Format a csv using regex

You can use the replaceAll method and save it into a new String:

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class Main {

    public static void main(String[] args) throws IOException {
        // Read file
        Path filePath = Paths.get("file.txt");
        byte[] arr = Files.readAllBytes(filePath);
        String content = new String(arr, Charset.defaultCharset());

        // Replace
        String newContent = new String(
                content.replaceAll(",([0-9]+)([0-9]{1})([0-9]{2})", ",$2/$3/$1$2$3"));

        System.out.println(newContent);
    }
}

This produces:

10003478,2/28/12228,4/79/7711479,8/89/15013889,1/20/3070120,2/62/5834262

You could have a pattern like this:

,((\d+)(\d)(\d{2}))

and do a replaceAll with:

,$3/$4/$1

Example in javascript:

console.log("10003478,12228,7711479,15013889,3070120,5834262".replace(/,((\d+)(\d)(\d{2}))/g, ",$3/$4/$1"));

I do not see why you should use regex. Instead you can just read the file and convert the strings and write back to file. Below is an example how it could look like using OpenCsv

import com.opencsv.CSVReader;
import com.opencsv.CSVWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.util.Arrays;
import java.util.List;


public class JavaApplication22 {

   public static void main(String[] args) throws IOException { 
       String path = "C:\\Users\\uzochi\\desktop\\numbers.csv";
       CSVReader reader = new CSVReader(new FileReader(path));
       List<String[]> myEntries = reader.readAll();

       for(String[] s : myEntries){
           for(int i = 1; i<=5; i++){
              String last3 = s[i].substring(s[i].length()-3,s[i].length()); 
              last3 = last3.charAt(0)+"/"+last3.substring(1)+"/";
              s[i] = last3+s[i];               
           }
           System.out.println(Arrays.toString(s));
       }

       CSVWriter writer = new CSVWriter(new FileWriter(path), ','); 
       for(String[] s : myEntries){       
          writer.writeNext(s);
       }
   writer.close();
   }    
 }

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