Parsing CSV files using Regex in Java

Question

I'm trying to create a program, which reads CSV files from a directory, using a regex it parses each line of the file and displays the lines after matching the regex pattern. For instance if this is the first line of my csv file

1997,Ford,E350,"ac, abs, moon",3000.00

my output should be

1997 Ford E350 ac, abs, moon 3000.00

I don't want to use any existing CSV libraries. I'm not good at regex, I've used a regex I found on net but its not working in my program This is my source code, I'll be grateful if any one tells me where and what I"ve to modify in order to make my code work. Pls explain me.

import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.CharBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.util.regex.Pattern;
import java.util.regex.Matcher;


public class RegexParser {

private static Charset charset = Charset.forName("UTF-8");
private static CharsetDecoder decoder = charset.newDecoder();
String pattern = "\"([^\"]*)\"|(?<=,|^)([^,]*)(?=,|$)";

void regexparser( CharBuffer cb)
{ 
    Pattern linePattern = Pattern.compile(".*\r?\n");
    Pattern csvpat = Pattern.compile(pattern);
    Matcher lm = linePattern.matcher(cb);
    Matcher pm = null;

    while(lm.find())
    {   
        CharSequence cs = lm.group();
        if (pm==null)
            pm = csvpat.matcher(cs);
            else
                pm.reset(cs);
        if(pm.find())
                     {

            System.out.println( cs);
                      }
        if (lm.end() == cb.limit())
        break;

        }

    }

public static void main(String[] args) throws IOException {
    RegexParser rp = new RegexParser();
    String folder = "Desktop/sample";
    File dir = new File(folder);
    File[] files = dir.listFiles();
    for( File entry: files)
    {
        FileInputStream fin = new FileInputStream(entry);
        FileChannel channel = fin.getChannel();
        int cs = (int) channel.size();
        MappedByteBuffer mbb = channel.map(FileChannel.MapMode.READ_ONLY, 0, cs);
        CharBuffer cb = decoder.decode(mbb);
        rp.regexparser(cb);
        fin.close();

    }




}

  }

This is my input file

Year,Make,Model,Description,Price

1997,Ford,E350,"ac, abs, moon",3000.00

1999,Chevy,"Venture ""Extended Edition""","",4900.00

1999,Chevy,"Venture ""Extended Edition, Very Large""","",5000.00

1996,Jeep,Grand Cherokee,"MUST SELL!

air, moon roof, loaded",4799.00

I'm getting the same as output where is the problem in my code? why doesn't my regex have any impact on the code?

"I don't want to use any existing CSV libraries" I suggest you elaborate on this point. Why not? In general Regex is the wrong tool for this job, there are good CSV parsing libraries for this purpose. — EdC
– EdC, Commented Sep 16, 2012 at 10:15
String.split ( docs.oracle.com/javase/1.4.2/docs/api/java/lang/String.html ) won't be enough for you? :) — m4tx
– m4tx, Commented Sep 16, 2012 at 10:17
@EdC By using Regex I'm trying to extend this to other file formats, like a generic reader, by using different Regex I can parse different formats. BTW why Regex is wrong tool for this can you expound more? — Niranjan Subramanian
– Niranjan Subramanian, Commented Sep 16, 2012 at 10:18
@m4tx full CSV support is more complicated than that, you have to deal with quoted content, rows that span multiple lines and escapes. — EdC
– EdC, Commented Sep 16, 2012 at 10:19
@niranjan-subramanian basically, CSV is a more complicated format than it seems, there are lots of edge cases to deal with. Regex is really good for searching strings. While it can be used for parsing, other tools do the job better. Not to put too finer point on it, but those CSV parsing libraries were written for a reason. — EdC
– EdC, Commented Sep 16, 2012 at 10:21

kovica · Accepted Answer · 2012-09-16 10:46:38Z

2

Using regexp seems "fancy", but with CSV files (at least in my opinion) is not worth it. For my parsing I use http://commons.apache.org/csv/. It has never let me down. :)

answered Sep 16, 2012 at 10:46

kovica

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1 Comment

Niranjan Subramanian Over a year ago

Ya I understand, reinventing the wheel isn't a good idea, but at least for learning purpose I need a solution for my program and that's why I've raised my question here.:)

Niranjan Subramanian · Accepted Answer · 2012-09-17 15:00:11Z

1

Anyway I've found the fix myself, thanks guys for your suggestion and help.

This was my initial code

    if(pm.find()
        System.out.println( cs);

Now changed this to

  while(pm.find()
  {
 CharSequence css = pm.group();
 //print css
   }

Also I used a different Regex. I'm getting the desired output now.

answered Sep 17, 2012 at 15:00

Niranjan Subramanian

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2 Comments

Arun Over a year ago

Hey can you please paste the Regex pattern here.

Niranjan Subramanian Over a year ago

\"([^\"]*)\"|(?<=,|^)([^,]*)(?=,|$)

Scis · Accepted Answer · 2012-09-16 10:23:12Z

0

You can try this : [ \t]*+"[^"\r\n]*+"[ \t]*+|[^,\r\n]*+ with this code :

try {
    Pattern regex = Pattern.compile("[ \t]*+\"[^\"\r\n]*+\"[ \t]*+|[^,\r\n]*+", Pattern.CASE_INSENSITIVE | Pattern.UNICODE_CASE | Pattern.MULTILINE);
    Matcher matcher = regex.matcher(subjectString);
    while (matcher.find()) {
        // Do actions
    } 
} catch (PatternSyntaxException ex) {
    // Take care of errors
}

But yeah, if it's not a very critical demand do try to use something that already working : )

answered Sep 16, 2012 at 10:23

Scis

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1 Comment

Niranjan Subramanian Over a year ago

Do I have to insert any escape sequence in your pattern, I'm getting misplaced constructs if I use your pattern

Community · Accepted Answer · 2017-05-23 10:34:53Z

0

Take the advice offered and do not use regular expressions to parse a CSV file. The format is deceptively complicated in the way it can be used.

The following answer contains links to wikipedia and the RFC describing the CSV file format:

field size limitation of csv file

edited May 23, 2017 at 10:34

CommunityBot

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answered Sep 16, 2012 at 21:15

Mark O'Connor

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1 Comment

Niranjan Subramanian Over a year ago

That's all fine, but I just want to know why isn't my code working, I"m not able to crack the wrong part in my code

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