如何解析多行上的字符串对？

Question

I'd like to parse content like:

tag = value
tag2 = value2
tag3 = value3

with the relaxation of allowing values over multiple lines and disregarding comments of the next tag. A tag is identified by not starting with the comment identifier '#' and starting at a new line. So this:

tag = value
  value continuation
tag2 = value2
  value continuation2
# comment for tag3
tag3 = value3

should parse the mapping:

tag : "value\nvalue continuation"
tag2 : "value2\nvalue continuation2"
tag3 : "value3"

How can I achieve this in a clean way? My current code for parsing one-line pairs looks sth like this:

while( std::getline( istr, line ) )
{
  ++lineCount;
  if( line[0] == '#' )
    currentComment.push_back( line );
  else if( isspace( line[0]) || line[0] == '\0' )
    currentComment.clear( );
  else
  {
    auto tag = Utils::string::splitString( line, '=' );
    if( tag.size() != 2 || line[line.size() - 1] == '=')
    {
      std::cerr << "Wrong tag syntax in line #" << lineCount << std::endl;
      return nullptr;
    }
    tagLines.push_back( line );
    currentComment.clear( );
  } 
}

Note that I don't require the results being stored in the types of containers that are currently used. I can switch to anything that fits better unless I get sets of (comment, tagname, value).

Answer 1

Generally regexs add complexity to your code , but in this case it seems a regex would be the best solution. A regex like this will capture the first and second parts of your pair:

(?:\s*#.*\n)*(\w+)\s*=\s*((?:[^#=\n]+(?:\n|$))+)

[ Live example ]

In order to use a regex_iterator on an istream you'll need to either slurp the stream or use boost::regex_iterator with the boost::match_partial flag. Say that istream has been slurped into string input . This code will extract the pairs:

const regex re("(?:\\s*#.*\\n)*(\\w+)\\s*=\\s*((?:[^#=\\n]+(\\n|$))+)");

for (sregex_iterator i(input.cbegin(), input.cend(), re); i != sregex_iterator(); ++i) {
    const string tag = i->operator[](1);
    const string value = i->operator[](2);

    cout << tag << ':' << value << endl;
}

[ Live example ]

This obviously exceeds the request in the original question; parsing out tags and values instead of just grabbing the line. There is a fair amount of functionality here that is new to C++, so if there are any questions please comment below.