What are regular expression Balancing Groups?

Question

I was just reading a question about how to get data inside double curly braces (this question), and then someone brought up balancing groups. I\'m still not quite sure what

Answer 1

Just a small addition to M. Buettner's excellent answer:

What's the deal with the (?) syntax?

(?x) is subtly different from (?<-A>(?x)). They result in the same control flow^*, but they capture differently.
For example, let's look at a pattern for balanced braces:

(?:[^{}]|(?{)|(?<-B>}))+(?(B)(?!))

At the end of the match we do have a balanced string, but that is all we have - we don't know where the braces are because the B stack is empty. The hard work the engine did for us is gone.
_{(example on Regex Storm)}

(?x) is the solution for that problem. How? It doesn't capture x into $A: it captures the content between the previous capture of B and the current position.

Let's use it in our pattern:

(?:[^{}]|(?{)|(?}))+(?(Open)(?!))

This would capture into $Content the strings between the braces (and their positions), for each pair along the way.
For the string {1 2 {3} {4 5 {6}} 7} there'd be four captures: 3, 6 ,4 5 {6} , and 1 2 {3} {4 5 {6}} 7 - much better than nothing or } } } }.
_{(example - click the table tab and look at ${Content}, captures)}

In fact, it can be used without balancing at all: (?).(.(?).) captures the first two characters, even though they are separated by groups.
(a lookahead is more commonly used here but it doesn't always scale: it may duplicate your logic.)

(?) is a strong feature - it gives you exact control over your captures. Keep that in mind when you're trying to get more out of your pattern.