diff --git a/Package.swift b/Package.swift
index f02ef1828..1047ad7d8 100644
--- a/Package.swift
+++ b/Package.swift
@@ -11,7 +11,11 @@ let availabilityDefinition = PackageDescription.SwiftSetting.unsafeFlags([
     "-Xfrontend",
     "-define-availability",
     "-Xfrontend",
-    "SwiftStdlib 5.8:macOS 9999, iOS 9999, watchOS 9999, tvOS 9999",
+    "SwiftStdlib 5.8:macOS 13.3, iOS 16.4, watchOS 9.4, tvOS 16.4",
+    "-Xfrontend",
+    "-define-availability",
+    "-Xfrontend",
+    "SwiftStdlib 5.9:macOS 9999, iOS 9999, watchOS 9999, tvOS 9999",
 ])
 
 /// Swift settings for building a private stdlib-like module that is to be used
@@ -128,7 +132,8 @@ let package = Package(
                 .product(name: "ArgumentParser", package: "swift-argument-parser"),
                 "_RegexParser",
                 "_StringProcessing"
-            ]),
+            ],
+            swiftSettings: [availabilityDefinition]),
         .executableTarget(
             name: "RegexBenchmark",
             dependencies: [
diff --git a/Sources/RegexTester/RegexTester.swift b/Sources/RegexTester/RegexTester.swift
index 970e47160..052de667f 100644
--- a/Sources/RegexTester/RegexTester.swift
+++ b/Sources/RegexTester/RegexTester.swift
@@ -14,7 +14,7 @@ import _RegexParser
 import _StringProcessing
 
 @main
-@available(macOS 9999, *)
+@available(SwiftStdlib 5.8, *)
 struct RegexTester: ParsableCommand {
   typealias MatchFunctionType = (String) throws -> Regex<AnyRegexOutput>.Match?