Fix IsMatched to correctly check balancing group state

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/libraries/System.Text.RegularExpressions/src/System/Text/RegularExpressions/Match.cs

@@ -233,10 +233,40 @@ internal void BalanceMatch(int cap)
         internal bool IsMatched(int cap)
         {
             int[] matchcount = _matchcount;
-            return
-                (uint)cap < (uint)matchcount.Length &&
-                matchcount[cap] > 0 &&
-                _matches[cap][matchcount[cap] * 2 - 1] != (-3 + 1);
+            if ((uint)cap >= (uint)matchcount.Length || matchcount[cap] == 0)
+            {
+                return false;
+            }
+
+            // If not balancing, the simple check suffices
+            if (!_balancing)
+            {
+                return _matches[cap][matchcount[cap] * 2 - 1] != (-3 + 1);
+            }
+
+            // When balancing is involved, we need to check if there are any real (non-negative) captures
+            // that would remain after TidyBalancing compacts the captures.
+            // TidyBalancing removes negative (balanced) captures, so we need to count positive ones.
+            int[] matcharray = _matches[cap];
+            int limit = matchcount[cap] * 2;
+            int realCaptureCount = 0;
+
+            for (int i = 0; i < limit; i += 2)
+            {
+                // Check if this is a real capture (start index is non-negative)
+                if (matcharray[i] >= 0)
+                {
+                    realCaptureCount++;
+                }
+                else
+                {
+                    // This is a balancing marker (negative index)
+                    // Balancing markers effectively "remove" a previous capture
+                    realCaptureCount--;
+                }
+            }
+
+            return realCaptureCount > 0;
         }
 
         /// <summary>