[Solved] How do i check if a string format is valid while reading from a text file in C++? [closed]

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When I give out my answer, it’s yet unclear that what name your input text file has. Let’s suppose the name of your text file is test.txt and it locates right within the same directory as the C++ source file, test.cpp.

test.txt

Car,Red,ZX342DC

test.cpp

#include <fstream>
#include <string>
#include <regex>
#include <iostream>

using namespace std;

int main()
{
    // Let's start with variable declaration and initialization

    // First you have to open the text file,
    // for which you need `ifstream`.
    // It is responsible for decoding text files.
    ifstream ifs("test.txt");

    // Provide a temporary storage for each line
    string line;

    // Regular expression pattern object
    // This is what you need
    // in order to validate each line
    // and extract data from each line,
    regex p("(\\w+),(\\w+),([A-Z]{2}[0-9]{3}[A-Z]{2})");

    // Match object
    smatch m;

    // Check if the file stream is opened
    // otherwise you might have some problems
    // to continue the following steps
    if (ifs.is_open())
    {
        // Then you need to traverse over each line in the file.
        for (int lineNum = 1;
                getline(ifs, line);
                lineNum++)
        {
            // Validate the line
            if (regex_search(line, m, p) && m.size() > 3)
            {
                cout << "Pattern matched." << endl;
                cout << endl;

                string vehicle = m.str(1);
                string color = m.str(2);
                string plate = m.str(3);

                cout << "Vehicle:\t"    << vehicle << endl;
                cout << "Color  :\t"    << color << endl;
                cout << "Plate  :\t"    << plate << endl;

                // You can insert some codes here to handle the results

                cout << endl;
            }
            else
            {
                cerr << "There's an invalid entry in line number - " << lineNum << "!" << endl;
            }
        }

        // Always remember to close a stream
        // before exiting the program
        // otherwise you may suffer from **memory leakage**
        ifs.close();
    }
    else
    {
        cerr << "Fail to open input file!" << endl;
    }

    return 0;
}

Reference:

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solved How do i check if a string format is valid while reading from a text file in C++? [closed]