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Parse large text file

  • Hi,
    i am trying to parse a large file that looks like this

    N14 X411 Y279.05
    N15 G2 X33 Y280.4 I.001 J4.00
    N16 G1 X408 Y280.4
    N17 G2 X402.3 Y280.09 I-3 J2.65
    N18 G1 X402.3 Y280.09
    N19 G2 X402.09 Y285.79 I2.7 J2.95
    N20 G1 X402.09 Y285.79

    I want to extract the X and Y values (rounded) for every line.
    I almost got it but cant figure out why i only get X values

    for exemple

    line read :  "N619 G1 X48 Y280.4"
    output    : x :  "48"
    but i also need y value

    This is my code, can someone tell me please why the while ((posY = rxY.indexIn(line, posY)) != -1) is not entered ?

    void ProgramParser::openFile(QString filePath,int ind) { // TODO Implement this
    ///   ... opening the file
        int minX, minY, maxX, maxY;
        const QRegularExpression re("X\\d+ Y\\d+");
        QRegExp rxX("\\X(\\d+)\\s");
        QRegExp rxY("\\Y(\\d+)\\s");
        QTextStream in(&file);
        while(!in.atEnd()) {
            QString line = in.readLine();
                int posX = 0;
                int posY = 0;
                while ((posX = rxX.indexIn(line, posX)) != -1)
                    QString lX = rxX.cap(1);
                    qDebug()<<"x : " << lX;
                    posX += rxX.matchedLength();
                while ((posY = rxY.indexIn(line, posY)) != -1)
                    QString lY = rxY.cap(1);
                    qDebug()<<"y : " << lY;
                     posY += rxY.matchedLength();

    Also, please suggest me a faster solution for this task if you know one.
    Thank you very much

  • Regular expression don't render very well in this forum, see

    Source of the regexp for the decimal number: (this is also the answer to @JonB, I don't type, I copy and paste, typing is for smart people)

  • @VRonin How do you type in this answer so fast --- do you have it pre-prepared? :)

  • Lifetime Qt Champion


    The expression you are looking for is "X(\\d+(?:\\.\\d+)?) Y(\\d+(?:\\.\\d+)?)".

    You should rather do the rounding with the captured data otherwise you will lose precession.

    You can use the QRegularExpression tool to test your expressions.

    [edit: Fixed RE missing . escaping SGaist]

  • You are awesome guys, thank you so much for quick/complete answers

  • Lifetime Qt Champion

    @LeLev take a look again at the expression. I forgot to escape the .. Therefore the expression was "wrong" in the sense that it will also find values like X1231a212.

  • @SGaist got it, thank you!

  • Hi !
    I have lot of troubles trying to use the tool @SGaist
    suggested to me. I want to edit my QRegularExpression so it also can catch lines where there is only X or only Y

    N9 G40 X411 Y7.62 // capture 
    N3 G40 Y6000 // << capture this also
    N1 G0 X411 Y7.52 // capture
    N3 G40 X-6000 // << capture this also
    // file 
     const QRegularExpression regx("X([-+]?\\d*\\.?\\d+)\\s+Y([-+]?\\d*\\.?\\d+)");
        QTextStream in(&file);
        while(!in.atEnd()) {
            QString line = in.readLine();
            const QRegularExpressionMatch reMatch = regx.match(line);
            bool iok;
                double lx = reMatch.captured(1).toDouble();
                double ly = reMatch.captured(2).toDouble();
                qDebug()<<"X : " << lx << " ly" << ly;
    //please tell me if there is a more efficient way to get maximum/minimum X  and  maximum/minimum Y in this file, 
                else if (lx>maxX) maxX=lx;
                else if (ly>maxY) maxY=ly;

    My final aim is to get maximum x and y And minimum x and y in the whole file
    Thank you

  • Qt Champions 2017

    When and/or if you use it, you must escape it properly for C++.

  • @kshegunov hi
    thak you.
    i only doubled the " \ "s

    // (X(-?\d+(?:\.\d+)?))?\s*(Y(-?\d+(?:\.\d+)?))? 
     const QRegularExpression regx("(X(-?\\d+(?:\\.\\d+)?))?\\s*(Y(-?\\d+(?:\\.\\d+)?))?");

    I tested your exemple on and it is doind what i need. But in My program i get Zeros only

  • This post is deleted!

  • Lifetime Qt Champion

    Because you are using way more capturing groups than you need and are not handling the case where there's no capture which is allowed since you make the whole thing optional.

    What about simplifying things a bit ?

    const QRegularExpression rx ("(?:[XY](-?\\d+(?:\\.\\d+)?))");
    while (!in.atEnd()) {
        QString line = in.readLine();
        QRegularExpressionMatchIterator iter = re.globalMatch(line);
        bool ok;
        while (iter.hasNext()) {
            QRegularExpressionMatch match =;
            double x_or_y = match.captured(1).toDouble(&ok);
            if (!ok) {
                   qDebug() << "Failed to convert" << match.captured(1);
            if (match.captured(0).startsWith(QLatin1Char('X')) {
                  minX = qmin(minX, x_or_y);
                  maxX = qmax(maxX, x_or_y);
            } else {
                  minY = qmin(minY, x_or_Y);
                  maxY = qmax(maxY, x_or_y);

    [edit: fixed rx, escaping was missing SGaist]

  • I'd use:

    const QRegularExpression regx("(?:([XY])([-+]?\\d*\\.?\\d+))");
    QTextStream in(&file);
    while(!in.atEnd()) {
        QString line = in.readLine();
        QRegularExpressionMatchIterator iter = re.globalMatch(line);
        while (iter.hasNext()){
            const QRegularExpressionMatch reMatch =;
            if(reMatch.captured(1).compare(QLatin1String("X"),Qt::CaseInsensitive)==0){ // matched X
                const double lx = reMatch.captured(2).toDouble();
                qDebug()<<"X : " << lx;
                else if (lx>maxX)
            else {// matched Y
                const double ly = reMatch.captured(2).toDouble();
                qDebug() << " ly" << ly;
                else if (ly>maxY)

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