Fix startingStackpos tracking for loop in loop

This extends the fix made earlier around proper handling of the backtracking stack in EmitLoop.  If this loop is inside of another loop, then we need to preserve the startingStackpos on the backtracking stack upon successfully completing the loop, as the outer loop might iterate and cause another occurrence of this loop to run, which will then overwrite our single startingStackpos local.  If that iteration backtracks, we then need to be able to pop the previous iterations startingStackpos and restore its value.

src/libraries/System.Text.RegularExpressions/gen/RegexGenerator.Emitter.cs

@@ -4146,8 +4146,10 @@ void EmitLoop(RegexNode node)
                         writer.WriteLine();
 
                         // Store the loop's state
-                        EmitStackPush(iterationMayBeEmpty ?
-                            new[] { startingPos!, iterationCount } :
+                        EmitStackPush(
+                            startingPos is not null && startingStackpos is not null ? new[] { startingPos, startingStackpos, iterationCount } :
+                            startingPos is not null ? new[] { startingPos, iterationCount } :
+                            startingStackpos is not null ? new[] { startingStackpos, iterationCount } :
                             new[] { iterationCount });
 
                         // Skip past the backtracking section
@@ -4158,8 +4160,10 @@ void EmitLoop(RegexNode node)
                         // Emit a backtracking section that restores the loop's state and then jumps to the previous done label
                         string backtrack = ReserveName("LoopBacktrack");
                         MarkLabel(backtrack, emitSemicolon: false);
-                        EmitStackPop(iterationMayBeEmpty ?
-                            new[] { iterationCount, startingPos! } :
+                        EmitStackPop(
+                            startingPos is not null && startingStackpos is not null ? new[] { iterationCount, startingStackpos, startingPos } :
+                            startingPos is not null ? new[] { iterationCount, startingPos } :
+                            startingStackpos is not null ? new[] { iterationCount, startingStackpos } :
                             new[] { iterationCount });
 
                         // We're backtracking.  Check the timeout.

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

@@ -4918,11 +4918,15 @@ void EmitLoop(RegexNode node)
                     if (analysis.IsInLoop(node))
                     {
                         // Store the loop's state
-                        EmitStackResizeIfNeeded(1 + (startingPos is not null ? 1 : 0));
+                        EmitStackResizeIfNeeded(1 + (startingPos is not null ? 1 : 0) + (startingStackpos is not null ? 1 : 0));
                         if (startingPos is not null)
                         {
                             EmitStackPush(() => Ldloc(startingPos));
                         }
+                        if (startingStackpos is not null)
+                        {
+                            EmitStackPush(() => Ldloc(startingStackpos));
+                        }
                         EmitStackPush(() => Ldloc(iterationCount));
 
                         // Skip past the backtracking section
@@ -4941,6 +4945,11 @@ void EmitLoop(RegexNode node)
                         // startingPos = base.runstack[--runstack];
                         EmitStackPop();
                         Stloc(iterationCount);
+                        if (startingStackpos is not null)
+                        {
+                            EmitStackPop();
+                            Stloc(startingStackpos);
+                        }
                         if (startingPos is not null)
                         {
                             EmitStackPop();

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.Match.Tests.cs

@@ -367,6 +367,10 @@ public static IEnumerable<object[]> Match_MemberData()
             yield return (@"(ver\.? |[_ ]+)?\d+(\.\d+){2,3}$", " Ver 2.0", RegexOptions.IgnoreCase, 0, 8, false, "");
             yield return (@"(?:|a)?(?:\b\d){2,}", " a 0", RegexOptions.None, 0, 4, false, "");
             yield return (@"(?:|a)?(\d){2,}", " a00a", RegexOptions.None, 0, 5, true, "a00");
+            yield return (@"^( |  )?((\w\d){3,}){3,}", " 12345678901234567", RegexOptions.None, 0, 18, false, "");
+            yield return (@"^( |  )?((\w\d){3,}){3,}", " 123456789012345678", RegexOptions.None, 0, 19, true, " 123456789012345678");
+            yield return (@"^( |  )?((\w\d){3,}){3,}", "  123456789012345678", RegexOptions.None, 0, 20, true, "  123456789012345678");
+            yield return (@"^( |  )?((\w\d){3,}){3,}", "   123456789012345678", RegexOptions.None, 0, 21, false, "");
 
             // Using beginning/end of string chars \A, \Z: Actual - "\\Aaaa\\w+zzz\\Z"
             yield return (@"\Aaaa\w+zzz\Z", "aaaasdfajsdlfjzzz", RegexOptions.IgnoreCase, 0, 17, true, "aaaasdfajsdlfjzzz");