How to replace QRegExp in a string?

Question

I have a string. For example:

QString myString = "Today is Tuesday";

The requirement is: when user types a string, if that string is contained in myString, then that part in the myString should be bold, and case insensitive (Qt::CaseInsensitive), but the format of myString should remain (upper case characters should be upper case and lower case characters should be lower case).

For example:

user types: tu -> Today is Tuesday
user types: ES -> Today is Tuesday
user types: aY -> Today is Tuesday

This is my function:

void myClass::setBoldForMatching( const QString &p_text )
{
  QRegExp regExp( p_text, Qt::CaseInsensitive, QRegExp::RegExp );
  if ( !p_text.isEmpty() )
  {       
    if ( myString.contains( regExp ) )
    {
      myString = myString.replace( p_text, QString( "<b>" + p_text + "</b>" ), Qt::CaseInsensitive );
    }
  }
}

This function is wrong because

user types t -> today is tuesday.

What I need is Today is Tuesday

How should I update my function?

u didn't describe full area of possible cases. What should happens if myString will be "Today is today" and user types "To" — borune, Aug 28 '18 at 10:25
Where is this text which supposed to be bold? Is it a `QTextEdit` or something else? Use of `QTextDocument` and `QTextCursor` can be more effective and simple than adding this html tags. — Marek R, Aug 28 '18 at 11:12

Toby Speight · Accepted Answer · 2018-08-28T13:45:45.117

6

We can use a different QString::replace(), which accepts a QRexExp, to substitute all occurrences. The key to this is that we need a capture group in order to replace the original text in the substitution, using a back-reference (\1):

#include <QRegExp>

QString setBoldForMatching(QString haystack, const QString& needle)
{
    if (needle.isEmpty()) return haystack;
    const QRegExp re{"("+QRegExp::escape(needle)+")", Qt::CaseInsensitive};
    return haystack.replace(re, "<b>\\1</b>");
}

Demo

#include <QDebug>
int main()
{
    qInfo() << setBoldForMatching("THIS DAY (today) is Tuesday.", "Day");
}

THIS DAY (today) is Tuesday.

edited Aug 28 '18 at 13:45

answered Aug 28 '18 at 10:55

Toby Speight

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1

wow, your solution made me read back-reference. That is new to me. thanks a lot! – songvan Aug 28 '18 at 12:02
I have one more question about this. What should I translate your answer, if I use QRegularExpression? – songvan Sep 13 '18 at 15:25
That's probably worth creating a new question for (if you can't find an existing one). – Toby Speight Sep 13 '18 at 15:34

score 2 · Answer 2 · edited Jun 20 '20 at 09:12

2

Solution

Change your setBoldForMatching like this:

void myClass::setBoldForMatching(const QString &p_text)
{
    QRegExp regExp(p_text, Qt::CaseInsensitive, QRegExp::RegExp);
    QString str = myString;
    
    if (p_text.isEmpty()) {
        label->setText(myString);
        return;
    }
    
    int count = 0;
    int pos = 0;
    QStringList matches;
    
    while ((pos = regExp.indexIn(str, pos)) != -1) {
        ++count;
        pos += regExp.matchedLength();
        matches.append(regExp.capturedTexts());
    }
    
    foreach (const QString &match, matches) {
        str.replace(match, "<b>" + match + "</b>");
    }
}

I have decided to keep the original content of myString, so the result is contained in str instead.

Example

I have prepared a small example for you in order to demonstrate the result. The full code is available on GitHub.

Result

edited Jun 20 '20 at 09:12

Community

1
1

answered Aug 28 '18 at 10:09

scopchanov

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10
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68

as i wrote, the question didn't describe full set of posiible cases. I think u should fully understand what is needed before post any answers – borune Aug 28 '18 at 10:27
@borune, yes, I have seen that. – scopchanov Aug 28 '18 at 10:28
1

Why `p_text.toLower()` rather than just `p_text`? I don't see that the conversion has any benefit. Also, problem statement suggests we want `QRegExp::FixedString` rather than `QRegExp::RegExp`. – Toby Speight Aug 28 '18 at 10:39

score 2 · Answer 3 · answered Aug 28 '18 at 10:40

Replacing a match with p_text will always change the case to the one of p_text. So you have to do the replacement step by step, like this:

void myClass::setBoldForMatching(const QString &p_text) {
    QRegExp regExp( p_text, Qt::CaseInsensitive, QRegExp::FixedString );

    QString start = "<b>";
    QString stop = "</b>";

    int i=-1;
    while (-1 != (i=myString.indexOf(regExp,i+1))) {
        myString.insert(i, start);
        i += start.size();
        i += p_text.size();
        myString.insert(i, stop);
        i += stop.size();
    }
}

As you can see, with this code, the start and stop tags will be inserted before and after the match, without changing the matched sub-string itself.

Here are some test cases:

Today is Tuesday + tu --> Today is Tuesday
Today is Tuesday + ES --> Today is Tuesday
Today is Tuesday + aY --> Today is Tuesday
Today is Tuesday + t --> Today is Tuesday
Today is today + To --> Today is today

How to replace QRegExp in a string?

3 Answers3

Demo

Solution

Example

Result

Linked