Question

I have the following string examples:

00001 1 12 123
00002 3 7 321
00003 99 23 332
00004 192 50 912

In a separate text file. Numbers are separated by tabs not spaces.

I tried to read the file and print each line if it matches a given RegExp, but I could not find the suitable RegExp for these lines.

private static void readFile() {
    String      fileName = "processes.lst";
    FileReader  file = null;
    String      result = "";


    try {
        file = new FileReader(fileName);
        BufferedReader reader = new BufferedReader(file);

        String line = null;
        String regEx = "[0-9]\t[0-9]\t[0-9]\t[0-9]";
        while((line = reader.readLine()) != null)  {
            if(line.matches(regEx)) {
                result += "\n" + line;
            }
        }
    } catch(Exception e) {
        System.out.println(e.getMessage());
    } finally {
        if(file != null)
            try {
                file.close();
            } catch(Exception e) {
                System.out.println(e.getMessage());
            }
    }

    System.out.println(result);
}

I ended up without any string being printed!!

Answer 1

The problem is that [0-9] matches exactly one digit, but your input often has multiple digits between the TAB characters. You need to use [0-9]+ to match each number. (The + means one or more repetitions of the previous …)

But a simpler solution would be to use String.split(...) … read the javadoc.