QRegExp a filename but its not matching

I'm trying to parse date time from a PNG file but can't quite get it with QRegExp

this_png_20211208_1916.png

  QDateTime Product::GetObstime()
    {
       QDateTime obstime;
       QString filename = FLAGS_file_name.c_str();
       QString year, month, day, hour, minute, second;


   QRegExp regexp = QRegExp("^.*\\w+_(\\d{4}\\d{2}\\d{2})_(\\d{2}\\d{2})\\.png$");

   VLOG(3) << " filename: " << filename.toStdString();
   if(regexp.indexIn(filename) !=-1)
   {
       VLOG(3) << " filename: " << filename.toStdString();
       QStringList dt_bits = regexp.capturedTexts();
       if(dt_bits.size() >=2)
       {
           year = dt_bits.at(1).mid(0, 4);
           month = dt_bits.at(1).mid(5, 2);
           day = dt_bits.at(1).mid(8, 2);

           hour = dt_bits.at(2).mid(0, 2);
           minute = dt_bits.at(2).mid(3, 2);
           second = dt_bits.at(2).mid(3, 2);
           VLOG(3) << " Year: " << year.toStdString()
                   << " Month: " << month.toStdString()
                   << " Day: " << day.toStdString()
                   << " Hour: " << hour.toStdString()
                   << " Min: " << minute.toStdString()
                   << " Sec: " << second.toStdString();
           QString datetime_str = year + "-" + month + "-" + day +
                   "T" + hour + ":" + minute + second + "00Z";

           obstime = QDateTime::fromString(datetime_str, Qt::ISODate);
           if (obstime.isValid())
           {
               VLOG(3)<<"Date iS VALID: "<<obstime.toString(Qt::ISODate).toStdString();
           }
           else
           {
               LOG(ERROR)<<" Error! Date Time bits did not match format.";
           }
       }

   }
   return obstime;
}

been using tools like https://regex101.com/

but to no avail. am I missing something?

Solution

You have the following errors in your code:

month = dt_bits.at(1).mid(5, 2); should be month = dt_bits.at(1).mid(4, 2); because the index is 0-based, not 1-based
day = dt_bits.at(1).mid(8, 2); should be day = dt_bits.at(1).mid(6, 2);
minute = dt_bits.at(2).mid(3, 2); should be minute = dt_bits.at(2).mid(2, 2);
second = dt_bits.at(2).mid(3, 2); should be second = "00"; because your filenames do not contain seconds

Generally I would recommend doing all of the work in the regex instead of doing some fancy splitting using QString::mid():

QRegExp regexp = QRegExp("^.*\\w+_(\\d{4})(\\d{2})(\\d{2})_(\\d{2})(\\d{2})\\.png$");

This gives you all the fields in separate groupings, no need for QString::mid() at all.