Fix regex test OOM on x86 checked coreclr

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

@@ -2713,7 +2713,7 @@ public async Task StressTestDeepNestingOfLoops(RegexEngine engine, string begin,
             }
         }
 
-        [ConditionalTheory]
+        [ConditionalTheory(typeof(PlatformDetection), nameof(PlatformDetection.Is64BitProcess))] // deep nesting exhausts address space on 32-bit
         [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "Fix is not available on .NET Framework")]
         [MemberData(nameof(RegexHelpers.AvailableEngines_MemberData), MemberType = typeof(RegexHelpers))]
         public async Task CharClassSubtraction_DeepNesting_DoesNotStackOverflow(RegexEngine engine)

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/RegexGeneratorHelper.netcoreapp.cs

@@ -146,7 +146,7 @@ internal static async Task<Regex[]> SourceGenRegexAsync(
             (string pattern, CultureInfo? culture, RegexOptions? options, TimeSpan? matchTimeout)[] regexes, CancellationToken cancellationToken = default)
         {
             // Un-ifdef to compile each regex individually, which can be useful if one regex among thousands is causing a failure.
-            // We compile them all en mass for test efficiency, but it can make it harder to debug a compilation failure in one of them.
+            // We compile them all en masse for test efficiency, but it can make it harder to debug a compilation failure in one of them.
 #if false
             if (regexes.Length > 1)
             {
@@ -159,6 +159,20 @@ internal static async Task<Regex[]> SourceGenRegexAsync(
             }
 #endif
 
+            // On 32-bit processes the ~2GB address space limit can cause OutOfMemoryException
+            // when Roslyn compiles thousands of patterns at once. Chunk into smaller batches
+            // on 32-bit to avoid OOM while keeping full batching on 64-bit.
+            const int MaxBatchSize = 200;
+            if (!Environment.Is64BitProcess && regexes.Length > MaxBatchSize)
+            {
+                var batchResults = new List<Regex>(regexes.Length);
+                foreach (var chunk in regexes.Chunk(MaxBatchSize))
+                {
+                    batchResults.AddRange(await SourceGenRegexAsync(chunk, cancellationToken));
+                }
+                return batchResults.ToArray();
+            }
+
             Debug.Assert(regexes.Length > 0);
 
             var code = new StringBuilder();