Issues with regex in java [duplicate]

I have started to learn regex since I have found it very usefull.

I have a task which I am trying to solve but my output fails. Read two files from keyboard, read first file and change all dots to exclamation marks and save the content of first file to second file. My solution:

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String file1 = reader.readLine();
        String file2 = reader.readLine();
        reader.close();

        FileReader fileReader = new FileReader(file1);
        FileWriter fileWriter = new FileWriter(file2);
        BufferedReader bufferedReader = new BufferedReader(fileReader);
        BufferedWriter bufferedWriter = new BufferedWriter(fileWriter);

        String a;
        while ((a = bufferedReader.readLine()) != null) {
            a.replaceAll("\\.","\\!");
            bufferedWriter.write(a);
        }

        bufferedReader.close();
        bufferedWriter.close();
    }
}