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Parse from end to start of a String to grab data before the third occurrence of a delimiter

September 29, 2022

I am working on some strings and trying to parse through the data and retrieve a string that lies before the third occurrence of " – " from the end of the string. This data comes as a String from the DB and there is some text "-NONE—-" that I would like to exclude while parsing.

Input (Below input is a String and not List)

String input1 = "-A123456-B987-013691-000-109264821"
String input2 = "-NONE----"
String input3 = "C1234567-A1241-EF-012361-000-18273460"

Output

String output1 = "-A123456-B987"
String output2= "-NONE----"
String output3 = "C1234567-A1241-EF"

Starting from the beginning of my string, I need to retrieve data before the third occurrence of
" – " (hyphen) is found, but I need to count " – " (hyphen) occurrence starting from end of string.

Any tips are appreciated.

>Solution :

You could match the three dashes from behind with the $ symbol and then extract everything that is in front of that. I created two capture groups, where the first one is what you want to extract:

private static String extractFront(String input1) {
    Pattern pattern = Pattern.compile("(.*)(-[^-]*){3}$");
    Matcher matcher = pattern.matcher(input1);
    if (matcher.find()) {
        return matcher.group(1);
    }

    return null;
}

Main to test:

public static void main(String[] args) {
    String input1 = "-A123456-B987-013691-000-109264821";
    String input2 = "-NONE----";
    String input3 = "C1234567-A1241-EF-012361-000-18273460";

    System.out.println(extractFront(input1));
    System.out.println(extractFront(input2));
    System.out.println(extractFront(input3));
}

Output: