[ANOTHER QUESTION] Can anyone explain me QRegExp?

Leon

Yes i have seen the documentation but it is kinda hard to understand.. I would like an explanation...
Lets say we have

@ QString leon="1\\\\\\\\n2\\\\\\\\n3\\\\\\\\ni want only the next line\\\\\\\\nfrom this line i want only --->Hello guys!<---\\\\\\\\nend here i dont want it";
@

how can i get the Hello guys! only?

so the code that must be used is something like

@ QRegExp regex("time=.* ");
regex.setMinimal(true);

    QStringList list;
    int pos = 0;

    while ((pos = regex.indexIn(leon, pos)) != -1)
    {
        list << regex.cap(0).remove(QChar(' '),Qt::CaseInsensitive);
        pos += regex.matchedLength();
    }
    if(!list.count()){
        //error
        return;
    }
     QString current=list.at(0);
    qDebug(current.toLocal8Bit());

@

i don't get what should i put in line 1 in the parentheses so the qdebug show Hello guys!

dmcr

something like @"--->()<---*"@ i guess

sidewinder

Regular expressions are powerful but require some love :). There are many engines with similar but not exactly the same future sets. For example, Qt 4 doesn't support look behinds.

Personally, I liked tutorial on "this":http://www.regular-expressions.info/tutorial.html page the most. For testing, there are many online RegEx testers like "this one":http://www.rubular.com/.

Now, back to your problem. Due to lack of look behinds, only reasonable way to extract "Hello guys" string I can see are following two steps:
@
QString leon="1\\n2\\n3\\ni want only the next line\\nfrom this line i want only --->Hello guys!<---\\nend here i dont want it";
QRegExp regex("--->.*(?=<---)");
if(regex.indexIn(leon) != -1){
QStringList list;
QString str;
list = regex.capturedTexts();
foreach(str,list)
qDebug() << str.remove("--->");
}
@

It is very likely someone will post one step regex only solution soon.

Since there are 2 steps anyway you could simply use QString::remove() to remove everything before and including ---> and after and including <---.
I'm not sure if there are significant difference in speed between 2 removes, or regex + remove version, but you can always benchmark it :).

Leon

[quote author="sidewinder" date="1343376961"]Regular expressions are powerful but require some love :). There are many engines with similar but not exactly the same future sets. For example, Qt 4 doesn't support look behinds.

Personally, I liked tutorial on "this":http://www.regular-expressions.info/tutorial.html page the most. For testing, there are many online RegEx testers like "this one":http://www.rubular.com/.

Now, back to your problem. Due to lack of look behinds, only reasonable way to extract "Hello guys" string I can see are following two steps:
@
QString leon="1\\\\n2\\\\n3\\\\ni want only the next line\\\\nfrom this line i want only --->Hello guys!<---\\\\nend here i dont want it";
QRegExp regex("--->.*(?=<---)");
if(regex.indexIn(leon) != -1){
QStringList list;
QString str;
list = regex.capturedTexts();
foreach(str,list)
qDebug() << str.remove("--->");
}
@

It is very likely someone will post one step regex only solution soon.

Since there are 2 steps anyway you could simply use QString::remove() to remove everything before and including ---> and after and including <---.
I'm not sure if there are significant difference in speed between 2 removes, or regex + remove version, but you can always benchmark it :).[/quote]

Thanks a lot! It works, but i would like an explanation for line 2.. what are
@.*(?=@
i quess they remove <-- but why not do something similar for the -->?

sidewinder

Step by step.
@"--->"@
matches "--->" :)

@.@
matches any number () of any characters (.)

@(?=)@
is called look ahead. It means "check if there is pattern ahead, but don't include it in results".

So
@.*(?=<---)@
checks if after any number of any characters <--- is present.

Qt doesn't support look behinds (?<=), so unfortunately something like that doesn't work:
@(?<=--->).*(?=<---)@

Hopefully it will work in Qt5 :).

Leon

Ok got it thanks ! :)

sierdzio

[quote author="sidewinder" date="1343388065"]Hopefully it will work in Qt5 :).[/quote]

Yes, but not without code changes. QRegExp was entangled too much into Qt code, it had to be left in Qt5. To solve regexp issues, a new class was introduced - "QRegularExpression":http://doc-snapshot.qt-project.org/5.0/qregularexpression.html. It uses a third party regexp engine that supports everything you might need and is also much faster :D

Leon

What if i only want to show the first capture?
For example

@QString lool="1/n3/n5/n6/n7/3/n5/n9";
QRegExp reg2("3.*5");
if(reg2.indexIn(lool) != -1){
QStringList list;
QString strq;
list = reg2.capturedTexts();
foreach(strq,list)
qDebug(strq.toLocal8Bit());
}@

it will show
@3/n5/n6/n7/3/n5@

while i want from it to show
@3/n5@

Leon

Anyone? Sorry for 2nd post..

alexandros

I can say that I have quite the same problem here... I want only the first occurrence of the match!

raaghuu

You have to turn off the 'greedy' mode for this... see "here":http://qt-project.org/doc/qt-4.8/qregexp.html#setMinimal