Regex for getting parentheses within parentheses infinitely

Question

So I have a String of text lets say "hello world () (foo bar) (foo bar 2 (this looks cozy)) (foo bar 3..." Is there a regex pattern I could use that will get the parentheses and include any parentheses inside them to nth depth. So the matches would be "()", "(foo bar)", "(foo bar 2 (this looks cozy))", ...?

Answer 1

Regex flavor in Java doesn't support recursion like some other flavors do. Instead you can write your own method which will build strings from characters only if they are:

• (
• )
• inside parenthesis.

To know if currently handled character is inside parenthesis we can create counter which will check parenthesis balance (you can also think of it as counter for nesting level). In short: if we saw more ( than ) then we are inside unclosed (open) parenthesis section, so we should add current character to resulting string.

Using that idea our code can look like:

String str = "hello world () (foo bar) (foo bar 2 (this looks cozy)) (foo bar 3...)";
List<String> result = new ArrayList<>();

int parenthesisNestingLevel = 0;
StringBuilder sb = new StringBuilder();
for (char ch : str.toCharArray()) {
    if (ch == '(') {
        parenthesisNestingLevel++;
        sb.append(ch);
    } else if (ch == ')') {
        parenthesisNestingLevel--;
        sb.append(ch);
        if (parenthesisNestingLevel == 0) {
            result.add(sb.toString());
            sb.delete(0, sb.length());//reset sb
        }
    } else if (parenthesisNestingLevel > 0) {//we are inside unclosed parenthesis
        sb.append(ch);
    }
}

result.forEach(System.out::println);

Output:

()
(foo bar)
(foo bar 2 (this looks cozy))
(foo bar 3...)