replace last occurrence of string in file

[peter m hayward]

I have a book entry system that used a number of text boxes to collect the data about the book, title author etc, once all the text boxes are filled the data is sent to mysql and also a text file for upload to a website
The possibility of an error means i need to be able to effectively edit the text file bearing in mind that last entry in the file is that of the current book
What i have so far is this

package findandreplace;

import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.logging.Level;
import java.util.logging.Logger;

/**
 *
 * @author pete
 */
public class FindAndReplace {

    /**
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        try {
            Path path = Paths.get("F:\\ebid.txt");
            Charset charset = StandardCharsets.UTF_8;
            
            String content = new String(Files.readAllBytes(path), charset);
                                         
            content = content.replaceFirst("pink elephant", "grey elephant"); //original replacement
            Files.write(path, content.getBytes(charset));
        
   
        
        } catch (IOException ex) {
            Logger.getLogger(FindAndReplace.class.getName()).log(Level.SEVERE, null, ex);
        }
    }

}

ideally i need content = content.replacelast  which does not exist then the error that has just taken place will be corrected so how can i convert content.replaceFirst to  content.replacelast

[Henry Wong]

peter m hayward wrote:
ideally i need content = content.replacelast  which does not exist then the error that has just taken place will be corrected so how can i convert content.replaceFirst to  content.replacelast

Well, one option would be to greedily grab everything (or as much as possible) before "pink elephant", leaving only the last "pink elephant" to be replaced. Sort of something like this...

content = content.replaceFirst("(.*)pink elephant", "$1grey elephant"); 

Henry

[Rob Spoor]

How about using $ to indicate the end of the string? I think you would need to add (?s) (Pattern.DOTALL) to the regex though:

content = content.replaceFirst("(?s)pink elephant$", "grey elephant");

(untested)

[Henry Wong]

Rob Spoor wrote:How about using $ to indicate the end of the string?

This would work only if the last occurrence is in the last position. It will not work if there is anything else after the last occurrence.

Henry

[peter m hayward]

Rob Spoor wrote:How about using $ to indicate the end of the string? I think you would need to add (?s) (Pattern.DOTALL) to the regex though:

content = content.replaceFirst("(?s)pink elephant$", "grey elephant");

(untested)

having read your reply it gave me an idea which seems to work here is the code just in case it helps some else

   public void FileControl(String thefile) throws FileNotFoundException, IOException {
        RandomAccessFile f = new RandomAccessFile("C:\\" + thefile + ".txt", "rw");
        byte b;
        long fileLength = f.length() - 1;// find the size of the file
        if (fileLength != 0)// lock it down prevent it trying to run with no file
        {
            f.seek(fileLength - 1);//point to the just before eof
            b = f.readByte();// grab the byte at the end of the file
            // now step backward to find a carrage return e.g end of previous row
            while (b != 13 && fileLength > 0)// find carrage return
            {
                fileLength = fileLength - 1;
                f.seek(fileLength);

                b = f.readByte();

            }

            f.setLength(fileLength + 1);// now set file lenght to one more so it skips eof
        }
       
    }

this removes the last entry into the file as it moves the pointer that defines the end of the file, it detects CR ascii 13 place at the end of the previous file

[Rob Spoor]

Henry Wong wrote:

Rob Spoor wrote:How about using $ to indicate the end of the string?

This would work only if the last occurrence is in the last position. It will not work if there is anything else after the last occurrence.

Henry

You're right of course.

[Junilu Lacar]

What I understand is that you'd like to roll back your changes to a text file if something goes wrong with writing to the database, correct? Then why don't you just cut to the chase and work at the file level instead? Either use a temp file for the transaction and only make it "official" when the transaction commits successfully or treat the previous version as a backup and revert to it on rollback. There are other options along this vein but that's the general idea.

[Junilu Lacar]

You can also make the write to the text file contingent on a successful write to the database. I think that would make the logic of ensuring the text file is in synch with the database simpler.

[peter m hayward]

Junilu Lacar wrote:What I understand is that you'd like to roll back your changes to a text file if something goes wrong with writing to the database, correct? Then why don't you just cut to the chase and work at the file level instead? Either use a temp file for the transaction and only make it "official" when the transaction commits successfully or treat the previous version as a backup and revert to it on rollback. There are other options along this vein but that's the general idea.

I posted the code so that others can benefit, the system has to create 3 different thing a mysql entry and two texts file with different layouts, i have now got this to work by just deleting the last entry to the text files and running a mysql update

thanks for trying to help