How to match string within parentheses (nested) in Java?

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As I said, contrary to popular belief (don’t believe everything people say) matching nested brackets is possible with regex.

The downside of using it is that you can only up to a fixed level of nesting. And for every additional level you wish to support, your regex will be bigger and bigger.

But don’t take my word for it. Let me show you. The regex:

\([^()]*\)

Matches one level. For up to two levels, you’d need:

\(([^()]*|\([^()]*\))*\)

And so on. To keep adding levels, all you have to do is change the middle (second) [^()]* part to ([^()]*|\([^()]*\))* (check three levels here). As I said, it will get bigger and bigger.

Your problem:

For your case, two levels may be enough. So the Java code for it would be:

String fortranCode = "code code u(i, j, k) code code code code u(i, j, k(1)) code code code u(i, j, k(m(2))) should match this last 'u', but it doesnt.";
String regex = "(\\w+)(\\(([^()]*|\\([^()]*\\))*\\))"; // (\w+)(\(([^()]*|\([^()]*\))*\))
System.out.println(fortranCode.replaceAll(regex, "__$1%array$2"));

Input:

code code u(i, j, k) code code code code u(i, j, k(1)) code code code u(i, j, k(m(2))) should match this last 'u', but it doesnt.

Output:

code code __u%array(i, j, k) code code code code __u%array(i, j, k(1)) code code code u(i, j, __k%array(m(2))) should match this last 'u', but it doesnt.

Bottom line:

In the general case, parsers will do a better job – that’s why people get so pissy about it. But for simple applications, regexes can pretty much be enough.

Note: Some flavors of regex support the nesting operator R (Java doesn’t, PCRE engines like PHP and Perl do), which allows you to nest arbitrary number of levels. With them, you could do: \(([^()]|(?R))*\).

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