Java Regular expression find and update

Question

I need to add couple of import statements in every file in java project. I have written regular expression to perform this activity.

File1.java
/**********************
*
*History Card
*
***********************/
package com.employee.details;

import java.io.BufferedReader;
import java.io.File;

public class File1
{

// java code
}

File2.java

package com.employee.details;
/**********************
*
*History Card
*
***********************/
import java.io.BufferedReader;
import java.io.File;

public class File1
{

// java code
}

Code to Perform the update:

String regEx = "^package .*;";
String pattern = "\0\n\nimport java.io.FileReader;\nimport java.nio.file.Path;";
textFile.replaceAll(regEx,pattern);

it's not working. whats wrong in my code? please help me

Answer 1

The problem is a) the comment at the beginning of the file, b) the greedy quantifier, and c) the missing variable assignment. Probably a removed package declaration as well. You should be using this code:

String regex = "(package .*?;)";
String replacement = "$1\0\n\nimport java.io.FileReader;\nimport java.nio.file.Path;";
textFile = textFile.replaceFirst(regex, replacement);

Strings in Java (in most languages, in fact) are immutable, and thus you can't change every instance of that string at once without a very large amount of code.

Answer 2

Your problem is the java uses the dollar sign flavour of back references, not the backslash flavour.

In your replacement, \\0 is a nul char (hex zero), not group zero. Change it to $0 :

String pattern = "$0\n\nimport java.io.FileReader;\nimport java.nio.file.Path;";
                  ^----- backslash changed to dollar sign

Answer 3

@RamenChef is almost right except that grouping (ie ( and ) ) is not necessary and backreference (ie \\0 ) too. So next code will work:

String regEx = "^package .*;";
String pattern = "$0\n\nimport java.io.FileReader;\nimport java.nio.file.Path;";
textFile = textFile.replaceAll(regEx,pattern);

Note only difference between your code and this is dollar sign before 0 and assigning of replaceAll result to textFile because String type is immutable .