using System;
using System.Text.RegularExpressions;
 
public class Example
{
   public static void Main()
   {
      var re = @"(?x)  # ignore spaces and comments
(?=
  (
    \(                 # first (
    (?:
      (?<open> \( )*   # open++
      [^()]+
      (?<-open> \) )*  # open--
    )+
    \)                 # last )
    (?(open)(?!))      # fail if unblanaced: open > 0
  )
)
\(                     # eat a (, to advance the match a char";
 
      var str = "a + ((b + (c + d)) + (e + f)) + (x + ((y) + (z)) + x)";
 
      var m = Regex.Matches(str, re);
 
      Console.WriteLine("Matched: ");
      foreach (Match i in m)
        Console.WriteLine(i.Groups[1]);
   }
}

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