This is the most trivial example of a `grep` I could write using Qt. All it does is read line by line, apply a `QRegularExpression` and prints any matching line.
```plaintext
#include <QCoreApplication>
#include <QFile>
#include <QRegularExpression>
#include <QDir>

// args are:
// pattern file1 [file2 ...]
// if file1 not specified, use stdin
QTextStream& qStdOut()
{
    static QTextStream ts( stdout );
    return ts;
}

QTextStream& qStdErr()
{
    static QTextStream ts( stderr );
    return ts;
}

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    QDir dir;

    qStdOut() << dir.currentPath() << "\n";
    qStdOut().flush();

    if( argc < 3 )
    {
        qStdOut() << "pmgrep <pattern> <files...>\n";
        qStdOut().flush();
    }
    else
    {
        // take pattern from argv[1]
        // iterate over args from 2 onwards
        const char* pattern = argv[1];

        const QRegularExpression regular_expression(pattern);

        for(int i=2; i<argc; i++)
        {
            const char* filename = argv[i];

            // open file argv[i]
            QFile inputFile(filename);
            if (inputFile.open(QIODevice::ReadOnly))
            {
               QTextStream in(&inputFile);
               while (!in.atEnd())
               {
                  QString line = in.readLine();

                  const QRegularExpressionMatch match = regular_expression.match(line);
                  if( match.hasMatch() )
                  {
                      qStdOut() << line << "\n";
                  }
                  qStdOut().flush();
               }
               inputFile.close();
            } else {
                // print to stderr
                qStdErr() << "Failed to open file: " << filename << "\n";
            }
        }
    }
}
```