterator(tempStr, search_string);
int count = std::distance(begin, {});
Which can be collapsed to a one-liner, with possible loss of readability.

Answer 4

This is a match counter function:
The author uses useless code: here is the equivalent code in std ( also boost )
unsigned int count_match( std::string user_string, const std::string& user_pattern ){
const std::regex rx( user_pattern );
std::regex_token_iterator< std::string::const_iterator > first( user_string. begin(), user_string.end(), rx ), last;
return std::distance( first, last );
}
and with std::regex_search it can be (also boost ):
unsigned int match_count( std::string user_string, const std::string& user_pattern ){
unsigned int counter = 0;
std::match_results< std::string::const_iterator > match_result;
std::regex regex( user_pattern );
while( std::regex_search( user_string, match_result, regex ) ){
user_string = match_result.suffix().str();
++counter;
}
return counter;
}
NOTE:
no need to use this part:
std::string::const_iterator start, end;
start = tempStr.begin();
end = tempStr.end();
Also
boost::match_results<std::string::const_iterator> what;
can be
boost::smatch what // a typedef of match_results<std::string::const_iterator>
no need:
boost::regex_constants::_match_flags flags = boost::match_default;
because by default regex_search has this flag
this:
start = what[0].second;
is for updating the iteration that can be:
match_result.suffix().str();
if you want to see what happen in the while loop use this code:
std::cout << "prefix: '" << what.prefix().str() << '\n';
std::cout << "match : '" << what.str() << '\n';
std::cout << "suffix: '" << what.suffix().str() << '\n';
std::cout << "------------------------------\n";