Why use magic number in the implementation of regex in java?

Question

The source code of constructor of Matcher class:

Matcher(Pattern parent, CharSequence text) {
    this.parentPattern = parent;
    this.text = text;

    // Allocate state storage
    int parentGroupCount = Math.max(parent.capturingGroupCount, 10);
    groups = new int[parentGroupCount * 2];
    locals = new int[parent.localCount];

    // Put fields into initial states
    reset();
}

Why we don't just use parent.capturingGroupCount*2 as the length of groups?

Answer 1

It's probably to make it easier to support backreferences (\0-\9) without having any special cases.