Balancing groups in variable-length lookbehind [duplicate]

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I think I got it.
First, as I mentioned in one of the comments, (?<=(?<A>.)(?<-A>.)) never matches.
But then I thought, what about (?<=(?<-A>.)(?<A>.))? It does match!
And how about (?<=(?<A>.)(?<A>.))? Matched against "12", A is captures "1", and if we look at the Captures collection, it is {"2", "1"} – first two, then one – it is reversed.
So, while inside a lookbehind, .net matches and captures from the right to the left.

Now, how can we make it capture from left to right? This is quite simple, really – we can trick the engine using a lookahead:

(?<=(?=(?<A>.)(?<A>.))..)

Applied to your original patten, the simplest option I came up with was:

(?<=
    ~[(]
    (?=
        (?:
            [^()]
            |
            (?<Depth>[(])
            |
            (?<-Depth>[)])
        )*
        (?<=(\k<Prefix>))   # Make sure we matched until the current position
    )
    (?<Prefix>.*)           # This is captured BEFORE getting to the lookahead
)
[a-z]

The challenge here was that now the balanced part may end anywhere, so we make it reach all the way to the current position (Something like \G or \Z would be useful here, but I don’t think .net has that)

It is very possible this behavior is documented somewhere, I’ll try to look it up.

Here’s another approach. The idea is simple – .net wants to match from right to left? Fine! Take that:
(tip: start reading from the bottom – that is how .net does it)

(?<=
    (?(Depth)(?!))  # 4. Finally, make sure there are no extra closed parentheses.
    ~\(
    (?>                     # (non backtracking)
        [^()]               # 3. Allow any other character
        |
        \( (?<-Depth>)?     # 2. When seeing an open paren, decreace depth.
                            #    Also allow excess parentheses: '~((((((a' is OK.
        |
        (?<Depth>  \) )     # 1. When seeing a closed paren, add to depth.
    )*
)
\w                          # Match your letter

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