Question

How would I parse the two numbers in the following string:

String fName = "Run_1_vs_2_pw_optimal_mapping.txt";

I tried it like this, but it doesn’t work:

    Pattern filePatt = Pattern.compile("Run_(\\d+)_vs_(\\d+)_", Pattern.CASE_INSENSITIVE);

    Matcher scanner = this.filePatt.matcher(fName);
    while (scanner.find()) {
            int groupSize = scanner.groupCount();
            if (groupSize == 2) {
                firstRun = Integer.parseInt(scanner.group(0));
                secondRun = Integer.parseInt(scanner.group(1));
            }
            break;
     }

However, this doesn’t work, because scanner.group(0) returns Run_1_vs_2. But why?

Answer 1

See the documentation.

Capturing groups are indexed from left to right, starting at one. Group zero denotes the entire pattern, so the expression m.group(0) is equivalent to m.group().

Use group(1) and group(2).