#include <QApplication>
#include <QtWidgets>
#include <QDebug>

class Highlighter : public QSyntaxHighlighter
{
public:
    Highlighter(QTextDocument *parent = nullptr)
        : QSyntaxHighlighter(parent)
    {
        multiLineCommentFormat.setForeground(Qt::red);
        commentStartExpression = QRegularExpression(QStringLiteral("/\\*"));
        commentEndExpression = QRegularExpression(QStringLiteral("\\*/"));
    }

protected:
    void highlightBlock(const QString &text) override
    {
        setCurrentBlockState(0);

        int startIndex = 0;
        if (previousBlockState() != 1)
            startIndex = text.indexOf(commentStartExpression);

        while (startIndex >= 0) {
            const QRegularExpressionMatch match = commentEndExpression.match(text, startIndex);
            const auto endIndex = match.capturedStart();
            int commentLength = 0;
            if (endIndex == -1) {
                setCurrentBlockState(1);
                commentLength = text.length() - startIndex;
            } else {
                commentLength = endIndex - startIndex + match.capturedLength();
            }
            setFormat(startIndex, commentLength, multiLineCommentFormat);
            startIndex = text.indexOf(commentStartExpression, startIndex + commentLength);
        }
    }

private:
    QRegularExpression commentStartExpression;
    QRegularExpression commentEndExpression;
    QTextCharFormat multiLineCommentFormat;
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QTextEdit editor;
    auto highlighter = new Highlighter(editor.document());
    Q_UNUSED(highlighter);
    editor.setPlainText("Here is /* a\n"
                        "multi-lines\n"
                        "comment in C++ */ !");

    const auto firstLine = editor.textCursor().block();
    const auto format1 = firstLine.layout()->formats().first();

    const auto secondLine = editor.textCursor().block().next();
    const auto format2 = secondLine.layout()->formats().first();

    qDebug() << "First line: From" << format1.start << "/" << format1.length << "chars";
    qDebug() << "Second line: From" << format2.start << "/" << format2.length << "chars";

    editor.show();
    return app.exec();
}

First line: From 8 / 4 chars
Second line: From 0 / 11 chars

First line: From 8 / 4 chars
Second line: From 0 / 12 chars