How to delete the specific lines that starts and ends with specific string in a file in java?

Question

I have a file abc.txt that has lines as

abc.txt

Ethernet 1/1 

Ethernet 1/2

interface 3

abs_mod_
jjj
kkkk
ll
abs_mod_

interface 6

interface 7
         


        

Answer 1

A naive approach:

package test;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public class ReadStringFromFileLineByLine {

    public static void main(String[] args) {
        try {
            File file = new File("abc.txt");
            FileReader fileReader = new FileReader(file);
            BufferedReader bufferedReader = new BufferedReader(fileReader);
            StringBuffer stringBuffer = new StringBuffer();
            String line;
            boolean flag = true;
            while ((line = bufferedReader.readLine()) != null) {
                if(line.contains("abs_mod_")){
                    if(flag)    flag = false;
                    else        flag = true;
                }else if(flag){
                    stringBuffer.append(line);
                    stringBuffer.append("\n");
                }
            }
            fileReader.close();
            System.out.println("Contents of file:");
            System.out.println(stringBuffer.toString().replace("\n\n\n", "\n\n"));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Using regex:

package test;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.IOException;

public class ReadStringFromFileLineByLine {

    public static void main(String[] args) {
        try {
            File file = new File("abc.txt");
            FileReader fileReader = new FileReader(file);
            BufferedReader bufferedReader = new BufferedReader(fileReader);
            StringBuffer stringBuffer = new StringBuffer();
            String line;
            while ((line = bufferedReader.readLine()) != null) {
                stringBuffer.append(line);
                stringBuffer.append("\n");
            }
            fileReader.close();
            System.out.println("Contents of file:");
            System.out.println(
                stringBuffer
                .toString()
                .trim()
                .replaceAll("[\\n]+(abs_mod_)(\\s|\\S)*(abs_mod_)", "")
            );
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Answer 2

Load the entire file into memory, then use regex to remove the lines you don't want.

Using Java 11+

String text = Files.readString(Paths.get("abc.txt"));
text = text.replaceAll("(?sm)^abs_mod_(?:.*?^abs_mod_)?.*?\\R", "");
System.out.println(text);

Using Java 7+

String text = new String(Files.readAllBytes(Paths.get("abc.txt")), StandardCharsets.UTF_8);
text = text.replaceAll("(?sm)^abs_mod_(?:.*?^abs_mod_)?.*?\\R", "");
System.out.println(text);

Output

Ethernet 1/1 

Ethernet 1/2

interface 3


interface 6

interface 7

Explanation

(?              Set flags:
  s               DOTALL     '.' matches any character, including a line terminator
  m               MULTILINE  '^' and '$' match just after/before a line terminator
)
^abs_mod_       Match 'abs_mod_' at beginning of line
(?:             Start optional non-capturing group
  .*?             Match any text (including line terminators) until:
  ^abs_mod_         Match 'abs_mod_' at beginning of line
)?              End optional section
.*?             Match any text up to:
\R                Match line terminator

Both .* have the extra ? making them "reluctant", so they don't cross the "ending" match. The . in the second .*? won't actually match a line terminator since the ending match is a line terminator.

The optional section is because you said: "delete the lines in between abs_mod_ and also lines starting with abs_mod_"

The regex is really these two blended together:

(?sm)^abs_mod_.*?^abs_mod_.*?\R   Lines between lines starting with 'abs_mod_' (inclusive)
(?m:^)abs_mod_.*\R                Single line starting with 'abs_mod_'