How to count numbers in the end of QString

Hello again, sorry for beginners question but I need to count numbers in the end of QString. The format of QString is different like B9_02 (must get result of 2 from here) or TP145 (and 3 from here).
What function should i use to get the count of numbers?

Start from the back of the string and check each (Q)char if it is a number (http://doc.qt.io/qt-4.8/QChar#isNumber). If it is, add 1 to a count.

the first non-number that you find (or start of string), then stop the count.

QChar::isDigit

ah yes, isDigit is better than isNumber in this case.

Trying such a code:

Code:

---

QString p=le2->text();
    int count = 0;
    for(int j = p.length(); j > 0; j--)
    {
       if (p.at(j).isDigit()==true)
       count++;
       if (p.at(j).isDigit()==false)
       break;
      }
    qDebug() << "count" << count;

---

But get such an error: ASSERT: "i >= 0 && i < size()" in file ../../../../Qt/Desktop/Qt/473/gcc/include/QtCore/qstring.h, line 702
Can you help me and write what i am doing wrong? using p[j] gives no error but gives no result (count =0 in any case).

If the string length is 5 then the character indexes are 0 through 4. Try starting with character length() - 1.